Abstract

Proton-transfer reactions provide the fundamental basis for acid-base chemistry in protic solvents. Many examples of excited-state proton-transfer reactions exist in the literature.¹ A mysterious red-shift in the emission spectrum with solution pH as first observed by Weber² in 1931 was subsequently realized by Förster³ and later Weller⁴ to be a characteristic signature of the occurence of an excited-state proton-transfer reaction. Förster and Weller pioneered in the development of steady-state and time-resolved emission spectroscopy as a powerful means for understanding and measuring the acid-base properties of electronically excited molecules. The pivotal importance of electronic structure in determining acid-base properties is evidenced by the remarkable changes in pK which occur upon electronic excitation.¹ Unfortunately, models based on steady-state spectral data and/or nanosecond timescale lifetime measurements do not always provide accurate descriptions of either the equilibrium or dynamics of excited-state protonation/deprotonation reactions, since such processes often occur on a picosecond timescale. Picosecond, time-resolved measurements on spectrally distinct acid-base pairs yield direct kinetic information and can thereby provide considerable insight into the effect of molecular structure on acid-base properties.⁵,⁶

© 1984 Optical Society of America