Fast Back Electron Transfer Research Articles

Unveiling a Three Phase Mixed Heterojunction via Phase‐Selective Anchoring of Polymer for Efficient Photocatalysis

AbstractThe ligand‐to‐metal charge transfer (LMCT) facilitated activation of TiO2 has noteworthy potential for solar energy harvesting. However, the fast back electron transfer from TiO2 to an oxidized sensitizer is a key limiting factor causing low photocatalyst efficiency. Herein, a new catalyst design to both increase LMCT efficiency and minimize the back electron transfer is presented. A phase‐selective modification of mixed‐phase TiO2 (anatase: rutile interface) with poly‐salophen organic polymer is developed. The salophen and salen family organic monomers are selectively bound and polymerized on the anatase phase but not the rutile phase, which results in the formation of a three‐phase system. Such a three‐phase system converts an unfavorable polymer TiO2 core‐shell structure to an intimately mixed blend morphology, consisting of interfaced crystalline rutile TiO2 and an amorphous polymer‐covered anatase‐phase TiO2. The developed mixed‐blend morphology poly‐S@P25 can produce H2 of 37 410 µmol h–1 g–1 of polymer, which is ≈3.4 times higher than core‐shell poly‐S@anatase TiO2. This approach overcomes the drawback of the traditional core‐shell structured system for efficient electron harvesting from the LMCT process.

Photocatalyst assemblies with two halide ions

A series of ruthenium polypyridyl photocatalysts bearing amide functional groups were designed that successfully promoted halide assembly in CH2Cl2 and CH3CN solution. In CH2Cl2, halide assembly was accompanied by a visible color change, and the spectral changes presented clear evidence for two halide binding events, yielding a 1:2 ruthenium:halide assembly. In the more polar solvent CH3CN, 1:2 assembly structures were also observed with chloride, bromide, and iodide, and large equilibrium constants were measured for association of the first and second halide (K11 = 0.04 – 2 × 106 M−1, K12 = 0.01 – 3 × 105 M−1). Varying the functional groups on the ancillary ligands tuned the excited-state reduction potentials (Ru2+⁎/+), resulting in a photocatalyst capable of performing iodide oxidation. Quenching of the photocatalyst excited state resulted in static and dynamic quenching, and a Stern-Volmer analysis yielded two linear regions at low and high iodide concentrations. The dynamic quenching rate constants (kq = 6.8 and 4.0 × 1010 M−1 s−1) and static quenching constants (KS = 2.4 and 0.13 × 104 M−1) at low and high iodide concentrations, respectively, were consistent with dynamic quenching of Ru2+ and [Ru2+,I−]+, and static quenching of [Ru2+,I−]+and [Ru2+,2I−]. Transient absorption spectroscopy revealed that the quenching reaction yielded a reduced ruthenium (Ru+) as the primary photoproduct and diiodide (I2•−) as a secondary photoproduct. The second-order rate constant for I2•− formation was measured to be 2.5 × 1010 M−1 s−1, a value consistent with the diffusion limited reaction. The transient absorption data indicates that oxidized halide photoproducts only result from the diffusional quenching reactions, and not from static quenching with an associated iodide ion. Fast back-electron transfer rates and low cage-escape yields in the ruthenium:iodide assemblies are invoked to explain why the static quenching pathway does not lead to measurable photoproduct yields.

Formation of a supramolecular charge-transfer complex. Ultrafast excited state dynamics and quantum-chemical calculations.

The formation of a supramolecular complex of bis(18-crown-6)stilbene (1) and 4,4'-bipyridine with two ammoniopropyl N-substituents (3) and the substitution reaction between 1·3 and alkali and alkaline-earth metal perchlorates have been studied using absorption, steady-state fluorescence, and femtosecond transient absorption spectroscopy. The formation of 1·(Mn+)2 complexes in acetonitrile was demonstrated. The weak long-wavelength charge-transfer absorption band of 1·3 completely vanishes upon complexation with metal cations because of disruption of the pseudocyclic structure. The spectroscopic and luminescence parameters, stability and substitution constants were calculated. The relaxation scheme of the 1·3 singlet state excited by a 25 fs laser pulse was proposed. It includes very fast vibrational relaxation and direct (τCT-d = 0.32 ps) and back (τCT-b = 0.51 ps) electron transfer resulting in complete fluorescence quenching. The quantum-chemistry calculations revealed the species taking part in the ET process and elucidated the mechanism of relaxation of the excited complex.

