376
Langmuir 1993,9, 376-380
Langmuir-Blodgett Films of a Rigidified 'I-Aminocoumarin Derivative and Their Absorption and Emission Properties A. S.Alekseev, T. V. Konforkina, and V. V. Savransky General Physics Institute of the Academy of Sciences of Russia, Vavilov str. 38, 11 7942 Moscow, Russia
M. F. Kovalenko Department of Chemistry, Moscow State University, 117334 Moscow, Russia
A. Jutila and H. Lemmetyinen' Department of Physical Chemistry, University of Helsinki, Meritullinkatu 1 C, 001 70 Helsinki, Finland Received April 28, 1992. In Final Form: December 8, 1992
Different types of LB films of a 7-aminocoumarin derivative bearing a rigidified amino group were prepared and investigated. Absorption spectra showed a shift to the blue range which increased when a monolayer was changed through z- and x-type multilayere to a y-type multilayer structure. Depending on the type of the films,both the absorption and the emission bands were in fresh samplesevidentlymade up of several aggregate componentawith differentrelative portions. The differences decreased with both time and UV irradiation and the absorption bands shifted to the blue and the emission bands to the red, showingan increasing portion of H aggregates in all film types. Finally a large Stokes shift was observed, 166 nm in a y-type structure. Polarized absorption and fluorescence measurementa showed that the absorption transition momenta of the coumarin chromophore were oriented parallel to each other and an angle between the absorption and emission transient momenta was about 63O.
Introduction Coumarin dyes with 7-amino substituents form an important class of fluorescent dyes in the blue-green spectral region. The photophysical and photochemical properties of these dyes are unique because of their strong charge separation in their excited states, which produces an increase in the dipole moments of the molecules. Fluorescence quantum yields, lifetimes, lasing output energies,and solvent-solute interactions have been widely investigated for dye molecules in which rotation of the 7-amino group is It can be assumed that these molecules exhibit a reduced level of multiple fluorescent activitie~,~1~-6 rotatory radiationless decay pathways: and solvent exciplex formation: when compared to molecules which are allowed to rotate freely. An efficient intersystem crossing process has been observed in dyes bearing a rigidified amino The optical and photophysical properties of coumarin dyes with a rigidified amino group have been considerably less studied in the solid state. New ways of investigating these dyes have been made possible with the LangmuirBlodgett technique for preparing highly ordered monomolecular films. The structure of aminocoumarin molecules allows for the formation of complex aggregates in
* Author to whom correspondence should be addressed. --
(1) Rettig, W.; Klock, A. Can. J. Chem. 1986,63, 1649. (2) Jones, G., 11; Jackson, W. R.; Choi, C. J. Am. Chem. SOC.1986,89,
294. _.
(3) Drexhage, K. H. In Topics in Applied Physics; ShSer, F. P., Ed.; Springer: Berlin, 1973; Vol. 1, p 144. (4) Wolfbeis, 0. S.; Rapp, W. Lippert, E. Monatsh. Chem. 1978,109, RQQ ---.
(5) Wolfbeis, 0. S.; Lippert, E. 2. Naturforsch. 1978,33a, 238. (6) Wolfbeis, 0. S.; Lippert, _ _ E.; Schwarz, H. Ber. Bunsen-Ges. Phvs. Chem. 1980,84, 1115. (7) Jones, G., 11; Jackson, W. R.; Choi, C.; Bergmark, W. R. J. Phys. Chem. 1986,89, 294. (8) Maailamani, V.: Saatikumar.. D.:. Nataraian. _ .S.:.Nataraian. - . P. O.D ~ . Commun. 1987,62, 389. (9) sahy~n,M. R. V.; Sharma, D. K. Chem. Phys. Lett. 1992,189,571.
0743-7463/9312409-0376$04.00/0
both the electrical ground and the excited state. Coumarin aggregates in highly structured films may possess several interesting nonlinear optical properties. These considerations make detailed investigations of the abilities of 7-aminocoumarin derivatives to order multilayer structures using the LB technique particularly interesting. In this studywe prepared different multilayer stractures of LB films using a 7-aminocoumarin derivative. The absorption and emission properties of different types of LB films, as well as their kinetics of fluorescence, were investigated.
