Azulene decay

Calculations explain many aspects of the complex S/Sq azulene decay. We have so far not been able to identify a third channel operating at low excess energies. This kind of disagreement is almost certainly related to the limitation of the MMVB potential and conld be overcome if it was possible to carry out dynamics at a mnch higher level of theory... 

Evidence that eliminates the triplet mechanism as the mode for the cis-trans isomerization of stilbene upon direct photolysis has been provided by azulene quenching studies.(48) Using the experimentally determined decay ratio a/(l — a) and the triplet mechanism, it is possible to calculate what the effect of azulene is upon the pss. The predicted and observed azulene effects on the direct photoisomerization are shown in Figure 9.6. The failure of the triplet mechanism in predicting the very small changes observed in the pss provides a crucial test that is the basis for rejecting the triplet mechanism. 

Emission from the upper electronic excited states of polyatomic molecules, in violation of Kasha s rule which allows emission only from the lowest excited states Q), have been observed in a reasonable variety of molecules (2 11). With notable exceptions of azulene and thioketone, however, such emission is usually very weak, because the rates of nonradiative decay processes greatly exceed the rates of radiative processes when excited states other than the lowest excited states are involved. 

Azulene is one of the most interesting molecules from the photophysical point of view. A picosecond measurement of the vibrational energy decay in matrix isolated polyatomic molecules shows at 4K that molecular modes in a polyatomic matrix does not affect decay in the 2 vibrational manifold . The S2 Sq fluorescence of pseudoazulenes has been studies in Shpolski... 

The low quantum yield indicates that intermediate complexes and diradicals decay unproduc-tively. In general, Norrish type II photoreactions and other hydrogen abstraction processes must be overcome in order to achieve successful cycloaddition. Only in a few reported cases is intramolecular hydrogen abstraction a serious competitive reaction path. For example, the cycloheptenyl-substituted ketone 5 yields an oxetane 6 in addition to a cyclobutanol derivative 758, whereas the unsaturated cycloheptanone 8 only gives oxetane products 9 and 10 on irradiation59. The main product 10 was converted in two steps to azulene in 25 % overall yield. The reaction sequence 11 - 12 - 13 also demonstrates the high synthetic potential of the intramolecular Paterno-Biichi reaction61. 

Two DHPs were obtained from tetramethoxystilbene [279]. Substitution by a nitro group in the meta or para position reduces distinctly [82], Saltiel et al. have questioned whether the values may be erroneous [105], On the basis of quenching measurements with azulene they proposed additional routes for bromostilbenes from c via excited states of DHP which may relax back to 3c or c. A consequence of a higher value of for the mechanism of cis -> tram isomerization is that the ratio of c decaying to the trans isomer may have to be reexamined. For trans-ct-bromostilbene and the / -phenyl substituted derivatives several photoreactions (e.g., debromination) compete with photocyclization [475]. Interestingly, no evidence for photocyclization could be found for several fluorinated stilbenes [481]. Rotamers can be distinguished in the cyclization of c/s-2,2 -DNE [482],... 

Attempts to study further some mechanistic aspects of the triplet decay by using additives with low triplet energies led initially to confusion. It turned out that two groups of quenchers exist. One group of classical triplet quenchers (e.g., azulene, ferrocene, and /i-carotene) quench preferentially 3t to 11 3c is not accessible because of its very short lifetime. Molecules of the second group (e.g., oxygen and di-tert-butyl nitroxide) quench preferentially 3p to p via a spin-exchange mechanism (Section... 

Figure 2.26 Reaction profile for the excited-state decay of azulene through a conical intersection.

Beer and Longuet-Higgins (Beer, M., Longuet-Higgins, H.C., J. Chem. Phys., 23,1390-1391,1955) were the first to observe that the Si state of azulene is nonfiuorescent and suggest that this was due to a nonradiative decay at a real crossing. 

In view of the well-documented ability of Oj to function generally as an acceptor of triplet energy, it had been expected that its presence should also increase ([t]/[c])3 ratios for the triplet sensitized photoisomerization of stilbene-like olefins. However, as was first noted for nitrostilbenes, the quenching of the olefin triplets by O2 does not alter ([t]/[c])s (42,43). For the parent stilbene it was shown that when azulene is used as a quencher of stilbene triplets the slope of the ([t]/[c])s-vs. -[Az] plot is strongly attenuated by O2 see Figure 5 (41,44). Since the intercept of the azulene plot was not influenced by O2, it was reasoned that O2 deactivates p without changing the decay firaction 8 as predicted by Schemes 1 and 2 (44). To account for stilbene triplet deactivation by O2 without change in 8 two possibilities were considered ... 

