Engineering tetraphenylethene-based Z and E stereoisomers: structural analysis and sensing applications

Abstract

The exploration of Z/E stereoisomers represents an interesting and significant endeavor in molecular engineering. We developed three pairs of tetraphenylethylene (TPE) derivatives modified with heteroatom-containing trivalent arylboron and/or tris(2-methylpyridyl)amine (TPA) to explore differences in the optical properties, morphologies, and sensing applications between the Z and E stereoisomers. These isomers exhibit similar photophysical properties in the solution state and obvious differences in the amorphous aggregate and crystal states. All E-isomers exhibit tighter molecular packing and spatial interactions compared with the Z-isomers, resulting in longer-wavelength fluorescence emission. For the first time, TPE-based Z/E-isomers were separated by using a strategy in which different crystals emit different fluorescence under ultraviolet lamps. The single-crystal X-ray structures reveal the molecular configurations and luminescence mechanisms, allowing for a discussion on the pattern of the through-space charge transfer. The ultramarine fluorescence emission of the Z-isomer (cis-BB) should originate from the staggered molecular pairs, and the green fluorescence of the E-isomer (trans-BB) should originate from J-type molecular aggregations. In addition, the E isomer (trans-BN) shows a highly selective and sensitive response to HPO₄²⁻ based on the restriction of intramolecular rotation-induced fluorescence enhancement of TPE and the synergistic coordination effect of triarylboron and metal ions chelated by TPA. This work extends the family of stereoisomers available in molecular engineering with new structures and correlated morphologies and functionalities and demonstrates the design, separation, and evaluation of the Z and E stereoisomers of TPE and their derivatives.