Boosting Photoelectric Conductivity in Porphyrin-based MOFs Incorporating C₆₀

Pratik, S. M.; Gagliardi, L.; Cramer, C. J.
J. Phys. Chem. C 2020, 124, 1878 (doi:10.1021/acs.jpcc.9b10834).

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 opto-electronic properties. A preferred slipped-parallel π-stacked interaction of C₆₀ that is predicted for NH₂-substituted DA-MOF and F-MOF fosters strong charge-transfer (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 π-stacked interfaces created by C₆₀ incorporation, a consequence of strong electronic coupling between initial and final diabatic states. Calculations indicate that photoinduced electron transfer (PET), 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- and infrared-based optoelectronics.