Structural Variations and Multiple Charge Transfer Transitions between Chloranil and Carbazole Derivatives

Abstract

Asymmetric charge transfer (CT) spectra from combinations of carbazole derivatives and chloranil are shown to consist of two bands originating from the highest (HOMO) and second highest (HOMO 2) energy oc- cupied molecular orbitals of carbazole. Symmetry arguments are used to indicate that of two possible (parallel plane) alignments of donor and acceptor, that which is totally symmetric gives rise to only the lower energy CT transition whereas the unsymmetrical alignment which is energetically preferred permits the two observable CT transitions. Low molecular weight model carbazoles all show the asymmetric C?' bands which have been resolved into two Gaussian components by means of a computer assisted analysis. Significantly, poly(N-vinylcarbazole) (PVCA) gives rise to CT hands much less asymmetric than corresponding model systems and it is concluded that steric interactions in PVCA greatly reduce the possibility for interaction of the carbazole units with chloranil in the 1:l asymmetric arrangement preferred by the model compounds. As expected from the theoretical argument, poly(N-ethyl-3-vinylcarbazole) and poly(N-ethyl-2-vinylcarbazole), both of which are derived from unsymmetri- cally substituted carbazoles, give with chloranil highly asymmetric CT spectra which are very similar to those of the appropriate model compounds.