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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2019-04, Vol.123 (13), p.2503-2516
• Main Authors: Chen, Kepeng, Zhao, Jianzhang, Li, Xiaoxin, Gurzadyan, Gagik G
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/acs.jpca.8b11828

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, V DA) on the photophysical properties of UV–vis absorption, fluorescence emission, especially spin–orbit charge transfer intersystem crossing. We found that the magnitude of V DA depends on the electron donating strength of the aryl moieties (V DA = 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.