6550
J. Phys. Chem. 1992, 96, 6550-6554
Excitation Energy Relaxation of Oxacyanine in Langmuir-Biodgett Monolayer Films: Picosecond Time-Resolved Fluorescence Studies Naoto TamaiJ Hiroshi Matwo,$ Tomoko Yamazaki, and Iwao Yamazaki* Department of Chemical Engineering, Faculty of Engineering, Hokkaido University, Sapporo 060, Japan (Received: February 19, 1992; In Final Form: April 20, 1992)
Picasecond timeresolved fluorescence spectra and fluorescence decay curves of N,h”dioctadecyloxacyanine in Langmuir-Blodgett (LB) monolayers have been measured in the dye concentration ranging from 0.02 to 17.3 mol %. At concentration lower than 0.2 mol %, oxacyanine monomers excited at 358 nm show approximately a single exponential decay with a 1.6-nslifetime. As the concentration is increased from 0.2 to 3.0 mol %, the excitation energy relaxation is interpreted in terms of the energy transfer from monomers to dimers. At concentrations higher than 3 mol %, oxacyanine dimers are excited and they undergo quenching at higher aggregates. It is found that oxacyanine in the ground state forms effectively a face-to-face dimer in which the long axis of oxacyanine is parallel and the short axis is perpendicular to the monolayer surfav. The oxacyanine LB monolayer is found to be essentially different in structures and in relaxatioin dynamics from the rhodamine B LB monolayers presented in our previous work (Can.J. Phys. 1990, 68, 1013).
Introduction Special attention has recently been focused on dynamics of the energy transfer and relaxation under restricted molecular geometries.’ Since the pioneering works were reported by Prof. H. Kuhn’s group in the 1960s: the energy relaxation of excited molecules incorporated in two-dimensional membranes has been studied by using LB films on quartz plates3+ and at air-water intcrfaces,1D-12 through the timeresolved fluorescencespectroscopy. The singlet energy relaxation of dye, rhodamine B in monolayers as an example, was interpreted in terms of energy migration among fractal-like distributed sites and energy trapping at higher aggregate~.~*’*~ In another example of a Langmuir-Blodgett (LB) monolayer containing 1 6 41-pyrenyl)hexadecanoicacid, excimer and dimer fluorescences should be considered throughout the relaxation pathways in LB monolayers.* Aggregation of chromophores occurs quite easily in monolayers not only on a quartz plate but also at air/N2-water interface~.~I-’~ Previous studies demonstrated that the excitation energy relaxation in LB films depends strongly on the surface morphology: dispersion of guest chromophores and orientation of chromophores in compressed monolayers. On the other hand, the structures of LB films containing dye molecules have been studied on the basis of the light reflection,14the polarized IR,lS and the total reflection FTIR measurements.16 Further studies are required to get a general picture of the relaxation dynamics of excited molecules in LB monolayers. In this paper, we report the picosecond timeresolved fluorescence study for the excited singlet state of a cyanine dye, N,”-dioctadecyloxacyanine perchlorate (hereafter referred to as oxacyanine) in LB monolayer films. The dye concentration dependence of the energy relaxation dynamics has been examined through the time-resolved fluorescence spectra and fluorescence decay curves. It is shown that the predominant pathway of energy relaxation is the excitation transfer from monomer to dimer sites formed in the ground state. This is in striking contrast to the case of rhodamine B LB films reported previously,’ where the excitation energy migrates among monomers of slightly different energy levels. The intermolecular interaction of dyes in LB monolayers will be discussed for these two cases.
Experimental Section Oxacyanine (Japan Research Institute for Photosensitizing Dyes Co.) was used without further purification. Arachidic acid and methyl arachidate (Wako Pure Chemical Co.) were purified by Present address: Masuhara Microconversion Project, ERATO,JRDC, Kyoto Research Park, Chudoji-minami, Simogyo-ku, Kyoto 600, Japan. *Faculty of Home Economics, Yamaguchi Women’s University. Sakurabatake, Yamaguchi 753, Japan.
repeated recrystallization from alcohol solution. A Milli-Q water purification system (Millipore Co.) was used for the purification of water for the subphase. The conditions of the subphase containing 3 X lo4 M CdC12 were maintained to be at 18 OC and pH 6.5 by adding NaHC03 buffer solution. A schematic illustration of the LB-deposited film is shown in Figure 1. Nonfluorescent quartz substrates were precoated with five layers of cadmium arachidate to make the surface hydrophobic and uniform. A chloroform solution of oxacyanine and arachidic acid/methyl arachidate (1:l molar ratio) was spread on the water subphase in a Langmuir trough (San-Yesu Instrument Co., FSD-20). A monolayer containing oxacyaninewas deposited on a substrate at a surface pressure of 25 mN/m and at a transfer rate of 5 mm/min. The concentration of oxacyanine was changed, ranging from 0.02 to 17.3 mol %. The monolayer surface was further coated with three layers of cadmium arachidate for protection of the film surface. Absorption spectra and fluorescence and excitation spectra (corrected) were measured with Hitachi U-3400 and Spex Fluorolog 2 spectrophotometers, respectively. Time-resolved fluorescence spectra and fluorescence decay curves were measured with a picosecond single-photon timing system using a synchronously-pumped, cavity-dumped dye laser.” A 6-pm microchannel-plate photomultiplier (Hamamatsu R2809U) was used to improve the time resolution of the measuring appanltus.18 Pyridine 1 (Lambda Physik) in benzyl alcohol/glycerol(4:1 volume ratio) was used as a dye laser to produce a sccond harmonic using a KDP crystal. The excitation wavelength is 358 nm.
