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The photophysical properties of closely-coupled, binuclear complexes formed by connecting two ruthenium(II) bis(2,2':6',2''-terpyridine) complexes via an alkynylene group are compared to those of the parent complex. The dimers exhibit red-shifted emission maxima and prolonged triplet lifetimes in deoxygenated solution. Triplet quantum yields are much less than unity and the dimers generate singlet molecular oxygen with low quantum efficiency. Temperature dependence emission studies indicate coupling to higher-energy triplet states while cyclic voltammetry shows that the metal centres are only very weakly coupled but that extensive electron delocalization occurs upon one-electron reduction. The radiative rate constants derived for these dimers are relatively low, because the lowest-energy metal-to-ligand, charge-transfer states possess increased triplet character. In contrast, the rate constants for nonradiative decay of the lowest-energy triplet states are kept low by extended electron delocalization over the polytopic ligand. The poor triplet yields are a consequence of partitioning at the second triplet level.

The need for alternative sources of energy has stimulated research into the storage of sunlight as chemical potential. Many systems have been investigated but photogeneration of H2 seems to be the most practical. The cyclic photodissociation of water has yet to be realized in homogeneous solution using visible light, although heterogeneous photosystems are known1–3. Efficient photogeneration of H2 in homogeneous conditions can be achieved4–7 at the expense of an added electron donor but such systems have little application in practical devices. Replacement of the sacrificial donor with waste material, for example sulphide8, offers a route for improving these systems and, here we describe efficient H2 photogeneration from a system using aqueous ethanol, as available from low-grade fermentation, as donor. To couple the photoproduction of H2 to the oxidation of ethanol we have used NADH/alcohol dehydrogenase as a relay.

A series of compounds has been synthesised having two identical viologen groups separated by organic bridges of varying degree of flexibility. Chemical or radiolytic reduction results in formation of the mono viologen π-radical cations, which undergo disproportionation to form the doubly reduced viologen. In this latter species both viologen units are reduced and there is a distinct stabilisation effect if the molecule can form an intramolecular cofacial dimer. The disproportionation constants have been determined by cyclic voltammetry and depend markedly upon the type of bridging group employed. Using pulse radiolysis it was shown that the mono radical could be stabilised by intramolecular association with the unreduced viologen. This is achieved by fairly slow conformational rearrangement in competition with bimolecular electron transfer to the preferred conformation. All the mono radicals disproportionate and the bimolecular rate constants have been measured. Although the viologens can store several electrons on one molecule, they do not reduce water to H2 in the absence of a catalyst.

The rate constants for quenching of chlorophyll-a singlets and triplets by a series of manganese complexes have been measured in fluid solution. Quenching efficiency correlates with the oxidation state of the central manganese atom : 4 > 3 > 2, and, for manganese (II) complexes, the degree of conjugation of the complexing ligand makes an important contribution towards the magnitude of the quenching rate constants. Ion-pair formation occurs with the higher oxidation state quenchers but not with manganese (II). Here, charge transfer interactions are implicated in the quenching reaction and the collision complex provides a means of non-radiative decay.

The bimolecular rate constant for quenching the triplet excited state of a metalloporphyrin by methyl viologen depends upon the electronic charge on the porphyrin ring. The rate constant increases with increased electrostatic attraction between the reactants and this is due to efficient formation of an encounter complex and a high thermodynamic driving force for formation of an ion-pair. However, dissociation of the ion-pair to separate ion products is favoured by strong electrostatic repulsion between the products due to electrostatic factors. In addition the yield of redox products depends upon the amount of electronic energy that must be dissipated in the form of heat, and the rate of internal conversion of the ion-pair decreases significantly with increased energy.

