Triplet-state probes

The use of excited triplet states as probes occurred in concert with the development of fluorescent probe methodologies but have not been employed to the same extent. Phosphorescence quantum yields in solution are generally much lower than fluorescence quantum yields, and when the emission of triplet states is measured, the signal-to-noise advantage of the fluorescent probes is lost. In most cases, triplet excited states are followed by their absorption spectra using laser flash photolysis. A second reason for the scarcer reports on the use of excited triplet states probes in supramolecular systems is the fact that the laser flash photolysis technique has not been as widespread as fluorescence techniques. This situation has changed over the past decade and we expect that the number of smdies which employ excited triplet states will increase. 

Table 24 Quenching of Extrinsic Triplet State Probes Complexed to Proteins...

Both energy and electron transfer quenchers have been employed to show that the quenching rates of the fullerene triplet state are decreased as a function of the size of the dendrimer shell [36]. These results further demonstrate that fullerene is an excellent functional group to probe the accessibility of a dendrimer core by external molecules. 

The quenching of benzophenone phosphorescence has been used by Mar and Winnik (1981) as a photochemical probe of hydrocarbon chains in solution. The bimolecular reaction for quenching the triplet state of 4-methoxy-carbonylbenzophenone [24] by 1-pentene occurs at rates which are below the diffusion limit by two to three orders of magnitude. Consequently, the intramolecular quenching reactions of to-alkenyl esters of benzophenone-4-carbo-xylic acid [25] occurs under conformational control. In [25] the point of... 

The quenching of a triplet carbene reaction with methanol is frequently used as the standard means of probing the singlet-triplet gap. It is widely believed that singlet carbenes insert readily into the O—H bonds of methanol, while the triplet states undergo hydrogen abstraction from the C—H bonds." The behavior of diarylcar-... 

Fig. 14. Schematic representation of energy levels and transitions for fluorescence and related processes kic, rate constant for interval conversion fcF, rate constant for fluorescence fcISC, rate constant for intersystems crossing fc[cp> rate constant for internal conversion from triplet state kp, rate constant for phosphorescence S, energy level for the first excited singlet state after solvent rearrangement for a polarity probe in a polar solvent.

A triplet state dimer of a benzofused 1,2,3-trithiole dication was postulated. However the occurrence of this high-spin dimer is strongly dependent on the nature of the solvent and the oxidant129. The 2,5-dimethoxy-4-methylphenyl moiety in which the additional charge and the single electron are stabilized in a quinoidal structure after one-electron oxidation has been utilized as a probe for electron delocalization when two of these moieties were connected either by a trimethylene, a 1,4-disubstituted phenylene or a biphenyl bridge130. 

This mechanism leads to a highly spin-polarized triplet state with a characteristic intensity pattern in the EPR spectrum, which is observed by time-resolved techniques (either transient or pulse EPR). The zero field splitting (ZFS) of the triplet state, which dominates the EPR spectrum, is an important additional spectroscopic probe. It can also be determined by optical detection of magnetic resonance (ODMR), for a review of the techniques involved and applications see reference 15. These methods also yield information about dynamical aspects related to the formation, selective population and decay of the triplet states. The application of EPR and related techniques to triplet states in photosynthesis have been reviewed by several authors in the past15 22-100 102. The field was also thoroughly reviewed by Mobius103 and Weber45 in this series. 

The obtained data clearly show that the g-anisotropy of the triplet states is larger than that of the respective cation-radical. A similar effect has been observed for the triplet states of the primary donors in PS II231 and in the bacterial RC.111112114 This can be explained by the fact that the triplet electrons probe the spin distribution in two different orbitals (HOMO and LUMO), and the latter has a rather large spin density at the nitrogens and the central magnesium (cf. references 216, 218), by which the spin-orbit coupling and the g-anisotropy is increased. 

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