Synthesis, structural characterization and fluorescence enhancement of chromophore-modified polyoxometalates

Intramolecular charge transfers between π-conjugated molecules and polyoxometalate (POM) clusters have been observed in donor-acceptor systems based on organic donors and inorganic POM acceptors, which unfortunately results in a general quenching of the chromophore luminescence. The development of POM-chromophore dyads that are capable of tackling the quenching process and enhancing the fluorescence intensity of such systems remains a highly challenging area of study. A family of organic-inorganic polyoxometalate hybrids, {[(n-C₄H₉)₄N]₃[(MnMo₆O₂₄){(CH₂)₃CR}₂]} [1, R = -NHCH₂C₁₄H₉, namely (anthracen-9-ylmethyl)amino; 2, R = -NHCH₂C₁₃H₉, (9H-fluoren-2-ylmethyl)amino; 3, R = -NHCH₂C₁₀H₇, (naphthalen-2-ylmethyl)amino; 4, R = -NHCH₂C₁₆H₉, (pyren-2-ylmethyl)amino], were synthesized by covalently tethering π-conjugated molecules onto an Anderson cluster. The resulting POM-chromophore dyads were fully characterized by various spectroscopic techniques, single-crystal X-ray diffraction analysis and ESI-MS. The fluorescence features of these dyads were studied in detail to verify a dramatic emission enhancement that can be achieved by fine-tuning the microenvironment in solution and suppressing the intrinsic photo-induced electron-transfer process.