Probing conformational dynamics in single donor-acceptor synthetic molecules by means of photoinduced reversible electron transfer

doi:10.1073/pnas.0406119101

Probing conformational dynamics in single donor-acceptor synthetic molecules by means of photoinduced reversible electron transfer

^*Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200 F, Heverlee 3001, Belgium; ^‡Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge MA 02138; and ^§Max-Planck-Institut für Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany

Communicated by Marye Anne Fox, University of California at San Diego, La Jolla, CA, August 19, 2004 (received for review June 25, 2004)

Abstract

We use single-molecule fluorescence lifetimes to probe dynamics of photoinduced reversible electron transfer occurring between triphenylamine (donor) and perylenediimide (acceptor) in single molecules of a polyphenylenic rigid dendrimer embedded in polystyrene. Here, reversible electron transfer in individual donor-acceptor molecules results in delayed fluorescence that is emitted with a high photon count rate. By monitoring fluorescence decay times on a photon-by-photon basis, we find fluctuations in both forward and reverse electron transfer spanning a broad time range, from milliseconds to seconds. Fluctuations are induced by conformational changes in the dendrimer structure as well by polystyrene chain reorientation. The conformational changes are related to changes in the dihedral angle of adjacent phenyl rings located in the dendritic branch near the donor transferring the charge, a torsional motion that results in millisecond fluctuations in the “through-bond” donor-acceptor electronic coupling. Polymer chain reorientation leads to changes in the local polarity experienced by the donors and to changes in the solvation of the charge-separated state. As a result, switching between different donor moieties within the same single molecule becomes possible and induces fluctuations in decay time on a time scale of seconds.

Footnotes

↵ ¶ To whom correspondence may be addressed. E-mail: frans.deschryver{at}chem.kuleuven.ac.be or xie{at}chemistry.harvard.edu.
↵ † M.C. and S.M. contributed equally to this work.
Abbreviations: SMD, single-molecule detection; ET, electron transfer; D-A, donor-acceptor; LES, locally excited state; CSS, charge-separated state; GS, ground state; PN, perylenediimide acceptor-triphenylamine donor; ZE, Zeonex; PS, polystyrene; MLE, maximum-likelihood estimator.
Freely available online through the PNAS open access option.