Preferential Solvation of Styrylpyridinium Dyes in Binary Mixtures of

J. Phys. Chem. B 2011, 115, 99–108

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Preferential Solvation of Styrylpyridinium Dyes in Binary Mixtures of Alcohols with Hexane, Dioxane, and Dichloromethane Mallika Panigrahi,† Sukalyan Dash,‡ Sabita Patel,§ and Bijay K. Mishra*,† Centre of Studies in Surface Science and Technology, School of Chemistry, Sambalpur UniVersity, Jyoti Vihar, Burla 768 019, India, Department of Chemistry, VSS UniVersity of Technology, Burla 768018, India, and Department of Chemistry, National Institute of Technology, Rourkela 769008, India ReceiVed: August 24, 2010; ReVised Manuscript ReceiVed: NoVember 24, 2010

The absorption maxima of N-alkyl(methyl, hexyl, and hexadecyl)-4-[(4-N,N-dimethylamino)styryl]pyridinium halides in binary solvents were analyzed for probing preferential solvation by any one of the solvents. The probes have a donor-acceptor system and the corresponding absorption bands are found to be solventsensitive. In neat solvents, excluding a few, reversal in solvatochromism was observed, identifying a polarity scale for solvatochromic switch, which appears around 45-50 ET(30) scale. The change in apolarity due to change in alkyl chain in the probe could not sway the scale, as the solvatochromism in neat solution is due to the chromophoric group only. When one solvent is considered as the parent solvent (hexane, dichloromethane, and dioxane) and the other as cosolvent (10 different alcohols), the solvation phenomena of these probes have been investigated. Due to solvent-solvent interactions, in some cases, hypo- and hyperpolarity in the solvent cage were observed. The preferential solvations of the dyes by these solvents were analyzed by consideration of the interactions of solvents with hydrophilic and hydrophobic groups of the probes. Tentative orientations of the solvent molecules around the probes were proposed. Disorder of the solvents around the probes was also considered for preferential solvation phenomena. Introduction Cyanine dyes are associated with many important properties, of which solvent-induced reversible color change or solvatochromism is one.1 These dyes, upon excitation, exhibit a large change in dipole moment due to the relative contributions of both dipolar zwitterionic benzenoid and neutral quinonoid forms and, hence, find application as probes for the establishment of empirical relationships of solvent polarity.2-4 Brooker et al.5 and Kiprianov et al.6 are pioneers of studies on solvatochromism in merocyanine dyes, which possess large negative solvatochromic shifts, second-order hyperpolarizability, and usefulness in diagnostics and therapeutics.7 The impact of solvents on the electronic spectral characteristics of cyanine dyes has been studied extensively.4,8-13 Reversal in the solvatochromism of some styrylpyridinium dyes, a class of cyanine dyes, has been reported by various workers.14-16 Earlier we investigated the relative stabilities of the electronic ground and excited states of some tailor-made styrylpyridinium dyes in different solvents and observed a deviation in steady solvatochromism. The changes in solvatochromism, referred to as solvatochromic switches, in these dyes with variable substituents were found to be around 45.00 in ET(30) scale.16 An analysis of absorption and fluorescence maxima for some symmetric and asymmetric ketocyanine dyes (polyenic bis-ω,ωaminoketones) showed that, for symmetric dyes (with two aminopolyenic fragments of the equal length), the absorption bands experienced a bathochromic shift compared to asymmetric dyes with the same total length of the polyenic chain.17 The * Corresponding author: e-mail [email protected]. † Sambalpur University. ‡ VSS University of Technology. § National Institute of Technology.

positive solvatochromism in these dyes was attributed to the zwitterionic structure of the ketocyanines having a positive charge on the amino group and a negative charge on the enolic group. Bagchi et al.18 made an extensive study on the solvatochromism/fluorosolvatochromism of some ketocyanine dyes, which proved to be good probes for monitoring micropolarity and-hydrogen bonding interactions, and for the investigation of microenvironmental characteristics of biochemical and biological systems.18-22 The solvatochromism and relaxation dynamics of some tricarbocyanine dyes in solution were compared by Yu et al.23 using absorption, fluorescence, and femtosecond magic-angle pump-probe experiments. The results indicated the existence of both nonpolar and polar solvatochromism, depending on the dipole moment of the dyes. Comparison between structurally related molecules with polar and nonpolar solvent responses provides an approach for measuring the change in solvation dynamics caused by charge redistribution. A similar observation was obtained by Bertolino et al.24 during the investigation on the effect of polymethine chain length and indolenine structure on the solvatochromic behavior of a series of indocyanine dyes. The solvatochromism was explained through semiempirical Pariser-Parr-Pople (PPP) calculations. Lepkowicz et al.25 performed a detailed experimental investigation and quantumchemical analysis of polymethine dyes with different chain lengths, in particular, in the tri- and pentathiacarbocyanines. An increase in the solvent polarity leads to polar solvatochromism, which is typical for the dyes that exhibit charge localization and a large permanent ground-state dipole moment. Cyanine dyes in general and styrylpyridinium dyes (also considered as hemicyanine dyes) in particular contain both donor and acceptor systems in the same species. In the case of a styrylpyridinium dye, the styryl group acts as the donor and the pyridinium moiety, containing quaternary nitrogen, acts as

