Browsing by Author "Gür M."

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A combined experimental and computational study of electrochemical and photophysical properties of new benzophenone derivatives functionalized with N-substituted-phenyl-1,3,4-thiadiazole-2-amine
(2020-03-05) Şener İ.; Şahin Ç.; Demir S.; Şener N.; Gür M.
New benzophenone derivatives with N-substituted-1,3,4-thiadiazole including different substituents were synthesized and characterized by using FTIR, NMR, UV/Visible, and fluorescence spectroscopies and cyclic voltammetry. The effect of –F, -Cl, –OCH3 and –NO2 substitutions at ortho-, meta-, para-positions on the photoluminescence properties of the molecules have been investigated. The results indicate that the electron-donating and electron-withdrawing moieties at the ortho-, meta-, and para-positions have an important effect on the photoluminescence properties of the molecules. The calculated the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels are in the range of (−5.55) to (−5.65) eV and (−2.83) to (−3.02), respectively. The lower LUMO energy levels were observed for the molecules with –OCH3 substituents due to the donor strength of the substituent. Excited states and electrochemical properties of the molecules were also comprehensively and quantitatively studied by Density Functional Theory and Time-Dependent Density Functional Theory calculations. Experimental trends were successfully revealed by calculated data.
Analysis of tautomeric equilibrium in (E)-4,6-dibromo-2-[(4-fluorophenylimino)methyl]-3-methoxyphenol compound
(2015-07-13) Kaştaş Ç.A.; Kaştaş G.; Gür M.; Muʇlu H.; Büyükgüngör O.
Abstract In this study, the tautomeric equilibrium between the phenol-imine and keto-amine structural forms of (E)-4,6-dibromo-2-[(4-fluorophenylimino)methyl]-3-methoxyphenol compound has been investigated with experimental (XRD, UV-vis and NMR) and theoretical (DFT and TD-DFT) methods. The results clearly show that structural preference of the compound is definitely depended on its state. Namely, the compound exists in phenol-imine form in the solid state while one or both of these forms can be seen in solvent media. For example, the compound prefers phenol-imine form in benzene while both forms exist in EtOH and DMSO solvents. Coexistence of two forms has been quantified with NMR studies, giving a ratio of 11:9 for phenol and keto structures of the compound in acetone-d6 solvent.
Anticancer, antimicrobial, and DNA protection analysis of novel 2,4-dihydroxyquinoline dyes
(2018-10-01) Şener N.; Mohammed H.J.A.; Yerlikaya S.; Celik Altunoglu Y.; Gür M.; Baloglu M.C.; Şener İ.
A new series of 2,4-dihydroxyquinoline derived disazo dyes has been synthesized by the reaction of 5-amino-4-phenylazo-3-methyl-1H-pyrazole derivatives with 2,4-dihydroxyquinoline. The structures of the obtained dyes were identified by various spectrophotometric methods such as FT-IR, 1H-NMR, and elemental analysis. The synthesized compounds were also examined for different biological activities, including DNA protection, antimicrobial, and anticancer activities. Compound 3b was found most effective on Gram-positive bacteria. A DNA protection assay was applied to all compounds, and it was found that compounds 3h and 3j had a high capacity for binding to DNA. Besides, compounds 3h and 3j showed cytotoxicity against HeLa and PC3 cancer cell lines. These compounds could potentially be used as drugs or drug additives based on their effects on bacterial and cancer cell lines.
Antidiabetic and Antioxidant Properties of Bilberry (Vaccinium myrtillus Linn.) Fruit and Their Chemical Composition
(2015-01-01) Güder A.; Gür M.; Engin M.
