J. Phys. Chem. 1992,96, 10125-10129
10125
Electron-Transfer Quenching of Carbazole Chromophore Incorporated In Polymer Monolayers by Pyrldlnlum Quenchers' Tom Yatsue,+ Minom Matsuda,$and Tokuji Miyasbita**t Department of Biochemistry and Engineering, Tohoku University, Aoba Aramaki, Aoba- ku, Sendai 980, Japan, and Institute for Chemical Reaction Science, Tohoku University, Katahira 2-1 -I, Aoba-ku, Sendai 980, Japan (Received: June 12, 1992; In Final Form: August 14, 1992)
Electron-transferquenching of the excited carbazole chromophore in poly(N-dodecylacrylamid~c~~-~yl~rbazole)s (DDA/Cz) polymer monolayers by three different N-stearyl para-substituted pyridinium derivatives (X-Py) (p-COCH,, p-COOCH3, and pCONH2) has been investigated in LangmukBlodgett (LB) assembly systems by using a steady-state emission quenching method. Three different mixed monolayers of the pyridiniums with stearic acid (St) are in direct contact with the DDA/Cz monolayers having various Cz contents to investigate the influence of the redox potentials of the quenchers and the effect of energy migration between the Cz chromophores on the electron-transfer quenching in the LB films. The emission of the excited carbazole was quenched effectively by a small amount of the pyridinium quencher with a help of energy migration between the Cz chromophores. The quenching efficiency was varied by the mole fraction of carbazole contents, which is related to the density of excimer site in the monolayer and by the redox potentials of the pyridiniums. The plots of In (Zo/Z - 1) (Iin the presence and Z, in the absence of the quencher) vs distance (R) between the quenchers gave a linear relationship for all the quenchen. The mechanism of electron-transferquenching in the LB film assembly is discussed on the basis of electron tunneling and energy migration.
Introduction Recently photoinduced electron transfer in molecular assembly systems has received much attention from the viewpoint of artificial
photarynthesis.2-s Various molecular organized assemblies have been attempted to realize an efficient photoenergy conversion ~ y s t e m . ~Supramolecules .~ in which donor and acceptor molecules/components are covalently linked were synthesized to examine key parameters of electron-transfer Langmuir-Blodgett (LB) film fabrication is also one of the most fascinating molecular assembly techniques for fabricating a controllable molecular array for efficient electron transport.sJ*12 Electron-transfer processes, however, in a solid matrix are complicated due to a nonuniform distribution and/or an aggregation of probe molecules. In this respect, the LB technique is the best tool for basic research of the electron-transfer process in solid films. The distance between donor and acceptor molecules and the two-dimensional densities can be controlled at a molecular level for the study of the electron-transfer process by using the LB technique. The incorporation of photofunctional chromophores into LB films has been done by various methods. Amphiphilic chromophores having the ability to form LB films have been synthesized.l39l4 As a more general method, a physical mixing of the chromophore with a long alkyl chain fatty acid forming a stable LB multilayer is employed.1sJ6 Recently, we have proposed the copolymerization method to incorporate various functional chromophores into polymer LB films as a comonomer of N-dodecylacrylamide (DDA), which has an excellent ability to form a stable LB m~ltilayer.'~-'~ In the previous study,20we have prepared stable polymer LB films containing a carbazole chromophore, which is known to show a wide range of photoactivity,21-2sas the form of a copolymer of DDA and N-vinylarbazole (Cz). The electron transfer from the excited carbazole chromophore in the copolymer monolayers to a stearylviologen (SV) acceptor incorporated in the adjacent monolayer has been studied in LB film assemblies by using an emission quenching method. It was shown that very effective quenching takes place due to the energy migration between the carbazole chromophora that are aligned regularly in the LB films and that the quenching rate was determined from results of measurements of competition of the energy migration to carbazole facing to SV quencher and the energy trapping at excimer formation site. In this work, to clarify the electron-transfer quenching Department of Biochemistry and Engineering. *Institutefor Chemical Reaction Science.
0022-3654/92/2096- 10125$03.00/0
mechanism in the LB assembly, the quenching of the carbazole chromophore as a function of mole fractions in the DDA/Cz polymer LB films by three kinds of N-stearyl para-substituted pyridiniums (X-Py, X = p-acetyl, p-carbomethoxy, pcarboxyamide derivatives; Figure 1) is investigated. The redox potentials of the quenchers can be varied without a large change in the molecular orientation, structure, and the distance in LB assembly systems. Through this work, the effect of energy migration between the carbazoles and moreover the influence of the redox potential in the electron-transfer quenching can be examined. Experimental Section
MateriaJs. The DDA/Cz copolymers were prepared by free radical polymerization of N-dodecylacrylamide (DDA) with N-vinylcarbazole (Cz) in benzene at 60 OC with 2,2'-azobis(isobutyronitrile) as a thermal initiator. The copolymem 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 a b sorption spectra. The number-average molecular weights were measured by gel permeation chromatography (GPC); the molecular weights for the copolymers with the compositions of DDA/Cz = 2.2/1,7/1, and 14.7/1 are 7.0, 1.0, and 1.6 X lo4, respectively. Three pyridinium quenchers were synthesized by a reaction of the corresponding para-substituted pyridines with stearyl bromide. The products were purified by recrystallization from ethanol. 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 pressurtarea isotherms ( F A isotherms) and preparation of LB films. All copolymers were dissolved in chloroform (spectroscopic grade) at a concentration of about M and spread on a water surface. The mixtures of N-stearylpyridiniumderivatives (X-b) with stearic acid were spread on the water subphase containing BaClz (3 X M) and KHC03 (4 X lo-" M) at 19 OC from a mixed solution of chloroform with 5% methanol. Distilled water (Millipore Milli-QII) was used. The quartz slides used for the deposition of monolayers were previously cleaned in a boiling H2S04-HN03 (2:l) solution and made hydrophobic with dichlorodimethylsilane and were 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 0 1992 American Chemical Society
10126 The Journal of Physical Chemistry, Vol. 96, No. 25, 1992
Yatsue et al.
CH3
H3COOC
