Suppression of HOMO-LUMO transition in a twist form of oligo(phenyleneethynylene) clamped by a right-handed helical peptide.

A donor-π-acceptor (D-π-A) system of oligo(phenyleneethynylene) (OPE) was twisted by clamping both ends of the OPE with a right-handed helical peptide (SSA8=OPE). The induced twist in OPE was in a right-handed way. SSA8=OPE showed a weaker HOMO-LUMO band in the absorption spectrum than that of a reference compound AcOPE without the helix bridge. The fluorescence quantum yield of SSA8=OPE was extremely low (0.0045-0.0165), which was in contrast to AcOPE with a moderate quantum yield of 0.355. The fluorescence life times of SSA8=OPE and AcOPE were nearly the same. Time-dependent density functional theory calculations (b3lyp/6-31G(d,p) level) on a twisted conformation of the D-π-A system qualitatively reproduced CD spectra and UV spectra of a weak oscillator strength of the HOMO-LUMO transition. Upon twisting the D-π-A system, the oscillator strength of the HOMO-LUMO transition is thus reduced.