J. Phys. Chem. 1993,97, 8269-8280
8269
Redox Limitations for the Spectral Sensitization of Silver Halide in the Infrared J. R. Lenhard,’ B. R. Hein, and A. A. Muenter Imaging Research Laboratories, Eastman Kodak Company, Rochester, New York 14652- 3208 Received: December 4, 1992; In Final Form: April 9, 1993
The efficiency of spectral sensitization of silver halide by cyanine dyes in the infrared region was found to be dependent on both the one-electron reduction and oxidation potentials of the dye. Correlation of sensitizing efficiency with reduction potential is expected, since this measurement reflects the electron-injecting ability of the dye’s excited state. The dependence on oxidation potential results from the correlation of this measurement with a dye’s propensity toward oxidation in the liquid state of the silver halide microcrystalline dispersions used to prepare photographic coatings. Oxidative decomposition of these dyes in such dispersions may lead to destruction of dye and creation of species that reduce spectral sensitizing efficiency and lower overall photographic sensitivity. Infrared diffuse-reflectance spectrophotometry was used to confirm that cationic tricarbocyanine sensitizing dyes undergo a one-electron oxidation in AgCl liquid dispersions to yield the corresponding dye radical dications. Correlations from spectra obtained for dye radical dications formed by electrochemical generation in acetonitrile solution were used to aid in identification of the dye-related species present in silver halide systems. The rate of sensitizing dye oxidation in a silver halide microcrystalline dispersion at 40 OC was found to depend on the dye oxidation potential and on the aggregation state of the unadsorbed dye. t
Introduction Interest in infrared-sensitivesilver halide materials has greatly expanded in recent years, mainly as a result of the development of exposing devices equipped with infrared-emitting laser diode sources. Because of the energy requirements for spectral sensitizationof AgX, the search for infraredsensitizersthat exhibit both thermodynamic stability and high photoefficiency when used with a silver halide material presents a formidable challenge. The sensitizing dye properties that are most important in this regard are the one-electron oxidation and reduction potentials of the dye. Relationships between the electrochemical, spectral, and photographic properties of an infraredsensitizer are illustrated in Figure 1, where the energies of the lowest vacant ELVand highest occupied EHO molecular orbitals of the dye are correlated with the dye reduction potential Erd and oxidation potential Eox, respectively, and hu is the spectral transition energy. In the simplified molecular orbital picture shown in Figure 1, theelectron-injectingabilityof theexcited stateof a given infrared dye is expected to depend on the energetic position of the lowest vacant level of the dye relative to the bottom of the silver halide conduction band. The electron-trapping ability of the ground state of the dye will also be correlated with the relative energy of this lowest vacant level. Dye reduction potentials have been shown tobelinearlyrelated toELv’Jand thusprovideaconvenient measure of the expected electron-injecting and electron-trapping characteristics of a given dye.3 Recent studies on series of carbocyanine dyes on AgBr or AgBrI have shown that the efficiency of electron injection from the dyes’ excited states, as measured by their photographically determinedrelative quantum yield for spectral sensitization, follows a Marcus-Levich type dependence on Ed.4.5 The cyanine dyes that we have considered for use as infrared sensitizers are those from the tricarbocyanine class. These dyes have reversible one-electron reduction potentials that range between -0.70 and -0.90 V vs the Ag/AgCl reference electrode.6 The energy of the silver halide conduction band edge will depend slightly on the halide composition of the crystal. A previous calibration of AgBr0.&,.02indicatesthe energy of the conduction band to be equivalent to the electron-injecting level of the excited state of a dye having a solution E d of -0.92V.4 The conduction
1
Conduction Band
h e -
E,
--+ Correlates
with
Dye Reduction Potential
ISilver Halide
Sensitizing Dye
E
t
+2.0 v
Figure 1. Energy diagram for infrared spectral sensitization of silver halide.
band for AgCl is expected to be at a slightly higher energy (more negative potential) than that for AgBrI.’ Given these electrochemical data, the electron-injecting levels of excited infrared dyes are considered to lie just below the level of the silver halide conduction band edge. Consequently, infrared sensitivity should be very sensitive to changes in dye reduction potential. In this paper, we examine explicitly the dependenceof infrared sensitivity on dye reduction potential using a series of cationic tricarbocyanine dyes and explore in detail the factors that limit dye sensitization at long wavelengths.
Experimental Section Materials. 3,3’-Diethylthiatricarbocyanineiodide was commercially available (Kodak Laboratory Chemicals). Other infrared sensitizing dyes were synthesized in the Dye Research Laboratory of Eastman Kodak Company and kept under refrigeration until use. Dyes obtained with halide counterions were converted to perchlorate or tetrafluoroborate salts for electrochemical studies; however, silver halidestudieswere carried out on bromide and iodide salts as well. Acetonitrile (CH&N, MCB spectrograde) was dried over 4-A molecular sieves (Kodak Laboratory Chemicals, baked at 400 “C). Tetrabutylammonium tetrafluoroborate (TBABF4, Kodak Laboratory Chemicals) was recrystallized three times from ethanol/water, vacuum dried at
0022-3654/93/2097-8269$04.00/0 0 1993 American Chemical Society
8270 The Journal of Physical Chemistry, Vol. 97, No. 31, 1993
