Photophysical properties of a surfactive long-chain styryl merocyanine dye as fluorescent probe

Journal of Luminescence 132 (2012) 2512–2520

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Photophysical properties of a surfactive long-chain styryl merocyanine dye as fluorescent probe L.F.M. Ismail n Al-Azhar University, Faculty of Science, Chemistry Department, Nasr City, 12 Ibrahim El-Nagar, El-Hegaz Sq. Heliopolis, Cairo 11315, Egypt

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 February 2009 Received in revised form 15 January 2011 Accepted 30 January 2012 Available online 8 March 2012

This work deals with detailed investigations of the photophysical properties of a styryl merocyanine dye, namely 1-cetyl-4-[40 -(methoxy) styryl]-pyridinium bromide (CMSPB) of molecular rotor type. The solvatochromic analyses of the data in different solvents using the Kamlet-Taft parameters (a, b, pn) were discussed. Optical excitation of the studied merocyanine dye populates a CT S1 state with about 22.64 folds higher dipole moment value relative to that in the S0 state. Moreover, the effect of solvent viscosity (glycerol at various temperatures (299.0–351.0 K)) on CMSPB fluorescent properties is analyzed to understand the molecular mechanisms of the characteristic increase in CMSPB fluorescence intensity. The results indicate that CMSPB exhibits fluorescent properties typical for molecular rotors. The results show that torsional relaxation dynamics of molecular rotors in high-viscosity solvents cannot be described by the simple stick boundary hydrodynamics defined by the Debye–Stokes– Einstein (DSE) equation. The fluorescence depolarization behavior in glycerol at various temperatures (299.0–351.0 K) shows that the molecular rotational diffusion is controlled by the free volume of the medium. Furthermore, excited state studies in ethanol/chloroform mixture revealed the formation of weak complex with chloroform of stoichiometry 1:1 with formation constant of 0.004l mol  1. Moreover, the increase of the quantum yield values in micellar solutions of CTAB and SDS relative to that of water indicates that the guest dye molecules are microencapsulated into the hydrophobic interior of host micelle. The obtained non-zero values of fluorescence polarization in micellar solution imply reduced rotational depolarization of dye molecules via association with the surfactant. Upon comparing the spectral data in micelles with those in homogeneous solvent systems, more can be learned of the structural details of the micellar environment, which have often been used as models for more complex bioaggregates. The results point out to a possible use of this dye as a fluorescence probe for microenvironmental parameters as well as in some micellar systems. & 2012 Elsevier B.V. All rights reserved.

Keywords: Styryl merocyanine dye Solvatochromic analyses Free volume Fluorescence polarization Microheterogeneous media

1. Introduction Merocyanine dyes have been intensively studied for secondharmonic generation, application for NLO materials [1,2], as sensitizers in solar cell [3] as photoconducting media [4], as well as in laser techniques as light pulse absorbers [5] and as potential sensitizers for photodynamic therapy [6]. Moreover, highly organized dye assemblies are crucial for the energy and electron-transfer events in natural photosynthesis [7,8]. Dye organization is also considered to be the key to advanced functional organic materials for electronics and photonics [9,10]. Nevertheless rational control of dye–dye interactions

n

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in supramolecular architectures is still a difficult task [11] and most known dye assemblies are constructed as simple one-dimensional stacks through aggregation of their p-systems [12]. Furthermore, several theoretical and experimental contribution [13–21] show that the rate of internal geometrical relaxation of S1 state of electron donor–acceptor [EDA] molecules depends critically on the kind of interactions exerted by environmental factors on the dynamics of molecular subunits (free-rotor mechanism). The size and shape of the relaxing groups, the nature of the interactions with the surrounding medium and the free volume necessary for motion are the most important factors. Thus, by studying the non-radiative and radiative relaxation of intramolecular charge transfer molecules (ICT), of the freerotor type in different media, information about the type of interaction and contribution of each mode of interaction can be specifically obtained at the molecular level. Moreover, the role played by such compounds as fluorescent probes in various systems is also very interesting [13–16].

L.F.M. Ismail / Journal of Luminescence 132 (2012) 2512–2520

This work reports on some photophysical properties of a styrylmerocyanine dye, namely 1-cetyl-4-[4-(methoxy)styryl]-pyridinium bromide (Scheme 1) of molecular rotor type. The photophysical behavior and the interplay between dye structure and solvent in homogeneous as well as microheterogeneous media have been explored. Studies are extended to various solvents, glycerol at various temperatures, mixed solvent (ethanol–chloroform) as well as micellar solutions of cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS). Moreover, fluorescence polarization behavior in glycerol at various temperatures and in micellar solutions will be also discussed.

