Benzene triplet state

The benzene triplet has not been observed to phosphoresce in either the gas or liquid phase, as noted previously but in view of the lifetime measured In glassy media at low temperature this is not surprising, since very small amounts of quenching impurities would reduce emission to undetectable levels. Some detailed investigations have shown that there is an intramolecular process which Induces radiationless decay of the benzene triplet state, other than impurity quenching. 

A benzene triplet state might be generated in the latter reaction which may account for the very negative entropy of activation in a cleavage reaction. This retro 2 + 2 reaction is entropically worse than that of the anti dimer which actually has a higher activation enthalpy. 

The kinetics and thermochemistry of the quantitative rearrangement of benzvalene (228) and bisdeuteriobenzvalene (229) into benzene and 1,2-dideuteriobenzene have been investigated. " Reactions are first order in the temperature range 313—330 K, with AH = 25.9 0.2kcalmol and AS = 1.6 + 0.7calK mol the heat of reaction for the Ag -ion catalysed process in chlorobenzene solution was estimated at 67.54 0.66 kcalmol" (i.e. more exothermic than the Dewar-benzene to benzene valence isomerization). In the presence of 9,10-diphenylanthracene or 9,10-dibromoanthracene (traps for benzene triplet states) the yield of excited benzene... 

Oxepin has also been converted photochemically to phenol in 74% yield. This reaction occurs under irradiation conditions by which benzene oxide is excited to a triplet state, e.g. by irradiation in acetone as solvent.207 A rare example for a nucleophilic catalysis of the aromatization of an oxepin/benzene oxide to a phenol has been reported for /err-butyl oxepin-4-carboxylate which undergoes a rearrangement reaction in the presence of trimethylamine to give a mixture of /m-butyl 3-hydroxybenzoate (94%) and 4-hydroxybenzoate (6%).243... 

In weaker acid systems, other reactions involving the triplet state supervene to the exclusion of dimerization. Photolysis of 85 in 3-3% sulfuric acid, 96-5% acetic acid, and 0-2% water gave as products tri-phenylmethane (93), 9-phenylfluorene (94), 6is-9-phenylfluorenyl peroxide (95) and benzophenone (96). When benzene was present, tetra-phenylmethane (97) was also formed in addition to the other products. When the triphenylmethyl cation is irradiated in 3-3% H2SO4, 80 1% HOAc, 16-4% toluene, and 0-2% H2O, the products observed were... 

Alcohols can be added to certain double-bond compounds (cyclohexenes, cycloheptenes) photochemically ° in the presence of a photosensitizer such as benzene. The mechanism is electrophilic and Markovnikov orientation is found. The alkenes react in their first excited triplet states. ... 

The quenching of the trans dimer with oxygen and ferrocene indicates that this product is formed almost entirely from the triplet state. It is possible to calculate the amount of triplet-derived product in benzene by subtracting the amount of product obtained in the presence of oxygen from the amount of product obtained in the absence of oxygen. Such a calculation indicates that acenaphthylene triplets in benzene give both trans and cis dimers in the ratio of 74 26. The triplet state accounts for almost all of the trans product and about 10% of the cis product. The break in the slope of the Stem-Volmer plot for the trans dimer (Figure 10.3) may be attributed to the presence of two excited species which are quenched at different rates. These two species could be (a) two different monomeric acenaphthylene triplet states 7 and T2 or (b) a monomeric acenaphthylene triplet state 7 and a triplet excimer. This second triplet species is of relatively minor importance in the overall reaction since less than 5% of the total product in an unquenched reaction is due to this species. 

Finally, one has to consider that an unknown impurity present in the benzene is responsible for quenching the triplet state/8,12,16 We then have four viable possibilities to explain this effect. 

