Side-chain engineering of 4-aryl-1,8-naphthalimides: Fine-tuning for mechnochromic and vapochromic response in dual-state emission mode

An alkyl chain engineering strategy is described to design mechanochromic and vapochromic response molecules with dual state emission. Based on a red emissive AIE-active fluorophore (NAPTPA-phenol), the alkyl chains with different length (C2 ∼ C8) are introduced to construct NAPTPA1-4 via an ester linker. All four compounds display dramatic emissions in both aggregation and solid states, which showed higher quantum yields than their precursor NAPTPA-phenol. Among these compounds, NAPTPA1 with the C2 alkyl chain shows the largest bathochromic shift (∼50 nm) from 560 nm (Φ = 92.2 %) to 610 nm (Φ = 59.2 %) between two solid states. The large shift might be assigned to the efficient intramolecular charge transfer between naphthalimide and triphenylamine and two different stacking modes (self-assembly and amorphous). The high fluorescence quantum yields in aggregation state should be attributed to restricted of intramolecular rotation of the rotor-shape 4-arylnaphthalimide fluorophores. Furthermore, the needle-like self-assembly structure is seen in the SEM pattern of NAPTPA1, which exhibited large difference comparing to NAPTPA-phenol and NAPTPA2 ∼ 4. After grinding, the amorphous forms were given to all compounds that led their similar emission bands in red region centered on 610 nm. With the dramatic dual emission color, NAPTPA1 with short side chain (C2) was used to perform a vapochromic reverse courses by using the common VOCs to fume the NAPTPA1-loaded filter paper. The test exhibits the obvious color transformation and fast reversing time (<20 s) indicating the test strip can be successfully implemented to monitor VOCs.