Langmuir 2003, 19, 8801-8806
8801
Solvatochromic Study of the Microenvironment of Surface-Bound Spiropyrans Rohit Rosario,† Devens Gust,‡ Mark Hayes,‡ Joseph Springer,‡ and Antonio A. Garcia*,§ Department of Chemical and Materials Engineering, Arizona State University, Tempe, Arizona 85287-6006, Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604, and Department of Bioengineering, Arizona State University, Tempe, Arizona 85287-9709 Received March 13, 2003. In Final Form: July 23, 2003
Spiropyrans are a group of organic molecules that undergo a reversible photoinduced transformation (i.e., photochromism) from a colorless, nonplanar spiropyran form to a colored, planar merocyanine form. Photochromism is accompanied by a large change in the structure and in the dipole moment. These changes suggest that such molecules might be useful in light-controlled, “smart surface” applications. This study examines the effect of the microenvironment near the surface-bound spiropyran on its photochemistry. The surfaces were designed to exhibit a mixture of hydrophobic and hydrophilic components by using a mixed silane chemistry on a glass substrate, and the spiropyran was covalently bound to the surface via amide linkages. The solvatochromic behavior of spiropyran derivatives was studied in solution using UV-vis absorption spectroscopy and fluorescence spectroscopy for comparison with the surface-bound species. Spiropyrans in solution and on the surface both exhibited negative solvatochromism. Correlations between emission maxima of the spiropyrans and Reichardt’s ET(30) polarity scale revealed that the surface-bound spiropyran experienced lower polarity than a solution model in solvents of low and medium polarities. Linear solvation energy relationships using the Kamlet-Taft polarity scales showed that hydrogen bonding played a prominent role in solvent stabilization of surface-bound spiropyrans in hydrogen-bonding solvents. The surface design used causes the spiropyran to interact significantly with the surface in solvents of lower polarity and to behave as if it were dissolved in solution in more polar, hydrogen-bonding solvents.
Introduction Spiropyrans are a well-known group of organic photochromes whose properties have been studied extensively.1 They undergo a reversible photoinduced transformation from a colorless, nonplanar “closed” spiropyran form to a colored, planar “open” merocyanine form as shown in Figure 1. The closed form of the spiropyran, which absorbs light primarily in the ultraviolet (UV) region of the spectrum, usually exists in nonpolar solvents in the dark or in solvents exposed to visible (vis) light. UV irradiation of spiropyran solutions causes an isomerization, producing the open, highly polar form that absorbs light in the visible region of the spectrum. This isomerization can be reversed either by vis light irradiation or thermally. The open form is a resonance hybrid of a neutral, quinoidal form and a charged, zwitterionic structure that is thought to be the major contributor to this isomer, as the aromaticity of the oxygen-bearing ring is lost in the neutral structure. The photoswitchability of spiropyrans has begun to be exploited as a means of reversible light modulation of properties for a variety of chemical assemblies. The large changes in molecular conformation and polarity between open and closed forms of spiropyrans have been utilized for photocontrol of enzyme activity,2-4 surface patterning,5 * Corresponding author. E-mail:
[email protected]. † Department of Chemical and Materials Engineering. ‡ Department of Chemistry and Biochemistry. § Department of Bioengineering. (1) Bertelson, R. C. In Photochromism; Brown, G. H., Ed.; WileyInterscience: New York, 1971. (2) Willner, I.; Katz, E.; Willner, B.; Blonder, R.; Heleg-Shabtai, V.; Buckman, A. F. Biosens. Bioelectron. 1997, 12, 337. (3) Weston, D. G.; Kirkham, J.; Cullen, D. C. Biochim. Biophys. Acta 1999, 1428, 463.
