Locked Planarity: A Strategy for Tailoring Ladder-Type π-Conjugated Anilido–Pyridine Boron Difluorides

Two-Coordinate Dinuclear Donor-Gold(I)-Acceptor Complexes Exhibiting Multiple Excitation Wavelength Dependent Phosphorescence

Two-coordinate Au(I) complexes with a donor-metal-acceptor (D-M-A) structure have shown rich luminescent properties. However, charge-neutral dinuclear donor-metal-acceptor type Au(I) complexes featuring aurophilic interactions have been seldom explored. Herein, we describe the structures and photoluminescence properties of two dinuclear Au(I) complexes, namely DiAu-Ph and DiAu-Me. Single crystal X-ray structural analysis of DiAu-Ph reveals a short intramolecular Au-Au distance of 3.224 Å. In dilute solution and doped films, excitation wavelength dependent multiple phosphorescence phenomena were observed for these dinuclear complexes. Theoretical calculations reveal that the aurophilic interaction causes increased contribution of the Au d orbital to the highest occupied molecular orbitals. Thus, the gap between singlet and triplet excited states (ΔEST) is enlarged, which disables the thermally activated delayed fluorescence (TADF). Moreover, the large energy separation (0.45-0.52 eV) and the different orbital configurations between the various excited states result in an inefficient internal conversion, accounting for their multiple phosphorescence properties.

Difluoro Dipyridomethene Boron Complexes: Synthesis, Characterization, and Ab Initio Calculations

Molecular engineering studies on the meso-cyano difluoro dipyridomethene boron complexes are presented and two series (a and b) of novel fluorophores are extensively studied. Halogenated derivatives were reacted under Suzuki-Miyaura or Sonogashira cross coupling reactions to introduce electron-donating or electron-withdrawing functional groups on positions 1 and 2 of the aromatic ligand. All derivatives were obtained in 14-90% yields and studied in detail by structural, photophysical, and computational analyses. Both series display excellent emissive properties in solution with blue to orange fluorescence emission upon blue light absorption and promising features as solid emitters. All the spectroscopic measurements are supported and confirmed by first-principles theoretical calculations combining TD-DFT and CC2. Series b, featuring an aryl substituent onto position 1 of the aromatic core, showed significantly large Stokes shifts values.

A series of boron difluoride complexes of azinylcarbazoles: synthesis and structure-property relationships

A series of boron difluoride (BF₂) complexes of azinylcarbazoles 1b-1h were synthesized, and the effects of the structure of azine moieties on the photophysical and electrochemical properties of the BF₂ complexes were clarified. UV-vis analysis of 1b with quinoline, 1c with isoquinoline, and fully fused 1d revealed that fusion with a benzene ring to a pyridylcarbazole BF₂ complex (1a) resulted in red shifts of longest-maximum absorption wavelengths (λ_max). UV-vis analysis of 1e and 1f with pyrimidine, 1g with pyridazine, and 1h with pyrazine revealed that substitution of a carbon atom to a nitrogen atom in 1a also resulted in red shifts of λ_max. The fluorescence quantum yields (Φ_f) decreased from 1a to 1b-1h, and especially, the fluorescence of 1e, 1g, and 1h was quenched in solution. At 77 K, the emission intensities of 1b-1h were significantly increased compared with those at ambient temperature, and they also exhibited phosphorescence with relatively narrow energy gaps between the singlet and triplet excited states. These results on the emission at 77 K indicate that the quench of fluorescence from 1e, 1g, and 1h at ambient temperature originates from both internal conversions and intersystem crossing. In the solid state, all of the complexes including 1e, 1g, and 1h exhibited emission. Distinctive aggregation-induced emission properties were observed for 1e-1h. Electrochemical measurements revealed that the replacement of the pyridine moiety in 1a with azine moieties reduced electrochemical gaps mainly due to a decrease in the LUMO levels. The effects of azine moieties on electronic structures were also discussed based on theoretical calculations.

