ORIGINAL ARTICLE Facile Synthesis and Photophysical Characterization of New Quinoline Dyes Giovanny Carvalho dos Santos1 & Aloisio de Andrade Bartolomeu1 & Valdecir Farias Ximenes1 & Luiz Carlos da Silva-Filho1 Received: 18 August 2016 /Accepted: 19 October 2016 /Published online: 27 October 2016 # Springer Science+Business Media New York 2016 Abstract This paper describes the synthesis of new quinoline derivatives, molecules that has been long interest in the organ- ic and medicinal chemistry. Through the Multicomponent Reaction (MCR), an important tool in modern synthetic meth- odology, that generate products with good structural complex- ity, in addition to economy of atoms and selectivity, we provide easy access to the preparation of quinoline de- rivatives. The reactions were promoted by niobium pentachloride, as a Lewis acid. Subsequently, the synthesis of new aminoquinoline derivatives with good yields was per- formed using Pd/C and hydrazine. The photophysical investi- gations of quinoline derivatives show the substituent effect on the optical properties characterization was done by absorption and photoluminescencemeasurements with quantum yields of up to 83%, the presence of the amino group at position 6 at the quinoline backbone was crucial for obtaining these increased quantum yields. Results show that these molecules may have potential use for a variety of applications and mainly attracts attention because of its wide potential of applicability in op- toelectronic devices. Keywords Niobium pentachloride .Multicomponent reactions . Quinoline derivatives . Photoluminescence . Fluorescence quantum yields Introduction The development of new synthetic strategies for the efficient production of organic compounds is very important, especial- ly when it comes to compounds such as quinoline derivatives which have a variety of applications in many areas and are present in various natural products and drugs [1, 2]. Thus, this compound has been the subject of numerous research groups. Besides the great biological applicability of the quinoline de- rivatives, such as anti-inflammatory [3, 4], anti-cancer [5–9], anti-hypertensive [10–13], antibacterial [14–17] and antifun- gal [18, 19] agents, there are also several studies aiming to take advantage of their excellent mechanical properties. For example, their application in polymer chemistry, or- ganic electronics and optoelectronics [20–24]. Also, the aminoquinolines that act as fluorophores show potential performance as organic semiconductors [25]. In addi- tion, some derivatives of substituted quinolines have been used as ligands for the preparation of phosphores- cent complexes used in organic light-emitting diodes (OLEDs) [26–28]. The diphenylaminoquinolines have shown to be a promising luminescent organic material with emission in blue range [29, 30]. Other derivatives are presented as potential candidates for applications in fiber op- tics, photonics and light emitting diodes [31–34]. The litera- ture shows that quinoline backbone is found in several natural products [35]. For dye applications in OLEDs, it is known that a higher value of short-circuit current density (Jsc) is assigned to mol- ecules bearing a broad and intense absorption spectrum [36]. This is the case of molecules containing the quinoline back- bone as π spacer, where the increase in the energy conversion efficiency reaches 6.07 % [36]. Zhang et al. showed a great improvement in OLEDs luminescence efficiency and bright- ness when quinoline derivatives were used [37]. Electronic supplementary material The online version of this article (doi:10.1007/s10895-016-1954-5) contains supplementary material, which is available to authorized users. * Luiz Carlos da Silva-Filho lcsilva@fc.unesp.br 1 Laboratory of Organic Synthesis and Processes (LOSP), Department of Chemistry, Faculty of Sciences, São Paulo State University (UNESP), 17033-360, Bauru, São Paulo, Brazil J Fluoresc (2017) 27:271–280 DOI 10.1007/s10895-016-1954-5 http://dx.doi.org/10.1007/s10895-016-1954-5 http://crossmark.crossref.org/dialog/?doi=10.1007/s10895-016-1954-5&domain=pdf Some classical methods of synthesis are described in the literature to give support for new synthesis [38–68]. Because of this potential applicability of quinoline derivatives, many research groups have geared their