# Realizing luminescent single crystals of covalent organic polymers with color-tunable emission through isomer engineering

## TL;DR

Scientists created glowing single crystals of a new polymer by using different molecular shapes, enabling color-tunable light emission and applications in data encryption.

## Contribution

A novel isomer engineering strategy to control luminescence in covalent organic polymer single crystals.

## Key findings

- 3,6-TPy-BCz produces luminescent crystals with a loose stacking model.
- 2,7-TPy-BCz forms non-emissive crystals with a close packing model.
- CityU-50 shows excitation-dependent and solvent-responsive luminescence for encryption.

## Abstract

Preparing luminescent single crystals of B–N-containing covalent organic polymers (COPs) is very important but highly challenging, since most conditions only produce luminescence-silent crystals. To address this issue, we employ an isomer engineering strategy to successfully prepare luminescent (CityU-50) and luminescence-silent (CityU-60) single crystals of B–N linked COPs based on two distinct chromophore isomers, 3,3′,6,6′-tetra(pyridin-4-yl)-9,9′-bicarbazole (3,6-TPy-BCz) and 2,2′,7,7′-tetra (pyridin-4-yl)-9,9′-bicarbazole (2,7-TPy-BCz). Single-crystal X-ray diffraction analysis reveals that 3,6-TPy-BCz can lead to a “loose” stacking model of helical polymer chains in CityU-50, effectively suppressing interchain aggregation and endowing CityU-50 with unprecedented visible-light-excitable luminescence, while 2,7-TPy-BCz results in a “close” packing model of helical polymer chains in CityU-60, leading to a non-emissive structure. This factor validates our design principle. We also find that CityU-50 can display excitation-dependent and solvent-responsive luminescence due to the different aggregation states within the crystal and solvent-triggered packing changes in the polymer chains of CityU-50. Based on these responses, we demonstrate the application of CityU-50 in advanced information encryption. This work not only delivers a new class of smart crystalline materials but also establishes a powerful strategy for engineering crystalline polymers with precisely tailored structures and properties.

Luminescent B–N linked polymer single crystals have been successfully synthesized through a molecular isomer engineering strategy. Moreover, advanced information encryption and optical storage applications have been developed.