Langmuir 1991, 7, 407-421
407
Molecular Orientation of Individual J Aggregates on Gelatin-Grown AgBr Tabular Microcrystals Joe E. Maskasky Corporate Research Laboratories, Eastman Kodak Company, Rochester, New York 14650-1 724 Received April 27, 1990. In Final Form: August 2, 1990 The epitaxial orientations of dye molecules making up J aggregates (two-dimensional crystals of dye having a narrow absorption peak with a large bathochromic spectral shift) were determined for the first time on AgBr (111)surfaces. By means of two different procedures, 14 cyanine spectral sensitizing dyes were grown into extremely large J aggregates on tabular AgBr grains. The polarization of individual aggregates was then measured at 77 K with a modified optical luminescence microscope. For a given dye and growth procedure, the emission polarization orientation of the aggregates relative to a (110) direction of the (1111surface formed a “narrow”distribution having a measurement standard deviation of no greater than f4’ showing that epitaxy had formed. For most of these dyes, the molecular orientation of the aggregate structure relative to the AgBr substrate could be readily determined from the polarization data; most formed aggregates made up of (110) or (321) oriented molecules. Since dyes having structures inappropriate for chemisorption still formed oriented aggregates and also since only the ( 110) orientation is consistent with chemisorption, it is concluded that physisorption alone is sufficient for epitaxy. However, these data do not rule out the possibility of a contribution from chemisorption for some of the dyes. During the course of this study, a number of interesting observations were made: (1) Two of the dyes yielded polarization data consistent with the more complex herringbone structure. (2) One of the three cationic dyes studied formed two different J aggregates that coexisted on the same grains. Their relative proportion depended upon the dye level and growth conditions. (3) The other two cationic dyes examined grew significantly larger aggregates when an increase was made in the concentration of multivalent metal cations, H+, or Ag+ or in the solution ionic strength. This is explained by a reduction in the relative number of nucleation sites and a slower rate of aggregate growth caused by less electrostatic attraction between the dye and the grain or its surrounding gelatin envelope. (4) The orientation of epitaxial aggregates of two of the dyes did not show the expected redundant symmetry relative to the (111)surface. This result implies a lower symmetry of this surface than that of the underlying AgBr (111)planes.
Introduction Photographic silver halide grains have an intrinsic absorption in the blue region and practically none beyond 500 nm. In order to spectrally sensitize them to longer wavelengths, dyes are used. The most useful spectral sensitizing dyes are cyanines (a methine conjugated carbon chain connecting two nitrogen atoms such as those of benzothiazole) that adsorb and aggregate on the grains’ surfaces. The J aggregate (probably named after Jelleyl) is the most commercially important dye assembly and is characterized by a narrow absorption peak occurring a t considerably longer wavelengths than that expected for nonaggregated dye adsorbed to the grains. An H aggregate forms for some cyanine dye structures. It is characterized by a hypsochromic absorption shift.2 The structure of an aggregate on a silver halide surface has never been directly measured but it has been inferred from other data, the most notable being molecular packing obtained from X-ray crystal structures of a few dyes known to aggregate,3+ theoretical approximations of aggregate wavelength shifts relative to molecular orientations,’* the measured amount of adsorbed dye needed to form monolayer coverage,l0I1l assumptions concerning chemisorp(1) Jelley, E. E. Nature 1936,138,1009. See also Scheibe, G.; Kandler, L.; Ecker, H. Naturwissenschaften 1937, 25, 75. (2) Herz, A. H. Adu. Colloid Interface Sci. 1977,8, 237. (3) Smith, D. L. Photogr. Sci. Eng. 1974, 18, 309. (4) Smith, D. L. Photogr. Sci. Eng. 1972, 16, 329. (5) Potenza, J.; Mastropaolo, D. Acta Crystallogr. 1974, B30, 2353. (6) Nakatsu, K.; Yoshioka, H.; Nishigaki, S.Nippon Shashin Gakkaishi 1983, 46, 89. (7) Norland, K.; Ames, A.; Taylor, T. Photogr. Sci. Eng. 1970,14,295. (8) Reich, C. Photogr. Sci. Eng. 1974, 18, 335. (9) Reich, C.; Pandolfe, W. D.; Bird, G. R. Photogr. Sci. Eng. 1973,17, 334.
(IO) Pandolfe, W. D.; Bird, G. Photogr. Sci. Eng. 1974, 18, 340.
0743-746319112407-0407$02.50/0
tion of the dye molecules to the silver halide ~ u r f a c e , ~ J ~ and polarization absorption measurements of aggregates formed on silver halide sheet ~ r y s t a l s ’ ~orJ ~ large cut single crystals15J6 There appears to be general acceptance that these two aggregate types (J and H)consist of the nearly planar dye molecules adsorbed to the silver halide surface along their long edge with their molecular planes stacked parallel to each other forming a two-dimensional crystal. According to theoretical calculations, whether a dye aggregate has a bathochromic or hypsochromic absorption shift depends on the angle of slippage between successive molecular planes. Large slippage (a< 32’)17 results in a bathochromic shift while small slippage (a> 32’) results in a hypsochromic shift.7 There has been considerable debate concerning the relative importance of chemisorption and physisorption in the adsorption of aggregating dye molecules to the silver halide surface. Either could result in discrete slip angles. The benzothiazole dyes, because they contain heterocyclic sulfur atoms, are the most generally considered candidates for chemisorption.9J8 Ligand bonds between the dye heteroatoms and silver ions along the (110) rows of an AgBr (111)surface could lead to a slip angle of 60°,30°, 19’12 (11) Gunther, E.; Mosiar, E. J. Photogr. Sci. 1965, 13, 280. (12) Bird, G. R.; Norland, K. S.; Rosenoff, A. E.; Michaud, H. B. Photogr. Sci. Eng. 1968, 12, 196. (13) Saunders, V. I.; Lovell, S. P. Photogr. Sci. Eng. 1980,24, 171. (14) Saunders, V. I.; Lovell, S. P. Photogr. Sci. Eng. 1980,24, 176. (15) Gray, W. E.; Brewer, W. R.; Bird, G. R. Photogr. Sci. Eng. 1970, 14, 316. (16) Yacynych, A. M.; Mark, H. B., Jr.; Giles, C. H. J. Phys. Chem. 1976, 80, 839. (17) This slippage is defined by the angle a,which is the angle between
the line-of-centers of a column of dye molecules and the long axis of any one of the parallel molecules. (18) Mastropaolo, D.; Potenza, J.; Bird, G. R. Photogr. Sci. Eng. 1974, 18, 450.