Bilberry [Vaccinium myrtillus Linn. (Ericaceae)] fruit (VMF) has been traditionally used for treatment of bladder stones, biliary disorders, scurvy, coughs, and lung tuberculosis. VMF may have some pharmaceutical properties owing to these uses, but in vivo and in vitro studies are limited for clarification of medicinal activity and its antidiabetic and antioxidant properties are not investigated in detail. Anti-amylase, antiglucosidase, and antioxidant activities of methanol (ME), ethanol (EE), acetone (AE), and water (WE) extracts of VMF were investigated. In addition, some chemical compositions were determined by using spectrophotometric methods. Antidiabetic and antioxidant activities of extracts were studied by using different tests such as α-amylase and α- glucosidase inhibitory, total antioxidant, DPPH scavenging activities according to in vitro methods at different concentrations (10-250 μg mL-1). Compared with the standards, ME, WE, and EE showed strong total antioxidant activities with IC50 (μg/mL) values of 24.46±0.34, 25.24±0.78, and 27.48±0.60, respectively. At the same time, ME (IC5061.38±1.40 μg mL-1) and EE (IC5065.52±1.19 μg mL-1) demonstrated very effective inhibitory activity against α-amylase and moderate inhibitory activity against α- glucosidase. All extracts also showed high reducing power, metal chelating activity, superoxide anion, DPPH radical, and H2O2scavenging activities. Important relationships were found between biological activity and chemical composition by statistical analyses. The VMF can be used as an antidiabetic and antioxidant source in medicinal and pharmaceutical areas due to its chemical composition. Anthocyanin contents may influence the anti-amylase inhibition activity more than phenolic and flavonoid contents.
Antimicrobial activities and some flavonoids in extracts of some medicinal plants
(2017-07-01) Şener I.; Gür M.; Verep D.; Güney K.; Altuner E.
Origanum majorana, Melissa officinalis, Anthemis cotula and Avena sativa were extracted by using 65% ethanol to isolate their active constituents. The antimicrobial activities of extracts were investigated against 15 microorganisms by using the disk diffusion method, MIC (Minimum Inhibitory Concentration), MBC (Minimum Bactericidal Concentration) and MFC (Minimal Fungicidal Concentration) tests. Furthermore, the presence of some flavonoids were analyzed by using HPLC. It was determined flavonoids in the extracts of O. majorana, M. officinalis, A. cotula and A. sativa. As a results it was observed that O. majorana was active against Staphylococcus aureus, Enterococcus faecium, Enterococcus faecalis, Salmonella typhimurium and Pseudomonas aeruginosa, where M. officinalis was active against Enterococcus faecium and Enterococcus faecalis. On the other hand, both A. cotula and A. sativa were observed to be active against Enterococcus faecium. The extracts of plant samples showed antibacterial activity against tested microorganisms at different levels.
Antimicrobial activity and chemical composition of coriander & galangal essential oil
(2017-07-01) Özkinali S.; Şener N.; Gür M.; Güney K.; Olgun Ç.
Essential oils were obtained from Coriandrum sativum and Alpinia officinarum through steam distillation method in this study. Antimicrobial activities of the obtained essential oils were investigated by using minimum inhibitory concentration (MIC) test by against 18 different species microorganisms. It was found that the C. sativum had strong antimicrobial activities against the 14 tested microorganism and A. officinarum oils showed strong antimicrobial activity against the 7 tested microorganisms. Furthermore, the contents of the essential oils were determined with GC-MS analyses. The most abundant compounds of essential oils of A. officinarum and C. sativum were determined as 28.4% eucalyptol and 69.4% linalool. As a result, it can be easily said that the essential oils of C. sativum and A. officinarum can be used as antimicrobial protector in related fields.
Antiproliferative properties and structural analysis of newly synthesized Schiff bases bearing pyrazole derivatives and molecular docking studies
(2021-10-05) Şener N.; Özkinali S.; Altunoglu Y.C.; Yerlikaya S.; Gökçe H.; Zurnaci M.; Gür M.; Baloglu M.C.; Şener İ.
New pyrazole Schiff bases containing azo groups were synthesized using the condensation reaction between p-nitrobenzaldehyde and (E)-4-(phenyl)-1-H-pyrazole-3,5-diamine in the molar ratio of 1:2. Characterization of the compounds was performed by spectroscopic techniques, including IR, UV-Vis, 1H-NMR and 13C-NMR. Biological activity of the compounds was evaluated by analyzing DNA protection, antimicrobial and anticancer properties. Compound 4 was effective for Salmonella typhimurium with the MIC concentration of 62.5 µg/mL. Moreover, this compound had the highest protection activity on DNA. Cytotoxic activity of compound 4 was determined on the HeLa cancer cell line with the IC50 concentration of 976.6 µM. The anti-cancer characteristic of compounds 4 and 5 for HeLa cancer was theoretically analyzed by molecular docking study as well. Among the tested compounds, compound 4 possessed significant results due to its in vitro cytotoxic properties. Therefore, it may be considered as a potential bioactive agent for cancer treatment studies. In addition, further in vivo analysis can be performed to indicate its anticancer property.