Experimental Section 7-Aminocoumarin. A coumarin 102 derivative (Figure l), synthesizedby Kirpichonokloespeciallyfor LB film preparation, was used in the investigations. Film Preparation. Special attention was paid to fiiding the optimal conditions for film preparation when investigating the ability of the coumarin molecule to assemble in organized multilayer structures. In order to do this, the speedof the barrier movement,rate of layer deposition,pH values, and temperatures were varied. It was found that in order to obtain a stable monolayer on a water surface, the pH value of the subphase played an important role in the production of reproducible films. This is probably dueto the protonation-deprotonation properties of nitrogen atoms both in the 7-position of the coumarinstructure and in the 3-position of the alkyl chain. Thus, low pH values produce a disturbance in monolayer structures on the water surface. The pH used in experiments was 6.0, which wae adjusted with a buffer solution of disodiumhydrogen phosphatepotassium dihydrogen phosphate. As our attempts to prepare multilayer structures of the coumarindye on a pure quartz substrate were unsuccessful,nine stearic acid layers were transferred onto quartz plates to act ae (10) Kirpichonok, M. A.; Gordeeva, N. A. Khim. Heter. Soed. (in Russian) 1990, 742.
Q 1993 American Chemical Society
Langmuir, Vol. 9, No.2, 1993 377
Letters I
c
bn
'I
m
v, :o-
'3
n
8
2 0 4 0
6
0
8
0
1
Figure 1. The 7-aminocoumarin derivative studied and ita surface pressurearea isotherm.
bottom layers. The quartz plates were cleaned in the usual mannerll in order to obtain hydrophilic surfaces. A KSV 5000 LB Center instrument was used for the LB films preparation. The x-, y-, or z-type depositions were used for preparations of coumarin multilayer structures on quartz plates with stearic acid sublayers. Special features of the instrument, with a two-section trough, allowed us to prepare different types of LB structures. For example, for the z-type deposition the substrate was fist dipped through the section with a pure subphase surface and raised through the coumarin Langmuir monolayer. This procedure was repeated several times. The deposition rates were found experimentally and depended on the type of the structure required. The lowest rate of 5 mm/min was used for the z-type structure, and the maximum rate of up to 80 mm/min was used for x-type deposition. These parameters yielded to values of 0.9-1.0 for the LB film transfer ratios and reproducible film properties. The coumarin molecules in chloroform (0.5-1.0 mg/mL) were spread on the water surface. The water was purified by SeparTec Setion 9OOO equipment. The subphase temperature was 21.0 O C . The pressurearea isoterm is presented in Figure 1. The barrier speed was 10mm/min, and the deposition surface pressure was 35 mN m-l. Spectroscopic Methods. Absorption spectra were recorded using a Shimadzu MPS-2000 spectrophotometer. Fluorescence and excitation spectra were obtained using a Shimadzu RF-2000 spectrofluorometer. Fluorescence decay times were measured by means of an Edinburgh Instrument 199, picosecond time-correlated single photon counting system. For the excitation (295 nm) a synchronously pumped, cavity-dumped dye laser from SpectraPhysics was used (Model 375 with a frequency doubler Model 390, pulse duration of 5 ps). As the pump source a Nd:YAG laser, with a repetition rate of 0.8 MHz and frequency doubled with a KDP crystal, was used. A 6-mm microchannel platephotomultiplier tube (Hamamatau R2809U), a time-to-amplitude converter (Ortec 567),and a constant fraction discriminator (Ortec 935) were used. The instrument response function was measured separately and the fluorescence kinetic curves were analyzed by the nonlinear least-squares method. The UV irradiations were done by using a 150-W xenon lamp and the monochromator of the Shimadzu RF-2000 spectrofluorometer.
Results The Film Structures. The type of the deposition does not always determine the final molecular arrangements. By X-ray diffraction measurements,it has been12J3shown that in the case of fatty acids both the x- and y-type depositionsyield the spacingbetween the hydrophilic head groups nearly the same and equal to twice the length of the hydrocarbon chain. Thus the molecules must rear-
s.
(11) Xu, X.; Era, M.; Tsutsui, T.; Saito, Thin Solid Films 1989,173, 541. (12) Bematein, S. J. Am. Chem. SOC.1938,60,1511. (13) Ehlert, R. C. J. Colloid Sci. 1966,20, 387.
