One- and Two-Photon Excitation of β-Carbolines in Aqueous Solution: pH-Dependent Spectroscopy, Photochemistry, and Photophysics

β-Carboline (βC) alkaloids are present in a wide range of biological systems and play a variety of significant photodependent roles. In this work, a study of the aqueous solution-phase photochemistry, photophysics, and spectroscopy of three important βCs [norharmane (nHo), harmane (Ho), and harmine (Ha)] and two βC derivatives [N-methylnorharmane (N-Me-nHo) and N-methylharmane (N-Me-Ho)] upon one- and two-photon excitation is presented. The results obtained depend significantly on pH, the ambient oxygen concentration, and the βC substituent and provide unique insight into a variety of fundamental photophysical phenomena. The data reported herein should not only help to understand the roles played by βC alkaloids in biological systems but should also help in the development of methods by which the photoinduced behavior of these important compounds can be controlled.