Pendant conjugated molecules based on a heterogeneous core structure with enhanced morphological and emissive properties for organic semiconductor lasing

Abstract

A family of oligofluorene-type pendant conjugated molecules (C1 and C2), consisting of a triazine center with two diphenylamine and one carbazole heterogeneous substituents to form a donor–acceptor (D–A) core structure, are synthesized and characterized. The relationship between the molecular architectures and the corresponding functional properties was systematically investigated by varying the fluorene lengths and the substituents on the triazine center and by comparing with the linear oligomer counterparts (xFCz), the pendant model compound (DNPhCzT) and the analogue with a homogeneous pendant core structure (T-m). The resulting pendant conjugated molecules exhibited a depressed crystallization tendency and thus enhanced morphology stability due to the incorporation of a bulky heterogeneous pendant core structure. Remarkably, enhanced photoluminescence quantum yields (PLQYs) (0.76 for C1 and 0.61 for C2) were afforded by virtue of the integration of bulky diphenylamine-carbazole heterogeneous core structures, in comparison with their homogeneous pendant analogue T-m (0.58). Solution-processed neat films demonstrated promising amplified spontaneous emission (ASE). The lowest ASE threshold (E^ASE_th) for the non-doped films was recorded as 5.0 μJ cm⁻² and 6.4 μJ cm⁻² for C1 and C2, respectively. The E^ASE_th was almost unchanged for C1 upon annealing from room temperature to 150 °C, while the E^ASE_th of the linear counterparts exhibited a sudden increase above 100 °C. Compared to C2 with extended oligofluorene arms, C1 with one fluorene arm exhibited a lower E^ASE_th and better optical stability at high temperature. The results suggest that excellent optoelectronic properties and enhanced thermal and morphological stabilities are afforded by constructing pendant conjugated molecules with a bulky heterogeneous core structure, and these materials have great potential as gain media for organic lasing.