Impact of Molecular Arrangement and Torsional Motion on the Fluorescence of Salophen and Its Metal Complexes

Salophen is a weakly emissive molecule with a flexible structure. The decrease in the flexibility of the molecule, which can be achieved by chemical or physical means, causes a significant increase in the emissivity and fluorescence lifetime. This phenomenon has been observed upon incorporation of salophen in the solid polymer matrix of poly­(methyl methacrylate) (PMMA). The enhancement in emission is even more prominent in the pure solid form of salophen. An enhancement of emission is also observed in the case of the zinc complex of salophen, SalZn, which is inherently more emissive than free salophen in solution. However, the enhancement in emission is greater in the PMMA matrix for the complex than in its solid form. Interestingly, a quenching of fluorescence is observed in the crystals of the aluminum complex of salophen (SalAl⁺), which is strongly emissive in solution phase. These apparently conflicting trends have been rationalized in the light of the molecular arrangement of salophen and its complexes in a solid matrix and in the pure solid forms. In the case of salophen, torsional motion provides major nonradiative channels of depopulation of its excited state in solution. These channels are blocked in the rigid environment provided of the polymer matrix and of the crystal, giving rise to aggregation induced enhancement of emission (AIEE). In the case of SalAl⁺, the torsional motion is restricted anyway due to complexation. The X-ray crystal structure indicates the possibility of π–π interaction between the planar ligands of two neighboring complex molecules, which could lead to aggregation-caused quenching (ACQ). This provides a justification for the lower emissivity of SalZn, as compared to SalAl⁺. SalZn is likely to exist as a dimer, in which intramolecular π–π interaction is possible. Thus, the emissivity of salophen and its complexes is found to be governed by interplay of torsional motion and intermolecular interaction. Experiments have been performed at liquid nitrogen temperature, whereby conformational motion is arrested, but additional intermolecular interactions are not brought in. Maximal fluorescence of each of the three species studied is observed in this condition