Photophysics of a Cationic Biological Photosensitizer in Anionic

J. Phys. Chem. B 2007, 111, 11169-11176

11169

Photophysics of a Cationic Biological Photosensitizer in Anionic Micellar Environments: Combined Effect of Polarity and Rigidity Paramita Das, Alok Chakrabarty, Arabinda Mallick,† and Nitin Chattopadhyay* Department of Chemistry, JadaVpur UniVersity, Calcutta 700 032, India ReceiVed: May 23, 2007; In Final Form: June 30, 2007

A steady-state and time-resolved photophysical study of a cationic phenazinium dye, phenosafranin (PSF), has been investigated in well-characterized biomimetic micellar nanocavities formed by anionic surfactants of varying chain lengths, namely, sodium decyl sulfate (S10S), sodium dodecyl sulfate (S12S), and sodium tetradecyl sulfate (S14S). In all these micellar environments, the charge transfer fluorescence of PSF shows a large hypsochromic shift along with an enhancement in the fluorescence quantum yield as compared to that in aqueous medium. A reduction in the nonradiative deactivation rate within the hydrophobic interior of micelles led to an increase in the fluorescence yield and lifetime. The present work shows the degree of accessibility of the fluorophore toward the ionic quencher in the presence of surfactants of different surfactant chain lengths. The fluorometric and fluorescence quenching studies suggest that the fluorophore resides at the micelle-water interfacial region. The enhancements in the fluorescence anisotropy and rotational relaxation time of the probe in all the micellar environments from the pure aqueous solution suggest that the fluorophore binds in motionally restricted regions introduced by the micelles. Polarity and viscosity of the microenvironments around the probe in the micellar systems have been determined. The work has paid proper attention to the hydrophobic effect of the surfactant chain length on photophysical observations.

1. Introduction In recent years, photophysical studies of dye-surfactant systems have drawn particular attention due to the diverse application of these systems in pharmaceutical science, analytical chemistry, biochemistry, luminescence, lasers, and photography.1 The dye-surfactant interaction is associated with many biological processes in large organic molecules and biomembranes.2,3 To understand the thermal and photoinduced processes in biomembranes, studies of the dye-surfactant interactions have vastly been used.4 Because of the potential application in designing electronic devices, interactions between ionic dyes and surfactants have gained wide interest.5-7 Among the various techniques, sensitive spectral techniques have been successfully exploited for the study of the dye-surfactant interactions in micelles. It is known that the local microenvironment surrounding a probe molecule influences its electronic structure and thus its photophysics.8 Changes in this local microenvironment can produce measurable spectral shift, which can be monitored spectroscopically. This property, known as solvatochroism, allows elucidation of the influence of the immediate environment of the molecule within the probed system and gives evidence of specific interactions. Aqueous micellar systems modify the microenvironment around the dye appreciably as compared to that in the bulk aqueous phase.3,5,6 Penetration of the probe into the micellar media from bulk water modifies the photoprocesses since the polarity and viscosity in the immediate environments around the probe are quite different from those of the bulk aqueous phase.8 Studies of such microenvironments and the * Corresponding author. Fax: 91-33-2414-6266; e-mail: pcnitin@ yahoo.com. † Present address: Division of Frontier Material Science, Graduate School of Engineering Science, Osaka University, Toyonaka 5608531, Japan.

corresponding modification of the photophysical processes of the probe (dye) remain the basic objectives of such studies. Phenazinium cationic dye, phenosafranin (PSF) (Scheme 1), has been widely recognized as a sensitizer in energy and electron-transfer reactions in homogeneous media9-11 and in semiconductors.12 As micelles are considered as simple model membrane systems, the study of this photosensitizing dye in micellar environments is utilized to understand the interaction, distribution, and localization of the dye in biological systems. Because of electrostatic interactions, the cationic dye can form stable complexes with the anionic micellar units.3 Apart from the electrostatic interaction, the hydrophobic interaction between the dye and the sodium alkyl sulfates also plays a role in the binding between the dye and the surfactants having different chain lengths. Although a reasonable amount of research has addressed to the electrostatic interaction between oppositely charged dye and surfactants, surprisingly, the effect of hydrophobic interactions has not received proper attention. Fluorometric technique has been advantageously exploited in the present study to monitor the interaction behavior of a homologous series of anionic sodium alkyl sulfate surfactants with this cationic probe. The photophysics of PSF have been studied in S10S, S12S, and S14S anionic micellar environments intending to assess the location of the probe and the micropolarity and microviscosity around the probe and to see the effect of a variation in the chain length of the surfactant constituting the micelle. The results reveal a combined effect of polarity and rigidity of the microenvironment on the fluorescence, steadystate fluorescence anisotropy, and the anisotropy decay of the entrapped fluorophore. 2. Experimental Procedures The dye phenosafranin (PSF) was purchased from Sigma and used as received. Its purity was confirmed from its absorption

