Photophysical Properties of Dioxolane-Substituted Pentacene Derivatives Dispersed in Tris(quinolin-8-olato)aluminum(III)

Abstract

Two novel dioxolane-substituted pentacene derivatives, namely, 6,14-bis-(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]pentacene (TP-5) and 2,2,10,10-tetraethyl-6,14-bis-(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]pentacene (EtTP-5), have been synthesized and spectroscopically characterized. Here, we examine the steady-state and time-resolved photoluminescence (PL) of solid-state composite films containing these pentacene derivatives dispersed in tris(quinolin-8-olato)aluminum(III) (Alq₃). The films show narrow red emission and high absolute photoluminescence quantum yields (φ_PL = 59% and 76% for films containing ∼0.25 mol % TP-5 and EtTP-5, respectively). The Förster transfer radius for both guest−host systems is estimated to be ∼33 Å. The TP-5/Alq₃ thin films show a marked decrease in φ_PL with increasing guest molecule concentrations, accompanied by dramatic changes in the PL spectra, suggesting that intermolecular interactions between pentacene molecules result in the formation of weakly radiative aggregates. In contrast, a lesser degree of fluorescence quenching is observed for EtTP-5/Alq₃ films. The measured fluorescence lifetimes of TP-5 and EtTP-5 are similar (∼18 ns) at low concentrations but deviate at higher concentrations as aggregation begins to play a role in the TP-5/Alq₃ films. The onset of aggregation in EtTP-5/Alq₃ films occurs at higher guest molecule concentrations (>1.00 mol %). The addition of ethyl groups on the terminal dioxolane rings leads to an increase in the intermolecular spacing in the solid, thereby reducing the tendency for π−π molecular stacking and aggregation.