Triplet energy transfer, collisional

Collisional deactivation and energy transfer play important roles in tropospheric chemistry. For example, electronically excited S02 in the 3B, state can be deactivated by 02 (as well as by N2 and H20) to the ground ( A,) state, with part of this process occurring via triplet-triplet energy transfer to generate singlet electronically excited states of 02 ... 

Triplet-triplet energy transfer, on the other hand, is a very important type of energy transfer observed with solutions of sufficient concentration. It has been established as occurring over distances of I-I.5 nm, comparable with collisional diameters. Steric effects were shown to be significant for in-... 

Other possible solvent effects depend on the heavy-atom effect, which may favor intersystem crossing, and on the viscosity, which may change the rate of diffusion and hence the collisional triplet-triplet energy transfer. These effects have already been mentioned in previous sections. 

Stern-Volmer Model. When the chromophores are sufficiently mobile as in fluid solution, the bimolecular quenching process of A by a quenching molecule, B, including the triplet energy transfer and some collisional quenching, will result in the single exponential... 

Sensitized photolysis is a much more efficient method to produce triplet nitrenes than the collisional deactivation method. This technique, which populates the triplet state of a nitrene exclusively by energy transfer, employs triplet sensitizers such as aromatic ketones with a high intersystem-crossing efficiency. Energy is... 

Absolute rate constants have been determined for aromatic triplet formation in acetone solutions of several aromatic compounds (5, 30). The formation curves were observed directly for anthracene and naphthalene triplet (5) and for diphenyl triplet. These rate curves were found to fit a first order rate law, and were interpreted as a bimolecular energy transfer process from a state of the solvent molecule which is probably the triplet, that is, by Reaction 11. These rate constants, as well as the triplet yields, are listed in Table VI. The rate constants for anthracene and naphthalene triplet formation appear to correspond to diffusion controlled rate constants. Two further points are of interest, which are in contrast with observations in other systems which will be discussed. In acetone, most of the yield of aromatic triplet (at concentrations of the aromatic compound of 5 X 10"3M or lower) is formed in diffusional processes such as collisional energy transfer. Any fast formation appears... 

Collisional energy transfer is frequently called sensitization. It is an important tool in photochemistry because it provides access to excited triplet states of compounds whose singlets do not intersystem cross efficiently (such as alkenes and conjugated polyenes). It also provides access directly to the triplet states of compounds for determination of the multiplicity of the state responsible for a photochemical reaction. 

Singlet oxygen is directly produced from triplet oxygen by collision with so-called photosensitizers A (electronically excited molecules), which transfer energy onto O2 in the ground state the third collisional partner M stabilizes the transfer complex ... 

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