2448
J . Phys. Chem. 1991, 95, 2448-2451
Polymer Langmuir-Blodgett FHms Containing Photofunctional Groups. 2.' Electron-Transfer Quenching in Polymer Langmulr-Biodgett Films Containing Carbazole Chromophore Tokuji Miyashita,* Tom Yatsue, Department of Biochemistry and Engineering, Tohoku University, Aoba Aramaki, Aoba- ku, Sendai 980, Japan
and Minoru Matsuda Chemical Research Institute of Nonaqueous Solutions, Tohoku University, Katahira 2-1 -1, Aoba-ku, Sendai 980, Japan (Received: May 1I, 1990: In Final Form: August 29, 1990)
N-Vinylcarbazole (Cz), being nonamphiphilic, has been introduced into Langmuir-Blodgett (LB) film as a comonomer of N-dcdecylacrylamide (DDA) which has an excellent ability to form monolayer and LB multilayer. The spreading behaviors of the DDA/Cz copolymers with various Cz contents on a water surface were investigated by a measurement of the surface pressuresurface area isotherms. The isotherms show that the DDA/Cz copolymers even with 30%Cz mole fraction form a stable condensed monolayer on pure water and the carbazole chromophore is dispersed uniformly having its own surface area on the water surface. The copolymer monolayer could be transferred onto solid supports successively by the Langmuir-Blodgett method (Y-type deposition). The electron transfer from the excited carbazole chromophore in the copolymer monolayer to a viologen acceptor incorporated in the adjacent monolayer has been studied in LB film assembly by using an emission quenching method. Very effective quenching occurred due to the energy migration between the carbazole chromophores. It was found that the electron transfer quenching efficiency was varied by the mole fraction of carbazole content, which is related to the density of excimer site, and the quenching was determined by the result of the competition of energy migration to the carbazole facing the viologen quencher and to the energy-trapping site (an excimer formation site).
Introduction The carbazole group is one of the most attractive chromophores in photochemistry and has been extensively studied. Poly(Nvinylcarbazole) (PVCz) has been utilized in various functional devices, for example, as a photoconductor, as a photoreceptor for the xerographics, and as a carrier of photogeneration," whereas monomers such as N-ethylcarbazole and N-isopropylcarbazole are poor photoconductors?-* The attempt to incorporate carbazole chromophore into uniformed thin films with controlled thickness is of great interest and important to refine the functions of the above devices. A Langmuir-Blodgett method has become one of the best methods to prepare ultrathin films in which chromophores are ordered regularly? A difficult synthesis of amphiphilic carbazole compounds is, however, necessary to introduce the carbazole chromophore into LB films.'O*'l Moreover, in the general case, such an amphiphilic compound alone could not form a stable monolayer and the compound is mixed with a long alkyl fatty acid monolayer such as stearic acid and arachidic acid to form the LB films. We have tried to prepare polymer LB films having various chromophores without the difficult synthesis of amphiphilic d y e ~ t u f f . ~ . ~Our * + ~recent ~ studiesleI6 have showed that N-al(1) Part 1: Miyashita, T.; Yatsue, T.; Mizuta, Y.; Matsuda, M. Thin Solid Films 1990, 179,439. (2) Mort, P. J. J . NonlCryst. Solids 1870, 4, 132. (3) Okamoto, K.; Itaya, A.; Kusabayashi. S. Polym. J . 1975, 7, 622. (4) Mort, J.; Chen. I.; Emerland, R. L.; Sharp, J. H. J . Appl. Phys. 1972,
453, 2285. (5) Yokoyama. M.; Hanabata, M.; Tamamura, T.; Nakano, T.; Mikawa, H.J . Chem. Phys. 1977.67, 1742. (6) Kim, N.; Stephen, E. W. Macromolecules 1985, 18, 741. (7) Kinjo, K.; Nagashima, S.;Yoshitake, K. Electrophotography 1961, 3, 29. ( 8 ) Sharp, J. H. J . Phys. Chem. 1967, 71, 2587.
