CENTRO DE INVESTIGACIN EN MATERIALES AVANZADOS S.C.

POSGRADO

Evaluacin de la eficiencia de una celda solar sensibilizada por colorante mediante el modelado molecular y la teora de funcionales de la densidad

ARTCULOS PUBLICADOS QUE PARA OBTENER EL GRADO DE DOCTOR EN CIENCIA DE MATERIALES

Presenta: Jess Adrin Baldenebro Lpez

DIRECTOR DE TESIS: Dr. Daniel Glossman Mitnik

Dr. Jos Castorena Gonzlez

CHIHUAHUA, CHIH. JUNIO, 2014

RESUMEN

Colorantes orgnicos basados en trifenilamina y complejos de cobre con ligandos

de bipiridina y biquinolina han sido analizados y discutidos desde el punto de vista

tericos a travs de la qumica computacional y la teora de funcionales de la

densidad (DFT). Diversos niveles de clculo han sido utilizados con la finalidad de

establecer una metodologa computacional confiable para el clculo de las

propiedades de inters tales como la estructura molecular, espectro de infrarrojo,

orbitales moleculares y sus niveles de energa, sitios y parmetros de reactividad

qumica a travs de DFT conceptual; tambin se incluye la prediccin de espectros

de absorcin y espectros de emisin con DFT dependiente del tiempo (TD-DFT).

Los sistemas moleculares son propuestos con potencial aplicacin en la fotovoltaica

y principalmente en celdas solares sensibilizadas por colorante (DSSC). Los

resultados abren la posibilidad de una proyeccin computacional de diversas

predicciones en la estructura electrnica y la respuesta ptica; pavimentando as el

camino a una ingeniera molecular eficaz de una mayor cantidad de sensibilizadores

reforzando las aplicaciones en celdas solares. Interesante trabajo para los

experimentalistas en el campo de las DSSC.

AGRADECIMIENTOS

Es importante mencionar la suerte que he tenido de trabajar con un conjunto de

personas a lo largo de mis estudios de doctorado, ya que no solo he podido aprender

a travs de ellos un conocimiento cientfico si no tambin moral y humano.

No puedo ni debo dejar pasar esta oportunidad para agradecer a mi familia todo su

apoyo durante este tiempo. A mi esposa Perla e hijos Esteban, Adriana y Elma

(tambin los llamo mis amores), quienes han sabido entender mis necesidades y

apoyado hasta el cansancio con su nimo. En la misma magnitud de importancia a

mis padres, Esteban Baldenebro y Elma Lpez, a quien debo absolutamente todo,

gracias por todo su cario y amor que siempre me han proporcionado; a mis

hermanos: Mirna, Francisco y Karely; por sus palabras de confianza y amor que en

todo momento me han expresado y que son por mi parte bien correspondidos.

Quiero mencionar de una forma muy especial a mis amigos y directores de tesis,

los doctores Daniel Glossman y Jos Castorena. Muchas gracias a ambos por sus

consejos e indicaciones en el trayecto de este trabajo de investigacin, adems del

entusiasmo que en todo momento han aportado y sabido transmitir a mi persona.

Cada uno de ellos ha contribuido de la mejor manera posible desde su rea de

conocimiento a la realizacin de esta tesis.

El agradecimiento para mi comit de sinodales, por sus valiosas aportaciones en la

revisin y evaluacin de mi trabajo, siempre sealando con mucha crtica

constructiva sus observaciones al respecto: Dra. Norma Flores, Dra. Luz Mara

Rodrguez, Dr Erasmo Orrantia y Dr. Francisco Espinosa.

A mis amigos y compaeros del tan querido NANOCOSMOS: Kathy, Nora, Linda, y

Rody, por su optimismo y comprensin, adems de siempre tener disponibilidad

para ayudarme en todo momento durante esta ardua tarea que representa realizar

estudios de posgrado.

INTRODUCCIN

Las fuentes convencionales de energa en el mundo (petrleo, gas natural y carbn)

tienen una vida finita y las previsiones actuales indican que las alternativas deben

aportar una contribucin importante en el futuro cercano. La energa solar es uno

de los recursos energticos ms prometedores del futuro. La conversin directa de

luz solar en energa elctrica mediante el uso de celdas solares es de particular

inters ya que se presentan muchas ventajas sobre otros mtodos de generacin

de energa elctrica, adems de evitar el escape de gases de efecto invernadero y

subproductos de residuos nucleares. Las celdas solares sensibilizadas por

colorante (DSSC) han atrado considerablemente la atencin, debido a su eficiencia,

proceso de fabricacin sencillo y bajo costo de produccin. En estas DSSC, un

sensibilizador debe ser qumicamente absorbido en la superficie porosa del xido

nanoestructurado. Tras la absorcin de un fotn, el electrn excitado en la molcula

de sensibilizador se transfiere a la banda de conduccin del xido nanoestructurado,

y esto se produce en un lapso de unos cientos de femtosegundos, seguido de un

proceso en el que el electrn se difunde a travs de la red porosa hasta llegar a un

electrodo. El sensibilizador en su estado oxidado, se reduce al estado original

mediante el suministro de electrones a travs de un electrolito lquido dentro de los

poros. En la actualidad, muchos grupos de investigadores de todo el mundo

participan activamente en la mejora de la eficiencia de cada uno de los procesos

involucrados en las DSSC. La eficiencia en la transferencia de carga de la molcula

de colorante al xido nanoestructurado es de vital importancia en el diseo de la

celda solar. Desde que Regal y Grtzel reportaron en su trabajo pionero, el

entendimiento con xito de este mecanismo ha requerido una investigacin

fundamental sobre los distintos fenmenos fsicos en escalas nanomtricas.

Investigaciones tericas de las propiedades fsicas y qumicas de los colorantes

sensibilizadores son muy importantes para conocer la relacin entre la estructura,

propiedades, desempeo y ayudar en el diseo y sntesis de nuevos colorantes

sensibilizadores.

La optimizacin de dispositivos como las celdas solares sensibilizadas por colorante

requiere un gran esfuerzo dirigido al desarrollo de nuevos diseos y a la elucidacin

de los procesos fotofsicos que producen la respuesta fotovoltaica. En la actualidad,

se realizan un gran nmero de investigaciones en el campo de este tipo de celdas,

tratando de llevar a cabo un mejor diseo que permita elevar la eficiencia. Sin

embargo, se han encontrado con la problemtica de no poder elevar dicho concepto

y para ese momento ya se ha invertido una cantidad importante de recurso

econmico, humano y de tiempo. Lo que nosotros planteamos en esta investigacin,

fue aplicar el modelado molecular con la finalidad de predecir o inferir tericamente

la eficiencia de una DSSC o establecer los parmetros ms relevantes que influyen

en su valor.

Las herramientas que proporciona la qumica computacional y el modelado

molecular, adems del creciente aumento en las capacidades de los equipos de

cmputo, permiten el desarrollo de objetivos que son cada vez ms demandados

por nuestra sociedad. Estas herramientas sern aplicadas al estudio y

caracterizacin por mtodos de qumica computacional de sensibilizadores

orgnicos y compuestos de coordinacin de complejos de cobre con ligandos de

tipo polipiridina, cuyos resultados obtenidos nos servirn para hacer la comparativa

con los datos experimentales y poder encontrar el concepto que permita evaluar o

inferir la eficiencia de una DSSC.

LISTA DE ARTCULOS

Baldenebro Lpez, J., Castorena Gonzlez, J., Flores Holgun, N., Caldern

Guilln, J., & Glossman Mitnik, D. (2012). Computational characterization of

the molecular structure and properties of Dye 7 for organic photovoltaics.

Journal of Molecular Modeling, 835-842.

Baldenebro Lpez, J., Castorena Gonzlez, J., Flores-Holgun, N., Almaral

Snchez, J., & Glossman Mitnik, D. (2012). Density Functional Theory

(DFT) Study of Triphenylamine-Based Dyes for Their Use as Sensitizers in

Molecular Photovoltaics. International Journal of Molecular Sciences, 4418-

4432.

Baldenebro Lpez, J., Castorena Gonzlez, J., Flores Holgun, N., Almaral

Snchez, J., & Glossman Mitnik, D. (2012). Computational Molecular

Nanoscience Study of the Properties of Copper Complexes for Dye-

Sensitized Solar Cells. International Journal of Molecular Sciences, 16005-

16019.

