First-Principles Study of the Nuclear Dynamics of Doped Conjugated Polymers

Abstract

Infrared-active vibrational (IRAV) modes are specific optical fingerprints to probe the density, dynamics, and spatial distribution of polarons in π-electron conjugated polymers. So far, the description of IRAV mode activation and selection rules, resulting from the local breaking of spatial symmetry induced by charge carriers, has been restricted to phenomenological lattice-dynamics models. Overcoming the classical picture, here we combine first-principles calculations with vibrational spectroscopy to study the nuclear dynamics of a model polymer system, poly(3-hexylthiophene) (P3HT). We assign and reproduce quantitatively the transition energies and intensities of vibrational normal modes in the ground and excited electronic states. By comparing the ground, polaronic, and excitonic states of regioregular (RR-) and regiorandom (RRa-) chains, we identify, for the first time, the vibrational fingerprints of neutral singlet excitations in the IRAV spectra of P3HT and highlight structure–property correlations. Within this new approach, vibrational spectroscopy provides a comprehensive tool to study not only polaron but also exciton density and dynamics and to better understand the influence of disorder on exciton and charge-carrier localization in functional organic systems

Delocalisation softens polaron electronic transitions and vibrational modes in conjugated polymers

“…Whilst some of them can be traced back to ground state modes, the distinct assignment to polaron-related modes has recently been invoked for the most prominent features. 10,11,16 Of the smaller features, some shift to lower energy (e.g. at 0.102 eV).…”

“…Also this classical explanation has recently been contested and it was suggested that pronounced vibrational signals should be considered as related to polarons instead of the ground state of the polymer. 10,11 The low energy signatures of polymer polarons have lately moved back into the focus of research interest as a means to extract information about the chain conformation in the environment of the charge carrier. The groups of Schwartz 12 and Salleo, 13 for example, studied the spectra of doped films of P3HT to investigate the location of doping-induced charge carriers.…”

Solvent tuning of photochemistry upon excited-state symmetry breaking

“…The S 2 ← S 1 mid-IR absorption observed here bears some resemblance with the low-energy polaron bands observed in the electronic absorption spectra of open-shell cations of conjugated oligomers and in doped conjugated polymers, as the latter report on the delocalization of the charge across the chain 62,63 . Charge localization breaks the symmetry leading to the transformation of Raman active modes into infrared-active vibrations (IRAVs) [64][65][66] .…”

Modeling organic electronic materials: bridging length and time scales

“…Common experiments used to validate electronic simulations include ultraviolet photoelectron spectroscopy (UPS) [43,44], ultraviolet-visible spectroscopy (UV-vis) [45][46][47] and infrared spectroscopy (IR) [46,48,49]. Each experiment validates a different observable of the DFT calculation.…”

“…This is difficult to reproduce with DFT; it is difficult to assign meaning to excited orbitals in DFT, as the theorems of DFT hold for the ground state only [50]. IR spectroscopy measures symmetry allowed molecular vibrations, which can be accurately simulated by finding the eigenvalues of the a matrix of interatomic force constants [48,49].…”