Langmuir 1988, 4, 1123-1128
1123
In-Plane Dichroisms of Phthalocyanine Langmuir-Blodgett Films Michiya Fujiki,*yt Hisao Tabei,t and Takashi Kuriharat NTT Basic Research Laboratories and NTT Opto-Electronics Laboratories, Tokai, Ibaraki, 319-11, J a p a n Received July 16, 1987. I n Final Form: April 5, 1988 Langmuir-Blodgett (LB) films of two new highly soluble nickel phthalocyanines involving four octadecyl amides (AmPcl and AmPc2) have been studied. The films have been prepared by both the vertical dipping and horizontal lifting techniques. The assembling numbers in AmPcl and AmPc2, which strongly depend on the the preparation condition of the LB films, are evaluated as being ca. 5-14 for the AmPcl and 5 for the AmPc2, using a modified molecular exciton theory. In-plane dichroisms in the LB films prepared by either the vertical dipping or the horizontal lifting technique can be invariably seen in UV-vis and IR regions. The dichroic ratios are sensitive to the condition of the film preparation (film pressures, transfer rates of substrates, additives, and film-transfer techniques). Molecular arrangement and orientation in the AmPcl LB film prepared by the vertical dipping method have been characterized by means of the polarized UV-vis and IR spectra, force-area isotherm, and mechanical stylus method.
Introduction Phthalocyanines (Pcs), which exhibit high chemical and thermal stabilities among several organic compounds, have been of particular interest in many fields of basic and applied research,' concerning solar cells,2photosensitizers,3 low-dimensional metals," gas sensor^,^ electrochromisms,6 lithium batteries,' electron beam negative resist,a and recently attractive photochemical hole-burning memoriesg and molecular electronic devices.1° However, these are generally limitations in arranging and organizing unsubstituted Pc moieties in a desired crystal structure and thickness onto solid substrates, because Pcs usually exhibit polymorphisms in the solid state and extremely poor solubilities in common organic solvents. In recent years the Langmuir-Blodgett (LB) technique has been noted as a useful means of obtaining tailored thin films of molecular size thickness for a variety of potential applications, such as electron beam resists, nonlinear optics, Josephson tunnel junctions, electroluminescences, and field effect transistors." Various surface-active molecules can form high-quality LB films, which can be characterized by new physical and spectral methods. The preparations, characterizations, and photo and electrical properties of LB films, based on soluble Pcs substituted with substituents such as tert-butyl, isopropylaminomethyl, dodecoxymethyl, octadecoxy, methoxy, octyloxy, and cumylphenoxy, have been studied.12 Unfortunately, little research on dichroisms has been done and only for three Pc systems.12k-m The control of lattice architecture and electronic delocalization in Pc LB films for applications such as gassensitive and photoactive devices is a primary interest. This purpose has several requirements. First, the Pcs should be soluble in a spreading solvent. Second, the Pcs should be arranged in a one-dimensional (1-D) linear stack of van der Waals thickness and arrayed in a preferential orientation on a suitable solid ~ u b s t r a t e . ~ J ~ Previous papers have reported that two new soluble PcNi compounds substituted with four octadecyl amides (Figure 1, abbreviated here as AmPcl and AmPc2) exhibit 1-D linear self-assembling and negative patterning properties, while maintaining sensitive conductivity changes to exposure of iodine g a ~ . ~This J ~ paper deals with the intense in-plane dichroisms observed in both UV-vis and N T T Basic Research Laboratories.