Baldenebro Lpez, J., Flores Holgun, N., Castorena Gonzlez, J., Almaral

Snchez, J., & Glossman Mitnik, D. (2013). Theoretical Study of Copper

Complexes: Molecular Structure, Properties, and Its Application to Solar

Cells. International Journal of Photoenergy, 1-7.

Baldenebro Lpez, J., Flores Holgun, N., Castorena Gonzlez, J., & Glossman

Mitnik, D. (2013). Molecular design of copper complexes as sensitizers for

efficient dye-sensitized solar cells. Journal of Photochemistry and

Photobiology A: Chemistry, 1-5.

Baldenebro Lpez, J., Castorena Gonzlez, J., Flores Holgun, N., & Glossman

Mitnik, D. (2014). Quantum chemical study of a new class of sensitisers:

influence of the substitution of aromatic rings on the properties of copper

complexes. Molecular Physics, 987-994.

Baldenebro Lpez, J., Flores Holgun, N., Castorena Gonzlez, J., & Glossman

Mitnik, D. (2014). Comparative study of copper complexes with different

anchoring groups by molecular modeling and its application to dye-

sensitized solar cells. Polyhedron, 33-36.

ORIGINAL PAPER

Computational characterization of the molecular structureand properties of Dye 7 for organic photovoltaics

Jess Baldenebro-Lpez & Jos Castorena-Gonzlez &Norma Flores-Holguin & Joel Caldern-Guilln &Daniel Glossman-Mitnik

Received: 6 February 2011 /Accepted: 5 May 2011 /Published online: 20 May 2011# Springer-Verlag 2011

Abstract Organic dyes have great potential for its use insolar cells. In this recent work, the molecular structure andproperties of Dye 7 were obtained using density functionaltheory (DFT) and different levels of calculation. Uponcomparing the molecular structure and the ultravioletvisible spectrum with experimental data reported in theliterature, it was found that the M05-2X/6-31G(d) level ofcalculation gave the best approximation. Once the appro-priate methodology had been obtained, the molecule wascharacterized by obtaining the infrared spectrum, dipolemoment, total energy, isotropic polarizability, molecularorbital energies, free energy of solvation in differentsolvents, and the chemical reactivity sites using thecondensed Fukui functions.

Keywords Molecular structure . Polarizability .G(solv) .Chemical reactivity

AbbreviationsDFT Density functional theoryTD-DFT Time-dependent density functional theoryIR InfraredUV-vis Ultraviolet Angstrom

max Wavelength of maximum absorptionTHF TetrahydrofuranHOMO Highest occupied molecular orbitalLUMO Lowest unoccupied molecular orbitalG(solv) Free energy of solvationIEF-PCM Integral equation formalism of the

polarized continuum model

Introduction

Photovoltaic devices have gained wide acceptance as aclean and renewable energy source [1]. These devices arebased on the concept of charge separation at an interfacebetween two materials with different conduction mecha-nisms [2, 3]. One important invention in this field is thephotovoltaic dye-sensitized solar cell (DSSC) [4], whichhas been the subject of intense research due to its ability toconvert solar energy into electrical energy [5, 6], as well asits low cost compared to solar cells that use polycrystallinesilicon [7]. There are four main factors that affect theperformance of a DSSC: the photosensitive dye, the anode,the cathode and the electrolyte. The dye plays a crucial rolein enhancing the efficiency of the cell, which is why it isone of the most intensely studied factors [8]. In the presentwork, a theoretical study of the molecular structure andproperties of a dye (Dye 7) was performed. This dyeconsists of a triphenylamine molecule that serves aselectron donor group [9, 10], a thiophene to adjust theabsorption spectra of the molecules [11], and a cyanoacrylicacid that acts as an acceptor group[12], as shown in Fig. 1.In the investigation described below, different levels oftheory were used in order to establish the most appropriatemethodology to study this dye. Besides optimizing the

J. Baldenebro-Lpez : J. Castorena-Gonzlez :J. Caldern-GuillnFacultad de Ingeniera Mochis, Universidad Autnoma deSinaloa. Prol. ngel Flores y Fuente de Poseidn, S.N,C.P. 81223 Los Mochis, Sinaloa, Mxico

N. Flores-Holguin :D. Glossman-Mitnik (*)Centro de Investigacin en Materiales Avanzados, SC,Complejo Industrial Chihuahua,Miguel de Cervantes 120,Chihuahua 31109, Mxicoe-mail: daniel.glossman@cimav.edu.mx

J Mol Model (2012) 18:835842DOI 10.1007/s00894-011-1120-6

geometry of Dye 7, the infrared and ultraviolet spectra werepresented, the dipole moment calculated, as were the totalenergy, isotropic polarizability, molecular orbital energies,and the free energy of solvation in different solvents;. Itschemically reactive sites were also discerned using thecondensed Fukui functions.

Molecular modeling

Density functional theory (DFT) was used in this study[13]. DFT was developed by Walter Kohn in the 1960s, andwas implemented in this study using the commercialsoftware Gaussian 03W [14]. The geometry of the moleculein its fundamental state was obtained by the establishedtechnique in Gaussian 03W. The force constants andvibrational frequencies were determined by calculatinganalytical frequencies for stationary points obtained afteroptimizing the geometry. Both calculations were done at thesame level of theory. The basis sets used in this study were3-21G(d) and 6-31G(d) (for more details, see [15]). Thedensity functionals used in this research were: BLYP [16,18], B3LYP [1619], PBE [20, 21], PBE1PBE [21], TPSS[22], TPSSh [23] and M05-2X [24]. A detailed descriptionof these density functionals can be found in the updatedbibliography of computational chemistry [2528]. Thecalculation of the ultraviolet spectrum of the moleculeDye 7 was done via time-dependent DFT equationsaccording to the method implemented in the Gaussianmolecular package 03W [25, 2932]. The equations weresolved for 20 excited states. The infrared (IR) andultraviolet-visible (UV-vis) spectra were analyzed andvisualized using the program SWizard [33]. In all cases,

the displayed spectra show the calculated frequencies andabsorption wavelengths.

The condensed Fukui functions were calculated usingAOMIX molecular analysis software [34, 35], starting fromsingle-point energy calculations.

Results and discussion

The molecular structure of Dye 7 was analyzed at differentlevels of theory, as mentioned above. The bond lengthsobtained from our calculations as well as experimental datareported in the literature on the systems that comprise ourdye are shown in Table 1 for the most representative bonds.It is clear that there is good agreement among the resultsobtained for different models for each bond. However, tocheck which methodology gives the most accurate resultsfor the molecular structure of Dye 7, a statistical techniqueknown as population standard deviation was applied(PSTD) to the results for the bond lengths. When applyingthis technique, the experimental result was used as areference for the average value; in this way, the modelsthat give the lowest deviation will give the best representationsof our study system. It is important to note that we have notconsidered a tolerance level for the deviation, as our prioritywas to establish the model that best fits the experimentalresults. The results from the PSTD are shown in Table 2,which indicates that the most accurate methodology for thetheoretical study is M05-2X/6-31G(d). The interatomic bondlengths () and the angles (in degrees) calculated at thislevel of calculation are shown in Fig. 2.

A second validation of the models involved comparingthe theoretical wavelengths of maximum absorption (max)

Fig. 1 Molecular structure of Dye 7: triphenylamine (donor), a thiophene (to adjust the absorption spectra of the molecules) and cyanoacrylic acid(acceptor)

836 J Mol Model (2012) 18:835842

Table 1 Bond lengths calculated at different levels of theory for Dye 7, and well as experimental data reported in the literature

Model C1C2 C3C4 C4C5 C1C6 C1H7 C13H12 C13C14 C16C19 C15C17 C23C24 C24C26

BLYP/3-21G* 1.415 1.404 1.408 1.404 1.089 1.089 1.415 1.404 1.408 1.421 1.395

B3LYP/3-21G* 1.403 1.394 1.397 1.394 1.082 1.082 1.403 1.394 1.397 1.408 1.387

PBE/3-21G* 1.412 1.401 1.406 1.401 1.092 1.092 1.412 1.402 1.406 1.418 1.393

PBE1PBE/3-21G* 1.400 1.391 1.395 1.391 1.084 1.084 1.400 1.391 1.395 1.405 1.385

TPSS/3-21G* 1.410 1.401 1.405 1.401 1.087 1.087 1.410 1.401 1.405 1.416 1.393

TPSSh/3-21G* 1.406 1.397 1.401 1.397 1.084 1.084 1.406 1.397 1.401 1.411 1.389

M05-2X/3-21G* 1.397 1.390 1.393 1.390 1.079 1.079 1.397 1.390 1.393 1.400 1.386

BLYP/6-31G(d) 1.414 1.404 1.407 1.403 1.092 1.092 1.414 1.403 1.407 1.420 1.395

B3LYP/6-31G(d) 1.403 1.394 1.397 1.394 1.085 1.085 1.403 1.394 1.396 1.408 1.387

PBE/6-31G(d) 1.410 1.400 1.403 1.399 1.094 1.094 1.410 1.400 1.403 1.416 1.392

PBE1PBE/6-31G(d) 1.399 1.390 1.393 1.390 1.086 1.086 1.399 1.390 1.393 1.403 1.384

TPSS/6-31G(d) 1.408 1.399 1.402 1.399 1.088 1.088 1.408 1.399 1.402 1.414 1.391

TPSSh/6-31G(d) 1.404 1.395 1.398 1.395 1.086 1.086 1.404 1.395 1.398 1.409 1.388

M05-2X/6-31G(d) 1.397 1.390 1.392 1.390 1.082 1.082 1.397 1.390 1.392 1.400 1.385

