Two novel dioxolane-substituted pentacene derivatives, namely, 6,14-bis-(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]p entacene (TP-5) and 2,2,10,10-tetraethyl-6,14-bis-(triisopropylsilyiethynyl)-1,3,9,11-tetrao xa-dicyclopenta[b,in]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)aluminium(III) (Alq(3)). The films show narrow red emission and high absolute photoluminescence quantum yields (phi(PL) = 59% and 76% for films containing similar to 0.25 mol % TP-5 and EtTP-5, respectively). The Forster transfer radius for both guest-host systems is estimated to be similar to 33 angstrom. The TP-5/Alq(3) thin films show a marked decrease in phi(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/Alq(3) films. The measured fluorescence lifetimes of TP-5 and EtTP-5 are similar (similar to 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 EtTP5/Alq(3) 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 pi-pi molecular stacking and aggregation.