Photoinduced Charge-separated States Research Articles

Photochromism and single-component white light emission from a metalloviologen complex based on 1,5-naphthyridine.

Metalloviologens, as emerging electron-transfer photochromic compounds, have shown intriguing properties such as radiochromism, photochromism and photoconductance. However, only a limited number of them have been reported so far. Exploration of new metalloviologens is strongly desired. Herein, we report a new solvothermally synthesized metalloviologen complex [CdCl2(ND)2]n (1, ND = 1,5-naphthalenes) that exhibits photochromic and intrinsic white light emission properties. Density functional theory calculation results reveal that the photochromism could be assigned to photoinduced electron transfer from chlorine atoms to ND molecules. The photoinduced charge-separated states are heat/air stable, attributed to the delocalization of ND and strong intermolecular π-π interactions. Besides, complex 1 consistently emits intrinsic white light when excited with 340-370 nm UV light, achieving high color rendering index (CRI) values (82.54-94.04). By adjusting the excitation wavelength, both "warm" and "cold" white light emission can be produced, making it suitable for the application of a white light emitting diode (WLED). Thus, this work demonstrates that the ND-based metalloviologen is not only helpful in producing photochromism, but also beneficial for creating white-light emission.

Confinement of Pyridine Derivatives into a Metal-Organic Framework Host as Electron Acceptors to Achieve Photoinduced Electron-Transfer.

The photoinduced electron-transfer (ET) process plays an irreplaceable role in chemical and biological fields exemplified by enzymatic catalysis, artificial photosystems, solar energy conversion, and so forth. Searching for a new photoinduced ET system is of great importance for the development of functional materials. Herein, a series of host-guest compounds based on a magnesium metal-organic framework (Mg-MOF) as a host and pyridine derivatives as guests have been presented. Notably, strong O-H···N hydrogen bond between the oxygen atom of μ2-H2O and the nitrogen atom of pyridine enables proton delocalization between water molecule and pyridine guest. Despite the absence of photochromic modules in these host-guest compounds, long-lived charge-separated states with distinct color changes can be formed after UV-light irradiation. The substituents in pyridines and the proton delocalization ability between the host and guests have a great influence on their photoinduced ET process to endow the MOF materials with tunable photoinduced charge-separated states.

Yield-not only Lifetime-of the Photoinduced Charge-Separated State in Iridium Complex-Polyoxometalate Dyads Impact Their Hydrogen Evolution Reactivity.

Covalently linked photosensitizer–polyoxometalate (PS‐POM) dyads are promising molecular systems for light‐induced energy conversion processes, such as “solar” hydrogen generation. To date, very little is known of their fundamental photophysical properties which affect the catalytic reactivity and stability of the systems. PS‐POM dyads often feature short‐lived photoinduced charge‐separated states, and the lifetimes of these states are considered crucial for the function of PS‐POM dyads in molecular photocatalysis. Hence, strategies have been developed to extend the lifetimes of the photoinduced charge‐separated states, either by tuning the PS photophysics or by tuning the POM redox properties. Recently, some of us reported PS‐POM dyads based on cyclometalated IrIII complexes covalently linked to Anderson‐type polyoxometalate. Distinct hydrogen evolution reactivity (HER) of the dyads was observed, which was tuned by varying the central metal ion M of the POMM (M=Mn3+, Co3+, Fe3+). In this manuscript, the photoinduced electron‐transfer processes in the three Ir‐POMM dyads are investigated to rationalize the underlying reasons for the differences in HER activity observed. We report that upon excitation of the IrIII complex, ultrafast (sub‐ps) charge separation occurs, leading to different amounts of the charge‐separated states (Ir.+‐POMM .−) generated in the different dyads. However, in all dyads studied, the resulting Ir.+‐POMM .− species are short‐lived (sub‐ns) when compared to reference electron acceptors (e.g. porphyrins or fullerenes) reported in the literature. The reductive quenching of Ir.+‐POMM .− by a sacrificial donor, triethyl amine (1 m), to generate the intermediate Ir‐POMM .− is estimated to be very efficient (70–80 %) for all dyads studied. Based on this analyses, we conclude that the yield instead of the lifetime of the Ir.+‐POMM .− charge‐separated state determines the catalytic capacity of the dyads investigated. This new feature in the PS‐POM photophysics could lead to new design criteria for the development of novel PS‐POM dyads.

