Triplet transfer efficiency

More than a decade ago, Frank et al. (1987) reported on the light-modulated EMR ofa number of synthetic porphyrin-polyene model systems that were linked by an amide group and in some cases by methylene spacers of various lengths. The triplet transfer efficiency increased as the link between the two pigments became shorter which is what one would expect, especially for Dexter type triplet exchange... 

Baldo et al. [ 164] used the platinum complex of 2,3,7,8,12,13,17,18-octaethyl-21 //,23//-porphine (PtOEP, 66) as efficient phosphorescent material. This complex absorbs at 530 nm and exhibits weak fluorescence at 580 nm but strong phosphorescence from the triplet state at 650 nm. Triplet transfer from a host like Alq3 was assumed to follow the Dexter mechanism. Dexter-type excitation transfer is a short-range process involving the exchange of electrons. In contrast to Forster transfer, triplet exciton transfer is allowed. 

Using laser flash photolysis with a frequency-quadrupled neodynium laser, Stevens and al 161b) measured the lifetime of the triplet state of fluoro- and pentafluoro-benzene in the gas phase along with the energy transfer efficiencies to cis-2-butane and oxygen. The triplet transient absorption decay was found to be predominantly first order with a... 

Direct evidence for triplet-triplet transfer has been provided by sensitizing both the isomerization of cis- or ira/w-olefins,28 38 79,80 and the dimerization of some cyclic olefins81-83 with carbonyl compounds. Furthermore, the phosphorescence of some carbonyl compounds can be quenched by olefins (for example, acetone with 2-pentene30 and phenyl-cyclopropyl ketone with 2-methyl-2-butene37). On the other hand, the phosphorescence of benzophenone is not quenched by 2-methyl-2-butene37 nor is the photoreduction of benzophenone quenched efficiently by cyclohexene (Table II). 

The most investigated system for energy transfer from the excited triplet states of fullerenes to an acceptor is the formation of singlet oxygen. Both the fullerenes, C6o and C70, in their triplet states efficiently transfer energy to 302 to... 

Porphyrin is one of the most widely studied macrocyclic systems suitable for complexation with lanthanide(III) ions. Porphyrins can absorb strongly in the UV-vis region so as to serve as efficient photo-sensitizers, making lanthanide(III)porphyrinate complexes ideal candidates for luminescence imaging agents. Indirect excitation of porphyrin antenna chromopheres in close proximity to lanthanide ions can make the energy in the triplet state of the porphyrin ligand transfer efficiently to the excited state of the lanthanide ion so as to sensitize the lanthanide luminescence, particularly NIR emission. 

Larson and O Neal reported a value of = 3.5 x 10 sec at 44 °C. Investigating the competition between decomposition and reaction with HBr of the ther-malized triplet acetone, these authors found that k. = 1.8 x 10 exp (-9450/Rr) l.mole . sec. A value of k 8 x 10 I.mole . sec" was reported for the quenching of the triplet state by oxygen. (For the triplet energy transfer efficiencies of organic molecules, see the references cited earlier.)... 

A FBrster-type transfer mechanism (48) was proposed to occur. Batochromic shifts and hyperchromicity were observed In the absorption spectra of systems [10b,1,3] and to the largest extent In [10b,2] (44,45). On specific excitation of the benzophenone chromophore, triplet transfer to the naphtalene moiety occurs with 100 percent efficiency. The rate of transfer is greater than lO s l g d not measurably dependent on chain length. Since the ratio of Intersystem crossing in benzophenone is of the order of (49), energy transfer will occur, after inter-... 

The fluorescence of the other chromophore was efficiently but not totally quenched in the bifunctional systems but not at all in equimolar mixtures of the monofunctional models. All phosphorescence, however, occurred from the parabutyrophenone part, indicating very efficient triplet transfer. Bathochromlc shifts and a hyperchromic effect in the longest wavelength band of the absorption spectra are an indication for ground state interaction, which might lead to nonplanar conformations of the molecule playing a role in the excitation transfer. 

In these types of blchromophorlc systems, linked by a short flexible chain, singlet transfer does occur efficiently in most cases, mostly through a dipole-dipole mechanism, while triplet transfer occurs with very high efficiency through an exchange mechanism. Due to very fast and almost total transfer, rates of transfer are difficult to measure. In several cases, there are hints at the possibility of complex formation being involved in excitation transfer. 

The distance between the chromophores Is approximately 21 8 as measured on molecular models and no ground state Interaction could be observed. In dioxane, the singlet energy transfer occurs by a Forster-type mechanism. On the basis of this mechanism, the distance can be calculated from the transfer efficiency. For [16a] a value of 21.211.6 X was found, while for [I6b] one obtained a value of 16.7 1.4 X. This latter discrepancy was explained on the basis of preferential orientation of the chromophores. That this phenomenon Indeed plays an Important role, was proved by Leermakers (59,60) In a series of indole alkaloids. The same author (59) also Illustrated the absence of triplet-triplet transfer at low temperature in compound [16b]. 

The mechanism of the efficient Chl-Car triplet-triplet transfer has been investigated by Damjanovic et al. [131], but quantitatively accurate calculations of the electronic couplings have yet not been possible. 

Ferrocene does not react with iron carbonyls under irradiation. Its low ionization potential [ 7—7.5 eV 387>] and its high triplet quenching efficiency 185> would cause a strong decrease of G pea(co)9 and G-[Fe(C0)4]3 if benzene triplets or any ions were involved in their formation. This is not the case. 0.54 mole/1 ferrocene causes Gj e2(co)9 to decrease by only 20%, and has no effect on G[Fe(CO)4]3 [0-21 mole Fe(CO)s/l benzene] 289>. This observation suggests that energy transfer from excited benzene singlets could play a role. 

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