Azulene, absorption fluorescence

This behaviour may be explained by considering that the azulene molecule has a relatively large S2-Si gap, which is responsible for slowing down the normally rapid S2 to Si internal conversion such that the fluorescence of azulene is due to the S2 —> S0 transition. The fluorescence emission spectrum of azulene is an approximate mirror image of the S0 — S2 absorption spectrum (Figure 4.6). 

Figure 4.6 Absorption (continuous line) and fluorescence (dashed line) spectra of azulene...

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. 

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. 

Figure 3.29 (a) Outline of the absorption, A fluorescence, F and phosphorescence, P spectra of a rigid polyatomic molecule. X = wavelength, vertical axis = absorbance (A) or emission intensity (F, P). (b) The Stokes shift of the absorption and fluorescence spectra is defined as the difference between their maxima. When this shift is small, there is a substantial spectral overlap between absorption and emission, (c) Jablonski diagram and outline of the absorption and fluorescence spectra of azulene, an exception to Kasha s rule. The energy gap between S0 and Sj is very small, that between Sj and S2 is very large... 

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... 

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. 

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-... 

Figure 5.18. Absorption (A) and fluorescence spectrum (P) of azulene in ethanol at 93 K. FP and AP(P) denote the polarization spectrum of fluorescence and the excitation-polarization spectrum of fluorescence, respectively (by permission from Ddrr, 1966).

The absorption and fluorescence spectra of azulene (1) and cycl[3.3.3]azine (2) are shown in Figures 4.22 and 4.23. The fluorescence emissions are from the second excited singlet state S2, in violation of Kasha s rule (Section 2.1.8), due to the large energy gap between the Si and S2 states in these compounds. 

Figure 4.22 Absorption (adapted from280) and fluorescence ( ) emission spectra of azulene (1)...

Exceptions to Kasha s rule can not only be found with azulene and other compounds [37], where emission from S2 is observed (typically because the energetic difference between the S2 and Si states is sufficiently large to reduce the S2-S1 internal conversion to values close to the S2-S0 radiative rate), but also when there is competition between vibrational relaxation and photochemistry, the so-called vibronic effect. This is an important concept that has been recently developed contrasting with the general wisdom in photochemistry, that only very few exceptions to Kasha s rule exist. The foundations of the vibronic effects were found in 1966 when Ralph Becker and Joseph Michl noticed that the fluorescence excitation spectrum of a photochromic compound, 2,2-diethylchromene (see Scheme 15.4), was significantly different from the absorption spectrum [38]. 

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