A series of porphyrins (2a)–(5a), unsymmetrically substituted in their meso-positions with 3,5-di-tbutyl-4-hydroxyphenyl(DtB4HP) and 4-hydroxyphenyl (4HP) groups, has been synthesised and characterised. The two di-DtB4HP di-4HP isomeric porphyrins, (3a) and (4a)(called ‘cis’ and ‘trans’, respectively), in particular are distinguishable by 1H n.m.r. spectroscopy. In basified dichloromethane (DCM) solutions, the porphyrins undergo drastic colour and u.v.–visible changes which, in 1 mol dm–3 methanolic potassium hydroxide, is indicative of irreversible aerial oxidation that is slower and less extensive than for the symmetrically tetrakis-DtB4H P substituted porphyrin (1). Cyclic voltammetry supports this, indicating that in basified DCM solutions, one-electron oxidation of porphyrins (2a)–(5a) becomes increasingly more difficult than for (1), as the DtB4HP groups are replaced by 4HP. An explanation of the relative redox activity of these porphyrins compared with (1), is offered in terms of the different electron-releasing properties of the two types of phenolic substituent.

The absorption spectra of paramagnetic transition metal porphyrins consist of (ππ*), charge-transfer and tripmultiplet transitions. The tripmultiplet states result from interaction between the metal d-orbitals and the porphyrin (ππ*) excited triplet states. For CuIITPP, luminescence occurs from the lowest energy tripquartet state whilst CrIIITPP shows luminescence from tripquartet and tripsextet states. Luminescence from MnIIITPP and FeIIITPP is much less efficient and originates from the second tripmultiplet state. The rate constants for deactivation of the tripmultiplet states depend markedly upon the nature of the central metal ion and are much faster for MnIII and FeIII than for CuII and CrIII. Luminescence was not observed for FeIITPP, possibly due to the presence of a low-lying c.t. state.

The quantum yield for formation of redox ions from the zinc(II) tetra(N-methyl-4-pyridyl)porphine-photosensitised reduction of methyl viologen is ca. 0.7, but the products undergo diffusion-controlled reverse electron transfer. Using ultrafine colloidal Pt particles, supported with polymers, waxes or surfactants, it is possible to intercept reverse electron transfer at modest concentrations of Pt, but the catalysts are not selective. Some degree of selectivity can be obtained if a positively charged Pt colloid is used in conjunction with a less hydrophilic viologen, but under such conditions steady-state irradiation does not produce H2.When a surfactant viologen (SV2+) is used as the support for Pt colloids, the reducing equivalent can be transferred rapidly to the Pt core. The lifetime of the reduction product (SV+) depends upon the number of electrons resident on the particle and increases with increased charge. This is due, at least in part, to dimerisation of the viologen radical. Again, steady-state irradiation in the presence of a neutral zinc porphyrin gives no detectable yield of H2.

Gold(III) porphyrins are easily reduced to the corresponding radical anions, which are stable in water over a wide pH range. Further reduction results in formation of the phlorins with relatively little tendency to form chlorins. Both radical anion and phlorin will reduce water to H2 on the surface of a colloidal Pt catalyst. From kinetic studies the radical anion is by far the better reducing species. Using NADH as reducing agent, a photosystem has been devised which results in the overall storage of visible light. Oxidation of the gold(III) porphyrins results in destruction of the porphyrin ring.

Metal-free and ZnTPP complexes exhibit both fluorescence and low temperature phosphorescence, MnIITPP shows only low temperature phosphorescence while NiIITPP is non-luminescent. The absorption spectra show that there is little interaction between the metal and the porphyrin π-system so that the excited states can be regarded as (ππ*) in nature. Where phosphorescence occurs, the Stokes shift is 3900 ± 100 cm–1 and decreases with increased metal–porphyrin interaction. The rate constants for intersystem crossing and for non-radiative decay from the lowest (ππ*) triplet state depend upon spin–orbital coupling properties. ZnII and NiI exhibit fairly weak spin–orbital coupling but the effectiveness of MnII is enhanced by the paramagnetic character and by favourable geometry. NiIITPP possesses a low energy (dd) excited singlet state which facilitates rapid internal conversion within the singlet manifold.

