Role of intermolecular coupling and electron-nuclear coupling in the photophysics of oligothiophenes

The role of intermolecular coupling and coherent electron-nuclear coupling in the photophysics of the conjugated compound α-quaterthiophene (α-4T) has been investigated. For this purpose, we have measured the photoluminescence (PL) spectra from α-4T as a thin film (80 nm) and a single crystal. Singlet and triplet excitons were brought into play using different excitation energies. By means of Franck-Condon (FC) analysis, we show that for the film, the PL is due to a single intrachain FC progression coupled to a single phonon mode (the C=C mode; ¯C=C = 0.18 eV). For the single crystal the PL profile is adequately reproduced by taking into account a pair of modes (C=C and C–C modes; ¯ hωC=C = 0.18 eV, ¯C–C = 0.15 eV). Moreover, the exciton band edge of the film exhibits a redshift (∼100 meV) as compared to that of the single crystal. This redshift has been attributed to an enhanced conjugation length due to an enhanced chain-chain contact. In the case of the single crystal, the latter effect cannot take place, yet the involvement of the second mode leads to an effective conjugation length. The direct targeting of the first triplet level of the single crystal induces an interchain intermolecular coupling, which in turn induces a coherent electron-nuclear coupling. This latter appears in the delayed fluorescence spectrum as shifted bands (∼100 meV) with respect to the bands observed in the prompt fluorescence.