Experimental 1.397 1.397 1.397 1.397 1.084 1.084 1.397 1.397 1.397 1.397 1.397

Model C26-C30 C24-H27 C2-N33 C30-C34 C34-C35 C34-H36 C35-C37 C37-C38 C37-S39 C38-C40 C38-H41

BLYP/3-21G* 1.425 1.089 1.448 1.459 1.371 1.095 1.444 1.412 1.764 1.409 1.088

B3LYP/3-21G* 1.411 1.082 1.434 1.456 1.355 1.088 1.442 1.397 1.741 1.404 1.080

PBE/3-21G* 1.422 1.091 1.436 1.454 1.369 1.098 1.440 1.412 1.748 1.405 1.090

PBE1PBE/3-21G* 1.407 1.083 1.424 1.452 1.352 1.089 1.440 1.395 1.726 1.401 1.081

TPSS/3-21G* 1.421 1.087 1.440 1.455 1.368 1.093 1.441 1.410 1.750 1.405 1.085

TPSSh/3-21G* 1.415 1.084 1.434 1.454 1.361 1.090 1.441 1.403 1.740 1.404 1.082

M05-2X/3-21G* 1.402 1.079 1.426 1.462 1.343 1.084 1.450 1.386 1.721 1.409 1.076

BLYP/6-31G(d) 1.423 1.092 1.439 1.455 1.374 1.097 1.443 1.410 1.766 1.405 1.091

B3LYP/6-31G(d) 1.410 1.085 1.427 1.453 1.358 1.089 1.442 1.396 1.743 1.400 1.084

PBE/6-31G(d) 1.419 1.093 1.428 1.449 1.370 1.098 1.438 1.408 1.748 1.400 1.092

PBE1PBE/6-31G(d) 1.405 1.085 1.417 1.449 1.353 1.090 1.439 1.392 1.727 1.397 1.084

TPSS/6-31G(d) 1.418 1.088 1.431 1.451 1.369 1.092 1.439 1.406 1.748 1.400 1.087

TPSSh/6-31G(d) 1.412 1.085 1.426 1.451 1.362 1.089 1.440 1.400 1.739 1.398 1.084

M05-2X/6-31G(d) 1.401 1.082 1.418 1.459 1.346 1.086 1.450 1.386 1.723 1.404 1.080

Experimental 1.397 1.084 1.418 1.475 1.334 1.099 1.475 1.370 1.714 1.423 1.079

Model C42-S39 C40-C42 C35-H44 C42-C45 C45-C46 C46-C48 C46-C49 C48-N50 C49-O51 C49-052 O52-53H

BLYP/3-21G* 1.781 1.412 1.094 1.425 1.383 1.417 1.488 1.182 1.244 1.400 1.010

B3LYP/3-21G* 1.758 1.397 1.086 1.421 1.367 1.411 1.477 1.168 1.231 1.376 0.996

PBE/3-21G* 1.763 1.411 1.096 1.422 1.380 1.412 1.481 1.183 1.243 1.390 1.008

PBE1PBE/3-21G* 1.742 1.395 1.087 1.419 1.363 1.408 1.472 1.167 1.228 1.366 0.992

TPSS/3-21G* 1.765 1.409 1.091 1.422 1.379 1.413 1.480 1.181 1.243 1.394 1.005

TPSSh/3-21G* 1.756 1.403 1.088 1.421 1.372 1.411 1.476 1.175 1.237 1.383 0.999

M05-2X/3-21G* 1.735 1.387 1.082 1.428 1.353 1.413 1.473 1.159 1.224 1.366 0.991

BLYP/6-31G(d) 1.784 1.410 1.095 1.426 1.389 1.427 1.492 1.179 1.230 1.375 0.986

B3LYP/6-31G(d) 1.761 1.396 1.088 1.423 1.372 1.424 1.483 1.165 1.216 1.353 0.975

PBE/6-31G(d) 1.765 1.408 1.097 1.422 1.385 1.422 1.486 1.179 1.227 1.364 0.985

PBE1PBE/6-31G(d) 1.743 1.393 1.088 1.421 1.367 1.420 1.479 1.164 1.212 1.343 0.972

TPSS/6-31G(d) 1.765 1.406 1.090 1.423 1.384 1.423 1.484 1.176 1.226 1.366 0.983

TPSSh/6-31G(d) 1.756 1.400 1.088 1.422 1.376 1.422 1.481 1.170 1.220 1.356 0.978

M05-2X/6-31G(d) 1.739 1.387 1.084 1.430 1.358 1.428 1.482 1.157 1.209 1.342 0.971

Experimental 1.714 1.370 1.099 1.475 1.334 1.475 1.475 1.172 1.200 1.334 0.970

J Mol Model (2012) 18:835842 837

for Dye 7 obtained using the models with the valueobtained experimentally in research conducted by Zhanget al. [4]. In their research, they found that max occurred at432 nm in the solvent tetrahydrofuran (THF). Knowing thisfact, it was possible to re-validate which of the models bestrepresented our molecular system. Thus, the UV-visspectrum of Dye 7 in THF solvent was calculated at theB3LYP/6-31G(d), BLYP/6-31G(d), M05-2X/6-31G(d),PBE1PBE/6-31G(d), PBE/6-31G(d), TPSS/6-31G(d) andTPSSh/6-31G(d) levels of theory using time-dependentDFT (TD-DFT). The results obtained are shown in Fig. 3.

The calculated UV-vis spectra are provided in Table 3,and it is quite apparent that the closest theoretical value ofmax to the experimental one is 444.44 nm, which wasafforded by M05-2X/6-31G(d).

The calculated value of max is an important parameterwhich indicates that this molecular system should beconsidered for use as a functional material (a dye in thiscase) in a DSSC, as the value of this parameter for Dye 7 fallswithin the range of the solar spectrum of visible light [36].

At all of the levels of theory tested, the observed signalcorresponded to the HOMO (highest occupied molecularorbital) to LUMO (lowest unoccupied molecular orbital)transition. Table 4 shows the results of TD-DFT calcu-lations performed using the functional M05-2X and thebasis set 6-31G(d), including the electronic state transitions,their corresponding wavelengths (in nm) and energies (ineV), as well as their assignments in terms of the orbitalsinvolved in the transitions.

Taking into account the results for the molecularstructure and UV-vis spectrum calculations, it is clear thatthe functional M05-2X and the basis set 6-31G(d) is the

most appropriate level of calculation to perform the rest ofthe characterization of Dye 7.

The infrared spectrum (IR) for Dye 7 calculated with M05-2X/6-31G(d) is shown in Fig. 4. The vibrational bands wereassigned using the molecular visualization software forWindows ChemCraft. CS stretching is observed as a peakat 584 cm1. At 796 cm1, a peak due to vibrations of carbon-chain hydrogens out of the plane of the aromatic rings can beseen, while another peak due to the corresponding vibrationsfor the bending of CH in thiophene is present at 1091 cm1.Vibrations due to the bending of the bond OH produce apeak at 1236 cm1. Other intense peaks include those due tothe stretching of the CN bond in the amine and the CC inthe thiophene occurring at 1392 cm1 and 1521 cm1

respectively; meanwhile, at 1677 cm1, a peak due to thedouble-bond stretching of C(45)=C(46) is noted. The peak at1872 cm1 represents the stretching of the double bond C=O,while the vibration at 2417 cm1 corresponds to the stretchingof the triple bond CN. The CH vibrations for the aromaticrings occur at 3244 cm1, and stretching of the OH bond isobserved at 3784 cm1.

