J . Phys. Chem. 1989, 93, 2533-2538
2533
Squaraine Chemistry. Effect of Orientation on the Aggregation of Surfactant Squaraines in Langmuir-Biodgett Films Kock-Yee Law* and Cindy C. Chen Xerox Webster Research Center, 800 Phillips Road, 01 141390, Webster, New York 14580 (Received: April 15, 1988: In Final Form: September 16, 1988)
Surfactant squaraines DSSQ (4-(distearylamino)phenyl-4'-(dimethylamino)pheny~squaraine), MSSQ (bis(.l-(methylstearylamino)phenyl)squaraine), and TSSQ (bis(4-(distearylamino)phenyl)squaraine), which were designed to orient the "bricklike" squaraine chromophore in three different orientations when organized in monolayers or Langmuir-Blodgett (LB) films, have been synthesized. Surface pressurearea isotherm studies show that the squaraine chromophores of DSSQ, MSSQ, and TSSQ orient as designed, namely, being laid down vertically along the long axis, vertically along the short axis, and flat, respectively, on water. Monolayers of TSSQ were found to collapse upon compression. Although stable monolayers can be obtained by mixing TSSQ with cadmium stearate, TSSQ was not studied in any great extent in this work due to the chemical decomposition of the squaraine chromophore by water at the air-water interface. Stable monolayers and LB films were obtained for DSSQ and MSSQ. The aggregation of the squaraine chromophore in these LB films was studied by absorption spectroscopy. Spectral data show that the absorption spectra of DSSQ and MSSQ in LB films not only are different from their solution absorption spectra but are also different from each other. By comparison of the absorption of model squaraine aggregates in the solid state, we conclude that the squaraine chromophores of DSSQ and MSSQ form aggregates in the LB films and that there is an orientationaleffect, which is induced by the LB film technique, on the aggregation. The stabilities of the LB films of DSSQ and MSSQ were also examined. While the LB film of DSSQ is very stable, a change in absorption maximum and a decrease in absorbance were observed in the LB film of MSSQ during storage. A model for the change in optical absorption is proposed and methodologies for preparing stable LB films of MSSQ are discussed.
Introduction Squaraines are 1,3-disubstituted products synthesized from squaric acid and N,N-dialkylanilines:'*'
squaric acid
N,N-dialkylaniline
Oe squaraine In solution, these compounds exhibit sharp and intense absorption ranging from 620 to -670 nm, in the visible region with ,A depending on the substituent in the phenyl ring and the substituent at the nitrogen atom. The molar extinction coefficients of these absorptions are very high, -3 X lo5 cm-' M-1.3 In the solid state, due to the extensive intermolecular interactions, their absorption becomes very broad and is red-shifted from the solution absorption. The solid-state absorption covers most of the visible region and extends to the near-IR, where the solid-state GaAs diode lasers emit. These optical characteristics have made squaraines very attractive for a number of industrial applications, e.g., xerographic photo receptor^,^-^ organic solar cells,4s1@'3optical (1) For nomenclature, see: West, R. Oxocorbon; Academic Press: New York, 1980; Chapter 10. (2) For reviews of the synthesis of squaraines, see: (a) Schmidt, A. H. Synthesis 1980, 961. (b) Sprenger, H. E.; Ziegenbein, W. Angew. Chem., Inr. Ed. Engl. 1968, 7 , 530. (c) Maahs, G.; Hegenberg, P. Angew. Chem., Inr. Ed. Engl. 1966, 5, 888. (3) Law, K. Y. J . Phys. Chem. 1987, 91, 5184. (4) Loutfy, R. 0.;Hsiao, C. K.; Kazmaier, P. M. Photogr. Sci. Eng. 1983, 27, 5. (5) Law, K. Y. J . ImagingSci. 1987, 31, 83. (6) Tam, A. C.; Balanson, R. D. IBM J . Res. Deu. 1982, 26, 186. (7) Wingard, R. E. IEEE Ind. Appl. 1982, 1251. ( 8 ) Tam, A. C. Appl. Phys. Left. 1980, 37, 978. (9) Melz, R. J.; Champ, R. B.; Chang, L. S.; Chiou, C.; Keller, G. S.; Liclican, L. C.; Neiman, R. B.; Shattuck, M. D.; Weiche, W. J. Photogr. Sci. Eng. 1977, 21. 73.
