How can aluminium(III) generate fluorescence?

In a previous paper [F. Launay, V. Alain, E. Destandau, N. Ramos, E. Bardez, P. Baret, J. L. Pierre, New J. Chem. 25 (2001) 1269-1280] [New J. Chem. 25 (2001) 1269], we showed that the hexadentate tripodal ligand O-TRENSOX (O-TR), incorporating three 8-hydroxy-5-sulfoquinoline subunits, was an efficient chelator of Al(III), quantitatively giving the 1:1 chelate in stoichiometric conditions even at the 10−5 mol L−1 concentration scale. However, the 1:1 Al:O-TR chelate turned out to be not significantly more fluorescent than the free ligand, whereas fluorescence enhancement by factors of at least 100 occurred either with the 3:1 Al:O-TR chelate, or with the 1:1 complex obtained with n-BUSOX, a ligand similar to one arm of O-TRENSOX. The present paper addresses the unresolved question of the magnitude of the fluorescence enhancement. Time-resolved fluorescence measurements, and additional complexation experiments carried out with the tripod TRENSOXCAMS2 (one 8-HQS and two 5-sulfocatechol subunits) and with n-BUCAMS analogous to one catechol arm of TRENSOXCAMS2, show that stoichiometry between Al(III) and the bound bidentate subunits is the key factor of fluorescence enhancement. The charge density on Al(III), tuned by the number of chelating groups and by their formal charges, influences the photoinduced charge transfer which tends to quench the fluorescence emission of the 8-hydroxyquinoline ligand. Transposition can be done to other bifunctional amphoterous ligands such as morin.