**4. Conclusions**

In summary, herein we discuss structural studies using the Hirshfeld molecular surface approach, as well as photophysical properties of four dyes: ethyl *N*-salicylideneglycinate (**1**), ethyl *N*-(5-methoxysalicylidene)glycinate (**2**), ethyl *N*-(5-bromosalicylidene)glycinate (**3**), and ethyl *N*-(5-nitrosalicylidene)glycinate (**4**).

The intermolecular H···H, H···C, and H···O contacts are the dominant contributors to the molecular surface of all the reported compounds. **3** is further described by a remarkable proportion of the intermolecular H···Br contacts. Furthermore, a distinct proportion of the C···C contacts was found in the molecular surface of **3** and **4**, which is explained by intermolecular π··· π stacking interactions between the phenylene rings.

The absorption spectra of **4** in THF and CH3CN exhibit a band for the *cis*-keto-enamine form, while only the enol-imine tautomer was found in the absorption spectrum of **4** in cyclohexane and in the absorption spectra of **1**–**3** in cyclohexane, THF, and CH3CN. Furthermore, in the absorption spectra of **4** in MeOH, EtOH, *i*PrOH, and *n*BuOH the *cis*-keto-enamine form is clearly observed, with the most remarkable one in MeOH, while the same band is absent in the spectrum of **4** in *n*PrOH, that might be explained by possible specific solute-solvent interactions. Dye **4** is emissive in *i*PrOH and *n*PrOH, arising from two conformers of the *cis*-keto-enamine\* form and from the *trans*-keto-enamine\* form, while the origin of emission in *n*PrOH is exclusively from different *cis*-keto-enamine\* conformers. Titration of the solutions of **3** and **4** in EtOH by NaOH leads to the gradual increasing of the band in the visible region of the corresponding UV-vis spectra. The same band gradually decreases and finally vanishes upon titration of the solutions of **3** and **4** in EtOH by CH3SO3H due to the protonation of the imine N-atom. Notably, upon addition of NaOH to the solution of **3** in EtOH, an emission band appeared and increased with increasing NaOH concentration.

We have also demonstrated that photophysical properties of a reported series of closely related compounds **1**–**4** can efficiently be tuned not only by changing the corresponding substituent in the phenolic ring, thus changing electronic properties, but also by the nature of a solvent (non-polar vs. polar aprotic vs. polar protic) and pH (NEt3 or NaOH, and CH3SO3H). All these factors allow to fine influence to the keto-enamine–enol-imine tautomerization both in the ground and excited states, yielding two or even three emission bands with a certain ratio and, as a result, different resulting emission color. No doubt, all these findings are of potential interest for molecular optics. Furthermore, the presence of the ethyl glycinate fragment in **1–4** might play a pivotal role for the application of these compounds, e.g., as luminescent sensors, in biological systems.

According to the DFT calculation results, it was established that all the tautomers of **1**–**4** each exhibit high electron-donating and low electron-accepting properties. The most pronounced nucleophilic centers of the enol-imine tautomers are located on the carbonyl and hydroxyl oxygen atoms, while the other negative electrostatic potential sites in the enol-imine form of **4** are located on the oxygen atoms of the NO2 group. In the *cis*-ketoenamine and *trans*-keto-enamine tautomers of **1**–**4**, the carbonyl oxygen atom attached to the aromatic ring is the most remarkable nucleophilic center, alongside both oxygen atoms of the NO2 group in **4**, while the carbonyl oxygen atom of the carboxyl group becomes a less pronounced nucleophilic center. As the most electrophilic region, the CH=N–CH2 fragment for the enol-imine tautomers and the CH–NH–CH2 fragment for the *cis*-ketoenamine and *trans*-keto-enamine tautomers are highlighted for the structures of **1**–**4**. The calculated absorption spectra of the fully optimized ground state geometry of all the three tautomers of **1**–**4** are in good agreemen<sup>t</sup> with experimental spectra.

**Supplementary Materials:** The following are available online, Figures S1–S4: 2D and decomposed 2D fingerprint plots of observed contacts for the crystal structure of **1–4**, Figures S5–S7: UV-vis spectra of **1–3** in the applied solvents, Figure S8: Emission and excitation spectra of **4** in *n*PrOH and *i*PrOH, Figure S9: UV-vis spectra of **4** in EtOH upon gradual addition of NEt3, Figure S10: Emission and excitation spectra of **3** in EtOH after addition of 5 eqv. of NaOH, Figure S11: The calculated UV-vis spectra of the ground states of the enol-imine tautomers of **1**–**4**, Figure S12: The calculated UV-vis spectra of the ground states of the *cis*-keto-enamine tautomers of **1**–**4**, Figure S13: The calculated UVvis spectra of the ground states of the *trans*-keto-enamine tautomers of **1**–**3**, Figures S14–S17: Energy levels and views on the electronic isosurfaces of the selected molecular orbitals of the ground state of **1**–**4**, Table S1: Values for the main maxima in the experimental UV-vis spectra of **1–4** in different solvents, and in the calculated UV-vis spectra for different tautomers of **1**–**4**, Table S2: Values for the

calculated UV-vis spectra of the ground state for the *cis*-keto-enamine tautomers of **1**–**4**, Table S3: Values for the calculated UV-vis spectra of the ground state for the *trans*-keto-enamine tautomers of **1**–**4**, Tables S4–S7: Cartesian atomic coordinates for optimized structures of **1**–**4**, obtained by using the DFT/B3LYP/6–311++G(d,p) method.

**Author Contributions:** Conceptualization, M.G.B. and D.A.S.; methodology, D.A.S.; formal analysis, L.E.A., M.G.B., and D.A.S.; investigation, L.E.A., M.G.B., and D.A.S.; data curation, L.E.A., M.G.B., and D.A.S.; writing—original draft preparation, L.E.A. and M.G.B.; writing—review and editing, L.E.A. and D.A.S.; supervision, D.A.S.; project administration, D.A.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not available.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compounds ethyl *N*-salicylideneglycinate (**1**), ethyl *N*-(5- methoxysalicylidene)glycinate (**2**), ethyl *N*-(5-bromosalicylidene)glycinate (**3**), and ethyl *N*-(5-nitrosalicylidene)glycinate (**4**) are available from the authors.
