Specific photoresponses of Langmuir-Blodgett films containing organized chromophores

COLLOIDS

AND ELSEVIER

Colloids and Surfaces A: Physicochemicaland EngineeringAspects 123-124 (1997)457 471

A

SURFACES

Specific photoresponses of Langmuir-Blodgett films containing organized chromophores Toshihiko Nagamura Crystalline Films Laboratory, Research Institute of Electronics Shizuoka University 3-5-1 Johoku, Hamamatsu 432, Japan

Received 18 July 1996; accepted 26 July 1996

Abstract

Various redox chromophores were highly organized in Langmuir-Blodgett (LB) films to show specific photoresponses such as colour changes due to photoinduced electron transfer (photoinduced electrochromism), anisotropic orientation of photogenerated radicals, molecular control of photocurrents, and dispersive deactivation of excited triplet states. Colour changes due to the photoinduced electron transfer reaction and molecular control of photogenerated radicals were achieved in LB films. Photoinduced electrochromism of a single monolayer LB film in amphiphilic 4,4'-bipyridinium was successfully detected by the optical waveguide (OWG) method. Transient absorption upon excitation of amphiphilic porphyrin in LB films with a nanosecond laser of 532 nm was also sensitively detected by this method. The excited triplet porphyrin decayed with a dispersive process in LB films indicating inhomogeneous distribution of chromophores. Anisotropic photoconduction and its molecular control were achieved in LB films. Keywords: Amphiphilic porphyrin; 4,4'-Bipyridinium; Dispersive deactivation of excited triplet state; Langmui~ Blodgett films; Optical waveguide (OWG) method; Photocurrents; Photoinduced electrochromism; Redox chromophores

1. Introduction

Organized molecular assemblies containing redox chromophores show specific and useful photoresponses which cannot be achieved in randomly dispersed systems. Ideal examples of such highly functional molecular assemblies can be found in nature in photosynthesis and vision. As shown by these elegant models, molecular design of functional compounds and molecular control of their arrangements will be essential to construct highly efficient and functional photoresponsive systems. Langmuir-Blodgett (LB) deposition is one of the best methods to prepare highly organized molecular thin films, in which various molecular parameters such as distance, orientation, extent of

chromophore interaction, or redox potential can be controlled in each monolayer. We have been studying photophysical and photochemical properties of LB films in order to construct molecular electronic and photonic devices in the near future [-1-38]. As schematically shown in Fig. 1, we have achieved various specific photoresponses in organized molecular assemblies including LB films [-1 38]. Those responses achieved in LB films utilizing the amphiphilic compounds and polymers shown together with their abbreviations in Fig. 2, include colour changes due to photoinduced electron transfer (photoinduced electrochromism) and control of molecular orientation of photogenerated radicals [-2-8], molar fraction dependent efficient energy transfer and amplified quenching

0927-7757/97/$17.00 Copyright© 1997 ElsevierScienceB.V. All rights reserved PH S0927-7757(96) 03779-X

458

T. Nagamura / Colloids"SutJaces A." Physicochem. Eng. A,spects 123 124 (1997) 457 471

chemical reactions I photochromism photopolymerization

Organized molecular assemblies (LB films, polymer films, supramolecules, silica, ) [electron, hole[ photoinduced

photochemical control of nonlinear optical properties optical amplification and switching highly amplified fluorescence quenching absorption and fluorescence spectral shift

electrochromism

ultrafast (fs) photon-mode recording and optical switching anisotropic photoconduction molecular control of photocurrents Fig. 1. Schematic representation of specific photoresponses of organized molecular assemblies including LB films.

of fluorescence [9 12], transient and steady photocurrents and their molecular control [13 19], the in-plane and out-of-plane orientation of various redox chromophores [20-26], and sensitive detection of colour changes and dispersive deactivation of excited triplet states in LB films [27]. In this article, several examples of photoresponsive LB films containing organized redox chromophores and their sensitive detection will be presented.

2. Photoinduced electrochromism and optical waveguide detection of photoresponses in LB films Various photochromic systems employing polymeric thin films or LB films have recently attracted much interest in view of their promising applicability to high-speed and high-density photonmode optical memory. The photochromism reported so far involves changes of chemical bonds such as heterolytic cleavage of a pyran ring in spiropyrans or cis-trans isomerization in azobenzenes. Recently we have reported novel photochromism (photoinduced electrochromism) between pale yellow and blue in various microenvironments including LB films [2-8], organic solutions [28,29], microcrystals [30,31], and polymer films [32-35] which was due only to the photoinduced electron

transfer reaction via the excited state of specific ion-pair charge-transfer (IPCT) complexes [36-38] of 4,4'-bipyridinium salts with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate [39] (abbreviated to TFPB ). The photochemical colouring and thermal fading due to the reverse electron transfer were highly reversible in all systems in a deaerated atmosphere [2-8,28-35]. The lifetime of the coloured (blue) state was found to depend markedly on the microenvironments and temperatures. From steady and laser flash photolysis results it has been strongly suggested that 4,4'-bipyridinium radical cations, released from the geminate reaction immediately after the photoinduced electron transfer upon IPCT excitation, became metastable owing to the bulk and chemical stability ofTFPB-, to the restriction of molecular motion by the microenvironment, and also probably to the very high exothermicity of the reverse reaction in the Marcus inverted region [40]. The photoinduced electron transfer upon IPTC excitation was found to occur very rapidly in a pulse width of a picosecond laser [41] or even of a femtosecond laser [42]. Highly sensitive detection of photoinduced electrochromism in ultra-thin LB and polymer films has also been achieved by the optical waveguide method [7,8]. In this section photoinduced electrochromism, sensitive detection of steady and

T. Nagamura / Colloids SurJdces A." Physicochem. Eng. Aspects 123 124 (1997) 457 471

R1

459

ZnTTOP RI=Rz-

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Fig. 2. The structure and abbreviations of the compounds used in the present experiment.

transient photoresponses, and molecular control of orientation of photogenerated radicals in LB films will be discussed.

