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Anthracene–Naphthalenediimide Compact Electron Donor/Acceptor Dyads: Electronic Coupling, Electron Transfer, and Intersystem Crossing

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journal contribution
posted on 12.03.2019, 00:00 by Kepeng Chen, Jianzhang Zhao, Xiaoxin Li, Gagik G. Gurzadyan
We attached different electron donors of phenyl, anthryl, and alkylamino moieties, to electron acceptor naphthalenediimide (NDI) to construct compact electron donor/acceptor dyads. The purpose is to study the effect of electron coupling (the magnitude is the matrix element, VDA) on the photophysical properties of UV–vis absorption, fluorescence emission, especially spin–orbit charge transfer intersystem crossing. We found that the magnitude of VDA depends on the electron donating strength of the aryl moieties (VDA = 0.22–0.55 eV), as well as the molecular conformation, based on steady state and time-resolved transient absorption spectroscopies. We also found that electron coupling does not show the add-up (or synergetic) effect. Solvent polarity-dependent intersystem crossing (ISC) was observed for the dyad/triads and singlet oxygen quantum yield decreases in polar solvents. Femtosecond transient absorption results indicate that the charge separation (CS) for 9-An-NDI-NH occurs on time scale of 0.83 ps (in toluene) or 0.71 ps (in acetonitrile). The charge recombination (CR) process (50 ps in toluene) produces triplet state with ΦISC = 19%. The triplet state lifetime is up to 22 μs. This result indicates that orthogonal geometry for a compact electron donor/acceptor does not lead to efficient ISC via CR. Other factors such as the energy gap between the CS state and triplet state also determine the ISC efficiency.