The molecular dipole moment is an experimentalmeasure of the charge distribution in a molecule. Theprecision of the global distribution of electrons in amolecule is difficult to quantify, since it involves allmultipoles. In this calculation, the values of the totalenergy of the system, the total dipole moment and theisotropic polarizability in the fundamental state obtained atthe M05-2X/6-31G(d) level of calculation are 1736.99 a.u.,6.7374 debye and 433.06 bohr3. Moreover, the calculatedenergies of the HOMO and LUMO are 6.29 eV and 1.85 eV,respectively. These results are of great importance, since theycan be used during synthesis to determine the solubility andchemical reactivity of the molecule, and they can also beemployed in organic electronics and photovoltaics, asreported in different works [3739].

The free energy of solvation G(solv) of the moleculewas calculated for Dye 7 using M05-2X/6-31G(d) coupledwith the integral equation formalism of the polarizedcontinuum model (IEF-PCM) for different solvents. Thesolubility of a molecule depends on several kinetic andthermodynamic factors. However, the magnitude and signof G(solv) can be used as an approximate index ofsolubility. In this sense, a negative sign and a largemagnitude indicates increased solubility. The results ofthis calculation for the studied molecule can besummarized as follows: cyclohexane=1.85 kcal mol1,chloroform=4.68 kcal mol1, water=5.02 kcal mol1,THF=5.30 kcal mol1, acetone=12.86 kcal mol1,ethanol=14.43 kcal mol1 and methanol=15.03 kcalmol1. Based on these results, it appears that the moleculeunder investigation will be most soluble in methanol andethanol.

Table 2 Results of the population standard deviation for the bondlengths obtained with different models

Model Population standard deviation

BLYP/3-21G* 0.0258

B3LYP/3-21G* 0.0189

PBE/3-21G* 0.0236

PBE1PBE/3-21G* 0.0175

TPSS/3-21G* 0.0232

TPSSh/3-21G* 0.0203

M05-2X/3-21G* 0.0150

BLYP/6-31G(d) 0.0238

B3LYP/6-31G(d) 0.0171

PBE/6-31G(d) 0.0214

PBE1PBE/6-31G(d) 0.0159

TPSS/6-31G(d) 0.0204

TPSSh/6-31G(d) 0.0179

M05-2X/6-31G(d) 0.0130

838 J Mol Model (2012) 18:835842

The HOMO and LUMO orbitals of Dye 7 calculated atthe M05-2X/6-31G(d) level of theory are shown in Fig. 5.The HOMO orbital density is located over the double bondsof the carbon chain and the nitrogen (N33); meanwhile, thedensity of the LUMO orbital is concentrated over the CCsingle bonds. This provides a good idea of the reactivity ofthe molecule.

The reactive sites can be identified through theseorbital densities. The calculated HOMO and LUMO

densities shown in Fig. 5 indicate that electrophilic attackmay occur preferentially at the C=C double bonds or atN33, while nucleophilic attack occurs at CC singlebonds.

The condensed Fukui functions can also be used todetermine the reactivity of each atom in the molecule. Thecorresponding condensed Fukui functions are f k qk N 1 % qkN (for nucleophilic attack), f %k qkN % qk N % 1 (for electrophilic attack) and f 0k

Fig. 2 Interatomic bond distances () and bond angles (in degrees) for Dye 7 obtained at the M05-2X/6-31G(d) level of calculation

J Mol Model (2012) 18:835842 839

qk N 1 % qk N % 1 '=2 (for radical attack), where qk isthe effective Mulliken charge of atom k in the molecule.

The calculations of the condensed Fukui functions fornucleophilic and electrophilic attacks were performed usingAOMIX (a molecular analysis program), which gave thefollowing results: f k 0:1876 and f %k 0:1906.

Electrophilic attack will occur at atoms that produce anegative charge, and where the Fukui function f %k is amaximum. This value confirms that the most probable siteof electrophilic attack is N33. Nucleophilic attacks, on theother hand, will occur at atoms that produce a positivecharge and where the Fukui function f k is a maximum.

Fig. 3 Ultraviolet-visible (UV-vis) spectrum of Dye 7 calculated using time-dependent DFT (TD-DFT) with the basis set 6-31G(d) and thefunctionals used in this research

Table 3 Values of the wavelength of maximum absorption by Dye 7calculated using the various models tested

Model max (nm)

BLYP/6-31G(d) 500.00

B3LYP/6-31G(d) 598.80

PBE/6-31G(d) 495.05

PBE1PBE/6-31G(d) 564.97

TPSS/6-31G(d) 480.77

TPSSh/6-31G(d) 666.67

M05-2X/6-31G(d) 444.44

Experimental 432.00

Table 4 Electronic transition states of Dye 7 (calculated with TD-DFT at the M05-2X/6-31G(d) level of theory)

State Wavelength (nm) Energy (eV) f Assignment (H = HOMO,L = LUMO)

1 444.1 2.79 1.7821 S H-0L+0(+73%) H-1L+0(11%)

2 325.2 3.81 0.0338 S H-1L+0(+61%) H-0L+1(+12%) H-0L+0(+8%)

3 295.5 4.2 0.2897 S H-0L+1(+64%) H-0L+0(12%) H-1L+0(9%)

4 280.2 4.42 0.0564 S H-0L+2(+79%)

5 271.3 4.57 0.2703 S H-0L+3(+81%) H-1L+3(+8%)

6 268.1 4.62 0.0851 S H-7L+0(+71%) H-4L+0(+12%)

7 252.1 4.92 0.0215 S H-0L+5(+33%) H-0L+4(9%) H-6L+0(+8%) H-1L+1(+8%) H-3L+0(+6%)

8 250.8 4.94 0.0052 S H-1L+1(+18%) H-4L+0(12%) H-0L+5(12%) H-7L+0(+9%) H-0L+4(6%)

9 243.4 5.09 0.0001 S H-9L+0(+67%) H-9L+8(12%) H-9L+1(9%)

10 241 5.15 0.0261 S H-0L+6(+57%) H-4L+0(+10%) H-1L+6(+8%) H-3L+3(5%)

11 239.2 5.18 0.0028 S H-1L+1(+24%) H-4L+0(+23%) H-2L+0(14%) H-0L+4(11%) H-0L+6(8%)

12 234.1 5.3 0.0824 S H-3L+0(+40%) H-0L+5(24%) H-6L+0(+14%)

13 227.8 5.44 0.001 S H-0L+7(+47%) H-1L+7(39%)

14 223.2 5.55 0.0095 S H-0L+4(+49%) H-1L+1(+26%) H-0L+1(+7%)

15 221 5.61 0.0054 S H-2L+0(+68%) H-4L+0(+17%)

16 215.4 5.76 0.0469 S H-6L+0(+51%) H-3L+0(33%)

17 209.6 5.92 0.0093 S H-5L+0(+88%) H-5L+1(+6%)

18 206.6 6 0.0012 S H-1L+10(+17%) H-15L+0(16%) H-0L+10(12%) H-9L+8(6%)

19 206 6.02 0.0678 S H-8L+0(+42%) H-4L+1(+15%) H-10L+0(13%) H-4L+0(+6%)

20 203.1 6.1 0.0158 S H-1L+2(+35%) H-2L+3(10%) H-2L+2(+6%) H-0L+5(+6%)

840 J Mol Model (2012) 18:835842

Thus, the atom most likely to suffer a nucleophilic attack isC45.

Conclusions

In this work, a general comparison of the optimizations ofthe molecular structure and the ultraviolet spectrum in THFsolvent achieved with different density functionals andbasis sets was performed. This comparison indicated thatthe functional that gave results that were closest to theexperimental results was M05-2X, along with the basis set6-31G(d), so this level of theory was then used to studyDye 7 molecule, which is intended for use in photovoltaicdevices. The total energy of this system, its dipole moment,

its isotropic polarizability, its molecular orbitals and itsinfrared spectrum were calculated using M05-2X/6-31G(d).

The free energy of solvation G(solv) of the molecule,calculated using the same level of theory along with theintegral equation formalism of the polarized continuummodel (IEF-PCM), indicates that the molecule is potentiallysoluble in methanol and ethanol.

The M05-2X/6-31G(d) methodology can be used as anuseful tool for studying the molecular structure andelectronic properties of Dye 7, as well as other structuresderived from it.

Acknowledgments This work was made possible by the support ofUniversidad Autnoma de Sinaloa through the Facultad de IngenieraMochis, by the PROFAPI2010/033 project, Centro de Investigacinen Materiales Avanzados, S.C. (CIMAV), and Consejo Nacional deCiencia y Tecnologa.

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Int. J. Mol. Sci. 2012, 13, 4418-4432; doi:10.3390/ijms13044418

International Journal of

Molecular Sciences ISSN 1422-0067

www.mdpi.com/journal/ijms

Article

Density Functional Theory (DFT) Study of

Triphenylamine-Based Dyes for Their Use as Sensitizers

in Molecular Photovoltaics

Jess Baldenebro-Lpez 1,2

, Jos Castorena-Gonzlez 2, Norma Flores-Holgun

1,

Jorge Almaral-Snchez 2 and Daniel Glossman-Mitnik

1,*

1 NANOCOSMOS Virtual Lab, Advanced Materials Research Center (CIMAV),

Miguel de Cervantes 120, Complejo Industrial Chihuahua, Chihuahua 31190, Mxico;

E-Mails: jesus.baldenebro@cimav.edu.mx (J.B.-L.); norma.flores@cimav.edu.mx (N.F.-H.) 2 Faculty of Engineering Mochis, Autonomous University of Sinaloa, Prol. ngel Flores y Fuente de

Poseidn, S.N., Los Mochis, Sinaloa 81223, Mxico; E-Mails: jhcg@uas.uasnet.mx (J.C.-G.);

jalmaral@correo.uasnet.mx (J.A.-S.)

* Author to whom correspondence should be addressed; E-Mail: daniel.glossman@cimav.edu.mx;

Tel.: +52-614-439-1151; Fax: +52-614-439-1130.

Received: 6 February 2012; in revised form: 4 March 2012 / Accepted: 20 March 2012 /

Published: 10 April 2012

Abstract: In this work we studied three dyes which are proposed for potential photovoltaic

applications and named Dye7, Dye7-2t and Dye7-3t. The Density Functional Theory (DFT)

was utilized, using the M05-2X hybrid meta-GGA functional and the 631+G(d,p) basis

set. This level of calculation was used to find the optimized molecular structure and to

predict the main molecular vibrations, the absorption and emission spectra, the molecular

orbitals energies, dipole moment, isotropic polarizability and the chemical reactivity

parameters that arise from Conceptual DFT. Also, the pKa values were calculated with the

semi-empirical PM6 method.

Keywords: molecular structure; absorption spectrum; polarizability; chemical reactivity;

dipole moment; triphenylamine; dye sensitizers

OPEN ACCESS

Int. J. Mol. Sci. 2012, 13 4419

1. Introduction

The worlds traditional energy sources (oil, natural gas and coal) have a finite life, and actual

predictions indicate that alternative sources must provide an important contribution in the near

future [1]. Solar energy is one of the most promising sources of energy in the future. The direct

conversion of sunlight into electric energy using solar cells is particularly interesting because it has a

lot of advantages over other methods, for example, it does not produce greenhouse gases nor nuclear

byproducts [2]. The dye sensitized solar cells (DSSC) have attracted attention due to their efficiency,

simple manufacturing and low cost [310]. In these DSSC, an organic sensitizer must be chemically

absorbed on the porous surface of the nanocrystalline oxide. After absorbing a photon, the excited

electron in the dye-sensitized molecule is transferred into the conduction band of nanocrystalline oxide,

followed by a process in which the electron diffuses through the electrode. The sensitizer in this

oxidized state is reduced to its normal state gaining electrons through a liquid electrolyte [1113].

Nowadays, many research groups from all over the world actively participate to improve the efficiency

of every single process involved in the DSSC [1416]. The charge transfer efficiency from the dye

molecule to the nanocrystalline oxide is extremely important in the solar cell design. Since Regal and

Grtzel published their pioneer study [17], the understanding of the mechanism has required

fundamental research about the diverse physical phenomena at nanometric scale [18].

Theoretical studies on physical and chemical properties of dye-sensitizers are very important to

understand the relationship between the structure, properties and performance in order to design and

synthesize new molecules for this purpose [1923]. To be useful in DSSC a sensitizer must meet

important requirements in its structure, such as: the electron-donating part [24,25], a unit to adjust the

absorption spectrum [26] and the electron-acceptor part [27].

In the synthesis of new DSSC dyes, the triphenylamine based structures have been widely

employed to build metal-free organic dyes and they have been successfully proven to show high

conversion efficiency in DSSC devices [2832] since they can both enhance the hole transporting

ability of the materials and inhibit the aggregation of the dyes with their non-planar structure [33]. The

tiophene molecule is included in the structure of the proposed systems due is one of the most

frequently used -spacer in organic dyes having a donor-(-conjugated bridge)-acceptor (D--A)

system in dye-sensitized solar cells (DSSCs) due to its high structural stability, ease of structural

modification, excellent optical and electronic properties and controllable electrochemical

behavior [3439]; the -conjugated derivates are generally efficient fluorophores, and as such, useful

for the fabrication of nanobiosensors; they can be used as an attractive building block for Organic

Molecular Materials [40]. The absorption spectra of the dye molecules can be easily tuned by changing

the length of the oligothiophene bridge [24], this modification of the size of the thiophene chain not

only will allow to evaluate the influence of these changes on the properties of absorption, but also on

the emission spectra and band gap, among others. Moreover, the electron acceptor part has significant

influences on the photovoltaic properties due to the fact that the excited electrons from the dye

molecules are injected into the semiconductor film through this component [41]; a viable candidate

that can efficiently act as a linker moiety is cyanoacrylic acid, since carboxylic acid binds strongly to

the TiO2 surface through a bridging which is not easily removed by rinsing, and the presence of the

Int. J. Mol. Sci. 2012, 13 4420

cyano group enhances the electron-withdrawing nature of the linker moiety [39]. Based on this, we

studied three sensitizers, which were named Dye7, Dye7-t2 and Dye7-t3 (Figure 1).

Figure 1. Sensitizer diagrams studied in this research (Dye7, Dye7-2t and Dye7-3t).

O

N

S

N

OH

N

S S

O

N

OH

N

S S

S

O

N

OH

Dye7

Dye7-2t

Dye7-3t

The aim of this work is to report the results of our research using molecular structure calculations

and properties of the three dyes according to the density functional theory [42], developed by Walter

Kohn in the 1960s. This method is implemented in the Gaussian 09W program package [43].

2. Results and Discussion

Once the molecular structures were proposed, the geometry optimization followed, which allowed

the lowest energy configurations to be obtained, which are shown in Figure 2 including the numbers of

atoms and symbols.

A selection of geometric parameters was made to clearly visualize how the geometric structures are

similar regarding both conformation and geometric data when they are optimized using the M05-2X

functional and the 631+G(d,p) basis set. Table 1 shows the selected values for bond length (), bond

angles and dihedral angles (in degrees).

Table 1. Dye 7, Dye7-2t and Dye7-3t selected bond lengths (angstroms), bond angles and

dihedral angles (degrees).

Dye7 Value Dye7-2t Value Dye7-3t Value

C2-N33 1.418 C55-N86 1.416 C125-N146 1.417

C14-N33 1.418 C67-N86 1.417 C115-N146 1.416

C23-N33 1.404 C76-N86 1.407 C139-N149 1.408

C2-C3 1.398 C55-C56 1.399 C125-C126 1.399

C15-C17 1.394 C70-C72 1.394 C118-C119 1.394

Int. J. Mol. Sci. 2012, 13 4421

Table 1. Cont.

C30-C34 1.460 C83-C87 1.463 C140-C147 1.463

C35-C37 1.452 C88-C90 1.454 C148-C151 1.454

C42-C45 1.430 C101-C105 1.427 C166-C172 1.431

C2-N33-C23 120.3 C55-N86-C76 120.4 C125-N146-C139 120.2

C14-N33-C23 120.6 C67-N86-C76 120.1 C115-N146-C139 120.3

C30-C34-C35 126.6 C83-C87-C88 126.6 C140-C147-C148 126.3

C42-C45-C46 129.2 C101-C105-C106 130.2 C166-C172-C173 129.0

C2-N33-C23-C25 35.16 C55-N86-C76-C78 37.71 C125-N146-C139-C137 37.52

C23-N33-C14-C16 44.61 C66-C67-N86-C76 44.26 C116-C115-N146-C139 42.86

C45-C46-C49-O51 0.21 C105-C106-C109-O111 0.22 C172-C173-C177-O178 0.27

Figure 2. Optimized molecular structures of Dye7, Dye7-2t and Dye7-3t.

In IR spectra calculations there also appears to be a similarity in molecular structure due to the

presence of vibrations with wave numbers close to each other. The O-H bond stretching vibration

appears from 3836 cm1

to 3843 cm1

. The vibration of the aromatic ring C-H bond is observed in the

range from 3237 cm1

to 3248 cm1

, and the range from 2406 cm1

to 2416 cm1

corresponds to the

stretching of the triple bond CN. Other intense peaks that correspond to the stretching of the double

bond carboxylic acid C=O and to the double bond between the thiophene molecule and the acid (C=C),

Int. J. Mol. Sci. 2012, 13 4422

appear in the ranges from 1835 cm1

to 1842 cm1

and from 1664 cm1

to 1678 cm1

, respectively. The

double bond C=C in the thiophene has a symmetric stretching vibration which presents from 1513

cm1

to 1517 cm1

. The vibration of the bond C-N in the amine occurs in the range from 1380 cm1

to

1391 cm1

. The corresponding vibration for the bending of the bond O-H produces a peak from 1201

cm1

to 1223 cm1

. The bending of C-H in thiophene is present from 1086 cm1

to 1093 cm1

. The out-

of-plane bending vibration associated with the aromatic rings appears from 721 cm1

to 726 cm1

. The

C-S bond stretching vibration is shown in the wavenumbers ranging from 581 cm1

to 598 cm1

.

Absorption spectra for the proposed dye molecules are shown in Figure 3. The calculated value of

max is an important parameter, which indicates that these molecular systems should be considered for

use as a functional material (as dye in this case) in a DSSC, the value of this parameter for Dye7,

Dye7-2t and Dye7-3t meets the requirements established in the literature [44].

Figure 3. Ultraviolet-visible (UV-vis) spectra of Dye7, Dye7-2t and Dye7-3t.

In all the UV-Vis spectra studied, the observed signal corresponds to the highest occupied

molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) transition. Tables 24

show the results of TD-DFT calculations performed using the M05-2X functional and the

631+G(d,p) basis set; including the electronic excited states, the corresponding wavelengths (in nm),

the energies (in eV), the oscillator strength (f) and the orbitals involved in the transitions.

Int. J. Mol. Sci. 2012, 13 4423

Table 2. Dye7 electronic excited states, showing wavelengths (nm), energies (eV),

oscillator strength (f) and the orbitals involved in the transitions; calculated with

Time-dependent density functional theory (TD-DFT) at M05-2X/631+G(d,p). Only

excited states with f > 0.03 are shown.

Number nm eV (f) Assignment; H = HOMO, L = LUMO, L + 1 = LUMO + 1, etc.

1 425.5 2.91 1.6808 S H-0- > L + 0 (+73%) H-1- > L + 0 (8%)

2 320.5 3.87 0.0361 S H-1- > L + 0 (+55%) H-0- > L + 1 (19%)

3 301.4 4.11 0.0797 S H-0- > L + 2 (+42%) H-0- > L + 1 (+27%)

H-1- > L + 0 (+10%) H-0- > L + 0 (+7%)

4 294.4 4.21 0.1563 S H-0- > L + 2 (+40%) H-0- > L + 1 (30%)

H-0- > L + 0 (7%) H-1- > L + 0 (6%)

5 281.2 4.41 0.2058 S H-0- > L + 3 (+80%) H-1- > L + 3 (+9%)

6 266.4 4.65 0.0517 S H-6- > L + 0 (+44%) H-7- > L + 0 (14%)

H-4- > L + 0 (+13%) H-1- > L + 1 (10%)

H-0- > L + 4 (+6%)

7 259.4 4.78 0.0365 S H-0- > L + 5 (+40%) H-0- > L + 4 (15%) H-7- > L + 0 (11%)

Table 3. Dye7-2t electronic excited states, showing wavelengths (nm), energies (eV),

oscillator strength (f) and the orbitals involved in the transitions; calculated with TD-DFT

at M05-2X/6-31+G(d,p). Only excited states with f > 0.03 are shown.

Number nm eV (f) Assignment; H = HOMO, L = LUMO, L + 1 = LUMO + 1, etc.

1 446.4 2.78 1.8564 S H-0- > L + 0 (+67%) H-1- > L + 0 (22%) H-0- > L + 1 (+7%)

2 322.8 3.84 0.2720 S H-0- > L + 1 (+41%) H-0- > L + 0 (25%)

H-1- > L + 0 (19%) H-1- > L + 1 (6%)

3 299.8 4.14 0.0368 S H-0- > L + 2 (+65%) H-0- > L + 3 (14%) H-1- > L + 2 (+8%)

4 286.8 4.32 0.0514 S H-1- > L + 1 (+36%) H-2- > L + 0 (22%)

H-8- > L + 0 (14%) H-0- > L + 3 (11%) H-5- > L + 0 (+6%)

5 283.2 4.38 0.2221 S H-0- > L + 4 (+77%) H-1- > L + 4 (+17%)

6 258.6 4.79 0.0496 S H-0- > L + 5 (+28%) H-8- > L + 0 (+12%)

H-5- > L + 0 (+11%) H-0- > L + 7 (7%)

7 248.9 4.98 0.0668 S H-0- > L + 6 (+42%) H-0- > L + 3 (12%) H-1- > L + 6 (+10%)

8 239.1 5.19 0.1066 S H-1- > L + 3(+15%) H-2- > L + 1(+11%) H-0- > L + 8(8%)

H-0- > L + 7(+8%) H-9- > L + 0(8%) H-0- > L + 9(+8%)

9 231.1 5.36 0.0553 S H-4- > L + 0(+32%) H-4- > L + 1(+15%)

H-0- > L + 7(+10%) H-7- > L + 0(10%) H-7- > L + 1(6%)

Int. J. Mol. Sci. 2012, 13 4424

Table 4. Dye7-3t electronic excited states, showing wavelengths (nm), energies (eV),

oscillator strength (f) and the orbitals involved in the transitions; calculated with TD-DFT

at M05-2X/6-31+G(d,p). Only excited states with f > 0.03 are shown.

Number nm eV (f) Assignment; H = HOMO, L = LUMO, L + 1 = LUMO + 1, etc.

1 448.4 2.76 2.1948 S H-0- > L + 0 (+47%) H-1- > L + 0 (26%) H-0- > L + 1 (+12%)

2 364.3 3.40 0.1536 S H-0- > L + 1 (+49%) H-1- > L + 0 (+19%)

H-0- > L + 2 (6%) H-2- > L + 0 (+6%)

3 339.3 3.65 0.3332 S H-0- > L + 0 (+47%) H-1- > L + 0 (+21%)

H-0- > L + 1 (12%) H-2- > L + 0 (+9%) H-1- > L + 1 (+5%)

4 309.6 4.00 0.0758 S H-1- > L + 1 (+39%) H-0- > L + 2 (19%) H-2- > L + 0 (12%)

5 300.2 4.13 0.0462 S H-0- > L + 3 (+70%) H-1- > L + 3 (+13%)

H-1- > L + 1 (+12%)

6 284.2 4.36 0.2165 S H-0- > L + 4 (+69%) H-1- > L + 4 (+18%)

7 278.2 4.46 0.0348 S H-0- > L + 2 (+29%) H-1- > L + 2 (27%)

H-1- > L + 1 (+15%) H-0- > L + 1 (+6%)

8 255.8 4.85 0.0597 S H-10- > L + 0 (+32%) H-9- > L + 0 (13%) H-6- > L + 0 (8%)

9 243.6 5.09 0.0752 S H-0- > L + 10 (13%) H-2- > L + 1 (+12%)

H-11- > L + 0 (+11%) H-0- > L + 5 (+10%)

H-5- > L + 0 (+9%) H-6- > L + 0 (+6%)

10 234.0 5.30 0.0496 S H-2- > L + 2 (+13%) H-1- > L + 5 (+10%)

H-0- > L + 11 (10%) H-9- > L + 0 (+6%)

To calculate the fluorescence spectra, the excited state optimization was carried out using the

CIS/6-31+G(d,p) model [45]. The TD-DFT results are shown in Figure 4. The wavelength

corresponding to the HOMO-LUMO transition is 483 nm for Dye7, 518 nm for Dye7-2t and 476 nm

for Dye7-3t. These results indicate that the studied molecules have fluorescence in the visible region,

and for this reason they constitute potential application in organic light emission diodes (OLEDs) [46].

Figure 4. Fluorescence (fluo) spectra of sensitizers calculated using time-dependent DFT

(TD-DFT) with the M05-2X/631+G(d,p) level.