Sahlstrom et al. [60] showed that the thermal detachment of an electron from an anion is observed readily in an ion mobility spectrometer. At an appropriate temperature, anions formed in the source decompose by thermal electron detachment in the drift region. The electrons move rapidly in the electrostatic field to the detector plate and their intensity at arrival time is a measure of the number of anions disappearing at that time. The resulting spectrum, of the form of Figure 13.2d, shows an elevated baseline that has a maximum at zero time, that is, for electron detachment at the shutter where the anion concentration is highest, and terminates at the peak for survivor anions. Examples of the mobility spectra obtained for thermal electron detachment from the azulene anion at different temperatures are shown in Figure 13.9. The Cl" peaks in the spectra are due to background ions formed in the source and do not interfere with the analysis. The exponential decay of the elevated baseline is described by... 

Figure 3.21 Azulene conical intersection for radiationless decay. Three VB structures are shown together with corresponding bond lengths (1) Si minimum, with quite localized double bonds around the ring and across the bridge, (2) So minimum, with the electron density completely delocalized around both rings but not across the transannular bond, and (3) the Si/So conical intersection which is a delicate balance between the previous two structures but dominated more by the Si structure. The most important structural change is associated with the transannular bond, which is a double bond in Si and a single bond in So-...

Bearpark M, Bemardi F, CUlibrd S, et al. The azulene SI state decays via a conical intersection a casscf study with MMVB dynamics. J Am Chem Soc. 1996 118 169-175. Bearpark M, Bemardi F, Ohvucci M, Rohh M. Potential energy surfaces ofpseudoar-omatic molecides an MMVB and CASSCF study of pentalene. IntJ Quantum Chem. 1996 60 505-512. 

Direct dynamics calculations of the type just described, with all degrees of freedom included, are very expensive if the local quadratic approximations to the potential energy surface are obtained from an ab initio computation. In applications we have used a hybrid parameterized quantum-mechanical/force-field method, designed to simulate the CASSCF potential for ground and covalent excited states. A force field is used to describe the inert molecular a-framework, and a parameterized Heisenberg Hamiltonian is used to represent the CASSCF active orbitals in a valence bond space. Applications include azulene and benzene excited state decay dynamics. 

Azulene is the best-known exception to Kasha s rule and serves as a model for other nonaltemant aromatic compounds, which also exhibit anomalous fluorescence from their second excited singlet states. This anomalous anission of the Sj state was first observed unambiguously by Beer and Longuet-Higgins [2] and has been confirmed many times in more recent studies [15,16]. The second excited singlet state of azulene has a lifetime of ca. 1 to 2 ns in both the gas phase and in solution, and exhibits dual emission, decaying radiatively to S, with a minute quantum yield (< 10" ) [17,18] and to So with a quantum yield most recently determined to be 0.041 (in ethanol at room temperature) [16]. Earlier studies placed the Sj - Sq fluorescence quantum yield near 0.03 [19,20]. Small et al also recently measured the quantum yield of azulene s Sj-Sq nonradialive decay using a completely independent... 

Most of the photophysical data for azulene and its simple derivatives have come from measurements of the quantum yields of Sj - Sq fluorescence, ( )s2, and time-domain measurements of the lifetimes of the fluorescent Sj state, Xjj, in fluid solution. The first order rate constants for the parallel radiative and nonradiative processes that depopulate Sj are then determined from k = ( )s2/Xs2 and Ek, = (1 - ( )s2)/ s2- Whereas Sj - Si internal conversion is clearly the major, and perhaps the exclusive, pathway for Sj s nonradiative decay in azulene itself, other nonradiative processes can be expected to occur in some substituted azulenes. 

The Sj radiationless transition rate is sensitive to both solvatochromic and substituent-induced shifts of the Sj - Sj energy gap. Figure 1.2 shows that an almost linear dependence of the logarithm of the rate constant of radiationless decay on the Sj - Sj energy gap is observed for a series of 1-and 1,3-fluorine-substituted azulenes in a range of solvents, as expected if the energy gap law of radiationless transition theory applies and internal conversion is the sole Sj decay process. 1,3-Difluoroazulene in ethanol... 

Photophysical Data for the Radiative and Nonradiative Decay of the S2 State of Azulene and its Derivatives in Several Solvents... 

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