efforts towards the development of new efficient and low cost synthetic methods. For the synthesis of quinoline derivatives, sev- eral types of catalysts may be used. It has also been shown that acid catalysts are superior to some of the basic types of reactions [44]. Therefore, in this work we describe the facile synthesis and the optical characterization of new nitroquinoline and aminoquinoline derivatives, compounds with potential appli- cation as dyes in organic electronic devices. Experimental Materials and Instrumentation All reactions were performed under air atmosphere, unless otherwise specified. Acetonitrile was distilled from calcium hydride. All commercially available reagents were used with- out further purification. The NbCl5 used was supplied by Companhia Brasileira de Metalurgia e Mineração (CBMM). Thin-layer chromatography was performed on 0.2 mmMerck 60F254 silica gel aluminum sheets, which were visualized with a vanillin/methanol/water/sulfuric acid mixture, molybdate or UV-365 nm irradiation. Bruker DRX 400 spectrometer was employed for the NMR spectra (CDCl3 solutions) using tetramethylsilane as internal reference for 1H and CDCl3 as an internal reference for 13C. A Bruker FTIR model VERTEX 70 was used to record IR spectra (neat). HRMS analyses were recorded in a micrOTOF (Bruker), with ESI-TOF detector working on positive mode. Absorption spectra in the UV-Vis region were obtained in an apparatus fromMolecular Devices (Model SpectraMax M2) using a quartz cuvette of 1 cm light path at room temperature. Fluorescence emission curves were obtained using a spectrophotometer BioTek microplate (Model Synergy H1). Quantum yields were analyzed by adjusting the solution absorption using the UV-Vis to ca. 0.05 at 325–393 nm wave- length, the output was measured using the luminescence spec- trophotometer at the same wavelength and comparing it to the known 9,10-diphenylanthracene standard using Eq. 1: Equation 1: Quantum yield calculation using 9,10- diphenylanthracene Φ f ¼ Φstdx Astd F AFstd x n2 n2std Φ is the fluorescence quantum yield, A is the absorption of the excitation wavelength, F is the area under the emission curve, and n is the refractive index of the solvents used. Subscript std. denotes the standard. The compounds were solubilized in ethanol and the concentration maintained at about 5 × 10−6 mol.L−1 to follow the protocol for analysis [69]. Synthesis General Procedure for the Synthesis of Nitroquinoline Derivatives To a solution of NbCl5 (50 mol%) in 7.0 mL of anhydrous acetonitrile, maintained at room temperature, under air atmo- sphere, we added a solution of p-nitroaniline (1.0 mmol), phenylacetylene (1.0 mmol) and benzaldehyde derivatives (2a–x) (1.0 mmol) in 3.0 mL of anhydrous acetonitrile. The reactionmixture was quenched with water (3.0 mL) after 96 h. The mixture was extracted with ethyl acetate (10.0 mL). The organic layer was separated and washed with saturated sodium bicarbonate solution (3 × 10.0 mL), saturated brine (2 × 10.0 mL), and then dried over anhydrous magnesium sulfate. The solvent was removed under vacu- um. The resulting mixture obtained was recrystallized in CH3OH. In some cases more than one recrystallization process was needed [70]. General Procedure for the Synthesis of Aminoquinoline Derivatives An ethanol suspension (20.0 mL) of quinoline derivative (4a–x) (1.0 mmol) was heated to 50.0 °C in the pres- ence of 10%Pd/C and 2 .00 mL of hydraz ine monohydrate was added over 30 min to this suspension. The reaction mixture became clear as the reaction proceeded. It was kept at reflux for another 12 h. Upon completion of reaction, the mixture was filtered over Celite twice to remove the Pd/C catalyst. Ethanol was removed under reduced pressure. The crude product was recrystallized from isopropanol. Note: the crystals grew slowly [25]. Results and Discussion Synthesis and Characterization Recently, our research group described the synthesis of quin- oline derivatives through multicomponent reaction (MCR) between arylaldehydes, anilines and alkynes catalyzed by Niobium Pentachloride [71]. In this work, we describe the synthesis of nitroquinoline derivatives by MCRs, using the promoter NbCl5, followed by the reduction of nitro groups for obtaining the aminoquinoline derivatives. 