Using spectroscopy to probe relaxation, decoherence, and localization of photoexcited states in π-conjugated polymers

• Published in: Faraday discussions 2020-01, Vol.221, p.281-298
• Main Authors: Barford, William, Gardner, John L. A, Mannouch, Jonathan R
• Format: Article
• Language: English
• Subjects: Chromophores; Computer simulation; Coupling; Covalent bonds; Depolarization; Excitons; Fluorescence; Localization; Molecular conformation; Polymers; Spectroscopy; Spectrum analysis; Wave propagation
• ISSN: 1359-6640; 1364-5498
• DOI: 10.1039/c9fd00054b

We use the coarse-grained Frenkel-Holstein model to simulate the relaxation, decoherence, and localization of photoexcited states in conformationally disordered π-conjugated polymers. The dynamics are computed via wave-packet propagation using matrix product states and the time evolution block decimation method. The ultrafast ( i.e. , t < 10 fs) coupling of an exciton to C-C bond vibrations creates an exciton-polaron. The relatively short ( ca. 10 monomers) exciton-phonon correlation length causes ultrafast exciton-site decoherence, which is observable on conformationally disordered chains as fluorescence depolarization. Dissipative coupling to the environment (modelled via quantum jumps) causes the localization of quasi-extended exciton states (QEESs) onto local exciton ground states (LEGSs, i.e. , chromophores). This is observable as lifetime broadening of the 0-0 transition (and vibronic satellites) of the QEES in two-dimensional electronic coherence spectroscopy. However, as this process is incoherent, neither population increases of the LEGSs nor coherences with LEGSs are observable. We use the coarse-grained Frenkel-Holstein model to simulate the relaxation, decoherence, and localization of photoexcited states in conformationally disordered π-conjugated polymers.