Langmuir 1991, 7, 24W2411
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Notes Photocontrol of Surface Activity and Self-Assembly with a Spirobenzopyran Surfactant Calum J. Drummond,’ Stefani Albers, D. Neil Furlong, and Darrell Wells CSIRO, Division of Chemicals and Polymers, Bayview Avenue, Private Bag 10, Clayton, Victoria, 3168, Australia Received April 22,1991. I n Final Form: May 24,1991
Introduction Water-soluble surfactants that can undergo reversible photoinduced molecular rearrangements, which can result in changes in surface activity and self-assembly,are ideal candidates for a number of specialized applications. Applications include the photocontrol of (i) surface wettability, (ii) solubilization, and (iii) chemical reactivity. Application i relies on the photocontrol of the surface activity, or alternatively the surface adsorption, of the surfactant. Applications ii and iii rely on the photogeneration of self-assembled surfactant aggregates creating an anisotropic distribution of solution species. For example, in the vicinity of photoinduced charged micellar surfaces there will be a depletion of co-ions and an enhancement of counterions. Nonelectrolyte solution species can partition into the micellar phase to varying degrees, depending on their relative molecular hydrophobicity/hydrophilicity. Water-insoluble species may become solubilized in the micellar phase. The change in solution species distribution can markedly alter chemical reactivity (e.g., micellar catalysis).’ Though not previously considered in this context, air/ water surface tension data reported for a series of l’dkylW3’,3’-dimethyl-&(l-pyridiniummetheno)-6-nitrospiro[ l-benzopyran-2,2’-indoline] chloride surfactants,2which have recently been extended for the 1’-dodecylhomologue (PSP-12): suggest that photochromic spirobenzopyran surfactants should be assessed as candidates for the aforementioned specialized applications. In this note we examine the surface activity and selfassembly of a new photochromic spirobenzopyran surfactant, viz. sodium 3’,3’-dimethyl-l’-dodecylspiro[2H-lbenzopyran-2,2’-indoline]-6-sulfonate(SSP-12;see Figure 1). The methyl homologue of SSP-12,viz. sodium 1’,3’,3’trimethylspiro[ W-l-benzopyran-2,2’-indoline]-6-sulfoPreaent address: Department of Applied Mathematics,Research School of Physical Sciences, The Australian National University, GPO Box 4, Canberra, ACT, 2601, Australia. (1) Fendler,J. H. Membrane Mimetic Chemistry; Wiley-Interscience: New York, 1982. (2) Tezuke, S.;Kurihara, S.; Yamaguchi, H.;Ikeda, T. J.Phys. Chem.
1987, 91, 249. (3) Drummond,C. J.; Albere,S.; Furlong,D. N.;Wells, D.,unpubliehed results. In an unbuffered aqueous solution at 25 OC, the cmc values for the open (nolight) and closed (254-nmirradiation) forms of PSP-12 are 1.7 X 1W and 1.2 X 1o-Smol dmd, respectively. The minimum area per molecule attained at the air/water interface for the open and closed forma of PSP-12are 0.99 and 0.75 nm*,respectively. The pK. of the phenoxide moiety of the open form of the 1’-methyl homologue of PSP-12 (PSP-1) is 1.47 0.08 in watet. At 2.5 X lo+ and 6.0 X lo-‘ mol dm”, PSP-12 thermally colors in water with firstorder rate constants of 2.5 X lo-‘ f 0.5 X 10-4 and 4.0 X 1o-S & 0.1 X 10” 8-1, respectively. When cycled between the open and closed forms, both PSP-12and PSP-1 exhibit very
high fatigue rates.
nate (SSP-1; see Figure l),is employed to help clarify the acid/base behavior of SSP-12. Thermal recovery is an important consideration in photocontrol applications. Therefore, rates for the thermal decoloration of SSP-12 are also reported. SSP-12 differs from PSP-12in anumber of ways.‘ SSP12 is an anionic surfactant that displays positive photochromism whereas PSP-12 is a cationic surfactant that displays negative photochromism. PSP-12 has a bulky substituent on the chromene section of the molecular skeleton, which may impose packing constraints that are not present in the case of SSP-12. Moreover, the synthesis of SSP-12 is far less complicated than that of PSP-12.