* N T T Opto-Electronics Laboratories. 0743-7463/88/2404-ll23$01.50/0
IR regions of AmPcl and AmPc2 LB films prepared by the vertical dipping and horizontal lifting techniques. Results and Discussion Both AmPcl and AmPc2 molecules possess so-called (1) (a) Boucher, L. J. Coordination Chemistry of Macrocyclic Compounds; Melson, G. A., Ed.; Plenum: New York, 1979. (b) Lever, A. B. Adu. Inorg. Chem. Radiochem. 1965,7,27-114. (c) Moser, F. H.; Thomas, A. L. The Phthalocyanine; CRC: Boca Raton, FL, 1983. (2)(a) Loutfy, R. 0.;Sharp, J. H. J. Chem. Phys. 1979,71,1211-1216. (b) Tang, C.W. Appl. Phys. Lett. 1986,48,183-185. (3)(a) Arishima, K.; Hiratsuka, H.; Tate, A.; Okada, T. Appl. Phys. Lett. 1982,40,279-281.(b) Kato, M.; Nishioka, Y.; Kaifu, K.; Kawamura, K.; Ohno, S. Appl. Phys. Lett. 1985,46,196-197. (c) Loutfy, R. 0.;Hor, A. M.; DiPaola-Baranyi, G.; Hsiao, C. K. J. Imag. Sci. 1985,29,116-121. (4)(a) Peterson, J. L.; Schramn, C. S.; Stojakovic, D. R.; Hoffman, B. M.; Marks, T. J. J . Am. Chem. SOC.1977,99,286-288.(b) Schramn, C. J.; Scaringe, R. P.; Stojakovic, D. R.; Hoffman, B. M.; Iben, J. A.; Marks, T. J. J. Am. Chem. SOC.1980,102,6702-6713.(c) Nohr, R. S.; Kuznesof, P. M.; Wynne, K. J.; Kenney, M. E.; Siebenman, P. G. J. Am. Chem. SOC. 1981,103,4371-4377. (5)(a) Sadaoka, Y.; Sakai, Y.; Aso, I.; Yamazoe, N.; Seiyama, T. Denki (in Japanese). (b) Honeybourne, C. L.; Ewen, Kagaku 1980,48,486-490 R. J.; Hill, C. A. S. J . Chem. Soc., Faraday Trans. 1. 1984,80,851-863. (6)(a) Moskalev. P. N.: Kirin. I. S. Russ. J . Phvs. Chem. 1972.46. 101911022. (b) Y k a m o t o ; H.; Sugiyama, T.; Tanaia, M. Jpn. J. Appl: Phys., Part 1 1985,24,L305-L307. (7) (a) Yamaki, J.; Yamaji, A. J. Electrochem. SOC.1982,129,5-9.(b) Wohrle, D.; Kirschenmann, M.; Jeeger, N. I. J. Electrochem. SOC.1985, 132,1150-1152. (8) (a) Tabei, H.; Fujiki, M.; Imamura. S.Jpn. J. Appl. Phys., Part 1, 1985,24,L685-L686. (b) Fujiki, M.; Tabei, H.; Imamura, S. Jpn. J . Appl. Phys., Part I 1987,26,1224-1229. (9)Gutierrez, A. R.; Friedrich, J.; Haarer, D.; Wolfman, H. IBM J. Res. Deuelop. 1982,26,198-208. (10)Molecular Electronic Deuices;Carter, F. L., Ed.; Marcell-Dekker: New York, 1982. (11)(a) Thin Solid Films 1983,99,1-330. (b) Langmuir-Blodgett Films; Barlow, W. A., Ed.; Elsevier: Amsterdam, 1980. (c) Thin Solid Films 1985,132-134. (12)(a) Baker, S.; Petty, M. C.; Roberts, G. G.; Twigg, M. V. Thin Solid Films 1983,99,53-59.(b) Baker, S.; Roberts, G. G.; Petty, M. C. IEE. Proc. 1. Solid State Electron Deuices 1983,130,260-263.(c) Hann, R. A.; Gupta, S. K.; Fryer, J. R.; Eyres, B. L. Thin Solid Films 1985,134, 35-42. (d) Kovacs, G. J.; Vincett, P. S.; Sharp, J. H. Can. J. Phys. 1985, 63,346-349. (e) Hua, Y. L.; Roberts, G. G.; Ahmad, M. M.; Petty, M. C.; Hanack, M.; Rein, M. Philos. Mag., [Part B] 1986,53,105-113.(fj Snow, A. W.; Jarvis, N. L. J . Am. Chem. SOC.1984,106,4706-4711.(9) Cook, M. J.; Daniel, M. F.; Dunn, A. J.; Gold, A. A.; Thomson, A. J. J . Chem. Soc., Chem. Commun. 1986,863-864.(h) Aroca, R.; Jennings, C.; Kovaa, G. J.; Loutfy, R. 0.; Vincett, P. S.J. Phys. Chem. 1985,89,4051-4054. (i) Roberts, G. G.; Petty, M. C.; Baker, S.; Fowler, M. T.; Thomas, N. J. Thin Solid Films 1985,132,113-123. 6)Barger, W. R.; Snow, A. W.; Wohltjen, H.; Jarvis, N. L. Thin Solid Films 1985,133,197-206. (k) Kalina, D. W.; Crane, S.W. Thin Solid Films 1985,134, 109-119. (1) Yoneyama, M.; Sugi, M.; Saito, M.; Ikegami, K.; Kuroda, S.; Iizima, S. Jpn. J . Appl. Phys., Part 1, 1986,25, 961-965. (m) Orthmann, E.; Wegner, G. Angew. Chem., Int. Ed. Engl. 1986,25,1105-1107. (13)Fujiki, M.; Tabei, H.; Kurihara, T. J . Phys. Chem. 1988, 92, 1281-1285.