10.1021/jp108002e  2011 American Chemical Society Published on Web 12/13/2010

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CHART 1

the acceptor. Binary mixture of solvents may lead to preferential solvation of these probes through different solvating factors like hydrogen bonding, apolar interactions, and dispersive forces. In the present work, we have studied the preferential solvation of some styrylpyridinium dyes associated with established donor-acceptor system as the probes in binary solvent mixtures. In addition, we have analyzed the effect of the aforesaid factors on the solvation pattern due to the presence of nonabsorbing flexible apolar units in the dyes.

Figure 1. Plot of λmax vs ET(30) of C1 dye in different organic solvents.

(µ), dielectric constant (ε), hydrogen-bond donocity (R), hydrogenbond acceptor ability (β), log P,27 and Taft’s π* values28 of the solvent with the corresponding absorption maxima were also found to be scattered (Supporting Information). When the ET(30) values of some solvents like benzene, toluene, chloroform, carbon tetrachloride, N,N-dimethylformamide (DMF), N,Ndimethylacetamide (DMA), and dimethyl sulfoxide (DMSO), responsible for the scattered behavior, were deleted from the plot, a distinct bilinear relationship was obtained for rest of the solvents (Figures 1-3). Initial steady positive solvatochromism was followed by negative solvatochromism at ET(30) values around 47, 48.5, and 49 for C1, C6, and C16 respectively. Owing to (i) a substantial solvatochromic shift in hexane, dichloromethane (DCM), and dioxane and (ii) the fact that these compounds have simple structure with no π-electron clouds, these three solvents were selected to investigate the effect of solvent composition in the binary mixtures with alcohols on the solvation shell formed around the chromophore during the solvation of the probe dyes. Solvatochromism in Binary Solvent Mixtures. In the presence of binary solvent mixtures, solvation of a dye probe (D) is contributed by solute-solute (DD), solute-solvent (DS), and solvent-solvent (SS) interactions. However, the orientation

Results and Discussion Solvatochromism in Pure Solvent. The probes, N-alkyl-4[(4-N,N-dimethylamino)styryl]pyridinium halides (C1, C6, and C16) with -NMe2 groups as the donor and quaternary nitrogen centers as the acceptor, flanked with styryl π-spacers, are cationic chromophores (Chart 1). The charge transfer in the probes is well recognized from a large solvatochromic shift, that is, 446 nm in hexane to 517 nm in dichloromethane (Table 1). The linearity of the plots of absorption maxima (λmax) of C1, C6, and C16 in 26 solvents of different polarities reveals the noninterference of the apolar alkyl tail in solvation of the chromophoric group. However, a consistent bathochromic shift due to the increase in alkyl chain length was observed. Albeit the plot of solvent parameter, ET(30) (derived from solvatochromism of Riechardt’s dye, which is a measure of the polarity of the medium)26 against the absorption maxima of the dyes, was a scattered one, the groups of solvents with linear relationships could be identified. The plots of the dipole moment

TABLE 1: Absorption Maxima of C1, C6, and C16 in Different Solvents and the Solvent Parameters λmax (nm) no.

solvent

C1

C6

C16

R

β

π

ET(30)

µ

ε

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

methanol ethanol 1-propanol 2-propanol 1-butanol 2-butanol pentanol hexanol octanol cyclohexanol ethylene glycol 2-methyl-2-propanol acetone hexane acetonitrile dioxane chloroform DCM CCl4 benzene toluene DMF DMA DMSO ethyl acetate water

475 480 484 482 485 489 487 488 482 491 479 483 477 446 471 459 490 517 469 479 477 470 471 470 463 449

480 485 488 487 490 489 492 491 486 496 485 487 476 449 473 460 492 521 470 477 476 474 477 476 458 448

480 485 488 489 490 489 492 491 486 495 485 487 477 451.2 475 460 493 519 470 477 477 475 477 475 458 445

0.98 0.86 0.84 0.76 0.84 0.69 0.84 0.80 0.77 0.66 0.90 0.42 0.08 0 0.19 0.00 0.20 0.13 0.00 0 0 0.00 0.00 0 0 1.17

