Langmuir 1992,8,2639-2643
2639
A Flat-Topped J-Aggregate Peak in Concentrated Dispersions of a Cyanine Dye Mark Wiltberger,? Ravi Sharma,**tPeter Martic,$ and Thomas L. Penned Materials Science and Engineering Division, Analytical Technology Division, and Corporate Research Laboratories, Eastman Kodak Company, Rochester, New York 14650 Received January 21,1992. In Final Form:August 6, 1992 Spectra of concentrated dispersions of a cyanine dye obtained using a Cary 2290 spectrometer were observed to be flat topped in the region usually assigned to J-aggregates. This observation is in contrast to the sharp peaks that are normally observed for J-aggregates. The flat top was found to disappear at electrolyte concentrations greater than 50 mM and also with the addition of pyrocatechol violet. In addition, spectra obtained using a tandem monochromator spectrometer,a spectrometer that only allows light of the same wavelength as the incident light to reach the detector, did not contain flat peaks. The J-aggregated dye was observed to exhibit fluorescence. These results indicate that the flat-top peak is a result of the superposition of transmitted and fluorescent emitted light. Introduction The absorbance spectra of dye solutions typically exhibit absorbance peaks for monomeric dye and aggregated dye. Two types of aggregates are known to exist, H-aggregates and J-aggregates.' Dye molecules aggregate by stacking on top of each other. They are said to be perfectly stacked when the dye molecules overlap each other completely, i.e., a stackingangle of 90°, which defines the angle between the transition dipole and the molecular axis of the aggregate. According to the point dipole model, the degree to which an absorbance peak is shifted depends on the stacking angle.'T2 When the stacking angle is 54O or less, the absorbance maximum of the aggregate is red-shifted, and called a J-aggregate. When the stacking angle is greater than 54O,the absorbance maximum of the aggregate is blue-shifted and called an H-aggregate. For example, the absorbance spectrum of a dilute aqueous solution of the cyanine dye studied in this report (compound 1) has the following assignments: monomer peak at -540 nm, H-aggregate peak at -500 nm, and J-aggregate peak at -619 nm (Figure 1). In recent investigations, absorbance spectra for concentrated dispersions of the cyanine dye (compound 1) were obtained. The spectra contained flat-topped peaks in the region where J-aggregates of this dye are expected to absorb (-619 nm, Figure 1). Typically J-aggregate absorbance peaks are sharp.ls3 In contrast, here, the absorbance peaks obtained were rather broad and flat topped. This puzzling phenomenon has not been reported previously. Three possible reasons for the flat peak were speculated upon. First, an obvious explanation for the flat peak shape was that the dye absorbance was large enough to exceed the limit of the spectrophotometer. Several factors indicated that this was not the case. (1)The flat peak was observed at an absorbance as low as 2.5,much lower than the instrument limit of 4.6 absorbance units. The instrument limit was verified by observing the absorbance
* To whom correspondence should be addressed.
t Materials Science and
Engineering Division.
* Analytical Technology Division.
Corporate Research Laboratories.
(1) Hen, A. H. Adv. Colloid Interface Sci. 1977, 8, 237. (2) McRae, E.;*ha, M. J. Chem. Phys. 1958,28, 721. (3)Daltrozo,E.; Schiebe, G.; Gechwind, K.;Haimerl, F.Photogr. Sei. Eng. 1974, 18, 441.
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Figure 1. Absorbancespectrum for dye (compound 1)dupereion (0.3 mM) in deionized water (broken line) and in 10 mM KNOa aqueoussolution (solid line). The peak at -500 nm (marked H) is assigned to H-aggregates, the shoulder marked M at -640 nm is monomeric dye, and the flat-topped peak at -619 nm (marked J) is assigned to J-aggregates. at which a cutoff occurred. For aqueous solutions of naphthol blue-black, a dye that absorbs in the same region as compound 1, it was found that the peak shape observed at lower concentrations (and therefore lower absorbances) was maintained until the absorbance was just below 4.5 absorbance unite, albeit noisy. At a slightly higher concentration (absorbance >4.6), a cutoff due to instrument limitations occurred. The absorbance peak was cut off with a horizontal line, as expected. These experiments confirmed that (i) the instrument limit is at about 4.6 absorbance units, and (ii) even at absorbance close to the instrument limit, peak shape is maintained; no flattening of the peak was observed. Thus, the cause of the flat peak is not due to detector limitations. (2)Close inspection showed that the flat-topped peak was slightly sloped; a cutoff due to having reached the limit of the instrument should be perfectly horizontal. The degree to which an absorbance maximum of an aggregate is shifted relative to the monomer peak dependa on the stackingangle. Thus, a second possible explanatioin for the flat J-aggregate peak is that J-aggregates with a range of stacking angles exist in the concentrated dye dispersion. The aggregate for each stacking angle gives rise to a narrow intense absorbance peak that is shifted
0743-7463/92/2408-2639$03.00/00 1992 American Chemical Society
Wiltberger et al.
