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Electron delocalization in the S1 and T 1 metal-to-ligand charge transfer states of trans -substitutedmetal quadruply bonded complexesBrian G. Alberding, Malcolm H. Chisholm1, Judith C. Gallucci, Yagnaseni Ghosh, and Terry L. Gustafson

Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, OH 43210-1185

Contributed by Malcolm H. Chisholm, March 15, 2011 (sent for review December 1, 2010)

The singlet S1 and triplet T 1 photoexcited states of the compounds

containing MM quadruple bonds trans -M2ðTi PBÞ2ðO2CC6H4-4-CNÞ2,

where Ti PB ¼ 2 ,4 ,6-triisopropylbenzoate and M ¼ Mo (I) or M ¼ W

(I0), and trans -M2ðO2CMeÞ2ððN½i PrÞ2CC≡ CC6H5Þ2, where M ¼ Mo

(II) and M ¼ W (II0), have been investigated by a variety of spectro-

scopic techniques including femtosecond time-resolved infrared

spectroscopy. The singlet states are shown to be delocalized me-

tal-to-ligand charge transfer (MLCT) states for I and I0 but localized

for II and II0 involving the cyanobenzoate or amidinate ligands,

respectively. The triplet states are MoMo δδ* for both I and II but

delocalized 3MLCT for I0 and localized 3MLCT for II0. These differ-

ences arise from consideration of the relative orbital energies of

the M2 δ or M2 δ* and the ligand π as well as the magnitudes of

orbital overlap.

excited state mixed valency ∣ vibrational spectroscopy ∣ potential energy

surface ∣ cyano group ∣ ethynyl group

Conjugated organic polymers have been intensely studied overthe past two decades because of their fascinating optoelectro-

nic properties. Aside from the sheer scientific curiosity that they aroused, it soon was recognized that a plastic electronics industry

was possible and a good deal of this expectation has already beenrealized. Conjugated organic polymers find commercial applica-tions as field-effect transistors (1), light emitting diodes (2), and

photovoltaic devices (3, 4). As an extension of this field there isconsiderable interest in incorporating metal ions into conjugatedorganic systems and numerous reports are to be found in theliterature concerning the role of metal ions in tailoring the optoe-lectronic properties of the organic conjugated systems. For exam-ple, the attachment of π-conjugated ligands to Ir(III) has allowedthe luminescence to cover the entire region of the visible spec-trum (5, 6) and the incorporation of Pt(II) into conjugatedethynylthiophenes has led to a significant enhancement of theefficiency of a bulk heterojunction photocell (7).

Knowledge of electronic structure is key to the understandingof these properties and the ability to manipulate electronic struc-ture by selection of metal-organic components holds the promiseof custom design. The MM quadruply bonded unit (MM ¼ Mo2,

MoW, or W2) has many attractive features due to the tunability of the energy of the M2δ orbital by selection of the metal and itsattendant ligands and due to M2δ to organic π-conjugation. Inthis report, we show how this can influence the charge dynamicsand delocalization of singlet and triplet photo-excited states thatmay be delocalized or localized (valence trapped) metal-to-ligandcharge transfer (MLCT) states or MMδδ * states. To achieve this

we have prepared the trans-substituted compounds M2ðTiPBÞ2ðO2CC6H4-4-CNÞ2, where TiPB ¼ 2,4,6-triisopropylbenzoateand M ¼ Mo (I) o r W (I

0), and M2ðO2CMeÞ2ððN½i PrÞ2CC≡CC6H5Þ2, where M ¼ Mo (II) or W (II

0), whose structures areshown in Fig. 1. Here, the bulky TiPB and amidinate ligands favorthe trans-substitution and allow the extended conjugation of thetwo trans-ligands via Lπ - M 2δ - Lπ conjugation. Evidence of this isseen in the molecular structures found in the solid-state that

reveal the near coplanarity of the aryl groups of the p-cyano-benzoate and amidinate ligands in I and II0, respectively.

The ground state geometry and symmetry of these discretecompounds presents a situation where added charge could resideon either of two interchangeable redox active ligand sites. Thissituation is, by definition, described by the term mixed valency.In the case of compounds I, II, and their tungsten analogs I

0

and II0, an electron can be added to the ligand sites by photoex-citation of the MLCT transition and they therefore can be furtherclassified as excited state mixed valence compounds (8). Depend-ing on how strongly the ligand sites interact with one another, theexcited electron can be completely localized (valence trapped)on one ligand, completely delocalized over both ligands, or only partially delocalized.

These various classifications of the mixed-valence states canbe visualized by the potential energy surfaces (PESs) shown inFig. 2. When the ligands are noninteracting, there are two isoe-nergetic PESs in the MLCT, S1 state. This is known as Class Iunder the Robin and Day scheme (9). As the electronic coupling, H ab, begins to increase, the ligand states interact to give a lower,in-phase (S1), and upper, out-of-phase (S2), combination of the

surfaces. When the coupling is small, two distinct ligand statesremain and the lower PES is a double well. An electron in thisstate can be said to be localized on a single ligand and is known as

Fig. 1. ORTEP diagram of I • 2THF (Left ) and II0 (Right ) drawn with 50%

probability displacement ellipsoids with hydrogen atoms omitted. Structure

I • 2THF contains a crystallographic inversion center. Structure II0 was

reproduced from ref. 26 (reproduced by permission of the Royal Society

of Chemistry).

Author contributions: M.H.C. designed research; B.G.A., J.C.G., and Y.G. performed

research; Y.G. contributed new reagents/reanalytic tools; B.G.A., M.H.C., J.C.G., and

T.L.G. analyzed data; and B.G.A., M.H.C., and T.L.G. wrote the paper.

The authors declare no conflict of interest.

Freely available online through the PNAS open access option.

Data deposition: The atomic coordinates have been deposited in the Cambridge

Crystallographic Data Center (CCDC number 801312).

1To whom correspondence should be addressed. E-mail: chisholm@chemistry.

ohio-state.edu.

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doi:10.1073/pnas.1103082108/-/DCSupplemental.

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Class II, shown in Fig. 2. An excited state transition can occur

within this mixed-valence state that overcomes the barrierbetween the two surfaces and transfers the electron between theligand sites, analogous to the intervalence charge transfer (IVCT)band of ground-state mixed-valence compounds (10). As thestrength of the coupling increases further, the two states havegreater mixing and the electron can begin to become delocalizedover both ligands. As a result, the PESs move apart and thepotential energy barrier between the two sites decreases. Even-tually, a point is reached where there is no longer a barrier andthe electron is nearly equally shared. This occurs at the class II/IIIborder and is shown in the right panel of Fig. 2. When the elec-tron finally becomes completely delocalized, class III, the PESsrepresent completely bonding and antibonding molecular orbitalsand the transition between S1 and S2 becomes like that for amolecular species. In this case the IVCT transition becomes

known as a charge resonance band (11).The electronic coupling of the two organic π-systems arises

from the interaction of the M2δ orbital with the carboxylate,CO2, or amidinate, NCN, π orbitals shown in A and B of Scheme 1, below. The orbital interactions shown in A involve thein- and out-of-phase combinations of the π

of the CO2 or NCNmoiety. The out-of-phase combination has a symmetry match

with the M2δ orbital and is a metal-to-ligand backbonding inter-action. On the other hand, the interactions in B involve the filledorbitals of the carboxylate or amidinate ligands and based onorbital energetics it is the former interaction, A, that is more im-portant. Given that O is more electronegative than N, the π

of the carboxylates is lower in energy than the amidinate and thus

the mixing with the M2δ orbital and the coupling between thetrans-ligands is greater in the p-cyanocarboxylates. Also, becausethe M2δ orbital in closely related compounds is roughly 0.5 eVhigher in energy for M2 ¼ W2 relative to Mo2, the interactionsin the tungsten complexes are greater.

These qualitative bonding descriptions are supported by den-sity functional theory (DFT) calculations that are described inSI Text and Fig. S1. The calculations reveal the energy splittingof the in- and out-of-phase ligand π* combinations due to mixing

with the M2

δ

orbital. Also consistent with the calculations, elec-trochemical studies reveal metal based oxidation and, for I and I0,ligand based reduction waves by cyclic voltametry. Reduction

waves for II and II0 were not seen within the solvent window

due to the higher energy of their π* orbitals.