2. Experimental 1-cetyl-4-[4-(methoxy)styryl]-pyridinium bromide (CMSPB, Scheme 1) was synthesized and purified according to the literature procedures [22]. Optically pure solvents were used. Cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) from BDH were used as received. Absorption spectra were recorded on a Unicam UV–visible double-beam spectrophotometer from Helios Company. Fluorescence and excitation spectra were recorded using a Shimadzu RF5301 (PC) spectrofluorophotometer in the range (290–750 nm) equipped with a temperature regulated cell holder. Fluorescence quantum yield (Ff) was determined by comparison against a quinine fluorescence standard (Ff ¼0.55 in 1 N H2SO4) using the following equation [20]:

Fu ¼ Fr ðn2 u =n2 r ÞðAr =Au ÞðF u =F r Þ

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by QCPE with the capability of error minimization by the sequential simplex statistical procedure [23].

3. Results and discussion 3.1. Effect of solvent 3.1.1. Effect of solvent polarity Table 1 contains a summary of the photophysical parameters of CMSPB in various solvents. Representative absorption and normalized fluorescence emission spectra are shown in Figs. 1 and 2, respectively. The visible absorption spectra of CMSPB exhibit a strong and broad absorption band. In the series of amphiprotic and hydrogen-bond donor (HBD) solvents of decreasing proton donor ability, the bandwidth at half height (Du1/2) shows a decrease, indicating a transition to an excited electronic state with a decrease in the geometric change between the ground- and excited- state electronic structure [24–27]. Moreover, lmax shows a bathochromic shift. This is a typical behavior of negatively solvatochromic dyes [26–28]. Interestingly, CMSPB shows positive solvatochromic behavior in aprotic solvents. Similar behavior has been noticed before [25] and was qualitatively attributed to specific solute–solvent

ð1Þ

where r and u stand for reference and unknown respectively. A is the absorbance at the exciting wavelength, F is the area under the emission spectrum, n is the solvent refractive index and Fr is the reference quantum yield. The least squares regression analysis and the fitting procedure of the spectral data to linear and multilinear equations were done using SIMFIT program supplied

H33C16

N

+

OMe

Br

Scheme 1. 1-cetyl-4-[4-(methoxy)styryl]-pyridinium bromide (CMSPB).

Fig. 1. Absorption spectrum of 6.54  10  6 M of CMSPB in different solvents.

Table 1 Absorption and fluorescence spectral data for CMSPB in different solvents. Absorption maxima (labs, nm), molar absorptivity (e, l mol  1 cm  1), half band width (Dn1/2, cm  1), fluorescence maxima (lflu, nm), Stokes shift (na–f cm  1), natural lifetime (t0 ¼1  10  4/e ns), quantum yield (Ff ), radiative lifetime (tr ¼ t0  Ff s), radiative rate constant (kr ¼1/tr s  1) and nonradiative rate constant knr ¼kr((1/Ff)  1 s  1). Solvent

labs (nm)

e (l mol  1 cm  1)

Dn1/2 (cm  1)

lflu (nm)

na–f (cm  1)

t0 (ns)

Ff  102

tr  1011 (s)

kr10  10(s  1)

lnknr (s  1)

1-H2Oa 2-CH3OH 3-C2H5OH 4-Isobutanol 5-Isopropanol 6-CH3CNb 7-DMSO 8-DMF 9-CHCl3a 10-Ethyacetatea 11-Dioxane 12-Cyclohexanea 13-n-Hexanea 14-n-Heptanea

371 377 377 378 378 378 375 370 370 375 370 376 372 374

10,319 17,285 15,479 15,479 15,479 19,736 14,447 15,995 16,511 17,801 15,995 16,898 16,769 17,156

5893.6 5460.5 5654.0 5173.7 5143.4 5429.0 3899.3 5220.2 5543.1 4797.7 5244.9 4797.7 4710.2 4536.1

490 495 488 480 485 495 500 500 480 500 480 460 470 475

7195.2 6323.2 6033.4 5621.7 5836.5 6253.0 6666.7 7027.0 4807.7 6666.7 6193.7 4856.6 5605.1 5685.3

9.69 5.78 6.46 6.46 6.46 5.06 6.92 6.25 6.05 5.61 6.25 5.91 5.96 5.82

2.23 1.62 1.52 1.40 1.23 0.67 3.95 2.72 1.42 1.56 1.59 0.63 0.50 0.59

21.61 9.37 9.81 9.04 7.94 3.39 27.34 17.01 9.69 8.76 9.94 3.72 2.98 3.43

0.46 1.06 1.01 1.11 1.25 2.94 0.36 0.58 1.03 1.14 1.01 2.68 3.35 2.90

26.03 27.19 27.21 27.38 27.64 29.10 25.21 26.07 27.17 27.30 27.15 29.07 29.52 29.22

DMSO ¼Dimethyl sulphoxide and DMF ¼ Dimethylformamide. a b

Omitting solvent from multilinear correlation with absorption and fluorescence maxima. Omitting solvent from multilinear correlation with lnKnr and lnFf.

