Time-resolved Transient Absorption Spectra Research Articles

Meso-meso linked diporphyrin functionalized single-walled carbon nanotubes

Meso-meso linked diporphyrins ([H 2Por] 2) covalently functionalized soluble single-walled carbon nanotubes ([H 2Por] 2-SWNTs) have been successfully prepared. As a light-harvesting chromophore, meso-meso linked diporphyrins have been incorporated into a photosynthetic electron-transfer model with SWNTs as an electron-acceptor. The steady emission characteristics revealed the existence of the effective energy and electron transfer between the excited porphyrin moiety and SWNTs, and the electron transfer process was confirmed by electrochemical study. The charge separation quenching process was supported by the results of time-resolved transient absorption spectra, and the lifetime of charge-separation state was observed to be 145 ns, which was 2.5 times of that of monoporphyrin modified SWNTs (TPP-SWNTs).

Cooperative enhancement of TPA in cruciform double-chain DSB derivation: a femtosecond transient absorption spectra study

Femtosecond time-resolved transient absorption (TA) spectra study was adopted to study the mechanism of the cooperative enhancement of two-photon absorption (TPA) cross section from the linear structure 1,4-di(4′-N,N-diphenylaminostyryl)benzene (DPA-DSB) to its cruciform double-chain dimer DPA-TSB. The results suggested that a non-emissive intramolecular charge-transfer (ICT) state, ICT’, was present upon excitation in the dimer, which was absent in the monomer. The existence of this non-emissive state, indicating the enhancement of the intramolecular charge-transfer of the dimer, should be the reason for the cooperative enhancement of the TPA cross section of the dimer compared to the monomer.

Direct determination of the excited state energies of the xanthophylls diadinoxanthin and diatoxanthin from Phaeodactylum tricornutum

Abstract Steady-state and femtosecond time-resolved transient absorption spectra of diadinoxanthin and diatoxanthin were measured in the visible and near-infrared (NIR) regions at 293 K. The difference in energy between the visible S 0 → S 2 transitions from the steady-state absorption measurements and the S 1 → S 2 transitions observed in the transient absorption spectra in the NIR region yields precise values of the S 1 energies. The data are important for evaluating the mechanism by which excess energy is dissipated by algal systems that interconvert these xanthophylls in response to changes in photon flux levels in the marine environment.

Ground-State Proton Transfer of 7-Hydroxyquinoline Confined in Biologically Relevant Water Nanopools

The ground-state reverse proton transfer of 7-hydroxyquinoline catalyzed by water confined in AOT reverse micelles has been investigated by measuring time-resolved transient-absorption spectra and kinetic profiles. The transfer time is profoundly retarded in water nanopools compared with that in bulk water (26 μs) although it diminishes with the size increase of the water nanopool. The spectral-shift time of tautomeric transient absorption agrees well with the proton transfer time. The probe molecule is subject to the local gradient of polarity, whose magnitude is altered with the sizes of water nanopools. Accordingly, the observations made in this study indicate the multidimensional character of reaction coordinates, in which solvent polarization coupled to charge transfer plays a seminal role in the control of overall proton-transfer dynamics. The retardation of proton transfer in water nanopools is ascribed to the increased formation energy of a charge-transferred optimal configuration, which is prereq...

Photoinduced Intramolecular Electron Transfer in Conjugated Perylene Bisimide-Dithienothiophene Systems: A Comparative Study of a Small Molecule and a Polymer

