Exploring the Photocyclization Pathways of Styrylthiophenes in the Synthesis of Thiahelicenes: When the Theory and Experiment Meet.

The introduction of thiophene rings to the helical structure of carbohelicenes has electronic effects that may be used advantageously in organic electronics. The performance of these devices is highly dependent on the sulfur atom topology, so a precise knowledge of the synthetic routes that may afford isomeric structures is necessary. We have studied the photocyclization pathway of both 2- and 3-styrylthiophenes on their way to thiahelicenes by experiment and theory. To begin with, the synthesis of stereochemically well-defined 2- and 3-styrylthiophenes allowed us to register first, and simulate later, the UV-vis electronic spectra of these precursors. This information gave us access through time-dependent density functional theory calculations to the very nature of the excited states involved in the photocyclization step and from there to the regio- and stereochemical outcome of the reaction. For the widely known case of a 2-styrylthiophene derivative, the expected naphtho[2,1-]thiophene type of ring fusion was predicted and experimentally observed by synthesis. On the contrary, 3-styrylthiophene derivatives have been seldom used in synthetic photocyclizations. Among the two possible structural outcomes, only the naphtho[1,2-]thiophene type of ring fusion was found to be mechanistically sound, and this was actually the only compound observed by synthesis.