Tuning of Fluorescence in Films and Nanoparticles of Oligophenylenevinylenes

Abstract

Oligophenylenevinylenes (OPV) with a series of distance-controlling, electron-donating, and/or electron-withdrawing substituents are deposited from vapor phase and solution as ultrathin films or nanoparticles with diameters of 20−200 nm. In some cases the systems are doped at levels of 10-5−10-3 with energy accepting OPV's of longer chainlengths. Absorption and fluorescence spectra, steady-state and time-resolved anisotropies, and radiative and nonradiative deactivation rates of these systems are investigated and compared to the corresponding properties in dilute solutions. Fluorescence yields of the parent oligomers and their alkyl or oxyalkyl derivatives are high in solution with an “infinite chain” limit of ΦF ≈ 0.5 and an upper radiative rate constant limit of kr∞ = (1 ± 0.3) × 109 s-1. Yields and kr decrease strongly in films and nanoparticles because of H-aggregate formation. However, doping with fluorescent acceptors can increase the yields up to ΦF → 0.7. Introduction of electron-withdrawing -CN and -SO2CF3 substituents reduces ΦF in low viscous dilute solutions almost to zero. High viscosities and condensation to solid phases will raise the yields up to ΦF → 0.6 because of suppression of nonradiative torsional deactivation and formation of J-aggregates with high kr.