Experimental Section 1’,3’,3’-Trimet hylspiro[ 2 8 - lbenzopyran-2,2’-indoline]6-sulfonic Acid. The method of Sunamoto et al.6 was employed to synthesize salicaldehyde-3-sulfonicacid polyhydrate. The product structure was confirmed by NMR. Salicaldehyde-3sulfonic acid polyhydrate was then reacted with 1,3,3-trimethyl2-methyleneindoline (Eastman Kodak), according to the method of Sunamoto et al.: to obtain 1‘,3’,3’-trimethylspiro[W-l-benzopyran-2,2’-indoline]B-sulfonicacid. Instead of the reddish brown product that has been reported: we obtained a bright orange HCl derivative which decomposed a t 279 OC. The NMR spectrum in trifluoroacetic acid showed the presence of both the open and closed forms in a ratio of ca. 3:l. 3’,3‘-Dimethyl- 1‘-dodecylspiro[ 2 8 - l-benzopyran-t$’-indoline]-6-sulfonic Acid. 2,3,3-Trimethylindolenine(Aldrich ChemicalCo.) anddodecyl iodide (Pfaltzand Bauer) were r e a d , according to the method of Gruda and L+eBlanc,B to give 3,3dimethyl- l-dodecyl-2-methyleneindoline.The NMR spectrum of the product was as expected. Salicaldehyde-3-sulfonicacid polyhydrate (2 g) and 3,3-dimethyl-l-dodecyl-2-methyleneindoline (3.4 g) were refluxed in methanol (20 mL) for 2 h. The mixture was diluted by adding ethanol (150 mL). While the mixture was boiled, water was added slowly until crystallization began to occur. More ethanol (5 mL) was added to redissolve all the crystals. After cooling, the brown crystals were filtered. The product was then recrystallized twice more by followingthe same procedure. The melting point was 135-137 “C. The NMR spectrum in DMSO showed both the open and closed forms in the approximate ratio 61. All experiments used water that was first deionized and then distilled twice; first from alkaline potassium permanganate. The water then had a conductivity of 450-nm light, a Varian VIX-300 UV 300-W Xenon illuminator with an Oriel cutoff filter was employed. In the case of irradiation with visible light, the Petri dish was located inside a glass cylinder that had a flat roof and a flat floor. The internal surface of the cylinder, including the roof and floor, was mirrored, except for a small window through which the surfactant solutions were irradiated.
Results and Discussion In the present study, we were primarily interested in the spirobenzopyran to merocyanine conversion (I I1 in Figure 1) and its effect on surface activity and selfassembly. Therefore, the requisite irradiation and solution conditions for this photochemical process needed to be determined. Acid/Base Behavior. SSP-1 and SSP-12 exhibit a wide range of pH-dependent molecular, and associated spectral, changes under different irradiation conditions. In water, the self-assembly and solubility of SSP-12varies with pH. Therefore, SSP-1 was employed to study the acid/ base behavior of the spirobenzopyran/merocyanine molecular skeletons in water. Most of the pH- and irradiation-induced spectral modifications can be interpreted with reference to the behavior of SP-16.7 Figure 2 shows typical spectral changes for the region 9.5 > pH > 4.8. These changes reflect the acid/base equilibrium of the phenoxide moiety in the open form (I1+ IV in Figure 1). The PKa for this acid/base equilibrium is 6.42 f 0.13, 6.62 f 0.09, and 6.92 f 0.09 under dark conditions, 506nm irradiation and 254-nm irradiation, respectively. The variation in the PKa value with the lighting conditions may reflect a variation in the open-form stereoisomer compo~ition.~ The open merocyanine form is degraded by hydroxide above a pH of 11.0. In the region 4.8 > pH > 1.0 there is another acid/base equilibrium. The PKa is 4.1 f 0.1 under dark conditions (very minor spectral changes with pH) and 3.4 f 0.2 under both 506- and 254-nm irradiation. It is difficult to isolate which group on the open-form skeleton is responsible for
I
I
I
Wavelength
I
(nrn)
Figure%.UV vis absorptionspectrum of the open form of SSP-1 (2.5x mo dm”; 254-nm irradiation) in water as a function of pH (9.5> pH > 4.8). As the pH is decreased the absorbance (Le., 506 nm) decreases. Spectra were recorded at pH at A, valuesof9.42,8.01,7.62,7.30,7.17,6.95,6.78,6.60,6.35,6.13,and 4.89.
