Proton transfer in the ground and excited state and photobehaviour of the positional isomers of E-5-[2-(pyrid-n-yl)ethenyl]oxazole's (n = 2, 3 and 4)

- The acid-base properties of the positional isomers of E-n-pyridyl,5-oxazolyl-ethene (n = 2, 3 and 4) in the ground and excited states were investigated.
- The pKa* of the S1 state, estimated by the Förster cycle and measured by fluorimetric titration for the longer-lived isomer with n = 3, showed that they become much stronger bases under excitation.
- The deactivating effect of the n,π* states of the pyridyl group, observed in non-polar solvent, disappears in the protonating medium for n = 2 and 4 which photoisomerize with high yield.
- The isomerization yield of the protonated isomer with n = 3 is instead quite low due to changes in the excited state character from ethenic to charge transfer.
- Quantum-mechanical calculations helped to describe the conformational equilibria and the charge transfer process from the oxazole group towards the pyridinium moiety.

The effect of protonation on the excited state properties of the three positional isomers of E-5-[2-(pyrid-n-yl)ethenyl]oxazole's (n-PyEt-5Ox with n = 2, 3 and 4) was studied in buffered water by spectrophotometric, fluorimetric and chromatographic measurements. The clear shift to the red of the absorption and emission spectra by protonation indicates that the basicity of these compounds increases strongly (by ∼7 pH units) under excitation. Ground state pKa values (around 5) were measured by absorption (for n = 2 and 4) and fluorescence (for n = 3) titration and excited state pKa* values were estimated by application of the Förster cycle. For the compound with n = 3, which displays a rather higher fluorescence lifetime, it was possible to perform a direct fluorimetric titration which showed two inflection points corresponding to pKa (4.77) and pKa* (12.27). Previous works on these compounds had shown a marked deactivating effect of the pyridine n,π* states on the emitting and reactive π,π* states (proximity effect) favouring relaxation by internal conversion with a marked reduction of the competitive fluorescence and trans-cis (E-Z) isomerisation yields, particularly for n = 2 and 4 in non-polar cyclohexane. The new results indicate that the aqueous solvent, and mainly protonation, affect sensibly the spectral, photochemical and photophysical behaviour increasing again the radiative and reactive parameters of compounds with n = 2 and 4. The isomer with n = 3, whose n,π* states play a much smaller deactivating effect, shows a peculiar behaviour with a very low isomerisation yield at acid pH, reasonably due to intramolecular charge transfer from the Ox ring to the PyH+ moiety, which hinders the twisting process. All these results are compared with those reported in the literature for the n-styrylpyridine analogues to point out the role of the oxazole group.

Strong increase of basicity under excitation of n-pyridyl,5-oxazolylethenes. The pKa* of S1 derived by the Förster cycle and directly measured by fluorimetric titration for the longer lived isomer with n = 3.95