50 OC, and stored in a desiccator. Ferric chloride hexahydrate (FeCl3, Kodak Laboratory Chemicals) and methanol (Kodak Laboratory Chemicals, spectrograde) were used as received. The silver halide substrate used in these studies consisted of AgCl cubes of 0.35-hm edge length and narrow size distribution, prepared by the double-jet precipitation methods and subjected to a standard treatment with sulfur- and gold-containing compounds to enhance light sensitivity.8b This preparation resulted in an aqueous microcrystalline dispersion of the AgCl cubes in gelatin and typically contained 14wt % AgCl and 5.1 wt % gelatin. Before additionof dye, the melted dispersion (melt) was pretreated at 40 OC with triazinylstilbenesulfonic acid (TSSA, 4.7 X 10-4 mol/mol AgCl), a phenylmercaptotetrazole (5.8 X lWmol/mol AgCl), and potassium bromide (0.01 mol/mol AgCl). Adjustments of dispersion pH were made by dropwise addition of 0.1 M NaOH or HN03. Dispersions used in photographic measurements had a pH of 5.6. In addition, the dispersion silver ion activity pAg was adjusted to 7.65 by careful addition of 1 M AgNO3 or NaCl. As will be discussed in the Results section, the TSSA and bromide additions were intended to modify adsorption characteristics of the dye. The phenylmercaptotetrazole, described as an antifogging compound in the photographic literature? was added to retard reduction of unexposed AgCl by the color developing agent used in the photographic measurements. Methanolic solutions of dyes were added to portions of the melt mol of dye/mol of AgCl, a to give a concentration of 3 X quantity corresponding to approximately 10% surface coverage of theAgC1, assuming complete adsorptionof the dye. To produce samplesfor photographic measurements, additionalwater, gelatin, and phenylmercaptotetrazole (1.23 X mol/mol of AgCl) were added to the melts. These melts were then coated at 240 mg of silver halide/m2 onto a polyethylene-coated paper base together with an equivalent quantity of cyan imaging coupler, a compound that forms cyan dye upon reaction with the oxidized color developer that results from reduction of the silver halide during the photographic development process. The quantity of cyan dye formed is directly proportional to the amount of silver halide reduced during development. CI
'S0,Na
NaS03/
N"
TSSA
Photographic Measurements. Photographic sensitivity in the region of inherent silver halide light absorption was determined by exposing the coated samples for 0.2 s to 365 nm radiation from a high-pressure mercury lamp. At the exposure plane, the light was filtered by a series of stepped neutral densities to give 3.0 log units gradiation in exposure intensity. Photographic sensitivity in the infrared was measured by exposing the samples for 4 s in a calibrated spectrograph having tungsten illumination. In this case, the stepped neutral density filter at the exposure plane was arranged to give gradations in exposure intensity perpendicular to the direction of spectral dispersion. The exposed samples were developed for 3.5 min in Kodak Ektaprint 2 developer/EP-2, bleached, fixed, and washed. The photographic sensitivityS was obtained as the reciprocal of the relative exposure (in erg/cm2) necessary to produce a cyan dye density of 1.0. Infrared sensitivitieswere calculated in IO-nm intervals from 600 to 900 nm to determine the wavelength of maximum sensitivity. In order to calculate the efficiency of electron injection from the excited state of an adsorbed dye to silver halide, a direct proportion is assumed between the photographic sensitivity and
Lenhard et al. the yield of electron injection from the dye. Further, it is assumed that photographic detection efficiency for an electron injected by the dye will be the same as that for a conduction band electron resulting from silver halide band gap excitation. The relative quantum efficiency for spectral sensitization is then the ratio of photographic efficiency for exposure at the dye wavelength X to the photographic efficiency for direct exposure of the silver halide at an instrinsically adsorbed wavelength such as 365 nm:
where S is the photographic sensitivity defined above and A is the fraction of light absorbed.10 Consequently,measurements of 4, require measurements of the light absorbed by the silver halide and by the dye in the photographic format used for the exposures. Unfortunately, several serious difficulties arose when trying to measure the light absorbed by these infrared dyes: (1) To minimize problems associated with dye desensitization of the overall silver halide response (see Discussion), the dyes were used at low surface coverage. This surface coverage, combined with the low silver halide laydown of the photographic format chosen, resulted in quite small expected values for the amount of light absorbed at the dye peak, in the range of a few tenths of a percent. (2) In contrast to measurements made on the liquid dispersions, the adsorbed infrared dyes were very easily photolyzed in the photographic coatings, causing distortions in the spectra as measurements were attempted. Because of these difficulties in measuring the light absorbed by the dyes, we did not attempt to make actual measurements of t$s for the dye series studied. Instead, for each dye, an infrared sensitivity ratio IRR was calculated according to eq 2a: Or, since photographic sensitivitesare more commonly expressed in logarithmic units, Comparison of eqs 1 and 2 shows that IRR will be closely proportional to t$8 if a series of dyes with similar absorption wavelengths and similar light absorptions are compared. The infrared dyes studied all had molar extinction coefficients near 20 X lo4 M-I cm-I (averagevalue = 21.5 X lo4 f 3.8 X lo4 M-1 cm-l) were all added to the silver chloride dispersion at the same concentration and were all expected to be well adsorbed to the silverchloride surface. Consequently,an assumption that changes in IRR would reflect changes in 9, seemed a reasonable starting point for our investigations. Note that, because the 365-nm light source and the spectrograph were not calibrated relative to each other, the absolute value of log IRR has no significance and was arbitrarily set equal to 2.6 log units for the most efficient dye measured (Dye 15). Electrochemistry. Electrochemical experiments were performed using a Princeton Applied Research Carp. (PAR) 173 potentiostat in conjunction with PAR Models 175 universal programmer, PAR 179 digital coulometer, and 124A lock-in amplifier. A Hewlett-Packard Model 239A low-distortion oscillator was used in ac measurements. Formal oxidation and reduction potentials for dyes were obtained via phase-selective second-harmonic ac voltammetry (quadrature component) at an applied frequency of 400 Hz (16-mV peak-to-peak signal) as described previously.6 Solutionsfor ac voltammetricexamination contained 0.1 M TBABF4 and ca. 5 X 10-4 M dye and were deaerated with argon prior to examination. The working electrode was a Pt disk (ca. 0.02 cm2) that was polished with 1-rmdiamond paste (Buhler Metadi), rinsed with water, and dried before each experiment. Current-voltage curves were recorded on a HewlettPackard Model 7045A X-Y recorder. All potentials were
