Indian Journal of Chemistry Vol. 58B, December 2019, pp 1361-1374 Synthesis, characterization and photophysical properties of novel thiazole substituted pyridine derivatives Manjusha Suryawanshi a, Varsha Gujar b, Divya Ottoor b & Vivek Bobade* a a Department of Chemistry, H.P.T Arts & R. Y. K. Science College, Nashik 422 005, India b Department of Chemistry, Savitribai Phule Pune University, Pune 411 007, India E-mail: [email protected] Received 3 July 2018; accepted (revised) 11 June 2019 Three series of isomeric 2-pyridyl 4-aryl thiazoles have been synthesized by reacting 2/3/4-pyridine thioamides derived from the corresponding nitriles with various 4-substituted phenacyl bromides using Hantzsch thiazole synthesis. Amongst the three isomeric series, 2-pyridyl and 4-pyridyl isomers are found to exhibit better photophysical properties than 3-pyridyl series. 4-Pyridyl isomer with methoxy substituent on phenyl ring is found to exhibit high luminescence quantum yield. The relationship between the structure and the photophysical properties have been studied using DFT calculations. Keywords : Hantzsch synthesis, photo physical properties, quantum yield, DFT Fluorescent dyes are widely used for detection and imine linkage and dative heteroatom that can be used monitoring in the fields of chemistry, biochemistry, in chemosensors 3,4 . Recently, these π-functional molecular biology, medicine and material sciences. materials 5 are of increasing interest for the studies of Due to sensitive and selective detection methods and organic electronics, such as organic photovoltaic's un-problematic toxicology, they have almost (OPVs), organic field effect transistors (OFETs) and completely replaced radioactive tags. Widely used organic light-emitting diodes (OLEDs). Symmetrical / representatives include dansyl chloride, fluoresceins, asymmetrical thiazole based heterocyclic aromatic rhodamines and boron-dipyrromethenes (BODIPYs) 1. fluorescent compounds with high thermal stability Molecular probes are widely used tools in chemical and good solubility, where various electron donating biology that allow tracing of bioactive metabolites and electron-withdrawing terminal groups have been and selective labelling of proteins and other bio- reported 6. Incorporation of electron-donating and macro-molecules. A successful class of fluorophores withdrawing groups at appropriate positions of used for probing in life science comprises the monocyclic hetero-aromatic compounds provides D- heterocyclic thiazoles. This structural element can be A molecules 7. Increasing attention has been paid to found in commercial products, such as thiazole fluorescent 1,3-imidazoles 8, oxazoles 9 and thiazoles 10 . orange, SYBR® Green I or TOTO®, which are The literature revealed that the pyridine containing used for DNA labelling. In these compounds the thiazole derivatives showing fluorescent properties thiazole ring is part of a benzothiazole. Based on are mainly 2,5-diaryl / heteroaryl substituted, where the luminescent properties of pyridyl-thiazoles, the the conjugation effect is clearly observed due to linear synthesis of BPT (4-(3-azidopropoxy)-5-(4- structure. We report herein the synthesis and bromophenyl)-2-(pyridin-2-yl)thiazole) with superior characterization of 2-pyridyl, 4-aryl-thiazole, where fluorescence properties have been reported 2. such conjugation is partly observed due to 1,3-Azoles are an important scaffold for providing nonlinearity in the structure and relative findings of fluorescent molecules due to presence of acceptor the effect of change in position from 5 to 4 of the thiazole ring on photo physical properties. —————— Abbreviations : BODIPYs: Boron-dipyrromethenes; BPT: 4-(3 - Experimental Section Azidopropoxy)-5-(4-bromophenyl)-2-(pyridin-2-yl)thiazole; ICT: General Remarks Intramolecular charge transfer; OFETs: Organic Field Effect Transistors; OLEDs: Organic Light-Emitting Diodes; OPVs: Melting points were determined in open capillaries Organic Photovoltaics; Q-TOF: Quadrupole Time Of Flight. on a Mel-Temp apparatus and are uncorrected. 1362 INDIAN J. CHEM., SEC B, DECEMBER 2019 All the reactions were monitored by thin layer completion of the reaction, the mixture was poured chromatography (TLC) on pre-coated silica gel into crushed ice, the crude product was filtered and 60 F254 (mesh); spots were visualized with UV light. washed extensively with water. The product was re- Merck silica gel (60-120 mesh) was used for column crystallized from ethanol to obtain pure product. chromatography. 