etc. In most of these device applications, squaraines are used as microcrystalline powder embedded in polymer matrices. The action spectra of these devices, which are parallel to the solid-state absorption spectra, suggest that aggregates of squaraines are responsible for the photoactivities observed. Recently, we1' reported an investigation on the effect of aggregation on the photogeneration efficiency of organic photogenerators in xerographic devices. Classes of organic photogenerators discussed include squaraines, metallophthalocyanines, and thiapyrylium salts. The general conclusion was that crystalline materials are better photogeneration materials than amorphous materials and that the precise molecular arrangement of photoconductive molecules in the crystalline form is critically important to the photogeneration efficiency. While the effect of dye aggregation on the quantum efficiency of photogeneration may have been qualitatively scoped, molecular details on the xerographic photodischarge process, which involves electron-hole pairs generation upon photoexcitation of the dye aggregate, electron transfer within the dye aggregate and electron transfer between the dye aggregate, and other electron-donor (or acceptor) molecules at the interface, remain a subject of further investigation.'* We felt that scientific understanding of the details at the molecular level is of particular value because only with such knowledge can the rational design and the synthesis of novel structures with optimal performance characteristics be achieved. Research efforts directed to understand the molecular details of the photogeneration process(es) of organic materials have been documented; however, most of these studies are focused on single-crystal systems for purity, structural, and orientation reasons. Single crystals of most of the organic compounds are very difficult to prepare unfortunately.19
(10) Morel, D. L. Mol. Crysf.Liq. Cryst. 1979, 50, 127. (1 1) Merritt, V. Y. IBM J . Res. Deu. 1978, 22, 353. (12) Morel, D. L.; Ghosh, A. K.; Feng, T.; Stogryn, E. L.; Purwin, P. E.; Shaw, R. F.; Fishman, C. Appl. Phys. Left. 1978, 32, 495. (13) Merritt, V. Y.; Hovel, H. J. Appl. Phys. Lett. 1976, 29, 414. (14) Gravesteijn, D. J.; Steenbergen, C.; Vander Veen, J. Proc. SPIE Inf. SOC.Opt. Eng. 1983, 420, 327. (15) Jipson, V. P.; Jones, C. R. J. Voc. Sci. Technol. 1981, 18, 105. (16) Jim'on, V. P.; Jones, C. R. IBM Technol. Discl. Bull. 1981, 24, 298. (17) Law, K. Y. J. Phys. Chem. 1988, 98, 4226. (1 8) For a discussion on the principle of the photodischarge of xerographic devices, see: Schaffert, R. M. Elecfrophorogrophy; Focal Press: London, 1980. Also see ref 9.
0022-365418912093-2533$01.50/00 1989 American Chemical Society
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Law and Chen
Our approach to circumvent this problem is to structurally modify a photoactive chromophore by hydrophobic hydrocarbon chains in such a fashion that the photoactive chromophore will form aggregates when the resulting amphipathic molecule is organized by the Langmuir-Blodgett (LB) technique. The aggregates formed will then be used to model the effects of aggregation and orientation on the photoconductivity and the electron-transfer reactions described above. As a result of this initiative, surfactant squaraines DSSQ, MSSQ, and TSSQ, which I
I
06
R1 \ R2
/ 00
DSSQ, R, = R2 CH3, R3 = R4 = n-Cl~Hj7 MSSQ, R, = Rj = CHj, Rz = R4 = n-ClsH37 TSSQ,R1 = Rz = R3 = R4 = n-ClsHj7
were designed to orient the squaraine chromophore in three different orientations when organized in monolayers and LB films, were synthesized. Here we describe the fabrication of LB films of DSSQ, MSSQ, and TSSQ.20,21 Surface pressure-area isotherms show that the squaraine chromophores of these three surfactant squaraines orient in three different orientations on water as designed. While TSSQ was found to be unstable chemically and structurally on water, DSSQ and MSSQ were found to be stable and monolayers of them could be transferred to hydrophobic glass substrates. The absorption spectra of DSSQ and MSSQ in LB films, which are different from their solution absorption spectra, are also different from each other. The results are discussed in terms of an orientation effect on the aggregation of the squaraine chromophore in the LB film. The thermal stabilities of the LB films of DSSQ and MSSQ are also reported.
Experimental Section Materials. DSSQ was synthesized by condensing l-(p-(dimethylamino)phenyl)-2-hydroxycyclobutene-3,4-dionewith an equivalent amount of N,N-dioctadecylaniline (from Pfaltz & Bauer) in a mixture of toluene and 1-butanol at reflux. The product was isolated and purified by column chromatography on silica gel by using chloroform as eluent. MSSQ was prepared by reacting squaric acid (Aldrich) and N-methyl-N-octadecylaniline with the procedures described by Spenger and Ziegenbein?6 The product was purified by solvent recrystallization from a mixture of methylene chloride and methanol. Satisfactory spectroscopic data and analytical data were obtained for DSSQ and MSSQ.27 Squaraine TSSQ was the same sample used in our previous study.3 Stearic acid and cadmium chloride were purchased from Aldrich and were recrystallized from ethanol (19) Gutman, F.; Lyon, L. E. Organic Semiconductors; Krieger: Malabar, FL, 1981; p 376. (20) A preliminary account of this work was presented at the 194th ACS National Meetings, New Orleans, August 30-September 4, 1987. (21) Compound MSSQ was synthesized and was used for LB film studies earlier by Swalen and c o - ~ o r k e r s . ~However, ~ - ~ ~ neither the analytical data of MSSQ nor the isotherm or optical properties of the LB film of MSSQ were reported. (22) Swalen, J . D.; Tacke, M . ; Santo, R.; Rieckhoff, K. E.; Fischer, J. Helu. Chim. Acta 1978, 61, 960. (23) Pockrand, I.; Swalen, J. D.; Santo, R.; Brillante, A,; Philpott, M. R. J . Chem. Phys. 1978, 69, 4001. (24) Philpott, M. R.; Brillante, A.; Pockrand, I.; Swalen, J. D. J . Mol. Struct. 1980, 61, 299. (25) Swalen, J . D.; Pockrand, I. J . Opt. SOC.A m . 1978, 68, 1147. (26) Spenger, H. E.; Ziegenbein, W. Angew. Chem., Inr. Ed. Engl. 1966, 5, 894 (27) Satisfactory spectroscopic and analytical data were obtained for DSSQ and MSSQ and these data are as follows: DSSQ, mp 203-204 "C; 'HNMR (CDzCl,) 6 0.8-1.5 (m, 70 H), 3.19 (s, 6 H, NCH,), 3.42 (t, 4 H, J = 6.4 Hz),6.77 (two overlapped doublets, 4 H), 8.30 (d, 4 H, J = 9.3 Hz). Anal. Calcd for CsH88N20~:C, 81.25; H, 11.12; N, 3.51. Found: C, 81.41; H, 10.69; N, 3.70. MSSQ, mp 146-149 OC; IH NMR (CD,CI,) 6 1.0-1.8 (m, -70 H), 3.12 (s, 6 H, NCH9). 3.40 (br t, 4 H, NCH2), 6.73 (br d, 4 H, J = 9.1 Hz), 8.26 (d, 4 H,J = 9.1 Hz). Anal. Calcd for C54H88NZ02: C, 81.25; H, 1 1 12; N , 3.51. Found. C, 81.41; H, 11.49; N, 3.13.
