Luminescent metal–organic complexes of pyrene or anthracene chromophores: energy transfer assisted amplified exciplex emission and Al³⁺sensing

Abstract

Using pyrene and anthracene monocarboxylate chromophores two metal–organic complexes, {[Cd (pma)₂(o-phen)₂]·2H₂O·MeOH)}_n (1) and {Cd₂(µ-H₂O)(amc)₄(o-phen)₂}_n (2) (Hpma = pyrene monocarboxylic acid; Hamc = 9-anthracene monocarboxylic acid; o-phen = orthophenanthroline) have been synthesized, respectively and characterized using a single crystal X-ray diffraction study. Compound 1 contains a seven-coordinated Cd²⁺ center connected by two o-phen and two pma, where one pma and o-phen pair stacks in a face-to-face fashion, and the other pma:o-phen pair is linked through C–H···π interaction. Compound 2 is a dimeric complex of Cd²⁺ reported previously, and it contains two pairs of face-to-face stacked amc:o-phen. Compound 1 shows a red-shifted bright cyan emission compared to pma monomer emission that can be attributed to pma:o-phen exciplex formation. This exciplex emission is further sensitized by another pma through a Förster resonance energy transfer (FRET) process. Similarly in the case of 2, amc:o-phen exciplex emission is sensitized through FRET from the other amc linker in the solid state, while in methanol such an energy transfer process is perturbed resulting in a dual emission related to a monomer of amc and exciplex of amc:o- phen. Interestingly, the blue emission of 2 dispersed in methanol changes to a bright cyan-green emission upon addition of Al³⁺ and remains almost unchanged or slightly affected with other metal ions leading to selective chemosensing of Al³⁺. Moreover, the density functional theory based calculations provide clear evidence of ground state interactions between pma:o-phen and amc:o- phen in 1 and 2, respectively.