Facile strategy for obtaining luminescent polymorphs based on the chirality of a boron-fused azomethine complex

Contribution of radiative rate constants to crystallization-induced emission enhancement in boron-fused azobenzene complexes

Analysis of emission enhancement in an aggregated state has been developed by focusing on the suppression of nonradiative processes by intermolecular interactions. On the other hand, it is difficult to evaluate the contribution of radiative processes despite its significant role in improving emission. Herein, the contribution of radiative processes to crystallization-induced emission enhancement (CIEE) is evaluated. By comparing optical properties between isolated and crystalline states at 77 K to minimize the structural relaxation, acceleration phenomena of the radiative process originating from the intermolecular interactions in crystal are clarified. Consequently, it is experimentally proved that a large refractive index in an organic crystal mainly promotes the radiative processes in CIEE even without significant intermolecular electronic interactions.

Near-Infrared Emissive Hypervalent Compounds with Germanium(IV)-Fused Azobenzene π-Conjugated Systems

A novel molecular design for showing near-infrared (NIR) emission is still required for satisfying growing demands for NIR-light technology. In this research, hypervalent compounds with germanium (Ge)-fused azobenzene (GAz) scaffolds were discovered that can exhibit NIR emission (λ_PL =690∼721 nm, Φ_PL =0.03∼0.04) despite compact π-conjugated systems. The unique optical properties are derived from the trigonal bipyramidal geometry of the hypervalent compounds constructed by combination of Ge and azobenzene-based tridentate ligands. Experimental and theoretical calculation results disclosed that the germanium-nitrogen (Ge-N) coordination at the equatorial position strongly reduces the energy level of the LUMO (lowest unoccupied molecular orbital), and the three-center four-electron (3 c-4 e) bond in the apical position effectively rises the energy level of the HOMO (highest occupied molecular orbital). It is emphasized that large narrowing of the HOMO-LUMO energy gap is achieved just by forming the hypervalent bond. In addition, the narrow-energy-gap property can be enhanced by extension of π-conjugation. The obtained π-conjugated polymer shows efficient NIR emission both in solution (λ_PL =770 nm and Φ_PL =0.10) and film (λ_PL =807 nm and Φ_PL =0.04). These results suggest that collaboration of a hypervalent bond and a π-conjugated system is a novel and effective strategy for tuning electronic properties even in the NIR region.

9,9-Bis[4-(N-aryl)phenyl]methylidene-xanthylidene Derivatives Displaying Mechano-, Crystallo-, and Thermochromism

Tetraphenylethylene (TPE) derivatives bearing a xanthene moiety are of interest because they have novel optical properties. 9,9-Bis[4-(N,N-diphenylamino)phenyl] and 9,9-bis[4-(9-carbazolyl)-phenyl]methylidene-xanthylidenes 3 and 4 were synthesized using Suzuki-Miyaura coupling of 9,9-dibromomethylidene-xanthylidene with the corresponding boronic acids. Diphenylamino derivative 3 exhibits mechanochromism and mechanofluorochromism (MC and MFC) reflected in absorption and fluorescence color changes. In contrast, carbazolyl derivative 4 displays thermo- and crystallo-chromism in addition to MC and MFC in the solid state. Powder X-ray diffraction and single crystal X-ray crystallographic analysis reveal that the solid state photophysical properties of these substances are governed by conformational changes rather by the creation of planar π-conjugation extended geometries.

Development of NIR emissive fully-fused bisboron complexes with π-conjugated systems including multiple azo groups

Development of novel near-infrared (NIR) emitters is essential for satisfying the growing demands of advancing optical telecommunication and medical technology. We synthesized elemental skeletons composed of robust π-conjugated systems including two boron-fused azo groups, which showed an intense emission in the red or near-infrared (NIR) region both in solution and solid states. Two types of bisboron complexes with different aromatic linkers showed emission properties with larger bathochromic shifts and emission efficiencies in solution than the corresponding monoboron complex. Transient absorption spectroscopy disclosed that the inferior optical properties of the monoboron complex can be attributed to fast nonradiative deactivation accompanied by a large structural relaxation after photoexcitation. The expanded π-conjugated system through multiple boron-fused azo groups can contribute to rigid molecular skeletons followed by improved emission properties. Moreover, the anti-form of the bisboron complex with fluorine groups in the opposite directions to the π-plane exhibited crystallization-induced emission enhancement in the NIR region. The molecular design by using multiple boron-fused azo groups is expected to be a critical strategy for creating novel NIR emitters.

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

The dynamic and reversible changes of coordination numbers between five and six in solution and solid states, based on hypervalent tin(IV)-fused azobenzene (TAz) complexes, are reported. It was found that the TAz complexes showed deep-red emission owing to the hypervalent bond composed of an electron-donating three-center four-electron (3c-4e) bond and an electron-accepting nitrogen-tin (N-Sn) coordination. Furthermore, hypsochromic shifts in optical spectra were observed in Lewis basic solvents because of alteration of the coordination number from five to six. In particular, vapochromic luminescence was induced by attachment of dimethyl sulfoxide (DMSO) vapor to the coordination point at the tin atom accompanied with a crystal-crystal phase transition. Additionally, the color-change mechanism and degree of binding constants were well explained by theoretical calculation. To the best of our knowledge, this is the first example of vapochromic luminescence by using stable and variable coordination numbers of hypervalent bonds.

Synthesis, Structure and Photophysical Properties of a New Class of Inherently Chiral Boron(III) Chelates-The tert-Leucine Complexes

A new family of boron(III) chelates is introduced whereby molecular chirality, confirmed by circular dichroism, is imported during synthesis such that isolation of the diastereoisomers does not require separation procedures. The photophysical properties of two members of the family have been examined: the N,O,O-salicylaldehyde-based derivative shows pronounced intramolecular charge-transfer character in fluid solution and is weakly fluorescent, with a large Stokes shift. The corresponding 2-methylamino-benzaldehyde-derived N,N,O-chelate absorbs and fluoresces in the visible region with a much smaller Stokes shift. Orange fluorescence is also observed for this compound as a cast film. Temperature-dependence studies show that decay of the fluorescent state is weakly activated but emission is less than quantitative at 77 K. Quite rare for boron(III)-based chelates, this derivative undergoes intersystem crossing to form a meta-stable triplet-excited state. X-ray crystal structures are reported for both compounds, along with simulated ECD spectra.

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

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 (λ_PL =758-847 nm) with high absolute photoluminescence quantum yield (Φ_PL =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 (λ_PL =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.

Two-photon excitable boron complex based on tridentate imidazo[1,5-a]pyridine ligand for heavy-atom-free mitochondria-targeted photodynamic therapy

We have synthesized a cyan fluorescent boron complex based on a tridentate imidazo[1,5-a]pyridine ligand. The boron complex was found to have potential applications as not only a chiroptical material but also a heavy-atom-free mitochondria-targeted photosensitizer for cancer treatment.

We have synthesized a cyan fluorescent boron complex based on a tridentate imidazo[1,5-a]pyridine ligand.

The effect of alkyl chain lengths on the red-to-near-infrared emission of boron-fused azomethine conjugated polymers and their film-state stimuli-responsivities

We present systematic studies of the dependence of the red-to-near-infrared emission and stimuli-responsive properties of boron-fused azomethine conjugated copolymers on the lengths of the alkyl chains.