0.66 0.75 0.90 0.84 0.84 0.80 0.86 0.84 0.81 0.84 0.52 0.93 0.48 0 0.40 0.37 0.10 0.10 0.10 0.1 0.11 0.69 0.76 0.76 0.45 0.47

0.60 0.54 0.52 0.48 0.47 0.40 0.40 0.40 0.40 0.45 0.92 0.41 0.62 -0.11 0.66 0.49 0.58 0.82 0.21 0.55 0.49 0.88 0.85 1.00 0.45 1.09

55.4 51.9 50.7 48.4 49.7 47.1 49.1 48.8 48.1 47.2 56.3 43.3 42.2 31.0 45.6 36.0 39.1 40.7 32.4 34.3 33.9 43.2 42.9 45.1 38.1 63.1

2.87 1.66 3.09 1.66 1.75 1.66 1.7 1.55 1.76 1.86 2.31 1.66 2.69 0.09 3.92 0.45 1.15 1.14 0 0 0.31 3.82 3.72 4.06 1.78 1.85

32.66 24.55 20.45 19.92 17.51 16.56 13.90 13.30 10.34 15.00 37.30 12.47 20.56 1.88 35.94 2.21 4.89 8.93 2.24 2.27 2.38 36.71 37.78 46.45 6.02 78.36

log P -0.82 -0.32 0.34 -0.04 0.75 0.71 1.4 2.03 3.15 1.23 -2.27 0.36 -0.24 3.9 -0.34 -0.42 1.94 1.15 --2.13 2.69 -1.10 -0.8 -1.35 0.73

Preferential Solvation of Styryl Pyridinium Dyes


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Figure 4. Absorption maxima of C1 in ethanol-hexane binary mixture.


of the solvent around the probe is guided by mostly DS interaction. The factors contributing to the solvation of the cyanine dyes (C1-C16) are mostly due to (i) charge on the chromophore, (ii) resultant dipole moment due to the donoracceptor system, (iii) polarity of the chromophoric unit, and (iv) apolar characteristics of the alkyl chain at the quaternary nitrogen center of the probe. The presence of charge in the chromophoric group decreases the solubility of the probe in nonpolar solvents like hexane, and the presence of a long nonpolar chain like the hexadecyl group decreases its solubility in water. In other polar solvents, however, the probes are found to be completely soluble, indicating the existence of a balance between the polar and nonpolar characteristics of the solvents and the probes. The effect of binary solvent mixtures on the electronic spectra of the probes was analyzed by assuming the substitution of a solvent molecule in the solvation shell around the dye by a molecule of cosolvent, present outside the solvation shell until equilibrium is reached. At equilibrium, the probe may experience the environment of (i) both the solvents present without having interaction with each other (ideal behavior of the solvents), that is, DS1 and DS2 present simultaneously; and/or (ii) a mixed solvent, that is, DS1S2. In the former case, with a change in composition of the solvent mixture, two different absorption maxima may exist with a clear isosbestic point, whereas the latter situation is characterized by shifting of the absorption maximum, which may appear between the absorption maxima of the solvents taken individually, in pure form. Similar solvation models have also been proposed by Soroka and co-workers.29 However, the absorption maxima sometimes appear beyond the range of both solvents and the phenomenon leads to hyper- or hypopolarity of the mixed solvents, which may be due to the synergistic solvent effect resulting from solvent-solvent

Figure 5. Absorption maxima of C1 in methanol-DCM binary mixture.

Figure 6. Absorption maxima of C1 in methanol-dioxane binary mixture.

interaction.30 In the present study, the absorption maxima in binary mixtures of alcohol-hexane and alcohol-DCM were found to be of the former category; that is, the absorption maxima in these solvent mixtures appeared between those in pure solvents, while in the alcohol-dioxane binary mixture, the absorption maxima appeared beyond the range of those in pure solvents (Figures 4-6). In dioxane-hexane binary mixture, a significant bathochromic shift was observed only for C1 (Figure 7). When a nonpolar solvent like hexane is taken as the medium of solvation, there exists a weak dye-hexane interaction. Hence it may be assumed that any polar solvent can easily replace hexane from the hexane-dye solvation shell. Alcohols with varied hydrophobic chains were introduced to analyze the effect of polar solvents on hexane-solvated dye.

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Figure 8. Plot of local mole fraction (x1L) vs mole fraction (x1) of alcohols in alcohol-hexane binary mixture in the solvation of dye C1. Figure 7. Absorption maxima of C1 in dioxane-hexane binary mixture.