Boosting Photocatalytic Hydrogen Production of Porphyrinic MOFs: The Metal Location in Metalloporphyrin Matters

Metal–organic frameworks (MOFs) have demonstrated great potentials toward catalysis, particularly in the establishment of structure–property relationships. Herein, an unusual OOP (out-of-plane) porphyrin-based MOF, synthesized by controlling the metal ion release with an unprecedented In(OH)3 precursor, possesses high stability and exhibits unexpectedly high photocatalytic hydrogen production activity, far surpassing the isostructural in-plane porphyrin-based MOF counterparts. In the MOF structure, indium ions not only form indium–oxo chains but also metalate the porphyrin rings in situ, locating above the porphyrin plane instead of fitting in a coplanar fashion into the cavity and affording an unusual OOP porphyrin. Control experiments demonstrate that the OOP In(III) ions readily detach from the porphyrin rings under light excitation, avoiding the fast back electron transfer and thus greatly improving electron–hole separation efficiency and photocatalytic performance. To our knowledge, this is an unprec...

Complexation of bis‐crown stilbene with alkali and alkaline‐earth metal cations. Ultrafast excited state dynamics of the stilbene‐viologen analogue charge transfer complex

AbstractThe complex formation of bis(18‐crown‐6)stilbene (1) and its supramolecular donor‐acceptor complex with N,N′‐bis(ammonioethyl) 1,2‐di(4‐pyridyl)ethylene derivative (2) with alkali and alkaline‐earth metal perchlorates has been studied using absorption, steady‐state fluorescence, and femtosecond transient absorption spectroscopy. The formation of 1∙Mn+ and 1∙(Mn+)2 complexes in acetonitrile was demonstrated. The weak long‐wavelength charge‐transfer absorption band of 1·2 completely vanishes upon complexation with metal cations because of disruption of the pseudocyclic structure. The spectroscopic and luminescence parameters, stability constants, and 2‐stage dissociation constants were calculated. The initial stage of a recoordination process was found in the excited complexes 1∙M+ and 1∙(M+)2 (M = Li, Na). The pronounced fluorescence quenching of 1·2 is explained by very fast back electron transfer (τet = 0.397 ps). The structure of complex 1·2 was studied by X‐ray diffraction; stacked (1·2)m polymer in which the components were connected by hydrogen bonding and stacking was found in the crystal. These compounds can be considered as novel optical molecular sensors for alkali and alkaline‐earth metal cations.

One‐Step Selective Hydroxylation of Benzene to Phenol

AbstractRecent developments in the one‐step thermal and photochemical catalytic hydroxylation of benzene to phenol using various oxidants are described, focusing on the catalytic mechanisms. Hydroxylation of benzene with hydrogen peroxide is catalyzed by homogeneous metal complexes and heterogeneous metal oxides to produce phenol when high‐valent metal‐oxo complexes are reactive species The selective hydroxylation of benzene to phenol is achieved by using mesoporous silica‐alumina as the catalyst support, which prohibits further oxidation of phenol due to the strongly acidic sites. Photocatalytic oxidation of benzene with dioxygen and water is made possible by using organic photocatalysts via photoinduced electron transfer from benzene to the excited states of the photocatalysts, followed by the hydroxylation of the benzene radical cation with water to yield phenol selectively. The further oxidation of phenol is prohibited because of the fast back electron transfer from the one‐electron reduced species of the photocatalyst to the phenol radical cation, whereas back electron transfer from the one‐electron reduced species to the benzene radical cation is slowed down because of the large driving force of the back electron transfer, which is in the Marcus inverted region.

Dynamics and mechanism of UV-damaged DNA repair in indole-thymine dimer adduct: molecular origin of low repair quantum efficiency.

