ORIGINAL ARTICLE Theoretical-Experimental Photophysical Investigations of the Solvent Effect on the Properties of Green- and Blue-Light-Emitting Quinoline Derivatives Giovanny Carvalho dos Santos1 & Roberta Oliveira Servilha1 & Eliézer Fernando de Oliveira2 & Francisco Carlos Lavarda2 & Valdecir Farias Ximenes1 & Luiz Carlos da Silva-Filho1 Received: 12 January 2017 /Accepted: 26 April 2017 /Published online: 11 May 2017 # Springer Science+Business Media New York 2017 Abstract This paper describes the investigations on the solvatochromic effect and the photophysical properties of quinoline derivatives, compounds with potential applicability in optoelectronic devices. Using an experimental and theoret- ical approach, the effect of the solvent and the insertion of the phenyl, nitro, amino and dimethylamino group in the quino- line backbone were investigated. The use of Density Functional Theory (DFT) calculations provided the bases for the understanding of the energetic transitions observed in the absorption and fluorescence experiments. In general, it was observed a change in the wavelength of maximum absorption and fluorescence quantum yield of the studied compounds caused by the substituents in the quinoline core. This effect was correlated with the solvent dielectric constants. Keywords Niobium pentachloride . Quinoline derivatives . Solvatochromic effect . DFT . Photoluminescence Introduction Over the last years, the Science community has turned its atten- tion to luminescent organic and organometallic compounds which have aroused great interest due to their potential applica- tions as organic solar cells (OSCs), biomolecular labels, organic light-emitting diodes (OLEDs), molecular probes and switches, laser dyes, and organic π-conjugated in solution-processed bulk- heterojunction (BHJ) organic photovoltaic devices [1]. Molecules such as quinolines have been of great interest in the fields of organic and medicinal chemistry since there is a large number of natural products and drugs that present this heterocyclic moiety in their structure. As a result, their use has been reported in several medical applications [2, 3]. In addi- tion, quinoline derivatives have also been used in polymer chemistry, organic electronics and optoelectronics. This is be- cause quinoline derivatives have excellent mechanical prop- erties, generating highly efficient materials in the electron transport [4], presenting crucial characteristics for their use in OLEDs, such as high electron mobilities, good thermal stability, high photoluminescence efficiencies and good film forming properties [5]. Therefore, a significant improvement in luminescence efficiency and brightness in OLEDs is ob- served when molecules contain quinolines [6]. Predominant advantages can be attributed in the competitive nature of small molecules in relation to the polymeric materials, including well-defined molecular structures, good flexibility, light weight, low cost, simple synthesis and fabrication-process- ing, an easier purification process and a better reproducibility [7]. Phenyl quinoline compounds are known to have an excel- lent photochemical stability even under high intensity laser irradiation [8]. We also know that a broad, intense absorption spectrum with molecules bearing the quinoline nucleus leads to a higher amount of short-circuit current density (Jsc). In addition, there is the possibility of modulating the emission wavelength of such compounds by introducing functional groups and/or extending the conjugation around the core [8]. The donor-acceptor (D-A) concept for band gap reduction was proposed by Havinga et al. in 1992 [9]. Through simple modifications, donors and acceptors groups can be inserted in the quinoline backbone. This makes this class of molecules * Luiz Carlos da Silva-Filho lcsilva@fc.unesp.br 1 Department of Chemistry, São Paulo State University (UNESP), School of Sciences, 17033-360, Bauru, São Paulo, Brazil 2 Department of Physics, São Paulo State University (UNESP), School of Sciences, 17033-360, Bauru, São Paulo, Brazil J Fluoresc (2017) 27:1709–1720 DOI 10.1007/s10895-017-2108-0 http://crossmark.crossref.org/dialog/?doi=10.1007/s10895-017-2108-0&domain=pdf important compounds to be studied more thoroughly for fu- ture applications in the field of organic electronics that con- tinues to evolve [10]. Yune and co-workers prepared the binder-free TiO2 colloidal pastes using quinoline backbone to produce robust photoanodes for dye-sensitized solar cells (DSSC) [11]. Bulkier heterocyclic bases as quinoline derivatives can also be tested for this purpose, but we need to know the influence of substituents in the photoproperties at the