Sensitised fluorescence

A detailed study of the photostability of the DAST-type agent Cl Fluorescent Brightener 85 on cellophane film was carried out recently. The initial fading reaction is a photo-sensitised trans-cis isomerisation of the stilbene grouping. The subsequent oxidative attack on the molecule is concentrated on the vulnerable ethene linkage at the centre of this moiety [39]. 

P-type delayed fluorescence is so called because it was first observed in pyrene. The fluorescence emission from a number of aromatic hydrocarbons shows two components with identical emission spectra. One component decays at the rate of normal fluorescence and the other has a lifetime approximately half that of phosphorescence. The implication of triplet species in the mechanism is given by the fact that the delayed emission can be induced by triplet sensitisers. The accepted mechanism is ... 

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). 

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. 

The exchange of electronic excitation between two atoms frequently results in sensitised fluorescence and one of the earlier examples was the discovery of emission of the fluorescence of atomic sodium, which occurs when a mixture of sodium and mercury vapour is irradiated with mercury resonance radiation at 2537 A... 

Figure 11.20 The ligand fluorescence arising from the sensitiser is much broader than the lanthanide-based lines (reproduced by permission of The Royal Society of Chemistry).

The tetraazatriphenylene chromophore attached to the cyclene ring acted as an efficient sensitiser for Eu3+ and Tb2+ emission but also intercalated between the base pairs of DNA. The complexes were tested as cellular imaging and reactive probes using the mouse fibroblast cell line. The complexes were quickly taken up by the fibroblast cells and localised in nucleus and around the cell membrane. The process was visualised by fluorescence microscopy. Photolysis at 340 nm and 350 nm damaged plasmid supercoiled DNA producing nicked (form II) and linear (form III) DNA. DNA damage is known to induce apoptotic cell death and these complexes may be therefore considered for the development as therapeutic probes, for example in the treatment of accessible tumours, such as skin melanoma. 

Polychlorinated dibenzofurans Insecticides and herbicides UV, fluorescence and sensitised room temperature fluorescence [104]... 

The triplet-photochrome labeling method has been used to study very rare encounters in a system containing the Erythrosin B sensitiser and SITC photochrome probe (Mekler and Likhtenshtein, 1986). Both types of the molecules were covalently bound to a-chymotrypsin. The photoisomerisation kinetics was monitored by fluorescence decay of the frans-SITS. The rate constants of the triplet-triplet energy transfer between Erythrosin B and SITS (at room temperature and pH 7) were found k,r = 2 xlO7 NT s-1 and ktT = 107 M V. It should be emphasized that the concentration of the triplet sensitiser attached to the protein did not exceed 10 7 M in those experiments, and the collision frequencies were close to 10 s 1 which are 8-9 orders of magnitude less than those measured with the regular luminescence or ESR techniques. 

Transient terahertz spectroscopy Time-resolved terahertz (THz) spectroscopy (TRTS) has been used to measure the transient photoconductivity of injected electrons in dye-sensitised titanium oxide with subpicosecond time resolution (Beard et al, 2002 Turner et al, 2002). Terahertz probes cover the far-infrared (10-600 cm or 0.3-20 THz) region of the spectrum and measure frequency-dependent photoconductivity. The sample is excited by an ultrafast optical pulse to initiate electron injection and subsequently probed with a THz pulse. In many THz detection schemes, the time-dependent electric field 6 f) of the THz probe pulse is measured by free-space electro-optic sampling (Beard et al, 2002). Both the amplitude and the phase of the electric field can be determined, from which the complex conductivity of the injected electrons can be obtained. Fitting the complex conductivity allows the determination of carrier concentration and mobility. The time evolution of these quantities can be determined by varying the delay time between the optical pump and THz probe pulses. The advantage of this technique is that it provides detailed information on the dynamics of the injected electrons in the semiconductor and complements the time-resolved fluorescence and transient absorption techniques, which often focus on the dynamics of the adsorbates. A similar technique, time-resolved microwave conductivity, has been used to study injection kinetics in dye-sensitised nanocrystalline thin films (Fessenden and Kamat, 1995). However, its time resolution is limited to longer than 1 ns. 

---