In addition to solute-solvent interaction, solvation characteristics in the binary solvent mixtures also depend on solvent-solvent interaction, that is, solvent nonideality as well, since solvent-solvent interaction creates a mixture with characteristics different from those of the individual parent solvents. During ideal solvation, any observed property (P) of a probe in the solvent mixture is the average of contributions of the component solvents (eq 1).31 Deviation from eq 1 indicates the existence of preferential solvation (PS) of the probe:

P)

∑ xiPi

(1)

where xi denotes the mole fraction of ith solvent in the solvent mixture. Maitra and Bagchi32 have extensively studied the PS of some ketocyanine dyes by binary and ternary solvent mixtures using electronic transition energy (ET) obtained from the relationship ET ) 28 951/λmax. From the deviation from ideality of the solvent mixture, they proposed a two-phase solvation model consisting of local region or solvation shell and outside the local region, or bulk. The solvent molecules present in the solvation shell experience the field due to solute molecules. The solvent composition in the solvation shell is different from that of the bulk. If the contribution of both solvents to the ET of the probe in the binary mixture is assumed to be a linear combination, then the mole fraction of the solvents in the solvation shell, otherwise termed as local mole fraction (x1L) of solvent 1 in the binary mixture of solvents 1 and 2 can be calculated from eq 2:

x1L ) [ET(S12) - ET(S2)]/[ET(S1) - ET(S2)]

(2)

where ET(S1), ET(S2), and ET(S12) refer to the electronic transition energy of the probe in solvent 1, solvent 2, and solvent mixture (solvents 1 + 2) respectively. In all the solvent mixtures in the present study, the more polar of the two component solvents is considered as solvent 1 and the other as solvent 2. Analysis of the plot of mole fraction (x1) of S1 in the S1-S2 solvent mixture versus the local mole fraction of S1 (xL) provides some useful information: (i) positive deviation from ideality, referring to PS by S1; (ii) negative deviation from ideality, referring to PS by S2; and (iii) collinearity of the plot with ideality, indicating similar composition of the binary solvents in the solvation shell as well as bulk. The xL values of various

solvents in the binary mixtures were determined for the present study by use of eq 2 and ET ) 28 951/λmax. Solvation by Alcohol-Hexane Binary Mixture. The plot of mole fraction of alcohol (x1) in the alcohol-hexane mixture versus local mole fraction of alcohol (x1L) for some of the alcohols such as ethanol, 1-propanol, and 1-butanol in the presence of C1 (Figure 8) exhibits a positive deviation throughout, indicating PS by these alcohols. With initial increase in mole fraction of these alcohols up to 0.20, the PS maintains almost the same trend. Beyond this mole fraction, the trend in PS decreases with decreasing polarity of the alcohols or increasing apolar characteristics. Above a mole fraction of 0.75, dye C1 is solvated by alcohols only. The change in the behavior of these three alcohols with the change in composition suggests the existence of some solvent-solvent interaction, though negligible. This interaction, however, is quite significant in the case of 2-propanol and 2-butanol, where the PS is due to hexane up to mole fractions of 0.55 and 0.75, respectively. Above these values the alcohols solvate the dye preferentially, which may be attributed to interaction of the polar hydroxy groups of the alcohols and the cationic chromophore of the dye. However, above 0.75 mol fraction of 2-butanol, the 2-butanol-hexane mixture behaves almost ideally. Thus, the alcohol-hexane interaction is due to the bulky alkyl groups of the alcohol and nonpolar hexane, which provides a similar environment as that of pure hexane. In case of both hexanol and cyclohexanol, PS by alcohol occurs up to a mole fraction of 0.25, beyond which the solvent-solvent interaction predominates, exhibiting PS by hexane up to a mole fraction of 0.50 and 0.60, respectively, above which alcohol-dye interaction predominates. The above results also support the hydrophobic characteristics of the dye as suggested by deBevilaqua et al.33 The results became interesting when longer alkyl chains are attached to the chromophoric group, leading to the addition of an apolar factor to the solvation phenomenon. While the cationic chromophore is more comfortable in a polar environment for solvation, the added alkyl groups can be solvated through apolar interaction only. In case of C6, addition of ethanol and 1-propanol led to an immediate expulsion of hexane from the solvent cage and the alcohols solvate the dye preferentially throughout the solvent composition. 1-Butanol could not exhibit PS up to a mole fraction of 0.45, suggesting ideality of the solvent mixtures. The alcohols with bulky groups such as 2-butanol, hexanol, and cyclohexanol could not expel hexane up to a mole fraction of 0.55-0.70, leading to PS by hexane (Figure 9). Above this mole fraction, alcohols exhibit PS. Similar is the observation for C16, where the polar interaction is found to be more prominent (Figure 10). In the 1-butanol-hexane mixture, PS initially is due to hexane only. However, with increasing mole fraction of alcohol, it is observed that C16 is



Figure 9. Plot of local mole fraction (x1L) vs mole fraction (x1) of alcohols in alcohol-hexane binary mixture in the solvation of dye C6.

