Radiationless decay rate constants

The MLCT emission properties of many rhenium(I) carbonyl diimine complexes have been used to demonstrate the energy-gap law. For example, the complexes [Re(N-N)(CO)3(L)] (N N = bpy, phen, 5-Ph-phen L = substituted pyridines or quinoline) displayed MLCT (d7r(Re)- 7t (N-N)) phosphorescence at room temperature. The radiationless decay-rate constants depended on the emission energy as described in the energy-gap law (Inknr A-B fern) From the Hammett constant of the substituent, both the emission energy and hfetime of related complexes can be predicted. ... 

We consider three decay channels for D in addition to injection Fluorescence (rate constant k ), intramolecular radiationless decay (rate constant k ), and energy transfer quenching within the adsorbed layer (rate constant kg) ... 

Marginal fluorescence quantum yields (1%) are generally observed though 25 and 33 fluorescence with 8% and 14% yields, respectively. Such low quantum yields are indicative of the effective competition of radiationless processes such as the Si —> Tj ISC and fast internal conversion (Si —> S0). The rate constants for radiative decay of Si (kF) range from 8 x 106 to 1.3 x 108 s-1, and the nonradiative decay rate constants (fcNR) range from 1.9 x 108 to 3.5 x 109 s / The nonradiative deactivation pathway is thus six times faster than the radiative one for 33 (anti) and about 110 times faster for 32 (syn). 

The rates of radiationless transitions between electronic states of porphyrins and their derivatives play a dominant role in their photochemistry because they are the major decay channels of the electronically excited states. Radiative channels, such as fluorescence, rarely exceed 10% of the overall decay rate constant at room temperature. The lifetimes of the lowest electronic states of free-base porph3nins and closed-shell metalloporphyrins vary by more than 10 orders of magnitude with the nature of the substituents. The understanding of such variations is essential to design and control the photochemistry of porphyrins and justifies an incursion on the fundamentals of radiationless transitions. 

The quantum yield can be close to unity if the radiationless decay rate is much smaller than the rate of radiative decay, that is, or < < r. We note that the energy yield of fluorescence is always less than unity because of Stokes losses. For convenience, we have grouped all possible nonradiative decay processes with the single rate constant Av. 

The magnitude of the values of 1/t indicates that the original assumption that radiationless decay of the triplet back to the ground state is unimportant in these reactions is correct since the triplet states of aliphatic ketones commonly have rate constants for radiationless decay from the triplet state on the order of 105 sec-1 (t = 10 5 sec).(8>... 

The development of comprehensive models for transition metal carbonyl photochemistry requires that three types of data be obtained. First, information on the dynamics of the photochemical event is needed. Which reactant electronic states are involved What is the role of radiationless transitions Second, what are the primary photoproducts Are they stable with respect to unimolecular decay Can the unsaturated species produced by photolysis be spectroscopically characterized in the absence of solvent Finally, we require thermochemical and kinetic data i.e. metal-ligand bond dissociation energies and association rate constants. We describe below how such data is being obtained in our laboratory. 

From Ah = 0.8x10 1 mole i s i and straightforward kinetics the rate constant for radiationless decay of triplet nitrobenzene could be derived Aat = 0.6 X 10 s i. Thus it is easily unterstood why oxygen quenching (assumed to be diffusion controlled does not affect photoreduction in 2-propanol to a large extent upon irradiation in air-saturated ([O2] 10 M) solutions, the quantum... 

Let Ac be the energy gap between the zero-order states intermolecular vibrational energy. Let k and k (k < k ) be the rate constants for the radiationless decay of these two states. Provided that vibrational... 

The energy stored in an excited state is dissipated by the unimolecular radiative and radiationless relaxations. For strongly allowed electronic transitions, Strickler and Berg have obtained an expression for the rate constant of the excited-state radiative decay (Equation 6.67).31... 

In the so-called statistical limit of radiationless transitions (where the molecule undergoes an irreversible, exponential decay), the rate constant knl of nonradia-tive decay from the initial electronic state. v) to the final electronic state /) is given by [36]... 

This explanation, as reported in a previous section, suggested by Farid [80] and Mataga [81], has been recently confirmed by Farid and Gould [83]. In other words, they claim that the rate constant kb for the singlet recombination is thought to decrease as the exothermidty increases. These results have also been explained in terms of the gap theory for radiationless decay and the Marcus inverted region of electron transfer [66, 83]. 

In the case of 7-diethylamino-4-(trifluoromethyl)coumarin ( coumarin-35 ), which has an amino group that is free to rotate, another competitive solvent-dependent decay path has been proposed rotation of the amino group of the planar ICT excited-state molecule can lead to a twisted intramolecular charge-iransfer (TICT) excited-state molecule, from which a radiationless decay to the ground-state molecule occurs [341], Solvent-dependent rate constants for both the radiative and nonradiative decay of excited-state coumarin dyes have been determined [341]. For critical discussions concerning the electronic structure of the excited states of 7-(dialkylamino)coumarins and 7-aminocoumarin ( coumarin-151 ), see references [341d, 341e]. 

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