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The creation of MFC-active smart molecules by tuning functionality has received considerable attention owing to its versatile applicability. Pyrene-based twisted donor–acceptor (D–A) dyes (PySS and PySP) have been synthesized and characterized. Here, the pyrene is directly connected with thiophene, and this unit is further linked terminally to photoactive species (thiophene/pyridine) via a four-carbon unit conjugated spacer. These molecules show excellent solvatochromic properties, with a substantial shifting of the emission wavelength (PySS- 147 nm and PySP- 130 nm). The lowest transition state contains a significant contribution from ICT characteristics, as evidenced by spectral analysis and computational calculations. Moreover, these are identified as 'aggregation-induced enhanced emission' (AIEE) active compounds and exhibit mechanofluorochromism (MFC). By grinding, PySS and PySP display MFC features with 50 nm and 54 nm red shifting, respectively. Interestingly, PySS shows a gradual emission change from green (510 nm) to orange emission (578 nm) by gradually changing the pressure with a hydraulic press (0 to 12.5 tons). The single crystal structure of both compounds was investigated to understand the structure–property relationship for MFC. The crystal packing shows that the twisted molecules (dihedral angle between pyrene and thiophene is 59.36° and 56.93° for PySS and PySP, respectively) are loosely bound with several weak interactions (C–H⋯π, C–π⋯H, H⋯H, C–H⋯O). Interestingly, it was observed that two molecules in a unit cell are arranged in an antiparallel fashion; these molecular pairs are linearly connected to another pair axially, forming a long one-dimensional chain-type arrangement. On applying pressure, these twisted molecular pairs may slowly planarize, leading the molecules to come closer, thus changing the molecular interaction and the emission properties. A feasibility study of the potentiality of using these compounds in data encryption–decryption and security ink has also been demonstrated. [ABSTRACT FROM AUTHOR]

Based on the explosive nature and harmful effects of nitro-based explosive materials on living beings and the environment, it is extremely important to develop luminescence-based probe molecules for their detection with excellent selectivity and sensitivity. Two AIPE (aggregation-induced phosphorescence emission)-active iridium(III) complexes (M1 and M2) were developed for the sensitive detection of TNT in both contact and non-contact modes. The aggregate solutions of both complexes (M1 and M2 in THF/H2O, 1/9 by volume) detected TNT at the pico-molar (pM) level. These complexes showed greatly enhanced emission intensity while embedded in a PMMA(polymethyl methacrylate) matrix film. The amplified quantum efficiency, improved phosphorescence lifetime, and enhanced porous network of M2-PMMA composite helps to improve the sesitivity of TNT vapor detection. Interestingly, the sensitivity of the detection of TNT by the M2 complex was significantly improved (5-fold) in a PMMA-incorporated complex (CP) with an observed limit of detection (LOD) of 12.8 ppb. From the BET analysis of CP, it was observed that the mesoporous network of CP has an average pore diameter of 8.52 nm and a surface area of 2.03 m2 g−1. The porous network of CP assists in trapping TNT vapor in a polymeric network containing an electron-rich probe (iridium(III) complex, M2), which helps to effectively trap TNT, thus enhancing electronic communication. As a result, significant emission quenching was observed. [ABSTRACT FROM AUTHOR]

Nitroaromatic compounds are extremely explosive materials that pose a national security risk and raise environmental concerns. The design and development of sensitive and selective compounds for explosive materials are highly desirable. 'Aggregation-Induced Emission' (AIE) active materials are best suited for sensing purposes because of their sensitivity, fast detection time, and easy operation. By rationally incorporating substituents on the cyclometalated (C^N) ligand, four different AIE active iridium(III) based monocyclometalated complexes with the general formula [Ir(PPh3)2(H)(Cl)(C^N)] were synthesized. The phenyl ring of the phenyl pyridine cyclometalated portion of an iridium(III) complex was substituted with the right substituents to adjust the FMO levels thus, leading to appropriate alignment of the energy levels. Each of the resulting complexes displayed a significant property known as 'Aggregation-Induced Phosphorescent Emission' (AIPE). The complexes were subjected to structural characterization, electrochemical analysis, and photophysical property studies. The synthesized complexes were employed for the detection of aromatic nitro explosive compounds such as trinitrophenol (TNP) and trinitrotoluene (TNT) in the aqueous phase with a high degree of sensitivity. The sensing capabilities of each complex were assessed for these nitro explosive compounds and compared to those of the unsubstituted iridium(III) complex (M). Notably, the best limits of detection for TNP and TNT have been achieved with iridium(III) complexes [M1 (489 pM) and M3 (3.6 nM)] within the literature reported until now. For detecting picric acid with M1, FRET was found to be the potential mechanism, and for TNT, PET was found to be the cause of emission quenching by M3. Furthermore, for low-cost detection, filter paper-based sensing was also found effective for each complex. Real-field sensing of PA in soil samples was also performed. [ABSTRACT FROM AUTHOR]

For the vast majority of solid organic luminescent compounds, the emission color depends critically on the molecular packing in their crystal structure, giving rise to different emission colors for different polymorphs. Since the number of possible polymorphs is limited, it is very unusual to obtain materials with emissions covering the entire visible range for the same molecule. In this work, we report a new organic compound containing pyrene and thiophene fragments that shows an extremely rare feature in which the tunable solid-state emission is wholly reliant on its particle size and surface morphological evolution, which can be controlled by adjusting the reaction time. This allowed us to obtain compounds of the same material emitting at different regions of the visible spectrum, from blue to orange-red. High-resolution transmission electron microscopy (HR-TEM) images and field emission scanning electron microscopy (FE-SEM) images of the compounds isolated at different reaction times reveal a distinct change in the aggregate size and morphology (1D assembly, which is slowly turned into 2D layers, that is further converted to 3D hierarchical flowery architectures) that can be unambiguously associated with an observed gradual red shift in the PL emission. Time-correlated single-photon counting (TCSPC), dynamic light scattering, Raman spectroscopy, powder X-ray diffraction and J-aggregates formation are used to complement the explanation of the observed phenomenon. [ABSTRACT FROM AUTHOR]