0 1988 American Chemical Society
Fujiki et al.
1124 Langmuir, Vol. 4, No. 5, 1988 Z
m
I
R
H = -CNCtt3H37
\,
/
/*p++y
AmPc 1
X
0
H R=-NCC18H37
z*
AmPc 2
0
Figure 1. Molecular structures of AmPcl and AmPc2.
X*
Figure 2. Space coordinates used in the present electronic and IR study: X,direction of meniscus; X*,polarized light plane; Y, transfer direction of substrate, Z*, molecular axis; 0, polarized angle of normal incident light; w and 4, mean axis angles of molecular axis.
degenerate Q-bands, which originate from n-a* transitions. These are comprised of M and L transitions with the same energies and oscillator strengths along the Pc ring,lb as shown in Figure 1. Therefore, because of their electronic absorption spectra, characteristic Q-bands become important in the discussion of AmPcl and AmPc2 molecular arrangement in LB films. In the molecular exciton appr~ximation,'~ the energy shift (AE) between the exciton bands for a 1-D finite chain stack of dyes and its monomer, when the end effects are neglected, is given as follows:
AE
= E , - E , = 41(
N)( N-1
$)(cos
a - 3 cos2 /3)1 (1)
where E, and E, are the respective assembly and monomer transition energies, N is the association number of the stacks, M the transition dipole strength, r the closest distance between the moments, a the torsion angle between the moments, and /3 the tilt angle. The theory also predicts the selection rule that when ' 0 C /3 C 54.7' the absorption band of assemblies will be red-shifted and when 54.7' C /3 C 90' the band will be blue-shifted. A previous paper has demonstrated the inadequacy of calculating the assembling number in the 1-D Pc stacks only from the monomeric Q-band parameters (such as M, r, a,and /3).13 The theory can qualitatively predict the trend of the exciton shift of the assemblies. According to a modified molecular exciton theory,13 the assembling number of 1-D Pc stacks can be evaluated simply and semiempirically, as follows: AE(N4-Q.) N = (2) AE(N--) - U(N) where U(N) is the exciton shift for the 1-D Pc stacks with an assembling number of N and AE(N--) is the exciton shift for the infinite 1-D Pc stacks. In eq 2, the A, value of the Q-band in the Pc assemblies relates directly to the assembling number. For evaluation of the orientation of the Pc ring in AmPcl and AmPc2 LB films using polarized UV-vis and IR spectroscopies, space coordinates used in the present study are shown in Figure 2. X,Y , and Z are the optical (14) (a) Kasha, M.; Rawls, R.; El-Bayoumi, M. S.Pure Appl. Chem. 1965, 11, 371-392. (b) Kasha, M. Spectroscopy of the Exited State; Bartolo, B. D., Ed.; Plenum: New York, 1976; pp 337-363. (c) Kasha, M. Physical Processes in Radiation Biology; Augenstein, L., Mason, R., Rosenberg, B., Eds.; Academic: New York, 1964; pp 17-42.
0
50 100 AREA(A2/molecule)
150
Figure 3. Forcearea isotherms of AmPcl and AmPc2 on pure water at 18 O C : a, AmPcl; b, AmPc2. axes. Y lies in the transfer direction of the substrate in the vertical dipping technique, X is parallel to the meniscus, w is the angle between the molecular axis of the Pc plate (Z*) and 2, 6 is the angle between the normal incident polarized light plane (X*) and X , and r$ is the angle between the mean molecular axis and X . A ( @ is the absorbance a t 6. The dichroic ratio ( D ) is defined as A(90)/A(O). A rather simple expression of biaxial orientation parameters for the degenerate transition molecular system on a substrate is given as follows:12 [(cos2 4) + (cos2 w sin2 4)] D= (3) [(sin2 4) + (cos2 w cos2 4 ) ] This is based on the assumption that the Pc ring is a flat circular plate with a uniformly distributed transition dipole, although a Pc ring has D a molecular symmetry. If a Pc ring is oriented perpendicularly to the substrate (w = goo), eq 3 can then be reduced as follows:
(4) The force-area isotherms of AmPcl and AmPc2 on pure water a t 18' C are shown in Figure 3. Both AmPcl and
Langmuir, Vol. 4, No. 5, 1988 1125
Dichroisms of Phthalocyanine LB Films 0
4
i
0; 00 0.1
model A
model B
Figure 4. Two possible models for AmPcl and AmPc2 molecular arrangements at the water-air interface (square and cylinder represent a Pc ring and an octadecyl amide unit). 0. t go"
AmPc2 form fairly stable and reproducible films a t the water-air interface. The observed occupied area per molecule is 120 and 112 A2 for AmPcl and AmPc2, respectively. The collapsed film pressures are obtained as 30 dyn/cm for AmPcl and 20 dyn/cm for AmPc2. AmPcl film a t the water-air interface can be transferred onto hydrophobic solid substrates by both the vertical dipping LB technique (abbreviated VDLB, Y-type) and the horizontal lifting LB technique (abbreviated HLLB, X-type). AmPc2 film can be transferred only by the HLLB technique. Two possible typical arrangement models for AmPcl and AmPc2 molecules a t the water-air interface are illustrated in Figure 4. Assuming that a Pc square plane has an edge of 12 8, and a ring spacing of 3.4 8, and that the long alkyl moiety has a cross section of 20 A2,the occupied area per molecule is evaluated as 121 A2for model A and 224 A2 for model B. Also, if alkyl chain axes are vertical to the water-air interface, the film thickness per layer is calculated as 37 8, for model A and 28 8,for model B. The previously reported 1-D self-assembling character of AmPcl and AmPc2 can take both models A and B.13 If the Pc assemblies develop as in model A, the Pc ring planes are vertically oriented to the water-air interface with a monolayer thickness. If the Pc assemblies develop as in model B, the Pc planes are parallel to the water-air interface with bimolecular thickness. This involves comparison between the observed and calculated limiting area values. Further polarized spectroscopic analysis is required. Typical polarized electronic absorption spectra for AmPcl VDLB, AmPcl HLLB, and AmPc2 HLLB films are illustrated in Figure 5. The angular dependence of the main Q-bands a t A, = 611 nm for the AmPcl VDLB film sample is shown in Figure 6 All these VDLB and HLLB films possess dichroisms in which the main Q-bands show maxima a t 8 = f 90° and a minimum a t 8 = Oo in the angular dependence of the main Q-band intensity. The electronic absorption spectra of AmPcl VDLB film are almost identical with that of AmPcl spin-cast film.13 In contrast, the electronic absorption spectra of AmPcl and AmPc2 HLLB films have a much stronger satellite band a t 680 nm than the corresponding spectra of the spin-cast film. Also, the main Q-bands of LB films are red-shifted by 11 and 1 2 nm, re~pective1y.l~ Since the Q-band intensity maximizes a t 0 = &goo, the Pc ring planes must be arrayed perpendicularly to the substrate plane. This is because the degenerate M and L transitions of the Q-bands lie in the Pc ring plane. If the Pc ring planes are parallel to the substrate plane, dichroism does not occur. In addition, Pc planes of AmPcl and
A
0.04-
0.03YI
m 4
0.02-
0.01-
300
400
500
6hO 700 WAVELENGTH(nm1
eo0
goo
t
Figure 5. Polarized electronic absorption spectra of AmPcl and AmPc2 LB films on SiOz substrates. (a, top) AmPcl LB film prepared by the vertical dipping method (80layers, f i pressure 15 dyn/cm, transfer rate of the substrate 7 mm/min, substrate is parallel to moving barrier). (b, middle) AmPcl LB film prepared by the horizontal lifting method (8 layers, film pressure 18 dyn cm, substrate is perpendicular to moving barrier). (c, bottom(AmPc2 LB fikn prepared by the horizontal liftii method (10 layers, film pressure 18 dyn/cm, substrate is perpendicular to moving barrier).