10.1021/jp073984o CCC: $37.00 © 2007 American Chemical Society Published on Web 09/05/2007

11170 J. Phys. Chem. B, Vol. 111, No. 38, 2007

Das et al.

SCHEME 1: Structure of PSF

and emission spectra in standard solvents. The surfactants, sodium decyl sulfate (S10S), sodium dodecyl sulfate (S12S), and sodium tetradecyl sulfate (S14S), were procured from Aldrich and were used as received. Prior to their use, it was checked that the surfactants did not contribute to either absorption or fluorescence in the region of interest. Triply distilled water was used to make the experimental solutions. The solvents, 1,4dioxane, glycerol, and methanol, used were of UV spectroscopy grade (Spectrochem). AR grade copper sulfate was purchased from SRL. The concentration of PSF was 4.0 × 10-6 mol dm-3 throughout the experiment. Absorption and steady-state fluorescence measurements were carried out using a Shimadzu MPS 2000 spectrophotometer and a Spex fluorolog-2 spectrofluorimeter equipped with DM3000F software, respectively. The steady-state fluorescence anisotropy measurements were performed with a PerkinElmer LS55 spectrofluorimeter model. The steady-state anisotropy, r, was defined by

r ) (IVV - GIVH)/(IVV + 2GIVH)

(1)

where IVV and IVH are the emission intensities obtained with the excitation polarizer oriented vertically and the emission polarizer oriented vertically and horizontally, respectively. The G factor was defined as

G ) IHV/IHH

(2)

The I terms refer to parameters similar to those mentioned previously for the horizontal position of the excitation polarizer. Fluorescence lifetime (τf) and anisotropy decay (r(t)) measurements were performed using time-resolved intensity decay analyses by the method of time correlated single photon counting (TCSPC) using a nanoLED at 403 nm (IBH, UK nanoLED07) as the light source. The typical response of this excitation source was 70 ps. The decay curves were analyzed using IBH DAS-6 decay analysis software. The goodness of fits were evaluated by χ2 criterion and visual inspection of the residuals of the fitted function to the data. For anisotropy decay measurements, emissions at parallel (I|) and perpendicular (I⊥) polarizations were collected by rotating the analyzer at regular intervals. The time-resolved anisotropy (r(t)) was calculated using the following relation:

r(t) ) [I|(t) - GI⊥(t)]/[I|(t) + 2GI⊥(t)]

(3)

All the experiments were performed at ambient temperature (300 K) with air-equilibrated solutions. 3. Results and Discussion The absorption spectrum of an aqueous solution of PSF shows a broad, unstructured band with a maximum at around 520 nm.13 The band maximum shifts slightly to the red upon addition of the surfactants, indicating that the environment around the probe becomes modified due to the formation of the micelles from pure aqueous solution. Discussions in the forthcoming sections reveal that a lowering in the polarity in the immediate environment around the probe is responsible for the spectral change.

Figure 1. Emission spectra of PSF solution as a function of added surfactant concentrations (λex ) 520 nm). Curves correspond to 0, 20, 26.2, 28.7, 30.2, 31.6, 33.1, and 64 mM S10S in panel a; 0, 5.5, 6.0, 7.0, 8.5, 12.0, 16.0, and 18.0 mM S12S in panel b; and 0, 1.8, 1.85, 1.9, 1.95, 2.01, 2.11, and 3.76 mM S14S in panel c. Insets show the variation of fluorescence intensities with surfactant concentrations.