Many biomimetic chemical systems for repair of UV-damaged DNA showed very low repair efficiency, and the molecular origin is still unknown. Here, we report our systematic characterization of the repair dynamics of a model compound of indole-thymine dimer adduct in three solvents with different polarity. By resolving all elementary steps including three electron-transfer processes and two bond-breaking and bond-formation dynamics with femtosecond resolution, we observed the slow electron injection in 580 ps in water, 4 ns in acetonitrile, and 1.38 ns in dioxane, the fast back electron transfer without repair in 120, 150, and 180 ps, and the slow bond splitting in 550 ps, 1.9 ns, and 4.5 ns, respectively. The dimer bond cleavage is clearly accelerated by the solvent polarity. By comparing with the biological repair machine photolyase with a slow back electron transfer (2.4 ns) and a fast bond cleavage (90 ps), the low repair efficiency in the biomimetic system is mainly determined by the fast back electron transfer and slow bond breakage. We also found that the model system exists in a dynamic heterogeneous C-clamped conformation, leading to a stretched dynamic behavior. In water, we even identified another stacked form with ultrafast cyclic electron transfer, significantly reducing the repair efficiency. Thus, the comparison of the repair efficiency in different solvents is complicated and should be cautious, and only the dynamics by resolving all elementary steps can finally determine the total repair efficiency. Finally, we use the Marcus electron-transfer theory to analyze all electron-transfer reactions and rationalize all observed electron-transfer dynamics.

Solvent-induced surface state passivation reduces recombination in semisquarylium dye-sensitized solar cells

The semisquarylium dye SY1T that is strongly bound to the surface of nanocrystalline TiO2 experiences very fast back-electron transfer of injected electrons to the SY1T cation, when the TiO2/SY1T interface is surrounded by ultrahigh vacuum. However, when located in methoxypropionitrile (MPN), which is frequently used as electrolyte solvent in dye-sensitized solar cells, the back-electron transfer is significantly retarded. Results are obtained both for picosecond and microsecond time scales using transient absorption spectroscopy. As solvent-induced interfacial energy level shifts can be excluded as possible cause, the role of TiO2 surface states in the beneficial retardation process is investigated. Highly surface sensitive synchrotron-induced photoelectron spectroscopy exhibits high densities of surface states on the pristine nanocrystalline TiO2 (nc-TiO2) surfaces. While SY1T dye-sensitization from a SY1T solution in tetrahydrofuran saturates about 30% of the surface states, the subsequent in-situ adsorption of MPN molecules at the TiO2/SY1T interface leads to further reduction by more than 50% of the remaining surface states. It is concluded that the saturation of TiO2 surface states hampers the otherwise efficient recombination of injected electrons with the SY1T dye cation.

Recombination inhibitive structure of organic dyes for cobalt complex redox electrolytes in dye-sensitised solar cells

A very high open-circuit voltage (Voc) has recently been achieved in dye-sensitised solar cells (DSSCs) using cobalt complex redox couple electrolytes because of the high positive redox potential of the electrolyte. However, the obtained Voc is lower than the expected value owing to fast back electron transfer reactions from the TiO2 surface to the redox species. Recombination may be retarded by introduction of steric barrier groups into the dye structure to block the approach of the cobalt complex towards the TiO2 surface. Herein, carbazole dyes with various structural features in each dye component, i.e., number of n-hexylthiophene units in the linker, bulky appendage in the donor group, and alkyl chain length in the thiophene linker moiety, were studied in order to retard the fast back electron transfer reactions. There was a positive relationship between Voc and the number of n-hexylthiophene units, bulkiness of the appendage group in the donor segment, and shorter alkyl chains in the thiophene linker moiety. The increased Voc was attributed to the retardation of charge recombination, demonstrating that longer and larger molecules exert better blocking function.

Direct Grafting of Long-Lived Luminescent Indicator Dyes to GaN Light-Emitting Diodes for Chemical Microsensor Development

Two new methods for covalent functionalization of GaN based on plasma activation of its surface are presented. Both of them allow attachment of sulfonated luminescent ruthenium(II) indicator dyes to the p- and n-type semiconductor as well as to the surface of nonencapsulated chips of GaN light-emitting diodes (blue LEDs). X-ray photoelectron spectroscopy analysis of the functionalized semiconductor confirms the formation of covalent bonds between the GaN surface and the dye. Confocal fluorescence microscopy with single-photon-timing (SPT) detection has been used for characterization of the functionalized surfaces and LED chips. While the ruthenium complex attached to p-GaN under an oxygen-free atmosphere gives significantly long mean emission lifetimes for the indicator dye (ca. 2000 ns), the n-GaN-functionalized surfaces display surprisingly low values (600 ns), suggesting the occurrence of a quenching process. A photoinduced electron injection from the dye to the semiconductor conduction band, followed by a fast back electron transfer, is proposed to be responsible for the excited ruthenium dye deactivation. This process invalidates the use of the n-GaN/dye system for sensing applications. However, for p-GaN/dye materials, the luminescence decay accelerates in the presence of O(2). The moderate sensitivity is attributed to the fact that only a monolayer of indicator dye is anchored to the semiconductor surface but serves as a demonstrator device. Moreover, the luminescence decays of the functionalized LED chip measured with excitation of either an external (laser) source or the underlying LED emission (from p-GaN/InGaN quantum wells) yield the same mean luminescence lifetime. These results pave the way for using advanced LEDs to develop integrateable optochemical microsensors for gas analysis.