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L.F.M. Ismail / Journal of Luminescence 132 (2012) 2512–2520

The effect of solvent on the fluorescence spectra of CMSPB is quit different from the effect on absorption spectra. A bathochromic shift relative to lf in cyclohexane, in fluorescence maximum is noticed in HBD and HBA solvents. Least-square fit of the fluorescence maxima to the multilinear Eq. [2] results in the following equation (r ¼0.9571): uflu ¼ 21950:1ð 7108:52Þ599:23ð 7 25:50Þa þ 397:00ð 7 20:81Þb2356:2ð 7 91:25Þpn :

Fig. 2. Normalized fluorescence spectrum at 490 nm of 6.54  10  6 M of CMSPB in different solvents (lex ¼ 370 nm).

Table 2 Solvent parameters used in establishing correlations. Solvent

ETN

a

b

pn

F2

F7

1-H2O 2-CH3OH 3-C2H5OH 4-isobutanol 5-Isopropanol 6-CH3CN 7-DMSO 8-DMF 9-CHCl3 10-Ethayacetate 11-Dioxane 12-cyclohexane 13-n-Hexane 14-n-Heptane

1.000 0.765 0.654 0.506 0.552 0.472 0.441 0.404 0.259 0.228 0.164 0.077 0.074 0.049

1.17 0.93 0.83 0.76 0.76 0.19 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

0.18 0.62 0.77 0.95 0.95 0.31 0.76 0.69 0.00 0.45 0.37 0.00 0.00 0.00

1.09 0.60 0.54 0.50 0.48 0.75 1.00 0.88 0.58 0.55 0.55 0.00  0.08  0.08

0.7155 0.6692 0.6165 0.5415 0.5801 0.6749 0.5742 0.5971 0.2219 0.3467  0.0249  0.0683  0.0544  0.0566

0.5846 0.5583 0.5542 0.5398 0.5466 0.5653 0.6111 0.5905 0.4130 0.4267 0.2742 0.2550 0.2278 0.2352

interaction. The results suggest that no single polarity parameter can be used to cover the spectral shifts in different types of solvents. However, successful correlation of the data in Table 1 with various solvent polarity parameters (Table 2) is carried out using the threeparameter approach of Kamlet, Abboud and Taft (Eq. [2])[29] (KAT). This method assigns each solvent three characteristic parameters: the hydrogen-bond donor (HBD) acidity (a), the hydrogen-bond acceptor (HBA) basicity (b) and pn, an index of solvent polarity/polarisability, which relates to the ability of the solvent to stabilize a charge or a dipole by virtue of its dielectric effect u ¼ u0 þ aa þ bb þ cpn

ð4Þ

A consequence of these correlations is that the interplay between solute–solvent interactions is mainly controlled by the solvent properties (polarity and hydrogen-bonding abilities). These established relationships suggest an analytical application of this dye as sensitive fluorescent probe for microscopic solvent properties [30]. Analysis of the SCF-CI-MOs involved in the lowest singlet electronic transition for a similar cyanine dyes reported previously [24] shows that the S1 state is well described by a single configuration, in which the electron charge is promoted from the highest occupied MO (HOMO), significantly localized on the donor part of the molecule, to the lowest unoccupied MO (LUMO) with a considerable contribution from the acceptor part of the molecule. Consequently, this transition results in population of the first singlet state with CT character; S1,CT state. Moreover, Fig. 3 displays the plot of the calculated maxima of absorption and fluorescence as a function of the corresponding experimental values for various solvents [31]. The calculated maxima are obtained from the multiparameter fit, using Eq. [2]. The line represents a perfect fit for which calculated and experimental values would be equal and each square represents the fitted value of u for a given solvent. Two important points can be noted. First, the maxima of absorption are grouped around a mean value of 26666 cm  1 while the fluorescence maxima (Fig. 3) are spread over a wavenumber range of 833.00 cm  1. The solvatochromism is thus much more important in fluorescence than in absorption, consistent with emission from a state of greater charge-transfer character than the ground state. Second, in absorption and fluorescence, the polarity as well as the H-bond-donating and -accepting properties of the solvent contribute to the stabilization of the excited state. However, this effect is much more pronounced on the fluorescence side, as shown by the much higher weights of the solvent parameter, as compared to the corresponding values on the

ð2Þ

where a, b and c are measures of the susceptibility of the absorption or emission spectra of the probe molecule to solvent parameters a, b and pn, respectively. Here u is the corresponding to the maximum of the absorbance or fluorescence in a given solvent while u0 corresponding to the condition where no intermolecular interaction occurs. Multiparameter linear regression of the correlation between the peaks of absorption and the tabulated parameters a, b and pn, (Tables 1 and 2) is carried out, the following regression equation is obtained (r¼0.7596): uabs ¼ 27135:12ð 7 161:01Þ605:94ð 7 30:13Þa þ 101:22ð 75:88Þb453:89ð 723:75Þpn :

ð3Þ

Fig. 3. Plot of the calculated maxima of absorbance and fluorescence for CMSPB as a function of the corresponding experimental maxima for various solvents.

