No abstract

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 (Φ Δ ) 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.

Heavy atom-free triplet photosensitizers (PSs) are particularly of interest concerning both fundamental photochemistry study and practical applications. However, achieving efficient intersystem crossing (ISC) in planar heavy atom-free aromatic organic compounds is challenging. Herein, we demonstrate that two perylenebisimide (PBI) derivatives with anthryl and carbazole moieties fused at the bay position, showing twisted πconjugation frameworks and red-shifted UV−vis absorption as compared to the native PBI chromophore (by 75−1610 cm −1 ), possess efficient ISC (singlet oxygen quantum yield: Φ Δ = 85%) and a long-lived triplet excited state (τ T = 382 μs in fluid solution and τ T = 4.28 ms in solid polymer film). Femtosecond transient absorption revealed ultrafast intramolecular charge-transfer (ICT) process in the twisted PBI derivatives (0.9 ps), and the ISC takes 3.7 ns. Pulsed laser excited time-resolved electron paramagnetic resonance (TREPR) spectra indicate that the triplet-state wave function of the twisted PBIs is mainly confined on the PBI core, demonstrated by the zero-field-splitting D parameter. Accordingly, the twisted derivatives have higher T 1 energy (E T 1 = 1.48−1.56 eV) as compared to the native PBI chromophore (1.20 eV), which is an advantage for the application of the derivatives as triplet PSs. Theoretical computation of the Franck−Condon density of states, based on excited-state dynamics methods, shows that the efficient ISC in the twisted PBI derivatives is due to the increased spin− orbit coupling matrix elements for the S 1 −T 1 and S 1 −T 2 states [spin−orbit coupling matrix element (SOCME): 0.11−0.44 cm −1 . SOCME is zero for native PBI], as well as the Herzberg−Teller vibronic coupling. For the planar benzoPBI, the moderate ISC is due to S 1 → T 2 transition (SOCME: 0.03 cm −1 . The two states share a similar energy, ca. 2.5 eV).