**1. Introduction**

Schiff base is a condensation product of aldehyde and primary amine. When salicylaldehyde derivative is used as an aldehyde and amine is a monoamine derivative, the condensation yields a NO-Schiff base compound that may produce complexes with a grea<sup>t</sup> variety of metal ions upon deprotonation. Thus, Schiff bases are still of grea<sup>t</sup> importance in coordination chemistry, although more than a century has passed since their discovery [1–3]. Schiff bases have been known to be the most actively used ligands thanks to their ease of synthesis, availability, structural versatility, and solubility in common solvents. Moreover, Schiff bases are not only efficient ligands to coordinate/chelate many elements but can stabilize elements in various and specific oxidation states. It is also noteworthy that Schiff bases, as well as their corresponding complexes, can possess pronounced bioactivity [4–6], including against coronavirus [7]. The latter is becoming even more important since these days humanity is in dire need of drugs for COVID-19 treatment.

Schiff bases fabricated from salicylaldehyde derivatives (*N*-salicylidene aniline derivatives) have been the main focus of our extensive studies due to their chromic properties and a broad color palette [8–18]. Particularly, these compounds are known to a possible intramolecular transfer of the OH proton to the imine N-atom affording tautomerization between the enol-imine and keto-enamine forms [19–28]. This phenomenon is usually responsible for the fascinating optical properties of these compounds. Notably, the enolimine and keto-enamine forms can adopt either the *trans*- or *cis*-isomers, yielding a rich color palette from colorless to red through yellow (Scheme 1) [29,30]. These colors can be called by different external stimuli (light irradiation, temperature, solvent, pH, etc.). Thus,

**Citation:** Alkhimova, L.E.; Babashkina, M.G.; Safin, D.A. A Family of Ethyl *N*-Salicylideneglycinate Dyes Stabilized by Intramolecular Hydrogen Bonding: Photophysical Properties and Computational Study. *Molecules* **2021**, *26*, 3112. https:// doi.org/10.3390/molecules26113112

Academic Editor: Mirosław Jabło ´nski

Received: 23 April 2021 Accepted: 21 May 2021 Published: 23 May 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/).

chromic properties are one of the main features of *N*-salicylidene aniline derivatives and are important for many practical applications [31–35].

**Scheme 1.** Isomeric forms and the corresponding color palette of *N*-salicylidene aniline derivatives.

Notably, due to the abovementioned tautomerization, *N*-salicylidene aniline derivatives can also possess the so-called ESIPT (excited state intramolecular proton transfer) [36], which can afford an equilibrium between the excited keto-enamine\* and enol-imine\* forms [29,30,37,38]. The keto-enamine form usually, emits at a lower energy than the enol-imine tautomer leading to dual fluorescence, which can be observed as two (or more) emission bands [29,30,37,38]. The ratio of these bands can be influenced by the nature of a solvent, which allows tuning the resulting emission color.

It should be noticed that we have recently initiated comprehensive studies of closely related *N*-salicylidene aniline derivatives **1**–**4**, derived from the ethyl ester of glycine (Chart 1) [17,18]. We found that **1**–**4** in the solid state each exhibits an enol-imine form. Notably, the electron-withdrawing NO2 substituent in **4** is mainly responsible for the formation of the *cis*-keto form isomer in EtOH, while dyes **1**–**3** adopt the enol-imine form in the same solvent. Additionally, only **4** is emissive under ambient conditions in the solid state, while **2** is emissive in EtOH.

**Chart 1.** Diagrams of the discussed dyes **1** (X = H), **2** (X = OMe), **3** (X = Br) and **4** (X = NO2).

Intrigued by the obtained results, herein, we continue our comprehensive studies of the dyes **1**–**4**. Particularly, to probe solvatochromism, we focused on the optical properties of these compounds in different solvents, as well as shedding more light on their crystal structures using the Hirshfeld surface analysis to examine in-depth the non-covalent interactions responsible for crystal packing, which might be responsible for the observed photophysical properties of **1**–**4** in the solid state.