The solution photophysical properties of two conjugated dithienothiophene (DTT)-perylene bisimide (PBI) systemsa polymer, poly{[N,N'-bis(2-decyl-tetradecyl)-3,4,9,10-perylene diimide-1,7-diyl]-alt-(dithieno[3,2-b:2',3'-d]thiophene-2,6-diyl)}, and a small molecule, 1,7-bis(dithieno[3,2-b:2',3'-d]thiophene-2-yl)-N,N'-bis(2-decyl-tetradecyl)-3,4,9,10-perylene diimidein solution have been investigated. Strong quenching of the fluorescence of the PBI moiety was observed in both DTT-PBI systems, suggesting the possibility of an efficient intramolecular electron-transfer process. The kinetics of photoinduced electron transfer in the DTT-PBI polymer and monomer in solutions were explored by femtosecond time-resolved transient absorption spectra. It was found that both the rates of charge separation and charge recombination in the DTT-PBI polymer were approximately double those in the small molecule. This indicates that electronic coupling plays an important role in the electron-transfer process in a polymer system.

Control of Photoinduced Electron Transfer in Zinc Phthalocyanine−Perylenediimide Dyad and Triad by the Magnesium Ion

Photoexcitation of a zinc phthalocyanine-perylenediimide (ZnPc-PDI) dyad and a bis(zinc phthalocyanine)-perylenediimide [(ZnPc) 2-PDI] triad results in formation of the triplet excited state of the PDI moiety without the fluorescence emission, whereas addition of Mg (2+) ions to the dyad and triad results in formation of long-lived charge-separated (CS) states (ZnPc (*+)-PDI (*-)/Mg (2+) and (ZnPc) 2 (*+)-PDI (*-)/Mg (2+)) in which PDI (*-) forms a complex with Mg (2+). Formation of the CS states in the presence of Mg (2+) was confirmed by appearance of the absorption bands due to ZnPc (*+) and PDI (*-)/Mg (2+) complex in the time-resolved transient absorption spectra of the dyad and triad. The one-electron reduction potential ( E red) of the PDI moiety in the presence of a metal ion is shifted to a positive direction due to the binding of Mg (2+) to PDI (*-), whereas the one-electron oxidation potential of the ZnPc moiety remains the same. The binding of Mg (2+) to PDI (*-) was confirmed by the ESR spectrum, which is different from that of PDI (*-) without Mg (2+). The energy of the CS state (ZnPc (*+)-PDI (*-)/Mg (2+)) is determined to be 0.79 eV, which becomes lower that of the triplet excited state (ZnPc- (3)PDI*: 1.07 eV). This is the reason why the long-lived CS states were attained in the presence of Mg (2+) instead of the triplet excited state of the PDI moiety.

Size Effects of Oligothiophene on the Dynamics of Electron Transfer in π-Conjugated Oligothiophene-Perylene Bisimide Dyads

The photophysical properties of a series of π-conjugated oligothiophene-perylene bisimide dyads nT-PBI (n = 1, 2, and 3) in solutions were investigated. A sharp fluorescence quenching of the perylene bisimide moiety in these dyads nT-PBI (n = 1, 2, and 3) was observed upon photoexcitation of the perylene bisimide moiety, indicating a fast and efficient electron-transfer process. The properties of photoinduced electron transfer of these dyads nT-PBI (n = 1, 2, and 3) in solutions were investigated by time-resolved fluorescence spectra and femtosecond transient absorption spectra in the visible region. It was found that as the number of thiophene repeat units, i.e., the size, increases, the rate constants of charge separation and charge recombination increase about 3 orders and over 1 order of magnitude, respectively, because of the combined effects of energetics and electronic coupling. These results demonstrate that a change in size of donor could change and modulate the dynamics of electron transfer.

Photophysical Properties of the Newly Synthesized Triad Based on [70]Fullerene Studies with Laser Flash Photolysis

N,N-Dimethylaniline-pyrazolinoC70-ferrocene has been prepared with the 1,3-dipolar cycloaddition reaction of a nitrile imine with C70. Although three regioisomers regarding the position of the pyrazolino group on the C70 were identified in the reaction products, molecular orbital calculations indicate that the stabilities and electronic properties of the three isomers are almost the same, which was confirmed by the sharp redox peaks. The photophysical properties of the triads have been investigated by measuring the time-resolved emission and transient absorption spectra showing that charge separation takes place efficiently via the photoexcited singlet state of the C70 moiety with accepting an electron from the donor moieties. It was found that the pyrazolino ring mediates a charge separation between the donor moieties and the photoexcited C70 moiety.