Room-temperature fluorescence spectrum of an aqueous solution of PSF shows a single, broad unstructured band peaking at 584 nm ascribed to the charge transfer (CT) emission.13 Figure 1 shows the fluorescence spectra of PSF in aqueous S10S, S12S, and S14S micellar environments as a function of surfactant concentrations. With an increase in the concentration of SnS in an aqueous solution of the fluorophore, the fluorescence intensity shows a small initial decrease followed by a substantial increase along with a noticeable hypsochromic shift (about 15 nm). Our observations with the PSF system qualitatively agree with the earlier reports from Pal et al. and Duemie and Baraka while investigating the photophysical properties of rhodamine derivatives in S12S micelles.14,15 The fluorescence intensity of the dye shows an initial decrease at lower surfactant concentrations. A similar decrease in the fluorescence yield is quite common at the lower concentrations of various surfactants and is ascribed to the formation of pre-micellar aggregates.14,16 Consistent with our earlier observation,16 the quantum of decrease in the fluorescence intensity as the chain length of the surfactant

Photophysics of a Cationic Biological Photosensitizer

J. Phys. Chem. B, Vol. 111, No. 38, 2007 11171

increases reveals that formation of the pre-micellar aggregates is more effective for the surfactant with a longer chain length. After cmcs are achieved, there is a huge enhancement in the emission of PSF in the SnS micellar environments. Plots of the fluorescence intensity against the surfactant concentrations lead to reliable and precise determination of the cmcs of the individual surfactants (insets of Figure 1). A comparison with the observation of a similar blue shift and an increase in the fluorescence quantum yield of PSF from water (φ ) 0.04) to methanol/ethanol (φ ) 0.20)17 proposes that the microenvironments around the fluorophore in the micellar solutions are quite different from the pure aqueous phase. The blue shift further suggests that the polarity of the micellar environment is less than the polarity of the bulk water. An explanation of the observation, in terms of the relative stabilities of the CT state and the ground states of the fluorophore, is available in the literature.8,18,19 As is known, micelles are characterized by three distinct regions: a nonpolar core formed by the hydrocarbon tails of the surfactant, a compact stern layer having mostly the headgroups, and a relatively wider Gouy-Chapman layer containing the counterions.20 Because of the difference in the properties on the core and headgroup regions of the micelles, the location of a probe depends both on the nature of the probe and on the nature of the micelle. Thus, a probe molecule can reside either in the nonpolar core of a micelle or in the micelle-water interface. The final position of the emission maximum of PSF in the three SnS micellar systems studied (at the stage of complete interaction) is observed to be at 567 nm. The proximity of the emission maxima of PSF in all three micelles suggests that the micropolarities around the probe in the micellar environments studied here are quite similar. The observed emission maxima of PSF in these micellar environments closely match the fluorescence maximum of the probe in the wateralcohol solution. In tune with the literature suggesting that the environment in the micellar interface resembles the wateralkanol system,16,21,22 the present observations indicate that the fluorophore resides in the micelle-water interfacial region and does not penetrate deep into the micellar core. The observation is well-rationalized, taking into consideration the fact that the fluorophore is cationic in nature and that the micelles under study are anionic since the electrostatic forces of the anionic micelle operative at the micelle-water interface are likely to trap the cationic probe in this region and thus restrict the penetration of it into the core of the micelle. 3.1. Dye-Micelle Binding. To understand the interaction between the dye and the micellar units, the binding constants of the probe in different SnS environments have been determined from the fluorescence intensity data following the method describe by Almgren et al.23 According to this method

[(IR - I0)/(Ic - I0)] ) 1 + (K[M])-1

(4)

where I0, Ic, and IR are the fluorescence intensities of PSF in the absence of surfactant, at an intermediate surfactant concentration, and at a condition of complete micellization, respectively, K is the binding constant, and [M] is the micellar concentration. [M] is determined by

[M] ) (S - cmc)/N

(5)

where S represents the surfactant concentration, and N is the aggregation number of the micellar system. The values of N and cmc for S10S, S12S, and S14S are presented in Table 1.

