Electronic spectra azulene

Pariser, R., J. Chem. Phys. 25, 1112, "Electronic spectrum and structure of azulene."... 

The lowest excited state of azulene is predicted to possess 2 symmetry, which is in agreement with the result obtained using the symmetry rule. A recent vibrational analysis of the longest wave-length absorption band in the electronic spectrum of azulene indicates that the lowest-excited state would possess C2 symmetry . ... 

As mentioned for the relationship between the PE spectrum of a parent molecule and the electronic spectrum of its radical cation, any close correspondence between the electronic spectra of anions and cations or their hyperfine coupling patterns holds only for alternant hydrocarbons. The anions and cations of nonalternant hydrocarbons (e.g., azulene) have significantly different hyperfine patterns. Azulene radical anion has major hyperfine splitting constants (hfcs) on carbons 6, and 4,8 (flH = 0-91 mT, H-6 ah = 0-65 mT, H-4,8 ah = 0-38 mT, H-2) in contrast, the radical cation has major hfcs on carbons 1 and 3 (ah = 1.065 mT, H-1,3 Ah = 0.152 mT, H-2 ah = 0.415 mT, H-5,7 ah = 0.112 mT, H-6). °°... 

The most striking feature of azulene and its derivatives is the intense blue colour. The electronic spectrum of azulene is complex and consists of weak, moderate and strong bands at progressively shorter wavelengths 580, 340, 270 nm] [41]. 

Determination of electrochemical oxidation potentials and electrochemical reduction of 13 p-phosphorylated acyclic nitrones shows that phosphorylated compounds have a clear anodic shift of potentials of both, oxidation (Ep 1.40 to 2.00 V versus SCE in CH3CN) and reduction (Ep—0.94 to —2.06 V). This is caused by a strong electron-acceptor influence of the diethoxyphosphoryl group (430). In contrast, a reversible one-electron oxidation of azulene nitrones (233) (Scheme 2.80) occurs 0.6 V below the Ep potential of PBN, that is at the value one observes the oxidation of AH -imidazole-1,3-dioxides (219) (428, 429). In other words, the corresponding RC (234) is 14 kcal more stable than the RC of PBN. Although the EPR spectrum of RC (234) was not recorded, RC (236) from dinitrone (235) turned out to be rather stable and gave an EPR spectrum (170). 

The electronic absorption spectrum of 77 is similar to that of azulene.32 This similarity also holds for the ESR spectra of the corresponding anions.54... 

Mason and Smith (1969) found that for a series of mono- and bicyclic aromatic hydrocarbons the changes in the fluorescence spectrum with acidity reflected the ground state protonation reaction. The p Sj )-values calculated for benzene, toluene, naphthalene, azulene, and indolizine do not correspond to observable processes since the rate of protonation is too slow to compete with deactivation of the Sj state. Photochemical deuterium and tritium exchange experiments in 1 mole dm-3 perchloric acid indicate that the radiative deactivation rate of an electronically excited aromatic hydrocarbon is faster than the rate of protonation by a factor >10s. 

In Figure 5.18 the absorption and emission spectra of azulene are shown. The anomalous fluorescence of azulene from the S, state is easy to recognize. The AP(F) spectrum exhibits a deep minimum at 33,900 cm. The small peak in the absorption spectrum at the same wave number is therefore not due to vibrational structure but rather to another electronic transition, the polarization of which had been predicted by PPP calculations. Figure 5.19 shows all four types of polarization spectra of phenanthrene. FP becomes negative at the vibrational maxima of the fluorescence the most intense vibration is not totally symmetric, in contrast to the one which shows up weakly. For all absorption bands, AP(P) = -0.3. The polarization direction of phosphorescence is perpendicular to the transition moments of all transitions lying in the mo-... 

The H-n.m.r. spectrum of azulene, with signals in the range 6 6.92-8.12, clearly indicates that it sustains a diamagnetic ring current [46]. The coupling constant.s are also in accord with this. The C-n.m.r. spectrum shows chemical shifts which correlate with the calculated electron densities for the different sites [17]. 

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... 

Excited-state absorption (ESA) is a type of electronic spectroscopy for which only a few theoretical studies have been performed (for a recent study on the singlet-singlet ESA spectrum of azulene, see Ref. 130 and for some benchmark calculations on small molecules, see Ref. 131). From the experimental point of view ESA is a very important topic in photophysical (kinetic) investigations of energy, electron or hydrogen-transfer processes. Although in principle all excited states of a molecule show a distinct electronic absorption spectrum, only the ESA spectra of the lowest excited state in each multiplicity (i.e.. Si or Ti) are usually accessible experimentally. Because of the short... 

Later, the [ , ] isomer 113 was synthesized by Oda and Kuroda s group using a modified Hafner azulene approach. A thermal ring-closure reaction offulvene 114 provides 113 in 2% yield (Scheme 6.27). The H-NMR spectrum of 113 indicates the presence of an aromatic 14-Jt electron ring current [60]. 

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