Luminescent Metal–Organic Complexes of Pyrene or Anthracene Chromophores: Energy Transfer Assisted Amplified Exciplex Emission and Al3+ Sensing

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.