9
82
WAVELENGTH ( n d
Figure 2. Absorption spectra of 7-aminocoumarin of fresh (a) and UV-irradiated old (b) samples: (1) monolayer; (2) z-type structure (fourlayers); (3)x-typestructure (fourlayers); (4) y-type structure (sevenlayers);(5) diluted y-type structure (sevenlayers); (6) chloroform solution. The bottom layers consisted of nine stearic acid layers.
range in the x-type films during or after the deposition. This phenomenon has mainly observed for fatty acids and for the x-deposition, where, in the deposition of the first layer, the interaction between the hydrocarbonchain and the hydrophobicsubstrate is weaker than interactions for they- or z-depositions. Both x- and z-type structureswere obtained for long chain azo dye materials by Blinov et al." by using the method we used, where the solid substrate was moving in one direction through the section with a pure subphase surface and in the opposite direction through the dye Langmuir monolayer. In addition, in the present work the y- and z-type coumarin films were deposited on a stearic acid layer with the hydrophilic surface pointing out from the solid substrate and the x-type films on a stearic acid layer with the hydrophobicsurface pointing out from the solid substrate. It should be noted, however, that the arguments presented above do not rule out the possibility of rearrangementsof monolayers during or after the deposition. More evidence for the film structures can be obtained from optical measurements. Absorption Spectra. Absorption spectra of different types of coumarin LB films, measured immediatelyafter their preparation (referred to as fresh samples) and chloroform solution are shown in Figure 2a. In chloroform solution (c = 0.08 pmol/L) the most intense band at 386 nm corresponds to the transition moment along the long axis (from the 7-nitrogen to the 2-oxygen) of the coumarin molecule.lS In multilayer LB structures the absorption bands are shifted to the blue. It is important to notice that the maxima of the z- and x-type films are at about the same wavelength, but for the y-type film the maximum is more shifted, indicating adifferent film structure. This kind of spectral behavior can be explained by considering the associations of the coumarin molecules in the films. The blue shift of the main band has been assigned to formation of H-type aggregates,16J7where the long axes (14) Blinov, L. M.; Dubinin, N. V.; Mikhnev, L. V.; Yudin, S. G. Thin Solid Films 1984, 120, 161. (15) Kubin, R. F.; Fletcher, A. N. Chem. Phys. Lett. 1985,99,49. (16) Hertz, A. H. Photogr. Sci. Eng. 1974,18,329. (17) Seki, T.; Ichimura, K. J. phy8. Chem. 1990, 94,3769.
Letters
378 Langmuir, Vol. 9,No. 2, 1993 of the chromophores are parallel and the chromophores have a side-by-side interaction. In order to confirm or disprove this arrangement, polarized absorption spectra were measured. It was found that both for horizontal and verticalpolarization to the deposition direction the changes in the band positions and intensities were negligiblysmall. The intensity maximum was obtained at an angle of 45' to the deposition direction. This indicates that the dipole transition moments of the coumarin chromophores were oriented parallel to each other, and their projections to the substrate were at an angle of about 45O to both the horizontal and vertical axis of the substrate. The differences of the absorption spectra of different types of fresh films can be explained by intra- and interlayer interactions between chromophores in the layers. The interaction was strongest in the y-type structure (Figure 2a, curve 4),when the distance between the polar heads of the molecules in adjacent layers was the shortest. In the x- and z-type structures or in the monolayers, only intralayer interactions are possible yielding smaller shifts. The absorption spectrumof the y-structure was different when measured several days after their preparation. The maximum of the main absorption band at about 380 nm was shifted to the shorter wavelength (346 nm), but a shoulder was still observable at about 400 nm. In the absorption spectra (Figure 2a, curve 5 ) of a fresh y-type multilayer of mixed films (30mol % coumarin + 70 mol % stearic acid as a matrix, referred to as the diluted film) and of the z- and x-type multilayers the only changes were decreases in intensities. Thus a self-organization process takes place only in the y-structure due to the interlayer interaction of the chromophores. Drastic changes in absorption spectra of fresh LB films were observed after 10min of UV irradiation (Figure 2b). In all LB structures the maxima were shifted to about 346 nm and the band at about 390nm disappeared. This could be associatedwith the formation of an H-aggregatadstate17 of coumarin molecules. A same kind of phenomenon