Multi-walled carbon nanotubes photochemistry: A mechanistic view of the effect of impurities and oxygen-containing surface groups

Continuous photolysis experiments and transient absorption spectroscopy were performed in combination with other techniques including HRTEM, XPS, Raman, TGA, and ESR spectroscopy, to investigate the role of residual metals and amorphous carbon on the photochemical process taking place after 350–355 nm light irradiation of as obtained commercial multi-walled carbon nanotubes, denoted as pCNT.The results indicate that 350–355 nm photolysis of pCNT leads to the oxidation of surface oxygen-containing groups and defects which in turn are eliminated leading to more graphitic –like multi-walled carbon nanotubes (MWCNT). Residual metal catalysts and oxygen containing amorphous carbon and oxidized C-functionalities of MWCNT play an important role in the generation of MWCNT photoinduced charge-separated states. The process of 350 nm excitation of pCNT leads to exciton formation followed by hole transfer to metal oxides and further oxidation of C-O functionalities. A plausible mechanism explaining the elimination of oxidized groups attached to pCNT graphene walls and amorphous carbon and leading to more graphite-like CNTs is discussed. The results presented may have implications in the nanoscale semiconductor materials for optoelectronics applications.

Characterization of photo-induced electron and hole transfer in a porphyrin based ambipolar organic molecule with cascade energy levels

A porphyrin based ambipolar organic molecule consisting of styrene based triphenylamine derivative (MTPA) as electron donor, s-triazine group (TRC) as electron acceptor 1 and metal-free tetraphenyl porphyrin (HTPP) as electron acceptor 2 was synthesized and characterized using computational methods and electrochemical and spectroscopic measurements. The kinetics analysis indicates that the photo-induced charge-separated states, MTPA.+-TRC-HTPP.-, were generated by sequential electron transfers from MTPA to TRC then to HTPP and/or a direct hole transfer from HTPP to MTPA. In toluene, the charge-separated states were formed with 54% through electron transfer and 46% through hole transfer once MTPA moiety was excited. However, in dichloromethane, they were formed with 75% through electron transfer and 25% through hole transfer. Furthermore, more charge-separated species were generated in dichloromethane than in toluene.

Enhancing Photoinduced Charge Separation through Donor Moiety in Donor–Acceptor Organic Semiconductors

Three systems were designed, synthesized, and characterized to understand decay processes of photoinduced charge separation in organic semiconductors that are imperative for efficient solar energy conversion. A styrene-based indoline derivative (YD) was used as donor moiety (D), a triazine derivative (TRC) as the first acceptor (A1), and 9,10-anthraquinone (AEAQ) as a second acceptor (A2) in constructing two systems, YD-TRC and YD-TRC-AEAQ. The lifetime of the photoinduced charge-separated states in YD-TRC, a D–A1 system, was found to be 215 ns and that in YD-TRC-AEAQ, a D–A1–A2 system, to be 1.14 μs, a 5-fold increase with respect to that of the YD-TRC. These results show that YD is a more effective donor in YD-TRC and YD-TRC-AEAQ systems at forming long-lived charge-separated states compared to a previously reported atriphenylamine derivative (MTPA) that generated charge-separated states with a lifetime of 80 ns in MTPA-TRC and 650 ns in MTPA-TRC-AEAQ. The third system was constructed using a metal-free...

Very Long-Lived Photoinduced Charge-Separated States of Triphenylamine-Naphthalenediimide Dyads in Polymer Matrices.

Photoinduced electron transfer was studied in dyads (dyad1 and dyad2) containing triphenylamine (MTA) and naphthalenediimide (MNDI) linked with oligo(phenyleneethynylene) dispersed in rigid polymer matrices of polystyrene (PS), poly(vinyl chloride), and poly(methyl methacrylate). Photoexcitation of these dyads yielded long-lived charge-separated (CS) states involving MTA+ and MNDI-. The quantum yields of charge separation in dyad1 and dyad2 were approximately 0.4 and 0.3, respectively, in the polymer matrices. The CS lifetime for dyad2 in PS was longer (400 ms) than those in poly(vinyl chloride) (120 ms) and poly(methyl methacrylate) (65 ms) at 298 K. In addition, CS state had a very long lifetime of 5.4 s in glassy toluene at 100 K. Below glass transition temperatures, polymer side chain motions with various relaxation rates should affect the charge recombination processes. The energy gap (ΔG) and outer-sphere reorganization energy (λ) in the charge recombination process were estimated using a slow-frequency component for dielectric constants. By use of ΔG and λ values, the matrix dependence of the CS lifetimes was successfully rationalized based on Marcus theory, and the charge recombination process in PS with low polarity and high polarizability should be in a deeper inverted region than the other polymer matrices. It also suggested that the rigidity of the polymer effectively suppressed intramolecular motions promoting the charge recombination process.

