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Ultrafast Electronic Energy Transfer Beyond the Weak Coupling Limit in a Proximal but Orthogonal Molecular Dyad Anthony Harriman, Ifor D. W. Samuel, Arvydas Ruseckas, Gordon J Hedley, and Andrew Christopher Benniston J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b08640 • Publication Date (Web): 28 Nov 2015 Downloaded from http://pubs.acs.org on December 1, 2015
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The Journal of Physical Chemistry
Ultrafast Electronic Energy Transfer Beyond the Weak Coupling Limit in a Proximal but Orthogonal Molecular Dyad Gordon J. Hedley,¶ Arvydas Ruseckas,¶ Andrew C. Benniston,§ Anthony Harriman§,* and Ifor D. W. Samuel¶,* ¶
Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife, KY16 9SS, United Kingdom
§
Molecular Photonics Laboratory, School of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne, NE1 7RU, United Kingdom KEYWORDS: Fluorescence . Photophysics . Förster theory . Oligothiophene . BODIPY dyes.
ABSTRACT: Electronic energy transfer (EET) from a donor to an acceptor is an important mechanism that controls the light harvesting efficiency in a wide variety of systems, including artificial and natural photosynthesis and contemporary photovoltaic technologies. The detailed mechanism of EET at short distances or large angles between the donor and acceptor is poorly understood. Here the influence of the orientation between the donor and acceptor on EET is explored using a molecule with two nearly perpendicular chromophores. Very fast EET with a time constant of 120 fs is observed, which is at least forty times faster than the time predicted by Coulombic coupling calculations. Depolarization of the emission signal indicates that the transition dipole rotates through ca. 640, indicating the near orthogonal nature of the EET event. The rate of EET is found to be similar to structural relaxation rates in the photo-excited oligothiophene donor alone, which suggests that this initial relaxation brings the dyad to a conical intersection where the excitation jumps to the acceptor.
INTRODUCTION Electronic energy transfer (EET) is known to play significant roles in numerous natural (e.g., photosynthesis, photolyase enzymes, DNA damage) and artificial (e.g., light harvesting antenna, advanced signaling protocols, organic solar cells) processes.1-5 This realization has led to a tremendous resurgence of interest in the generic mechanisms that underpin EET in both solid and liquid phases,6,7 fueled by recent observations of energy transfer without spectral overlap in single-molecule fluorescence experiments8 and the possible involvement of quantum coherence.9 Most notable among the emerging research trends has been the detailed examination of EET across short (i.e.,