this acid/base equilibrium. It is not likely to be the sulfonate group as these groups are usually much more acidic.1° Though not pursued by us, there is also some evidence that the protonated open forms may undergo cis/trans photoisomerization. From the foregoing, it is clear that the pH range from 9.0 to 11.0 is the optimum for an investigation of the spirobenzopyran to merocyanine conversion. Photochromism. Both SSP-1 and SSP-12 display positive photochromism; i.e., UV irradiation (254 nm) causes part of the spirobenzopyran population to be converted to the open merocyanine form and reversion of this part of the population to the closed form occurs by visible irradiation (open form) ,A or thermally (seeFigure 1). Figure 3 shows the photostationary photochromic response of SSP-1. Note that both photostationary states have some open form present. The photochromic response of SSP-12 is similar to that of SSP-1. The relative proportion of open to closed form obtained with visible irradiation is approximately one-third that obtained with UV irradiation. The fatigue resistance of both SSP-1 and SSP-12 is extremely low. The photochromic response is sequentially diminished with each spirobenzopyran/merocyaninecycle and is quickly exhausted.
~~
(9) Furlong, D. N.;Freeman, P. A.; Metcalfe, I. M.; White, L. R. J. Chem. SOC.,Faraday Trans. 1 1983, 79,1701.
(IO)Zollinger, Hch.; Buchler, W.; Wittwer, C.Helu. Chim Acta 1953, 36,1711.
Notes
Langmuir, Vol. 7, No.10,1991 2411 0.5
I
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0.L
\Navelength ( n m )
Figure 3. UV/vis absorption spectrum of SSP-l(2.5 X l t 6mol dm-9) in water (pH 9.5). Spectrum a was obtained when the solution was irradiated with 254-nm light and spectrum b was obtained when the solution was irradiated with 506-nm light.
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-5.8 -5.4 -5.0 -4.6 -4.2 Log [SSP-12]
Figure 4. Surface tension (y) as a function of the logarithm of the SSP-12concentration (mol dm") in an aqueous pH 11buffer
solution. Data represented by 0 were obtained when the solution was continuously irradiated with >450-nm light and w when the solution was continuously irradiated with 254-nm light.
Surface Activity and Self-Assembly. Figure 4 shows the air/water surface tension data for SSP-12 under conditions of both UV and visible irradiation. The SSP12 solutions were maintained at pH 11 by employing a disodium hydrogen phosphate buffer ([Na+] = 0.054 mol dm-3).11 pH 11was chosen because self-assembly of SSP12 may result in a large increase (possibly as much as 3 pH units)7 in the apparent PKa of the open-form phenoxide moiety. There is an extended [SSP-12]region where the surface activity can be photocontrolled. When irradiated with visible light, SSP-12 has a cmc of 6.3 X 104 mol dm-3. When irradiated with UV light, SSP-12 has a cmc of 1.7 X 10" mol dm-3. In the region between these two cmc values self-assembly can be photocontrolled. By assuming that the aqueous SSP-12 solutions at pH 11 can be treated like 1:l ionic surfactant solutions with a swamping amount of electrolyte (buffer),the minimum average area per molecule at the air/water interface (AminaIW;in nm2) may be derived from the relationships12
rmar= ( 2 . 3 0 3 ~ ~ )(-6y/6 -' log [ssP-I~]),
(1)
where F- is the maximum surface excess concentration (in mol cm-2), R is the universal gas constant, T is the thegradient absolute temperature, (-6y/61og [SSP-l2])~is of the surface tension vs log [SSP-12]in the region just before the cmc is reached, and N is Avogadro's number. It is assumed that the extra f charge character of the (11)Handbook of Chemistry and Physics, 60th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1979; p D-148.
