Dynamic Modulation of Circularly Polarized Luminescence in Photoresponsive Assemblies

A photochromic molecule composed of a helical tetrathiazole framework and two pyrene units bridged by amino acid spacers exhibited on-off photoswitching of circularly polarized luminescence (CPL) of a pyrene-excimer-like state in a molecularly dispersed state. The pyrene-modified chiral tetrathiazole is employed as a building unit of supramolecular aggregates and self-assembled into spherical nanoparticles in a methylcyclohexane-based mixed solvent system. The spherical assemblies showed an enhancement of CPL activity compared to that of the molecularly dispersed state. UV light irradiation of the self-assembled state gave rise to a 6-electrocyclization reaction in the photoresponsive tetrathiazole framework. The photoreaction of the self-assembly component led to efficient emission quenching via energy transfer from the pyrene unit to the colored photochrome center in the aggregates. The CPL activity was diminished along with this emission quenching. The photoreaction also brought about the clouding of the solution due to a secondary aggregation of spherical assemblies. The CPL activity as well as the secondary aggregation were reversibly modulated by successive photoirradiation using UV and visible (>440nm) light.