Research paper Intermolecular vibrations and vibrational dynamics of a phenolmethanol binary complex studied by LIF spectroscopy Deb Pratim Mukhopadhyay, Souvick Biswas, Tapas Chakraborty ⇑ Department of Physical Chemistry, Indian Association for the Cultivation of Science, 2A Raja S C Mullick Road, Jadavpur, Kolkata 700032, India article info Article history: Received 14 January 2017 In final form 15 February 2017 Available online 17 February 2017 abstract Low-frequency intermolecular vibrations in S 0 and S 1 states of p-fluorophenol (pFP) methanol binary complex have been studied using laser induced fluorescence (LIF) spectroscopy in a supersonic jet expan- sion. Vibrational fundamentals of five such modes show up in fluorescence excitation (FE) spectrum, and corresponding ground state frequencies are obtained measuring disperse fluorescence (DF) spectra. Signatures of strong coupling between the hydrogen bond stretching fundamental (r 0 1 ) and a ring mode of pFP moiety in S 1 state are revealed. In comparison with the analogous pFP-water complex, the present system displays very low threshold (170 cm 1 ) for vibrational energy relaxation in S 1 state. Ó 2017 Published by Elsevier B.V. 1. Introduction Small clusters of phenol with selected number of protic solvent molecules bound via hydrogen bonded (H-bonded) networks have been used over many years as convenient models to study the sol- vation behavior of this important class of molecules at the micro- scopic level [1–23]. Phenol is the smallest aromatic alcohol and is used extensively in different industrial applications. From photo- chemical viewpoint it is also the smallest photo-acid [24]. Many halophenols used as pesticides [25], are found in soil sediments, and their light induced chemically relevant processes are signifi- cant from atmospheric viewpoints [26]. In biology, the phenolic chromophore is the light absorbing moiety of the amino acid tyro- sine, and it acts as a catalyst in many enzymatic reactions, e.g., light-induced water splitting in photosystem-II [27]. The photophysical studies carried out with size-selected clus- ters over the past several decades indicate that dynamical behavior of solutes solvated with limited number of solvent molecules, in general, are different compared to the bulk behavior [3–11,28– 33]. The contrasts are more evident in the case of hydrogen bonded clusters, where apart from the mere size effects, the solvent mole- cules are bound strongly via electron rich centers of the solutes and thus can exert much stronger electronic effects. Furthermore the clusters can have well defined low frequency intermolecular vibra- tional modes, which in the bulk solutions are blurred into complex rotational and translational motions leading to exchange. There- fore, in small clusters, the specific molecular influence and size effects on photophysical behavior becomes more distinct. Recently we have reported the intermolecular vibrations in S 0 and S 1 states of the binary pFP-water complex [34]. The goal of the present study is to see how these low frequency vibrations and vibrational dynamics are affected due to introduction of a methyl group on the solvent counterpart. With this in mind, in the present study, we have measured the laser-induced FE and DF spectra of the 1:1 com- plex of methanol with pFP. Shown in Scheme 1 are the theoretically predicted (HF/6-311+ +GG**) structures of the two complexes and we have attempted to address the following issues related to the photophysical behav- ior of the complex. Solvent molecules in the two complexes are bound via OHO hydrogen bonds, and in the energetically pre- ferred geometries, pFP is the hydrogen bond donor. In the present study we have address the following issues. (1) The optimized structures show that although the overall binding energies of the two complexes are similar, the water complex possesses a C s sym- metry, which is lost in methanol complex. This symmetry loss in the latter complex can have a profound effect on Franck-Condon active band structures in the vibronically resolved electronic spec- tra. (2) Methanol has additional low-frequency modes including the rotor mode that contribute to increase in overall density of states with increase in vibrational energy in an electronic state, but such modes are absent in water. (3) Whether the solvation shift of the solute in S 1  S 0 electronic transition is determined by solute-solvent dipolar interaction. Dipole moment of a metha- nol molecule (1.69 D) being smaller compared to water (1.85 D), the spectral shift of the electronic transition of the binary complex with methanol is expected to be smaller if the dipolar interaction dominates [35]. On the other hand, if the quantum mechanical hyperconjugative charge transfer interaction between lone pair electron orbitals (n) of the solvent and anti-bonding r / (OAH) http://dx.doi.org/10.1016/j.cplett.2017.02.059 0009-2614/Ó 2017 Published by Elsevier B.V. ⇑ Corresponding author. E-mail address: pctc@iacs.res.in (T. Chakraborty). Chemical Physics Letters 674 (2017) 71–76 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett