STERIC HINDRANCE-INDUCED DUAL FLUORESCENCE OF CONGESTED BENZENEHEXACARBOXYLATES

Alkyl benzenetetra- and benzenehexacarboxylates were shown to fluoresce weakly in fluid solutions, in sharp contrast to entirely nonfluorescent mono-, di-, or tricarboxylates. Interestingly, benzenehexacarboxylates with bulky alkyl groups provide a novel dual-fluorescing molecular system involving a mechanism differing from the well-documented excimer formation, protonation/deprotonation, or twisted intramolecular charge transfer. Thus hexacarboxylates of isopropyl, tert-butyl, (-)-bornyl, (-)-menthyl, and 1-adamantyl alcohols show unambiguous, bulkiness-dependent dual fluorescence peaks with both small and substantially large Stokes shifts at room and/or low temperatures, whereas the hexamethyl ester and the corresponding tetracarboxylates with bulky alkyl groups never show such unusual fluorescence behavior. This dual fluorescence phenomenon is attributed to the decelerated conformational relaxation in the excited state owing to steric hindrance between adjacent bulky alkyl groups.