Antiproliferative-antimicrobial properties and structural analysis of newly synthesized Schiff bases derived from some 1,3,4-thiadiazole compounds
(2020-11-05) Gür M.; Yerlikaya S.; Şener N.; Özkınalı S.; Baloglu M.C.; Gökçe H.; Altunoglu Y.C.; Demir S.; Şener İ.
The 1,3,4-thiadiazole core has been mainly used as a pharmacological scaffold in medicinal chemistry. A series of Schiff bases derived from 5-substituted-1,3,4-thiadiazole-2-amine were designed and synthesized to investigate their biological activities. Structures of compounds were clarified with FTIR, 1H NMR and elemental analysis. Due to the importance of this core in pharmacology, all these newly synthesized compounds were tested for different biological properties at the same time. Compound 3A ((E)-N-(2,5-dimethoxybenzylidene)-5-(4-methoxyquinolin-2-yl)-1,3,4-thiadiazol-2-amine) and compound 4A ((E)-N-(2,5-dimethoxybenzylidene)-5-(3-methylbenzofuran-2-yl)-1,3,4-thiadiazol-2-amine) possessed high DNA protective ability against oxidative Fenton mixture. Compound 1A ((E)-N-(2,5-dimethoxybenzylidene)-5-(benzo[b]thiophen-2-yl)-1,3,4-thiadiazol-2-amine) and compound 2B ((E)-2-((5-(1H-indol-2-yl)-1,3,4-thiadiazol-2-ylimino)methyl)-6-methoxyphenol) showed strong antimicrobial activity against S. epidermidis. The most effective compound was detected as compound 3A which exhibited cytotoxicity on both PC-3 and MDA-MB-231 cancer cell lines. The IC50 of this compound was calculated as 370.7 μM and 505.1 μM for MDA-MB-231 and PC-3 cells, respectively. Molecular docking studies were also performed to examine the understanding of the mechanism behind the anti-cancer and anti-bacterial properties. For further study, compound 3A has the potential for utilization with chemotherapy drugs to establish a more efficient therapy strategy with minimum cytotoxicity against cancer cells.
Chemical composition and antimicrobial activity in cold press oil of fennel, anise, white and black mustard seeds
(2017-07-01) Olgun Ç.; Özkan O.; Güney B.; Pattabanoglu E.; Güney K.; Gür M.
In this study, the cold press oil components and antimicrobial activities of fennel (Foeniculum vulgare) and anise (Pimpinella anisum) and white mustard (Sinapis alba) and black mustard (Brassica nigra) species seeds, which are widely used by the people for alternative medicine, were determined. F. vulgare, P. anisum, S. alba and B. nigra species seeds were obtained from cultivated areas in central Anatolia in Turkey. The oil was extracted by using a screw press (MP-001 Cold Press, Turkey), and the volatile oil components and fatty acid components in these oils were analysed by GCMS and total phenolic content, total flavonoid content and antioxidant activities by DPPH and FRAP (%) method were determined. Antimicrobial activities of obtained oils were investigated by using minimum inhibitory concentration (MIC) test by against 18 different species microorganisms. In the GCMS results, F. vulgare and P. anisum oils were found to be the most abundant components which were anethole (89.74%, 88.95%, respectively). According to these results, the plants oils didn’t show any antimicrobial activities against tested microorganisms. However especially white and black mustard oils showed strong antioxidant activity when compared with artificial antioxidants.
Chemical composition and antioxidant properties of some industrial tree bark extracts
(2019-08-01) Hamad A.; Ates S.; Olgun Ç.; Gür M.
Wood bark is a residue of forestry production that is used as a fuel source. The chemical composition of tree bark is similar to that of the harvested wood, and it contains a variety of useful compounds. To determine the chemical composition and antioxidant activities of different barks, fir (Abies nordmanniana), beech (Fagus orientalis), pine (Pinus sylvestris), poplar (Populus alba), and oak (Quercus robur) barks were selected because they are used for industrial purposes in Turkey. The dried bark powders were extracted using a 65:35 methanol-water mixture (v/v) to determine the total phenolic content, the flavonoid content, and the antioxidant properties (2,2-diphenyl-1-picrylhydrazyl (DPPH), ferric reducing antioxidant power (FRAP), metal chelating, and H2O2 scavenging). The flavonoid components were analyzed by high performance liquid chromatography (HPLC) and extracted by hexane to analyze the volatile components by gas chromatography-mass spectrometry (GC-MS). The poplar bark extracts had the highest total phenolic content, highest total flavonoid content, and highest antioxidant content. The poplar bark extracts were rich in myricetin (87.761 mg/L), which is a flavonoid with rich antioxidant properties. The presence of valuable extracts suggests that barks may have uses as valuable raw materials for chemical applications such as cosmetics, perfumes, and food preservatives.
Chemical contents and antifungal activity of some durable wood extractives vs. Pleurotus ostreatus
(2015-01-01) Ates S.; Gür M.; Özkan O.E.; Akça M.; Olgun Ç.; Güder A.
The total phenolic content (TPC), total flavonoid content (TFC), phenolic compounds, and antifungal activity of olive (Olea europaea var. sylvestris) and juniper (Juniperus foetidissima) sapwood and heartwood extractives were examined. The extractives were obtained using methanol solvents. The different compounds in extractives were identified and quantified. The antifungal activities of different parts of the olive and juniper wood extractives were determined in vitro. Pleurotus ostreatus mycelium was used for the antifungal activity experiment. Extractive compounds obtained from olive and juniper woods were found to be effective, natural antifungal agents.
Determination of antimicrobial activity and chemical composition of pimento & ginger essential oil
(2017-07-01) Şener N.; Özkinali S.; Gür M.; Güney K.; Özkan O.; Khalifa M.
The essential oil was obtained from pimento (Pimenta racemosa Mill.) and ginger (Zingiber officinale Rosc.) through hydrodistillation method. The chemical composition of pimento and ginger essential oils were analyzed by GC-MS. It was found that the most abundant components were methyleugenol (52.33%) and zingiberene (16.32%) in essential oils of pimento and ginger, respectively. The antimicrobial activities of essential oils were investigated against 18 microorganisms with minimum inhibitory concentration (MIC) method. While pimento essential oil behaved strong antimicrobial against all bacteria apart from S. epidermidis, ginger essential oil exhibited poor activity against all bacteria. As a result of this study; it can be suggested that pimento essential oil is a preservative in the medicinal application and food industry.
Determination of some flavonoids and antimicrobial behaviour of some plants’ extracts
(2017-07-01) Gür M.; Verep D.; Güney K.; Güder A.; Altuner E.
C. sativa, C. intybus, L. stoechas, V. officinalis and G. glabra plants were extracted by using 65% ethanol to isolate their active constituents. The antimicrobial activities of extracts were investigated against 15 microorganisms by using the disk diffusion method, MIC (Minimum Inhibitory Concentration), MBC (Minimum Bactericidal Concentration) and MFC (Minimal Fungicidal Concentration) tests. Furthermore, the presence of eight flavonoids were analysed by using HPLC. It was found that C. sativa is active against C. albicans, E. faecalis, S. enteritidis and S. typhimurium with MIC values of 26.02 µg/mL, 13.01 µg/mL, 416.25 µg/mL and 832.50 µg/mL respectively, where C. intybus is active against C. albicans and E. faecalis, with MIC values of 13.01 µg/mL and 6.50 µg/mL, respectively. On the other hand, L. stoechas and V. officinalis were observed to be active against only S. enteritidis with MIC values of 52.03 µg/mL and 26.02 µg/L respectively, where G. glabra was active against only E. faecalis, with a MIC value of 52.03 µg/mL. The extracts of plant samples showed antibacterial activity against tested microorganisms at different levels. But the activities against C. albicans and E. faecalis is noteworthy. The flavonoids were determined at different amounts in extracts.
Effect of processing temperature on the chemical composition and antioxidant activity of vaccinium arctostaphylos fruit and their jam
(2014-01-01) Güder A.; Engin M.; Yolcu M.; Gür M.