Water-soluble, diamagnetic metalloporphyrins have been prepared which contain either zinc(II), palladium(II) or tin(IV) ions as the central metals and their photophysical properties have been measured in dilute aqueous solution. All the compounds undergo efficient intersystem crossing to form long-lived excited triplet states that can participate in electron-transfer reactions. Thus excitation of the metalloporphyrin in dilute aqueous solution containing an appropriate electron acceptor, such as iron(III), may result in formation of the metalloporphyrin π-radical cation in quite high yield. These π-radical cations are powerful oxidants, in some cases E° > 1 V vs NHE, but they undergo secondary reactions that lead to formation of π-dications and isoporphyrins. Despite the high redox potentials, it has not been possible to couple the one-electron reduction of the π-radical cations to the four-electron oxidation of water to molecular oxygen, even in the presence of a redox catalyst such as RuO2. Therefore these compounds appear to possess little promise as water oxidants in homogeneous photosystems.

We have confirmed an earlier report by Kalanasundaram and Gratzel that a positively charged, water-soluble zinc porphyrin photosensitises the reduction of water to H2 with high efficiency. Using MV2+ as electron relay and EDTA as sacrificial electron donor, the quantum yield for production of ½H2 is ca. 0.6. The reaction mechanism involves reduction of MV2+ by triplet porphyrin and the porphyrin π-radical cation so produced is reduced by EDTA. The concentrations of reactants have been optimised for production of H2 and to limit destruction of the porphyrin. Under optimised conditions the turnover with respect to the porphyrin can reach 6000.In addition, some consideration has been given to ways of improving oH2 and of increasing the fraction of sunlight that can be harvested. In this respect, Cd porphyrins may possess suitable properties.Finally, we note that the porphyrin π-radical cation may possess the thermodynamic capacity to oxidise water to O2, but this is very much borderline.

Methyl viologen quenches the singlet excited state of a positively charged magnesium phthalocyanine but not the triplet excited state. In aqueous solution, the singlet quenching rate constant depends upon ionic strength but at zero ionic strength it approaches the diffusion-controlled limit. Overall, the quenching act leads to net electron transfer but the probability of separation of the singlet ion-pair is very low. The efficiency of this separation process is related to the electronic charge on the reactants, the spin-multiplicity of the ion-pair and to the amount of free energy that must be dissipated by the competing internal conversion process.Although the system possesses the capability of collecting ca. 45% of the solar spectrum, irradiation in the presence of EDTA and a colloidal Pt catalyst does not produce H2.

Water-soluble manganese(III) porphyrins undergo a well defined, one-electron reduction to the corresponding manganese(II) derivative for which the overall reversibility is very high. The intrinsic electrochemical rate constant (k0) was (1.2 ± 0.4)× 10–3 cm s–1 and it remained essentially independent of pH. The nature of the porphyrin ligand affected the magnitude of E½ but caused no significant differences in electrode behaviour. One-electron oxidations were observed for which both E½ and k0 depended markedly upon pH. At pH > 13, oxidation approached the fully reversible case but as the pH was decreased oxidation became progressively more difficult and less reversible. These differences are associated with acid–base equilibria of axially co-ordinated water molecules.

Flash excitation of chlorophyll-a in outgassed ethanol solution gives a high yield of the long-lived triplet state. Various types of aromatic nitrogen compound have been found of quench the triplet via an electron transfer reaction. Thus, nitrobenzenes and aromatic heterocyclic compounds act as electron acceptors with triplet chlorophyll-a and quenching leads to formation of chlorophyll-a radical cations and quencher radical anions. Decay of the transient absorption observed by flash spectroscopy is due to back electron transfer. On the other hand, aromatic amines react with triplet chlorophyll-a to yield the chlorophyll-a radical anion and the amine radical cation. Here, reduced chlorophyll-a decays via oxidation with an impurity within the system.

Phosphorescence and fluorescence studies of a series of N-substituted phthalimides suggest that the photochemical cyclization reaction of dialkylaminoalkyl substituted compounds probably occurs through the (n,π*) singlet excited state. Flash photolysis of these and other, simpler phthalimides reveals an unrelated reaction involving two types of transient intermediate. One is a triplet state and the other a much longer-lived species which is postulated to be an isomer of the imide produced by a 1,3-shift. The isomer reverts to the imide, and the effect of substituent and of solvent on the decay is discussed.

