Luminescent hexagonal microtubes prepared through water-induced self-assembly of a polymorphic organoboron compound: formation mechanism and waveguide behaviour

Tetra-coordinated organoboron (TCOB) compounds are promising candidates for developing high-performance optical devices due to their excellent optoelectronic performance. Fabricating TCOB-based nanomaterials of controlled and defined morphology through rapid and easy-to-execute protocols can significantly accelerate their practical utility in the aforesaid applications. Herein, we report water-induced self-assembly (WISA) to convert a polymorphic TCOB complex (HNBI-B, derived from a 2-(2'-hydroxy-naphthyl)-benzimidazole precursor) into two unique nanomorphologies viz. nanodiscoids (NDs) and fluorescent microtubes with hexagonal cross-sections (HMTs). Detailed electron microscopic investigations revealed that oriented assembly and fusion of the initially formed NDs yield the blue emissive HMTs (SSQY = 26.7%) that exhibited highly promising photophysical behaviour. For example, the HMTs outperformed all the crystal polymorphs of HNBI-B obtained from CHCl₃, EtOAc and MeOH in emissivity and also exhibited superior waveguide behaviour, with a much lower optical loss coefficient α' = 1.692 dB mm^-1 compared to the rod-shaped microcrystals of HNBI-B obtained from MeOH (α' = 1.853 dB mm^-1). Thus, this work reports rapid access to high performance optical nanomaterials through WISA, opening new avenues for creating useful nanomaterial morphologies with superior optical performance.