Solid-State Near-Infrared Emission of π-Conjugated Polymers Consisting of Boron Complexes with Vertically Projected Steric Substituents

Abstract

The development of solid-state near-infrared (NIR)-emissive π-conjugated polymers (CPs) has been still difficult due to lack of valid guidelines for avoiding aggregation-caused quenching. To obtain solid-state emission, we designed new strong electron acceptors by employing boron-fused azobenzene complexes with vertically projected bulky substituents at boron, which can prevent the π-surface from intermolecular interactions. Herein, we demonstrate donor–acceptor-type CPs with NIR emission properties. In summary, the CP with a bithiophene donor bearing the bulkiest substituent exhibited excellent solid-state NIR emission (λFL = 806 nm, ΦFL = 7.5%). In addition, the longest emission maximum was obtained from the CP containing cyclopentadithiophene (λFL = 923 nm, ΦFL = 0.9%). The concept for the introduction of vertically projected bulky substituents at boron opens a new research field on solid-state NIR-emissive materials.

UV-to-NIR Wavelength Conversion of π-Conjugated Polymers Based on Pyrene-Substituted Boron-Fused Azobenzene Complexes

“…Both polymers showed good solubility in moderately polar organic solvents such as THF, chloroform, and dichloromethane, while they were slightly soluble in nonpolar solvents such as toluene probably due to strong π–π interactions of pyrene moieties. This property was different from previously reported similar polymers with bulky substituents which also showed good solubility in toluene . The chemical structures of all new compounds in this study were confirmed by 1 H, 13 C­{ 1 H}, 11 B NMR spectroscopies, high-resolution mass spectrometry, and elemental analyses (see the Supporting Information).…”

“…This property was different from previously reported similar polymers with bulky substituents which also showed good solubility in toluene. 38 The chemical structures of all new compounds in this study were confirmed by 1 H, 13 C{ 1 H}, 11 B NMR spectroscopies, high-resolution mass spectrometry, and elemental analyses (see the Supporting Information). In addition, we clarified the single-crystal structure of 1-py-Br whose crystal with sufficient size for the analysis was obtained by slow evaporation from acetonitrile solution (Figure S1 and Table S1).…”

“…To investigate the electronic properties in the ground state, UV–vis–NIR absorption measurements of 1-py-BT and 2-py-BT were conducted with a diluted solution (1.0 × 10 –5 M per repeating unit in chloroform) (Figure a and Table ). We also examined the previously reported polymer Ph-BT for comparison. , All polymers showed the broad absorption bands around 650 nm (A1 band, 500–750 nm). In addition, the synthesized polymers ( 1-py-BT and 2-py-BT ) exhibited the stronger absorption band attributed to S 0 → S 2 transition of pyrene around 345 nm (A2 band, 300–400 nm) than Ph-BT .…”

“…In addition, BAz had an unique structure: (1) the substituent at the boron center is perpendicularly protruded to the π-conjugated plane, which can work as a better steric hindrance than the twisted structures apparent in boron complexes with bidentate ligands; and (2) the fused ring structure is less planar than BODIPY, which can also contribute to avoiding ACQ. As a result, the D–A polymers based on BAz as the electron acceptor exhibited NIR luminescence in not only solution but also film. , Furthermore, we have clarified that the solid-state luminescence can be enhanced by introducing further steric substituents at the boron center. , Herein, to develop efficient UV-to-NIR WCMs, we have designed the novel BAz derivative-attached pyrene, which had both bulky structure and excellent UV absorption capability, at the boron. Compared to the system based on the subphthalocyanine and N 2 O-type BODIPY , scaffolds which had similar structural features to our design, the system using the BAz unit has advantages in terms of polymerization for NIR emission.…”

“…35,36 Furthermore, we have clarified that the solid-state luminescence can be enhanced by introducing further steric substituents at the boron center. 37,38 Herein, to develop efficient UV-to-NIR WCMs, we have designed the novel BAz derivative-attached pyrene, which had both bulky structure and excellent UV absorption capability, at the boron. Compared to the system based on the subphthalocyanine 39 and N 2 O-type BODIPY 40,41 scaffolds which had similar structural features to our design, the system using the BAz unit has advantages in terms of polymerization for NIR emission.…”