2.1. Reversible photoinduced radical formation and molecular control of radical orientation in LB films TFPB salts of N,N'-dihexadecyl-4,4'-bipyridinium (HV2+) and N-ethyl-N'-(2-ethylamide)N',N'-dihexadecyl-4,4'-bipyridinium (AV 2+) were prepared from corresponding bromide salts and Na + T F P B . Monolayer properties of several mixtures of AA with T F P B - salts of HV 2 + or AV 2 + were studied on an aqueous subphase containing 0.25 mM CdC12 and 0.05 mM NaHCO3, pH 6.3,

at 18°C. LB films were deposited at 20 m N m 1 and 18°C on a quartz plate for UV/vis or on a poly(ethylene terephthalate) film for ESR and small angle X-ray diffraction measurements from 1:1 and 4:1 mixtures of AA and H V 2 + ( T F P B - ) 2 or AVZ+(TFPB-)2. The deposition ratio was almost unity during 30 deposition cycles for all mixed monolayers. For steady photolysis these samples were irradiated in the degassed condition by a Hamamatsu 150 W Xe-Hg lamp equipped with a Toshiba L-39 cut off filter (,~e×> 365 nm) and an IR-cut filter to excite their IPCT absorption band alone. The incident angle dependences of both sand p-polarized absorption for 4,4'-bipyridinium radical cations were measured in degassed con-

72 Nagamura / Colloids" Surfaces A." Physicochem. Eng. Aspects 123-124 (1997) 457 471

460

ditions together with the polarization angle dependence at normal incidence. The ~ A isotherms are shown in Fig. 3 for three mixtures of AVZ+(TFPB-)2 and AA, which exhibited several transitions before forming a solid condensed phase. A similar ~-A isotherm was observed for a mixture of HV 2+ (18.8%) and AA [3,4]. The apparent limiting area observed at each transition for mixtures of AV2+(TFPB )2 and AA corresponded well with the calculated values based on the molecular area of T F P B (1.4nm 2) and 4,4'-bipyridinium ion (0.82nm 2) for a stepwise squeezing-out of 4,4'-bipyridinium ion and TFPB , which does not dissolve in water, as schematically shown in the inset of Fig. 3. From an X-ray analysis on a single crystal of N,N'-dimethyl-4,4'-bipyridinium tetraphenylborate (TPB-), Moody et al. [43] reported that N,N'-dimethyl-4,4'-bipyridinium ion was sandwiched between two T P B - ions. IPCT complexes of TFPB salts were expected to have similar configuration from several spectroscopic studies and MOPAC calculations [28,31,37]. Such a structure of IPCT complexes corresponds well to that schematically shown in the inset (A) of Fig. 3 based on the limiting area. AVZ+(TFPB )2 and AA systems showed larger molecular areas than HV2+( TFPB )2 and AA systems in all corre-

"T

E z60

1

E

~40

m L/1 i1J

• 20

0=0

i..i//../~//

sponding mixtures. This result may reflect the different orientation of 4,4'-bipyridinium ions as mentioned below. Upon irradiation of an IPCT band in the degassed condition (Z~,> 365 nm), the colour of both LB films changed from pale yellow to blue. The UV/vis absorption spectrum after irradiation is shown in Fig. 4, which is characteristic of a 4,4'-bipyridinium radical cation monomer [44]. Coloured species photogenerated in mixed LB films of AV2+(TFPB-)2/AA or HV2+(TFPB-)2/AA systems decayed reversibly and almost exponentially in the dark in vacuo with a lifetime of about 4 h at 20°C [2,3]. The lifetime of 4,4'-bipyridinium radical cations in LB films was almost the same as that in microcrystalline films [30], which indicated the microenvironment around photogenerated radical cations in LB films is similar to that in microcrystals. Such photochemical colouring and thermal fading was repeated reversibly. A broad single line ESR spectrum was observed upon irradiation of both LB films, whereas well-resolved hyperfine structures were observed for the same 4,4'-bipyridinium salts irradiated in solutions. Polarized absorption spectra of photogenerated 4,4'-bipyridinium radical cations were measured in vacuo for LB films of HVZ+(TFPB-)2/AA and AV2+(TFPB-)2/AA systems as a function of polarization angle and incident angle. The thermal decay of radicals during measurements of polarized

S

/ z" /

5

0q

i

i

02.

i

I

I

[

I

I

x 4

./3

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I

0

,

i

i

,

|l

\

\

\

i

0.4 0.6 0.8 Molecular area /nm 2

4,...

1.0 300

Fig. 3. The rc A isotherms for mixtures of AA with AV 2+ at a molar fraction of (a) 0.10, (b) 0.188, (c) 0.50 at pH 6.3 and 18°C. The inset shows the schematic representation of surface monolayers during the compression process ( C ) ~ ( B ) ~ (A). The circle and rectangle represent the T F P B and 4,4'-bipyridinium groups of AV 2+, respectively.

~,*~"

, , ..,--'"' . . . . .

400 500 600 Wavelength /rim

•

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700

Fig. 4. Absorption spectrum of mixed LB films (60 x 2) of HV 2 ÷ and AA (1:4) after excitation (> 365 nm) in the degassed condition at 20°C for 10 rain using non-irradiated LB films as a reference.

22 Nagamura / ColloMsSmfaces A. Physicochem. Eng. Aspects 123 124 (1997) 457 471 absorption spectra was corrected by their lifetime [ 2 - 5 ] . The different optical path length in the incident angle dependence measurements was also corrected from an apparent incident angle dependence of s-polarized absorption in a way similar to that used for amphiphilic porphyrin [21,22]. No polarization angle dependences were observed at normal incidence in either LB film. The incident angle dependences of p-polarized absorption of 4,4'-bipyridinium radical cations at 400 nm, which corresponds to the short-axis transition, are shown in Fig. 5 for (a) H V 2 + ( T F P B )2(18.8%)/AA and (b) A V i + ( T F P B )2(18.8%)/AA. Fig. 5 shows a minimum absorbance in H V i + ( T F P B )2/AA and a maximum in A V i + ( T F P B )2/AA at normal incidence. The solid lines in Fig. 5 are calculated by the least squares method taking the angle (~b) distribution of the transition dipole moments with respect to the surface normal into account. The best fit curves gave the following values of ¢; 4 5 c < ~ b < 4 6 ° for H V 2 + ( T F P B - ) e / A A and HV/AA

.25 C

l

I

I

I

I

I

I

,

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/

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-60

i

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.25

I

I

uC .20 m

~ .15 0 m

.a .10 .05 0 -9(

_6~0 ,

, ~ i I -30 0 30 60 90 Incident angle, f/degrees

Fig. 5. The p-polarized absorbance of 4,4'-bipyridinium radical cations in LB films at 400 nm after correction for the decay and optical path length for photoexcited systems:(a) HV2+/AA; {b) AV2+/AA. The solid lines are calculated dependences.

