Near-Infrared→Visible Light Upconversion in a Molecular Trinuclear d-f-d Complex

摘要

Molecular nonlinear-optical (NLO) phenomena are currently being exploited for the design of visible emitting bioprobes with unprecedented properties that result from 1) the transparency of living tissues toward low-energy near-infrared (NIR) incident radiation and 2) the improved focusing of the light from an excitation laser beam within nanometric volumes. Nonresonant multiphoton absorption produces a single emission during the irradiation; this emission corresponds to a multiple of the energy of the incident beam, as, for example, in second-harmonic generation (Figure 1 a), while resonant multiphoton absorption populates a real excited state of the chromophore, which relaxes and fluoresces with the same characteristics as if sensitized by a linear excitation process (Figure 1b). Beyond the optimization of polarizable push-pull π-aromatic molecules for NLO optical response, and the related development of upconverted fluorescence signals in polyaromatic platforms produced by metal-sensitized triplet-triplet annihilation photochemistry, Le Bozec, Maury, Andraud, and their respective co-workers demonstrated that trivalent lanthanide ions (Ln~(III)) may efficiently contribute to the polarization of coordinated aromatic ligands for resonant multiphoton absorption. Moreover, the wealth of accessible long-lived metal-centered luminescent emissive levels can be exploited for two-photon-excited time-gated fluorescence analyses of biological tissues.