Synthesis of highly stable luminescent molecular crystals based on (E)-2-((3-(ethoxycarbonyl)-5-methyl-4-phenylthiophen-2-yl)amino)-4-oxo-4-(p-tolyl)but-2-enoic acid Chimica Techno Acta LETTER published by Ural Federal University 2021, vol. 8(4), № 20218411 eISSN 2411-1414; chimicatechnoacta.ru DOI: 10.15826/chimtech.2021.8.4.11 1 of 5 Synthesis of highly stable luminescent molecular crystals based on (E)-2-((3-(ethoxycarbonyl)-5-methyl-4-phenylthiophen-2- yl)amino)-4-oxo-4-(p-tolyl)but-2-enoic acid N.A. Zhestkij a, E.V. Gunina a, S.P. Fisenko b, A.E. Rubtsov c, D.A. Shipilovskikh d, V.A. Milichko ae* , S.A. Shipilovskikh ac* a: ITMO University, 197101 Kronverksky pr., 49, St. Petersburg, Russia b: A. V. Luikov, Heat and Mass Transfer Institute, NASB, 220072 Brovki st., 15, Minsk, Belarus c: Perm State University, 614068 Bukireva st., 15, Perm, Russia d: Perm National Research Polytechnic University, 614077 Komsomolsky pr., 29, Perm, Russia e: Universite de Lorraine, CNRS, IJL, F-54000 Nancy, France * Corresponding authors: v.milichko@metalab.ifmo.ru (V.A. Milichko), s.shipilovskikh@metalab.ifmo.ru (S.A. Shipilovskikh) This short communication (letter) belongs to the MOSM2021 Special Issue. © 2021, The Authors. This article is published in open access form under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Abstract The synthesis of (E)-2-((3-(ethoxycarbonyl)-5-methyl-4- phenylthiophen-2-yl)amino)-4-oxo-4-(p-tolyl)but-2-enoic acid was performed. This organic compound was used as a building block for the organic molecular crystals with highly stable photoluminescence at ambient conditions, which has been established during 10 years of exploitation. Keywords organic molecular crystal photoluminescence structure rigidity substituted 2,4-dioxobutanoic acids Gewald thiophenes Received: 09.12.2021 Revised: 14.12.2021 Accepted: 14.12.2021 Available online: 15.12.2021 1. Introduction Luminescent molecular crystals (MCs) based on organic molecules are the cornerstones of modern organic elec- tronics and luminescent technologies. The utilization of such crystals as active components of displays and lasers [1–5] significantly decreases energy consumption and makes such devices recyclable. Generally, organic crystals (OCs) are based on organic molecules packed in a specific order, where weak intermolecular interactions maintain the integrity of the crystal structure. For such a structure, the nature of luminescence is defined by intrinsic elec- tronic transitions of individual organic molecules and gen- eralized electronic states, which are directly determined by the type of molecular packing [6–8]. Demonstrating unprecedented efficiency, light spec- trum control [1, 3], scalability [2], flexibility [9] and en- hanced time of emission (phosphorescence) [5, 7, 10–12], MCs still suffer from aging and poor structural stability [8, 11, 13, 14] at ambient conditions (in air atmosphere and room temperature). The reasons for this are, on the one hand, the violation of the integrity of the organic mol- ecules during long-term excitation. This is due to the heat- and photoinduced destruction of certain chemical bonds. On the other hand, the distortion of the weak intermolecu- lar interactions under normal conditions disrupts the ag- gregation of molecules and negatively affects both the emission spectrum and the luminescence efficiency [13, 14]. What is important is that the latter effect is ob- served at lower excitation parameters at ambient condi- tions and, hence, more significantly affects the working capacity of corresponding organic devices. 2. Experimental 2.1. Chemical experiments The yields are given for the isolated products showing one spot on a TLC plate and no impurities detectable in the NMR spectrum. The identity of the products prepared by differ- ent methods was checked by comparison of their NMR spec- tra. 1H and 13C NMR spectra were recorded at 400 MHz for 1H and 100 MHz for 13C NMR at room temperature; the chemical shifts (δ) were measured in ppm with respect to http://chimicatechnoacta.ru/ https://doi.org/10.15826/chimtech.2021.8.4.11 https://orcid.org/0000-0002-8461-0804 https://orcid.org/0000-0002-8917-2583 http://creativecommons.org/licenses/by/4.0/ Chimica Techno Acta 2021, vol. 8(4), № 20218411 LETTER 2 of 5 the solvent (CDCl3, 1Н: δ = 7.26 