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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusion
  7. Acknowledgements
  8. References
  9. Supporting Information

Photoreaction of trans-2-[4′-(dimethylamino)styryl]benzothiazole (t-DMASBT) under direct irradiation has been investigated in dioxane, chloroform, methanol and glycerol to understand the mechanism of photoisomerization. Contrary to an earlier report, isomerization takes place in all these solvents including glycerol. The results show that restriction on photoisomerization leads to the increase in fluorescence quantum yield in glycerol. The results are consistent with the theoretically simulated potential energy surface reported earlier using time-dependent density functional theory (TDDFT) calculations. DFT calculations on cis isomers under isolated condition have suggested that cis-B conformer is more stable than cis-A conformer due to hydrogen-bonding interaction. In the ground state, cis-DMASBT is predominantly present as cis-B. The fluorescence spectra of the irradiated t-DMASBT suggested that photoisomerization follows not the adiabatic path as proposed by Saha et al., but the nonadiabatic path.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusion
  7. Acknowledgements
  8. References
  9. Supporting Information

Due to wide applications, fluorescing donor–acceptor substituted push–pull styryl dyes have received considerable attention [1-5]. trans-2-[4′-(Dimethylamino)styryl]benzothiazole (t-DMASBT, Chart 1) is an interesting push–pull styryl dye, and was suggested as a good microsensor to study the biological functions as well as biomimicking systems [6]. Fayed et al. showed that t-DMASBT emits from an intramolecular charge transfer (ICT) state in polar solvents [7]. Later Saha et al. [8] studied the spectral characteristics of t-DMASBT and suggested that it emits from the twisted ICT (TICT) state in polar solvents. They also proposed that the polarity of the S3 state increases by torsional rotation of the dimethylamino group and at 90° it forms the TICT state. In the gas phase, the energy of the TICT state is higher than that of the planar S1 state, but polar solvents stabilize the TICT state and make it as the emitting state. However, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) reported by them for the twisted geometry are not consistent with the formation of a charge transfer state. There was a large overlap between the HOMO and LUMO and in the HOMO the electron density that is supposed to be localized on the dimethylamino group (donor) is localized on the other part of the molecule. Thus, we reinvestigated the photochemical nature of t-DMASBT theoretically [9]. Our calculations predicted that t-DMASBT is present in two conformeric forms, trans-A and -B (Fig. 1). Their relative populations and spectral characteristics and nature of the excited states are predicted to be nearly the same. Our calculations also suggested that twisting of the dimethylamino group is energetically favored compared with twisting of dimethylanilino or dimethylanilinostyryl moieties. With torsional rotation of the dimethylamino group, the energy of the S3 state decreases and those of the lower energy states increase. The S3 state crosses the S2 state, but there is an avoided crossing between S3 and S1 states. At the perpendicular geometry, the emitting state is described by HOMO–LUMO excitation. The HOMO and LUMO are decoupled with the HOMO localized on the donor dimethylamino group and the LUMO localized on the other part of the molecule. Thus, the donor lone pair becomes available for charge transfer to the decoupled LUMO that results in the formation of the TICT state.

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Figure 1. UV–Visible spectra of t-DMASBT at different irradiation times (using 420 nm cutoff filter) in (a) chloroform (b) dioxane (c) methanol and (d) glycerol (dotted line shows the spectrum of the 420 nm cutoff filter).

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image

Chart 1. Structures of different conformers of t-DMASBT.

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t-DMASBT has been shown to act as an excellent sensor for a variety of heterogeneous systems [10-16]. It was used as a surface probe to monitor the premicellar aggregation in sodium dodecylsulfate, cetyltrimethyl ammonium bromide and Triton X 100 [10] and brijs [11]. It was also found that t-DMASBT induces the formation of cylclodextrin nanotubular suprastructures [12-14]. Recently, Purkayastha et al. used t-DMASBT as a fluorophore to demonstrate the targeted drug delivery of nanoparticle [15]. They also utilized t-DMASBT to study the ionic surfactant created peripheral confined water around silver nanoparticle [16]. Despite the fact that the sensitivity of the photophysics and photochemistry of t-DMASBT to environment is responsible for its wide application in various systems, the mechanism of nonradiative decay and photoisomerism are still controversial.

