Positional Effect of π-extension in Triple Helicenes

The targeted application of multiple helicenes in photo-optical applications requires their rational design. Toward this goal, we report on the synthesis of pyrene-based triple helicene 1 and investigate the positional effect of extension of the π-conjugated system on the photo-chemical and chiro-optical properties of triple helicenes. The conformational and aggregatory behavior of 1 were studied both experimentally using VT NMR spectroscopy and computationally using high-level DFT computations. Although π-extension was observed to have a marked effect on the spectroscopic properties of triple helicenes, comparison with other known π-extended helicenes reveals that the position at which π-extension is introduced is only of nominal importance. Our results thus suggest that the presence of a particular helicene motif is dominant in dictating the physico-chemical properties of triple helicenes.