Figure 11. Plot of disorder vs volume percent hexane for the solvation of C1, C6, and C16 dyes in alcohol-hexane binary mixture.

SCHEME 1

Figure 10. Plot of local mole fraction (x1L) vs mole fraction (x1) of alcohols in alcohol-hexane binary mixture in the solvation of dye C16.

extensively solvated by the alcohol at a lower mole fraction than in the case of C6, which in turn is lower than that in the case of C1. The solvation is due to both polar and apolar interactions and, hence, both types of solvents contribute to the solvatochromism. Further, when one solvent is considered as the cosolvent (S2) in a binary mixture, an attempt was made to investigate the effect of the cosolvent on the disorder (or chaos) of the solvation shell formed due to solvent S1. The effect of the increase in hexane (S2) content in the solvation shell created by alcohols (S1) was monitored through the change in absorption maxima of the probe (Table 2). If a similar effect of S2 on all the hydroxylic alcohol solvents is assumed, the absorption maxima in a binary mixture with 1 vol % hexane should be collinear with those in the binary mixture with other volume percentages of hexane. Deviation from linearity or the appearance of scattered points with low correlation coefficient (R2) may be considered as the disorder () 1 - R2) of the solvent cage due to addition of the cosolvent. Plots of disorder against the volume

percent of hexane in the solvation of C1, C6, and C16 are presented in Figure 11. The disorder is found to be maximal around 0.30 and 0.90 mol fractions of hexane, indicating two transitions (T1 and T2) occurring in solvation of the dye by the binary mixtures (Scheme 1). The shifting of disorder in C6 dye to higher mole fraction of hexane may be attributed to the compatibility of the chain length of alkyl tail of the dye with the chain length of cosolvent (hexane). For C6 dye, the solvation shell due to the alcohols is stabilized to a higher mole fraction of hexane as the initial entry of hexane to the solvent cage is accommodated around the N-alkyl group. Solvation by Alcohol-DCM Binary Mixture. The probes C1, C6, and C16 in DCM experienced large bathochromic shifts in their electronic transitions, compared to other organic solvents (up to 71 nm compared to hexane). The solvation of C1 by DCM is assumed to be a polar-polar interaction. When methanol was added to a DCM-solvated C1 dye, it could not replace the solvent from the solvation shell. Hence, PS throughout the binary mixture is due to DCM only, albeit not significant (Figure 12). The mixture of ethanol and 1-propanol with DCM behaves ideally, which may be due to similar solute-solvent interactions. In the presence of 2-propanol and 1-butanol, PS by alcohol was noted with increasing alcohol proportion, followed by ideal behavior beyond a mole fraction of 0.70. This ideality in solvation behavior may be attributed to the orientation of the alcohols around the substrate, providing an isopolar environment of DCM. Due to relatively stronger interaction in dye-alcohol than in alcohol-DCM, the alcohol molecules at a lower mole fraction solvate the dye with the hydroxylic groups oriented toward the dye. However, with

TABLE 2: Absorption Maxima of C1 in Different Alcohol-Hexane Binary Mixtures and Disorder absorption maximum (nm) hexane (vol %)

EtOH

1-PrOH

2-PrOH

1-BuOH

2-BuOH

pentanol

cyclohexanol

hexanol

disorder

0 10 20 30 40 50 60 70 80 90 100

480 480 480 480 480 479 477 472 464.5 456.2 446

484 484 484 483 482 480 475 467 462 456 446

482 481 480 480 479 471 464 456 453 450 446

485 487 486 486 482 476 468 463 459 455 446

489 487 483 479 470 462 457 453 451 449 446

491 490 489 488 473 466 459 457 458 452.4 446

488 485 482 478 473 465 460 457.2 457.4 452 446

480 480 480 480 480 479 477 472 464.5 456.2 446

0 0.169 0.493 0.921 0.437 0.379 0.359 0.519 0.795 0.774 0

104


Figure 12. Plot of local mole fraction (x1L) vs mole fraction (x1) of alcohols in alcohol-DCM binary mixture in the solvation of dye C1.


increasing alcohol content, the solvent-solvent interactions due to DCM-alcohol and alcohol-alcohol increase so that the hydroxyl groups will be oriented toward the bulk, thereby decreasing the polar environment in the solvation shell. Such an environment provided by the solvation shell may have characteristics of the ideal mixture of DCM and alcohol. This proposition gets support from analysis of the plots of x1L versus x1 for alcohols with larger hydrophobic groups, like 2-butanol, cyclohexanol, and hexanol. PS by these alcohols with bulky groups has a similar trend as that of 2-propanol and 1-butanol; that is, initially there is a higher PS, which decreases with increasing mole fraction of the alcohols. PS at the higher mole fraction may be attributed to the apolar characteristics of the dye-solvent interaction. The effect of apolar tail on the PS in the solvent mixtures can be seen from Figures 12-14. When C6 is taken as the probe, the binary mixtures of DCM with methanol, ethanol, 1-propanol, 2-propanol, or 1-butanol behave almost ideally, thus providing a similar environment of the solvation shell as that of the bulk (Figure 13). With increasing size of the alcohols, their partition to the solvation shell from the bulk is found to be greater, indicating a strong dye-alcohol interaction and weak solvent-solvent interaction. Such an observation may be attributed to the influence of the apolar tail in the dye. In C16 dye, higher alcohols have a greater degree of PS, whereas in a methanol-DCM mixture, PS is completely by DCM at all proportions (Figure 14). Within 0.18 mol fraction, methanol is almost nonexistent in the solvation sphere. Thus, the apolar tail in the probe contributes significantly to preferential solvation by both DCM and alcohols. The disorder of the solvent cage of alcohols due to addition of DCM was determined as mentioned earlier. Figure 15 represents the plot of disorder (1 - R2) versus volume percent of DCM. The addition of DCM could induce a single transition

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Figure 15. Plot of disorder vs volume percent of DCM for solvation of C1,C6, and C16 dyes in alcohol-DCM binary mixture.

in the disorder of the solvent cage around a volume fraction 65% for C1, 75% for C16 and 90% for C6. The early disorder in C1 may be attributed to the affinity of the relatively more polar probe for both the polar solvents. However, with increasing apolar characteristics of the probe, the interference of DCM in the solvent cage is less due to its accommodation in the alkyl tails (C6 and C16) and thus DCM extends the disorder to a higher volume percent. Solvation by Alcohol-Dioxane Binary Mixture. In dioxane medium, C1 absorbs at 459 nm, while in methanol medium it absorbs at 475 nm. With increasing chain length or number of electron-donating methylene groups of the alcohols, the absorption maxima increase up to 488 nm in hexanol medium and 491 nm in cyclohexanol medium. If the effect of solvation shell on the absorption maxima is due to the dipole moment and, hence, polarity of the constituent solvent, then the orientation of dioxane in the solvation shell around the probe is such that the methylene groups of the dioxane molecule, existing in its boat form, are oriented toward the chromophoric unit of the dye and its ethereal oxygen atoms are oriented toward the bulk. Seth et al.34 have proposed the involvement of both the chair and boat forms of dioxane molecules in the solvation of a cyanine dye. The boat form, having a higher dipole moment, during the solvation of a polar solute, discharges dipole-dielectric stabilization energy that overcomes the energy difference between the chair and boat forms.35 This nonideal behavior has also been reflected in a large nonideal quadrupolar charge distribution of dioxane itself.36 During solvation of the dyes by the alcohols, the orientation of the alcohols around the dye is due to an interaction of their hydroxy -O- groups with the cationic chromophoric group of the dye. With increasing number of methylene units in the alcohols, the dipole increases, resulting in stronger binding with the dye molecules. It is reflected in the bathochromism in the spectral characteristics of the dyes in the medium containing



SCHEME 2: Schematic Representation of Orientation of the Solvent Molecules around the Probe in the Binary Mixture of Methanol and Dioxane

Figure 16. Plot of local mole fraction (x1L) vs mole fraction (x1) of alcohols in alcohol-dioxane binary mixture in the solvation of dye C1.