Figure 2. Plot of (IR - I0)/(Ic - I0) against [M]-1 in (a) S10S, (b) S12S, and (c) S14S.

TABLE 1: Literature Values of cmc and Aggregation Numbers of Investigated Surfactant Systemsa

a

surfactants

cmc (mM)

N

S10S S12S S14S

30 8.1 1.8

64 92 120

Data are taken from refs 16 and 24.

Figure 2 shows the plot of [(IR - I0)/(Ic - I0)] against [M]-1 for PSF in three micellar environments, and the binding constant values have been determined from the slopes of the individual plots. The binding constant values are thus obtained as 7.67 × 104, 6.45 × 105, and 1.35 × 106 L mol-1, and the free energy change values are -28.0, -33.3, and -35.2 kJ mol-1 for the probe-micelle binding process in the S10S, S12S, and S14S environments, respectively. Higher values of the binding constants in the present case as compared to the binding constants with the neutral probes25 is justified considering the electrically opposite characters of the probe and the micelles (cationic probe and anionic micelle). 3.2. Metal-Induced Fluorescence Quenching Study. Metalinduced fluorescence quenching of PSF has been studied in SnS environments using Cu2+ as an ionic quencher to assign the probable location of the fluorophore and to understand how the degree of accessibility of the probe molecule varies with an increase in the surfactant chain length.26,27 The idea behind the experiment is the following. It is known that the Cu2+ ion is preferentially available in polar regions, namely, in the micellewater interface and in the bulk aqueous phase. It is not available in the hydrophobic core of the micelle.8 For anionic surfactants, as the surface charge is negative, a cationic quencher is expected to be attracted toward the micelle-water interface. Hence, had


Das et al.

Figure 3. Stern-Volmer plots for fluorescence quenching of PSF by Cu2+ ions in aqueous micellar solutions. Concentrations of the micelles are given in Table 2.

TABLE 2: Photophysical Data of PSF in Different SnS Environments environment

SV constant (M-1)

steady-state anisotropy (r)

viscosity (cp)

Water S10S (80.6 mM) S12S (22.0 mM) S14S (4.6 mM)

3.6 118 398 2130

0.028 0.065 0.076 0.083

0.895 1.75 2.14 2.54

the fluorophore been located in the micellar core, there would have been no appreciable fluorescence quenching due to the lack of availability of the quencher (Cu2+). On the other hand, if the probe is located in the micelle-water interface, the Cu2+induced quenching of its fluorescence is supposed to be remarkably more than that in pure aqueous medium. In the three micellar (SnS) environments, appreciable quenching of PSF fluorescence was observed at the saturation level of PSFmicelle interactions. The specific concentrations of the surfactants used for the quenching experiments are indicated in Table 2. The quenching of the fluorescence of PSF with the addition of quencher (Cu2+) followed the Stern-Volmer relation

F0/F ) 1 + KSV[Q]

(6)

where F0 and F are the fluorescence intensities in the absence and in the presence of quencher, respectively, [Q] is the molar concentration of the quencher, and KSV is the Stern-Volmer quenching constant. Figure 3 depicts the Stern-Volmer plots for the quenching of PSF fluorescence in the three micellar environments. The slope of each plot gives the Stern-Volmer quenching constant (Ksv). The higher the slope of the plot, the greater the degree of exposure is since there is not much difference in the fluorescence lifetime of the probe in the three micelles (see section 3.6). The KSV values are collected in Table 2. From Figure 3 and Table 2, it is clear that an increase in the surfactant chain length increases the accessibility of the probe molecule toward the quencher. The quenching experiment reveals that the degree of exposure of the probe toward the quencher is gradually increased from S10S to S14S through S12S. The quenching of PSF in all micellar environments is significantly higher than that in the pure aqueous phase presumably due to higher local concentrations of both the cationic fluorophore and the cationic quencher near the micellar surfaces because of the mutual electrostatic attractions.25,28,29 The relative increase in the fluorescence quenching efficiency with an increase in the chain length of the surfactants constituting the micelles can be rationalized in terms of the compactness of the headgroups and the aggregation number of the micelles. It is known that water can enter micelles up to a certain depth depending on the compactness of the micellar units.8,30,31 Micelles with compact headgroups suffer smaller water penetration as compared to the micelles with less compact

