Azulene triplet states

The photochemistry of a-methylstilbene (5) resembles stilbene photochemistry in many ways. However, as pointed out earlier, both the cis and trcms isomers are nonclassical acceptors of triplet excitation. This suggests that both the cis and trcms triplet states correspond to high-energy vibrational levels of the twisted or phantom triplet. Azulene does not alter the photo-... 

While a complete study of the photoreactions of 4-bromostilbene (6) has not been published yet, preliminary azulene quenching results(1) suggest that upon direct photoreaction about 3570 of the tram singlets intersystem cross to the triplet state ... 

Calculated Charge Densities for the Ground State (Sg), the Lowest Excited Singlet State (Sj), and the Lowest Triplet State (Tj) of Azulene (Pariser, 1954)... 

Photoreduction was quenched by high concentrations of biacetyl, slightly retarded by iodonaphthalene, but not affected by azulene or anthracene.113 These observations led to the unsatisfying conclusion that reduction proceeded via a triplet state which could be only selectively quenched. However, later work114 using flash photolysis showed that the benzophenone ketyl radical was generated upon irradiation of solutions of benzophenone and acridine, and that its predominant mode of disappearance was by reaction with... 

In compiling the information in this chapter, I have relied heavily on several very comprehensive reviews that have appeared over the past few years [1-7]. In particular, the 1978 review by T irro et al. [1] is extremely thorough in describing the intra- and intermolecular photophysics and chemistry of upper singlet and triplet states. In fact, rather than reproduce the same details here, I direct the reader to this review for a summary of upper state behavior reported prior to 1978. (A description of azulene and thione anomalous fluorescence is included since these systems are the best-known systems that display upper state behavior.) I also direct readers to the reviews by Johnston and Scaiano [2] and Wilson and Schnapp [3] which focus on the chemistry of both upper triplet states and excited reaction intermediates as studied by laser flash photolysis (one- and two-color methods) and laser jet techniques. Also, Johnston s thorough treatment of excited radicals and biradicals [4] and the review of thioketone photophysics and chemistry by Maciejewski and Steer [5] are excellent sources of detailed information. 

The 1,2-cycloaddition reaction can take place in an intramolecular manner (equation 63), although in this example the initial excitation involves the aromatic group . A reaction of a different type is thought to be involved in the first stage of the formation of azulene or naphthalene photodimers from diphenylacetylene (equation 64), though here it is claimed that an intermediate benzocyclobutadiene species has been detected . The intermediate isomer of diphenylacetylene is formed via the triplet state and is relatively long-lived at — 10 °C. The major dimers formed are 1,2,3-triphenylazulene and 1,2,3-triphenylnaphthalene hexaphenylbenzene and octaphenylcubane are also produced . 

Using azulene as a triplet quencher it has been shown that triplet states are not involved in the cis-trans isomerization of stilbene on direct excitation. Triplet-sensitized cis-trans isomerization, however, is observed and proceeds in both directions through a minimum in the triplet potential energy surface at a twisted geometry, often referred to as the triplet phantom state P. ... 

Nickel, B., Klemp, D., The Lowest Triplet State of Azulene hs, and Azulene d% in Liquid Solu tion. II. Phosphorescence and E Type Delayed Fluorescence, Chem. Phys. 1993,174, 319 330. 

The quantum chain process has also been observed in benzophenone-sensitized isomerization of 2,6-dimethyl-2,4,6-octatriene (25) [120]. The effect of azulene on the photostationary state isomer composition suggests that both tt and tc are the stable conformers in the triplet state and are equilibrated. However, equilibration of all excited intermediates is not complete within the lifetime of the excited triplet state ( 50 ns). The triplet lifetime of 1,3,5-hexa-triene (26) is reported as 100 ns (Table 5) [121]. 

In several excellent surveys, Saltiel and co-workers have presented the advances in interpretation of the azulene effect [25, 26, 105, 194, 195]. The evaluation of results from comparative quenching measurements of fluorescence and cis trans photoisomerization under direct and sensitized excitation conditions confirms a singlet pathway for stilbene. For bromo-substituted stilbenes, however, participation of the triplet state has been... 

In the presence of azulene the photostationary state is shifted to the trans side [26,194], It follows that the lowest triplet state is quenched to the trans... 

The azulene effect is sensitive to substituents introduced into stilbene. A comparison of slope/intercept ratios is given for stilbene and several bromostilbenes (Table 8). The results show clearly that the triplet state is... 

With ferrocene as quencher and triphenylene as sensitizer the cis trans isomerization of several 4-nitrostilbenes has been examined [201], From linear plots of ([t]/[c])sens versus [Q], larger slope/intercept ratios were found as compared to direct excitation conditions (Table 8). A cis -> trans pathway partly bypassing the triplet state accounts for this difference (Section VI). Plots of ([t]/[c])scns as a function of the azulene or ferrocene concentrations are shown in Figure 10 for three stilbenes. 

Other quenchers that have been used in the benzophenone-sensitized cis trans isomerization of stilbene are /9-carotene, oxygen, and di-tert-butyl nitroxide. /9-Carotene shifts the photostationary state to the trans side similar to the azulene effect [237]. In the presence of di-tert-butyl nitroxide, a radical quencher, the photostationary state is slightly shifted to the cis side Caldwell and Schwerzel [226] have suggested the involvement of the twisted triplet state and a quenching mechanism other than energy transfer, probably vibrational relaxation to the ground state caused by spin exchange. 

Introduction of one or two phenyl groups in a-position shifts AT to the red (and prolongs tt) [299], Two transients with different AT and rT for 2-StN were attributed to two conformers [410,421], as already indicated by the effect of azulene on the sensitized photoequilibrium [422]. Detailed studies of the triplet of 1-StN and its C1-, Br-, and N02-derivatives have been published [122,199], Differences in the T-T absorption spectra of 2,2 -DNE at low temperatures have been ascribed to changes in the rotameric composition [423], T-T absorption spectra of various ADBs are available [142, 143,145, 146, 235, 411], For example, from a comparison with 3,4-dihydrophenanthrene (iT = 430, 580nm) a transient of an ADB with aryl = 2-naphthyl (1T = 400, 430nm) has been attributed to the trans triplet state [33],... 

This sequence (Eq. 14) can be excluded since on direct excitation of fram-stilbene in fluid solutions, the generation of 3p has been observed neither by laser flash photolysis nor by quenching experiments with azulene [27]. Sensitized excitation of m-stilbene leads to the trans triplet state. This shows that 3p cannot be populated selectively without formation of 3t [412]. Actually, upon sensitized excitation of cis- or trans-stilbene one sees a common triplet state which may well be 3p. Saltiel et al. [194] have argued that 3p is the stable geometry of the stilbene triplet. Whatever the... 

A kinetic analysis of the behaviour of the styrene (40) upon irradiation using oxygen and azulene as quenchers has shown that the lifetime of the triplet state is... 

Early discrepancies between the lifetime of the triplet state of ftwts-stilbene (3/) obtained from pulse radiolysis and azulene quenching studies have been reconciled by a recognition that azulene quenches the twisted triplet (3p) with... 

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