Dual fluorescence, excited-state conformation

One of the possible explanations for the kinetics of racemic III fluorescence (in vihich dual exponential decay in the monomer region is observed) is shown in Scheme 4 Fig. 32, in which the estabMunent of an equilibrium between GG and TT ground-state and excited state conformation nrecedes formation of a TG excimer from TT. Intramolecular excimer formation in dinaphthyl alkanes has been similarly owm to be related to the conformation of the ground-state molecule and the facility for conformational relaxation to the excimer state Thus koM (s Scheme 1) for meso 2,4-dinaphthyl pentane was much greater than that for racemic 2,4-dinaphthyl pentane, as in 111 above. 

Figure 5.2. Grabowski s model of TICT formation in DMABN the locally excited (LE) state with near-planar conformation is a precursor for the TICT state with near perpendicular geometry. The reaction coordinate involves charge transfer from donor D to acceptor A. intramolecular twisting between these subunits, and solvent relaxation around the newly created strong dipole. Decay kinetics of LE and rise kinetics of the TICT state can be followed separately by observing the two bands of the dual fluorescence. For medium polar solvents, well-behaved first-order kinetics are observed, with the rise-time of the product equal to the decay time of the precursor, but for the more complex alcohol solvents, kinetics can strongly deviate from exponentiality, interpretable by time-dependent rate constants. 52 ...

The nature of this dual fluorescence phenomenon has been the subject of intense interest. The currently most widely accepted explanation assumes that dielectric polarization of the solvent permits excited-state rotational isomerization, leading to a highly polar fluorescent TICT state with a conformation of the D+ and A moieties close to perpendicular [12, 13, 15-24]. This model predicts that the 7r-electronic decoupling of the D+ and A subunits leads to full charge separation and, consequently, to a large dipole moment and a considerable solvent reorientational en-... 

Very fast electron transfers from P+ to bacteriochlorophyl (Bchl) and from (Bchl)- to QA do not depend on media dynamics and occur via conformationally non-equilibrium states (Fig.3.18). The dual fluorophore-nitroxide molecules (D-A) are also convenient objects for analysing the activity-dynamics relationship. The marked irreversible photoreduction of the nitroxide fragment of the dual probe incorporated into the binding site of HSA only took place when the nanosecond dynamical processes around the probe traced by ESR and fluorescence methods were detected (Rubtsova et al., 1993, Fogel et al, 1994 Likhtenshtein, 1986 Lozinsky et al., 2002). Similar results were reported for another model protein system, i.e. a-chymotrypsin with spin labeled methionin-92 groups (Belonogova et al., 1997). In the latter enzyme, the excited tryptophan group serves as an electron donor. 

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