1 Introduction

Investigating photophysical properties of fluorescent probes have been a subject of interest for many researchers due to their potential applications as sensors, optoelectronics, and in biomedical field [1]. A change in dielectric constant and polarizability of the molecular environment brings substantial change in the emission characteristics of fluorescent probes, and affects their ground and excited states [2]. Thus, the analysis of the resultant dipole moments of electronically excited species in different solvents has been explored for various functionalized heterocyclic architectures towards designing nonlinear optical materials and fluorescence tools for sensing and bioimaging [3, 4]. In spite of reasonable advancement in the development of fluorescent probes, the demand for newer environment-sensitive solvatochromic push\(--\)pull probes remains a prime area of research in medicinal and material chemistry [5].

It is often anticipated that when the charge is transferred from the donor to the acceptor group after light absorption in such fluorophores, a highly dipolar excited state gets created, which relaxes through interaction with the dipoles of solvent and shifts its emission to longer wavelengths in more polar solvents.  In particular, fluorophores, whose emission properties vary with the polarity [6] or viscosity of their microenvironment [7], have attracted considerable attention due to their fast and non-destructive abilities to detect changes in their local environment.  Thus, such fluorophores have been used as probes for determining the critical micelle concentrations (cmc) of various surfactants [8], viscosity measurements of biological membranes [9], and interaction monitoring of protein bindings [10].

In this realm, various substituted coumarins have exemplified numerous applications as photo-initiators, probes, photosensitizing agents in the biological study and photodimerization in polar and nonpolar solvents [11]. Moreover, coumarins are potential candidates for the apoptosis targeted therapy and have gained much attention due to their ability to preferentially kill cancer cells [12]. In addition, diversity has been introduced by fusing it with other heterocycles to afford polycyclic fluorescent architectures that have exhibited interesting photophysical and biological profile [13,14,15,16].

In striking analogy, quinoline scaffold is another privileged pharmacophore present in several natural products, and complex bioactive entities. Quinoline-fused heterocycles have displayed a wide range of biological activities including, antidepressant, antimalarial, anticancer, antiviral, and antipsychotic [17]. Coumarin and quinoline-based dyes have been widely used as fluorophores owing to their favourable biocompatibility and photophysical properties [18, 19]. Interestingly, tremendous efforts have been devoted in the past towards the construction of coumarin-fused quinolines, as they constitute an integral part of natural products such as santiagonamine, and several anticancer and antibacterial agents, dyes for mitochondrial imaging, fluorescent sensors, estrogen receptor \(\beta \)-selective ligands, glucocorticoid modulators and selective non-steroidal progesterone receptor modulators [20].

In spite of reasonable applications of different coumarin-fused quinolines, their solvatochromic and photophysical studies remained unexplored. In the light of these facts and in continuation to our studies [21,22,23,24], we planned to synthesize a new push–pull coumarin-fused quinoline probe, and study its ground- and excited-state dipole moments using solvatochromic shift method.

2 Experimental

2.1 Materials, methods and instrumentation

All reagents and solvents were purchased from commercial sources and were used as received. Required solvents for reaction were dried by standard procedures prior to use. Reactions were performed under air atmosphere, unless otherwise noted. \(^{\mathrm {1}}\hbox {H}\) NMR spectrum was recorded on 400 MHz spectrometer, and the chemical shifts are reported in \(\delta \) units, parts per million (ppm), and referred to the internal standard TMS set as 0.00 ppm, relative to residual chloroform (7.26 ppm) in the deuterated solvent. Data are reported as follows: the following abbreviations were used to describe peak splitting patterns: \(\hbox {s} = \hbox {singlet}, \hbox {d} = \hbox {doublet}, \hbox {t} = \hbox {triplet}, \hbox {dd} = \hbox {doublet}\) of doublet and m \(=\) multiplet. Coupling constants J were reported in Hz. The \(^{\mathrm {13}}\)C NMR spectrum was reported in ppm relative to deuterochloroform (77.0 ppm). Melting points was determined on a capillary point apparatus equipped with a digital thermometer and are uncorrected. High-resolution mass spectra were recorded with a TOF analyser spectrometer by using electrospray mode. Flash column chromatography was performed on silica gel, 100−200 mesh. Analytical and preparative thin-layer chromatography was carried out on silica gel 60 F-254 plates. Product was visualized using UV and shown yellow colour under UV light. Absorption spectra were recorded using dual-beam Thermo Scientific Evolution 201 UV–visible Spectrophotometer, and fluorescence spectra were recorded using Shimadzu, RF-5301PC spectrofluorometer. The data were analysed using related software. The spectral shifts obtained with different sets of samples were identical in most of the cases, and values were within \(\pm 1.0\,\hbox {nm}\). The concentration of fluorescent probe in all the solutions prepared in different solvents was \(10^{\mathrm {-4}}\, \hbox {M}\), and all measurements with the solvent systems were carried out at room temperature.

2.2 General procedure for the synthesis of 10-chloro-3-(diethylamino)-9-fluoro-6H-chromeno[4,3-b]quinolin-6-one (3)

A mixture of 4-chloro-7-(diethylamino)-2-oxo-2H-chromene-3-carbaldehyde (1, 0.35 mmol, 1.0 equiv), 4-chloro-3-fluoroaniline (2, 0.43 mmol, 1.2 equiv) was heated in DMSO (4.0 mL) at \(100\,^{\circ }\hbox {C}\) under ambient conditions for 8 h. On completion of the reaction as indicated by TLC, the reaction mixture was cooled to room temperature and diluted with ice water. The mixture was extracted with ethyl acetate (\(3 \times 20\, \hbox {mL}\)), and the organic layer was separated and dried over \(\hbox {Na}_{{2}}\hbox {SO}_{\mathrm {4}}\). The organic layer was concentrated, and the crude product was subjected to silica gel column chromatography [\(\hbox {SiO}_{\mathrm {2}}\) (100–200 mesh), with (hexanes/EtOAc, 19:1)] as the eluent to afford pure product (3).

Yellow solid; yield: 106.08 mg (80%); mp: 185–187 \( ^{\circ } \hbox {C}\); \(^{\mathrm {1}}\hbox {H}\) NMR (400 MHz, \(\hbox {CDCl}_{\mathrm {3}}) \delta \) 9.00 (s, 1H), 8.43–8.40 (m, 1H), 8.21 (d, \(J = 7.1\, \hbox {Hz}\), 1H), 7.62 (d, \(J = 8.6\, \hbox {Hz}\), 1H), 6.73 (dd, \(J = 9.0, 2.5\, \hbox {Hz}\), 1H), 6.53 (t, \(J = 2.2\, \hbox {Hz}\), 1H), 3.48 (q, \(J = 7.1\, \hbox {Hz}\), 4H), 1.28 (t, \(J = 7.1\, \hbox {Hz}\), 6H); \(^{\mathrm {13}}\hbox {C}\) NMR (100 MHz, \(\hbox {CDCl}_{\mathrm {3}}) \delta \) 161.7, 154.7, 151.4, 148.4, 139.9, 139.8, 130.6, 126.2, 125.2, 125.1, 115.4, 113.2, 113.0, 109.2, 106.8, 97.9, 44.8, 12.5; HRMS (ESI-TOF) (m/z) calculated \(\hbox {C}_{\mathrm {20}}\hbox {H}_{\mathrm {17}}\hbox {ClFN}_{\mathrm {2}} \hbox {O}_{\mathrm {2}}^{{+}}\): 371.0957; found 371.0954 [M\(+\)H]\(^{\mathrm {+}}\).

Scheme 1
scheme 1

Synthesis of 10-chloro-3-(diethylamino)-9-fluoro-6H-chromeno[4,3-b]quinolin-6-one

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2.3 Computational details

A comprehensive theoretical simulation using sophisticated DFT [25, 26] based method was performed to understand the relative stability, electronic environment and the photo-physical behaviour of the coumarin-fused quinoline (3). The DFT-based calculations are efficient and reliable computational tools for providing useful insights into the experimental findings. In the present work, DFT-based calculations were performed using Gaussian09 [27] package at B3LYP [28, 29] level using \(6\text {-}31+\hbox {G(d,p)}\) basis sets for all atoms. We used density functional theory (DFT) [30, 31]-based calculation to compute the ground-state and excited-state dipole moment along with the Onsager radii of the compound 3. Implicit solvent environment for water was taken into account under self-consistent reaction field (SCRF) [30] methodology using Integral Equation Formalism-Polarizable Continuum Model (IEFPCM) [31] formalism as implemented in Gaussian09 program suit. All the structures were optimized at \(\hbox {B3LYP}/6\text {-}31+\hbox {G(d,p)}\) level, followed by corresponding frequency calculations. The visual graphics related to the calculations were created using GaussView [32] visualization programs. Time-dependent DFT (TDDFT) calculations were also performed at the same level as that of the optimization. As many as 30 lowest vertical excitation energies on the optimized geometries, the compound (in their default spin states) was analysed using TDDFT formalism.

3 Results and discussion

The dihalo-substituted coumarin-fused quinoline (3) used in the present study was synthesized in a one-pot manner by coupling 4-chloro-7-(diethylamino)-2-oxo-2H-chromene-3-carbaldehyde and 4-chloro-3-fluoroaniline in DMSO at \(100\, ^{\circ } \hbox {C}\) under ambient conditions for 8 h. The compound was completely characterized using \(^{\mathrm {1}}\hbox {H}\) NMR, \(^{\mathrm {13}}\hbox {C}\) NMR and HRMS.

3.1 Solvent effect on electronic absorption and emission spectra

Generally, the electronic spectra of organic compounds are affected by their immediate environment, particularly solvent effects. A change of organic solvent is usually associated with a change in dielectric constant, polarizability and polarity of the surrounding medium. Hence, the solvent affects the ground-state and excited-state dipole of the compound in numerous ways. In order to understand the effect of solvent nature on compound 3, its electronic absorption and emission spectra were recorded and analysed in different solvents. The absorption spectra of 3 showed two broad bands centred around 360 nm and430 nm. Clearly, a bathochromic shift in both the bands was observed on increasing the solvent polarity from non-polar to aprotic polar (Fig. 1). In particular, 7–8 nm spectral shift was observed in the absorption band on changing the solvent from \(\hbox {CCl}_{\mathrm {4}}\) to DMF, which indicated that the absorption band has charge transfer character. This intramolecular charge transfer (ICT) interaction possibly is a result of the molecular configuration of 3, which consists of electron donor (N, N-diethyl group on coumarin moiety) and electron acceptor (dihalo-substituted quinoline moiety) groups. The magnitude of these shifts suggested that the ground state of the molecule is sensitive towards the polarity of solvent. Further, the fluorescence maximum also shifts to the longer wavelengths with the increase in the polarity of the solvent. However, larger shift (\(\sim 67\, \hbox {nm}\)) in the emission spectra was observed as compared to absorption spectra on changing the solvent from \(\hbox {CCl}_{\mathrm {4}}\) to DMF. It is clearly observed that the shifts in the emission bands with change in solvent polarity are more pronounced than those observed in absorption bands. This provides an indication of more stabilization and higher dipole moment in the excited state than in the ground state. At the same time, it was also observed that 3 demonstrated the maximum fluorescence intensity in non-polar solvents as compared to polar solvents. The low fluorescence intensity of compound 3 in aprotic polar solvents is likely due to radiation-less decay of 3 due to hydrogen bonding. As the solvent polarity increased, the emission wavelength was red-shifted and the fluorescence intensity decreased.

Fig. 1
figure 1

a Absorption and b emission spectra of compound 3 in different pure solvents at [concentration of 3] \(= 0.1\,\upmu \hbox {M}\)

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3.2 Stokes’ shift and experimental calculations of ground-state and excited-state dipole moments

Stokes’ shift values of 3 in all the solvents used in this work are given in Table 1. On increasing the solvent polarity, the Stokes’ shift value enhances from \(5376.76\, \hbox {cm}^{{-1}}\) (for \(\hbox {CCl}_{\mathrm {4}})\) to \(6887.58\,\hbox {cm}^{{-1}}\) (for DMF). Typically, the large magnitude of stokes’ shifts is an indicative of intramolecular charge transfer. These data imply that the molecule is influenced by parameters such as polarity, hydrogen bond donor-acceptor strength of the solvent. A large Stokes’ shift shows that the excited-state conformation may differ from the ground state. A glance in Table 1 revealed that the Stokes’ shift exhibited by compound 3 gradually increases with increasing solvent polarity, indicating enhanced stabilization of the charge transfer species in the polar solvents. An increase in Stokes’ shift with solvent polarity showed that the dipole moment increases on excitation.

Table 1 Different solvent parameters and spectral data of compound 3
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Analysis of solvatochromic behaviour allows us to estimate the dipole moment in ground and excited state. The solvatochromic parameters of compound3 were determined by making use of well-known solvatochromic shift method as described in literature [33,34,35]. Figure 2a shows \(\overline{v_{a} } -\overline{v_{f} }\) versus \({F}_{{1}}(\varepsilon ,\eta )\), while Fig. 2b shows \({F}_{\mathrm {2}}(\varepsilon ,\upeta )\) versus \(\frac{\overline{v_{a} } +\overline{v_{f} } }{2}\). The linear behaviour of the solvent polarity versus Stokes’ shift demonstrated solvent effects as a function of refractive index and dielectric constant. The slopes of the fitted lines from Fig. 2a, b were found to be \({S}_{\mathrm {1}} = 1476.27\) and \({S}_{\mathrm {2}} = -2995.95\), respectively. The solute cavity radius (\({a}_{\mathrm {o}})\) of 3 was found to be 6.15 Å (obtained from computational calculations). With the methodology and equations demonstrated in the literature [3, 33, 35], the experimental ground-state and excited-state dipole moments of 3 were computed to be \(\mu _{g}= 2.85\) D and \(\mu _{e} = 8.39\) D, respectively (Table 2). This confirmed that the compound 3 is more polar in their excited singlet state than in the ground state, and this is in agreement with the better stabilization under excitation that is consistent with Stokes’ shifts data. The intramolecular charge transfer resulted in a large dipole moment in the excited state as compared to that of the ground state, rendering its fluorescence emission sensitive to environment and solvent polarity.

Fig. 2
figure 2

a Plot for Stokes’ shift versus solvent polarity function \({F}_{\mathrm {1}}\). b Plot for arithmetic average of absorption and fluorescence wavenumbers versus solvent polarity function \({F}_{\mathrm {2.}}\) (1) DMF, (2) acetonitrile, (3) DCM, (4) THF, (5) ethyl acetate,(6) chloroform, (7) diethyl ether, (8) o-xylene, (9) toluene, (10) 1,4-dioxane and (11) carbon tetrachloride

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4 Theoretical computations

The gas phase optimized structure of the compound 3 is reported in Fig. 3a. It was observed that the presence of solvent environment did not introduce any significant changes to the gas phase optimized structure, as evident from the DFT calculations performed in different solvent environments.

We can develop some analytical understandings on the experimental intramolecular charge transfer prospect of the system from the quantum mechanical calculations. The isosurface HOMO-LUMO orbital pictures of the system are included in Fig. 3b. The figure accounts for substantial variation in the distribution of electron density associated with the electronic excitations from HOMO to LUMO and \(\hbox {LUMO}+1\). As evident from Fig. 3b, the HOMO is primarily defused throughout the system. However, there are relatively higher concentrations of electronic densities on the benzene ring containing the two N,N-diethyl groups. On the other hand, the LUMOs are located on the (1H-pyrano[4,3-b]quinolin-1-one) moiety.

Apparently, this observation determines the possibilities of intramolecular charge transfer during electronic excitations and corroborates the experimental outcomes. It is also important to note that the computational study interprets moderate reduction in the HOMO-LUMO gap in the excited state of the molecule, and this could also expedite the electronic excitations.

The dipole moment of the system provides some important insights into the electronic anisotropy of the system. The arrow represents the direction of dipole moment in Fig. 4a. We have reported the graphical representation of the computed ground-state and excited-state dipole moment values for the system in Fig. 4b. A large discrepancy for the experimental and calculated ground-state dipole moment values was obtained. It is worth noting here that the inherent theoretical limitations of DFT formalism are responsible for the considerable variation in the computed ground-state dipole moments. As we know, DFT methods could not effectively account the weak non-bonding (long-range) interactions in the system to counterbalance the additional perturbations in the electronic wave-function of the model system. Subsequently, this attributes a virtual increase in the system’s polarity, which might be substantially different from the real system. However, it is encouraging to note that the qualitative trend for the computed ground-state and excited-state dipole moment correlates well with the experimental findings.

Table 2 Dipole moment of compound 3 molecule in ground and excited states
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Fig. 3
figure 3

a DFT optimized structure of 3 at \(\hbox {B3LYP}/6{-}31+\hbox {G(d,p)}\) level, b corresponding FMOs of the system in the ground and excited state

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Fig. 4
figure 4

a The orientation of dipole moment (red arrow) in the system, b computed ground-state and excited-state dipole moment values for the compound 3 in different solvents

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Fig. 5
figure 5

a The molecular electrostatic potential (MEP) map of the compound 3, b electron density differences between the \({S}_{{1}}\) and \({S}_{{0}}\) states. Cyan corresponds to negative values (higher electron density in the ground state), while purple corresponds to positive ones (higher electron density in the excited state), c DFT simulated adsorption spectrum of the compound in different solvents

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Table 3 Vertical excitation energies calculated for the lowest lying singlet states of the compounds
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If we consider the excitation from the ground state, \(\hbox {S}_{\mathrm {0}}\) to the first excited singlet state \(\hbox {S}_{\mathrm {1}}\), our results (Table 3) indicate that the two major absorption peaks at around 350 nm and 450–500 nm for the complexes are typically due to the HOMO to LUMO\(+\)1 and HOMO-LUMO types of electron promotion, respectively. Consequently, the representation of the molecular orbitals involved in the transition (Fig. 3b) depicted the characteristic intramolecular charge transfer accounted from the \({\varvec{\pi }} \rightarrow {\varvec{\pi }} \mathbf * \) electronic transition. The calculated excitation energies are practically consistent with the experimental absorption spectra, as evident from Fig. 5c. The nature of the intramolecular charge transfer character of these transitions can be comprehensively interpreted by plotting the difference between the charge densities of the two states involved in the transition. The density difference plot for the compound is reported in Fig. 5b. From the figure, it can be assessed that the most prominent charge transfer in the \(\hbox {S}_{{0}}-\hbox {S}_{{1}}\) excitation responsible for the absorption process, is between the benzene ring with two N,N-diethyl groups to (1H-pyrano[4,3-b]quinolin-1-one) with halogen substituents. This particular assessment is further supported by the computed molecular electrostatic potential (MEP) map reported in Fig. 5a.

5 Conclusions

The present article demonstrates the detailed solvatochromic behaviour of a newly synthesized coumarin-fused quinoline push–pull probe in neat solvents of varying polarity. The absorption and the emission spectral traces suggested the positive solvatochromism behaviour of 3 on going from nonpolar to aprotic polar solvents, i.e. \(\sim 8\, \hbox {nm}\) and \(\sim 67\, \hbox {nm}\) red shift in absorption and emission spectra, respectively. Changes in spectral behaviour can be attributed to the interaction of 3 with the solvents studied. The ground-state and the excited-state dipole moment values for 3 were found to be 2.85 D and 8.39 D, respectively, as obtained from the solvatochromic shift method. The fourfold increase in the dipole moment in the excited state of 3 is due to greater polar nature of the excited state. In the excited state, it is likely that the charge separation increases, resulting in a large dipole moment than in the ground state. Theoretically obtained values of excited-state dipole moment are also higher than the values of ground-state dipole moment of compound 3. These observations correlated well with the experimental study.