Spectral Sensitization of Silver Halide measured vs the NaCl saturated calomel electrode at 22 OC and converted to the AgjAgCl reference by adding 40 mV. Transmission Spectra of Radicals and Aggregates. Solutions of dye radical dication were prepared by chemical or electrochemical one-electron oxidation. Chemical oxidations were performedby adding a slight excess of a ferricchloride/acetonitrile solution to a dilute (ca. 10-6 M) solution of dye in acetonitrile. Electrooxidized dye solutions were prepared by controlledpotential coulometry with a PAR 337A (three compartment) coulometric cell system equipped with a 67 cm2 area Pt gauze working electrode. (Electrolysis time for >98% oxidation is ca. 2 min.) In some experiments, the cell used was a tubular electrochemical flow cell of a design similar to that reported by Miner and Kissinger except that the working electrode was a compressedPt gauze(Fisher).ll In either experiment the working electrode was poised at a potential 200 mV beyond the reversible oxidation potential of the dye. The electrolyzed solutions were rapidly pumped through a 1- or 10-mm quartz spectral flow cell (Helma) for absorption measurement. Solutions containing a mixture of dye in the monomeric and aggregate form were prepared at a constant total dye concentration by careful dilution of methanol solutions of dye with deionized water. Transmission spectra were measured with a Varian 2400 UV-vis-IR spectrophotometeror with a Hewlett-Packard 8450A UV-vis diode array spectrophotometer. Diffuse Reflectance Spectroscopy. Stabilities of the dyes in the AgCl dispersion melts were assessed by diffuse reflectance spectrophotometry using a Varian 2400 spectrophotometer fitted with an integrating sphere. The dispersion had been pretreated as described above and had a composition of 8.0 wt % AgCl and 2.8 wt % gelatin. Dispersion pAg was 7.65, and pH was typically 4.8 but was adjusted to higher values for some experiments. Melt samples were prepared by adding 1 mL of dye stock solution (in methanol) to 4 mL of stirred AgCl dispersion. Equimolar dye stock solutions were used to achieve a concentration of 3.5 X 10-5 mol of dyejmol of AgC1. The dyed melt-concentrate was held for a minimum of 3 min at 40 OC to facilitate adsorption of the dye to the silver halide grain surface. The dispersion was subsequently diluted with 9.5 mL of 4.3% gelatin or, in some experiments, 0.1 M phosphate buffer at pH 5.5. Dye stability results were largely independent of the diluent used. Dilution with gelatin gave a final dispersion that contained 3.6 wt % of gelatin. Capped "Opticlear" vials (Kimble, 4 dram) served as convenient, disposable cells for incubation and for recording reflection spectra of turbid dispersionmelt samples. Dyed samples were incubated in the dark at 40 OC using a Lauda/Brinkman MS-20 constant-temperature bath. Spectra were recorded for each sample as a function of incubation time and were measured vs an electronically stored spectrum of an undyed dispersion. In general, there was no change in reflectancespectrum of dispersions after several successive spectral recordings, indicating that any photoexposure incurred during routine measurement contributed little to the observed spectral changes.
Results and Discussion
Infrared Sensitivity Measurements. We examined the dependence of infrared sensitivity on dye reduction potential using coatings of the 0.35-rm AgCl microcrystals containing a series of cationic tricarbocyanine dyes. The structures of these dyes, together with their electrochemical potentials, are listed in Table I. As discussed in the Experimental Section, the liquid microcrystallineAgCl dispersionwas pretreated with potassium bromide to enhance the adsorption of the dye and with a triazinylstilbenesulfonic acid supersensitizer to inhibit aggregation of the dye on the AgCl surface.12 Dyes were added to the pretreated dispersion at 3.0 X 10-5 mol/mol of AgCl, a level that corresponds to a surface coverage of ca. 10%. This low surface coverage of dye, together with the presence of the stilbene, ensured that the
The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 8271
3ia) m15
0 .-+
2-
E
'
2 .-w> .v)
10 11
.5 I12
.4
6
a3
.7
C
a
2 E c c
0,
0
Figure 2. Dependence of infrared sensitivity ratio IRR on redox potential for dyes adsorbed to 0.35-pm cubic AgCl microcrystals. (a) Plot of log IRRvsdye reduction potential, E d . (b) Plot of log IRRvsthediffcrcnce (Eox- E d ) in dye redox potentials.
sensitizing dye was adsorbed primarily in the monomeric state. Logarithmicvalues of the infrared sensitivity ratio IRR measured for this dye series are also contained in Table I and are plotted vs Erd in Figure 2a. As discussed in the Experimental Section, the values for IRR should be proportional to the relative quantum efficiency for spectral sensitization, and consequently are expected to depend on Erd, since this quantity is correlated with the position of the electron-injecting levels of the dyes' excited states. Considering that the electron-injecting levels of the excited infrared dyes are estimated to fall below the conduction band edge, and because the Erd values encompass a relatively narrow range of potentials, a near linear relationship is predicted between log IRR and Erd.5aContrary to this expectation, the results on AgCl indicate that no clear correlation exists between infrared photographic sensitivity and dye reduction potential. Instead, photographic performance depends on both E d and Eox. As shown in Figure 2b, an approximately linear relationship exists between the log of the infrared sensitivity ratio and the difference (Eox- Erd) in the redox potentials of a dye. Since, as seen in Figure 1, the spectral absorption energy hu of a dye is directly related to the quantity ( E , - Erd),minimum infrared sensitivity is associated with sensitizations intended for the longer infrared wavelengths.l 3 This dependenceof photographicperformanceon E,, is thought to be the result of the correlation of this measurement with a dye's propensity toward oxidation in the liquid silver halide dispersion. Depending on the type and extent of substitution, tricarbocyanine dyes have oxidation potentials in the range 0.30.5 V vs Ag/AgCl. Dyes with oxidation potentials lower than 0.5 V are thermodynamically unstable with respect to silver ion and are, furthermore, prone to aerial oxidation. As described by eqs 3-5, chemical reactivity of these dyes in silver halide dispersions may lead to destruction of dye, which will obviously reduce infrared ~ensitivity.'~In addition, both the oxidized dye radicals and subsequent reaction byproducts may serve as traps and/or recombination centers for electrons produced by light exposure. This loss of electrons causes a suppression of overall photographic sensitivity known as desensitization. Further, to the extent that these traps interact selectively with electrons generated from the excited infrared dye, the infrared sensitivity
Lenhard et al.
8272 The Journal of Physical Chemistry, Vol. 97, No. 31, 1993
TABLE I: Electrochemical and Photographic Data for Infrared Sensitizing Dyes* no.
EOW V
Eredv V
nm
1
0.322
-0.83d
860
2
0.382
-0.768
850
1.97
3
0.315
-0.793
885
1.25
0.340
-0.796
865
.49
0.374
-0.765
860
.77
0.407
-0.761
835
1.66
7
0.32
-0.863
840
1.13
8
0.327
-0.873
850
1.88
9
0.37d
-0.85d
850
.76
10
0.37d
-0.803
850
.96
0.38d
-0.797
850
1.95
dye
structure
I
Et
4
U
Et
5
6 I
Et
Et
IRR
I
Et
Et
log c
U
I
Et
Et
SMe
ms. 11 I
Et
I
Et
The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 0273
Spectral Sensitization of Silver Halide
TABLE I: (Continued) dye no.
structure
Et
0.400
-0.760
860
1.68
0.422
-0.793
820
2.14
0.450
-0.740
820
2.06
0.46d
-0.840
760
2.60
0.46e
-0.862
780
2.33
0.W
-0.860
740
2.24
Et
13 Et
Et
14 I
Et
Et
15 Et
Et
16 Et
Et
17
Potentials are reported vs the Ag/AgCl referenceelectrode at 25 OC. Wavelength of maximum spectral sensitivity, determined at 10-nm intervals. Infrared sensitivity ratio, see text. 10.01 V. f0.03 V.
will be suppressed more severely than the intrinsic sensitivity, resulting in a decrease in the relative quantum efficiency of the infrared sensitization. Ag'
- + + -
+ dye'
0, + 2dye'
2H'
dyeg2'
Ago dyeo2'
2dye"'
desensitizer
+ H20,
\-0.15
N (3)
(4) (5)
If the reactivity of the infrared dyes in the silver halide dispersions were a simple function of oxidation potential only, a very close correlation of the infrared sensitivity ratio with (Eox - E d ) might be expected. The fact that some scatter remains in Figure 2b suggests that the relationship of dye reactivity with oxidation potential is more complex. To investigate this relationship in moredetailand to assess its impact on the photographic performance of infrared dyes, we have examined the oxidation chemistry of these dyes, first in solution and then in AgC1 liquid dispersions. Oxidationof Tricarbocyaninesin Solution. The one-electron oxidation of a cationic tricarbocyanine dye initially yields the corresponding radical dication. The electron density distribution in the singly occupied molecular orbital (SOMO)of the radical was calculated using MOPAC as described previously15 and is schematicallydepicted in Figure 3. The odd-electron density at a given atom is proportional to the squareof the orbital coefficient
0.23
0
0.11
u -0.36
0.49
N
w -0.49
0.36
-0.23
Figure 3. Diagram for electron density distribution in SOMO of a thiatricarbocyanine radical dication.
and is represented in Figure 3 by the size of the circle. For a thiatricarbocyanine radical dication, the electron density in the SOMO is seen to be symmetricallydistributed and found to reside primarily on the even-alternantcarbon atoms of the polymethine chain (C8, C10, C12,and C14)and toa lesser extent on the nitrogen atoms. Results from cyclic voltammetry experiments indicate that tricarbocyanineradical dication stability is markedly dependent on the type and extent of substitution in the polymethine chain. Radical dicationsderived from dyes that contain alkyl substituents at the even-numbered carbons of the methine chain were found to exhibit a modest degree of stability, presumably because these substituentsstabilizethe radical and/or sterically inhibit radicalradical coupling. Chemical structures of various dyes for which solutions of radical dication could be prepared in methanol or acetonitrile by electrochemical one-electron oxidation at a Pt anode or by chemical oxidation using FeCl3 are shown in Table 11.
The thiatricarbocyanine dye 18, which contains hydrocarbon bridges at the 8, 10, 12, and 14 positions of the methine chain,
Lenhard et al.
8214 The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 TABLE II: Solution Spectral Data for Tricarbocyanine Dyes and Radical DicationS.
radical dication
dye
Emax,
3LmaxV
nm
x 10"
nm
Emax. x 10"
804
27.7
568
13C
2
785
20.0
554
13.2
3
824
19.3
600
-C
809
20.7
54 1
9.5
708
26.8
528
10.7
762
29.6
604
13.3
stnlcauleb
dye
3 L m a x l
no.
1 Et
Et
18
U V
19
20
Q q p R
-
R
a Radical dications were generated by chemical oxidation with FeCI3 or coulometry in CH&N/O.l M TBABFd. b R is sulfoethyl. C Radical dication instability prevents accurate assessment of extinction coefficient.
exhibits reversible cyclic voltammetry and forms the most stable radical dication. Results from spectroelectrochemical experiments are also consistent with formation of a persistent radical. Figure 4a shows, for example, absorption spectra of a 17 pM solution of dye 18 obtained before and after exhaustive oneelectron oxidation at 0.65 V vs Ag/AgCl. The radical dication ofdye 18exhibitsanabmrptionmaximumat541 nminacetonitrile and a molar extinction coefficient emu of 9.5 X 104 M-1 cm-l. Immediate one-electron reduction of the solution of Figure 4a, at 0.2 V,regenerates ca. 95% of the parent dye 18 (A= 809 nm, e= 20.7 X lo4 M-1 cm-1). The shape of the principal absorption band for the radical closely resembles that for the parent dye, except for a small (ca. 170 cm-1) shift in the position of the vibronic transition that appears as a shoulder on the shortwavelength side of the dyes's 0 0 transition. The similarity in spectral band shapes supports the conclusion that there are no gross differences in the overall structure (bond lengths, molecular conformation, etc.) between dye and radical. The stability of the radical species derived from the remaining dyes of Table I1 was found to depend on molecular structure and concentration of radical. The radical dications are susceptible to radical-radical dimerization at the unsubstituted, evennumbered methine carbon atoms as depicted in Scheme I.16Under
-
cyclic voltammetry conditions, where the concentration of dye is ca. 5 X 1W M, the radical dications have a half-life of about 1 s. More stable solutions of radical dication could, however, be prepared at extreme dilution (1V M)via chemical oxidation using ferric chloride. Spectral absorption data are listed in Table 11. The radical dications of these dyes exhibit absorption maxima in the range 528-604 nm and have molar extinction coefficients that are slightly less than one-half that measured for the parent dye, UV-vis spectra for the radical dications of dyes 1, 2, 19, and 20 are very similar to that shown for dye 18 (Figure 4a). Dyes containing the 4,S-benzo (dye 3) or the 5,6-dithiomethyl (dye 5 ) substituent, on the other hand, exhibit a much broader radical dication absorption feature as shown in Figure 4b. All radical dicationscould be convertedback to dye by electrochemical one-electron reduction or by chemical reduction with aqueous ascorbic acid. In contrast to the dyes of Table 11, tricarbocyanine dyes that contain no alkyl substituents on the even-methinecarbon atoms, e.g., dyes 7-17 of Table I, exhibit completely irreversible, oneelectron cyclic voltammograms in acetonitrile. The electrochemical data are consistent with a rapid and irreversible dimerization of the corresponding radical dications.17 The
Spectral Sensitization of Silver Halide
The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 8275
C
S
I
500
600
700
800
900
1000
Wavelength, nm Figure 5. Reflection spectra recorded for an AgCl dispersion melt containing dye 18 as a function of time at 40 OC: (-) 0 h, (- - -) 1 h, (---) 5 h.
