Fluorescence spectra for the microcrystals and thin films of trans,trans,trans-1,6-diphenyl-1,3,5-hexatrienes

The absorption and fluorescence spectra, fluorescence quantum yields (phi(flu)), and lifetimes (tau(s)) for the microcrystals and vacuum deposited thin films of the pp'-disubstituted trans, trans, trans- 1,6-diphenyl- 1,3,5-hexatrienes (DPHs) 1-10 have been measured and compared with those in dilute solutions at room temperature and in organic glasses at 77 K. The positions of the peaks at the shortest wavelengths in the fluorescence main bands, which are mainly determined by effective pi-conjugation lengths (ECLs) in the excited state, are tuned in the visible spectral range 463-587 nm by introduction of various substituents. The fluorescence main bands shift to longer wavelengths as the strength of electron-donating/withdrawing substituents on the aromatic ring increases. The fluorescence spectra of the microcrystalline samples are more strongly dependent on substituents than those of samples in solution, because of strong charge transfer (CT) type intermolecular interactions induced by substituents in the solid state. The solid-state fluorescence spectra of the derivatives with electron-donating NN-dimethylamino and methoxy groups (1 and 2, respectively) are red-shifted and structured, probably due to the formation of ground-state aggregates. The spectrum for the thin film of 2 shows a close resemblance to that for the thin film of high molecular weight poly(1,4-phenylenevinylene) (PPV), suggesting similar ECLs for 2 and PPV in the excited states. The spectra of the derivatives with electron-withdrawing substituents, on the other hand, are red-shifted and structureless. The broad fluorescence observed for the diformyl derivative (6) is assigned to excimer emission.