Anal. Chem. 1980, 61, 391-398
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Analysis of Spectral Sensitizing Dyes in Photographic Films by Enhanced Raman Scattering Spectroscopy E. Steven Brandt Photographic Products, Research Laboratories, Eastman Kodak Company, Rochester, New York 14650-2132
Enhanced Raman scatterlng has been used to oMaln specHk and quantltatlve analysis of three aggregated spectral sensitlzlng dyes In common photographlc film coating formats, lndudkrg (1) single- and muttiple-layer coatings contalnlng one dye per layer and (2) comblnatlons of multlple dyes In slngle layers. Partial conversion of the gelatin-dispersed AgBr mlcrocrystab to sliver metal by conventlonai photographlc processing b requked to separate the Raman slgnal from a large fluorescence background. Llnear Raman Intensity vs coverage responses are observed between 0.05 and 1 monolayer for all three [red (5,5'dkhloro-3,3',9-trlethytthlacarbocyanine bromide), green (5,5'dlphenyl-3,3',9-trlethyloxacarbocyanlne bromkle), and blue (5,5'-dlchloro-3,3'-dlethylthlacyanlnetosylate)] dyes. From quantitative results obtalned by hlghperformance llquld chromatography (HPLC) for the post-prck cess dye content of the flhns, detectlon llmlts are determlned to be ca. 2 X lo-'' mol of dye. An enhancement factor of ca. 1 X lo4 Is estimated for partlally developed photographlc fllms. The resutts can be explained by molecular resonance enhancement by the chromophore wlthout evoklng large surface enhancements due to either chemlcal or electromagnetic mechanlsms lnvolvlng the developed sllver.
INTRODUCTION Few common commercial products are as chemically complex as photographic films. The ability of these materials to capture photons and to reproduce both color and black-andwhite images depends on many organic and inorganic systems working both independently and in concert ( 1 ) . One such system involves combining the spectral and charge-donating characteristics of an organic chromophore, or dye, with the charge carrier trapping efficiency of silver chloride or bromide to obtain a film that can capture photons with energies that are outside the range of the intrinsic photoresponse of the silver halide. This process is called spectral sensitization ( 2 ) and is one of the first steps in the mechanism of capturing both color and black-and-white photographic images on film. Historically, a number of techniques including spectrophotometry, luminescence, photographic response, and electrochemistry ( 3 ) have been employed to investigate the spectral sensitization process and spectral sensitizing dyes. Although much useful information has been obtained with these approaches, they do not, in general, provide a means to study spectral sensitization on a molecular level within a film. Molecular information is necessary to fully explore fundamental processes such as adsorption, aggregation, and charge transfer, which are intimately associated with spectral sensitization efficiencies. Until recently, vibrational spectroscopic probes have been limited by the amount of dye used in spectrally sensitizing photographic films (ca. 1 mg/ft2 of fii) and the intense and complex background associated with the silver halide dispersion medium, gelatin. In a recent report, we described the use of Raman scattering (RS)spectroscopy to obtain molecular spectra of an aggregated 0003-2700/89/0361-0391$01.50/0
spectral sensitizing dye within a gelatin-based photographic film (4). Partial reduction (development) of the silver halide to filamentary silver was necessary to quench the high fluorescencebackground from the adsorbed dye aggregate and observe a measurable Raman signal. The results were interpreted in terms of combining surface-enhanced Raman scattering (SERS) ( 5 )mechanisms involving the rough morphology of filamentary silver with the molecular resonance Raman scattering (RRS) enhancement of the aggregated dye to obtain a high degree of discrimination among the possible interferences. Following the results of this preliminary work, we have now extended our investigations into the use of RS as an in situ probe of chemicals added to photographic systems to modify the inherent photographic response of silver halide microcrystal dispersions (addenda) to include films containing multiple spectral sensitizing dyes in both single- and multiple-layer (multilayer) coating formats, which are commonly employed in practical systems. Combining conventional photographic exposure and development techniques with resonance excitation of the aggregated dye, we have found that it is possible to "tune" the experiment to a particular dye, even in the presence of other chromophores and gelatin. In addition, the Raman response from the dye is linear over a wide range of concentrations and processing conditions. In the time elapsing between our preliminary report and the present work, our experimental procedures have been expanded and refined to include complementary analytical information concerning the amounts of dye, silver, and silver halide remaining in the films after photographic processing. Combining these additional analytical results with the Raman response from a wider variety of samples, we find that the Raman activity from spectral sensitizing dyes in partially developed photographic films can be explained without evoking large SERS enhancements by the developed silver.
