A new mechanism for spectral sensitization of silver halide is described, which can potentially double the sensitivity of photographic emulsions. The photooxidized sensitizing dye is trapped using an organic donor molecule, which fragments to form a cation and a reducing radical, which injects an electron into the conduction band of the silver halide. In this way, two conduction-band electrons can be produced for each absorbed photon

Farid, S.

Godleski, Stephen A.

Gould, I.

Lenhard, J. R.

Muenter, A. A.

English

The College at Brockport, State University of New York: Digital Commons @Brockport

The College at Brockport: State University of New York
Digital Commons @Brockport
Chemistry Faculty Publications Chemistry and Biochemistry
2001
New Approach to Silver Halide Photography Using
Radical Cation Chemistry
I. Gould
Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ
J. R. Lenhard
Research Laboratories, Eastman Kodak Company, Rochester, NY
A. A. Muenter
Research Laboratories, Eastman Kodak Company, Rochester, NY
Stephen A. Godleski
The College at Brockport, sgodlesk@brockport.edu
S. Farid
Research Laboratories, Eastman Kodak Company, Rochester, NY
Follow this and additional works at: https://digitalcommons.brockport.edu/chm_facpub
Part of the Chemistry Commons
Citation/Publisher Attribution:
Gould, I.; Lenhard, J.R.; Muenter, A.A.; Godleski, S.A.; Farid, S. New Approach to Silver Halide Photography Using Radical Cation
Chemistry. Pure Appl. Chem. 2001, 73, 455-458. Available on publisher's site at: http://iupac.org/publications/pac/
This Article is brought to you for free and open access by the Chemistry and Biochemistry at Digital Commons @Brockport. It has been accepted for
inclusion in Chemistry Faculty Publications by an authorized administrator of Digital Commons @Brockport. For more information, please contact
kmyers@brockport.edu.
Repository Citation
Gould, I.; Lenhard, J. R.; Muenter, A. A.; Godleski, Stephen A.; and Farid, S., "New Approach to Silver Halide Photography Using
Radical Cation Chemistry" (2001). Chemistry Faculty Publications. 5.
https://digitalcommons.brockport.edu/chm_facpub/5
Pure Appl. Chem., Vol. 73, No. 3, pp. 455–458, 2001.
© 2001 IUPAC
455
New approach to silver halide photography
using radical cation chemistry*
I. R. Gould1,†, J. R. Lenhard2, A. A. Muenter2, S. A. Godleski2, 
and S. Farid2
1Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ
85287-1604, USA; 2Research Laboratories, Eastman Kodak Company, Rochester,
NY 14650-2109, USA
Abstract: A new mechanism for spectral sensitization of silver halide is described, which can
potentially double the sensitivity of photographic emulsions. The photooxidized sensitizing
dye is trapped using an organic donor molecule, which fragments to form a cation and a
reducing radical, which injects an electron into the conduction band of the silver halide. In
this way, two conduction-band electrons can be produced for each absorbed photon.
INTRODUCTION
The primary step in photography involves excitation of a sensitizing dye that is adsorbed to the surface
of the silver halide crystal [1]. The excited dye injects an electron into the conduction band of the sil-
ver halide, which is trapped in the form of a metallic silver atom [1]. A single silver atom is unstable
with respect to return electron transfer to the oxidized dye. With more photon absorption events, how-
ever, multiple electrons can be injected, and a cluster of silver atoms can form. A cluster as small as
four silver atoms is stable and can function as a latent image [2]. In the development process, the latent
image catalyzes a series of redox reactions resulting in reduction of the entire silver halide crystal and
formation of millions of image dye molecules. It is this unique amplification process that distinguishes
photography from other forms of imaging.
Despite the enormous inherent amplification, there remains a need for improvement in the sensi-
