Unimolecular Deactivation Processes

The excess eneigy taken up by light absorption can be dissipated through unimolecular processes either as radiation (emission) or by radiationless transitions. It can also be transferred to other molecules through bimolecular processes. The relative importance of these various processes depends on the molecular structure as well as on the surroundings of the molecules. 

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Obviously, if is small, the term (.Keq + 1)/reduces to 1/. feq- Eqn (2.7) can assume a number of different forms depending on whether activation is catalyzed and the nature and concentration of G and its state of association with the active chain end. The form presented here applies to an uncatalyzed (thermal) activation process and a unimolecular deactivation process characteristic of, for example, ionic polymerizations in which the active species is a paired ion. This represents the simplest case alternative forms can be more complicated, but the principles to be demonstrated are essentially the same. For example, if the activation process is catalyzed, the following approximation, derived by Muller,holds for [ P ] [CE] ... 

The efficiency of these radiative processes often increase at low temperatures or in solvents of high viscosity. Consequently emission spectra are generally run in a low-temperature matrix (glass) or in a rigid polymer at room temperature. The variation in efficiency of these processes as a function of temperature and viscosity of the medium indicates that collisional processes compete with radiative and unimolecular nonradiative processes for deactivation of the lowest singlet and triplet states. 

This equation shows that the reaction rate is neither first-order nor second-order with respect to species A. However, there are two limiting cases. At high pressures where [A] is lar e, the bimolecular deactivation process is much more rapid than the unimolecular decomposition (i.e., /c2[A][A ] /c3[A ]). Under these conditions the second term in the denominator of equation 4.3.20 may be neglected to yield a first-order rate expression. 

Additional experimental investigations and theoretical treatments of collisional deactivation processes have recently been reported from several laboratories,250 253 Temperature effects on the lifetimes of intermediate adducts formed in the 0 -C02 interaction and in other relatively simple processes have been examined by Meisels and co-workers.252 254 Here the theoretical treatment involves application of a modified RRKM approach to the unimolecular dissociation of the adduct and/or of the termolecular collision complex consisting of the adduct plus the deactivating species M,. 

Thus the measured unimolecular radiative lifetime is the reciprocal of the sum of the unimolecular rate constants for all the deactivation processes. The general form of the equation is given by... 

The quantum yield of H atoms (4> ) is measured as the quantum yield of C3H6(<1>C3H ) i-n limit of zero pressure where the col-lisional deactivation process (eq. 32) is negligible compared to the unimolecular decomposition process (eq. 31). Therefore, under appropriate experimental conditions, <1 torr, C3H6 acts as a quantum counter of the H atoms formed. The kinetics involved in reactions 30-33 are expected to give a linear Stern-Volmer plot of 1/ c3H6 vs ptotal This butene scavenger/unimolecular decomposition method has been successfully tested. 

The potential energy surface of the isomerization is discussed in terms of adiabatic and diabatic processes [1-4,12,18,71]. Two-way isomerization in the triplet manifold without a quencher takes place as a diabatic process by deactivation at p. However, as mentioned above, photochemical cis- trans one-way isomerization in the triplet state proceeds by an adiabatic process where the excited state of a starting material, c, undergoes adiabatic conversion to the excited state of the product, t, followed by either unimolecular deactivation to the, ground state of the product, t, or energy transfer to c to give t and c. The isomerization of 5b also proceeds partly by way of an adiabatic process. Deactivation from t occurs as an adiabatic process, but that from p proceeds as a diabatic process [25]. Therefore, two-way photoisomerization usually takes place as a diabatic process, whereas one-way photoisomerization and isomerization... 

DiphenyIbutadiene (19) undergoes ZZ- ZE and ZZ->EE one-way photoisomerization in the triplet state in a quantum chain process [94]. On the other hand, ZE and EE isomers undergo mutual isomerization. However, ZE - EE isomerization proceeds in a quantum chain process, since its quantum yield increases linearly with the total concentration of 19 on 9-fluorenone sensitization. On 9-fluorenone sensitization, all the isomers exhibited the same T-T absorption = 390 nm, Tt 1.6/is) attributable to EE at 1 /is after the laser pulse [94,95]. Furthermore, the photostationary mixture becomes richer in EE with increased initial ZE concentration. Therefore, the isomerization between ZE and EE takes place similarly to that of 5b with a dual mechanism. Thus the triplet state of 19 is composed of EP and EE in equilibrium, where EP means the perpendicular geometry at one double bond and E geometry at the other double bond of the diene (Scheme 5) unimolecular deactivation from EP gives EE and ZE, and bimolecular deactivation from EE with ZE isomer gives solely EE. 

