High quantum yield and pH sensitive fluorescence dyes based on coumarin derivatives: fluorescence characteristics and theoretical study

Abstract

The fluorescence coumarin derivatives were synthesized by an efficient one-pot, three-component reaction in 80–90% yields. These fluorescence dyes exhibited high fluorescent intensity and quantum yield where compound 4e, with p-methyl substituted on the phenyl ring, had the maximum fluorescence intensity and also the fluorescence quantum yield reached 0.83. These coumarin derivatives display different fluorescence in acidity and alkaline conditions and the fluorescent colour changed from blue to yellow-green when the pH of the solution turned from acidic to alkaline. The pH sensitive fluorescence properties of the coumarin derivatives showed a large shift from 441 to 538 nm in wavelength. Experimental results were confirmed with DFT and TDDFT calculations.

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Introduction

Coumarins consist of pyrone and benzene¹ which are important components of versatile applications in various fields of science and technology² such as perfumes,³ cosmetics,⁴ disperse dyes,⁵ cationic dyes,⁶ charge-transfer agents,⁷ solar energy collectors,⁸ and potential applications in organic light-emitting diodes (OLEDs)⁹ etc. In addition, coumarin derivatives have played an important role in medicinal chemistry such as the platelet inhibition agent, anti-thrombus agent,¹⁰ hepatoprotective drugs, and so on.

Coumarins also are well-known laser dyes for the blue-green spectral region¹¹ whose derivatives are one of the most favorable fluorophore groups used to develop fluorescence agents¹² because of their high quantum yields and high photostability.¹³ It was reported¹⁴ that the electron donating groups in the 7-position and electron-withdrawing groups in the 3-position were able to enhance the fluorescence intensity of coumarins¹⁵ based on an intramolecular charger transfer process.¹⁶ Among different coumarin dyes, Xu's group reported that 7-(N,N-diethylamino)coumarin-3-aldehyde was selected as a fluorophore and ionophore for the construction of Cu²⁺ selective fluorescence sensors.¹⁷ In 2006 Li's group reported that 7-aminocoumarin derivative dyes underwent change in their dipole moment on excitation from ground to the excited S1 state transition.¹⁸ Thus, these molecules are suitable sensors for investigating many physical and physiochemical processes in a condensed phase,¹⁹ making use of their fluorescence properties.

Optical methods²⁰ via fluorescent imaging and sensing are attracting attention for pH measurements due to their rapid response time,²¹ simplicity, non-invasiveness, and good sensitivity.²² Currently, a large number of fluorescence sensors for pH measurement have been reported in the literature and are commercially available due to their extensive applications. As an example, Liu²³ published on coumarin sensors that have optical responses under acid conditions. Their studies disclosed that these coumarin derivatives could be used for quantitative fluorescence imaging in detection of intracellular pH by the formation of an intramolecular charge transfer (ICT) platform in an acidic environment, leading to colour changes. In addition, Oliveira's group²⁴ designed a new ratiometric fluorescent pH coumarin quinoline derivative sensor where in acidic pH it exhibited a strong fluorescence which was very useful for monitoring pH variations from neutral to acidic conditions in living cells. Also, a series of highly fluorescence quantum yield coumarin derivatives were synthesized using a very simple, facile, and economically cheap route which was based on literature from Ahmed's group²⁵ and the optical properties were investigated under various conditions. There was high fluorescent intensity and quantum yield in DMSO. Moreover, we describe a different phenomenon between acid and alkaline conditions in DMSO and we used DFT and TDDFT calculations to confirm the experimental results.

Experimental section

General methods

All chemicals used in the current study were purchased from commercial vendors and used as received without further purification, unless otherwise noted. All solvents were purified and dried using standard methods prior to use. Nuclear magnetic resonance (¹H, ¹³C) spectra were recorded on a Bruker AM 500 spectrometer (Switzerland) with chemical shifts reported as ppm at 500 and 125 MHz, respectively (in DMSO with TMS as the internal standard). Fluorescence spectra were obtained with an F-7000 fluorescence spectrophotometer (Hitachi) in a solution of 1.0 × 10⁻⁵ mol L⁻¹. UV-Vis absorption spectra were measured on a UV-2550 (Shimadzu). The absorption and emission studies at different pH were all measured in a DMSO–water mixture (DMSO : H₂O = 9 : 1) by a PHS-3C digital pH-meter (Youke, Shanghai).

General procedure for the synthesis of 4-aminocoumarin

A mixture of powdered 4-hydroxycoumarin (1.07 g, 0.0066 mol) and ammonium acetate (7.87 g, 0.100 mol) was melted in an oil bath (max. 130 °C) and stirred for 3 h. After cooling to ambient temperature, water was added and the crude product was isolated as yellow crystals by simple filtration. 4-Aminocoumarin was obtained by recrystallized from EtOH and water as yellow crystals (0.98 g, 92.2%).²⁵

General procedure for the synthesis of compound 4a–4e

A mixture of benzaldehyde (0.106 g, 1 mmol), 1,3-cyclohexanedione (0.112 g, 1 mmol) and 4-aminocoumarin (0.161 g, 1 mmol) in acetic acid (6 mL) was stirred for 2–3 h at 110 °C; the completed reaction was detected by TLC analysis and then cooled to room temperature. Then, the reaction mixture was washed with H₂O (20 mL) and the crude product was recrystallized from DMSO to give product 4a (0.295 g, 86.3%) as light yellow solid.²⁵

7-Phenyl-9,10,11,12-tetrahydro-6H-chromeno[4,3-b]quinoline-6,8(7H)-dione (4a). Light yellow solid. Yield 86%. Mp 296–298 °C. ¹H-NMR (DMSO, 500 MHz) δ (ppm): 9.76 (s, 1H), 8.32 (d, J = 8.0 Hz, 1H), 7.62–7.65 (m, 1H), 7.44 (t, J = 7.5 Hz, 1H), 7.38 (d, J = 8.3 Hz, 1H), 7.11–7.24 (m, 4H), 7.09 (t, J = 7.2 Hz, 1H), 4.99 (s, 1H), 2.82–2.87 (m, 1H), 2.67–2.73 (m, 1H), 2.24–2.32 (m, 2H), 1.98–2.02 (m, 1H), 1.87–1.88 (m, 1H). ¹³C NMR (DMSO, 125 MHz) δ (ppm): 194.93 (1C), 160.33 (1C), 152.02 (1C), 151.67 (1C), 145.90 (1C), 142.05 (1C), 131.92 (1C), 128.02 (2C), 127.65 (2C), 126.18 (1C), 123.97 (1C), 122.93 (1C), 116.85 (1C), 113.03 (1C), 111.86 (1C), 101.78 (1C), 36.66 (1C), 34.12 (1C), 26.37 (1C), 20.71 (1C).

7-(4-Nitrophenyl)-9,10,11,12-tetrahydro-6H-chromeno[4,3-b]quinoline-6,8(7H)-dione (4b). Light yellow solid. Yield 90%. Mp 260–262 °C. ¹H-NMR (DMSO, 500 MHz) δ (ppm): 9.88 (s, 1H), 8.34–8.36 (m, 1H), 8.08–8.10 (m, 2H), 7.65–7.68 (m, 1H), 7.51–7.54 (m, 2H), 7.45–7.48 (m, 1H), 7.40 (t, J = 7.7 Hz, 1H), 5.11 (s, 1H), 2.87 (t, J = 9.4 Hz, 1H), 2.73 (t, J = 3.9 Hz, 1H), 2.27–2.33 (m, 2H), 1.98–2.02 (m, 1H), 1.88 (s, 1H). ¹³C NMR (DMSO, 125 MHz) δ (ppm): 194.89 (1C), 160.21 (1C), 153.12 (1C), 152.31 (1C), 152.14 (1C), 145.94 (1C), 142.60 (1C), 132.22 (1C), 129.10 (2C), 124.06 (1C), 123.26 (2C), 123.12 (1C), 116.92 (1C), 112.83 (1C), 110.97 (1C), 100.63 (1C), 36.50 (1C), 34.96 (1C), 26.38 (1C), 20.61 (1C).

7-(4-Methoxyphenyl)-9,10,11,12-tetrahydro-6H-chromeno[4,3-b]quinoline-6,8(7H)-dione (4c). Light yellow solid. Yield 88%. Mp 276–278 °C. ¹H-NMR (DMSO, 500 MHz) δ (ppm): 9.72 (s, 1H), 8.30–8.32 (m, 1H), 7.61–7.65 (m, 1H), 7.42–7.45 (m, 1H), 7.37 (t, J = 8.2 Hz, 1H), 7.13–7.14 (m, 2H), 6.75–6.77 (m, 2H), 4.93 (s, 1H), 3.66 (s, 3H), 2.81–2.86 (m, 1H), 2.65–2.72 (m, 1H), 2.24–2.32 (m, 2H), 1.98–2.03 (m, 1H), 1.84–1.90 (m, 1H). ¹³C NMR (DMSO, 125 MHz) δ (ppm): 194.90 (1C), 160.32 (1C), 151.98 (1C), 151.32 (1C), 141.74 (1C), 138.23 (1C), 131.80 (1C), 128.59 (3C), 123.91 (1C), 122.86 (1C), 116.79 (1C), 113.38 (2C), 113.05 (1C), 112.09 (1C), 102.06 (1C), 55.99 (1C), 36.67 (1C), 33.19 (1C), 26.33 (1C), 20.74 (1C).

7-(3-Chlorophenyl)-9,10,11,12-tetrahydro-6H-chromeno[4,3-b]quinoline-6,8(7H)-dione (4d). Light yellow solid. Yield 82%. Mp 267–269 °C. ¹H-NMR (DMSO, 500 MHz) δ (ppm): 9.82 (s, 1H), 8.33–8.35 (m, 1H), 7.46 (s, 1H), 7.39–7.41 (m, 1H), 7.36–7.38 (m, 3H), 7.28 (m, 1H), 5.26 (s, 1H), 2.85–2.88 (m, 1H), 2.68–2.72 (m, 1H), 2.29–2.33 (m, 2H), 2.00–2.02 (m, 1H), 1.83 (s, 1H). ¹³C NMR (DMSO, 125 MHz) δ (ppm): 194.96 (1C), 160.30 (1C), 152.11 (1C), 149.12 (1C), 140.13 (1C), 133.39 (1C), 132.14 (1C), 131.21 (1C), 128.34 (1C), 126.83 (1C), 123.95 (1C), 121.08 (1C), 120.23 (1C), 116.82 (1C), 114.08 (1C), 113.04 (1C), 111.01 (1C), 101.09 (1C), 36.61 (1C), 33.93 (1C), 26.37 (1C), 20.65 (1C).

7-(p-Tolyl)-9,10,11,12-tetrahydro-6H-chromeno[4,3-b]quinoline-6,8(7H)-dione (4e). Light yellow solid. Yield 83%. Mp 304–305 °C. ¹H-NMR (DMSO, 500 MHz) δ (ppm): 9.72 (s, 1H), 8.30–8.32 (m, 1H), 7.61–7.63 (m, 1H), 7.41–7.45 (m, 1H), 7.36 (t, J = 8.2 Hz, 1H), 7.12 (d, J = 8.05 Hz, 2H), 7.00 (d, J = 7.95 Hz, 2H), 4.95 (s, 1H), 2.82–2.86 (m, 1H), 2.69–2.71 (m, 1H), 2.18–2.31 (m, 2H), 2.19 (s, 3H), 2.0–2.02 (m, 1H), 1.82–1.89 (m, 1H). ¹³C NMR (DMSO, 125 MHz) δ (ppm): 194.86 (1C), 160.28 (1C), 151.99 (1C), 151.40 (1C), 143.04 (1C), 141.87 (1C), 135.15 (1C), 131.81 (1C), 128.54 (2C), 127.52 (2C), 123.90 (1C), 122.87 (1C), 116.79 (1C), 113.02 (1C), 112.00 (1C), 101.91 (1C), 36.66 (1C), 33.66 (1C), 26.34 (1C), 20.72 (1C), 20.51 (1C).

Computational methodology

All the structure optimizations were based on density functional theory (DFT) with the B3LYP functional and 6-311G basis set. The fluorescence emission wavelength and the excitation wavelength of the compounds were calculated with the time-dependent density functional theory (TDDFT). All these calculations were performed with Gaussian 09W.²⁶

Results and discussion

As shown in Scheme 1, the coumarin fused dihydropyridine derivatives 4 were synthesized by an efficient three-component reaction of 1,3-cyclohexadione and aryl aldehydes and products 4 were obtained in 86% (4a), 90% (4b), 88% (4c), 82% (4d), and 83% (4e) yields, respectively. With a para-nitro substituent on the benzene unit the largest yield (90%) of the desired product was obtained. These compounds were all characterized by ¹H-NMR and ¹³C-NMR.

Scheme 1 Synthesis of coumarin fused dihydropyridine derivatives 4.

UV-Vis absorption and fluorescence emission spectra

The absorption and fluorescence emission spectra of coumarin-fused dihydropyridine derivatives 4 were detected and summarized in Fig. 1. The fluorescence properties of the coumarin derivatives 4 were examined under different conditions including different organic solvents and with different pH conditions.

Fig. 1 Left: Absorption spectra of compound 4c (1.0 × 10⁻⁵ mol L⁻¹) in different solutions (PMT voltage: 900 V). Right: Fluorescence emission spectra of compound 4c in different solutions (PMT voltage: 500 V).

The solvent effect on absorption and fluorescence properties of 7-(4-methoxyphenyl)-9,10,11,12-tetrahydro-6H-chromeno[4,3-b]quinoline-6,8(7H)-dione (4c) were examined (Fig. 1, Table 1). As shown in Fig. 1, 4c showed a maximum sharp absorption peak at approximately 308 nm accompanied by a shoulder peak at near 375 nm and these were barely affected by solvent polarity. The molar absorption coefficient (ε) of 4c underwent a slight change in different organic solvents; in MeCN it had the highest value.

Table 1 Photophysical properties of compound 4a–4e

Compounds	λabs^a (nm)	λem^a (nm)	ε (M^−1 cm^−1)	ΦF^a	SS^b (nm)
a Measurements were performed in DMSO solvent. Fluorescence quantum yields were measured with quinine sulfate (ΦF = 54.6% in 0.05 M H2SO4) as the reference.b Stokes shift.
4a	368	440	14 000	0.41	72
4b	373	441	18 700	0.014	68
4c	361	441	20 800	0.26	80
4d	373	443	15 200	0.60	70
4e	366	441	15 300	0.83	75

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Obviously, 4c shows the fluorescence emission wavelength at 438 nm with 375 nm as the excitation wavelength in different organic solvents, except for a slight blue shift in CH₃CN (410 nm). It's worth noting that the fluorescence intensity of 4c is much higher in DMSO than other solvents and thus we can anticipate high fluorescence quantum yields in DMSO. The Stokes shift is 63 nm in DMSO. Therefore, we selected DMSO as the best solvent for studies on the fluorescent properties of compound 4.

The absorption and fluorescence emission spectra of compounds 4a–4e in DMSO are detailed in Table 1 and Fig. 2. They show similar absorption, but have a slight change in intensity with different substituents on the phenyl ring. Similarly, the fluorescence emission wavelength of 4 was at about 440 nm and showed a strong blue fluorescence, but fluorescence intensity was significantly different with the different groups substituted on the phenyl ring of the coumarin-fused dihydropyridine derivatives of 4. As shown in Fig. 2, compound 4e, with p-methyl substituted on the phenyl ring, had the maximum fluorescence intensity and also the fluorescence quantum yield reached 0.83. Compounds 4a (H), 4c (4-OMe), and 4d (3-Cl) all had strong blue fluorescence and the fluorescence quantum yields were respectively, 0.41, 0.26, and 0.60. It is worth noting that compound 4e shows an unexpected higher fluorescence quantum yield (Φ_F = 0.83) than most of the fluorophores, and the Stokes shift of 4e is 75 nm. Compound 4b, with p-nitro substituted on the phenyl ring, displayed the maximum absorption and the minimum fluorescence intensity, so it's fluorescence is weak and the fluorescence quantum yield is only 0.014. In addition, through the TD-DFT calculation, the HOMO–LUMO energy gap of compound 4b is largest, which makes the transition hard to occur. As shown in Table 1, the Stokes shift of compounds 4a, 4b, 4c, 4e, and 4d, respectively were 72, 68, 80, 70, and 75 nm in DMSO.

Fig. 2 Left: Absorption spectra of 4a–4e (1.0 × 10⁻⁵ mol L⁻¹) in DMSO (PMT voltage: 900 V). Right: Fluorescence emission spectra of 4a–4e in DMSO (PMT voltage: 600 V).

On the other hand, we also examined absorption and fluorescence properties of compound 4 under different pH from 2 to 12 in a DMSO–water mixture (DMSO : H₂O = 9 : 1). As shown in Fig. 3 and 4, compound 4a exhibited different properties in UV-Vis absorption and fluorescence spectra. A new absorption peak appeared at 455 nm, which is situated in the visible region, as the pH value gradually increased from 7 to 12 in a DMSO–water mixture. The solution was colorless under acidic conditions and turned yellow under basic conditions. The fluorescence intensity of 4a was obviously larger under alkaline conditions than under acidic conditions. The maximal emission wavelength of 4a had an obvious red shift (103 nm) under different pH conditions; it displayed blue fluorescence at 450 nm under acidic conditions (pH: 2–7) and yellow-green fluorescence at 553 nm under basic conditions (pH: 7–12). This effect causes significant changes in fluorescence color; blue in acid solution and yellow-green in an alkaline environment (Fig. 4). Moreover, the substituted compounds 4c–4e exhibited the same property accompanied by the change of pH value, and the maximal absorption wavelength displayed a red shift from 375 nm to 450 nm when the solution changed from acid to alkaline. The fluorescence intensity varied with the pH and the fluorescence intensity reached a maximum when the pH value of the solution was pH 11. However, the fluorescence emission wavelength of compound 4b did not change significantly in alkaline condition; rather it happened due to an intermolecular charger transfer (ICT) process (Fig. 5).²⁷

Fig. 3 Left: The absorption spectra of compound 4a with different pH (1.0 × 10⁻⁵ mol L⁻¹) in DMSO–water mixture (DMSO : H₂O = 9 : 1, PMT voltage: 900 V). Right: Fluorescence emission spectra of compound 4a (1.0 × 10⁻⁵ mol L⁻¹) with different pH in DMSO–water mixture (DMSO : H₂O = 9 : 1, PMT voltage: 500 V).

Fig. 4 Left: Compound 4a (1.0 × 10⁻⁵ mol L⁻¹) under visible light with different pH in DMSO–water mixture (DMSO : H₂O = 9 : 1). Right: Compound 4a (1.0 × 10⁻⁵ mol L⁻¹) under a UV lamp (365 nm) with different PH in DMSO–water mixture (DMSO : H₂O = 9 : 1).

Fig. 5 Left: The absorption spectra of compounds 4a–4e (1.0 × 10⁻⁵ mol L⁻¹) with pH = 11. Right: The fluorescence emission spectra of compounds 4a–4e (1.0 × 10⁻⁵ mol L⁻¹) with pH = 7 and pH = 11 in DMSO–water mixture (DMSO : H₂O = 9 : 1, PMT voltage: 550 V).

DFT calculations: rationalize the excitation and emission spectra. Recently, theoretical calculations have become attractive for studying fluorescence or molecular probes.²⁶ In order to support our assumptions, we made the compound's geometric optimizations using the density functional theory (DFT) method as implemented in Gaussian 09W suits of the program employing basis sets B3LYP/6-311G (see Fig. 6.); the most stable structures of compounds 4a–4e indicated that the precursor of these compounds contained a large conjugated system²⁸ which is useful for their fluorescence properties.²⁹ However, the benzene ring, which is depicted at the bottom of the pyridine structure, is not in the same plane due to steric effects of the carbonyl groups.

Fig. 6 Optimized geometry of compounds 4a–4e at B3LYP/6-311g level with Gaussian 09W.

TD-DFT calculations were performed during this study to further understand the mechanism of the fluorescence intensity with different groups from the energy and charge transfer model (see Fig. 7). In addition, we calculated the fluorescence emission spectra and the HOMO–LUMO energy gap³⁰ by the TD-DFT method as implemented (see Table 2). From this calculation, compound 4b requires the most energy (ΔE = 0.66 eV) to finish the HOMO–LUMO transition and compound 4e requires the least energy (ΔE = 0.31 eV) to finish HOMO–LUMO transition. Therefore, the photochemical property of compound 4b (Φ_F = 0.014) is worse than compound 4e (Φ_F = 0.83).

Fig. 7 Frontier molecular orbitals of compound 4b and 4e.

Table 2 Calculation data of compounds 4a–4e

Compounds	λcal^a (nm)	λexp^b (nm)	λcal^c (nm)	λexp^d (nm)	f^e	Transition character	Energy gap (eV)
a Theoretical calculations of basis sets B3LYP/6-311G without solution (the calculated emission maximums).b λmax in DMSO solvent (the experimental emission maximums).c Theoretical calculations of basis sets B3LYP/6-311G without solution (the calculated absorption maximums).d λmax in DMSO solvent (the experimental absorption maximums).e Oscillator strength coefficients.
4a	450	440	372	368	0.0122	HOMO → LUMO	0.44
4b	473	441	375	373	0.0093	HOMO → LUMO	0.66
4c	455	441	368	361	0.0154	HOMO → LUMO	0.44
4d	447	443	373	373	0.0189	HOMO → LUMO	0.39
4e	459	441	367	366	0.0215	HOMO → LUMO	0.31

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Conclusions

The coumarin fused dihydropyridine derivatives were prepared by an efficient one-pot, three-component reaction and obtained in good yield. Optical properties detections indicated that these coumarin derivatives had excellent fluorescence with high fluorescent quantum yields and exhibited significant emission changes in their fluorescence spectra with changes of pH values. TD-DFT calculations were performed with this study to support our experiment results. Findings will be useful for design of new pH probe fluorescent dyes with high fluorescent quantum yields and chemical sensors for detecting pH environments.

Acknowledgements

We gratefully acknowledge the financial supported by the Natural Science Foundation of China (21176223) and the National Natural Science Foundation of Zhejiang (LY13B020016) and the Key Innovation Team of Science and Technology in Zhejiang Province (2010R50018).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra05996a

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