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Reports: UR452071-UR4: Hydrogen-Bonding Control of Solvatochromism and Non-Radiative Decay in the Fluorescence of PRODAN Derivatives

Christopher J. Abelt, PhD, College of William and Mary

The effects of H-bonding on the fluorescence of PRODAN and derivatives have been the subject of one publication acknowledging PRF support.  In this paper the two 2,5-PRODAN derivatives shown below were shown to be highly sensitive sensors of low solvent acidity (SA). 

 

 

slope = 4.5 ± 0.1 R2 = 0.97 slope = 4.2 ± 0.1 R2 = 0.97  

 

The magnitudes of the slopes of the plots of quenching vs. solvent acidity are both greater than four.  These slopes are at least twice as large as for our previously reported twisted-carbonyl derivatives of 2,6-PRODAN.  The compounds above are sensitive in the SA range of 0.15-0.40 (t-butanol to ethanol), whereas the twisted-carbonyl compounds are sensitive in the range of 0.25-1.1 (isopropanol to water).  Preferential solvation studies confirmed that two H-bonds are required for quenching just as with the 2,6-PRODAN derivatives. 

Syntheses of model compounds incorporating one H-bond are nearly complete.  Switching the alcohol protecting group from benzyl to methoxymethyl proved efficacious.  The timing of placement of the MOM-ether and the Weinreb amide is critical.  Placing the MOM-ether first activates the ester carbonyl towards nucleophilic addition by isopropyl magnesium chloride in the amidation reaction.  If the hydroxyl group is not protected, then one equivalent of iPrMgCl creates an alkoxide which is not as activating, and prevents the unwanted addition side-reaction.