Oligosilane Chain-Length Dependence of Electron Transfer of Zinc Porphyrin−Oligosilane−Fullerene Molecules

A new series of zinc porphyrin-fullerenes bridged by flexible oligosilane chains ZnP-[Sin]-C60 (n = 1-5) was synthesized, and the photophysical properties of these molecules were investigated using steady-state and time-resolved spectroscopic methods. The spectral observations can be well explained by assuming the coexistence of extended conformers and folded conformers, that is, the observed emissions are from the extended conformers while the folded conformers form very short lifetime nonfluorescent excited-state charge-transfer (CT) complexes. Time-resolved transient absorption spectra suggest the generation of intramolecular radical-ion pairs that have sub-microsecond lifetimes. With the number of silicon atoms of the bridged oligosilane, the lifetimes of the radical-ion pairs do not vary regularly, indicating that intramolecular collision of the radical-cation moiety with the radical-anion moiety controls the charge-recombination rate. The attenuation factor of the electron transfer of the silicon chain was evaluated by the bridge-length dependence of charge-separation rate to be 0.16 A-1 on the basis of the oligosilane chain-length dependence of fluorescence lifetimes. This is the first evaluation of the attenuation factor for the one-dimensional Si-Si chain to the best of our knowledge.

Intermolecular photoinduced electron-transfer processes between C 60 and aniline derivatives in benzonitrile

The quenching behavior of the triplets of C 60 by various aniline derivatives ( 1a– d and 2a– e) was investigated by means of laser flash photolysis in benzonitrile at 293 K. Electron transfer process was proposed to be the main mechanism because of the direct detection of radical ions of aniline derivatives and C 60 in time-resolved transient absorption spectra. The quenching rate constants ( k q) of 3 C 60 * by different substrates determined at 740 nm approach or reach the diffusion-controlled limit. DFT method was employed to calculate the unknown oxidation potentials of substrates in solution. With these E ox values, free energy changes (Δ G) were obtained through Rehm–Weller equation. Dependence of observed quenching rate constants on the free energy changes further indicates the photoinduced reactions between 3C 60 * and substrates proceed through an electron transfer mechanism. Obtained k q values for the aniline derivatives are impacted obviously by ground-state configurations and the kinds substituents quantified by Hammett σ constant. Good correlation between log k q and σ values conforms to the empirical Hammett equation. A more negative ρ value (−3.356) was gained for anilines ( 2a– e) than that of N, N-dimethylanilines ( 1a– d) (−1.382), which suggests a more susceptible reactivity for the former substrates. Charge density distribution of reaction center “N” originated from quantum calculation supports this suggestion. In addition, a relationship between quenching rate constants and solvent viscosity was gained from C 60/dimethyl -p-toluidine system in altered mixtures of acetonitrile and toluene.

Studies on Photoinduced H-Atom and Electron Transfer Reactions of o-Naphthoquinones by Laser Flash Photolysis

The quenching of the triplets of 1,2-naphthoquinone (NQ) and 1,2-naphthoquinone-4-sulfonic acid sodium salt (NQS) by various electron and H-atom donors was investigated by laser flash photolysis measurement in acetonitrile and benzene. The results showed that the reactivities and configurations of 3NQ* (3NQS*) are governed by solvent polarity. All the quenching rate constants (kq) measured in benzene are larger than those in acetonitrile. The SO3Na substituent at the C-4 position of NQS makes 3NQS* more reactive than 3NQ* in electron/H-atom transfer reactions. Large differences of kq values were discovered in H-atom transfer reactions for alcohols and phenols, which can be explained by different H-abstraction mechanisms. Detection of radical cations of amines/anilines in time-resolved transient absorption spectra confirms an electron transfer mechanism. Triplets are identified as precursors of formed radical anions of NQ and NQS in photoinduced reactions. The dependence of electron transfer rate constants on the free energy changes (DeltaG) was treated by using the Rehm-Weller equation. For the four anilines with different substituents on the para or meta position of amidocyanogen, good correlation between log kq values with Hammett sigma constants testifies the correctness of empirical Hammett equation. Charge density distributions, adiabatic ionization/affinity potentials and redox potentials of NQ (NQS) and some quenchers were studied by quantum chemistry calculation.

Photoinduced electron-transfer dynamics and long-lived CS states of donor–acceptor linked dyads and a triad containing a gold porphyrin in nonpolar solvents

An electron donor–acceptor linked compound containing Zn(II) and Au(III) porphyrins ( ZnPQ - AuPQ + PF 6 - ) has been designed and synthesized to examine its electrochemical and photophysical properties. Time-resolved transient absorption spectra of ZnPQ–AuPQ + were measured by nanosecond laser photolysis in toluene and cyclohexane. The observed transient absorption bands at 600–800 nm are assigned to the charge-shift state (ZnPQ + –AuPQ). The charge-shifted state decays via back electron transfer (BET) to the ground state rather than to the triplet excited state. The BET rate was determined from the disappearance of the absorption band at 750 nm due to ZnPQ + in ZnPQ + –AuPQ. The decay of the absorption band obeys first-order kinetics. The k BET value in cyclohexane is determined as 1.0 × 10 5 s −1, which corresponds to a lifetime of 10μs, whereas the lifetime becomes much shorter (250 ps) in benzonitrile. The charge-shift state is also observed for a zinc porphyrin–free base porphyrin–gold porphyrin triad (ZnPQ–2HPQ–AuPQ +), which has a longer lifetime of 14 μs in cyclohexane. The detailed photodynamics of ZnPQ–AuPQ + and ZnPQ–H 2PQ–AuPQ + are also reported. We have successfully attained long-lived charge-shift states in nonpolar solvents using the ZnPQ–AuPQ + dyad and the ZnPQ–H 2PQ–AuPQ + triad. This photoinduced electron-transfer in low dielectric environment mimics a key feature of photosynthetic reaction centers.

Photochemical Reaction Mechanism of 2,2′-Pyridil in Alcoholic Media

Abstract Time-resolved transient absorption spectra of photo-irradiated 2,2′-pyridil were measured in PVA films. The broad bands at 552 and ∼455 nm, appearing just after photo-excitation of 2,2′-pyridil, are assigned to the skewed triplet species. The triplet species changes into 1-hydroxy-2-oxo-1,2-di(2-pyridyl)ethyl (pyridoil) radical via the abstraction of a hydrogen atom from the solvent. The radical produced by the photo-irradiation of 2,2′-pyridil is confirmed to be the pyridoil radical from the measurement of the polarized absorption spectrum, with the aid of the stretched polymer film technique and MO calculations. The time-resolved transient absorption spectra of 2,2′-pyridil irradiated by a xenon flash lamp in a 1:1 (v/v) mixed solvent of methanol and ethanol show that the pyridoil radical further abstracts a hydrogen atom from the solvent to afford cis-enediol (cis-1,2-di(2-pyridyl)-1,2-ethenediol) and that cis-enediol is converted into trans-enediol (trans-1,2-di(2-pyridyl)-1,2-ethenediol). 2,2′-Pyridil shows a structured Tn ← T1 absorption spectrum in a 1:1 (v/v) mixed solvent of methylcyclohexane and 2-methylbutane, indicating that 2,2′-pyridil takes a planar structure in the lowest triplet excited state in a nonpolar medium.

