Chemical modulation of the electronic properties of polydiacetylenes

We have investigated the effects of the chemical substitution of long alkyl chains in the carbazolyl group of a polycarbazolyldiacetylene on its electronic properties. The electronic absorption and Raman spectra of the unsubstituted unsoluble polycarbazolyldiacetylene polyDCHD and those of the alkyl-substituted soluble polymers (polyDCHD-S, polyDCHD-HS) in the blue form are very similar indicating that the backbone conjugation length is practically not affected by this substitution. Very different instead is the nature of the long-lived photoinduced excitations of these polymers. Charged carriers as well as triplet excitons are formed in the unsubstituted blue polymer while triplet excitons dominate the photoinduced spectrum when long alkyl chains are attached to the aromatic ring. Only triplet excitons are observed in the red form of the soluble polydiacetylenes. We have interpreted these findings as due to the different molecular and supramolecular structure of these polymers. The larger interchain separation achieved in the substituted polycarbazolyldiacetylenes favours the photogeneration of triplet excitons relative to charged species. This effect is even more pronounced for the red form of these polymers where the twisted conformation further decreases the interchain coupling. The femtosecond-picosecond time evolution of the excited state dynamics of the red form of polyDCHD-S and -HS in a polyethylene matrix has also been measured by the pump-probe technique. Films of this type in which the polymer backbones of the different chains are isolated and in the twisted conformation are particularly suitable for this investigation. We believe in fact that these conditions can allow a meaningful comparison of the experimental data with the results of highly correlated quantum mechanical calculations carried out on oligomers, The transient differential transmission spectra display characteristic features depending upon the probe time delay. Photobleaching at 2.3 eV and photoinduced absorptions at 1.8 and 1.5 eV an present at shea probe delay. The 1.5 eV feature, present also at large probe delays, is assigned to triplets which are formed by singlet fission as a pair with an overall zero spin. These pairs, when survived to their recombination and trapped in defect sites, can live up to milliseconds. On the basis of the quantum mechanical calculations on oligomers, we believe that the state responsible for the fission is the optically forbidden 2(1)A(g) level below the first allowed 1(1)B(u) state. (C) 2000 Elsevier Science B.V. All rights reserved.