Three tetraphenyl imidazole derivatives functionalized by an α-cyanostilbene unit (3a–3c) have been designed and synthesized. A combination of the representative aggregation-induced emission enhancement fluorophore and propeller sharp tetraphenyl imidazole unit in the same molecule achieved the integration of their function. 3a–3c emitted weakly in dilute organic solvents and showed an evident solvatochromic effect caused by strong intramolecular charge transfer (ICT), which has been confirmed by density functional theory (DFT) calculations. Meanwhile, in the solid state, they exhibited obvious fluorescence and stimuli-responsive emission. The main emission peak of compound 3a was red-shifted from 519 nm to 550 nm after grinding. The reason could be explained that the destruction of the crystalline structure leads to the planarization of molecular conformation or the increase of conjugation.

A new Λ-shaped α-cyanostilbene derivative (TSA) containing an active amino group with fluorescence emission both in solution and the solid state was designed, synthesized and characterized. Its colour and fluorescence emission can be switched by external stimuli including acid/base, trinitrophenol (picric acid, PA) and grinding. The ¹H NMR spectra of TSA were recorded before and after alternately adding trifluoroacetic acid (TFA) and triethylamine (TEA), which confirmed that acidichromism originated in the transformation of protonation/deprotonation between TSA and TSA-H⁺. Powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) revealed the crystalline–amorphous state transformation between the original and ground compounds, and this transformation could reasonably account for the mechanochromic (MFC) luminescence properties. The results revealed that TSA can be used as a fluorescence sensor for acid/base and PA, and as a rewritable information storage material. Moreover, it can also be applied in white light-emitting diodes (w-LEDs) based on its desirable fluorescence emission in the solid state.

A highly fluorescent emissive difluoroboron complex (TC) was designed, synthesized and characterized. The absolute fluorescence quantum yields of TC are 33.0% in the solid state and 26.0% in the solution state. The results of X-ray diffraction analysis demonstrate that the rigid structure, the intermolecular hydrogen bonding and the well-ordered as well as loose molecular packing are the major reasons for the high fluorescence emission. Moreover, the colors and fluorescence emission of TC can be switched by external stimuli including grinding and acids/bases. Powder X-ray diffraction (PXRD) investigation verifies that the crystalline–amorphous state transformation between the original and ground states might be attributed to the mechanochromic luminescence properties. The high fluorescence emission and multi-stimuli-responsive fluorescence properties reveal that TC can be used as rewritable information storage, security ink and a fluorescence indicator for acids/bases.

Two novel imidazole derivatives (M-1 and M-2) containing an α-cyanostilbene unit were synthesized and characterized by FT-IR and NMR spectroscopy, mass spectrometry and X-ray crystallography. Compounds M-1 and M-2 exhibited aggregation-induced emission enhancement (AIEE) properties due to intermolecular interactions and the restriction of intramolecular rotation in the aggregated state. The relationships between their structures and photophysical properties were studied via spectroscopic methods and density functional theory (DFT) calculations. It is worth noting that these materials can change their fluorescence emission between green and yellow under gaseous acidic (TFA) and basic conditions (Et₃N), and they show reversible acid/base stimulated fluorescence switching behavior in the solid state. The mechanism was further confirmed by ¹H NMR spectroscopy in chloroform. In addition, M-1 and M-2 were utilized for picric acid (PA) and 2,4-dinitrophenol (2,4-DNP) detection in a THF–water mixture (f_w = 90%) and the detection limits were as low as 10⁻⁶ mol L⁻¹.

Organic luminogens (OLs) that are capable of yielding strong fluorescence in both solution and the aggregated state are referred to as dual-state emission luminogens (DSEgens). They are able to achieve multiple functions within onefold materials owing to their applicability in both states, and are highly desirable though less explored compared to conventional and aggregation-induced-emission (AIE) OLs. In this work, cyanostilbene (CS) and naphthalimide (NI) were incorporated to construct novel DSEgens, which were functionalized by various alkoxyl chains at the CS unit (NICS-X, X = M, E, P, and B, respectively) by balancing the design principles for conventional and AIE OLs. The photophysical properties of NICS-Xs were investigated by steady-state and transient spectroscopies, which showed strong emission with quantum yields of luminescence (φ) above 0.70 in both solution and solids. In particular, the attachment of different terminal alkoxyl chains finely tuned the emission wavelength of NICS-Xs with a difference of up to ∼59 nm. Single-crystal structure analysis and theoretical calculations of these materials suggested that simply changing the length of the terminal alkoxyl chains could effectively adjust the molecular conformation and packing structure, thus inducing large shifts of the photoluminescence (PL) spectra. In addition, NICS-Xs show two-photon absorption behaviors in the dual states with a cross section of up to 238 Goeppert-Mayer. On the one hand, our study reported a new family of DESgens and a straightforward strategy to finely tune their photophysical properties. On the other hand, the correlations between the photophysical properties and the molecular/solid-state structures were revealed, which suggested new design ideas for developing novel high-performance DSEgens.