Phenothiazine-, Dihydroacridine-, and Acridone-Based Boron Difluoride Complexes: Synthesis and Structure-Property Relationships

Boron difluoride complexes of phenothiazine, 9,10-dihydroacridine, and acridone derivatives with a pyridyl group were synthesized. Their structures, dynamic conformational behaviors, and photophysical properties, including solid-state fluorescence, were disclosed. The effects of the oxidation state of the sulfur atom in the phenothiazine derivatives on their properties were also clarified.

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.

BN Embedded Polycyclic π-Conjugated Systems: Synthesis, Optoelectronic Properties, and Photovoltaic Applications

In the periodic table of elements, boron (B, atomic number, 5) and nitrogen (N, atomic number, 7) are neighboring to the carbon (C, atomic number, 6). Thus, the total electronic number of two carbons (12) is equal to the electronic sum of one boron (5) and one nitrogen (7). Accordingly, replacing two carbons with one boron and one nitrogen in a π-conjugated structure gives an isoelectronic system, i.e., the BN perturbed π-conjugated system, comparing to their all-carbon analogs. The BN embedded π-conjugated systems have unique properties, e.g., optical absorption, emission, energy levels, bandgaps, and packing order in contrast to their all-carbon analogs and have been intensively studied in terms of novel synthesis, photophysical characterizations, and electronic applications in recent years. In this review, we try to summarize the synthesis methods, optoelectronic properties, and progress in organic photovoltaic (OPV) applications of the representative BN embedded polycyclic π-conjugated systems. Firstly, the narrative will be commenced with a general introduction to the BN units, i.e., B←N coordination bond, B-N covalent bond, and N-B←N group. Then, the representative synthesis strategies toward π-conjugated systems containing B←N coordination bond, B-N covalent bond, and N-B←N group will be summarized. Afterwards, the frontier orbital energy levels, optical absorption, packing order in solid state, charge transportation ability, and photovoltaic performances of typical BN embedded π-conjugated systems will be discussed. Finally, a prospect will be proposed on the OPV materials of BN doped π-conjugated systems, especially their potential applications to the small molecules organic solar cells.

Effects of the Substituents of Boron Atoms on Conjugated Polymers Containing B←N Units

Organoboron chemistry is a new tool to tune the electronic structures and properties of conjugated polymers, which are important for applications in organic optoelectronic devices. To investigate the effects of substituents of boron atoms on conjugated polymers, we synthesized three conjugated polymers based on double B←N-bridged bipyridine (BNBP) with various substituents on the boron atoms. By changing the substituents from four phenyl groups and two phenyl groups/two fluorine atoms to four fluorine atoms, the BNBP-based polymers show blue-shifted absorption spectra, decreased LUMO/HOMO energy levels, and enhanced electron affinities, as well as increased electron mobilities. Moreover, these BNBP-based polymers can be used as electron acceptors for all-polymer solar cells. These results demonstrate that substituents of boron atoms can effectively modulate the electronic properties and applications of conjugated polymers.

4,7-Bis[3-(dimesitylboryl)thien-2-yl]benzothiadiazole: Solvato-, Thermo-, and Mechanochromism Based on the Reversible Formation of an Intramolecular B-N Bond

4,7-Bis-[3-(dimesitylboryl)thien-2-yl]benzothiadiazole (1) and monoborylated derivative 2 were synthesized and their chromic behavior was investigated. Photophysical measurements, single-crystal XRD analysis, and theoretical calculations revealed that an intramolecular B-N coordination bond formed reversibly. The equilibrium of this reversible bond formation depends on the solid-state structure, solvent, temperature, and mechanical forces, and leads to significant changes in the electronic structure and chromic behavior of these molecules. The responsiveness toward external stimuli, resulting in the reversible formation of open and closed forms of this system, is achieved through weak intramolecular B-N coordination bonds induced by the steric bulk of the mesityl groups on the boron centers.

Carbazole-based BODIPYs with ethynyl substituents at the boron center: solid-state excimer fluorescence in the VIS/NIR region

Carbazole-based BODIPYs 1–6 with several different substituents at the boron atom site were synthesized.