Results and Discussion

Photophysical Studies: Electronic Spectroscopy. All of the com-pounds are strongly colored due to MLCT and their electronicabsorption spectra are shown in Fig. 3. Though allowed by sym-metry, the 1MM δδ* transition is weak due to the relatively pooroverlap of the metal d orbitals. In each case, the lowest energy transition is characterized as MLCT (12, 13). Although the planar D2 h structure of these molecules is the ground state structure,thermal energy leads to a Boltzmann distribution of confor-

mers/rotamers in which the aryl groups of the ligand deviate fromcoplanarity with the carboxylate CO2 or amidinate NCN moi-eties. The room temperature solution spectra represent the com-bined absorptions of an ensemble of molecules, which together

with vibronic features arising from the displacement of theground and excited state potential energy surfaces leads to thebroadening of these MLCT bands. Upon cooling to 77 K in a2-methyltetrahydrofuran glass, these bands sharpen with the peak shifting to lower frequency and the vibronic features becomingbetter resolved, as shown in Figs. S2 and S3. At higher energy to these MLCT bands associated with the conjugated ligands,

we also observe weaker MLCT bands involving the CO2 moietiesof the TiPB (in I

0) and acetate ligands (in II0) and ligand based

ππ* transitions.

All of the compounds are emissive and compounds I

(Figs. S3and S4), II (14), and II0 (14) each emit from both the S1 and T 1states. In compound I

0, the T 1–S0 energy gap is estimated to besmall and this favors nonradiative decay so only emission fromthe S1 state is observed (Figs. S3 and S4). Interestingly, the S1states of both II and II

0 show a solvent dependent emission whereas the MLCT absorptions do not. Furthermore, the phos-phorescence of the tungsten complex (II

0) shows a solvent depen-dence but that of the molybdenum compound (II) does not. Themore limited range of solubilities of compounds I and I0 made thesolvent dependent study less clear. However for II , our interpre-tation of the solvent dependence is that the initial absorptionoccurs without a significant change in dipole moment whereasthat associated with the emission from the S1 states is significant(14). Because the ground state molecules are symmetrical ( D2 h)

and have a center of inversion, the Franck –Condon absorptionconserves the point group symmetry to place the excited electron

Fig. 2. Potential energy surface representation of the mixed-valence states

for a symmetrical system. S 1 represents the state where an excited electron

occupies the in-phase L-π and S 2 the state where an excited electron occu-

pies the out-of-phase L-π combination. (Left ) Class I; weakly coupled loca-

lized. (Right ) Class III strongly coupled, delocalized. (Center ) Class II-II/III;

0 < a < 1. Modified from ref. 11.

Fig. 3. Electronic absorption spectra showing the molar absorptivity (ϵ) of I

(red) I 0 (purple), II (green), II 0 (blue) at room temperature in THF.Scheme 1.

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delocalized over both ligands. Further relaxation of the Franck –Condon S1 states in polar solvents leads to the localization of theexcited electron mostly on an individual ligand, increasing thedipole moment of the S1 state and causing the emission solventpolarity dependence.

Similarly, T 1 emission for II0 (W) is solvent dependent but

emission from T 1 for II (Mo) is not. At low temperature, theemissive T 1 state for II shows vibronic features assignable to theMoMo stretching frequency with νðMoMoÞ ∼ 400 cm−1 (14). The

vibronic spacing together with the lack of solvent dependence inthe T 1 state emission for II is consistent with the assignment of this state as the 3MoMoδδ state. Furthermore we should notethat the lowest energy emission for II0 also shows solvent depen-dence and suggests an assignment of 3MLCT for the T 1 state of this complex. Again, our evaluation of the electron delocalization

within the MLCT excited states is further substantiated by thefemtosecond time-resolved infrared (fsTRIR) spectroscopy ex-periments, below, which also confirm our assignments of the S1

and T 1 states as MLCT or δδ*.

Time-Resolved Studies. We have examined the compounds by fem-tosecond and nanosecond transient absorption spectroscopy (fsTA and nsTA), detecting both the S1 and T 1 states in all cases.The lifetimes for the S1 MLCT states measured for these types of quadruply bonded compounds are generally on the order of 10 ps(15–17) and the compounds under investigation here range fromapproximately 3–20 ps. On the other hand, the lifetimes of the T 1states generally range from 0.010–100 μs depending largely uponthe metal and give an indication of their nature as 3MLCT or 3δδ (12, 13, 16, 18). For carboxylate complexes, if the T 1 state is3MLCT, the lifetime is on the order of hundreds of nanosecondsor less. In the case of I0, the T 1 state is detected in the fsTA as along-lived transient but the lifetime is shorter (τ < 10 ns) thanthe time resolution of the nsTA experiment. On the other hand,if the T 1 state is 3δδ , the lifetimes are longer, lasting for approxi-mately 100 μs. In amidinate complexes, the lifetimes are observedto increase as a result of the ligand based orbitals originating athigher energy compared to the carboxylate ligands. In II

0, the T 1

state has τ ¼ 4.6 μs and in II it is approximately 105 μs. In general we find that T 1 for Mo complexes is 3

δδ and the lifetimes are

3–4 orders of magnitude longer than T 1 for W complexes thatare 3MLCT. The transient absorption spectra along with repre-sentative kinetic traces for I and I 0 are shown in Fig. S5 and thosefor II and II

0 are shown in ref. 14. Based upon the absorption,emission, and transient absorption results we can formulate aJablonski diagram describing the general photophysics of the fourcompounds, and this is shown in Fig. 4.

