ortho-Phenylene oligomers with terminal pushpull substitutionThis article is part of an Organic & Biomolecular Chemistry web theme issue on Foldamer Chemistry.Electronic supplementary information (ESI) available: supplemental figures referred to in the text; NMR spectra of oP(DA)6; energies for optimized geometries; synthetic details for oP(DA)2, oP(DA)3, oP(D)4, and oP(A)4; NMR spectra of all novel compounds; Cartesian coordinates for optimized geometries; and complete ref. 58. See DOI: 10.1039

ortho -Phenylenes are an emerging class of helical oligomers and polymers. We have synthesized a series of pushpull-substituted o -phenylene oligomers (dimethylamino/nitro) up to the octamer. Conformational analysis of the hexamer using a combination of low-temperature NMR spectroscopy and ab initio predictions of 1 H NMR chemical shifts indicates that, like other o -phenylenes, they exist as compact helices in solution. However, the substituents are found to have a significant effect on their conformational behavior: the nitro-functionalized terminus is 3-fold more likely to twist out of the helix. Protonation of the dimethylamino group favors the helical conformer. UV/vis spectroscopy indicates that the direct charge-transfer interaction between the pushpull substituents attenuates quickly compared to other conjugated systems, with no significant charge-transfer band for oligomers longer than the trimer. On protonation of the dimethylamino group, significant bathochromic shifts with increasing oligomer length are observed: the effective conjugation length is 9 repeat units, more than twice that of the parent oligomer. This behavior may be rationalized through examination of the frontier molecular orbitals of these compounds, which exhibit greater delocalization after protonation, as shown by DFT calculations. Substituent effects on conformational behavior and electronic spectra have been examined in the first series of pushpull o -phenylenes.