0 1991 American Chemical Society
Maskasky
408 Langmuir, Vol. 7, No. 2, 1991 and 90°, 41°,23O,and 16°.3 Alignment of thedye molecules along (110) rows seems reasonable since the spacin between these rows is 3.54A,fortuitously close to the 3.37graphitic packing distance. Of these possible angles, the preferred slip angle of an aggregate of a given dye must also depend on intermolecular steric interactions and attractive forces between adjacent dye molecules. However, as has been pointed out, steric hindrance between benzothiazole dyes and the idealized flat (Ill)surface would preclude the formation of specific S-.Ag+ bonds.3 While the actual structure of the { 111)surface is not known, this surface must exist in a reconstructed form to avoid being composed of ions of only one sign, which would have a prohibitive electrostatic p ~ t e n t i a l and , ~ ~this ~ ~ recon~ structed surface could perhaps be more accommodating to such bonding by allowing the closer approach of the dye molecules. In fact, the adsorption of dye may even further alter the (111)surface. Atomic force microscopy may soon reveal the nature of this surface but atomic scale resolution has not yet been reported.21 For either chemisorption or physisorption of dye molecules on silver halide surfaces, oriented aggregates could r e ~ u l t .There ~ is evidence for such epitaxy from polarized light absorption measurements on unique AgBr surfaces. If it is assumed that an aggregate has a crystallographic structure involving one molecule per unit cell or more than one molecule per unit cell but all having parallel electronic transition dipoles, then dichroism measurements can reveal the orientation of the dye molecules relative to the silver halide surface. The plane of polarization of the absorbed radiation depends on the direction of the transition dipole. This dipole is always in the plane of the molecule for transitions involving the T orbitals of planar molecules. Also, arguments have been presented that the transition dipole for the strong and long wavelength absorption band of a cyanine dye aggregate, the J band, is parallel to the long axis of the molecules.12 Gray et al. measured the dichroism for aggregates of 5,5’,6,6’-tetrachloro-l,l’,3,3’-tetraethylbenzimidazolocarbocyanine adsorbed to an AgBr large crystal cut a t an angle slightly off from the (111)face in such a way as to preserve a single (110)intercept in the plane of the cut. They interpreted their results in terms of stepped micro (111)surfaces on which the dyes initially aligned along terrace steps parallel to the (110)intercept and then grew such that, whenever possible, the heteroatoms formed ligand bonds to nearby silver ions.15 (The extent of any chemisorption should be small because this dye has its nitrogen atoms blocked by ethyl groups.) Later, Smith pointed out that such alignment might occur due to the alternating sign of the electrostatic charge of the (111) steps along the [ 1121 direction and the uniform charge character along a given [110]line.3 Another dichroism study involved dye adsorption to AgBr sheet crystals that had a wide variety of characterized crystallographic surfaces. Upon examination of 11 cyanine dyes, Saunders and Love11 concluded that there was no preference of J aggregates for orientation parallel to (110)directions and the dichroism they observed was primarily a result of physisorption forces.13J4 Neither of these studies found any significant dichroism on the I1111 surface and, in fact, none had been expected since both groups were measuring the polarization as an average of a large number of small aggregates and this
1
(19) Hamilton, J. F.; Brady, L. E. Surf. Sci. 1970, 23, 389. (20) Baetzold,R. C.;Tan,Y.T.;Tasker,P.W.Surf.Sci. 1988,195,579. (21) Meyer, E.; Heinzelmann, H.; Grutter, P.; Hidber, H. R.; Giintherodt, H. J.; Steiger, R. J . Appl. Phys. 1989, 66, 4243.
surface would permit multiple equivalent aggregate orientations that when averaged would have no net dichroism. Thus they focused their studies on other surfaces having a single (110)direction. However, information obtained from these surfaces may not be relevant to actual emulsion grains bound by the (111)surface because the spacing between nearest (110)lines of silver ions is closest to the graphitic packing distance for the (111)surface and is greater for other surfaces. Also, for unknown reasons, the (111)surfaces of melt-grown sheet crystals, even having been treated with gelatin, behaved differently toward dye aggregation than those of emulsion grains grown in gelatin.14 The aim of this study is to grow extremely large J aggregates onto large gelatin-grown silver bromide tabular emulsion grains and then measure their molecular orientation relative to the grains’ six surface (110)directions. This should provide information necessary to determine if J aggregates grow epitaxially on {lll) silver halide surfaces and, if so, whether the aggregate molecules are oriented along these (110)directions. These data may help resolve the relative importance of chemisorption and physisorption. Since even the relatively strong and narrow J band absorbs only 10% of the light a t its peak absorption,13J4 luminescence microscopy was considered desirable over conventional transmission microscopyto obtain the needed precision in the polarization data. J aggregates on silver halides usually do not luminesce at room temperature but do emit very strongly at liquid nitrogen temperature. The emission is in the form of a narrow peak having a very small Stokes shift from the absorption peak.22 The J-band absorption and emission involve the same electronic transition. In order to carry out this study, a low-temperature polarized-luminescence microscope had to be built. Lowtemperature stages capable of liquid nitrogen (77 K) or lower temperatures have been custom built for specific applications. Typically their design uses windows that prevent the use of high magnification objective^.^^ There are windowless designs such as that of Silaevet al.?4 which places the microscope objective under vacuum along with the sample and heat exchanger. Recently I reported on a simple, windowless low-temperature stage in which the sample is placed on a thermally conductive holder (Le., polished silver), which is in turn partially immersed in liquid nitrogen. Frost is not a problem because the objective and sample are shrouded in the evaporated dry nitrogen under a thin plastic cover. Samples can be changed easily and rapidly, reaching liquid nitrogen temperature in about 90 s.25 This stage has been used to study the growth and twinning of silver bromide tabular grains that had been banded with small amounts of iodide to produce luminescent growth ringseZ6Detail as firle as -0.3 pm was resolved and photographed, using exposure times of 1 min, showing the good vibrational stability of the system. This simple stage was practical since for this application epi-illumination was used and neither
-
-
(22) Makio, S.;Kanamaru, N.; Tanaka, J.Bull. Chem. SOC. Jpn. 1980, 53, 3120. (23) Cosier,J.; Wood, I. G.J.Phys.E: Sci. Instrum. 1983,16,687. van Kempen, H.; Mischgofsky, F. H.; Wyder, P. Reu. Sci. Instrum. 1972,43, 1209. Kessler, G.; Rudman, M. R. Prakt, Metallogr. 1971,8,40. Carroll, K. J.; Liebenberg, D. H.; Overton, W. C. Rev. Sci. Instrum. 1967,38,260. Clothier, W. C. J . Sci. Instrum. 1967, 44, 535. (24) Silaev, V. I.; Belyaeva, A. I.; Stel’makhov, Yu. N. Prib. Tekh. Eksp. 1977, 260 (in Russian); Instrum. Exp. Tech. 1977, 20, 1206 (in English). (25) Maskasky, J. E. J . Imaging Sci. 1988, 32, 15. (26) Maskasky, J. E. J. Imaging Sci. 1987, 31, 15.