Spectroscopic Studies on Inclusion Properties of Fullerenocalix[4]arene Conjugates with Metal Ions

Metal-chelating properties-in the ground and excited states-of fullerenocalix[4]arenes containing two malonamide substituents at the upper rim and four alkyl ester chains at the lower rim have been studied by means of steady-state absorption, fluorescence spectroscopy, and time-resolved transient absorption spectra. In particular, the influence that Ag+ enforces on the fullerene electronic spectra is due to direct interactions between Ag+ and the surface of C60. The effects stemming from Na+, Mg2+, and Ba2+, on the other hand, are indirect and are introduced through chelating the metal ions to the calix[4]arene moiety. They strongly depend on the molecular structure of the fullerenocalix[4]arenes. No spectroscopic evidence was obtained for any influence caused by Mn2+, although the malonamide groups provide good chelating ability even for this transition metal ion.

Photoinduced Charge Separation and Charge Recombination of Oligothiophene−Viologen Dyads in Polar Solvent

Photoinduced intramolecular charge separation and charge recombination of oligothiophene−methyl viologen dyads (Tn−MV2+·2X-, n = 4 or 8, X- = I- or PF6-), which were designed to generate positively charged radical ion pairs (Tn•+−MV•+·2X-), have been investigated by the time-resolved fluorescence and transient absorption spectra in CH3CN. Upon excitation of the T8 moiety in T8−MV2+·2X-, the radical cation pair (T8•+−MV•+·2X-) was produced via the excited singlet state of T8 (1T8*), because the fluorescence lifetime of the 1T8* moiety in T8−MV2+·2X- becomes short compared with pristine 1T8*. The lifetimes of T8•+−MV•+·2X- were evaluated to be 720−980 ns depending on the counteranion, I- or PF6-. On the other hand, T4−MV2+·2X- leads to lifetimes of T4•+−MV•+·2X- of 210−240 ns, although the charge separation seems to occur via the excited triplet state of T4 (3T4*), because the fluorescence quenching of the 1T4* moiety in T4−MV2+·2X- was not observed. As factors to achieve these relatively long lifetimes of Tn•+−MV•+·2X-, the charge recombination between of the positive charges (holes) on the Tn and MV moieties and the center-to-center distance between the positive charges (holes) on the Tn and MV moieties may be considered, in addition to the triplet spin character of Tn•+−MV•+·2X-.

Laser flash photolysis detection of mono- and biradical mesitylene

Time-resolved transient absorption spectra have been obtained first for mesitylene by 355 nm laser flash photolysis at ambient temperature. Mono- and biradicals were observed in the experiment, in which the transient absorption peak at about 370 nm was assigned to the biradical and the peak at about 580 nm to the monoradical; the biradical has a longer lifetime than the monoradical. According to the representative value of the C-H bond dissociation energy for aromatic compounds monoradical formation might be a single-photon process. For comparison with the absorption spectra the time-resolved fluorescence spectra were obtained and also the biradical was assigned to about 393 nm.

Effects of metal ions on photoinduced electron transfer in zinc porphyrin-naphthalenediimide linked systems.

