Nonradiative decay enhancement

Herein, F is the radiative decay rate and km is the nonradiative decay rate, which comes from quenching. It has been demonstrated that silica nanomatrixes can change the fluorescence quantum yield and lifetime of fluorophores. Several groups have reported that both quantum yield and lifetime of fluorophores increased in DDSNs [27, 28, 52, 65-67]. However, the mechanisms regarding this enhancement were reported differently. 

The long lifetime has important consequences on the decay rates. First, we consider what affects the nonradiative rates (knr) which change the yields of fluorescence and phosphorescence. The nonradiative decay rate is often enhanced in molecules which have flexible constituents (the so-called loose-bolt effect). Therefore, both fluorescence and phosphorescence yields are generally larger for rigid molecules than flexible molecules. For the same reason, a rigid environment will increase the emission yields hence both fluorescence and phosphorescence yields often increase with increasing viscosity. 

Stabilization of the exciplex by the estimated bond energy of the amine dimer cation radical, 0.7 eV (129), would render intersystem crossing to form t endothermic by 0.2 eV as shown in Fig. 9 and enhance the rate of nonradiative decay. 

The emission properties of lumophores change when included within the microenvironment of a supramolecule bucket. Nonradiative decay processes are generally curtailed within the confines of the bucket interior and luminescence intensity is therefore increased [138,208,209], Because CDs present a more protected microenvironment than calixarenes, the binary complexes of the former supramolecule have been examined most extensively. Spurred by Cramer s pioneering observation that the spectral properties of a lumophore are perturbed by complexation within a CD [210], a large body of work has sought to define the influence of CDs on the photophysics of bound lumophores. Different factors contribute to the enhanced luminescence of 1 1 CDilumophore complexes. These include the following. 

The conformational flexibility of several bucket chemosensors is reduced upon analyte binding. In the rigidified state, the luminescence from the reporter site is enhanced. Scheme 11 pictorially represents the transduction process. The precise mechanism by which nonradiative decay is disrupted in chemosensors of this type is usually ill defined. 

Merkle et al. (1981) discussed three possible mechanisms of the pressure-enhanced decay rate an increase of the radiative decay rate of each Nd3+ ion, an increase of the nonradiative decay rate of each Nd3+ ion, or an increase in the interaction between Nd3+ ions leading to luminescence quenching. The nonradiative decay rates for the 4F3/2 multiplet were estimated to contribute less than 20% to the total decay rate (Powell et al., 1980) at ambient pressure. 

It has been also reported that the emission intensity is enhanced as the emitting chromophore is diluted by other inert polymers in solid state [79]. The emission intensity of the blend polymer is increased as MEH-PPV is diluted by DSiPV because the nonradiative decays, especially intermolecular quenching, are reduced by the dilution effect. 

The NIR emission intensity of the lanthanide porphyrinate complexes follows the trend Yb > Nd > Er. This agrees with observations on other luminescent lanthanide complexes and reflects the fact that the efficiency of nonradiative decay increases as the energy of the luminescent state decreases. The emission yields of the ternary lan-thanide(III) monoporphyrinate complexes with hydridotris(pyrazol-l-yl)borate or (cyclopen-tadienyl)tris(diethylphosphito)cobaltate as a co-ligand are generally higher than those of other Yb(III), Nd(III), and Er(III) complexes because the coordination environment provided by the porphyrinate in combination with the tripodal anion effectively shields the Ln + ion from interacting with solvent (C-H) vibrational modes that enhance the rate of nonradiative decay. 

To understand the importance of spectral overlap to metal-enhanced fluorescence, it is useful to review the basics of metal-enhanced fluorescence. Metal nanostructures can alter the apparent fluorescence from nearby fluorophores in two ways. First, metal nanoparticles can enhance the excitation rate of the nearby fluorophore, as the excitation rate is proportional to the electric field intensity that is increased by the local-field enhancement. Fluorophores in such "hot spots" absorb more light than in the absence of the metal nanoparticle. Second, metal nanoparticles can alter the radiative decay rate and nonradiative decay rate of the nearby fluorophore, thus changing both quantum yield and the lifetime of the emitting species. We can summarize the various effects of a nanoparticle on the apparent fluorescence intensity, Y p, of a nearby fluorophore as ... 

The model of the photophysics we have advocated does not take into account spin-orbit coupling effects associated with silver as a heavy atom that might affect phosphorescent and nonradiative decay rates for the triplet state. The theoretical justification for this is that heavy atom effects are quite short range since they require wavefunction overlap. Effects of the silver are in any case likely to be much smaller than those of the Pt atom embedded in the porphyrin. Experimentally, we can rule out the importance of these effects since we do not observe phosphorescence enhancement on top of uniform evaporated silver films nor on films that become essentially continuous as for the thickest films in Figure 19.4. 

The emission color and efficiency of PAVs is primarily affected by two structural elements - the effects of substituents, and the degree of conjugation along the backbone. Intermolecular effects such as aggregation of polymer chains in the solid state can also affect the emission spectrum (aggregation generally produces a red-shift) as well as the photoluminescence efficiency (aggregation tends to enhance nonradiative decay pathways). 

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