AmPc2 seem to take an edge-on configuration a t the water-air interface, because of their inherent intense 1-D self-assembling nature even in hot dilute s ~ l u t i o n . ' The ~ observed film thickness of AmPcl (36 8,/layer) is consistent with the estimated value for model A. These results support the idea that AmPcl and AmPc2 take model A as the molecular arrangement a t the water-air interface. This is also consistent with the assumption that, in eq 4, a Pc ring is oriented perpendicularly to the substrate with w = goo. There has been little research on dichroisms in Pc LB films. To date, there are two monomeric and one oligom-
1126 Langmuir, Vol. 4, No. 5, 1988
Fujiki et al.
Table I. Characteristic Parameters in AmPcl and AmPc2 LB Films Prepared by the Vertical Dipping and Horizontal Lifting Techniques F,' dyn/cm T,d mm/min De ,A,, nm position of substrate' Ng Pc methodn E/m/nb 1/0/0
15 15 15 15 18 18 18 30 30 30 20 20 18 18 18 18
VD
AmPcl
1/4/0h 1/0/2'
AmPcl
HL
AmPc2
HL
1.8 7 7 28 3.5 7 14 7 7 28 7 28
i
2.4 2.4 2.1 1.8 2.0 2.0 1.2 1.2 1.7 2.4 2.0 1.1 1.1 1.4 1.3
j 608 608 608 612 611 611 611 617 615 608 611 614 614 620 620
I1 It
I I1 II II 11 /I I1 /I /I I1 II
I II I
14.4 14.4 14.4 7.8 8.9 8.9 8.9 5.0 5.8 14.4 8.9 6.4 6.4 5.4 5.4
"VD and HL mean the vertical dipping and horizontal lifting techniques. * l , m, and n mean the molar composition ratio for AmPcl, n-nonadecanoic acid, and n-hexadecane, respectively. e Film pressure. Transfer rates of a substrate in the VD method. e Dichroic ratio for the main Q-bands, defined as Abs(0 = *90°)/Abs(B = 0'). f 11 and I mean that a meniscus is parallel and perpendicular to the moving values of the main Q-bands calculated by using eq 2. hCollapsed film pressure 35 barrier, respectively. #Values based on the observed A,, dyn/cm. Collapsed film pressure 22 dyn/cm. No transfer. J
0 4;
! 9.31 e 0
m
e
l
1
0 2J,
e
a
e
a
0 1; I I
I
I
3
/
-goo
C 0
I
t
,
+90a
Figure 6. Angular dependence of the main Q-band intensity (A= 608 nm) for the AmPcl VDLB film (data are taken from Figure 5a).
