  Citation: Marinescu, M.; Cintez ˘ a, L.-O.; Popa, C.-V.; Gîfu, I.C.; Burlacu, S.G.; Petcu, C. Synthesis, Characterization and Antimicrobial Activities of Some Schiff Bases with Non-Linear Optical Applications. Chem. Proc. 2022, 7, 40. https:// doi.org/10.3390/chemproc2022007040 Academic Editors: Mihaela Doni, Florin Oancea, Zina Vuluga and Radu Claudiu Fierăscu Published: 11 March 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 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/). Abstract Synthesis, Characterization and Antimicrobial Activities of Some Schiff Bases with Non-Linear Optical Applications † Maria Marinescu 1, * , Ludmila-Otilia Cinteză 2, * , Claudia-Valentina Popa 1,3 , Ioana Cătălina Gîfu 4 , Sabina Georgiana Burlacu 4 and Cristian Petcu 4 1 Department of Organic Chemistry, Biochemistry and Catalysis, University of Bucharest, 90-92 Sos Panduri, 050663 Bucharest, Romania; valentina.popa@chimie.unibuc.ro 2 Faculty of Chemistry, Department of Physical Chemistry, University of Bucharest, Bd. Regina Elisabeta 4-12, 0330018 Bucharest, Romania 3 “Cantacuzino” National Military Medical Institute for Research and Development, 103 Splaiul Independent , ei, 050096 Bucharest, Romania 4 National Institute for Research & Development in Chemistry and Petrochemistry, Splaiul Independent , ei 202, 060021 Bucharest, Romania; gifu_ioanacatalina@yahoo.com (I.C.G.); sabina.nitu@icechim-pd.ro (S.G.B.); cpetcu@icf.ro (C.P.) * Correspondence: maria.marinescu@chimie.unibuc.ro (M.M.); ocinteza@gw-chimie.math.unibuc.ro (L.-O.C.) † Presented at the 17th International Symposium “Priorities of Chemistry for a Sustainable Development” PRIOCHEM, Bucharest, Romania, 27–29 Oct 2021. Keywords: organic synthesis; Schiff bases; antimicrobial; nonlinear optical properties Introduction: Schiff bases or imine compounds are considered a very remarkable category of organic compounds because of their π-delocalized structure and because of their pharmacological properties. Thus, the delocalization of the π-electrons defines the classic structures with an optical response due to the large hyperpolarizabilities which result from a “push–pull” system. Furthermore, imine compounds have been revealed to be promising leads for the design of more competent antimicrobial agents [1–3]. Materials and methods: Organic commercial and synthetic materials were used for the synthesis of the heterocyclic compounds. All compounds were characterized with physicochemical techniques (elemental analysis, 1 H, 13 C, FTIR and UV-Vis spectroscopy) [4]. The SHG capability of samples was measured by using an experimental set-up [1,5]. Results: A series of Schiff bases containing heterocyclic compounds (pyrazoles, pyrimidines, benzimidazoles, benzothiazole, etc.) was synthesized and characterized. The nonlinear optical (NLO) response of some Schiff bases is investigated by the static hyperpolarizability coefficients (β), calculated using the semi-empirical quantum chemistry algorithms (MOPAC software). All compounds were evaluated by qualitative and quantitative methods against a panel of selected bacterial and fungal strains [6,7]. Conclusions: It was found that relationship between donor/acceptor moieties, the dihedral angles around the azo bridge (-N=N-) transmitter group, the pass length as well as the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), strongly influenced the NLO response. Author Contributions: Conceptualization, M.M. and L.-O.C.; methodology, M.M.; software, L.-O.C.; validation, M.M., L.-O.C. and C.P.; formal analysis, C.-V.P.; investigation, I.C.G.; resources, S.G.B.; data curation, M.M.; writing—original draft preparation, M.M.; writing—review and editing, M.M.; visualization, L.-O.C.; supervision, M.M.; project administration, L.-O.C.; funding acquisition, M.M. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by a grant of the Ministry of Research, Innovation and Digiti- zation, CCCDI—UEFISCDI, project number PN-III-P2-2.1-PED-2019-3009, within PNCDI III. https: //uefiscdi.gov.ro, accessed on 10 March 2022. Any errors may affect your future funding. Chem. Proc. 2022, 7, 40. https://doi.org/10.3390/chemproc2022007040 https://www.mdpi.com/journal/chemproc