Formation of Excited Triplet States

Calculations are in agreement with the formation of excited triplet state, and this intermediate can evolve to the formation of the azirine via the biradical intermediate [99H(50)1115]. 

A particularly interesting example with BP/maleic anhydride (MAH) described by Hoyle and coworkers [PAN 04] pointed out that BP could also be used as an efficient photosensitizer for the photografting of MAH onto PP. The occurrence of the MAH grafting could be ascribed via the formation of excited triplet state of MAH sensitized by BP. 

Formation of excited triplet states of the donor T-T annihilation of Ar )... 

Computational results are reported for the isomerization of 1,4,5-trimethyl-imidazole (99MI233). They show that the isomerization occurs through the Dewar isomer arising from the excited singlet state. The formation of the triplet state is energetically favored however, the biradical intermediate cannot be produced because it has higher energy than the excited triplet state. 

Figure 8.8 Kinetics of formation of the triplet state of 1-ethylnaphthalene after excitation of its charge-transfer complex with O2...

A major advantage of benzophenones is that they can be excited at wavelengths of 350-360 nm, just like diazirines. The possible reaction pathways of benzophenones after photolysis are shown in Scheme 6. Absorption of a photon of the proper wavelength by a benzophenone (55) initially results in the formation of a triplet state benzhydril diradical (56). The formation of the triplet diradical is reversible... 

Tetracyanoethylene forms a charge-transfer complex with 1,1 -binaphthyl which exhibits two charge-transfer absorption bands (Yorozu et al., 1978). Excitation via the band of higher energy leads to the formation of the triplet state of binaphthyl which, unlike the ground state, is planar. When optically active 1,1 -binaphthyl is used, this change in geometry can be measured by the fact that the production of the triplet state is attended by racemisation. 

Ru(bipy)3 + is the prototype of a very large family of MLCT species. In the standard model of the photoprocesses of this compound, a photon excites the molecule to an initial Frank-Condon singlet state, MLCT, that rapidly transforms to a triplet, MLCT, with a quantum yield of near unity. Femtosecond pump probe experiments have established a half-life of about 100 fs for the formation of the triplet state. Recent studies utilizing femtosecond time-resolved fluorescence emission spectroscopy has observed fluorescence emission from the Frank-Condon state itself and the hfetime of this state has been estimated to be 40 15 fs. ... 

These observations show that energy transfer may occur from the triplet acetone to the aldehyde molecule, and that triplet biacetyl can be formed as a result of energy transfer from the excited aldehyde. However, the data available at present leave unsettled the queastion as to whether reaction (5), or (5 ) followed by (5 ), is responsible for the formation of the triplet state in the photolysis of -butyr-aldehyde. 

Photonucleophilic substitution of fluoro- and chloro-anisoles has been the subject of three reports within the year. Cornelisse and co-workers have studied the photocyanation and photohydrolysis of 4-fluoro- and chloro-anisoles by laser spectroscopy and report that the initial step of the reaction involves formation of a triplet state transient complex composed of a ground state and an excited state aromatic molecule. Only in the presence of water does the complex yield radical ions and it is this process which determines the product quantum yield. The radical cation then reacts with the nucleophile to give a neutral radical which yields the substituted arene in a single step. Liu and Weiss report on anomalous effects during photonucleophilic aromatic substitution of 2- and 4-fluoroanisoles and also on the photo-... 

The above discussions may be summarized as follows when the acceptor side of the reaction center is oxidized, i.e., it is in the [Pd] QA-state, light activation produces the P -state, i.e., the [P" -r]-Q -state, where I is the transient intermediary electron acceptor, namely a BO molecule. When the reaction center is pre-reduced to the [Pdj QA -state, light activation produces the P -state, i.e., the [ P-l]-Q -state with a lifetime of 10 ns. Most of the radical pairs recombine to reform the original [P-I] state, but some form the triplet state of P. In the initial excited singlet state [P 4 ], the spins on and r are antiparallel. During the 10-n lifetime of the excited singlet state, the spins of the unpaired electrons on the radical pair interact with nuclear spins on the two molecules, or with the electron spins on or the nonheme iron atom, but in any case there is a rephasing of these two unpaired spins. Recombination of these radical pairs, now with a predominantly triplet character, leads to the formation of the triplet state of P, i.e., theP -state [P -r] QA" [P -H-Qa" -> Pdl-QA. ... 

Little to no carbazole was observed upon triplet photosensitized photolysis of 2-azidobiphenyl. Azo formation was suppressed by isoprene, a quencher of excited triplet states. 

---