Single‐Step Synthesis of Dithieno[3,2‐ <i>b</i> :2′,3′‐ <i>e</i> ]Pyridines and Their Acid‐Dependent Optical and Photosensitising Properties

Dithieno[3,2-b:2',3'-e]pyridine (DTP) is a heterocycle consisting of a central pyridine ring fused with two thiophene rings and can be regarded as a thiophene-based analogue of the classic N-heterocycle acridine. Unlike acridine-based dyes, the fundamental chemistry of DTP has received relatively little attention. Here, we report the straightforward synthesis of a library of DTP molecules and an investigation of their electronic properties using UV/vis absorbance and emission spectroscopy supported by computational insights, with particular focus on the influence of protonation and methylation of the central N atom on their properties. While the absorbance of the DTPs studied was confined to ultraviolet wavelengths, protonation or N-methylation of the pyridine group led to significant visible light absorption, which was exploited for the visible light photosensitisation of oxygen.