Exciplex formation as an approach to selective Copper(II) fluorescent sensors

Abstract The fluorescence of TB-cyclen (1,4,7,10-tetrabenzyl-1,4,7,10-tetraaza-cyclododecane) and its complexes with CuII, CdII, PbII, ZnII, HgII and AgI are reported in 50% (v/v) CH3OH/H2O. It appears, as supported by NMR studies, that the TB-cyclen undergoes slow protonation reactions in CH3CN with 0.01 M HClO4 present. The non-protonated TB-cyclen adopts the ++-- conformer that is sterically shielded from protonation and complex-formation, and the slow formation of the protonated forms of the ligand, and formation of the ++++ conformer, is required before complex-formation can occur. Strong emissions at 352 nm are oberved on formation of TB-cyclen complexes with CuII, CdII, PbII, but not with ZnII, HgII and AgI: it is suggested that these are exciplex emissions. The selectivity of the fluorescence of CuII with TB-cyclen compared to ZnII suggests that ligands of this type might be useful in sensing CuII in neural tissue, as in the case of studying Alzheimer’s disease, where potentially interefering metal ions such as CdII or PbII should not be present. The structure of [Pb(TB-cyclen)](ClO4)2 is reported, which shows that Pb···C π contacts are formed to all four N-benzyl groups. Structures of [Cd(TB-cyclen)CH3CN](ClO4)2 and [Hg(TB-cyclen)CH3CN](ClO4)2 are reported, that show M···C π contacts to only two of the N-benzyl groups, but the non-formation of four π contacts may be due to the presence of coordinated CH3CN molecules. The structure of [TB-cyclenH]ClO4 shows that the cyclen moeity of the ligand has the ++++ conformer required for complex-formation with metal ions. It is suggested that in the protonated forms of the free ligand, the ++++ conformer is stabilized with the N-benzyl groups in an upright position by the formation of N-H···C π contacts, and C-H···C π contacts between the upright N-benzyl groups. DFT calculations are carried out that show the structure of the exciplex that can be formed in the free (non-protonated) form of DB-cyclen, and show that its ++-- conformer is the lowest energy form.