Azulene excitation energy

Equally interesting is the situation in the second class of compounds studied (analogues of non-alternant hydrocarbons), which is best divided into two sub-groups analogues of the tropylium ion and analogues of azulene. The empirical correlation of experimental and theoretical excitation energies studied requires a further subdivision into compounds with one heteroatom (e.g. thiopyrylium ion) and two heteroatoms, either adjacent (e.g., 1,2-dithiolium ion) or non-adjacent (e.g., 1,3-dithiolium ion). Experimental and theoretical data are presented in Table VII. Table VIII summarizes data for the derivatives of dithiolia. Figure 15 shows the absorption curves of 1-benzo-... 

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. 

Flints The assumption that smaller HOMO-LUMO gaps will produce more strongly colored products is true to a first approximation, but not good enough for this problem. In the compounds under study, the HOMO-LUMO gap is largest in naphthalene, which could well explain its lack of color. However, the gap is very similar in anthracene and azulene. To understand their color differences, we need a much more accurate estimate of their excitation energies. 

Another example that illustrates the effect of the overlap density i/z/i/zy upon the magnitude of A /y involves two conjugated hydrocarbons. Listed in Table 8.1 are the experimental values of the first ionization potential (IP), the electron affinity (EA), the first singlet excitation energy (E Eq), and the first triplet excitation energy (Ex - Eg) for azulene 8.7 and anthracene 8.8. For simplicity of notation, the... 

TABLE 8.1. The Ionization Potentials, Electron Affinities, and Excitation Energies of Azulene and Anthracene... 

Despite the fact that the HOMO and LUMO energies of the two molecules are virtually identical, azulene is blue but anthracene is colorless (i.e., s — Eg = I -8 and 3.3 eVfor 8.7 and 8.8, respectively). To explore the cause of this difference we note from equations 8.42 and 8.48 that the singlet excitation energy is given by... 

This prediction was substantiated experimentally for several donors with triplet excitation energies in the 45-68 kcal per mole region. Besides establishing the existence of a trans triplet in solution, the results with azulene allowed the evaluation of the ratio kn/(kis - - kig) = 50 liter mole. Since, by analogy to the quenchii of anthracene (Ware, 1962) and benzophenone (Hammond and Leermakers, 1962a) by azulene, the value of k would be expected to be at least close to that for a diffusion-controlled process, it is possible to estimate the effective lifetime of stilbene triplets, (kjg -j- fc ) to be of the order of 10 sec. 

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-... 

Fluorescence always occurs from the lowest singlet state even if the initial excitation is to higher energy state (Kasha s rule). Azulene and some of its derivatives are exceptions to this rule. Because of vibrational relaxation of initially excited vibronic state, the fluorescence spectrum may appear as a minor image of the absorption spectrum for large polyatomic molecules. The shape of the emission spectrum is independent of the exciting wavelength. 

The photodissociation of aromatic molecules does not always take place at the weakest bond. It has been reported that in a chlorobenzene, substituted with an aliphatic chain which holds a far-away Br atom, dissociation occurs at the aromatic C-Cl bond rather than at the much weaker aliphatic C-Br bond (Figure 4.30). This is not easily understood on the basis of a simple picture of the crossing to a dissociative state, and it is probable that the reaction takes place in the tt-tt Si excited state which is localized on the aromatic system. There are indeed cases in which the dissociation is so fast (< 10-12 s) that it competes efficiently with internal conversion. 1-Chloromethyl-Np provides a clear example of this behaviour, its fluorescence quantum yield being much smaller when excitation populates S2 than when it reaches Figure 4.31 shows a comparison of the fluorescence excitation spectrum and the absorption spectrum of this compound. This is one of the few well-documented examples of an upper excited state reaction of an organic molecule which has a normal pattern of energy levels (e.g. unlike azulene or thioketones). This unusual behaviour is related of course to the extremely fast dissociation, within a single vibration very probably. We must now... 

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. 

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