Molecular Design of Sexiphenyl-Based Liquid Crystals: Towards Temperature-Stable, Nematic Phases with Enhanced Optical Properties

This research introduces a novel liquid crystal molecular design approach based on the para-sexiphenyl (6P) structure. Six new liquid crystalline materials were synthesized, incorporating an alkyl terminal and lateral substitutions of the sexiphenyl core to achieve temperature-stable and broad nematic phases. The synthetic pathway involved cross-coupling, resulting in derivatives with strong nematogenic characteristics. Optical investigations demonstrated that the tested material had high birefringence values, making it promising for optical and electronic applications. These results open up new avenues of research and offer potential practical applications in electronics, photonics, optoelectronics and beyond.

Contrasting the excited state properties of different conformers of trans- and cis-2,2'-bipyridine oligomers in the gas phase

In this article, we present conformation-dependent photophysical and excited state properties of trans- and cis- BPY oligomers. Oligomers up to tetramers for three conformers, namely, o-, m-, and p-, are constructed and optimized at the B3LYP-D3/def2-SVPD level. The photophysical and excited state properties are interpreted in terms of UV and CD spectra at the RI-ADC(2)/def2-TZVPD level. The UV spectra of oligomers of the m-conformer show high-intensity and red-shifted UV bands compared to o- and p-oligomers. The CD spectra of p-oligomers show intense CD bands compared to o- and p-oligomers in the case of trans-structures. In contrast, oligomers of each conformer of cis-structures show high-intensity CD bands. The excited states of (BPY)2 and (BPY)4 are also characterized by analysis of one-electron transition density matrix considering three descriptors: ωCT, dexc, and PRNTO. The ωCT values of dimers are in the range of 0.06-0.32, which indicates the excited states are mainly LE states, whereas, for (BPY)4, the ωCT values range from 0.17 to 0.53, indicating the possibility of partial CT in the excited states. These observations are also explained using the NTOs and e-h correlation plots.

Conformational Effect on the Excitonic States of 2-Phenylpyridine Oligomers: Ab Initio Studies and Analysis

In this work, we report the effect of different conformations of 2-phenylpyridine oligomers ((PhPy)n=1-5) on the excited state properties from the results obtained at the RI-ADC(2)/def2-TZVP level. Three different conformers, namely, A, B, and C, are considered for each oligomer. All the oligomers of conformer A have linear-type structures, whereas conformers B and C form helical structures at n = 5 and n = 3, respectively. The differences in the geometries of the three conformers are reflected in the UV and CD spectra. The UV spectra of conformer A show high-intensity peaks compared to the conformers B and C, for each oligomer. While the helical oligomers of conformers B and C show high-intensity CD bands, the intensities of CD bands for all of the oligomers of conformer A are weaker. Analysis of the properties of the first five excited states in (PhPy)5 is carried out using three descriptors, and the results reveal that these are partially charge transfer states.

Synthesis of Dendronized Polymers on the Au(111) Surface

Dendronized polymers (DPs) consist of a linear polymeric backbone with dendritic side chains. Fine-tuning of the functional groups in the side chains enriches the structural versatility of the DPs and imparts a variety of novel physical properties. Herein, the first on-surface synthesis of DPs is achieved via the postfunctionalization of polymers on Au(111), in which the surface-confinement-induced planar conformation and chiral configurations were unambiguously characterized. While the dendronized monomer was synthesized in situ on Au(111), the subsequent polymerization afforded only short, cross-linked DP chains owing to multiple side reactions. The postfunctionalization approach selectively produced brominated polyphenylene backbone moieties by the deiodination polymerization of 4-bromo-4″-iodo-5'-(4-iodophenyl)-1,1':3',1″-terphenyl on Au(111), which smoothly underwent divergent cross-coupling reactions with two different isocyanides to form two types of DPs as individual long chains.

Modeling and Characterization of Exciplexes in Photoredox CO2 Reduction: Insights from Quantum Chemistry and Fluorescence Spectroscopy

Interactions between excited-state arenes and amines can lead to the formation of structures with a distinct emission behavior. These excited-state complexes or exciplexes can reduce the ability of the arene to participate in other reactions, such as CO₂ reduction, or increase the likelihood of degradation via Birch reduction. Exciplex geometries are necessary to understand photophysical behavior and probe degradation pathways but are challenging to calculate. We establish a detailed computational protocol for calculation, verification, and characterization of exciplexes. Using fluorescence spectroscopy, we first demonstrate the formation of exciplexes between excited-state oligo-(p-phenylene) (OPP), shown to successfully carry out CO₂ reduction, and triethylamine. Time-dependent density functional theory is employed to optimize the geometries of these exciplexes, which are validated by comparing both emission energies and their solvatochromism with the experiment. Excited-state energy decomposition analysis confirms the predominant role played by charge transfer interactions in the red shift of emissions relative to the isolated excited-state OPP*. We find that although the exciplex emission frequency depends strongly on solvent dielectric, the extent of charge separation in an exciplex does not. Our results also suggest that the formation of solvent-separated ionic radical states upon complete electron transfer competes with exciplex formation in higher-dielectric solvents, thereby leading to reduced exciplex emission intensities in fluorescence experiments.

