J. Phys. Chem. 1993,97, 12862-12869
12862
Orientation Change of Porphyrin in Langmuir-Blodgett Films Caused by a Trigger Molecule Reiko Azumi,. Mutsuyoshi Matsumoto, and Yasujiro Kawabata National Institute of Materials and Chemical Research, Higashi, Tsukuba, Ibaraki 305, Japan
Shin-ichi Kuroda and Michio Sugi Electrotechnical Laboratory, Umezono, Tsukuba, Ibaraki 305, Japan
Lionel G. King CSIRO Division of Food Science and Technology, P. 0. Box 52 North Ryde, New South Wales 21 13, Australia
Maxwell J. Crossley School of Chemistry, The University of Sydney, New South Wales 2006, Australia Received: June 16, 1993; In Final Form: August 16, 1993"
The orientation of the functional molecule PM, tetrakis(3,5-di-t-butylphenyl)porphinatocopper(II), was controlled by adding a small amount of the trigger molecule, n-hexatriacontane ( C H ~ ( C H Z ) ~ & Hin~the ) mixed LangmuirBlodgett (LB) films with cadmium icosanoate. The orientation of P M was investigated by using the electron spin resonance (ESR) and UV/vis absorption spectroscopies for mixed LB films with the molar mixing ratio PM/cadmium icosanoate/n-hexatriacontane = 1.5/10/x in the range 0 I x I5 . The ESR spectra showed that there were two P M components, having different orientation, in each of the LB films. Without n-hexatriacontane, the major component had an orientation tilted with the most probable angle 00 = 5 8 O between the plane normal of the molecule and the film surface, while the macrocycle plane of the major component was oriented almost vertically (00 = 80°) with respect to the film surface for the LB film with the mixing ratio x = 0.5 of n-hexatriacontane. This mixing ratio means that the number of n-hexatriacontane molecules can be as small as one-third the number of P M molecules to control the orientation of PM. Polarized UV/vis absorption spectra indicated that doubly degenerated transition dipole moments of the Soret band of the porphyrin give rise to the two absorption bands due to the aggregation of molecules. They are J-like and H-like bands resulting from the interaction between the in-plane (parallel to the film surface) and out-of-plane (tilted with respect to the film surface) components of transition dipole moments, respectively. The dichroic ratio ASIA,of each band obtained by using UV/vis polarized light a t the incident angle 4 5 O is well explained by taking into account the molecular orientation determined with the ESR measurements. Study of thedependence of the mixing ratio x of n-hexatriacontane showed that there is a transition in the orientation of P M from the 00 = 58' to the 00 = 80° state in such a manner that the population of the former species decreases with increasing x and becomes indiscernible a t x = 0.5.The X-ray diffraction analyses and I R absorption spectroscopy showed that the orientation of cadmium icosanoate is not affected significantly by the presence of n-hexatriacontane, suggesting that the orientation of P M is controlled through the molecular interaction with n-hexatriacontane. At x = 2, the orientation of P M is slightly different from the one a t x = 0.5 probably due to the change in the structure of the LB film caused by the introduction of an excess amount of the nonamphiphilic molecule, n-hexatriacontane,
Introduction Porphyrin derivatives have been extensively studied from the viewpoint of modeling electron-transfer processes in biological systems.' The important parameters of electron-transfer processes are the distance, relative orientation between the donor and acceptor, symmetry and energetic match of the molecular orbitals involved, and the polarity of the medium. In this respect, it is interesting to fix the position and the orientation of porphyrins by incorporating them into molecular assemblies.24 The Langmuir-Blodgett (LB) method has been widely studied as one of the most versatile techniques for fabricating organic thin films with well controlled compositions, structures, and Due to these features the LB technique has been used to construct prototypes of molecular electronic devices.9-18 To realize molecular electronics or molecular devices, one of the key issues is how to handle and assemble the functional molecules *Abstract published in Advance ACS Abstracts. November 1, 1993.
0022-3654/93/2097-12862$04.00/0
in a desired manner, especially when different molecules with different functions are assembled to fabricate the devices. Typical LB films consist of amphiphilic molecules with long alkyl chains. The alternative is to employ lightly-substituted molecules or molecules without long alkyl chains, which enables dense packing of functional molecules in the films.Is23 This strategy makes the orientation of the chromophores different from that obtained for the usual LB films of amphiphilic molecules. Such nonamphiphilic molecules have been mixed with filmforming molecules like fatty acids to obtain good LB films.2C26 A great deal of effort has been made to control the orientation of functional molecules in LB films. Chemical modification seems to be very efficient in this respect. The orientation of anthraquinone has been controlled by changing the position of the attached alkyl chains.2' Physical methods have also been employed for this purpose. The orientation of 7,7,8,8-tetracyanoquinodimethane(TCNQ) has been controlled by the subphase temperature from which monolayers are transferred, and the lateral conductivity of the film is affected by this change in the 0 1993 American Chemical Society
Orientation Change of Porphyrin in LB Films
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12863
Figure 1. Molecular structure of PM.
orientation of TCNQ.** The structure of LB films depends on the surface pressure for the monolayer transfer when a phase transition takes place during c o m p r e s s i ~ n . ~ ~ . ~ ~ We have reported that a small amount of long chain n-alkane, when added in the preparation of monolayers, can drastically change theorientation of a porphyrin molecule in the LB films.” This means that by using a trigger molecule, which in this case is a long chain n-alkane, we can control the orientation of the functional molecules in the LB films. In this report we will show how the structure of the LB film changes with the addition of the trigger molecule. This is done by elucidating the orientation of the porphyrin molecule in the LB films by using electron spin resonance (ESR) spectroscopy and the polarized UV/vis absorption spectroscopy.