optical signal transduction,6 and membrane permeability.7 We have recently shown that the water contact angle on spiropyran-coated surfaces is altered by the wavelength of irradiation, permitting photonic modulation of microfluidic flow through coated capillaries.8 All these applications required the spiropyran molecule to be attached to surfaces such as glass or gold or entrapped within polymeric matrixes. This is significant because the polarity of the microenvironment of the spiropyran dramatically affects its photochromism. It is known that in polar environments the open, merocyanine form is stabilized.9 The reversible photoswitching of spiropyrans trapped within a silica matrix was found to require the presence of both polar and nonpolar regions within the matrix.10 Thus, the polarity of the spiropyran microenvironment is an important factor to be taken into account during the design of any command surfaces using spiropyrans. Solvatochromism refers to the change in position, and sometimes intensity, of the absorption bands of solutes when measured in different solvents.11 These changes are caused by intermolecular interactions between the solute and solvent that modify the energy gap between the ground (4) Aizawa, M.; Namba, K.; Suzuki, S. Archives of Biochemistry and Biophysics 1977, 182, 305. (5) Willner, I.; Blonder, R. Thin Solid Films 1995, 266, 254. (6) Willner, I. Acc. Chem. Res. 1997, 30, 347. (7) Willner, I.; Rubin, S.; Shatzmiller, R.; Zor, T. J. Am. Chem. Soc. 1993, 115, 8690. (8) Rosario, R.; Gust, D.; Hayes, M.; Jahnke, F.; Springer, J.; Garcia, A. A. Langmuir 2002, 18, 8062. (9) Tagaya, H.; Nagaoka, T.; Kuwahara, T.; Karasu, M.; Kadokawa, J.; Chiba, K. Microporous Mesoporous Mater. 1998, 21, 395. (10) Hori, T.; Tagaya, H.; Nagaoka, T.; Kadokawa, J.; Chiba, K. Appl. Surf. Sci. 1997, 121/122, 530. (11) Nigam, S.; Rutan, S. Appl. Spectrosc. 2001, 55, 362A.
10.1021/la0344332 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/17/2003
8802
Langmuir, Vol. 19, No. 21, 2003
Rosario et al.
Figure 1. Molecular structure of spiropyran upon irradiation with light. Zwitterionic and quinoidal structures contribute to the open form.
and excited states of the absorbing species.12 The openchain merocyanine forms of spiropyrans have been found to be negatively solvatochromic, meaning that their absorption and fluorescence emission bands undergo a hypsochromic (blue) shift in solvents of increasing polarity.13 The solvatochromism of the open-chain merocyanine forms of spiropyrans has been studied primarily for its potential use as an empirical indicator of solvent polarity.14-17 Spiropyrans have also been used to estimate the polarity of silica gel surfaces on which they are absorbed.18 Both the absorption and the emission wavelengths of the open form of the spiropyran may be used to indicate polarity of the microenvironment of the solvatochromic species. The microenvironment of the surface-bound spiropyran may be composed of solvent molecules, substituent groups on the spiropyran, and surface components. In this work, the solvatochromic properties of spiropyran derivatives in solution were compared with those of surface-bound spiropyran in a series of solvents in order to determine the polarity of the environment surrounding the surface-bound spiropyran molecules. The surface coating was prepared by treating glass surfaces sequentially with tert-butyldiphenylchlorosilane (TBDS) and (3aminopropyl)triethoxysilane (ATES) and then linking the photochromic spiropyran 1′-(3-carboxypropyl)-3′,3′dimethyl-6-nitrospiro[2H-1]benzopyran-2,2′-indoline (SPacid) to the free amino groups via an amide bond. The result is a dilute layer of spiropyran on a silanized surface (SP-surface) that can be switched between open and closed forms using light of different wavelengths.8 A series of spiropyrans with substitution at the heterocyclic nitrogen (shown in Figure 1) were prepared. These were 1′-methyl3′,3′-dimethyl-6-nitrospiro[2H-1]benzopyran-2,2′-indoline (SP-methyl), 1′-(3-carbomethoxypropyl)-3′,3′-dimethyl6-nitrospiro[2H-1]benzopyran-2,2′-indoline (SP-ester), SPacid, and 1′-(3-(propylcarbamyl)propyl)-3′,3′-dimethyl-6nitrospiro[2H-1]benzopyran-2,2′-indoline (SP-amide). The structure most relevant to the SP-surface is SP-amide. A comparison between the solvatochromic properties of these (12) Reichardt, C. Solvents and solvent effects in organic chemistry, 2nd ed.; Weinheim: Basel, 1988. (13) Song, X.; Zhou, J.; Li, Y.; Tang, Y. J. Photochem. Photobiol. A: Chem. 1995, 92, 99. (14) Botrel, A.; Aboab, B.; Corre, F.; Tonnard, F. Chem. Phys. 1995, 194, 101. (15) Zhou, J.; Li, Y.; Tang, Y.; Zhao, F.; Song, X.; Li, E. J. Photochem. Photobiol. A: Chem. 1995, 90, 117. (16) Keum, S.; Choi, Y.; Lee, M.; Kim, S. Dyes Pigments 2001, 50, 171. (17) Flannery, J., Jr. J. Am. Chem. Soc. 1968, 90, 5660. (18) de Mayo, P.; Safarzadeh-Amiri, A.; Wong, S. Can. J. Chem. 1984, 62, 1001.