L.F.M. Ismail / Journal of Luminescence 132 (2012) 2512–2520

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absorption side. This is indicative of a large charge transfer character in the excited state [31]. Moreover, on excitation it is thus expected that CMSPB should exhibit a dipole moment increase in the S1,CT state relative to the S0. An important support for this predication can be obtained by estimating the relative values of the static dipole moment mg and me, in the S0 and S1,CT, respectively. The spectral shift method is used (using the fluorescence and absorption maxima [2432]). The bulk dielectric constant (D) and refractive index (n) functions employed are represented below, given in the same form as reported previously [32] F 2 ðD,nÞ ¼ f½ðD21Þ=ðD þ 2Þ½ðn2 1Þ=ðn2 þ2Þ½ð2n2 þ 1Þ=ðn2 þ 2Þg

ð5Þ

F 7 ðD,nÞ ¼ 0:5 F 2 ðD,nÞ þ 1:5½ðn4 1Þ=ðn2 þ 2Þ2 

ð6Þ

Excluding amphiprotic and chlorinated solvents as recommended to minimize specific interactions [32] and complex formation [33,34], the usual statistical treatment of the data (Eq. (7)) yields the following (Eqs. (8) and (9) and Figs. 4 and 5): Y ¼ ax þ b

ð7Þ

ðna nf Þ ¼ 1545:19ð 7506:10Þ F 2 ðD,nÞ þ 5951:23ð 7 210:17Þ

ð8Þ

ðna þ nf Þ=2 ¼ 1688:00ð 7 230:36Þ F 7 ðD,nÞ þ 24432:67ð 7106:14Þ

Fig. 5. Plot of (ua  uf) vs. F2 for CMSPB (r ¼0.9074).

ð9Þ with correlation coefficients higher than 0.96. Slope ratio can be readily converted to mg/me relative values [¼ (1þslope ratio)/ (1–slope ratio)], where the slope ratio ¼[(me–mg)2/(mg–me)], giving me ¼22.64 mg. With this method the solvent cavity radius is not needed [24]. The result thus leads to the conclusion that the optical excitation of CMSPB populates a CT S1 state with a dipole moment higher than the S0 state. It is worth mentioning here that such a conclusion based on the spectral shifts method using a limited number of solvents must be used with caution [24,25,34]. The fluorescence quantum yield is a photophysical parameter sensitive to almost all types of solvent effect [35]. The relation between lnFf of CMSPB and multiparameter equation (Eq. (2)) exhibits dependence on the various microscopic solvent property parameters (with r ¼0.991, Eq. (10)). ln Ff ¼ 5:12ð 70:17Þ þ 0:23ð 70:13Þa þ 0:46ð 7 0:30Þb þ 1:28ð 70:28Þp

n

Fig. 4. Plot of (ua þ uf)/2 vs. F7 for CMSPB (r¼  0.9819).

ð10Þ

Moreover, correlating the logarithm of the rate constants of radiationless decay calculated using the formula knr ¼ kr ðð1=Ff Þ21Þ

ð11Þ

where kr, the radiative rate constant is given by the reciprocal of tr (radiative lifetime), with various solvent polarity parameters using Eq. (2) yields good relationship (r ¼0.993, Eq. (12)) ln knr ¼ 29:12ð 70:08Þ þ0:45ð 7 0:12Þa 0:44ð 70:16Þb3:31ð 70:15Þpn :

ð12Þ

The above results show that geometric relaxation can be controlled via solvent effect. 3.1.2. Effect of solvent viscosity: At RT, changing the solvent from non-viscous ethanol, characterized by ultra-fast reorientation on the picosecond time scale [36] to glycerol has the expected effect on the fluorescence properties. For the given dye molecule, the fluorescence quantum yield is enhanced markedly (Ff ¼0. 218 for glycerol) owing to a decrease in the rate of non-radiative emission induced by increasing frictional forces and a decrease in solvent-free volume [35,37,38] demanded by this dye for free rotations around the single bonds and trans/cis isomerization about the double bonds [35]. Furthermore, decrease of the glycerol viscosity induced by heating is accompanied by dramatic decrease in the fluorescence quantum yield (Table 3). Such behavior indicates that a viscositydependent nonradiative decay channel exists for the excited state of CMSPB. We suggest that the deactivation channel is connected with large-amplitude torsional motion in the molecule followed by internal conversion to the ground state. Same behavior is noticed previously for similar molecules (molecules whose fragments rotated relative to each other in the excited state known as molecular rotors) [18,19,21,38–42]. The most notable feature of molecular rotors is the dependency of the twisted state formation rate on the local microenvironment, predominantly the microviscosity of the solvent. In the case of molecular rotors that exhibit nonradiative relaxation from the twisted state, the fluorescent quantum yield increases in higher-viscosity solvents [36,43,44].

