Azulene, fluorescence from

According to Kasha s rule, fluorescence from organic compounds usually originates from the lowest vibrational level of the lowest excited singlet state (Si). An exception to Kasha s rule is the hydrocarbon azulene (2) (Figure 4.5), which shows fluorescence from S2. 

These discussions provide an explanation for the fact that fluorescence emission is normally observed from the zero vibrational level of the first excited state of a molecule (Kasha s rule). The photochemical behaviour of polyatomic molecules is almost always decided by the chemical properties of their first excited state. Azulenes and substituted azulenes are some important exceptions to this rule observed so far. The fluorescence from azulene originates from S2 state and is the mirror image of S2 S0 transition in absorption. It appears that in this molecule, S1 - S0 absorption energy is lost in a time less than the fluorescence lifetime, whereas certain restrictions are imposed for S2 -> S0 nonradiative transitions. In azulene, the energy gap AE, between S2 and St is large compared with that between S2 and S0. The small value of AE facilitates radiationless conversion from 5, but that from S2 cannot compete with fluorescence emission. Recently, more sensitive measurement techniques such as picosecond flash fluorimetry have led to the observation of S - - S0 fluorescence also. The emission is extremely weak. Higher energy states of some other molecules have been observed to emit very weak fluorescence. The effect is controlled by the relative rate constants of the photophysical processes. 

Some fluorescence lifetimes are observed in ps times, although these are unusual cases. In organic molecules the Sj—S0 fluorescence has natural lifetimes of the order of ns but the observed lifetimes can be much shorter if there is some competitive non-radiative deactivation (as seen above for the case of cyanine dyes). A few organic molecules show fluorescence from an upper singlet state (e.g. azulene) and here the emission lifetimes come within the ps time-scale because internal conversion to S and intersystem crossing compete with the radiative process. To take one example, the S2-S0 fluorescence lifetime of xanthione is 18 ps in benzene, 43 ps in iso-octane. 

Azulene has weak absorption in the visible region (near 7000 A) and more intense band systems in the ultraviolet. The first ultraviolet system, which commences at about 3500 A, has been examined in substitutional solid solution in naphthalene (Sidman and McClure, 1956) and in the vapour state (Hunt and Ross, 1962), and can be observed in fluorescence from the vapour (Hunt and Ross, 1956). Theory predicts that the transition is 1Al<-lAl(C2K), i.e. allowed by the electronic selection rules with polarization parallel to the twofold symmetry axis (see, e.g., Ham, 1960 Mofifitt, 1954 Pariser, 1956b). The vibrational analysis shows that the transition is allowed but does not establish the axis of polarization. The intensity distribution among the vibrational bands indicates a small increase in CC bond distance without change in symmetry. 

The well known anomalous fluorescence from S2 has been interpreted in terms of a much slower radiationless transition out of S2 than Si, such that for Si the fluorescence lifetime is severely shortened relative to the radiative lifetime. The anomaly is related to the unusual energy disposition of the two lowest excited singlet states. Hochstrasser and Li wished to ascertain whether the spectral linewidths were consistent with this interpretation and also whether the Si linewidths of azulene-ds were narrowed in comparison, as theoretically predicted. Their results are listed in Table 1. The spectral resolution was claimed to be <0.15 cm-1 as linewidths in the S2 system corresponding to the observed fluorescence lifetime are of the order of 10-4 cm-1, the linewidths of 0.50 cm-1 measured must be considered crystal-imposed. It is assumed that the maximum crystal inhomogeneity contribution to the Si linewidth is similarly 0.50 cm-1. This leads to a line broadening due to rapid nonradiative electronic relaxation of 1.61 (-hs) and 1.27 (-da) cm-1 as compared to 0.64 cm-1 (-hs) determined by Rentzepis 50> from lifetime studies of azulene in benzene solution at 300 K. 

Y. Zhang, K. Aslan, M. J. Previte, and C. D. Geddes. Metal-enhanced S2 fluorescence from azulene Chemical Physics Letters 2006,432, 528-532. 

Zhang, Y. X., Aslan, K., Previte, M. J. R., and Geddes, C. D. (2006). Metal-enhanced S-2 fluorescence from azulene Chemical Physics Letters 432 528-532. 

The raditionless processes S wS, and T a -T are usually so fast that lifetimes of higher excited states are very short and quantum yields of emission from higher excited states are very small. In the vast majority of cases luminescence is observed exclusively from the lowest excited state. This so-called Kasha s rule is of course relative in that what is observed depends on the sensitivity of the detector. For benzenoid aromatics, fluorescence from higher excited states in addition to fluorescence from the lowest excited state was observed for the first time in 1969 (Geldorf et al., 1969), whereas the fluorescence from the S2 state of azulene had been known for quite some time. (See below.)... 

A case study resonance Raman scattering and fluorescence from Azulene in a Naphthalene matrix... 

Klemp, D., Nickel, B., Relative Quantum Yield of the S2 > S, Fluorescence from Azulene,... 

The photophysical properties of polyacene molecules depend markedly on the number of rings and the fluorescence behaviour of hexacene has now been compared with that of earlier members of the series. A similar comparison has been made of the photophysical properties of catacondensed aromatic poly-cycles. Fluorescence from an upper-excited singlet state has been described for benz[a]azulene derivatives while the fluorescence properties of some antiaromatic molecules have been described in detail. Several thiopyrylium and pyrylium salts have been studied and the effects of various substituents attached to the heterocycle have been examined in terms of the triplet yield. A full evaluation of the photophysical properties of 4-aminonaphthalimide, and its... 

The phosphorescence and fluorescence spectra of compound (23) have been recorded <89CBi 119>. The electronic and fluorescence spectra have been measured for azuleno[l,2-f>]furan (144), which is an iso-/7-electron system of benz[a]azulene and presents anomalous fluorescence from the second excited singlet state, as does azulene <87SA(A)1377). Fluorescence spectral properties of a series of 2-(4-substituted phenyl)benzofurans have been investigated to determine their applicability as organic reagents for analysis <85bcj2192>. 

So far, we have considered only the excitation to 5i. If the molecule is excited to a higher electronic state S2, S3, etc.), in the overwhelming majority of cases, it reaches the lowest excited singlet state S on the picosecond timescale by a cascade of nonradiative (vibration relaxation) processes and only then can the emission of a photon occur. There exist only a few exceptions from this rule, e.g., azulene [7], which exhibits fluorescence from 2 ... 

The molecule often cited as the exception that proves Kasha s Rule is azulene, that fluoresces preferentially to So from its second excited singlet, [4, pp. 8,22-23], [6, 147-148]. The anomaly has been ascribed to a rather large S1-S2 energy gap and to a remarkably weak fluorescence from Sj, that cannot compete with vibronically induced internal conversion to So and subsequent relaxation to its vibrational ground-state. It is clear, however, that orbital symmetry cannot be an insignificant factor. 

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... 

Fujimori and coworkers [42 4] studied several azulene derivatives, including azulene[l,2-b] furan, azulenopyridines, and azulenofuran, all of which exhibit anomalous fluorescence from the second excited singlet state. The emission quantum yields observed for these compounds were quite small, in the 10" to 10" range. 

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