Figure 1. n-A curve of DSSQ on water
before use. Chloroform (HPLC grade, pentene stabilized) was bought from Fisher. Distilled water was in-house deionized water purified by passing through a Millipore Q system and was doubly distilled before use. Glass substrates (2 in. X 2 in.) were obtained from Corning (7059) and were degreased by a mixture of hot sulfuric acid and nitric acid (1 :1) and further cleaned by distilled water before use. All glass substrates were then made hydrophobic by silanization with a tetrahydrofuran solution of N-(trimethylsily1)acetamide (10% by weight). Preparation and Characterization of LB Films. Monolayers of DSSQ, MSSQ, and TSSQ were obtained by spreading chioM) roform solutions of the three surfactant squaraines ( w onto an aqueous subphase, which contained cadmium chloride (3 X M) and sodium bicarbonate (5 X lo4 M, pH 6.8), at -20 "C in a Lauda film balance equipped with a solid Teflon trough. The spreading solutions were usually air-dried for -40 min before compression. While the monolayer of TSSQ was structurally unstable upon compression and was decomposed by water at the interface (see Results and Discussion), monolayers of DSSQ and MSSQ were stable under our experimental conditions. LB films of DSSQ and MSSQ were obtained by transferring these monolayers onto hydrophobic glass substrates by using a vertical dipping technique at a surface pressure of 25 mN/m. The aggregation of the squaraine chromophore in LB films was studied by absorption spectroscopy using a Hewlett Packard 845 1A diode array spectrophotometer. The uncoated side of the same glass substrate was used as reference throughout this work.
Results and Discussion Surface Pressure-Area Isotherms. DSSQ. The surface pressure-area isotherm (n-A curve) of pure DSSQ on water surface is given in Figure 1. Results shows that the n-A curve is very steep and exhibits only one transition with a limiting area of 52 ~z/molecule. From single-crystal X-ray structural data X-ray powder difof bis(4-methoxyphenyl)squaraine
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(l),2s329
0e
Oe 2
The Journal of Physical Chemistry, Vol. 93, No. 6,1989 2535
Orientation Effect on Dye Aggregation in LB Films
i
5141
i
OOt
0
30
60 AREA
90
MOLECULE,
120
I50
Figure 2. x-A curve of MSSQ on water.
1
,
0
40
80
,
1
I
120
160
200
AREA (82/MOLECULEl
Figure 3. T-A curve of TSSQ on water.
