Colorless Near-Infrared Absorbing Dyes Based on B-N Fused Donor-Acceptor-Donor π-Conjugated Molecules for Organic Phototransistors

The introduction of a colorless function to organic electronic devices allows responses to light in the near-infrared (NIR) region and is expected to broaden the applications of these devices. However, the development of a colorless NIR dye remains a challenge due to the lack of a rational molecular design for controlling electronic transitions. In this study, to suppress the π-π* transitions in the visible region, polycyclic donor-acceptor-donor π-conjugated molecules with boron bridges (Py-FNTz-B and IP-FNTz-B) are designed and synthesized, which contain pyrrole or indenopyrrole as donor units with fluorinated naphthobisthiadiazole (FNTz) as an acceptor unit. The pyrrole end-capped Py-FNTz-B shows an absorption band in the NIR region without distinct visible-light absorption, which has led to the establishment of colorless characteristics. The indenopyrrole end-capped IP-FNTz-B shows a narrow optical energy gap of 0.87 eV in films. Time-resolved microwave conductance and field-effect transistors demonstrate the semiconducting characteristics of these molecules, and Py-FNTz-B-based devices function as NIR phototransistors. Theoretical analyses indicate that the combination of a polyene-like electronic structure with orbital symmetry is important to obtain NIR wavelength-selective absorption. This study suggests that a molecular design based on electronic structures can be effective in the development of colorless NIR-absorbing dyes for organic electronics.

Metal-Ligand Exchange Coupling Alters the Open-Shell Ligand Electronic Structure in a Bis(semiquinone) Complex

The electronic structure of the bis(dioxolene) bridging ligand -SQ2Th2- is responsive to metal-ligand magnetic exchange coupling. Comparison of the crystal structure of (NiSQ)2Th2 to that of (ZnSQ)2Th2 indicates an open-shell biradical ground state for the dinuclear Ni(II) complex compared to the closed-shell quinoidal character found in the dinuclear Zn(II) complex. Consistent with a comparison of bond lengths obtained by X-ray diffraction, the analysis of the variable-temperature magnetic susceptibility data for crystalline (NiSQ)2Th2 yields reduced SQ-SQ radical-radical magnetic exchange coupling (JSQ-SQ = -203 cm-1) compared to that of (ZnSQ)2Th2 (JSQ-SQ = -321 cm-1). The reduced SQ-SQ exchange coupling in (NiSQ)2Th2 derives from an attenuation of the SQ spin densities, which in turn is derived from the Ni-SQ antiferromagnetic exchange interactions. This reduction in SQ--SQ exchange that we observe for (NiSQ)2Th2 correlates with an effective lengthening of the bridge unit by ∼2.1 Å relative to that of (ZnSQ)2Th2. This magnitude of the effective increase in the bridge distance is consistent with the (NiSQ)2Th2 JSQ-SQ value lying between those of (ZnSQ)2Th2 and (ZnSQ)2Th3. The ability to modulate spin populations on an organic radical via pairwise Ni-SQ magnetic exchange interactions is a general way to affect electronic coupling in the Th-Th bridge. Our results suggest that metal-radical exchange coupling represents a powerful mechanism for tuning organic molecular electronic structure, with important implications for molecular electronics and molecular electron transport.

Fluorinated Dihydropentalene-1,4-Dione: A Strong Electron-Accepting Unit with Organic Semiconductor Characteristics

The development of electron-accepting units is of significant importance because the construction of donor (D)-acceptor (A) configurations is an effective strategy for tuning the electronic properties of π-conjugated systems. Although doubly fused pentagons represented by diketopyrrolopyrrole (DPP) have been used as an effective electron-accepting unit, the relatively high-lying frontier molecular orbital levels (FMOs) leave room for further improvement. We report herein the synthesis of a fluorinated dihydropentalene-1,4-dione (FPD) derivative as a strong electron-accepting unit and the development of D-A-D π-extended molecules. X-ray analyses revealed that the presence of fluorine atoms contributed to the formation of high planar structures and slipped-stacked packing. Electrochemical measurements indicated that the FPD derivatives showed relatively lower FMO energy levels than the corresponding DPP-containing derivatives. The D-A-D molecule based on terthiophene and FPD showed semiconducting responses. This study demonstrates that the FPD unit can function as a new acceptor unit for organic semiconductors.

