Electron transfer and intersystem crossing in the coumarin-anthracene electron donor-acceptor dyads

A series of compact coumarin (Cou)-anthracene (An) electron donor-acceptor dyads were prepared, in which the two units are directly connected at either 2- or 9- position of the An unit and the 3-position of the Cou unit to study the relationship between the molecular configuration, electronic coupling and the intersystem crossing (ISC) efficiency. Single crystal X-ray diffraction results indicate that the dihedral angles are 88.0° and 85.7°, respectively, in the dyads, in which the donor and acceptor are connected by 9- position of the An unit (Cou-9An and Cou-9AnCN). However, for Cou-2An the optimized ground state geometry indicates a more coplanar configuration with a dihedral angle of 38.6°. The solvent polarity-dependent singlet oxygen quantum yields (ΦΔ) of the orthogonal dyads (Cou-9An and Cou-9AnCN) are 39% (in acetonitrile) and 64% (in dichloromethane), respectively. whereas ΦΔ is negligible for the coplanar dyad (Cou-2An). These results indicated that orthogonal geometry is beneficial to spin−orbit charge transfer ISC (SOCT-ISC) mechanism. Nanosecond transient absorption spectra demonstrated that the lowest triplet state was mainly localized on An unit in Cou-9An and Cou-9AnCN, with the intrinsic triplet state lifetime of 234 and 207 μs, respectively. Triplet state delocalization was observed for Cou-2An, which prolongs the triplet state lifetime to 742 μs. Femtosecond transient absorption spectra results indicated fast charge separation (CS, 0.36−0.41 ps) and slow charge recombination (CR, 2.1−2.8 ns) in the orthogonal dyads. Time-resolved electron paramagnetic resonance (TREPR) spectra further confirmed the SOCT-ISC mechanism in the orthogonal dyads (Cou-9An and Cou-9AnCN), however, the SOCT-ISC and spin orbit ISC (SO-ISC) mechanism are both responsible for the triplet state formation in the coplanar dyad (Cou-2An). Electron spin polarization (ESP) pattern of the triplet state TREPR spectra of Cou-9An and Cou-9AnCN is (a,e,a,e,a,e), however, ESP phase pattern of (e,a,e,a,e,a) and (e,e,e,a,a,a) were observed for the two triplet states required for the simulation of the TREPR spectrum of Cou-2An, which indicated that ESP patterns of triplet state not only depend on the molecular geometry, but also on the structure of the electron donor and acceptor.