value (59.5 A2/molecule) based on the reported X-ray structural fraction patterns of a number of bis(4-(dimethylamino)phenyl)data of s q ~ a r a i n e s ~ ,and ~ * -the ~ ~assumption that the squaraine squaraine derivatives (2),7930 and molecular modeling studies, the chromophore of MSSQ is residing vertically along its short axis dimension of the squaraine chromophore can be estimated to be on water. The agreement thus indicates that MSSQ is organized -11 A x -1 A x -3.5 A. as designed on water (see inset in Figure 2). Thus if the squaraine chromophore of DSSQ residues vertically Recently Matsumoto and co-workers reported their LB film along its long axis on water upon compression, the expected limstudies of squaraine 3.35-37Despite the similarity of structure iting area would be -25 A2/molecule. The observed limiting area, which is -52 A2/molecule, is about twice that of the expected value. The large limiting area observed suggests that the squaraine chromophore may be tilted on water. This, however, is inconsistent with the absorption spectral data presented below. For example, the squaraine chromophore of DSSQ is shown to form aggregates in the LB film, and the major intermolecular interaction in the aggregate is C - O dipole-dipole interactions. The only geometry that allows these kind of intermolecular interactions to occur is R R to have the squaraine chromophore standing vertically along its 3, R = n-CI8H3, long axis on the glass substrate. We thus consider the possibility 4, R = CH3 of having the squaraine chromophore tilted unlikely because this between MSSQ and 3,the monolayer of 3 collapses upon comrequires reordering of the squaraine chromophore during the pression. The instability of the monolayer of 3 is probably due transfer of the monolayer to the glass substrate. In earlier studies of the pressure-area isotherms of n-fatty to the nonplanarity of the squaraine chromophore in 3 according alcohols, Nutting and Harkins3' and Harkins and C ~ p e l a n d ~ ~ to single-crystal X-ray data by Kobayashi et al. on squaraine 4.38 reported limiting areas of -21 A2 for the saturated hydrocarbon Our finding in this work is consistent with that reported by Swalen chains of these alcohols on water. Similarly, the limiting areas and c o - w o r k e r ~ , who ~ ~ - also ~ ~ reported the preparation of the LB films from MSSQ. a-A curve of MSSQ was not reported and of n-fatty acids on water were found to be -20-25 Az/molecule.33 A wider spread of limiting areas from 20 to 30 A2/molecule was a comparison of results could not be made, however. TSSQ. Figure 3 shows the a-A curve of TSSQ on water observed when the stearyl chain is attached to different "not too surface. The compression isotherm is broad and has two transitions large" head groups.34 These observations suggest that if the two of limiting areas of 126 and 85 A2/molecule. Since the calstearyl chains are the controlling factor in determining the limiting culated molecular area of the squaraine chromophore of TSSQ area of DSSQ on water, a limiting area of 50 f 10 A2/molecule would be obtained. Since our observed value is within this range, is 119 A2/molecule (excluding the hydrocarbon chains), our our data thus suggest that the compression isotherm of DSSQ results suggest that the squaraine chromophore lays flat on the is controlled by the two hydrocarbon chains rather than the water surface upon compression (Figure 3, inset). As the surface squaraine chromophore. The steepness of the a-A curve in Figure pressure increases ( > l o mN/m), the monolayer of TSSQ col1 , which is analogous to those of fatty acids and fatty alcohols, lapses. Since the limiting area of the second transition is smaller is certainly consistent with this interpretation. than the molecular area of a squaraine chromophore, the *-A MSSQ. The P A curve of MSSQ is given in Figure 2. Unlike curve suggests that the squaraine chromophores may become tilted that seen in Figure 1, the compression curve of MSSQ is broader, and stack on each other at surface pressures higher than 10 implying that MSSQ has more flexibility in the gaseous state on mN/m. Or alternatively, monolayers of TSSQ may transform water. The limiting area of the isotherm is -60 A2/molecule. into a supermonomolecular structure analogous to that reported The observed value is in excellent agreement with the calculated Further experimentation by Matsumoto et al. on squaraine 3.35-37 is needed to differentiate these two possibilities. While the monolayer of TSSQ is unstable for transfer to glass substrates, we have been able to prepare stable, transferable mixed (28) The sin le-crystal X-ray structure of 1 was first reported by Farnum
-
and co-~orkers!~ Recently, we redetermined the crystal structure of 1 and more accurate structural and packing data of 1 in solid were obtained: Ziolo, R. F.; Law, K. Y., unpublished results. (29) Farnum, D. G.; Neuman, M. A.; Suggs, W. T. J . Cryst. Mol. Struct. 1974,4, 199. (30) Law, K. Y.; Bailey, F. C. J . Imaging Sci. 1981,31, 172. (31) Nutting, G. C.; Harkins, W. D. J . Am. Chem. SOC.1939,61,1180. (32) Harkins, W. D.; Copeland, L. E. J . Chem. Phys. 1942, IO, 272. (33) Nutting, G. C.; Harkins, W. D. J . Am. Chem. SOC.1939,6J,1182. (34) Gaines, G. L. Insoluble Monolayers at Liquid Gas Interfaces; Interscience: New York, 1966; p 249.
-
(35) Matsumoto, M.; Nakamura, T.; Tanaka, M.; Sekiguchi, T.; Komizu, H.; Matsuzaki, S.Y.; Manda, E.; Kawabata, Y . ;Saito, M.; Iizima, S.;Sugi, M. Bull. Chem. SOC.Jpn. 1987,60,2737. (36) Kawabata, Y.; Sekiguchi, T.; Tanaka, M.; Nakamura, T.; Komizu, H.; Honda, K.; Manda, E. J . Am. Chem. SOC.1985,107, 5270. (37) Kawabata, Y.; Sekiguchi, T.; Tanaka, M.; Nakamura, T.; Komizu, H.; Matsumoto, M.; Manda, E. Thin Solid Films 1985,133, 175. (38) Kobayashi, Y.; Goto, M.; Kurahashi, M. Bull. Chem. SOC.Jpn. 1986, 59, 311.
