Spiro Rhodamine-Perylene Compact Electron Donor–Acceptor Dyads: Conformation Restriction, Charge Separation, and Spin–Orbit Charge Transfer Intersystem Crossing

Spiro rhodamine (Rho)-perylene (Pery) electron donor–acceptor dyads were prepared to study the spin–orbit charge transfer intersystem crossing (SOCT-ISC) in these rigid and sterically congested molecular systems. The electron-donor Rho (lactam form) moiety is attached via the N–C bond to the electron acceptor at either 1- or 3-position of the Pery moiety (Rho-Pery-1 and Rho-Pery-3). Severe torsion of the Pery moiety in Rho-Pery-1 was observed. The fluorescence of the two dyads is significantly quenched in polar solvents, and the singlet oxygen quantum yields (ΦΔ) are strongly dependent on solvent polarity (4–36%). Femtosecond transient absorption spectra demonstrate that charge separation (CS) takes 0.51 ps in Rho-Pery-1 and 5.75 ps in Rho-Pery-3, and the charge recombination (CR)-induced ISC is slow (>3 ns). Nanosecond transient absorption spectra indicate that the formation of triplet states via SOCT-ISC takes 24–75 ns for Rho-Pery-1 and 6–15 ns for Rho-Pery-3, and the distorted π-framework of the Pery moiety results in a shorter triplet lifetime of 19.9 vs 291 μs for the planar analogue. Time-resolved electron paramagnetic resonance spectroscopy confirms the SOCT-ISC mechanism.