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Synthesis and spectroscopic study of 2,7-diethylamino-2-oxo-2H-chromen-3-yl benzothiazole-6-sulfonyl chlorides and its derivatives
⁎Corresponding author. Fax: +968 2441469. alkindy@squ.edu.om (Salma M.Z. Al-Kindy)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
2,7-Diethylamino-2-oxo-2H-chromen-3-yl benzothiazole-6-sulfonyl chlorides dye (Coumarin 6-SO2Cl) was used to synthesize anilines and amino acid derivatives.
The electronic absorption and emission spectra of the label and its derivatives in organic solvents of different polarity, micellar systems and in aqueous buffered media are investigated. The label and its derivatives exhibited more or less the same excitation and emission wavelength around 470 and 520 nm, respectively. Maximum fluorescence quantum yield for aniline derivative was observed in ethyl acetate while minimum yield was observed in water. In micellar systems, maximum quantum yield was observed in the presence of Tw-20 for the label while proline derivative gave maximum enhancement in the presence of Tw-80. The results reflect the importance of medium effect on the fluorescence intensity and molar absorptivity of the label and its derivatives.
Keywords
Coumarin-6
Derivatives
Electronic absorption
Emission spectra
Surfactant
1 Introduction
Coumarin and its derivatives occur widely in nature, and have been extensively studied due to their commercial applications in biological, chemical and physical fields. They possess excellent biological activity, such as anticancer and anticoagulant activity (Abraham et al., 2010). Moreover, this series of dye compounds has outstanding optical properties, including an extended spectral range, high quantum yields of fluorescence, superior photostability and good solubility in common solvents and hence has been found useful as optical brighteners, laser dyes, fluorescence indicators and sunburn preventive materials. Due to their analytical and biological uses, the study of coumarin derivatives has special topical importance. The photophysical and spectroscopic properties of coumarin derivatives can be readily modified by the introduction of substituents in the coumarin ring, giving them more flexibility to fit well in various applications (Kitamura et al., 2007; Chen et al., 2010; Sheng et al., 2008; Gao et al., 2000). The unsubstituted coumarin molecule is non-fluorescent but it exhibits intense fluorescence on substitution of various functional groups at different positions. In general, electron-donating substituents in positions 3- or 7-of the coumarin ring tend to enhance emission intensity accompanied by bathochromic shifts of the absorption maximum while electron-withdrawing substituents tend to diminish it. The improvement of the photophysical properties of coumarin derivatives directed toward a bathochromic shift of the fluorescence maximum, large Stokes shift and high fluorescence quantum yields is of interest in many laboratories. On the other hand, the demand for trace analysis in various fields has shifted the requirements of analytical determination to increasingly lower levels of detection. Fluorimetric methods seem to meet such a demand being typically 1000-fold more sensitive than the corresponding UV methods. Since only a small minority of organic compounds show analytically useful intrinsic fluorescence, most methods involve labeling of the analyte with a suitable fluorophore. Fluorescence derivatization is one of the most versatile and sensitive techniques employed in analytical laboratories to overcome low detection limits as it takes advantage of the fluorescence properties of the reagent used. The synthesis of coumarins and their derivatives has attracted considerable attention for many years as a large number of natural products contain this heterocyclic nucleus. Recently, our group synthesised coumarin-sulfonyl chloride (C6SCl) and used it as a pre-column derivatization reagent. A quantitative derivatization reaction of C6SCl with phenols and amino acids has been achieved giving stable derivatives which possess a reasonable molar absorptivity and a large Stokes shift (Al-Kindy and Miller 1989a, 1989b; Suliman et al., 2006). The luminescent properties of these derivatives met well with the requirements of an ideal label.