(9) Kuhn, H.; Mbbius, D.; Bncher, H. Physical Methods ofchemistry; Weissberger, A., Rossiter, B., Eds.; Wiley: New York, 1972; Vol. 1, p 577. (10) Tamai, N.; Yamazaki, T.; Yamazaki, I. J . Phys. Chem. 1987,91,841. ( 1 1) Yamazaki, 1.; Tamai, N.; Yamazaki, T.; Murakami, A.; Mimuro, M.; Fujita, Y. J. Phys. Chem. 1988, 92, 5035. (12) Murakata, T.; Miyashita, T.; Matsuda, M. Macromolecules 1988,21, 2730.
TABLE I: Spreading Bebrvior of the Monolayer of DDA/Cz copolYmers
Cz mole fracn. 76
surf. area for Cz, nm*/monomer
collapse press., mN/m
31.3 12.5 6.4
0.35 0.37 0.31 av 0.34 f 0.02
38.5 42.5 49.0
kylacrylamides have an excellent ability to form monolayer and LB multilayers. In a previous paper,' we have reported that preformed copolymers of N-octylacrylamide or N-dodecylacrylamide with N-vinylcarbazole form a stable monolayer on the water surface and the monolayers can be transferred onto a solid support forming Y-type polymer LB films. In the present work, N-dodecylacrylamide copolymers with various mole fractions of Nvinylcarbazole (Figure 1) were prepared and the spreading behaviors of the copolymers on the water surface were investigated. Moreover, the mechanism of electron transfer from the excited carbazole chromophore in the copolymer monolayers to a viologen acceptor incorporated in the adjacent monolayer is also discussed. Experimental Section Materials. The copolymers were prepared by free-radical polymerization of N-dodecylacrylamide (DDA) with N-vinylcarbazole (Cz) in benzene at 60 'C. The copolymers were purified by dissolution in chloroform, followed by filtration and precipitation in a large excess of acetonitrile. The molar ratios of the carbazole in the copolymers were determined by measuring nuclear magnetic resonance (NMR) spectroscopy and UV-visible absorption spectra. The number average molecular weights were measured by gel permeation chromatography (GPC);the mo(1 3) Murakata, T.; Miyashita, T.; Matsuda, M. Macromolecules 1989.22, 2706. (14) Miyashita, T.; Yoshida, H.; Murakata, T.; Matsuda, M. Polymer 1987, 28, 31 1. (15) Miyashita, T.; Yoshida, H.; Itoh, H.; Matsuda, M. Nippon Kagaku Kaishi 1987, 2169. (16) Miyashita, T.; Mizuta, Y.; Matsuda, M. Br. Polym. J . 1990, 22,
0022-3654191 /2095-2448SO2.50/0 0 1991 American Chemical Societv
E T Quenching in Polymer LB Films
The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 2449 03s
9
m2
2 II
CH3 Figure 1. Chemical structure of DDA/Cz copolymers. 60
I
5540 m L
a n m n L
n
f
-
20
c L
03
H
9 Surface area I n i Z / i o n o i a r 1
Figure 2. Sui ace pressurearea isotherms of DDA/Cz copolymer monolavers at 3 OC: (a) DDA/Cz = 2.2/1, (b) DDA/Cz = 7/1, (c) D D A / ~ Z= 141.7/ 1.
lecular weights for the copolymers with the compositions of DDA/Cz = 2.2/1,7/1, and 14.7/1 are 7.0 X lo4, 1.0 X 104, and 1.6 X lo4, respectively. Preparation of Langmuir-Blodgett Films. An automatic working Langmuir trough (Kyowa Kaimen Kagaku HBM-AP with a Wilhelmy-type film balance) was used for the measurements of surface pressure-area isotherms ( r A isotherms) and the preparation of LB films. All copolymers were dissolved in chloroform (spectroscopic grade) at a concentration of about M and spread on a water surface. Distilled water was used (Millipore Milli-QII). The quartz slides used for the deposition of monolayers were previously cleaned in boiling H$04-HN03 (21) solution, made hydrophobic with dichlorodimethylsilane, and coated in advance by four layers of poly(N-dodecylacrylamide) to prepare a uniform surface and to remove the influence of the bare quartz slide. Fluorescence spectra and UV-visible absorption spectra were measured with a Hitachi 850 spectrofluorophotometer and a Shimadzu UV-160 UV-visible spectrophotometer, respectively.