From u.v. absorption and fluorescence measurements it is suggested that N-(dialkylaminomethyl)phthalimides fluoresce(weakly) from the second excited singlet state (π,π*), and that the lowest energy singlet state (n, π*) is non-fluorescent. Phosphorescence occurs from the lowest triplet state at 77 K, and in ethanol solution this state is unambiguously assigned a (π,π*) configuration. The second triplet state (n, π*) is quite close in energy to the first. The luminescence results point to the operation of an initial electron transfer step in the photochemical cyclization reaction of these phthalimides.

Studies with aqueous and organic solvents and with micelles, membranes, and microemulsions showed that the environment had little effect upon the spectroscopic and chemical properties of MnII and MnIII porphyrins. However, for oxidation of a MnIII porphyrin, the environment makes important contributions towards the ease of oxidation and the nature of the oxidation process. In alkaline aqueous solution, oxidation was facile and resulted in formation of MnIV and MnV porphyrins. Identical, but less efficient, oxidation processes were observed in membranes and positively charged micelles but in both negatively charged micelles and microemulsions oxidation was extremely inefficient and the MnV porphyrin was not observed. In CH2Cl2 solution, reaction with phenoxathiinylium hexachloroantimonate gave the MnIII porphyrin π-radical cation whilst reaction with iodosylbenzene resulted in formation of the MnIV porphyrin µ-oxo-dimer.

Iridium oxide hydrosols, containing both IrIII and IrIV sites within a complex oxyhydroxide structure, have been prepared and their interaction with water-soluble zinc porphyrin radical cations has been studied. The colloids possess a negative surface charge so that the positively charged zinc porphyrins are bound to the colloid by electrostatic attraction whilst negatively charged porphyrins are repelled from the surface. Oxidation of IrIII sites can be accomplished with fairly mild oxidants and, at most pH values, the zinc porphyrin radical cations are capable of driving this process. The resultant IrIV sites can undergo further redox reactions, the outlet for any stored oxidizing equivalents being dependent upon the type of porphyrin used. With positively charged porphyrins, discharge of the colloid involves oxidation of surface-bound porphyrin. Using negatively charged porphyrins, the colloid catalyses the oxidation of water to O2 with remarkably high efficiency. Evolution of O2 is restricted, however, to pH > 7 owing to thermodynamic factors associated with the porphyrin. The photo-oxidation of water to O2, under sacrificial conditions, is found to proceed with an initial quantum efficiency of 72% for an optimized system.

The photophysical properties of a water-soluble 9-oxothioxanthene have been determined in dilute aqueous solution. The first excited singlet state, which is of charge-transfer character, is strongly fluorescent and possesses a relatively long lifetime. Inefficient triplet-state formation occurs, but the triplet state, which is also of charge-transfer character, has a very long lifetime in the absence of oxygen. Neither excited state is able to abstract hydrogen atoms from organic materials, but both oxidative and reductive electron-transfer reactions can be observed following irradiation in the presence of viologens, iron(III) ions or amines. Irradiation at 400 nm in the presence of triethylamine and colloidal Pt produces H2 with an optimised quantum yield of 1.02. The total yield of H2 obtained upon exhaustive photolysis and the rate of H2 formation compare favourably with any photosystem reported previously.

The metal centre in osmium porphyrinates (OsP) can exist in a wide range of oxidation states, each being stabilised by axial complexation with a particular ligand. Thus, OsIIP is stabilised by ligation with a CO molecule. Oxidation occurs readily to form the corresponding OsIIIP which retains the CO ligand and can be reduced quantitatively to the original OsII complex. Further oxidation gives OsIVP which is stabilised by axial ligation of two alcohol or water molecules. These OsIV complexes are stable to both oxidation and reduction. Treating the OsIIP with an organic peroxide results in formation of an OsVIP complex which is stabilised by two oxo ligands but can be reduced to the OsIVP species. Photolysis of OsIIP and OsIIIP causes loss of the CO ligand, the naked porphyrin species so formed undergoes oxidative reactions with the solvent, O2, or trace impurities, in most cases forming an OsIVP complex. The [OsVIP(O)2] species does not oxidise alkanes or alkenes in the dark at room temperature but does oxidise cyclohexene under visible light irradiation.