Int. J. Mol. Sci. 2012, 13 4425

In this work, a summary has been made based on the total dipole moment (for the ground state), the

isotropic polarizability, and pKa values (in water as solvent). This information is shown in Table 5.

Figure 5 shows the HOMO-LUMO molecular orbitals energetic position. These results are of great

importance, since they can be used during synthesis to determine the solubility and chemical reactivity

of the molecule, and they can also be employed in organic electronics and photovoltaics, as reported in

different studies [4749].

Table 5. Dipole moment (), polarizability () and pKa values.

Molecule (Debye) (Bohr3) pKa

Dye7 7.15 479.04 0.17

Dye7-2t 6.42 578.60 0.24

Dye-3t 7.13 684.24 0.39

Figure 5. Molecular orbitals energy levels diagram.

The energy values of the orbitals indicate that these dyes can be used in a DSSC; this is a result of the

sensitizer LUMO level and the conduction band of nanocrystalline oxides that are commonly used in

such devices.

The molecular orbitals HOMO and LUMO calculated at M05-2X/631+G(d,p) level of theory are

shown in Figure 6. The reactive sites can be identified via orbital densities. The HOMO orbital density

is located over the double bonds of the carbon chain and the nitrogen of the triphenylamine molecule,

which indicates that in these sites an electrophilic attack can occur. Meanwhile, the density of the

LUMO orbital is concentrated over the C-C single bonds; therefore, these are most likely sites for a

nucleophilic attack.

Int. J. Mol. Sci. 2012, 13 4426

Figure 6. HOMO and LUMO orbitals of Dye 7, Dye7-2t and Dye7-3t calculated at the

M05-2X/631G(d) level of theory.

The site for electrophilic attack will occur at atoms that produce a negative charge, and where the

Fukui function fk is a maximal. The sites for nucleophilic attack will be those atoms that produce a

positive charge and where the Fukui function fk is a maximal. The condensed Fukui function results

for nucleophilic and electrophilic attacks were obtained with the AOMIX (a molecular analysis

program) and are shown in Table 6.

Table 6. Dye7, Dye7-2t and Dye7-3t nucleophilic and electrophilic attack sites.

Molecule Site for Electrophilic Attack Site for Nucleophilic Attack

Dye7 N33 C45

Dye7-2t N86 C105

Dye7-3t N146 C172

As can be seen, these values confirm that the nitrogen atom in the triphenylamine molecule is the

most likely site for the electrophilic attack, and the carbon atom that joins the thiophene chain to the

acid is the site for the nucleophilic attack.

Chemical reactivity parameters such as ionization potential (I), electron affinity (A),

electronegativity (), chemical hardness () and electrophilic index () for the studied molecular

systems (Table 7) were obtained by energy calculations (neutral and ionic state), taking into account

the ground state geometry optimization.

Int. J. Mol. Sci. 2012, 13 4427

Table 7. Dye7, Dye7-2t and Dye7-3t chemical reactivity parameters calculated with

M05-2X/631+G(d,p) using DFT descriptors.

Molecule Conceptual DFT

I (eV) A (eV) (eV) (eV) (eV)

Dye7 6.844 1.672 4.258 2.586 3.506

Dye7-2t 6.693 1.748 4.220 2.473 3.602

Dye7-3t 6.625 1.826 4.226 2.399 3.721

3. Experimental Section

Computational calculations were carried out with the Gaussian 09W molecular modeling software

obtaining the ground state molecular geometry for each dye. The strength constants and vibrational

frequencies were determinated via analytic frequency calculations on stationary points obtained after

geometry optimization. Both calculations were carried out at the same theory level. The chemical

model utilized for the calculations was the 631+G(d,p) basis set [5053] and the M05-2X hybrid

meta-GGA functional [54], which have been proved to yield good results when modeling these kinds

of structures.

Ultraviolet and fluorescence spectra in gas phase were carried out using Time-dependent density

functional theory (TD-DFT) equations according to the method implemented in the molecular package

Gaussian 09W [5558]. The equations were solved for 20 excited states. The infrared (IR),

ultraviolet-visible (UV-vis) and fluorescence (fluo) spectra were analyzed using the program SWizard [59].

The wavelength of maximum absorption and emission are shown for UV-vis and fluo spectra.

In this work we calculated the total dipole moment () and the isotropic polarizability (). The pKa for

the hydrogen atom attached to the oxygen atom was calculated using the MOPAC 2009 program [60].

In this program, the pKa is calculated using the O-H distance calculated using PM6 [61]. Molecular

dipole moment is an experimental measure of the charge distribution in a molecule. It is difficult to

evaluate accurately the global electron distribution in a molecule because it involves all the multipoles.

The polarizability contributes in a significant way to the understanding of the response of the system

facing an external field.

The reactive sites can be identified using orbital densities. The condensed Fukui functions can also

be used to determine the reactivity of each atom in the molecule. The corresponding condensed

Fukui functions are given by NqkNqkfk 1 (nucleophilic attack), 1 NqkNqkfk (electrophilic attack) y 2/110 NqkNqkfk (radical attack), where qk is the effective

Mulliken charge of atom k in the molecule. The condensed Fukui functions were evaluated with the

AOMix molecular analysis program [62,63].

On the other hand, using the DFT framework makes it possible to find the chemical reactivity

descriptors values, such as: electron affinity, ionization potential, electronegativity, hardness and

electrophilic index. All these values were obtained using system energy calculations.

Int. J. Mol. Sci. 2012, 13 4428

4. Conclusions

In this work, the molecular structure and properties of three molecules proposed as sensitizers in

solar cells have been calculated. The applied methodology for this study is based on the density

functional theory, using the M05-2x hybrid meta-GGA functional and the 631+G(d,p) basis set.

The molecular systems characterization includes the calculation of vibrations of functional groups,

ultraviolet-visible and fluorescence spectra, total dipole moment, isotropic polarizability, pKa values,

molecular orbitals, chemical reactivity parameters and attack sites. The molecular orbitals energy

indicates a smaller energy gap in the Dye7-3t sensitizer. For all three sensitizers, the LUMO orbital

energies are similar, the difference being approximately 0.14 eV. Analyzing these data makes it

possible to find potential applications for these dyes in photovoltaic devices.

Acknowledgments

This work has been supported by Consejo Nacional de Ciencia y Tecnologa (CONACYT) and

Centro de Investigacin en Materiales Avanzados, S.C. (CIMAV), and Universidad Autnoma de

Sinaloa and Direccin de Investigacin y Posgrado by PROFAPI 2011/043. J.B.L. gratefully

acknowledges a fellowship from CONACYT. D.G.M and N.F.H. are researchers of CIMAV

and CONACYT.

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Int. J. Mol. Sci. 2012, 13, 16005-16019; doi:10.3390/ijms131216005

International Journal of

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Computational Molecular Nanoscience Study of the Properties

of Copper Complexes for Dye-Sensitized Solar Cells

Jess Baldenebro-Lpez 1,2

, Jos Castorena-Gonzlez 2, Norma Flores-Holgun

1,

Jorge Almaral-Snchez 2 and Daniel Glossman-Mitnik

1,*

1 Centro de Investigacin en Materiales Avanzados, S.C., Miguel de Cervantes 120,

Complejo Industrial Chihuahua, Chihuahua 31190, Mxico;

E-Mails: jesus.baldenebro@cimav.edu.mx (J.B.-L.); norma.flores@cimav.edu.mx (N.F.-H.) 2 Universidad Autnoma de Sinaloa, Prol. ngel Flores y Fuente de Poseidn, S.N., Los Mochis,

Sinaloa 81223, Mxico; E-Mails: kstor28@yahoo.com.mx (J.C.-G.); jalmaral@gmail.com (J.A.-S.)

* Author to whom correspondence should be addressed; E-Mail: daniel.glossman@cimav.edu.mx;

Tel.: +52-614-439-1151; Fax: +52-614-439-1130.

Received: 20 August 2012; in revised form: 9 October 2012 / Accepted: 12 November 2012 /

Published: 28 November 2012

Abstract: In this work, we studied a copper complex-based dye, which is proposed for

potential photovoltaic applications and is named Cu (I) biquinoline dye. Results of electron

affinities and ionization potentials have been used for the correlation between different

levels of calculation used in this study, which are based on The Density Functional Theory

(DFT) and time-dependent (TD) DFT. Further, the maximum absorption wavelengths of

our theoretical calculations were compared with the experimental data. It was found that

the M06/LANL2DZ + DZVP level of calculation provides the best approximation. This

level of calculation was used to find the optimized molecular structure and to predict the

main molecular vibrations, the molecular orbitals energies, dipole moment, isotropic

polarizability and the chemical reactivity parameters that arise from Conceptual DFT.