Time Resolved EPR Study on the Photoinduced Long-Range Charge-Separated State in Protein: Electron Tunneling Mediated by Arginine Residue in Human Serum Albumin.

To elucidate how local molecular conformations play a role on electronic couplings for the long-range photoinduced charge-separated (CS) states in protein systems, we have analyzed time-resolved electron paramagnetic resonance (TREPR) spectra by polarized laser irradiations of 9,10-anthraquinone-1-sulfonate (AQ1S(-)) bound to human serum albumin (HSA). Analyses of the magnetophotoselection effects on the EPR spectra and a docking simulation clarified the molecular geometry and the electronic coupling of the long-range CS states of AQ1S(•2-)-tryptophan214 radical cation (W214(•+)) separated by 1.2 nm. The ligand of AQ1S(-) has been demonstrated to be bound to the drug site I in HSA. Molecular conformations of the binding region were estimated by the docking simulations, indicating that an arginine218 (R218(+)) residue bound to AQ1S(•2-) mediates the long-range electron-transfer. The energetics of triad states of AQ1S(•2-)-R218(+)-W214(•+) and AQ1S(-)-R218(•)-W214(•+) have been computed on the basis of the density functional molecular orbital calculations, providing the clear evidence for the long-range electronic couplings of the CS states in terms of the superexchange tunneling model through the arginine residue.

Geometries, Electronic Couplings, and Hole Dissociation Dynamics of Photoinduced Electron-Hole Pairs in Polyhexylthiophene-Fullerene Dyads Rigidly Linked by Oligophenylenes.

To shed a light on fundamental molecular functions of photoinduced charge conductions by organic photovoltaic materials, it is important to directly observe molecular geometries of the intermediate charges just after the photoinduced electron-transfer reactions. However, highly inhomogeneous molecular environments at the bulk heteojunction interfaces in the photoactive layers have prevented us from understanding the mechanism of the charge conductions. We have herein investigated orbital geometries, electronic couplings, and hole-dissociation dynamics of photoinduced charge-separated (CS) states in a series of poly(3-hexylthiophene)-fullerene linked dyads bridged by rigid oligo-p-phenylene spacers by using time-resolved EPR spectroscopy. It has been revealed that one-dimensional intramolecular hole-dissociations exothermically take place from localized holes in initial CS states, following bridge-mediated, photoinduced charge-separations via triplet exciton diffusions in the conjugated polymer-backbones. This molecular wire property of the photoinduced charges in solution at room temperature demonstrates the potential utility of the covalently bridged polymer molecules applied for the molecular devices.

Nonparallel Stacks of Donor and Acceptor Chromophores Evade Geminate Charge Recombination.

We report a nonparallel stacked arrangement of donor–acceptor (D–A) pairs for prolonging the lifetime of photoinduced charge-separated states. Hydrogen–hydrogen steric repulsion in naphthalimide-naphthalene (NIN) dyad destabilizes the planar geometry between the constituent units in solution/ground state. Sterically imposed nonplanar geometry of the dyad allows the access of nonparallel arrangement of the donor and acceptor stacks having triclinic space group in the crystalline state. Antiparallel trajectory of excitons in nonparallel D–A stacks can result in lower probability of geminate charge recombination, upon photoexcitation, thereby resulting in a long-lived charge-separated state. Upon photoexcitation of the NIN dyad, electron transfer from naphthalene to the singlet excited state of naphthalimide moiety results in radical ion pair intermediates that survive >10,000-fold longer in the aggregated state (τcra > 1.2 ns) as compared to that of monomeric dyad (τcrm < 110 fs), monitored using femtosecond transient absorption spectroscopy.

