Spin-orbit charge-transfer intersystem crossing (SOCT-ISC) is useful for the preparation of heavy atom-free triplet photosensitisers( PSs). Herein, as eries of perylene-Bodipy compact electrond onor/acceptor dyads showing efficient SOCT-ISC is prepared. The photophysical properties of the dyads were studiedw ith steady-state and time-resolved spectroscopies. Efficient triplet state formation (quantum yield F T = 60 %) waso bserved, with at riplets tate lifetime (t T = 436 ms) much longert han that accessed with the conventional heavy atom effect (t T = 62 ms). The SOCT-ISC mechanism wasu nambiguously confirmed by direct excitation of the charget ransfer (CT) absorption band by using nanosecond transienta bsorption spectroscopy and time-resolved electronp aramagnetic resonance (TREPR) spectroscopy.T he factors affecting the SOCT-ISC efficiency include the geometry,t he potential energy surfaceo ft he torsion, the spin density for the atoms of the linker,s olvent polarity,a nd the energym atchingo ft he 1 CT/ 3 LE states. Remarkably,t hese heavya tom-free triplet PSs were demonstrated as an ew type of efficient photodynamic therapy (PDT) reagents (phototoxicity,E C 50 = 75 nm), with an egligibled ark toxicity (EC 50 = 78.1 mm)c ompared with the conventionalh eavy atom PSs (dark toxicity,E C 50 = 6.0 mm, light toxicity, EC 50 = 4.0 nm). This study provides in-depthu nderstanding of the SOCT-ISC, unveils the design principles of triplet PSs based on SOCT-ISC, andu nderlines their applicationa sanew generationo fp otent PDT reagents.[a] Dr.[h] Prof. M. Di Donato INO, Istituto Nazionale di Ottica Largo Enrico Fermi 6, 50125F lorence (Italy)[ + + ] These authorscontributed equally to this work.Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.

Phenothiazine (PTZ)–anthracene (An) compact electron donor/acceptor dyads were synthesized. The molecular conformation was constrained by rotation restriction to achieve an orthogonal geometry between the electron donor (PTZ) and the electron acceptor (An), with the aim to enhance the spin–orbit charge-transfer intersystem crossing (SOCT–ISC). The substitution positions on the PTZ and An moieties were varied to attain dyads with different mutual orientations of the donor/acceptor as well as different rotation-steric hindrances. The electronic coupling strengths between the electron donor and the acceptor were quantified with the matrix elements (V DA, 0.04–0.18 eV); the smallest value was observed for the dyad with orthogonal geometry. Charge-transfer absorption and fluorescence emission bands were observed for the dyads, for which the intensity varied, manifested by the V DA values. The fluorescence of the An moiety was significantly quenched in the dyads, efficient ISC, and the formation of the triplet state were confirmed with nanosecond transient absorption spectroscopy (ΦΔ = 65%, τT = 209 μs). The rotation-steric hindrance was analyzed with potential energy curves, and PTZ was found to be an ideal electron donor to attain SOCT–ISC. Time-resolved electron paramagnetic resonance spectra revealed the electron-spin polarization (ESP) of the triplets of the dyads, which is drastically different from that of An, thus confirming the SOCT–ISC mechanism. Moreover, we found that the ESP patterns of the dyads strongly depend on the topological features of the molecules and the structure of the electron donor, thus indicating that the relationship between the molecular conformation and the ESP parameters of the triplet state of the dyads cannot be described solely by the orthogonal geometry, as was previously observed.

No abstract

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.