Abstract

On the basis of a thorough study of 7-aminocoumarin dyes in different solvents, the methodology of amplified spontaneous emission (ASE) gain spectroscopy at pumping threshold is established and has been applied to study the complexation of 7-diethylamino-4-methylcoumarin (C1) in different co-solvent systems. The advantage of ASE gain spectroscopy over conventional spectrofluorimetry is the ease of separating heavily overlapping emission spectra of 'free' and complex species, which is important for the study of complexation in biological system. The photophysical behaviour of conformationally sensitive 7-aminocoumarin dyes in different solvents and their cyclodextrin (CD) complexes in water is studied using picosecond time-resolved transient absorption and emission techniques. These studies offer some important information about the possible twisted intramolecular charge transfer (TICT) state of C1 (not the rigid C102) in protic solvents. The structures of C1/CD complexes in water are deduced from both static and dynamic studies and are supported by the study of these dyes in pure or binary solvents. The time-resolved emission spectral study of C1 and C102 in protic solvents, shows that the solvation dynamics can be explained by a multi-state model in kinetics rather than a single excited state undergoing a continuous solvent relaxation process. This has been confirmed by (i) the simulation results, and (ii) the shape of the decay associated spectrum which is the normal preexponential distribution of the exponential time-dependent fluorescence (TDF). One of the wavelength-independent decay times of TDFD is closer to the longitudinal relaxation time of solvent than that of the single state model. Time-resolved pump-probe technique has been also used to investigate the poorly understood excited state photophysical behaviour of nonfluorescent nitrobenzene derivatives in solution. A remarkable solvation effect is found in the ESA study of dimethoxynitrobenzenes. The lowest excited state observed on ESA spectra is attributed to an intramolecular charge transfer state. The rate constant of the time-dependent blue-shift ESA spectra is comparable to the rate constant of the longitudinal relaxation of solvent. This observation implies an alternative method to study the solvation dynamics. A relationship between excited state absorption (ESA) spectra of some alkylnitrobenzenes and the structure of substituted nitrobenzene is found. Finally, in order to understand and to control the photochemical process, the biradical intermediate of o-nitrobenzyl p-cyanophenyl ether in acetonitrile has been identified from ESA spectra which provides information about the partitioning singlet/triplet pathway of the photochemical reaction.