Figure 2, Diagram of the cascade deactivation process note the compe-tit n between colUsional stabilization arid unimolecular reaction...

Now, let us find a little flaw in the theory equation (2-86) predicts only first-order behavior for the unimolecular reaction, something we know in fact is not true at low pressures. The reason for this failure in TST is the assumption of universal equilibrium between reactants and the transition state complex. At low pressures the collisional deactivation process becomes very slow, since collisions are infrequent, and the rate of decomposition becomes large compared to deactivation. In such an event, equilibrium cannot be established nearly every molecule which is activated will decompose to product. However, the magnitude of the rate of decomposition of the transition complex is much larger than the decomposition of the activated molecule in the collision theory scheme, so one must resist the temptation to equate the two. Since the transition state complex represents a configuration of the reacting molecule on the way from reactants to products, the activated molecule must be a precursor of the transition state complex. 

In general, if there are i first-order, unimolecular decay processes that deactivate an excited singlet state, each with rate constant kf, then the observed singlet lifetime (ts) is... 

T = quantum efficiency for triplet, T, formation Kp = rate of phosphorescence Kuo = rsite of radiationless unimolecular Ti deactivating processes Kpq = rate of phosphorescence bimolecular quenching to = intensity of the excitation radiation 8/c=absorbance of the sample solution c=analyte concentration /= pathlength through sample for absorption 8 = molecular absorptivity. 

Some examples of potential converging LH nanoantennas based on QDs as ET acceptors were investigated by Matloussi and coworkers (Figure 29). Different organic donors were examined, and the ET process resulted to be, in aU cases, too slow to compete with the unimolecular deactivation of the donors and hence quite inefficient. 

Fig. 1 Unimolecular and bimolecular deactivation processes of an excited state.

Those giving deactivation by reversible coupling and involving a unimolecular activation process as shown in Scheme 62. is a propagating radical (an active chain). The deactivator (X) is usually, though not always, a stable radical. However, X may also be an even electron (diamagnetic) species, for example, diphenylethylene. In this case Pn-X would be a persistent radical, or a transition metal complex, for example, a low-spin cobalt (II) complex. These systems are discussed in Section 3.04.6.4. Possibly the best-known process is NMP (Section 3.04.6.4.2). 

Excited molecules Pf and A (e) can participate in various deactivation processes upon collisions with other molecules. Usually unimolecular reactions involving excited molecules and A (e) are carried out under such experimental conditions where no collision with other molecules occurs within the time of the unimolecular reaction. However, electronically excited molecule A can participate in non-radia-tive intramolecular transitions flx)m one potential ener surface to another. Let us assume for determinacy that in process 1 molecule A is formed in the first singlet electronic state (designate A" as A(5))). Molecule A(5i) can participate in non-adia-batic non-radiative processes and transform into the triplet electronic state Ti and ground electronic state... 

It is useful to interpret Eqs. (2.6)-(2.9). The dissociation rate at low pressure is equal to /ci[M], the rate of collisional activation [Eq. (2.6)]. At high pressures, the collisional activation/deactivation processes establish an equilibrium ratio of AB and AB, described by the rate-coefficient ratio ki/k-i, and the unimolecular dissociation process of AB, reaction (2), becomes rate determining [Eq. (2.8)]. In recombination at low pressures, association and redissociation of AB are much more frequent than collisional stabilization, such that an equilibrium between A, B, and AB is established, as described by the rate-coefficient ratio fc 2//c2 Collisional stabilization of AB, reaction (-1), is then rate determining [Eq. (2.7)]. At high pressures, collisional stabilization is so frequent that the rate of association of A and B, reaction (2), determines the recombination rate [Eq. (2.9)]. 

When only unimolecular (first order) processes serve to deactivate the fluorescent state (e.g., at very low concentrations) the quantum yield becomes... 

Therefore, the overall rate of deactivation of Si, QJtotai, is given by the sum of the rates of the unimolecular and bimolecular processes ... 

Rate constants of unimolecular processes can be obtained from spectral data and are useful parameters in photochemical kinetics. Even the nature of photoproducts may be different if these parameters change due to some perturbations. In the absence of bimolecular quenching and photochemical reactions, the following reaction steps are important in deactivating the excited molecule back to the ground state. 

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