Transition metal complexes display a number of charge-transfer bands in the absorption spectra. Optical excitation of the metal complexes produces a variety of products depending upon the nature of the excited state. Cobalt (III) ammine complexes on excitation in the CTTM bands produce cobalt(II) and oxidised ligand. Ruthenium(II) complexes on excitation in the cm. band leads to the oxidation of the metal centre. In certain reactions participation of the solvent in the primary photoredox reactions has also been reported. In recent years extensive investigations have been undertaken to utilize the photoredox systems of coordination compounds to convert solar energy to electricity or hydrogen.

Cyclic voltammetric studies were carried out for dioxygen-bridged dinuclear cobalt(III) complexes using platinum electrodes at different values of pH and ionic strength. Reversible electrode reactions occur particularly for the dioxygen complexes [(phen)₂Co(μ-O₂,NH₂)Co(phen)₂] (ClO₄)₄2H₂O and [(bpy)₂Co(μ-u-O₂, NH₂)Co(bpy)₂] (ClO₄)₄H₂O with 1,10-phenanthroline (phen) and 2,2-bipyridine (bpy) as the non-bridging ligands. For complexes with ammonia (NH₃) ethylenediamine (en) as the terminal ligands, there is quasi-reversible electrode behaviour at pH < 3 due to protonation of the peroxo complexes which subsequently undergo equilibration with isomeric forms. Such behaviour is well-marked for the peroxo complex [(en)₂Co(μ-O₂ NH₂)Co(en)₂](NO₃)₃. The decaammine complex [(NH₃₅Co(μ-O₂Co(NH₃)₅](NO₃)₅ shows only a cathodic peak due to intramolecular charge transfer decomposition in solution after reduction at the electrode. Diffusion coefficients for all the dioxygen complexes were determined from the plots ofI_p vs.v^1/2.

In cobalt(III) amine complexes containing amino acids, photolysis in the chargetransfer bands leads to the formation of alkyl cobalt(III) complexes. Steady and flash photolysis of complexes [Co(LL)₂X]²⁺ and [Co(LL)₂Y₂]⁺ (LL = ethylene diamine, polypyridyl or propylenediamine, X = amino acid and Y = nitro ligand) were carried out. Steady irradiation of [Co(bpy)₂(gly)]²⁺ ion (bpy = bipyridine, gly = glycine) in aqueous medium (pH 1–7) produces the six-coordinated complex [Co(bpy)₂(CH₂NH₂)]²⁺. Flash photolysis of the complex under identical conditions gives an alkyl complex. The alkyl complex formed in the case of [Co(bpy)₂(gly)]²⁺ ion is stable and could be isolated; the corresponding alkyl complex formed from [Co(bpy)(en)(gly)]²⁺ ion leads to the labilisation of the ethylenediamine ligand in acidic solutions. Photolysis of the nitro complexes of the type [Co(en)₂(NO₂)₂]⁺ ion in acetonitrile, methanol and ethanol produces aπ-bonded transient nitro complex which reacts with molecular oxygen to give the mononuclear superoxo, dinuclear superoxo and dinuclear peroxo complexes in sequence. The formation of these products from the free radical coordinated cobalt(II) complex has been followed by the flash photolysis method and the rate constants have been determined. The details of the mechanisms are discussed.

The phenosafranine adsorbed onto the micro and mesoporous materials prepared by ion exchange method and interaction of the dye with host materials were studied by X-ray photoelectron spectroscopy to elucidate the influence of the host matrix on the binding energy of N 1s orbital. Core level N 1s X-ray photoelectron spectroscopy reveals the interaction between the dye and the solid surface through the hydrogen bonding between the hydrogen atoms of primary amino groups in dye molecule and the oxygen atom of surface hydroxyl groups. The strength of the hydrogen bonding depends on the nature of the solid surface. In the dye adsorbed onto the micro and mesoporous materials the interaction between adsorbed phenosafranine and the surfaces of the porous materials are found to modify the optical spectra and the excited state dynamics of the confined phenosafranine molecules. The change in photophysical properties of phenosafranine adsorbed on to the host materials on dehydration at elevated temperatures is attributed to the modification of host surface during dehydration process.

The photoredox reactions of cobalt(III), nickel(II) complexes and organic dyes are reviewed with particular emphasis on using flash photolysis methods to identify the transients in the time scale from milliseconds to subpicoseconds. The excitation flash radiation initially was limited to flash lamps with a time resolution to study the decay of the excited states and the fast reactions of photo produced transients in milliseconds and microseconds. Some of the photolysis reactions investigated provide details in real time including the thermal reactions of molecular oxygen with cobalt(II) amine systems and the cobalt (III) alkyl complex formation on photolysis of cobalt(III) aminoacid complexes. Photoredox reactions of thiazine and safranine dyes are investigated using time resolved absorption and emission techniques in solution and on encapsulation in micro and mesoporous host materials. Picosecond and femtosecond diffuse reflectance fluorescence investigations in the porous solid host materials show intramolecular proton transfer reactions of the dyes and deprotonation of the dyes in the excited states. Photosensitization of nanoparticles of semiconductors, anatase TiO₂ and ZnO by the excited states of dyes in meso and microporous silicate hosts is investigated using pico and femtosecond fluorescence and transient absorptions techniques and the results are reviewed.