Highly Bright Fluorescence from Dispersed Dimers: Deep‐Red Polymorphs and Wide‐Range Piezochromism

Abstract

applicability. [1b,2] At present, the practical requirements of flaw detection, haptic sensing, and deformation detection have promoted research into PCMs with strong penetrability and clear color differences (large spectral shifts). Compared with visible light, deep-red to infrared (IR) fluorescence has lower energy and longer PL wavelengths, giving it stronger penetrability. [3] Surprisingly, there are very few reports on PCMs with PL spectra that cover the deepred/near-IR regions and have clear color differences (>200 nm, Figure 1). [3b,4] In general, donor (D)-acceptor (A) luminophors generate an intramolecular charge transfer (CT) excited state in polar environments due to the twisting of the D-A bond. Interestingly, the CT state, which is highly sensitive to changes in the external environment, can induce tremendous shifts in the PL spectra. [4a,5] For instance, the He's group investigated the piezochromic behavior of two propeller-shaped boron difluoride complexes under high pressure.[5c] The intramolecular CT-type luminophor presented a larger color difference and higher sensitivity. However, such a CT state generally causes forbidden electronic transitions, leading to PL quenching. [5a,6] Thus, preparing near-IR luminophors with high PL quantum yields (PLQYs) is quite difficult. To solve this problem, Ma and another group reported D-A luminophores showing very bright near-IR fluorescence arising from the presence of a hybridized local and charge transfer (HLCT) state, which is a special excited state that possesses the characteristics of a locally excited (LE) state and CT character. [7] This combination is a "win-win" tactic: the former contributes to high PLQYs, while the latter guarantees high sensitivity to external stimuli. Accordingly, HLCT-type cyano-substituted oligo(p-phenylene vinylene) (CN-OPV) demonstrated a clear bathochromic shift (Δλ max > 145 nm) in its PL color from the visible to the near-IR region (λ max = 765 nm) upon exposure to high pressure. [8] Clearly, D-π-A-type luminophors with high brightness have great potential to show the clear color differences and strong penetrability characteristic. Nevertheless, during the compression, the PL intensity of PCMs is dramatically weakened as a consequence of conformational planarization and enhanced intermolecular π-π interactions, which is not conducive to further improving the piezochromic properties. [3b,9] Therefore, to realize high-performance piezochromic behavior, designing Near-infrared piezochromic materials (PCMs), which show a fluorescence response with a clear color difference and strong penetrability, have important potential applications, but few such organic luminophores have been reported. High-brightness donor (D)-acceptor (A) luminophores present great potential for preparing near-IR PCMs with clear color differences. However, the intermolecular π-π interactions are sharply enhanced at high pressures, which is not beneficial for the improvement of the piezochromic properties. Herein, two crystalline polymorphs (...

Effect of Connecting Units on Aggregation-Induced Emission and Mechanofluorochromic Properties of Isoquinoline Derivatives with Malononitrile as the Terminal Group

“…Furthermore, the fluorescence intensity of the mixture of IQ-BM decreases gradually when f w ≤ 80% (Figure 4a,b), which originates from the enhanced ICT effect through the increased polarity of the mixture. 50 It should be pointed out that the fluorescence of the mixture suddenly increases at f w = 90% (Φ F = 37.2%), considering that the increase of the content of glycerol does not lead to the enhancement of fluorescence of the glycerol− methanol mixed solvent of IQ-BM (Figure 4d); the enhanced fluorescence should be due to the loose stacking caused by the formation of aggregates rather than the RIR mechanism. These results show that the double bond is more conducive to AIE property than the single bond as a connecting unit.…”

Ultrahigh‐Sensitive Compression‐Stress Sensor Using Integrated Stimuli‐Responsive Materials

“…Imaging and measuring mechanical stresses, such as, compression, friction, and tensile stresses, are some of the challenging goals in recent materials science. [ 1–21 ] Mechano‐responsive materials exhibit visible and/or fluorescent color changes through microscopic structural changes including the chromophore at the molecular scale with the application of macroscopic mechanical stresses. Transfer of macroscopic mechanical stresses to microscopic structures is important for the design of mechano‐responsive color‐changing materials.…”

“…However, compression stress was detectable in the range 10 6 –10 10 Pa, namely the order of MPa and GPa, using mechano‐responsive materials in previous works ( Figure e and Figure S1, Supporting Information). [ 7–11,13,15,17,20,21,27–33 ] The facts indicate that the molecular motion leading to color changes is not easily induced by weak compression stress. New strategies are required for the measurement of weak stresses beyond approaches in molecular sciences.…”

“…The aggregation states of the chromophores were directly changed owing to the application of mechanical stresses, such as compression, tensile, and shear stresses. [7][8][9][10][11][12][13][14][15][16] The conjugation length of the chromophores changed through the cleavage of the chemical bond with the application of a tensile stress. [17][18][19][20][21] Previous studies indicate that sufficient mechanical stress strength is required to induce structural changes at the molecular scale, leading to color changes.…”

Dark to bright fluorescence state by inter-connecting fluorophores: concentration-dependent blue to NIR emission and live cell imaging applications

“…[52][53][54][55] Special strategies such as integration of thermally delayed fluorescence, structural engineering of squaraine dye and isolated dimer formation in crystals have also been reported for achieving strong deep red emission. [56][57][58] Despite these efforts, it is still a formidable task to develop efficient deep red/NIR fluorescent molecules.…”