model compounds and the SP-surface can provide a quantitative spectroscopic measure of the environment of the surface-bound species. Materials and Methods Sample Preparation. The synthesis of the spiropyrans and the preparation of spiropyran-coated glass surfaces have been described previously.8,19 At each stage of the glass-coating process, water contact angle measurements were used to characterize the surface coating. Glass slides were well-cleaned using a 1:1 volume ratio of methanol/concentrated hydrochloric acid solution, followed by extensive washing in deionized water (contact angle ) 5 ( 1°). They were then treated with toluene solutions of TBDS (contact angle ) 57 ( 2°) and ATES and cured at 140 °C (contact angle ) 72 ( 4°). The silane-treated slides were incubated in an ethanolic solution of SP-acid (1 mM) in the presence of the coupling agent, 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide (10 mM), washed sequentially with ethanol and water, and dried under vacuum (contact angle ) 74 ( 3° under visible irradiation). Toluene, tetrahydrofuran (THF), acetone, 1-butanol, 2-propanol, ethanol, and methanol were of analytical grade and used as received from the supplier (Mallinckrodt Baker, Kentucky) for all spectroscopic measurements. Solutions (10 µΜ) of SPmethyl, SP-acid, SP-ester, and SP-amide were prepared in these solvents and kept in the dark until used. Aqueous solutions of SP-acid and SP-amide were prepared with doubly distilled water. Quartz cuvettes (Starna Spectrosil Far UV, 3-Q-10) having a 10-mm path length were used in all measurements. The treated glass slides were cut to precisely fit diagonally into the cuvettes for measurement of their emission spectra. Fluorescence Spectroscopy and UV/VIS Absorption Spectroscopy. The steady-state fluorescence spectra were obtained with a computer-controlled Photon Technology International spectrofluorimeter (Model QM-2000-4). The fluorescence of spiropyran solutions was measured by using a 1-nm excitation band-pass, 8-nm emission band-pass, 2-nm step size, and a 0.2-s integration time. The signal-to-noise ratio of the surface-bound spiropyran fluorescence was increased by using 8-nm band-pass excitation and emission slits, a 10-nm step size and a 10-s integration time. The emission spectra were collected at 90° to the excitation, and in the case of the surface-bound spiropyran, the treated slide was held at a 45° angle to both the excitation beam and the detector. The fluorescence spectra were corrected for instrumental response after subtraction of the background. Absorption spectra were recorded on a Schimadzu UV2100U spectrometer using 5-nm slits, a 2-nm step size, and medium speed scans. All measurements were performed at room temperature (25 °C). The absorption and emission spectra of the spiropyran solutions were measured after illuminating them with either UV or vis light (for about 10 min) in order to confirm that the spiropyran was switching between its open and closed forms in (19) Garcia, A. A.; Cherian, S.; Park, J.; Gust, D.; Jahnke, F.; Rosario, R. J. Phys. Chem. A 2000, 104, 6103.
Microenvironment of Surface Bound Spiropyrans
Langmuir, Vol. 19, No. 21, 2003 8803 Table 1. Solvent Polarity Parametersa solvent
r
n
ETN
π*
a
b
toluene tetrahydrofuran acetone 1-butanol 2-propanol ethanol methanol water
2.38 7.58 20.56 17.51 19.92 24.55 32.66 78.30
1.497 1.407 1.359 1.399 1.377 1.361 1.328 1.333
0.099 0.207 0.355 0.602 0.546 0.654 0.762 1.000
0.54 0.58 0.71 0.41 0.48 0.54 0.60 1.09
0 0 0.08 0.68 0.76 0.83 0.93 1.17
0.11 0.55 0.48 1.01 0.95 0.77 0.62 0.18
a , n, and E N are dielectric constant, refractive index, and r T normalized Reichardt’s solvent polarity factor, respectively. π*, R, and β are the Kamlet-Taft scales for solvent dipolarity-polarizability, hydrogen bond donor acidity, and hydrogen bond acceptor basicity, respectively.12
Figure 2. Spectra of SP-amide in 2-propanol. Curves a and b are the absorption spectra after UV and vis irradiation, respectively. Curves c and d are the corrected emission spectra after UV and vis irradiation, respectively. solution. The UV light source was a model UVGL-25 Mineralight Lamp in the 366-nm wavelength setting. This generated a power of 2 mW cm-2 at a distance 2 cm from the source. The vis actinic light source was a 50 W high-intensity illuminator (Series 180, Dolan Jenner) coupled to a water flow-through IR filter to reduce heat. It generated a power of 4.8 mW cm-2 at a distance 2 cm from the source. Only the spectra obtained after UV irradiation were used to calculate the absorption and emission peaks for the solvatochromic analysis, as only the open merocyanine form of the molecule exhibits easily detectable fluorescence. In the case of the spiropyran-coated slide only the fluorescence spectra were obtained, as the surface concentration of spiropyran molecules proved to be below the detection limit of the single-pass absorption spectrophotometer used. The fluorescence spectrum of the coated slide was obtained by placing the slide diagonally in the quartz cuvette, filling the cuvette with the solvent of interest, irradiating the cuvette with the appropriate wavelength of light (for about 15 min), and then measuring the emission. A glass slide treated only with TBDS and ATES and immersed in the specific solvent was used as the background for the surface-bound spiropyran fluorescence spectra.
