High Stokes Shift Anilido-Pyridine Boron Difluoride Dyes

Thiazole Boron Difluoride Dyes with Large Stokes Shift, Solid State Emission and Room-Temperature Phosphorescence

The small Stokes shift and weak emission in the solid state are two main shortcomings associated with the boron-dipyrromethene (BODIPY) family of dyes. This study presents the design, synthesis and luminescent properties of boron difluoro complexes of 2-aryl-5-alkylamino-4-alkylaminocarbonylthiazoles. These dyes display Stokes shifts (Δλ, 77-101 nm) with quantum yields (ϕ_FL ) up to 64.9 and 34.7 % in toluene solution and in solid state, respectively. Some of these compounds exhibit dual fluorescence and room-temperature phosphorescence (RTP) emission properties with modulable phosphorescence quantum yields (ϕ_PL ) and lifetime (τ_p up to 251 μs). The presence of intramolecular H-bonds and negligible π-π stacking revealed by X-ray crystal structure might account for the observed large Stokes shift and significant solid-state emission of these fluorophores, while the enhanced spin-orbit coupling (SOC) of iodine and the self-assembly driven by halogen bonding, π-π and C-H^… π interactions could be responsible for the observed RTP of iodine containing phosphors.

Enhanced Optical Properties of Azaborole Helicenes by Lateral and Helical Extension

The synthesis and characterization of laterally extended azabora[5]-, -[6]- and -[7]helicenes, assembled from N-heteroaromatic and dibenzo[g,p]chrysene building blocks is described. Formally, the π-conjugated systems of the pristine azaborole helicenes were enlarged with a phenanthrene unit leading to compounds with large Stokes shifts, significantly enhanced luminescence quantum yields (Φ) and dissymmetry factors (g_lum ). The beneficial effect on optical properties was also observed for helical elongation. The combined contributions of lateral and helical extensions resulted in a compound showing green emission with Φ of 0.31 and |g_lum | of 2.2×10^-3 , highest within the series of π-extended azaborahelicenes and superior to emission intensity and chiroptical response of its non-extended congener. This study shows that helical and lateral extensions of π-conjugated systems are viable strategies to improve features of azaborole helicenes. In addition, single crystal X-ray analysis of configurationally stable [6]- and -[7]helicenes was used to provide insight into their packing arrangements.

Recent Advances in BODIPY Compounds: Synthetic Methods, Optical and Nonlinear Optical Properties, and Their Medical Applications

Organoboron compounds are attracting immense research interest due to their wide range of applications. Particularly, low-coordinate organoboron complexes are receiving more attention due to their improbable optical and nonlinear optical properties, which makes them better candidates for medical applications. In this review, we summarize the various synthetic methods including multicomponent reactions, microwave-assisted and traditional pathways of organoboron complexes, and their optical and nonlinear properties. This review also includes the usage of organoboron complexes in various fields including biomedical applications.

Conjugated Molecular Nanotubes

Molecular compounds with permanent tubular architectures displaying radial π-conjugation are exceedingly rare. Their radial and axial delocalization presents them with unique optical and electronic properties, such as remarkable tuning of their Stokes shifts, and redox switching between global and local aromaticity. Although these tubular compounds display large internal void spaces, these attributes have not been extensively explored, thus presenting future opportunities in the development of materials. By using cutting-edge synthetic methodologies to bend aromatic surfaces, large opportunities in synthesis, property discovery, and applications are expected in new members of this family of conjugated molecular nanotubes.