1H NMR (400 MHz) and 13 C NMR Yield: 80-90%. (100 MHz) spectra were recorded on a Bruker Avance II 400 MHz NMR spectrometer in CDCl 3/DMSO-d6 General procedure for the synthesis of isomeric 4- solution using tetramethylsilane as an internal phenylthiazol-2-pyridyl derivatives 2a-h/3a-h/4a-h standard. All chemical shifts were recorded in δ To a solution of pyridine thioamide 2/3/4 (1 g,7.2 (ppm) and the following abbreviations are used: s, mmol) in ethyl alcohol (4 mL), 4-substituted phenacyl singlet; d, doublet; dd, doublet of doublet; t, triplet; m, bromide (7.2 mmol) was added and the reaction was multiplet. The mass spectra were recorded on Waters, refluxed for 3 h. After the completion of the reaction, Q-TOF Micromass /ESI-MS at 70eV. as monitored on TLC, the reaction mixture was poured in cold water. The solid product obtained was Spectrophotometric measurements filtered, washed with water, dried and purified by UV–Vis absorbance measurements were performed column chromatography using hexane: ethyl acetate using UVPC-39 Shimadzu spectrophotometer. as eluent. Fluorescence and relative fluorescence quantum 4-Phenyl-2-(pyridin-2-yl) thiazole, 2a : Yield 87%. yield data were obtained on JASCO FP 8300 Yellowish green, m.p.198°C. 1H NMR (400 MHz, spectrofluorometer, equipped with a xenon lamp CDCl ): δ 7.07-7.09 (m, 1H, phenyl), 7.10-7.12 (dd, −1 3 (150 W) as the source. Scan speed of 1000 nm s was J = 8.0 and 7.6 Hz, 2H, phenyl), 7.33-7.34 (ddd, maintained for all measurements. The slit width for J = 7.8, 4.6 and 1.0 Hz, 1H, pyridine), 7.42 (s, 1H, both excitation and emission was kept at 5 nm. thiazole), 7.76-7.79 (dt, J = 1.0 and 7.8 Hz, 1H, Concentrations of samples were kept constant at 2.5 × pyridine), 7.85-7.86 (dd, J = 8.0 and 2.3 Hz, 2H, −6 10 M for all the experiments. phenyl), 8.10-8.20 (dd, J = 7.8 and 1.0 Hz, 1H, pyridine), 8.33-8.34 (dd, J = 4.6 and 1.0 Hz, 1H, General procedure for the synthesis of isomeric 13 δ pyridine thioamides 2,3,4 pyridine); C NMR (100 MHz, CDCl 3): 116.80, A solution of pyridine carbonitrile 1 (Scheme I) 117.29, 117.40, 121.90, 125.60, 129.02, 129.12, (5 g/5 mL) in pyridine (15 mL) and triethyl amine 139.06, 146.36, 151.43, 157.70, 164.20. (3 mL) was stirred for 15 min. H 2S gas was then 4-(4-Fluorophenyl)-2-(pyridin-2-yl) thiazole, 2b : passed into the reaction mixture. The reaction was Yield 89%. Green, m.p.114°C. 1H NMR (400 MHz, monitored on TLC. After stirring for about 2 h the CDCl 3): δ 7.12-7.16 (dd, J = 10.6 and 8.0 Hz, 2H, solution turned into a greenish yellow solid. On phenyl), 7.32-7.36 (dd, J = 7.6, 4.8 and 1.0 Hz, 1H, Scheme I — Synthesis of 2-pyridyl-4-phenyl thiazoles SURYAWANSHI et al .: THIAZOLE SUBSTITUTED PYRIDINE DERIVATIVES 1363 13 pyridine), 7.53 (s, 1H, thiazole), 7.80-7.85 (dt, J = 1.0 pyridine); C NMR (100 MHz, CDCl 3): δ 55.10 and 7.6 Hz, 1H, pyridine), 7.95-7.99 (dd, J = 8.0 and (−OMe), 113.60, 114.14, 119.85, 124.43, 127.51, 7.4 Hz, 2H, phenyl), 8.30-8.32 (dd, J = 7.9 and 1.0 Hz, 127.66, 136.99, 149.44, 151.55, 156.50, 159.72, 1H, pyridine), 8.62-8.63 (dd, J = 4.8 and 1.0 Hz, 1H, 168.61; ESI-MS: m/z (70 eV): 269.0817 [M+1]. 13 δ pyridine); C NMR (100 MHz, CDCl 3): 114.86, 4-(2-(Pyridin-2-yl) thiazol-4-yl) benzonitrile, 2g : 115.58, 115.66, 115.79, 119.86, 124.59, 128.06, Yield 93%. White, m.p. 168°C. 1H NMR (400 MHz, 128.14, 128.35, 137.07, 149.48, 155.67, 158.12; ESI- CDCl 3): δ 7.21-7.22 (d, J = 7.9 Hz, 2H, phenyl), 7.43- MS: m/z (70 eV): 257.0612 [M+1]. 7.44 (ddd, J = 7.2, 4.3 and 1.0 Hz, 1H, pyridine), 7.86 4-(4-Chlorophenyl)-2-(pyridin-2-yl) thiazole, 2c : (s, 1H, thiazole), 7.78-7.79 (dt, J = 1.0 and 7.2 Hz, Yield 81%. Light Brown, m.p.136°C. 1H NMR 1H, pyridine), 8.00-8.10 (d, J = 7.9 Hz, 2H, phenyl), (400 MHz, CDCl 3): δ 7.14-7.16 (d, J = 7.8 Hz, 2H, 8.20-8.21 (dd, J = 7.2 and 1.0 Hz, 1H, pyridine), 8.65- phenyl), 7.42-7.45 (ddd, J = 7.6, 4.4 and 1.0 Hz, 1H, 8.66 (dd, J = 4.3 and 1.0 Hz, 1H, pyridine); 13 C NMR pyridine), 7.59 (s, 1H, thiazole), 7.86-7.87 (dt, J = 1.0 (100 MHz, CDCl 3): δ 114.02, 114.25, 115.27, 118.70 and 7.6 Hz, 1H, pyridine), 7.91-7.94 (d, J = 7.8 Hz, (-CN), 124.07, 129.12, 129.24, 138.17, 146.2, 151.25, 2H, phenyl), 8.33-8.34 (dd, J = 7.6 and 1.0 Hz, 1H, 157.06, 164.23. pyridine), 8.65-8.66 (dd, J = 4.4 and 1.0 Hz, 1H, 4-Phenyl-2-(pyridin-3-yl) thiazole, 3a : Yield 13 1 pyridine); C NMR (100 MHz, CDCl 3): δ 114.86, 98%.