Understanding the absorption spectrum of mesityl oxide dye in solvents of different polarities Henrique M. Cezar ⁎, Sylvio Canuto, Kaline Coutinho Universidade de Sao Paulo, Instituto de Fisica, Rua do Matao 1371, Sao Paulo, SP 05508-090, Brazil abstract article info Article history: Received 9 December 2019 Received in revised form 10 March 2020 Accepted 17 March 2020 Available online 21 March 2020 Keywords: Solvatochromism QM/MM Explicit solvent Monte Carlo simulations Understanding the solvent contribution to the UV–Vis absorption spectrum of mesityl oxide (MOx) in different solvents is still challenging because of the interplay between the two possible conformations. The syn:anti rela- tive stability changes according to the solvent polarity. In this work a sequential quantum mechanics/molecular mechanics based on a configuration biased Monte Carlo and the explicit use of solvent molecules is adopted to describe in detail the solvent effects. The contributions of the conformation changes, the polarization effects of the solvent molecules in MOx and the explicit solvation are separated and analyzed. The excitation energies are obtained using time-dependent density functional theory (TDDFT B3LYP/6-311++G(d,p)) calculations on statistically relevant configurations of MOx in acetonitrile, methanol and water solutions. Considering the explicit solvent molecules the calculated solvatochromic shift of −120 cm −1 , −1410 cm −1 and −2500 cm −1 are ob- tained for acetonitrile, methanol and water, respectively. These results are in due agreement with the experimen- tal values. The separated contributions of conformation changes, polarization effects and the explicit solvation add to a consistent description of MOx UV–Vis absorption spectra in solution. The delocalization of the valence orbitals into the solvent region is also considered. The results emphasize the role of the solvent polarity and the limitations of methods that do not include explicit solvent molecules and the thermodynamic condition that characterizes a liquid system. © 2020 Elsevier B.V. All rights reserved. 1. Introduction The understanding of solvent effects and solvent polarity on reaction rates, conformational stability, UV–Vis absorption spectra is of great im- portance in optimization of several processes [1]. For example, solute- solvent systems are a fundamental part in the light harvesting process for dye sensitized solar cells [2] and for water splitting [3]. In general, the solvent affects not only the electronic structure of the solute, but also the molecular structure, changing bond lengths and in some cases even changing the dominant conformer. For this reason, the spectros- copy of molecules is often solvent dependent, and polarity scales such as E T (30), Y scale and Z scale, have been proposed to characterize the solvents and also provide insights on how the solvent effects affect the absorption spectra [1,4]. In the validation of the Z scale of solvent polarity, Kosower used the dye mesityl oxide (MOx) [(CH 3 ) 2 C=CHC(=O)CH 3 , 4-methyl-3- penten-2-one, shown in Fig. 1] to examine the sensitivity of the scale [5]. It was shown that for MOx, the n — π ∗ transition blue shifts, while the π — π ∗ transition red shifts when the solvent polarity increases. These results correlated very well with the Z scale [5] and the E T (30) [6] for several solvents, such as isooctane, ethylene dichloride, acetoni- trile, isopropyl alcohol, n-butyl alcohol, ethanol, 95% ethanol, methanol, ethylene glycol, tetrafluoropropanol and water. For the π — π ∗ transi- tion, specifically, the red shift was attributed to a higher dipole moment of the molecules excited state in comparison with the dipole moment of the ground state. Additionally, the author mentioned that the equilib- rium of syn and anti conformers is solvent dependent, but did not ex- plore the conformational effects on the spectroscopy. Experimental data using different techniques pointed the syn con- former (Fig. 1a) as the favored conformer in low polarity solvent. Mea- surements of UV–Vis and IR spectra of MOx in cyclohexane and tetrachloroethylene solution, respectively, were used to establish the syn conformer as more favorable [7]. An independent measurement of the IR spectra in tetrachloromethane, another low polarity solvent, showed the same [8]. Similarly, the NMR spectra [9–11] and Raman spectra [12] of MOx in tetrachloromethane arrived at the same conclu- sion, that the syn conformer is dominant in low polarity solvents. Using the lanthanide induced shift technique, the ratio between syn and anti conformers was estimated to be of 72% of syn conformers in deuterated-chloroform [13]. To the best of our knowledge, no experimental information on the syn:anti conformational ratio in polar solvents is available. Previous Journal of Molecular Liquids 307 (2020) 112924 ⁎ Corresponding author. E-mail addresses: henrique.cezar@usp.br (H.M. Cezar), canuto@if.usp.br (S. Canuto), kaline@if.usp.br (K. Coutinho). https://doi.org/10.1016/j.molliq.2020.112924 0167-7322/© 2020 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq