Langmuir 1992,8, 2240-2246
2240
Pressure-Induced Changes in the Absorption Spectrum of Monolayers at the Air/Water Interface: Comparison of Calculations with Experiments Masafumi Adachi, Mitsuru Yoneyama, and Shinichiro Nakamura* Mitsubishi Kasei Corporation, Research Center, 1000 Kamoshida-cho, Midori-ku, Yokohama 227, Japan Received January 13,1992. In Final Form: April 29,1992 A shift to longer wavelengths and a change in the intensity of the Soret band spectrum of monolayers @-C&“P) at the airlwater interface were of 5-(N-tetradecyl-4-pyridino)-10,15,20-tri-p-tolylporph~in observed when the film was compressed. Considering the T-A curve and the polarized absorption measurement, this change is interpreted as a rearrangement of the molecular packing within the layer. We investigated the mechanism of this spectral change by semiempirical MO INDOIS using a model porphyrin dimer system in which substituents on the pyridinium and benzene rings were replaced with hydrogen atoms. We calculated the spectra for various geometrical conformationsproduced by rotation and translation of one ring relative to the other. On the basis of the calculated spectral changes of the dimers and the observed polarization absorption measurement, a molecular level interpretation of the spectral change is presented.
Introduction Langmuir-Blodgett (LB) films have been a subject of increasingattention due to their potential use in fabricating functional devices. LB film research also provides a valuable tool in the search for a basic understanding of the properties of molecules at an interface.’ A molecular level understanding is a requisite for the construction of highly ordered macromolecular assemblies. Chromophoresprovide ameans to probe molecular level events in LB films at an interface. LB films of porphyrin derivativesincludingphthalocyanineshave been studied,’ since they have prominent spectra. For example, Miibius et al. have presented a study on the orientation and aggregation of various dye monolayers at an aidwater interface using absorption and polarized light absorption spectra,lc and they have also examined the pH dependence of porphyrin monolayers.le Schicket al. have reported the angle-resolvedpolarized light absorption spectroscopy of porphyrin monolayer assembles.lf Four different porphyrin derivatives, including zinc and magnesium porphyrin, have been studied byM6hwald et al.ld Substituent effects have been reported by DBsormeaux et al., where they have shown how small changes in the molecular structure of porphyrins strongly affect the spectral properties of the porphyrin films.lh An understanding of the monolayer assemblies of porphyrin-like molecules also providevaluable knowledge,forwardingthe understanding of the biological and catalytic features of such mo1ecules.l Changes in the spectral features, such as wavelength, intensity, and band shape, reflect changes in the environment around a chromophore. The F A curve, one of the most important observable8 in LB film research, reflects the macroscopic aspect of the phase. One way to
* To whom correspondence should be addressed.
(1) (a) Roberts,G.,Ed.LangmuiA3lodgett Film;Plenum Prees: New York, 1990. (b) Ruaudel-Teixier, A.; Barraud, A.; Belbeoch, B.;Roulliay, M. Thin Solid F i l m 1983,99,33. (c) Orrit, M.; MBbius, D.; Lehmann, U.; Meyer, H. J. Chem. Phys. 1986,85,4966. (d) MBhwald, H.; Miller, A.; Stich, W.; Knoll, W.; Ruaudel-Teixier, A.; Lehmann, T.; Fuhrhop, J.-H. Thin Solid Film 1986,141,261. (e) Loschek,R.;MBbius, D. Chem. Phys. Lett. 1988,151,176. (fl Schick, G. A.; Schreiian, 1. C.; Wagner, R. W.; Lindsey, J. S.; Bocian, D. F. J. Am. Chem. SOC.1989,111,1344. (g) Ouyang, J.; Lever, A. B. P. J. Phys. Chem. 1991,95,5272. (h) D6sormeaus, A,; Ringuet, M.; Leblanc, R. M. J . Colloid Interface Sci. 1991, 147, 57.
0143-1463/92/2408-2240$03.00/0
approach a molecular level understanding is to correlate these two aspects. In this connection, molecular orbital (MO) calculations are a useful aid in the interpretation of spectroscopicobservations,sincethe calculationscan help in determining the physical origin of spectral changes. Previously, MO calculations relating the orientation of porphyrinsto properties such as the absorption spectrum, and the excited state structures of the porphyrins within molecular dimers or larger aggregations, have been reported. A central theme was the mechanism of the photosynthetic reaction Specific examples include porphyrin dimer by PPP calculation,2 dimers of magnesium porphyrin and porphyrin by ab initio calculation: magnesium bacteriochlorophylldimer by QCFF/ PI: CNDO/S,Sand INDOIS calculations,@ the specialpair in the photosynthetic reaction center of Rodopseudomonas viridis by free electron model? CNDO/S,8and INDO/ S9calculations, and six chromosphore systems of Rodopseudomonas viridis by PPP,’ O QCFF/PI,ll and INDO/ S2J3 calculations. In these calculations, the geometries used were based on the experimental structure data, while the influence of geometricalvariation on the spectroscopic properties has received less attention. (2) (a) Kuz’mitakii,V. A.; Kravchuk, 0.V.; Solov’ev, K. N. Zh. Prikl. Spectrosk. 1981,33, 326. (b) Kuz’mitakii, V. A.; Solov’ev, K. N. Dokl. Akad. Nauk BSSR 1983,27,794. (c) Kuz’mitakii, V. A.; Solov’ev, K. N. Zh. Prikl. Spectrosk. 1983,38,267. (d)Kuz’mitakii, V. A. Zh. Prikl. Spectrosk. 1983,38, 424. (e) Kuz’mitakii, V. A. Zh. Prikl. Spectrosk. 1984, 43,959.
(3) (a) Petke, J. D.; Maggiora, G. M. Chem. Phys. Lett. 1983,97,231. (b) Petke, J. D.; Maggiora, G. M. J. Chem. Phys. 1986,84,1640. (4) Warshel, A. J. Am. Chem. SOC.1979,101,744. (5) Datta, S. N.; Priyadarshy, S. Chem. Phys. Lett. 1990,173, 360. (6) Thompson, M. A.; Zerner, M. C.; Fajer, J. J.Phys. Chem. 1990,94, 3820. (7) Kuhn, H. Phys. Rev. A 1986,34,3409. (8) (a) Kaebring, B.; Larsson, S. Chem. Phys. Lett. 1987,138,76. (b) Maslov, V. G. Biofirika 1990,35,373. (9) (a) Thompson, M. A.; Zerner, M. C. J. Am. Chem. SOC.1988,110, 606. (b)Thompson, M. A.; Zerner, M. C.; Fajer, J. J.Phys. Chem. 1991, 95,5693. (10) (a) Fischer, S. F.; Scherer, P. 0. J. Chem. Phys. 1987,115,151. (b) Scherer, P. 0. J.; Fiecher, S. F. Chem. Phys. Lett. 1987,141, 179. 1987,109,6143. (11) (a) Warshel,A.; Parson, W. W. J. Am. Chem. SOC. (b) Parson, W. W.; Warshel, A. J. Am. Chem. SOC.1987,109,6152. (12) Scherer, P. 0.J.; Fischer, S. F. Chem. Phys. 1989, 131, 115. (13)(a) Thompson, M. A.; Zemer, M. C. J. Am. Chem. SOC.1990,112, 7828. (b) Thompson, M. A.; Zerner, M. C. J. Am. Chem. Soc. 1991,113, 8210.
0 1992 American Chemical Society
Pressure-Induced Changes in Monolayers
Langmuir, Vol. 8, No. 9, 1992 2241
"
Figure 1. p-C&"P
(a) chemical structure (b) space filling
model.
We have studied the spectral aspect on Soret band (B band) absorption wavelength shifts and intensity changes in 5-(N-tetradecyl-4pyridino)-l0,15,20-tri-p-tolylporphyrin (p-Cl4PyTTP; Figure 1)monolayersspread on a water surface as a result of compression of the film. Considering the shape of the T-A curve (and the results of polarized absorptionmeasurement),where abovementioned spectral behavior was observed before the collapse pressure, this change was postulated to be due to some rearrangement inside the layer such as the rotation and translation of porphyrin macrocycles relative to one another. Such a spectral change, we would expect, is the result of an ensemble of interactions: the solvation of the hydrophilic parts by the subphase, interactions among the hydrophobic long chains, interactions between the long chains and the hydrophilic parts, and interaction within the hydrophilic parts. It would be relevant to start by analyzing the interaction of the porphyrin chromophores (the hydrophilic portion) since the absorptions being examined are localized at the porphyrin. We have examined in the present paper the interaction of two porphyrin chromophores. Semiempirical molecular orbital INDO/S14calculations of a model porphyrin dimer system (H2-porphine, called H2-P hereafter) are presented. In this model system, the substituents on the pyridinium and benzenerings present inp-CI&T"P were replaced with hydrogen atoms. These calculations, in contrast to the abovementionedstudies, followthe spectral behavior as a function of changesin the various geometrical conformationsthat may be adopted by a dimer aggregate.
I
-
7
I I I
I
Figure 2. Dimer structure of Hrporphine and geometric parameters used in the model.
Calculations Calculations were performed by the INDO/S method (modified to perform spectral calculation^).^^ The electronic repulsion integral was determined by the Nishimoto-Mataga f0rmu1a.l~ All SCF calculations were executed at the closed shell Hartree-Fock level (RHF). Configurationinteraction (CI) calculations included single excited configurationsfrom the ground state, 12(occupied) X 12 (virtual) for H2-P monomer, 24 (occupied) X 24 (virtual) for H r P dimer, and 10 (occupied) X 15 (virtual) for p-ClPyTTP (where -C14H29 was replaced with -CHs in the model molecule). The moleculargeometryof the H r P monomer wastaken from X-ray data,I6 and that of p-C1PyTTP was adapted ~ J ~infrom the X-ray data for a similar m ~ l e c u l e . ~The terplanar distance of p-CI.&TTP moleculesat the water surface was assumed to be 4 A based on the interplanar distance found in the crystal structure (3.64.0A)of similar porphyrins.17J8 We have assumed the porphyrin rings to be parallel. As shown in Figure 2,the relative orientation of pairs of porphine molecules A and B is represented using the coordinates of molecule B (X,Y,2, e), where 8 is the rotation angle round the z' axis.
Results and Discussion Spectral Changes Upon Monolayer Compression. Figure 3 showsthe typical pressure-dependent absorption spectra of a p-Cl4Py"P monolayer measured at normal incidence. At the film is compressedup to about a pressure Experimental Section of 30 mN/m, the absorption spectrum remains virtually The structure of the porphyrin molecule used in LB film unchanged, having a similar band shape as the monomer experiments,5-(N-tetradecyl-4-pyridinio)-10,15,20-tri-p-tolylpor- but being slightly blue shifted. The spectrum at a film phyrin (pC14PfI'TP), is shown in Figure 1. This compound pressure of 20 mN/m is shown by the solid line in Figure was dissolved in spectroscopicgrade chloroformand then spread 3. As the compression is continued from 30 mN/m, the onto a purified aqueous subphase (pH 6.5,23"C)to form surface absorption peak becomes slightly red shifted and the band monolayers. Surfacepressurearea (FA) isothermsof the monoshape becomes broader and somewhat lower in intensity layers were measured with a compression speed of about 0.5 Biz (Figure 3, dotted line). In this region the spectral shape molecule-' s-l. Visibleabsorption spectra of the monolayers were monitored as a function of surface pressure using a multichannel is dependent on the pressure up to about 40 mN/m. Then, spectrophotometer equipped with optical fiber probes (MCPDwhen the pressure is increased beyond 40 mN/m, the 110, Otsuka Electronics). Transmission spectra were obtained spectrum again becomes stable until the collapse pressure as follows: the incident light traversing the monolayer was of 46.6 mN/m is reached (see Figure 4). The absorption reflected by a mirror under the water surface and detected by maximum in this region is further red shifted, but the a fiber probe following a second pass through the monolayer. band has narrowed again (Figure 3, dashed line). This Polarized absorption spectra were measured at an incident angle of 45O for both normal polarized (s-polarization) and parallel polarized (p-polarization) light. (14) (a) Ridley, J. E.; Zemer, M.C. Theor. Chim. Acta 1973,32,111. (b)Bacon, A. D.; Zemer, M. C. Theor. Chim.Acta 1979,53,21. (c) Zemer, M. C.; Loew, G. H.; Kirchner, R. F.; Mueller-Westerhoff, U. T. J. Am.
Chem. SOC.1980,102,589.
(15) (a) Nishimoto, K.; Mataga, N. 2. Phys. Chem. (Frankfurt am Main) 1957, 12, 335. (b) Mataga, N.; Nishimoto, K. 2.Phys. Chem. (Frankfurt am Main) 1957,13, 140. (16) Chen, B. M. L.; Tulinsky, A. J. Am. Chem. SOC.1972,94,4144. (17) Hamor,M. J.; Hamor, T. A,; Hoard, J. L. J. Am. Chem. SOC.1964, 86, 1938. (18) Silvers, S. J.; Tulinsky, A. J. Am. Chem. SOC.1967,89,3331.
Adachi et al.
2242 Langmuir, Vol. 8, No. 9,1992 0.06
0.08 -
8
0.04I
E e
5
e P 9
s
0.06
8 0.04
9 0.02
0 400
500
600
400
500
Wavelength (nm)
Wavelength (nm)
Figure 3. Absorption spectra of monolayer of p-C&TIT a function of the f i i pressure.
aEE
6o
$
40
-
600
as
Figure 5. Polarized absorption spectrum at 20 mN/m.
-. \
9.
v)
E
a
0
20
01 06
I
-
08
1.0
1.2
1.4
16
”
400
Wavelength (nm)
1.8
Area (nm2imoiecuie)
(points indicate where absorption spectra were measured (see Figure 3)).
600
500
Figure 6. Polarized absorption spectrum at 45.5 mN/m.
Figure 4. r A curve of p-C&”P
-
spectral behavior of the B bands, intense/narrow up to about 30 mN/m less intense/broad at about 35 mN/m intense/narrow at about 45.5 mN/m, was reproducible in all the samples (- 10) and, furthermore, repeatedlywith compression and decompression of the same sample. On the other hand the Q bands are virtually unchanged throughout the compression process. Also, throughout the process the surface layer remains homogeneous to the eye when observed through an optical microscope. Polarized Light Absorption Measurement. The ratio of the optical densities for s-polarized light to ppolarized light,AJA,, has been shown to be directly related to the angular distribution of transition dipoles in monolayer and multilayer a s a e m b l i e ~ . ~ ~This J ~ Jtechnique ~ can be used to estimate the chromophore orientation at the airlwater interface. We attemptad to obtain the average tilt angle of the porphyrin ring with respect to the water surface by applying the method developed by Orrit et al.lC They have treated the reflection from a monolayer on a water surface as well as the transmission through a monolayer on aglass substrate. However,the situation discussed in the present paper is the transmission with a double pass through a monolayer on a water surface. Therefore, we evaluated the relationship between the experimental quantity A$A, for a double pass and the orientation parameter P which is defined as
-
(19)Vaudevyver, M.; Barraud, A.; Raudel-Teixier; Maillard, P.; Gianotti, C.J. Colloid Interface Sci. 1982,86, 571.
P = (cos2,) (1) where B is the porphyrin tilt angle, i.e., angle between surface normal and porphyrin ring normal. By use of this definition of the orientation parameter, the average polarizability tensor, a11and (YL,can be related to P as a
(1/2)(1
+ PI,
aA a (1-P)
(2)
On the basis of this equation, together with eq 16 in ref IC relating the polarizability tensor with the reflection and transmission amplitudes of the dye monolayer, the behavior of the ratio AJA, as a functionof P was calculated by using the multireflection scheme.lc The porphyrin orientation presented below has been obtained in this way. Figures 5 and 6 show the typical polarized absorption spectra of a p-&PyTTP monolayer measured at 20 and 45.5 mN1m. Identical spectra were obtained irrespective of the azimuthal angle of the plane of incidence,indicating the occurrence of a nearly isotropic in-plane distribution of the porphyrin molecules on a lengthscale smaller than the size of the incident beam (-1 mm). At low film pressures (40 mN/m) the polarized spectrawere highly reproducible both in intensity
Pressure-Induced Changes in Monolayers
Langmuir, Vol. 8, No. 9, 1992 2243
t
-0.04
Low
pressure
High pressure
Figure7. Models of the molecular organization at the aidwater
-0.06 I
interface.
c"
Table I. Absorption Properties of the Monomers ObsaNedoP - c ~ s f l P calculated (INDO/S) 4L/ P-C1PYTrP Hz-porphine A-(nm) mol-cm) A-(nm) fb h-(nm) f Qx 650 (0-0) 5300 721.1 (0.032) 720.3 (0.021) 591 (0-1) 6300 QY 558(0-0) lo600 546.4 (0.064) 593.1 (0.036) 518(0-1) 14800 380.9 (0.731) 369.0 (1.396) Bs 418 198000 375.2 (2.187) 347.4 (2.431)
-
2
BY
a
Ethanol solution (3.14 X 0.6
: e
0.4
0
0.2
0
A 400
lV M). Oscillator strength.
500
600
Wavelength (nm)
Figure 8.
p . C J " I T P absorption spectrum in ethanol.
and in peak wavelength. The ratio A$A, was 1.5 as seen in Figure 6, indicating that the porphyrin rings lie nearly pardlel to the water surface. The occupied molecular area at 45.5 mN/m was about 91 A2 (Figure 41, which is less thanhalfof the planar area of the porphyrin (-200 A2).lb Such a small area per molecule may be explained by an arrangement of porphyrin rings parallel to the water surface if a stacked structure is formed at the air/water interface, with the porphyrin rings overlapping one another (Figure 7). Spectrum of Monomer. As a basis for the comparison of spectra of the compacted molecules on the water surface, we have measured and calculated the spectral data for the free, nonaggregated porphyrin. The absorption spectrum of p-C&"P in ethanol is shown in Figure 8 and the absorption spectra of p-C1PfITP monomer and H r P monomer calculatedby INDO/ S are shown in Table I. As is well-known, INDO/S reproduces quite well the spectrum of free porphyrin.20 The low intensity peak at 650 nm (e 5300) and 558 nm (e 106oO) with vibrational progressions are assigned to be the Q bands (Figure 8), and they are in good agreement with calculated spectral bands of 721 and 546 nm (Table I). The absorption band a t ,A 418 nm (e 198 O00) for p-ClrPfITP, assigned to be the Soret band (Bband),2l (20) Edwards, W. D.; Weiner, B.; Zerner, M. C . J. Am. Chem. SOC. 1986,108,2196, and references therein. (21) (a) Goutarman, M.. J. Mol. Spectrosc. 1961,6,138. (b) Goutermau, M. In The Porphynm; Dolphin, D., Ed.;Academic Press: New York, 1978; Vol. III,pp 1-166.
virtual
1
-0.22 ' occupied
m
c
-0.23
, ,
'
-0.24'
-0.25 ' monomer A
dimer
monomer 0
Figure 9. Frontier molecular orbitals (MO) of Hz-porphine for the monomer and dimer (dimer: 2 = 4 A,X = Y = 0 A, 6 = 0"). Table 11. Calculated Absorption Properties of Q and B Bands in HrP Dimer (2-4k X = Y = 0 A,e = oo) h- (nm) P transition t w e transition character 746.6 ERb (0.OOO) Qx 721.1 (0.029) ER Qx 619.0 ER (0.OOO) QY 590.2 (0.041) ER QY 481.5 CRc (0.OOO) Qx 476.0 CR (0.002) Qx 467.7 CR (0.OOO) QY 465.5 (0.001) CR QY 431.2 CR (0.OOO) BY 427.9 CR (0.OOO) B, 426.4 (0.003) CR BY 417.5 CR (0.001) B, 393.7 ER (0.OOO) BZ 385.0 ER (0.OOO) BY 352.3 ER (1.163) B, 315.6 ER (3.363) BY 0 Oscillator strength. Exictonic resonance. Charge resonance.
corresponds to the calculated values of 375 and 381 nm.22 Compared to the film at low pressure, the shape is quite similar, but in the film ,A, is slightly blue shifted. Spectrum of Dimer. In an attempt to shed light on the ensemble interactions, we have isolated from the whole system the minimum unit of interaction, the porphyrin dimer system (Figure 2). The orbital interactions that arise from dimer formation are shown in Figure 9. The two pairs of occupied orbitals a, and bl, interact to generate four occupied orbitale. The same is true for the two virtual orbitals, bz, and b3g. The original B, and Bybands, as a result, each give rise to a pair of transitions. Table I1 shows the calculatad &, Q B,, and Bytransitions for an interplanar distance of 4 (X= 0 in Figure 2). The two calculated B bands (352 and 316 nm in Table 11)are slightly blue shifted relative to the monomer's values (369 and 347 nm in Table I). In Table 11,ER indicates the excitonic resonance (transition in one molecule)? and CR indicates the charge resonance (transition of intermolecular).2 On account of our purpose to investigate the mechanism of spectral change upon compression, we shall concentrate on the B, and By bands whose spectral features were observed to change upon compression (Table 11). The changes in the absorption spectrum of hematoporphyrin and protoporphyrinon dilution in 0.02 M NaOH were reported by Gallagher and Elliott,%where the dimer-
x
(22) Ds symmetry is adopted for the calculations. (23) Gallagher, W. A.; Elliott, W. B. Ann. N.Y. Acad. Sci. 1973,206, 483.
Adachi et al.
2244 Langmuir, Vol. 8,No. 9, 1992
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I
f
2
E
X
C
E
x
m
5
BY
-E
v
i
350
350
5 m
4
;
-m ._
q 1
P BY
300
300
0
2
4
6
0
0
(A) Figure 10. Spectral change of the B band of porphine dimer with changes in the interplanar distance (INDO/S).
Rotational angle
Interplanar distance Z
ization was discussed. A similar reslt on dilution of protoporphyrin was also reported by White and Plane." The B band was found to be far more sensitive to change from dimer to monomer than was the Q band. The spectra of B bands change from sharp to broad on dimer formation with slight blue ~ h i f t This . ~ ~blue ~ ~shift ~ is consistent with the calculated change from monomer (Table I) to dimer (Table 11). The dimerization mechanism in solution is probably different from that in an oriented film at an air/water interface; however the assumption to limit the source of spectral change on chromophore is indirectly supported since the order of magnitude of the spectral change is comparable. Especially, the blue shift and the decrease of intensity due to stacking in a dimer formation are consistent with the calculated results (Figure 10). Spectral Change in Various Dimer Conformations. We have examined the spectral change of the dimer as a function of four geometrical variations: change of the interplanar distance (Zin Figure 21,rotation (e), translation (X,Y), and the combination of rotation and translation. A. Interplanar Distance. Changes in the spectrum as a function of the interplanar distance (2)(the other geometric parameters of Figure 2 are set to be zero) are shown in Figure 10. Starting at a distance of 8 A, the transition characters of B, and Byare essentially those of the free monomer. As the interplanar distance decreases, the interaction energy increases. As a result, the absorption maximum (Amm) moves to high energy (shorter wavelength) and the oscillator strength decreases until the interplanar separation is about 4 A, which coincides with the distance found in the crystal ~tructure.~~J8 When the compression continues to less than 3 A, strong orbital interactions start. Repulsive forces become dominant,and the structure is not stable. On the other hand, theQ bands are only slightly perturbed by this motion 720-728
(a,
(24) White, W. I.; Plane, R. A. Bioinorg. Chen. 197.4,4, 21.
0 180
90
e
(")
Figure 11. Spectral change of the B band of porphine dimer as a result of rotation (INDO/S). nm; Qy,590-599 nm). This is consistent with the report25 that the B state exciton interactions are much larger than the Q state interactions, being proportinal to the oscillator strength. B. Rotation. The effect of rotation (e) on the spectrum, while keeping Z = 4 A and fixing the other parameters to zero, is shown in Figure 11. The Q bands are influenced only slightly (721-732 nm and 590-608 nm). As the rotation angle 6 is changed from 0" toward No, the B, absorption shifts to shorter wavelengthsand the oscillator strength increases, whereas the By absorption shifta to longer wavelengths and the oscillator strength decreases. At the rotation angle 90",the B, and By transitions come to be degenerate due to the symmetry of the pair. With an increase of the rotation angle from 90"to 180°, the B, (at 0")transition changes ita character to By,and vice versa, the By(at 0") changes to Bz.26Thus the spectral change in this region corresponds to that in the region of ' 0 to 90". It should be noted that the model H2-P has Dur symmetry, while the real p-C1J'yTTP has at most C, symmetry. As a result the periodicity of the change is different. C. Translation. Next, translations within the 2-y plane are considered, with the other parameters fixed; 2 = 4A and 6 = 0". The calculated spectra of the x-directed translation is shown in Figure 12,the y-directed translation in Figure 13,and the translation directed along the line X = Y in Figure 14. Upon translation, both the absorption wavelength and oscillator strength are increased in all casesfor the B bands. Q bands are again almost unchanged 719-721nm; &., 590-594 nm). Near the translation distance of 7 A, the
(a,
(25) Gouterman, M.;Holten, D.; Lieberman, E. Chem.Phya.1977,25,
139. (26) In detail, with an increase of the rotation angle larger than Oo, there no longer exists any symmetry elements. Then, the original By transitionsbegintomiwith 4 transitions (whichareoriginallywithout intensity) through an accidentaldegeneracy. In order to avoid complexity, in Figure 11 only a t the range from 30° to 90°, the mainly Bycharacter band is shown.
Pressure-Induced Changes in Monolayers
Langmuir, Vol. 8, No. 9,1992 2245
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1
-E2
350
E
A
t
300
0
300
2
6
4
8
10
2
0
Translation distance X (A)
-
Figure 12. Spectral change of the B band of porphine dimer with translation along the direction of the x axis (INDO/S).
4
6
0
10
Translation distance R (A)
Figure 14. Spectral change of the B band of porphine dimer with translationalong the direction of the line X = Y (INDO/S).
-o--o-
1;j-
*
hax f
u
4A
BY
4
-
c
Bx BY
300
350
t 1
0
4
6
8
600
700
800
Fi ure 15. Absorption spectrum of porphine dimer translated 7 along the x axis (INDO/S).
d
’0
2
500
Wavelength (nm)
BX
300
400
10
Translation distance Y (A)
Figure 13. Spectral change of the B band of porphine dimer with translation along the direction of the y axis (INDO/S).
absorption,, ,A coincides with that of the monomer and continuing the translation causes the ,A, to continue to shift to the red. In the x-directed translation (Figure 12), the change in B, is particularly pronounced, and this is similarly true for the Bychange in the y-directed translation (Figure 13). In the 45O directed translation, an intermediate behavior to the x - and the y-directed translations is obtained (Figure 14). Some points not in line with the monotonic trend have appeared in Figure 13 (at 3 A) and in Figure 14 (at 5 A). They are assigned to be due to the mixing with other transitions, mainly v u * ,
which are originallyvery small in intensity and accidenkly degenerate with the B bands in this geometry. D. Rotation and Translation. The possibility of rotation combined with translation was examined. Analysis of the B bands becomes complicated because the symmetry of the system has been reduced and almost all the transitions are allowed. The Q bands remain essentially unchanged (720-723 nm and 591-595 nm). The general trend for changes in the wavelength and oscillator strength is demonstrated by a comparison of the effect of a translation without and then with rotation. First, in Figure 15,the result of a 7-A translation in the x direction without a rotation is shown. Second, in Figure 16 is shown the result of the 7-A n-directed translation now coupled with a 30° rotation. The two distinct, intense bands (Figure 15) transform upon rotation into a set of less intense bands (Figure 161,because the reduction of the symmetry leads to mixing with what were originally symmetry forbidden bands. The average value of the wavelength remains approximately the same. Although we have shown the result of rotation occurred in only the x-directed translation (Figures 15 and 161,the
Adachi et ai.
2246 Langmuir, Vol. 8, No. 9, 1992
300
400
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600
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Wavelength (nm)
Fi ure 16. Absorption spectrum of porphine dimer translatad
7 falong the z axis and rotated 30° round the z' axis (INDO/S).
same conclusion has been verified for the y-directed and the 45O X = Y directed translation, that is, the band splits with a loss in its intensity and remains at about the same wavelength. Mechanistic Consideration. We have obtained two pieces of information: (i) the tilt angle of the chromophore plane at the aidwater interface varies from an angle 4045O to Oo as the film pressure is increased and (ii) the calculated spectral changes for various conformational changes of the chromophore dimer. On the basis of these two, we propose that the observed spectral changes caused by increasing the film pressure may be interpreted as illustrated in Figure 7. At low pressures, 0-30 mN/m in Figure 4, polarized absorption spectroscopy shows chromophore molecules are canted 40-45O (on the average) with respect to the air/water interface. This is consistent with the fact that because the planar area of one molecule is 200 A22,1bthe rising point at 120 A2 in the PA curve means that chromophore cannot be completely flat at the aidwater interface. The spectral band shape (Figure 3) resembles that of the monomer, slightly blue shifted relative to Am= = 418 nm of the monomer. This suggests that there is some interaction between chromophores, but it is weak as the magnitude of the effect is slight. And considering that the absorption spectrum remains the same, independent of the pressure in this region, we postulate a certain type of stable aggregate, presumably a dimer, is the*majority species, rather than a homogeneously distributed nonaggregated configuration (see the left side of Figure 7). As the pressure increases, free spaces between these stable aggregates decrease, but the relative geometric orientation between chromophoresis not much affected and the free spaces are probably not yet exhausted. At intermediate pressures, 30-40 mN/m in Figure 4, the spectrum becomes unstable. It depends on the given pressure until 40 mN/m. This suggests that free spaces between aggregates have been closed and the chromophores are being farced into new conformations with different absorption spectra That is, in this region increases in film pressure alter the relative orientation of chromophores rather than alter the free volume in the monolayer. As the intermediate step in the two extremes N
of Figure 7, there would be more randomness than is depicted in the right side of Figure 7. What conformations may result in a broadening and a shift in the spectrum that was observed (Figure 3)? The calculated spectra suggest that the band intensity changes may be due to rotations of the chromophores relative to one another (see B. Rotation and Figures 15 and 161,and to produce the red shift,translated arrangements are suggested (Figures 12,13,and 14). In the highest pressure region, from 40 mN/m up to the collapse point (Figure 4),again a stable spectrum that has been red shifted and become narrow is observed. Polarized light absorption spectroscopy shows the chromophore is approximately parallel to the aidwater interface. The observed red shift may be explained by the calculations showing a red shift occurs upon translation (see Figures 12, 13, and 14). The observed sharpening also may be interpreted by the intensity increase that occurs on translation (seeFigures 12,13,and 14). As it isjust before the collapse,the number of possible conformationsbecome restricted. A simplified view of the monolayer structure is depicted in the right side in Figure 7. This picture of a double layer structure of the porphyrins stacked at the interface is based on (i) the collapse point occurring at the calculated area of 90 &molecule, which is roughly half the size of the planar area of the porphyrin, and (ii) polarized absorption spectroscopy indicating the porphyrin ring is parallel to the interface. Conclusion A shift to longer wavelengths and a change in intensity of the Soret band spectrum of P - C I ~ ~ T monolayer TP on water surface was observed when the f h w a s compressed. The tilt angle of the molecular plane to the air/water interface was estimated by polarized absorption spectroscopy. By using the INDO/Smethod,we investigated the mechanism of this spectral change caused by ensemble interactions. A model porphyrin dimer system was selected as the minimum unit of the interaction of chromophores. We examined the spectra of various geometrical conformations produced by rotation and translation. The Q bands were almostunaffected by these elementalmotions, due to the smallexciton interaction. The observed Q bands also remained unchanged. The B bands' wavelength shift and change in intensity are discussed, as related to the structural features of the monolayer. We believe that the present results of the calculation using the minimum unit of the interacting chromophorescombined with polarized absorption measurements reflect some important mechanistic features of the changes occurring in LB monolayers being compressed. Acknowledgment. The authors thank Professor M. C. Zerner for the release of the semiempirical INDO/S MO program and Dr. D. Albagli for valuable discussions and the proofreading of the draft. This work was performed under the management of FED (the R&D Association for Future Electron Devices) as a part of the R&D of Basic Technology for Future Industrim supported by NED0 (New Energy and Industrial Technology Development Organization).
