Sensitised emission

Energy Transfer and Excitons are, as we have already mentioned, perhaps the most interesting and in any case the most characteristic photophysical processes in molecular crystals. The investigation of these processes began in 1934, when A. Winterstein, U. Schon and H. Vetter [5] were able to explain the green fluorescence radiation from anthracene crystals, which had been described as due to the effect of an unknown chrysogen , in terms of sensitised fluorescence. This fluorescence is emitted by tetracene molecules which are present at very low concentration in the anthracene. Pure anthracene fluoresces in the crystalline phase just as in solution with a blue-violet colour. This observation set off a large number of spectroscopic studies of the sensitised emission from mixed crystals. Very soon, J. Franck and E. Teller [6] pointed out in a summary report of this field that there was an important cormection here to the primary processes of photosynthesis and other biophysical processes. 

Sensitised emission in isotopic mixed crystals at higher (> 1%) concentrations Exdton transfer in dilute exdton systems kfjG D in dilute exdton systems S,T Intensity of the guest emission with varying trap depth in isotopic mixed crystals as... 

Fig. 15.3 Sensitised emission spectra of singlet oxygen in aerated toluene solutions of IH-phenalen-l-one and a bis(naphthalene)-oligothiophene at 293 K. Reproduced with permission from Ref. [12], Copyright 2009, the Royal Society of Chemistry...

The overall efficiency of sensitised emission is given by Equation 1.77. 

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 ... 

Sensitisation by a high-energy donor, such as triplet xanthone, can populate both the Qi and the D, states by energy transfer (Figure 10.4). This results in both the photosolvation reaction and phosphorescence emission. 

Sensitisation with lower-energy donors, such as triplet Ru(bpy)3+, on the other hand, means that energy transfer is only possible to the Di state (Figure 10.4). This results in phosphorescence emission, but the photosolvation reaction does not occur. This shows that the Qi state must be responsible for the photosolvation reaction. 

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... 

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).

Shavaleev, N. M., Moorcraft, L. P., Pope, S. J. A., Bell, Z. R., Faulkner, S., Ward, M. D., Sensitised near-infrared emission from lanthanides using a covalently-attached Pt(U) fragment as an antenna group. Chem. Commun. 2003, 1134-1135. 

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. 

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)... 

Note that in the case where the Forster mechanism concerns an organic species transferring its energy to the lanthanide center to which it is bound, this is often referred to as the antenna effect and the ligand is sometimes called a sensitiser. Such an effect becomes efficient provided that the organic chromophore possesses a triplet excited state close to but at least 1700 cm-1 above that of the lanthanide emissive state (Parker and Williams, 1996). 

Ronson, T.K., Lazarides, T., Adams, H., et al. (2006) Luminescent PtII(bipyridyl)(diacetylide) chromophores with pendant binding sites as energy donors for sensitised near-infrared emission from lanthanides structures and photophysics of PtII/LnIII assembhes. Chemistry - A European Journal, 12, 9299. 

Chen, F.-F., Bian, Z.-Q., Lou, B., et al. (2008) Sensitised near-infrared emission from lanthanides using an iridium complex as a ligand in heteronuclear Ir2Ln arrays. Dalton Transactions, 5577. 

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