Wavelength, nm Figure 4. Absorption spectra for tricarbocyanine dyes recorded before (- -) and after (-) oxidation. Spectrum of oxidation product after treatment with base (-). Absorbance scale is arbitrary; see text for
-
extinction values.
SCHEME I Dye
Radical Dication
I
kd
Dimer
majority of tricarbocyanine radicals with this pattern of chain substitution exhibit lifetimes under 50 ms in CH&N/O.l M TBABF, with structurally dependent rates of decomposition. Absorption spectra recorded during the chemical (FeC13) oxidation of a 10-6 M solution of dye 13 (Amx = 764 nm, ,t = 18.1 X 104 M-1 cm-I), which is bridged across the 9, 11 carbons of the heptamethine chain, also indicate the radical dication (Amx = 540 nm) to be transient. The oxidation product of dye 13, which can be isolated as a solid by counterion-exchange precipitation, exhibits an absorption maximum at 39 1 nm in acetonitrile
(Figure 4c) and may be assigned to the dimer form of dye 13 (t, = 2.9 X 104 M-1 cm-l). Similarly, chemical oxidation of dye 21, a multiply bridged analog of dye 13 with a structure given in later discussion (Table IV), gives a transient radical (A, = 542 nm) and a stable dimer product (A, = 410 nm). This pattern of reactivity observed for tricarbocyanine radical dications is completely analogous to that reported by Parton and Lenhard for dicarbocyanine dyes.l* Depending on reaction conditions, dimeric products formed during the one-electron oxidation of tricarbocyanine dyes are susceptible to further reaction. For example, treatment of the UV-absorbing dimer of dye 13 with triethylamine gives a species whose spectral properties (Figure 4c) are similar to those of the parent dye. This behavior is consistent with deprotonation of the methine carbons of the dimer at the sites of radical coupling to yield the corresponding bis dye (A, = 764 nm). Oxidative Decomposition of Dyes in Silver Halide Dispersion Melts. The reactivity of various infrared sensitizingdyes in liquid silver halide dispersions was measured by diffuse reflectance spectrophotometry using the dispersion preparation containing 0.35-pm AgCl microcrystals. Experimental data were obtained as plots of reflection absorbance (-log R ) vs wavelength. For quantitative analysis of dye spectra in these turbid dispersion systems the Kubelka-Munk function (eq 6) was utili~ed:I~-~l
where R, is the reflectivity of an infinitely thick sample, S is the scattering coefficient for the silver halide, and C and t are the molar concentration and molar absorptivity coefficients of the dye, respectively. Experimental -log R values were used to calculate R , and F(R,) ratios according to eq 6 so that changes in reflectivity could be proportionally related to changes in the surface concentration of dye. A calibration plot of F(R,) vs dye concentrationdemonstrated this linear proportionality and verified that conditions of infinite reflectivity were adequately satisfied in these experiments. Figure 5 shows reflectance spectra periodically recorded for the AgCl dispersion (pH 6), which was incubated at 40 OC and contained the tricarbocyanine dye 18. The initial spectrum shows two strong bands at 860 and 590 nm and a relatively weak band at 460 nm. The spectral band at 860 nm is due to the monomericallyadsorbed dye and is bathochromically shifted ( 5 1 nm) and broadened relative to the spectrum of the dye in solution. This bathochromic shift is related to the refractive index of the silver halide and also depends on the spectral transition energy of the dye.20 Most infrared sensitizers exhibit a 50-60-nm shift
8276 The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 on adsorption. Having a very low oxidation potential, Eox= 0.1 8 V, dye 18 was especially reactive in the melt environment, and the rapid loss of the dye absorption band at 860 nm was accompanied by increases in absorption at 590 nm. The latter spectral band occurs 49 nm bathochromic of the absorption maximum of the dye 18 radical dication in solution. Spectral changes similar to that displayed in Figure 5 could also beobtained by deliberatechemical oxidation of the AgCl dispersioncontaining dye 18. Treating the dispersion sample with FeCl3 resulted in bleaching of the dye and generation of a spectral band at 590 nm. This correspondencein the spectral and reactivity features between solution and silver halide systems permits assignment of the absorption band at 590 nm to the adsorbed radical dication of dye 18. As the reflectance spectra of Figure 5 indicate, the oxidative decomposition of dye 18 to yield radical dication is more than 90% complete after 5 h. A kinetic analysis of dye 18 oxidation was obtained by converting the -log reflectance values of Figure 5 to surfacedyeconcentrationsusing the Kubelka-Munk equation. As illustrated in Figure 6, a plot of In [dye] vs time is linear, indicating that the oxidation reaction proceeds in a manner that is first-order with respect to the concentration of dye. Although the data of Figure 5 clearly demonstrate the formation of radical dication species,conclusionsregarding the participation of radical-radical coupling in the overall mechanism of dye decomposition in the dispersion environment are less obvious. Dimer products for tricarbocyaninedyes can be expected to absorb in the 390-450-nm rangedepending on thespecific dimer structure and strength of adsorption to AgCl. Due to spectral absorption and light scattering by the dispersion at wavelengths less than 450 nm, formationof tricarbocyaninedimer productscannot easily be verified by reflectance spectrophotometric methods. A comparison of the dye structures related to the data of Figure 2 indicates, however, that neither the lack of correlation in Figure 2a nor the scatter in the data of Figure 2b can be attributed simply to differencesin dye radical coupling chemistry. As noted below, dye structural features that affect the rate of dye oxidation are more important than the radical coupling chemistry in determining the observed infrared photographic sensitivity. To identify the important dispersion-related variables that influence spectral sensitizing dye stability, reflectance spectra analogous to those of Figure 5 were qualitatively compared as a function of dispersion composition. Higher dispersion pH, lower temperature, and the presence of added bromide ion decreased sensitizing dye reactivity. Among these factors, pH was the dominant variable controlling dye stability. Increases in dye oxidation rate were associated with lowering the dispersion pH prior to dye addition. For example, reflectance data for dye 3 in an AgCl dispersion that was adjusted to pH 7.5 indicate that ca. 40% of thedye had undergone oxidation within 20 min, whereas nearly 55% of the dye was oxidized during this same time period in a companion dispersion that was adjusted to pH 4.5. This sensitivity to pH is most likely related to the hydrogen ion dependence of the oxygen electrode reaction and suggests that dye decomposition in a dispersion melt is largely driven by the aerial oxidation reaction represented by eq 4.22 Dye Structure-Reactivity Relationships. The relative melt stabilities of approximately 20 infrared sensitizerswere measured by reflectancespectroscopyin an effort to examine the relationship betweendyeE,anddyestability. Attempts toobtainquantitative data for the rate of sensitizing dye oxidation in the dispersion melt using the kinetic analysis as shown in Figure 6 were largely unsuccessful. Complications related to the appearance of broad and overlapping spectral bands, together with the presence of the various aggregate bands observed for many dyes, prevented the determination of the necessary dye surface concentrations. The formation of aggregated dye in the solution (aqueous) phase of the dispersion was predominant during the initial stages of the
Lenhard et al.