headgroups, and the compactness of the headgroup increases with an increase in the surfactant chain length. Here, as we move from S10S to S14S, the increased surfactant chain length enhances the compactness of the headgroups gradually, which in turn decreases the water penetration as well as the probe penetration. An increase in the aggregation number of the SnS micelles with an increase in the surfactant chain length leads to an increase in the surface charge (and also charge density), facilitating the trapping of both the cationic fluorophore and the cationic quencher and hence contributes to a greater degree in fluorescence quenching. 3.3. Polarity of the Micellar Environments around the Fluorophore. For a few decades, fluorescent probes have been serving a unique role in the determination of the microscopic polarity of biological and biomimicking environments.8,32-34 The polarity determined through different photophysical parameters of the probe gives a relative measure of the polarity of the microenvironment next to the probe. Local polarity around a probe in biomimetic environments such as micelles can be estimated by comparison of the spectral properties of the fluorophore in that environment with those of the probe in pure solvents or in solvent mixtures of known polarities.8,32-34 It is true that the polarity of a homogeneous environment is not exactly the same as the polarity in a microheterogeneous medium. However, to have an estimate of the micropolarities in different environments such as micelles, reverse micelles, proteins, lipids, and cyclodextrins, the Lippert-Mataga polarity scale35,36 or the ET(30) scale as developed by Reichardt et al. are often used.37,38 While the former one is based on rigorous descriptive models, the latter one is based on the experimental transition energy for the solvatochromic intramolecular charge transfer absorption of the betaine dye 2,6-diphenyl(2,4,6triphenyl-1-pyridino)phenolate. The Lippert-Mataga scale evaluates the polarity aspect of the solvents but does not satisfy the experimental observations for protic solvents. Taking the nonprotic and protic solvents together, the scale often becomes nonlinear.17,39 The ET(30) scale, however, remains linear throughout, presumably considering both factors together (a rigorous descriptive model is still awaited). The CT emission maximum of PSF is observed to move from 584 nm in water to 560 nm in 1,4-dioxane. As already mentioned, the fluorescence maxima of PSF in all the micelles under study are observed at 567 nm. Thus, one can presume that the micropolarities around the probe in these environments lie between the polarities of the two aforesaid media. We have studied the fluorescence behavior of PSF in a water-dioxane mixture of varying composition to determine the polarity around the probe in the micelles. To obtain the micropolarity around the probe in the micellar environment, we have constructed the calibration curves based on both ET(30) and Lippert-Mataga scales whereby the Stokes shift and emission maxima of PSF in the dioxane-water mixtures are plotted against the polarity parameters ET(30) and Lippert-Mataga (∆f), respectively. Figure 4a shows the normalized emission spectra of PSF in solutions of dioxane-water mixtures with different compositions. The plot of the fluorescence maxima of PSF in the waterdioxane mixtures against ET(30) of the solutions establishes a linear correlation between the two (inset of Figure 4a). Plots of both the Stokes shift and the emission maxima of PSF in waterdioxane mixtures against ∆f reveal a nonlinear relationship (Figure 4b). Interpolating the values of the emission maxima and Stokes shift of PSF in S10S, S12S, and S14S micellar systems on the calibration curves (Figure 4a,b), we have determined the micropolarities around the probe. The estimated polarities in


J. Phys. Chem. B, Vol. 111, No. 38, 2007 11173

Figure 5. Normalized fluorescence spectra of PSF in methanolglycerol mixtures with different compositions. Curves (i)-(v) correspond to 0, 10, 30, 50, and 70 wt % glycerol in the mixture. Inset shows the variation of the emission maximum as a function of wt % glycerol in the mixture.

