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Temperature Dependence of Charge Separation and Recombination in Porphyrin Oligomer–Fullerene Donor–Acceptor Systems

Axel Kahnt†, Joakim Kärnbratt†, Louisa J. Esdaile‡, Marie Hutin‡, Katsutoshi Sawada‡, Harry L. Anderson‡, and Bo Albinsson†*

Physical Chemistry, Department of Chemical and Biological Engineering, Chalmers University of Technology, Kemivägen 3, 412 96 Göteborg, Sweden

Chemistry Research Laboratory, Department of Chemistry, University of Oxford, Mansfield Road, Oxford OX1 3TA, U.K.

J. Am. Chem. Soc., 2011, 133 (25), pp 9863–9871

DOI: 10.1021/ja2019367

Publication Date (Web): May 19, 2011

balb@chalmers.se

Section:

Radiation Chemistry, Photochemistry, and Photographic and Other Reprographic Processes

Abstract

Electron-transfer reactions are fundamental to many practical devices, but because of their complexity, it is often very difficult to interpret measurements done on the complete device. Therefore, studies of model systems are crucial. Here the rates of charge separation and recombination in donor–acceptor systems consisting of a series of butadiyne-linked porphyrin oligomers (n = 1–4, 6) appended to C₆₀ were investigated. At room temperature, excitation of the porphyrin oligomer led to fast (5–25 ps) electron transfer to C₆₀ followed by slower (200–650 ps) recombination. The temperature dependence of the charge-separation reaction revealed a complex process for the longer oligomers, in which a combination of (i) direct charge separation and (ii) migration of excitation energy along the oligomer followed by charge separation explained the observed fluorescence decay kinetics. The energy migration is controlled by the temperature-dependent conformational dynamics of the longer oligomers and thereby limits the quantum yield for charge separation. Charge recombination was also studied as a function of temperature through measurements of femtosecond transient absorption. The temperature dependence of the electron-transfer reactions could be successfully modeled using the Marcus equation through optimization of the electronic coupling (V) and the reorganization energy (λ). For the charge-separation rate, all of the donor–acceptor systems could be successfully described by a common electronic coupling, supporting a model in which energy migration is followed by charge separation. In this respect, the C₆₀-appended porphyrin oligomers are suitable model systems for practical charge-separation devices such as bulk-heterojunction solar cells, where conformational disorder strongly influences the electron-transfer reactions and performance of the device.

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This article has been cited by 1 ACS Journal articles (1 most recent appear below).

Molecular Fluorescence, Phosphorescence, and Chemiluminescence Spectrometry
Susmita Das, Aleeta M. Powe, Gary A. Baker, Bertha Valle, Bilal El-Zahab, Herman O. Sintim, Mark Lowry, Sayo O. Fakayode, Matthew E. McCarroll, Gabor Patonay, Min Li, Robert M. Strongin, Maxwell L. Geng, and Isiah M. Warner
Analytical Chemistry2012 84 (2), 597-625
- Molecular Fluorescence, Phosphorescence, and Chemiluminescence Spectrometry
  Susmita Das, Aleeta M. Powe, Gary A. Baker, Bertha Valle, Bilal El-Zahab, Herman O. Sintim, Mark Lowry, Sayo O. Fakayode, Matthew E. McCarroll, Gabor Patonay, Min Li, Robert M. Strongin, Maxwell L. Geng, and Isiah M. Warner
  Analytical Chemistry2012 84 (2), 597-625
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