Nitrogen, triplet state excitation

For the investigation of triplet state properties a laser flash photolysis apparatus was used. The excitation source was a Lambda Physik 1 M 50A nitrogen laser which furnished pulses of 3.5 ns half-width and 2 mJ energy. The fluorescence decay times were measured with the phase fluorimeter developed by Hauser et al. (11). 

The photoelimination of nitrogen from 1-pyrazolines is one of the most thoroughly investigated photoreactions and it has been used extensively in the synthesis of cyclopropane derivatives.334 Both stereospecific and non-stereospedfic processes have been observed and these are believed, at least in simple 1-pyrazolines, to correspond to singlet and triplet excited states, respectively. Two reaction pathways have been proposed in the azoalkane 405335 direct excitation via a thermally activated S, state affords the C6H6 isomers 406 to 409, whereas triplet-sensitized excitation results in a tem-... 

Three of the four photophores, listed in Scheme 3, eliminate nitrogen upon excitation while the biradicaloid triplet state of benzophenone can be reversibly activated, and creates covalent linkage upon excitation-relaxation cycling. This fact and their extremely low reactivity towards protic solvents make them very efficient in the majority of the cases. [7, 19, 20]. Unsaturated ketones can be activated by a similar mechanism, although the secondary processes are much more complex [21]. That mechanism is mainly utilized in steroid hormones possessing unsaturated ketones as intrinsic photophores. 

The reaction of atoms, radicals or excited triplet states of some molecules with silicon hydrides is the most important way for generating silyl radicals [1,2]. Indeed, Reaction (1.1) in solution has been used for different applications. Usually radicals X are centred at carbon, nitrogen, oxygen, or sulfur atoms... 

A study of the photochemistry of 4-acetyl- and 4-benzoyl-5-methyl-1,2,3-triazoles shows that the nature and lifetime of the lowest triplet state depends on the nature of the 1- and 4-substituents. 4-Benzoyl-5-methyl-1,2,3-triazole has a high rate constant for triplet deactivation, which is attributed to interaction of the nitrogen lone pairs with the excited carbonyl function. The compound forms a pinacol derivative when irradiated in propan-2-ol and undergoes cycloaddition, involving the carbonyl group, with 2-methylpropene, giving an oxetane derivative. 

Since quenching experiments with piperylene and naphthalene showed no effect, it was suggested that the triplet state of the thiatriazole is not involved in the reactions.22 It was later shown that the photoreactions leading to nitrile, isothiocyanate, sulfur, and nitrogen actually take place from a singlet excited state of the thiatriazole.20 Thus the photoreduction of benzophenone in isopropyl alcohol is effectively quenched by addition of 5-phenylthiatriazole, but analysis revealed that all thiatriazole could be recovered from the photolysis mixture, indicating the lack of photoreactivity of the triplet state. 

The excitations of lower triplet states of benzene in these two approximations have been calculated in Ref. 133. A similar study of excitation cross sections for triplet states of a nitrogen molecule has been done in Ref. 134. 

The H(D) atom abstraction rate constants in durene crystals by the impurity molecules quinoline, isoquinoline, quinoxaline, and quinozaline in their excited triplet state were measured by Hoshi et al. [1990] using the phosphorescence method described above. The transfer occurs in the fragment CH N formed by a methyl group of durene and a nitrogen atom of the impurity molecule. In the interval 300-100 K the activation energy drops from 3.5 kcal/mol to 1.6 kcal/mol. Deuteration reduces the... 

An excited aryl halide may also transfer charge to oxygen and thereby become activated towards attack by nucleophiles. The photoamination of 1-amino-2,4-dibromoan-thraquinone by rc-butylamine, in which the 4-Br atom is replaced, is supposed to proceed via direct interaction between the triplet state and the amine when the reaction is performed under nitrogen. In air atmosphere, however, it proceeds via both the T7, and SY states. From the singlet excited state and oxygen, an exciplex or collisional complex [AQ ()/ ] is formed which undergoes the animation639. 

Therefore, the radiationless deactivation process for most emitting species in their excited triplet states is so efficient that phosphorescence spectra can be observed only when the system including the emitting species is frozen at liquid nitrogen temperature. Of course, the environmental aspects of radiationless deactivation do not determine all the whole properties. Pyridine molecules show neither fluorescence nor phosphorescence at room temperature or in the rigid glass state at liquid nitrogen temperature. 

Molecular nitrogen, N2, is one of the most extensively studied diatomic molecules and optical spectroscopy has provided a wealth of information about its ground and excited electronic states. Molecular beam magnetic resonance studies of N2 in its ground state have yielded information about 14N nuclear spin dipolar and quadrupole interactions. Similar studies of N2 in its electronically excited A 3LU state were described in two very extensive papers by Freund, Miller, De Santis and Lurio [43] (paper I) and De Santis, Lurio, Miller and Freund [44] (paper II). We will describe their results and analysis in detail, but first note in passing that, strictly speaking, the lowest excited triplet state should be labelled the a state the label A has been used by all concerned in the past, so we will continue to do so. 

The triplet state di-TC-methane reactivity of the methanoquinoline systems (61, 62) have been studied. The results of the irradiations are shown in Schemes 3 and 4 where it can be seen that each compound usually affords two products. The results indicate that the pyridine nitrogen does not manifestly alter the photochemical behaviour of the compounds. A detailed analysis of the reasons for the observed regioselectivity is made on the basis of molecular orbital theory. The di-ic-methane reactivity of the benzonorbornadienes (63) still excites considerable interest. Paquette and Burke have studied the triplet state reactivity of the derivatives (63) in an attempt to establish the influence of bridgehead substitution. Thus the irradiation of (63a) results in the formation of both isomers (64a) and <65a) in 42 and 58 It respectively. Similar yields are shown for the irradiation of (63b) when the two products are obtained in similar yields to the above. However, irradiation of the derivative (63c) results in the formation of only the cyclized product (64c) while the derivative with the bridgehead cyano (63d) yields both (64d) and (65d) but in 10 and 90 X yields respectively. The authors present arguments to explain the observed specificities leading to products of either bridgehead control or vinylic control of the biradical... 

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