Vaccinium arctostaphylos fruit (VAF) has long been an important source of food and pharmaceutical ingredients and is considered to have high antioxidant potential. Growing V.arctostaphylos in Turkey is a substantial food, but information about its antioxidant activities is scarce. Other pharmacological properties of V.arctostaphylos, specifically antioxidant activity, contents of total phenolic, total flavonoid, total anthocyanin and ascorbic acid, are also not well explored. This study was designed to explore the in vitro antioxidant activities of ethanol extracts and chemical compositions such as ascorbic acid, total phenols, flavonoids and anthocyanins in V.arctostaphylos fruit and their jam (VAJ). Antioxidant activity, total phenolics, flavonoids, anthocyanins and ascorbic acid content changed during processing of jam depends on temperature. The results obtained in this study indicate that V.arctostaphylos fruit and their jam are potential sources of natural antioxidant. © 2013 Wiley Periodicals, Inc.
Essential oil of oregano and savory; chemical composition and antimicrobial activity
(2017-07-01) Özkan O.; Güney K.; Gür M.; Pattabanoğlu E.; Babat E.; Khalifa M.
The essential oils of oregano (Origanum vulgare ssp. hirtum) and savory (Satureja thymbra) plants were obtained through steam distillation method. The chemical compositions of their essential oils were analyzed via GC-MS technique. Furthermore, their antimicrobial activities were studied by using minimum inhibitory concentration (MIC) method. Their chemical composition was found as carvacrol (63.97%), p-cymene (12.63%) and linalool (3.67%) in oregano essential oil as major compounds and also, as carvacrol (42.7%), o-cymene (17.98%), linalool (9.65%), caryophyllene oxide (5.25%) and γ-terpinene (4.22%) in savory essential oil as major compounds. As a result, it can be said that that oregano and savory essential oil may be useful in the medicinal application and food industry as a preservative against harmful microorganisms.
Investigation of two o-hydroxy Schiff bases in terms of prototropy and radical scavenging activity
(2017-02-15) Albayrak Kaştaş Ç.; Kaştaş G.; Güder A.; Gür M.; Muğlu H.; Büyükgüngör O.
Two Schiff bases, namely (E)-4,6-dibromo-3-methoxy-2-[(phenylimino)methyl]phenol (1) and (Z)-2,4-dibromo-6-[(4-buthylphenylamino)methylene]-5-methoxycyclohexa-2,4-dienone (2), have been investigated by considering solvent, substituent and temperature dependence of prototropy, and scavenging activities. Experimental (X-ray diffraction, UV–vis and NMR) and computational (DFT) techniques have been used to obtain key data on prototropy and other properties of interest. X-ray and UV–vis results underline the variability in the structural preferences of the compounds with respect to the phase and solvent media conditions. This kind of tautomeric behavior has been elaborated by 1H NMR and 13C NMR experiments performed at room and low temperatures. Radical scavenging properties of two compounds have been probed for their usage potentials as therapeutic agent and ingredient in medicinal and food industries, respectively. For this purpose, three different test methods (DPPH, ABTS•+ and DMPD•+) have been used. It has been found from in vivo and in vitro studies that the compound 2 could be interesting as an active component in pharmaceutical industry or as an additive in food industry when its antiradical activity is considered.
Investigations of Painting Techniques: Edirne Süleyman Pasha Mosque Wall Paintings
(2022-01-01) Yetiş E.; Eroğlu M.; Eskici B.; Gür M.; Deniz K.; Kadıoğlu Y.K.
Decorative wall paintings created in the Ottoman era are called kalem işi, and they are generally created using various pigments and binding media on dry rendering, or via lime-paint technique. However, detection of some decorative wall paintings that were applied on wet plaster in Süleyman Pasha Mosque located in Edirne, Turkey, has led to a more detailed analytical examination. Our study aims to research the techniques used on some painting fragments which were found while removing cement renderings during the restoration of the Mosque. The fragments were revealed in 2019 and were taken under protection as these were stylistically different from the existing paintings of the Mosque. The original location of the fragments on the wall could not be located and in situ conservation is not possible, but research continues. Initially, traces of preparatory drawing, which is thought to be the incising technique, were found on the paintings drawn on the rendering fragments. Then, SEM-EDX, FTIR spectroscopy, confocal Raman spectroscopy, and PED-XRF analyses were carried out along with examinations by optical microscopy. Following these results, experimental reconstructions of the paintings were made on test materials and a story about the painting techniques was revealed, indicating that these paintings were drawn on wet rendering (fresco). Therefore, it is considered that there may be different wall painting techniques in Ottoman architecture and that research in this direction should be increased.
