Electronic structure calculations show that guest C60 in the porphyrin-containing metal organic frameworks Zn2(TCPB)(DA-ZnP) (DA-MOF; H4TCPB = 1,2,4,5-tetrakis(4-carboxyphenyl)benzene, DA-ZnP = [5,15-bis[(4-pyridyl)ethynyl]-10,20-diphenylporphinato]zinc(II)) and Zn2(TCPB)(F-ZnP) (F-MOF; F-ZnP = [5,15-di(4-pyridyl)-10,20-bis(pentafluorophenyl)porphinato]zinc(II)) engenders high photoelectrical conductivity due to efficient donor-acceptor charge-transfer (CT) interactions. Structural modifications at the meso position of the porphyrin influence the preferred positions of C60 within the frameworks, giving rise to host-guest interactions with different anisotropic structural, electronic, and optoelectronic properties. A preferred slipped-parallel Ï-stacked interaction of C60 that is predicted for NH2-substituted DA-MOF and F-MOF fosters strong CT transitions and lowers band gaps by 1.0 eV compared to the pristine DA-MOF and F-MOF. Hopping rates computed using Marcus theory are found to be anisotropic and accelerated by multiple orders of magnitude across Ï-stacked interfaces created by C60 incorporation, a consequence of strong electronic coupling between initial and final diabatic states. Calculations indicate that photoinduced electron transfer, as well as direct CT from porphyrin to C60 upon irradiation, triggers a charge separation process that leads to the formation of what should be long-lived electron-trapped states at the heterojunctions. Design principles revealed here for the control of photophysical and electron-transfer processes will be useful for constructing new MOF-derived visible- A nd infrared-based optoelectronics.
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This research was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences under Award DE-FG02-17ER16362. Computer resources were provided by the Minnesota Supercomputing Institute at the University of Minnesota.
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