Spectral diffusion within the inhomogeneously broadened n-π* singlet-triplet transition of the orientationally disordered solid of 4-bromo-4′-chlorobenzophenone

Spectral diffusion within the inhomogeneously broadened n-π* singlet-triplet transition is investigated for an orientationally disordered organic solid, 4-bromo-4′-chlorobenzophenone, at 4.2 and 1.8 K. The observed 0-0 band of the triplet absorption profile shows a large inhomogeneous broadening, presumably due to the orientational disorder in the position of the halogen substituents. The absorption profile fits to a Gaussian shape on the low-energy side and a Lorentzian shape on the high-energy side. The steady-state studies along with the time-resolved experiments establish the presence of a phonon sideband in each emission spectra. The analysis of the phonon sideband reveals that the exciton-phonon coupling is independent of site. The time-resolved experiments reveal that (i) the spectral diffusion is unidirectional and site dependent, (ii) the spectral diffusion from sites on the low-energy side of the absorption profile is thermally activated, (iii) the spectral diffusion process occurs in the form of energy cascading in steps, (iv) upon excitation at extremely low-energy sites, the energy remains localized and no spectral diffusion is observed. The observed spectral diffusion is explained in terms of a phonon-assisted unidirectional (high- to low-) energy transfer process, where the density of acceptor states plays an important role. The excitation localization at the extremely low-energy sites suggests the absence of any long-range interaction responsible for the energy transfer.