Photophysical properties of 9,9‐dimethyl‐9,10‐dihydroacridine‐functionalized salen–indium complexes: Effects of structural rigidity and number of donor substituents

Abstract Indium–salen complexes with electron‐donating 9,9‐dimethyl‐9,10‐dihydroacridine (DMAC) groups at positions 4 ( DMACIn1 ) and 4 and 6 ( DMACIn2 ) were synthesized and characterized to investigate the effect of the substituents number and structural rigidity on photophysical properties. The single crystal structure of DMACIn1 revealed highly twisted arrays (83–89°) between the DMAC groups and salen moieties and a nearly square‐pyramidal geometry around the indium center. Both complexes exhibited green fluorescence in toluene at 298 K and in rigid states (in toluene at 77 K and in a film), which originates from intramolecular charge transfer (ICT) transitions. The absolute photoluminescence quantum yields (PLQYs) of DMACIn1 and DMACIn2 were low in solution but high in the rigid states. The film‐state PLQY of DMACIn2 (59.5%) was more than five‐fold higher than that of DMACIn1 (11.1%). A similar result was observed in toluene at 77 K. These findings were rationalized in terms of the beneficial effects of structural rigidity and higher number of DMAC donors on ICT‐based radiative decay. The experimental results agreed with those of computational studies.