Triplet-energy transfer

WIGNER SPIN CONSERVATION RULE DELAYED LUMINESCENCE TRIPLET-TRIPLET ENERGY TRANSFER TRIPLET-TRIPLET TRANSITIONS TRITIUM TRITON TRITIUM... 

The photochemical configurational isomerization of previtamin D (5) into tachysterol (6) (and back) differs from the photochemical constitutional isomerization of provitamin D (1) or lumisterol (2) into previtamin D (5) (and back see Scheme 3) not only in the reaction type, but also in which of the jr jT-spin isomers is involved. Unlike the constitutional isomerization, the configurational isomerization can proceed not only under direct light excitation (singlet mechanism), but also by triplet-triplet energy transfer (triplet mechanism), with the aid of photosensitizers. 

Singlet energy transfer Triplet energy transfer... 

Recently, Bach and Schroder " reported the photocycloaddition of a-alkyl-substituted enecarbamates 41 to benzaldehyde affording 3-aminooxetanes 42 in moderate to good yields (46 to 71%). The a-phenyl-substituted enecarbamate did not lead to a photocycloaddition product presumably due to rapid energy transfer (triplet sensitization) from the electronically excited aldehyde. An increase in steric bulk of the alkyl substituent R shifted the diastereomeric ratio cis/trans in the direction of the thermodynamically more stable cis product. 

Once the excited molecule reaches the S state it can decay by emitting fluorescence or it can undergo a fiirtlier radiationless transition to a triplet state. A radiationless transition between states of different multiplicity is called intersystem crossing. This is a spin-forbidden process. It is not as fast as internal conversion and often has a rate comparable to the radiative rate, so some S molecules fluoresce and otliers produce triplet states. There may also be fiirther internal conversion from to the ground state, though it is not easy to detemiine the extent to which that occurs. Photochemical reactions or energy transfer may also occur from S. ... 

Fig. 4. Chemistry of poly(vinyl cinnamate) negative-acting resist. Initial light absorption by the photosensitizer is followed by energy transfer to produce a pendant cinnamate group in a triplet electronic state. This combines with a second cinnamate on another polymer chain, forming a polymer—polymer...

The energy of the triplet state of the sensitizer ( Sens ) must be greater than that of the reactant. If this condition is not met, the energy transfer becomes endothermic and cannot compete with other transformations of Sens. ... 

The common case, and the one that will be emphasized here, is triplet sensitization. In this case, the intersystem crossing of the sensitizer must be faster than energy transfer to the reactant or solvent from the singlet excited state. 

TTie photoreduction can be quenched by known triplet quenchers. The effecti e quenchers are those which have T] states less than 69kcal/moI above S,. Quenchers with higher triplet energies are ineffective because the benzophenone n-n triplet is then not sufficiently energetic to effect energy transfer. 

Aromatic compounds such as toluene, xylene, and phenol can photosensitize cis-trans interconversion of simple alkenes. This is a case in which the sensitization process must be somewhat endothermic because of the energy relationships between the excited states of the alkene and the sensitizers. The photostationary state obtained under these conditions favors the less strained of the alkene isomers. The explanation for this effect can be summarized with reference to Fig. 13.12. Isomerization takes place through a twisted triplet state. This state is achieved by a combination of energy transfer Irom the sensitizer and thermal activation. Because the Z isomer is somewhat higher in energy, its requirement for activation to the excited state is somewhat less than for the E isomer. If it is also assumed that the excited state forms the Z- and -isomers with equal ease, the rate of... 

Hydroxy Benzophenone P-Naphthoxy Energy Transfer Processes Triplet State Intramolecular Excimers" ... 

A rather important aspeet that should be eonsidered is that interfaeial quenching of dyes does not neeessarily imply an eleetron-transfer step. Indeed, many photoehemieal reactions involving anthracene oeeur via energy transfer rather than ET [128]. A way to discern between both kinds of meehanisms is via monitoring the accumulation of photoproducts at the interfaee. Eor instance, heterogeneous quenehing of water-soluble porphyrins by TCNQ at the water-toluene interfaee showed a elear accumulation of the radical TCNQ under illumination [129]. This system was also analyzed within the framework of the exeited-state diffusion model where time-resolved absorption of the porphyrin triplet state provided a quenehing rate eonstant of the order of 92M ems. ... 

---