On the other hand, the arylsulfonamido group was introduced into the fluorescent dyes containing 2-(7-Diethylamino-2-oxo-2H-chromen-3-yl)-benzothiazole (coumarin 6) in order to improve the photostability of the dyes (Christie et al., 2008). To our knowledge, coumarin-6 containing an arylsulfonyl chloride group has not been used as a fluorigenic label before. A combination of the reactivity and versatility of the arylsulfonyl chloride group, with the improved quantum yield and bathochromic shift of the benzothiazole moiety incorporated in the coumarin nucleus, may result in a fluorigenic, label with improved properties when compared to the sulfonyl chloride labels based on unsubstituted coumarin. Herein, we report the synthesis of 2-(7-Diethylamino-2-oxo-2H-chromen-3-yl)-benzothiazole-6-sulfonyl chloride (Coumarin 6-SO2Cl) (Scheme 1) and explore the possibility of using it as a fluorigenic label for the derivatization of amino acids, and anilines.
It is well documented, however, that the fluorescence properties of a molecule do not only depend on the structural characteristics of the molecule but are also a function of its environment. Knowledge of possible environmental effects on the spectral characteristics is therefore necessary for the utilization of such fluorescent labels to their maximum potential. Thus this work also aims at investigating the effect of the medium on the spectral behavior of Coumarin 6-SO2Cl and its derivatives in homogenous solutions of varying polarity and buffer solutions of different pH values. In addition, the spectral characteristics in aqueous micellar systems of anionic and cationic surfactants were investigated.
2 Experimental
2.1 Materials and methods
(See Schemes 1 and 2).
2.2 General methods
All reactions sensitive to air were performed under Argon atmosphere in flame-dried flasks, and all the reagents were introduced by syringes. All solvents were dried by standard methods. Starting materials were commercially available and were used without further purification. Coumarin 6, chlorosulfonic acid, sodium carbonate, hydrochloric acid, benzyl bromide, sodium chloride, sodium hydroxide pellets, sodium dihydrogen phosphate, butan-1-ol, hexan-1-ol, dichloromethane, diethyl ether, ethyl acetate, acetone and aniline were purchased from BDH chemicals Ltd (UK, Pool). All solvents used are of HPLC or spectroscopic grade unless otherwise indicated. Ultra pure water from Millipore corporation was used throughout. Products were purified by flash chromatography on silica gel (230–400 mesh, Merck) using Hexane: ethylacetate (3:1) as eluent. 1H NMR [CDCl3 (δ = 7.25 ppm) or TMS (δ = 0.00 ppm) as internal standard] were recorded on Bruker AC 500 (400 MHz). Integrals are in accordance with assignments; coupling constants are given in Hz. IR spectra were measured with an FTIR spectrometer (Perkin–Elmer BX FT-IR-spectrometer) with DTGS-detector. Mass spectra were recorded with High performance Liquid Chromatography-Tandem Quadrupole Mass Spectrometry [HPLC-MS/MS, Quattro Ultima Pt (MS spec). Melting points were measured on a Gallenkamp melting point microscope and were uncorrected. Thin layer chromatography was performed on TLC plates on Merck silica gel 60.
Absorbance spectra were measured using a Varian CARY 50 UV–visible spectrophotometer. A quartz cell of 1 cm path length was used. The reference cell contained pure solvent of the solution to be investigated. Perkin Elmer LS55 luminescence spectrometer (UK) was used to record both the emission and the excitation spectra. A quartz cell of 1 cm path length was used for measurement. The pHs of the solutions were measured on a JENWAY 3015 pH meter (UK). The pH meter was calibrated before use with buffer solutions prepared from pH buffer powder of pH 4.0, 7.0 and 9.0.
2.3 Synthesis of coumarin 6 derivatives
2.3.1 2,7-Diethylamino-2-oxo-2H-chromen-3-yl benzothiazole-6-sulfonyl chloride (2)
Coumarin 6-sulfonyl chloride (2) and aniline derivatives were synthesized according to the literature method (Christie et al., 2008). The structures of the products were verified using NMR and mass spectroscopic tools.
