Spectroscopic characterization of p-phenylene vinylene (PV) oligomers. Part II: Selected 2,5-diheptyl substituted PV-oligomers

Selected, metathetically synthesized, monodisperse all-trans oligomers of 2,5-diheptyl-p-divinylbenzene (DHep-OPVns, with n the number of aromatic rings) are investigated for n=1-6 in solution at (i) ambient temperature, (ii) matrix isolated in a 2MeTHF glass at 10 and 80 K and (iii) as spin coated thin layers on quartz substrates. These materials are compared to their diheptyloxy substituted analogs (DHepO-OPVs), described in the preceding paper, involving absorption and emission as well as site-selective fluorescence (SSF) spectroscopy at various temperatures and as a function of concentration and chain length. Due to both, lower electron density along the conjugated backbone and the presence of methylene groups, which are directly linked to the aromatic rings, the diheptyl substituted oligomers adopt a less planar conformation and, concomitantly, are more disordered as evidenced by the larger inhomogeneous spectral broadening. Consequently, their tendency toward formation of nanoaggregates, though occurring in solution and matrix isolated, is diminished. For both types of oligomers, the tendency of aggregation increases with increasing concentration and chain length and decreases with increasing temperature. Site-selective fluorescence (SSF) at low temperature bear out a finite Stokes shift of 270 and 260 cm-1 for the diheptyl substituted DHep-OPV3 and DHep-OPV6, respectively, while in the case of DHepO-OPV3, the Stokes shift is vanishing small. This shows that rigidity of the oligomer backbone and molecular relaxation upon excitation are correlated