An Unexpected 4,5‐Diphenyl‐2,7‐naphthyridine Derivative with Aggregation‐Induced Emission and Mechanofluorochromic Properties Obtained from a 3,5‐Diphenyl‐4H‐pyran Derivative

Abstract

For a specific fluorescent molecule, the increase of molecular conformation distortion is beneficial to endow it with aggregation-induced emission (AIE) and mechanofluorochromic (MFC) properties. Herein, 3,5-diphenyl-4H-pyran derivative 5 and 4,5-diphenyl-2,7-naphthidine derivative 7 with highly twisted conformations were synthesized. For compound 5, although the introduction of phenyl rings with large steric hindrance at 3 and 5 positions of the 4H-pyran skeleton realized the transformation from aggregation-induced quenching (ACQ)-active molecule to AIE-active molecule, it only showed a low-contrast MFC activity. Compound 7 was accidentally obtained from compound 5 and n-butylamine via a ring-opening and subsequent intramolecular ring-closing mechanism. Compound 7 was confirmed to have a highly twisted molecular conformation by the crystal structural analysis and exhibited AIE activity originated from the restriction of intramolecular rotation. Furthermore, compound 7 exhibited reversible high-contrast MFC activity. Upon grinding, the change of solid-state fluorescence color from orange to yellow was confirmed to be due to the partial destruction of crystal structure. This work provides new ideas for the design and synthesis of novel AIE-active and MFC-active fluorescent molecules based on ACQ-active parent molecules.

Mechanochromic Luminescence of 2,6-Bis(4-biphenyl)isonicotinic Acid via Interconversion of Classical/Frustrated Brönsted Pair

“…These materials have attracted considerable attention because of their potential applications in mechanosensors, fluorescence switches, and security systems . There are several approaches to design mechanochromic materials, such as incorporation of mechanophores, regulation of molecular conformation, and molecular stacking patterns . Although mechanochromic materials have developed rapidly in the past 20 years, the types of interactions are still very limited.…”

Unexpected synthesis, delayed emission and solid-state acidochromism of novel 2,7-naphthyridine derivatives obtained from 2-(3,5-diaryl-4H-pyran-4-ylidene)malononitrile

“…In order to explain this phenomenon, the possible mechanisms for obtaining DPNp (Scheme S2, ESI †) and DBNt (Scheme S3, ESI †), respectively, are proposed. According to previous reports, 3,6,7 the reaction should start with the nucleophilic attack of benzylamine on 2- (3,5-diaryl-4H-pyran-4-ylidene) malononitrile which causes the ring opening of 4H-pyran. The intramolecular and thus intermolecular nucleophilic addition reactions of the amino group on benzylamine to two cyano groups lead to the formation of intermediates 9 (Scheme S2, ESI †) and 10 (Scheme S3, ESI †).…”

“…Inspired by the plasticity of the 4H-pyran ring in reactions, in this work, the reaction of 2- (3,malononitrile and benzylamine is investigated (Scheme 1f ). Herein, phenyl and 1,1′-biphenyl were introduced at the 3 and 5 positions of the 4H-pyran ring, which would be conducive to the ring-opening reaction of pyran due to the further enhancement of the twisted molecular structure.…”

“…Using 1,3-diphenylpropan-2-one (1) as the starting material, 2,6-dimethyl-3,5-diphenyl-4H-pyran-4-one (2) and the key intermediate 2-(2,6-dimethyl-3,5-diphenyl-4H-pyran-4-ylidene) malononitrile ( 3) were synthesized according to a previous synthetic method (Scheme S1, ESI †). 3 Using 2-(4-bromophenyl) acetic acid (4) as the starting material, 1,3-bis (4-bromophenyl) propan-2-one (5), 3,5-bis(4-bromophenyl)-2,6-dimethyl-4Hpyran-4-one ( 6), and 3,5-di([1,1′-biphenyl]-4-yl)-2,6-dimethyl-4H-pyran-4-one (7) were obtained by a previous synthetic method. 8 Another key intermediate, 2- (3,5-di([1,1′-biphenyl]-4yl)-2,6-dimethyl-4H-pyran-4-ylidene)malononitrile (8), was synthesized by reacting compound 7 with malononitrile in acetic anhydride and its chemical structure was confirmed by singlecrystal X-ray diffraction (Fig.…”

“…2 However, they are used only in the liquid phase, and to the best of our knowledge, only one case of a 2,7-naphthyridine-based solid-state fluorescent material has been reported to date. 3 The possible reason for this phenomenon is that its planar and rigid structure is vulnerable to the negative effects of solid-state fluorescence quenching caused by aggregation. Additionally, the lack of simple methods to construct 2,7-naphthyridines is also an important obstacle, 4 which deserves further study.…”