Figure 4. (a) Normalized fluorescence spectra of PSF in dioxanewater mixtures with different compositions. Curves (i)-(v) correspond to 0, 10, 30, 60, and 90 vol % dioxane in the mixture. Inset of panel a shows the variation of the emission maximum in dioxane-water mixtures and micellar solutions with ET(30). (b) Plot of stokes shift (∆ν, in cm-1) against ∆f in dioxane-water mixtures and micellar solutions. Inset of panel b shows the variation of the emission maximum against ∆f in dioxane-water mixtures and micellar solutions. Solid circles represent the interpolated points for the micelles.

both the scales suggest that the micropolarities in all three micellar environments are very similar and correspond to that of a ∼65% dioxane-water mixture (ET(30) ) 51.5 and ∆f ) 0.2873) or of an ethanol solvent (ET(30) ) 51.940 and ∆f ) 0.3). The estimated ET(30) values of the immediate environments around PSF suggest that the probe resides in an environment whose polarity is appreciably less than that of the aqueous phase (ET(30) ) 63.1)40 and therefore rules out the residence of the probe in bulk water. Similarly, a large difference between the estimated polarity in the micelles and the polarities of hydrocarbons (e.g., ET(30) ) 31.1 for n-heptane)40 negates the possibility of penetration of the fluorophore into the core of the micelles. Considering the cationic nature of the probe and the anionic character of the micellar units, it is rational to assign the micelle-water interface (Stern layer) to be the most probable location of the fluorophore. A similar value of the estimated polarity of the microenvironments around the fluorophore in all the SnS micellar solutions studied goes against our normal expectation from a polarity point of view that with an increase in the hydrophobicity (as the surfactant chain length increases), the emission maximum should move toward blue. A rationalization toward this strange observation could be that the polarity and rigidity factors might be operative in opposition to balance each other. It is known that the rigidity of the headgroups of SnS anionic micelles increases with an increase in the chain length of the surfactant.16 An increase in the rigidity might restrict the penetration of the fluorophore inside the micellar unit. This argument is strengthened from our fluorescence quenching study as well (see section 3.2.). To see, in general, if the rigidity of environment plays a role in controlling the fluorescence maximum, we studied the emission behavior of PSF in mixtures of two solvents with comparable polarity but differing widely in terms of viscosity. Methanol (ET(30) ) 55.4) and glycerol (ET(30) ) 57.0) were chosen as the pair of solvents. Figure 5 shows the normalized emission spectra of PSF in different weight percentages of glycerol in the methanol-glycerol mixtures; the inset gives the

Figure 6. Variation of the fluorescence anisotropy (r) of PSF as a function of S10S (i), S12S (ii), and S14S (inset) concentrations.

corresponding variation in the emission maximum. The study reveals a nearly 6 nm red shift of the fluorescence maximum as we move from pure methanol to an 80-20 wt % glycerolmethanol mixture in spite of the fact that the polarity of the media does not differ appreciably. The observation thus suggests that an increase in the rigidity of the environment leads to a red shift in the emission maximum. 3.4. Steady-State Fluorescence Anisotropy Study. The measurement of fluorescence anisotropy serves an important role in biochemical research owing to the fact that any factor affecting size, shape, or segmental flexibility of a molecule will affect the parameter.26 Such studies reflect how the micellar environments impose motional restriction on the probe. An increase in the rigidity of the neighboring environment of a fluorophre results in an increase in its fluorescence anisotropy. Thus, the study of fluorescence anisotropy has a great impact on determining the probable location of the probe in microheterogeneous environments such as proteins,26,41 micelles,8 reverse micelles42 etc. In the present study, fluorescence anisotropy provides a picture of the impact of the increasing surfactant chain length and compactness of the micelles toward the motional restriction of the probe. Figure 6 depicts the variation of the fluorescence anisotropy with surfactant concentrations in the different micellar environments. The figure leads to two interesting observations. First, the anisotropy increases sharply until the cmcs of the individual micelles are attained, leveling off thereafter. Second, in the fully micellized condition, the anisotropy value gradually increases as the chain length of the surfactant increases from S10S to S14S (Table 2). The first point is understandable from the consideration of the formation of the micellar aggregates. Before micelle formation, the surfactant units remain in a rather scattered and unorganized pattern. As the surfactant concentration increases, the monomers are arranged more orderly, and consequently, the motion of the probe is restricted gradually. Above the cmc, micellar units are formed, the probe is trapped in the micelles, and the motion is restricted to its maximum. The second observation is explained in terms of increasing motional


Das et al.

