Dicyanopyrazine capped with tetraphenylethylene: polymorphs with high contrast luminescence as organic volatile sensors

Chao Ge, Yang Liu,* Xin Ye, Xiaoxin Zheng, Quanxiang Han, Jie Liu and Xutang Tao

Mater. Chem. Front., 2017, 1, 530--537

Abstract

Recently the polymorphism and phase-transition between polymorphs have attracted special attention
in organic luminescent materials. In this report, we employed propeller-like tetraphenylethylenes to
functionalize large conjugated and electron-withdrawing dicyanopyrazine units, constructing novel
organic luminescent polymorphs with high contrast fluorescence. Through an in-depth study of the
luminescence-related properties, the more planar structured 2DQCN was found to generate close
intermolecular packing and demonstrated essentially different solid state luminescence behaviour
compared with that of 1DQCN. It shows crystallization-induced red-shift fluorescence, in contrast to the
crystallization-induced blue-shift of 1DQCN and many other AIE (aggregation-induced emission)
materials. The morphology transformation of 2DQCN can be realized via solvent fuming and thermal
annealing, where the transformation among its three forms was manifested by DSC measurements.
Utilizing the high contrast luminescence of 2DQCN polymorphs, we demonstrated their real applications
as selective visual sensors for detecting volatile organic compounds. The results, revealing insights into
the structure–property relationship of luminescent polymorphs, should provide useful directions for the
understanding, design and utilization of luminescent colour-switching materials.