Synthesis, structural, and photophysical properties of the first member of the class of pyrene-based [4]helicenes

A convenient route to a new class of pyrene-based [4]helicenes is presented. Wittig reaction of 7-tert-butyl-1,3-dimethyl-5-formylpyrene with benzyltriphenylphosphonium salts in the presence of nBuLi afforded 7-tert-butyl-1,3-dimethyl-5-(phenylethenyl)pyrenes, from which 4,5-naphthalene annulated aromatic [4]helicenes, namely 7-tert-butyl-1,3-dimethyl-13- methoxydibenzo[ij,no]tetraphene and 7-tert-butyl-1,3,12,14- tetramethyldibenzo[ij,no]tetraphene, were obtained by photoinduced intramolecular cyclization. The chemical structures of these [4]helicenes were determined on the basis of their elemental analyses and spectroscopic data. The helicity in the synthesized [4]helicene induced by the presence of the second methyl group in the fjord region is discussed in detail. The photophysical and electrochemical properties of these newly developed [4]helicenes were fully investigated by UV/Vis absorption and photoluminescence spectrphotometry, and cyclic voltammetry (CV), and the crystal structures were determined for two examples. A convenient route to a new class of pyrene-based [4]helicenes is presented along with their optoelectronic properties. The introduction of two methyl groups in the fjord region of the [4]helicene gives a more distorted structure and leads to a remarkable redshift of the absorption band. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim