Facile Functionalization of Ambipolar, Nitrogen-Doped PAHs toward Highly Efficient TADF OLED Emitters

Despite promising optoelectronic features of N-doped polycyclic aromatic hydrocarbons (PAHs), their use as functional materials remains underdeveloped due to their limited post-functionalization. Facing this challenge, a novel design of N-doped PAHs with D-A-D electronic structure for thermally activated delayed fluorescence (TADF) emitters was performed. Implementing a set of auxiliary donors at the meta position of the protruding phenyl ring of quinoxaline triggers an increase in the charge-transfer property simultaneously decreasing the delayed fluorescence lifetime. This, in turn, contributes to a narrow (0.04-0.28 eV) singlet-triplet exchange energy split (ΔEST) and promotes a reverse intersystem crossing transition that is pivotal for an efficient TADF process. Boosting the electron-donating ability of our N-PAH scaffold leads to excellent photoluminescence quantum yield that was found in a solid-state matrix up to 96% (for phenoxazine-substituted derivatives, under air) with yellow or orange-red emission, depending on the specific compound. Organic light-emitting diodes (OLEDs) utilizing six, (D-A)-D, N-PAH emitters demonstrate a significant throughput with a maximum external quantum efficiency of 21.9% which is accompanied by remarkable luminance values which were found for all investigated devices in the range of 20,000-30,100 cd/m2 which is the highest reported to date for N-doped PAHs investigated in the OLED domain.

Synthesis and Photophysical Properties of Novel Meta-Conjugated Organic Molecules with 1,3,5-Benzene Branching Units

The synthesis and photophysical investigation of three novel meta-conjugated molecules based on 3,1,2-benzothiadiazole and thiophene-2,5-diyl derivatives linked through 1,3,5-benzene branching units are described. Each of them is a symmetrical molecule with two branching units, four identical lateral thiophene-containing fragments, and one central benzothiadiazole-containing fragment. To study the effect of the chemical structure on their photophysical properties, the molecules with different linearly conjugated lateral and central fragments due to incorporation of additional thiophene rings were synthesized and compared. It was shown that absorption spectra of the meta-conjugated molecules can be represented as a sum of absorption bands of model compounds for their peripheral and central fragments containing a common benzene ring being branched at the 1,3,5-benzene unit in the meta-conjugated molecules. Therefore, they cannot be considered simply as isolated π-conjugated systems of their peripheral and central fragments. Instead, DFT calculations showed that several transitions between the orbitals located in different regions of the meta-conjugated molecule are responsible for the formation of their absorption spectra, and they strongly depend on the degree of their overlapping. Theoretical absorption spectra reconstructed from the DFT data demonstrated a good agreement with the experimental results: the transitions with larger oscillator strength correspond to the bands with higher molar extinction coefficients and vice versa. It was shown that luminescence spectral maxima of the meta-conjugated molecules monotonically shift to the lower energy from 489 to 540 and 613 nm with increasing the number of thiophene rings in the peripheral and central fragments, respectively. However, luminescence quantum yield of the meta-conjugated molecules critically depends on the length of linearly conjugated fragments in its structure decreasing from 24% to 1.3% with increasing the number of thiophene rings in the lateral fragments but increasing to 90% in the molecule with more thiophene rings in both types of the fragments. The results obtained are well correlated to the ratio of radiative and nonradiative deactivation rate constants of the meta-conjugated molecules that indicates a high rate of internal conversion between the excited states corresponding to different fragments of the molecule. The CV measurements allowed estimating the HOMO, LUMO, and bandgap values of the target and model compounds, which confirm the presence of meta-conjugation within the molecules investigated. Thus, connection of linearly conjugated fragments through meta-positions (meta-conjugation) of a benzene ring leads to an intermediate option between fully conjugated and nonconjugated molecules due to partial delocalization of electron density through the 1,3,5-substituted benzene branching center.

3',5'-Di-chloro-N,N-diphenyl-[1,1'-biphen-yl]-4-amine

The title tri-phenyl-amine derivative, C24H17Cl2N, featuring a 3,5-di-chloro-1,1'-biphenyl moiety has been synthesized and structurally characterized. The mol-ecular structure shows rotations of the phenyl rings in the range of 37-40° from the amine plane. In the crystal, the mol-ecules inter-act by van der Waals inter-actions.

