Abstract
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-b]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-b]thiophene type of ring fusion was found to be mechanistically sound, and this was actually the only compound observed by synthesis.
Highlights
Helicenes display a robust, fully conjugated helical architecture that makes them prototypes of chiral carbon nanostructures, with increasing applications in organic electronics.1 With the inclusion of thiophene rings into the helical structure, two noticeable effects occur
These two aspects are highly dependent on the sulfur atom position and bond topology generated therefrom within the thiahelicene,6,7 so synthetic routes must be explicit in that respect
Terminated helicenes with special regard to the topology well-defined 2- and 3-styrylthiophene precursors were required originated from the sulfur atom positioning in the key to trace the following key photochemical step of the synthesis
Summary
Fully conjugated helical architecture that makes them prototypes of chiral carbon nanostructures, with increasing applications in organic electronics. With the inclusion of thiophene rings into the helical structure, two noticeable effects occur. Focusing on the central features of this photocyclization and on its backbone scaffold (Scheme 1b), it has been reported to afford naphtho[1,2-b]thiophenes and its isomeric naphtho[1,2-c] counterpart, which may leave doubts about the actual natural outcome of this reaction This is a critical yet obscure point that gives rise to different topologies in the sulfur atom positioning, and with it, to the entire electronic structure of the corresponding thiahelicene. This is an important matter for many reasons, but in particular, if the objective of the synthesis is to build conductive yet robust nanocontacts between a helicene and external metallic electrodes.
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