Jinghua Yu, Haixia Chang, Wendan Luo, Liping Ding,* Taihong Liu,* and Yu Fang. Adv. Optical Mater. 2025, e02848. DOI: https://doi.org/10.1002/adom.202502848

Two-dimensional covalent organic framework materials (2D COFs) have shown broad prospects in the fields of sensing, catalysis, and optoelectronic devices due to their adjustable pore structure and electronic properties. However, a majority of COFs are non-emissive or weakly emissive in the solid state owing to the intramolecular rotation and vibration together with strong π-π interactions. Therefore, achieving precise donor-π-acceptor (D-π-A) strength modulation for high-performance fluorescence sensing remains a significant challenge.

Figure 1. a) Molecular structures of the monomers and schematic description of the four nanofilms, b) Different D-π-A strength properties along the molecular skeletons, c) Schematic description for preparing the nanofilms based on the liquid-liquid interfacial condensation strategy. d) Photographs of the as-fabricated four nanofilms floating on the water surface under UV light.

In this work, four fluorescent nanofilms IDT-TAPB, IDT-TAPP, IDT-TAPM and IDT-TAPT were successfully prepared via the Schiff-base condensation reaction of 4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7-dicarbaldehyde (IDT-2CHO) and one of the heterocyclic triphenyl-amines with different nitrogen atoms, namely 1,3,5-tris(4-aminophenyl)-benzene (TAPB), 4,4',4''-(pyridine-2,4,6-triyl)trianiline (TAPP), 4,4',4''-(pyrimidine-2,4,6-triyl)trianiline (TAPM), and 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT) under liquid-liquid interfacial condition. Four nanofilms showed the uniform structure, smooth surface, adjustable thickness and different D-π-A strength. Through incorporating diverse electron-withdrawing acceptors, an atomically precise approach was demonstrated to tune D-π-A strength, which affected the energy level structure, intramolecular charge transfer (ICT) efficiency and fluorescence performance of the nanofilms. The experimental and theoretical calculations showed that the IDT-TAPM nanofilm had the strongest electron affinity and proton binding ability due to its central pyrimidine ring structure, and exhibited the most significant fluorescence enhancement and red shift when exposed to DCP vapor, which was an optimal sensing material. In the test of the laminated sensor based on the nanofilm, IDT-TAPM nanofilms showed fast response to DCP (fluorescence “turn on” within 3 s), wide detection range (0.1 ppb-132 ppm) and low detection limit (0.066 ppt), significantly lower than the Immediately Dangerous to Life and Health (IDLH) level of sarin. In addition, IDT-TAPM nanofilms remained stable over 55 cycles, with excellent photochemical stability and reversibility.

This study presented a novel strategy for enabling turn-on, ultrasensitive fluorescence detection through precise D-π-A modulation in interfacial nanofilms, thereby paving the way for next-generation, portable detectors with high reliability for chemical warfare agents. These sensing nanofilms hold significant potential for broad applications in military protection, environmental surveillance, and public safety.

Figure 2. Sensing performance analysis based on the laminated sensor platform

First Author: Yu Jinghua, Doctoral Candidate, Shaanxi Normal University.

Correspondence Authors: A/Prof. Liu Taihong, Prof. Ding Liping, Shaanxi Normal University

Full Text Link: https://doi.org/10.1002/adom.202502848