The effect of the anion size and electronegativity of halide-based anions (Cl − , Br − , I − , and BF 4 − ) on the interionic interaction in 1-ethyl-3-methylimidazolium-based ionic liquids (ILs) C 2 mim X (X = Cl, Br, I, and BF 4 ) is studied by a combined approach of experiments (Raman, IR, UV−vis spectroscopy) and quantum chemical calculations. The fingerprint region of the Raman spectra of these C 2 mim X ion-pairs provides evidence of the presence of the conformational isomerism in the alkyl chain of the C 2 mim + cation. The Raman and IR bands of the imidazolium C 2 −H stretch vibration for C 2 mim X (X = Cl, Br, I, and BF 4 ) were noticeably blue-shifted with the systematic change in size of anions and the electronegativity. The observed blue shift in the C 2 −H stretch vibration follows the order C 2 mim BF 4 > C 2 mim I > C 2 mim Br > C 2 mim Cl, which essentially indicates the strong hydrogen bonding in the C 2 mim Cl ion-pair. DFT calculations predict at least four configurations for the cation−anion interaction. On the basis of relative optimized energies and basis-setsuperposition-error (BSSE) corrected binding energies for all ion-pair configurations, the most active site for the anion interaction was found at the C 2 H position of the cation. Besides information about the C 2 H position, our DFT results give insights into the anion interaction with the ethyl and methyl chain of the cation, which was also confirmed experimentally [Chem. Commun. 2015, 51, 3193]. The anion interaction at the C 2 H site of the cation favors a planar geometry in C 2 mim X for X = Cl, Br, and I; however, for BF 4 , the system prefers a nonplanar geometry where the anion is located over the imidazolium ring. TD-DFT results were used to analyze the observed UV−vis absorption spectra in a more adequate way giving insights into the electronic structure of the ILs. Overall, a reasonable correlation between the observed and the DFT-predicted results is established.

Photogenerated polaron-pair ultrafast dynamics in poly(3-hexylthiophene)-based devices are found to be influenced by external electric fields via delayed field-induced singlet exciton dissociation, yielding a bimolecular decay contribution.

Metal halide-based perovskite semiconductors exhibit excellent optoelectronic properties such as a sharp absorption edge, high absorption coefficients, and a small recombination rate. Mixed compositions result in a variation of the structure of these perovskite materials, which also influences their electronic properties. Even though huge progress in synthesis and device fabrication has been made, still systematic investigations of structural properties of lead halide-based perovskites are missing. Here, we systematically investigate the vibrational features of lead bromide-based perovskites using Raman spectroscopy and density functional theory (DFT). We have performed these investigations using MA + , FA + , and Cs + as cations in the lead bromide structures and determined the vibrational modes both from Raman experiments and DFT simulations. We find a clear dependence of the Raman band wavenumbers on the chosen cations. The structural differences are reflected in the different line-width broadening of Raman bands, charge distribution on the cations and the extent of their interactions with the bromide anions. K E Y W O R D S density functional theory (DFT), perovskites, Raman spectroscopy Debkumar Rana and Bapi Pradhan contributed equally. This paper is dedicated to late Prof. Derek A. Long whose important works in the field of Raman spectroscopy contributed fundamentally to the development of this technique.

We present a study of the impact of water on the size-and electronegativitydependent hydrogen bonds between cation and anion of imidazolium-based ionic liquids (ILs) with halide anions. We have combined Raman spectroscopy with density functional theory (DFT) computations to characterize the intermolecular associations of cation-anion-water moieties. The Raman band at~1,422 cm −1 , which is assigned to the ν(N─CH 2 ─CH 3 ) vibration of the ethyl chain in the neat ion pairs (pure ion pairs), is blue shifted by +7, +4, and +3 cm −1 for 1-ethyl-3-methylimidazolim (C 2 mim) cation-based C 2 mim X ion pairs with X = Cl, Br, and I, respectively, in a water-rich environment. Simultaneously, upon dilution, a new peak at~1,438 cm −1 starts to appear at the high-wavenumber side with continuously increasing intensity. These significant perturbations of the ν(N─CH 2 ─CH 3 ) vibrational mode in water environment are a clear indication of the change in the alkyl chain conformation. Further, the relative intensities of the Raman bands at~2,946 and~2,950 cm −1 corresponding to ν s (CH 2 ) and ν as (CH 3 ) of the ethyl chain, respectively, were significantly changed upon dilution, which also confirms the conformational change of the alkyl chain. The Raman band belonging to the C 4/5 ─H stretch mode was significantly blue shifted by +33, +41, and +42 cm −1 going from the studied neat ion pairs having Cl, Br, and I anions to the corresponding water mixtures, respectively. Significant changes were also observed for the ν(C 2 ─H) vibrational mode of the ion pairs in water environment;here, we observed blue shifts of +60, +51, and +50 cm −1 for ion pairs having Cl, Br, and I anions, respectively. These results confirm the weakening of the ion-pair interaction due to the interaction with water molecules. In contrast to the ILs with halide anions, C 2 mim BF 4 does not show any variation in the Raman peak positions upon dilution. Experimental results are nicely supported by the results of DFT calculations, and a sensible connection was established. K E Y W O R D Sionic liquids, hydrogen bonds, Raman spectroscopy, density functional theory calculations Deepak K. Pandey and Paridhi Sanchora Authors have contributed equally