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L.F.M. Ismail / Journal of Luminescence 132 (2012) 2512–2520

Table 3 Fluorescence quantum yields (Ff) for CMSPB in pure glycerol at different temperatures, viscosities (Z), fluorescence anisotropy (r), depolarization (P) and free-volume fractions (f) of pure glycerol at different temperatures. T (K)

Z (cP)

Ff

r

P

1/P

1/e

299.0 309.3 315.7 324.3 329.2 337.3 346.0 351.0

934.12 449.95 317.96 156.17 109.33 75.27 49.73 36.95

0.218 0.194 0.148 0.104 0.090 0.064 0.048 0.042

0.307 0.292 0.275 0.280 0.271 0.268 0.265 0.247

0.365 0.351 0.319 0.269 0.229 0.201 0.195 0.152

2.745 2.851 3.133 3.712 4.372 4.979 5.141 6.564

13.62 12.85 12.38 11.83 11.54 11.08 10.63 10.37

Eq. [10], has been proposed by Loutfy and Arnold [19] to describe fluorescence quantum yield dependence of p-(dialkylamino) benzylidenemalononitrile dyes on bulk viscosity in highly viscous solvents or in polymers above the glass transition temperature. Loutfy and Arnold have shown that the torsional relaxation dynamics of molecular rotors in medium- to high-viscosity solvents (Z 42 cP) cannot be described by the simple stick boundary hydrodynamics defined by the Debye–Stokes–Einstein (DSE) equation. For CMSPB, a plot of Eq. [14] is nonlinear, and nonzero intercepts are obtained, suggesting deviation from the DSE theory (data not shown). Theoretical explanations of this fact ¨ were proposed by Forster and Hoffmann [45]. Our results indicate that CMSPB exhibits fluorescent properties typical for molecular rotors, and its photophysics may be reasonably described by two competing deactivation pathways: (1) fluorescence emission and (2) torsional relaxation, leading to internal conversion to the ground state. Therefore, we may conclude that the rate of torsional relaxation of the CMSPB fragments in the excited state, leading to charge transfer and nonradiative deactivation, is mainly determined by solvent viscosity. The results show the unique suitability of CMSPB molecular rotors as fluorescent viscosity sensors. Moreover, the steady state anisotropy (r) can be represented as [44] r ¼ ðIVV GIVH Þ=ðIVV þ 2GIVH Þ

ð16Þ

where IVH and IVV are the intensities obtained from the excitation polarizer oriented vertically and the emission polarizer oriented in horizontal and vertical positions, respectively. The factor G is defined as G ¼ IHV =IHH Fig. 6. Dependence of lnF/(1  F) vs. ln(Z/T) for CMSPB glycerol at different temperatures (lex ¼385 nm, slop ¼ 0.74 70.03 and r ¼0.994).

Supposing that the rotational diffusion obeys the Debye– Stokes–Einstein (DSE) equation, its rate kj should be proportional to the temperature T and inversely proportional to the viscosity Z [43,44]. kj  T=Z

ð13Þ

ð17Þ

Emission anisotropy (r) data for the CMSPB in glycerol at different temperatures is collated in Table 3. The plot of r vs. Z/T shows linear dependency (Fig. 7). r values of CMSPB in glycerol are 0.25–0.31 (Table 3). In general, in the viscous medium where the molecules do not within the fluorescence lifetime, r value takes the region of  0.2  r o0.4. Namely, when the direction of the linear oscillators for the absorption and emission are parallel to each other, r is o0.4; and when those are perpendicular to each other, r is  0.2; [46]. The positive and the value of anisotropy (r, 0.25–0.31) suggests that the emission dipoles of

The fluorescence quantum yield F is related to the radiative and nonradiative rate constants by the expression F ¼ kr/(kr þknr) and suggesting that knr Ekj, one can obtain [43,44]

F=ð12FÞ ¼ kr =kj  ðZ=TÞ:

ð14Þ

However, plot for CMSPB in glycerol (viscosity change is induced by heating) deviates from linearity, and data points may be fitted using the following equation (Fig. 6) with a ¼0.7470.03.

F=ð12FÞ  ðZ=TÞa

ð15Þ

The fractional value of a testifies that torsional movement in this case is less affected by friction of surrounding solvent molecules than it is predicted by DSE equation [43,44]. Indeed, the DSE equation is valid only for rotational diffusion of spherical particle in homogeneous medium with bulk viscosity Z, and it fails when sizes of rotating probe moiety and of solvent molecules are comparable. Spatial inhomogeneity and existence of cavities in the solvent, so-called effect of the solvent free volume must be taken into account. It must be noted that the rate of torsional relaxation and change of F/(1  F) value are connected rather with microviscosity (Zmicro) but not with macroscopical viscosity (Z) of the solvent. Moreover, the empirical relation, similar to

Fig. 7. Plot of fluorescence anisotropy (r) vs. Z/T for CMSPB in glycerol (lex ¼ 385 nm and lem ¼485 nm).

L.F.M. Ismail / Journal of Luminescence 132 (2012) 2512–2520

CMSPB at 485 nm are collinear with the absorption dipole at 385 nm [47]. Furthermore, fluorescence polarization measurements are sensitive to and can provide information about intermolecular interactions over the length scale of the probe molecule [48]. The steady state fluorescence polarization (SSFP, P) has been defined according to the following equation: P ¼ ðIVV 2GIVH Þ=ðIVV þ GIVH Þ:

ð18Þ

Moreover, it has been known for many years that the polarization of fluorescence increases with viscosity of the medium containing the fluorophore. A quantitative theory connecting the phenomenon with the molecular rotational diffusion of the emitting species was formulated by Perrin in 1929 and has been applied successfully by many workers [14,49–52]. The rate of molecular motion can be estimated by determining the degree of polarization P. According to the Perrin equation [51] l=P1=3 ¼ ðl=P o 1=3Þfl þ ðRT=V ZÞtg:

ð19Þ

It is thus expected that a plot of l/P vs. T/Z should give a straight line with a slope of (l/Po) (Rt/V) and an intercept of (l/Po), where Po is the characteristic value for the same fluorophore in vitrified solution, R is the gas constant and t is the fluorescence lifetime of the fluorophore of molecular volume V. Thus, varying the ratio T/Z for glycerol by temperature change [53] and studying the rotational diffusion using fluorescence polarization technique should assist in establishing the dominant solvent relaxation process and provide insight into the role of molecular structure and its sensitivity to the average medium microviscosity. Plotting 1/P against T/Z (Viscosity values were interpolated or extrapolated from the linear least-squares equation ln(1/Z)¼4.838– 7312/ T based on viscosity values taken from Ref. [54]) yields the straight line depicted in Fig. 8 in temperature range 299.0–351.0 K. The linear least squares analysis gives the following equation: 1=P ¼ 2:75 þ0:363T=Z

ð20Þ

The result shows high degree of sensitivity of CMSPB to viscosity, suggesting its promising use as a probe to explore the microscopic fluidity to the interior of industrially and biologically

Fig. 8. Plot of 1/P vs. T/Z for CMSPB in glycerol.

2517

important systems such as polymers, proteins and micelles [14,16,50,51]. Furthermore, to test the applicability of the free-volume concept in describing the dynamics of this fluorophore the change in the free-volume of glycerol with temperature can be calculated using the following equation: f ¼ f g þ aðTT g Þ

ð21Þ

where fg ¼0.025, a ¼4.4  10  4 K  1 and Tg ¼189 K for glycerol [19]. On correlation (1/P) and the calculated 1/f values for glycerol at different temperatures (299.0–351.0 K, Fig. 9 and Table 3) the following Arrhenius-like regression equation is obtained [55, 56] 1=P ¼ 170:71 exp ð0:32=f Þ

ð22Þ

This finding points to the applicability of the free-volume concept in describing the molecular rotational diffusion of this dye. It is worth mentioning that the results are consistent with the finding [19] that the temperature-variation studies in a high viscosity solvent should provide more consistent data and a less ambiguous test of dominant solvent relaxation mechanism. Moreover, the dependence of the orientational relaxation times (and consequently, the Ff) for some dyes of different volumes on Z/T has been confirmed before [19]. In high viscosity solvents the free-volume concept has been found to provide an accurate description of the solvent-temperature behavior and the probe torsional relaxation dynamics. Since depolarization (l/P) is a thermally activated process involving molecular rotational diffusion [30,56], it can be expressed in an Arrhenius form as 1=P ¼ A expðDEa =RTÞ

ð23Þ

where A is the pre-exponential factor and DEa is the activation energy of depolarization. The plot of ln(l/P) vs. l/T is linear (in the temperature range 299.0–351.0 K, Fig. 10); from the slope of this plot the value of the activation energy of depolarization for CMSPB is calculated; least-squares analysis of the data listed in Table 3 gives the value 4.08 kcal mol  1. This value compared with the activation energy of glycerol flow (14.25 kcal mol  1 [19]) implies that the solute molecule is rotating relatively unprohibited within a solvent cage (free-volume) arising from multiple-hydrogen bonding sites in glycerol [56].

Fig. 9. Graphical representation of the noticed variation of degree of fluorescence polarization of CMSPB with f  1 for glycerol at different temperature.