Forster (1968) points out that R0 is independent of donor radiative lifetime it only depends on the quantum efficiency of its emission. Thus, transfer from the donor triplet state is not forbidden. The slow rate of transfer is partially offset by its long lifetime. The importance of Eq. (4.4) is that it allows calculation in terms of experimentally measured quantities. For a large class of donor-acceptor pairs in inert solvents, Forster reports Rg values in the range 50-100 A. On the other hand, for scintillators such as PPO (diphenyl-2,5-oxazole), pT (p-terphenyl), and DPH (diphenyl hexatriene) in the solvents benzene, toluene, and p-xylene, Voltz et al. (1966) have reported Rg values in the range 15-20 A. Whatever the value of R0 is, it is clear that a moderate red shift of the acceptor spectrum with respect to that of the donor is favorable for resonant energy transfer. 

Pulse radiolysis is used also for preparation of excited states of dienes and polyenes. This is done by irradiation of the diene/polyene in toluene solution. The radiolysis of toluene yield high concentration of molecules in the triplet excited state of the solute. Wilbrandt and coworkers61 pulse-radiolysed 1 mM solution of al I -lrans-1,3,5-heptatriene in toluene solution and observed the absorption spectra of the triplet state of the heptatriene with a maximum at 315 nm. The same group62 produced and measured the absorption spectra of several isomeric retinals in their lowest excited triplet state by pulse irradiation of their dilute solution in Ar-saturated benzene containing 10 2 M naphthalene. Nakabayashi and coworkers63 prepared the lowest triplet states of 1,3-cyclohexadiene,... 

Woodruff and coworkers65 produced triplet states for several carotenoid pigments by pulse radiolysing benzene solution of the carotenoids with higher concentration of naphthalene. The initially produced excited states of benzenes (both singlet and triplet) are... 

The transient triplet carbene formed from irradiation of DABA in benzene can be observed to react with styrene. For this process laser spectroscopy reveals a bimolecular rate constant (ksty) equal to 1.2 x 107M-1s-1. The product of the reaction is the expected cyclopropane. This observation clearly supports the spectral assignment of the carbene made above. When deuteriated a-methylstyrene is substituted for styrene as a trap for the carbene, the cyclopropane that results is a 1 1 mixture of stereoisomers, (20) (Table 6). This finding indicates that BA is reacting in this sequence exclusively from its ground triplet state. 

El-Sayed 28> has reported on the phosphorescence spectrum of [2.2]paracyclophane. The emission differs both in wavelength ( 4700 A) and in duration (3.3 s) from that of benzene ( 3400 A, 6 s) hence a favorable intersystem crossing from the lowest singlet to the emitting triplet state was inferred. The emission spectrum also indicates that interactions take place between the two aromatic nuclei in the triplet state. 

The triplet state of 4-nitrobiphenyl has been observed in laser flashed benzene solution (. max 540 nm, t 10 ns at room temperature) 32). 4-Nitrobiphenyl and 4,4 -dinitrobiphenyl have been photoreduced by sodium formate in buffered aqueous methanolic solution 43i>) 15% 4-aminobiphenyl and 11% 4,4 -azobi-phenyl as well as 49% 4-amino-4 -nitrobiphenyl and 20% 4,4 -(p-nitrophenyl)-azoxybenzene, respectively, could be isolated and identified by comparison with authentic samples. 

In contrast to borazine, the three corresponding excited singlet states of benzene have a much wider spread of absorbing wavelengths and exhibit easily distinguished vibrational fine structure. Many photolysis experiments have been performed using laser lines tuned to selective excite a particular vibrational level of a particular excited state of benzene. Such experiments are more difficult with borazine. The triplet states of benzene have been located experimentally and quantum yields for fluorescence and phosphorescence at various wavelengths and pressure conditions have been determined. 

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... 

From the materials just mentioned earlier, one can conclude that mutual meta orientation (meta through a benzene) of the spin-bearing moieties is an indispensable condition for the existence of triplet states in aromatic di- or tri-(cation-radical)s. However, in fact, these systems have both singlet and triplet forms, and the questions are about what is the difference in the corresponding energy and which state is more stable. Stability of the polyion-polyradicals is also a very important factor, especially in the sense of practical application. Let us consider several relevant examples. 

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