272 J Fluoresc (2017) 27:271–280 The MCRs were conducted between p-nitroaniline (1) (1.0 eq.), benzaldehyde derivatives (2a–x) (1.0 eq.) and phenylacetylene (3) (1.0 eq.) under air atmosphere, room temperature, constant stirring and using anhydrous ace- tonitrile (CH3CN) as solvent. NbCl5 was used in the proportion of 50 % for each mol of benzaldehyde derivative used. Reduction of nitro group in the nitroquinoline derivatives was conducted with hydrazine monohydrate in the pres- ence of 10 % Pd/C (Scheme 1). The results are summarized in Table 1. The MRCs in the presence of NbCl5 showed good yields for the production of nitroquinoline derivatives, regardless of the benzaldehyde derivatives used. The results were better using derivatives containing electron withdrawing substitu- ents, which improve the coordination of the benzaldehyde with the Lewis acid (NbCl5) and facilitate the subsequent ad- dition of the amine group of p-nitroaniline to the carbonyl group of the benzaldehyde [72]. The reactions with m- nitrobenzaldehyde and p-nitrobenzaldehyde (4v and 4x) showed lower yields due to the poor solubility of these com- pounds. To support the good results of the first step, in Table 2 Scheme 1 Synthesis of aminoquinolines 5a–x in two steps Table 1 Benzaldehyde derivatives utilized and reaction yields Aldehyde R1 R2 R3 R4 R5 Nitroquinoline yield(%)a Aminoquinoline yield(%)a 2a H H H H H 93 (4a) 92 (5a) 2b F H H H H 87(4b) 91 (5a) 2c Cl H H H H 92(4c) 94 (5a) 2d Br H H H H 81(4d) 93 (5a) 2e H F H H H 75(4e) 84 (5a) 2f H Cl H H H 98(4f) 92 (5a) 2g H Br H H H 79(4 g) 90 (5a) 2h H H F H H 98(4 h) 84 (5a) 2i H H Cl H H 86(4i) 80 (5a) 2j H H Br H H 98(4j) 87 (5a) 2k OCH3 H H H H 93 (4 k) 74 (5 k) 2l H OCH3 H H H 79 (4 l) 90 (5l) 2m H H OCH3 H H 71 (4m) 85 (5 m) 2n CH3 H H H H 83(4n) 65 (5n) 2o H CH3 H H H 78(4o) 70 (5o) 2p H H CH3 H H 82 ) (4p) 72 (5p) 2q H H C6H5 H H 98 (4q) 92 (5q) 2r H H N(CH3)2 H H 70 ) (4r) 91 (5r) 2s H H C(CH3)3 H H 78(4s) 93 (5s) 2t CH3 H CH3 H CH3 79 (4t) 89 (5t) 2u H H SCH3 H H 75 (4u) 91 (5u) 2v H H NO2 H H 56(4v) 84 (5v) 2x H NO2 H H H 54(4x) 82 (5x) a Yields of isolated products after recrystallization J Fluoresc (2017) 27:271–280 273 are shown the yields when compared to others catalysts [24, 73–80], we note that the NbCl5 promotes MCRs with good yields in milder conditions. The efficiency of this catalyst is highlighted by the conduction of reactions at room tempera- ture and pressure, and air atmosphere. These factors could reduce the cost of large-scale production. The second reaction step, the reduction of the nitro group in all nitroquinoline derivatives, was successfully obtained and presented good yields. An exception was the halogenated nitroquinoline derivatives, in which the reductive reactional condition also resulted in the dehalogenation of the products, obtaining only the compound 5a as product. An explanation for this fact is the small amount of reagents used in our exper- iments, a condition different from those described in the liter- ature. Indeed, Pd-catalyzed hydrogenolysis of carbon-halogen bonds with hydrazine as a hydrogen donor is a long known method, but it is usually performed with large amounts of catalyst and/or reducing agent [81–85]. In short, this study has shown that the conversion of polysubstituted quinolines with fluorides, bromides and chlorides into corresponding quinoline dehalogenated can be efficiently performed with ethanol in the presence of hydrazine as hydrogen donor and catalytic amounts of Pd/C. The chemical structures of the resulting nitroquinoline and aminoquinoline deriva- tives were confirmed by 1H NMR, 13C NMR, IR and HRMS spectra. Photophysical Properties As it is well known, substituents have a key effect on the properties of fluorophores. In this study, we add different types of substituents at various positions to examine the ef- fects on absorption, emission and fluorescence quantum yield. The photophysical characteristics were investigated in CH3CH2OH solutions. UV-Vis Absorption Properties The data of UV-Vis absorption are summarized in Table 3 in 10−3 mol. L−1EtOH solution. The absorption spectra of the nitroquinoline derivatives in ethanol are characterized by strong absorption peaks centered Table 2 Comparison of the data reported in literature and the products obtained in the synthesis of 6-nitro-2,4-diphenylquinoline (4a) Catalyst