Tuning of the Emission Efficiency and HOMO–LUMO Band Gap for Ester-Functionalized {Al(salophen)(H 2 O) 2 } + Blue Luminophors
Résumé
A series of [AlL(H2O)2(NO3)] complexes, with L standing for an ester substituted salophen-type ligand, has been synthesized, and the luminescence properties have been investigated. These derivatives differ by the nature of the ester-R group introduced at the C5 position of their salicylidene rings (i.e., phenyl, 7a,a′; naphthyl, 7b,b′; pentafluorophenyl, 7c,c′; and p-nitrophenyl, 7d) and by the bis-imino bridge (i.e., 1,2- phenylene, 7a–d; and 1,2-naphthalene, 7a′–c′). All the complexes are characterized by luminescence in the blue range, the chemical diversity having no effect on the emission wavelength (480–485 nm). However, the emission efficiency was found to be strongly dependent on the Schiff-base ligand with quantum yields ranging from ϕ = 22% to 44%, the highest values being for the salophen derivatives with the electron-withdrawing ester-R groups (7a, 34%; 7a′, 23%; 7b, 31%; 7b′, 22%; 7c, 40%; 7c′, 29%, and 7d, 44%). Both the electrochemical data and DFT calculations show that the HOMO–LUMO band gap is modified as a function of the ester R group (from 2.92 to 3.16 eV, based on the redox potentials). The crystal structures for the N,N′-bis(5-(phenoxycarbonyl)salicylidene)-1,2-phenylenediamine and the N,N′-bis(5-(p-nitrophenoxycarbonyl)salicylidene)-1,2-phenylenediamine aluminum complexes (7a and 7d) are reported.