Dithia[9]helicenes: Molecular design, surface imaging, and circularly polarized luminescence with enhanced dissymmetry factors

We have carried out the synthesis of two dithia[9]helicenes by means of a LED-based double photocyclization. The compounds have sulfur atoms placed at the terminal rings of the helicene, and they display two alternative C2-symmetrical arrangements named exo (1) and endo (2). Separation of enantiomers of opposite helicity allowed the complete characterization of their chiroptical properties in solution, in silico, by X-ray crystallography, and adsorbed on gold. The theoretical analysis confirms the unexpected finding that endo-dithia[9]helicene displays an experimental dissymmetry factor in CPL (glum) larger than its isomer, exo-dithia[9]helicene. This enhanced glum factor ranks among the largest for a helicene-type molecule. Comparison with smaller analogues, namely exo and endo-dithia[7]helicenes (10 and 11, respectively), is also presented. AG and CU thank the financial support by the Spanish MICIIN (PID2019-109539GB-C41), the Generalitat Valenciana (PROMETEO/2021/017), and the University of Alicante (VIGROB-285). CS thanks for the financial support by the Generalitat Valenciana through CDEIGENT2018/028. The computational resources provided by the Department of Applied Physics of the University of Alicante are greatly appreciated. JA and PM thank the National Science Foundation, grant CHE-1855470, for support of the theoretical component of this study, and the Center for Computational Research (CCR) at UB for providing computational resources. J.C. thanks the the Ministère de l’Education Nationale, de la Recherche et de la Technologie and the Centre National de la Recherche Scientifique (CNRS).