C 60 solutions in benzonitrile have been found to show concentration dependent optical absorption behaviour. At lower concentrations (<100 µM) the UV-vis absorption characteristics of C 60 in benzonitrile are exactly similar to those in benzene and decalin. At higher concentrations (>100 µM), however, the C 60 solutions in benzonitrile show very broad absorption tail, extending beyond 900 nm. At higher concentrations the solutions are also visually opaque. From picosecond laser flash photolysis experiments it is seen that the triplet quantum yield of C 60 in benzonitrile at higher concentrations (∼400 µM) is much less than unity and increases with the dilution while in decalin and benzene it is always close to unity and independent of the C 60 concentration. Dynamic light scattering experiments indicate the presence of particles of mean size of about 250 nm in C 60 solutions in benzonitrile with concentration >100 µM, while in <100 µM solutions no such particles have been observed. Such particles are also not observed for C 60 solutions in benzene and decalin, even up to ∼500 µM. Scanning electron microscopy also shows particles of size ∼250 nm. It is inferred that C 60 forms aggregates in benzonitrile at concentrations >100 µM and that the aggregated and the monomeric form of C 60 are in equilibrium.

The unusual solvatochromism of C70 is investigated in a variety of solvent mixtures by optical absorption and fluorescence techniques. Distinct reversible color change from pink to purple is seen in the solvent mixtures studied. Such changes are seen also for C60 solutions in some solvent mixtures. Formation of clusters is found to be responsible for the observed optical changes. Light scattering studies are carried out to confirm the presence of clusters which show that the particle size varies from ∼100 to ∼1000 nm depending on the concentration of the fullerene. It is found that the solubility of the fullerene in the solubilizing solvent and that in the solvent mixtures are the major factors governing the aggregation behavior of the fullerene. Polarity of the solvent plays a minor role in the formation of aggregates.

Time-resolved studies in the pico- and nanosecond time domain have been performed to characterize the triplet states of monohydroxy-substituted benzophenones, namely, para- (p-), meta- (m-), and ortho- (o-) hydroxybenzophenones (HOBP). Due to a very fast intersystem crossing (ISC) process, only the triplet states have been detected in the subnanosecond time domain. Spectral characteristics and lifetimes of the triplet states of HOBP have been seen to be extremely sensitive to the position of the OH group in the phenyl ring as well as the solvent characteristics. In case of m-HOBP and p-HOBP, the excited triplet state in non-hydrogen-bond-forming solvents has an nπ* configuration and is capable of abstracting a hydrogen atom from another unexcited molecule to form ketyl and phenoxy type radicals. But in hydrogen-bond-forming solvents, the triplet state, which is strongly associated with the solvent molecules as a hydrogen-bonded complex, is very short-lived due to fast nonradiative relaxation via hydrogen-stretching vibrations in intermolecular hydrogen bonds with the solvents and is capable of abstracting a hydrogen atom neither from the solvent molecule nor from another unexcited HOBP molecule. In the case of o-HOBP, due to strong intramolecular hydrogen bonding, the internal conversion in the excited singlet state is very efficient and hence the yield of the triplet state is low (<15%) and also the triplet state is very short-lived. However, in methanol and DMSO, due to disruption of the intramolecular hydrogen bond, the triplet yield is higher and also the generation of the phenolate ion via excited-state proton transfer is a significant process. Deprotonation reactions probably taking place from both the excited singlet state as well as the triplet state of the other two derivatives also have been found to be significant in polar solvents.

Phenothiazine and 10-methylphenothiazine have been found to form weak complexes with the chloroalkanes, such as CCl4 and CHCl3, both in the ground and excited singlet states as investigated by spectrophotometric and fluorimetric techniques. Steady-state and time-resolved fluorescence quenching experiments indicate that the strength of interaction of the excited singlet (S1) state of the phenothiazines with the chloroalkanes increases as the reduction potentials of the latter become less negative or the oxidation potentials of the phenothiazines become less positive. These results indicate that the electron-transfer or charge-transfer type of interaction is operative in these systems. In less polar solvents, such as cyclohexane and ethyl acetate, weak exciplex formation between the S1 state of the phenothiazines and the chloroalkanes has been observed, whereas in a more polar solvent, such as acetonitrile, direct electron transfer takes place from the S1 state of the phenothiazines to the chloroalkanes, leading to dissociation of the C−X bond of chloroalkanes. Picosecond flash photolysis studies carried out on these systems showed the presence of the cation radical of the phenothiazines and support the photoinduced charge separation and other processes proposed.

Studies on radical equilibria and the spectral evaluation have been carried out for three monohydroxy-substituted benzophenones (HOBP's) in aqueous solution. The transient ketyl or anion radicals were generated via hydrated electron (eaq -) reaction or dimethyl ketyl radical reaction in a pulse radiolysis experiment. The reactivity of both the undissociated (HOBP) as well as dissociated (-OBP) forms toward these reducing agents were studied. eaq - was found to react with the HOBP's and -OBP's with diffusion-controlled rates which are of the order 3 × 1010 dm3 mol-1 s-1. In the case of the para derivative, two acid−base equilibria were identified (pK a 9 and 11) in the pH range 6−13, one corresponding to the protonation−deprotonation at the OH site of the ketyl radical and the other for the protonation−deprotonation at the carbonyl site. For the ortho derivative, the intramolecularly hydrogen bonded structure of the radical anion has been found to be stable even in strongly alkaline solution (pH 13). Three different radical forms for the meta derivative have been identified at different pH conditions. The spectral evaluation of these radical species at different pH conditions was carried out. Reactions of dimethyl ketyl radicals with HOBP's and -OBP's were found to be different at different pH conditions. At acidic pH, it forms an adduct with the HOBP's, whereas at alkaline pH, the reaction with the ortho and meta derivatives occurs by H atom transfer. However, no reaction is seen with the para derivative in alkaline solution. Detailed spectral and kinetic data on the formation and decay of the various transient intermediates have been obtained.