Experimental and theoretical investigations on the optical and electrochemical properties of π-conjugated donor-acceptor-donor (DAD) compounds toward a universal model

A series of nine (9) donor-acceptor-donor (DAD) π-conjugated small molecules were synthesized via palladium catalyzed Stille aromatic cross-coupling reactions by the combination of six (6) heterocycle building blocks (thiophene, furan, thiazole, 2,1,3-benzothiadiazole, 2,1,3-pyridinothiadiazole, thienothiadiazole) acting as electron donating (thiazole, furan, thiophene) and electron deficient (benzothiadiazole, pyridinethiadiazole, thienothiadiazole) units. These model compounds enable determining the correspondence between the theoretical and experimental optical and electrochemical properties for the first time, via Density Functional Theory (DFT), time-dependent DFT, UV-Vis spectroscopy, and cyclic voltammetry, accordingly. The obtained theoretical models can be utilized for the design and synthesis of new DAD structures with precise optical bandgaps, absorption maxima, and energy levels suitable for different optoelectronic applications.

Folding of ortho-Phenylenes

In nature, the folding of oligomers and polymers is used to generate complex three-dimensional structures, yielding macromolecules with diverse functions in catalysis, recognition, transport, and charge- and energy-transfer. Over the past 20-30 years, chemists have sought to replicate this strategy by developing new foldamers: oligomers that fold into well-defined secondary structures in solution. A wide array of abiotic foldamers have been developed, ranging from non-natural peptides to aromatics. The ortho-phenylenes represent a recent addition to the family of aromatic foldamers. Despite their structural simplicity (chains of benzenes connected at the ortho positions), it was not until 2010 that systematic studies of o-phenylenes showed that they reliably fold into helices in solution (and in the solid state). This conformational behavior is of fundamental interest: o-Arylene and o-heteroarylene structures are found embedded within many other systems, part of an emerging interest in sterically congested polyphenylenes. Further, o-phenylenes are increasingly straightforward to synthesize because of continuing developments in arene-arene coupling, the Asao-Yamamoto benzannulation, and benzyne polymerization. In this Account, we discuss the folding of o-phenylenes with emphasis on features that make them unique among aromatic foldamers. Interconversion between their different backbone conformers is slow on the NMR time scale around room temperature. The (1)H NMR spectra of oligomers can therefore be deconvoluted to give sets of chemical shifts for different folding states. The chemical shifts are both highly sensitive to conformation and readily predicted using ab initio methods, affording critical information about the conformational distribution. The picture that emerges is that o-phenylenes fold into helices with offset stacking between every third repeat unit. In general, misfolding occurs primarily at the oligomer termini (i.e., "frayed ends"). Because of their structural simplicity, the folding can be described by straightforward models. The overall population can be divided into two enantiomeric pools, with racemization and misfolding as two distinct processes. Examination of substituent effects on folding reveals that the determinant of the relative stability of different conformers is (offset) aromatic stacking interactions parallel to the helical axis. That is, the folding of o-phenylenes is analogous to that of α-helices, with aromatic stacking in place of hydrogen bonding. The folding propensity can be tuned using well-known substituent effects on aromatic stacking, with moderate electron-withdrawing substituents giving nearly perfect folding. The combination of a simple folding mechanism and readily characterized conformational populations makes o-phenylenes attractive structural motifs for incorporation into more-complex architectures, an important part of the next phase of foldamer research.

Avogadro: an advanced semantic chemical editor, visualization, and analysis platform

Background

The Avogadro project has developed an advanced molecule editor and visualizer designed for cross-platform use in computational chemistry, molecular modeling, bioinformatics, materials science, and related areas. It offers flexible, high quality rendering, and a powerful plugin architecture. Typical uses include building molecular structures, formatting input files, and analyzing output of a wide variety of computational chemistry packages. By using the CML file format as its native document type, Avogadro seeks to enhance the semantic accessibility of chemical data types.

Results

The work presented here details the Avogadro library, which is a framework providing a code library and application programming interface (API) with three-dimensional visualization capabilities; and has direct applications to research and education in the fields of chemistry, physics, materials science, and biology. The Avogadro application provides a rich graphical interface using dynamically loaded plugins through the library itself. The application and library can each be extended by implementing a plugin module in C++ or Python to explore different visualization techniques, build/manipulate molecular structures, and interact with other programs. We describe some example extensions, one which uses a genetic algorithm to find stable crystal structures, and one which interfaces with the PackMol program to create packed, solvated structures for molecular dynamics simulations. The 1.0 release series of Avogadro is the main focus of the results discussed here.

Conclusions

Avogadro offers a semantic chemical builder and platform for visualization and analysis. For users, it offers an easy-to-use builder, integrated support for downloading from common databases such as PubChem and the Protein Data Bank, extracting chemical data from a wide variety of formats, including computational chemistry output, and native, semantic support for the CML file format. For developers, it can be easily extended via a powerful plugin mechanism to support new features in organic chemistry, inorganic complexes, drug design, materials, biomolecules, and simulations. Avogadro is freely available under an open-source license from http://avogadro.openmolecules.net.