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

Spectroscopic characterization of p-phenylene vinylene (PV) oligomers. Part I: A homologous series of 2,5-diheptyloxy substituted PV oligomers.

A homologous series of metathetically synthesized, monodisperse all-trans oligomers of 2,5-diheptyloxy-p-divinylbenzene1 (DHepO-OPVns, with n the number of aromatic rings) is characterized for n=1–7. Experiments were carried out in solution at ambient temperature, under matrix-isolated conditions (dispersed in a 2-methyltetrahydrofuran (2MeTHF) glass) at low temperatures (10 and 80 K) and as spin coated thin layers on quartz substrates at various temperatures. The investigations involve absorption and emission as well as site-selective fluorescence (SSF) spectroscopy. DHepO-OPVns (n2) form nanoaggregates and the tendency of the formation increases with increasing concentration and chain length and decreases with increasing temperature. Absorption spectra of isolated chains as well as aggregated species are simulated and agree nicely with the experimental data. Furthermore, the absorption spectra of the aggregates can be correlated with the spectra of the corresponding spin coated oligomer on a quartz substrate. The singlet (S1¿S0) transition energies decrease with increasing conjugation length and converge for the longer oligomers towards a limiting value. The Huang–Rhys parameter S associated with the S1¿S0 electronic transition and its evolution with conjugation length has been determined via analysis of the vibronic fine structure. At low temperatures, the values determined can be attributed to aggregate formation. At ambient temperature, S decreases with increasing chain length. Low-temperature SSF spectroscopy is employed to study morphological effects on the conformation of DHepO-OPV3. A vanishing Stokes shift is recorded for the resonantly excited fluorescence spectra