Stokes shift, excited-state conformation

In methanol/DMSO solvent mixtures the fluorescence spectrum of TIN (A.max = 400 nm) displays a normal Stokes shift indicating that this emission arises from a non proton-transferred, excited state of TIN. The fluorescence excitation spectrum for this emission coincides with the absorption spectrum of the resolved non-planar species suggesting that this conformer is the ground-state precursor responsible for the observed emission. As the amount of DMSO in the mixture increases the fluorescence maximum undergoes a bathochromic shift from 415 nm in pure methanol to 440 nm in pure DMSO. 

The fluorescence spectrum of the nonsteroidal anti-inflammatory agent piroxicam 21 has been determined in a variety of solvents (Scheme 7) <1999PCP4213>. The key observations are that the molecule exists with a strong H-bond between the phenolic OH and the adjacent amide. A very high Stokes shift in the excited state was observed and attributed to the proton-transfer event (tautomerization) between the phenolic and amide oxygens (cf. 21 —>63). In the case of protic solvents, such as water, the open conformation 64 was observed. 

Fig. 5A). The emission spectra of benzo[a]phenothiazine and the three methyl derivatives exhibited generally a broad and structureless band located at 500-527 nm in water/ethanol mixture (99 1, v/v), 500-507 nm in ethanol and 512-519 nm in 11P-f>-CD (Fig. 5B). Enhancement of the Stokes shift in HP-fi-CD solutions was interpreted by a greater stabilization of the tt-tc solvated excited singlet state [21], due to the reduced rotational and conformational motions of the BPHT molecules within the HP-fi-CD cavity [82], Comparable weak excitation or emission spectral shifts upon adding CD to an aqueous medium have also been reported in the case of unsubstituted phe-nothiazine [21], azure A [83] and several types of aromatic molecules [84,85]. 

Fischer, G., Seger, G., Muszkat, K. A., Fischer, E., Emissions of Sterically Hindered Stilbene Derivatives and Related Compounds. IV. Large Conformational Differences Between Ground and Excited States of Sterically Hindered Stilbenes. Implications Regarding Stokes Shifts and Viscosity or Temperature Dependence of Fluorescence Yields, J. Chem. Soc., Perkin Trans. 2 1975, 1569 1576. 

The difference between absorption and emission maxima, the Stokes shift, also increases with HH dyad content, which was attributed to greater relief from conformational strain in the first excited state. 

Dorfman et al have used theoretical models to examine the probabilities of photoinduced charge separation in solid matrices which are relatively concentrated in acceptors ([donor) [acceptor]). " Hoffman et al have presented a readable discussion of the coupling between slow nuclear motions of environmental species, such as might be the case for some conformational changes, and electron transfer processes.Such coupling has been labeled gated electron transfer. Simon and Su have used the fluorescence Stokes shift, band shape, and decay kinetics of 6w-(dimethylaminophenyl)-sulfone, which has a twisted intramolecular charge transfer excited state, to examine... 

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