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

Abstract

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.

Making triplets from photo-generated charges: observations, mechanisms and theory

“…The authors show a generic scheme for triplet formation in donor-acceptor dyads (Fig. 9) that complies to the work of El-Sayed in 1974, 40 the work of Van Willigen, 24 Wasielewski, 23,37,41 and Filatov, Senge 29,31,35 and Buck et al 39 According to the authors, the recent studies conducted by Filatov et al, 29,31,35 Zhao et al, 30,34,36 lack magnetic field effects (MFE) measurements clearly indicating that SOCT-ISC is the main transition to populate triplet excited states. Buck et al proceeded to carry out studies to answer one question: in relation to Fig.…”

“…Using DFT calculations, they explained that the alkyl groups increased the S CT and reduced the electron accepting ability of the BODIPY moiety. Hou et al 36 synthesized phenothiazine(PTZ)-anthracene (An) dyads ( Fig. 8) with the aim of achieving orthogonal geometry thus enhancing SOCT-ISC.…”

“…As discussed in section 2, Weber and co-workers have been able to apply computational chemistry to determine important aspects for the triplet formation by charge recombination for compound 11 and 12. 36 By using experimental data from charge-transfer absorption and charge-transfer emission properties they determined a rather large V DA value of 0.16 eV (= 1290 cm −1 ) for 12 (related to the charge separation). By using DFT methods (TD-DFT and the Dalton code) they were able to calculate the magnitude of the spin-orbit coupling interaction matrix element (SOCME) to be 2.37 cm −1 for compound 12, and 0.37 cm −1 for compound 11.…”

Efficient Intersystem Crossing in the Tröger's Base Derived From 4‐Amino‐1,8‐naphthalimide and Application as a Potent Photodynamic Therapy Reagent

“…The 1,7‐methyl groups on the boron dipyrromethene (Bodipy) moiety play a similar role in Bodipy‐derived electron donor/acceptor dyads showing SOCT‐ISC . We found that the presence of peri ‐H(C) of electron donor phenothiazine is helpful for conformation restriction . However, in all these molecular structural motifs, rotation about the C−C or the C−N linker is not fully restricted, and various conformations are still accessible with thermal energy.…”

“…[22][23][24][25][26] We found that the presence of peri-H(C) of electron donor phenothiazine is helpful for conformation restriction. [24,27,28] However,i na ll these molecular structural motifs, rotation about the CÀCo rt he CÀNl inker is not fully restricted, and various conformationsa re still accessible with thermale nergy.D eviation from the orthogonal geometry may be detrimental to the SOCT-ISC efficiency.T hus, fully rigid linkers are desired for linking the electron donora nd acceptor in these dyads, but this kind of linker is difficult to obtain from the point of view of synthesis.…”