A Highly Efficient Near‐Infrared‐Emissive Copolymer with a N=N Double‐Bond π‐Conjugated System Based on a Fused Azobenzene–Boron Complex

Abstract

Fused azobenzene-boron complexes (BAzs) show highly efficient near-infrared (NIR) emission from the nitrogen-nitrogen double bond (N=N) containing π-conjugated copolymer. Optical measurements showed that BAz worked as a strong electron acceptor because of the intrinsic electron deficiency of the N=N double bond and the boron-nitrogen (B-N) coordination which dramatically lowered the energy of the lowest unoccupied molecular orbital (LUMO) of the azobenzene ligand. The simple donor-acceptor (D-A) type copolymer of bithiophene (BT) and BAz exhibited intense photoluminescence (PL) in the NIR region both in the dilute solution (λ =751 nm, Φ =0.25) and in the film (λ =821 nm, Φ =0.038). The BAz monomer showed slight PL in the dilute solution, and aggregation-induced emission (AIE) was detected. We proposed that N=N double bonds should be attractive and functional building blocks for designing π-conjugated materials.

Unique Substitution Effect at 5,5′‐Positions of Fused Azobenzene–Boron Complexes with a N=N π‐Conjugated System

“…Synthesis of the 5,5'-dibromo-substituted fused BAz complex (BAz-5Br)w as performed according to our previous report (Scheme1). [10] Initially,a no xidative coupling reactiono f5bromo-2-methoxyaniline in the presenceo fM nO 2 was carried out to obtain azobenzene derivative Az-5Br.T he methoxy groups of 1 were deprotected with BBr 3 to afford tridentate ligand 2.Amixture of ligand 2 and BF 3 ·Et 2 Oi nt oluene and Et 3 Nw as reacted at 100 8Cf or 1h,a nd then the target complex BAz-5Br was synthesized. Next, with BAz-5Br as am onomer,D -A-type p-conjugated copolymers with ab ithiophene comonomer and af luorene comonomer were prepared (Scheme 2).…”

“…As model compoundsf or investigating substitutione ffects, compounds BAz-H and BAz-4Br were prepared according to our previousw ork;t he structures of these compounds, as well as that of P-BAz-4BT,a re shown in Scheme2. [10] All synthesized compounds showed good solubility in common organic solvents, such as chloroform, dichloromethane, and tetrahydrofuran, and were characterized by meanso f 1 H, 13 C, and 11 BNMR spectroscopy;M S; and elemental analyses (see the Supporting Information). According to these characterization data, we concluded that the products possessed the designed structures.…”

“…[11] In contrast, it was unveiled that the fused azobenzene-boron (BAz) complex (Figure1)p ossessed an outstandingly low LUMO energy level and the resulting D-A-type p-conjugated copolymers howedh ighly efficient NIR emission. [10] From these analyses, it was suggestedt hat substitution at the 4,4'-positions of the BAz played ak ey role in expanding the p-conjugated system ande xpressing intense emission, even in the presenceo ft he azobenzene unit. Future interests are directed to expressd iversity in electronic structures based on the BAz skeleton by alteringt he positions of connections.…”

“…As another example, it was reported that BÀNc oordination in the p-conjugated system effectively lowered the LUMO level of the p-conjugated unit. [10] Although well knownf or azobenzene derivatives, owing to emission annihilation through molecular motions in the excited state, such as cis-trans photoisomerization, so far a limited number of luminescent molecules containing N=N bonds have been reported. [11] In contrast, it was unveiled that the fused azobenzene-boron (BAz) complex (Figure1)p ossessed an outstandingly low LUMO energy level and the resulting D-A-type p-conjugated copolymers howedh ighly efficient NIR emission.…”

“…It was shownf rom optical and electrochemical measurements that an arrower band gap energy should be realized in the 5,5'-substitutedB Az, compared with those of 4,4'-substituted derivatives. [10] Theoretical calculations at the DFT level proposed that the HOMO and LUMO energy levels were individually perturbed through the substituent effect. Furthermore, it was found that delocalized HOMO and localizedLUMO weresimultaneously realized in the 5,5'-substitutedB Az polymers.…”