Area per
/ nmz
Abs(v) = K1/[1 +4((~-K2)/K3)’] +K4/[1 +4((~-K5)/K6)’] (1)
Experimental Section Tetrakis( 3,5-di-t-butylphenyl)porphmatocopper(II), PM3Z(Figure l), was synthesized by slight modification of the literature synthesis.33J4 Icosanoic acid (Eastman Kodak) and n-hexatriacontane (CH3(CHz)&H3, Wako Chemicals) were purchased and used without further purification. A chloroform solution of molar ratio PM/icosanoic acidlnhexatriacontane = 1.5/1O/x (0 5 x I5 ) was spread onto an aqueous subphase of pH 6.0, containing 4.0 X 10-4 M of CdClz and 5.0 X 10-5 M of KHCO3 a t 17 OC. Icosanoic acid was converted to cadmium salt (C20) on this subphase. All the monolayer experiments were performed at 17 OC on a Lauda Filmwaage. The monolayers were transferred onto a solid substrate by the vertical dipping method at a surface pressure of 25 mN m-I. The dipping speed was 10 and 15 mm min-’ for upward and downward strokes, respectively. A poly(ethy1ene terephthalate) sheet precoated with five monolayers of cadmium icosanoate was used as a substrate for electron spin resonance (ESR) measurements,35 a quartz plate hydrophobized with l , l , 13,3,3-hexamethyldisilazanefor UV/vis absorption spectroscopy, a glass plate for X-ray diffraction measurements, and a CaF2 plate for IR absorption spectroscopy. The transfer ratio around unity was obtained for any of the substrates. The X-band ESR spectra were obtained with a Varian E-4 ESR spectrometer at room temperature. A 40-layered film was placed in the ESR cavity with the film surface perpendicular or parallel to the external magnetic field. TheUV/vis absorption spectra were obtained for a four-layered LB film and an n-hexane solution, with a JASCO HSSP-1 UV/ vis spectrophotometer. To investigate the in-plane anisotropy, the LB films were mounted at an incident angle of Oo, using polarized light with the electric vector parallel or perpendicular to the dipping direction. For the out-of-plane anisotropy, the films were mounted at an incident angle of 45’ using p- and s-polarized light with the electric vectors parallel and perpendicular to the incident plane. The curve fitting of the absorption spectra was performed for the 45O-incident polarized spectra using the program IGOR (Wavemetrics) on a Macintosh IIci as follows. (1) The abscissa of each spectrum is reduced to a wavenumber unit in the region of (22-27) X lo3 cm-1 (370-454 nm). (2) Each absorption spectrum was assumed to be expressed as a sum of two Lorentzian peaks or two Gaussian peaks as follows:
C20
Figure 2. Surface pressure-area isotherms of the mixed monolayerswith PM/cadmium icosanoateln-hexatriacontane= 1.5/10/x: (a) x = 0; (b) x = 0.5; (c) x = 2.0.
Abs(v) = K1 exp(-(v - K2)2/K3) + K4 exp(-(v - K5)2/K6) (2) where v represents a wavenumber. The parameters Kl-K6 were determined by the least-squares fitting method. (3) The values Kl-K6 were optimized for the p-polarized spectra. (4) The values K1 and K4 were optimized for the s-polarized spectra using the values K2, K3, K5, and K6 obtained for the corresponding p-polarized spectra. ( 5 ) The values K1 and K4 were used to obtain the dichoric ratio ASIA,of each peak. Polarized IR absorption spectra of the 41-layered LB films with and without n-hexatriacontane were measured a t an incident angle 45O using a Shimadzu FTIR-4000, X-ray diffraction patterns were also obtained for the 41 -layered LB films with and without n-hexatriacontane using a Philips PW 1800.