Triarylborane-Appended Anils and Boranils: Solid-State Emission, Mechanofluorochromism, and Phosphorescence

Herein, the design, synthesis, optical properties, and mechanofluorochromism characteristics of a series of conjugates having covalently linked triarylborane (TAB) and anil/boranil units (TAB-anil: 1 a-3 a and TAB-boranil: 1-3) are reported. The electronic interactions between TAB and anil/boranil in 1 a-3 a and 1-3 were fine-tuned by changing the boryl moiety's position on the phenyl spacer connecting the BMes₂ (Mes=mesityl) and anil/boranil units. A boryl moiety at the meta position (1 a) of the phenyl spacer stabilizes the enolic form (E-OH), whereas a boryl moiety at the para position (2 a and 3 a) stabilizes the keto form (Z-NH) in the solid state. However, in solution 1 a, 2 a, and 3 a exhibit keto-enol tautomerism in both ground and excited states. Compounds 1 a-3 a and 1-3 show red-shifted absorption compared with 4 a and 4, which are devoid of TAB moieties, which indicate effective participation of an empty p orbital on the boron center in 1 a-3 a and 1-3. Compounds 1 and 2 showed fluorescence variations in response to external stimuli such as mechanical grinding. Long phosphorescence lifetimes of 18-46 ms were observed for compounds 1-3. The observed optical properties of 1 a-3 a and 1-3 are rationalized in the context of quantum mechanical calculations.

Optoelectronic, Aggregation, and Redox Properties of Double-Rotor Boron Difluoride Hydrazone Dyes

We develop the chemistry of boron difluoride hydrazone dyes (BODIHYs) bearing two aryl substituents and explore their properties. The low-energy absorption bands (λ_max =427-464 nm) of these dyes depend on the nature of the N-aryl groups appended to the BODIHY framework. Electron-donating and extended π-conjugated groups cause a redshift, whereas electron-withdrawing groups result in a blueshift. The title compounds were weakly photoluminescent in solution and strongly photoluminescent as thin films (λ_PL =525-578 nm) with quantum yields of up to 18 % and lifetimes of 1.1-1.7 ns, consistent with the dominant radiative decay through fluorescence. Addition of water to THF solutions of the BODIHYs studied causes molecular aggregation which restricts intramolecular motion and thereby enhances photoluminescence. The observed photoluminescence of BODIHY thin films is likely facilitated by a similar molecular packing effect. Finally, cyclic voltammetry studies confirmed that BODIHY derivatives bearing para-substituted N-aryl groups could be reversibly oxidized (E_ox1 =0.62-1.02 V vs. Fc/Fc⁺ ) to their radical cation forms. Chemical oxidation studies confirmed that para-substituents at the N-aryl groups are required to circumvent radical decomposition pathways. Our findings provide new opportunities and guiding principles for the design of sought-after multifunctional boron difluoride complexes that are photoluminescent in the solid state.

Facile Synthesis and Properties of 2-λ(5)-Phosphaquinolines and 2-λ(5)-Phosphaquinolin-2-ones

Treatment of 2-ethynylanilines with P(OPh)3 gives either 2,2-diphenoxy-2-λ(5)-phosphaquinolines or 2-phenoxy-2-λ(5)-phosphaquinolin-2-ones under transition-metal-free conditions. This reaction offers access to an underexplored heterocycle, which opens up the study of the fundamental nature of the N=P(V) double bond and its potential for delocalization within a cyclic π-electron system. This heterocycle can serve as a carbostyril mimic, with application as a bioisostere for pharmaceuticals based on the 2-quinolinone scaffold. It also holds promise as a new fluorophore, since initial screening reveals quantum yields upwards of 40%, Stokes shifts of 50-150 nm, and emission wavelengths of 380-540 nm. The phosphaquinolin-2-ones possess one of the strongest solution-state dimerization constants for a D-A system (130 M(-1)) owing to the close proximity of a strong acceptor (P=O) and a strong donor (phosphonamidate N-H), which suggests that they might hold promise as new hydrogen-bonding hosts for optoelectronic sensing.

Synthesis, structures and electrochemical and photophysical properties of anilido-benzoxazole boron difluoride (ABB) complexes

Four-ring-fused anilido-benzoxazole boron difluoride (ABB) dyes were synthesized, and exhibited very bright luminescence in solution (Φ_f = 0.45–0.96 in CH₂Cl₂) and in the solid-state (Φ_f = 0.07–0.37).