Decelerating Charge Recombination Using Fluorinated Porphyrins in N,N-Bis(3,4,5-trimethoxyphenyl)aniline - Aluminum(III) Porphyrin - Fullerene Reaction Center Models

Niloofar Zarrabi, Sairaman Seetharaman, Subhajyoti Chaudhuri, Noah Holzer, Victor S. Batista, Art Van Der Est, Francis D'Souza, Prashanth K. Poddutoori

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Abstract

In supramolecular reaction center models, the lifetime of the charge-separated state depends on many factors. However, little attention has been paid to the redox potential of the species that lie between the donor and acceptor in the final charge separated state. Here, we report on a series of self-assembled aluminum porphyrin-based triads that provide a unique opportunity to study the influence of the porphyrin redox potential independently of other factors. The triads, BTMPA-Im→AlPorFn-Ph-C60 (n = 0, 3, 5), were constructed by linking the fullerene (C60) and bis(3,4,5-trimethoxyphenyl)aniline (BTMPA) to the aluminum(III) porphyrin. The porphyrin (AlPor, AlPorF3, or AlPorF5) redox potentials are tuned by the substitution of phenyl (Ph), 3,4,5-trifluorophenyl (PhF3), or 2,3,4,5,6-pentafluorophenyl (PhF5) groups in its meso positions. The C60 and BTMPA units are bound axially to opposite faces of the porphyrin plane via covalent and coordination bonds, respectively. Excitation of all of the triads results in sequential electron transfer that generates the identical final charge separated state, BTMPA•+-Im→AlPorFn-Ph-C60•-, which lies energetically 1.50 eV above the ground state. Despite the fact that the radical pair is identical in all of the triads, remarkably, the lifetime of the BTMPA•+-Im→AlPorFn-Ph-C60•- radical pair was found to be very different in each of them, that is, 1240, 740, and 56 ns for BTMPA-Im→AlPorF5-Ph-C60, BTMPA-Im→AlPorF3-Ph-C60, and BTMPA-Im→AlPor-Ph-C60, respectively. These results clearly suggest that the charge recombination is an activated process that depends on the midpoint potential of the central aluminum(III) porphyrin (AlPorFn).

Original languageEnglish (US)
Pages (from-to)10008-10024
Number of pages17
JournalJournal of the American Chemical Society
Volume142
Issue number22
DOIs
StatePublished - Jun 3 2020

Bibliographical note

Funding Information:
This work was supported by startup grants from the University of Minnesota Duluth, to PPK, and by the National Science Foundation (Grant Nos. CHE-1401188 and CHE-2000988 to FD) and by an NSERC Discovery Grant (2015-04021) to AvdE. VSB acknowledges support from the U.S. Department of Energy, Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science (Grant DEFG02- 07ER15909).

Publisher Copyright:
Copyright © 2020 American Chemical Society.

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