Rigid environments

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. 

Table 1. Intrinsic Photophysical Species in Aryl Vinyl Polymers in a Rigid Environment...

In some crosslinked polymer systems, like air-cured alkyd polyester paints, two separate Tlfi decay processes are observed. Longer Tlcs in these systems is associated with a more rigid environment for the spins the fractions of the two T1 populations indicate the amount of rigid and mobile protons, respectively 82). 

With this information in hand the photophysical properties of the complex are explained. In the relaxed ground state, the MQ+ ligand is twisted and the dir (Re) —> tt (bpy) MLCT state is lowest in energy. In fluid solution, near UV photoexcitation produces the Re - bpy MLCT state and rapidly thereafter intramolecular bpy —> MQ+ ET occurs to produce the dir (Re) —> tt (MQ+) MLCT state (Scheme 6). By contrast, in rigid environments (77 K solvent glass or 298 K... 

Both the emission quantum yields and lifetimes are significantly increased on cooling the solution to 77 K, implying that the radiative decay pathways are favored in the more rigid environment the emission lifetimes are typically governed by the energy gap law [17, 20, 74, 75], Table 3 summarizes the excited-state decay parameters for the MLCT excited states of /ac-tClRe1, 4 -(X)2-bpy)(CO)3] complexes (where 4,4 -(X)2-bpy is 4,4 -disubstituted-2,2 -bipyridine). 

Av in SC cannot be explained by polarity and hydrogen-bonding of the surrounding medium alone. Rather the results are explained better in terms of hindered fluorophore reorientation in the excited state, similar to results obtained with other lipid systems [7,11-13]. Thus, the unusual Stokes shifts seen with AF probes in SC suggests a rather rigid environment in the lipid lamellae, which hinder the fluorophore reorientation. 

The same approach applies to photoprocesses, with one significant difference. In the ground state, when no chemical reaction obtains, the lifetime of any A species is governed only by the interconversion rate between the various solvates. In excited states, competition between the photophysical steps, e.g., Af => A(, and solvent motion must be considered. Two extreme limits can be treated (1) a rigid environment where there is no environmental movement on the time scale of any excited state process, and (2) a very fluid environment where the converse prevails. The luminescence of complexes that occupy multiple sites in crystals is an example of the first limit, while long-lived luminescence in low viscosity solvents conforms to the second limit. 

On 2e oxidation of phenols such as 467, the resulting phenoxonium ions are expected to undergo intramolecular nucleophilic substitutiou by the tertiary hydroxyl group, which is in a relatively rigid environment imparted by the presence of the (Z)-alkenyl group, to afford highly functionalized bicyclic compounds. Thus, oxidatiou of 467 with PhI(OAc)2... 

Energy transfer in polymers has been studied in the pure solid state, in heterogeneous systems (e.g. polymer blends), in liquid solutions and in solid solutions. The last case, which will be considered here, provides relatively simple and clear experimental conditions since interactions between the macromolecules can be excluded by dilution and molecular movement is severely restricted by low temperature and rigid environment. Thus, excitonic energy transfer can be studied without competing molecular movement. The luminescence of dilute, solid solutions of aromatic polymers is not dominated by excimers - in sharp contrast to the other modes of observation - so that side group fluorescence and phosphorescence can be observed. This does not mean, however, that exciton trapping processes are absent in these systems. 

The adsorption system is in many respects similar to biological systems. Especially the smallness of the solvent reorientation energy, with Ag < and -AGg = A, of a donor-acceptor system with ion-paired product state in a diffusionless, semi-rigid environment, opens many possibilities to study certain aspects of electron transfer which are of importance to biological systems. 

Fluorescence of porphycene embedded in rigid media was found to be depolarized, both at room temperature in the poly(vinyl butyral) matrix [79] and at low temperatures in glasses [30, 80[ (Fig. 8.11). In a rigid environment, where the reorientation of an excited chromophore is not possible, the direction of the Sq-Sj transition moment should be the same in absorption and emission, leading, for excitation into Sj, to the anisotropy value of 0.4. Instead, the observed values... 

A NMR Mouse (Mobile Unit for Surface Exploration) and a CPMG spin echo sequence were used to assess T2 as a function of tin content. Figure 6 and Figure 7 show that as the catalyst levels in the reaction mixture are increased, the level of chain restiction is increased and the T2 is lowered. These results suggest that the tin affects the mobility of the more rigid environments (short T2, such as crosslinks) as well as the mobility of chain ends and mid chain components (long T2). 

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