461

89' < ¢ < 90 ~ for AV > ( T F P B - ) 2 / A A systems, respectively. Similar incident angle dependences were observed at 614 nm which is due to a longaxis transition of 4,4'-bipyridinium radical cations. From these results and the simulation of angular dependences, it was shown that both the long and short axes of 4,4'-bipyridinium radical cations lay almost fiat in LB films o f A V 2 + ( T E P B )2/AA and inclined by about 46 ° to the substrate surface in LB films of H v e + ( T F P B - ) e / A A as schematically shown in Fig. 6. These results demonstrated that the molecular orientation of photogenerated radicals was controlled in LB films by the structure of substituents in the 4,4'-bipyridinium ions. 2.2. Highly sensitive detection of photoinduced electrochromism and transient species in ultra-thin .fihns by the optical waveguide method It is very interesting and important to observe colour changes in LB films of a single monolayer or a few monolayers thick in the context of a sensing application with fast response, studying dynamics of photoinduced reactions between organized chromophores, or the easy preparation of good quality LB films. We have applied an optical waveguide (OWG) method for such purposes [-7,8]. The electric fields of light propagating through the O W G layer have an exponentially decreasing value as evanescent waves beyond the surface of the OWG. Evanescent waves have been used to sensitively detect and characterize adsorbates and thin films on the O W G . Thin films (S) deposited on the surface of the O W G were degassed by a rotary pump in a small chamber and irradiated with a X e - H g lamp through appropriate filters (2ex > 365 rim) as shown schematically in Fig. 7. A linearly polarized H e - N e laser (632.8nm) was used as a monitor light, since photogenerated 4,4'-bipyridinium radical cations show adsorption at this wavelength as shown in Fig. 4. A 150-fold better sensitivity of the O W G method as compared with the conventional normal incidence method was demonstrated from the colour change measurement in an approximately 180nm thick film of p v e + ( T F P B )2 containing 4,4'-bipyridinium groups as part of the main chain. The absorbances calculated from the O W G signal

T. Nagamura /' Colloids Surfaces A: Physicochem. Eng. Aspects 123-124 (1997) 457 471

462

\

< <

\

1 a

x

Fig. 6. Schematic representation of the orientation of 4,4'-bipyridinium radical cations in: (a} HVZ+/'AA; (b) AVZ+/AA LB films. Counter anions (TFPB } and AA are not shown for simplicity.

to v a c u u m line

M

,,

,

C L ~']iM""

at 632.8 nm, using the absorbance before irradiation as a reference, are plotted in Fig. 8 against the irradiation time for pV2+(TFPB-)2 thin films of (a) 10.0, (b) 40.4, (c) 64.9, (d) 95.5, and (e) 179.6 nm. It is clearly seen that the number of photogenerated 4,4'-bipyridinium radical cations increased linearly with irradiation time. The rate of absorbance change was proportonal to the film thickness in the range studied. These results strongly suggested

,,--'

,0 u C

.4

e

.3 0 I/1

.2

d c

.1

b a

0

0

1000

2000

3000

4000

Time / s

Fig. 7. Schematic representation of the O W G system for detecting photoinduced electrochromism of ultra-thin films (S) in an evacuation chamber shown in the inset.

Fig. 8. Changes in the OWG absorbance of PVZ+(TFPB -)2 thin films of various thicknesses during IPCT excitation in the degassed condition: (a) 10.0; (b) 40.4; (c) 64.9; (d) 95.5; (e) 179,6 nm,

T Nagamura/Colloids Surfaces A: Physicochem. Eng. Aspects 123-124 (1997) 45~471

that all p V 2 + ( T F P B - ) 2 thin films are homogeneous and that 4,4'-bipyridinium groups are distributed randomly throughout the polymer films. Photoinduced colour change in a singlemonolayer LB film was successfully detected as shown in Fig. 9 for the H V 2+ ( T F P B - ) 2 / A A system [8]. Comparison of this result with Fig. 4, observed with a conventional normal incidence method, demonstrated again that the sensitivity of the O W G method is more than 120 times higher. Changes of O W G absorbance are also shown for Y-type LB films deposited on glass slides covered with three monolayers of cadmium arachidate; (b) 2, (c) 4, and (d) 6 monolayers. The absorbance changes increased with the number of monolayers deposited. In contrast with the almost linear increase in absorbance of polymer films as shown in Fig. 8, the absorbances in LB films tended to saturate at longer irradiation time. The "saturated" absorbances increased almost proportionally to the number of monolayers. Similar results were obtained for LB films of AVZ+(TFPB-)z/AA systems. In LB films the 4,4'-bipyridinium ions are not distributed randomly but are confined to, and aligned in, a layer a few angstrom thick periodically distributed in the direction of the surface normal. The long spacing of LB films of H V 2 + / A A was evaluated as 55 A by small angle X-ray scattering [2,4]. Such structural properties and much smaller

.4

.3

O m .13

2L -1

C

b 0

-0~ ' : ' '

1200

2/,00

3600

Time / s

Fig. 9. Changes in the OWG absorbance of LB films of HV2+/AA (1:4) with various numbers of monolayers during IPCT excitation in the degassed condition: (a) 1; (b) 2; (c) 4; (d) 6 monolayers.

