Zusammenfassung
Heavy atom-free triplet photosensitizers (PSs) are particularly of interest concerning both fundamental photo-chemistry 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 ...
Zusammenfassung
Heavy atom-free triplet photosensitizers (PSs) are particularly of interest concerning both fundamental photo-chemistry 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 pi-conjugation 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: Phi(Delta) = 85%) and a long-lived triplet excited state (tau(T) = 382 mu s in fluid solution and tau(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-T1 = 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).
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