Photophysical Studies: Vibrational Spectroscopy (fsTRIR). Particularly convenient for studying the nature of these types of photoexcitedstates is the presence of the IR active C≡N and C≡ C functionalgroups within the ligands. As a consequence, the compounds have

also been subjected to fs time-resolved, visible pump, IR probe(fsTRIR) spectroscopy.

The p-cyanobenzoates, I and I0, show νðC≡NÞ ∼ 2;230 cm−1

in the ground state and upon excitation a single new band occursthat is shifted to lower energy by approximately 60–70 cm−1, asshown in the time-resolved infrared (TRIR) spectra above, Fig. 5.In the case of I (Mo), this band decays completely with a lifetimeof approximately 3.9 ps, similar to that of the 1MLCT state de-termined by fsTA, and no new transient band is observed there-after. In the case of I0, however, this band decays with a lifetime of 7.2 ps (again similar to that of the 1MLCT state lifetime by fsTA)to give rise to a long-lived transient that is present after 1,000 psat nearly the same wavenumber. This result is a clear indicationthat for I

0 (W) both the S1 and T 1 states are MLCT and furthersuggests that the charge distribution in both the singlet and tripletstates is similar. The spectroscopic features of the ligands inMLCT excited states commonly resemble those of the reducedligand. For this reason, we have completed DFT calculations onthe model anions to predict the change in νðC≡NÞ upon reduc-tion of the ligand. Our expectation is that, because the anion cal-culations were done at the optimized ground state geometry with D2 h symmetry, the calculated shift predicts the experimental shiftin the situation that the photoexcited electron is delocalized overboth ligands (Class III). For both compounds I and I0, the calcu-lated shift is approximately 65 cm−1. Because the experimental

values are similar, it implies that the 1MLCT excited states forboth complexes as well as the 3MLCT excited state for I

0 are de-localized or Class III mixed-valence (MV) ions in the Robin andDay scheme. The results are also consistent with the expectation

that in a fully delocalized excited state both of the trans-ligandsare equivalent and both should have identical stretching frequen-cies. A summary of the TRIR experimental and DFT calculatedC≡N frequencies are shown in Table 1 and representativekinetic traces from the fsTRIR experiments are shown in Fig. S6.

Fig. 4. Jablonski diagram showing excited states in compounds I (red), I0

(purple), II (green), and II 0 (blue) observed after MLCT photoexcitation.

Fig. 5. TRIR spectra for I (Upper ) and I0 (Lower ) in THF at room temperature.

Table 1. Summary of νðC≡NÞ values (cm−1)

Experimental (TRIR) Calculated (DFT)

Ground state Excited state Shift Neutral Anion Shift

Compound νðC≡NÞ νðC≡NÞ Δν νðC≡NÞ νðC≡NÞ− Δν

I (1MLCT) 2,230 2,157 73 2,345 2,278 67I0 (1MLCT) 2,225 2,164 61 2,343 2,279 64I0 (3MLCT) 2,225 2,162 63 — — —

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Relatively few examples occur in the literature of excited stateinfrared spectra for molecules involving cyano groups. One exam-ple that does exist is 4-dimethylaminobenzonitrile (DMABN).It is generally agreed that, in polar solvents, the lowest excitedstate of DMABN is described by the transfer of an electron fromthe amino group to benzonitrile and there is a 90° dihedral anglebetween these groups, which has been named the twisted-intra-molecular charge transfer state (TICT) state (19). There are tworeports of the TRIR in the C ≡N stretching frequency region of the TICT state for this molecule (20, 21) In THF, the shift down-

ward is 102 cm−1, significantly larger than that observed for I andI0

. This further supports the view that the excited electron is de-localized across two cyanobenzoate ligands in I and I 0 whereas inDMABN the electron can occupy only one benzonitrile unit.