Results and Discussion Monolayer and Multilayer Formation. The T-A isotherms of DDA/Cz copolymers on a water surface at 19 "C are shown in Figure 2. The isotherms have a steep rise in surface pressure and also have a high collapse pressure, which decreased with an increase in the mole fraction of carbazole in the copolymer (Table I). This indicates that stable and condensed monolayers are formed, in spite of the lack of surfactant substituents attached to the carbazole moiety. The average limiting surface areas per monomer in the copolymers are estimated to be 0.28-0.30 nm2/monomer by extrapolating the steep region of the isotherms to zero surface pressure. The surface area for the carbazole moiety could be calculated from the average areas by assuming the area of DDA to be 0.28 nm2/monomer.16 The obtained area for the carbazole moiety was almost constantly 0.34 f 0.02 nm2/molecule regardless of the mole fraction of carbazole in the copolymers. This means that the carbazole chromophore is dispersed, having its own surface area in the copolymer monolayer. The surface area is consistent with the value estimated from the CPK model where the polymer chain is laid horizontally on the water surface and the dodecyl substituent and the carbazole ring are oriented vertically to the water surface (Figure 3). The copolymer monolayers could be transferred onto a solid support by both
Figure 3. Proposed orientation of the copolymer chain and carbazole
moiety.
Wavelength ( n i l Figure 4. Emission and excitation spectra of DDA/Cz LB films: (a) DDA/Cz = 2.2/1, (b) DDA/Cz = 7/1, (c) DDA/Cz = 14.7/1.
downward and upward strokes at a dipping speed of 10 mm/min with a transfer ratio of unity (Y-type deposition). The emission and the excitation spectra of the LB films of the DDA/Cz copolymers are shown in Figure 4. The emission spectrum of the LB film for the copolymer with a low content of carbazole (DDA/Cz = 14.7/1) shows the structured fluorescence characteristic of monomeric carbazole chromophore with a strong (0,O)band at 350 nmI7J8 and no excimer emission was observed. The spectra for the LB films of the copolymers with higher carbazole content show both monomer and excimer emission, and the intensity of the excimer emission increases with the content of carbazole. In chloroform solution, however, the emission spectra of all copolymers show only monomer fluorescence which is identical with spectrum c in Figure 4. The appearance of excimer emission in the LB films can be explained by the energy migration between the carbazole chromophores in the monolayer, followed by the trapping by an excimer formation site of the nearest-neighbor carbazole-carbazole pair existing in the polymer chain. The emission spectra were not changed with the number of layers deposited and the emission intensity increases linearly with the number of layers. In addition, the excitation (17) Johnson, G . E.J . Phys. Chem. 1974, 78, 1512. (18) Miyashita, T.; Ohsawa, M.;Matsuda, M.Macromolecules 1986, 19, 585.
2450 The Journal of Physical Chemistry, Vol. 95, No. 6, 1991
Miyashita et al.
DDAICz
,
u
Figure 5. Schematicillustration of the structure of LB film assembly for
the quenching.
= o
“
u
I
0
0.2
2
0.4
V i o l o g c n d e n s i t y lnm-21
Fipyre 7. Plots of relative emission intensity (I/[,,) against viologen density: (a) DDA/Cz = 2.2/1, (b) DDA/Cz = 7/1, (c) DDA/Cz = 14.71I.