K⁺ Channels and Some Familiar Antiepileptic Drugs: Evaluation of Their the Structure-Activity Relationships with Molecular Docking Analysis
(2023-05-31) Çakmak E.N.; Gür M.; Kiran B.
This study includes the structure-activity relationship of active molecules that are commonly used in the treatment of convulsive seizures in epileptic diseases. Well-known epileptic active molecules studied are: Vigabatrin, Lokosamidine, Zonisamide, Oxcarbazepine, Levetiresetam, Tiagabine, Topiramate, Lamotrigin, Gabapentin, Felbamat, Ethosuximide, Valproic Acid, Mesuximide, Ethotoin, Primidon, Trimethadion, Phenytoin, Remasemide, Mephenytoin. These molecules, which were selected considering the physiopathological mechanisms of action of epileptic disease, were considered suitable for molecular docking studies since they were used as a potential antiepileptic agent. In addition, it was focused on the potassium channels, which were prominent in the mechanisms of epilepsy. During the action potential that triggers seizure formation, inward rectifying potassium channels (KIR3.2) make a important role providing the flow of K+ ions. Thus, PDB ID: 4KFM receptor was chosen for molecular docking study, since its act as an agonist according to its activity on the canal in the case of epileptic seizures formation. The result of molecular docking analysis demonstrated that Phenytoin gave the best binding affinity for 4KFM with a value of-6.2 kcal/mol. Other analysis in descending order (as kcal/mol); Oxcarbazepine (-6,0), Remasemide (-5.9), Topiramate and Primidon (-5.8), Tiagabine, Felbamat and Mesuximide (-5.7), Lamotrigin (-5.6) Zonisamide, Ethotoin and Mephenytoin, Lokosamidine (-5.5), Gabapentin (-4.8), Trimethadion (-4.7), Ethosuximide (-4.6), Levetiresetam (-4.5), Vigabatrin (-4.0), Valproic Acid (-3.9) determined as.
New 1,3,4-thiadiazole compounds including pyrazine moiety: Synthesis, structural properties and antimicrobial features
(2017-07-05) Gür M.; Şener N.; Muğlu H.; Çavuş M.S.; Özkan O.E.; Kandemirli F.; Şener İ.
In the study, some new 1,3,4-thiadiazole compounds were synthesized and we have reported identification of the structures by using UV-Vis, FT-IR, 1H NMR, 13C NMR and Mass spectroscopic methods. Antimicrobial activities of the compounds against three microorganisms, namely, Candida albicans ATCC 26555, Staphylococcus aureus ATCC 9144, and Escherichia coli ATCC 25922 were investigated by using disk diffusion method. These thiadiazoles exhibited an antimicrobial activity against Staphylococcus aureus and Candida albicans. The experimental data was supported by the quantum chemical calculations. Density functional theory (DFT) calculations were carried out to obtain the ground state optimized geometries of the molecules using the B3LYP, M06 and PBE1PBE methods with 3–21 g, 4–31 g, 6–311++g(2d,2p), cc-pvtz and cc-pvqz basis sets in the different combinations. Frontier molecular orbitals (FMOs) energies, band gap energies and some chemical reactivity parameters were calculated by using the aforementioned methods and basis sets, and the results were also compared with the experimental UV-Vis data.
Novel Disubstituted Calix[4]arenes Containing Chromogenic Groups on the Lower Rim: Synthesis, Structural Identification, and Absorption Properties
(2018-04-01) Şener N.; Erişkin S.; Gür M.; Şener İ.
5,11,17,23-Tetra-tert-butyl-25,26,27,28-tetrahydroxycalix[4]arene and 25,26,27,28-tetrahydroxycalix[4]arene were synthesized based on previous literature. Firstly, several azo dyes were synthesized, and then, these dyestuffs were chlorinated through reaction with thionyl chloride. Secondly, the azo compounds bonded with methylene chloride and reacted with 25,26,27,28-tetrahydroxycalix[4]arene, and then, six novel disazocalix[4]arene derivatives 4(a–f) were achieved. Their structures were characterized as Fourier transform infrared, 1H-NMR, and elemental analysis. In this study, we investigated the solvatochromic properties of the compounds and the acid–base effect on their ultraviolet–visible absorption by using six solvents.