Spectral sensitisation

Because they exhibit excellent fluorescence on excitation, the polymethine cyanine dyes are dealt with in greater detail in Chapter 3 (section 3.5.1.7). They are also used as spectral sensitisers in photography (see section 4.5.3). 

The dyes that perform this function are called spectral sensitisers because they are used to sensitise the photographic emulsion to light across the whole visible spectrum and beyond. Since silver halides are naturally blue sensitive, the green and red spectral sensitiser dyes must also be able to reduce the natural sensitivity of the silver halide emulsion to blue light. The most important class of compounds used in this... 

Figure 4.19 Schema for spectral sensitisation of silver bromide.

Fisher followed Hamer to Kodak in 1934, and they worked together co-authoring a total of seven publications and several patents. In 1939, Fisher was asked to transfer to Kodak s laboratories in Australia.70 Kodak (Australia) needed a specialist organic research chemist to provide expertise in preparing emergency quantities of vital spectral sensitisers, in case supplies were restricted during the Second World War. 

In summary, photoexcitation can lead to photoionisation of colour centres and conseqnent generation of free charge carriers that can participate in both physical and chemical processes. The spectral limits of photochemical and photophysical processes are red-shifted in photocoloured solids compared with their initial states. Thns, photocolouration leads to spectral sensitisation of solids in heterogeneous systems (RyabchukandBurukina, 1991 Emeline eta/., 1998a, 1998b, 1999c). 

Merocyanine based spectral sensitising dye for photographic material... 

The mechanism of spectral sensitisation has been shown to involve the direct transfer of an electron from the excited state of the dye into the AgX conduction band (see Fig. 11.8). In confirmation of this mechanism, excellent correlation has been demonstrated between the sensitisation capability of dyes and their electrochemical reduction potential. For the most efficient dyes, the quantum yield of the electron transfer step is close to 1.0. Using picosecond laser pulse excitation to measure the fluorescence decay rates of dyes adsorbed to AgX crystals, it has been shown that the electron transfer is very fast, with rate constants in the region of 10 s having been reported [24]. Once an electron has been transferred into the AgX conduction band, the latent image formation process ensues more or less as it does following direct absorption by AgX itself. The difference in the case of the dye-sensitised process is that the positive holes are trapped at the dye molecules. 

The emitted light is efficiently harvested by spectrally sensitised film. In medical applications this increased sensitivity is an important contribution to reducing the doses of radiation suffered by the patient and/or to improving image resolution. 

Various types of the photoconductive polymers are available now. The photoconductivity of such materials may be essentially increased by means of the chemical and spectral sensitization [12-14]. Spectral sensitization is connected with the appearance of the photosensitivity in the new spectral bands and the chemical sensitization with the increase of the proper sensitivity. As a rule both types of sensitisation may take place in the photoconductor at the same turn. The first data about chemical and spectral sensitization in organic photoconductors appeared in [19, 20]. The example of the chemical and spectral sensitization of the photoconductivity by dyes in polymeric copper-phenyl-acetylenide is presented in Fig. 2. Later on it was proposed that not only low molecular weight compounds but polyconjugated polymers could also be used as sensitizers [21] having broad absorption tends and high thermostability compared with dyes. Now it is clear that various types of molecules may be used as a photosensitizers. 

Figure 8.6 Spectral response of the photocurrent for mesoscopic injection solar cells. Grey curve bare Ti02 film, blue and black curves same film sensitised by the N3 and black ruthenium dyes, respectively.

When considering organic dyes for use in DSSCs, porphyrins and phthalocyanines have attracted particular attention, the former because of the analogy with natural photosynthetic processes, the latter because of their photochemical and phototherapeutic applications. However, porphyrins cannot compete with the N3 or black dye sensitiser due to their lack of red light and near-lR absorption. Phthalocyanines, on the other hand, show intense absorption bands in this spectral region. However, problems with aggregation and the unsuitable energetic position of the LUMO level, which is too low for electron transfer to the TiOa conduction band, have turned out to be intractable for the moment. 

Figure 11.3 Visible and near-IR transient absorption spectra of N3-sensitised TiOi film at time delays of 0.5 ( ), 10 (o), and 150 ps (A) between the pump and probe pulses. Clear spectral signatures of triplet excited state and oxidised dye with corresponding dynamics were observed. Inset transient absorption kinetics of oxidised N3 cation-TiOi measured at 860 nm. Symbols are measured data, while the curve is a fit with the following time constants and amplitudes rise within the laser pulse (20 5%) 28 3 fs (50%), 1 0.1 ps (11%), 9.5 1 ps (12%), and 50 5 ps (7%). Reprinted with permission from J. Am. Chem. Soc. 124, 489 (2002), Fig. 2. Copyright (2002) American Chemical Society.

Intraoperative fluorescence is induced by a UV light source at 370-440 nm, which spectrally matches the main absorption peak of the sensitiser for fluorescence excitation and is delivered via a liquid light-guide to the surgical microscope (Figure 4). 

Fig. 6.20 Sensitised fluorescence, schematic representation, a the hostcrystai H contains a low concentration of guest moiecuies, C. The crystal Is excited by light which is mainiy absorbed by the host. The fiuorescence contains mainly guest light, b the emitted fluorescence light quanta from the host and the guests, Qh and Qq, differ spectrally in their wavelengths and can therefore be detected separately. The quantum ratio QcIQh plotted against the guest concentration Cq.

Compared with the classical DSSC Ru dyes, their extinction coefficients are higher and the spectral response is shifted to the red. The solar cell efficiency of these types of sensitisers has increased continuously over the last 2 years and efficiencies above 12% have been reported. There are sometimes arguments that DSSCs have not developed much since the... 

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