Results Figure 2 shows the surface pressurearea isotherm of mixtures of oxacyanine and arachidic acid/methyl arachidate (1:l molar ratio) with various concentrations of oxacyanine. Sharp increases of the surface pressure are seen in every sample of different concentrations, indicating formation of compressed monolayers. In higher concentration,however, plateaus appear at low surface pressure, indicating that dye chmophores reorient in an eqmded monolayer from parallel to perpendicular to the surface during compnssion.4 The molecular area of oxBcyaninc in the compressed monolayer was estimated to be 36 f 4 A2,under an assumption that the limiting area of arachidic acid or methyl arachidate is 19.8 A* and these fatty acids cannot occupy the top of the surface of oxacyanine. Absorption spectra of oxacyanine in LB monolayers are shown in Figure 3a, together with the spectrum in dichloromethane solution. The spectra in LB films are essentially the same as that in CH2C12solution. Two absorption maxima appear at 363 and 382 nm in LB films of dye concentration lower than 2 mol %, and at 365 and 380 nm in CH2C12 solution. On increasing the con-
0022-3654/92/2096-6550$03.00/0@ 1992 American Chemical Society
Excitation Energy Relaxation of Oxacyanine in LB Films
The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6551 I
,
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Figure 1. Schematic illustration of the LB film. On a quartz plate, five layers of cadmium arachidate (a), a monolayer containing oxacyanine (b), and three layers of cadmium arachidate (c) are deposited.
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Figure 4. Wavelengths of absorption and fluorescence maxima as a function of oxacyanine concentration. The concentration range is classified into three groups.
4
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arachidic acid
N, N’- dioctsdecyloxacyanine
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Figure 2. Surface prcssure-area isotherms of mixtures of N,N’-dioctadecyloxacyanine(oxacyanine) and arachidicacid/methyl arachidate (1: 1 in molar ratio). Curves a, b, and c are for the concentrations of oxacyanine of 0.58, 11.2, and 17.3 mol W ,respectively.
200
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Wavelength ( nm )
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400
450 500 Wavelength ( nm )
550
Figure 3. (a) Absorption spectra of oxacyanine in LB monolayers (curves 1-5) and in CHICll solution (curve 6). The concentrations of oxacyanine in LB films are (1) 17.3, (2) 5.90, (3) 2.84, and (4) and (5) 1.24 mol W. Spectrum 5 is obtained by extending spectrum 4 by 6.7 times. (b) Fluortscmce spectra of oxacyanine in LB monolayers excited at 355 nm. The concentrationsof oxacyanine are (1) 17.3, (2) 2.84, (3) 0.25, and (4) 0.05 mol 96.
centration of the dye, the first absorption band shifts to the red: 388 nm at 17.3 mol 96. Figure 3b shows fluorescence spectra of LB monolayers with various oxacyanine concentrations. At concentrations lower than 0.1 mol 96, the fluorescence spectrum
-39 400
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400
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- - 29
p5
550
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Figure 5. Normalized time-resolved fluorescence spectra of oxacyanine in LB monolayers excited at 358 nm. The time zero corresponds to the
time in which the excitation laser pulse reaches the maximum intensity. shows a peak a t 412 nm. At the concentration of 0.1-0.4 mol %, the peak shifts to the red, and it constantly locates a t 427 nm in higher concentrations. The peak wavelengths in absorption and fluorescence spectra are plotted in Figure 4 as a function of concentration. It is seen that the fluorescence spectrum shifts sharply between 0.1 and 0.4 mol %, while the absorption spectrum shows no change in the concentration range up to 3 mol %. We assume that the two distinct fluorescence bands with peaks at 412 and 428 nm are due to a monomer and a dimer of oxacyanine, respectively, and that the absorption band a t 382 nm is due to a monomer, and shifts to the red (388 nm) associated with the dimer formation. Then the difference between the curves of absorption and the fluorescence can be explained as due to the fluorescence emission taking place after the energy transfer from the monomer site to the dimers. Another interesting feature of the fluorescence spectrum is the appearance of a sharp band a t 403 nm a t higher concentrations (curve 1 in Figure 3). This band can be ascribed to the fluorescence from the well-known J-aggregate of oxacyanine.2Je22 This band appears in the time-resolved spectra of 17.3 mol 96 just after the pulsed excitation as shown below. Normalized time-resolved fluorescence spectra are shown in Figure 5 for 1.24 and 17.3 mol % oxacyanine. In the spectra of 1.24 mol I, a fluorescence band appears with a peak around 412 nm in an earlier time region. This band corresponds well to the monomer band obtained under steady excitation at concentrations lower than 0.1 mol %. Subsequently, the fluorescence spectrum is changed so that the maximum locates a t 428 nm after 1 ns. The spectrum in the long time region is very similar to the dimer spectrum observed in the steady-excitation spectrum at high concentrations. The dimer fluorescence beecomes dominant after 5 ns. The spectrum of 17.3 mol 7% shows a sharp fluorescence band due to a J-aggregate at 403 nm in the short time region (-39 to -29 ps) as well as the dimer band at 428 nm. The J-aggregate fluorescence decays within 100 ps. Furthermore, a very broad
6552 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 Ions
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Tamai et al.
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Figure 6. Fluorescence maximum wavelength as a function of time, obtained from the time-resolved spectra. The concentrations of oxacyanine are (a) 0.25, (b) 1.24, (c) 2.84, and (d) 17.3 mol %. TABLE I: Ratio of the Number Density of Aggregates to That of Monomers 8od Delay Times (7,") of Changing from the Monomer to the Dimer Fluorescence concn of oxacyanine, [aggregates]/ T ~6 ~ , range I range I1 range 111
1
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mol %
[monomers]'
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0.12 0.25 0.58 1.24 2.84 5.90 11.2 17.3
0.004 0.008 0.02 0.05 0.10 0.21 0.66 1.32
3800 2100 1300 640 55