2640 Langmuir, Vol. 8,No. 11,1992
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Sample Cell Monochromator
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Incident Light (I,)
U
Detector
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+ve Absorbance Peak
A = l o . It
Emitted Light
Figure 2. Schematicrepresentation demonstrating how a flattopped absorbance peak results by the superposition of an emission peak (negative) and 'an absorbance peak (positive). Zt = intensity of transmitted light. slightly from the absorbance peak of another aggregate of a slightly different stacking angle. The overlap and s u m of these peaks give rise to the flat absorbance peak for the J-aggregates. But this explanation is unsatisfactory since it would take a miraculous combination of aggregate orientations and aggregate structures to produce a flat peak. Third, this observation could be explained if the sample were capable of emitting light since this would tend to cancel the absorbance peak in a single monochromator instrument. Thisseemed to be plausible since J-aggregates are known to fluoresce.' A schematic representation of how this might occur is presented in Figure 2. In this phenomenological study, we present a compilation of resulta obtained from several diagnostic experiments that were conducted in order to verify whether fluorescence originating from the aggregated and agglomerated dye was responsible for the observed flat peak.
Experimental Section Materials. Dye (compound 1, see structure below) was synthesized using methods common in the synthesis of cyanine dyes outlined elsewhere.'Vs The resulting dye was a dark green solidwith amelting point of 260 O C . When dissolvedin methanol, the dye solution exhibited an absorbance maximum at 542 nm (A, = 542 nm) and an optical density at A, of 7.58 X 10.'. TAI (compound 2, see structure below) was obtained by neutralizing 5-methyl-s-triazol0(1,5-a)pyrimidin-7-01 (Aldrich) with sodium hydroxide (Kodak).Pyrocatechol (or catechol), pyrocatechol Et
I
so,-
CH,-C-H
I
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(E~)~NH+
1 (anhydro-5,6-dichloro-l-ethyl-3-(3-suH~utyl)-3'-(3-sull~opyl)-
4',5'-benzcbenzimidazolothiaca1bocyaninehydroxide, triethylamine salt)
cH3y-p 0-
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Wavelength (nm) Figure 3. Dependence of peak shape on dye concentration. The dye is dissolved in an aqueous medium containing 6 w t 9% deionized gelatin and 8.7 mM TAI. Dye concentrationsare 0.025 mM (l), 0.075 mM (21, 0.15 mM (3), 0.25 mM (4), and 0.3 mM (5). Flat-topped peaksonly appear at dye concentrationsgreater than 0.075 mM.
Photographicgradelimeprocessed"deionized"bone gelatinwith an isoelectric point equal to pH 6.0 was used. The deionized gelatin had a residual amount of ions such as Cas+, Mg2+,SO2-, and Nos-, each present at 50 mM), we observe that flat-topped absorbance peaks are rounded and of lower intensity. The increased rounding of the absorbance peak is consistent with a reduced amount of emitted light caused by an increase in agglomerate size with electrolyte concentration (see shielding and shadow effects discussed below). The decreased absorbance with electrolyte concentration is also consistent with increasing agglomerate size. Furthermore, it can be observed in Figure 4 that the absorbance at 700 nm increases with electrolyte concentration. This is good evidence that the turbidity increases with electrolyte concentration since none of the solution components absorb at this wavelength. The increase in turbidity is most likely due to an increase in the size of scattering units since it is well recognized that electrolytes encourage aggregation. As electrolyte concentration increases, the size of the scattering units increases at the expense of the number of scatterers. It follows, therefore, that absorption and emitted intensity would decrease since fewer molecules are excited per unit volume; molecules in the core of the agglomerate are not likely to receive much light (this is called a “shielding”effect in Figure 9). In addition, there is a “shadow” effect which also reduces absorbed and emitted light (seeFigure 10). In Figure4, it can be observed
Absorbed Light
from Neighboring Aggregate
Incident Light
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Figure 9. Schematic representation of shielding effect. Dye molecules in the core of a dye agglomerate are shielded from incidentlight becauseouter moleculeshave absorbed or scattered all light. Dye Aggregate Suspension
Incident
Cell Wall
Partially Eclipsed Dye Aggregate
Cell Wall
Figure 10. Schematicrepresentation of shadoweffect. Became
agglomerates are opaque, agglomerates positioned behind the agglomerates closest to the wall first receivingincident light will either be partially or fully eclipsed depending on relative size.
that generally there is a decrease in peak absorbance with increasing electrolyte concentration. The observed decrease in absorbance and transformation to rounded peaks at higher electrolyte concentrations can be attributed to shielding and shadow effects. This effect can also explain the rounded peak shape for compound 1 that had previously been observed in dry regular gelatin films.12 Under these conditions one would not expect flat-topped peaks because of the highly aggregated state of the dye in the dry film.
Conclusion Flabtopped absorbance peaks for concentrated dispersions of a cyanine dye were observed. A transformation to a rounded peak was observed at higher dye concentrations and at high electrolyte concentrations. This was attributed to a reduction of fluorescence by the agglomerated dye due to shielding and shadow effects. Rounded peaks were also observed when the fluorescent emitted light was prevented from reaching the detector in a tandem monochromator spectrometer. Addition of small amounta of PCV to the dye dispersion was shown to reduce fluorescence and also resulted in rounded absorbance peaks. All these observations point to the fact that when emitted light from the sample interferes with the absorbance, a modified absorbance peak, which was flat-topped in this case, can result. Although the distortion of absorbance spectra by fluorescence has been recognized in other systems: this is the first report in which the distortion was so severe that flat-topped peaks resulted. Acknowledgment. We thank Professor David Whitten of the University of Rochester and the following members of Kodak Research and Development Laboratories for help in facilitating the experiments and for helpful discussions: John Mee, Nancy Armstrong, Samir Farid, Ken Harbison, Roger McCleary, Don Brumbaugh, and Alfred Marchetti. (12) Penner, T. L.; Gilman, P. B., Jr. Photogr. Sci. Eng. 1976,22,97.