Exposure of manganese(III) porphyrin solutions to 60Co γ-rays at 77 K gave the manganese(II) derivatives, with g⊥(apparent)≈ 6, A(55Mn)≈ 77 G and |D| 0.3 cm–1. The spectra are very similar to those for MnII derivatives prepared chemically. Weak features in the g= 2 region for these complexes were obscured by features from a symmetrical MnII derivative of unknown origin. These features grew in intensity on melting and re-freezing.These results suggest that low-temperature irradiation coupled with e.s.r. spectroscopy may be a suitable technique for detecting MnIII derivatives in biological systems.

CdO is an inherent n-type semiconductor with a band-gap of ca. 2.3 eV and a flat-band potential located at –0.25 V vs SCE in 0.1 mol dm–3 NaOH solution. Electrodes fabricated by compressing CdO powder showed an exchange current density of 5 × 10–8 A cm–2 for O2 liberation in 0.1 mol dm–3 NaOH and this was increased to 9 × 10–6 A cm–2 by depositing small amounts of RuO2 onto the CdO surface. Irradiation of CdO powder suspended in alkaline solution resulted in O2 formation only when an electron acceptor, such as ferricyanide, was present in solution. The presence of RuO2 deposits reduced the yield of O2.

A novel mechanism, based upon photoisomerisation, is proposed to account for the efficient and selective photodynamic therapeutic activity of Merocyanine 540, an anionic cyanine dye.

The irradiation of bipy3Ru2+ in acidic aqueous solution containing iron(III) and colloidal RuO2·2H2O, resulting in formation of O2 with a quantum efficiency of ca. 48%, has been studied. At pH 1.9, the rate of O2 formation was 2.6 × 10–6 mol dm–3 min –1 and the total yield of O2 obtained upon prolonged irradiation was 9.1 × 10–4 mol dm–3. With mercury(II) as electron acceptor, O2 formation was less efficient but it could be observed at pH 3.

Tetrasulphophthalocyanine and the ZnII and CuII complexes have intense absorption in the near i.r. region but the compounds aggregate in aqueous solution. Aggregation is hindered by addition of organic solvents, such as pyridine, and here H2PcS4– and ZnPcS4– show strong fluorescence. Photophysical data have been collected for both compounds and for the CuII complex and the observed values related to the ideal properties of a chromophore for a three-component [chromophore/donor/methyl viologen (MV2+)] H2-producing system. The ZnII complex appears as the most attractive chromophore but irradiation in the presence of EDTA, MV2+ and colloidal Pt gives no H2 production due to efficient reverse electron-transfer. The yield of H2 is increased in the absence of MV2+ but the sulphonated phthalocyanines compare unfavourably with metalloporphyrins.

The photophysical properties of a new series of tripyrroledimethine-derived ‘expanded porphyrins’(‘texaphyrins’) are reported; these compounds show strong low energy optical absorptions in the 730–770 nm spectral range as well as a high triplet quantum yield, and act as efficient photosensitizers for the production of singlet oxygen in methanol solution.

Aluminium(III) tetraphenylporphine possesses a tightly bound axial ligand, the nature of which influences the photophysical properties. As the spin–orbital coupling character of the ligand increases (OH– < OOCCH–3 < F– < Cl– < Br– < I–), the singlet excited state lifetime and the fluorescence quantum yield decrease whilst the quantum yield for formation of the excited triplet state increases. The observations can be interpreted in terms of an internal heavy-atom enhanced intersystem-crossing mechanism, and for these compounds internal conversion is unimportant. However, for the µ-oxo dimer internal conversion accounts for some 46% of the decay route for the excited singlet state although there is little evidence to suggest pronounced exciton coupling between the two porphyrin rings.