ppm, 13C: δ = 77.16 ppm; [D6] DMSO, 1Н: δ = 2.50 ppm, 13C: δ = 39.52 ppm). The cou- pling constants (J) are given in Hertz. The splitting patterns of apparent multiplets associated with averaged coupling constants were designated as s (singlet), d (doublet), t (tri- plet), q (quartet), sept (septet), m (multiplet), dd (doublet of doublets) and br (broadened). The melting points were determined with a «Stuart SMP 30», the values are uncor- rected. Flash chromatography was performed on silica gel Macherey Nagel (40–63 m). The elemental analysis was performed on a Leco CHNS-932 instrument. The reaction progress was monitored by GC/MS analysis and thin layer chromatography (TLC) on aluminum backed plates with Merck Kiesel 60 F254 silica gel. The TLC plates were visualized either by UV radiation at a wavelength of 254 nm or stained by exposure to a Dragendorff’s reagent or potassium permanganate aqueous solution. All the reactions were carried out using dried and freshly distilled solvent. 2.2. Synthesis of (Z)-2-hydroxy-4-oxo-4-(p-tolyl)but- 2-enoic acid 3 A cooled mixture of 29.2 g (0.2 mol) diethyl oxalate and 26.8 g (0.2 mol) 1-(p-tolyl)ethan-1-one was slowly added with stirring to a solution of freshly prepared (0.4 mol) sodium methoxide in 100 ml of methanol. After one day, the precipitate that formed was dissolved in warm water (60 °С) and the solution was acidified with concentrated hydrochloric acid to pH = 3. The formed precipitate was filtered off and recrystallized from acetonitrile (Scheme 1). Scheme 1 Synthesis of (Z)-2-hydroxy-4-oxo-4-(p-tolyl)but-2-enoic acid 3 Beige crystals (36.3 g, 88%), m.p. 141–143 °С (141– 142 °С [15]). 1H NMR (CDCl3, 400 MHz) δ (ppm): 2.48 (s, 3H, Me), 7.16 (s, 1H, C=CH), 7.35 (m, 2H, HAr), 7.94 (m, 2H, HAr). 13C NMR (CDCl3, 100 MHz) δ (ppm): 21.8, 94.8, 128.0, 129.7, 129.8, 145.6, 161.6, 174.6. Found, %: C 64.05, H 4.93. C11H10O4. Calculated, %: C 64.08, H 4.89. 2.3. Synthesis of ethyl 2-amino-5-methyl-4- phenylthiophene-3-carboxylate 6 In a 100 ml three-necked flask equipped with a reflux con- denser, a dropping funnel, an internal thermometer and a magnetic stirrer a solution of 13.4 g (0.1 mol) of propiophe- none and 11.3 g (0.1 mol) of ethyl 2-cyanoacetate in 40 ml of ethanol was placed. To the resulting solution 3.2 g (0.1 mol) of finely ground sulfur was added. While stirring, 4 ml of morpholine was added dropwise to the resulting mixture, making sure that the reaction mixture did not overheat. After the end of the exothermic reaction, the mixture was heated in a water bath until the sulfur was completely dis- solved. After cooling the solution to 0 °C, ethyl 2-amino-5- methyl-4-phenylthiophene-3-carboxylate precipitated in the form of yellow crystals. The resulting product was filtered off and recrystallized from methanol (Scheme 2). Scheme 2 Synthesis of ethyl 2-amino-5-methyl-4- phenylthiophene-3-carboxylate 6 Yellow crystals (22.47 g, 86%), m.p. 91–93 °С (93 °С [16]). 1H NMR (CDCl3, 400 MHz) δ (ppm): 1.35 (t, J = 7.1 Hz, 3H, Ме), 2.30 (s, 3H, Ме), 4.29 (q, J = 7.1 Hz, 2H, СН2), 6.05 (s, 2H, NH2), 7.29 (m, 5H, HAr). Found, %: C 64.30, H 5.72, N 5.31. C14H15NO2S. Calculated, %: C 64.34, H 5.79, N 5.36. 2.4. Synthesis of (E)-2-((3-(ethoxycarbonyl)-5- methyl-4-phenylthiophen-2-yl)amino)-4-oxo-4-(p- tolyl)but-2-enoic acid 7 To a solution of 2.06 g (0.01 mol) of (Z)-2-hydroxy-4-oxo- 4-(p-tolyl)but-2-enoic acid 3 in 10 ml of ethanol was added 2.61 g (0.01 mol) of a solution of ethyl 2-amino-5-methyl- 4-phenylthiophene-3-carboxylate 6 in 10 ml of ethanol. Аfter the resulting solution had been heated to boiling, it was refluxed. The resulting saturated red solution was kept for 24 hours at –18 °C, then the formed precipitate was filtered off and recrystallized from ethanol (Scheme 3). The compound 7 was obtained according to the previously described method [17]. The new MCs and the old MCs were obtained by the same method and re- peated after 10 years. Red crystals (3.7 g, 82% «old MCs»), (3.6 g, 80% «new MCs»), m.p. 171–172 °С. 1H NMR (CDCl3, 400 MHz) δ (ppm): 0.87 (t, J = 7.1 Hz, 3Н, Me), 2.13 (s, 3H, Me), 2.39 (s, 3Н, Me), 4.00 (q, J = 7.1 Hz, 2Н, CH2О), 6.58 (s, 1H, C=CH), 7.20 (m, 2H, HAr), 7.36 (m, 5H, HAr), 7.92 (m, 2H, HAr), 12.74 (s, 1Н, NH). Found, %: C 66.84, H 5.21, N 3.10. C25H23NO5S. Calculated, %: C 66.80, H 5.16, N 3.12. 2.5. Optical experiment The single MCs were placed on a 6.45 cm2 (0.2 mm thick- ness) glass substrate under normal conditions (in air at- mosphere, room temperature, and 30% humidity). The absorption and PL spectra have been measured using a home-made confocal microscope setup [18, 19]. The single crystals have been irradiated by incoherent (halogen light source AvaLight-HAL-S-Mini, 300-900 nm spectral range, for absorbance) and coherent light (for PL) via 100x/0.9NA Mitutoyo objective. For PL measurement, femtosecond laser system (Laser Pharos PH1-SP-20W, 1030 nm pump, 220 fs pulse duration, 1 MHz repetition rate), associated with an optical parametric amplifier Or- pheus HP to emit 400, 450, and 500 nm (with 10 nm band width), has been utilized. Chimica Techno Acta 2021, vol. 8(4), № 20218411 LETTER 3 of 5 Scheme 3 Synthesis of (E)-2-((3-(ethoxycarbonyl)-5-methyl-4-phenylthiophen-2-yl)amino)-4-oxo-4-(p-tolyl)but-2-enoic acid The PL signal was collected in the reflection regime via the same objective and then analyzed using a confo- cal Raman Spectroscopy system HORIBA Labram with 600 g/mm diffraction gratings and a water-cooling camera ANDOR. The absorption spectra A for the single crystals have been obtained in the transmission regime by transmission T spectroscopy (A = 1 − T) under the assumption that signals reflected from the crystal sur- face and scattered on its defects were small compared with the transmission signal. 3. Results and discussion Here we demonstrate the synthesis of a highly luminescent MCs based on (E)-2-((3-(ethoxycarbonyl)-5-methyl-4- phenylthiophen-2-yl)amino)-4-oxo-4-(p-tolyl)but-2-enoic acid. The photoluminescent (PL) behavior of the old crystal has been verified over 10 years at ambient conditions. We discov- ered that aging of the crystals is accompanied by a 30 nm red shift of the absorption spectrum, while the shape and PL peaks positions have not changed after 10 years. Fig. 1 The absorption spectra for single MCs (a); optical images of the corresponding single MCs and PL images of the corresponding thin films on 1 inch2 glass, excited by 400 nm; scale bars, 100 m (b); PL spectra for MC_new and MC_old compounds excited by 400, 450 and 500 nm with corresponding 0.25, 1.5 and 2 W laser power (c, d) Chimica Techno Acta 2021, vol. 8(4), № 20218411 LETTER 4 of 5 The observed decrease in PL intensity by 30% can be explained by an enhanced self-absorption effect. Also, an analysis of PL signal versus pumping wavelength (400 to 500 nm) showed an increase in efficiency when the aged crystal was excited with a green light. These results pave the way for utilization of new MCs based on (E)-2-((3- (ethoxycarbonyl)-5-methyl-4-phenylthiophen-2-yl)amino)-4- oxo-4-(p-tolyl)but-2-enoic acid as promising extended-life materials for optical application under normal operating conditions. The optical analysis of the MCs is summarized in Fig. 1 in detail. As one can see, aging caused a 30 nm red shift in the absorption spectrum (Fig. 1a) and the disappearance of small interference beats for approximately the same MC thickness. This process can be described by a partial viola- tion of the long-range order in the crystal, which can cause an increase in absorption in the red region of the spectrum [20]. However, the shape of the PL spectrum and the positions of the peaks (610 and 660 nm) during aging were found to remain the same. This indirectly indicates both the molecular nature of the PL and the relative stabil- ity of the molecules packed in the MCs over time. In this case, the aging process is described by an approximately 30% decline in the integral PL intensity and a relative in- crease in the red tail by 10% (Fig. 1c, d). Also, we found that aging of the MCs causes a threefold increase in the intensity of PL excited by 500 nm (blue curves in Fig. 1c, d). This effect can be also explained by a red shift of the absorption band and, hence, an increase in the absorption coefficient for the old MC. Finally, the PL signal has been stimulated by extremely low laser power (0.25 W for 400 nm, 1.5 W for 450 nm, and 2 W for 500 nm) and ob- served up to 0.5 mW, confirming the MC structure rigidity with a change in the pump laser power by three orders of magnitude. 4. 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