Limonene magic: noncovalent molecular chirality transfer leading to ambidextrous circularly polarised luminescent π-conjugated polymers
New Journal of Chemistry
Solvent chirality transfer using (S)- and (R)-limonenes, which are candidates for renewable volatile bioresources (bp 176 °C/760 Torr or bp 94 °C/68 Torr), allowed for the successful production of optically active poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-bithiophene] (F8T2) particles with circular dichroism (CD) and circularly polarised luminescence (CPL) properties. The particles were rapidly produced by CD-silent F8T2 with the aid of solvent chirality transfer at 25 °C. The present paper demonstrates the following: (i) through weak intermolecular forces, such as π/π, van der Waals and CH/π interactions, CD-/CPL-active F8T2 aggregates successfully emerge in a chiral tersolvent system of chloroform (a good solvent), alkanol (a poor solvent) and limonene (a chiral solvent); (ii) the alkanol and the enantiopurity of the limonene greatly affect the magnitude and sign of CD-/CPL-signals; (iii) aggregate size considerably affects the magnitude of the induced CD amplitude; (iv) clockwise and counter-clockwise stirring during preparation do not affect the magnitude of these signals; (v) stirring speed weakly affects the induced CD amplitude and (vi) the order of addition of limonene and methanol to the chloroform solution of F8T2 greatly affects the magnitude of the induced CD amplitude. To prove the renewability of limonenes, we re-used (S)-limonene purified by distilling very impure (S)-limonene-containing chloroform, methanol and F8T2, which was used and stored in a number of limonene chirality transfer experiments. As expected, the CD-/UV-vis spectra of F8T2 particles utilizing the renewed (S)-limonene gave similar CD-/UV-vis spectra to those using the fresh (S)-limonene. Moreover, limonene chirality transfer was possible to obtain two CD-active polymers from CD-silent, poly(9,9-di-n-octylfluorenyl-2,7-diyl) (F8) and poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-thiophene] (F8T1). The protocol may provide an environmentally friendly, safe and mild process to rapidly produce ambidextrous light-emitting polymers with a minimal loss of starting polymers at ambient temperature, from CD-silent polymers without any specific chiral substituents or chiral catalysts.
J-725, V-570, FP-6500, CPL-200
Circular dichroism, Absorption, Fluorescence, Polymers, Circularly polarized luminescence, Chemical stability, Stereochemistry, Materials