Saha et al. [6] investigated the effect of temperature on the fluorescence spectral characteristics of t-DMASBT. They hypothesized that in a highly viscous polar medium, the restricted rotation of the dimethylamino group causes a greater extent of donation of charge toward acceptor, thereby stabilizing the TICT state and the temperature-induced TICT fluorescence quenching is observed without any isomerization. However, in low polarity solvents the decrease in fluorescence was attributed to the temperature-dependent cistrans isomerization. According to Saha et al., the isomerization is supposed to take place in the excited state i.e. by adiabatic process and the so formed cis isomer should relax radiatively. On the other hand, our time-dependent density functional theory (TDDFT) calculations predicted that the photoisomerization of t-DMASBT occurs via a phantom state which decays to cis and trans isomers nonradiatively [9]. Therefore, we have investigated the photochemistry of t-DMASBT in different solvents. The main objectives are to determine whether (1) t-DMASBT isomerizes in polar viscous media such as glycerol or not and (2) the isomerization follows the adiabatic or nonadiabatic path.

Materials and Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusion
  7. Acknowledgements
  8. References
  9. Supporting Information

t-DMASBT was synthesized by following the procedure reported by Fayed et al. [7]. A solution of p-dimethylaminobenzaldehyde (0.002 mmol) in dry dimethyl formamide (DMF) (4 mL) was added dropwise to a solution of 2-methylbenzothiazole (0.003 mmol) and powdered KOH (0.02 mmol) in 8 mL of dry DMF with continuous stirring. The mixture was stirred at 140°C. After 48 h the mixture was cooled to room temperature and dilute HCl (10%) was added to it to make it weakly acidic. The compound was extracted with dichloromethane. The compound was purified first by column chromatography and further by preparative thin layer chromatography using a hexane–ethyl acetate mixture.

Dioxane, chloroform, methanol all HPLC grade and glycerol AR grade were used as received. CDCl3 was used for NMR experiments. p-dimethylaminobenzaldehyde, 2-methylbenzothiazole and CDCl3 were procured from Sigma–Aldrich. All other solvents were procured from Rankem, India and were tested for spurious absorbance/fluorescence in the region of spectral measurements. Irradiations were carried out in a standard quartz cell/NMR tube at room temperature in presence of air with light from a 350 W ozone free Xe-arc lamp. In addition to water filter, Scotch cutoff filters were used when required. For irradiation experiments, the concentrations of the solutions were 3 × 10−5 to 5 × 10−5 m (in 1 cm quartz cell) and 10−2 m (in NMR tube). For fluorescence measurement, the absorbance of the solutions was kept below 0.1 at the wavelength of excitation. The irradiation of the solutions in quartz cells and NMR tube were followed by UV–Visible absorption and NMR spectrometers respectively. UV–Visible absorption and fluorescence spectra were recorded on a Varian Cary 100 spectrometer (Varian Australia Pty Ltd, Palo Alto, Australia) and Jobin-Yvon Spex Fluoromax 4 fluorometer (Horiba Instruments Incorporated, Edison, NJ, USA) respectively. 1H NMR was recorded using a 400 MHz Varian instrument (Varian Mercury plus, Palo Alto, CA, USA).

All the calculations were carried out using the GAUSSIAN 03 program [17]. The ground state geometries were obtained by full optimization of structural parameters using DFT employing the 6-31G(d,p) basis set using spin-restricted shell wavefunctions [18, 19]. Geometry optimizations were carried out using Becke's three-parameter hybrid functional B3, [20] with nonlocal correlation of Lee-Yang-Parr, LYP [21], abbreviated as B3LYP. The minimum energy nature of the stationary points was verified from vibrational frequency analysis. The excitation energies were obtained by vertical excitations of optimized ground states using TDDFT/B3LYP/6-31G(d,p) calculations [22, 23].