463

thickness of LB films most probably contributed to the "saturation" tendency shown in Fig. 9. Tetraphenyl-type amphiphilic porphyrin (ZnPC4AB) and zinc tetratolylporphyrin ( Z n T T O P ) were used for O W G detection of photogenerated transient species in LB and polymer films. LB films of ZnPC4AB in a 1:5 mixture with 2C18NB were deposited at 3 0 m N m -~ and 23°C as polyion complexes with dextran subphase on a hydrophobic surface of the OWG, treated with a silane coupling agent [27]. For the kinetics measurements in LB films, 2C18NB and a 1:5 mixture of ZnPC4AB and 2C18NB were alternately deposited to form heterogeneous Y-type LB films in order to avoid face-to-face deactivation of excited porphyrins. Polymer thin films containing Z n T T O P with a 200:1 weight ratio were deposited on the surface of the O W G by spin coating from chloroform solutions at various concentrations. The thickness of spin-coated polymer films was determined by the Surface Plasmon Resonance (SPR) method. These films were degassed by a rotary pump in a small chamber in a similar manner to that shown in Fig. 7 and irradiated with the second harmonics of a nanosecond pulsed Nd : YAG laser (532.0 nm, 10 ns, 2 10 mJ per pulse) from a surface normal direction. The pulsed laser beam was focused to a 2 mm x 20 mm area with convex and concave cylindrical lenses to excite thin films between two coupling 45 ° LaSF8 ( n = 1.8785) prisms. No damage to samples was observed by repeated excitation at 532 nm. An Ar + laser (457.9, 476.5, 488.0, 496.5, 514.5nm), H e - N e laser (632.8 nm), and diode lasers (670-830 nm) were used as probe lights. Then intensity of the O W G signal was detected with a photomultiplier (Hamamatsu Photonics R928) through appropriate filters to cut the 532 nm pulse and was recorded with a digital storage oscilloscope (SonyTektronix TDS540A, 8-bit A/D) with or without averaging. The transient absorbance (A Absorbance) was calculated from the O W G signals after laser excitation relative to the average signal level before excitation. Transient absorption spectra of ZnPC4AB in methanol solution (50 pM) upon nanosecond laser excitation were observed with a streak camera (Hamamatus Photonics C2830S). The absorption spectra of ultra-thin films

Z Nagamura / Colloids" Surfaces A" Pl ysicochem Eng. Aspects 123 124 (1997) 457 471

464

prepared on quartz plates were recorded by a JASCO Ubest-55 spectrophotometer with an accuracy of +0.001 absorbance unit. LB films containing ZnPC4AB with 2-24 monolayers one each side of a substrate were prepared. The absorbance of these LB films at the excitation laser wavelength (532 nm) ranged from about 0.002 to 0.020. No transient absorption was observed even in LB films with 24 x 2 monolayers by the conventional laser flash photolysis mainly due to the extremely small optical path length (total thickness is about 120 nm). Recently Tran-Thi and coworkers [45,46] reported transient absorption measurements in LB films containing heterodimers of cationic porphyrin and anionic phthalocyanine upon excitation with nanosecond and subpicosecond pulsed lasers at 532-565 nm. They prepared LB films with 150-400 monolayers on each side of a substrate and observed transient absorption or photobleaching with maximum absorbance changes of only about 0.02-0.08. In the O W G measurement, excited chromophores in LB films were detected only on one side of the substrate. An example of the time dependence of transient absorbance is shown in Fig. 10 for LB films with 16 monolayers monitored at 476.5 nm upon a single shot laser excitation at 532 nm. It is rather noisy but shows clearly the photogeneration and decay of transient species, excited triplet porphyrins as mentioned below, in LB films. This result indicated much increased sensitivity of the present O W G method in timeresolved measurements as compared with the con-

J

~

~

ventional method. The signal-to-noise (S/N) ratio of transient absorbance was improved considerably by averaging as shown in Fig. 11 on a logarithmic scale for the same LB sample monitored at 457.9 nm after 10 laser shots. The decay shown in Fig. 11 followed neither first order nor second order kinetics. This was due to the dispersive process as a result of the change of rate constants over time, which indicated that the microenvironment and/or the intermolecular interaction of excited porphyrins are not identical in LB films. The time dependence in such dispersive processes is known to be expressed by

I(t)=Ioexp(-kU)

t

c

40

(1)

with the dispersion parameter c~ being a measure of the deviation from pure exponential decay. The value was estimated to be 0.2 and 0.5 for Y-type LB films with 1 : 5 and 1 : 50 mixtures of ZnPC4AB and 2C 18NB, respectively. The sensitivity of transient O W G measurements will be affected by various parameters such as the molar extinction coefficient and quantum yield of transient species, dark noises of detector and amplifier, or fluctuation of the probe laser. The dependence of the transient initial absorbance upon nanosecond laser excitation on the wavelength of probe light is shown in Fig. 12 together with a transient absorption spectrum of ZnPC4AB in methanol solution 50 gs after excitation. They corresponded very well with each other, which indicated that the transient absorption detected by the O W G method in LB films is attributed to the

-i

O

x

(0<~<1)

i

i

200

300

ID {J ¢.. 0~

20

O

0.018

kI

o

0

..Q

<

.,~

-20 0

50 Time/~

10o

15o

s

Fig. 10. Time dependence of transient absorbance for LB films with 16 monolayers monitored at 476.5 nm following a single shot laser excitation at 532 nm.

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0

100

400

Time/gs Fig. II. Logarithmic plot of averaged transient absorbance for LB films with 16 monolayers monitored at 457.9 nm after 10 shots of laser excitation at 532 nm.

T. Nagamura / Colloids Surfaces A." Physicochem. Eng. Aspects 123 124 (1997) 457 471

0.6

'

'

,

u.~u

G.I

r-

0.4

lo/ )

0.2

< "~

0.0

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I ~

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600

0.10 ~"

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Wavelength/nm Fig. 12. Wavelength dependences of transient absorption in LB films by the O W G method (O) and in methanol ( --- ) following nanosecond laser excitation at 532 nm.

excited triplet porphyrin [-47]. The quantum yield of the excited triplet is very high, 0.90 for zinc tetraphenyl-type porphyrins [-47]. This indicates that the sensitivity for this system becomes higher at the wavelengths of the Ar + laser than at those of H e - N e or diode lasers due to the increased molar extinction coefficient of the excited triplet porphyrin [47]. The transient absorbances monitored at 457.9 and 476.5 nm are plotted against the number of monolayers in Fig. 13 which showed almost a linear dependence. The minimum transient absorbance to be detected in the present system with the 8 bit A/D is 1.7 x 10 -3. For the excited triplet of amphiphilic prophyrin ( 16.7 mol.%) in mixed LB films, the detection limits were two monolayers as shown in Fig. 13.