molecule. As it can be seen, substituents have a key effect on the properties of fluorophores. Thus, in this paper we report the photophysical investigation of the solvent effect on quinoline derivatives, synthesized through multicom- ponent reaction (MCR) among arylaldehydes, anilines and al- kynes catalyzed by Niobium Pentachloride, as described recent- ly by our research group [12]. In addition, to better understand the photophysical properties of substituted quinoline deriva- tives, computational simulations using the Density Functional Theory (DFT) were performed. Experimental Materials and Instrumentation The complete synthesis and full spectral characterization (NMR, MS, IR and other techniques) of all studied com- pounds have been reported previously [12]. All reactions were performed under air atmosphere, unless otherwise specified. Acetonitrile was distilled from calcium hydride. The quinoline backbone (compound 1) is commercially available in Sigma- Aldrich. 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. UV-Vis absorption spectra were obtained in a SpectraMax M2 spectrophotometer (Molecular Devices, USA) using a 1.0 cm light path quartz cuvette at room temperature. Fluorescence emission spectra were ob- tained using a Synergy2 Multi-Mode reader (BioTek, USA). Quantum yields were analyzed by adjusting the solution absorption using the UV-Vis to ca. 0.05 at 272–388 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 Quantum yield calculation using 9,10-diphenylanthracene: Φ f ¼ Φstdx Astd F AFstd x n2 n2std ð1Þ 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 eth- anol and the concentrat ion mainta ined at about 5 × 10−6 mol.L−1 to follow the protocol for analysis [13]. Computational Details All theoretical calculations were performed with GAUSSIAN09 program [14]. Ground state geometries were fully optimized by Density Functional Theory (DFT), employing the Becke three-parameter Lee − Yang − Parr exchange-correlation functional (B3LYP) [15] and the basis set functions 6-31G(d) [16]. The solvation effects were simu- lated by the Polarizable Continuum Model (PCM) [17, 18]. Today, there are large amounts of available functionals within the DFT theory and our option for the B3LYP is based on the fact that it has already been successfully used in other studies with quinoline derivatives [19–24]. The equilibrium geome- tries were confirmed by vibrational spectrum calculations since no imaginary frequencies were found. Results and Discussion The Solvent Effect on UV-vis Absorption of Quinoline and Its Derivatives Firstly, to investigate the solvent effect in quinoline 1 and its derivatives 2, 3 and 4 (Fig. 1), the UV-Vis absorption of these Fig. 1 Quinoline and derivatives 1710 J Fluoresc (2017) 27:1709–1720 compounds was examined in various solvents with different polarities. This investigation was necessary, due the fact that electron charge distribution and the twist angle of the mole- cules can be explained by the solvent effect, mainly because of stacking alignment and intra-charge transfer of compounds [25]. This evaluation is important to study applications in optoelectronic devices [26]. Aiming to study the effect of the insertion of substituents in the quinoline moiety, the measurements of UV-Vis absorption in different solvents were performed to know the influence of the solvent, so it would not limit our explanation of the change in the values only in connection with the twist angle of the substituents. Figure 2 shows the UV-Vis absorption spectra of quinoline 1. It can be seen in Fig. 2 that the solvent altered the wave- length of maximum absorption (λmax) of 1. However, by com- paring only the dielectric constant values of the solvent, it was not possible to find out a pattern for the alteration in λmax. As well-established in the literature, the absorption phenomenon depends on factors that cause changes in the electronic struc- ture of molecules such as viscosity, polarity and, therefore, solubility (Table 1) [27]. In Fig. 2, we hid the first absorption band because of a strong influence of the solvents cut off wavelength. The second and third bands have differences in shift and intensity. These bands show π-π* and n-π* transi- tions. These transitions are sensitive to solvent polarity, but in the quinoline backbone (1), they did not exhibit significant changes in the λmax. Although it is not the only parameter responsible for chang- es in values, a bathochromic shift by increasing the solvent polarity indicates that the ground state tends to be better sol- vated by polar solvents. This behavior was observed with hexane (λmax = 261 nm) and DMSO (λmax = 274 nm), which have a difference of 13 nm in the λmax. It can be said that the ground state is better solvated by solvents with higher polarity in this compound [27]. 