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The Journal of Physical Chemistry, Vol. 93, No. 6, 1989
Law and Chen
_----,’ ’\ I
0 00
300
400
500
600
700
800
300
WAVELENGTH i n m l
DSSQ). 0055,
2 0025
i
m
0 000
----
300
/
\
I
L
400
500 600 NAVELENGTH (nm)
700
800
Figure 5. Absorption spectrum of a four-layer LB film of MSSQ on glass (inset: solution absorption of MSSQ in chloroform,schematic of intermolecular interactions of the squaraine chromophore in the LB film of MSSQ).
monolayers of TSSQ with cadmium stearate (1:3 molar ratio). This result is very similar to that reported by Matsumoto and co-workers in their studies of the mixed LB films of squaraine 3.35-37As shall be discussed later, monolayers of TSSQ and mixture of TSSQ with cadmium stearate are chemically unstable on water due to chemical reaction between the squaraine chromophore and water. Reproducible preparations of the LB and the mixed LB films of TSSQ have been difficult. In this work, we concentrate our study to the LB films of DSSQ and MSSQ. Solution Absorption Spectra. DSSQ, MSSQ, and TSSQ exhibit sharp and intense solution absorption in the visible region in chloroform (for spectra of DSSQ and MSSQ, see insets of Figures 4 and 5). Their absorption maxima are at 634, 632, and 640 nm, respectively, and the absorption coefficients are -3 X IO5 cm-’ M-I. These spectral results indicate that our surfactant squaraine compounds are in monomeric forms in dilute chloroform
solution^.^ Effect of Orientation on the Aggregation of Squaraines in LB Films. Pure DSSQ and MSSQ form stable monolayers on water surface, and these monolayers can be transferred to hydrophobic glass substrates with a transfer ratio of 1.O. The aggregational states of the squaraine chromophore in the LB films of DSSQ and MSSQ were studied by absorption spectroscopy, and the spectral data are given in Figures 4 and 5. Results show that the absorption spectra of the squaraine chromophore in LB films not only are different from their corresponding solution spectrum (see insets) but also are different from each other. We attribute the broad absorption observed to aggregates of the squaraine chromophore in LB films. Since controlled experiments showed that similar absorption spectra could be obtained with single layers of DSSQ and MSSQ on glass substrates and that the absorbance increases as the number of layers increases (in case of MSSQ, see Figures 5 and 6 for the increase in absorbance between four and eight layers of MSSQ), we conclude that aggregates of the
-
600
700
800
WAVELENGTH ( nm 1
Figure 4. Absorption spectrum of a four-layer LB film of DSSQ on glass (inset: solution absorption of DSSQ in chloroform, schematic of intermolecular interactions of the squaraine chromophore in the LB film of
$ 1
500
400
Figure 6. Absorption spectrum of an eight-layer LB film of MSSQ as a function of storage time (a, t = 0 h; b, t = 1 h; c, t = 2 h; d , t = 4 h; e, t = 5 h; f, t 2 20 h).
squaraine chromophore of DSSQ and MSSQ are formed as a result of intermolecular interactions within a single layer. The optical absorption of the squaraine aggregates in the LB films of DSSQ and MSSQ has been examined by polarized light. Neither change in A, nor absorbance was observed when the polarized (both parallel and perpendicular) absorption spectra of the aggregates were taken. Examination of the LB films of DSSQ and MSSQ by light microscope reveals that the LB films are optically smooth. These results suggest that the size of the aggregates in LB films is probably CO.1 Mm, and that they are isotropic on the glass substrate. Very similar results were obtained by Bird et al.,39 who reported that the J-aggregates of 3,3’dioctadecyl-9-ethylthiacarbocyanineare also isotropic, on a glass substrate. The Bird, Debuch, and Mobius report further showed that a total orientation of the J-aggregates can be induced by using a gypsum substrate, and we are definitely interested in preparing films of DSSQ and MSSQ with total orientation in the near future. The LB film of DSSQ exhibits an absorption maximum at 530 nm and an absorption shoulder at -656 nm. The absorption in LB film is blue-shifted by 100 nm from the solution absorption. ,Although the effect of aggregation on the absorption of squaraine in solution is not known, the effect of aggregation of squaraine chromophore in the solid-state and in LB films has been documented. For example, Loutfy et aL4 reported the absorption spectra of a number of squaraines in methylene chloride solution and in solid and found that the solid-state absorption spectra are broad and red-shifted as compared to the solution spectra. Very similar results were also obtained by Eckhardt and co-workers,4w2 who reported detailed studies on the structural properties and the optical spectra of a number of squaraines. In case of LB films, Matsumoto and co-workers r e p ~ r t e d ~ a~ thorough -~’ study on the effect of aggregation on the absorption of 3 in mixed LB films. Again, these authors found that the absorption spectra of squaraine aggregates are broad and red-shifted as compared to the solution absorption. All these authors attributed the red-shift and the broad absorption to intermolecular charge-transfer interactions between the donor (aniline) and the acceptor (the four-membered ring) groups in squaraine. The observed blue-shift in Figure 4 indicates that such intermolecular charge-transfer interactions are not occurring in the LB film of DSSQ. In fact, such interactions are physically improbable, unless the squaraine chromophore is significantly tilted (>70°)or vertically moved during the transfer of the monolayer to a glass substrate. Very recently, we studied the X-ray crystal structure and the solid-state absorption of squaraine l.17 Results showed that, in contrast to all known examples, the solid-state absorption of 1 is blue-shifted (from 538 nm in solution to 480 nm in solid) as N
(39) Bird, G. R.; Debuch, G.; Mobius, D. J . Phys. Chem. 1977,81.2657. (40) Bernstein, J.; Tristani-Kendra, M.; Eckhardt, C. J. J . Phys. Chem. 1986, 90, 1069. (41) Tristani-Kendra, M.; Eckhardt, C . J. J . Chem. Phys. 1984, 81, 1160. (42) Tristani-Kendra, M.; Eckhardt, C. J.; Bernstein, J.; Goldstein, E. Chem. Phys. Lett. 1983, 98, 57.
