Controlling Energy Gaps of π‐Conjugated Polymers by Multi‐Fluorinated Boron‐Fused Azobenzene Acceptors for Highly Efficient Near‐Infrared Emission

Abstract

We demonstrate that multi‐fluorinated boron‐fused azobenzene (BAz) complexes can work as a strong electron acceptor in electron donor‐acceptor (D‐A) type π‐conjugated polymers. Position‐dependent substitution effects were revealed, and the energy level of the lowest unoccupied molecular orbital (LUMO) was critically decreased by fluorination. As a result, the obtained polymers showed near‐infrared (NIR) emission (λPL=758–847 nm) with high absolute photoluminescence quantum yield (ΦPL=7–23%) originating from low‐lying LUMO energy levels of the BAz moieties (−3.94 to −4.25 eV). Owing to inherent solid‐state emissive properties of the BAz units, deeper NIR emission (λPL=852980 nm) was detected in film state. Clear solvent effects prove that the NIR emission is from a charge transfer state originating from a strong D‐A interaction. The effects of fluorination on the frontier orbitals are well understandable and predictable by theoretical calculation with density functional theory. This study demonstrates the effectiveness of fluorination to the BAz units for producing a strong electron‐accepting unit through fine‐tuning of energy gaps, which can be the promising strategy for designing NIR absorptive and emissive materials.

Vapochromic films of π-conjugated polymers based on coordination and desorption at hypervalent tin(iv)-fused azobenzene compounds

“…[47][48][49] Based on this concept, we have developed main-chain-type p-conjugated molecules and polymers with boron-fused azo (NQN) or azomethine (CQN) groups. 25,[50][51][52][53][54][55][56][57] They showed various functions such as film-state visible to NIR emission, 25,[51][52][53][54]57 stimuliresponsiveness, 25 aggregation-induced emission (AIE), 53,54,57,58 and crystallized-induced emission enhancement (CIEE). 51,52,[59][60][61][62][63] Hypervalent compounds are a class of molecules in which over eight electrons are formally assigned around a valence shell of a main group element beyond the limit of the Lewis octet rule.…”

Vapochromic Luminescent π‐Conjugated Systems with Reversible Coordination‐Number Control of Hypervalent Tin(IV)‐Fused Azobenzene Complexes

“…[28][29][30] As examples, we discovered cubic silica and boron cluster compounds named polyhedral oligomeric silsesquioxane (POSS) [31,32] and ocarborane, [33,34] respectively, have been applicable to main scaffolds for creating functional materials showing superior luminescence behaviors with high durability or stimuli-responsiveness. We recently reported the series of solid-state luminescent complexes and polymers containing boron-fused azomethine (À C=NÀ ) [35][36][37][38][39] and azo (À N=NÀ ) [40][41][42][43][44] complexes with the tridentate ligands, o-salicylideneaminophenol or 2,2'-dihydroxyazobenzne scaffold, respectively. [45] These materials showed intense emission in the condensed state, and highlyefficient polymer film emission [38] and near-infrared (NIR) absorptive and emissive characteristics [40][41][42]44] were obtained.…”

“…We recently reported the series of solid-state luminescent complexes and polymers containing boron-fused azomethine (À C=NÀ ) [35][36][37][38][39] and azo (À N=NÀ ) [40][41][42][43][44] complexes with the tridentate ligands, o-salicylideneaminophenol or 2,2'-dihydroxyazobenzne scaffold, respectively. [45] These materials showed intense emission in the condensed state, and highlyefficient polymer film emission [38] and near-infrared (NIR) absorptive and emissive characteristics [40][41][42]44] were obtained. It should be emphasized that unique environment-sensitive luminescent properties, such as aggregation-induced emission (AIE), [46,47] crystallization-induced emission enhancement (CIEE), [48] in which enhanced emission can be observed only in aggregation and crystal, respectively.…”

Effects of Hypervalent Bismuth on Electronic Properties of the Azobenzene Tridentate Ligand and Roles of Lewis Acidity in Controlling Optical Properties

“…The introduction of heteroatoms or inorganic elements such as nitrogen and boron is recognized as a valid strategy to improve the performance of electronics . Previously, in our group, the azobenzene tridentate ligand was selected as a π-conjugated scaffold because it allowed the coordination of various elements. − The introduction of tin for azobenzene tridentate ligands especially illustrated interesting properties such as a narrower energy gap and vapochromism for Lewis basic molecules. , This phenomenon was caused by the unique electronic properties of the hypervalent state of tin and its Lewis acidity. By introducing other elements, we have expected the structural diversities and the novel electronic effects of each hypervalent state.…”