Time, h
Figure 6. Kinetic plot of In [dye]vs time for the oxidative decomposition of a dye 18 in an AgCl dispersion melt at 40 OC.
Wavelength, nm F i p e 7. Absorption spectra for tricarbocyanine dyes in methanol/ water mixtures. Methanol content is 20% -), 40% (- - -), 60% (.-), and 100% (-); [dye] = 4 rM. (-a
dye adsorption process and, furthermore, played an integral role in the dye oxidation mechanism. Compared to cyanine dyes of shorter methine chain length, tricarbocyanines are very insoluble in water and are known to rapidly form hypsochromically shifted aggregates (H-aggregates) when introduced to an aqueous solution.23 An increase in the concentration of dye or an increase in the water/solvent ratio of the medium causes a shift in the aggregate equilibrium away from the monomer and toward thedimer, trimer, and higher H-aggregate forms. Figure 7 shows the progression in the absorption maximum of the aggregate, from 750 to ca. 600 nm, that results from a gradual increase in the water content of solutions of dyes 2 and 3. For each dye there exists a characteristic water/methanol ratio wherein one-half of the dye is in an aggregated state. When obtained at equivalent dye concentrations, these half-aggregation solvent ratios can be used as a measure of the relative aggregation propensities for various dyes. The less water that is required to achieve this specified degree of aggregation, the higher the tendency for the particular dye to aggregate. Table I11 lists spectral data and half-aggregation
The Journal of Physical Chemistry, Vo1. 97, No. 31, 1993 8217
Spectral Sensitization of Silver Halide TABLE IIk Solution Physical Data for Tricarbocyanine Dyes I_\
I_\
0.2 0.1 L a b
L x a
Y
dye no. 2 6
X R MeOH H-agg H Me Me 784 634 H 782 Ph Et 658 22 5,6-Sme Me Et 824 5 5,6-Sme Ph Et 816 640 3 4,S-benzo Me Et 824 644 4 4,5-benzo Ph Et 817 684 a Monomer absorption maximuminmethanol. Absorption maximum of limiting H-aggregate. e Percentagewater in MeOH/H2O mixture that yields a solution that is half-aggregated for a 4 pM solution of dye. r
8c
s0
0.2
Q,
;5
uQ,
0.1
-8
I
0.2 0.1
500
600
700
800
900
Wavelength, nm
E
Figure 9. Reflectance spectra for AgCl dispersions at pH 4.8 recorded after melt-hold times of 0 h (-), 0.5 h (- - -), and 2.0 h
-0 0.2-
(-e.).
I
0.1
I
500
I
!
600
700
,
,
800
I
900 1000
Wavelength, nm Figure 8. Reflectance spectra for an AgCl dispersion melt recorded at various times after addition of dye. (a) dye 2 at (-) 1 min and (- -) 3 min, pH 4.8; (b) dye 3 at (-) 1 min and (- -) 5 min, pH 7.5. Each
.
sample melt was diluted with phosphate buffer before measurement. solvent ratios for a series of related dyes. A comparison of these data reveals that tricarbocyanines containing the 4,4’,5,5’-benzo or the 5,5’,6,6’-thiomethyl substituents are more prone to H-aggregation than similar dyes that do not contain these functional groups. These aggregation phenomena influence the dynamics of dye adsorption to AgCI, as illustrated by the data of Figure 8. Reflectance spectra are shown for dyes 2 and 3 wherein the time allowed for dye adsorption, that is, the period for which the dyed dispersion was held in the concentrated form, was varied from 1 to 5 min. The data obtained for dye 2 at pH 4.8 show that, in the earliest stages of adsorption, distinct spectral bands are observed for the adsorbed dyemonomer (845 nm) adsorbed radical dication (625 nm). The spectral bands for the solution monomer and aggregate are convoluted with the bands for the adsorbed forms of the dye and, as such, appear as a broad, featureless spectral band that ranges from about 680 to 780 nm. Continued adsorption results in a decrease in spectral absorption in the 6 8 b 780-nm region and a concomitant growth of the bands associated with the adsorbed monomer and radical dication. After 3 min, the spectral band for the adsorbed monomer reaches a maximum and the adsorption process is essentially complete. In comparison, Figure 8b shows that the solution H-aggregate is much more pronounced for 4,4’,5,5‘-benzo-~ubstituteddye 3 (aggregate X,
= 640 nm) and the initial amount of adsorbed monomer is relatively After 5 min, the amount of adsorbed monomer is significantly increased at the expense of the solution aggregate. The data of Figure 8 and other results indicate that, for most cationic tricarbocyanines, a maximum of adsorbed monomeric dye is obtained during a 6-min melt hold at 40 OC. Although quantitative rate data could not be readily extracted from reflectance spectra,valuable structure-reactivity information for dyes was obtained from a qualitativeanalysisof spectral results. Figure 9 shows, for example, that significant stability differences exist between a family of similarly structured dyes that vary in oxidation potential and aggregation propensity. In each of the spectra (a-c), reflection-absorption bands that correspond to the adsorbed dye monomer, aggregated dye, and radical dication have been indicated by M, A, and R, respectively. Data from Tables I1 and I11 were used in the identification of aggregate and radical species. In spectrum c, the aggregate band is presumed to be buried under the spectral band of the radical dication. As given by the relative magnitude of the spectral bands for dye and radical, the4,4‘,5,5’-benzo-substituteddye 3, which has thelowest oxidation potential of this group of dyes (Em= 0.3 15 V), is much less stable than the 5,5’,6,6’-thiomethyl-substituteddye 22 or the unsubstituted parent dye 2. Interestingly, the thiomethylsubstituted dye 22 (Eox= 0.35 V) appears slightly more stable than the unsubstituted dye 2 (Eox= 0.382 V). Solution studies indicate that the 4,4’,5,5’-benzo-substituteddye and the 5,5‘,6,6’thiomethyl-substituted dye possess higher aggregation tendencies in solution as compared to the less substituted analog dye 2. Furthermore, the Ag+ affinity of the thiomethyl group provides for enhanced adsorptivity to silver halide for dyes bearing this functional group.25 Thus, dye 22 would be expected to be more tightly adsorbed to the AgCl crystal surface than would dyes 2 and 3. Collectively,thedataof Figure9 illustratethatdyestability is a complex function not only of the dyes’ oxidation potential and aggregation propensity but also of the adsorptivity to silver halide. Table IV lists reflectance spectral data obtained for other variously structured tricarbocyanine dyes adsorbed to the AgCl
8278 The Journal of Physical Chemistry, Vol. 97, No. 31, 1993
Lenhard et al.
