Theory of exciton transfer and diffusion in conjugated polymers
Abstract
We describe a theory of Förstertype exciton transfer between conjugated polymers. The theory is built on three assumptions. First, we assume that the lowlying excited states of conjugated polymers are Frenkel excitons coupled to local normal modes, and described by the FrenkelHolstein model. Second, we assume that the relevant parameter regime is ℏω < J, i.e., the adiabatic regime, and thus the BornOppenheimer factorization of the electronic and nuclear degrees of freedom is generally applicable. Finally, we assume that the Condon approximation is valid, i.e., the excitonpolaron wavefunction is essentially independent of the normal modes. The resulting expression for the exciton transfer rate has a familiar form, being a function of the exciton transfer integral and the effective FranckCondon factors. The effective FranckCondon factors are functions of the effective HuangRhys parameters, which are inversely proportional to the chromophore size. The BornOppenheimer expressions were checked against DMRG calculations, and are found to be within 10% of the exact value for a tiny fraction of the computational cost. This theory of exciton transfer is then applied to model exciton migration in conformationally disordered poly(pphenylene vinylene). Key to this modeling is the assumption that the donor and acceptor chromophores are defined by localmore »
 Authors:

 Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford OX1 3QZ (United Kingdom)
 Publication Date:
 OSTI Identifier:
 22310723
 Resource Type:
 Journal Article
 Journal Name:
 Journal of Chemical Physics
 Additional Journal Information:
 Journal Volume: 141; Journal Issue: 16; Other Information: (c) 2014 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 00219606
 Country of Publication:
 United States
 Language:
 English
 Subject:
 37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; BORNOPPENHEIMER APPROXIMATION; CENTEROFMASS SYSTEM; COMPUTERIZED SIMULATION; DEGREES OF FREEDOM; DIFFUSION; DIFFUSION LENGTH; EMISSION; EXCITATION; EXCITED STATES; EXCITONS; FACTORIZATION; GROUND STATES; MONTE CARLO METHOD; PEAKS; POLYMERS; SPECTRA; WAVE FUNCTIONS
Citation Formats
Barford, William, Tozer, Oliver Robert, and University College, University of Oxford, Oxford OX1 4BH. Theory of exciton transfer and diffusion in conjugated polymers. United States: N. p., 2014.
Web. doi:10.1063/1.4897986.
Barford, William, Tozer, Oliver Robert, & University College, University of Oxford, Oxford OX1 4BH. Theory of exciton transfer and diffusion in conjugated polymers. United States. doi:10.1063/1.4897986.
Barford, William, Tozer, Oliver Robert, and University College, University of Oxford, Oxford OX1 4BH. Tue .
"Theory of exciton transfer and diffusion in conjugated polymers". United States. doi:10.1063/1.4897986.
@article{osti_22310723,
title = {Theory of exciton transfer and diffusion in conjugated polymers},
author = {Barford, William and Tozer, Oliver Robert and University College, University of Oxford, Oxford OX1 4BH},
abstractNote = {We describe a theory of Förstertype exciton transfer between conjugated polymers. The theory is built on three assumptions. First, we assume that the lowlying excited states of conjugated polymers are Frenkel excitons coupled to local normal modes, and described by the FrenkelHolstein model. Second, we assume that the relevant parameter regime is ℏω < J, i.e., the adiabatic regime, and thus the BornOppenheimer factorization of the electronic and nuclear degrees of freedom is generally applicable. Finally, we assume that the Condon approximation is valid, i.e., the excitonpolaron wavefunction is essentially independent of the normal modes. The resulting expression for the exciton transfer rate has a familiar form, being a function of the exciton transfer integral and the effective FranckCondon factors. The effective FranckCondon factors are functions of the effective HuangRhys parameters, which are inversely proportional to the chromophore size. The BornOppenheimer expressions were checked against DMRG calculations, and are found to be within 10% of the exact value for a tiny fraction of the computational cost. This theory of exciton transfer is then applied to model exciton migration in conformationally disordered poly(pphenylene vinylene). Key to this modeling is the assumption that the donor and acceptor chromophores are defined by local exciton ground states (LEGSs). Since LEGSs are readily determined by the exciton centerofmass wavefunction, this theory provides a quantitative link between polymer conformation and exciton migration. Our Monte Carlo simulations indicate that the exciton diffusion length depends weakly on the conformation of the polymer, with the diffusion length increasing slightly as the chromophores became straighter and longer. This is largely a geometrical effect: longer and straighter chromophores extend over larger distances. The calculated diffusion lengths of ∼10 nm are in good agreement with experiment. The spectral properties of the migrating excitons are also investigated. The emission intensity ratio of the 00 and 01 vibronic peaks is related to the effective HuangRhys parameter of the emitting state, which in turn is related to the chromophore size. The intensity ratios calculated from the effective HuangRhys parameters are in agreement with experimental spectra, and the timeresolved trend for the intensity ratio to decrease with time was also reproduced as the excitation migrates to shorter, lower energy chromophores as a function of time. In addition, the energy of the exciton state shows a logarithmic decrease with time, in agreement with experimental observations.},
doi = {10.1063/1.4897986},
journal = {Journal of Chemical Physics},
issn = {00219606},
number = 16,
volume = 141,
place = {United States},
year = {2014},
month = {10}
}