Dynamic quenching of porphyrin triplet states by two-photon absorbing dyes: Towards two-photon-enhanced oxygen nanosensors

Two-photon-enhanced dendritic nanoprobes are being developed for two-photon (2P) laser scanning microscopy of oxygen [R.P. Briñas, T. Troxler, R.M. Hochstrasser, S.A. Vinogradov, J. Am. Chem. Soc. 127 (2005) 11851–11862]. In these molecular constructs, phosphorescence of metalloporphyrins is coupled to two-photon absorption (2PA) of electronically separate antenna dyes via intramolecular Förster-type resonance energy transfer (FRET). In the originally developed probes, competing electron transfer (ET) between the antennae and the long-lived triplet states of metalloporphyrins partially quenched the phosphorescence, reducing the probe's sensitivity and dynamic range. The rate of such ET can be reduced by tuning the redox potentials of the chromophores. In order to identify the optimal metalloporphyrin-2P antenna pairs, we performed screening of several phosphorescent Pt porphyrins (FRET acceptors) and 2P dyes (FRET donors) using dynamic quenching of phosphorescence. Phosphorescence lifetimes of Pt porphyrins were measured as a function of the dye concentration in organic solutions. The obtained Stern–Volmer quenching constants were correlated with the corresponding ET driving forces (ΔGET), calculated using the Rehm–Weller equation. FRET pairs with minimal quenching rates were identified. The developed approach allows convenient screening of candidate-compounds for covalent assembly of 2P-enhanced triplet nanodevices. Systematic electrochemical measurements in a series of Pt porphyrins with varying peripheral substitution and conjugation pathways are presented.