J Aggregates on AgBr Tabular Microcrystals
Langmuir, Vol. 7, No. 2, 1991 409
Table I. Cyanine Dye Structures
DyeA
I
CH2CH3
CH2CH,
Br
-
precise nor variable temperature regulation was necessary. For this study of polarized luminescence, the lowtemperature Dewar assembly was unaltered but modifications on the basis microscope were needed.
Experimental Section AgBr Tabular Grains. The silver bromide tabular grain host emulsion was prepared under red safelights using oxidized gelatinz7and a procedure similar to that previously described.26 Deionized gelatins low in calcium (-20 ppm) and magnesium (-2 ppm) were used exclusively. The final emulsion had a pH of 5.6, a pBr of 3.2, and -20 g of gelatin/mol of Ag and was (27) Maskasky, J. E. J. Imaging Sci. 1989, 33, 13.
relatively low in soluble salts, having a conductivity of 0.35 mS/ cm a t 40 "C after being diluted from its storage concentration of 1.7 mol of Ag/kg to 1.0 mol of Ag/kg. (Making this sample -20 mM in NaN03 raised its conductivity to 2.7 mS/cm.) The tabular grains ranged in thickness from 0.1 to 0.4 pm, had an equivalent circular diameter of 17 pm, and had an average surface area of 337 m2/mol, determined by using dye A, Table I, and the Langmuir adsorption equation.28 Dye Purity. The dyes used in this study were prepared and checked for purity by organic chemists a t the Research Laboratories of Eastman Kodak Company. The three asymmetrical dyes (dyes L, M, and N)and the dye that showed two distinct aggregates (dye E) were rechecked for purity by high-pressure (28) Maskasky, J. E. J. Imaging Sci. 1988, 32, 95.
410 Langmuir, VoZ.7, No. 2, 1991
Mas kas ky
liquid chromatography. Other than solvents, dye E contained 2 7; of the 9-methyl derivative and was 98 % pure, dye L contained 0.3% 9-methyl derivative and 0.5% starting material and was 99.2% pure, dye M contained less than 1%impurities, and dye N contained 8.7% of the symmetrical benzothiazole dye as a major but sole contaminant. A Method of Growth. To 25 mmol(25 g) of the silver bromide host emulsion a t 90 "C was added the solution of dye (usually in 1 mL of methanol) with stirring. The stirred and covered emulsion was held a t 90 "C for 15 min and then was allowed to cool to room temperature a t -0.4 "C/min. A portion of the dyed emulsion was diluted 10-fold with distilled water and the grains were allowed to settle overnight a t 4 "C. The supernatant, which contains the bulk gelatin, was discarded. The grains were resuspended in distilled water and coated onto a polished silver sample holder for examination with the low-temperature luminescencemicroscope. Sample preparations were conducted under appropriate safelight conditions to avoid photodecomposition. M Method of Growth. To 25 mmol(25 g) of the silver bromide host emulsion a t 65 "C were added, with stirring, 5 mL of distilled water and 40 mL of methanol. A methanol solution of dye was added and then the temperature of the water bath was raised to 75 "C and the methanol was allowed to evaporate from the uncovered stirred emulsion. Usually 75 min was sufficient, resulting in a reduction of the final weight of the emulsion to 12 g. The sample workup was similar to that described above for the A method. Conventional Method of Growth and Absorptance Measurements. To 25 mmol(25 g) of the stirred silver bromide emulsion a t 40 "C was added a methanol solution of the dye (usually in 1mL of methanol). After 30 min of stirring a t 40 "C, the resulting emulsion was coated onto clear support and dried. Absorptance measurements were made by use of a spectrophotometer equipped with an integrating sphere for 1-R-T measurements. Effect of Environment on Aggregate Growth. The effects on aggregate growth of Ca(N03)2 and NaNO3 were tested for each dye using the A and M methods. Emulsion portions were diluted to 1.0 mol of Ag/kg and then made 1.5 mM in added Ca(N03)2 or 24 mM in added NaN03 before aggregate growth. The effects on aggregate growth of 1.5 mM of the nitrate salts of Mg, Sr, Ba, and C0(en)3~+ were also determined for dye A by using the M method. The effects of low pH and low pAg on the aggregate growth of dyes A and D, using the M method, were examined. To make the low pH emulsion, HN03 was added to a portion of the host emulsion to lower the pH to 2.0, then the emulsion grains were centrifuged and redispersed in sufficient 0.38 mM NaBr solution to make a suspension having a concentration of 1.0 mol of Ag/ kg. This washing procedure was necessary to restore to the emulsion a low solution ionic strength and an excess bromide ion environment. A control emulsion was prepared similarly but without the pH adjustment. To make the low pAg emulsion, a portion of the host emulsion was centrifuged and resuspended in a similar quantity of distilled water twice to remove excess bromide ions and was then adjusted to a pAg of 3.3 with AgNO3 solution. The Microscope. The low-temperature polarized-luminescence microscope used the same Dewar assembly that had been described previously25(Figure 1). However, changes in other parts of the Olympus Vanox trinocular microscopewere necessary in order to examine polarized luminescence. An X and Y translation stage and Dewar assembly were mounted on a custommade air bearing assembly (8 in. o.d., 4 in. i.d., 1.5 in. thick) (Dover Instrument Corp., Westboro, MA), which permitted smooth rotation of the sample without loss of focus even a t the highest magnifications. This simplified the visual determination of the midpoint of the angular range where the aggregate emission disappeared (complete aggregate disappearance usually occurred). A precision 0-360" scale was mounted on the circumference of the stage. Two 0.5-in. Glan-Thompson polarizing prisms (Newport Corp., Fountain Valley, CA) were precision mounted in removable mounts. One was used for polarized excitation and the other for polarized emission. For polarized excitation studies, the polarizer was placed between the field and aperture diaphragms of the modified Olympus vertical fluorescence illuminator, Model
-
Flat and Polished Surface Silver Sample Holder Magnetic Stainless Steel
7 Titanium Pla&
Glass Vacuum Dewar
-
Nylon Screw for Height Adjustment
Figure 1. Low-temperature luminescence microscope Dewar assembly. AH-RFL-LB. A companion 5X photoeyepiece with fixed reticle was permanently mounted in a ring that had alignment pins that mated with holes placed in the body of the microscope so that, once aligned, the reticle always indicated the experimentally determined polarization axis. By design, this axis was perpendicular to the tilt axis of the dichroic mirror (beam splitter) in the vertical illuminator. For polarized emission studies, another modified 5X photoeyepiece was used. This photoeyepiece contained both the reticle and polarizing prism, which, once aligned, were permanently mounted relative to each other (Figure 2). This unit also contained alignment pins so that the polarization axis was always perpendicular to the tilt axis of the dichroic mirror. Alignment of the reticle relative to the polarization axis of this prism was accomplished using transmission polarized light and needle-shaped crystals of ammonium sulfate (which are known to have parallel extinction), following the procedure described elsewhere.29 It is estimated from examination of a number of crystals that the reticle was aligned with the polarization axis to better than &lo. Depending on the type of polarization measurement, either of these photoeyepieces was used to make visual polarization measurements. By use of the monocular camera port to make polarization measurements, brighter images were obtained than were possible if the light had been diverted through the split beam of the binocular part of the microscope. A secondary advantage is that for illustrations, the polarization axis would be indicated in photomicrographs. The microscope was also equipped with a microspectrophotometer (Farrand Optical Co., Inc., Valhalla, NY) capable of scanning the luminescence emission from an area of the sample as small as -1 pm2. A wavelength calibration was made on a regular basis using the 546-nm Hg line from the high-pressure Hg microscope illuminator. Wavelength scale linearity was checked by using the lines from a low pressure Hg lamp and that from a 633-nm He-Ne laser. Sample emission peaks were measured to an accuracy of better than f 4 nm. Samples were examined by using bright field epi-illumination with appropriate excitation and blocking filters. Most of the dyes examined gave the most intense emission using the 546-nm Hg line, but excitation using the broad 490-nm peak or the sum of the 577 + 579-nm lines was found best for some of the dyes. (The interference filter used to excite with the broad 490-nm peak had 82 96 transmittance a t 490 nm and a full width a t half maximum of 36 nm.) (29) Chamot, E. M.; Mason, C. W. Handbook of ChemicalMicroscopy, 3rd ed.; John Wiley and Sons, Inc.: New York, 1958; Vol. I, p 275.