tivity of silver halide materials. For example, the number of photons that are required to form the latent
image is typically ca. 15–20, which means that most of the absorbed photons are wasted [3]. More effi-
cient latent image formation would result in an overall increase in sensitivity without sacrificing grain
size or other properties of the photographic emulsion. The standard approach to increasing photoeffi-
ciency of silver halide emulsions involves the addition of dopants to the silver halide crystal which trap
the injected electrons, and thus minimize return electron transfer [4]. Much less attention has been paid
to trapping the oxidized sensitizing dye to achieve the same effect. Here we describe a novel mecha-
nism for increasing the photoefficiency of latent image formation based on trapping of the oxidized dye,
two-electron sensitization, which has the potential for more than doubling the photosensitivity of silver
halide emulsions.
BASIC CONCEPT
In conventional photography, the oxidized sensitizing dye (Sens•+) can undergo return electron transfer
in competition with latent image formation. The return electron transfer process wastes the photon ener-
*Lecture presented at the XVIIIth IUPAC Symposium on Photochemistry, Dresden, Germany, 22–27 July 2000. Other presenta-
tions are published in this issue, pp. 395–548.
†Corresponding author
gy and reduces the overall photographic sensitivity. In contrast, the two-electron concept uses the chem-
ical potential contained in the Sens•+ in a productive way. The concept is illustrated in Scheme 1.
Excitation of the sensitizing dye as in conventional photography, results in formation of a reduced sil-
ver ion and the radical cation of the sensitizer, Sens•+. In the two-electron sensitization scheme, an elec-
tron donor compound, X–Y, is added, which can reduce the Sens•+ to form the radical cation X–Y•+.
This reaction physically separates the oxidized species and the trapped electrons, and thus could slow
return electron transfer. In photographic systems, however, return electron transfer may occur over
many timescales, from fractions of seconds to days. Thus, it seems unlikely that this simple charge-sep-
aration process alone will be particularly beneficial. In the subsequent step, however, the X–Y•+ under-
goes fragmentation to form a radical X• and a cation Y+. If the cation Y+ is sufficiently stable, return
electron transfer can be endothermic and thus be essentially eliminated. Finally, if the X–Y molecule is
properly designed, the radical X• is highly reducing and can irreversibly inject another electron into the
silver halide conduction band. In this way, two electrons can be injected into the silver halide for each
absorbed photon.
ENERGETIC, KINETIC, AND STRUCTURAL REQUIREMENTS
For the two-electron sensitization scheme to work, the X–Y molecule must satisfy three requirements.
The first is that it must donate an electron to the oxidized dye. This is easily accomplished by ensuring
that the oxidation potential of the X–Y molecule is lower than that of the sensitizing dye. For example,
sensitizing dyes that absorb in the blue region of the spectrum typically have oxidation potentials
around 1.4 V vs. SCE [5]. The oxidation potential should not be too low, however, since the compound
might then be susceptible to oxidation by air or other adventitious oxidants. Also, X–Y compounds with
low oxidation potentials may be unreactive and thus not meet the second requirement, which is that the
X–Y•+ be capable of undergoing fragmentation within a reasonable timescale to form a stable cation Y+.
The rate of fragmentation of the X–Y•+ should be high enough so that it can react before being reduced
by an electron donor in the matrix, or by return electron transfer from the silver halide. The timescales
for these processes are not known, but it would seem reasonable that the lifetime of X–Y•+ should be
