RADIATIVE DYNAMICS IN SOLUTION AND IN MOLECULAR ASSEMBLIES OF AN H-AGGREGATE-FORMING STILBAZOLIUM AMPHIPHILE

Radiative dynamics of the hemicyanine dye -(3-sulfopropyl-4-(p-dioctyl aminostyryl)pyridinium (I) have been studied in primary alcohols and i n Langmuir-Blodgett (LB) monolayer films by using time-resolved laser- induced fluorescence spectroscopy. In solution, the deactivation of th e optically excited state is dominated by a radiationless transition v ia an activated barrier crossing to the twisted intramolecular charge transfer (TICT) state, and the decay rate decreases substantially with increasing n in the series CH3(CH2)(n)OH. Observed biexponential deca ys are consistent with the three-level scheme describing the horizonta l transition on the excited-state potential surface. In monolayer film s the decay dynamics are similar whether the initial excitation occurs into the monomer or aggregate bands. The kinetics are interpreted in terms of diffusion of excitons in the aggregate with eventual energy t ransfer to the monomer. A Monte Carlo simulation was used to character ize the motion of the excitons within aggregates and to correlate the aggregate size with exciton lifetime. The finite size of the aggregate in monolayer films was estimated under two limiting cases: one in whi ch exciton transport on the aggregate is the rate-limiting step in emi ssion and another in which aggregate-monomer excitation transfer limit s the radiative rate. Comparison of experimental and simulated results yielded an estimate of the size of an H-aggregate in monolayers of I to be ca. 45 molecules in the first case and 50 molecules in the secon d. A comprehensive kinetic scheme is presented that unifies both the s olution and monolayer film behavior observed in these studies.