J Aggregates on AgBr Tabular Microcrystals
Langmuir, Vol. 7, No. 2, 1991 411
Measurement of the Dye Aggregate's Polarization. For a single aggregate, the mid-angle in the angular range resulting
in the complete "extinction" (i.e., disappearance) of its luminescence and, when present, the long axis of the aggregate crystal were measured relative to the edge of the host tabular crystal ( ( 110)direction). Aggregateson the same grain having different extinction angleswere measured and counted but not those having the same angle. A minimum of 20 unique aggregates on a number of different grains were measured. For the determination of the average polarization angle, 0, 90" was added to each measured extinctionangle and the resulting value reduced to a *30° range by adding or subtracting 60° increments. Plotting angle vs frequencyclearly showed those dyes having a narrow distribution centered at 0" or 30";for distributions not centered at either of these two angles,the mean and measurement standard deviation were calculated from the absolute values of the angles. When present, the long axis of the aggregate crystal was measured and reduced to a 30" range relative to a (110) direction. Then the mean angle for the long axis of the aggregate, 4, and the measurement standard deivation were calculated. The polarization direction relative to the aggregate long axis f o r each aggregate was calculated, and these values were averaged and reduced to a 90" range, e-@. These values are given in Table 11. Note that the measurement standard deviation shows the variation among the individual measurements and is different from the standard error of the mean. The measurement standard deviations ranged from 1 to 7 for 6 , 4, and 6-4. The standard errors of the sample means were 0.2 pm). Two growth methods were used. The A method relies on reduced dye solubility caused by slow cooling from 90 "C and the M method relies on reduced dye solubility caused by slow methanol evaporation from a partially aqueous environment. The solution environment of the M method is least like a conventional photographic spectral sensitization but is a milder technique than the A method because of its lower temperature. It was the preferred method for benzoxazole dyes, which are easily hydrolyzed. By use of these two methods, polarization data were successfully obtained on aggregates made by using 14 different green and red spectral sensitizing cyanine dyes (Tables I and 11). Approximately twice as many structurally similar dyes were examined, but polarization data could not be obtained on nearly half because either the aggregate size was too small to be resolved or luminescence was not observed. I t was generally found that the less water- or methanol-soluble the dye, the less likely it would be to grow resolvable aggregates. The lack of luminescence could be a result of impurities causing quenching or of the limited available selection of excitation wavelengths of the mercury light source (i.e., the peaks a t 490,456, and 577 + 579 nm). The dye aggregates were found to grow into three general shapes: blotchy, cigar, and rod. The difference between the cigar and rod shapes is simply that, relative to the length growth, the width growth is faster for the cigar (30) It is important to also note that the average values of 8-6 that are very close to the possible limits of Oo or 90° may have a unique error. If an actual value for 0-4 was at a limit, then the distribution of its determinations would be bound by that limit and the average value of the determinations could not be that of the limit. Those values for 8-6 that have a measurement standard deviation close to or greater than the difference between the mean and a limit may have a correct value which is at the limit itself. A similar problem should be minimal for the mean values of 0 and $I since the shape and symmetry of their distributions were considered before the final reduction of the data to a Oo-30° range.
Reticle Poladzer
Removable Photoeyepiece Assembly
-
I ;Lr' Polarizer
-ExciterFiner
Liquid Nitrogen
Figure 2. The low-temperature polarized-luminescencemicroscope. Note that the removable polarizers could be placed either in the excitation light path or in the emission light path. type than for the rod type. Usually a sample consisted of primarily one shape, but some samples did contain mixtures. Two different methods of preparation sometimes resulted in different aggregate shapes for the same dye (Table 11). All three shape types were polarized. Of the dyes examined, a few grew into better defined aggregates (i.e., larger and more uniform in shape and/or sharper polarized extinction) while others grew into less well defined aggregates when Ca(N03)~ (1.5 mM) or NaN03 (24 mM) was added to the host emulsion (Figure 3B). One even formed two differently emitting aggregate types (Figure 4); their relative concentration depended on the growth conditions. The growth behavior of specific dyes and probable explanations are given in the Appendix. For the record, the data listed in Table I1 for samples 2,6-10, and 15 were obtained from emulsions having added Ca(NO& and for samples 4,5, and 16 were obtained from emulsions having added NaN03.
