Sensitised triplet states

Due to the relatively long lifetime of the sensitiser triplet state and the possibility of integrating data on the stilbene photoisomerisation, the apparent characteristic time of the method can reach hundreds of seconds. This unique property of the cascade system and, therefor triplet-photochrome technique, allows the investigation of slow diffusion processes, including encounters of proteins in membranes using very low concentrations of both the triplet and photochrome probes. 

There is a very large energy difference between the 1(it,7t ) and 3 (71,71 ) states for alkenes and so intersystem crossing is very inefficient and a triplet sensitiser is needed to populate the triplet state. Thus, different reaction conditions are required to form the excited singlet state (by direct irradiation) and the excited triplet state (by sensitised irradiation). 

Figure 8.5 Effect of the sensitiser triplet energy on the percentage of cis-stilbene in the photostationary state...

Intersystem crossing in aromatic ketones is very efficient, so that their photochemistry is dominated by the triplet-state processes. The efficient formation of triplet states, the small singlet-triplet energy and the accessible long-wavelength absorption make the aryl ketones excellent triplet sensitisers. 

If the products formed from the direct and sensitised reactions differ then the triplet state formed on sensitisation is not the reactive excited state formed by direct irradiation. This situation is illustrated in Sections 8.2 and 8.3, where direct and photosensitised cis-trans isomerisation of alkenes was considered. 

If the same products are formed from the direct and sensitised reactions then the triplet state formed on sensitisation may be the state responsible for reaction on direct irradiation. It is important to understand that it is possible for the triplet state to give the same products as the reactive singlet state. 

Type II involves electron transfer from the excited triplet state of the sensitiser to triplet oxygen producing the highly oxidative singlet oxygen. This mechanism has already been discussed (see Figure 4.14). 

Since PDT photosensitisers locate preferentially in tumours they should be in theory of value in the diagnosis of the disease. The procedure of diagnosis would involve measuring the fluorescence of the tumour area before and after treatment with the sensitiser (see Chapter 3, section 3.5.6). For this to work effectively the fluorescence quantum yield should be as high as possible. However, the most effective photosensitisers are those with a high triplet-state quantnm yield and hence a low fluorescence quantum yield. Therefore it is unlikely that the more active photosensitisers can be used for this purpose. 

Scheme 11.5 Photophysics of sensitised lanthanide sensors sens-Ln such as 11.32 and 11.33. Key k = rate constant, em = emission, eT = electron transfer, q = quenching, isc = inter-system crossing to triplet state, (B)ET = (back) energy transfer (reproduced with permission from Reference 24).

Fig-1 Illustration of energy transfer to the lanthanide ion in sensitisation process for Tb(III), from triplet state of the antenna and the corresponding emission spectra for Tb(III)... 

Hviid and co-workers have recently found a way around this limitation by using the method of intermolecular triplet sensitisation to generate the locally excited triplet state of the donor chromophore in a D-A dyad.222 The sensitiser scheme is described by equations (35)-(38) and is also illustrated graphically in Fig. 36 for the case of triplet sensitisation of a donor chromophore (S is the sensitiser molecule). The principal advantage of this method is that the locally excited triplet state of one of the chromophores (in this case, the donor) is populated without invoking its... 

This method has been applied to the rigid 3-bond dyad, DNM[3]M, (Fig. 37), in which the maleate ester group (M) serves as the acceptor (the DCV acceptor group cannot be used because the energies of the singlet and triplet +DMN[3]DCV CS states all lie above the 3[ DMN[3]DCV] locally excited triplet state, see Fig. 29). Benzophenone (BP) was used as the sensitiser, and a highly polar solvent... 

Fig. 37 (a) Depiction of the triplet sensitisation of DMN[3]M by benzophenone, to give the locally excited donor triplet state, 3( DMN[3]M),... 

An additional step in the cascade reaction scheme is the quenching of the sensitizer triplet state with relatively low-concentration radicals (Fig. 1.5) (Papper et al 1999, 2000 Papper and Likhtenshtein, 2001). The entire investigated reaction that is shown in Fig. 1.5 is the sequence of the four kinetic processes and serves as a basis for the spin-triplet-photochrome labeling technique. This technique combines the three types of biophysical probes stilbene photochrome probe, triplet probe and stable nitroxide-radical spin probe, which depresses the sensitiser exited triplet state. 

The alkyl aryl ketone sensitised photolysis (313 nm) of hexane solutions of t-butyl Cl reported by HarrimaniZ also gave HCl as a major product together with 2-methyl propene. Quantum yields ranged from 0.19 when benzophenone was the sensitiser to 0.313 for EtC=0 and the involvement of the triplet state was confirmed by the quenching of the reaction by 2,5-dimethyl-hexa-... 

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