0 0. I
0.6
1.1
Su r f a c e a r e a I n n 2 /no i c c u i e 1
Figure 6. Surface pressure-area isotherms for the mixed monolayers of stearylviologen with barium stearate at 19 OC: (a) St/SV2+ = lo/], (b) 5/1, (c) 3/1, (d) 2/1, (e) 1/1.
spectrum of the LB film is in agreement with the absorption spectrum of monomeric carbazole chromophore in solution. These results indicate that the carbazole chromophore is dispersed molecularly in the monolayer, and the regular successive deposition of the copolymer monolayers, which yields the uniform copolymer LB films, is achieved. Photoinduced Electron Transfer in the Polymer LB Film System. The electron transfer from the photoexcited carbazole chromophore to viologen acceptor in the polymer LB films was investigated by the emission quenching method using the LB film assembly shown in Figure 5 , where four DDA homopolymer monolayers are coated on a bare quartz slide and then the copolymer monolayer, the viologen quencher monolayer, and finally again four the homopolymer monolayers are coated. Stearyl viologen (SV2+)was used as the viologen quencher. The surface pressure-area isotherms for the mixed monolayer of SV2+with barium stearate (St) indicate that stable condensed mixed monolayers are obtained for more diluted mixed monolayers than St/SV2+ = 5/1 molar ratio (Figure 6). The intensity of the monomer emission at 350 nm from the excited carbazole in the DDA/Cz copolymer monolayer decreased due to the presence of stearyl viologen in the adjacent monolayer of the LB assembly. The emission quenching is due to electron transfer from the photoexcited carbazole to the viologen quencher; this process is an exothermic process (AG = -2.13 V).
+
-
+
Czt svz+ cz*+ SV’+ The extent of the decrease in the emission intensity due to the electron-transfer quenching depended on the two-dimensional density of the viologen in the monolayer, which can be varied by changing the mixing ratio of the viologen and the inert barium stearate matrix. The relative steady-state emission intensities Illo (I in the presence and Io in the absence of the viologen) plotted against the density of the viologen for the copolymer monolayers with various carbazole contents are shown in Figure 7. The average separation distance of the viologan (2R)can be calculated from the surface area allotted to one SV2+ molecule on the basis of the results that the area for stearic acid is 0.2 nm2 and the area for the viologen 0.4 nm2 from the isotherms of the mixed monolayer. The half-quenching distance (&), which is defined as the distance giving half the emission intensity (I/Io = OS),was ob-
a :sv
*
:cz
Figure 8. Schematic illustration of the mechanism of the photoinduced
electron transfer in the LB assembly. tained to be ca. 1.8 nm (St/SV2+ = 47/1) and ca. 2.7 nm (St/ SV2+= 112/1) for the copolymer monolayers of DDA/Cz = 2.2/1 and 7/1, respectively, from the plots in Figure 7. A half-quenching of the emission is achieved even with a small amount of the viologen quencher, apparently indicating that very effective quenching occurs. The relative emission intensity (Z/Io),however, did not decrease to zero value. About 30% emission of carbazole for the monolayer of DDA/Cz = 7/1 copolymer, for example, was left without quenching. This is because the carbazole chromophore and the viologen quencher each locates at a different layer. It would be difficult to quench the emission completely even if the density of the viologen quencher increases up to the limit. The half-quenching distance for the copolymer of DDA/Cz = 14.7/1 could not be obtained in the present experiment, because the stable mixed monolayer of viologen with barium stearate is formed only under the condition where the content of stearyl viologen is lower than about 20% mole fraction. It is very interesting that the half-quenching distance is not correlated linearly with the mole fraction of carbazole in the copolymers, because the probability that the carbazole chromophore just facing viologen quencher is excited should increase with the mole fraction of carbazole. The quenching, however, in the copolymer monolayer of DDA/Cz = 7/1 was the most effective. The mechanism of the photoinduced electron-transfer process in the copolymer LB films can be considered as follows: the excited energy absorbed by carbazole moiety migrates between the carbazole chromophores in the DDA/Cz copolymer monolayer, and when the migrating energy arrives at the carbazole moiety facing the viologen, electron-transfer quenching may occur (Figure 8). When the migrating energy arrives at an energy-trapping site (an excimer formation site), the energy would be trapped and the electron transfer from the site would be difficult. The mean distances between carbazole moieties along the copolymer chain in the monolayers are estimated to be 0.54, 0.86, and 1.20 nm for the ratios of DDA/Cz = 2.2/1, 7/1, and 14.7/1, respectively. The critical transfer distance of energy migration for a carbazolecarbazole chromophore pair is reported to be 1G2.0 nm.19 Thus, the effective energy migration is not expected for the copolymer ~~
(19) Berlman, I. B.Energy Transfer Parameters of Aromatic Compounds; Academic: New York, 1973.