TABLE IV: Electrochemical and Reflectance Spectral Data for Tricarbocyanine Dyes*
0.42
790
600
stable
0.422
810
600
stable
0.374
860
0.450
810
590
less stable
6
0.407
825
610
less stable
2
0.382
845
625
less stable
23
0.43
720
565
less stable
18
0.183
860
590
unstable
3
0.315
875
670
unstable
7
0.37
860
670
unstable
Et
Et
13 I
5 Et
U
Et
14 Et
kt
stable
Et
I
Et
Spectral Sensitization of Silver Halide
The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 8279
TABLE IW (Continued) dye
structure
no.
1 I
Et
U
0.39
835
0.322
860
650
unstable
unstable
I
Et
Redox potential was measured in CH,CN/O.l M TBABF,. Relative stability was determined on an AgCl dispersion at pH 4.8 for dispersion concentrates that were diluted with buffer solution. bye and And are the wavelengths of maximum reflection-absorption of the dye and radical dication, respectively.
microcrystals at pH 4Xz6 Each dye is categorized on a relative basis as being stable, less stable, and unstable, as gauged by similarities in dispersion spectra with those of Figures 5 and 9. These data lead to the following general conclusions: (i) Other factors being constant, the more reactive dyes are those with the lowest electrochemical oxidation potential. Examples of this trend are dye 18 compared to dye 6 and dye 1 compared to dye 2. (ii) Dyes that exhibit thegreater tendency toformH-aggregates in solution are more reactive than those that do not readily aggregate. In particular, dyes containing a 4,4',5,5'-dibenzo or 6,6',7,7'-dibenzo group fit into this category. (iii) For dyes of similar redox potential, those dyes that contain functional groups (e.g., thiomethyl) that facilitate adsorption to silver halide are more stable than analogous dyes in which the group is absent. Dye 5 compared to dye 6 is an example of this trend. (iv) Methine chain substituents, which determine the coupling chemistry of dye radical dications, have no apparent influence on the observed dye stability beyond that attributable to a change in redox potential. Given these findings regarding dye structure and propensity for oxidative decay, it is informative to reexamine the relation of IRR vs (Eox- Erd)as plotted in Figure 2b. As noted earlier, the deviation of the individual data points from the solid line is thought to be due to the complexity of the Eox-oxidation rate relationship for the various dyes. Interestingly, the dyes that are regarded as stable or relatively less stable tend to cluster above the solid line of Figure 2b, whereas the methoxy-containing dyes and unstable, dibenzo-substituted dyes tend to fall below the line. Dye 17, which is the most deviant from the IRR -(Eox - E d ) relation, represents the only dye that contains thequinoline nucleus and suggests that dyes of this class are particularly susceptible to oxidation. Mechanistic Aspects. Guided by the above generalizations, we propose Scheme I1 to describe the processes associated with the addition of a methanolic solution of an infrared spectral sensitizing dye to an aqueous silver halide dispersion melt. According to this scheme, the initial state of the sensitizing dye is that of a dissolved, monomeric species. Upon addition to the aqueous dispersion melt, the dye monomer is susceptible to aggregation to yield unadsorbed dimer-aggregate and H-aggregate complexes. For simplicity, these equilibria are considered as one process and are described by Kawee. The aggregation reactions compete with the silver halide adsorption process for locally available dye monomer. Since the silver halide microcrystal surface is pretreated with TSSA, the dye is essentially
SCHEME I1 DYE-MONOMER (methanol)
1
AgX: TSSA
DYE-MONOMER (adsobed)
K dye ads
Kaggreg
DYE-MONOMER (solution)
'1
DYE-AGGREGATE (solution)
I
RADICALMONOMER (adsorbed)
& rad ads
kox'
RADICAL-MONOMER (solution)
adsorbed in the monomeric state. The factor KabdYcrepresents the dye adsorption equilibrium constant. Sensitizing dye molecules in each of the defined physical states, Le., solution monomer, solution aggregate, and adsorbed monomer, can undergo one-electron oxidation to yield the corresponding dye radical ion. The individual rates of oxidation are given by koxl,kox*,and ko2. It is assumed in Scheme I1 that, during the oxidation of a dye-aggregate species in solution, the aggregate undergoes a rapid dissociation to give radical ion monomers. Dissociation of the oxidized solution-aggregate is expected for Coulombic reasons and is supported by the fact that experimental efforts to deliberately prepare or measure radical-aggregates have been unsuccessful. Finally, radical ions that are formed via processes in the dissolved state may adsorb to the silver halide microcrystal according to the equilibrium designated Kabrad. In the ideal AgX dispersion system, the adsorption of infrared spectral sensitizing dyes to the surfaces of microcrystals would proceed without the complication of aggregation and oxidation of the dye molecules in the solution phase. The results presented here suggest that the rate of oxidation of the adsorbed dyemonomer species is lower than that for the solution-monomer and solution-aggregate forms, i.e., koxl > kox3and kOx2> koxS. Accordingly, factors that promote adsorption of the dye to the silver halide surface, such as the presence of added bromide ion in the dispersion and substitution of dye with thiomethyl groups, have a stabilizing influence on the dye. Conclusion
Infrared spectral sensitizingdyes from the tricarbocyanine class are prone to oxidative decomposition in liquid silver halide microcrystalline dispersions to give the corresponding monooxidized radical dication forms of the dyes. Although both silver ion and molecular oxygen may serve as oxidizing agents in the dispersion environment, the observed decrease in dye oxidation
8280 The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 rate with higher pH suggests that the predominant mechanism of dye decay involves aerial oxidation. Other factors that reduce the extent of dye oxidation include a lower propensity for dye aggregation and increased adsorption to the silver halide surface. Because oxidation in dispersion melts destroys the infrared chromophore and creates additional desensitizing species, the oxidation potential of the dye is as important as its reduction potential in determining spectral sensitizing ability of infrared dyes.