Keywords: molecular structure; absorption spectra; polarizability; chemical reactivity;

dipole moment; copper complex; dye-sensitized

OPEN ACCESS

Int. J. Mol. Sci. 2012, 13 16006

1. Introduction

The current warming of the global climate is the result of an increase in greenhouse gas (GHG)

emissions, particularly CO2. Global average atmospheric CO2 has increased from 280 ppm in the

1750s to 389 ppm in 2010 [13]. An increasing demand for energy in the emerging economies and

energy crisis worldwide has stimulated a growing number of researches on renewable energy, in that

the utilization of renewable energy can help reduce fossil fuel consumption and alleviate

environmental problems. Renewables-based power systems provide an opportunity to generate cleaner

electricity with a lower cost of energy [4]. It is thought that the transition from fossil fuels to a

diversified energy matrix can be accelerated by governments by means of adequate policies and

instruments that support the creation of incentives for mitigation of greenhouse gases (GHG) emissions

and investments in renewable energy technology research and development. Solar energy is one of the

most promising sources of energy in the future and one of the renewable energy resources that has long

played a dominant role in the field of energy research with its wide application and great potential [5,6].

Recently, dye sensitized solar cells (DSSC) [79], considered as a credible alternative to conventional

inorganic silicon-based solar cells, have attracted attention due to their efficiency, simple

manufacturing and low cost [1013]. In these DSSC, an organic sensitizer must be chemically

absorbed on the porous surface of the nanocrystalline oxide. After absorbing a photon, the excited

electron in the dye-sensitized molecule is transferred into the conduction band of nanocrystalline

oxide, followed by a process in which the electron diffuses through the electrode. The sensitizer in this

oxidized state is reduced to its normal state gaining electrons through a liquid electrolyte [1416].

Nowadays, many research groups from all over the world actively participate to improve the efficiency

of every single process involved in the DSSC [1719]. The charge transfer efficiency from the dye

molecule to the nanocrystalline oxide is extremely important in the solar cell design. Since Regal and

Grtzel published their pioneer study [7], the understanding of the mechanism has required

fundamental research about the diverse physical phenomena at nanometric scale [20]. Theoretical

studies on physical and chemical properties of dye-sensitizers are very important to understand the

relationship between the structure, properties and performance in order to design and synthesize new

molecules for this purpose [2124].

Ruthenium(II) complexes as dyes have been extensively used as sensitizers in DSSCs owing to their

strong absorption in the visible range and relatively long-lived excited states [25]. The strong

absorptivity of these complexes is due to a metal-to-ligand charge transfer (MLCT) transition [26].

These complexes have reached over 12% power conversion efficiency [27]; but the rarity and high

cost of the Ru may limit their practical usage. Since initial reports of a [Ru(bpy)3]2+

-based

(bpy = 2,2'-bipyridine) photosensitizer, the dyes reported in the literature have predominantly been

ruthenium(II) complexes. We have recently become interested in the study and optimization of

copper(I)-based DSSC, literature reports of which are scarce. Sauvage and co-workers [28] discovered

that copper(I) complexes have similar photo-physical properties with Ru complexes, indicating that the

iterative chemical optimization of common metal complexes sensitizers can be comparable to that of

Ru complexes [29]. Copper(I) complexes display a wide variety of excited states and especially

photophysical and photochemical processes. Particularly, copper(I)poly-pyridine complexes exhibit

low-lying MLCT transitions that can participate, among others, in electron transfer processes [30]. In

Int. J. Mol. Sci. 2012, 13 16007

this research, we propose the study of a molecular system of this type, such as [Cu(LL)2]+

(LL = 2,2'-biquinoline-4,4'-dicarboxylic acid) which is shown in Figure 1 (Cu(I) biquinoline), in order

to define from the theoretical point of view a suitable calculation methodology for obtaining structural

parameters, as well as electrical and optical properties using the density functional theory (DFT) [3133]

and time-dependent (TD) DFT [3436]. These methods are implemented in the Gaussian 09W

program package [37]. In this paper, we have found very interesting properties with the proposed

ligand to increase the level of conjugation, which has not been reported as an article in the DSSC field.

Figure 1. Molecular structure of copper complex.

2. Results and Discussion

Once the molecular structure was proposed, the geometry optimization was calculated, followed by

the frequency analysis to confirm that the species had the minimum energy conformation. These

calculations were performed in the presence of methanol as solvent.

The electron affinity (A) and the ionization potential (I) were obtained by energy calculations

(neutral and ionic state), taking into account the ground state geometry optimization. The aim of this

was to establish the correlation of results between the different levels of calculation, since there are no

experimental results reported for this system; this is a contribution of our research. Table 1 shows the

values obtained and where it seems as if some of these have variations between them.

Figure 2 helps to visualize more clearly the dispersion of these values. M06-HF/LANL2DZ-DZVP,

M06-2X/6-31G(d) + DZVP and M06-HF/6-31G(d) + DZVP levels of theory show the greatest

dispersion; therefore, the values are ignored for the calculation of some measures of central tendency,

such as the mean and median. The red line represents the mean value, in the case of electron affinity, it

is located at 4.75 eV and for the ionization potential it is equal to 9.75 eV; meanwhile, the medium has

the values of 4.79 eV and 9.80 eV, respectively. This led to results of the population standard deviation

of 0.1555 (in A) and 0.1768 (in I) with a coefficient of variation of 3.27% and 1.81%, which are values

of high quality. On this basis, we can establish that the model chemistry M06/LANL2DZ + DZVP and

PBE0/LANL2DZ + DZVP represent excellent approximations in this context.

Int. J. Mol. Sci. 2012, 13 16008

Table 1. Electron affinity and the ionization potential of Cu(I) biquinoline with different

levels of theory.

Basis set Functional Electron affinity (eV) Ionization potential (eV)

LANL2DZ + DZVP M06 4.79 9.80

M06-2X 4.65 9.93

M06-HF 4.58 11.86

M06-L 4.98 9.55

B3LYP 4.88 9.84

PBE0 4.79 9.79

LANL2DZ M06 4.82 9.90

M06-2X 4.64 10.01

M06-HF 4.57 9.63

M06-L 5.00 9.48

6-31G(d) + DZVP M06 4.49 9.87

M06-2X 4.26 11.01

M06-HF 4.12 11.61

M06-L 4.69 9.49

Figure 2. Electron affinity and ionization potential for the levels of theory used in this

study and the dispersion between the values. The red line represents the mean value, in the case

of electron affinity it is located at 4.75 eV and in the case of ionization potential it is equal

to 9.75 eV.

Int. J. Mol. Sci. 2012, 13 16009

Another fundamental property as a potential sensitizer for DSSC is the maximum absorption

wavelength (max); according to the levels of theory selected, the UV-Vis spectrum and the max were

calculated. The results of max are shown in Table 2.

The results in Table 2 show that max varies from a high value to a low value when the functional

increases the percentage of Hartree-Fock exchange. The experimental result indicates that max is

553 nm [38]. Comparing this result with our calculation, we can conclude that the calculation

methodology with more precision is M06/LANL2DZ + DZVP. Similarly, by comparing the results of

A, I and max, this methodology will be used to study other properties of the dye under investigation.

Another aspect to consider is that using a LANL2DZ basis set better describes the behavior of the

excited states of the ligand molecule.

Table 2. Maximum absorption wavelength of Cu(I) biquinoline dye using various models.

Model chemistry max (nm) Model chemistry max (nm)

M06/LANL2DZ + DZVP 556 M06-HF/LANL2DZ 282

M06-2X/LANL2DZ + DZVP 386 M06/LANL2DZ 543

M06-HF/LANL2DZ + DZVP 279 M06-2X/LANL2DZ 378

M06-L/LANL2DZ + DZVP 641 M06-L/6-31G(d) + DZVP 645

B3LYP/LANL2DZ + DZVP 614 M06-HF/6-31G(d) + DZVP 279

PBE0/LANL2DZ + DZVP 578 M06/6-31G(d) + DZVP 488

M06-L/LANL2DZ 629 M06-2X/6-31G(d) + DZVP 328

Figure 3 shows the UV-Vis spectra at the M06/LANL2DZ-DZVP level of calculation and its

corresponding max