Results and Discussion Spiropyrans in Solution. The absorption and corrected emission spectra of the four spiropyran derivatives were recorded in solvents of different polarity. A typical set of absorption and emission spectra after UV and vis irradiation of the solution is shown in Figure 2. As expected, UV irradiation of the spiropyran solution resulted in the open merocyanine form that absorbs light in the visible and fluoresces when excited. Visible irradiation of the solution resulted in the closed form that neither absorbs visible light nor fluoresces when excited at 360 nm. Table 1 lists the solvents and values of the relevant solvent polarity scales used for the analysis of the solvatochromic behavior of the spiropyrans.12 The terms r, n, and ETN refer to the dielectric constant, refractive index, and normalized Reichardt’s solvent polarity factor, respectively, while π*, R, and β are the Kamlet-Taft20-22 scales for solvent dipolarity-polarizability, hydrogen bond donor (HBD) acidity, and hydrogen bond acceptor (HBA) basicity. The absorption and fluorescence peaks of the spiropyrans in each solvent are summarized in Table 2. A number of idealized theories predict r to be a quantitative measure of solvent polarity.12 A hypsochromic (20) Kamlet, M. J.; Taft, R. W. J. Am. Chem. Soc. 1976, 98, 377. (21) Kamlet, M. J.; Abboud, J. L.; Taft, R. W. J. Am. Chem. Soc. 1977, 99, 6027. (22) Taft, R. W.; Kamlet, M. J. J. Am. Chem. Soc. 1976, 98, 2886.
shift with increasing solvent polarity was found in the absorption, λabs, and fluorescence, λem, maxima of all four spiropyrans tested. When the absorption and fluorescence maxima (in terms of v, cm-1) of the SP-amide are plotted against r (Figure 3) it is seen that the shift is greater in the case of absorption as compared to fluorescence. This shift can be interpreted by observing that the open chain spiropyran is a merocyanine dye with an electron-donating group linked to an electron-accepting group via a polymethine bridge. The molecule is a resonance hybrid of a highly polar charge-separated zwitterionic state and a less polar noncharged state. Thus, the hypsochromic shift may be explained if the dipole moment of the molecule decreases upon excitation. If this happens, a more polar solvent would serve to stabilize the ground state more effectively than the excited state, leading to a greater separation between ground and excited states. A similar effect explains the well-documented behavior of merocyanines, which display a blue shift in the absorption and emission maxima with increasing solvent polarity.18,23 The Stokes shift between absorption and fluorescence spectra for the spiropyrans was plotted as a function of the polarity parameter f(r,n)
f(r,n) )
(
r - 1 n2 - 1 - 2 2r + 1 2n + 1
)
(1)
in order to determine the applicability of the LippertMataga equation24 for the solvatochromic behavior of spiropyrans (Figure 4). For all the spiropyran molecules studied, the plots of (va - vf) produced nonlinear plots against the function f(r,n). The Lippert-Mataga equation assumes that the solvent may be described as a dielectric continuum. The nonlinearity of the Stokes shift against the polarity parameter f(r,n) implies that this is not a valid assumption for the spiropyran systems studied. Therefore, the Stokes shift cannot be explained solely in terms of the change in permanent dipole moments.25 This is consistent with solvent stabilization of ground and excited states of merocyanine dyes which are due to a variety of solute-solvent interactions such as hydrogen bonding and dipole-dipole interactions.23,26 Since the use of simple physical models for solvents to describe the solvatochromism of spiropyrans appears to be inadequate, Reichardt’s empirical ET(30) polarity scale is considered next. This scale is based on the solventdependent spectral shifts experienced by pyridinium N-phenoxide betaine dye (ET-30) and has been shown to (23) Vandewyer, P.; Hoefnagels, J.; Smets, G. Tetrahedron 1969, 25, 3251. (24) Cunderlikova, B.; Sikurova, L. Chem. Phys. 2001, 263, 415. (25) Suppan, P. J. Photochem. Photobiol. A: Chem. 1990, 50, 293. (26) Keum, S.; Lee, K. Bull. Korean Chem. Soc. 1993, 14, 16.