Figure 7. Variation of fluorescence anisotropy of PSF as a function of the composition of the glycerol-water mixtures. Anisotropy values in the micellar media are inserted onto the curve.

restriction imposed on the fluorophore with an increase in the compactness of the micelles as the chain length of the surfactant increases. 3.5. Viscosity of the Micellar Environments around the Probe. The viscosity of the microenvironment around the probe is related to the steady-state fluorescence anisotropy.8,41 Determination of the viscosity of a heterogeneous environment depends on the choice of probes, since depending on the electrical and dipolar characteristics, different probes can be located in different microscopic regions of such environments.43 Thus, microviscosity is often estimated from a comparison of the fluorescence anisotropy of a fluorophore in an environment to those of the probe in environments of known viscosities.8,42,44,45 Fluorescence anisotropy of PSF in glycerol-water mixtures having different weight percentage compositions and in the micellar environments under consideration at the saturation level has been measured and is shown in Figure 7. From the constructed calibration curve based on the data set in glycerol-water environments and interpolating the measured anisotropy values in the micellar environments, it is shown that, from a viscosity point of view, the S10S, S12S, and S14S environments are comparable with the 32, 36, and 39% glycerol-water mixture, respectively. A consultation with the available viscosity data in the glycerol-water system gives the microviscosities in these three micellar environments.46 The estimated values are presented in Table 2. The table reveals that the viscosity in the immediate environment of the probe increases slightly in the order S10S < S12S < S14S in spite of the fact that in all three micelles, PSF is located in the micellewater interface. This corroborates the proposition that the compactness of the anionic micelles increases in the same order.16,47 3.6. Time-Resolved Studies. (a) Fluorescence Lifetime Study. The fluorescence lifetime serves as a sensitive parameter for exploring the local environment around a fluorophore, and it is sensitive to excited-state interactions.48-52 It also provides information relating to the probe-micelle interactions.48-51 The fluorescence lifetimes of PSF have been measured in water and at the saturation level of the probe-micelle interaction as probed by the steady-state observations. The fluorescence decay of PSF was found to be single exponential in water as well as in all the SnS micelles studied. The decay profiles of PSF in aqueous and different micellar environments are shown in Figure 8. The fluorescence lifetimes of PSF in water and in the SnS environments are presented in Table 3. Table 3 reflects that in all three micellar environments, the fluorescence lifetime of PSF is found to be almost the same, although this differs appreciably from the fluorescence lifetime of the probe in pure aqueous medium. The insignificant difference between the lifetimes of PSF in different SnS environments reflects that a variation in the alkyl chain length of the surfactants does not affect its fluorescence decay characteristics. The single exponential fluorescence decay in the micellar environments suggests that the fluorescent species

Figure 8. Time-resolved fluorescence intensity decay of PSF in water and in SnS micelles. Inset shows the respective environments.

TABLE 3: Radiative and Nonradiative Rate Constants of PSF in Aqueous and Aqueous Micellar Environments environment

φf

τf (ns)

kr × 10-9 (s-1)

knr × 10-9 (s-1)


0.04 0.213 0.288 0.202

0.93 2.30 2.31 2.36

0.043 0.092 0.125 0.085

1.032 0.341 0.308 0.337

is located in a single type of site. From the differences in the fluorescence lifetime of PSF in water and in the SnS environments, it is logical to rule out the assignment of the bulk aqueous phase to be the single site in the immediate neighbor around the probe. A close resemblance between the fluorescence lifetime of PSF in ethanol/methanol (2.6/2.1 ns) and that observed in the micelles (∼2.3 ns) thus corroborates the proposition the previous discussions made that the fluorophore binds with the Stern layer of the SnS micelles. The strong electrostatic attraction between the cationic fluorophore and the oppositely charged micellar surface possibly dictates the single site binding of PSF. A fluorescence quenching study and the polarity study also support this conjecture. From the values of quantum yield (φf) and lifetime (τf) of PSF in aqueous and aqueous micellar environments, we can calculate the radiative and nonradiative rate constants of the CT state using eqs 7 and 8

kr ) φf/τf

(7)