has been recently reported for LB films of a spiropyran derivative. Thus it can be inferred that fresh structures of 100% coumarin films contain several types of molecular organizations. During the UV irradiation the aggregation form having an absorption band at the longer wavelength is mainly changed to another having the band at 346 nm. The changes are caused by intralayer forces. The new band is sharper but still rounded, indicating an existence of at least two organizations. Fluorescence Spectra. The fluorescence and excitation spectra of the coumarin dye molecules in a chloroform solution, in LB monolayer and in multilayers of different LB structures, measured immediately after preparation, are presented in Figure 3a. For the fresh multilayer structures the maxima of the fluorescencewere shifted 40-60 nm to the red aa compared with the emission band of the molecule in the chloroform solution. After the UV irradiation larger shifts were observed for the all multilayer Structures (Figure 3b). The absorption maxima were shifted to about 350 nm and the positions of the fluorescence maxima depended slightly on the film structure. The emission bands are still rounded, indicating an existence of several organizations. The large shifts may be connected with strong interactions between coumarin molecules in organized aggregated structures. As it was for the absorption spectra, the emission spectra of the x-
and z-type films are about the same (curves 2 and 3 in Figure 3a), but differ from that of the y-type film (curves 4 and 5). It is important to note that after the UV irradiation the Stokes shifts of the y-type LB structures were about 165 nm as compared with that in chloroform solution, where it was about 50 nm (Figures 3, curves 5). A large Stokes shift is typical for the H-aggregate and the short shift for the J-aggregate. Fluorescence Anisotropy. The fluorescence anisotropies of y-, x-, and z-type multilayers as well as x- and z-type monolayers with stearic acid bottom layers were measured. The measurements were doneusingexcitation wavelengths of 360,370,380, and 390 nm and several different emission wavelengths in the range from 420 to 525 nm. In most of these experimenta an incident angle of 30° between the excitation beam and the quartz substrate was used. The y-type film was measured also using an incident angle of 60°,but the obtained anisotropy values were, however, the same. The anisotropies were measured and calculated by the well-known method.'9 All multilayers and monolayers had an average anisotropy value of 0.16. As shown by polarization measurements, the projections of the absorption transition moments form an angle of 45O to the axis of the substrate. Thus it is easy to show1*that the value for the photoselection is 0.2 and an equation r, = 0.2 (3 cos2 a - 1)/2 can be used to calculate the value for the angle a between the absorption and emission dipoles. The angle was 63O. Fluorescence Kinetics. The fluorescence kinetics of the coumarin dye monitored at various wavelengths in
(18) Hibino, J.; Moriyama, K.; Suzuki, M.; Kishimoto, Y. Thin Solid Films 1992,2101211, 562.
(19) Lakowicz,J. R. Principles ofFluorescence Spectroscopy; Plenum Press: New York, 1983; p 112.
I
I
100 500 WAVELENGTH (nm)
t
Figure 3. Fluorescence ( d i d line, at absorption maxima) and excitation (dotted line, A, at emission maxima) spectra of 7-aminocoumarinof fresh (a) and UV irradiated old (b) samples: (1) monolayer; (2) z-type structure (four layers); (3) x-type structure (four layers); (4) y-type structure (seven layers); (5) diluted y-type structure (seven layers);(6) chloroform Solution. The bottom layers consisted of nine stearic acid layers.
Langmuir, Vol. 9, No. 2, 1993 379
Let tera
Table I. Fluorescence Lifetimer and Relative Amplitudes of 7-Aminocoumarinin Chloroform, Monolayer and Multilayer Structures Am=460*10nm A, = 490 10nm Xm=620*10nm hex = 295 hex = 295 nm a, = 295 nm nmlifetimes (ne) and lifetimes (ns) and lifetimes (ne)and re1 amplitudes ( % ) re1 amplitudes (%) re1 amplitudes ( % ) sample chloroform 0.56 2.82 solution (34) (66) monolayer 0.50 2.45
*
z-type film (100%) y-type film (100%) y-type film (diluted) y-type film (100%) after UV y-type film (diluted) after UV
3.26
(44) 3.24 (47) 3.17 (70)
(24) 0.68 (31) 0.63 (28)
(76) 3.24 (55) 3.30 (52)
0.12
(11)
0.70 (30) 0.73 (27)
3.36 (59) 3.22 (62)
0.14 (20)
0.73 (39)
3.38 (41)
0.13 (14)
0.71 (37)
3.43 (49)
(11) 0.10
increased for the longest living component. After UV irradiation, both for the 100% and the diluted y-type structures, the same three components were observed (Table I). Unfortunately, a direct comparison between the relative portions of fresh and irradiated films cannot be done due to the changes in absorption intensities at the excitation wavelength.
