Langmuir 1989,5, 1407-1409
x* vi ( K
yi
8 7
+
total electric potential (\E* a*), statvolts relative diffusivity, Di/D3 dimensionless radial coordinate reciprocal Debye length, cm-' relative adsorption capacity, Nt/acio dimensionless time, t/(12/D3) tortuosity factor
1407
Subscripts 0 i j
s
condition in the external bulk solution ionic species ionic species that complexes with the surface of the support condition near the pore surface
Letters Fluorescence Spectral Change of LB Films Containing W-( 1-Pyreny1)alkanoicAcids Induced by an Excimer Laser Akira Itaya,*Vt Hiroshi Masuhara,**+Yoshio Taniguchi,$ and Shuji Imazekis Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Kyoto 606, Japan, and Advanced Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185, Japan Received February 24, 1989. I n Final Form: May 31, 1989 Langmuir-Blodgett (LB) films of a mixture of w-(1-pyreny1)alkanoicand arachidic acids showed fluorescence spectral changes upon irradiation with an excimer laser and a Xe lamp, although absorption spectral change was not observed. This characteristic phenomenon depended upon irradiation light source and the number of methylenes in the alkanoic acids. The fluorescence spectral change induced by laser irradiation was attributed to structural change of fluorescent pyrenyl aggregates. The new aggregates of 8-(1-pyreny1)octanoicacid have the common local structure among vacuum-deposited, cast, and LB films. Introduction Pyrene is the most representative molecule which forms excimer in concentrated solutions and molecular assemblies. Recently, we reported that vacuum-deposited films of o-(1-pyreny1)alkanoic acid (PyC, where n represents the number of carbon atoms in the substituent) gave two kinds of new fluorescence: a weak band at 420 nm and a structured fluorescence band with three peaks in the same wavelength region as that of excimer.lS2 These fluorescence states are metastable and easily modified by weak perturbations. Actually, we have found interesting phenomena of fluorescence spectral changes of these deposited films caused by irradiation with an excimer laser and a Xe lamp.3 A similar phenomenon was also observed for cast films of PYC,.~ These fluorescence spectral changes were confirmed to be attributed to laser annealing, and its molecular aspects were investigated by using fluorescence spectroscopic methoda3s4 The result indicated that the spectral change is due to a structural change
' Kyoto Institute of Technology.
Advanced Research Laboratory.
(1) Taniguchi, Y.; Mitauya, M.; Tamai, N.; Yamazaki, I.; Masuhara, H. Chem. Phys. Lett. 1986,132,512. (2) Mitauya, M.; Kiguchi, M.; Taniguchi, Y.; Masuhara, H. Thin Solid Films 1989,169, 323. (3) Itaya, A.; Kawamura, T.; Masuhara, H.; Taniguchi, Y.; Mitsuya, M. Chem. Phys. Lett. 1987, 133, 235. (4) Itaya, I.; Kawamura, H.; Masuhara, H.; Taniguchi, Y.; Mitauya, M., unpublished results.
of pyrenyl aggregates caused by a thermalization process from higher excited singlet states of pyrenyl chromophores and that the latter states are formed through mutual interactions between excited states and successive multiphoton ab~orption.~ A mixture of PyC, and alkanoic acid such as arachidic acid forms a Langmuir-Blodgett (LB) film. It is a twoand three-dimensional molecular assembly of a thin monolayer or its multilayers which is prepared by transferring a monolayer spread on water surface onto a substrate. While deposited and cast films of PyC, form an amorphous state, which is due to mutual penetration of the long chain of aliphatic acids, LB films have oriented and aligned structure. Utilizing these molecular assemblies, investigations of two- or three-dimensional5 and sequential excitation energy transfer' have been carried out. However, no report has been given for laser annealing of such LB films. Hence, it is very interesting to investigate whether structural change of LB films is induced by laser irradiation in a similar way as for deposited and cast films. We can easily detect structural change by monitoring the change of the fluorescence spectrum. Some LB films are also investigated for practical development of photofunctional materials for optoelectronics (5) Yamazaki, I.; Tamai, N.; Yamazaki, T. J. Phys. Chem. 1987,91, 3572 and references cited therein. (6) Yamazaki, I.; Tamai, N.; Yamazaki, T.; Murakami, A.; Mimuro, M.; Fujita, Y. J. Phys. Chem. 1988,92, 5035.
Q743-7463/89/2405-14Q7$Q1.50/00 1989 American Chemical Society
1408 Langmuir, Vol. 5, No. 6, 1989
Letters
and optics. If laser annealing of LB films is induced quite easily, it may call our attention to applying the laser to LB films. On the contrary, from the standpoint of application of laser annealing phenomena, it may show a possibility of production of new functional organic thin films, as in the case of the laser annealing of silicon. However, very little is known about the investigation of the laser annealing of LB films. Here we report new interesting phenomena of fluorescence spectral changes of the LB films of a mixture of PyC, and arachidic acid induced by laser irradiation.
