Spiro rhodamine-coumarin compact electron donor–acceptor dyads: synthesis and spin–orbit charge transfer intersystem crossing

We prepared spiro rhodamine (RB)-coumarin (Cou) compact electron donor-acceptor dyads (RB-Cou-CF3 and RB-Cou-CN), to study the charge transfer (CT) and spin-orbit CT intersystem crossing (SOCT-ISC). The π-conjugation planes of the rhodamine and coumarin units in both dyads are in nearly orthogonal geometry (dihedral angle: 86.3°). CT state emission was observed for RB-Cou-CF3 (at 550 nm) and RB-Cou-CN (at 595 nm). Although the fluorescence of the pristine coumarin units (fluorescence quantum yields ΦF = 59%) was quenched in the dyads (ΦF = 0.5 ~ 1.1% in n-hexane), the triplet state quantum yields of the dyads are also low (singlet oxygen quantum yield, ΦΔ = 2.3-7.5% in n-hexane). Nanosecond transient absorption spectra show that the 3Cou* state was formed, which shows a triplet state lifetime of 11-15.6 μs. The proposed photophysical path for the dyads is as follows: RB-1Cou* → RB+•-Cou-• → RB-3Cou*. The low SOCT-ISC yield is attributed to the slightly lower charge-transfer state energy (1.94 eV in toluene) as compared to the 3Cou* state energy (2.23 eV) and the shallow potential energy curve (PEC) at energy minima of the dyads. This work indicates that orthogonal conformation of donor-acceptor units is inadequate for achieving efficient SOCT-ISC. These results are useful for studying charge separation and intersystem crossing of electron donor/acceptor dyads.