Synthesis, Characterization, Photophysical and DFT Studies of Coumarin Schiff Bases and Their Dimethylgallium Complexes

Reactions of trimethyl gallium ether adduct with five different coumarin Schiff bases in benzene yielded dimethyl gallium complexes of the general formula [Me2Ga{–O–(C9H3O–(2-O)–(4-Me))–CH=N–R}] (R = tBu, C6H5, C6H4Me-4, C6H4Cl-4 and C6H4Br-4). These complexes have been characterized by elemental analysis, IR, UV–vis, NMR (1H and 13C{1H}) spectroscopy. The molecular structures of three gallium complexes, viz. [Me2Ga{–O–(C9H3O–(2-O)–(4-Me))–CH=N–C6H5}], [Me2Ga{–O–(C9H3O–(2-O)–(4-Me))–CH=N–C6H4Me-4] and [Me2Ga{–O–(C9H3O–(2-O)–(4-Me))–CH=N–C6H4Br-4] were unequivocally established by single crystal X-ray crystallography. The gallium atom acquires a distorted tetrahedral geometry. These complexes exhibit emission in the visible region and their emission maxima are blue shifted with respect to that of the corresponding free ligand. DFT calculations reveal that the HOMO–LUMO gap value becomes slightly smaller with electron withdrawing substituent like Cl and Br.