eric Pc systems. The monomeric Pc compounds include eight dodecoxymethyl and three isopropylaminomethyl groups.12kJ21The oligomeric system is 1-D linear stack 0-linked PcSi substituted with tetramethoxy-tetraoctyloxy groups (assembling number is 15).lZmIn the monomeric Pc systems, the dichroic ratios of the Q-band are 2.1 for the dodecoxy Pc and 1.4 for the isopropylaminomethyl Pc. In the oligomeric Pc system, the dichloric ratio of the Q-band is 2.7 for an as-prepared film and becomes 5.8 after thermal annealing. In the present Pc systems, dichroism in AmPcl and AmPc2 LB films can consistently be seen in both the UV-vis and IR regions. In particular, the dichroic ratio of AmPcl ( N = 14.4) is 2.4 a t 608 nm. Except for the Pc with three isopropylamino groups, the other four Pc systems, which are symmetricallysubstituted with liquid-crystalline-like alkyl moieties, are fixed into a 1-D assembled structure by the Si-0 covalent linkage or the hydrogen bonding of the secondary amide. We suppose that the coexistence of such flexible alkyl moieties and highly fixed Pc ring arrays is very important to achieve a preferential orientation of Pc molecules on substrates. From Figure 5, the dichroic ratio of AmPcl is determined to be 2.4. This corresponds to ( 4 ) = 33". This means that the mean stacking axis of the AmPcl assem-
blies is tilted by 33" to the meniscus (X axis). Similarly, ( 4 ) = 41" is obtained for the AmPc2 HLLB film by using D = 1.35 at 620 nm. Dichroisms of the alkyl amide moieties in a typical AmPcl VDLB film, exhibiting D = 2.0 at 608 nm, can also be seen in the IR region. The angular dependence on the intensities of the characteristic alkyl amide bands is shown in Figure 7.15 The amide I1 band a t 1536 cm-', which is a combination band of i3N-H + u C - ~ ,maximizes a t 8 = 90' and has D = 1.6. The amide I11 band at 1305 cm-', which is assigned to be U N C H ~ , maximizes at 8 = 90° and exhibits D = 1.8. The band uN-H a t 3240 cm-' maximizes a t 8 = 0' and exhibits D = 0.59. The amide I band due to UC+ a t 1635 cm-' shows a weak dichroism of D = 0.93 that maximizes at 8 = 0". The band due to P C - H a t 2922 cm-l has no dichroism. Such dichroisms in the IR region closely relate to the Pc ring arrangement. Further discussion on the amide moiety arrangement, however, is rather difficult, because of the unresolved geometric isomers in AmPcl and AmP~2.l~ The molecular arrangement and orientational models of the AmPcl VDLB film on the substrate are illustrated in Figure 8. AmPcl showing A, = 611 nm corresponds to 1-D assemblies with N = 8.9 from the modified exciton theory.13 The Pc rings of the assemblies are perpendicularly oriented to the substrate. The mean stack axis of the assemblies and the mean direction of the hydrogen bonding band are oriented to the direction of the meniscus. The dichroic ratios and absorption maxima of the main Q-bands are sensitive to film pressure, transfer rate of a substrate, additives, and film deposition methods. For AmPcl and AmPc2 LB films prepared by the VDLB and HLLB methods, the value of dichroic ratios, ,A,, and the evaluated assembling number based on the A, value and eq 2 are summarized in Table I. In AmPcl single-component VDLB systems (llmln = 1/0/0; where I , m, and n represent the molar composition ratios for AmPcl, n-nonadecanoic acid, and n-hexadecane, respectively), the dichroic ratio value decreases markedly from 2.4 to 1.2 with increasing film pressure from 15 to 30 dyn/cm. Also, the ,A, values tend to be red-shifted with increasing film pressure and substrate transfer rate. The red-shift of the main Q-bands indicates that the as(15) Socrates, G. Infrared Characteristic Group Frequencies; Wiley: Chichester, UK, 1980; Chapter 10.
Langmuir, Vol. 4,No. 5, 1988 1127
Dichroisms of Phthalocyanine LB Films
I
0
1
I
I
,
,
1
I
,
L
0
0
Figure 7. Angular dependence of the characteristic alkyl amide intensities in the AmPcl VDLB film (dichroic ratio 2.0 a t 611 nm).