EDTA transfers an electron to the triplet excited state of ZnTMPyP4+ in aqueous solution and the one-electron reduction product of the zinc porphyrin reduces water to H2. A Pt catalyst is required to promote this latter reaction and it was found that the nature of the support used to stabilise the colloidal Pt particle against flocculation had a great effect upon the efficiency of the Pt catalyst. The most effective support was Carbowax 20M and irradiation of ZnTMPyP4+ in the presence of EDTA (0.1 mol dm–3) and Pt–Carbo (10–4 mol dm–3) resulted in formation of H2 with an optimum quantum yield (ϕ½ H2) of 0.07.The average lifetime of the reduced zinc porphyrin was increased by binding the porphyrin to a polymer and by using a surfactant derivative of the porphyrin. In the latter case, the surfactant porphyrin was used as a support for colloidal Pt particles and upon irradiation in the presence of EDTA H2 was produced with ϕ½ H2 of ca. 0.004; this was increased significantly when methyl viologen was present in the aqueous solution.

Manganese(III) porphyrins (MnIIIP) are easily oxidised to the corresponding MnIVP in alkaline aqueous solution. At pH < 5 the oxidation product is a MnIIIP π-radical cation. These oxidised metalloporphyrins have limited stability in water and they revert to the original MnIIIP upon standing in the dark. The rate and mechanism of this inherent reduction process depends upon pH, with lower pH giving the higher rates. The inherent reduction appears to involve disproportionation and rearrangement of the MnIVP but it does not lead to formation of molecular O2. Addition of colloidal RuO2·2H2O, a good O2-evolving catalyst, has a pronounced effect upon the reduction process. The oxidised metalloporphyrin is bound to the catalyst particles by electrostatic forces, and at pH < 11 the bound material decays more slowly than the free compound. For 8 < pH < 11, decay of the bound metalloporphyrin involves oxidation of water to O2, but the yield of O2 is much less than the stoichiometric value.

Zinc tetraphenylporphine shows fairly weak fluorescence and phosphorescence but intense, long-lived triplet absorption. Benzo-1,4-quinone quenches the singlet and triplet excited states of ZnTPP and the quenching probability increases with increased polarity of the solvent. Quenching leads to formation of a complex which, for the triplet reaction in toluene, is sufficiently long-lived to be observed by flash spectroscopy. In polar solvents, quenching leads to production of separated ions, the yield being a function of the solvent dielectric constant.

The photoredox reaction between thionine and iron (II) has been studied by flash photolysis techniques. The principal reaction intermediate is the semithionine radical anion which decays via a dismutation process forming ground state thionine and the fully reduced leucothionine. Subsequently, leucothionine is reoxidised slowly by iron (III) which is formed in the primary electron transfer reaction. Rate constants for all the reaction steps have been determined.The reaction has been extended to include reduction with cobalt (II) and manganese (II). Here, the ground state quenchers do not possess the necessary reducing power to form the semithionine radical anion, and reaction leads to enhanced deactivation of triplet thionine. However, when the transition metal ion is excited directly, full electron transfer takes place with concomitant formation of the radical ions.

Colloids of RuO2· 2H2O and MnO2 have been produced by chemical and radiolytic methods. The colloids slowly aggregate upon standing in water over several weeks but radiolysis gives much smaller particles. The colloids are active in catalysing water oxidation under chemical and photochemical conditions although there is some corrosion. The chemically formed RuO2· 2H2O colloids are quite efficient catalysts for water reduction to H2 but the radiolytically formed colloids do not promote H2 formation. Colloidal MnO2 is a poor catalyst for both H2 and O2 generation and it tends to aggregate under operating conditions. The RuO2· 2H2O colloids obtained by gamma radiolysis are selective for oxidation processes. They are stable and catalyse O2 formation with high efficiencies. Such materials appear to be the best catalysts available for use in model systems for the photodissociation of water.

Zinc porphyrins (1) and (2), their triplet excited states and their triplet excited states and their cation and anion radicals, are remarkably stable in 1 M nitric acid: the consequences for photosensitised water reduction, oxidation, and the photogalvanic effect, are discussed.

A1TPP exhibits intense fluorescence and low-temperature phosphorescence for which the observed properties are entirely consistent with (ππ*) excited states of a diamagnetic metalloporphyrin. In contrast, the paramagnetic CrTPP does not show normal (ππ*) fluorescence, but instead extremely weak triplet-state fluorescence from a tripquartet state is observed at room temperature. The lifetime of this tripquartet state is 295 ps, and its deactivation involves population of a lower-energy tripsextet state which shows inefficient low-temperature phosphorescence. Since the two states are in thermal equilibrium, delayed triplet fluorescence is observed at room temperature.