1/τf ) kr + knr

(8)

where kr and knr are the radiative and nonradiative rate constant, respectively, and φf is the fluorescence quantum yield. All these photophysical parameters are presented in Table 3. Table 3 reflects that a significant decrease in the nonradiative decay rate (knr) in the micellar media as compared to that in pure aqueous phase is responsible for the increase in the radiative decay rate (kr) and hence in the net fluorescence quantum yield in these microheterogeneous environments. (b) Fluorescence Anisotropy Study. The time dependent decay of the fluorescence anisotropy provides additional information about the rotational motion and/or rotational relaxation of the fluorophore in organized assemblies.26,51 To see how the rotational relaxation dynamics of the probe is affected when we go from bulk water to the micellar environments, the fluorescence anisotropy decays of PSF in aqueous and SnS micellar environments have been measured. In aqueous medium as well as in the SnS micellar media, the anisotropy decays are found to be single exponential. Figure 9 illustrates the fluores-


J. Phys. Chem. B, Vol. 111, No. 38, 2007 11175 and Kelvin temperature, respectively. Following Maiti et al., the hydrodynamic radii (rh) of the micelles are calculated using the relation, rh ) core radius + headgroup radius (2 Å) + two layers of water (2 Å) associated with the charged micelles (SnS).53 The estimated rh values of S10S, S12S, and S14S micelles and the corresponding rotational relaxation times are presented in Table 4. Table 4 reflects that the rotational relaxation times of the micelles are remarkably higher than the corresponding depolarization times of fluorescence in the media and confirms that the relaxation of the fluorescence anisotropy results from the rotation of the dye only and not of the micelles. Conclusion

Figure 9. Fluorescence anisotropy decays of PSF in aqueous and aqueous SnS micellar environments.

TABLE 4: Decay Parameters of Fluorescence Anisotropy of PSF in Water and in Micelles environment

τr (ns)

rh (Å)

τM (ns)


0.120 0.726 0.680 0.727

18.1 20.7 23.2

5.4 8.0 11.3

cence anisotropy decays of PSF in aqueous and in aqueous micellar environments. All the anisotropy decay parameters in aqueous as well as in aqueous micellar environments are collected in Table 4. Figure 9 and Table 4 reveal that in all the SnS micellar media, the rotational relaxation times (τr) of the probe are comparable (∼700 ps), and they are appreciably longer than that in water (∼120 ps). However, all these relaxation times are less than the fluorescence lifetime of the probe in the corresponding environments, reflecting that the relaxation processes are completed within the lifetime of the photoexcited state of the fluorophore. A higher value of the rotational relaxation time of the probe in the micelles as compared to that in pure water solution reveals that the probe molecule experiences motionally restricted environments in the micelles. Comparable values of the fluorescence depolarization times of PSF in the three SnS micelles indicate that in spite of a gradual increase in the surfactant chain length from S10S to S14S through S12S, the rigidity of the microenvironment around the fluorophore does not differ much so as to have a perceptible effect on the dynamic parameters. This also corroborates the near constancy of the fluorescence lifetimes of PSF in the three micellar environments (see Table 3). Relaxation of the fluorescence anisotropy in a micellar phase might lead to the following possibilities: (i) the fluorophore rotates within the micelle, (ii) the entrapped fluorophore cannot rotate, but the micelle carrying the probe rotates, or (iii) both rotations are possible. Time-resolved fluorescence depolarization studies can resolve this problem. The last option should result in a biexponential decay pattern, while the former two should lead to single exponential anisotropy decays. The observed single exponential anisotropy decays of PSF fluorescence in aqueous as well as in aqueous SnS micelles, as mentioned previously, therefore rule out the last option. To confirm one from the former two options, we have determined the rotational relaxation times of the micelles using the Stokes-EinsteinDebye equation.26,53

τM ) 4πηrh3/3kT

(9)

where η is the viscosity of water in poise, rh is the hydrodynamic radius of the micelle, and k and T are the Boltzmann constant

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Photophysics of a Cationic Biological Photosensitizer in Anionic

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