alcohols with increasing chain length. The deviation in the case of alcohols with branched chains may be attributed to steric hindrance introduced by the chains, resulting in decreased dye-solvent interaction. Addition of dioxane to methanolsolvated C1 results in an increase in the absorption maxima, which reveals a stronger interaction of methanol with the dye. The possible different species in the mixture of dioxane (Dx), methanol (Me), and C1 are Dx-C1, Me-C1, and DxMe-C1. The first two species absorb at 459 and 475 nm, respectively. Hence, the bathochromic shift may be assigned to the third species. The bathochromism in this system extends up to the “butanol-C1” system, leading to a proposition that DxMe interaction can lead to a butanol-type environment. It can happen only when the ethereal oxygen atoms of the dioxane molecules, present in its boat form, can trap the -H of the alcoholic group (Scheme 2). The local mole fraction of alcohols exceeds 1.00 during the solvation by dioxane-alcohol binary mixtures. Plots of x1L versus x1 show that methanol preferentially solvates the dye throughout the solvent mixture of methanol and dioxane (Figures 16-18). Figure 16 shows analogous behavior of ethanol-dioxane mixture for the solvation of C1. All the alcohols exhibit PS up to a mole fraction of 0.15, beyond which (i) 1-propanol and 2-propanol experience ideality up to a mole fraction of around 0.4, followed by PS by alcohols; (ii) 1-butanol, 2-butanol, 1-pentanol, 1-hexanol, and cyclohexanol are overtaken by dioxane up to a mole fraction value of 0.70, followed by PS by the alcohols again; and (iii) in octanol-dioxane mixture, dioxane solvates preferentially. These results support the proposition that apolar interaction contributes significantly to the solvation of the dye. With increasing number of methylene groups in the alkyl chain, the behavior of the solvents in the binary mixture toward the probe does not change significantly. However, some deviations from the trend during the solvation of C1 have been recorded: (i) dioxane preferentially solvates the dyes C6 and C16 in the binary mixture with octanol in all proportions, and (ii) except methanol and ethanol, the other alcohols are overtaken by dioxane in the course of solvation beyond an alcohol mole fraction value of 0.15, followed by PS due to the alcohols at different mole fractions. The crossing over from ideality by these higher alcohols from the PS by dioxane in C6 obeys a trend, 1-propanol ) 2-propanol < 1-butanol < 1-pentanol < cyclohexanol < 2-butanol < hexanol, which is in accordance with the increasingly apolar characteristics of the alcohols (Figure 17). Probe C16, which possesses a long alkyl chain, is found to be initially preferentially solvated by dioxane in the binary mixtures with octanol, hexanol, and 2-butanol, followed by a crossover in ideality in the same trend as that for C6 (Figure 18).



The disorder in the solvent cage of alcohols due to incorporation of dioxane was found to be similar to that due to incorporation of hexane. As the polarities of both solvents are similar, when compared to alcohols [refer to Table 1 for ET(30) values], the similar pattern in the graph leads to the conclusion that the polarity of the cosolvent contributes to the disorder of the solvent cage (Figure 19). Solvation by Dioxane-Hexane Binary Mixture. From earlier studies it was found that, in binary mixtures of alcohol-dioxane and alcohol-hexane, the solvation shell changes drastically with structure of the probe. The difference in absorption maxima due to change in the N-alkyl apolar tail in a single solvent is found to be within 7.0 nm (for 2-propanol), albeit in some solvents no significant change is observed (Table 3). In dioxane-hexane a blue shift of 20 nm is observed due to a change in alkyl group from C1 to C6. With further increase

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Figure 19. Plot of disorder vs volume percent dioxane for the solvation of C1, C6, and C16 dyes in alcohol-dioxane binary mixture.

Figure 20. Plot of local mole fraction (x1L) vs mole fraction (x1) of dioxane in dioxane-hexane binary mixture in the solvation of dye C1.

TABLE 3: Absorption Maxima of C1, C6, and C16 in Different Solvents absorption maximum (nm) solvent

C1 probe

C6 probe

C16 probe

hexane dioxane water methanol dioxane/hexane (1:1 v/v) dioxane/water (1:1 v/v) dioxane/MeOH (1:1 v/v)

446 459 449.5 475 477 472 482

449 460 447.8 480 457 479 486

451 460 447.8 480 457 482 486

in alkyl chain, that is, to C16, no change is observed. For dioxane-water and dioxane-methanol mixtures, the absorption maxima underwent a red shift to 7 and 4 nm for C1 f C6 and 3 nm and no change for C6 f C16, respectively. Further, the absorption maxima suffer bathochromism in all binary mixtures with respect to the neat solvents. The bathochromism in binary mixtures compared to the neat solvent is attributed to solvent-solvent interactions, while the hypsochromism in dioxane-hexane and bathochromism in dioxane-water with increasing apolar alkyl chain length in the probe is due to solute-solvent interactions. The absorption maxima of C6 and C16 in dioxane-hexane mixture and neat dioxane are almost the same, indicating a dioxane-type environment of the binary mixture. The plots of x1 versus x1L in dioxane-hexane mixture for all dyes indicate PS of the dyes by dioxane in all proportions (Figures 20-22). It is obvious because inside the solvation shell, the hexane molecules will be oriented toward the apolar tail, allowing the chromophoric group to be solvated by the dioxane only. For C1, as there is no significant apolar alkyl tail, the solvation is due to a mixture of dioxane and hexane, which provides an environment similar to that of methanol and acetone. Due to interaction of hexane with the methylene units of dioxane, the probability of existence of boat conformation increases, leading to the proposition that ethereal oxygen may be responsible for binding of the dye with the solvent. A probable orientation of the solvents in the binary mixture of dioxane-hexane is presented in Scheme 3. Orientation of Solvents in Solvation Shell. Due to the donor-acceptor characteristics of the chromophoric group, it may be assumed that the polar part, like ethereal oxygen of dioxane, hydroxyl group of alcohols, and chloro group of DCM, will be oriented toward the dye. Addition of any hydrophobic units to the solvent mixture will decrease the binding of these groups with the dye and concomitantly increase the absorption maxima. Thus the positive solvatochromism is due to a balance of polar and nonpolar groups of the solvent in the solvation process as well as to the orientation of solvent molecule around