220 240 260 280 300 320 0,0 0,2 0,4 0,6 0,8 1,0 1,2 N 1,4 1,6 A b s o r p ti o n Wavelength (nm) Hexane Ethanol THF DMSO Chloroform Acetonitrile Dichloromethane 1 Fig. 2 UV- Vis absorption of 1 in various solvents, 10−3 mol.L−1 Table 1 Maximum wavelength of the quinoline in some solvents ordered by dielectric increasing dielectric constant Solvent Relative Polaritya,b Dipole Moment Viscosity Dielectric Constant λmax (nm) of 1 Hexane 0,009 NP 0,00 0,33 1,88 261 CHCl3 0,259 NP 1,04 0,57 4,81 270 THF 0,207 PA 1,75 0,55 7,5 269 DCM 0,309 PA 1,60 0,44 9,1 270 EtOH 0,654 PP 1,69 1,20 30 272 CH3CN 0,460 PA 3,92 0,37 37,5 268 DMSO 0,444 PA 3,96 2 46,7 274 aValues of relative polarity were taken from the solvent displacement measurements in the absorption spectrum and were taken from the book of Christian Reichardt, Solvents and Solvent Effects in Organic Chemistry, Wiley-VCH Publishers, 3. ed., 2003. b NP – Non-Polar; PA - Polar Aprotic; PP - Polar Protic J Fluoresc (2017) 27:1709–1720 1711 Aiming to study the effect of the insertion of the phenyl, nitro, amino and dimethylamino groups in the quinoline moi- ety, the measurements of UV-Vis absorption in ethanol for compounds 2, 3 and 4 were performed to observe the λmax shift (Fig. 3). The substituents at the quinoline backbone caused a change in the electron charge distribution and consequently a shift in the λmax. The values observed were: quinoline (1) λmax = 272 nm, nitroquinoline (2) λmax = 335 nm, aminoquinoline (3) λmax = 375 nm and dimethylamine quinoline (4) λmax = 388 nm. When compared to the quinoline core (1), the insertion of phenyl groups increased the electron delocalization and the consequent bathochromic shift, which could be an important property for applications of these molecules as dyes. The ami- no group and the dimethylamine group also caused a red shift. These groups with free electrons contribute toward the mobil- ity of electrons in the structure. Although the phenyl substitu- ents reduced the structural rigidity, this molecule still present- ed a high fluorescence quantum yield which was significantly higher compared to the parent molecule (1) [12]. 400300 0,0 0,2 0,4 0,6 0,8 1 2 1,0 1,2 1,4 Wavelength (nm) A b s o r p ti o n 1 2 3 4 3 4 Fig. 3 UV- Vis absorption of quinoline, nitroquinoline, aminoquinoline and dimethylamine quinoline, concentration = 10−3 mol.L−1 400 450 500 550 600 650 0 100 200 300 400 500 1 4321 N O N2 H N N 2 H N N N 2 N 2 3 4 In te n s it y ( u .a ) Wavelength (nm) Fig. 4 Fluorescence emission of quinoline and derivatives in ethanol 1712 J Fluoresc (2017) 27:1709–1720 The fluorescence quantum yield and Stokes shift of nitroquinoline 2, aminoquinoline 3 and dimethyllamine quin- oline 4 were also studied (Fig. 4). The results showed that quinoline backbone (1) do not exhibit fluorescence, nitroquinoline showed 0.25%, aminoquinoline 65.33% and dimethylamino quinoline 57.70% of fluorescence quantum yield. This demonstrates the importance of inserting the phe- nyl groups to increase the conjugation of the molecule and consequent emission, but also demonstrates the key effect of the amino group on the quinoline backbone. The optical data for 1–4 in ethanol solution are summarized in Table 2. Theoretical-Experimental Solvent Effect in Quinoline Derivatives 3 and 4 As showed in section 3.1, substituents had a key effect on the fluorescent properties of the quinoline derivatives. Hence, for a better understanding of their effects, theoretical calculations were used in this section to understand the effect of the amino (compound 3) and dimethylamine (compound 4) substituents. Compound 3 was chosen because it does not contain any substituent in the phenyl ring in position 2 and it is useful to know the effect of the substituent group. Compound 4 was chosen by the known effect of the dimethylamino substituent, as well as the result achieved by its absorption. This group can better contribute to the resonance structures of the quinoline Table 2 Photophysical data of quinoline derivatives Compound λmax λem Δλst Φfx (%) 1 272 --- --- --- 2 335 410 75 0.25 3 375 470 95 65.33 4 388 475 87 51.70 340 360 380 400 420 440 0 1 b a A b s o r p ti o n Wavelength (nm) Hexane Toluene Et2O Dioxane THF EA CHCl3 DCM DCE Acetone DMF DMSO ACN EtOH MeOH H N2 N Fig. 5 a UV-Vis absorption of aminoquinoline 3 in various solvents. b Solvatochromism shown by emission of compound 3 under UV-lamp (365 nm) J Fluoresc (2017) 27:1709–1720 1713 moieties upon excitation. Thus, these molecules proved to be good candidates for further photophysical