EXPERIMENTAL SECTION Film Coatings. Spectral sensitizing dyes were adsorbed from a MeOH solution to 0.34-pm (measured as equivalent circular diameter) octahedral (predominantly (111)surface planes) AgBr microcrystals dispersed in an aqueous solution containing deionized alkali-derived ossein gelatin. The dispersionscontaining the dyed microcrystalswere then diluted to suitable coating levels (1 X mol of AgBr/ft2) by addition to warmed aqueous solutions containing higher concentrations of gelatin. These were then machine-coated at 40 "C onto a continuously moving strip (web) of poly(ethy1ene terephthalate) (PET),which served the coating support. After a partial hardening of the dispersion layer, a protective overcoat of gelatin was applied to complete the film package. A total of four different coating formats were used. The first was a multilayer (ML) coating in which each dye was confined to a separate layer according to photographic convention, with the red-sensitized layer on the PET support, followed by an overlay of the green-sensitized layer, with the blue-sensitizedlayer coated on top of the other two layers. The second was a single-layer coating (excluding overcoat) containing a 1:l:l mixture of microcrystals in which each fraction was dyed before mixing (mixed fractions (MF)). (Total AgBr coverage was maintained mol/ft2.) The third type of coating was also a at ca. 1 X 0 1989 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 5, MARCH 1, 1989
single-layer coating, but one in which the three sensitizing dye solutions were premixed in a 1:l:l ratio before addition to the AgBr dispersion (mixed dyes (MD)). For this coating, the individual dye levels were adjusted to one-third of the concentration per unit of AgBr surface area in the other formats to obtain approximately the same total dye coverage per unit area of AgBr. The final format was simply a single dye (SD) in a single layer coated at various coverages. Coverages were calculated on the basis of the following projected areas of the three dyes: RSD (5,5’-dichloro-3,3’,9-triethylthiacarbocyanine bromide), 72 A2/ molecule; GSD (5,5’-diphenyl-3,3’,9-triethyloxacarbocyanine bromide), 84 A2/molecule;and BSD (5,5’-dichloro-3,3‘-diethylthiacyanine tosylate), 65 A2/molecule. (The abbreviations stand for red, green, and blue sensitizing dyes, respectively. The structures of these dyes are given in Figure 2, along with their corresponding counterions.) Prior to analysis by Raman spectroscopy,the single-layer fiis were exposed to ambient fluorescent laboratory light for ca.1min and then chemically reduced (developed) at room temperature to produce an optical density of ca. 1.5, which generally corresponded to ca. 50% molar conversion of the AgBr to silver metal (see text). For these coatings, the development consisted of 30 s in Kodak Developer D-76 (Kodak Developer D-76 contains the reducing agents hydroquinone and elan (N-methyl-p-aminophenol), sodium sulfite, and borax buffer) (6),followed by a 2-min rinse in Milli-Q purified water. This procedure is referred to as “partial” or “arrested” development in the text because (1)all of the silver halide was not converted to silver metal and (2) the excesa AgBr was not removed by a suitable Ag+ complexing agent (fixing). The ML coating required more control over the amount of total silver developed to reduce the attenuation of light by developed silver in the intervening layers. Confining development to the layer containing the dye of interest was achieved by a 1-s daylight filtered exposure on a Kodak Model IV, Type 1B sensitometer through an appropriate Kodak Wratten filter followed by darkroom development in either D-76 or an ascorbic acid developer containing Kodak Elon developing agent. Chemicals. Gelatin, dyes, and processing chemicals used in this work were of Kodak photographic grades and were used without further purification. The remainder of the compounds were of reagent grade and were obtained from a number of suppliers. The water for solutions and processing was obtained from a Milli-Q water purification system. All analyses were performed at room temperature, 22 OC f 2 O . Instrumentation. The Raman instrumentation has been described in detail previously (4). For some experiments involving coatings containing BSD, a Liconix Model 4240 NB He/Cd laser was added to the Raman system to provide 441.6-nm excitation. Analyses of the dyes by HPLC were performed on a Hewlett-Packard 1090 liquid chromatograph equipped with a diode array detector and using a 15-cmHamilton PRP-1 column (0.5-pm polystyrene/divinyl benzene stationary phase). The A solvent was 0.1% trifluoroacetic acid in water, and the B solvent was a 1:l acetonitrile/MeOH mixture. Elution conditions were varied to accomplish the best chromatographicseparation and peak shape for a particular dye. Dyes were removed from the film coating by enzymatic digestion of