Synthesis and Properties of an Electropolymer Obtained from a Dimeric Donor/Acceptor System with a 4,4′-Spirobi[cyclopenta[2,1-b:3,4-b′]dithiophene] Core

The design and synthesis of two potentially electropolymerizable electron donor–acceptor dyes characterized by a cruciform 4,4′-spirobi[cyclopenta[2,1-b:3,4-b′]dithiophene] core is here outlined. The new molecules feature two perpendicularly aligned cyclopentadithiophene branches, each one having an electrodimerizable, electron donor triphenylamine unit and an electron acceptor dicyanovinylene group. By merging these structural elements, the light absorption ability of the conjugated cyclopentadithiophene chromophore is extended to a broad region of the visible spectrum. The photoelectroactive film obtained from one of the dyes by electrochemical deposition technique retains the ability to generate photoinduced charge-separated states and to transport holes, turning this material into a unique example of donor–acceptor polymer with potential application in the development of organic optoelectronic devices.

Complete Monitoring of Coherent and Incoherent Spin Flip Domains in the Recombination of Charge-Separated States of Donor-Iridium Complex-Acceptor Triads.

The spin chemistry of photoinduced charge-separated (CS) states of three triads comprising one or two triarylamine donors, a cyclometalated iridium complex sensitizer and a naphthalene diimide (NDI) acceptor, was investigated by transient absorption spectroscopy in the ns-μs time regime. Strong magnetic-field effects (MFE) were observed for two triads with a phenylene bridge between iridium complex sensitizer and NDI acceptor. For these triads, the lifetimes of the CS states increased from 0.6 μs at zero field to 40 μs at about 2 T. Substituting the phenylene by a biphenyl bridge causes the lifetime of the CS state at zero field to increase by more than 2 orders of magnitude (τ = 79 μs) and the MFE to disappear almost completely. The kinetic MFE was analyzed in the framework of a generalized Hayashi-Nagakura scheme describing coherent (S, T0 ↔ T±) as well as incoherent (S, T0 ⇌ T±) processes by a single rate constant k±. The magnetic-field dependence of k± of the triads with phenylene bridge spans 2 orders of magnitude and exhibits a biphasic behavior characterized by a superposition of two Lorentzians. This biphasic MFE is observed for the first time and is clearly attributable to the coherent (B < 10 mT) and incoherent (10 mT < B < 2 T) domains of spin motion induced by isotropic and anisotropic hyperfine coupling. The parameters of both domains are well understood in terms of the structural properties of the two triads, including the effect of electron hopping in the triad with two donor moieties. The kinetic model also accounts for the reduction of the MFE on reducing the rate constant of charge recombination in the triad with the biphenyl bridge.

Porphyrin-Fullerene Based Donor-Acceptor Conjugates for Photocontrol of Cell Membrane Potentials

Optical control of cellular functions is a promising approach for therapeutic application because it can realize precise control with resolutions of submicrosecond temporally and submicrometer spatially.1 Among various key factors for controlling cellular functions, cell membrane potential is one of the most important ones because it triggers various biological events such as neuron fire and cell signaling. For maintaining membrane potentials, ion channels play crucial roles to keep or to change cell membrane potentials. Therefore, artificial control of cell membrane potential and selective inhibition/activation of ion channels are considered to be promising therapeutic approach.2 Meanwhile, in electron donor-acceptor linked molecules, photoirradiation triggers intramolecular electron transfer and then charge separated (CS) state is generated. The molecules in CS state demonstrate high oxidizing and reducing abilities, and may induce large dipole which can generate large electric field in nanometer scale. Our group has focused on the photoinduced CS states in the donor-acceptor linked molecules because it may perturb an environment in the cell membrane and may influence function of the proteins therein. Recently, we have reported that such molecules can induce depolarization in the membrane potential of PC12 cells as well as inhibition of the potassium ion flow across the membrane by illumination.3 In this study, a series of amphiphilic electron donor-acceptor linked molecules based on fullerene and porphyrin were systematically designed, synthesized and investigated for photocontrol on cell membrane potentials. Their molecular structures and unique aggregate structures revealed their characteristic photoinduced excited states of the molecules. Utilizing fullerene moiety was found to be a key for formation of the unique aggregates in highly polar solvent and introduction of porphyrin moiety provides a benefit for using light energy efficiently. Successful design of a CS molecule was found to suppress photocytotoxicity of the molecule remarkably and demonstrated its ability for modulation on the membrane potentials which can increase the frequency of the action potential and trigger neuron firing in rat hippocampal neurons by photo irradiation.