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusion
  7. Acknowledgements
  8. References
  9. Supporting Information

t-DMASBT was irradiated in chloroform using a 420 nm cutoff filter and the reaction was monitored with a UV–Visible absorption spectrometer (Fig. 1a). Upon irradiation, the 400 nm absorption band of t-DMASBT gradually blue shifted with a hypochromic effect. The photostationary state was reached in 35 s of irradiation and an isosbestic point was observed at 327 nm. The isosbestic point revealed a simple one-to-one conversion of t-DMASBT to cis product. To confirm the formation of cis isomer further, we followed the reaction by 1H NMR in CDCl3. Figure 2 shows the NMR spectra of t-DMASBT at different irradiation times. One of the signals corresponding to a trans olefinic proton of t-DMASBT overlaps those of the aromatic protons, but the other olefinic proton appears as a doublet with a coupling constant of 16.0 Hz at δ 7.14 ppm. This olefinic proton was used to monitor the reaction. Upon irradiation, the intensity of the doublet at δ 7.14 decreases, and an olefinic proton of cis isomer appears at δ 6.91 (d, = 12.0 Hz, 1H). Other peaks corresponding to the cis isomer also emerge. This confirms that the blueshifted photoproduct obtained is the cis isomer of DMASBT. Figure 1 also shows the irradiation in nonviscous solvents dioxane (non polar) and methanol (polar) and also in a viscous polar solvent glycerol. In all the solvents, formation of cis isomer is observed as in chloroform. Without determining the spectrum of the cis isomer, it is difficult to predict the effect of solvent on the photostationary state. However, comparison of the panels in Fig. 1 suggests higher trans contents in dioxane and glycerol. These changes may be real or may be due to spectral shifts that change the incident light excitation-ratio of the two isomers. The absorbance ratio of the sample after irradiation (i.e. the photostationary state) and before irradiation (i.e. the trans isomer) at absorption maxima, Aai/Abi was calculated. Aai/Abi ratio follows the order methanol (0.43) < chloroform (0.44) < dioxane (0.53) < glycerol (0.67). However, the absorption maxima of the trans isomers follows the order dioxane (394 nm) < chloroform (400 nm) < methanol (402 nm) < glycerol (416 nm). Therefore, it may be inferred that the content of trans is larger in dioxane and glycerol at photostaionary state due to solvent effect.

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Figure 2. 1H NMR spectra of t-DMASBT in CDCl3 at different irradiation times (using 420 nm cutoff filter; the intensity of the peaks is expanded in the aromatic region for clarity, * denotes chloroform peak).

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Saha et al. [6] hypothesized that in a highly viscous medium such as glycerol the restricted rotation of the dimethylamino group causes a greater extent of donation of charge toward the acceptor thereby stabilizing the TICT state responsible for the large Stokes shift in the fluorescence. Only in low polarity solvents, where TICT emission is very weak, the molecule undergoes a temperature-dependent photoisomerization. The temperature-induced TICT quenching occurs in a polar viscous medium without any isomerization. But our studies clearly establish that t-DMASBT photoisomerizes in all the media including glycerol. The molecule is predicted to have planar geometry in the ground state by both our DFT calculations [9] as well as by AM1 calculations [6, 8]. The corresponding Franck–Condon state has to have planar geometry and the molecule has to relax from the planar Franck–Condon state to the TICT state. Therefore, any restriction on rotational motion is expected to hinder not only the reverse process but also the formation of the TICT state. Therefore, the enhancement in fluorescence quantum yield of t-DMASBT in glycerol is due to restriction on twisting of the olefinic double bond that leads to isomerization.

In t-DMASBT, the TICT state is formed by the rotation of the smaller dimethylamino group, but the competing torsional rotation around the olefinic double bond involves the rotation of dimethylanilino group or the benzothiazole group. Fluorescence quantum yields of trans-stilbene were also shown to be temperature and viscosity dependent due to the twisting motion about the central bond of phenyl ring [24-33]. Although the fluorescence quantum yield decreases with increase in temperature, it increases with increase in viscosity. The observed marked increase in fluorescence quantum yield of trans-stilbene in glycerol on lowering of temperature was attributed to the fact that the processes of fluorescence and isomerization are coupled with changing temperature in highly viscous solvents [26-28]. It was also reported that the normally very weakly fluorescent cis-stilbenes [29-31] and the weakly fluorescent trans-stilbenes become strongly fluorescent in highly viscous media [32]. Our present results on t-DMASBT also indicate that photoisomerization competes with fluorescence.