The vibrational spectroscopic features associated with thecompounds II and II

0 are even more interesting. In the groundstate these compounds show weak IR bands close to 2;200 cm−1

associated with νðC≡ CÞ. Upon photoexcitation, the S1 states of these compounds have two distinct transient bands that haveboth been shifted to lower wavenumber than that in the groundstate, see Fig. 6. One band has weaker intensity at approximately 2;150 cm−1 and the other, with much stronger intensity, occurs atmuch lower wavenumber between 1,950 and 2;000 cm−1. Similarto the p-cyanobenzoates, these initial transient bands decay withlifetimes 18.8 and 6.8 ps for II and II

0 (Fig. S7), respectively, which is in agreement with the 1MLCT lifetimes determined

by fsTA. For II (Mo), the transient decays completely and nonew transient band is observed thereafter. On the other hand,the decay of the initial transients at 2,150 and 1;990 cm−1 for

II0 (W) is accompanied by the formation of a new set of transient

bands at 2;115 cm−1 and 1;940 cm−1 that are long-lived and per-sistent. The presence of these persistent transient bands in II

0

clearly indicates that the T 1 state is 3MLCT and the absenceof these bands in II implies that the T 1 state can be assignedto 3MoMoδδ .

Another interesting feature of the TRIR spectra for II0 is

that the transient bands associated with νðC≡CÞ in the 3MLCTstate are shifted to even lower wavenumber than those for the

corresponding 1

MLCT state. This suggests that in the T 1 state thenegative charge is more localized on the CC triple bond relativeto the S1 state. It is also consistent with the general view that sing-let states are more diffuse than triplet states, which are lower inenergy and more tightly bound.

To evaluate the electronic coupling within the Robin and Day scheme for the compounds of type II, a set of calculations on themodel anions were again completed. These calculations predict ashift for νðC≡CÞ of approximately 130 cm−1 to lower wavenum-ber in the reduced anion. This shift is notably larger than what

was calculated for the p-cyanobenzoates. In Fig. S6, the molecu-lar orbital (MO) picture for the lowest unoccupied molecular or-bital (LUMO), as determined from DFT, shows that the electrondensity has a much greater contribution from the C≡C anti-bonding orbital in II 0 than that contribution from the C≡N anti-

bonding orbital in I0. Therefore, the observed shifts are larger forthe ligands with C ≡ C moieties compared to the C≡N moiety.

As stated above, the MO calculations pertain to the delocalizedmodel or MV ion of Class III. The observed TRIR shift of themajor peak in the compounds II and II

0 is notably greater thanthe calculated value, which together with the solvent dependentemission implicates a polarized MLCT state where one ligandhas been mostly reduced by the photoexcited electron with only a small “spillover” to the other ligand. In other words, the ami-dinate complexes, II and II

0, exist as only partially delocalizedmixed-valence Class II ions in both their relaxed 1MLCT and3MLCT states. The observation of two distinct transient peaksin the TRIR spectra is an experimental signature of this situation.

Also, recall the earlier statement that the Franck –Condon ab-

sorption is to a delocalized state. Because the initial spectra afterexcitation contain two peaks, it appears that the charge localiza-tion process that places the excited electron mostly on one ligandoccurs more rapidly than can be detected and within the lifetimeof the 1MLCT states.

The specific C≡C stretching wavenumbers determined forcompounds II and II0 are collected in Table 2 along with thecalculated wavenumbers for the model compounds and their cor-responding anions. There it can be seen that Δν2 for the 1MLCTstate of II is notably larger than that for II0, which may be anindication of the more C≡ C triple bond localized nature of thecharge in the 1MLCT state for the molybdenum compound.

There have been several reports of excited state infraredspectra for square-planar transition metal complexes of platinuminvolving phenylacetylide ligands that monitor the C≡ C triplebond stretching wavenumber (22–25). Of these, the most notablecomparison to compounds II and II

0 is trans-Pt½ðCCPhÞ2ðPBu3Þ2in its 3MLCT state(22). This molecule has a highest occupied mo-

Fig. 6. TRIR spectra for II (Upper ) and II0 (Lower ) in THFat room temperature.

Table 2. Summary of νðC≡ CÞ values (cm−1)

Experimental (TRIR) Calculated (DFT)

Ground state Excited state Shift Neutral Anion Shift

Compound νðC≡ CÞ ν1ðC≡ CÞ ν2ðC≡ CÞ Δν1 Δν2 νðC≡ CÞ νðC≡ CÞ− Δν

II (1MLCT) 2,200 * 2,155 1,959 45 241 2,293 2,152 141II0 (1MLCT) 2,180 2,152 1,989 28 191 2,281 2,153 128II0 (3MLCT) 2,180 2,116 1,942 64 238 — — —

*Value obtained from Raman spectrum due to weak IR intensity.