Photoredox Chemistry with Organic Catalysts: Role of Computational Methods

Organic catalysts have the potential to carry out a wide range of otherwise thermally inaccessible reactions via photoredox routes. Early demonstrated successes of organic photoredox catalysts include one-electron CO₂ reduction and H₂ generation via water splitting. Photoredox systems are challenging to study and design owing to the sheer number and diversity of phenomena involved, including light absorption, emission, intersystem crossing, partial or complete charge transfer, and bond breaking or formation. Designing a viable photoredox route therefore requires consideration of a host of factors such as absorption wavelength, solvent, choice of electron donor or acceptor, and so on. Quantum chemistry methods can play a critical role in demystifying photoredox phenomena. Using one-electron CO₂ reduction with phenylene-based chromophores as an illustrative example, this perspective highlights recent developments in quantum chemistry that can advance our understanding of photoredox processes and proposes a way forward for driving the design and discovery of organic catalysts.

meta-Terphenyl linked donor-π-acceptor dyads: intramolecular charge transfer controlled by electron acceptor group tuning

A series of meta-terphenyl linked donor-π-acceptor (D-π-A) dyads were prepared to understand the electronic effects of a meta-terphenyl linker according to the electron-accepting ability change. The energy band gaps of the dyads were controlled by tuning the accepting ability, which resulted in emission colors ranging from blue-green to red. In the Lippert-Mataga plots, intramolecular charge transfer (ICT) behavior was observed, which showed gradually increased ICT characteristics as the accepting ability was increased. On the other hand, in the absorption spectra, a red shift of the ICT transition was observed differently from the electron-accepting ability tendency. Thus, the experimental results show that the ICT is determined by steric hindrance rather than the acceptor ability in the ground state due to the lack of π-conjugation of the terphenyl linker by the electron node in the meta-position, whereas ICT in the excited state is controlled by electron-accepting ability.

Computational Analysis of Electron Transfer Kinetics for CO2 Reduction with Organic Photoredox Catalysts

We present a fundamental description of the electron transfer (ET) step from substituted oligo(p-phenylene) (OPP) radical anions to CO₂, with the larger goal of assessing the viability of underexplored, organic photoredox routes for utilization of anthropogenic CO₂. This work varies the electrophilicity of para-substituents to OPP and probes the dependence of rate coefficients and interfragment interactions on the substituent Hammett parameter, σ_p, using constrained density functional theory (CDFT) and energy decomposition analysis (EDA). Large electronic couplings across substituents indicates an adiabatic electron transfer process for reactants at contact. As one might intuitively expect, free energy changes dominate trends in ET rate coefficients in most cases, and rates increase with substituent electron-donating ability. However, we observe an unexpected dip in rate coefficients for the most electron-donating groups, due to the combined impact of flattening free energies and a steep increase in reorganization energies. Our analysis shows that, with decreasing σ_p, flattening OPP LUMO levels lower the marginal increase in free energy. EDA reveals trends in electrostatics and charge transfer interactions between the catalyst and substrate fragments that influence free energy changes across substituents. Reorganization energies do not exhibit a direct dependence on σ_p and are largely similar across systems, with the exception of substituents containing lone pairs of electrons that exhibit significant deformation upon electron transfer. Our study therefore suggests that while a wide range of ET rates are observed, there is an upper limit to rate enhancements achievable by only tuning the substituent electrophilicity.

New generations of spirobifluorene regioisomers for organic electronics: tuning electronic properties with the substitution pattern

In the present feature article, we present the new generations of spirobifluorenes for organic electronics and we detail the impact of positional isomerism on the electronic properties and device performance.

Twist sense control in terminally functionalized ortho-phenylenes

Chiral groups induce opposite twist senses of o-phenylene helices depending on their positions in dynamic mixtures.

Many abiotic foldamers are based on achiral repeat units but adopt chiral geometries, especially helices. In these systems, there is no inherent preference for one handedness of the fold; however, it is well-established that the point chirality of substituents can be communicated to the helix. This capability represents a basic level of control over folding that is necessary for applications in molecular recognition and in the assembly of higher-order structures. The ortho-phenylenes are a structurally simple class of aromatic foldamers that fold into helices driven by arene–arene stacking interactions. Although their folding is now reasonably well-understood, access to o-phenylenes enriched in one twist sense has been limited to resolution, yielding conformationally dynamic samples that racemize over the course of minutes to hours. Here, we report a detailed structure–property study of chiral induction from o-phenylene termini using a combination of NMR spectroscopy, CD spectroscopy, and computational chemistry. We uncover mechanistic details of chiral induction and show that the same substituents can give effective twist sense control in opposite directions in mixtures of interconverting conformers; that is, they are “ambidextrous”. This behavior should be general and can be rationalized using a simple model based on sterics, noting that arene–arene stacking is, to a first approximation, unaffected by flipping either partner. We demonstrate control over this mechanism by showing that chiral groups can be chosen such that they both favor one orientation and provide effective chiral induction.

Electronic Structure Tunability by Periodic meta-Ligand Spacing in One-Dimensional Organic Semiconductors

Designing molecular organic semiconductors with distinct frontier orbitals is key for the development of devices with desirable properties. Generating defined organic nanostructures with atomic precision can be accomplished by on-surface synthesis. We use this "dry" chemistry to introduce topological variations in a conjugated poly( para-phenylene) chain in the form of meta-junctions. As evidenced by STM and LEED, we produce a macroscopically ordered, monolayer thin zigzag chain film on a vicinal silver crystal. These cross-conjugated nanostructures are expected to display altered electronic properties, which are now unraveled by highly complementary experimental techniques (ARPES and STS) and theoretical calculations (DFT and EPWE). We find that meta-junctions dominate the weakly dispersive band structure, while the band gap is tunable by altering the linear segment's length. These periodic topology effects induce significant loss of the electronic coupling between neighboring linear segments leading to partial electron confinement in the form of weakly coupled quantum dots. Such periodic quantum interference effects determine the overall semiconducting character and functionality of the chains.

syn-BN-heteroacene cored conjugated oligomers with finely tuned blue-violet luminescent properties

Starting from dibrominated syn-BN-heteroacenes, a series of BN-containing conjugated oligomers with blue-violet luminescence was synthesized.

Sequence-defined oligo(ortho-arylene) foldamers derived from the benzannulation of ortho(arylene ethynylene)s

A Cu-catalyzed benzannulation reaction transforms ortho(arylene ethynylene) oligomers into ortho-arylenes.

A Cu-catalyzed benzannulation reaction transforms ortho(arylene ethynylene) oligomers into ortho-arylenes. This approach circumvents iterative Suzuki cross-coupling reactions previously used to assemble hindered ortho-arylene backbones. These derivatives form helical folded structures in the solid-state and in solution, as demonstrated by X-ray crystallography and solution-state NMR analysis. DFT calculations of misfolded conformations are correlated with variable-temperature ¹H and EXSY NMR to reveal that folding is cooperative and more favorable in halide-substituted naphthalenes. Helical ortho-arylene foldamers with specific aromatic sequences organize functional π-electron systems into arrangements ideal for ambipolar charge transport and show preliminary promise for the surface-mediated synthesis of structurally defined graphene nanoribbons.

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.

Extended conjugated microporous polymers for photocatalytic hydrogen evolution from water

Conjugated microporous polymers (CMPs) have been used as photocatalysts for hydrogen production from water in the presence of a sacrificial electron donor.

How does tetraphenylethylene relax from its excited states?

Photocyclization play a key role in the deactivation mechanism of tetraphenylethylene.

Assessment of Approximate Coupled-Cluster and Algebraic-Diagrammatic-Construction Methods for Ground- and Excited-State Reaction Paths and the Conical-Intersection Seam of a Retinal-Chromophore Model

As a minimal model of the chromophore of rhodopsin proteins, the penta-2,4-dieniminium cation (PSB3) poses a challenging test system for the assessment of electronic-structure methods for the exploration of ground- and excited-state potential-energy surfaces, the topography of conical intersections, and the dimensionality (topology) of the branching space. Herein, we report on the performance of the approximate linear-response coupled-cluster method of second order (CC2) and the algebraic-diagrammatic-construction scheme of the polarization propagator of second and third orders (ADC(2) and ADC(3)). For the ADC(2) method, we considered both the strict and extended variants (ADC(2)-s and ADC(2)-x). For both CC2 and ADC methods, we also tested the spin-component-scaled (SCS) and spin-opposite-scaled (SOS) variants. We have explored several ground- and excited-state reaction paths, a circular path centered around the S1/S0 surface crossing, and a 2D scan of the potential-energy surfaces along the branching space. We find that the CC2 and ADC methods yield a different dimensionality of the intersection space. While the ADC methods yield a linear intersection topology, we find a conical intersection topology for the CC2 method. We present computational evidence showing that the linear-response CC2 method yields a surface crossing between the reference state and the first response state featuring characteristics that are expected for a true conical intersection. Finally, we test the performance of these methods for the approximate geometry optimization of the S1/S0 minimum-energy conical intersection and compare the geometries with available data from multireference methods. The present study provides new insight into the performance of linear-response CC2 and polarization-propagator ADC methods for molecular electronic spectroscopy and applications in computational photochemistry.