Non-radiative decay mechanisms

Among the non-radiative decay mechanisms is coupling to vibrational overtones of H, N H, and C-H bonds. This is a particularly effective route if O-H containing solvent molecules are coordinated directly to the metal center. As such, this effect can impart information on solution-state structures of complexes. This is discussed further in Section III.A.2. 

However, in the last two decades it has been shown experimentally [1,7, 8,12-14] and theoretically [15-18] that in many wide-gap insulators including alkali halides the primary mechanism of the Frenkel defect formation is subthreshold, i.e., lattice defects arise from the non-radiative decay of excitons whose formation energy is less than the forbidden gap of solids, typically 10 eV. These excitons are created easily by X-rays and UV light. Under ionic or electron beam irradiations the main portion of the incident particle... 

The quantum mechanical approach is based on time-dependent perturbation theory and is derived from Fermi s Golden Rule for non-radiative decay processes [1]. 

Figure 4 Orbital scheme illustrating the quenching of a photo-excited fluorophore FI by a nearby metal centre M via an electronic energy transfer (ET) mechanism. A simultaneous exchange of two electrons takes place, one from FI to M, one from M to FI. Following this circular electron motion, FI is deactivated. The excited M centre which is obtained can emit and relax to its ground state, but in most cases undergoes a non-radiative decay.

The copper(I) complex Cu(dmp)2 (dmp = 2,9-dimethyl-l,10-phenanthroline) displays MLCT luminescence in ambient temperature CH2CI2 solutions [85]. This emission has been shown to be quenched by various Lewis bases (B), and the mechanism proposed is addition of B to the MLCT state at the metal center to give an exciplex which decays rapidly (Eq. 6.39). The validity of this mechanism was tested by comparing, in the presence and absence of Lewis base quenchers, the pressure effects on the emission lifetimes of Cu(dmp)2 vvith those on the emission lifetimes of the bulkier 2,9-diphenyl-phen analog Cu(dpp) [86]. The lattice ions should not be as susceptible to reaction of the copper center with B. For both ions, emission quantum yields are small (<10 ) at ambient T and unimolecular photoreactions are not observed, so the pressure sensitivity of x reflects non-radiative deactivation mechanisms. 

A quantum-mechanical tunnelling model of non-radiative decay 492 has been proposed. Several papers have been concerned with the choice of basis set for treatments of non-radiative decay,498 and it has been shown in the paper of Sharf that higher-order vibronic terms are comparable in magnitude with those of the first-order term. It is thus not sufficient to make the usual assumption that interactions describing non-radiative decay are independent or linearly dependent on nuclear co-ordinates. The other papers reinforce this point, and... 

Sobolewski AL, Domcke W On the mechanism of rapid non-radiative decay in intramolecularly hydrogen-bonded n systems. Chem Phys Lett 1999, 300(5-6) 533-539. 

The ability to survive electronic excitation is a fundamental property of DNA [1]. While excited states of DNA nucleobases are important in mutation and repair processes, the average lifetime of these states is 1 ps [2]. DNA excited states dissipate in a variety of ways with the dominant mechanism being non-radiative decay. Other decay pathways are important due to their health implications ... 

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