8804
Langmuir, Vol. 19, No. 21, 2003
Rosario et al.
Table 2. Effect of Solvent on Position of Absorption and Fluorescence Maxima of Spiropyransa SP-ester
SP-methyl
SP-acid
SP-amide
SP-surface
solvent
λabs, nm
λem, nm
λabs, nm
λem, nm
λabs, nm
λem, nm
λabs, nm
λem, nm
λem, nm
toluene tetrahydrofuran acetone 1-butanol 2-propanol ethanol methanol water
601 584 566 552 557 543 537 NS
670 664 660 648 652 647 646 NS
596 574 562 548 550 534 525 NS
655 654 652 650 650 646 644 NS
582 577 564 545 549 535 530 499
664 654 654 644 644 643 639 623
573 564 558 546 549 540 532 505
652 648 646 640 638 636 626 614
662 654 651 642 646 639 630 615
a
λabs and λem are the absorption and fluorescence emission maxima, respectively. NS ) not soluble.
Figure 3. Correlation of solvent dielectric constant, r, and the absorption (filled squares) and emission maxima (empty squares) of SP-amide, expressed in terms of v (cm-1).
Figure 4. Correlation of solvent polarity function f(r,n) and the Stokes shift of SP-amide. The va and vf are the absorption and fluorescence maxima in cm-1.
take into account both the dipolarity and the hydrogen bond donating acidity of the solvent.12 Thus, this scale can be used to probe the influence of hydrogen bond donating acidity on solvent stabilization of spiropyrans. The absorption and emission peaks of all four spiropyrans studied were found to give reasonably good linear correlations with the normalized ET(30) in all cases (r > 0.90). The fluorescence maxima of the four spiropyrans have been plotted against the normalized Reichardt’s scale, ETN (Figure 5). As both merocyanines and ET-30 are meropolymethines, a good correlation between the solvatochromic behaviors of the two was expected. The positive slopes of the plots indicate that solvent stabilization increases with solvent polarity, and the linear fit implies that there is a hydrogen bond contribution to this effect.
A comparison of the slopes of the different spiropyran plots indicates that the solvent stabilization effect is much more pronounced in the case of SP-amide, SP-acid, and SP-ester than for SP-methyl. Thus, the carboxylic acid, ester, or amide group present as a substituent on the heterocyclic nitrogen increases the sensitivity of the spiropyran to solvent effects relative to the methyl group. In the more polar solvents (such as the alcohols and water) the energy of the emission maxima decreased in the order SP-amide > SP-acid > SP-methyl > SP-ester. This finding is consistent with the report that n-alkyl substituents of increasing length on the nitrogen group of benzothiazoline spirobenzopyrans had the effect of hindering the solvation of the molecule.23 We have found that the presence of more polar substituents on the SP-amide and SP-acid, as compared to the SP-methyl and SP-ester, increase the solubility of the spiropyrans, making them soluble even in water (Table 2). This enhanced solubility suggests that the spiropyran ground state is stabilized in polar solvents, and this in turn would lead to enhanced negative solvatochromism. Thus, the choice of substituent groups on the heterocyclic nitrogen is an important variable in the design of nanoscale spiropyran-coated surfaces with controlled solvation shell polarity. Surface-Bound Spiropyrans. The emission spectra of spiropyran-coated surfaces were recorded in a range of solvents after either UV or vis irradiation. A typical set of corrected fluorescence spectra of the SP-surface in 2-propanol after UV and vis irradiation is shown in Figure 6. The fluorescence observed from the UV irradiated slide is attributed to the open, merocyanine species on the surface, and the lack of fluorescence from the vis irradiated slide is consistent with the spiropyran switching to the closed form. The corrected fluorescence maxima of the UV-irradiated SP-surface in a range of solvents is tabulated in Table 2. The fluorescence maximum of