Towards highly luminescent phenylene vinylene films

Fluorescence and electronic absorption spectra, fluorescence decay curves and fluorescence quantum yields of a series of oligo(p-phenylene vinylenes) are investigated in solution, nanoaggregates and vapour-deposited or cast ultrathin films. The film constituting molecules are varied in chain length and modified by electron donating and withdrawing substituents and bulky alkyl spacers. PPP-MO calculations serve to rationalize the resulting spectral changes. In dilute solutions, fluorescence yields of the short oligomers with alkyl or oxyalkyl substituents approach the region of unity. The yields decrease with chain length, reaching a long-chain limit of Phi(F)=0.4-0.7. Introduction of electron withdrawing -CN or -SO2CF3 groups can reduce the yields to almost zero, due to facilitated excited-state torsions around the vinylene double bonds. In films, the situation changes drastically. Fluorescence yields of the parent compounds become very low because of molecular exciton coupling which reduces the radiative rates and increases the nonradiative rates of charge separation or internal conversion. Introduction of bulky or polar substituents reduces excitonic coupling, but keeps the molecular environment rigid enough to suppress nonradiative torsional deactivation, so that finally the substituted oligophenylene vinylenes produce highly luminescent films with a present maximum of Phi(F) approximate to 0.6 for 1,4-bis(alpha-cyanostyryl)-2,5-di-n-hexylbenzene (HTCo).