No abstract

Protolytic photodissociation of some hydroxyaromatic compounds, ArOH (1-and 2-naphthol, chlorosubstituted naphthols), was studied in micellar solutions and phospholipid vesicles by fluorescence spectra and kinetics. Experimental results give evidence of two localization sites of naphthols in the microphase of these systems. In lipid bilayer membranes of vesicles there are two comparable fractions of ArOH molecules, one of which undergoes photodissociation but the other does not dissociate. In micelles, a minor fraction (few percent) of ArOH molecules, which are located probably in the core of the micelle, do not take part in excited-state proton-transfer reaction. These phenomena refllect heterogeneous structure and dynamic properties of lipid bilayer membranes and micelles.

Diffusion-controlled rates of formation were found from the temperature dependence of apparent quenching rate constants for exciplexes, when the driving force of excited-state electron transfer -0.1 < deltaG(ET)* < +0.1 eV. This is inconsistent with the conventional mechanism of electron-transfer reactions, involving preliminary reorganisation of the medium and reactants, and provides strong support for the mechanism of medium reorganisation stimulated by strong electronic coupling of locally excited and charge-transfer states.

Kinetics of excited-state proton-transfer reactions and proton-induced fluorescence quenching of 1-naphthol (1N) and 2-octadecyl-1-naphthol (2O1N) in micellar solutions of cetyltrimethylammonium bromide (CTAB), polyoxyethylene(23) lauryl ether (Brij 35), and sodium dodecyl sulfate (SDS) was studied by using stationary and time-resolved fluorescence techniques. The ground-state acidity constant of 2O1N in cationic micelles of CTAB was found to be significantly smaller than that of the parent compound (ΔpK = 0.5). However, similar rate and equilibrium constants of the protolytic dissociation were obtained for 1N and 2O1N in the singlet excited state. Effects of nonionic micelles of Brij 35 closely resemble those of CTAB. In anionic micelles of SDS, the protolytic photodissociation was much slower for 2O1N than for 1N. The protonation rate for the excited anions in micellar solutions increases by approximately 2 orders of magnitude in the series CTAB, Brij 35, SDS. Excited-state kinetics was rationalized within the framework of a pseudophase model, which included micellar effects on the proton-transfer equilibrium and interfacial diffusion of hydronium ions. The electrostatic surface potential of charged micelles was estimated from the acidity constants of naphthols.

Exciplex emission spectra and rate constants of their decay via internal conversion and intersystem crossing are studied and discussed in terms of conventional radiationless transition approach. Exciplexes of 9-cyanophenanthrene with 1,2,3-trimethoxybenzene and 1,3,5-trimethoxybenzene were studied in heptane, toluene, butyl acetate, dichloromethane, butyronitrile, and acetonitrile. A better description of spectra and rate constants is obtained using 0-0 transition energy and Gauss broadening of vibrational bands rather than the free energy of electron transfer and reorganization energy. The coincidence of parameters describing exciplex emission spectra and dependence of exciplex decay rate constants on energy gap gives the evidence of radiationless quantum transition mechanism rather than thermally activated medium reorganization mechanism of charge recombination in exciplexes and excited charge transfer complexes (contact radical ion pairs) as well as in solvent separated radical ion pairs. Radiationless quantum transition mechanism is shown to provide an appropriate description also for the main features of exergonic excited-state charge separation reactions if fast mutual transformations of loose and tight pairs of reactants are considered. In particular, very fast electron transfer (ET) in tight pairs of reactants with strong electronic coupling of locally excited and charge transfer states can prevent the observation of an inverted region in bimolecular excited-state charge separation even for highly exergonic reactions.