Dynamics and mechanism of repair of ultraviolet-induced (6–4) photoproduct by photolyase

One of the detrimental effects of UV radiation on DNA is the formation of the (6-4) photoproduct (6-4PP) between two adjacent pyrimidines1. This lesion interferes with replication and transcription and may result in mutation and cell death2. In many organisms a flavoenzyme called photolyase uses blue light energy to repair the 6-4PP3. The molecular mechanism of the repair reaction is poorly understood. Here, we use ultrafast spectroscopy to show that the key step in the repair photocycle is a cyclic proton transfer between the enzyme and the substrate. By femtosecond synchronization of the enzymatic dynamics with the repair function, we followed the function evolution and observed direct electron transfer from the excited flavin cofactor to the 6-4PP in 225 ps but surprisingly fast back electron transfer in 50 ps without repair. Strikingly, we found that the catalytic proton transfer between a histidine residue in the active site and the 6-4PP, induced by the initial photoinduced electron transfer from the excited flavin cofactor to 6-4PP, occurs in 425 ps and leads to 6-4PP repair in tens of nanoseconds. These key dynamics define the repair photocycle and explain the underlying molecular mechanism of the enzyme’s modest efficiency.

The effect of molecular aggregates over the interfacial charge transfer processes on dye sensitized solar cells

The electron transfer reaction between the photoinjected electrons in the nanocrystalline TiO2 mesoporous sensitized films and the oxidized electrolyte in dye sensitized solar cells (DSSC) plays a major role on the device efficiency. In this communication we show that, although the presence of molecular aggregates on the free base porphyrin DSSC limits the device photocurrent response under illumination, they form an effective hydrophobic barrier against the oxidized electrolyte impeding fast back-electron transfer kinetics. Therefore, their drawback can be overcome by designing dyes with peripheral moieties that prevent the formation of the aggregates and are able to achieve efficiencies as high as 3.2% under full sun.

Electrochemical and Photosensitized Redox Reactions of a Prussian Blue Film Layered on a WO3/[Ru(bpy)3]2+/Polymer Hybrid Film

A hybrid film of WO(3)/tris(2,2'-bipyridine)ruthenium(II) ([Ru(bpy)(3)](2+))/poly(sodium 4-styrenesulfonate) (PSS) (denoted as a WRP hybrid film) was prepared as a base layer on an indium tin oxide electrode substrate by cathodic electrodeposition from a colloidal ternary solution containing peroxotungstic acid, [Ru(bpy)(3)](2+), and PSS. Prussian blue, Fe(III) (4)[Fe(II)(CN)(6)](3) (Fe(II)-Fe(III)) was cathodically electrodeposited on the WRP hybrid film from a Berlin brown (Fe(III)-Fe(III)) colloidal solution to give a WRP/Fe(II)-Fe(III) bilayer film. Spectrocyclic voltammetry measurement of the WRP/Fe(II)-Fe(III) bilayer film reveals that Prussian white (Fe(II)-Fe(II)) is oxidized to Fe(II)-Fe(III) by electrogenerated Ru(III), and Fe(II)-Fe(III) is re-reduced to Fe(II)-Fe(II) by electrogenerated H(x)WO(3). Visible-light irradiation of the WRP hybrid film generates a small photocurrent (approximately 8 nA cm(-2)) at 0.4 V of an applied potential, whereas irradiation of the WRP/Fe(II)-Fe(II) bilayer film (Fe(II)-Fe(III) is electrochemically reduced to the Fe(II)-Fe(II) state) significantly generates a steady photoanodic current of 2.0-1.1 microA cm(-2) under the same conditions, thus demonstrating that the photoanodic current is produced by the layered Fe(II)-Fe(II) film. The photoaction spectrum of the bilayer film reveals that the photoanodic current is based on the photoexcitation of [Ru(bpy)(3)](2+). The photogeneration of Fe(II)-Fe(III) from Fe(II)-Fe(II) is shown by the absorption spectral change of the bilayer film on irradiation. These results corroborate the notion that Fe(II)-Fe(II) is oxidized by photogenerated Ru(III) to generate Fe(II)-Fe(III). However, the rate of photogeneration of Fe(II)-Fe(III) is slow, which could be ascribed to the fast back electron transfer (ET) from WO(3) to Ru(III), comparable with the forward ET from Fe(II)-Fe(II) to Ru(III). The fast back ET could be a crucial problem for the [Ru(bpy)(3)](2+)-sensitized reaction in the hybrid film.