In aqueous solution under an atmosphere of H2, the pH at which the concentrations of methyl viologen (MV2+) and its radical cation (MV.+) are equal depends upon the type of Pt catalyst used but is close to the Nernst value of pH 7.6. With polymeric viologens, it is closer to pH 5.6 owing to dimerisation of the radical cation. Controlled potential electrolysis at pH 4.7 shows that MV.+ gives H2 in almost quantitative yield but that the polymeric viologens give low yields of H2. Similarly, the yield of H2 obtained from photochemical systems is very much lower when a polymeric viologen replaces MV2+. However, if the viologen is attached to a macroreticular bead then modest yields of H2 are obtained and the reduced viologen can be isolated and transported (under anaerobic conditions).

Photoinduced electron transfer from the singlet excited state of a zinc porphyrin to co-bound silver(I) ions within a polymetallic macrotetracyclic cryptate forms a long-lived charge-separated state.

Water-soluble derivatives of benzophenone are readily reduced to the pinacol or alcohol upon irradiation with near-ultraviolet light in aqueous solution. The hydrogen donor can be a simple organic compound (e.g. methane, alcohol, aldehyde, amine, amide, sulphide, ether, carboxylic acid etc.) but not water. In most cases the photochemistry proceeds rapidly with high quantum yield. If an active Pt colloidal catalyst is added to the solution, the intermediary radicals can be used to reduce water to H2 on the metallic surface. Thus the organic substrate is effectively dehydrogenated under ambient conditions. In some cases the reaction can be used for synthesis of useful organic materials.

Metal complexes of the title porphyrin were prepared and their behaviour in basified dichloromethane solution examined using u.v.–visible and e.s.r. spectroscopy. The free base, ZnII, FeIII, and CoII complexes undergo drastic colour changes with rapid and irreversible aerial oxidation. Acidification does not regenerate the porphyrin spectrum, but shaking with aqueous metabisulphite or dithionite does. The VIVO, MnIII, CuII, SnIV, PdII, and PtII complexes also change colour but their metalloporphyrin u.v.–visible spectra largely return on acidification. Some deterioration over time in the returned B band intensity is noted, which is greatest for the NiII complex and is most likely due to autoxidation. E.s.r. spectra, arising from oxidation or autoxidation of the complexes to radical species, were observed for the MnIII, NiII, ZnII, and SnIV complexes, the latter in neutral dichloromethane. This gave a triplet e.s.r. spectrum that decayed with second-order kinetics. Addition of base to the SnIV complex quenched the e.s.r. spectrum. An explanation is offered in which the metal modulates electron transfer onto the macrocycle, from the phenoxy meso-substituents, via back-bonding and changes in porphyrin conformation.

In aqueous solution at pH 9, lead(II) tetrakis(N-methylpyridyl)porphyrins are oxidised rapidly to the metalloporphyrin π-radical cations by Br˙–2, under pulse radiolytic conditions. The π-radical cations disproportionate at a rate close to the diffusion-controlled limit. The doubly oxidised metalloporphyrin so formed can be assigned to the lead(IV) porphyrin, on the basis of absorption spectroscopy. These highly oxidised metalloporphyrins decay via first-order kinetics (k= 0.93 ± 0.05 s–1) at pH 9. Even in the presence of colloidal RuO2·2H2O, this decay process does not oxidise water to O2.

The photochemical properties of a series of water-soluble benzophenones have been evaluated in dilute aqueous solution. The compounds possess lowest-energy singlet and triplet excited states demonstrating considerable n,π* character. As such, irradiation of the compounds in aqueous solution containing ethanol (2% v/v) results in pinacol formation via a triplet-state hydrogen-abstraction process. In the presence of a colloidal Pt catalyst, the intermediate ketyl and 1-hydroxyethyl radicals can be used to reduce water to H2. The rate of H2 formation and its total yield depend upon the nature of the substituent used to solubilise the benzophenone. The rate at which the ketyl radical transfers an electron to the Pt particles can be rationalised in terms of thermodynamic and electrostatic factors.