Theoretical investigation of the charge injection effects on the electronic properties of substituted oligodiacetylenes

We discuss the charge-induced modifications in the ground state geometry and in the electronic properties of two types of fully carbazolyl-substituted oligodiacetylenes, named CBDn and CHDn respectively, which are taken as models of the corresponding polymers. Quantum chemical calculations have been performed on the isolated oligomers carrying substituents, either directly linked to the backbone (CBDn) or connected to it through a methylene spacer (CHDn). The collective electronic oscillator (CEO) approach is used to compute the excitations in terms of the one-electron transition density matrixes (CEO-modes). The simulated photoinduced absorption spectra which are obtained show interesting features, and the allowed low-energy transitions compare favorably with experimental results where available. Two-dimensional plots in real space of the CEO-modes allow us to analyze the physical nature of the excitons in terms of the photoinduced electron/hole pairs motion. For charged oligomers, two different types of electronic interactions between the ð systems of the backbone and of the substituents are seen to occur in the two types of oligomers. Even in the presence of a methylene spacer, a charge flow is unexpectedly observed from the backbone toward the carbazolyl substituents, and a manifold of allowed low-energy excitations rises. In this way, the experimental peaks in the photoinduced absorption spectra of polyDCHD (the polymer corresponding to CHDn) find a new explanation, which is different from (and more convincing than) that previously given by us on the basis of results obtained on unsubstituted oligomers.