As reported earlier [9], t-DMASBT is present in two conformeric forms, trans-A (μ = 5.2 D) and trans-B (μ = 5.1 D). The energy difference between the two conformers is only 0.2 kcal mol−1 and thus both conformers are predicted to be equally probable. As the difference in dipole moments between the two conformers is negligible their relative population is nearly the same in all the media. Such conformers equilibrate in the ground state, but not in the excited state [34]. Consequently, the presence of the two conformers may be reflected in λexc dependence fluorescence spectra [35]. The shifts in the fluorescence spectra with λexc are very small (see Supporting information). It should be noted that the molecular parameters predicted by the theoretical calculations for both the conformers are nearly identical and the emission energies predicted are the same [9].

trans-A and -B conformers upon one bond twist give cis-A and -B conformers, respectively (Scheme 1). Unlike organic glasses or other restricted media, the flexibility of the media in our reaction conditions should allow thermal equilibration in all these environments. The absorption spectra were recorded after such equilibration between the conformers. As the molecular parameters of trans conformers were already reported [9], we calculated the molecular parameters of cis conformers under isolated condition and the optimized structures are shown in Fig. 3 and the data are compiled in Table 1. The calculations predict that cis-B is more stable than cis-A by 0.1295 eV (ca 3 kcal mol−1). Although the trans isomer is present as both A and B conformers, the cis isomer is predominantly present in the B form and the population of cis-A is less than 1%, even in the most polar environment. cis-B is more stabilized by the presence of pseudohydrogen bonding between thiazole nitrogen and one of the phenyl hydrogen (Fig. 3). The presence of hydrogen bond in cis-B also planarizes the molecule (Table 1). In protic solvents such as methanol and glycerol, this hydrogen bond may break to form intermolecular hydrogen bond. However, the transition energies of both conformers are nearly same and are described by excitation of an electron from highest occupied molecular orbital to lowest unoccupied orbital in both conformers. The smaller oscillator strengths of the cis conformers (Table 1) compared to those of the trans conformers (oscillator strength ca 1.3; [9]) explain the observed hypochromic effect of the absorption spectra on photoisomerization.

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Figure 3. Optimized structures along with HOMO and LUMO of cis-A and -B.

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Scheme 1. Photoisomerization of t-DMASBT.

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Table 1. Optimized geometrical parameters for cis-conformers in the ground state
Parameterscis-Acis-B
  1. a

    With respect to ground state energy of trans-A.

  2. b

    Dihedral angles defined in Chart 1.

Relative Energya(kcal mol−1)6.753.77
Transition energy (nm)392394
Oscillator strength0.54770.8420
Dipole moment (D)4.83.8
Dihedral angleb(°)
φ19.40.0
φ2174.40.0
φ338.20.1
φ47.45.2

Thus, the ca 23-fold increase in fluorescence quantum yield of t-DMASBT in glycerol observed by Saha et al. may be attributed to fact that the viscous medium enhanced the torsional barrier leading to photoisomerization. Saha et al. also reported that the fluorescence quantum yield of t-DMASBT decreases with increase in temperature in both dioxane and glycerol [6]. Our earlier theoretical calculations [9] had predicted a small barrier for twisting in the first excited state of t-DMASBT to attain a perpendicular geometry. Therefore, the thermally activated nonradiative de-excitation process that competes with fluorescence may be assigned to isomerization.