the SP-surface was found to exhibit a hypsochromic shift in more polar solvents, indicating that the spiropyran on the surface was negatively solvatochromic. To evaluate the extent of interaction of the surface with the bound spiropyran, the fluorescence maxima of the SPsurface was plotted against the normalized ET(30) scale along with the trend lines for SP-amide and SP-acid in solution (Figure 7). Overall, the fluorescence maxima of the SP-surface indicate negative solvatochromism, similar to that shown by the various model compounds in solution. Thus, the spiropyran bound to the surface clearly experiences the solvent environment to some extent. Inspection of Figure 7 reveals that at high values of ETN, the behavior of the SP-surface resembles that of SP-amide, which would seem to be the most relevant chemical model compound for the surface-bound spiropyran. This suggests that in these solvents, the spiropyran molecule experiences an
Microenvironment of Surface Bound Spiropyrans
Langmuir, Vol. 19, No. 21, 2003 8805
Figure 5. Plot of emission maxima of UV-irradiated spiropyrans against normalized ET(30) scale. (× represents SP-amide, - represents SP-acid, 9 represents SP-ester, and 2 represents SP-methyl.)
Figure 6. Corrected fluorescence spectra of the SP-surface in 2-propanol after UV (broken line) and vis (solid line) irradiation.
environment that is dominated by the solvent, rather than the surface. However, at lower values of ETN, the SP-surface exhibits emission maxima at lower frequencies than would be expected for the solvent environment, based on the SPamide model compound. This demonstrated that the spiropyran experiences an environment that is influenced not only by the solvent but also by the surface. It also indicated that this surface environment is “less polar” than the solvent, in the Reichardt sense. This is a reasonable conclusion, as the surface environment surrounding each spiropyran consists of very nonpolar regions of TBDS groups bound to the surface as well as other spiropyran molecules. Most of the relatively polar surface silanol groups must have reacted with either the TBDS or ATES reagents, as suggested by the conversion of the glass surface with a low water contact angle (5°) to the treated surface with a high contact angle (∼70°).8 The contact angle results and the negative solvatochromism are completely consistent. Why, then, do the SP-surface molecules exhibit a relatively tight association with the surface in nonpolar solvents and a highly solvated environment in solvents with high ETN values? Clearly, this cannot be simply due to a preference of the spiropyran for solvents with a higher
Figure 7. Plot of emission maxima of the open form of SPsurface against the normalized ET(30) scale. Also shown in broken lines are the trend lines for SP-amide and SP-acid in solution.
dielectric constant, since in solvents of low ETN, the molecules are strongly influenced by the even less “polar” surface. It seems reasonable to suggest that the explanation lies in the hydrogen bonding ability of the solvents with high ETN. If the solvent is able to form strong hydrogen bonds with the SP-surface molecules, the molecules tend to experience a solvent-dominated environment. Otherwise, their environment is strongly influenced by the surface. The Kamlet-Taft solvatochromic parameters, used in multiparameter correlations called linear solvation energy relationships, can help quantify the relative effects of solvent dipolarity/polarizability and hydrogen bonding on solvent stabilization.12 According to this model,21 the fluorescence energy maximum, vf, is expected to follow the relationship
vf ) v0 + sπ* + aR + bβ
(2)
where π*, R, and β are the Kamlet-Taft scales for solvent dipolarity-polarizability, hydrogen bond donor (HBD) acidity, and hydrogen bond acceptor (HBA) basicity,
8806
Langmuir, Vol. 19, No. 21, 2003
Rosario et al.