study. We decided to study compound 4 as an unsymmetrical Donor-Acceptor- Donor (D-A-D) type of organic small molecule, with quino- line acting as an electron-acceptor moiety. As demonstrated in the literature, these groups contribute toward the mobility of π-electrons of the quinolines and cause different spectroscopic behavior [28]. Once these molecules could increase their absorption band through intramolecular charge transfer (ICT), and consequently encompass a larger region of the solar spectrum, then they would be able to achieve a high PCE in organic solar cells devices [29]. A variation in the intensity, shape and peak position of absorption spectra can be used to know the interactions be- tween solute and solvent. When the compound is dissolved in different solvents and the phenomenon of change in color occurs, as well as their absorption and emission spectrum, this effect is called solvatochromism. Compounds with a large change in their permanent dipole moment upon excitation exhibit a strong solvatochromism. In compounds 3 and 4, this was observed in different intensity [30]. Compound 3 is a yellow solid and 4, a dark orange solid. They showed green to blue emission under UV-lamp (365 nm) (Figs. 5 and 6). The UV-visible absorption (Figs. 5 and 6) was recorded in different solvents at room temperature using 10−6 M solutions. Compound 3 showed λmax in the range of 355–390 nm while compound 4 was in the range of 378– 403 nm with shoulder in the range of 474–483 nm in some solvents (Table 3). Some dependence in the absorption spectra indicates a good interaction with the solvent molecule and ground state of quinoline derivatives. The solvatochromic effect is less pronounced in compound 3. Whereas, in compound 4, the non-bonding electrons on the dimethylamino group partici- pate in the π-electron of quinoline skeleton and decrease the value of dipole moments in ground and excited states. 350 400 450 500 550 0,0 0,5 A b s o r p ti o n Wavelength (nm) Hexane Toluene Et2O Dioxane THF EA CHCl3 DCM DCE Acetone DMF DMSO ACN EtOH MeOH b a H N2 N N Fig. 6 a UV-Vis absorption of aminoquinoline 4 in various solvents. b Solvatochromism shown by emission of compound 4 under UV-lamp (365 nm) 1714 J Fluoresc (2017) 27:1709–1720 The amine and the dimethylamine group act as donator units and the quinoline backbone act as an acceptor building unit, making this an asymmetric D-A-D system. The extended absorption via n-π* intramolecular D-A-D charge transfer is a possible explanation for the shoulder ranging from 473 to 483 nm. Compound 4 (Fig. 5) shows intramolecular charge transfer (ICT) observed in solvents THF, ACN, CHCl3, DCM and DCE. This effect that the chlorinated solvents CHCl3, DCM and DCE cause happens because they best solvate in the excited state. Chlorine, which is strongly electronegative, has an interaction with the positive charges formed after the excitation of the molecule in the dimethylamine group. THF and ACN also stabilize the positive charge of the molecule. The fluorescence of these dyes in most solvents is so intensive that it is visible to naked eyes in the irradiation under normal daylight. In order to evaluate theoretically and experimentally the solvent-dependent electronic structures in the quinolines and correlate the results, computations were performed using DFT/ PCM/B3LYP/6-31G(d), as described in Section 2.3. The exper- imental and theoretical results of the λmax were described in Table 3. The geometries of the optimized structures in vacuum of 3 and 4 are depicted in Fig. 7. As it can be seen, the main quinoline structure is planar with the attached benzenes a little twisted. Both, the lowest unoccupied and the highest occupied molecular orbitals (LUMO and HOMO, respectively), are delocalized through the molecule, indicating a good overlap between HOMO and LUMO, which favor the optical transi- tions between ground and excited states. In HOMO orbitals the π-electrons are able to delocalize over the entire quinoline backbone, including amine group and phenyl group at position 2 (terminal substituents). In LUMO orbital the π-electrons are delocalized extensively over whole π-conjugated systems, including the phenyl group at position 4. This delocalization is important to lower the HOMO-LUMO gap, as it can be seen in Fig. 7 and in Tables 4 and 5 where all the computational results are summarized. The comparative increase and decrease in the energy of the highest occupied molecular orbital (HOMO’s) and lowest un- occupied molecular orbitals (LUMO’s) give a qualitative idea of the excitation properties. Lower LUMO and higher HOMO levels were observed. On the basis of the reduction potentials, the lowest unoc- cupied molecular orbital (LUMO) energy levels of molecule 3 were calculated to be in the range of −1.37 to −1.60 eVand of molecule 4 in the range of −1.14 to −1.47. These results prove that these molecules have high electron-accepting ability. When compared to tris(8-hydroxyquinolinate)-aluminium (Alq3) (−2.3 V), which is one of the most widely used struc- tures in electron-transporting materials, the LUMO energy Table 3 Maximum wavelength of 3 and 4 in various solvents Solvent Relative polarity a,b Dipole moment Viscosity Dielectric constant (20 °C) Compound 3 Compound 4 Exp. λmax (nm)c Calc. λmax (nm)d Exp. λmax (nm)c Calc. λmax (nm)d Hexane 0.009 NP 0 0.31 1.9 365 364.18 378 401.24 Toluene 0.099 NP 0.4 0.59 2.4 370 366.34 388 403.61 THF 0.207 PA 1.75 0.55 7.6 355 374 385 (483) 413.14 Dioxane 0.164 NP 0.4 1.37 2.2 378 365.69 389 402.83 ACN 0.460 PA 3.2 0.38 37.5 381 378.14 392 (481) 418.38 CHCl3 0.259 NP 1.1 0.57 4.8 366 371.53 387 (476) 410.03 DCM 0.309 PA 1.8 0.44 9.1 369 374.8 389 (482) 414.14 DCE 0.327 PA 1.8 0.79 10.4 371 375.28 390 (474) 414.76 EA 0.228 PA 1.7 0.45 6.0 375 372.92 388 411.79 Acetone 0.355 PA 2.9 0.36 20.6 378 377.25 391 417.26 MeOH 0.762 PP 1.7 0.59 32.6 374 378.02 391 418.24 DMF 0.386 PA 3.8 0.92 36.7 386 376.87 400 418.44 DMSO 0.444 PA 3.96 2.24 46.6 390 377.09 403 418.76 EtOH 0.654 PP 1.7 1.1 22.4 375 377.61 388 417.71 Et2O 0.117 NP 1.3 0.24 4.3 378 370.85 389 409.17 a Values of relative polarity were taken from the solvent displacement measurements in the absorption spectrum and were taken from the book of Christian Reichardt, Solvents and Solvent Effects in Organic Chemistry, Wiley-VCH Publishers, 3. ed., 2003. b NP – Non-Polar; PA - Polar Aprotic; PP - Polar Protic c Experimental values d Theoretical values J Fluoresc (2017) 27:1709–1720 1715 levels of 3 and 4 shifted positively [31]. The HOMO values of 3 were calculated to be in the range of -5.24 to −5.39 eV, and of 4 in the range of −4.68 to −4.91 eV. Bandgap energy is an essential tool for constructing optical devices. The energy gap was calculated to be in the range of 3.78 to 3.86 eV for 3 and 3.43 to 3.53 eV for 4 [31]. In molecules 3 and 4, a similar behavior is noted, in which the solvents with the highest dielectric constant lead to a lower energy of HOMO, LUMO and band gap. Another fact to be emphasized is that the HOMO energies of molecule 4 are higher than molecule 3; this is due to the dimethylamino group, which is a strong electron donor and tends to destabi- lize the orbitals energies. Hence, dimethylamine groups can contribute to the resonance structures of the quinolines back- bone upon excitation. Therefore, the contribution of the pos- itive charge on the quinoline amine group, in the resonance structures of compound 4 spectrum appears in lower energies as compared to that of 3 in the same media. The aromaticity of the dye may sometimes be strongly affected by changes in the dihedral angles and it is noted that the difference between calculated and experimental values becomes larger as seen for azobenzene, hydrazine and cationic dyes in a study reported by Guillaument and Nakamura [32, 33]. It is inferred that the dye adopts different conformations with various solvents, which is also supported by results from quantum chemical calculations. Table 4 Theoretical results of Compound 3 from DFT/PCM/B3LYP/6-31G(d) solvent Dielectric constant θ1 (°) dif vacuo θ2 (°) dif vacuo θ3 (°) dif vacuo HOMO (eV) LUMO (eV) GAP (eV) λ max (nm) vacuo --- 29.169 --- 127.755 --- 17.830 --- −5.240 −1.375 3.865 358.98 Hexane 1.9 28.939 −0.229 127.858 0.103 18.340 0.510 −5.284 −1.442 3.841 364.18 Toluene 2.4 28.850 −0.318 127.932 0.177 18.612 0.782 −5.300 −1.466 3.833 366.34 THF 7.6 28.560 −0.608 128.388 0.633 20.199 2.369 −5.359 −1.554 3.804 374 Dioxane 2.2 28.878 −0.291 127.907 0.152 18.522 0.691 −5.295 −1.459 3.835 365.69 ACN 37.5 28.477 −0.692 128.761 1.006 20.861 3.030 −5.391 −1.602 3.788 378.14 CHCl3 4.8 28.621 −0.547 128.210 0.455 19.703 1.873 −5.340 −1.525 3.814 371.53 DCM 9.1 28.544 −0.625 128.452 0.697 20.340 2.509 −5.365 −1.563 3.801 374.8 DCE 10.4 28.534 −0.634 128.492 0.737 20.421 2.591 −5.369 −1.569 3.800 375.28 Ethyl acetate 6.0 28.584 −0.585 128.307 0.552 19.996 2.165 −5.350 −1.541 3.809 372.92 Acetone 20.6 28.495 −0.674 128.671 0.916 20.731 2.901 −5.384 −1.591 3.792 377.25 MeOH 32.6 28.479 −0.689 128.749 0.994 20.844 3.014 −5.390 −1.600 3.789 378.02 DMF 36.7 28.476 −0.693 128.766 1.011 20.868 3.037 −5.391 −1.602 3.788 376.87 DMSO 46.6 28.471 −0.698 128.792 1.037 20.902 3.072 −5.393 −1.605 3.787 377.09 EtOH 22.4 28.487 −0.681 128.707 0.952 20.785 2.954 −5.387 −1.596 3.790 377.61 Et2O 4.3 28.644 −0.525 128.165 0.410 19.549 1.719 −5.334 −1.518 3.816 370.85 Fig. 7 Optimized geometry in vacuum of (a) 3 and (b) 4 by DFT/B3LYP/6-31G(d). It is presented a top view (left) and the HOMO (middle) and LUMO (right) orbitals 1716 J Fluoresc (2017) 27:1709–1720 As previously mentioned, it is important to know the influence of the solvent on the quinoline core without substituents, so we do not limit our explanation of the change in values only in relation with the twist angle of substituents (θ1, θ2, θ3 and θ4). The dihedral angles are shown in Fig. 8. All the results presented by com- putational studies are summarized in Table 4 and Table 5. Theoretical results showed that for compounds 3 and 4, angle θ1, even in different solvents, does not present major changes. On the other hand, when we observe the dihedral angles between phenyl rings and quinoline base, it is possible to see that, when changing the solvent, torsions up to 3° were observed in dihedral angles θ2 and θ3; when the dielectric constant is increased, the changes in θ2 and θ3 also increase, indicating a greater interaction between the solvent and the molecule. As for θ4 of 4, we can see variations up to 2.3°, with greater variations in those with higher dielectric constants. The mobility that the molecule presents makes the structure favorable to the effect of the solvent type and thus hinders the correlation between theoretical and experimental values for maximum absorption length. However, we found a trend in the behavior of these molecules. Initially, with the data obtain- ed theoretically, we observed a correlation between the dielec- tric constant of the solvent and the λmax. A first correlation was found between the dielectric constant and the experimen- tal λmax (Fig. 9), and a second correlation was found between the dielectric constant and the theoretical λmax (Fig. 10). The value of R indicates a strong correlation between experimental and theoretical studies [34]. Figures 9 and 10 show strong correlation between λmax and dielectric constant. The tendency is to increase the dielectric constant, shifting the λmax batochromically. The errors occur because the absorption phenomenon depends on other factors that cause changes in the elec- tronic structure of the molecule such as viscosity, a bFig. 8 Compound 3 (a) and 4 (b) and the dihedral angles considered for computations Table 5 Theoretical results of Compound 4 from DFT/PCM/B3LYP/6-31G(d) solvent Dielectric constant θ1 (°) dif vacuo θ2 (°) dif vacuo θ3 (°) dif vacuo θ4 (°) dif vacuo HOMO (eV) LUMO (eV) GAP (eV) λ max (nm) vacuo --- 30.078 --- 127.667 --- 12.712 --- 9.474 --- −4.683 −1.149 3.534 393.65 Hexane 1.9 30.014 −0.064 127.830 0.163 14.249 1.537 8.860 −0.614 −4.753 −1.247 3.506 401.24 Toluene 2.4 29.964 −0.114 127.915 0.248 14.664 1.952 8.630 −0.844 −4.778 −1.281 3.496 403.61 THF 7.6 29.653 −0.425 128.239 0.572 15.202 2.489 7.750 −1.723 −4.864 −1.405 3.459 413.14 Dioxane 2.2 29.981 −0.097 127.889 0.222 14.555 1.843 8.700 −0.774 −4.770 −1.271 3.499 402.83 ACN 37.5 29.447 −0.631 128.437 0.770 15.027 2.315 7.241 −2.233 −4.908 −1.470 3.438 418.38 CHCl3 4.8 29.768 −0.310 128.129 0.462 15.157 2.445 8.041 −1.433 −4.837 −1.365 3.471 410.03 DCM 9.1 29.614 −0.464 128.276 0.609 15.193 2.481 7.655 −1.819 −4.873 −1.418 3.455 414.14 DCE 10.4 29.590 −0.488 128.298 0.631 15.182 2.470 7.596 −1.878 −4.878 −1.425 3.452 414.76 Ethyl acetate 6.0 29.704 −0.374 128.190 0.523 15.196 2.483 7.878 −1.596 −4.852 −1.388 3.464 411.79 Acetone 20.6 29.492 −0.586 128.393 0.726 15.094 2.381 7.354 −2.120 −4.899 −1.456 3.442 417.26 MeOH 32.6 29.453 −0.625 128.431 0.764 15.037 2.325 7.256 −2.218 −4.907 −1.468 3.438 418.24 DMF 36.7 29.444 −0.633 128.439 0.773 15.023 2.311 7.235 −2.239 −4.909 −1.471 3.437 418.44 DMSO 46.6 29.432 −0.646 128.452 0.785 15.001 2.289 7.203 −2.271 −4.911 −1.475 3.436 418.76 EtOH 22.4 29.474 −0.604 128.411 0.744 15.069 2.357 7.308 −2.166 −4.903 −1.462 3.440 417.71 Et2O 4.3 29.798 −0.280 128.100 0.433 15.125 2.413 8.120 −1.354 −4.829 −1.354 3.474 409.17 J Fluoresc (2017) 27:1709–1720 1717 polarity, and, therefore, solubility [27]. Theoretical stud- ies minimize these errors relative to dielectric constant. In these studies, the interaction of the electron cloud between the solvent and the compound is considered and the effect of viscosity and others previously men- tioned is not considered. But even so, what probably plays a major role in the geometry of the molecule, in the solvatochromism, is the electrostatic and orbital in- teractions with the solvent. Another observation regarding the theoretical correlation is that reaching the high values of dielectric constant seems to be saturation, and wavelength does not follow a linear increase. Further investigations of this class of D-A-D systems, as well as the synthesis and modification of groups to know photovoltaic properties in optoelectronic devices, will be studied and will be reported in future publications. Conclusion The solvatochromic effect and photophysical properties of quinoline derivatives in various solvents were studied to know their relation with each substituent in the structure. Among the substituents, we highlighted the dimethylamine group. In ad- dition, it is possible to observe the highlighted effect on the absorption bathochromic shift and fluorescence displayed by the compounds having amine group at position 6 of the quin- oline core. This demonstrates the importance of inserting the phenyl groups, but also confirms the key effect of the amino group at position 6 of the quinoline backbone. In theoretical calculations, both effects of solvation and electrostatic factors were taken into consideration. Thus, mol- ecules 3 and 4were further studied by the properties highlight- ed based on an earlier study. A correlation between theoretical and experimental maximum absorption length and the Fig. 10 Theoretical results. a Compound 3, correlation between wavelength and dielectric constant R = 0.76. b Compound 4 R = 0.80 Fig. 9 Experimental results. a Compound 3, correlation between wavelength and dielectric constant R = 0.70. b Compound 4 R = 0.80 1718 J Fluoresc (2017) 27:1709–1720 dielectric constant was observed. In general, the λmax can be predicted with good accuracy. It is difficult to be precise as to when the molecule is more flexible because the structure is more difficult to be predicted, therefore the geometry of the excited state can greatly differ from the ground state. The solvatochromic effect was studied in the two molecules, being more pronounced in molecule 4, a D-A-D molecule type. All results obtained in this study are important for quantum chem- istry practical applications in the design of functional dyes. Ackowledgements The authors would like to thank Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) (Procs. 2013/08697–0, 2012/24199–8, 2012/21983–0, 2014/20410–1, 2015/00615–0 and 2016/01599–1), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) (Proc. 302,753/2015–0) and Pró- Reitoria de Pesquisa (PROPe-UNESP) for their financial sup- port. We would also like to thank CBMM – Companhia Brasileira de Mineralogia e Mineração for the NbCl5 samples. This research was also supported by resources supplied by the Center for Scientific Computing (NCC/GridUNESP) of the São Paulo State University (UNESP). References 1. Balijapalli U, Iyer SK (2015) Synthesis and optical properties of a series of green-light-emitting 2-(4-Phenylquinolin-2-yl) phenol– BF2 complexes (Boroquinols). Eur J Org Chem 23:5089–5098 2. Michael JP (1997) Quinoline, quinazoline and acridone alkaloids. Nat Prod Rep 14(6):605–618 3. Markees DG, Dewey VC, Kidder GW (1970) Antiprotozoal 4- aryloxy-2-aminoquinolines and related compounds. J Med Chem 13(2):324–326 4. Dumouchel S, Mongin F, Trécourt F, Quéguiner G (2003) Tributylmagnesium ate complex-mediated bromine–magnesium exchange of bromoquinolines: a convenient access to functional- ized quinolines. Tetrahedron Lett 44(10):2033–2035 5. Goel A, Kumar V, Singh SP, Sharma A, Prakash S, Singh C, Anand RS (2012) Non-aggregating Solvatochromic bipolar benzo-[f]- quinolines and benzo-[a]-acridines for organic electronics. J Mater Chem 22(30):14880–14888 6. Zhang X, Kale DM, Jenekhe SA (2002) Electroluminescence of multicomponent conjugated polymers. 2. Photophysics and en- hancement of electroluminescence from blends of polyquinolines. Macromolecules 35(2):382–393 7. Lin LY, Chen YH, Huang ZY, Lin HW, Chou SH, Lin F, Chen CW, Liu YH, Wong KT (2011) A low-energy-gap organic dye for high- performance small-molecule organic solar cells. J Am Chem Soc 133(40):15822–15825 8. Choi HJ, Choi HB, Paek SH, Song KH, Kang MS, Ko JJ (2010) Novel organic sensitizers with a quinoline unit for efficient dye- sensitized solar cells. Bull Kor Chem Soc 31(1):125–132 9. Havinga EE, ten Hoeve W, Wynberg H (1992) A new class of small band gap organic polymer conductors. Polym Bull 29(1–2):119–126 10. Kumar V, Gohain M, Van Tonder JH, Ponra S, Bezuindenhoudt BCB, Ntwaeaborwa OM, Swart HC (2015) Synthesis of quinoline based heterocyclic compounds for blue lighting application. Opt Mater 50:275–281 11. Yune JH, Karatchevtseva I, Evans PJ, Wagner K, Griffith MJ, Officer D, Triani G (2015) A versatile binder-free TiO2 paste for dye-sensitized solar cells. RSC Adv 5(37):29513–29523 12. dos Santos GC, Bartolomeu AA, Ximenes VF, da Silva-Filho LC (2016) Facile synthesis and Photophysical characterization of new Quinoline dyes. J Fluoresc. doi:10.1007/s10895-016-1954-5 13. Fery-Forgues S, Lavabre D (1999) Are fluorescence quantum yields so tricky to measure? A demonstration using familiar statio- nery products. J Chem Educ 76(9):1260 14. Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman, JR, …, Nakatsuji H (2009) Gaussian 09, revision A. 1. Gaussian Inc., Wallingford 15. Becke AD (1993) Density-functional thermochemistry. III The role of exact exchange J Chem Phys 98(7):5648–5652 16. Ditchfield RHWJ, Hehre WJ, Pople JA (1971) Self-consistent mo- lecular-orbital methods. IX An extended Gaussian-type basis for molecular-orbital studies of organic molecules. J Chem Phys 54(2):724–728 17. Miertuš S, Scrocco E, Tomasi J (1981) Electrostatic interaction of a solute with a continuum. A direct utilizaion of AB initio molecular potentials for the prevision of solvent effects. Chem Phys 55(1): 117–129 18. Mennucci B (2012) Polarizable continuum model. Wiley Interdiscip Rev: Comput Mol Sc 2(3):386–404 19. Akbar R, Baral M, Kanungo BK (2015) Experimental and DFT assessment on the development of Tris (methoxymethyl)-5-oxine. J Chem Eng 60(11):3236–3245 20. Ebenso EE, Kabanda MM, Arslan T, Saracoglu M, Kandemirli F, Murulana LC, Quraishi MA (2012) Quantum chemical investiga- tions on quinoline derivatives as effective corrosion inhibitors for mild steel in acidic medium. Int J Electrochem Sci 7:5643–5676 21. Kumru M, Küçük V, Kocademir M (2012) Determination of struc- tural and vibrational properties of 6-quinolinecarboxaldehyde using FT-IR, FT-Raman and dispersive-Raman experimental techniques and theoretical HF and DFT (B3LYP) methods. Spectrochim Acta A 96:242–251 22. Fazal E, Jasinski JP, Anderson BJ, Kaur M, Nagarajan S, Sudha BS (2015) Synthesis, crystal and molecular structure studies and DFT calculations of phenyl quinoline-2-carboxylate and 2- methoxyphenyl quinoline-2-carboxylate; two new quinoline-2 car- boxylic derivatives. Crystals 5(1):100–115 23. Stefanou V, Matiadis D, Melagraki G, Afantitis A, Athanasellis G, Igglessi-Markopoulou O, Markopoulos J (2011) Functionalized 4- hydroxy coumarins: novel synthesis, crystal structure and DFT cal- culations. Molecules 16(1):384–402 24. Mirjafary Z, Saidian H, Sahandi M, Shojaei L (2014) Efficient synthesis of novel pyranoquinoline derivatives from simple acetan- ilide derivatives: experimental and theoretical study of their physi- cochemical properties using DFT calculations. J Brazil Chem Soc 25(7):1253–1260 25. Pavia DL, Lampman GM, Kriz GS (2001) Introduction to spectros- copy: a guide for students of organic chemistry, 3rd ed. Thomson Learning Inc., Bellingham 26. Nedeltchev AK, Han H, Bhowmik PK (2010) Photoactive amor- phous molecular materials based on quinoline amines and their synthesis by Friedländer condensation reaction. Tetrahedron 66(48):9319–9326 27. Lakowicz JR (2006) Principles of fluorescence spectroscopy. Springer, New York 28. Park KK, Park JW, Hamilton AD (2007) Solvent and pH effects on the fluorescence of 7-(Dimethylamino)-2-Fluorenesulfonate. J Fluoresc 17(4):361–369 29. Chang DW, Ko SJ, Kim JY, Dai L, Baek JB (2012) Multifunctional quinoxaline containing small molecules with multiple electron- J Fluoresc (2017) 27:1709–1720 1719 http://dx.doi.org/10.1007/s10895-016-1954-5 donating moieties: Solvatochromic and optoelectronic properties. Synthetic Met 162(13):1169–1176 30. Nazim M, Ameen S, Seo HK, Shin HS (2015) Effective DAD type chromophore of fumaronitrile-core and terminal alkylated bithiophene for solution-processed small molecule organic solar cells. Sci Rep 5:11143 31. Guillaumont D, Nakamura S (2000) Calculation of the absorption wavelength of dyes using time-dependent density-functional theory (TD-DFT). Dyes Pigments 46(2):85–92 32. Burrows PE, Shen Z, Bulovic V, McCarty DM, Forrest SR, Cronin JA, Thompson ME (1996) Relationship between electroluminescence and current transport in organic heterojunction light-emitting devices. J Appl Phys 79(10): 7991–8006 33. Gao N, Cheng C, Yu C, Hao E, Wang S, Wang J, Wei Y, Mu X, Jiao L (2014) Facile synthesis of highly fluorescent BF2 complexes bearing isoindolin-1-one ligand. Dalton Trans 43(19):7121–7127 34. Mukaka MM (2012) A guide to appropriate use of correlation co- efficient in medical research. Malawi Med J 24(3):69–71 1720 J Fluoresc (2017) 27:1709–1720 Theoretical-Experimental... Abstract Introduction Experimental Materials and Instrumentation Computational Details Results and Discussion The Solvent Effect on UV-vis Absorption of Quinoline and Its Derivatives Theoretical-Experimental Solvent Effect in Quinoline Derivatives 3 and 4 Conclusion References