465

In ultra-thin polymer films (820 nm thick) containing Z n T T O P with a weight ratio of 200:1, similar transient absorption was observed upon nanosecond laser excitation at 532 nm. The decay behaviour and the lifetime z of excited triplet porphyrins shown in Fig. 14 for a 820 nm thick polymer film, which showed a single exponential decay with ~ = 37 ms, were very different from those in LB films as shown in Fig. 11. The lifetime of excited triplet porphyrins in polymer films was more than 20 times longer than in solutions, most probably due to the homogeneous dispersion and restriction of molecular motions in a P M M A matrix which is in a glassy state at room temperature. The decay behaviour did not depend on the film thickness. The ground state absorbance of an about 160nm thick polymer film ( Z n T T O P : P M M A = 1:200) is almost the same as that of LB films with two monolayers (ZnPC4AB : 2C18NB = 1 : 5), whereas the observed initial transient O W G absorbance of the former is more than 10 times higher than in LB films. This is most probably due to the short lifetime of excited triplet porphyrins in LB films in which porphyrins are distributed inhomogeneously and in different microenvironments as schematically shown in Fig. 15.

3. Photoelectric responses in LB films

The LB technique is relevant to fabricating various molecular electronics or photonics devices.

8 0 x l 0 -3

(3

0 l/)

1

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< <

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0.01 I

5

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115

ZO

Number of layers Fig. 13. The transient absorbances monitored at (closed circles) 457.9 and (open circles) 476.5 nm in LB films plotted against the number of monolayers.

-20

,liil, 0

20

40

60

80

Time/ms Fig. 14. Logarithmic plot of transient O W G absorbance at 476.5nm of porphyrins in P M M A films (1:200) following nanosecond laser excitation at 532 nm.

466

T. Nagamura / Colloids Surfaces A: Physicochem. Eng. Aspects 123 124 (1997) 45~471

porp,hyrins

C

O

I AlternateY-typeLB films I

I Polymerfilms }

Fig. 15. Schematic representations of porphyrins in (A) LB films and (B) P M M A films. The circles show porphyrins. The polymer chains are not shown for simplicity.

Among possible applications of these devices, the storage or processing of information using excess electrons at the molecular layer is very interesting. To achieve this, control of slow interlayer and rapid intralayer electron transfer is required. LB films containing redox chromophores work as a kind of organic multi-quantum well. Several reports have recently been published on transient photoelectric responses in LB films [ 14-17,48-56]. The mechanism of electron transfer in LB films will be analysed at the molecular level from such measurements. The rate of electron transfer is controlled by the distance, orientation of chromophores or height of multiple-quantum wells. In this section the control of photocarrier conduction in LB films by incorporating redox chromophores and their interactions will be discussed.

3.1. Control of photocarrier conduction in LB films by redox chromophores We have already reported transient photocurrents in LB films consisting of monolayers of

amphiphilic porphyrin (AMP) and monolayers of long-chain fatty acids [14]. From this result and steady-state photocurrent measurements it was shown that most photocarriers generated from excited prophyrins decayed in adjacent monolayers of fatty acids. Transient and steady photoelectric responses of LB films composed of AMP and redox chromophores such as 1,3dehexadecylalloxazine (DHA) or 11-(9-carbazolyl)undecanoic acid (CUA) were studied to control photocarrier conduction at the molecular level [15 17]. A M P was mixed with AA in several molar ratios. Five to seven mixed monolayers were deposited at 20 or 50 mN m - 1 on an ITO plate at 18 °C to form stable Y-type LB films by a vertical dipping method at a speed of 8 mm min- 1 with a deposition ratio of unity. Aluminium was carefully evaporated onto the LB films as a sandwich-type electrode at 10 - 6 Torr. The effective junction area was about 0.2 cm 2. These LB films were set in a cryostat and evacuated by a rotary pump until dark currents became less than 10 pA. They were irradiated by a 150 W Xe lamp through an IR-cut

"12Nagamura / Colloids Surfaces A. Physicochem. Eng. Aspects 123-124 (1997) 457 471

filter, an L-39 filter, and a monochromator. Steady photocurrents were measured by an electrometer with or without bias voltages on the top A1 electrode. The action spectra of photocurrents were normalized at the incident irradiance of 3 W m-Z by measuring the light intensity at each wavelength with a photodiode in a way similar to that previously reported [2-6]. On five monolayers of a 1 : 4 mixture of AMP and AA were deposited two additional monolayers of DHA or palmitic acid (PA) alone, mixtures of CUA and (PA) with a fraction of chromophores, f~=0.02-0.25, or a mixture of DHA and AA with f~=0.17, as schematically shown in Fig. 16 for five AMP/AA and two CUA/PA (f~=0.05) monolayers with base ITO and top A1 electrodes. These LB films are abbreviated to PCUA0, PCUA2, PCUA5, PCUA25, PD0, PD17, PD100, respectively, corresponding to the molar percentage of CUA or DHA. These LB films set in a cryostat were irradiated by a 150 W lamp through appropriate filters and a monochromator. Transient photocurrents upon excitation with a nanosecond dye laser were measured with a 2-CH digital memory through a fast current amplifier. The action spectra of steady photocurrents corresponded well with the absorption spectrum

\ \

467

of AMP in LB films for all systems at about 350-700 nm except PD17 and PD100 which gave additional photocurrents below about 425nm due to alloxazine chromophores [15,16]. Steady photocurrents in LB films containing redox chromophores depended on the nature and molar fractions of chromophores. They decreased in the order of PCUA5 > PCUA25 > PCUA2 > PCUA0 in LB films containing carbazolyl chromophores [ 16,17]. Meanwhile, they increased with increasing molar fraction of DHA in the case of LB films containing alloxazine derivatives [15,17]. Transient photocurrents upon excitation of AMP in LB films with a pulsed dye laser showed a fast rise controlled by the time resolution (4-6 ps) of the current amplifier and much slower decay in all LB films. No transient photocurrents were observed for LB films of PA or AA alone. The dependences of transient photocurrents on the wavelength of the dye laser corresponded well with the absorption spectrum (Q-band) of AMP. These results clearly indicated that the observed transient photocurrents were due to the movement of photocarriers generated by the excitation of porphyrin in LB films. The decay was found to depend on the bias voltage, and the nature and fraction of chromophores in mixed monolayers adjacent to porphyrin LB monolayers. Logarithmic plots of transient photocurrents are shown in Fig. 17 for short circuit photocurrents in (a) PD100, (b) PD17, and (c) PD0

77Y \

/// / / / / / / / / /

11

/// ///

base 1To,

/ / / / / /

\

/ / / /// / / / / / /

top Al

/// \ \

/ / / /

/ / / /

/ / / /

a C .D L,.

