A combined MD/QM and experimental exploration of conformational richness in branched oligothiophenes

Abstract

Infrared (IR) absorption and vibrational Raman spectra of a family of branched oligothiophenes have been determined experimentally as well as theoretically. The molecular spectra have been compared to those of the linear analogues, with identification made of spectral features due to structural properties that are valued in organic solar cell applications. The theoretical spectra have been obtained through a newly developed method in which individual conformer spectra, calculated at the time-dependent DFT level in this work, are weighted by statistics extracted from classical molecular dynamics trajectories. The agreement with experiment for the resulting averaged spectra is at least as good as, and often better than, what is observed for Boltzmann-weighted spectra. As the weights are available before the costly step of spectrum calculation, the method has the additional advantage of enabling efficient approximations. For simulating the molecular dynamics of the studied α,β-linked thiophenes and 2-methylthiophenes, high quality parameters have been derived for the CHARMM force field. Furthermore, the temperature dependence of the IR and Raman spectra has been investigated, both experimentally and theoretically.

Structure determination of oleanolic and ursolic acids: a combined density functional theory/vibrational spectroscopy methodology

“…A theoretical spectrum for each terpenoid acid can be obtained based on a Maxwell-Boltzmann population distribution [2,43,44]. This spectrum can be recreated considering the proportional weight of each tetrameric conformer following the expression:…”

Computing the Absorption and Emission Spectra of 5-Methylcytidine in Different Solvents: A Test-Case for Different Solvation Models

“…Inclusion of Quantum vibronic effects, resulting in a further increase of the computed band-width, would partially recover this discrepancy, in line with the promising results recently obtained on oligothiophenes by using mixed Quantum/Classical approaches. [100][101][102][103][104] These approaches (whose application is well beyond the scope of the present work) are based on the separation of high-frequency 'molecular' vibrational modes, which are treated at a fully Quantum vibronic level, and low-frequency 'environmental' (large amplitude motion of the solute, solvent fluctuations) modes, whose effect is included by using classical MD simulations.…”

Synergistic Approach of Ultrafast Spectroscopy and Molecular Simulations in the Characterization of Intramolecular Charge Transfer in Push-Pull Molecules

“…For predicting conformational transitions occurring in large-sized systems, all-atom simulations combined with enhanced sampling are to date the computational technique that well balance accuracy and computational cost [127][128][129][130][131][132][133]. In this framework, excited state potentials have been developed in the years to study molecular conformations in a given electronic state in oligothiophene [134], as well as for predicting the absorption properties of the anthocyanidine dye in solution [135] or thermally activated delayed fluorescence emitters [136]. More recently, a scheme pioneered by some of us [137] introduced enhanced sampling simulations in the framework of well-tempered [138] metadynamics [139] with excited-state torsional potentials in combination with Free-Energy Perturbation (FEP) theory to estimate free-energy gaps from the ground to the excited states.…”