Onset 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 spectral diffusion in the presence of both orientational and substitutional disorders is investigated at 4.2 and 1.8 K. Our systems of study are different compositions of solid solutions of 1-bromo-4-chloronaphthalene (BCN) and 1,4-dibromonaphthalene (DBN). BCN is known to form an orientationally disordered solid. We used a wide range of concentrations within which the DBN crystal structure prevails. The phosphorescence of BCN was monitored as a function of the concentration of BCN. The analysis of our results shows that (i) the spectral diffusion is concentration dependent and, at very low concentration, the excitation is completely localized; (ii) as the concentration of BCN is increased, the onset of spectral diffusion is observed; (iii) above a critical concentration, the spectral diffusion rate increases rapidly; (iv) the process of spectral diffusion is highly temperature dependent. The concentration dependence behavior is found to be consistent with both the diffusion and the percolation models. From the analysis of the results, we conclude that the spectral diffusion in this system is phonon assisted and it involves cascading down in steps from higher to lower energy sites. Also, the theoretical analysis suggests that the excitation transport topology involves 2-dimensional exchange interactions.