0
c)
Number of shots
Experimental Procedure Samples PyC,, PyC,, (KSV),PyC,,, PyC,, (MolecularProbes), and arachidic acid (Gasukuro Kogyo) were used as received. A mixture of arachidic acid and PyC, dissolved in chloroform was spread on the surface of the water subphase of pH 6.5 containmol/L). Mixed monolayers were deposited ing CdC1, (4 x on a quartz glass plate under a constant surface pressure of 15 mN/m. The details of film preparation are the same reported before.' Monolayers of pure PyC, were unstable. The stabilities of the monolayers of PyC, were, however, increased by mixing with arachidic acid. The quality of LB films used in this experiment was good. Laser irradiation was performed by using a Lumonics T E 430T-2 excimer laser (308 nm, 6-11s full width a t halfmaximum). The irradiation area was limited to 7 X 11 cm', and the irradiation power was measured with a GENTEC ED 200 power meter. The laser repetition rate was 5 Hz. The irradiation intensity was adjusted by using several pieces of glass plate. Steady-state irradiation was performed by using a 500W Xe lamp (Wacom XD-501s) with a condensing lens, an aqueous solution fiiter (7 cm), and a HOYA HA-50 fiiter. This restricts the excitation wavelength from 310 to 800 nm. Fluctuations of the irradiation intensity were always monitored.
Results and Discussion Figure l a shows fluorescence spectral changes of LB film containing PyC, (PyC,:arachidic acid = 1:5, 15 layers on each side of quartz glass plate) induced by excimer laser irradiation under vacuum. The fluorescence spectrum of the unirradiated LB film consists of monomer and sandwich excimer (around 470 nm) fluorescence, while dimer fluorescence and a weak structureless band at 420 nm are not observed clearly for the present sample. These characteristics are consistent with a detailed investigation of LB film containing P$,, by Yamazaki et al.5 Irradiation with a few tens of laser shots resulted in the decrease of the fluorescence intensity. After irradiation with 35 shots of laser pulse, a shoulder appeared around 445 nm. Upon further irradiation, new vibrationally structured bands with peaks at 445.5,472.5, and 507 nm (shoulder) appeared. While this fluorescence spectral change was observed, the absorption spectrum did not change. After irradiation with 1000 shots of laser pulse, a change of the absorption spectrum became observable. The same behavior was also observed for a LB film with 25 layers. The appearance of t h e new fluorescence bands was observed also under atmosphere. Although the intensity of the new structured fluorescence did not decrease by further irradiation under vacuum, it decreased under atmosphere. Judging from the fact that the absorption spectrum did not change, the decrease of the fluorescence is considered to be attributed to the formation of a nonfluorescent aggregate of pyrenyl chromophores. Concerning the latter aggregation, we reported its important role in the fluorescence behavior of vacuum-deposited films of PYC,.~ Due to the laser irradiation on these films, their fluorescence spectral shape changed, and finally (7) Imazeki, S.; Takeda, M.; Tomioka, Y.; Kakuta, A.; Mukoh, A.; Narahara, T. Thin Solid F i l m 1985, 134,27.
-0 .---. 35 -._. 65
--- 200
1
4 00
I
500 Wavelength I nm
Figure 1. Fluorescence spectral changes of (a) LB film containing PyC,, (b) PyC deposited f i b , (c) PyC, cast film induced by excimer laser irra&ation. Irradiation laser fluence is (a) 9.2, (b) 14.0, and (c) 7.2 mJ/cm2 per pulse. Number of shots
=-!
c .v)
c CJ
c
G CJ lJ
c CJ V
v)
P 0
7
ii 400
5 00 Wavelength I n m
Figure 2. Fluorescence spectral change of LB film containing PyC,, induced by excimer laser irradiation. Irradiation laser fluence is 9.6 mJ/cm2 per pulse.
its intensity decreased. We consider that similar quenching sites are responsible for the present decrease of the fluorescence intensity. On the other hand, LB film containing PyCl0 showed a quite different behavior from LB film containing PyC,. The original fluorescence intensity of LB films containing PyC,, (PyC,,:arachidic acid = 1:5, 15 layers) decreased monotonously by laser irradiation, as is shown in Figure 2. The structured fluorescence did not appear
Langmuir, Vol. 5, No. 6, 1989 1409
Letters Irradiation ti melmin.
