Highly blue-emissive CBZ-functionalized salen–In complexes: Influence of structural rigidity and donor substituent quantity

Abstract

Indium–salen complexes with electron-donating carbazole (CBZ) groups at positions 4 (CBZIn1) and both 4 and 6 (CBZIn2) were synthesized and characterized to explore the impact of the number of substituents and the structural rigidity on their photophysical properties. The single-crystal structure of CBZIn2 revealed highly twisted arrangements between the CBZ groups and salen moieties, with torsion angles of 80°–81° at 6-position and 45°–47° at 4-position. The In center adopted a nearly square-pyramidal geometry. Both complexes displayed blue fluorescence in toluene at 298 K and in rigid environments (toluene at 77 K and in films), with emission originating from intramolecular charge transfer (ICT) transitions. The photoluminescence quantum yields (PLQYs) of CBZIn1 and CBZIn2 were low in solution but significantly higher in rigid states. Notably, the film-state PLQY of CBZIn2 (78.7%) was over eight times greater than that of CBZIn1 (9.0%). A similar trend was observed in toluene at 77 K. These results highlight the positive influence of structural rigidity and a great number of CBZ donors on ICT-based radiative decay. The experimental observations were consistent with computational predictions, further supporting the proposed mechanism.