Aggregate Characterization. It is generally accepted that aggregates grown by the conventional method (dye rapidly added a t -40 "C) on silver halide surfaces are two-dimensional, being only one molecule thick, and that the magnitude of the spectral shift of an aggregate depends on its structure. Since the aggregatesgrown with the same dye by conventional, A, or M methods gave, in most instances, very similar spectral absorptance maxima (Table 11), then those made by the A or M methods are perhaps also monolayer in thickness. Consistent with monolayer thick aggregates, but not proof, are the shadowed carbon replicas examined by electron miscroscopy of the 21 samples of Table 11; no three-dimensional surface structures were observed that were attributable to the aggregates, indicating that these aggregates must have a thickness of less than 100 A. Also consistent with monolayer thick aggregates, the amount of average grain area producing dye luminescence seemed to be roughly equiv-
412 Langmuir, Vol. 7, No. 2, 1991
Maskasky
Table 11. S u m m a r y of Data for J Aggregates on A u B r 11111 T a b u l a r G r a i n S u r f a c e s
calcd % monolayer covermethod aggregate no. of age of shape determns of dye preprn added4 measdb of 0
sample no. 1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
A A B
A
M M
C C
A
D D E E
A
M
M A
25 25 50 15 75 100 75 35 25 25 50 25 25 4 50 50 50
N
A
50
E F G G
H
M M A
M M A A
M M
I
A
J K L L
M M A
R C R,C R,C R
40 40 25
R R R B B
R,C
R R
C, B R,B B R,C B
C C B
R C
59 54 66 22 33 33 30 30 20 29 67 90 75 108 72 36 41 45 101 50 222 77 225
8,
XC
2 13 f 2 14 f 2 16 f 2 26 f 2 Of2 Of2 Of2 Of3 30 f 3 30 f 4 9f3 22 f 3 2f4 Of3 Of3 15 f 2 20 f 3 2f1 20 f 4 20 f 2 (26 f 2)h (22 f 4)h 19 f 3
4, deg (approx) 0 12 f 3 0 13 f 4 27 f 2 0 0 20 f 6 90 30f2 90 30f 2 90 30rt 2 90 29 f 3 90 90 50 25 f 5 0 23 f 3 0 4f3 10 f 4 0 0 3f4 0 0 13 f 5 0 0 11f6 15f6 0
8-4,
2 2
w,
0
3f3 3f2 13 f 4 9f5 88 f 1 88f1 88 f 1 87 f 3 -
sample coating longest wavelength and complexity 1u m in esof cence absorption peak peaks 1-R-T, (fwhm), nm nm 572 (8) 571 (6) 574 (6)
621 (10) 620 (9) 616 (12) 615 (9) 605 (14) 606 (10) 18 f 7 623 (8) 3f3 643 (11) 5f3 652 (7) 10 f 4 654 (7) 4f3 652 (7) 625 (16)s 540 (7) 7f4 546 (7) 10 f 6 586 (11) 6f4 586 (9) 588 (12) 2 0 f 2 (3 f 3)h 590 (13) 23 f 2 18 f 4 621 (20)
control coating longest wavelength peak conventional dying a t 30', 40 O C 1-R-T, nm
absn max for
a%, nm
5750 5730 580e
573 573 576
523 523 515
623 623 622 623' 60kY 606 + 624' 624 649d 656d 656d 654d 622e 546e 54ae 591e 58ge 559 f 583'
612 612 618 sh 618 sh 609 609 609 646 650 650 646 621 546 542 586 586 557 (sh 580)
543 543 540 540 546 546 546 549 550 550 550 559 496 505 522 522 519
617f
572 (sh 605)
521
*
Assumed area occupied per molecule was 57 A2 for dye A and 75 A2 for all others.2s B, blotches; C, cigars; R, rods. X is excitation wavelength: x , broad 490-nm peak; y, 546-nm line; 2,577- + 579-nm lines. Two values mean t h a t either would give the same polarization data. Only one peak. Two peaks but one is significantly smaller and centered within 10 nm of the wavelength maximum for t h e dye in methanol. Multiple peaks. g A large and broad peak a t 650 nm, believed to be caused by phosphorescence, was also present. This dye produced aggregates that had a n unusual polarization behavior. Exciting a t 546 n m did not result in polarized emission but did result in polarized excitation, while exciting a t 577 579 n m resulted in polarized emission and polarized excitation which were 90° apart. This polarized emission had 0 and 0-$ similar to those of the 546-nm polarized excitation. This is the only sample reported in this table in which polarized excitation was used to obtain 8 and 8-$.
+
alent to the calculated percent of monolayer coverage of dye added to the emulsion, especially after considering that some of the aggregates would be expected to weakly luminesce or even not luminesce a t a11.31-35 (31) The failure of some aggregates to luminesce and the grain-tograin variation in aggregate luminescence intensity, which was observed for these samples on the polished silver sample holder, was caused, in part, by the location of the aggregates relative to the standing waves generated by the interference of the direct and reflected excitation beams. An aggregate, if at a node, would not absorb the radiation and therefore could not luminesce and, if a t an antinode, would absorb a maximum amount and luminesce most strongly.3z The grain thickness of the host emulsion used in this study, and hence the approximation separation between the sample holder and the aggregates on the top face of these grains, ranged from 0.1 to 0.4 pm. In AgBr, using 546-nm illumination, nodes would be expected at -0.023,0.098,0.219, and 0.341 Fm from the silver sample holder surface and antinodes at 0.038, 0.159, 0.280, and 0.401 um from this surface. Thesevalueswerecalculatedassumingparallel illumination and that a node would be located 0.043 X below the silver surface.33 (The fact that such interference effects existed in these samples was apparent by the presence of interference fringes that run across aggregate arrays on grains resting not parallel to the holder surface.) Other causes for lack of aggregate luminescence and intensity variations are known and are probably also important for these samples. The direct and reflected fluorescent light can interfere and an amplification or attenuation can be obtained depending on the distance between the aggregate and mirror (polished silver) surface and on the angle of Also, dye aggregates that are close to a silver surface (optimally 18 nm) can transfer their energy to plasmons a t the metal surface.36 It is likely that aggregates on the bottom surface of the tabular grains would be a t an appropriate distance for such energy transfer since, before applying the grains to the sample holder, most of the bulk gelatin had been removed, leaving an adsorbed gelatin film that is estimated to be 5-15 nm in dry thickness. Finally, since aggregates on the top surface of a grain rely on heat transfer through the grain, dyes in which the aggregate luminescence is particularly thermally sensitive may have diminished luminescence on extremely thick grains.
-
In all cases, the aggregates that were studied were of the J type, as is apparent from absorption and luminescence data. Comparing the absorption maximum of the dye in methanol to that of the longest wavelength peak obtained for samples dyed by using the three aggregate growth procedures, A, M, or conventional methods, showed a bathochromic shift of a t least 50 nm, consistent with the J aggregate36(Table 11). The slightly longer wavelengths often observed for the absorption of the A and M prepared samples, compared to the respective conventional controls, may indicate a longer coherent aggregate size.lo Four samples had a considerably longer wavelength absorption than their conventionally dyed controls; they are samples 4,5,10,and 21. For these, a different J-aggregate structure must have formed. The luminescence measured and reported in Table I1 is usually that for a single aggregate unless the sample consisted of small and weakly luminescent aggregates, making it necessary to measure a small cluster of them. But in either case, these aggregates were typical in size and shape to the ones used to obtain the polarization data. The luminescence of these aggregates was narrow and very similar in peak position to that of the sample's absorption. (32) Bucher, H.; Drexhage, K. H.; Fleck, M.; Kuhn, H.; Mobius, D.; Schafer, F. P.; Sondermann, J.; Sperling, W.; Tillmann, P.; Wiegand, Mol. Cryst. 1967, 2, 199. (33) Jenkins, F.; White, H. Fundamentals of Optics, 3rd ed.; McGrawHill Book Co., Inc.: New York, 1957; p 527. (34) Drexhage, K. H. J . Lumin. 1970, 1(2) 693. (35) Pockrand, I.; Brillante, A.; Mobius, D.; Chem. Phys. Lett. 1980, 69, 499. (36) Leubner, I. H. Photogr. Sci. Eng. 1978, 22, 270.