Results Surface PressureArea Isotherms of the Mixed LB Films. Monolayer measurements give an average vlaue of the area occupied by P M a t the air-water interface. Figure 2 shows the surface pressure-area isotherms of the mixed monolayers with and without n-hexatriacontane. The limiting area per PM molecule is estimated to be ca. 1.1 nmz for the mixed monolayer without n-hexatriacontane, which is consistent with the molecular size of PM roughly expected from the space-filling model.36 The change in structure of the LB film with an increasing amount of n-hexatriacontane is reflected in the occupied area at the air-water interface. Figure 3 shows the limiting area and the area at 25 mN m-1 per cadmium icosanoate as a function of the mixing ratio of n-hexatriacontane, x. Both of the areas show similar behaviors: almost constant up to ca. x = 1 and linearly increasing above that point. The constant values below x = 1 indicate that n-hexatriacontane does not contribute significantly to the observed areas, suggesting that n-hexatriacontane fills the vacant space present in the LB films. Subsequent increases in the areas suggest that such vacant space for the accommodation of n-hexatriacontane is not available for the films with x > 1. ElectronSpin ResonanceSpectroscopy. The ESR Spectroscopy is a powerful tool for elucidating the structure of the LB films especially for investigating the orientation and angular distribution of paramagnetic species in the LB films.2*3593743One of the advantages of this technique is that we can obtain the distribution
Azumi et al.
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993
0.42
TABLE I: Optimized Parameters Employed in the Simulation of ESR Spectra
-
0 0.5 2.0
0.40-
0.77 0.91 0.87
58 80 80
14 12 15
20 30 30
10 10 10
#Error: &5O.
1
I
0
2
1
3
5
4
6
mixing value x
Figure 3. Limiting area (m) and the area at 25 mN m-’
( 0 )per cadmium icosanoate as a function of the mixing ratio of n-hexatriacontane, x .
function of the orientation of the probe molecule compared with other spectroscopic techniques such as polarized UV/vis and I R spectroscopies which give only the averagevalue of the orientation of the probe molecule. For the planar copper(I1) complexes such as porphyrins or phthalocyanines, the hyperfine interaction of the unpaired 3d electron of copper ion with its own nucleus causes the signal to split into four lines.4 This hyperfine coupling as well as the g value are highly anisotropic due to the fourcoordinated ligandation. The gvalue and the hyperfine coupling constant are expressed as follows: g(cp)’
= g,,’cos’
A(+,)* =
cos’
cp
+ g,’
cp
+ A,* sin2 cp
sin’ cp
(3) (4)
where cp is the angle of the external magnetic field with respect to the normal of the macrocycle plane, gll and gl are the gvalues when the external magnetic field is parallel and perpendicular to the axial direction, respectively, and All and A , are the corresponding coupling constants. For usual copper porphyrins, these values are obtained as follows: A , 3 X 10-3cm-1, gl 2.18, All 2 X cm-l, and41 2.05.4 These values do not vary so much by substitution at the peripheral positions. Such features have been used to determine the orientation of planar copper complexes with respect to an external magnetic field. Figure 4a shows the spectra of the mixed LB film without n-alkane, with the external magnetic field perpendicular (HI) and parallel (Hll) to the film surface. The out-of-plane anisotropy is clearly observed. In the HI spectrum the hyperfine splitting is smaller than that in the Hll spectrum and the signal moves to a higher field, indicating that the macrocycle has a certain orientational order with respect to the surface normal of the film. The small peak located in the low-field region in the HIspectrum is assigned to another component lying rather flat on the film
- -
-
-
surface, as isshown below. Nosuperhyperfinestnrctureassociatbd with four pyrrole nitrogens was observed due to the high concentrationof thecopper comple~.4**~~ Nor was thereobserved any AM = f2 transition arising from the magnetic dipole coupling between the neighboring copper ions.46~~~ The orientation of PM molecule in the LB film without n-hexatriacontane is elucidated by simulating both the HI and the Hll spectra. The simulation was performed according to the previously described procedure,3- assuming the angular distribution function is expressed as follows:
P(0) = t[exp{-sin2(0 - 6,)/2 sin’ So) 6,)/2 sin2 So)]
+ exp(-sin2(0 +
- 0,)/2 sin‘ 6,) + exp{-sin’(B + 0 ~ sin’2 s,)] ( 5 )
+ (1 - t)[exp{-sin2(6
where 6 represents the angle between the surface normal of the film and the plane normal of the porphyrin ring, 00 is the angle for the most probable orientation, and bo is the distribution parameter. This distribution function takes into account that there is no in-plane anisotropy observed in the polarized UV/vis absorption spectra shown later. The second term of the righthand side of this equation represents another component with the most probable angle and the distribution 1 3 ~ . (1) In the simulation, an isotropic line width of the Lorentzian line shape was assumed. (2) The difference of the hyperfine constants of 63Cuand 65Cu as the second-order shift of the hyperfine splitting was neglected for simplicity. The best values of the parameters in eq 5 are summarized in Table I. The simulated spectra agree well with the experimentally observed spectra shown by a dashed line in Figure 4, when we assume a certain amount of species with the macrocycle plane lying rather flat on the film surface, which corresponds to the second term of the right-hand side of eq 5. These results show that there are two components of PM in the LB film without n-hexatriacontane: the major one with 00 = 58O and the minor one with 61 = 20°. This simulation also gives the optimized values A I 2.6 X 10-3 cm-1, g, 2.1835,All 2.16 X 1P2 cm-I, gl 2.0365. The simulation assuming a one-component system ( t = 1 in eq 5) or a two-component system in which the second component has a random orientation did not give good results.
--
-
-
Mpre 4. First derivative ESR spectra of the LB film (40 layers on both sides) at the angles of Oo (HI) and 90’ (Hll) between the external magnetic field and the film surface. Dashed lines represent simulated spectra: (a) x = 0; (b) x = 0.5.
Orientation Change of Porphyrin in LB Films
V
The Journal of Physical Chemistry, Vol. 97, No. 49, I993
0.4
t
I
0.0 350
AV
x=2.0 ..
"I/------
- v
200G
Figwe 5. First derivative HI ESR spectra of the LB film (40layers on
both sides) with varying value x.
The spectra of the LB film containing n-hexatriacontane (Figure 4b) is quite different from that for the LB film without n-hexatriacontane (Figure 4a). The All component is hardly observed in the HI spectrum, and the signal is almost a singlet with unresolved hyperfine splitting. On the other hand, the splitting due to the All component is clearly observed in the Hi1 spectrum. The results of the simulation are also summarized in Table I. The distinct features are as follows: (1) ThevalueofBoforthemajor component isca. 80°, indicating the orientation of the macrocycle is almost perpendicular with respect to the film surface. (2) The value of r is larger than that of the LB film without n-hexatriacontane,showing the dominance of the first component. That is, the higher regularity of the film can be attained by adding n-hexatriacontane. (3) The value of 01 is 30°, indicating the presence of the minor component. It should be noticed that this orientation change starts with a small amount of n-hexatriacontane. Figure 5 shows the change of the shape of the HI spectrum with varying value of x . The change in orientation is already recognized at the value as small as x = 0.1. In the range 0.1 I x I O S , the intensity of the hyperfinecomponent decreases without changing its position and the edge-on component increases with the increasing value of x. These results suggest that, aside from the flat-on species, there is a transition in the orientation of the porphyrin from the Bo = 58" state to the Bo = 80" state in such a manner that the fractions of the two species change: the fraction of the latter species increases with increasing x, and the fraction of the former species is indiscernible at x = 0.5. These results will not be explained by a model in which the value of 00 gradually increases with increasing x since such a model requires the values of g and A tochange gradually, as shown in eqs 3 and 4, which is not observed in the present case. At x = 2.0, however, the signal is slightly different from the one at x = 0.5. The simulation of the spectra shows that in the LB film with x = 2.0, the distribution of the angle of the major component (6o/deg) is larger, and the amount of the major component (t) is smaller than that with x = 0.5 (Table I). It is probably due to the change in the structure of the LB film caused by the introduction of an excess amount of nonamphiphilic molecule (n-hexatriacontane). U V / V i Absorption Spectroscopy. The orientation of porphyrins in the LB films has been investigated by using polarized UV/vis absorption spectroscopy at oblique incidence. The Soret band of porphyrins is assigned to two degenerate transition moments which are perpendicular to each other in a monomeric
12865
400
450
500
550
600
Wavelength / nm
Figure 6. Absorption spectra of the mixed LB film (four layers on both sides) with x = 0 (-) and 0.5 (- -). The dotted line is the absorption spectrum of PM in an n-hexane solution (0.52 X 1od mol L-').
~ t a t e .The ~ transition moment lies in the direction starting from one nitrogen atom to the opposite one with respect to copper. We use the Soret band to determine the orientation of PM in the LB films. Figure 6 shows the absorption spectra of PM in an n-hexane solution and in the mixed LB films with and without n-hexatriacontane at normal incidence. The intense band of PM in both of the LB films was slightly broadened and red-shifted compared with the Soret band of PM in an n-hexane solution (A, = 415 nm), indicating the electronic interaction between porphyrin molecule^.^^^* No in-plane anisotropy was observed for the orientation of PM in the polarized UV/vis absorption spectra of the LB films at normal incidence. In order to obtain further information on the structure of the PM molecule in the mixed LB films, we measured the polarized absorption spectra of these mixed LB films at an incident angle of 45" with respect to the surface normal. In thespectra of the LB film without n-hexatriacontane (Figure 7a), an intense peak is observed at 421 nm with its dichroic ratio AJA, around 1.7 and a shoulder at ca. 404 nm whose dichroic ratio is below unity. The large dichroic ratio at 421 nm indicates that the transition moment of this peak is directed almost parallel to the film surface, while the transition moment of the peak at 404 nm is rather tilted from the film surface, as is seen in the small dichroic ratio. The presence of two absorption bands at 404 and 421 nm suggests theexcitonsplittingoftheSoretbanddueto the formation of aggregate of PM mole~ules.~9.50 Similiar splitting of the Soret band and the anisotropy of the absorption bands in the LB film were reported and analyzed by Schick et al!9 The polarized absorption spectra of the mixed LB films were simulated well using the curve-fitting method, assuming two Lorentzian peaks as shown in Figure 7b,c. The dichroic ratio ASIApof the absorption at 421 nm is estimated to be ca. 1.69, and that for the 404-nm peak is ca. 0.61, as summarized in Table 11. Figure 8a shows the polarized absorption spectra of mixed LB films containing n-hexatriacontane ( x = 0.5). The major peak is slightly red-shifted to 423 nm with its dichroic ratio being almost the same as that for the film without n-hexatriacontane. On the contrary, the shoulder is blue-shifted to 398 nm with a very small dichroic ratio. According to the curve-fitting (Figure 8b,c), the dichroic ratio ASIApof the absorption at 423 nm is estimated to be ca. 1.73 and that for the 398-nm peak is ca. 0.30. The real dichroic ratio for the latter can be smaller than the simulated value, as is seen in Figure 8c, because only the symmetrical Lorentzian waves were used in the simulation. These results show that the trigger molecule, n-hexatriacontane, controls not only the orientation of PM but also the aggregate state of PM.
