TOWARDS HIGHLY LUMINESCENT PHENYLENE VINYLENE FILMS

Fluorescence and electronic absorption spectra, fluorescence decay cur ves and fluorescence quantum yields of a series of oligo(p-phenylene v inylenes) are investigated in solution, nanoaggregates and vapour-depo sited or cast ultrathin films. The film constituting molecules are var ied in chain length and modified by electron donating and withdrawing substituents and bulky alkyl spacers. PPP-MO calculations serve to rat ionalize the resulting spectral changes. In dilute solutions, fluoresc ence 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 electro n withdrawing -CN or -SO2CF3 groups can reduce the yields to almost ze ro, due to facilitated excited-state torsions around the vinylene doub le bonds. In films, the situation changes drastically. Fluorescence yi elds of the parent compounds become very low because of molecular exci ton coupling which reduces the radiative rates and increases the nonra diative rates of charge separation or internal conversion. Introductio n of bulky or polar substituents reduces excitonic coupling, but keeps the molecular environment rigid enough to suppress nonradiative torsi onal deactivation, so that finally the substituted oligophenylene viny lenes 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-hexylbenz ene (HTCo).