Langmuir 1988,4, 583-588
H2°
I "L
["
HP' I HO
2
Figure 4. Schematic representation of the local environment of the Cu(I1) ion on the surface of K. pneumoniae. The suggested
structure involves a bidentate chelate coordination with histidine bound to a surface peptide chain.
FT-ESEEM on paramagnetic species the exact zero-field NQR frequencies are not obtained as, for instance, in the corresponding diamagnetic metal complexes. Two reasons might be responsible for these deviations. Firstly, the proton on the remote 14N of metal-ligated His may be hydrogen bonded to the protein structure; in the case of model compounds there can be a hydrogen bond to the solvent, which is H20 in our case. It should be noted that the two lower NQR frequencies (yo, u-) are shifted by more than the high-frequency value (v+) in relation to the values obtained for systems when there is no substitution on the tricoordinated nitrogen site.18 Secondly, and presumedly, the most striking deviation stems from some difficulties of which ESEEM spectroscopy suffers in disordered systems: anisotropic broadening leads to destructive interference of the echo modulation,
583
which resulta in distorted line shapes in the frequency domain.24 Reijerse and K e i j ~ e r sshowed ~~ by spectra simulation that intense low-frequency components may arise, caused by anisotropic hyperfine interactions in combination with an anisotropy of the modulation intensity, which yield somewhat shifted quadrupole values. This might also be the case in our system. Furthermore, it has ale0 been pointed out%that the high-frequency component provides helpful information about the validity of the assumption that contact and nuclear Zeeman interaction are of comparable magnitude. For U l 4 ~N 1 MHz, this transition should occur at about 4 MHz, which is sufficiently confirmed by our experimental data (cf. Table 11). Summarizing all results leads to the assumption that the Cu(I1) ion is coordinated bidentally to histidine with evidence of remaining H20molecules occupying axial as well as equatorial positions. The tentative structure of the complex formed by Cu(I1) on the bacterial cell surface is shown in Figure 4. In this work it is shown that by means of complementary magnetic resonance data it is possible to elucidate the local environment of adsorbed metal ions (e.g., Cu(I1)) on unknown biological surfaces. On the basis of the available data an assignment of the most likely surface functional group was achieved.
Acknowledgment. This research has been supported by a grant of the Board of the Swiss Federal Institutes of Technology. Registry No. Cu(His), 77280-83-2; Cu(Hi&, 13870-80-9; C U ( H ~ O ) 14946-74-8; ~~+, His, 71-00-1. (24) Astashkm, A. V.; Dikanov, S. A.; Tsvetkov, Yu. D. Chem. Phys. Lett. 1987, 136,204. (25) Reijerse, E. J.; Keijzers, C. P. J. Mugn. Reson. 1987, 71,83.
Absorption and Fluorescence Properties of Rhodamine B Derivatives Forming Langmuir-Blodgett Films M. Van der Auweraer,* B. Verschuere, and F. C. De Schryver Chemistry Department KULeuven, Celestijnenlaan 200F, 3030 Leuuen, Belgium Received June 4, 1987. In Final Form: November 20, 1987 Surface pressurearea isotherms for mixed films of diodadecylrhodamine B and arachidic acid with varying mixing ratios were recorded. Monolayers and multilayers were deposited on hydrophylic glass slides. Absorption spectra, fluorescence spectra, and relative quantum yields of fluorescence were examined as a function of the mixing ratio in the film. The type of aggregates formed in homogeneous solution was not observed, and the relative quantum yield of fluorescence decreases with increasing amount of dye in the layer. From these results a packing in the Langmuir-Blodgett monolayer can be proposed. Two dye molecules form a dimer in which the transition dipoles and the vector linking the centers of the molecules form an angle of 55O, leading to a dimer without spectral shift. The similarity between the photophysical properties of adsorbed rhodamine B molecules at high coverages and of the monolayers investigated in this contribution could point to a similar packing of the chromophores.
Introduction Although the aggregation of rhodamine' in solution has been observed several years ago, the nature of the aggregates and their photophysical properties are still a controversial topic.2 FBrster and KBnig3showed that in an (1) Avnir, D.; Levy, D.; Reisfeld, R. J. Phys. Chem. 1984, 88, 5956. ojeda, R. P. Chem. Phys. Lett. 1982,87,656. (2) Lopez-Arbeloa,I.; 0.;
aqueous solution of rhodamine B Beer's law is valid up to a concentration of 5 X lo* M; at higher concentrations a new band at the hypsochromic side was observed. The band at low concentrations is due to absorption of the monomers, while the hypsochromic band was attributed to the absorption of dimers. Dimerization leads to a blue (3) Ffster, Th.; KBnig, E. 2.Electrochem. 1967, 61,344.
0743-7463/88/2404-0583$01.50/00 1988 American Chemical Society
584 Langmuir, Vol. 4, No. 3, 1988
Van der Auweraer et al.
shift of the absorption band the absorption spectrum of the dimer consists of a hypsochromic band (an allowed transition) and a less intense bathochromic band (forbidden transiti~n).~ This change in the absorption spectrum was accompanied by a decrease of fluorescence quantum yield and decay time a t higher concentrations. This observation is quite general and is known as "concentration quen~hing".~Later it was observed that the dimer formed by rhodamine B' in water was a nonfluorescent sandwich dimer. The quenching of the emission of the rhodamine B monomers was then attributed to excitation energy transfer from monomer to dimer.l Rhodamine B has been used to investigate the properties of molecules adsorbed on various organic and inorganic substrates. When the adsorption takes place from a dilute solution a red shift of the absorption m a x i " is observed for all the substrates used (clay particles,6 q ~ a r t zinor,~ ganic semiconductors: polystyrene? or organic single crystaldo). The bathochromic shift is diagnostic for the transition of the dye molecule from the aqueous phase to the adsorbed state and is the consequence of the change in the polarity and refractive index of the environment of the dye due to the adsorption process. Garoff et al?bhave observed analogous results for rhodamine 6G adsorbed on glass. Adsorption from more concentrated solutions leads to a higher surface coverage of the adsorbed molecules. Under those circumstances however no band corresponding to the absorption of the solution dimers is observed. Fluorescence spectra of adsorbed rhodamine molecules are red shifted compared to those in solution. Higher surface coverage leads here to a dramatic decrease of the fluorescence intensity and a small broadening and red shift of the emission maximum.11J2 Kemnitz et a1.12cascribe this red shift to dimers where the coupling between the monomers is considerably smaller than in the solution dimers. They assume that a broad distribution of structurally slightly different dimers is formed, leading to a simultaneous presence of a slightly red- and blue-shifted absorption. In emission only the transition from the lowest state of each dimer is observed, which leads to a red shift of the emission. When rhodamine B is adsorbed on an anthracene13 single crystal or an inorganic semi~onductor'~ @noz,TiOJ, photosensitized electron and hole currents can be observed. This charge carrier injection process is accompanied by fluorescence quenching."JZb While on perylene single
crystals and inorganic semiconductors the quantum yield of those currents does not depend upon the coverage,16it decreased3 strongly on anthracene single crystals, indicating that in this system the excited dimers inject holes with a considerably lower efficiency. Furthermore,l6 a detailed study of the electric field dependence of the sensitized hole current shows that a t higher dye concentration species (dimers) are present that act as efficient recombination centers. This effect is, however, not accompanied by the appearance of the absorption band of the solution dimer in the action spectrum of the sensitized hole current. Such action spectra must however be considered with sufficient caution, as they indicate the state of aggregation of the adsorbed dye molecules only if the energy transfer from monomer to dimers has an efficiency of one or if monomer and dimer have the same injection effi~iency.'~The way in which the photosensitized currents are quenched by an increase of the surface coverage of the dye is however difficult to understand without the assumption of ground-state dimers to which efficient energy transfer occurs. This dimer formation is very efficient as it occurs already at very low (lo4 M) concentrations of the dye in solution. As the local concentration of the dye on the surface is considerably higher than the average concentration in solution, it is not straightforward that the tendency for aggregation is 1argeP on the crystal. These observations indicate that a detailed knowledge of the structure and photophysical properties of the aggregates of adsorbed dye molecules is important for the study of photosensitization processes. In order to be able to study those effects of dimerization upon sensitized hole currents in a more controlled way, the adsorbed dyes were replaced by dioctadecylindolocarbocyanine and dioctadecy10xacarbocyanine,''J9 which could be incorporated into Langmuir-Blodgett monolayers. The extra information available about the packing of the dye molecules in the monolayer limits the possible intermolecular orientations in the dimers and aggregates. When a monolayer-forming derivative of rhodamine Bm is incorporated in a Langmuir-Blodgett monolayer that can be deposited on a solid surface, more information will be available about the packing of the rhodamine B molecules in eventual aggregates. In a monolayer the rhodamine B molecules are forced to have at least a partially predictable orientation in the aggregates. Comparison of the photophysical properties of those aggregates with those of adsorbed rhodamine B molecules will eventually allow one to obtain more information about the latter species.
(4) (a) Chambers, R. W.; Kajiwana, T.; Keans, D. J. Phys. Chem.1974, 78,380. (b) Gal, M. E.; Kelly, G. R.; Kurucaev, T. J. Chem.SOC., Faraday Trans. 2 1973,69, 395. (5) Koizumi, M. Photochemistry 1963, 205. (6) (a) Gollnick, K.; Franken, T.; Fouda, M. F. R. Tetradedron Lett. 1982,22,4049. (b) Grauer, Z.; Malter, A. B.; Yariv, S.; Avnir, D. Colloids.
Experimental Section Nfl-Bis(ethyloctadecy1)rhodamine B (SRB)was prepared according to ref 20. It was purified by column chromatography
Surf.,submitted.
(7) (a) Kemnitz, K.; Tamai, N.; Yamazaki, I.; Yoshihara, K. J. Phys. Chem. 1986, 90,5094. (b) Garoff, S.; Stephens, R. B.; Kanson, C. D.; Sorenson, G. K. Opt. Commun. 1982,41, 257. (8) Spitler, M.; Calvin, M. J. Chem. Phys. 1977, 67, 5193. (9) (a) Kemnitz, K.; Murao, T.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. Chem. Phys. Lett. 1983, 101(4,5), 337. (b) Nakashima, N.; Yoshihara, K.; Willig, F. J . Chem. Phys. 1980, 73, 3553. (10) Mulder, P. J. Philips Res. Rep. 1967, 22, 553. (11) Itoh, K.; Chiyokawa, Y.; Nakao, M.; Honda, K. J. Am. Chem. SOC. 1984,106, 1620.
(12) (a) Kemnitz, K.; Murao, T.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. Chem. Phys. Lett. 1983, 101(4,5), 337. (b) Nakashima, N.; Yoshihara, K.; Willig, F. J.Chem. Phys. 1980, 73, 3553. (c) Kemnitz, K.; Tamai, N.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. J . Phys. Chem. 1986,90,5094. (13) (a) Willig, F.; Michel-Beyerle, E. Photochem. Photobiol. 1972,16, 371. (b) Willig, F.; Gerischer, H. Top. Curr. Chem. 1976, 61, 331. (14) Tributach, H.; Gerischer, H. Ber. Bunsen-Ges. Phys. Chem. 1969, 73, 251.
on silica gel using chloroform as eluent. Arachidic acid (Aldrich p.a.) was recrystallized from ethanol. Monolayers were spread M solution of CdClz (Aldrich gold label) in water on a 5 X purified by a Milli-Q system. The pH was adjusted to 5.5 by addition of HCl. A circular trough of the Fromherz type was used (15) Fujishima, A.; Hayashitani, E.; Honda, K. Seisan Kenkyu 1971, 23, 363. (16) Willig, F.; Mfiller, N.; CharlC, K.-P. Electrochim. Acta 1979,24, 463. (17) Van der Auweraer, M.; Willig, F. Zsr. J. Chem. 1986, 25,274. (18) Killesreiter, H. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 503. (19) (a) Van der Auweraer, M.; Verschuere, B.; Biesmans, G.; De Schryver, F. C.; Willig, F. Langmuir, in press. (b) W X i , F.; CharlC, K.-P.; Van der Auweraer, M.; Bitterling, K. Mol. Cryst. Liq. Cryst. 1986, 137, 229. (c) Van der Auweraer, M.; W a g , F.; CharlB, K.-P. Chem. Phys. Lett. 1986, 128, 214. (20) (a) Hurd, C. D.; Schmreling, L. J . Am. Chem. SOC.1937,59,112. (b) Ioffe, I. S.;Shapiro, A. L. J. Org. Chem. USSR 1970, 6, 356.
Absorption and Fluorescence of Rhodamine B Derivatives
0
20
60
140
100 e 2
Average area/ m o l e c u l e ( A
Figure 1. Surface areaadace preasure isotherms of SRB diluted with arachidic acid with varying mixing ratios. to prepare and compress the monolayers. The monolayers were deposited on glass slides (10 X 50 mm) cleaned by heating for 2 h at 110 O C in sulfochromic acid, followed by repeated rinsing in Milli-Q purified water and an overnight stay in a lod M solution of NaOH in Milli-Q purified water. Monolayers of rhodamine B were compressed at a rate of 6.4 cm2 s-’ at a total surface of about 150 cm2;they were deposited with a rate of 1.6 mm min-’ at a pressure of 30 dyn cm-’ for the diluted systems and 27 dyn cm-’ for the pure dye monolayer. Absorption spectra were recorded on a Perkin-Elmer 550 S spectrophotometer. Fluorescence emission and excitation spectra were recorded on a Spex fluorolog. A sample compartment was built to record the spectra under vacuum t o prevent photooxidation.21
Results Pressure-Area Isotherms. In a mixed layer with a long-chain fatty acid, e.g., arachidic acid (C19H&OOH), the following observations suggest that the paraffin substituents of the dye are incorporated into the layer by interaction with the chain of the fatty acid. The hydrophobic chromophore is at the interface with the aqueous solution. Monomolecular films with a mixing ratio dyelarachidic acid varying from pure dye to 11400 are spread on a water surface. In Figure 1 the surface pressure of the mixed films is plotted against the average area per molecule determined by the total area of the monolayer and the total number of molecules in the monolayer according to eq 1:22 A,, =
(Adye
+ NAaJ/(N + 1)
(1)
where A, is the average area per molecule, N the number of arachidic acid molecules per dye molecule, A, the area per arachidic acid molecule, and Adre the area per dye molecule. From the average area per molecule the area per dye molecule at a predetermined pressure can be calculated according to eq 1, assuming that there are no arachidic acid molecules between the two apolar chains of the dye molecule. This arrangement was already observed for monolayers of dioctadecylbenzothiazolemonomethincyanine diluted with octadecane.22 A t 30 dyn cm-’ a value of 60 A2 is found for all the mixing ratios, a value which is comparable with the 8 A for the average molecular radius of rhodamine B calculated by Nakashima et a1.12 The (21) Van der Auweraer, M.; Biesmans, G.; Verbeek, G.; Verschuere, B.; De Schryver, F. C., unpublished results. (22) Kuhn, H.; MBbius, D.; BOcher, H. Technology in Chemistry I , Part III; Wiley: New York, 1972; pp 552-702.
Langmuir, Vol. 4, No. 3, 1988 585
value of 60 A2 is larger than the values calculated for other dyes,12larger than the cross section of two hydrocarbon chains, but smaller than the 120 A2 calculated for dioctadecylbenzothiazolemonomethincyanine monolayers in which the chromophores lye flat in the plant of the monolayer. In this system the chromophores are oriented with their short axis perpendicular to the surface and the long axis parallel to the surface of the water. The phenyl group in meso position is assumed to be perpendicular to the xanthene moiety. Assuming a thickness of 3.6 A for the r-system, this would correspond to a length of 16 A. When a molecular model is considered this could agree with the projection of the aromatic system, the two nitrogens, and the first methylene group of each octadecyl chain on the long axis (15 A) of the chromophore. For the pure monolayer there is only a condensed phase up to about 32 dyn cm-’ (at 32 dyn cm-l there is a plateau with an area per molecule between 20 and 50 A2). At higher pressures there is a collapse of the layers for which there exists already a tendency in the 115 monolayer. At higher dilutions the surface pressure isotherms show the same characteristics of those of the pure fatty acids. Two different multilayer systems are built up with the Langmuir-Blodgett technique23for all the mixing ratios dyelarachidic acid (Figure 2). In the first case the dye molecules are in a tail to tail arrangement. This is called a “monolayer” because of the impossibility of aggregate formation between the chromophores present in different monomolecular layers. The head to head arrangement is called a “double layer” because of the possibility to have interactions between chromophores pressent in two adjacent monomolecular layers. With the same technique it was posssible to build up to system with 11, 15, and 21 monomolecular layers on glass, resulting in a system with 1 monolayer and 5 or 10 double layers, respectively. The deposition ratios are nearly equal to 1 for the successive monomolecular layers. The optical densities depend linearly on the number of layers (Figure 3), as has been expected from the assumption that the double layer behaves like two monolayers. All the measurements are performed on monolayer systems (shown in Figure 2) and double-layer systems in which on each side of the glass slide 15 layers are deposited in order to have a better signal to noise ratio. Spectroscopic Properties. The spectroscopic properties of the rhodamine B derivative have been determined in homogeneous solution. They are similar to those of N,N‘-diethylrhodamine B. In methanol the absorption maximum is at 546 nm and the emission maximum at 572 nm. In both cases the fwhm amounts to 1210 cm-’ f 3%. The Stokes shift amounts to 835 cm-’, which indicates that So and S1differ not largely in geometry and solvation. In chloroform there is an absorption maximum at 552 nm and an emission maximum at 567 nm. The Stokes shift amounts to 480 cm-l and the fwhm to 2050 cm-l f 5%. The absorption spectra are shown in Figure 4. In all the systems the absorption spectra have a maximum between 558 and 564 nm. There is a slight bathochromic shift with increasing dye concentration, due to an increase of the refractive index.12 Even in pure dye multilayers, no hypsochromic transition corresponding to an aggregation of the type that occurs in aqueous solution at high concentrations is observed. For all the systems the fwhm amounts to 2325 cm-’ f 5 % . The emission spectra (Figure 5) are red shifted compared to an ethanol solution of rhodamine B. For the (23) (a) Gaines, J. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Wiley: New York, 1966. (b) Blodgett, K. J. Am. Chem. SOC.1935, 57, 1007.
586 Langmuir, Vol. 4, No. 3, 1988
Van der Auweraer et al.
DYE MOLECULE
ARACHIDIC
ACID MOLECULE
Figure 2. Schematic representation of the “monolayer”system and the “double layer” systems of SRB, deposited on hydrophylic
glass.
96.0 0OL t
c -
x
m
79 9
+--
z
VI
iL
0
*
3 02
--J
4
63 8
-c
r
L? 7
CL
0
0 ‘
k’l
I
C
I
I 5
I I
I
I
I 1 I 1
10 NUMBER
I
I I I I I 1 1 I 1
15
OF
20
I 25
31 6
LAYERS
570
Figure 3. Dependence of the optical density at the maximum on the number of dye layers for a coverage Y = 0.02.
1
C 0395
0 0316
610 6 50 690. Wavelength ( nm 1
730
1
l C800 oo -
0 0237
4
0 0300
LOO 0 2632 0 2706
500 600 Wavelength I nm)
700
1
5 00
and the double-layersystems (b, bottom) on glass, all normalized in the maximum of the spectra: 1, Y = 0.0074; 2, Y = 0.029; 3, Y = 0.057; 4, Y = 0.13; 5, Y = 0.38; 6,Y = 1.0.
0 1979
9 L
E
9 CI
70C
Figure 5. Emission spectra of the monolayer systems (a, top)
a
;
500 Wavelength ( n m )
3 1052
0 0526
0 OOCO LOO
500
600
70 0
Wavelength I n m )
Figure 4. Absorption spectra of the monolayer systems (a, top) and the double-layersystems (b, bottom) on glass: 1, Y = 0.0074; 2, Y = 0.029; 3, Y = 0.057; 4, Y = 0.13; 5, Y = 0.38; 6,Y = 1.0.
The absorption is reduced to one monolayer.
1/400mololayer system the fwhm amounts to 1540 cm-l. Upon increasing concentration dye in the monolayer there is a broadening of the emission band and a maximum that shifts from 588 to 600 nm. The broadening of the band can be attributed to the occurrence of aggregates. The fine structure in the bathochromic part of the spectrum obtained at high concentrations of rhodamine B must be considered with caution, as the low quantum yield for the emission from these monolayers leads to a low signal to noise ratio. This is indicated by the fact that this structure
Langmuir, Vol. 4, No. 3, 1988 587
Absorption and Fluorescence of Rhodamine B Derivatives
e@ 1
::::I, , ,
1-3 00
,
I
- 6 00 - 6 00 - 5 00
-400
,
I
I
-300 - 2 00
, , I
-100
000
I
, 100
Ln Y
__c
0
Monolayers
X
Doublelayers
Figure 6. Influence of the surface coverage Yon the relative quantum yield 0 for the monolayer and 15 double layers.
disappears when the signal to noise ratio is increased by using multilayers (15 or 21 layers) instead of monolayers or double layers. The use of multilayers does however not influence the features of the absorption band. When the absorption and fluorescence spectra of the monolayers are compared to those of rhodamine B in solution, it is observed that the Stokes shift (990 cm-l f 7%) is considerably larger than that in homogenous solution. The red shift of the fluorescence maxima is larger than that of the absorption maxima. According to Kemnitz this phenomena could be explained by a distribution of dimer geometries. On the surface (glass or arachidic acid layers) a variety of different dimer species can exist. Some species contribute to a red shift in absorption, others will give a blue shift. The superposition of all the absorption spectra creates an overall absorption spectrum which resembles the monomer absorption spectrum, due to the superposition of red- and blue-shifted spectra, but with an enhanced fwhm.After absorption of a photon by a monomer or a dimer, energy transfer is possible to the most redshifted dimer. All the species in the same monomolecular layer contribute to a red shift of emission, while only part of the dimers contributes to a red shift of the absorption spectrum. Therefore the red shift in the emission spectra is larger than in the absorption spectra. When the emission is analyzed at 590 or 670 nm the excitation spectra are all the same, identical with the absorption spectra. The quantum yields of fluorescence were determined for the monolayers and the double layers (head to head); the system with mixing ratio 1/400 was used as reference. Figure 6 shows a decrease of the quantum yield with increasing surface coverage. The surface coverage Y is calculated by eq 2 and represents the fraction of the surface occupied by the dye molecules:
Discussion In the absorption spectra of the monolayers and double layers there is no evidence for aggregation of dye molecules as is known in homogeneous solution, even not at high surface coverages (no new band hypsochromic to the monomer band is observed). The emission spectra are largely dependent on the surface coverage. The emission spectra have a maximum between 588 and 600 nm with a Stokes shift of 990 cm-l f 7%. The emission spectrum of the 1/400 monolayer system has a fwhm of 1540 cm-' while with increasing surface coverage the emission band becomes broader. This can be attributed to the emission of dimers, although the band, characteristic for the dimer formed in homogeneous solution, is not observed.24 The
"
Go MONOMER LEVELS
6
5 L 7' DIMER LEVELS
90'
Figure 7. Exciton band energy diagram for a molecular dimer or a double molecule with coplanar transition dipoles inclined to the interconnecting axis over an angle 8.
dimers do emit, but their relative quantum yield is at least 100 times smaller than that of the monomers, assuming that in a mixing ratio fo 1/400 only monomers are present. The energy transfer from monomers to dimers occurs in the mdtilayers either directly or by a hopping mechanism, which leads to a broad emission band due to emission from different dimer traps. It is not surprising that the absorption or emission spectra of the solution spectra are not observed. Due to the forced orientation of the chromophores in the Langmuir-Blodgett films, which makes a tail to tail overlap impossible, the geometry of the dimers is different of the dimers in solution. Furthermore, the double layers show no interaction between two adjacent monomolecular layers, due to steric hindrance of the phenyl groups in the meso position, which prohibits an overlap between the xanthane moieties. The double layer system behaves as two individual monolayer systems. Although no spectral shifts are observed, it is difficult to explain the rapid decrease of the fluorescence quantum yield observed upon increasing the concentration of the chromophores in the monolayers without the assumption of the formation of ground-state dimers to which energy transfer occurs. If the photons absorbed by dimers would show a low fluorescence quantum yield, the intensity could not decrease faster than the fraction of molecules present as monomers. This fraction would decrease much slower with the increase of the concentration of the chromophores. This would also be the case if the quenching occurs by interaction between excited dye molecules and groundstate dye molecules. The occurrence of rhodamine B dimers without spectral shift is a peculiar phenomenon. A dimer with parallel coplanar inclined transition dipoles leads to the exciton energy diagram shown in Figure 7.26 The exciton band splitting is given by eq 3: e
= (2~M~2/ril)/(l - 3 cos2 e)
(3)
This case describes the influence of the variation of the angle 0 between the polarization axes and the vedor linking the centers of the molecules (0 = 90°, Oo corresponds to ~
(24) (a) Lopez-Arbeloa, I. J. Photochem. 1982, 18, 161. (b) LopezArbeloa, I. Thermochim. Acta 1983,60, 216. (25) Knsha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371.
588 Langmuir, Vol. 4 , No. 3, 1988
Figure 8. Schematical representationof SRB dimers, occurring in a Langmuir-Blodgett system.
parallel and in-line transition dipoles, respectively). For an angle of 54.7O the exciton splitting is zero, and thus the dipole-dipole interactions become zero, irrespective of the intermolecular distance pi In a monolayer system t i e dimer geometry as known in solution is not probable (the type a, c, or d dimers of Kemnitd2): the large octadecyl groups must be all at the same side of the monolayer packing away from the polar interface. Space-filling molecular models indicate that a dimer geometry in which one dye molecule is shifted over 2.4 A parallel to its long axis (one benzene ring) is possible. This situation is represented in Figure 8. The phenyl groups in the meso position are perpendicular to the xanthene moiety. This makes it impossible to form an aggregate with the transition dipoles perpendicular to the vector linking the center of the molecules. The shift of one benzene ring from the one rhodamine B molecule to the other results in an angle between the transition dipoles of about 55O, a dimer geometry without spectral shift.26 If the 54.7O dimers emit, kf will have the same order of magnitude a8 tzf from the monomers. In this case it is possible to conclude that the fluorescence decay time will decrease with decreasing quantum yield of fluorescence. In the case of a bathochromic dimer kfcan be smaller than kf of the monomer. This makes it impossible to predict values for the fluorescence decay time when only the quantum yield of fluorescence is taken into account. The broadening and the red shift of the dimer emission at higher concentrations suggest that a certain distribution of dimer geometry exists. At high coverages energy transfer will occur to those dimer sites emitting at the (26) (a) Szikkely, V.; Faraterling, H. D.; Kuhn, H. Chem. Phys. Lett.
(b) Biicher, H.; Kuhn, H. Chem. Phys. Lett. 1970,6,183. (c) McKay, R. B. Trans. Faraday SOC.1965, 6, 1787. (d) SundstrBm, V.; Gillbro, T. J. Chem. Phys. 1985, 83, 2733. 1970,6,11.
Van der Auweraer et al. longest wavelength, therefore those sites will contribute more to the emission spectra than to the absorption spectra. As no changes in the absorption spectrum are observed the broadness of this distribution will be limited, and only a very small fraction of the dimers will be present in a packing with 0 deviating strongly from 54.7O. Especially at high coverages ( Y > 0.4), where the monolayer packing becomes less defined, as is expressed by the surfacepressure isotherm of the pure dye, it becomes possible that a very small fraction of the dye molecules is present in a packing resembling the packing of the solution dimers. Energy transfer to those sites will yield an emission resembling those of the solution dimers. As an alternative, a dimer where the long axes of the dye molecules are coplanar but perpendicular cannot be excluded completely. As such a packing would lead to much smaller van der Waals interactions between the dye molecules or a smaller change of the interactions between dye molecules and arachidic acid molecules, it is not clear why such a packing would be more probable. Furthermore, for the orientation of the dye molecules deduced from the pressure-area isotherms the distance between the centers of the dye molecules would be large (at least 9 A). Also a packing analogous to that of the type b dimers of KemniW is improbable. Such dimers could only be present in double layers and not in monolayers while experimentally no differences are observed between double layers and monolayers. Between the monolayers containing the rhodamine B derivative and the adsorbed rhodamine, the following similarities exist when the surface coverage is increased: (1)Except for a small red shift the absorption spectrum does not change. (2) Although the fluorescence quantum yield decreased dramatically, which points to energy transfer to a weakly emitting species, only a small red shift and a band broadening of the emission are observed. (3) In the adsorbed dimer the electronic interaction (overlap) between the dye molecules is considerable as it is a more efficient hole trap27than the monomer. These observations indicate that in both cases dimerization occurs and that a large fraction of the dimers is characterized by a small splitting. In both cases efficient energy transfer to the dimers occurs, and both dimers are characterized by a small fluorescent quantum yield. Therefore it is not unreasonable to conclude that both dimers have approximately the same structure. The dimer structure proposed for the monolayer is also possible for adsorbed rhodamine B molecules independently whether they are adsorbed edge on or lying flat. For the adsored dye molecules the structure described here, which is in agreement with the spectroscopic observations, leads to a larger decrease of the area of the dye molecules in contact with water than a loose dimer,l*where the edges of the molecules touch each other. Acknowledgment. We are indebted to the I.W.O.N.L. and to the N.F.W.O. for continuous financial support to the laboratory and for fellowships to B.V. and M.V., respectively. Registry No. Arachidic acid, 506-30-9; N,N'-bis(ethy1octadecy1)rhodamine B,106853-81-0.