C ~-

~7 _

..2.2_ ~._.

o

/// \

/ / / 11/

31

Fig. 16. Schematic representation of LB films composed of charge-generating A M P and charge-transporting redox monolayers sandwiched between two electrodes.

'

,

0.25

0.5 Time / ms

0.75'

Fig. 17. Logarithmic plots of short circuit transient photocurrents for: (a) PD100; (b) PD17; (c) P D 0 LB films excited at 525 n m with a pulsed laser.

468

Z Nagamura / CoUoids Smfaces A: Physicochem. Eng. Aspects 123 124 (1997) 457 471

systems, all of which showed an exponential decay. The decay became slower by incorporating DHA in a cadmium arachidate matrix. In the PD100 system which contained two monolayers of pure DHA, the decay became markedly retarded. With increasing bias voltages, the decay became slower and the extent of changes depended on the molar fraction of DHA. Similar results were obtained for LB films containing CUA. These results cannot be explained by a simple tunneling mechanism in which no decay of carriers was assumed before reaching the electrodes [14,15]. They strongly suggest that most photocarriers disappeared or are trapped during transfer processes. As long as the relaxation time required for tunneling conduction, estimated to be 1 10 ms for arachidate monolayers or their equivalents [57,58], is longer than the "lifetime" of the photocarriers determined by various decay processes, the apparent lifetime observed will increase as the probability of capture of photocarriers decreases. The increase of lifetime with bias voltages can be explained qualitatively by assuming that the increased velocity of photocarriers at high bias voltage decreased the capture probability of the photocarriers by recombination sites or "traps" [14]. Such decay of photocarriers observed as transient photocurrents was shown to control the steady photocurrents [14]. The incorporation of redox chromophores in LB films composed of AA or PA without changing the thickness of the monolayers increased the lifetime of photocarriers as shown in Fig. 17. Redox chromophores are thus concluded to effectively retard the decay of photocarriers during their transfer processes presumably by a superexchange mechanism which works in long distance electron (hole) transfer in rigid matrices or in biological systems [59-61]. It is based on wave function propagation via nearest-neighbour exchange interactions. Aromatic chromophores containing K-electrons as a bridge have been reported to facilitate the electron transfer reaction by a superexchange mechanism between donors and acceptors linked by them [62]. Redox chromophores in LB films placed between the carrier-generating porphyrin layer and electrode are expected to cause the transfer of photocarriers by through-bond electronic exchange interactions

and to form traps that are much less "deep" than would be expected in long-chain fatty acid monolayers alone.

3.2. Control of photoelectric properties by a supermonomolecular structure The mixtures of AMP and AA showed two solid condensed phases above and below about 30 mN m -1 [21,23]. A loosely stacked structure of two porphyrins was proposed for LB films prepared at surface pressures higher than 30 mN m- 1 as schematically shown in Fig. 18, caused by the squeezing-out of a monomolecular structure formed at lower surface pressure [21,23]. The dependence of steady photocurrents on the excitation wavelength corresponded well with the absorption

I@D@ H !

!

a

4 b Fig. 18. Schematic representation of LB films containing A M P deposited at: (a) 20; (b) 50 mN m ~.

72 Nagamura / Colloids' Sur/iwes A: Physicochem. Eng. Aspects 123 124 (1997) 457-471

spectra of A M P in LB films independently of the molar ratio or the surface pressure of deposition. Much higher photocurrents were observed at the same bias voltage in LB films deposited at 50 mN m -~ (film A) than in those deposited at 20 mN m -~ (film B) [18]. Fig. 19 shows bias voltage dependences of the ratio of steady photocurrents at 560 nm for film A and film B, I5o/I2o, deposited from 1 : 5 and 1 : 10 mixtures. The ratio was almost independent of the mixing ratio and was much higher than the absorbance ratio; 1.86 and 1.45 for 1:5 and 1:10 mixtures, respectively [21,23]. It decreased with increasing bias voltages above 0.2 V and showed an asymptotic value of about 11 at higher bias voltages. The ratio of steady photocurrents in two LB films deposited at 50 and 20 mN m-~ is proportional to the ratio of intralayer (rp~l) and interlayer (rpe0 tunneling times based on anisotropic intermolecular tunneling in LB films proposed by Donovan et al. [55] who took into account the intralayer bimolecular recombination of photocarriers and the interlayer tunneling conduction; I50/120 = const('CpalSO/Zper50)(A20/A50)/('Epal20/'Cper20)

(2) where Azo/Aso is the ratio of molecular areas of AMP and is two for the present mixtures from the ~-A isotherm [21,23]. The suffixes, 50 and 20, denote the surface pressure of deposition for film

