Biphenyl, triplet state

The photodecarbonylation of a series of dibenzyl ketones was studied by Robbins and Eastman/63 The results of this study are presented in Table 4.5. The data in Table 4.5 indicate that the presence of a p-methyl or a p-methoxy group has little effect on the quantum yield for this reaction. p-Cyano groups, on the other hand, essentially totally eliminated the decarbonylation. Since the reaction could also be quenched (inefficiently) by benzonitrile or biphenyl, it was concluded that the decarbonylation occurs from a short-lived triplet state. The effect of the p-cyano groups then could result from internal triplet quenching. 

In alkanes, excited states are produced (not exclusively) by ion neutralization. Singlet states are also produced, but exact pathways are debatable. In alkane solutions, excited states of solutes, produced by ion neutralization, are mainly triplets, but some singlets are also formed with biphenyl as a solute. Mechanisms of formation and decay of triplet states have been treated in detail... 

It has been suggested that this increase in yield is due to the heteroatom which reduces the energy difference between the singlet and triplet states of the common, biphenyl nucleus. - - The lifetime and quantum yields of triplet state formation have been measured at 25° and interpreted in the context of spin-orbital coupling. -... 

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. 

Fig. 3. Schematic potential energy diagram for the ground- and first-triplet states of biphenyl as a function of the angle of twist between the rings. The vertical lines represent allowed spectroscopic T <-> S0 transitions, while the slanted line represents the nonvertical transition possible during energy transfer.

Another example is the photoreaction of tctrazolo[ 1,5- ]pyrimidinc 14 with aromatic compounds in the presence of trifluoroacetic acid (TFA) to give 2-(2-, 3-, and 4-substituted anilino)pyrimidines together with 2-aminopyridine and biphenyl or diarylmethanes. Here the key intermediate is also the 2-pyrimidylnitrenium ion originating from the photochemical decomposition of an imidoyl azide. This ion, depending on the reaction conditions, is present either in the singlet or triplet state and this governs the composition of the products (Scheme 11) <1998JP0478>. 

Stem-Volmer analysis (measuring the magnitude of the biphenyl 7-7 absorption produced by the second pulse as a function of the concentration of added biphenyl) yielded an estimate of -600 ps for the lifetime of the upper triplet state of 71. 

Figure 1 Energy transfer from the upper triplet state of 71 to biphenyl.

The linear correlation with slope AR/Ap = —0.36 shows that PPP bond orders can be used for excited-state relaxation. The comparable graph of AR/Ap for trans-stilbene has a linear correlation with slope —0.25. Similarly, it is seen in biphenyl[37] that the lowest triplet state has a bond order pattern corresponding to the quinonoidal structure, implying a planar geometry in the triplet state in conformity with the prediction of Lewis and Kasha[56]. 

A basis for understanding the photochemistry of polybromobiphenyl photochemistry is provided by a consideration of the mechanistic features of the phototransformation of 4-bromobiphenyl (6 = BpBr) at 300 nm in various solvents [9], 4-Bromobiphenyl is expected to undergo efficient intersystem crossing due to the heavy atom effect, and involvement of the triplet state of 4-bromobiphenyl has been shown earlier by various workers [10, 11, 12]. Since the reported quantum yield of the intersystem crossing process for biphenyl is 0.81 [13], BpBr was expected to have a quantum yield for intersystem crossing... 

The question of whether it, it triplets of aromatic ketones have some intrinsic chemical property has been a controversial subject in organic photochemistry (130). Recently, the correlation of structure and reactivity of some excited triplet biphenyl ketones was approached by a combined esr, CIDEP, and CIDNP study (129). The low-temperature esr confirmed the assignment of the ir, ir triplet as the lowest triplet state in these biphenyl ketones. 

As was pointed out in the introduction, optical detection of the Zeeman transitions of the triplet state preceded the optical detection in zf. Since these former experiments resemble those in ionic crystals, researchers in this field called this technique MODR (microwave-optical double resonance). The assignment of the zf transitions as well as the relative order of the zf levels could be concluded also from the MODR techniques as in the PMDR technique. The first reported MODR experiment was made by Sharnoff (15), in which the Am = 2 transition of the C10D8 tr Plet state in a biphenyl host, using amplitude modulation of the microwave power. A few months later Kwiram reported the optical detection of the Am = + 1 for phenanthrene in biphenyl (16). The experiments were... 

Oxygen quenching of phenanthrene phosphorescence involves an exchange mechanism which also operates for triplet state energy transfer involving phenanthrene in biphenyl 2i. Radiationless processes of excited states of 5,6-diazaphenanthrene in hexane and hexafluoroisopropanol have been studied by both triplet and singlet lumi ne s cen ce 2 2 extremely fast radiationless transition in the... 

The photochemistry of 2-fluoroaniline and 2-bromoaniline is also similar to that of 2-chloroaniline, but the formation of 2-aminophenol is more efficient with 2-fluoroaniline than with the other two species. In contrast, the formation of biphenyl derivatives is more efficient for 2-bromoaniline than 2-chloroaniline, and it does not occur for 2-fluoroaniline214. The absence of biphenyl derivatives in the presence of oxygen indicates that the reaction involves the excited triplet state. 

SET Induced Reactions - Further studies on ring opening reactions of 1,2-diarylcyclopropanes have focused on compounds (83) bearing an acetyl functional group.All of the cyclopropane derivatives studied show efficient cis.trans isomerism with reasonable quantum efficiency. The isomerism reaction involves an acetophenone-like triplet state with lifetimes shorter than 1 ns. The formation of ring opened products also takes place but inefficiently to give a mixture of the two alkenes (84) and (85). A study of the electron-transfer-induced photochemical reactivity of the cyclopropane (86) has been reported in the co-sensitisation system (biphenyl/phenanthrene/DCA). ... 

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