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

Abstract

Beta-carboline (betaC) 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 betaCs [norharmane (nHo), harmane (Ho), and harmine (Ha)] and two betaC 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 betaC 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 betaC 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.

Fluorescence Quenching by Oxygen: “Debunking” a Classic Rule

“…For many organic molecules, this appears to be sufficiently efficient; indeed, O 2 quenching of S 1 generally occurs at the diffusion-controlled limit. [6,11] In contrast, our data show that, for the bCs, channels 3 and 4 play a more dominant role. Indeed, our data are consistent with an increase of at least~50-60 % in the efficiency of S 1 !…”

“…[6] Moreover, available data indicate that the chemical reaction channel becomes less important with an increase in [O 2 ], which only further supports our thesis (see Supporting Information). Given the relation F F + F ISC + F IC = 1, where conservation demands that the sum of the efficiencies of all S 1 deactivation channels must equal unity, and where we assume that these terms include both unimolecular and O 2 -dependent bimolecular processes, our data clearly indicate that IC becomes more efficient under conditions where O 2 deactivates S 1 .…”

“…We recently suggested that b-carbolines, bCs, may provide substantive evidence that an O 2 -induced S 1 !S 0 transition can actually occur. [6] We now demonstrate that the O 2 -dependent photophysics of selected bCs indeed provide an "exception to the rule" and that O 2 -induced S 1 !S 0 IC can efficiently compete with O 2 -induced S 1 !T 1 ISC. The present data, point to the possibility that other molecules will respond to O 2 like the bCs, limiting T 1 -derived chemistry or behavior which, in turn, opens new options for the design and application of unique photochemical, photophysical, and photobiological systems.…”

“…This is expected with the bimolecular quenching of S 1 by O 2 . Indeed, these compounds have an S 1 lifetime (~20-25 ns) [6][7][8] that is long enough for O 2 -induced deactivation to play a prominent role.If O 2 indeed induces S 1 !T 1 ISC, one would then expect to see a corresponding increase in the triplet state yield. This was not the case.…”

“…[6] Although the S 1 !S 0 phenomenon presented herein is observed in both alkaline and acidic media, it is more pronounced in the latter and, as such, we limit our discussion to data recorded at pH 5. We also limit our discussion to Ho; data for nHo and NMeHo can be found in the Supporting Information.…”