401 - - - 35

I

I

--I

469

A and film B. Eq. (2) holds for the recombination of electrons and holes as a result of the intralayer diffusion occurring simultaneously with the interlayer transfer. The value of rpal will be independent of electric fields, since porphyrin rings lie almost flat in both LB films [21,23]. The bias voltage dependence of 15o/12o is then proportional to 2"per20/"Cper50. From the results in Fig. 19, the perpendicular intermolecular tunneling rate (Zpe~5o)in LB films prepared from a solid condensed phase at 5 0 m N m ~ is thus shown to be much larger than that of LB films prepared from a solid condensed phase at 20 mN m - 1. The value of Z'h-1 hig in LB films containing a loosely stacked pair can be estimated by the intermolecular tunneling rate from a porphyrin pair p to a porphyrin pair q as schematically shown in Fig. 20. The value of qow -1 can be estimated by the intermolecular tunneling rate for monomolecularly dispersed porphyrins in LB films prepared at lower surface pressures. The observed value of "Cper20/'Cper50at higher bias voltages is estimated to be 5.5 from the asymptotic value in Fig. 19 and Eq. (2) as mentioned above. The calculated values of 271ow/Z'high for porphyrins in adjacent hydrophilic regions and at an average lateral distance in 1:5 and 1:10 mixtures corresponded well with the observed value of 2"per20/'Cper50 at higher bias voltages. This result indicates that the average distribution of porphyrins in LB films contributes to

+

a0~ O

//

z /

"B 25

/ ~ /

//

Lr~ //

/

/

~

/ /

,

20 /

15 10 -1.0

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o.o

0.5

1.

/

/

/ / - /

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/

/

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Applied bias voltage I V pair p Fig. 19. The ratio of steady photocurrents for LB films of 1 : 5 (closed circles) and 1:10 (open circles) mixtures deposited at 50 m N m-~ to those at 20 m N m - 1 as a function of applied bias voltage.

Fig. 20. Schematic representation of the intermolecular tunneling interaction from a pair p to a pair q in LB films containing loosely stacked porphyrins (ellipses).

470

z Nagamura / Colloids"Surfaces"A: Physicochem. Eng. Aspects 123-124 (1997) 457-471

the p h o t o c o n d u c t i o n at higher bias voltages where intermolecular tunneling of photocarriers occurs rather easily because of a decreased tunneling barrier, as suggested by the Poole-Frenkel theory. Meanwhile, the observed value of Tper20/Tper50 at lower bias voltages corresponds with the calculated values of Zlow/Zhigh for porphyrins in adjacent hydrophilic regions and in nearest neighbour arrangements in a plane. This result strongly suggests that only porphyrins located much closer than the average distribution value can contribute to the p h o t o c o n d u c t i o n at lower bias voltages, probably due to a large intermolecular tunneling time because of the much lower barrier height. The present model of intermolecular tunneling is thus supported for photocurrents in LB films deposited from two solid condensed phases with different molar fractions of amphiphilic porphyrins which were loosely stacked or monomolecularly dispersed, depending on the surface pressure. The supermonomolecular structure of amphiphilic porphyrin in LB films is concluded to greatly facilitate intermolecular photoconduction.

4. Conclusion Stable radical formation with remarkable and reversible colour changes were observed upon excitation of 4,4'-bipyridinium I P C T complexes. Molecular orientation of photogenerated radicals was controlled by substituents in the LB films. M u c h larger steady photocurrents were observed in supermonomolecular LB films containing loosely stacked amphiphilic porphyrins prepared at surface pressures higher than 30 m N m - 1. F r o m comparison of observed and calculated ratios of photocurrents in LB films, with different molar fractions of amphiphilic porphyrins deposited from two solid condensed phases, it was concluded that the loosely stacked porphyrin pairs in LB films prepared at higher surface pressures contributed a great deal to the p h o t o c o n d u c t i o n controlled by intermolecular tunneling. The present study demonstrated that the photoelectric characteristics of LB films can be controlled by the molecular organization. The O W G m e t h o d was shown to be very useful not only in steady photolysis, but also

in nanosecond laser flash photolysis. It will become possible to discuss the dynamics even in a single m o n o l a y e r LB films by the present O W G m e t h o d with a time resolution of about 100 ps if higher molar extinction coefficient and q u a n t u m yield of transient species, higher resolution of A / D conversion, and lower dark noise are achieved.

References [1] T. Nagamura, in T. Kajiyama and M. Aizawa (Eds.), New Developments on Construction and Functions of Organic Thin Films, Elsevier, (1996) 247. [2] 1". Nagamura and K. Sakai, J. Chem. Soc., Chem. Commun., (1988) 1035. [3] T. Nagamura and K. Sakai, Thin Solid Films, 179 (1989) 375. [4] T. Nagamura, Y. Isoda and K. Sakai, J. Chem. Soc., Chem. Commun., (1990) 703. [5] T. Nagamura, Y. Isoda, K. Sakai and T. Ogawa, Thin Solid Films, 210/211 (1992) 617. [6] T. Nagamura, Mol. Cryst. Liq. Cryst., 224 (1993) 75. [7] T. Nagamura, H. Sakaguchi, K. Suzuki, C. Mochizuki and K. Sasaki, J. Photopolym. Sci. Technol., 6 (1993) 133. [8] T. Nagamura, H. Sakaguchi, K. Sasaki, C. Mochizuki and K. Suzuki, Thin Solid Films, 243 (1994) 660. [9] T. Nagamura, K. Toyozawa, S. Kamata and T. Ogawa, Thin Solid Films, 178 (1989) 399. [ 10] T. Nagamura, S. Kamata, K. Toyozawa and T. Ogawa, Bet. Bunsenges. Phys. Chem., 94 (1990) 87. [11] S. Kamata and T. Nagamura, J. Photopolym. Sci. Technol., 7 (1994) 133. [12] T. Nagamura, K. Toyozawa and S. Kamata, Colloid Surfaces, 102 (1995) 31. [13] T. Nagamura, K. Matano and T. Ogawa, Bet. Bunsenges. Phys. Chem., 91 (1987) 759. [14] T. Nagamura, K. Matano and T. Ogawa, J. Phys. Chem., 91 (1987) 2019. [15] T. Nagamura, S. Kamata, K. Sakai, K. Matano and T. Ogawa, Thin Solid Films, 179 (1989) 293. [16] T. Nagamura, K. Toyozawa, S. Kamata and T. Ogawa, Thin Solid Films, 210/211 (1992) 160. [17] T. Nagamura, S. Kamata, K. Toyozawa and T. Ogawa, Mol. Cryst. Liq. Cryst., 227 (1993) 171. [18] S. Kamata and T. Nagamura, Colloid Surfaces, 103 (1995) 257. [19] T. Nagamura, Hyomen, 28 (1990) 1. [20] T. Nagamura, K. Kamata and T. Ogawa, Nippon Kagaku Kaishi, (1987) 2090. [21] T. Nagamura, T. Koga and T. Ogawa, Denki Kagaku, 57 (1989) 1223. [22] T. Nagamura and S. Kamata, J. Photochem. Photobiol. A: Chem., 55 (1990) 187.