Solvent Temperature (C°) Time (min) Yield (%) 1 NbCl5 CH3CN r.t. 5760 93 2 YCl3 ---- 180 (MW) c 8 63 3 K5CoW12O40.3H2O ---- 90 (MW) 10 90 4 Fe(OTf)3 ---- 100 180 69 5 In/HCl H2O Reflux 1080 77 6 H2SO4 CH3COOH Reflux 240 93 7 K-10 a ---- 100 (MW) 10 72 8 Zn(OTf)2 [hmim]PF6 b 90 120 90 9 In(OTf)3 ---- 110 (MW) 300 87 10 FeCl3 CH3CN Reflux 45 61 aMontmorillonite [(Na,Ca)0,3(Al,Mg)2Si4O10(OH)2.nH2O] b 1-hexyl-3-methylimidazolium hexafluorophosphate cMW- Microwave irradiation Table 3 Maximum absorption wavelength (λmax) for quinoline derivatives Compound λmax(nm) ethanol Compound λmax(nm) ethanol 4a 335 5a 375 4b 326 5a 375 4c 375 5a 375 4d 332 5a 375 4e 331 5a 375 4f 332 5a 375 4g 332 5a 375 4h 337 5a 375 4i 341 5a 375 4j 341 5a 375 4k 343 5k 370 4l 341 5l 373 4m 357 5m 373 4n 379 5n 370 4o 341 5o 373 4p 356 5p 373 4q 360 5q 373 4r 393 5r 388 4s 344 5s 373 4t 358 5t 370 4u 369 5u 376 4v 332 5v 385 4x 325 5x 373 274 J Fluoresc (2017) 27:271–280 at 250–280 nm and 325–393 nm probably due to π-π* and n-π* transitions, which are characteristics of conjugated quin- oline backbone and phenyl side chains with or without sub- stituents. The absorption spectra of ethanolic solutions (10−3 mol.L−1) of nitroquinoline derivatives are depicted in Fig. 1. Analyzing the effect of the substituent groups using 4a (6- nitro-2,4-diphenylquinoline) as the reference compound, it can be observed a general tendency by which the compounds containing electron withdrawing substituents showed lower values of λmax when compared to electron donors. We also observed that electron withdrawing substituents have greater absorption when in para position. Electron withdrawing groups such as nitro (NO2), in meta or para position showed hypsochromic shift. The same was observed in the halogen substituents (F, Cl and Br) in ortho and meta positions, except chlorine in ortho position. A bathochromic shift was observed for these halogens in para. On the other hand, electron donor Fig. 2 UV-Vis absorption of aminoquinoline derivatives (5a–x) in ethanol Fig. 1 UV-Vis absorption of nitroquinoline derivatives (4a–x) in ethanol J Fluoresc (2017) 27:271–280 275 substituents, such as methoxy (341–375 nm) and methyl (341–379 nm) in any position, tert-butyl group (344 nm) and thioether group (369 nm), also exhibited a bathochromic shift. The compound that showed higher λmax was 4r (393 nm) having dimethylamine as substituent, an electron donating group. This phenomenon may be explained taking into account the presence of a pair of non-bonding electrons that are capable of interacting with π electrons of the aromatic ring. Phenyl substituent had a bathochromic shift (360 nm) because it has effectively extended the conjugation of the molecule. The change of electron withdrawing groups (NO2) by the electron donating group (NH2) had profound influence in the spectral properties. As it can be observed, a difference of 40 nm was seen between molecule 4a and 5a, a bathochromic shift due to the change of the electron withdrawing groups (NO2) by the electron donating group (NH2) at position 6 at the quinoline backbone. In general, due to the strong influence of amino group at position 6, no significant alteration was observed by changing the substituents on the phenyl ring in position 2 (370 nm to 376 nm) (Fig. 2). Molecules 5n and 5r were exceptions when compared to 4n and 4r, exhibiting a slight hypsochromic shift. There was a large bathochromic shift in molecules with the amino groups 5v (385 nm) and 5x (373 nm). Again, para position showed higher values com- pared to other positions. Emission Fluorescence Properties Figure 3 shows the fluorescence emission of quinoline derivatives. Similar to the absorption effects, the fluorescence behavior was also affected by the substituents and their positions. Molecules 4a–x mostly show a similar behavior in the shift. When the difference in fluorescence intensity was analyzed, molecules 4n and 4g presented greater intensity. Fluorescence emission spectra of derivatives 5a–x exhibited high intensity and showed similar values even by altering the substituents. a b Fig. 3 a Fluorescence emission of nitroquinoline derivatives (4a–x) in ethanol. b Fluorescence emission of aminoquinoline derivatives (5a–x) in ethanol Table 4 Photophysical data obtained fromUV-Vis absorption and fluo- rescence emission