the gelatin, followed by extraction of the residue in a.smal1 quantity of B solvent. RESULTS AND DISCUSSION Electronic Spectra of Adsorbed Spectral Sensitizing Dyes. RRS excitation frequencies were determined from the absorptance ( % A = 100 - % T - %R; where T and R are the transmitted and reflected light, respectively) curves of the spectrally sensitized film coatings. The spectra of the coatings containing all three dyes in ML, MF, and MD formats are shown in Figure 1, along with a composite of representative spectra of the individual dyes (SD format). The maxima of the adsorbed dyes (X,,(ads)) are shifted bathochromically ca. 40-100 nm from their corresponding solution values in MeOH (X,,(sol)) (Table I) due to the formation of extended two-dimensionallattice structures (J aggregates) on the surface of AgBr. The formation of dye J aggregates is also responsible for the relatively narrow bandwidths (fwhm of ca. 20-30 nm)
1 A; 400
480
i
, / : 560
; I ;
;
640
~
720
, 800
X/nm
spectra of single and multilayer spectrally sensitized photographic coatings used in this work: ML, muttilayer with RSD, 0, and BSD confined to separate layers; MF, three indkriduaily dyed fractions of AgBr microcrystals in a single layer; MD, AgBr microcrystals coated with a mixture of ail three dyes; SD, % A spectra of 30% equivalent monolayer coverage dispersions for ail three dyes. Arrows indicate Raman excitation wavelengths (nm) used to obtain the spectra in Figures 2 and 3: 1, 465.8; 2, 5, 540.0; 3, 647.1; 4, 441.6; 6, 615.0. Ail spectra are on the same % A scale. Figure 1. % A
Table I. Comparison of &(sol) with k,(ads) for the Red, Green, and Blue Spectral Sensitizing Dyes Used in This Work dye
Xma.(sol),nm
RSD GSD BSD
553 501 427
X,,(ads), 650 540 470
nm
A, nm
97 39 43
and for the extinction coefficients a t X,,(ads) 10-20 times the values observed for a similar number of dye molecules in solution. The electronic spectra of the ML and MF coatings have X,,(ads) and band shapes that are virtually identical with those of the % A curves of the individual dyes (cf. Figure 1, SD). The composite character of these curves is indicative of the irreversibility of dye molecule adsorption to the respective host AgBr microcrystals during the dispersion mixing and coating operations. By constrast, the aggregate peaks associated with BSD and RSD in the MD coating are hypsochromically shifted toward their corresponding solution monomer X,(ads). This behavior is characteristic of the formation of diluted dye aggregates on the surface of the AgBr microcrystal, as these dyes compete for available adsorption sites during the dyeing step. In the diluted aggregates, the absorbed electronic energy is less delocalized via excitonic (7)channels, whose energy levels scale with n, the number of molecules in the lattice. The aggregate peak for GSD, on the other hand, shifts slightly (ca. 5 nm) to the red under these coating conditions. The direction of this shift can be explained by the mixing of the electronic
ANALYTICAL CHEMISTRY, VOL. 61, NO. 5, MARCH 1, 1989
energy levels of the GSD molecules with those of the RSD molecules to form excitonic energy levels that lie between those of the individual dye aggregates. Raman S p e c t r a of Spectrally Sensitized Films. Spectral sensitizing dyes in unprocessed (raw stock) photographic film coatings often exhibit strong fluorescence with laser excitation near the X,(ads) of the aggregate, which is commensurate with the high fluorescence quantum efficiencies observed for many dyes in this class. For example, with 5 mW of resonance excitation and a spectrometerbandpaw of 2 cm-', the spectrally sensitized raw stock films in this work produced fluorescence background count rates generally in excess of lo6 counts/s, which overwhelmed the relatively weak Raman scattering from these dyes. As described in ref 4, the fluorescence background can be decreased by ca. lo3 by first exposing a film to light within the absorption band of the aggregated spectral sensitizing dye and then partially developing the film to convert a fraction of the silver halide to filamentary silver metal (see Experimental Section). When this procedure was performed on the SD coatings in this series, the RRS spectrum of the BSD, GSD, and RSD aggregates could be obtained in situ with a relatively high S/N ratio and with no evidence of bands attributable to either madsorbed developer or the gelatin matrix surrounding the partially developed microcrystals (Figure 2). The quenching of dye fluorescence by silver occurs only after metallic silver has been developed from the photographic latent image (defined as developable photolytic silver) formed by exposure of the films to actinic radiation. While photolytic silver is formed a t room temperature by high-intensity laser illumination of the film during Raman analysis, this silver is composed of clusters of relatively few (
