Phosphorescence of 4-bromobenzophenone polymorphs

The integrated and time-resolved phosphorescence spectra of the monoclinic (M-form) and triclinic (T-form) polymorphs of 4-bromobenzophenone have been measured at three temperatures 293, 77 and 1.6 K as well as the parameters of the emission kinetics of the most important bands have been determined. The room-temperature integrated spectra of both polymorphs have a clear monomeric structure with quite close values of the band energies. The integrated spectra at 77K are also monomeric and have close decay times of the 0-0 band, however the spectra feature another monomeric series, less intensive compared with the main one and considerably shifted to red. The 0-0 band decay times in both polymorphs at 77 K are close, unlike at room temperature, at which the decay in the T-form is three times slower than in the M-form. The general shape of integrated spectra of both polymorphs at 1.6 K is in general monomeric, yet the contour of the 0-0 bands has essential and meaningful differences. Based on the actual crystal structure of the polymorphs, a one-dimensional model is suggested and, within the tight-binding approach the corresponding energy spectra of triplet excitons have been calculated to conclude that the exciton band width Δ in the T-form is roughly four times wider than in the M-form. Therefore, the tunnel diffusion coefficient, which is proportional to Δ2, in the T-form is expected to exceed that in the M-form by an order of magnitude. Comparative analysis of the integrated phosphorescence spectra at 1.6 K gives grounds to assert that the diffusion of triplet excitons in the T-form is substantially faster, in agreement with the qualitative conclusions of our theory.