sembling number of AmPcl in LB films decreases from 14.4 to 7.8. Although in AmPcl composite VDLB systems (Z/m/n = 11410) the dichroic ratio increases with increasing transfer rate, the, , A values of the main Q-bands are red-shifted by 7-9 nm compared to those of the singlecomponent system. This indicates that the assembling number further decreases to 5.0. The dichroic ratios decrease as the, A value increases. fn contrast, the dichroic ratios and, , A values in the presence of nonoleophilic molecules such as n-hexadecane (l/m/n = 11012) are almost identical with those of the corresponding singlecomponent systems. Even in the HLLB films, weak dichroisms can be seen. AmPc2 HLLB films have larger dichroic ratios than AmPcl HLLB films. For AmPc2, the main Q-bands in HLLB films are red-shifted by 13 nm and have intense satellite Q-bands compared to cast films.13 For AmPcl, the main Q-bands in HLLB films are also red-shifted 3-11 nm more than those in cast and VDLB films. This indicates that assembling numbers decrease to 6.4 for AmPcl and 5.4 for AmPc2. There are two possible steps in producing orientatation of Pc assemblies in LB films. One is the transfer step a t the meniscus, and the other is the compression step by the
moving barrier. If the former process is the cause of the orientation, the dichroic ratio and its phase are independent of the relative position between the moving barrier and the meniscus in the dipping process. In contrast, the latter leads to a phase shift of the dichroic ratio: though the dichroic ratio value is independent of the relative position between the meniscus and substrate, the Q-bands maximize a t 8 = *90° when the meniscus is parallel to the moving barrier, but the Q-bands maximize a t 8 = Oo when the meniscus is perpendicular to the barrier. Actually, in all AmPcl VDLB and AmPcl and AmPc2 HLLB films, the respective Q-bands, in which the dichroic ratio values in the parallel position are equal to those in the perpendicular position, are independent of the relative positions and show maxima at only 8 = &goo. This strongly indicates that orientation of Pc assemblies occurs during the transfer step of the meniscus but not during the compression process by the barrier. The, A values of the main Q-bands in LB films, which tend to be blue-shifted with increasing dichroic ratios, can be closely related to the assembling number, as discussed previously. From observation of dichroisms in VDLB and even in HLLB films, both transfer steps of the meniscus and reorganization of AmPcl and AmPc2 assemblies seem to be important during the up and down strokes of the meniscus in generating a preferential orientation. In this situation, an intense shear stress is applied to the meniscus during deposition of the monolayer. The increase in stress leads to decomposition of Pc assemblies and prevents their reorganization at the meniscus. This also restricts a preferential orientation of 1-D Pc stacks. Stress in the HLLB technique seems much stronger than that in the VDLB technique since dichroic ratios in the HLLB process are smaller than those in the VDLB process under the same film pressure. Increase in film pressure might repress the reorganization of assemblies a t the meniscus. Also, addition of hydrogen-bonding oleophilic molecules, which may strongly interact with amide moieties of assemblies, markedly decreases the dichroic ratio and assembling number. Such information may lead to the control of preferential orientation of desired molecules on suitable substrates.
Conclusion Langmuir-Blodgett (LB) films of two new types of highly soluble nickel phthalocyanines substituted with four odadecyl amides (AmPcl and AmPc2) have been prepared by using the vertical dipping and horizontal lifting tech-
S u b s t r a t e
I,
3 7 A
1
. H y d r o g e n
,,I
B o n d ' n g
_'
Figure 8. Molecular arrangement models of the AmPcl VDLB film on the substrate.
1128
Langmuir 1988, 4 , 1128-1130
niques. The assembling numbers of AmPcl and AmPc2, which depend on the preparation condition of LB films, are evaluated by using a modified molecular exciton theory as ca. 5-14 for AmPcl and ca. 5 for AmPc2. LB film dichroisms prepared by either the vertical dipping or horizontal lifting can be consistently seen in UV-vis and/or IR regions. Dichroic ratios and ,A, values of &-bands that correlate to the asssembling number are sensitive to LB film preparation conditions, such as film pressures, transfer rate of substrates, additives, and film-transfer techniques. Molecular arrangement and orientation models of AmPcl LB film prepared by vertical dipping are proposed, on the basis of polarized UV-vis and IR spectra, force-area isotherms, and mechanical stylus methods. The transfer steps at the meniscus, which are involved in the vertical dipping and horizontal lifting techniques, strongly suggest the cause of the in-plane dichroism of AmPcl and AmpC2 LB films. They suggest that dichroic ratios and their phases are independent of the relative position between the moving barrier and meniscus.
