We studied the ISC mechanism of novel SOCT-ISC photosensitizers, and applied them as novel potent PDT reagents.
Spin-orbit-charge-transfer intersystem crossing (SOCT-ISC) is a particularly interesting ISC mechanism, useful for preparation of heavy atom-free triplet photosensitizers (PSs) with simple molecular structures. Herein we prepared a series of perylene-Bodipy compact electron donor/acceptor dyads showing efficient SOCT-ISC. The photophysical properties of these systems were studied with steady state and time-resolved transient optical and magnetic spectroscopies. Efficient triplet state formation (quantum yield FT = 66%) was observed, with triplet state lifetime (t=196 ms) longer than that accessed with the conventional heavy atom effect (t=62 ms). The SOCT-ISC mechanism was unambiguously confirmed by direct excitation of the charge transfer (CT) absorption band using nanosecond transient absorption spectroscopy and time-resolved electron paramagnetic resonance (TREPR). Our studies highlight the factors affecting the SOCT-ISC efficiency, which include the geometry of the electron donor/acceptor pair, the potential energy surface of the torsion between the electron donor and acceptor, the spin density for the atoms of linker, solvent polarity, as well as the energy matching of 1CT/3LE states. Remarkably, these heavy atom-free SOCT-ISC triplet PSs were demonstrated as a new type of efficient photodynamic therapy (PDT) reagents, with a negligible dark toxicity (EC50 = 85.1 mM) as compared to the conventional heavy atom PS (dark toxicity, EC50 = 3.1 mM). This study provides new in-depth understanding of the SOCT-ISC mechanism, unveils the design principles of triplet PSs based on SOCT-ISC, and outlines their applications as a new generation of novel and potent PDT reagents. It is also important for fundamental photochemical study of charge separation and ISC.
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Wang, Z., Ivanov, M., Gao, Y., Bussotti, L., Foggi, P., Zhang, H., Russo, N., Dick, B., Zhao, J., Di Donato, M., Mazzone, G., Luo, L., Fedin, M. Spin–Orbit Charge-Transfer Intersystem Crossing (ISC) in Compact Electron Donor–Acceptor Dyads: ISC Mechanism and Application as Novel and Potent Photodynamic Therapy Reagents (2020) Chemistry - A European Journal, 26 (5), pp. 1091-1102. DOI: 10.1002/chem.201904306