open merocyanineform does not change the relationships. Note that in the determination of A-ah values, zwitterionic surfactants are generally treated the same way as nonionic surfactants.12-14 TheA,,&Wvalue for SSP-12irradiated with visible light is 0.16 nm2. The value for SSP-12irradiated with UV light is 0.59 nm2. Interestingly, surface pressure-area isotherms obtained with insoluble monolayers of SP-16 spread at the air/water interface indicate a limiting area per molecule of 0.14 f 0.03 nm2 for the closed form and a limiting area per molecule of 0.54 f 0.02 nm2 for the open that is areas close to those of the soluble SSP-12. The closed spirobenzopyranform consists of a chromene ring othogonal to a heterocycle ring. By contrast, the open merocyanine form has a planar configuration. Disregarding electrostatic interactions, molecular models suggest that the closed spirobenzopyran form should occupy an area of 0.40-0.50 nm2and that the open merocyanine form should occupy an area of 0.70.90131112.18 The value for SSP-12irradiated with visible light is significantly less than the minimum possible for a vertically orientated hydrocarbon chain. This suggests that a monomolecularfilm of SSP-12does not form a t the air/water interface. Crystallization of the closed form of the surfactant at the air/water interface may be occurring.15-18 The crystalline domains may be respread by irradiating the surface with UV lightale The value for SSP-12under UV irradiation is consistent with a monomolecular film that is a mixture of both closed spirobenzopyran and open merocyanine forms. There may also be some J aggregation (i.e., head-to-tail aggregation, rather than side-by-side aggregation (sandwich; H aggregation)) of the open merocyanine form. Thermal Decoloration. The thermal decoloration of the open form of SSP-12 in water is a first-order rate process. Below the cmc values ([SSP-12]=2.5 X lo4 mol f 0.2 dm-3; pH 11 buffer) the rate constant is 1.0 X X s-l. Above the cmc values ([SSP-12]= 5.0 X mol dm-3; pH 11buffer) the rate constant is 2.5 X f 0.5 X 10-3 5-1. Therefore, thermal decoloration of SSP-12 is faster in the self-assembled state. This is consistent with the micellar interface having a lower effective polarity.
Conclusion Photocontrol of both surface activity and self-assembly has been demonstrated with SSP-12. The low fatigue resistance of SSP-12, and probable UV-induced crystallization, preclude practical application. Irrespective of the fatigue and the presumed crystallization, SSP-12does not have an ideal photoresponse. The ideal photochromic surfactant would have one photoisomerself-assembled at a concentration where the other photoisomer had yet to register a surface tension reduction. Other candidates for the photocontrol of surface activity and self-assembly include surfactant molecules modified with spiroxazine, fulgide, and azo moieties. These surfactants may be significantlymore fatigue resistant than the spirobenzopyrans.19 (12) Rosen, M. J. Surfactants and Interfacial Phenomena; Wdey: New York, 1978; p 59. (13)Dahanayake, M.; Roeen, M. J. In Relation between Structure and Performance of Surfactants; Rosen, M. J., Ed.; ACS Symposium Series 263;American Chemical Society: Washington, DC, 19W, p 49. (14) Zhao, F.; Rosen, M. J. J. Phys. Chem. 1984,88,6041. (15) McArdle, C. B.: Blair, H.; Barraud, A.: Ruaudel-Teider, A. Thin Solid F i l m 1983, 99, 181. (16) McArdle, C. B.; Blair, H.S. Colloid Polym. Sci. 1984,262, 481. (17) Holden, D. A.:. Rinmdorf. . H.:. Deblauwe, V.: Smets, G. J. Phre. Chem. 1984,8& 716. (18)Polymeropoulos,E. E.; Mobius, D. Ber. Bunsenges. Phys. Chem. 1979,83, 1215. (19) Photochromism; Brown, G. H., Ed.; Wiley: New York, 1971.