0
2
4
6
8
10
TIME (ns)
Figure 4. Fluorescence decay curves of y-type multilayer = 295 nm) of 7-aminocoumarin: (a) structure (eight layers) X, = 460 nm;(b)X, = 520 nm; (c) X, = 560 nm. The excitation profile (d) and the exponential fitting curves are shown.
the range from 460 to 560 nm was nonexponential both in chloroform solution and in different types of LB structures. The kinetics in the chloroform solution could be approximated by a two-exponential response function with lifetimes of 0.6 and 2.8 ns (Table I) showing of some kind aggregationof coumarin molecules in the ground state. This was also observed in a chloroform solution by the emission (Figure 3a) and absorption spectra (Figure 2a), where the bands were rounded. The aggregation of dye molecules in the chloroform solution was due to the nonpolar nature of the solvent molecules. When ethanol was used as a solvent, the decay was single exponential with a lifetime of 1.9 ns. For LB monolayers and for a fresh diluted y-type structure, a two-exponential decay was observed. The kinetics of fresh multilayer LB structures (100% coumarin in z- and y-type films, with the excitation of 295 nm) were more complicated. The fluorescence decay kinetics could be described at different wavelength by three-exponential response functions (Figure 4) with the same lifetimes but with different relative amplitudes (Table I). These decreased for the shortest living component, remained about constant for the middle-aged component, and
Conclusions According to the stationary and time-resolved absorptions and emissionsseveraltypes of aggregateswere present in all structures of the studied films. It ie worth noting three important phenomena to be found in the 7-aminocoumarin LB films studied in the present work. Firstly, the method of the film deposition has a big influence on primary structures of fresh films due to interlayer interactions of the coumarin molecules during the deposition. Secondly, the 7-aminocoumarin molecules are organized with time by interlayer forces between the chromophores in adjacent layers yielding a high degree of intermolecular interactions between the chromophores in same layers. As a result the main absorption band is shifted remarkably to the blue in the spectra of y-types of multilayers, as compared to what is observed for the molecules in solution. Thirdly, under the UV irradiation the intralayer forces between the molecules in excited states organize all types of multilayers in the same manner. The intermolecular interactionscan be quite clearlyseen in the emission spectra. The Stokes shift of the emission band of an organized y-type multilayer is about 166 nm, which is unusual for the emission band of excited monomer molecules. The shiftsfor both the x- and z-typemultilayers are 155 nm and that for the monolayer is about 120 nm. The ground- and excited-state aggregations are evident in all cases. The blue shifts of absorption bands and the red shifte of emission bands for all the structures compared with dye molecules in solution, the same final positions of the absorption band maxima as well 88 the emission maxima after UV irradiation of the samples, together with large Stokes shifts, supported the existence of H-aggregates in multilayer LB structures. The observed fluorescence bands of all LB structures were composed of several emission bands due to the different configurations of chromophores. Using timeresolved fluorescence spectroscopy it was shown, that the emission bands, with the maxima at 520 nm, are made up
380 Langmuir, Vol. 9, No. 2, 1993 of three components with different lifetimes and amplitudes when measured at a wavelength range from 460 to 520 nm. The large Stokes shift supports the notion that the emissions originate from aggregated species. This assumption was confirmed,at least partly, by the polarized absorption measurements, which showed that the absorption transient momenta are oriented parallel to each other and by the fluorescence anisotropy measurementa, which showed, that the angle between the absorption and the emission transient momenta was about 63'. Thus, if the absorption moment aligned along the long axes of the coumarin chromophore corresponds to the lowest excited
Letters
singlet state, then the emission originates from an aggregated system with the emission dipole moment aligned at an angle of 63O to the coumarin plane. Acknowledgment. We thank Professor M.A. Kirpichonok for preparing the molecule studied, Dr. N. V. Tkachenko for his help with the kinetics measurements, and Mr. V. I. Chucharev for his assistance. We gratefully acknowledge the finanical assistance of the Technology Development Centre and the Academy of Finland for their support of our studies into photochemical processes in organic thin films.