Multiscale Simulation of the Ground and Photo-Induced Charge-Separated States of a Molecular Triad in Polar Organic Solvent: Exploring the Conformations, Fluctuations, and Free Energy Landscapes

The approach of a multiscale simulation that combines quantum chemical calculations, classical molecular dynamics simulations, and statistical physics to integrate the information of the electronic states of a conformationally complex molecule into its structural distribution over an ensemble was performed to understand the influence of a polar organic solvent on the structural stability of carotene-porphyrin-fullerene molecular triad in both the ground and the photoinduced charge-separated excited states. The excited states of the triad were computed with the ab initio quantum chemical calculations using the algebraic diagrammatic construction through the second order correction (ADC(2)) method and the time-dependent density functional theory (TDDFT). The replica exchange molecular dynamics was used for the enhanced sampling of the ensemble in order to explore the phase space of the ground state and the photoinduced charge-separated excited state of triad in explicit tetrahydrofuran (THF) solvent. An importance sampling was strategically employed to bridge the gap between the two computational methods that aim to explore distinct realms of dynamics. We analyzed the free energy landscape, the structural fluctuations, the solvent arrangements, the static dielectric constant, and the interactions between the triad and the solvent molecules. The analysis of the free energy landscape of the triad indicates that the charge-separated excited state of the triad is thermodynamically stable in a linearly extended geometry, while the ground-state triad explores several extended and bent conformations that are populated in the local free energy minima separated by low free energy barriers at an order of thermal fluctuation (k(B)T). The stabilization of a linearly extended structure of the charge-separated excited state triad is predominantly due to the solvation interactions (van der Waals and electrostatic interactions) between the triad and the THF molecules as well as the interactions within the THF molecules. Differences in the charge distribution on a molecular triad induce slight changes in the dielectric property of THF near the triad. Our study suggests that by measuring the differences in a dielectric response of solvent near the triad, it is possible to provide insight into the population of the charge-separated electronic state of the triad relative to that of the ground state.

Initial Molecular Photocurrent: Nanostructure and Motion of Weakly Bound Charge-Separated State in Organic Photovoltaic Interface

To elucidate mechanism of the efficient photocarrier generation by the photoactive bulkheterojunction layers of the organic photovoltaic (OPV) devices, we have employed the time-resolved electron paramagnetic resonance method on solid blends composed of [6,6]-C61-butyric acid methyl ester (PCBM) and of poly(3-hexylthiophene-2,5-diyl) (P3HT) with different regioregularities. The photoinduced charge-separated (CS) states have been detected at the boundary regions between the P3HT and PCBM domains at T = 77 K. We have characterized molecular geometries, electronic couplings, and molecular motions of the long-range CS states. From the CS structure, it is indicated that the pentagonal or hexagonal aromatic rings of the buckyball in PCBM directly face the aromatic plane of the π-stacked P3HT surfaces. It has been concluded that the distant CS states are produced via fast hole-delocalization process from the contact charge-transfer (CT) states. Such hole dynamics is explained by a coupling of the hole to librations of chains in the conjugated polymer. It has been concluded that both the enthalpy stabilization and the enhancement of the entropy occur through the orbital delocalization by the electron–phonon coupling, overcoming the initial CT binding to generate the molecular photocurrent.

Protein–Ligand Structure and Electronic Coupling of Photoinduced Charge-Separated State: 9,10-Anthraquinone-1-sulfonate Bound to Human Serum Albumin

To elucidate how the protein-ligand docking structure affects electronic interactions in the electron-transfer process, we have analyzed time-resolved electron paramagnetic resonance spectra of photoinduced charge-separated (CS) states generated by light excitation of 9,10-anthraquinone-1-sulfonate (AQ1S(-)) bound to human serum albumin at a hydrophobic drug-binding region. The spectra have been explained in terms of the triplet-triplet electron spin polarization transfer model to determine both the geometries and the exchange couplings of the CS states of AQ1S(2-•)-histidine-242 radical cation (H242(+•)) and AQ1S(2-•)-tryptophan-214 radical cation (W214(+•)). For the CS state of the former, it has been revealed that, due to the orthogonal relationship between the singly occupied molecular orbitals of AQ1S(2-•) and H242(+•), the electronic coupling (5.4 cm(-1)) is very weak, contributing to the prevention of energy-wasting charge recombination, even at a contact edge-to-edge separation.

