Conformational control of oligo(p-phenyleneethynylene)s with intrinsic substituent electronic effects: Origin of the twist in pentiptycene-containing systems

Abstract

Dependence of the backbone planarity of oligo(pphenyleneethynylene) s (OPEs) on the intrinsic electronic character of substituents and on the nature of the solvent has been experimentally demonstrated with a series of center-symmetrical five-ring systems, pentiptycene-pentipty-cene-arene-pentiptycene-pentiptycene, differing in the substituents on the central arene. In frozen 2-methyltetrahydrofuran (MTHF), the adjacent pentiptycene units prefer to be in a mutually twisted orientation when the substituents are electron-withdrawing (F and amido), resulting in a TPPT or TTTT conformation, whereas a planarized PPPP backbone is favored in the case of electron-donating substituents (alkyl and alkoxy). The propensity to adopt the PPPP form is generally enhanced by replacing MTHF with either methylcyclohexane or mixed ethanol/methanol as solvent. These observations reveal that the twist between adjacent pentiptycene units in OPEs is a consequence of the electronic rather than steric effects of iptycenyl substituents. The electronic effect of iptycenyl substituents is manifested in decreased phenylene πpolarizability as the net effect of both electron-donating hyperconjugation and an electron-withdrawing inductive effect. Variable-temperature electronic absorption and emission spectroscopies are the critical tools for this work. Our findings provide important guidelines for conformational and electronic engineering of OPEs and for the design of novel iptycene-based organic electronic materials.