Langmuir, Vol. 7, No. 2, 1991 413
J Aggregates on AgBr Tabular Microcrystals
Figure 3. An illustration of the effect of Ca on the size and shape of the aggregates of dye A grown by using the M method. (A) Without added calcium, the luminescence consisted of large nonpolarized regions (probably very small aggregates), small polarized blotchyshaped aggregates, and a very small number of small polarized rod-shaped aggregates. No cigar-shaped aggregates were observed. (€3) With 1.5 mM added Ca(NO& -30% of the luminescence area consisted of large polarized cigar-shaped aggregates. (C) With a higher level, 6.0 mM, the luminescence consisted of 100% extremely large polarized cigar or distorted (due mostly to crowding) cigar-shaped aggregates.
-
-
Figure 4. The two types of polarized aggregates of sample 9 made by using 100 7; monolayer coverage of dye E. The yellow blotchy-shaped polarized aggregates emit a t 606 nm and the close array of polarized red-orange rod-, cigar-, and blotchy-shaped aggregates emits a t 623 nm. These two aggregate types are believed to differ in their two-dimensional molecular structure. (Since commercial color films were insensitive to this wavelength difference, this photograph had to be made from two black-and-white negatives exposed through 606- and 623-nm interference filters.) The triangular grain has an edge length of 22 pm.
J aggregates are known to have a narrow J-band luminescence that has a very small Stokes shift.n (Note that the 77 K J-band luminescence observed for these samples was often a t slightly shorter wavelength than the room temperature absorption of the sample. This is probably the result of the temperature difference and not the result of the excitation intensity since as an experiment, the
intensity on sample 12 was reduced by 0.1X and this had no effect on the wavelength maximum of its emission.) Before polarization data on individual aggregates could be interpreted, assumptions about their molecular structures had to be made. These were made based on the complexity of the samples’ absorptance spectra, Table 11. The absorptance spectra (1-reflection-transmission) of
Maskas k y
414 Langmuir, Vol. 7, No. 2, 1991 [i2i1
7 Example 01 " g a l s Symmmrywrl 1111) Plane
20' - 200' 40' - 220'
80' - 280'
100'.280'
\
/
Ill
\
/
140'- 320' 160' - 340'
Figure 5. Symmetry of the (1111lattice plane. This plane has an axis of %fold symmetry and three mirror planes so that, for example, if a dye aggregate grew epitaxially onto this surface having a polarization of 20° (and 200') with respect to the [Oll] direction, other similar aggregates of the same sample could have any of the six equivalent pairs of polarization directions given in the illustrative table. The same symmetry considerations also apply to the orientation of the long axis of aggregates epitaxially grown on this surface.
coatings of the samples were divided into three types having the following significance. The first type is those having one major dye absorption peak, suggesting that the aggregates have only one molecule per unit crystallographic ce1112 or, if more than one, that the electronic transition dipoles are parallel (i.e., long axes of molecules are parallel). The second type is those having two peaks with the shorter wavelength peak significantly smaller and centered within 10 nm of the wavelength maximum for the dye in methanol. This type is probably most often the result of an aggregate having one major absorption peak along with some nonadsorbed dye. The third type is those having two or more major peaks. This could be the result of more than one type of aggregate present in the emulsion coating or may be the result of a crystallographically more complex aggregate with two or more molecules per unit cell. In the latter case, the polarization axis would not represent the molecular axis, thus making the polarization data more difficult to interpret. Aggregate Orientation. Gelatin-grown silver bromide tabular grains were the ideal substrate for this optical microscopy study of dye aggregates because these grains can be grown sufficiently in size to enable large aggregates to form on their two parallel major (111)surfaces. Unlike the surfaces of sheet crystals, they are a true photographic emulsion grain type surface. For silver halides, the (111) type of surface probably does not exist as an idealized flat plane, which would consist of ions of only one sign. Surface structures consisting of approximately half-layer of ions have been p r o p o ~ e d , ' ~but t ~ ~the actual structure of this surface remains a mystery; adsorption of dye may even uniquely alter it. However, it seems reasonable initially to assume that the symmetry of this surface should be related to that of the underlying (111)lattice planes. Therefore, in order to understand the possible orientations of dye aggregates and the polarization properties of these aggregates on the (111)surface, the symmetryof an internal lattice plane needs to be considered. The (111)lattice plane has an axis of %fold symmetry and three mirror planes (Figure 5). With these symmetry elements, any linear property of oriented dye aggregates (such as polarization direction or axis of extended growth) can be uniquely defined within a 30' arc measured from either a (211) or (110) direction. For this paper, the (110)
direction was used as the reference direction for all reported angular data. A good illustration of how this symmetry affects epitaxially deposited aggregates is shown in Figure 6. Note that the polarization angles for aggregates of dye N before being reduced to a 30" angular range form six sets of distributions between 0' and 180' and the relative location of these distributions is related by the expected mirror symmetry. Figure 7 shows these data reduced to a 30' range (0 = 19'). This same effect of the lattice symmetry on epitaxial aggregates is pictorially represented in Figure 8 for dye L by the collection of photomicrographs of a single tabular grain taken at different sample-polarizer orientations. The angle between the upper left edge of the grain and the polarization axis (reticle) is marked beside each view and shows that there are six angles in a 180" arc where at least one aggregate has completely disappeared as marked by the white arrows. These six angles reduce to 0 = 19', 20°, 19', 20°, 15', 19'. (The one low value of 15' is probably the result of experimental error in picking the mid angle of complete aggregate disappearance.) The average polarization angle of the aggregates of a dye, 0, should give useful information about their molecular orientation for most of the dyes examined. The values for 0 listed in Table I1 were measured from the aggregates' emission (except for the two aggregate types of sample 20, which were measured from their 546-nm excitation). Also given are the measurement standard deviations of the distribution of measured angles (reported as la). In general, the distribution increased for samples having a broader extinction minimum or having relatively low emission intensity, suggesting that much of the distribution could be a result of uncertainty in making the measurement and does not necessarily imply variations in the orientation of the aggregates. The difference between the polarization angles measured by emission and excitation, w , was obtained for 12 typical aggregates of each sample and is given in Table I1 as an approximate average value. Of the 21 samples studied, 9 had w # 0. The significance of w # 0 is that it indicates either that the sample has been excited by irradiating into a transition not parallel to the a a* J band (e.g., n a*)or that the aggregate is made up of nonparallel molecules with excitation into a transition other than that responsible for the longest wavelength band. This latter possibility would mean that, for such a sample, the emission polarization direction is not a direct measure of the molecular orientation. Samples that had w # 0 also had multiple peak type absorptance spectra. But for most of the dyes, studied, Table 11,the polarization data could be analyzed to obtain information about molecular orientation of the aggregates relative to the AgBr substrate. However, the polarization data could not be analyzed for dyes A, E, or M for the following reasons. The crystal structure of dye A has been discussed in the literature; this dye is not a planar molecule. The lack of planarity is a result of the steric repulsion of the hydrogen atoms a t the 3 and 3' positions that causes the two quinoline rings to be twisted 50.6' relative to each other.3 This dye still readily forms a J aggregate, however. Also, the free-standing aggregates precipitated from solution have been reported to contain more than one molecule per unit Considering this information, the polarization angle, 8, obtained for aggregates of this dye probably does not directly indicate the molecular orientation despite the apparent simplicity of the absorptance spectra (Table 11).
