Triplet sensitisers

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 ... 

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). 

For rigid alkenes, triplet sensitisation brings about photocycloaddition via the 3(Jt,7t ) state. These reactions are neither concerted nor stereospecific. Cyclopentene produces a tricyclic dimer ... 

Triplet-sensitised photocycloaddition of norbornadiene (N) leads to formation of the energy-rich quadricyclane (Q) by an intramolecular reaction ... 

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. 

Triplet-sensitisation studies can provide information regarding ... 

Ultraviolet irradiation of oxadiazoline (38d) at 333.6 nm (or irradiation using benzophenone as a triplet sensitiser) gave 2-diazopropane and methyl acetate. A triplet biradical intermediate formed by cleavage of the C(OMe)—N bond was postulated <90TL863>. Oxadiazolinone (42) underwent nucleophilic attack at the carbonyl group by methyllithium to give acetate (41) after treatment of the product with acetyl chloride <89CJC1753>. 

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... 

The population of the locally exited donor (or acceptor) triplet is achieved simply by excitation of an added external triplet sensitiser molecule whose singlet energy lies below that of both donor and acceptor chromophores, but whose triplet energy lies above that of either the donor or acceptor (Fig. 36). 

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),... 

In summary, we have demonstrated a proof of principle that long-lived triplet CS states in dwarf dyads - that is, dyads possessing short rigid saturated hydrocarbon bridges - may be achieved using the triplet sensitised approach. This sensitisation method offers at least two important advantages over other methods discussed above. These are ... 

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. 

Sens = Sensitiser, Sens = Excited singlet sensitiser, Sens = Triplet sensitiser, Subs = Substrate and = Singlet oxygen]... 

They also obtained a similar CIDEP spectrum by triplet sensitisation with triplet benzophenone [13]. The polarization was found to consist of two net component, one reflecting the polrization of the triplet sensitizer due to the p-type TM, and the other being attributable to the d-type TM. 

In the triplet sensitised stepwise photopolymerisation of a polymethylene having terminal dibenz(b,f) azepine groups indicates... 

One very useful class of molecules in this respect are Ni(II) chelates which quench both triplet sensitiser and singlet oxygen. It is interesting though that diamagnetic square planar complexes are more effective in stabilising polypropylene... 

The preparation of ODNs containing a non-adjacent cis-syn-cyclobutane thymine dimer has been reported. Triplet-sensitised irradiation of a duplex containing a d(TCT) sequence gave rise to the dimer between the two thymines. 

The photolysis of triarylsulphonium salts yields diarylsulphides and products of lateral nuclear shift reactions which are ortho, meta, and para aryl substituted diaryldisulphides such as (342). A by-product in these reactions is a proton because of this these reactions have been applied to photoinitiation of cationic polymerisations. A full paper describing a detailed study of the reaction mechanism has been published.In addition, the product distribution obtained by photolysis of triphenylsulphonium salts in films of the polymer of 4-(tert -butoxycarbonyloxy)styrene has been compared with that obtained in solution.The synthesis of some new triarylsulphonium salts and their application for photoinitiation of cationic polymerisation has also been reported.The formation of the products arising from lateral nuclear shifts in sulphonium salts occurs under direct photolysis but not under triplet sensitisation. 

Cyclopropane derivatives are the major products of photoelimination of nitrogen from 1-pyrazolines. Conversion of the fused pyrazoline (10) into the cyclopropane (11) was achieved in this way in 85% yield by irradiation in the presence of acetophenone and constitutes a valuable step in a synthesis of a diquinane alcohol. The short-lived triplet 1,3-cyclopentadiyl biradical, generated by benzophenone-sensitised irradiation of diazabicyclo-(2.2.1]hept-2-ene, has been trapped as a Jbis-alkoxyamine by a nitroxide. The diazabicycloheptenes (12) gave in a similar fashion the bicydo[2.1.0]pentanes (13) on triplet-sensitised photolysis, whereas laser/liquid jet excitation of the same compounds gave in addition the cyclopentenes (14), derived by 1,2-hydrogen shift evidence for a two-photon process is described. The results of a time-resolved spectroscopic study of the photodecomposition of 2,3-diazabicyclo[2.2.l]hept-2-ene in the vapour phase have also been reported.Photolysis of 2,3-diaza-5-methylenebicyclo[2.2.l]hept-2-ene affords the semi-localised... 

Photochemistry.—Singlet oxygen is very efficiently quenched by /5-carotene. If the polyene system is shortened the triplet sensitiser is also quenched. However, the fact that extension of the system has little effect suggests a diffusion-controlled process. This phenomenon suggests that one of the functions of carotenoids in nature is to protect the organism against photochemical oxidation. 

The photo-isomerisation of trimethylsilyl ethers of tachysterola (724) and previtamin Dj (725) in benzene is triplet-sensitised, and leads to an equilibrated mixture (20 80, respectively). Since no cyclisation to 5,7-dienes was observed, it seems likely that cyclisation would require singlet excitation. 

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