Dye sensitized photoluminescence in silver halides

Journal of Luminescence 24/25 (1981)881—884 North-Holland Publishing Company

881

DYE SENSITIZED PHOTOLIJMINESCENCE IN SILVER HALIDES H. Hediger, P. Junod, R. Steiger CIBA - GEIGY CH-1700 Fribourg Switzerland

Measurements of the dye sensitized photoluminescence on iodide doped Ag Hal layers have shown that this luniinescen— ce does not decrease drastically at dye coverages above about half a monolayer, contrary to the photographic sensitivity at room temperature. Thus the decrease in photographic sensitivity may originate in a reoxidation of the latent image centres by photoholes percolating in the dye layer. The possibility of increasing sensitivity by FOERSTER energy transfer is investigated by luminescence in a system of organized monolayer assemblies. INTRODUCTION In photography the silver halides are spectrally sensitized with organic dyes (e.g. cyanines) in the spectral region where they do not absorb (x~500 nm). If the dye coverage on the silver halides exceeds about half a monolayer, a drastic decrease in photographic sensitivity is generally observed [1].In order to get a deeper understanding, we have investigated the photoelectron injection yield versus dye coverage. At low temperatures silver halides show photoluniinescence. This allows a study of this problem by measuring the dye sensitized luminescence of silver halides L2], see Fig. 1. Since at 4.5 K ionic processes (Ag~)arefrozen in, the electronic processes can be observed separately. EXPERIMENTS Two different kinds of silver halide substrates have been used: AgBr(I) and AgC1(I). AgBr was doped with iodide in a molar fraction [AgI~/ [AgBr]= l.5~l0

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At 4.5 K, AgBr(I) shows an emission at 2.37 eV which is due to the recombination of excitons at iodide pair centers as first described by C~NZAKI ~ The sample has been laminated by means of two quartz cylinders to a thickness of 0.2 mm, and was then annealed. Dyes were adsorbed on silver halides by immersion in aqueous methanolic solutions. The adsorption behaviour of the cyanine dye used (Fig. 1) was reproducible in this case. The luminescence emission was measured with a photon counting system. Because of the long decay times of the silver halide luminescence, a rotating can chopper (3500 r.p.e.) can be used to make sure that no direct dye fluorescence (~nanoseconds)is observed. The stray light is also strongly reduced. The excitation spectra (Fig. 2) show the following features In the absorption region of the adsorbed dye, the molecular band (M) is clearly visible, while the dimer transition (D) is badly resolved as it falls into the spectral region of the emission (2.37 eV) where measurements were disturbed by stray—light. With increasing dye coverage, the sensitized luminescence increases (excitation: 2.0 2.5 eV) while the intrinsic AgBr(I) luminescence (excitation 2.7 3.0 eV) diminishes. These results are summarized in Fig. 3 for various dye coverages. The results in the intrinsic region are qualitatively comparable with the blue desensitization, which is generally observed in photography at higher dye coverages.

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In the spectral region of the dye monomer, the number of emitted photons rises with dye coverage and attains saturation at about one monolayer. However, by taking into account the higher dye absorption with increasing coverage, the relative quantum efficiency of this process passes through a maximum at coverage around 0.5 monolayer. This result is in qualitative agreement with the variation of the photographic speed with coverage in the dye spectral region. However, the decrease between 0.5 and 1.0 monolayer obtained by luminescence measurements (factor about 2) is not as dramatic as the decrease of the photographic speed in photographic emulsions (factor 30).

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The second system under investigation was evaporated AgCl(I) sensitized by the same dye. The silver chloride was doped nominally with 0.05 mole % iodide and shows a luminescence emission at 2.63 eV [3g. The disadvantage of this substrate is that the dye adsorption at low dye coverages is hardly reproducible. On the other hand, the stray light (5) disturbs the excitation spectra (Fig. 4) only on the high energy side of the dye dimer. The dependence of the integrated excitation spectra on dye coverage is given in Fig. 5 and shows an increasing tendency. DISCUSSION No decrease of the dye sensitized luminescence for high dye coverage is observed in either system. Thus, the drastic decrease of photographic sensitivity is not due to lack of injected photoelectrons. We suppose that it originates in a reoxidation of the latent image centres by photoholes percolating in the more compact dye layer. INCREASE OF SENSITIVITY BY ENERGY TRANSFER The dye coverage on Ag Hal cannot be increased beyond half a monolayer without a serious loss in photographic sensitivity. As only a small fraction of the incident light is absorbed by the dye in this case, it is of interest to try to increase sensitivity by using a concerted energy-electron transfer system 4~.A cyanine dye 0 which can act as an energy donor for an acceptor A is separated from the acceptor by an insulating layer. Conditions for energy transfer from D to A: good overlapping between emission of D and absorption of A and high fluorescence quantum yield of 0 [5]. A is in contact with the AgC1(I) surface. D is excited and transfers excitation energy to A which is able to sensitize AgC1(I) by electron transfer. No electron loss processes

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Fig.!. Excitation spectra of AgCl(I) sensitized with a donoracceptor system should be possible if the insulating layer between B and A inhibits electron transfer and if the coverage of A is below half a monolayer. In our experiment (Fig. 7), these conditions are realized with the aid of organized monolayer assemblies of A and D, separated by the hydrocarbon chains (C18H37) of the dye molecules (Fig. 6). The excitation spectrum of AgCl(I), sensitized with this energy— electron transfer system, shows the contribution of energy transfer due to the donor D at distance from A (Fig. 7). Its efficiency can be evaluated in terms of a critical distance d0 between D and A for which half the excitation energy is transferred to A. d0 = 30 + 10 ~ as determined from luminescence measurements is in good agreement with the value calculated from spectroscopic data of this system and the fluorescence quantum yield of the donor (d0 = 40 ~ 5 ~), showing that no major electron loss processes occur during the electron injection. This d0 corresponds to an additional sensitizing effect by D by a factor of about 2 compared with that of the acceptor alone. Preliminary experiments showed that a higher value of d0 (8O~)can be achieved if B is organized mainly as monomers instead of dimers as in the present case. A factor of 6 instead of 2 is then expected. These luminescence measurements prove that this complex system works, but it still has to be realized in Ag Hal emulsions. REFERENCES [1] P. Junod, H. Hediger, B. Kilchbr and R. Steiger, Photogr. Sci. Eng. 23 (1979) 266

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[2j L. Costa, F. Grum and P.B. Gilman, Photogr. Sci. Eng. 18 (1974) 261

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[3] H. Kanzaki and S. Sakuragi, Photogr. Sci. Eng. 17 (1973) 69 -19977 9JO)p. [4] R. H. Steiger, Kuhn, in H.DyeHedigere sensitization, Press (l [5] P. Junod, Focal H. Kuhn and London D. Möbius, Photogr. Sci. Eng. 24 (1980) 185 - 195