Deep‐Red Fluorescence from AIE‐Active Luminophore: High‐Brightness and Wide‐Range Piezochromism**

Abstract

Driven by the rapidly increasing demand for intelligent materials, the sensitivity of organic materials to pressure has been intensively investigated in recent years. Many examples describe material responses to both mechanical grinding and hydrostatic pressure, but only few materials have been identified with clear colour difference as well as strong penetrability, especially under high pressure, limiting development of pressure sensors. In this work, an asymmetric luminophore, MTBA was developed by end‐capping a (Z)‐3‐(benzo[c][1,2,5]‐thiadiazol‐4‐yl)‐2‐phenylacrylonitrile (TPAN) core with propeller‐like triphenylamine (TPA) and rigid 12b‐methyl‐5,12b‐dihydroindeno[1,2,3‐kl]acridine (MeIAc) units, revealing aggregation‐induced emission (AIE) behaviour. In high‐pressure and grinding experiments, MTBA exhibits a dramatically large colour difference of up to 164 nm and 112 nm in deep‐red and near‐infrared regions, respectively, for piezochromic performance that is among the best reports for pure organic materials. The experimental and theoretical analyses indicated that the excellent piezochromic performance of MTBA is due to its highly twisty and rigid conformation, which weakens intermolecular π‐π interaction and obstructs emission quenching during compression.

Near‐infrared Piezochromic Materials at High Pressure

“…It can well modulate the lowest unoccupied molecular orbital (LUMO) energy level of the molecule and the combination with the acrylonitrile part can constitute a sizeable electron-accept plane, which can further reduce the molecular energy gap and modulate the aggregated structure. [50][51][52][53][54] As expected, our group employed this structure to construct some NIR-emitting PCMs, which exhibit the high PLQYs due to the distinct aggregation-induced emission and hybridization-local charge transfer excited states properties. In fact, these structures do exhibit satisfactory piezochromic properties, some with significant color differences, some more pressure-resistant, and some with higher sensitivity.…”

The crucial role of planar π-conjugated segment in the molecular conformation of diphenyldibenzofulvene derivatives in crystals contributing to reducing emission energy

“…As a result, at ambient pressure, intermolecular coupling should not be the crucial factor to impact the emission color ofS4 . [60][61][62][63] Contrastively, the molecular conformation of crystal S5 states the phenyl blades with slightly less torsion, 64.3 0 of θ 5 and 49.4 0 of θ 6 . In the planar moiety (conformation torsion ¡ 10.7 0), it reveals the longer ethylenic C 1 =C 2 bond (1.355 ), larger θ 1 (105.2 0), and the related shorter r 1 (1.492 ) and r 2 (1.489 ), as collected in Table 1.…”

“…As well, some investigations are highlighting the piezochromic luminescence of AIEgens in the near-infrared range and the AIE-to-ACQ transformation. [60][61][62][63][64][65][66][67][68] These investigations seem to establish a common rule that the piezochromic luminescence of AIEgens with raising pressure principally performs an initially enhanced emission tendency and then follows a gradually decayed tendency. It is assigned to the AIEgens with flexible molecular conformations.…”

“…Because the close distance among molecules leads to intense hydrogen bonds and the strong steric hindrance to block the non-radiative transition. [60][61][62][63] Moreover, it also can be concluded that the closer molecular stacking without validπ -π stacking could not cause the lower emission energy. To gain the lower energy emission of S3 , a pressure larger than 3.0 GPa should be applied.…”

“…Nevertheless, the piezochromic luminescence of materials that suffer the obvious π -π face-to-face stacking and closing distance to surrounding molecules display a continuously decayed emission intensity and the shift of emission center with the raising pressure but do not appear an initial increase window of emission intensity. [60][61][62][63] Herein, to decipher the intrinsic factors responding to the emission energy of AIEgens, we cautiously investigated the photophysical properties of a series of DPDBF derivatives in crystals. The utilized DPDBF derivatives include 9-Benzylidenefluorene to produce crystalS1 , DPDBF to generate crystal S2 , 9-(4-Methoxyphenylphenylmethylene)-9H-Fluorene to obtain crystalS3 , and 9-[di(4-methoxyphenyl)methylene]-9h-fluorene to grow crystals S4 and S5 .…”