2.3.2 1-[2-(7-Diethylamino-2-oxo-2H-chromen-3-yl)-benzothiazole-6-sulfonyl]-pyrrolidine-2-carboxylic acid (7)
To a solution of l-proline (0.10 g, 0.80 mmol) dissolved in sodium carbonate hydrate (0.25 M (20 mL), Coumarin 6 sulfonyl chloride (0.10 g, 0.22 mmol) in acetonitrile (20 mL), was added. The mixture was stirred for 3 h at 20 °C. Then, it was extracted with ethylacetate (3 × 10 ml) to remove unreacted starting material. It was then acidified with concentrated HCl and extracted with ethylacetate again (3 × 10 ml). After that, the solvent was removed using rotary evaporator. The reaction yielded red crystals (0.08 g, 68%); max (KBr)/cm−1 3448 (NH), 1620 (C⚌O), 1502 (Ar C–C), 1354, 1163 (SO2NH); m/z (EI): 528 (M+, 100%), 509 (M+-H2O, 40); 1H NMR (400 MHz, DMSO-d6) δ = 1.13 (6H, t, N(CH2CH3)2), 1.53–2.2 (m, 2H, β-H, γ-H), 2.8 (m, 1H, δ-H), 3.39 (4H, q, N(CH2CH3)2), 3.55 (1H, m, α-H), 6.55 (1H, dd, J = 8.0, 2.0 Hz, 6-H), 6.58 (1H, d, J = 2.0 Hz, 8-H), 7.55 (1H, d, J = 8.0 Hz, 5-H), 8.14 (1H, s, H-4), 8.22 (1H, d, J = 8.0 Hz, 5′-H), 8.79 (1H, s, H-8′), (1H, d, J = 8.0 Hz, 6′-H).
3 Results and discussion
3.1 Synthesis
There are several reports which described the synthesis of Compound 1 (British patent, 1963; Hausermann and Voltz, 1961; Kendall et al., 1961; Harnisch, 1976) which is commercially available. In this study compound 1 was subjected to the standard chlorosulfonation procedure which furnished dye 2 in excellent yield (Scheme 1). When aniline 3 was mixed with dye 2, the expected derivative 5 was obtained in 72% yield. Similarly, the reaction of p-toludine 4 with dye 2 afforded the derivative 6 in 83% yields (Scheme 1). Gratifyingly, dye 7 was obtained in a reasonable yield when dye 2 was reacted with the amino acid proline (Scheme 2).
3.2 Spectroscopic properties of the label and its derivatives
The absorption and fluorescence spectra of coumarin 6-SO2Cl, proline, aniline and p-toludine derivatives were measured in methanol and ethanol as solvents. The parent Coumarin 6 molecule exhibited an absorption band at 457 nm in methanol. Chlorosulfonation of Coumarin 6 resulted in a bathochromic shift of maximum wavelength from 457 to 470 nm in methanol. All the derivatives exhibited more or less the same maximum absorption wavelength at 470 nm. A slight blue shift in wavelength was observed in proline derivative to 465 nm. Maximum absorbance was exhibited by the label (coumarin 6-SO2Cl). Similar behavior was observed in ethanol, but with an increase in the molar absorptivities of the label and the derivatives. This long wavelength absorption band has been assigned as the intramolecular charge transfer transition due to the transfer of the charge between the electron donor groups (diethyl amino group) to the carbonyl oxygen center in the molecule (Turki et al., 2006). The label exhibited an emission wavelength of 516 nm when excited at 470 nm. A slight red shift was observed in the emission wavelengths of the derivatives in methanol. In methanol, the aniline derivative exhibited maximum fluorescence intensity followed by the label, proline and p-toludine derivatives. While using ethanol as the solvent, maximum emission was exhibited by the aniline derivative followed by the proline, Coumarin 6-SO2Cl and p-toludine derivatives. The fluorescence excitation and emission spectra were almost mirror images of each other in the label and the studied derivatives. This behavior may indicate that the geometry of the So state is similar to that of the S1 state.
