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

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.