Photophysical process rate

The competing intramolecular photophysical processes that can occur from Si(v0) are fluorescence, intersystem crossing and internal conversion, with first-order rate constants of kf, kisc and kic, respectively (Figure 3.3). 

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. 

Observable photophysical parameters, their relationship to rate constants of various photophysical processes and sources of their information. 

The rate constants for unimolecular photophysical processes in few representative organic molecules are given in the Table 5.4. These give an idea of the order of magnitude expected for various processes. 

Rate constants for unimolecular photophysical processes in some organic molecules... 

Kinetics of Energy and Electron Transfer. A semi-quantitative estimate for the rate constants of the various photophysical processes can be obtained from fluorescence quenching. Based on the quenching ratios of the OPV fluorescence and the OPVn singlet excited state lifetimes, the rate constants for energy transfer reactions in toluene solutions were estimated to lie between 1.1 x 1012 and 2.1 x 1012 s-1 for OPV3 Cgo and OPV4 Cgo (Table... 

In equation (1) K y is referred to as the Stern-Volmer constant Equation (1) applies when a quencher inhibits either a photochemical reaction or a photophysical process by a single reaction. <1>° and M° are the quantum yield and emission intensity (radiant exitance), respectively, in the absence of the quencher Q, while <1> and M are the same quantities in the presence of the different concentrations of Q. In the case of dynamic quenching the constant K y is the product of the true quenching constant kq and the excited state lifetime, t°, in the absence of quencher, kq is the bimolecular reaction rate constant for the elementary reaction of the excited state with the particular quencher Q. Equation (1) can therefore be replaced by the expression (2)... 

Because the spin-lattice relaxation between the three ZF levels (Ty, j = x, y, z) is much slower than other photophysical processes at 1.6 K, a set of rate equations may be derived for the following biphotonic mechanism ... 

Table 5.1 Relations Between Quantum Yield, Lifetime, and Rate Constcmt of Unimolecular Photophysical Processes...

Table 5.2 Observable Photophysical Parameters and their Relationship to Rate Constants of Various Photophysical Processes and Sources of their Information ...

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