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L.F.M. Ismail / Journal of Luminescence 132 (2012) 2512–2520

Fig. 11. Fluorescence emission spectra of 6.54  10  6 M in ethanol as a function of increasing chloroform concentration (lex ¼ 370 nm). Inset upward curvature of Stern–Volmer plot. Fig. 10. Plot of ln 1/P vs. 1/T for CMSPB.

3.1.3. Effect of binary solvent (ethanol/chloroform) Fig. 11 represents the fluorescence emission spectrum of 6.54  10  6 M of CMSPB in ethanol at different chloroform concentrations. It is clearly evident from Fig. 11, increasing chloroform concentration led to progressive decrease in the emission intensity concomitant with a slight hypsochromic shift of the maximum fluorescence wavelength. This could be attributed to the complexation of the dye molecule with chloroform. Such interactions between chloro solvents with many aromatic molecules have already been reported [57]. The hypsochromic spectral shift with increasing chloroform concentration indicates that the solvent composition is ethanolrich at low concentration. An increasing chloroform concentration increases the fraction in the solvation sphere [58]. Hypsochromic shift of fluorescence bands is apparently due to the fact that at transition to more or less polar solvents reversed solvatochromism can occur [59,60]. Such type of solvatochromism is detected in polymethine dye chromophores, which can reach the ideal polymethine state in weakly polar solvent [61]. Similar phenomena have been observed for other dyes also [62–65], where the replacement of solvents with low polarity by highly polar solvent, or vice versa, leads to a deviation from the ideal polymethine state and changes the energy of the electronic transition. It should be noted that the study of dye are salts, and in polar solvents may be in the form of dissociation, to form ionic complexes between the cation dye, its anion and solvent molecules which manifest themselves in the absorption spectra as a bathochromic or hypsochromic shifts [66]. The cation and the anion can be preferentially solvated by the components of the mixed system. A similar relationship was observed earlier for cyanine dyes in little polar solutions [67], and also in the works of other authors [68,69]. Moreover, the steady-state fluorescence quenching can be analyzed by Stern–Volmer [70] equation (Eq. (24)) ðF 0 =FÞ1 ¼ K sv ½Q 

ð24Þ

where F0 and F are the fluorescence intensity of CMSPB in absence and presence of quencher, respectively, Ksv is the Stern–Volmer constant and equal to Kqt and [Q] is the quencher concentration. The Stern–Volmer plot (Eq. [24], inset of Fig. 11) exhibits upward curvature indicating that chloroform preassociate with the dye molecule (i.e. static contribution in addition to dynamic one). A linear plot of (F0/F)  1 vs. [CHCl3] is obtained at low chloroform

Fig. 12. Benesi-Hildebrand plot for 6.54  10  6 M of CMSPB in EtOH as a function of chloroform concentration (r ¼0.9997).

concentration from which the Stern–Volmer constant Ksv can be calculated (Ksv ¼ 0.04 l mol  1). The low value of Ksv suggests the formation of a ground state complex [71]. Moreover, a second solution was obtained corresponding to the complex formation using the following equation [72]: 1=ðF 0 FÞ ¼ 1=ðF 0 F c Þ þ1=ðK formation =½CHCl3 n =ðF 0 F c ÞÞ

ð25Þ

where F0 represents the fluorescence intensity in absence of chloroform, F is the fluorescence intensity in presence of certain concentration of chloroform, Fc is the fluorescence intensity of the complex, Kformation is the formation constant of the formed complex and [CHCl3] is the chloroform concentration. Linear plot of 1/F F0 vs. 1/[CHCl3] is obtained suggests that only a 1:1 complex formed between CMSPB and chloroform molecules in the concentration range measured (Fig. 12), and from the intercept and the slope the formation constant is calculated which revealed the formation of weak complex with chloroform of stoichiometry 1:1 (Kformation ¼0.004 l mol  1). 3.2. Effect of microheterogeneous solutions In biological systems the dyes can encounter hydrophobic environments marked by much higher viscosity, lower dielectric

