Spin-Orbit Charge Recombination Intersystem Crossing in Phenothiazine-Anthracene Compact Dyads: Effect of Molecular Conformation on Electronic Coupling, Electronic Transitions, and Electron Spin Polarizations of the Triplet States

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 (VDA, 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 VDA 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 (δ = 65%, δT = 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.