-
-
(37) Scherer, P. 0. J.; Fischer, S. F. Chem. Phys. 1984,86, 269. (38) Duschl, C.; Frey, W.; Knoll, W. Thin Solid Films 1988,160,251.
Langmuir, Vol. 7, No. 2, 1991 415
J Aggregates on AgBr Tabular Microcrystals
"I 12
R
30
g
20
?
LL
10
0 5
0
IO
i5
20
25
30
Polarization Angle Relative to a e1 10> Direction, (e). Polarization Angle Relative to a Direction
Figure 6. An example of polarization emission data for 225 aggregates of dye N on AgBr tabular grains before being reduced to a 30" range to collapse the redundancy brought about by the symmetry of the 11111lattice plane. The angles are relative to a (110) direction.
The two types of aggregates of dye E had unusual combinations of b' and w for which possible molecular structures were not obvious. The rod- and the blotchy-shaped aggregates of dye M, which both appeared in the same sample, had w 90' for 577- 579-nm excitation, and in addition had a peculiar wavelength-dependent polarization behavior (see footnote h in Table 11) that suggests a complex band structure for these aggregate types. The polarization angles of the remaining 11 dyes are probably the same as their molecular orientations if it is assumed that the complex absorption spectra that were observed for some dyes were due to mixtures of aggregate species. (This assumption was not made for dyes C and D for reasons that will be discussed later.) These polarization angles, representing the molecular orientations, can then be compared to the AgBr in-plane (110) and (321) directions (which are Oo and 19O, respectively). The measured polarization data are believed to be accurate to f 2 O since the polarizer calibration and the standard error of the sample mean were each within *lo. With this range as limits, the molecules making up the aggregates of dyes C, D, G, H, and K were aligned with the (110) directions and dyes J, L, and N with the (321) directions (Table 11). Those making up the aggregates of dye F and the major population of aggregates of dye B39had average polarization angles just 3' from the (321) directions. Finally, those making up the aggregates of dye I and the minor population of aggregates of dye B had average polarization angles 4 ' and 7O from the (321) directions, respectively, and are considered to be significantly greater than the experimental error limits. These molecular alignments can be considered in terms of chemisorption and physisorption. Chemisorption of dye molecules along AgBr (110) directions, as mentioned previously, is unlikely on an idealized flat surface due to steric considerations, but a reconstructed or stepped surface could alleviate the steric hindrance problems. Chemisorption of dye molecules along AgBr (321) directions on a nonreconstructed surface is unlikely due, in part, to the large silver ion separation. Of the five dyes found to have a (321) orientation, dye F is the only
+
(39) The polarization angles of the aggregates of dye B formed two distributions. Also, it is interesting that this anionic dye did not form aggregates composed of molecules aligned along (110) directions, since the structurally similar cationic dye (1,1',3,3'-tetrachloro'5,5',6,6'-tetramethylbenzimidazolocarbocyanineiodide) was predicted to align along these directions based on its crystal packing obtained from single-crystal X-ray data.3
Figure 7. Distribution of polarization angles versus number for the 225 determinations of dye N shown in Figure 6 but reduced to a 30" range, yielding an average angle of 0 = 19'. A similar treatment of the long axis of the cigar-shaped aggregates yielded 6 = 23". The polarization angle relative to the long axis of the aggregate determined for each of the 225 aggregates was reduced to a 0-90" range to yield 0-6 = 18" (Table 11).
symmetrical benzothiazole dye and, since it contains divalent sulfur atoms, is the most likely candidate for chemisorption. The Ag ion sites along a (321) direction are 10.8 A apart, a distance substantially greater than that needed to interact with the two sulfurs separated by 6.168 A as reported for the analogous dye, 5,5'-dichloro-3,3',9triethy1thiacarbocyanine.ls This Ag ion separation is, in fact, close to the separation of the dye's two 7 and 7' hydrogens, thus preventing close approach of the dye for this orientation unless reconstruction of the surface left these Ag ion sites vacant. Also, on the basis of chemisorption alone, it is difficult to explain the fact that the analogous 5,5'-difluorobenzothiazole dye, dye G, aligned along ( 110) directions while dye F adopted this (321) alignment. Steric considerations should be similar for these two dyes since there is only a relatively small difference in the van der Waals radii of hydrogen and fluorine (1.2 Avs 1.35 A). While the electron distributions within these two dye structures should be different, possibly resulting in physisorption and dyedye interaction differences, there should be little effect on any S-Agf interactions. Particularly relevant to the question of the importance of chemisorption is the fact that dyes B, J, and K, which are unlikely to chemisorb based on their lack of appropriate interacting groups, also formed epitaxial aggregates. Dye B is a benzimidazole dye that has its nitrogens blocked by alkyl groups. Dyes J and K are benzoxazole dyes and, because of the poor strength of O-Ag+ ligand would not be expected to chemisorb; thus the alignment of dye J along (321) directions and dye K along (110) directions is probably a result of physisorption. The fact that so many dyes examined formed epitaxial aggregates even though many had structures not expected to chemisorb shows that physisorption alone is sufficient for epitaxial growth, but a contribution from chemisorption by some dyes cannot be ruled out. Comparable AgBr lattice network and possible aggregate networks could allow either (110) or (321) oriented epitaxy with no lattice misfit. This epitaxy could form solely as a result of physisorption. The (110) molecular fit has already been c o n ~ i d e r e d ~ ~ but ~ ~ the ~ J ~(321) J ~ J fit ~ has ~
~~
~~
(40) Laing, D. K.; Pettit, L. D. J . Chem. SOC.,Dalton Trans. 1975, 2297. (41) Barnes, D.; Laye, P. G.; Pettit, L. D. J . Chem. SOC.A 1969,2073. (42) Ahrland, S.;Chatt, J.;Davies, N. R.; Williams,A. A.J. Chem. SOC. 1958, 264.