Azumi et al.
l a 8 6 6 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993
TABLE II: Dichroic Ratio AJA, of Each Component ASIApfor given peak position (nm) 398 nm
X
0 0.30 0.16'
0.5 a
by
360
400
440
480
Wavelength I nm
0.4
t
404 nm
421 nm
0.6 1 0.580
1.69
423 nm
1.73
Dichroic ratio simulated by using the distribution function obtained
ESR data.
measurements, suggesting a change in population of the two states with increasingx. Abovex = 0.5, theabsorptionspectrumslightly changes, probably due to the change in the structure of the film caused by introducing an excess amount of n-hexatriacontane. Similar phenomena are also observed in the ESR spectra of Figure 5. X-ray Diffraction and IR Spectroscopy. X-ray diffraction patterns were obtained for the LB films with and without n-hexatriacontane. The LB films showed patterns similar to the one obtained for the LB film of cadmium icosanoate, and no significant difference was discerned for the LB films with and without n-hexatriacontane (data not shown). Polarized IR absorption spectra were measured for the LB films with and without n-hexatriacontane, The spectra were similar to those of the LB film of cadmium icosanoate since the absorption bandsattributable toPM arevery weak. No significant changes in the spectral shape and the AI/A,values were observed for the absorption bands assigned to cadmium icosanoate in the LB films with and without n-hexatriacontane (data not shown).
Discussion
L/' .L,J
0.0
--
360
400
440
480
Wavelength I nm
0.0
I i I-& ,
380
400
440
- - - - -. 460
Wavelength I nm Figure 7. Polarized absorption spectra of the mixed LB film (four layers on both sides) with x = 0: (a) ppolarized light (-) and s-polarized light (- -)withan incident angleof 4 5 O ; (b) simulated ppolarization spectrum;
(c) simulated s-polarization spectrum.
The change in electronic state of PM with variation of the mixing ratio x is investigated by monitoring the p-polarization spectrum at an incident angle of 4S0, as is shown in Figure 9. The s-polarization spectrum is not appropriate in this respect since the absorption band with transition moment perpendicular to the film surface does not appear in the s-polarization spectrum. In the region 0.1 Ix I0.5, there are two isosbestic points around 400 and 422 nm, which agrees with the results of ESR
The above results have shown that ESR and UV/vis absorption spectroscopies when combined together are very powerful in elucidating the orientation of porphyrins in organized molecular assemblies. In the present case, ESR spectroscopy has revealed that there are two differently oriented components in both of the LB films with and without n-hexatriacontane. The most probable polar angle with the distribution parameter was obtained for each of the components. The UV/vis absorption spectroscopy has indicated that the porphyrins are in an aggregated state in the LB films. Polarized absorption spectra with oblique incidence gave the dichroic ratios A,/A, of a J-like and a H-like bands, which alone cannot be reduced to the most probable polar angle 4 since the contribution of the second component should be considered. The orientation model of the major component of PM in the LB films with and without n-hexatriacontane can be made, taking into account the following features: (1) the average angle 00 obtained from the simulation of the ESR spectra; (2) the splitting of the Soret band, resulting in the red-shifted and the blue-shifted bands; (3) the dichroic ratio A,/A, of each band. The values of ASIA, 1.7 for the red-shifted bands mean that the transition dipole moments are parallel to the film surface (in-plane component). The smaller values for the blue-shifted bands show that the transition dipole moments are tilted with respect to the film surface (out-of-plane component). The proposed models are shown in Figure 10a,b. In both of the models, the two transition moments, which are degenerate when PM is in a monomeric state, exist in aggregated states, each of the two showing electronic interaction with the corresponding transition moments, having the same orientation, in other molecules within one aggregate. The in-plane component should exhibit J-like arrangement, as is shown in the peak position (red-shifted). On the other hand, the out-of-plane component should exhibit H-like arrangement. Thenearlyvertical orientation of the macrocycle plane with respect to the film surface in the
-
Orientation Change of Porphyrin in LB Films
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12867
1
0.2
0.1
I
00 350
x-
0-
0.1
0.2 ..... 0.3 --0.4 OS
400
450
500
Wavelength / nm
Figure 9. p-polarization absorption spectra of the mixed LB films (four layers on both sides) with varying value of x at an incident angle of 45’.
