Radiationless conversion

Si emission at 823 nm when excited at 340 nm. The relationship between the number of sulfur atoms and emission strength suggests that the Si excited state rapidly decays through radiationless conversion to sulfur n-ji states and then to the ground state (50). 

Until recently the lifetimes of the triplet states of aromatic hydrocarbons in fluid solution at room temperature had been investigated exclusively by the technique of flash absorption spectroscopy. The lifetimes reported for many hydrocarbons, e.g., anthracene or phenan-threne, had been below 1 msec, and it had been assumed that radiationless conversion processes were so rapid under these conditions that the competing radiative triplet-singlet transition would be too slow to per-... 

At low rates of light absorptionn (Ia) the rate of triplet self-quenching (eq. 31) is small compared with the rate of its radiationless conversion (eq. 30) and under these conditions ... 

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. 

G. Porter, Reactivity radiationless conversion and electron distribution in the excited state in Reactivity of Photoexcited Organic Molecules, Proc Thirteenth Conf. on Chem. at the Univ. of Brussels, October 1965. New York Wiley, 1967. p. 80. 

The condition EM implies a small displacement for each normal mode and therefore for the potential surfaces of the electronic states. This condition is recognized as the weak coupling limit of electronic states and Equation 6.75 gives the rate of radiationless conversion in this limit. 

Note the similarity to the energy gap law for radiationless conversion of an excited state. Compare normal re on. 

Absorption spectra of electronically excited states may be observed in flash photolysis studies. Porter has established the existence of the triplet state in a wide range of organic compounds in the liquid and gaseous phases. For example, the first triplet state of anthracene is populated by radiationless conversion from a photochemically excited singlet molecule, and may be observed by the absorption to the second triplet level. Absolute measurements of the triplet concentration may be made by determinations, from the absorption spectra, of the depletion of the singlet state. Similar results have been obtained with a variety of hydrocarbons, ketones, quinones and dyestuffs. 

We have presented here the first observation of transient molecular reorientation induced in a liquid crystal by a -switched laser pulse. The response time of molecular reorientation in the nematic phase is of the order of 10—100 psec. Although this is 10 —10 times longer than the duration of the laser pulse, transient molecular reorientation is still strong enough to yield an easily detectable phase shift in the probe beam. Residual al> sorption and subsequent very rapid radiationless conversion into heat can result in a temperature rise in the medium which decays via heat diffusion with relaxation times in the 10—200 msec range. The temperature rise also induces a refractive-index change in the medium and hence a phase shift in the probe beam. This thermal effect and the molecular reorientation are initiated simultaneously by the pulsed laser excitation. They are in general coupled... 

In Figure 16,1, we also show radiationless conversion of T to So, but this is not common. Similarly uncommon is direct absorption from Sq to Ti. It is technically not impossible, just extremely inefficient, with e values on the order of 10 -10. In a few cases it has been useful for mechanistic studies, but it is generally not preparatively useful. 

Fig. 6.2. Radiationless conversion in a molecule with a high density of vibronic states. As Qf Qi, k. See text for estimates of Qf as a function of the number of atoms in a molecule.

Fig. 6.3. Electronic energy levels in anthracene. The radiationless conversion rate is much larger than that for 7 reflecting the effect of energy differences.

If excitation is to a repulsive state (Fig. 6.4a), dissociation is immediate, occurring within one vibrational period, t < 10 sec. Another rapid process involves predissociation (Fig. 6.4b). Here the molecule is excited to a bound state which, because there is a resonant continuum level of the repulsive electronic state, undergoes radiationless conversion followed by immediate dissociation. Excitation could also be to a metastable vibronic state (Fig. 6.4c) which dissociates via tunneling. The dissociation probability would be very sensitive to the height and the width of the barrier to be overcome, and the decomposition rate greatly enhanced by vibrational excitation in the excited state. Finally, as in Fig. 6.4d, excitation may be to a bonding state but into a continuum level above the dissociation limit decomposition is again immediate. 

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