Acknowledgment. The authors thank J. Krieger for technical assistance in obtaining the photographic data and A. Cameron for computer calculations. References and Notes (1) West, W.; Gilman, P. B. In The Theory of the Photographic Process; James, T. H., Ed.; Macmillan: New York, 1977; pp 251-290. (2) (a) Tani, T.; Kikuchi, S.Phoiogr. Sci. Eng. 1967,l I , 129. (b) Tani, T.;Honda,K.;Kikuchi,S.J. Elecirochem.Soc.Jpn. 1969,37,17. (c) Sturmer, D. M.; Gaugh, W. S.Phoiogr. Sci. Eng. 1973, 17, 146. (3) Because of differences in higher order terms in the energies of the parent dye and of the radical resulting fromelectron-trapping, the exact energy of the lowest vacant level is not identical for the two species. However, this distinction is not important for the correlations discussed in the text. (4) Muenter, A. A.; Gilman, P. B.; Lenhard, J. R.; Penner, T. L. Mechanisitc Consequences of Anomalously Efficient Spectral Sensitizaiion by DesensitizingDyes; The International East-West S y m p i u m on the Factors Influencing Photographic Efficiency, 1984; Paper C-21. ( 5 ) (a) Tani, T. J . Phys. Chem. 1990,94, 1298. (b) Tani, T. J. Imag. Sci. 1990, 34, 143. (6) Lenhard. J. J . Imae. Sci. 1986. 30. 27. (7j Berry, C. R. Photogr. Sci.-Eng. 197$, 19, 93. (8) (a) Berry, C. R.; Skillman, D. C. Photogr. Sci. Eng. 1962, 6, 159. (b) Harbison, J. M.; Spencer, H. E. In The Theory of ihe Photographic Process; James, T H., Ed.; Macmillan: New York. 1977: DD 152-156. (9) James, T. H. In The Theory of the Photographic P/iess; James, T. H., Ed.; Macmillan: New York, 1977; pp 396-397. (IO) Spence, J.; Carroll, B. H. J. Phys. Colloid Chem. 1948, 52, 1090.
Lenhard et al. (1 1) Miner, D. J.; Kissinger, P. T. Eiochem. Pharmacol. 1979,28,3285. association (12) TSSA complex readily withadsorbs the dyetoon silver the Agx halidesurface, and is thought to form an
(13) For tricarbocyanine dyes, this relationship has the form [Em- E d ] = (1240V~nm)/~-0.4V,whereA,istheabsorptionmaximuminmethano1 solution. (14) In assuming the infrared sensitivity ratio IRR to be proportional to the relative quantum efficiency #, it was assumed that the light absorption by all the dyes was approximately equal. As dye is destroyed by oxidation, this assumption loses validity. (15) Fullyoptimized (RHF)structur*l werecomputedwithMOPACusing the MNDO/pm3 parameter set. Unpaired electron densities in the highest occupied molecular orbitals of the radical dications were calculated using the INDO/rohf approximation. Calculation details are further described in: Lenhard, J. R.;Cameron, A. D. J. Phys. Chem. 1993, 97, 4916. (16) This scheme outlines a general oxidation route for this class of dyes and shows only the 8,IW isomer. The particular isomer or isomer distribution obtained for a given dye will depend on its chain substitution pattern. (17) Electroreduction of some tricarbocyanine dimers at negative applied potentials (5-0.3 V) results in partial regeneration of dye. (18) Parton, R. L.; Lenhard, J. R. J. Org. Chem. 1990,55,49. (19) Wendlandt; W. W.; Hecht, G. G. Reflectance Spectroscopy; Interscience: New York, 1966; pp 46-90. (20) Herz, A.; Danner, R.; Janusonis, G. Adsorption from Aqueous Solution. Advances in Chemistry Series No. 79; American Chemical Society: Washington, DC, 1968; p 173. (21) Gade, R.; Kaden, U.; Fassler, D. J. Chem. Soc., Faraday Trans. 1987, 83, 2201. (22) When compared at similar concentrations and temperatures, tricarbocyanine dyes were, in general, much more reactive in the AgX dispersion than in methanol or methanol (80%)/water (20%) solutions. This increase indye reactivityis probablydue tosimplepHdifferencesbetween thedispenion and solvent systems. (23) Herz, A. H. Adv. Colloid Interface Sci. 1977,8, 237. (24) The degree of dye 3 oxidation was intentionally minimized in the experiment of Figure 8b by the use of a higher dispersion pH (7.5). (25) Dyes containing specific Ag+ binding sites are known to exhibit enhanced adsorptivity to silver halide; see ref 23, pp 286 and references cited therein. (26) The small differences in the values for maximum sensitivity, Table I, and peakabsorption wavelength,Table IV,are the result of the low resolution of the photographic measurement and a 5-nmoffset to long wavelength in the infrared calibration of the photographic spectrograph.