SCHEME 3: Schematic Representation of Orientation of the Solvent Molecules around the Probe in Dioxane-Hexane Binary Mixture

the dye. The similar spectral characteristics of the dyes, in hexane and water and as hexane solvates the dye through van der Waals interaction only, reveal that water structure remains unaffected in the presence of the dye, depicting the dye to be hydrophobic. However, methanol provides an environment wherein the dye can sense both hydrophobic and hydrophilic sites, and due to a balance contribution of both, it absorbs at higher wavelength (475 nm for C1 dye) (Scheme 2). With increasing alkyl chain length in alcohols, the absorption maxima increase due to increasing apolar characteristics of the solvent

Preferential Solvation of Styryl Pyridinium Dyes SCHEME 4: Schematic Representation of Orientation of the Solvent Molecules around the Probe in the Binary Mixture of Butanol and Dioxane

J. Phys. Chem. B, Vol. 115, No. 1, 2011 107 (Bengal Chemicals), 1-propanol, 2-propanol, 1-butanol, 2-butanol, 1-pentanol, cyclohexanol, dichloromethane, hexane, acetonitrile (Qualigens), 1-hexanol (Loba Chemie), and 1,4dioxane (Merck), were purified by standard procedures and were distilled just before use.38 Spectroscopic measurements were carried out at 298 K in a Hitachi U-3010 recording UV-vis spectrophotometer fitted with thermostatic cell holders. The mixtures of dye solutions were prepared by mixing the dye solutions in different solvents with appropriate dye concentration. Acknowledgment. B.K.M. thanks University Grants Commission, New Delhi, and Department of Science and Technology, New Delhi, for financial assistance through DRS and FIST programmes, respectively. M.P. thanks the Council of Scientific and Industrial Research, New Delhi, for a senior research fellowship.

cage. The solvents are orientated in such a way that both the hydroxyl and alkyl groups remain close to the chromophoric unit. In dioxane, as described earlier, the ethereal oxygens are assumed to be oriented toward the dye. Upon addition of an equal amount of alcohol, the ethereal oxygen of dioxane forms a hydrogen bond with the alcohol, and in its hydrogen-bonded form; it behaves as a different solvent with an enriched apolar characteristic to experience a bathochromic shift (up to 482 nm). Water with dioxane can have a similar hydrogen-bonding phenomenon, but due to less apolar interaction, the dye absorbs at 472 nm. However, with hexane, the apolar enrichment is due to the interaction of methylene group of dioxane with that of hexane, with preferential solvation by dioxane (Scheme 3). With increasing alkyl chain in the alcohols, that is, for butanol, the nonpolar alkyl chain forces the hydrogen-bonded group away from the dye and thus the ethereal oxygens of dioxane remain away from the dye (Scheme 4). The environment is mostly due to apolar characteristics and thus there is a decrease in the absorption maxima (473 nm). Conclusion In conclusion, the [(N,N-dimethylamino)styryl]pyridinium unit, though charged, is found to behave as a hydrophobic unit with high sensitivity toward changes in the apolar characteristics around its environment. The dye is found to be insensitive toward polarity only, which is revealed from the absorption maxima in hexane and water. The balancing of both polar and apolar characetrisitcs in the solvent cage could show significant solvatochromism. The binary mixtures, upon tuning such characteristics, have led to enrichment of both polar and apolar environments for the dye. Materials and Methods Some tailor-made styrylpyridinium dyes (C1, C6, and C16) were prepared by the sequential reaction of 4-methylpyridine with alkyl halide (methyl iodide, hexyl bromide, and hexadecyl bromide), followed by condensation with N,N-dimethylaminobenzaldehyde in the presence of piperidine in ethanol medium.37 The purity of the synthesized compounds was checked by IR, UV-vis, and NMR spectral data (Supporting Information) and also by thin-layer chromatography. The various solvents used in the study, such as methanol (Qualigens), ethanol

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Preferential Solvation of Styrylpyridinium Dyes in Binary Mixtures of

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