L.F.M. Ismail / Journal of Luminescence 132 (2012) 2512–2520

constant and polarity, and poorer hydrogen bond donor capabilities, compared with that of free solutions [73]. Investigation of the photophysical properties in micellar media, which can mimic some of these conditions, can provide useful information on the behavior of the dyes in such environments. In view of this, the absorption and fluorescence properties of CMSPB are examined in the presence of cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS). Representative emission spectrum of CMSPB in water and aqueous micellar solutions of CTAB and SDS above their critical micellar concentrations (CMC¼8  10  4 M and 1  10  2 M for CTAB and SDS, respectively) is shown in Fig. 13, and Table 4 reveals the relevant photophysical parameters. Inspections of the absorbance maxima indicate that the ground state of CMSPB is not perturbed to a great extent by the presence of micelles in solution (Table 4). The environment-dependent changes in emission maxima for this chromophore are more pronounced. When compared to aqueous medium, emission of CMSPB is found to be red shifted by 5 and 10 nm in CTAB and SDS, respectively and an increase in the intensity of emission are observed. The increase in fluorescence intensity can be attributed to microencapsulation of the dye into the hydrophobic interior of the micelle [74]. The observed increase of fluorescence when encapsulated into hydrophobic domains makes this dye especially useful as a probe for mapping such domains in biological systems. However, due to strong charge separation in the structure of hydrophilic cyanines, such dyes (in an all-transform configuration) should possess great dipole moments and can attractively interact with both positive and negative charges [75] (cationic and anionic micelles). This interaction causes alignment of the dye molecules along the electrostatic field created by charged micellar interfaces, leading to the conversion of the cis isomers of dye into its trans counterpart, the rigidization of the trans isomers in charged micelles and to the high fluorescence quantum yield

2519

(cis–trans isomerization of cyanines in micelles has already been reported in Ref. [74]). Moreover, micelles themselves can provide a high viscosity microenvironment for fluorophores [48]. As a result the molecular rotation of the dye molecules decreases and the fluorescence quantum yield increases relative to that of water. The results show that the fluorescence quantum yields of CMSPB in water, CTAB and SDS increase in the order SDS 4CTAB4water (^f ¼0.022, 0.067 and 0.154 for water, CTAB and SDS, respectively). This result indicates that CMSPB molecules are located inside the micellar aggregates. Similar behavior was reported previously for some cyanine dyes [49]. However, cyanine dyes contain a flexible polymethine chain and hence exhibit nonradiative deactivation (NRD) via rotation around the C–C bonds of this chain, which results in photoisomerization [75]. The twisting that leads to NRD occurs with intramolecular charge transfer; hence, NRD is strongly stimulated by polar solvents. In contrast, the intramolecular charge transfer is much less pronounced for classical cyanines, for which the viscosity of the medium is the main factor determining NRD [67–69]. For hydrophilic cyanines there is an additional factor that suppresses NRD in micelles, namely, a strong electrostatic field at charged micellar interfaces, which aligns and rigidizes the dye molecules. Moreover, the calculated values of the fluorescence polarization of CMSPB in CTAB and SDS are listed in Table 4. The obtained non-zero values of P imply reduced rotational depolarization of dye molecules via association with the surfactants. Difference in the measured polarization between CTAB and SDS can be attributed almost to the decrease of molecular rotation of CMSPB in SDS as a result of the attractive coulombic interaction between dye molecules and oppositely charged head groups of SDS [66]. Thus information on the microviscosity of organized assemblies is accessible in a relatively simple way, as shown above in the work on the Z(T) behavior of high viscosity solvents using the fluorescent probe technique [19,55,56]. Moreover, using the linear dependence of 1/P on Z(T) function above established, the determined 1/P values for CMSPB in CTAB and SDS solutions (above CMC) are 2.99 and 3.27, respectively (Table 4) and are readily converted to the average microviscosities in the range of 18–30 cP which are comparable with these in the literature [55,56].

4. Conclusion

Fig. 13. Fluorescence emission spectrum of 6.54  10  6 M of dye1 in water and micellar solutions.

Table 4 Absorption and fluorescence spectral data for CMSPB in water, CTAB and SDS and fluorescence polarization data in CTAB and SDS. Medium

labs (nm)

e (l mol  1 cm  1)

lflu (nm)

F2f  10

P

1/P

1-H2O 2-CTAB 3-SDS

371 375 376

10319.0 15867.0 17028.0

490 495 500

2.23 6.70 15.40

– 0.306 0.335

– 3.27 2.99

The steady state fluorescence emission as well as the fluorescence polarization is used to obtain information on the long chain styryl merocyanine chromophore (CMSPB) in homogeneous and microheterogeneous media. CMSPB has been found to be sensitive to solvent polarity and viscosity. The polarity as well as the H-bond-donating and accepting properties of the solvent contribute to the stabilization of the excited state. However, this effect is much more pronounced on the fluorescence side. This is indicative of a large charge transfer character in the excited state. Moreover, in high viscous solvent such as glycerol, the freevolume concept proposed by Loufty et al. has been found to provide the accurate description of the solvent’s viscosity and temperature-dependent torsional relaxation of CMSPB. The torsional motion in the molecule responsible for inducing the nonradiative decay processes is hindered by the viscous drag of the solvent that results in an increase in fluorescence quantum yield in highly viscous solvents. Therefore, CMSPB can be used as a potential microscopic probe molecule that can provide valuable information about the hydrodynamic interaction in various solvents and a useful microviscosity sensor to study the biological systems where many cellular and organismal functions are dependent on the viscosity of their environments.

2520

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