Wavelength / nm
0.5r 0.4
t
Wavelength / nm
Figure 10. Schematic view of the models of PM aggregates in the mixed LB film (a) without n-hexatriacontane and (b) containing n-hexatriacontane. (c) The definitions of the angles 0 and 6.
0.57 surface normal of the film is given by using the procedure slightly modified from the one reported by Yoneyama et al.S1
A J A , = sin’ @/(cos2/3 sin’ @
+ 2 sin’ /3 cos’ @)
(6)
where /3 is the refraction angle inside the layer for 4 5 O incidence. The value /3 = 3 8 O is obtained for various organic compounds in LB films.51 Using this value, the angle between the film normal and the transition dipole moment of the red-shifted (in-plane) band with the dichroic ratio A J A , 1.7 is estimated a t ca. 90°, i.e., almost parallel to the film surface as mentioned above. Here we will use a further modified procedure for estimating the orientation of the out-of-plane band, taking into account the distribution of the angleand theexistenceof theminor component. This is done by first obtaining the intensities of the absorption bands in each s-polarization and p-polarization spectra, respectively, as a function of the angle 4 between the transition moment of the out-of-plane band and the surface normal (Figure 1Oc).
-
Wavelength / nm Figure 8. Polarized absorptionspectra of the mixed LB film (four layers on both sides) with x = 0.5: (a) ppolarized light (-) and s-polarized light (- -) with an incident angle of 45’; (b) simulated ppolarization spectrum; (c) simulated s-polarization spectrum.
LB film with n-hexatriacontane seems to render the blue shift of the H-like band larger and its dichroic ratio smaller. To valdify the orientation models of P M in the LB films, we have simulated the dichroic ratios ASIApfor the out-of-plane bands (blue-shiftedbands in the present case) by using the results of ESR summarized in Table I. The average angle @ between the transition moment and the
Zs(4) = kE2M2sin’ 4
(7)
1,(4) = ~ E ~ M ~ ( BCsin’ O S4~+ 2 sin’ B cos’ 4)
(8)
Here ZI(4)and Ip(4)are the absorbance in the s-polarization and ppolarization spectra, respectively. k is a constant which is proportional to the density of the transition moment. E and M represent the electric vector and the transition moment, respec-
12868 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993
tively. The dichroic ratio ASIApis given as follows:
where P’(t$) is the normalized distribution function obtained from P(0) simulated for the ESR spectra. Here we assume cp = 90° - 8, since the in-plane component of each spectrum can be regarded almost parallel to the film surface. The results are summarized in Table 11. The values obtained by using eq 9 agree well with the ones simulated by the curve-fitting method. These results show that ESR and UV/vis spectroscopies have complementary features in this case. It is sometimes not straightforward to assume that the polar angle 4 is obtained by simply substituting the experimentalvalues into eq 6. If we try to get the value of 4 of PM in the LB film at x = 0.5, without taking into account that there are two components with certain distributions, we obtain the value cp = 2 8 O for the 398-nm peak, which is not consistent with the results of the ESR spectra. The important question is how the orientation change of PM occurs with an increasing amount of n-hexatriacontane. The answer is given by investigating the results shown in Figures 5 and 9, which were obtained by using ESR and UV/vis absorption spectroscopies with variation in the mixing ratio x of n-hexatriacontane. Both spectroscopies gave similar results: (1) The change in orientation of PM starts at the value as small as x = 0.1. (2) In therange0.1 I x I0.5,thereisachangeintheorientation of PM from the 8, = 5 8 O to the 00 = 80° state in such a manner that the population of the former species decreases with increasing x and becomes indiscernible at x = 0.5, not that the orientation of each molecule changes continuously. The fact that there are only two orientation states for the major component of PM suggests that the change in the orientation of PM is caused by a phase transition between these two states. The present case is reminiscent of the order4sorder transition of alloys, so-called X transition, in which the fraction of the second state increases gradually and cooperatively before reaching the transition point.52 The other important question is how the trigger molecule controls the orientation of the functional molecule. The results of X-ray diffraction analyses and I R absorption spectroscopy suggest that the orientation of the matrix molecule is not affected significantly by the introduction of the trigger molecule. The most plausible explanation is that theorientation of the functional molecule is controlled through the molecular interaction with the trigger molecule. This effect seems to be microscopic, not macroscopic, since the population of the controlled portion increases gradually with the increasing value of x. At x = 0.5, which means that the number of n-hexatriacontane molecules is as large as one-third the number of PM molecules, most of the PM molecules are oriented with their macrocycle planes perpendicular to the film surface. The constant area per molecule of PM up to x = 1 suggests that, in this region, the trigger molecule fills the vacant space present in the LB films. These phenomena may be relevant to the case in which a long chain n-alkane such as octadecane has been used to fill the vacancy in the hydrophobic portion to form a densely packed monolayer when the hydrophilic part of the amphiphilic molecule is larger than its hydrophobic part .53-56 In the region x > 1, the orientation of the functional molecule is slightly different from the one a t x = 0.5, probably due to the change in the structure of the LB film caused by the introduction of an excess amount of nonamphiphilic molecule, n-hexatriacontane, which is shown both in the ESR and UV/vis absorption spectra in Figures 5 and 9.