less than a millisecond. As discussed below, the most likely structure class for X–Y is an amine.
Fragmentation reactions that have been reported for amine radical cations which take place on various
I. R. GOULD et al.
© 2001 IUPAC, Pure and Applied Chemistry 73, 455–458
456
Scheme 1
timescales include deprotonation, desilylation, decarboxylation, and carbon–carbon bond cleavage [6].
Importantly, these different reactions occur via different mechanisms, and respond to external condi-
tions such as solvent polarity, nucleophilicity, and basicity in different ways. This suggests that a range
of different X–Y molecules could be used, and that the reactivities of these molecules can be manipu-
lated to suit the conditions of the silver halide gelatin matrix.
The final requirement is that the X• radical should inject an electron into the conduction band of
silver halide. Experiments with sensitizing dyes of varying redox potentials have shown that silver
halide has an effective reduction potential of ca. –0.9 V vs. SCE [4b]. From this we can assume that if
X• has an oxidation potential of –0.9 V or more negative, it will be capable of injecting an electron.
Measurements of oxidation potentials for radicals have been reported by Griller and Wayner et al. [7].
Typical hydrocarbon radicals such as benzyl are not very reducing, having positive oxidation potentials
(0.73 V vs. SCE). A significant increase in reducing power is obtained by the addition of an oxygen
atom in the position α to the radical center. To lower the reduction potential to –1.0 V, however, the lit-
erature data suggest that a nitrogen atom has to be present in the α position. α-Amino radicals are high-
ly reducing because the products of their oxidation are the very stable iminium ions. To generate an 
α-amino radical, the molecule X–Y should clearly be an amine. The pKa’s of most aliphatic amines are
typically in the range of 9–10, whereas those for aromatic amines are lower, typically 3–4 [8]. At the
pH of the gelatin medium (ca. 5.5) [9], most aliphatic amines will be protonated, which suggests that
the amine should be aromatic.
BINDING MODES
For the X–Y molecule to be effective, it should be physically located close to the silver halide surface,
so that it can reduce the oxidized dye and inject the second electron. This could be accomplished in sev-
eral ways, as illustrated in Scheme 2. The X–Y molecule could be covalently attached to each dye mol-
ecule (A). Electron transfer between moieties separated by a few atoms has been shown to be quite effi-
cient and can occur in picoseconds [10]. Trapping of the oxidized dye should occur with very high effi-
ciency under these conditions. Alternatively, the X–Y molecule could be attached to only a few of the
dye molecules, (B). In this case, the “hole” on the oxidized dye has to migrate among the dyes until it
arrives at the one with the X–Y, where rapid electron transfer to form X–Y•+ would occur. This scheme
has the advantage that most of the sensitizing dyes are “conventional”, and need not be subject to the
synthetic procedure required for the linked molecules. Additionally, less disruption of dye aggregation
by the attached X–Y groups may occur. Also, because it would be present in sufficiently low concen-
trations, the dye to which the X–Y is linked need not be identical to the other sensitizing dyes. In a third
scheme (C), the X–Y is covalently attached to a compound that adsorbs to the surface of the silver
halide crystal. This scheme also relies on hole hopping, but has the advantage that the X–Y compound
is not attached to a light absorbing molecule. Finally, the X–Y molecule can simply be added at rela-
tively high concentration (D), with the assumption that the molecule itself will adsorb sufficiently to the
surface of the silver halide, or that it will be present at a sufficiently high concentration so that electron
