**1. Introduction**

This paper dwells on the studies of one of the most popular photo-thermochromic compounds, 2-[(E)-(phenylimino)methyl]phenol (N-salicylideneaniline), from the group of the ortho-hydroxy aryl Schiff bases. The first compounds of this type were synthesized in 1864 by Hugo Schiff [1] and have attracted attention ever since [2,3]. The ortho-hydroxy aryl Schiff bases, and materials based on them, demonstrate a number of interesting and useful characteristics. For example, they manifest polymorphic properties [4], ionic liquids' properties [5], elastic bending capability [6] and recognised anti-cancer properties [7]. The chiral aldimines possess the photochromic feature in the crystalline state, caused by photoinduced proton transfer in the intramolecular OHN hydrogen bond [8,9]. The chiroptical and optical anisotropic properties of photomechanical Schiff bases were previously studied under UV irradiation [10]. Such properties as ferroelectricity, piezoelectricity and second-order optical non-linearity were reported [11,12].

Many of the abovementioned features are linked to the existence of an intramolecular OHN hydrogen bond and to various conformational changes that ortho-hydroxy aryl Schiff

**Citation:** Hetma ´nczyk, Ł.;

Goremychkin, E.A.; Waliszewski, J.; Vener, M.V.; Lipkowski, P.; Tolstoy, P.M.; Filarowski, A. Spectroscopic Identification of Hydrogen Bond Vibrations and Quasi-Isostructural Polymorphism in N-Salicylideneaniline. *Molecules* **2021**,

*26*, 5043. https://doi.org/10.3390/ molecules26165043

Academic Editor: Mirosław Jabło ´nski

Received: 31 July 2021 Accepted: 16 August 2021 Published: 20 August 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

bases could undergo. The conformational changes in the excited state were first studied in the year 1962 [13] and continue to attract attention to date. The development of new synthetic routes led to a significant increase in the number of available compounds and allowed novel types of liquid crystals and photo-optical switches to be obtained [14]. Such characteristics are usually conditioned by the isomerization of the imine fragment [15–18]. Besides, isomerization of an ortho-hydroxy aryl Schiff base attached to a BODIPY chromophore was used to tune its UV absorption spectra [19].

A long discussion takes place in the literature [20–30] on what intramolecular changes are responsible for the emergence and fine tuning of photo-thermochromic properties of ortho-hydroxy aryl Schiff bases: isomerization of the hydroxyl- and imine- groups (Chart **a**, Figure 1) or intramolecular proton transfer with the consequent isomerization of the amino group (Chart **b**, Figure 1) or aniline ring rotation (Chart **c**, Figure 1).

**Figure 1.** Conformational equilibria of 2-[(E)-(phenylimino)methyl]phenol.

The recent comprehensive studies seem to indicate that the proton transfer in the intramolecular OHN hydrogen bond and the isomerization of the aldimine fragment (Chart **b**, Figure 1) is in charge of the appearance of photo-thermochromic properties in ortho-hydroxy aryl Schiff bases [28]. Notably, the strength of the hydrogen bond and tautomeric equilibrium in it are linked to the observation of certain physical-chemical properties. In cases where the OH tautomeric form is prevailing, it is possible to observe OH isomerization, leading to the absence of photo-thermochromic properties, Chart **a**. Recently, Mielke et al. [29,30] have discovered the existence of the trans-OH form in the specific condition of matrix isolation. However, the prevailing of the NH tautomeric form makes it possible to observe the isomerization of the amino group (Chart **b**) and, therefore, the emergence of photo-thermochromic properties. It is important to underline the contribution by Ogawa et al. [24] who were the first to show the existence of trans-NH form in the solid state by X-ray method. The possibility of rotation of aldimine ring (Chart **c**) was first demonstrated by Cohen et al. [13,20], and later it was associated with the appearance of polymorphism of the studied compound.

The conformational changes, occurring in ortho-hydroxy aryl Schiff bases in different conditions and states, were investigated mostly by the methods of vibrational spectroscopy in the middle spectral range [29–36], while the low- and high-frequency hydrogen bridge vibrations have never been in the focus of the spectroscopic studies before. Therefore, in order to fill in the gap, we have synthesized 2-[(E)-(phenylimino)methyl]phenol (Nsalicylideneaniline, **SA**) and its deutero-derivatives (**SA-OD** and **SA-C6D5**, see Figure 2) to analyse the vibrational spectra measured by IINS, Raman and IR methods. The quantummechanical DFT and DFPT calculations, as well as Potential Energy Distribution (PED) analysis, have been conducted to assign the spectral bands. The isotopic replacements of the bridged hydrogen for deuterium (O-H···N → O-D···N) and the hydrogen atoms for the deuterium atoms in aldimine ring (NC6H5 → NC6D5) were performed to additionally confirm the assignment. To that end, the Isotopic Spectral Ratios (ISRs) were used, which are defined for a given vibrational mode as the ratio of frequencies for non-deuterated and deuterated species. The ISR values are well studied for high-frequency stretching and bending proton vibrations [37], but poorly studied for the low-frequency hydrogen bridge vibrations.

**Figure 2.** Structures of studied 2-[(E)-(phenylimino)methyl]phenol and its isotopologues.

Moreover, this paper delves into the spectroscopic manifestations of polymorphism of N-salicylideneaniline and its isotopologues in a wider temperature range in 20–320 K. N-salicylideneaniline was studied earlier in papers [27,28] in the 100–320 K temperature range.