3.3 Effect of various solvents
The effects of solvent on the absorption and fluorescence properties were studied using the aniline derivative as a representative. For all fluorescence measurements the dye concentration was around 1 × 10−8 M and absorbance at the excitation wavelength was below 0.05. The fluorescence was measured in solvents of different polarities.
The results are summarized in Table 1. A slight blue shift in the absorption maximum was observed with an increase in the hydrogen bonding ability of the solvent along with a decrease in the molar absorptivity of the derivative. The spectroscopic properties of these dyes are dominated by the intramolecular charge transfer (ICT) states associated with the coumarin moiety. The less pronounced absorption shift with increasing solvent polarity implies that the ground state energy is not affected to a greater extent, possibly due to stronger hydrogen bonding with the solvent by the derivatives in the ground state or to Franck–Condon effects arising from different equilibrium geometries in ground and lowest excited singlet states, respectively. The absorption spectra show a single absorption band around 475 nm in CH2Cl2. This band was not very sensitive to the solvent polarity. The position of absorption maximum is blue shifted to 460 nm in the presence of water with a decrease in the molar absorptivity. It has been reported; that the absorption peak is blue shifted in the presence of some donor–acceptor charge transfer systems in protic solvents (Mahanta et al., 2008). Other protic solvents such as ethanol and methanol gave rise to a much smaller blue shift when compared to that in water. This is because the hydrogen bonding strength of ethanol and methanol solvents is comparatively less.
| Solvent | λmax | λex (nm) | λem (nm) | ε | ϕ | Stoke shift (cm−1) |
|---|---|---|---|---|---|---|
| Water | 457 | 460 | 523 | 3.42 × 104 | 0.22 | 151515 |
| Methanol | 466 | 473 | 519 | 1.01 × 105 | 0.58 | 188679 |
| Ethanol | 467 | 473 | 514 | 1.15 × 105 | 0.60 | 212766 |
| Propanol | 469 | 473 | 513 | 1.23 × 105 | 0.78 | 227273 |
| Butanol | 468 | 480 | 510 | 1.31 × 105 | 0.82 | 238095 |
| Acetonitrile | 472 | 470 | 521 | 1.74 × 105 | 0.77 | 204082 |
| Ethylacetate | 467 | 460 | 508 | 1.34 × 105 | 0.96 | 243902 |
| CH2Cl2 | 475 | 480 | 503 | 1.77 × 105 | 0.87 | 357143 |
ϕ, Quantum yield; ε, molar absorptivity L mol−1 cm−1.
In general, the fluorescence quantum yield of the aniline derivative increased with a decrease in the polarity of the solvent accompanied by a blue shift in emission wavelength. This is clearly observed in a series of alcohol where the quantum yield increased as the polarity decreased. The correlation is not perfect however, since the fluorescence quantum yield in acetonitrile was found to be higher than that of methanol and ethanol, although the dielectric constant of acetonitrile is higher than that of methanol. The decrease of the fluorescence quantum yield in polar solvents has been previously reported to be due to the deactivation of the excited state by a twisted internal charge transfer (TICT) mechanism (Ammar et al., 2003). This consists of the appearance of localized charges in the excited state, accompanied by the rotation of a molecular moiety to reach a right angle. This mechanism has been known to occur in coumarins with a strong donor group in the 7-position and a strong acceptor group (often in the 3-, 4- and 6-positions) (Ammar et al., 2003; Dahiya et al., 2005). Solvents seem to play an important role in stabilizing the highly dipolar TICT state. Therefore it is formed only in polar solvents. It had been reported that the parent coumarin-6 molecule exists in a non-planar structure in non-polar solvents, where its diethylamino moiety adopts a pyramidal conformation. In polar solvents, the molecule adopts a planar ICT structure (Satpati et al., 2005). Due to reduced resonance of the amino lone pair with the benzopyrone moiety, the non-planar structure is expected to be less polar in character when compared to the planar ICT form (Satpati et al., 2005).The excitation wavelength on the other hand, exhibited a red shift with a decrease in dielectric constant of the solvents.
On considering a series of alcohols, the Stokes' shifts increase with an increase in solvent polarity as shown in Table 1. This behavior is indicative of a charge transfer transition. Similar results have been previously reported for other coumarins (Raikar et al., 2006) which were found to exhibit large Stokes' shifts in going from non-polar cyclohexane to polar ethanol. The charge transfer nature of the ππ∗ state makes it more polarized than the ground state leading to more stabilized excited states than the ground state in polar solvents (Kurashuge et al., 2007). The general increase in the Stokes' shift value with increasing solvent polarity shows that there is an increase in the dipole moment on excitation. Similar results were previously reported (Satpati et al., 2005) for the fluorescence behavior of the parent coumarin 6 nucleus.
3.4 Effect of pH
Since the molecules bear ionizable groups their spectroscopic properties are dependent upon the pH, the UV–visible absorption and fluorescence properties of coumarin 6-SO2Cl were measured in acetate buffer in the pH range (1–12) (Figs. 1 and 2). At pH 1 the spectra display a single band at λmax = 523 nm with absorbance of 0.066. On increasing the pH to 2.0, the band at 523 nm decreased while a new band at 482 nm emerged. The band at 523 nm is attributed to the protonation of the nitrogen atom of the benzothiazole ring. On increasing the pH further, the benzothiazole nitrogen is de-protonated and a shift in wavelength is observed. It has been reported that the protonation of the benzothiazole nitrogen ring occurs at low pH < 2.0 (Jones and Jimenez, 2001). For the pH range 2–5, an increase in absorbance was observed with a continuous blue shift from 482 nm at pH 2.0 to 472 nm at pH 5.0. The increase in absorbance with an increase in pH at this range may be explained to be due the de-protonation of the tertiary nitrogen in the N-Et group. The pKa of the diethyl amino group is expected to occur in the range of 4.0–6.0. The existing species at the pH range 6–9 may be due to the neutral form of the molecule. On increasing the pH to 10.0, the absorbance increased. This behavior is attributed to the ring opening of the coumarin nucleus at high pH giving rise to an anionic species. A further increase in the pH resulted in a decrease in the absorbance of the dye. It has been reported that the monocation of coumarin 6 parent compound possesses a higher molar absorptivity compared to the neutral form (Corrent et al., 1998). Clearly as has been previously reported the donor–acceptor charge transfer system shows characteristic spectral change in the presence of acid (Mahanta et al., 2008).
Absorption spectra of 2.7 × 10−7 M Coumarin 6-SO2Cl in different buffer solutions pH range 1–6 (1) pH 5. (2) pH 4 (3) pH 3, (4) pH 6 (5) pH 2 (6) pH 1.
Absorption spectra of 2.7 × 10−7 M Coumarin 6-SO2Cl in different buffer solutions pH range 7–12. (1) pH 10 (2) pH 11 (3) pH 9, (4) pH 7 (5) pH 12 (6) pH 8.
The fluorescence spectra of 1.00 × 10−8 M of coumarin 6-SO2Cl solutions were measured at λex = 470. On increasing the pH from 1–6, the fluorescence intensity increased (Fig. 3). The maximum fluorescence intensity was observed at pH 6. The intensity decreased on increasing the pH to 7.0 and then increased up to pH 10. A decrease in fluorescence intensity of the dye was observed with a further increase in pH (Fig. 4). Using similar reasoning as for the absorption spectra, the low emitting species at pH 1.0 is due to the protonated benzothiazole ring nitrogen. Similarly, the increase in emission intensity for the pH range 2.0–6.0 is due to the de-protonation of the lone pair of the tertiary amine group in the diethyl amine chain. With an increase in pH up to pH 8, the neutral species of the tertiary amine predominates. From pH 9.0 to 12.0, the emission is due to the anionic form of the molecule which occurs by ring opening of the coumarin nucleus. This is a typical result of the coumarin nucleus. Similar results were previously reported for CSCl and its phenol derivatives (Suliman et al., 2006). On increasing the pH further, a decrease in fluorescence intensity was observed. The excitation and emission wavelength exhibited slight blue shift with an increase in pH from λex 480 at pH 1.0 to 462 nm at pH 6.0 and the emission wavelength on the other hand exhibited emission at λem 534 at pH 1.0 and 520 nm at higher pH. The pH behavior observed in the excited state matches well with that reported in the ground state above.
Fluorescence spectra of 1.0 × 10−8 M. of Coumarin 6-SO2Cl in different buffers pH range 1–6. (1) pH 1 (2) pH 2 (3) pH 3 (4) pH 5 (5) pH 6 λem = 520 nm, λex = 470 nm. Excitation and emission slit widths = 3 nm.
The Fluorescence spectra of 1.0 × 10−8 M Coumarin 6-SO2Cl in various pH solutions pH range 9–12. (1) pH 9 (2) pH 12 (3) pH 11 (4) pH 10. λem = 520 nm, λex = 470 nm. Excitation and emission slit widths = 3 nm.
3.5 Effect of micellar system
The absorption spectra of 5 × 10−6 M of coumarin 6-SO2Cl and its proline derivative were measured in water, anionic, cationic and neutral micellar system. The concentration of the surfactants was maintained at 0.1% which is above the critical micelle concentration of the surfactants (CMC). The absorption spectra show a well-defined band at 470 nm. On addition of surfactants, the absorbance was increased slightly in all cases except for the addition of CTAB where a decrease in absorbance was observed (Table 2 and Table 3). Maximum absorbance was exhibited in nonionic surfactants for both coumarin 6-SO2Cl and the proline derivative. Maximum absorbance of coumarin 6-SO2Cl was observed in the presence of TW-80, followed by TritonX-100, TW-20, and SDS. But in proline derivative the maximum absorbance was observed in the presence of TW-20 followed by TW-80, TritonX-100 and SDS. A slight decrease in absorbance was observed in the presence of CTAB surfactant. When compared to coumarin 6-SO2Cl, proline derivative exhibited less absorbance in water as well as the different miceller media.
| Surfactant | λabs/nm | A | λex/nm | λem/nm | ϕ |
|---|---|---|---|---|---|
| Water | 460 | 0.34 | 470 | 520 | 0.59 |
| Triton X-100 | 468 | 0.43 | 468 | 510 | 0.65 |
| SDS | 464 | 0.38 | 470 | 518 | 0.66 |
| TW-80 | 465 | 0.48 | 463 | 515 | 0.74 |
| CTAB | 447 | 0.09 | 460 | 510 | 0.52 |
| TW-20 | 464 | 0.42 | 463 | 515 | 0.79 |
ϕ = fluorescence quantum yield. For UV measurement [C-6 SO2Cl] = 5 × 10−6M and for fluorescence measurement [C-6 SO2Cl] = 5 × 10−8M at slit width 4 nm. Concentration of surfactant = 0.1%.
| Surfactant | λabs/nm | A | λex(nm) | λem (nm) | ϕ |
|---|---|---|---|---|---|
| Water | 468 | 0.0970 | 470 | 520 | 0.40 |
| TritonX-100 | 468 | 0.1539 | 468 | 510 | 0.58 |
| SDS | 471 | 0.1226 | 470 | 518 | 0.52 |
| TW-80 | 465 | 0.1739 | 463 | 515 | 0.77 |
| CTAB | 444 | 0.0344 | 460 | 516 | 0.35 |
| TW-20 | 465 | 0.1934 | 463 | 515 | 0.67 |
These results suggest that non-ionic surfactants provide a much more favorable microenvironment for the molecules to reside. Since λmax is almost similar in the non-ionic and anionic surfactants, suggest that both the label and proline derivative reside in the same microenvironment in almost all those surfactants. On the other hand a hypsochromic shift in the presence of CTAB accompanied by a decrease in the molar absorptivity indicates that the derivative and the parent molecule reside in a similar microenvironment in the presence of cationic surfactants.