Experimental Section Preparation of nickel 4,4’,4”,4”’-tetrakis((n-octadecy1amino)carbony1)phthalocyanine(AmPcl) and nickel 4,4‘,4‘‘,4’‘’-tetrakis(n-nonadecanoy1amino)phthalocyanine(AmPc2)was described in the literature.’, Kyowa Kaimen Kagaku (Model HBP-AP) was used as a film balance and consists of a Teflon-coatedtrough, a Wilhelmy-type glass float, and vertical dipping and horizontal lifting units. Subphase temperature was regulated at 18 “C by using a Lauda RCS2O thermostat. Doubly distilled pure water was used as the
subphase. Optically polished fused quartz and CaFzwere used as solid substrates. Hydrophobic quartz was obtained by treatment with octadecyltrichlorosilane. A CHCl, solution containing either AmPcl or AmPc2 and/or other additives was passed through a Millipore 0.45-wm filter and spread on water. Two techniques were employed for building up films at a desired constant film pressure. One is the conventional LB technique in which only Y-type multilayers are deposited by the vertical dipping technique. The other is a modified LB technique in which only X-type multilayers can be formed by the horizontal lifting technique with a small tilt angle.I6 The transfer ratio of AmPcl by the vertical dipping LB method was equal to unity. Electronic absorption intensities of the built-up f i i s were proportional to the dipping or lifting number. An LB film thickness per layer was evaluated by using a mechanical stylus method (Sloan, model dektak 3030; stylus force, 1 mg; stylus radius, 12.5 pm) to obtain the film thickness of a multilayer. Electronic absorption spectra and their second-derivative spectra were recorded by using a Hitachi 330 UV-vis/near-infrared spectrophotometer. IR absorption spectra were recorded with a Perkin-elmer 1800 FT IR spectrometer. Polarized electronic and IR absorption spectra, in which the incident angle of the light beam is normal, were recorded by using a Glan prism, a gold wire grid polarizer, and a goniometer. In order to obtain an absorption spectrum of the film, the difference spectral technique was employed with a single-ratiotechnique. In the method, two spectra, of a substrate and of the substrate with film stored in the respective memories, and an absorption spectrum of the film was calculated from them. Registry No. AmPcl, 106725-59-1;AmPc2, 99144-77-1. (16) Nakahara, H.; Fukuda, K. J . Colloid Interface Sci. 1979, 69, 124-133.
Domain Size in Ordered Packing of Monodispersed Particles S. Y. Chang and T. A. Ring* Department of Chemical Engineering, University of Utah, Salt Lake City, Utah 84112 Received May 18, 1987. I n Final Form: April 14, 1988 Diffraction line broadening has been adapted to light diffraction to measure ordered domain size. Domain sizes of -25 pm were observed for TiOz powders and of -40 pm for SiOz powders. Scanning electron microscopy was used to confirm the ordered domain size of both SiOz and TiOz.
Introduction Using light diffraction, Hiltner and Krieger’ showed that the interparticle spacing in an ordered packing of latex particles could be obtained by using the Bragg equation (commonly used for X-ray analysis of crystalline materials): nX = 2d sin 8 (1) where n is the order of diffraction, X is the wavelength of the incident light, d is the distance between the diffracting planes, and 0 is the angle between the incident light and the diffraction planes. In the usual experiment X is known and 0 is measured, allowing the calculation of d when n is assumed to be 1. In this paper we extend a phenomena called line broadening in X-ray diffraction to light diffraction. Line broadening is used in powder X-ray dif(1)Hiltner, P. A.; Krieger, I. M. J.Phys. Chem. 1969, 73,2386-2389.
fraction to determine the crystalite size; the smaller the diffracting crystals the broader the diffraction peak.
Theory The first treatment of line broadening was due to Scherrer,2 who derived the equation kX p(28) = L cos 0 where /3 is the peak breadth at half-height in radians, L is the crystal size (the volume average crystal dimension perpendicular to the diffracting planes), and k is a constant about which there has been considerable disagreement. Values given include 0.94 (Scherrer, 1920), 0.89 (Bragg, 1933), 0.92 (Seljalkow, 1925), and 1.42 (Laue, 1926; Jones, (2) Scherrer, Kolloidchemie by Zsigmondy, 3rd ed.; Leipzig: Berlin, 1920; p 394.
0743-7463188/ 2404-1128$01.50/0 0 1988 American Chemical Society