Unraveling two distinct polymorph transition mechanisms in one n-type single crystal for dynamic electronics

Cooperativity is used by living systems to circumvent energetic and entropic barriers to yield highly efficient molecular processes. Cooperative structural transitions involve the concerted displacement of molecules in a crystalline material, as opposed to typical molecule-by-molecule nucleation and growth mechanisms which often break single crystallinity. Cooperative transitions have acquired much attention for low transition barriers, ultrafast kinetics, and structural reversibility. However, cooperative transitions are rare in molecular crystals and their origin is poorly understood. Crystals of 2-dimensional quinoidal terthiophene (2DQTT-o-B), a high-performance n-type organic semiconductor, demonstrate two distinct thermally activated phase transitions following these mechanisms. Here we show reorientation of the alkyl side chains triggers cooperative behavior, tilting the molecules like dominos. Whereas, nucleation and growth transition is coincident with increasing alkyl chain disorder and driven by forming a biradical state. We establish alkyl chain engineering as integral to rationally controlling these polymorphic behaviors for novel electronic applications.

Tuning Diradical Properties of Boron-Containing π-Systems by Structural Isomerism

Tuning diradical character is an important topic for organic diradicaloids. Herein, we report the precise borylation enabling structural isomerism as an effective strategy to modulate diradical character and thereby properties of organic diradicaloids. We synthesized a new B-containing polycyclic hydrocarbon that has the indeno[1,2-b]fluorene π-skeleton with the β-carbons bonding to two boron atoms. Detailed theoretical and experimental results show that this bonding pattern leads to its distinctive electronic structures and properties in comparison to that of its isomeric molecule. This molecule has the efficient conjugation between boron atoms and π-skeleton, resulting in downshifted LUMO and HOMO levels. Moreover, it exhibits smaller diradical character and thereby inhibited diradical properties, such as significantly blue-shifted light absorption, larger energy bandgap and weak para-magnetic resonance. Notably, this B-containing polycyclic hydrocarbon possesses much stronger Lewis acidity and its Lewis acid-base adducts display enhanced diradical character, demonstrating the positive effects of Lewis coordination on modulating diradical performance.

Linear Response Functions of Densities and Spin Densities for Systematic Modeling of the QM/MM Approach for Mono- and Poly-Nuclear Transition Metal Systems

We applied our analysis, based on a linear response function of density and spin density, to two typical transition metal complex systems-the reaction centers of P450, and oxygen evolving center in Photosystem II, both of which contain open-shell transition metal ions. We discuss the relationship between LRF of electron density and spin density and the types of units and interactions of the systems. The computational results are discussed in relation to quantum mechanics (QM) cluster and quantum mechanics/molecular mechanics (QM/MM) modeling that are employed to compute the reaction centers of enzymes.

Tunability of Open-Shell Character, Charge Asymmetry, and Third-Order Nonlinear Optical Properties of Covalently Linked (Hetero)Phenalenyl Dimers

