Chiral triptycene-pyrene π-conjugated chromophores with circularly polarized luminescence†
A pair of optically pure triptycene derivatives ((R,R)- and (S,S)-3) containing fluorescent pyrene-based π-conjugated pendant groups attached through amide spacers were prepared via a resolution step using chiral high-performance liquid chromatography. Their absorption, circular dichroism, photoluminescence and circularly polarized luminescence (CPL) properties were investigated under various solution conditions. (R,R)- and (S,S)-3 exhibited clear solvent- and concentration-dependences of the optical and chiroptical properties as a result of the interconversion between molecularly dispersed and aggregate states. We also observed that (R,R)- and (S,S)-3 emitted left- and right-handed circularly polarized light, respectively, upon UV irradiation under aggregation conditions, and their dissymmetry factors were found to be greater than 1.0 × 10−3. Based on the contrasting result that almost no CPL signal appeared in the monomeric solution state, the resulting CPL was considered to arise from the supramolecular chirality induced in the hydrogen-bonded aggregate, wherein the pyrenyl pendants of 3 were most likely arranged in a preferred-handed twisting structure.