2451
J . Phys. Chem. 1991, 95, 2451-2456 of DDA/Cz = 14.7/1, and then the effective quenching with the help of the energy migration was not be observed. On the other hand, the LB film of the copolymer with DDA/Cz = 2.2/1 showed a strong excimer emission, suggesting that the migration energy would be trapped by the excimer formation site in the monolayer, resulting in the inefficient quenching. The mean distance between carbazole moieties of DDA/Cz = 7/1 is at a favorable distance for the energy migration. However, the amount of the nearestneighbor carbazole pair which is the excimer formation site is not so large, as confirmed by a relative lower intensity in the excimer emission. The energy migration to the chromophore facing the viologen quencher is more favorable than to the energy-trapping site. As a result, the quenching in the copolymer LB assembly with DDa/Cz = 7/1 becomes the most effective. Mobius et al. reported the half-quenching distances for various dye donor and viologen acceptor couples in similar LB assemblies. The half-quenching distance for the photoexcited bipyridine-ruthenium complex donor to stearylviologen quencher was about 1.0 nm, while the distance was 3.0, 6.0, and 7.5 nm for different
cyanine dyes used instead of the ruthenium complex; that is, the correlation of the half-quenching distance with the energy difference between the excited donor and the acceptor was obser~ed.~ The influence of energy migration on the electron-transfer quenching was not demonstrated in the ruthenium complex LB assembly.20 The arrangement of carbazole chromophores along the polymer chain and their orientation in the LB film in the present polymer LB system would be favorable to the energy migration. In conclusion, it can be said that the efficiency of quenching in the copolymer monolayer is determined by the result of the competition of energy migration between the energy-trapping site (excimer formation site) and the carbazole chromophore facing the viologen quencher. Registry No. (Cz)(DOA) (copolymer), 125976-02-5;SV2+,6405517-0; St, 6865-35-6. _
_
_
~
~~
(20) Seefeld, K-P.; Mobius, D.; Kuhn, H. Helu. Chim. Acru 1977, 60, 2608.
