Polymorphism-induced multi-functional crystal photonics achieved by a highly luminescent benzofuranyl molecule having a tetrafluorophenylene core

Abstract

Luminescent organic single crystals are attractive as components of miniaturized photonic integrated circuits such as optical waveguides, lasers, and optical resonators. Various functions have been discovered by designing chemical or crystal structures. The optical characteristics can be modulated by designing the crystal structures. Various polymorphic compounds have been reported, and differences in luminescence colors have been well-explained by the differences in the crystal structures. However, drastic modulation of the photonics functions has not been reported. We report here multi-functional photonics achieved by a highly luminescent and polymorphic compound, 1,4-bis(benzofuran-2-yl)-2,3,5,6-tetrafluorophenylene, named BFTFP. BFTFP exhibited three types of crystal structures: BFTFP_α: a flexible fiber, BFTFP_β: a rigid block, and BFTFP_γ: a plate. Depending on each crystal's morphology or emission properties, specific photonic functions assigned for each crystal, BFTFP_α, BFTFP_β, and BFTFP_γ, respectively, were developed. BFTFP_α exhibited elastic flexibility with optical waveguiding. Although elastic organic single crystals tend to be less luminescent, the BFTFP_α crystal possessed 52% of Φ_PL which was one of the highest among previously reported elastic organic single crystals. BFTFP_β exhibited amplified spontaneous emission under excitation using a nanosecond pulsed laser due to their rigidity and monomeric luminescence. Platelet crystals of BFTFP_γ exhibited intense luminescence from their basal facets, making them ideal media for highly luminant photonic devices such as vertical cavity surface emitting lasers.