Orientation Effect on Dye Aggregation in LB Films SCHEME I
aggregate B
aggregate A
compared to its solution absorption. The spectral characteristic is identical with that seen in the LB film of DSSQ. Since X-ray structural data of 1 showed that the major intermolecular interactions in solid is C-0 dipole-dipole interactions, the similarity in spectral change from solution to LB film for DSSQ suggests that the major intermolecular interaction between the squaraine chromophore in the LB film of DSSQ is C - 0 dipole-dopole interactions also. This is simply accomplished by slipping the squaraine chromophore by 1.4A along its short molecular axis. A schematic showing the intermolecular interaction of the squaraine chromophore in the LB film of DSSQ is given in the inset of Figure 4. MSSQ exhibits an absorption maximum at -650 nm and an absorption shoulder at -590 nm in the visible region in LB film. The absorption is broad and red-shifted as compared to the solution absorption. From the absorption spectral data of various aggregates of squaraines in the s ~ l i d - s t a t and e ~ in ~ ~LB ~ film^:^-^' we attribute the absorption of MSSQ in LBfilm to the aggregate of the squaraine chromophore resulted from an intermolecular charge-transfer interaction between the donor-acceptor groups. A schematic showing the intermolecular interaction is depicted in the inset of Figure 5. We also conclude that the difference in absorption spectra between the squaraine aggregates in the LB films of DSSQ and MSSQ is an aggregational effect induced by the LB film technique. Chemical Stability of the Monolayers and the LB Films of DSSQ, MSSQ, and TSSQ. As noted earlier, a monolayer of TSSQ is not stable on water. The squaraine chromophore decomposes in 15-20 min after the spreading of the squaraine solution. Since TSSQ appears to be indefinitely stable in chloroform solution, the instability of TSSQ on water is likely caused by interactions of the squaraine chromophore with water at the air-water interface.43 In this work, we also examined the stabilities of monolayers of DSSQ and MSSQ on water: we found that monolayers of DSSQ and MSSQ are stable for >4 and -2 h, respectively, on water. Since all the LB films used in this work were collected about 40 min after the spreading of the chloroform solution, the chemical instability of DSSQ and MSSQ on water imposes no serious fabrication problem in this work. The thermal stabilities of the LB films of DSSQ and MSSQ were studied by absorption spectroscopy. Results show that the LB film of DSSQ is stable over several months, and neither a decrease in absorbance nor a change in absorption maximum was observed. On the other hand, a visible change from blue to purple was observed for the LB film of MSSQ. The time-dependent spectra are presented in Figure 6. Spectral data show that, within the first 3 h, there is a decrease in absorbance for the absorption
-
(43) Recent spectroscopic results showed that squaraines form solutesolvent complexes in organic solvents and that the complexation constant increases as the polarity of the solvent increases (see ref 3 and references cited therein). It is thus very likely that the squaraine chromophore of DSSQ, MSSQ, and TSSQ forms complexes with water at the air-water interface during film fabrication. The extent of the complexation probably depends on the duration of the exposure of the chromophore at the air-water interface, the area of contact, and other fabrication conditions. In any event, the “squaraine watern complex may react to form a hydrated product, which will lead to the destruction of the squaraine chromophore. A very similar observation had been reported by Merritt” for the complex between squaraine and alkylamine in alkylamine solvents.
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989 2537 maximum at 656 nm, accompanied by the development of an absorption maximum at -520 nm. An isosbestic point at -545 nm was observed along with this change. Since the ,A, 520 nm absorption is a characteristic of squaraine aggregates having C-0 dipole-dipole interaction^,"^^^ the observed change in optical absorption and the occurrence of an isosbestic point within 3 h suggest that there is a change in aggregation (from aggregate A to aggregate B) in the LB film of MSSQ (Scheme I). After 3 h, a further decrease in absorbance was observed at ,,A 656 nm, while the absorbance at 520 nm remains essentially unchanged. Since water is known to cause decomposition of the squaraine chromophore, we suspect that a small amount of water may be present in the LB film and causes a slow decomposition of the squaraine chromophore in the LB film of MSSQ during storage. Even though the instability of the LB film of MSSQ appears to preclude immediate investigation of the photoconductivity, the electron/hole transporting properties, and various interfacial electron-transfer reactions of these films, the ability of the squaraine chromophore of MSSQ to form different aggregates is nevertheless an exciting finding. Our current plans on MSSQ are 2-fold. We plan to stabilize the type A aggregate of MSSQ. Since water causes decomposition of the squaraine chromophore, we shall attempt to minimize this damage by, e.g., changing the surface pressure, the ambient temperature, or even the subphase to a hydrocarbon. We also plan to manipulate our fabrication conditions to synthesize the type B aggregate of MSSQ.M If these efforts were successful, we would be able to study the effect of aggregation on various device parameters using MSSQ alone. Since the type B aggregate of MSSQ is very similar in the LB film of DSSQ, we may also be able to address the orientational effect on the photoconductivity, the transporting properties, and various electron-transfer reactions as well.
