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 (Phi(Delta)) 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 p-framework of the Pery moiety results in a shorter triplet lifetime of 19.9 vs 291 mu s for the planar analogue. Time-resolved electron paramagnetic resonance spectroscopy confirms the SOCT-ISC mechanism.

Automated summary

Electron donor-acceptor dyads Rho-Pery were prepared to study the spin-orbit charge transfer intersystem crossing in these rigid and sterically congested molecular systems. The fluorescence of the dyads is significantly quenched in polar solvents, and the singlet oxygen quantum yields are strongly dependent on solvent polarity. Femtosecond transient absorption spectra show rapid charge separation and slow charge recombination, while nanosecond transient absorption spectra reveal the formation of triplet states via SOCT-ISC with distorted p-framework resulting in shorter triplet lifetime. Time-resolved electron paramagnetic resonance spectroscopy confirms the SOCT-ISC mechanism.