Photophysical Behavior of β-Carboline-3-Carboxylic Acid N-Methylamide

The solvent and temperature effect of fluorescence and phosphorescence and its polarization characteristics for β-carboline-3-carboxylic acid N-methylamide were studied. From the results obtained it is concluded that the first excited singlet state changes from the ππ* type in a polar solvent (EPA) to the nπ* type in a nonpolar solvent (MC) and that the lowest singlet excited states (ππ* type and nπ* type) interact by vibronic coupling. In the nonpolar solvent (MC) the lowest singlet excited states are very close in energy and consequently the vibronic coupling is stronger. It has been found that the 0–0 band of the phosphorescence emission in EPA is polarized out of plane, using the excitations corresponding to absorptions into the first and second excited ππ* singlet states. This fact and the order of lifetime measured indicate an emitting triplet that is ππ*. The origin of this polarization is very likely \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $$^1 (n\pi ^* )\xrightarrow{{SO}}^3 (\pi \pi ^* )$$ \end{document} mixing as a result of spin–orbit coupling. However, the vibrational structures and changing polarization outside the 0–0 band indicate that second-order effects involving spin–orbit coupling and vibronic interactions are important and the mixing scheme appears to be \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $$^1 (\pi \pi ^* )\xrightarrow{{VIB}}^1 (n\pi ^* )\xrightarrow{{SO}}^3 (\pi \pi ^* )$$ \end{document} and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $$^1 (\pi \pi ^* )\xrightarrow{{SO}}^3 (n\pi ^* )\xrightarrow{{VIB}}^3 (\pi \pi ^* )$$ \end{document}. The “proximity effect” can qualitatively explain the absence of phosphorescence and the stronger vibronic interaction between the nπ* and the ππ* states of β-carboline-3-carboxylic acid .N-methylamide in a hydrocarbon solvent (MC).