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J Phys Chem A. 2016 Feb 4;120(4):615-20. doi: 10.1021/acs.jpca.5b08764. Epub 2016 Jan 21.

Polarons in π-Conjugated Polymers: Anderson or Landau?

Author information

1
Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford , Oxford, OX1 3QZ, United Kingdom.
2
Magdalen College, University of Oxford , Oxford, OX1 4AU, U.K.
3
University College, University of Oxford , Oxford, OX1 4BH, U.K.

Abstract

Using both analytical expressions and the density matrix renormalization group method, we study the fully quantized disordered Holstein model to investigate the localization of charges and excitons by vibrational or torsional modes-i.e., the formation of polarons-in conformationally disordered π-conjugated polymers. We identify two distinct mechanisms for polaron formation, namely Anderson localization via disorder (causing the formation of Anderson polarons) and self-localization by self-trapping via normal modes (causing the formation of Landau polarons). We identify the regimes where either description is more valid. The key distinction between Anderson and Landau polarons is that for the latter the particle wave function is a strong function of the normal coordinates, and hence the "vertical" and "relaxed" wave functions are different. This has theoretical and experimental consequences for Landau polarons. Theoretically, it means that the Condon approximation is not valid, and so care needs to be taken when evaluating transition rates. Experimentally, it means that the self-localization of the particle as a consequence of its coupling to the normal coordinates may lead to experimental observables, e.g., ultrafast fluorescence depolarization. We apply these ideas to poly(p-phenylenevinylene). We show that the high frequency C-C bond oscillation only causes Landau polarons for a very narrow parameter regime; generally we expect disorder to dominate and Anderson polarons to be a more applicable description. Similarly, for the low frequency torsional fluctuations we show that Anderson polarons are expected for realistic parameters.

PMID:
26756225
DOI:
10.1021/acs.jpca.5b08764

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