Fluorescent analogs, quantum mechanical

The second form of relaxation is called spin-spin relaxation. This form involves any change in the quantum state of the spin. Thus, any of the transitions shown in Figure 3.1 can cause spin-spin relaxation. In particular, the exchange of magnetization between spins via a zero quantum transition is a very effective mechanism for spin-spin relaxation. Thus, spin-spin relaxation is analogous to fluorescence energy transfer. Because spin-spin relaxation limits the lifetime of the excited state, it affects the line width of the observed resonance lines due to the uncertainty principle shortlived states have ill-defined frequencies. The actual relationship between the spin-spin relaxation rate and the line width (Av) is given by R2, the rate of spin-spin relaxation T2 is the time constant for spin-spin relaxation,... 

From Table 13.11, the variation of the lifetimes and fluorescence quantum yields in the series of compounds shows the clear increase of homo-chromophore interactions in the excited states when the distance between the chromophores diminishes. The rate and efficiency of the energy transfer in hetero-dimers does not seem to be metal dependent. The distance dependence of the energy transfer rate has been analyzed using Forster and Dexter theories. Harvey and Guilard have established that in 135-Zn-H2 and 136-Zn-H2, energy transfer is dominated by a Forster mechanism, while in the case of hetero-dimers 137, 138, and 139, it proceeds mainly via a Dexter mechanism. The critical distance at which the Dexter mechanism becomes inoperative is estimated between 5 and 6 By analogy with what has been discussed earlier in the case of linearly arranged covalent dimers, it should be noted that for compounds 135-139, no electron density should be present on the meso carbons involved in the covalent connection to the spacer. 

When a photoexcited molecule undergoes chemical reaction the fluorescence quantum efficiency is reduced, i.e., it is quenched. Fluorescence quenching provides an excellent example of how competition between chemical and physical processes may be used to establish both rate law and mechanism. The reaction of photoexcited acridine (A ) with amines in aqueous solution has been thoroughly studied.Addition of amines reduces the fluorescence efficiency. Data for an analogous system are presented as a Stern-Volmer plot in Fig. 6.6. The relative fluorescence efficiency, (pf c=0)l(pf c), is equal to the ratio of fluorescence intensities without and with quenchers lf c=0)jlf c). If this latter quantity is graphed against amine concentration, c, it appears that... 

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