Vectorial Electron Transfer in Donor–Photosensitizer–Acceptor Triads Based on Novel Bis‐tridentate Ruthenium Polypyridyl Complexes

The first examples of rodlike donor-photosensitizer-acceptor arrays based on bis-2,6-di(quinolin-8-yl)pyridine Ru(II) complexes 1a and 3a for photoinduced electron transfer have been synthesized and investigated. The complexes are synthesized in a convergent manner and are isolated as linear, single isomers. Time-resolved absorption spectroscopy reveals long-lived, photoinduced charge-separated states (tau(CSS) (1a)=140 ns, tau(CSS) (3a)=200 ns) formed by stepwise electron transfer. The overall yields of charge separation (> or = 50% for complex 1a and > or = 95% for complex 3a) are unprecedented for bis-tridentate Ru(II) polypyridyl complexes. This is attributed to the long-lived excited state of the [Ru(dqp)(2)](2+) complex combined with fast electron transfer from the donor moiety following the initial charge separation. The rodlike arrangement of donor and acceptor gives controlled, vectorial electron transfer, free from the complications of stereoisomeric diversity. Thus, such arrays provide an excellent system for the study of photoinduced electron transfer and, ultimately, the harvesting of solar energy.

Effect of anion binding on charge stabilization in a bis-fullerene–oxoporphyrinogen conjugate

Presence of strongly binding anion, F(-) stabilizes the photo-induced charge-separated states of a bis-fullerene-substituted oxoporphyrinogen due to the large shift in the oxidation potential of the oxoporphyrinogen moiety upon anion binding through hydrogen bonding at its core.

Phthalocyanines: From outstanding electronic properties to emerging applications

This review paper gives a brief overview on how the outstanding chemical and physical properties of phthalocyanines and phthalocyanine derivatives are being studied and employed in order to construct state-of-the-art technological devices. In a first instance, a short account on how the nature of the phthalocyanine structure and its organization in condensed phases play an important role in their conducting and ultraviolet-visible absorption properties is presented. Consequently, these basic electronic and photophysical features of phthalocyanines allow us to explain why phthalocyanine-based multicomponent covalent or noncovalent donor-acceptor systems may give rise to very interesting photophysical properties, in particular in terms of their ability to generate very long-lived photoinduced charge-separated states. A concise survey on the organization of these multifunctional systems shows how a profound understanding of the morphology at the nanometer-scale of these phthalocyanine-based molecular materials is needed in order to control their physical properties in condensed phases. All the previously mentioned chemical and physical features combined together led us to the description of the latest attempts at incorporating phthalocyanines into photovoltaic devices for solar energy conversion and onto quantum dots for photodynamic therapy or quantum computing.

Syntheses and Properties of Bimetallic Chromophore-Quencher Assemblies Containing Ruthenium(II) and Rhenium(I) Centers

We have prepared a family of six new bimetallic complexes containing electron-donating trans-{RuIICl(pdma)2}+ [pdma = 1,2-phenylenebis(dimethylarsine)] centers linked to trans-[RuIICl(pdma)(2,2‘-bpy)(NO)] (bpy = bipyridyl) or fac-{ReI(biq)(CO)3}+-based (biq = 2,2‘-biquinolinyl) electron acceptors. The extended bridging units are based on 4,4‘-bpy or bpe [E-1,2-bis(4-pyridyl)ethylene] ligands, and these assemblies are designed to display long-lived photoinduced charge-separated states. A number of new monometallic precursor complexes have also been synthesized and fully characterized. The electronic absorption spectra of the bimetallic species are dominated by intense, visible d(RuII) → π*(4,4‘-bpy/bpe) metal-to-ligand charge-transfer (MLCT) bands and d(ReI) → π*(biq) absorptions in the near-UV region. Cyclic voltammetric studies show both metal-based oxidation and ligand-based reduction processes and provide evidence that these assemblies contain thermodynamic driving forces for charge separation. Single-crystal X-ray structures have been determined for the monometallic complex salts fac-[ReI(biq)(CO)3(MeCN)]CF3SO3·CHCl3, fac-[ReI(biq)(CO)3(4-BrCH2C5H4N)]PF6, fac-[ReI(biq)(CO)3(4-HCO2CH2C5H4N)]PF6·Me2CO, fac-[ReI(biq)(CO)3(ISNE)]CF3SO3 (ISNE = ethylisonicotinate), and fac-[ReI(biq)(CO)3(NC5H4CO2)]2·HPF6·3MeCN. Ultrafast time-resolved infrared and Raman spectroscopic measurements will be employed to investigate the photoexcitation properties of the bimetallic species and of monometallic reference complexes.