Decay intramolecular nonradiative

If, therefore, l/iium is plotted as a function of [M], the ratio of slope to intercept provides a value of kq/A, even if Iium is measured in arbitrary units and Jabs is not determined. Thus, if the Einstein A factor is known, or can be measured, the value of the quenching rate constant can be calculated. The A factor can be calculated from the B factor by use of the v3 relationship presented as Eq. 9 (and B itself can be calculated from the measured integrated extinction coefficient for the absorption band, as implied by Eq. 15). It is also possible, under suitable conditions, to measure A directly by observation of the decay of emission after suddenly extinguishing the illuminating beam. As will be explained at the end of this section, the fluorescence or phosphorescence lifetime may be shorter than the natural radiative lifetime as a result of intermolecular and intramolecular nonradiative energy degradation, so that due care must be taken in the interpretation of emission decay measurements. 

Let the basis set still be the BO states starting points. Sim we wish to focus upon all the diverse molecular phenomena which are classify as involving radiationless processes, it is necessary to center attention upon th molecule. This focus is best obtained by considering the effective Hamiltoniar Hett, for the molecule which accounts for all relaxation mechanisms other tha the intramolecular nonradiative decay. (The use of effective Hamiltonians is popular in considering the relaxation processes associated with studies of magnetic resonance 37L) For the present case, the effective Hamiltonian is 16>17)... 

In the following discussion, we consider first the intramolecular nonradiative decay channels, which can occur for isolated oligomers, then intermolecular nonradiative decay channels, which may also operate in solid state thin films, where the oligomers or polymers are densely packed. We also consider the effects of interring torsion and coplanarity of the 7r-conjugated chains, which give rise to both intramolecular and intermolecular effects. 

Yoshihara s group has also measured the lifetimes of SA rotamers in a supersonic free jet expansion. The fluorescence decays of rotamer 11 were too fast to measure with their experimental time resolution (<1 ns) however, the excitation energy dependence of decay rates of rotamer I fluorescence could be observed. At low energy, the fluorescence decay was measured to be single exponential with a lifetime of 9.6 ns (12.0 ns for MSA). On the other hand, at higher energies above -1100 cm, the decay curves show bi-exponential behavior with lifetimes decreasing to -4.3 ns due to an efficient intramolecular nonradiative decay process, as observed in solution, which may be attributed to the proton transfer. The 9.6-ns component is nonlinearly dependent on SA concentration and is attributed to the dimer. 

Viscosity-dependent fluorescence (VDF) typically occurs in molecules which, following absorption of excitation light, undergo nonradiative decay by intramolecular twisting or torsional motions (12-19). In ordinary low-viscosity solvents, VDF compounds exhibit... 

A similar but smaller intramolecular quenching effect was seen by Phillips and co-workers 44,4S) for 1-vinylnaphthalene copolymers incapable of excimer fluorescence. The monomer fluorescence lifetime of the 1-naphthyl group in the methyl methacrylate copolymer 44) was 20% less than the lifetime of 1-methylnaphthalene in the same solvent, tetrahydrofuran. However, no difference in lifetimes was observed between the 1-vinylnaphthalene/methyl acrylate copolymer 45) and 1-methylnaphthalene. To summarize, the nonradiative decay rate of excited singlet monomer in polymers, koM + k1M, may not be identical to that of a monochromophoric model compound, especially when the polymer contains quenching moieties and the solvent is fluid enough to allow rapid intramolecular quenching to occur. 

The imbedded nature of the potential curves in Figure 6 for electron transfer in the inverted region is a feature shared with the nonradiative decay of molecular excited states. In fact, in the inverted region another channel for the transition between states is by emission, D,A -> D+,A + hv, which can be observed, for example, from organic exciplexes,74 chemiluminescent reactions,75 or from intramolecular charge transfer excited states, e.g. (bipy)2Rum(bipyT)2+ - (bipy)2Run(bipy)2+ + hv. 

The fluorescence quantum yield of 448 is 0.14, a sixfold increase relative to that of the parent. In comparison, the fluorescence quantum yield of 449 (0.01) is comparable to that of the parent compound. The phosphorescence emission quantum yield of 449 is 0.56 in a frozen matrix as expected as a result of the intramolecular heavy atom effect. In contrast, the phosphorescence is effectively shut off in the anti-isomer where the quantum yield is only 0.04. This observation suggests that the electronic excited state structures and nonradiative decay channels very considerably with constitution of the isomers. The optical gap energy was 3.1 (3.3) eV for 448 (449). 

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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