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lecular orbital (HOMO) that is composed of ligand π orbitals with some admixture of Pt 5 d xy and the LUMO is totally ligandπ*. Photoexcitation is thus a mixture of ππ* and MLCT andthis yields a triplet state that has a transient IR band assignableto νðC≡ CÞ that is shifted approximately 360 cm−1 to lower wa-

venumber and another that is shifted to higher wavenumber ap-proximately 10 cm−1 relative to the ground state νðC≡ CÞ value.This is consistent with a localized triplet state arising from ππ* +MLCTwhere one phenylacetylide is in the photoexcited state andthe other feels the positive charge on the platinum that lessensthe Pt 5d to ligand π* backbonding. It is thus evident that the

higher energy M2δ orbitals and the smaller HOMO-LUMO gapfor MM quadruply bonded complexes of Mo or W allows forgreater coupling of the two ligand π* systems.

Concluding RemarksThe nature of the photoexcited states in compounds I, II, andtheir tungsten analogs can be correlated with the electronic struc-tures of the molecules in their ground states. First, the lowerenergy of the Mo2 δ and δ* orbitals leads to the T 1 states being3MMδδ

and this can be expected to be generally the case unlessthe ligand π* orbital is very low in energy and the 1MLCT absorp-tion falls in the near infrared. Conversely, the higher energy of the W 2 δ and δ* will lead to T 1 being 3MLCT unless the energy of the ligand π* orbital is notably high in energy and the 1MLCTabsorption is in the high energy region of the UV/visible spec-

trum. Second, the coupling of the two trans ligands is dependent

primarily on the energy separations between the M2δ and theligand based LUMO π* orbital. The smaller the energy gap thegreater the coupling and this can lead to fully delocalized S1and T 1 MLCT states. As the coupling of the two ligands viathe M2δ decreases, the photoexcited state can take on mixed va-lence Class II characteristics. The initial photoexcitation may beto a delocalized state that rapidly relaxes to the charge localizedClass II MV state. This is seen for the amidinate compoundsand their ground state and photoexcited state potential energy

surfaces can be represented by Fig. 2, above. Third, the extentof charge delocalization in the singlet and triplet MLCT statesis in general expected to be different with the higher energy S1 state being more diffuse than the lower energy T 1 state. This

was very nicely revealed in the TRIR spectra of II0.Collected in Table 3 is a summary for compounds I , I 0, II, and

II0 of the S1 and T 1 state assignments and their classifications

under the Robin and Day scheme. It is particularly appealingthat these four molecules have revealed these limiting cases.Furthermore, based on considerations of orbital energies invol-

ving the M2δ and the ligand π* we anticipate these findings willbe applicable to the photoexcited states of conjugated polymersincorporating MM quadruple bonds (15).

Materials and MethodsAll reactions were carried out in an inert atmosphere using Schlenk linetechniques whereas samples for measurements were prepared inside a glove

box. All solvents were dried, distilled, and degassed prior to use. Steady state

absorption, emission, and infrared spectra, nanosecond and femtosecond

transient absorption spectra, electrochemistry, electronic structure calcula-

tions, fsTRIR, and crystal structure details are presented in the SI section

materials and methods. Details for the synthesis of compounds I and I0

can also be found in the SI Text . Compounds II and II0 were synthesized

according to published procedures (26).

ACKNOWLEDGMENTS. This material is based on work supported by theNational Science Foundation under Grant CHE-0957191. The work wasperformed at The Ohio State University, partly in the Center for Chemicaland Biophysical Dynamics. We thank The Ohio State University Institutefor Materials Research and the Wright Center for Photovoltaic Innovationand Commercialization for financial support and the Ohio Supercomputing

Center for computational resources.

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Table 3. Summary of photophysical assignments and Robinand Day classification

Compound (M) State Assignment Classifi cation

I (Mo) S 11MLCT Class III

T 13δδ N.A.

I0 (W) S 11MLCT Class III

T 13MLCT Class III

II (Mo) S 11MLCT Class II

T 13δδ N.A.

II0

(W) S

1

1

MLCT Class IIT 13MLCT Class II

Alberding et al. PNAS Early Edition ∣ 5 of 5

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