Surface-Enhanced Resonance Raman Scattering Study on the Structure of a Merocyanine Dye, 442 4 4-Hydroxyphenyl)ethenyl)-l-methylpyridinium, Adsorbed on Silver Surfaces in Water and in Acetonitrile Yasushi Mineo and Koichi Itoh* Department of Chemistry, School of Science and Engineering, Waseda University, Shinjuku- ku, Tokyo 169, Japan (Received: May 18. 1990; In Final Form: September 26, 1990)
Surface-enhanced resonance Raman scattering (SERRS) spectra were measured for a merocyanine dye, 4-(2-(4-hydroxypheny1)ethenyl)-1-methylpyridinium (MH+), and its base form (M) adsorbed on silver colloids and electrodes in water as well as in acetonitrile (ACN). The surface spectra indicated that M adsorbed on the silver surfaces takes on a trans conformation. The central C-C stretching frequency (vC4), which is a good marker for estimation of the relative contribution of the resonance structures, the benzenoid and quinoid forms, were measured for M adsorbed on the silver electrode in 0.1 mol/L KCI and M on the electrode in 0.1 mol/L LiCIO,; on sweeping the electrode potential from -0.2 to -0.5 V (vs Ag/AgCl) the former adsorbate shows a shift of vc4 from 1571 to 1561 cm-l (Avc4 = 10 cm-I) while the latter shows a shift from 1571 to 1567 cm-' (AvcF = 4 cm-'). The frequency shifts to the lower frequency side mean a decrease in the contribution of the benzenoid form. Simple Coulombic interaction and polarization effect due to coadsorbed chloride ions on the electrode surface are proposed to explain the results. Especially, the larger shift observed for the adsorbate on the electrode in 0.1 mol/L KCI is explained as due to the reduction in the polarization effect on the stabilization of the benzenoid form. The SERRS spectrum proved that MH+ adsorbed on the silver electrode and colloid surfaces takes on a trans conformation. The adsorbate on the colloid gives rise to a far stronger SERRS band near 890 cm-l due to a CH out-of-plane bending vibration of the central olefinic bond compared to that observed for the adsorbate on the electrode surface; the result suggests that the adsorbate on the colloid exists in a flatter orientation and/or in more direct interaction with the electrode surface. It was also found that irradiation of the electrode surface by a Xe lamp (500 W) causa an orientational change of the adsorbate to a flatter one, which is in direct contact with the surface.
Introduction Surface-enhanced Raman scattering (SERS) and surface-enh a n d rmnance Raman scattering (SERRS) spectroscopies have now h e n established as a powerful tool for investigating surface geometries and surface reactions of adsorbates on electrodes and colloids of coinage metals such as silver and Marked changes in SERS or SERRS features are generally observed on changing solvents, pH, coexisting electrolytes, and electrode potentials; detailed analyses of the surface spectral changes give ample information about how these conditions affect the structures (1) Surfuce Enhanced Rumun Scarrering, Chang, R. K., Furtak, T. E., Eds.; Plenum Press: New York, 1982. (2) Moskovits, M. Rev. Mod. fhys. 1985, 57, 783. (3) Campion, A. In Vibrutionul Spectroscopy of Molecules on Surfaces; Yates, J. T. Jr., Madey, T. E., Eds.; Plenum Press: New York, 1987; p 345. (4) Creighton, J. A. In Specrroscopy of Surfaces; Clark, R.J. H., Hester, R. E., Eds.; Wiley: Chichester, U.K., 1988; p 37. (5) Cotton, T. M. In Spectroscopy of Surfaces; Clark, R.J. H., Hester, R. E., Eds.; Wiley: Chichester, U.K.,1988; p 153.
of adsorbates and the mode of their interaction with metal surfaces; then the analysis reveals new aspects of chemistry on metal surfaces. This is exemplified by the SERS studies on 2,2'-bipyridine adsorbed on silver electrodes and colloids.6~' In the present paper we measured the SERRS spectra of a typical merocyanine dye, 4-(2-(4-hydroxyphenyl)ethenyl)-lmethylpyridinium in neutral (or base) and protonated forms (the neutral form is abbreviated to M and the protonated one to MH+; see Figure 1) adsorbed on silver colloid as well as on silver electrodes. M has been extensively studied because of its unusual solvatochromic behavior in the visible region6-" and large sol(6) Kim, M.; Itoh, K. J. Elecrrounul. Chem. 1985, 188, 137. (7) Kim,M.; Itoh, K. J . fhys. Chem. 1985, 91, 126. (8) Benson, H. G.; Murrell, J. N. J. Chem. Soc., Furuduy Trans. 2 1972, 68, 137. (9) Botrel, A.; Beuze, A,; Jacques, P.; Strub, H. J . Chem. Soc., Furuduy Trans. 2 1984,80, 1235. (IO) Donchi, K.F.; Robert,G. P.; Temai, B.; Derrick, P. J. Ausr. J. Chem. 1980, 33, 2199.
0022-3654/91/2095-245 1%02.50/0 0 1991 American Chemical Society