120
c c
.-c
Wavelength I nm
Figure 3. Fluorescence spectral change of LB film containing PyC, induced by Xe lamp irradiation. I
Wavelength lnm
Figure 4. Fluorescence spectra of LB film containing PyC,: (a) before laser irradiation, (b) after 300 shots of laser pulse, and (c) a crystallized film formed by melting the laser-irradiated film.
even if the original fluorescence intensity decreased by about one-tenth by irradiation with 700 shots of laser pulse. This monotonous decrease of the original fluorescence intensity was observed also by laser irradiation under atmosphere. The rate of the decrease under atmosphere was larger than that under vacuum. LB films containing PyC,, or Pyc16 showed the same behavior as the case for PyC,,. In the case of irradiation with an intense Xe lamp, the fluorescence intensity of all LB films used in this experiment decreased monotonously. When LB film containing PyC, was irradiated under vacuum, the decrease was little even if irradiation time was long, as is shown in Figure 3. On the ot,her hand, irradiation under atmosphere for 80 min decreased the intensity to one-twentieth of the original one. Under both conditions, an appearance of the structured fluorescenceband was not observed. The fact that the fluorescence change induced by irradiation depends upon the kind of irradiation light source suggests that a multiphoton process plays an important role in the fluorescence spectral change induced by laser irradiation. We can reject a possibility that the new vibrationally structured fluorescence of LB film containing PyC, is ascribed to photoproducts formed during laser irradiation. Figure 4 shows fluorescence spectra of unirradiated film, film irradiated with 300 shots of laser pulse, and crystallized film formed by melting the laser-irradiated film. The film formed by melting did not show the structured fluorescence but normal excimer fluorescence. Thus, the fluorescent species giving the new structured band are considered to be destroyed during the melting process of the irradiated film.
It is worth noting that the new structured fluorescence induced by laser irradiation is quite similar to those of vacuum-deposited and cast films (Figure 1). The spectral change of both vacuum-deposited and cast films was attributed to structural change of pyrenyl aggregates. This phenomenon was induced by a thermalization process from higher excited singlet states of pyrenyl chromophores, which were formed through mutual interactions between excited states and successive multiphoton absorption.'-4 Both vacuum-deposited and cast films consist of PyC, only, and they are amorphous and have no ordered structure. This is because mutual penetration of long aliphatic chains and a hydrogen-bonding network prevent the molecular motion leading to crystallization. On the other hand, LB film is composed of a mixture with arachidic acid and PyC,, and pyrenyl chromophores of PyC, are buried among arachidic acids with long methylene chains. However, PyC, forms aggregates which show dimer and excimer fluores~ence.~ The new structured fluorescence induced by laser irradiation is considered to be attributed to the change of the local structure of the pyrenyl aggregates in LB films. Thus, the similarity of the new fluorescence induced by laser irradiation among cast, vacuum-deposited, and LB films strongly suggests that excimer laser irradiation on these films produces the higher excited singlet state of pyrenyl chromophore, and the thermalization process from it results in the common local structure in pyrenyl aggregates of these films. Pyc6 and Pyc, cast films showed the similar structured fluorescence as described here but not cast films of PyC, and PYC,,.~ In the case of vacuum-deposited films, PyC,, PyC,, and PyC,, systems showed the laserinduced behavior in problem, while PyC,, did not.' These results indicate that molecular motions leading to the new aggregates are more easily induced for PyC, with short methylene chains than for PyC, with long methylene chains. In the present case, LB films containing PyC, showed the structured fluorescence, but LB films containing Pyc,,, Pyc,,, and PYC!6 did not. Since the methylene chain length of arachidic acid is longer than that of PyC, used in this experiment, pyrenyl chromophores of PyC, are buried among methylene chains of arachidic acid. Therefore, because of the bulkiness of pyrenyl chromophores, PyC, with short methylene chains tends to disturb the oriented and aligned structure as compared with PyC, with long methylene chains. That is, the oriented and aligned structure of LB films containing PyC, is weak compared with that of LB films containing PyC,,, PyC,,, and PyC,,. These are the reason why the new aggregate giving the structured fluorescence is formed by laser irradiation for LB film containing PyC,. In conclusion, the change of the local structure of pyrenyl aggregate induced by laser irradiation was made clear for the first time by means of the fluorescence probe method. We consider that a thermalization process from a higher excited singlet state of pyrenyl chromophores is responsible for the present behavior and leads to the common local structure among cast, vacuum-deposited, and LB films.
Acknowledgment. We are indebted to T. Kawamura and S. Takada for help with the experiments. The present work was partly supported by the Grant-in-Aid for Scientific Research on Priority Area for Macromolecular Complexes (63612510)and on Special Project Research for Photochemical Processes (63104007) and the Grantin-Aid for Scientific Research (63430003) from the Japanese Ministry of Education, Science, and Culture.