Zinc porphyrin-naphthalenediimide (ZnP-NIm) dyads and zinc porphyrin-pyromellitdiimide-naphthalenediimide (ZnP-Im-NIm) triad have been employed to examine the effects of metal ions on photoinduced charge-separation (CS) and charge-recombination (CR) processes in the presence of metal ions (scandium triflate (Sc(OTf)(3)) or lutetium triflate (Lu(OTf)(3)), both of which can bind with the radical anion of NIm). Formation of the charge-separated states in the absence and in the presence of Sc(3+) was confirmed by the appearance of absorption bands due to ZnP(.) (+) and NIm(.) (-) in the absence of metal ions and of those due to ZnP(.) (+) and the NIm(.) (-)/Sc(3+) complex in the presence of Sc(3+) in the time-resolved transient absorption spectra of dyads and triad. The lifetimes of the charge-separated states in the presence of 1.0 x 10(-3) M Sc(3+) (14 micros for ZnP-NIm, 8.3 micros for ZnP-Im-NIm) are more than ten times longer than those in the absence of metal ions (1.3 micros for ZnP-NIm, 0.33 micros for ZnP-Im-NIm). In contrast, the rate constants of the CS step determined by the fluorescence lifetime measurements are the same, irrespective of the presence or absence of metal ions. This indicates that photoinduced electron transfer from (1)ZnP(*) to NIm in the presence of Sc(3+) occurs without involvement of the metal ion to produce ZnP(.) (+)-NIm(.) (-), followed by complexation with Sc(3+) to afford the ZnP(.) (+)-NIm(.) (-)/Sc(3+) complex. The one-electron reduction potential (E(red)) of the NIm moiety in the presence of a metal ion is shifted in a positive direction with increasing metal ion concentration, obeying the Nernst equation, whereas the one-electron oxidation potential of the ZnP moiety remains the same. The driving force dependence of the observed rate constants (k(ET)) of CS and CR processes in the absence and in the presence of metal ions is well evaluated in terms of the Marcus theory of electron transfer. In the presence of metal ions, the driving force of the CS process is the same as that in the absence of metal ions, whereas the driving force of the CR process decreases with increasing metal ion concentration. The reorganization energy of the CR process also decreases with increasing metal ion concentration, when the CR rate constant becomes independent of the metal ion concentration.

Investigation of photoinduced electron transport in electron sensitizer/relay/acceptor hetero-LB films using transient absorption technique

Photoinduced electron transport processes in hetero-Langmuir–Blodgett (LB) films composed of electron sensitizer (S), relay (R) and acceptor (A) molecules were investigated by femtosecond transient absorption technique. 7,8-Dimethyl-10-dodecyl isoalloxazine (flavin), N-allyl- N′-[3-propylamido- N″, N″-di( n-octadecyl)]-4,4′-bipyridium dibromide (viologen) and N-docosilquinolinium tetracyanoquinodimethane (TCNQ) were employed as S, R and A units, respectively. Based on the time-resolved transient absorption spectra of S/R/A series hetero-LB films with different numbers of R layers, the optimum number of R layers for efficient photoinduced charge transport from S to A layers via R was determined to be two. Over two layers of R molecules in the S/R/A series LB films, the charge transport rate became slower due to an increase in the back electron transport from R to A layers. S(30 layers)/A(31 layers) (S(30)/A(31)) hetero-LB film exhibits nearly the same charge transport rate as S(30)/R(2)/A(31) hetero-LB film. But the charge recombination from A to S occurred in S(30)/A(31) hetero-LB film. In the S(30)/A(31) hetero-LB film, charge separated species were directly monitored in the transient absorption spectra. At 660 nm, a long decay component become manifest due to the contribution of bulk (TCNQ) 2 − species. At 740 and 840 nm, on the other hand, a slow rise and long-time decay components were observed due to the contribution of TCNQ − monomer existing at the interface between S and A layers.

Linkage Dependent Charge Separation and Charge Recombination in Porphyrin-Pyromellitimide-Fullerene Triads

