Dynamics of singlet-triplet transitions in 1,4-dihalonaphthalene neat crystals and solid solutions

In the 1,4-dihalonaphthalene series, 1,4-dichloronaphthalene and 1-bromo-4-chloronaphthalene are isomorphous but 1,4-dibromonaphthalene has a different crystal structure. It is found that the effect of halogen substitution on the singlet-triplet transition energy is smaller than the effect due to a change in the crystal structure. The singlet-triplet transitions of these halonaphthalenes show similar vibronic features, no factor group splitting, and weak exciton-phonon coupling. In the 1-bromo-4-chloronaphthalene crystal, which shows disorder with respect to the halogen, a large manifestation of this disorder is found on the line width of the singlet-triplet transition; yet no Anderson localization is observed. Binary solid solutions of these 1,4-dihalonaphthalenes also exhibit exciton migration.