Azumi et al. Conclusions This work has shown that the orientation of the functional molecule in the molecular assemblies can be controlled by the trigger molecule, n-hexatriacontane in this case. The addition of a small amount of n-hexatriacontane changes the orientation of the lightly-substituted porphyrin in the LB films. With the number of n-hexatriacontane molecules equal to one-third the number of PM molecules, a t the mixing ratio x = 0.5, the major component of PM has an orientation with its macrocycle plane almost perpendicular to the film surface. Without n-hexatriacontane the major component of PM takes a tilted orientation with respect to the film surface. The ESR spectra have indicated that in the LB film there are two components having different orientations with certain distributions. The UV/vis absorption spectra have revealed the intermolecular interaction between PM molecules but failed to distinguish between the two components. With the two techniques we have proposed the orientational models of PM in the LB films which reconcile all the experimental data available. Further investigation is now in progress to extend this technique for controlling the orientation of functional molecules to a series of porphyrin derivatives and other functional molecules without long alkyl chains in the molecular assemblies. This will also allow us to clarify the mechanisms involved in these processes. Acknowledgment. We thank Dr. S.Abe for helpful discussions on the UV/vis absorption spectra. References and Notes (1) Dolphin, D., Ed.The Porphyrins; Academic Press: New York, 1978. (2) Vandevyver, M.; Barraud, A.; Ruaudel-Teixier, A.; Maillard, P.; Gianotti, C. J . Colloid Interface Sci. 1982, 85, 571, (3) Flbrsheimer. M.: Mahwald. H. Thin Solid Films 1988. 159. 115. (4 Mabius, D.;Cordroch, W.; Loschek, R.; Chi, L. F.; Dhathathreyan, A.; Vogel, V. Thin Solid Films 1989. 178, 53. (5) Bergeron, J. A.;Gaines, G. L., Jr.;Bellamy, W. D. J. Colloid Interface Sci. 1967. 25. 97. (6) Tredgold, R. H.; Evans, S.D.; Hodge, P.; Jones, R.; Stocks, N. G.; Young, M. C. J. Br. Polym. J. 1987,19, 397. (7) Roberts, G. G., Ed.Longmuir-Blodgeit Films; Plenum Press: New York, 1990. (8) Proceedings of the Fifth International Conference on LangmuirBlodgett Films. Thin Solid Films 1992, 210-21 I . (9) Fujihira, M.; Nishiyama, K.; Yamada, H. Thin Solid Films 1985, 132. 77. (10) Sakai, K.; Matsuda, H.; Kawada, H.; Eguchi, K.; Nakagiri, T. Appl. Phys. Lett. 1988, 53, 1274. (11) Seki, T.; Tamaki, T.; Suzuki, Y.; Kawanishi, Y.; Ichimura, K. Macromolecules 1989, 22, 3505. (12) Tachibana,H.; Nakamura,T.;Matsumoto,M.; Komizu, H.; Manda, E.; Niino, H.; Y a k , A.; Kawabata, Y. J. Am. Chem. Soc. 1989, I l l , 3080. (13) Tachibana, H.; Goto, A.; Nakamura, T.; Matsumoto, M.; Manda, E.; Niino, H.; Yabe, A.; Kawabata, Y. Thin Solid Films 1989, 179, 207. (14) Tachibana,H.;Azumi, R.;Nakamura,T.;Matsumoto,M.;Kawabata, Y. Chem. Lett. 1992, 173. (15) Liu, Z. F.; Hashimoto, K.; Fujishima, A. Nature 1990, 347, 658. (16) Iwamoto,M.; Majima,Y.;Naruse,H.;Noguchi,T.;Fuwa,H.Nature 1991, 353, 645. (17) Sakaguchi, H.; Nagamura, T.; Matsuo, T. Jpn. J. Appl. Phys. 1991, 30, L377. (18) Metzger, R. M.; Panetta, C. A. New J . Chem. 1991, 15, 209. (19) Roberts,G. G.; McGinnity, T. M.; Barlow, W. A.; Vincett, P. S. Solid Stare Commun. 1979. 32, 683. (20) Baker, S.;Roberts, G. G.; Petty, M. C.; Twigg, M. V. Thin Solid Films 1983. 99. 53. (21) Era, M.; Hayashi, S.; Tsutsui, T.; Saito, S. J . Chem. Soc., Chem. Commun. 1985, 557. (22) Fujiki, M.; Tabei, H. Synth. Mer. 1987, 18, 815. (23) Obeng, Y. S.;Bard, A. J. J. Am. Chem. Soc. 1991, 113,6279. (24) Schoeler, U.; Tews, K. H.; Kuhn, H. J . Chem. Phys. 1974,61,5009. (25) Warren,J.G.;Cr*iswell,J.P.;Petty,M.C.;Lloyd,J.P.;VitukhnovsLy, A.; Sluch, M. I. Thin Solid Films 1989, 179, 515. (26) Nakamura,T.;Tachibana,H.; Yumura, M.; Matsumoto, M.; Azumi, R.; Tanaka, M.; Kawabata, Y. Longmuir 1992,8,4. (27) Fukuda, K.; Nakahara, H.; Kato, T. J. Colloid Interface Sci. 1976, 54, 430. (28) Nakamura, T.; Tanaka, M.; Sekiguchi, T.; Kawabata, Y. J . Am. Chem. Soc. 1986, 108, 1302.