A homologous series of zincporphyrin (ZnP)-pyromellitimide (Im)-C60 linked triads where the pyromellitimide (Im) moiety is incorporated as an intermediate acceptor between the above two chromophores with a linkage of different spacers, ZnP-Im-CH2-C60, ZnP-Im-C60, and ZnP-CH2-Im-C60 as well as the reference dyads (ZnP-Im-CH2-ref, ZnP-Im-ref, and ZnP-CH2-Im-ref) have been prepared to investigate linkage dependence of photoinduced electron transfer (ET) and back ET to the ground state in the triads. Time-resolved transient absorption spectra of the triads measured by picosecond laser photolysis as well as the fluorescence lifetimes in THF reveal the occurrence of photoinduced ET from the singlet excited state of the ZnP to the Im moiety to give the initial charge-separated state, i.e., the zincporphyrin radical cation (ZnP•+)-imide radical anion (Im•-) pair, followed by a charge shift (CSH) to produce the final charge-separated state, the ZnP•+-C60•- pair. The rate constants of photoinduced ETs in ZnP-Im-C60 (1.8 × 1010 s-1) and ZnP-Im-CH2-C60 (1.3 × 1010 s-1) in THF are much larger than those in ZnP-CH2-Im-C60 (2.9 × 109 s-1) and ZnP-CH2-Im-ref (1.9 × 109 s-1). The larger charge separation (CS) rates in the former case are ascribed to the relatively strong electronic coupling because of the absence of a methylene linkage between the ZnP and the Im moieties in ZnP-Im-C60 and ZnP-Im-CH2-C60 as compared to the triad and dyad with the methylene linkage. The transient absorption spectra of the final charge-separated state, the ZnP•+-C60•- pair, have been also measured by nanosecond laser photolysis. It has been found that the rate constants of charge recombination (CR) of ZnP•+-Im-C60•- are temperature independent, but that the CR rate constants of ZnP•+-Im-CH2-C60•-exhibit an Arrhenius-like temperature dependence with an activation energy of 0.13 eV which corresponds to the energy difference between ZnP•+-Im•--CH2-C60 and ZnP•+-Im-CH2-C60•-. This indicates that the relatively strong electronic coupling without methylene linkage in ZnP-Im-C60 results in the preference of the tunneling superexchange ET over the sequential ET in the CR process which requires the thermal activation to reach the higher energy state (i.e., ZnP•+-Im•--C60), whereas the sequential ET predominates in the triads with the methylene linkage.

The first steps of the light-induced biradical and zwitterion formation from the clusters [Os 3(CO) 10(α-diimine)] studied with ultrafast time-resolved absorption spectroscopy

The (sub)picosecond time-resolved transient absorption spectra of two triangular [Os 3(CO) 10(α-diimine)] clusters have been studied to establish the primary photoprocesses responsible for the formation of biradicals and zwitterions. The TA spectra of [Os 3(CO) 10( iPrAcPy)] obtained by excitation into its visible absorption band, show a bleach due to the disappearance of the parent cluster and a new absorption with a maximum at 630 nm. In a non-coordinating solvent the bleach and absorption decay with a lifetime of 25±2 ps but do not disappear completely. The bleach decays to approximately 30% of the initial signal and the transient absorption changes into a much broader absorption without a distinct maximum. The initial transient absorption is assigned to the excited state of the cluster having predominant σ(OsOs)→π*( iPrAcPy) character. From the relaxed excited state the cluster partially decays to the ground state and partially produces biradicals. The lifetime of the excited state does not depend on the solvent as long as it is non-coordinating, but it depends on the energy of this 3σπ* excited state, as observed for [Os 3(CO) 10(dmb)]. This effect is attributed to a lowering of the barrier for the reaction from the 3σπ* state. In coordinating acetonitrile (MeCN) the excited state of [Os 3(CO) 10( iPrAcPy)] decays double-exponentially. The longer lifetime ( τ=21.4 ps) matches that observed in non-coordinating solvents and is assigned to biradical formation. In agreement with previous observations that zwitterion formation in coordinating solvents must occur in the picosecond time domain, the second and faster process ( τ=2.9 ps) is assigned for zwitterion formation. These zwitterions are formed by heterolytic splitting of an OsOs bond induced by coordination of MeCN to the Os(CO) 2( iPrAcPy) moiety in the excited state of the cluster. Time-resolved absorption studies in the microsecond time domain showed that the MeCN-coordinated biradicals convert with a lifetime of 13.7 μs into zwitterions. The unique result of this study is that coordinating solvents such as MeCN may induce both homolytic and heterolytic cleavage of a metalmetal bond in such clusters.