The potential energy surface constructed by Saha et al. has a huge barrier for rotation (77 kcal) in the ground state and it is also considerable (36 kcal) in the excited state without a phantom state [6]. Although their potential energy surface predict no photoisomerism, they hypothesized that the trans isomer (t*) undergoes torsional rotation in the first excited state to form cis isomer (c*) in the excited state and that the cis isomer relaxes radiatively. Contrary to the adiabatic path proposed by Saha et al. [6], the DFT calculation carried out by us predicted a nonadiabatic path [9]. There is a small barrier for twisting in the trans isomer in the S1 state to reach the perpendicular state (p*). The molecule decays from the p* state by a nonradiative path to the ground state trans and cis isomers. There is little or no barrier for the cis isomers to reach the p* state. Therefore, the cis isomer is expected to fluoresce only in a rigid environment and to be nonfluorescent in fluid solution at room temperature. The fluorescence spectra of t-DMASBT were recorded in nonpolar dioxane, polar aprotic acetonitrile and polar protic viscous glycerol as a function irradiation time. In all the solvents the fluorescence intensities decrease, but no shift is observed in the fluorescence spectra (Fig. 4). The decreases in fluorescence intensities indicate the formation of cis isomer. If the adiabatic mechanism predicted by Saha et al. is correct, the radiative decay from excited cis isomer should result in blueshift upon irradiation. In fact, Saha et al. attributed the hypsochromic shift in the fluorescence spectrum of t-DMASBT in dioxane upon increasing the temperature to thermal isomerization [6]. The absence of a spectral shift in the fluorescence spectrum of t-DMASBT upon isomerization (Fig. 4) substantiates the nonadiabatic path predicted by our TDDFT calculation for t-DMASBT. Our studies clearly establish that no shift is observed in the fluorescence spectra of t-DMASBT upon isomerization. Therefore, we attribute the small 4 nm hypsochromic shift observed in the fluorescence spectrum of t-DMASBT in dioxane by Saha et al. upon increasing the temperature to a thermochromic shift commonly observed in fluorescence spectra of dyes.

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Figure 4. Fluorescence spectra of t-DMASBT in (a) dioxane, (b) acetonitrile and (c) glycerol (inserts show the normalized spectra) in the course of the irradiation time (all irradiations are carried out in fluorescence instrument with λexc = 395 nm and 10 nm slit width).

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Conclusion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusion
  7. Acknowledgements
  8. References
  9. Supporting Information

Contrary to the report of Saha et al., the trans–cis photoisomerization of t-DMASBT competes with fluorescence not only in nonpolar solvents but also in polar and polar viscous solvents such as methanol and glycerol. The relatively high fluorescence quantum yield of t-DMASBT in glycerol is due to restriction by increased viscosity on the twisting of carbon–carbon double bond that leads to isomerzation and not to restricted twisting from the TICT state as proposed earlier. The photoisomerization of t-DMASBT does not occur by an adiabatic path as previously proposed, but by a nonadiabatic path and the cis isomer is nonfluorescent in fluid solution at room temperature. DFT calculations predict that cis-B is stabilized by intramolecular hydrogen bonding under isolated condition.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusion
  7. Acknowledgements
  8. References
  9. Supporting Information

The authors thank the Department of Science and Technology (DST), India and the Council of Science and Industrial Research (CSIR), India for the financial support. The authors are also thankful to the reviewers for their valuable suggestions.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusion
  7. Acknowledgements
  8. References
  9. Supporting Information
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Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusion
  7. Acknowledgements
  8. References
  9. Supporting Information
FilenameFormatSizeDescription
php1227-sup-0001-FigureS1.docWord document80KFigure S1. λexc dependence emission spectra of t-DMASBT (normalised) in chloroform (λexc value varies from 375 to 430 nm).
php1227-sup-0002-FigureS2.docWord document79KFigure S2. λexc dependence emission spectra of t-DMASBT (normalised) in dioxane (λexc value varies from 375 to 430 nm).
php1227-sup-0003-FigureS3.docWord document71KFigure S3. λexc dependence emission spectra of t-DMASBT (normalised) in acetonitrile (λexc value varies from 375 to 430 nm).
php1227-sup-0004-FigureS4.docWord document73KFigure S4. λexc dependence emission spectra of t-DMASBT (normalised) in methanol (λexc value varies from 375 to 430 nm).
php1227-sup-0005-FigureS5.docWord document63KFigure S5. λexc dependence emission spectra of t-DMASBT (normalised) in glycerol (λexc value varies from 375 to 430 nm).

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