Fast back electron transfer prevents guanine damage by photoexcited thionine bound to DNA.

The phenothiazinium dye thionine has a high excited state reduction potential and is quenched by guanine on the femtosecond time scale. Here, we show by gel electrophoresis that irradiation of thionine with 599 nm light in the presence of an oligonucleotide duplex does not produce permanent DNA damage. Upon photoexcitation of thionine weakly associated with guanosine-5'-monophosphate, the reduced protonated thionine radical and neutral guanine radical are detected by transient absorption spectroscopy, indicating that the quenching of thionine by guanine occurs via an electron-transfer mechanism. The observation of radical formation without permanent guanine damage indicates that fast back electron transfer plays a critical role in governing the yield of damage by DNA-binding molecules.

Light-driven tyrosine radical formation in a ruthenium-tyrosine complex attached to nanoparticle TiO2.

We demonstrate a possibility of multistep electron transfer in a supramolecular complex adsorbed on the surface of nanocrystalline TiO(2). The complex mimics the function of the tyrosine(Z)() and chlorophyll unit P(680) in natural photosystem II (PSII). A ruthenium(II) tris(bipyridyl) complex covalently linked to a L-tyrosine ethyl ester through an amide bond was attached to the surface of nanocrystalline TiO(2) via carboxylic acid groups linked to the bpy ligands. Synthesis and characterization of this complex are described. Excitation (450 nm) of the complex promotes an electron to a metal-to-ligand charge-transfer (MLCT) excited state, from which the electron is injected into TiO(2). The photogeneration of Ru(III) is followed by an intramolecular electron transfer from tyrosine to Ru(III), regenerating the photosensitizer Ru(II) and forming the tyrosyl radical. The tyrosyl radical is formed in less than 5 micros with a yield of 15%. This rather low yield is a result of a fast back electron transfer reaction from the nanocrystalline TiO(2) to the photogenerated Ru(III).

Photo-Induced Electron Transfer from Excited Tris[2,2′-bipyridine-κN1,κN1′]ruthenium(2+) Ions to Dyadic Electron Acceptors That Can Be Protected by Host-Guest Interaction against Fast Back Electron Transfer, Characterization of the Guest Molecules

Anthraquinone and viologen moieties were combined to dyadic molecules with two redox centers. These dyades and their constituents were used as acceptors in photo-induced electron-transfer reactions. The experiments show that a caveat is necessary if one tries to derive the properties and the reactions of the dyades from those of their constituents: the spectral properties appear to be independent superpositions of those of the constituents. However, the redox potentials of the two redox centres in the dyades deviate from that of their constituents, the methylene bridge can not suppress a considerable interaction between the two redox centres. This is especially true when the redox potentials of the constituents are close to one another. From the quenching experiments, it can be concluded that electrolyte cations like Na+ are engaged in the transition states of the electron-transfer reactions. In this way, they can control the fate of the transferred electron.

Photoinduced Charge Migration in the Picosecond Regime for Thianthrene-Linked Acridinium Ions

The thianthrene ring system has a novel redox chemistry that has been the subject of study for three decades. The sulfur heterocycle undergoes a reversible on-electron oxidation (E{sub 1/2} = 1.23 vs SCE, CH{sub 3}CN), yielding a radical cation species that is stable enough to be isolated for independent investigation; magnetic resonance studies on the cation have included ESR, ENDOR, and CIDEP measurements. Photoinduced electron transfer involving thianthrene (TH) as an electron donor has been studied, including a report of trapping of the radical cation. As part of a study of linked donor-acceptor systems that employ the acridinium ion as electron acceptor, the authors have prepared the first linked thianthrenes appropriate for investigation of intramolecular electron transfer involving TH as an electron donor. Principal findings include the observation of very fast forward and back electron transfer (charge shift, CSH) in the picosecond time domain involving TH and acridinium subunits.

