Time-resolved studies of dynamics of triplet state spectral diffusion in the presence of both orientational and substitutional disorders: binary solid solutions of 1-bromo-4-chloronaphthalene and 1,4-dibromonaphthalene

The dynamics of spectral diffusion is studied in the presence of both orientational and substitutional disorders at 4.2 and 1.8 K by time-resolved and site-selective excitation methods. The systems of study are different compositions of solid solutions of 1-bromo-4-chloronaphthalene (BCN) and 1,4-dibromonaphthalene (DBN). We used a wide range of concentrations within which the DBN crystal structure prevails. The phosphorescence of BCN is monitored as a function of the excitation energy (i.e. acceptor concentration). The site-selective experiments, along with the time-resolved studies, reveal that (i) the spectral diffusion is acceptor concentration dependent; at very low acceptor concentration the excitation is completely localized; (ii) as the excitation energy is increased, the onset of spectral diffusion is observed; (iii) above a critical acceptor concentration, the spectral diffusion rate increases rapidly. A theoretical analysis based on a kinetic model suggests that the results of both the site-selective excitation and the time-resolved experiments are consistent with a two-dimensional excitation exchange interaction mechanism.