Highly Emissive Far Red/Near-IR Fluorophores Based on Borylated Fluorene-Benzothiadiazole Donor-Acceptor Materials

Stille, Suzuki-Miyaura and Negishi cross-coupling reactions of bromine-functionalised borylated precursors enable the facile, high yielding, synthesis of borylated donor-acceptor materials that contain electron-rich aromatic units and/or extended effective conjugation lengths. These materials have large Stokes shifts, low LUMO energies, small band-gaps and significant fluorescence emission >700 nm in solution and when dispersed in a host polymer.

Low Band Gap Coplanar Conjugated Molecules Featuring Dynamic Intramolecular Lewis Acid-Base Coordination

Ladder-type conjugated molecules with a low band gap and low LUMO level were synthesized through an N-directed borylation reaction of pyrazine-derived donor-acceptor-donor precursors. The intramolecular boron-nitrogen coordination bonds played a key role in rendering the rigid and coplanar conformation of these molecules and their corresponding electronic structures. Experimental investigation and theoretical simulation revealed the dynamic nature of such coordination, which allowed for active manipulation of the optical properties of these molecules by using competing Lewis basic solvents.

Enhancing electron affinity and tuning band gap in donor-acceptor organic semiconductors by benzothiadiazole directed C-H borylation

Electrophilic borylation using BCl3 and benzothiadiazole to direct the C-H functionalisation of an adjacent aromatic unit produces fused boracyclic materials with minimally changed HOMO energy levels but significantly reduced LUMO energy levels. In situ alkylation and arylation at boron using Al(alkyl)3 or Zn(aryl)2 is facile and affords boracycles that possess excellent stability towards protic solvents, including water, and display large bathochromic shifts leading to far red/NIR emission in the solid state with quantum yields of up to 34%. Solution fabricated OLEDs with far red/NIR electroluminescence are reported with EQEs > 0.4%.

Series of Carbazole-Pyrimidine Conjugates: Syntheses and Electronic, Photophysical, and Electrochemical Properties

A series of carbazole-pyrimidine conjugates 1-17 were synthesized by Pd-catalyzed cross-coupling, oxidation, and nucleophilic aromatic substitution reactions. In 1-17, the carbazole moieties are connected at the 4,6-positions of the pyrimidine ring either directly or via ethynylene or vinylene spacers, and various electron-donating or electron-withdrawing substituents are introduced at the 2-position of the pyrimidine ring. The effects of structural variations on the electronic, photophysical, and electrochemical properties of 1-17 were comprehensively investigated. Compounds 1-17 exhibit intramolecular charge-transfer (ICT) states, which essentially lead to moderate-to-strong fluorescence emission with large Stokes shifts depending on the solvent polarity. These compounds tend to show significant changes in optical and fluorescence properties upon addition of trifluoroacetic acid. The electron-accepting ability of these compounds can be tuned by both substituents on the pyrimidine moiety and spacers. The ethynylene spacer lowers both the HOMO and LUMO levels, while the vinylene spacer elevates the HOMO level and lowers the LUMO level. The X-ray crystal structures of 3, 6, 11, and 14 are also disclosed.

Excited states of ladder-type π-conjugated dyes with a joint SOS-CIS(D) and PCM-TD-DFT approach

First-principle simulations aimed at accurately reproducing the excited state properties of a large series of ladder-type π-conjugated organic molecules containing heteroatoms (Si, S, B, O, and N) have been performed. In particular, time-dependent density functional theory (TD-DFT) calculations relying on several global and range-separated hybrid functionals have been carried out in conjunction with three variations of the polarizable continuum model (PCM), namely, the linear-response (LR), corrected linear-response (cLR), and state-specific (SS) approaches. For this series of molecules, similar to many borate derivatives, the cLR-PCM-TD-M06-2X approach can be used to reproduce the auxochromic effects that tune the 0-0 energies. However, TD-DFT yields rather large absolute deviations with respect to the experimental 0-0 energies. These systematic errors can be reduced by more than 0.1 eV when scaled opposite spin-configuration interaction singles with a double correction [SOS-CIS(D)] vertical calculations are combined to the PCM-TD-DFT results. This study demonstrates that such a "hybrid" scheme, where the geometrical and vibrational parameters, as well as the solvation effects, are determined with PCM-TD-DFT, whereas the transition energies are obtained with a wavefunction-based method, offers a useful compromise between accuracy and computational cost.