Concluding Remarks Surfactant squaraines DSSQ, MSSQ, and TSSQ, which were designed to orient the “bricklike” squaraine chromophore in three different orientations, were found to organize as designed on water surface by studying their surface pressure-area isotherms. Pure DSSQ and MSSQ form stable monolayers on water surface, which are transferrable to hydrophobic glass substrates. Pure TSSQ was found to collapse upon compression; the stability of the monolayer of TSSQ on water can be enhanced by mixing TSSQ with cadmium stearate. Owing to the decomposition of the squaraine chromophore by water at the air-water interface, effort on TSSQ has been abandoned. The aggregation of the squaraine chromophores of DSSQ and MSSQ in LB films was studied by absorption spectroscopy. By comparison with the absorption spectra of model squaraine aggregates, we conclude that the squaraine chromophores of DSSQ and MSSQ in LB films form different aggregates. While the squaraine aggregate in the LB film of DSSQ is controlled by C-0 dipole-dipole interactions, the aggregate in the LB film of MSSQ is controlled by intermolecular charge-transfer interactions between the electron-donor and -acceptor groups of squaraines. The difference in the aggregational behavior is an orientational effect induced by the LB film technique. Stability studies reveal that the LB film of DSSQ is perfectly stable upon storage and that there is a change in the aggregational state and a slow decomposition of the squaraine chromophore in the LB film of MSSQ. We are now in the progress of identifying conditions to control and stabilize various aggregates formed in the LB film of MSSQ, and preliminary results are promising. Plans have been made to use these aggregates to model the xerographic photodischarge process at a molecular level. Future investigations will include the study of the effects of aggregation and orientation on the quantum efficiency of photogeneration, the rate of electron-transfer within a dye aggregate, and the rate (44) Preliminary results show that the type B aggregate can be synthesized in the LB film of MSSQ under certain conditions and the aggregate appears to be stable. Chen, C.; Law, K. Y . , work in progress.
2538
J . Phys. Chem. 1989, 93, 2538-2542
of electron-transfer between the dye aggregate and other electron donor (or acceptor) molecules at the interface.
Acknowledgment. We thank F. c. Bailey for his assistance in the squaraine synthesis, Dr. J. Gold for preliminary experiments,
and Dr. T. W. Smith for his encouragement throughout this work. Registry No. DSSQ, 118418-01-2;MSSQ, 68149-27-9;TSSQ, 109976-92-3;I-@-(dimethylamino)phenyl)-2-hydroxycyclobutene-3,4dione, 107885-39-2;cadmium stearate, 2223-93-0.
Electrochemical Investigation of Dihexadecyl Phosphate Vesicle Incorporated Colloidal CdS Particles An-Cheng Chang and Janos H. Fendler* Department of Chemistry, Syracuse University, Syracuse, New York 13244-1 200 (Received: April 19, 1988; In Final Form: July 18, 1988)
Photocurrents, generated upon irradiation of dihexadecylphosphate (DHP)incorporated cadmium sulfide (CdS) solutions containing glucose, have been determined in a three-compartment electrochemical cell with a 3.0-mm2platinum-disk working electrode in the absence of supporting electrolytes. Addition of methylviologen, MV2+, as electron acceptor significantly increased the photocurrent. The onset of the photocurrent, pH, = 10.1 f 0.1, and the location of the quasi-Fermi level for electrons, nFf* = -0.85 V vs SCE,have been determined for DHP vesicle incorporated CdS particles containing glucose and MV2+from pH-dependent photocurrent measurements. Substantially greater steady-state photocurrents were observed in irradiating 50-& instead of loo-& diameter DHP vesicle incorporated CdS particles. The electrochemical mechanism has been discussed in terms of (i) production of conduction-band electrons and valence-band holes, (ii) hole depletion by sacrificial electron transfer from glucose, (iii) MV2+reduction to M V " by conduction-band electrons, and (iv) reoxidation of MV" at the anode.
