Electron rich substituted β-carboline derivatives: Synthesis and photophysical properties

We report the synthesis and characterisation of known and novel analogues of β-carboline, a naturally occurring alkaloid. Striving to induce changes in the optical properties of carboline, we first focus on substitutions at position 1 before exploring other positions. The β-carbolines were synthesized using, on the one hand, post-synthetic functionalization of biosourced products and, on the other hand, total synthesis of the desired compounds. Our strategy mainly relies on the well-known Pictet-Spengler reaction, followed by aromatization using different techniques. Post functionalization were performed by Knoevenagel condensation or pallado-catalyzed cross-coupling. The measured UV–visible and fluorescence spectra show unexpected results with large shifts obtained when changing a few substituents only. For instance, the replacement of the aryl by a styryl substituent induces a bathochromic shift as large as 70 nm. This allowed controlling the position of the emission on a broad range (from 370 to 650 nm). Additionally, protonation of the compounds leads to profound changes in the spectroscopic properties. We indeed observe for some compounds a bathochromic shift in the emission as large as 200 nm upon protonation. The main experimental results are rationalized using theoretical calculations.