Tunability of the open-shell character, charge asymmetry, and third-order nonlinear optical (NLO) properties of covalently linked (hetero)phenalenyl dimers are investigated by using the density functional theory method. By changing the molecular species X and substitution position (i, j) for the linker part, a variety of intermonomer distances R and relative alignments between the phenalenyl dimers can be realized from the geometry optimizations, resulting in a wide-range tuning of diradical character y and charge asymmetry. It is found that the static second hyperpolarizabilities along the stacking direction, γ_yyyy , are one-order enhanced for phenalenyl dimer systems exhibiting intermediate y, a feature that is in good agreement with the "y-γ correlation". By replacing the central carbon atoms of the phenalenyl rings with a boron or a nitrogen, we have also designed covalently linked heterophenalenyl dimers. The introduction of such a charge asymmetry to the open-shell systems, which leads to closed-shell ionic ground states, is found to further enhance the γ_yyyy values of the systems having longer intermonomer distance R with intermediate ionic character, that is, charge asymmetry. The present results demonstrate a promising potential of covalently linked NLO dimers with intermediate open-shell/ionic characters as a new building block of highly efficient NLO systems.

A structure-property interplay between the width and height of cages and the static third order nonlinear optical responses for fullerenes: applying gamma density analysis

To reveal a new structure-property relationship regarding the nonlinear optical (NLO) properties of fullerenes that are associated with gamma (γ) density, fullerenes I (C40, C50, C60 and C70), whose heights range from 4.83 to 7.96 Å, and II (C24, C36, C48 and C72), whose widths range from 4.45 to 8.22 Å, have been the research objects. Calculation of their geometric and electronic structures, absorption spectra, and the second hyperpolarizability (γ) and the γ density analysis have been performed. It is found that the electronic spatial extent and the polarizability (α) value increase linearly as the fullerenes increase by every 12 carbon atoms. Similarly, the γ values are also proportional to the fullerene size. It is worth noting that the relative magnitude of γ_xxxx and γ_zzzz was exactly consistent with that of the width and height of fullerenes. The analysis of γ density provides the essential reason for this result, that is, the magnitude of the contribution to γ values associated with γ densities is proportional to the density amplitudes multiplied by the distance between them. Larger fullerenes possess larger density amplitudes and longer distances, resulting in larger γ values with respect to smaller fullerenes. This work presents a new structure-property interplay between the width and height of the fullerenes and their second hyperpolarizability γ. Moreover, the γ density analysis provides a new insight to explore the nature of the relationship between the structure and the NLO properties.

Third-Order Nonlinear Optical Properties of One-Dimensional Quinoidal Oligothiophene Derivatives Involving Phenoxyl Groups

The diradical characters (y) and third‐order nonlinear optical (NLO) properties of open‐shell quinoidal oligothiophene derivatives with phenoxyl groups, and the corresponding reduced (hydrogenated)‐state oligomers, are investigated by using the broken‐symmetry density functional theory method. The oxidized (dehydrogenated) states are predicted to have an open‐shell singlet ground state and their y values increase with the number of units. Static second hyperpolarizabilities (γ) of the open‐shell oligomers with intermediate y are shown to be enhanced significantly compared with those of the closed‐shell analogues. Furthermore, owing to the effective diradical distances, the γ values of open‐shell oligomers are found to exceed that of s‐indaceno[1,2,3‐cd;5,6,7‐c′d′]diphenalene, which is known as an organic molecule with the largest two‐photon absorption cross‐section in this size of the pure hydrocarbons. This feature extends the range of efficient open‐shell third‐order NLO materials to a novel class of one‐dimensional conjugated oligomers with redox‐based high tunability of third‐order NLO properties.

Diradical Character-Based Design for Singlet Fission of Bisanthene Derivatives: Aromatic-Ring Attachment and π-Plane Twisting

We demonstrate a diradical character-based molecular design for singlet fission using polycyclic aromatic hydrocarbons, bisanthene derivatives. Two types of chemical modifications-aromatic-ring attachment and π-plane twisting-are examined in order to satisfy the energy level matching condition for singlet fission. Detailed analysis of the electronic structures of the model molecules using nucleus-independent chemical shift, molecular orbitals, and their energies has demonstrated the usefulness of the relationship between the resonance structure and aromaticity and that between nonplanarity of π-conjugated systems and reduction of orbital overlap for tuning the diradical character. This result provides a novel design guideline for polycyclic aromatic hydrocarbons toward efficient singlet fission.