Breaking Bonds and Forming Nanographene Diradicals with Pressure

Anthraquinodimethane-Based Molecular Switches Tethered by Four-Arm Star-like Polymers

Molecular switches that reversibly change their structures and physical properties are important for applications such as sensing and information processing at molecular scales. In order to avoid the intermolecular aggregation that is often detrimental to the stimuli-responses of molecular switches, previous studies of molecular switches have been often conducted in dilute solutions which are difficult for applications in solid-state devices. Here we report molecular design and synthesis that integrates anthraquinodimethane as molecular switching units into polymers with amenable processibility in solid states. Optical and electron spin resonance characterizations indicate that the four-arm polymers of poly(ϵ-caprolactone) or poly(D,L-lactide) tethered from anthraquinodimethane slow down the dynamics of the conformational switching between the folded and the twisted conformations, enhance the photoluminescence in solid states and impart materials with a small energy gap from singlet ground state to thermally accessible triplet state.

A highly efficient open-shell singlet luminescent diradical with strong magnetoluminescence properties

Developing open-shell singlet (OS) diradicals with high luminescent properties and exceptional single-molecule magnetoluminescence (ML) performance is extremely challenging. Herein, we propose a concept to enhance luminescent efficiency by adjusting the donor conjugation of OS diradicals, thereby achieving a highly luminescent diradical, DR1, with outstanding stability and making it a viable option for use in the emitting layer of organic light-emitting diodes (OLEDs). More importantly, the 0.5 wt%-DR1 doped film demonstrates significant single-molecule magnetoluminescence (ML) properties. A giant ML value of 210% is achieved at a magnetic field of 7 T, showing the great potential of DR1 in magneto-optoelectronic devices.

Spin-Orbit Couplings of Open-Shell Systems with Restricted Active Space Configuration Interaction

In this work we perform electronic structure calculations to unravel the origin of spin-orbit couplings (SOCs) in open-shell molecules. For that, we select systems displaying di or polyradical character, e.g., trimethylene, and analyze the changes in the magnitude of SOC constants along molecular distortions of ethylene and in the presence of intermolecular interactions between open and closed-shell moieties in the O₂-C₂H₄ system. Calculations were performed by using nonrelativistic wave functions obtained with the restricted active space configuration interaction (RASCI) method, in conjunction with a recent implementation for the calculation of SOC based on the spin-orbit mean field approximation. Our results demonstrate the suitability of RASCI in the calculation of SOCs of open-shell systems, while providing a deep understanding of the relationship between couplings and the nature of the electronic states. Moreover, we introduce a new definition of the SOC constant for the study of molecular aggregates.

Short-range DFT energy correction to multiconfigurational wave functions for open-shell systems

Electronic structure methods emerging from the combination of multiconfigurational wave functions and density functional theory (DFT) aim to take advantage of the strengths of the two nearly antagonistic theories. One of the common strategies employed to merge wave function theory (WFT) with DFT relies on the range separation of the Coulomb operator in which DFT functionals take care of the short-distance part, while long-range inter-electronic interactions are evaluated by using the chosen wave function method (WFT-srDFT). In this work, we uncover the limitations of WFT-srDFT in the characterization of open-shell systems. We show that spin polarization effects have a major impact on the (short-range) DFT exchange energy and are of vital importance in order to provide a balanced description between closed and open-shell configurations. We introduce different strategies to account for spin polarization in the short range based on the definition of a spin polarized electron density and with the use of short-range exact exchange. We test the performance of these approaches in the dissociation of the hydrogen molecule, the calculation of energy gaps in spin-triplet atoms and molecular diradicals, and the characterization of low-lying states of the gallium dimer. Our results indicate that the use of short-range DFT correlation in combination with a (full-range) multiconfigurational wave function might be an excellent approach for the study of open-shell molecules and largely improves the performance of WFT and WFT-srDFT.

Switching of Redox Properties Triggered by a Thermal Equilibrium between Closed-Shell Folded and Open-Shell Twisted Species

Thermally switchable redox properties have been reported to be due to a change in the spin state of newly designed overcrowded ethylenes, which can adopt closed-shell folded and open-shell twisted forms. In this study, tetrathienylanthraquinodimethane derivatives were designed to be in thermal equilibrium between a more stable folded form and less stable but more donating twisted diradical in solution, so that the oxidation potential can be controlled by heating/cooling. This is the first example of a switching of redox properties based on a thermally equilibrated twisted diradical, which can be more readily oxidized to the twisted dication.

From cyclic nanorings to single-walled carbon nanotubes: disclosing the evolution of their electronic structure with the help of theoretical methods

We systematically investigate the relationships between structural and electronic effects of finite size zigzag or armchair carbon nanotubes of various diameters and lengths, starting from a molecular template of varying shape and diameter, i.e. cyclic oligoacene or oligophenacene molecules, and disclosing how adding layers and/or end-caps (i.e. hemifullerenes) can modify their (poly)radicaloid nature. We mostly used tight-binding and finite-temperature density-based methods, the former providing a simple but intuitive picture about their electronic structure, and the latter dealing effectively with strong correlation effects by relying on a fractional occupation number weighted electron density (ρFOD), with additional RAS-SF calculations backing up the latter results. We also explore how minor structural modifications of nanotube end-caps might influence the results, showing that topology, together with the chemical nature of the systems, is pivotal for the understanding of the electronic properties of these and other related systems.