Table 3. Regression Fit to Kamlet-Taft Solvatochromic Parameters Using Equation 2 ν0, 103 cm-1 s, 103 cm-1 a, 103 cm-1 b, 103 cm-1 SP-ester SP-methyl SP-acid SP-amide SP-surface
14.65 15.08 14.58 15.06 14.68
0.48 0.37 0.85 0.58 0.85
0.41 0.26 0.41 0.53 0.56
0.25 -0.04 0.28 -0.03 0.11
r 0.98 0.98 0.99 0.99 0.98
solvents that have this capability. The HBD acidity (represented by the coefficient a) appears to play a significant role in stabilizing all the spiropyrans examined. The magnitude of a for the SP-surface is almost identical to that of its solution model SP-amide, implying that in strongly hydrogen bonding solvents the surface-bound spiropyran would behave like SP-amide in solution. Physically, this would be possible if the spiropyran was in a more solvent-like environment due to movement of the molecule away from the nonpolar surface and/or increased clustering of the solvent molecules around the spiropyran. This again correlates well with the findings of the ET(30) scale analysis. These results show that this particular spiropyran surface design causes the molecule to interact with the surface in solvents of low polarity and to behave as if it were dissolved in solution in more polar, hydrogen bonding solvents. Conclusions
Figure 8. Fluorescence emission energy of SP-surface as a function of π*. Comparison between measured values (empty circles) and values calculated (filled circles) using the KamletTaft multiparameter approach.
respectively, and the intercept v0 and coefficients s, a, and b depend on the nature of the dye and are determined by multiple regression analysis. The regression coefficients s, a, and b measure the relative susceptibility of vf to the solvent parameters. Since the π*, R, and β scales are normalized from 0.0 to about 1, the relative magnitudes of the regression coefficients are expected to quantify the relative contribution of the respective solvent parameter to solvent stabilization. The results for the multiple regression analysis of the spiropyrans in solution and on the surface are presented in Table 3. In all cases the correlation coefficients are very high (r > 0.98). The calculated fluorescence energies using these fit equations are graphically compared to the measured energies for SP-surface (Figure 9). Based on the relative magnitudes of the correlation coefficients s, a, and b it appears that in all cases the HBD acidity of the solvent played a significant role in solvent stabilization. The HBA basicity of the solvent seems to have a smaller effect, or no effect in some cases, on the fluorescence energy transition. The dipolarity-polarizability (represented by the coefficient s) is the primary means of solvent stabilization in solvents of low polarity which lack the capacity for specific interactions such as hydrogen bonding. The value of s for the SP-surface is significantly higher than that of SPamide, suggesting that in solvents of low polarity the behavior of the surface-bound spiropyran will deviate significantly from that of SP-amide, due to an increased contribution of the nonpolar surface to its solvation shell. This is in agreement with the findings of the ET(30) scale analysis. On the other hand, hydrogen bonding is expected to play a significant role in stabilizing the spiropyran in
A mixed surface coating consisting of silyl groups and spiropyran groups was prepared on glass substrates. The effect of the surface on the photochemistry of the surfacebound spiropyran was evaluated by comparing its behavior with that of a solution model, SP-amide, and other spiropyrans in solution. In solution the spiropyrans exhibit negative solvatochromic behavior resulting in hypsochromic shifts in their absorption and fluorescence maxima, with increasing solvent polarity. The Stokes shift was found to be nonlinear with the polarity parameter f(r,n), implying that it cannot be explained solely in terms of the change in permanent dipole moments. A good correlation was found between the absorption and fluorescence energies of the spiropyran in solution and the ET(30) parameter, indicating that hydrogen bonding plays a role in solvent stabilization. The presence of more polar substituents on the heterocyclic nitrogen of SP-amide and SP-acid, as compared to SP-methyl and SP-ester, increase the spiropyran solubility, consistent with enhanced solvent stabilization. The SP-surface exhibits negative solvatochromism consistent with spiropyran behavior in solution. In solutions of low polarity (as measured by ET(30)) the SP-surface solvatochromism indicates significant effects due to interaction of the spiropyran with the surface environment, whereas in more polar solvents it behaves much like the SP-amide model in solution. The behavior in solvents of low polarity can be explained if the spiropyran interacts relatively strongly with the surface, allowing the nonpolar surface components to influence the polarity of the solvation shell. At higher solvent polarities the surface plays a lesser role in stabilizing the surface-bound spiropyran ground state. The multiple regression analysis of the peak fluorescence energy with the multiparameter Kamlet-Taft equation produced a correlation coefficient close to 1. The relative magnitudes of the regression coefficients confirm that hydrogen bonding plays a prominent role in stabilizing the SPsurface in polar solvents and that the surface has an increased role in less polar solvents. Thus, this particular spiropyran surface design causes the molecule to interact with the surface in solvents of low polarity and to behave as if it were dissolved in solution in more polar, hydrogen bonding solvents. Acknowledgment. We acknowledge the National Science Foundation (Grant CTS-0102680) for financial support. LA0344332