The fluorescence spectra of coumarin 6-SO2Cl and proline derivative were determined in the same type of surfactants as for the absorption measurement. In most cases, the maximum fluorescence intensity was enhanced in the presence of surfactants except CTAB surfactant. The decrease in quantum yield in the presence of CTAB may be attributed to the formation of aggregates of the fluorophore at the low concentration of the surfactant. The positively charged surfactant may exhibit electrostatic attraction to the lone pair of the nitrogen atom in the benzothiazole nucleus. The positively charged surfactant molecules bring the fluorophore molecules close together and cause aggregation. It has been reported that the aggregation enhanced by micelles could lead mainly to static quenching, however, the presence of a dynamic quenching cannot be ruled out (Mishra et al., 2004). Another possible explanation for the quenching in the presence of CTAB may be due to intersystem crossing from singlet to triplet state brought about by the presence of bromine in the surfactant molecule. For coumarin 6-SO2Cl maximum fluorescence quantum yield was observed in the presence of TW-20, followed by TW-80, TritonX-100, and SDS (Table 2). However for proline derivative a maximum enhancement was observed in the presence of TW-80, followed by TW-20, TritonX-100, and SDS (Table 3). These results suggest that an increase in fluorescence emission in the presence of TritonX-100, TW-80 and TW-20 may be due to an increase in the molar absorptivity of these compounds.
Clearly maximum enhancements were obtained in non-ionic surfactants and moderate enhancement in anionic surfactants. Slight enhancement in fluorescence intensity is observed in anionic surfactants while a quenching of fluorescence emission is observed in cationic surfactant. A plausible explanation for this miceller effect on coumarin 6-SO2Cl and its proline derivative may be due to the solubility of the molecules in micellar microenvironment having different hydrophobicity. Accordingly the fluorescence enhancement observed with TW-80, TW-20 and TritonX-100 may reflect a more hydrophobic microenvironment of coumarin 6-SO2Cl compared to that observed with SDS and CTAB micelles. In addition the fluorescence enhancement may be due to a decrease in fluorescence quenching of coumarin 6-SO2Cl single state in the presence of non-ionic surfactants.
Another interesting spectral feature observed when both coumarin 6-SO2Cl and its proline derivative are incorporated into micellar media is that a blue shift in wavelength of maximum fluorescence. With the maximum shift of 10 nm observed in the presence of TritonX-100, a shift of 5 nm was observed in the presence of TW-80 and TW-20 and 4 nm in the presence of SDS, respectively (Tables 2 and 3). The exact location in the micelles at which coumarin 6-SO2Cl and its proline derivatives occur is dictated by the type of interactions occurring between surfactants and solubilizate. Therefore it seems that coumarin 6-SO2Cl and its proline derivatives are experiencing a similar microenvironment in both TW-80, TW-20 due to the similar blue shift observed in these two systems (5 nm). On the other hand, the spectral blue shift of 10 nm in the presence of Triton X-100 is reflecting a different solubilizate microenvironment.
4 Conclusion
Coumarin 6-SO2Cl was synthesized by direct chlorosulfonation of coumarin-6 nucleus. The label was used to derivatize anilines and an amino acid. Studies of the photophysical properties of Coumarin 6-SO2Cl and its derivatives in solvents of different polarity, pH and micellar media reveal the importance of the medium effect on the absorbance and fluorescence properties of the molecule.
Acknowledgment
The financial support from SQU, Grant #IG/SCI/CHEM/09/01, is gratefully acknowledged.
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