Aggregation of Naphthobisthiadiazole-Based Donor-Acceptor-Donor Dyes That Restrict Quenching in Solution and Emit Red Light in Polar Aqueous Media

Water Fraction Dependence of the Aggregation Behavior of Hydrophobic Fluorescent Solutes in Water-Tetrahydrofuran

This work investigates the water fraction dependence of the aggregation behavior of hydrophobic solutes in water-tetrahydrofuran (THF) and the elucidation of the role of THF using fluorescence microscopy, dynamic light scattering, neutron and X-ray scattering, and photoluminescence measurements. On the basis of the obtained results, the following model is proposed: hydrophobic molecules are molecularly dispersed in the low-water-content region (10-20 vol %), while they form mesoscopic particles upon increasing the water fraction to ∼30 vol %. This abrupt change is due to the composition fluctuation of the water-THF binary system to form hydrophobic areas in THF, followed by THF-rich droplets where hydrophobic solutes are incorporated and form loose aggregates. Further increasing the water content prompts the desolvation of THF, which decreases the particle size and generates tight aggregates of solute molecules. This model is consistent with the luminescence behavior of the solutes and will be helpful to control the aggregation state of hydrophobic solutes in various applications.

Reducing the internal reorganization energy via symmetry controlled π-electron delocalization

The magnitude of the reorganization energy is closely related to the nonradiative relaxation rate, which affects the photoemission quantum efficiency, particularly for the emission with a lower energy gap toward the near IR (NIR) region. In this study, we explore the relationship between the reorganization energy and the molecular geometry, and hence the transition density by computational methods using two popular models of NIR luminescent materials: (1) linearly conjugated cyanine dyes and (2) electron donor–acceptor (D–A) composites with various degrees of charge transfer (CT) character. We find that in some cases, reorganization energies can be significantly reduced to 50% despite slight structural modifications. Detailed analyses indicate that the reflection symmetry plays an important role in linear cyanine systems. As for electron donor–acceptor systems, both the donor strength and the substitution position affect the relative magnitude of reorganization energies. If CT is dominant and creates large spatial separation between HOMO and LUMO density distributions, the reorganization energy is effectively increased due to the large electron density variation between S₀ and S₁ states. Mixing a certain degree of local excitation (LE) with CT in the S₁ state reduces the reorganization energy. The principles proposed in this study are also translated into various pathways of canonically equivalent π-conjugation resonances to represent intramolecular π-delocalization, the concept of which may be applicable, in a facile manner, to improve the emission efficiency especially in the NIR region.

The reorganization energies may be significantly reduced by molecular symmetry effect.

Long-wavelength visible to near infrared photoluminescence from carbon-bridged styrylstilbene and thiadiazole conjugates in organic and aqueous media

Donor–acceptor–donor conjugates composed of electron-donating carbon-bridged styrylstilbene (COPV2) and electron-accepting thiadiazole derivatives equipped with carbazolyl (Cz) terminators, Cz-COPV2-A-COPV2-Cz (A = benzothiadiazole (BTz), naphthobis(thiadiazole) (NTz), or benzobis(thiadiazole) (BBTz)), were newly synthesized and found to serve as efficient and stable long-wavelength photoluminescent dyes in organic and aqueous media. In particular, Cz-COPV2-BBTz-COPV2-Cz showed photoluminescence in the near infrared region (895–927 nm) with a photoluminescence quantum yield (PLQY) of up to 0.19 in cyclohexane and of 0.02–0.03 in THF/water mixtures. Its analogues with weaker acceptors, Cz-COPV2-BTz-COPV2-Cz and Cz-COPV2-NTz-COPV2-Cz, showed yellow to deep-red emission in organic solvents, with PLQYs of up to 0.71 in organic solvents and 0.45 in THF/water mixtures.

Efficient long-wavelength visible to NIR-emitting materials have been synthesized by use of rigid planar styrylstilbene as a donor component.

A Simple and Strong Electron-Deficient 5,6-Dicyano[2,1,3]benzothiadiazole-Cored Donor-Acceptor-Donor Compound for Efficient Near Infrared Thermally Activated Delayed Fluorescence

Despite the success of thermally activated delayed fluorescent (TADF) materials in steering the next generation of organic light-emitting diodes (OLEDs), effective near infrared (NIR) TADF emitters are still very rare. Here, we present a simple and extremely high electron-deficient compound, 5,6-dicyano[2,1,3]benzothiadiazole (CNBz), as a strong electron-accepting unit to develop a sufficiently strong donor-acceptor (D-A) interaction for NIR emission. End-capping with the electron-donating triphenylamine (TPA) unit created an effective D-A-D type system, giving rise to an efficient NIR TADF emissive molecule (λ_em =750 nm) with a very small ΔE_ST of 0.06 eV. The electroluminescent device using this NIR TADF emitter exhibited an excellent performance with a high maximum radiance of 10020 mW Sr^-1  m^-2 , a maximum EQE of 6.57% and a peak wavelength of 712 nm.

Amphiphilic triphenylamine–benzothiadiazole dyes: preparation, fluorescence and aggregation behavior, and enzyme fluorescence detection

Change of aggregate stabilization based on removal of the galactopyranose moiety leads to an emission enhancement to detect β-galactosidase activity.

Mechanochromic fluorescence based on a combination of acceptor and bulky donor moieties: tuning emission color and regulating emission change direction

Mechanochromic fluorescence based on a conventional strategy using a donor–acceptor structure bearing nonplanar three-dimensional donor moieties was studied.

Turn-on-type emission enhancement and ratiometric emission color change based on the combination effect of aggregation and TICT found in the hexaazatriphenylene-triphenylamine dye in an aqueous environment

Turn-on-type emission enhancement and a ratiometric emission color change were achieved simultaneously due to the aggregation and TICT of a donor–acceptor-type dye.

Amphiphilic benzothiadiazole–triphenylamine-based aggregates that emit red light in water

An amphiphilic donor–acceptor dye can provide red light emission in water in an aggregate state.