of quinoline derivatives Compound λem Δλst Φfx (%) Compound λem Δλst Φfx (%) 4a 410 75 0.25 5a 470 95 65.33 4b 410 84 1.50 5a 470 95 65.33 4c 430 55 0.58 5a 470 95 65.33 4d 415 83 2.05 5a 470 95 65.33 4e 415 84 0.76 5a 470 95 65.33 4f 415 83 1.10 5a 470 95 65.33 4g 430 98 0.91 5a 470 95 65.33 4h 415 78 0.37 5a 470 95 65.33 4i 405 64 0.37 5a 470 95 65.33 4j 395 54 0.23 5a 470 95 65.33 4k 415 72 0.43 5k 466 93 69.92 4l 425 84 0.18 5l 469 96 69.57 4m 430 73 0.24 5m 469 96 70.18 4n 430 51 3.15 5n 466 93 53.64 4o 435 94 0.80 5o 469 96 80.32 4p 435 79 0.50 5p 469 96 59.53 4q 445 85 0.35 5q 466 96 72.92 4r 450 57 0.45 5r 475 87 51.70 4s 430 86 0.26 5s 469 96 73.03 4t 430 72 0.32 5t 466 96 49.16 4u 425 56 0.57 5u 472 99 83.48 4v 415 83 0.23 5v 472 87 24.15 4x 410 85 0.25 5x 469 90 14.66 276 J Fluoresc (2017) 27:271–280 These high values are evidenced when compared to several standards of fluorescence as 9,10-diphenylanthracene which was the standard used for measures (Φfx = 90 % in ethanol) [70]. These results confirmed the strong influence of the ami- no group in the derivatives. The photophysical data of all synthetized compounds are showed in Table 4. The fluorescence quantum yield (Φf) is one of the most fundamental and important properties for materials with po- tential application in fluorescence imaging, optical devices, analysis and biosensing [86]. Here, we observed that the mol- ecules 4a–x with nitro group in position 6 do not present significant values ofΦf. In contrast, the presence of the amino group in this position was crucial for obtaining increased Φf, as can be seen for aminoquinolines (5a–x) [25]. These high values of fluorescence quantum yield of fluorescence con- firmed the importance of the amino group and demonstrated that fluorescence is not detected in aromatic compounds con- taining the nitro group, as seen in molecules 4a–x. This is likely due to the existence of transitions n → π*, causing an efficient intersystem crossing process, and the great speed of the internal conversion processes S0 → S1 [87]. Values of Stokes shifts are between 51 and 98 nm for com- pounds containing the nitro group (4a–x) and 87 to 100 nm for the aminoquinolines (5a–x). This shows energy loss in the excited state due to rearrangements or structural changes of molecules. In Fig. 4, compounds irradiated with ultraviolet light showed blue fluorescence for the aminoquinolines. Conclusion In conclusion, NbCl5 proved to be an excellent catalyst for MCRs. The reactions were conducted in mild conditions, with low production cost and with good yields. The optical prop- erties were dependent on the substituents, showing lower values of maximum absorption for the electron withdrawing substituents when compared to electron donors. The substitu- ents positions were also important. Compounds 4a–x exhibit- ed low values ofΦf and aminoquinolines (5a–x) showed high values. In this context, compounds like 5r are potential can- didates for organic electronic devices. The results also show that the diamine quinoline derivatives 5v and 5x could be used as asymmetric monomers for the preparation of high perfor- mance polymers [25]. Due to the unquestionable importance of quinoline derivatives in several areas as the development of pharmaceuticals, dyes, chemical polymers and in electronic and organic optoelectronics devices, the MCRs methodology developed here is of great interest for those who study these molecules. Acknowledgments The authors would like to thank Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) (Procs. 2013/08,697–0, 2012/24,199–8, 2015/00,615–0 and 2016/01,599–1), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) (Proc. 302,753/2015–0) and Pró-Reitoria de Pesquisa (PROPe-UNESP) for their financial support. 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Thomson Learning Inc., Bellingham (USA) 280 J Fluoresc (2017) 27:271–280 Facile Synthesis and Photophysical Characterization of New Quinoline Dyes Abstract Introduction Experimental Materials and Instrumentation Synthesis General Procedure for the Synthesis of Nitroquinoline Derivatives General Procedure for the Synthesis of Aminoquinoline Derivatives Results and Discussion Synthesis and Characterization Photophysical Properties UV-Vis Absorption Properties Emission Fluorescence Properties Conclusion References