transfer to the oxidized dye can take place.
© 2001 IUPAC, Pure and Applied Chemistry 73, 455–458
Silver halide photography using radical cation chemistry 457
Scheme 2
SUMMARY
A fundamentally new mechanism for spectral sensitization of silver halide is proposed which has the
capability of delivering two conduction-band electrons per absorbed photon. The scheme uses an organ-
ic electron donor compound X–Y which must meet the following criteria: 1) the oxidation potential
must be lower than that of the sensitizing dye, 2) the radical cation of X–Y must undergo fragmenta-
tion with a faster rate than those of competing electron-hole recombination reactions, 3) the oxidation
potential of the radical X• must be more negative than the effective electrochemical potential of the sil-
ver halide conduction band, and 4) the pKa of the X–Y compound must be such that at the pH of the
gelatin medium, it is not protonated.
Details of the thermodynamic and kinetic properties of X–Y compounds which meet these
requirements and can be used in the two-electron sensitization scheme, together with photographic eval-
uations of their effectiveness, are given elsewhere [11].
REFERENCES
1. W. West and P. B. Gilman. In The Theory of the Photographic Process, T. H. James (Ed.), p. 393,
Macmillan, New York (1977).
2. T. H. James. In The Theory of the Photographic Process, T. H. James (Ed.), p. 373, Macmillan,
New York (1977).
3. R. K. Hailstone, N. B. Liebert, M. Levy, J. F. Hamilton. J. Imag. Sci. 31, 185 (1987).
4. (a) R. S. Eachus, R. O. Baetzold, Th. D. Pawlik, O. G. Poluektov. Phys. Rev. B 59, 8560 (1999);
(b) R. S. Eachus, A. P. Marchetti, A. A. Muenter. Annual Rev. Phys. Chem. 50, 117 (1999).
5. (a) J. Lenhard. J. Imag. Sci. 30, 27 (1986); (b) J. R. Lenhard, B. R. Hein, A. A. Muenter. J. Phys.
Chem. 97, 8269 (1993).
6. See, for example: (a) Z. Su, P. S. Mariano, D. E. Falvey, Ung C. Yoon, S. W. Oh. J. Am. Chem.
Soc. 120, 10676 (1998); (b) Z. Su, D. E. Falvey, Ung C. Yoon, P. S. Mariano. J. Am. Chem. Soc.
119, 5261 (1997); (c) U. C. Yoon, P. S. Mariano, R. S. Givens, B. W. Atwater. Adv. Electron
Transfer Chem. 4, 117 (1994); (e) E. R. Gaillard and D. G. Whitten. Acc. Chem. Res. 29, 292
(1996).
7. (a) D. D. Wayner, D. J. McPhee, D. Griller. J. Am. Chem. Soc. 110, 132 (1988); (b) D. Griller and
D. D. M. Wayner. Pure Appl. Chem. 61, 717 (1989).
8. G. Kortum, W. Vogel, K. Andrussow. Dissociation Constants of Organic Acids in Aqueous
Solution, Butterworth, London (1961).
9. P. I. Rose. In The Theory of the Photographic Process, T. H. James (Ed.), p. 51, Macmillan, New
York (1977).
10. See, for example: M. R. Wasielewski. In Photoinduced Electron Transfer, M. A. Fox and M.
Channon (Eds.), Part A, p. 161, Elsevier, Amsterdam (1988). 
11. (a) S. Y. Farid, J. R. Lenhard, C. H. Chen, A. A. Muenter, I. R. Gould, S. A. Godleski, P. A.
Zielinski. U.S. Patent 5,747,235, 1998; (b) S. Y. Farid, J. R. Lenhard, C. H. Chen, A. A. Muenter,
I. R. Gould, S. A. Godleski, P. A. Zielinski, C. H. Weidner. U.S. Patent 5,747,236, 1998; (c) S. Y.
Farid, J. R. Lenhard, C. H. Chen, A. A. Muenter, I. R. Gould, S. A. Godleski, P. A. Zielinski, C.
H. Weidner. U.S. Patent 5,994,051, 1999; (d) S. Y. Farid, J. R. Lenhard, C. H. Chen, A. A.
Muenter, I. R. Gould, S. A. Godleski, P. A. Zielinski. U.S. Patent 6,010,841, 2000; (e) A. Adin,
J. J. Looker, S. Y. Farid, I. R. Gould, S. A. Godleski, J. R. Lenhard, A. A. Muenter, L. C.
Vishwakarma, P. A. Zielinski. U.S. Patent 6,054,260, 2000. 
I. R. GOULD et al.
© 2001 IUPAC, Pure and Applied Chemistry 73, 455–458
458


New Approach to Silver Halide Photography Using Radical Cation Chemistry

https://digitalcommons.brockport.edu/cgi/viewcontent.cgi?article=1004&amp;context=chm_facpub

New Approach to Silver Halide Photography Using Radical Cation Chemistry

Abstract

Similar works

Full text

Available Versions

The College at Brockport, State University of New York: Digital Commons @Brockport