Orientation Change of Porphyrin in LB Films (29) Kawabata, Y.; Sekiguchi, T.; Tanaka, M.; Nakamura, T.; Komizu, H.: Honda. K.: Manda.,E.:.Saito.. M.: . Suni.. M.:. Iizima. S . J. Am. Chcm. Soc.
ids,io7,’52io.
-
(30) Matsumoto, M.; Nakamura, T.; Tanaka, M.; Sekiguchi, T.; Komizu, H.; Matsuzaki, S.Y.; Manda, E.;Kawabata, Y.; Saito, M.;Iizima, S.;Sugi, M. Bull. Chcm. Soc. Jpn. 1987,60, 2737. (31) Ammi, R.; Matsumoto, M.; Kawabata, Y.; Kuroda, S.;Sugi, M.; King, L. G.; Crossley, M.J. J. Am. Chcm. Soc. 1992,114, 10662. (32) Crossley, M.J.; Burn, P. L. J. Chcm. Soc., Chcm. Commun. 1987, 39. (33) Adler, A. D.; Longo, F. R.;Finarelli, J. D.; Goldmacher, J.; Assour, J.; Korsakoff, L. J. Org. Chcm. 1967, 32, 476. (34) Adler, A. D.; Longo, F. R.;Kampas, F.; Kim, J. J. Inorg. Nucl. Chcm. 1970,32, 2443. (35) Kuroda, S.;Sugi, M.;Iizima, S . Thin Solid Films 1983, 99, 21. (36) Crossley, M.J.; Bum, P. L. J. Chem. Soc., Chcm. Commun. 1991, 1569. (37) Messier, J.; Marc, 0. J. Phys. (Fr.) 1971, 32, 799. (38) Kuroda, S.; Ikegami, K.; Sugi, M.; Iizima, S . Solid State Commun. 1986,58,493. (39) Kuroda, S.;Ikegami, K.; Sugi, M. Mol. Cryst. Liq. Cryst. 1990,190, 111. (40) Kuroda, S.;Ikegami, K.; T a b , Y.;Saito, K.; Saito, M.;Sugi, M. Phvs. Reo. 1991.843.2531. i41) Cook, hi. J.; Daniel, M. F.; Dunn, A. J.; Gold, A. A,; Thomson, A. J. J . Chcm. Soc., Chcm. Commun. 1986, 863.
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12869 (42) Cook, M. J.; Dunn, A. J.; Gold, A. A.; Thomson, A. J.; Daniel, M. F. J. Chem. Soc.. Dalton Trans. 1988. 1583. (43) Paw, M: D.; Barger, W. R.; Snow, A. W. Lungmuir 1989,5,973. (44) Lin, W. C. The Porphyrins; Academic Press: New York, 1979; Vol. IV, p 355. (45) Abkowitz, M.; Monahan, A. R. J. Chcm. Phys. 1973,58,2281. (46) Boas, J. F.; Pilbrow, J. R.; Smith, T. D. J. Chcm. Soc. A 1969,721. (47) Skorobogaty, A.; Lanchashire, R.;Smith, T. D.; Pilbrow, J. R.; Sinclair, G. R.J. Chcm. Soc., Faraday Trans. 2 1983, 79, 1123. (48) Bcswick, R. B.; Pitt, C. W. Chem. Phys. Lett. llp88, 143, 589. (49) Schick, G. A.; Schreiman, I. C.; Wagner, R. W.; Lindsey, J. S.; Bocran, D. F. J. Am. Chem. Soc. 1989, I l l , 1344. (50) Gouterman, M.; Holten, D.; Lieberman, E. Chem. Phys. 1977, 25, 139. (51) Yoneyama, M.; Sugi, M.;Saito, M.; Ikegami, K.; Kuroda, S.;Iizima, S. Jpn. J. Appl. Phys. 1986, 25, 961. (52) Atkins, P. W. Physical Chemistry; Oxford University Press: Oxford, U.K., 1978; Chapter 7. (53) Steiger, R.; Kitzing, R.;Junod, P. J. Photogr. Sci. 1973, 21, 107. (54) Sotnikov, P. S.;Berzina, T. S;Troitsky, V. I.; Valter, R. E.; Karlivan, G. A.; Neiland, 0.YA. Thin Solid Films 1989, 179, 267. (55) Kuhn, H. Thin Solid Films 1983, 99, 1. (56) Shimidzu, T.; Iyoda, T.; Ando, M.; Ohtani, A.; Kaneko, T.; Honda, K. Thin Solid Films 1988, 160, 67.