Synthesis, Characterization, Photophysics, and Photosensitization Studies of Squaraine−Bipyridinium Diads

Two squaraine−bipyridinium diads designed to study electron transfer between the squaraine chromophore and the bipyridinium group have been synthesized. The two diads, denoted as C4Sq−By and C12Sq−By, consist of amphiphilic squaraines covalently linked to a bipyridinium group by a trimethylene chain. The squaraine chromophores in the diads do not form charge-transfer complexes with the bipyridinium group in the ground state. Both C4Sq−By and C12Sq−By show fluorescence yields about 100 times weaker than those of alkyl substituted squaraines, which is attributed to quenching by an intramolecular electron transfer from the excited squaraine chromophore to the bipyridinium; the intramolecular electron-transfer rate is estimated to be 4 × 1011 s-1. Assuming the short lifetime for the undetectable transient produced is due to very fast back electron transfer to the radical cation of squaraine from the reduced bipyridinium ion, the rate of the back intramolecular electron transfer is estimated as >3 × 1010 s-1. The two diads synthesized in this work have been used as photocurrent enhancers for the monolayers of DSSQ (4-(distearylamino)phenyl-4‘-(dimethylamino)phenylsquaraine). C4Sq−By as a solution additive is 5 times more effective in sensitizing the cathodic photocurrent generation of the DSSQ monolayer-modified SnO2 electrode compared to methylviologen. Analogously, C12Sq−By is 3 times more effective in sensitizing the DSSQ monolayer-modified SnO2 electrode compared to 4-tetradecyl-4‘-methylbipyridinium dichloride when it is incorporated upon spreading in the monolayer of DSSQ. The increase in cathodic photocurrent generation efficiency is postulated to result from fast electron transfer from the excited squaraine to the bipyridinium group in the diad. The fast electron transfer facilitates electron separation in the photogeneration step and consequently increases the quantum efficiency of the cathodic photocurrent generation process.

ChemInform Abstract: Photochemical Dehydrofragmentation Reactions: Importance of Donor and Acceptor Structure in Determination of Reactivity in Radical Ion Pairs Formed in Electron‐Transfer Photoreactions.

Photoinduced electron-transfer reactions are the subject of considerable interest due both to the versatility of net chemical reactions which can be achieved as well as to the variety of intermediates and mechanistic complexities encountered in their study. For initially neutral donors and acceptors quenching of excited singlet states is frequently dominated by formation of a geminate ion pair, A/sup -//D/sup +/, which can subsequently decay by back electron transfer or, in moderately polar to polar solvents, by cage escape to form free ions. In nonpolar solvents the back electron transfer process acts as an effective clock to limit the lifetime for reaction of the geminate pair to the ns time; consequently only relatively rapid reactions can occur efficiently from the geminate pair and these frequently involve participation of the solvent or other reagents present in high concentration. The authors have previously reported the chemically clean and moderately efficient C-C bond photofragmentation reactions of ..beta..-amino-alcohols initiated by electron transfer to excited acceptors. They report here a mechanistic study where this reaction is restricted to the geminate pair and which emphasizes the complimentary roles of reduced acceptor and oxidized donor in facilitating chemical reaction in competition with fast back electron transfer. Amore » key result is that rapid fragmentation is coincident with acceptor radical anion induced deprotonation of the donor cation radical and thus strongly dependent on acceptor structure.« less

Photocatalytic systems. LXIII. Intervalence transfer (IT) behaviour and IT photochemistry of mixed-valence compounds with cyanometallates

Cyanometallates such as octacyanomolybdate(IV), octacyanotungstate(IV), hexacyanoruthenate(II), and hexacyanoferrate(II) form mixed-valence compounds with appropriate copper(II), iron(III), uranium(VI) and vanadium(IV) compounds. Detailed spectroscopic investigations in solution show that the mixed-valence compounds under discussion are characterized by changes in the IT transitions. The IT behaviour is discussed within the framework of the conceptual approach of Hush. Photochemical investigations of the mixed-valence compounds described show low photoreactivity only, caused by fast back electron transfer processes. Both chemical and physical scavenging processes, the latter in the form of a new type of sequential two-photon processes, may be helpful in quenching the back electron transfer. The results obtained enable us to realize the concept of static spectral sensitization of photocatalytic systems based on light-sensitive coordination compounds.