T. Nagamura / Colloids SurJitces A: Physicochem. Eng. Aspects 123 124 (1997) 457 471 [23] T. Nagamura, T. Koga and T. Ogawa, J. Photochem. Photobiol. A: Chem., 66 (1992) 119. [24] T. Nagamura, S. Kamata, K. Toyozawa and T. Ogawa, Ber. Bunsenges. Phys. Chem., 96 (1992) 74. [25] T. Nagamura, T. Fujita, M. Takasaka, H. Takeshita, A. Mori and T. Nagao, Thin Solid Films, 243 (1994) 602. [26] T. Koga, T. Nagamura and T. Ogawa, Thin Solid Films, 243 (1994) 606. [27] T. Nagamura, D. Kuroyanagi, K. Sasaki and H. Sakaguchi, SPIE Proc., 2547 (1995) 320. [28] T. Nagamura and K. Sakai, J. Chem. Soc., Faraday Trans. 1, 84 (1988) 3529. [29] T. Nagamura and S. Muta, J. Photopolym. Sci. Technol., 4 (1991) 55. [30] T. Nagamura and K. Sakai, J. Chem. Soc., Chem. Commun., (1986) 810. [31] T. Nagamura and K. Sakai, Ber. Bunsenges. Phys. Chem., 93 (1989) 1432. [32] T. Nagamura, Mol. Cryst. Liq. Cryst., 224 (1993) 75. [33] T. Nagamura and Y. Isoda, J. Chem. Soc., Chem. Commun., (1991) 72. [-34] T. Nagamura, Polym. Int., 27 (1992) 125. [35] T. Nagamura, S. Muta and K. Sakai, J. Photopolym. Sci. Technol., 5 (1992) 561. [36] T. Nagamura and K. Sakai, Chem. Phys. Lett., 141 (1987) 553. [37] T. Nagamura and K. Sakai, Ber. Bunsenges. Phys. Chem., 92 (1988) 707. [38] T. Nagamura, S. Muta and K. Shiratori, Chem. Phys. Lett., 238 (1995) 353. [39] H. Nishida, N. Takada, M. Yoshimura, T. Sonoda and H. Kobayashi, Bull. Chem. Soc. Jpn., 57 (1984) 2600. [40] R.A. Marcus, J. Chem. Phys., 24 (1956) 966. [ 4 l ] T. Nagamura, H. Sakaguchi, S. Muta and T. Ito, Appl. Phys. Lett., 63 (1993) 2762. [42] T. Nagamura, H. Sakaguchi and S. Muta, SPIE Proc., 2514 (1995) 241. [43] G.J. Moody, R.K. Owusu, A.M.Z. Slawin, N. Spencer, J.F. Stoddart, J.D.R. Thomas and D.J. Williams, Angew. Chem. Int. Ed. Engl., 26 (1987) 890. [44] E.M. Kosower and J.L. Cotter, J. Am. Chem. Soc., 86 (1964) 5524.

471

[45] T.H. Tran-Thi, J.F. Lipskier, D. Houde, C. P6pin, R. Langlois and S. Palacin, J. Chem. Soc., Faraday Trans., 88 (1992) 2529. [46] T.H. Tran-Thi, J.F. Lipskier, M. Simoes and S. Palacin, Thin Solid Films, 210/211 (1992) 150. [47] K. Kalyanasundarum and M. Neumann-Spallart, J. Phys. Chem., 86 (1981) 5163. [48] F. Willig, R. Eichberger, K. Bitterling, W.S. Durfee. W. Storck and M. Van der Auweraer, Ber. Bunsenges. Phys. Chem., 91 (1987) 869. [49] J.K. Severn, R.V. Sudiwala and E.G. Wilson, Thin Solid Films, 160 (1988) 171. [50] P.E. Burrows, K.J. Donovan and E.G. Wilson, Thin Solid Films, 179 (1989) 129 [51] K.J. Donovan, R.V. Sudiwala and E.G. Wilson, Mol. Cryst. Liq. Cryst., t94 ( 1991 ) 337. [52] K.J. Donovan, R. Paradiso, K. Scott, R.V. Sudiwala, E.G. Wilson, R. Bonnet, R.F. Wilkins, D.A. Batzel, T.R. Clark and M.E. Kenny, Thin Solid Films, 210/211 (1992) 253. [53] K.J. Donovan, R.V. Sudiwala and E.G. Wilson, Thin Solid Films, 210/211 (1992) 271. [54] K.J. Donovan, K. Scott, R.V. Sudiwala, E.G. Wilson, R. Bonnett, R.F. Wilkins, T.R. Clark, D.A. Batzel and M.E. Kenny, Thin Solid Films, 232 (1993) 110. [55] K.J. Donovan, K. Scott, R.V. Sudiwala, E.G. Wilson, R. Bonnett, R.F. Wilkins, R. Paradiso, T.R. Clark, D.A. Batzel and M.E. Kenny, Thin Solid Films, 244 (1993) 923. [56] K.J. Donovan, R. Paradiso, R.F. Wilkins, D.A. Batzel, T.R. Clark and M.E. Kenny, Thin Solid Films, 244 (1994) 92. [57] M. Sugi, K. Nembach, D. M6bius and H. Kuhn, Solid State Commun., 15 (1974) 1867. [58] H. Kuhn, J. Photochem., 10 (1979) 111. [59] H.J. McConnel, J. Chem. Phys., 35 (1961) 508, 515. [60] J.R. Miller and J.V. Beitz, J. Chem. Phys., 74 (1981) 6746. [61] M.E. Michel-Beyerle, M. Plato, J. Deisenhofer, H. Michel, M. Bixon and J. Jortner, Biochim. Biophys. Acta, 932 (1988) 52. [62] H. Heitele and M.E. Michel-Beyerle, J. Am. Chem. Soc., 107 (1985) 8286.