Introduction The potential of colloidal semiconductor particles for solar energy conversion devices and, more generally, for photocatalysis has been rmgnized by the rapidly growing number of publications originating from laboratories around the Irradiation (1 ) Organic Phototransformations in Nonhomogeneous Media; Fox, M. A,, Ed.; American Chemical Society: Washington, DC, 1985. (2) Ramsden, J. J.; Gratzel, M. J . Chem. SOC.,Faraday Trans. 1 1984, 80, 919. (3) Serpone, N.;Sharma, D. K.; Jamieson, M. A,; Gratzel, M.; Ramsden, J. J. Chem. Phys. Let?. 1985, 115, 473. (4) Moser, J.; Gratzel, M. J . A m . Chem. SOC.1984, 106,6557. ( 5 ) Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1982, 86, 302. (6) Henglein, A,; Gutierrez, M. Ber. Bunsen-Ges. Phys. Chem. 1983,87, 852. (7) Weller, H.; Koch, U.; Gutierrez, M.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1984,88, 649. (8) Foitik, A.; Weller, M.; Koch, U.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 969. (9) Rossetti, R.; Nakahara, S.; Brus, L. E. J . Chem. Phys. 1983, 79, 1986. (IO) Brus, L. E. J . Chem. Phys. 1984, 80, 4403. ( 1 1) Rossetti, R.; Ellison, J. L.; Gibson, J. M.; Brus, L. E. J . Chem. Phys. 1984, 80, 4464. (12) Ramsden, J. J.; Webber, S. E.; Gratzel, M. J . Phys. Chem. 1985, 89, 2240. (13) Dimitrijevic, N. M.; Savic, D.; Micic, 0. I.; Nozik, A. J. J . Phys. Chem. 1984.88, 4278. (14) Williams, F.;Nozik, A. J. Nature (London) 1984, 312, 21. (15) Nozik, A. J.; Williams, F.; Nenadovic, M. T.; Rahj, T.; Micic, 0. I. J . Phys. Chem. 1985, 89, 397. (16) Nau, A. W.-H.; Huang, C.-B.; Kakuta, N.; Bard, A. J.; Campion, A,; Fox, M. A.; White, J. M.; Webber, S. E. J . Am. Chem. SOC.1984, 106,6537. (17) Meissner, D.; Memming, R.; Kastening, B. Chem. Phys. Lett. 1983, 96, 34. (18) Kuczynski, J. P.;Millosajevic, B. H.; Thomas, J. K. J . Phys. Chem. 1984, 88, 890. (19) Furlong, D. N.; Wells, D.; Sasse, W. H. F. J . Phys. Chem. 1985, 89, 626, 1922. (20) Meyer, M.; Wallberg, C.; Kurihara, K.; Fendler, J. H. J . Chem. Soc., Chem. Commun. 1984, 90. (21) Tricot, Y.-M.; Fendler, J. H. J . A m . Chem. SOC.1984, 106,7359. (22) Rafaeloff, R.; Tricot, Y.-M.; Nome, F.; Fendler, J. H. J . Phys. Chem. 1985, 89, 533. (23) Tricot, Y.-M.; Emeren, A.; Fendler, J. H. J . Phys. Chem. 1985, 89, 4721. (24) White, J. R.;Bard, A. J. J . Phys. Chem. 1985, 89, 1947
0022-3654/89/2093-2538$01 S O / O
of the semiconductor results in the formation of conduction-band electrons, e-, and valence-band holes, h+ve. These species may nonproductively recombine in the interior or at the surface of the colloidal semiconductor. Alternatively, they can mediate reductions and oxidations of appropriate dissolved substrates. Size and uniformity are important considerations. The smaller the dispersed particles, the larger their surface areas and, hence, their ability to harvest photons. There is a limit, however, to which the sizes of semiconductors could be decreased. Below a certain size they lose their band structure and cease to be semiconductors. Colloidal particles have many unique properties at the size range distributions where they are about to become semiconductors. Quantum confinement of the charge carriers in ultrasmall colloidal semiconductor particles results in increased band-gap energy and in the development of absorption peaks due to exciton formati~n.""~'~In addition to inherent interest, quantum-sized colloidal semiconductor particles often manifest altered redox and oxidation potentials that can advantageously be utilized in driving photocatalytic r e a c t i o n ~ . ~For ~ example, band-gap irradiation of C02-saturated solutions of dispersed colloidal CdSe particles in sizes smaller than 50 A produced formic acid. Conversely, excitation of large-particle-sized CdSe colloids under identical conditions did not yield formic acid.28 Generating and maintaining monodisperse semiconductor particles in controlled sizes are experimentally demanding. Nonaqueous solvents, low temperature, and stabilizers have been used to maintain ultrasmall semiconductors. Techniques have been developed in our laboratory for the in situ generation and stabilization of semiconductor particles in surfactant (25)Tricot, Y.-M.; Fendler, J. H. J . Phys. Chem. 1986, 90, 3369. (26) Tricot, Y.-M.; Furlong, D. M.; Sasse, W. H.; Daivis, P.; Snook, I.; Van Megen, W. J . Colloid Interface Sci. 1984, 97, 380. (27)Carmona-Ribeiro, A. M.; Yoshida, L. S.; Chaimovich, H. J . Phys. Chem. 1985,89, 2928. (28) Nemeljkovic, J. M.; Nenadovic, M. T.; Micic, 0. I.; Nozik, A. J. J . Phys. Chem. 1986, 90, 12. (29) Kano, K.; Romero, A.; Djermouni, B.; Ache, H.; Fendler, J. H. J. Am. Chem. SOC.1979, 101,4030. (30)Carmona-Ribeiro, A. M.; Yoshida, L. S.; Sesso, A,; Chaimovich, H. J . Collid Interface Sci. 1984, 100,433.
0 1989 American Chemical Society