Maskasky
416 Langmuir, Vol. 7, No. 2, 1991
I
A
347
4 9 O l
YO0
70'
131' Figure 8. A series of color photomicrographs of one AgBr tabular grain showing the luminescence of aggregates of dye L (sample 18), a t 77 K. The first photo is without a polarizer in the emitted light path. The remaining series of photos shows aggregate disappearance (location indicated by white arrows) a t six different polarization angles. The number next to each photo is the angle between the upper left edge of the tabular grain ( ( 110) direction) and the reticle that indicates the polarizer axis and was set a t the estimated mid-range of sample rotation over which an aggregate was no longer visible. These aggregates are an example of the cigar-shaped type and measure -7.5 X 2 pm. The grain is 20.0 Fm across as measured between parallel edges and is estimated to be 0.1-0.2 pm in thickness.
J Aggregates on AgBr Tabular Microcrystals
Figure 9. A representation of dye molecules (-19 A long) on an idealized flat 1111)AgBr surface aligned along a (321) direction, R = 1 9 . 1 O . The angle of slippage, a, is 19O and the separation between parallel rows of dye molecules is 3.74 A. This aggregate is drawn with steps along its two long edges, which result in 4 = 1 3 O and R-4 = 6'.
not. On an idealized flat (111)surface, alignment of dye molecules along (321) rows in a regular repeat network relative to the AgBr lattice could result in a slip angle of 19' and a separation between adjacent molecular planes of 3.742 A (Figure 9). This small slip angle should result in a bathochromic spectral shift of the dye absorption consistent with the J aggregate.' The molecular plane separation is only 11%larger than the 3.37-A graphitic packing distance. A rotation from vertical of the dye molecules about their long axis by 26" would decrease the molecular plane separation to the graphitic packing distance. In addition to measuring aggregate polarization, the orientation of the long axis of the rod- and cigar-shaped aggregates relative to a (110) direction on the silver bromide surface (4, which can have values of 0-30') and the average of the orientation of the long axis of each aggregate relative to its polarization direction (9-4, which can have values of 0-90') were measured. Both 4 and 0-4 show the preferred direction of two-dimensional aggregate growth. This preferred direction of growth can be influenced by both dye-dye and dye-surface interactions. The values are given in Table I1 and are averages for the same individual aggregates whose values of 9 are also reported (except for a few aggregates in which a long axis could not be distinguished). The nonzero values obtained for 0-4 show that for those aggregate samples in which the polarization direction is the molecular orientation, the long edges of the aggregates must be composed of stepped sides. For aggregates having a slip angle consistent with the J aggregate (a C 32°),7aggregate orientations with 9-4 N 7' can be obtained with regular steps as illustrated in Figure 9. Similar small angles for 9-4 were measured for the aggregates of all the emulsions having w = 0. (Note that the aggregates of some samples found to have a very small 9-4 angle may actually be zero, meaning that the molecular and aggregate axes are collinear. See ref 30.) Large average angles that were very close to the possible limit of 90' (Le., 87' and 88') were calculated from the distributions of 9-4 for the aggregates of samples 4, 5, 6, and 7 made from the two9-methylbenzothiazole dyes, dyes C and D. (The symmetry of each of these distributions was that expected for 9-4 equal to exactly 90'). These aggregates had w = 90'. They were rod shaped with their long axis at an angle of 30' to a (110) direction (Figure 10). They also had an emission polarization angle along a (110) direction and an excitation polarization perpendicular to that of the emission. All of these facts strongly suggest a herringbone structure8for these aggregates. This structure consists of two molecules per unit cell with each aligned along adjacent (110) directions (i.e., 60' apart). It seems reasonable that the long axis of such an aggregate would be exactly between the two columns (i.e., 4 = 30')
Langmuir, Vol. 7, No. 2, 1991 417 (Figure 11). When two molecules are nonparallel, two different transitions, polarized at right angles to each other, will occur in the absorption spectrum as a result of the addition and subtraction of their nonparallel transition dipoles (Davydov splitting). If these two transitions are coupled (perhaps weakly in this case), then excitation into the higher energy band would produce emission from the lower energy band and thus, at appropriate excitation wavelengths, there would be a polarization angle of 90' between the excitation and emission (Le., w = 90'). Additionally, from considerations of the coulombic interactions between the two nonparallel transition dipoles, the lower energy transition is probably the out-of-phase transition, which would have a direction 90" from the center line running between the two columns (Le., 0 = 0' and 0-4 = 90'). Interestingly, Scrutton found that dye D, when adsorbed to AgBr (loo\ faces, produced two main absorption peak whose relative areas appeared to be unaffected by changes in the dye concentration. He suggested that a herringbone structure had formed. He also found that on (111)faces the spectra were different from those obtained from (100) faces and they changed depending on dyeing conditions; he suggested that a herringbone structure did not form on these (111)faces.43 Unfortunately, in the present study, the absorption spectra of the sample coatings of dyes C and D suspected of containing herringbone aggregates were too complex to identify a companion short wavelength absorption band. Perhaps such a band could be found by adding the capability to measure excitation spectra of single aggregates to the low-temperature polarizedluminescence microscope. A particularly puzzling observation was made for two of the dyes. Between 0 and 180°, there should be six equivalent values of nonreduced 0 (and 4 when present), which should be randomlyrepresented on individualgrains unless these values happen to be at an axis or plane of symmetry (Le., O', 30°, 60O . . . ) , Figure 5. But with dye I, most individual tabular grains had only every alternate possible value for 9, and with dye M, only alternate values for 9 and for (Figures 1 2 and 13). Since these tabular grains would most often contain two twin planes parallel to the two major faces,26 observing only one symmetry network is possible only if aggregates on just one side of each grain were seen and if there were no mirror planes perpendicular to the 1111)surface. As previously discussed, the aggregates on one face of a tabular grain should have diminished emission intensity as a result of optical interference effects and the emission of those on the bottom face could even be quenched by energy transfer to the silver sample holder. To confirm the lack of perceptible luminescence on one side of individual tabular grains, a simple experiment was performed observing the rod-shaped aggregates of sample 20. Gelatin (-2.5 pm dry thickness) was coated onto one half of the surface of a silver sample holder; after the gelatin hardened, grains from sample 20 were coated onto this holder. On the side of the sample holder not coated with gelatin, less than 1% of the grains showed aggregate orientations from both symmetry networks (as observed by 4). But on the gelatin side, about 3070 of the grains showed both symmetry networks. This result demonstrates that the aggregates of this dye were visible on only one side of the grain. This conclusion is further supported by the general lack of overlapping blotchy-shaped aggregates of sample 20 (Figure 12). Note also that for the blotchy-shaped (43) Scrutton, S. L. J. Photogr. Sci. 1974, 22, 69.
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418 Langmuir, Vol. 7, No. 2, 1991
Figure 10. The rod-shaped aggregates of sample 5 that are oriented 30" to a (110) direction. These aggregates are believed to consist of molecules packed in a herringbone arrangement. The large grain measured 23 p m between opposing parallel edges.
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