Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 2011-12-15

pH-dependent spectral properties of para-aminobenzoic acid and its derivatives.

Mitchell P Thayer, Colin McGuire, Elana M S Stennett, Mary Kate Lockhart, Daniela Canache, Marnie Novak, Sarah J Schmidtke

Index: Spectrochim. Acta. A. Mol. Biomol. Spectrosc. 84(1) , 227-32, (2011)

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Abstract

The local environment dictates the structural and functional properties of many important chemical and biological systems. The impact of pH on the photophysical properties of a series of para-aminobenzoic acids is examined using a combination of experimental spectroscopy and quantum chemical calculations. Following photoexcitation, PABA derivatives may undergo an intramolecular charge transfer (ICT) resulting in the formation of a zwitterionic species. The thermodynamics of the excited state reaction and temperature-dependence of the radiative emission processes are evaluated through variable temperature fluorescence spectroscopy carried out in a range of aqueous buffers. Quantum chemical calculations are used to analyze structural changes with modifications at the amine position and different protonation states. The ICT is only observed in the tertiary amine, which calculations show has more sp(2) character than the primary or secondary amines. Thermodynamic analysis indicates the ICT reaction is driven by entropy.Copyright © 2011 Elsevier B.V. All rights reserved.