An Important Key to Design Molecules with Small Internal Reorganization Energy: Strong Nonbonding Character in Frontier Orbitals

For an electron to move between molecules (reactants), structural reorganizations of the reactants and their surrounding molecules are needed. The energy cost of the reorganizations, which is determined by structural and electronic features of molecules involved, contributes to the energy barrier of an electron transfer reaction. Finding the factors affecting the energy cost is of fundamental and technological importance. It is believed that extended π-conjugation and a rigid molecular framework are beneficial for minimizing the energy cost. We prove with phenalenyl and phthalocyanine derivatives that the extent of local nonbonding character in frontier molecular orbitals is in fact more crucial than extended π-conjugation; unprecedented small energy cost for reorganization has been found with the help of the nonbonding character. This finding provides a much better understanding of the literature data, as well as a new focus of the molecular design of cutting-edge organic electronics materials

Molecular Orbital-Based Design of π‑Conjugated Organic Materials with Small Internal Reorganization Energy: Generation of Nonbonding Character in Frontier Orbitals

One of the key parameters determining the rate of electron transfer is reorganization energy, an energy associated with geometry change during electron/hole transfer between molecules. To achieve efficient electron transfer, molecules with small reorganization energy are pursued, but the design guidelines remain elusive. It has been shown that a π-conjugated organic molecule with strong local nonbonding character in frontier orbitals may have small internal reorganization energy (λ). To explore how one can introduce such character in frontier orbitals so as to design high-performance materials, in this study we employed fragment molecular orbital analysis and pairing theorem to understand why the frontier orbital of phenalenyl radical had perfect local nonbonding character. The principles learned from phenalenyl radical lead to the design of various closed-shell π-conjugated skeletons with small λ. Functionalization of these skeletons afforded potential n-type materials with small λ (<100 meV) and large electron affinity. Overall, the present work showed that one can design molecules with desired λ by assembling common π-conjugated building blocks in a rational way directed by simple qualitative molecular orbital theory

Substituent Effect on the Structural Behavior of Modified Cyclodextrin: A Molecular Dynamics Study on Methylated β-CDs

A series of methylated and non-methylated β-cyclodextrin (β-CD) structures in three macrocyclic configurations (<i>a</i>–<i>c</i>) were studied with molecular dynamics (MD) simulations to elucidate the dynamic behavior of the different CD structures using a continuum water model with the AMBER* force field. A set of parameters were defined to describe the geometric dimensions of the CD, such as its cavity shape, the upper and lower rim sizes, and the tilting of each of the glucose rings. Correlation analyses between the different parameters were carried out, and they have provided insights into the different dynamic behaviors for the different CD structures. Detailed analyses on the crystal structures of the different methylated and non-methylated β-CD complexes were also carried out using the defined parameters. Correlation of parameters from crystal structures and MD simulations has allowed us to identify the effect that crystal packing/guest inclusion has on the CD geometries. The overall analysis approach can be a useful tool for other related macrocyclic structures, such as modified α-, β-CDs or even calixarenes

Intriguing Electrochemical Behavior of Free Base Porphyrins: Effect of Porphyrin–<i>meso</i>-Phenyl Interaction Controlled by Position of Substituents on <i>meso</i>-Phenyls

Electrochemical properties of substituted free base <i>meso</i>-tetraphenylporphyrins (H₂T­(<i>o</i>,<i>o</i>′-X)­PP, H₂T­(<i>o</i>-X)­PP, and H₂T­(<i>p</i>-X)­PP, where X = OCH₃, CH₃, H, F, or Cl on the phenyl rings) are examined by cyclic voltammetry. When a substituent is located only at the para position of the <i>meso</i>-phenyl group, the difference between the first and second oxidation potentials (Δ<i>E</i>^ox, i.e., <i>E</i>₂^ox – <i>E</i>₁^ox), is generally significantly smaller than those of the H₂TPPs with bulky o,o′-substituents on the phenyl group. This trend is elucidated with density functional theory calculations and attributed mainly to the <i>sterically controlled</i> π-conjugation of the <i>meso</i>-phenyl groups to the central porphyrin ring, rather than the often discussed deformation of porphyrin

Redox-Gated Tristable Molecular Brakes of Geared Rotation

<i>p</i>-Bis­(arylcarbonyl)­pentiptycenes <b>2</b> (aryl = 4-(trifluoromethyl)­phenyl) and <b>3</b> (aryl = mesityl) have been prepared and investigated as redox-gated molecular rotors. For <b>2</b>, rotations about the pentiptycene–carbonyl bond (the α rotation) and about the aryl–carbonyl bond (the β rotation) are independent, and the rotation barriers are 11.3 and 9.5 kcal mol^–1, respectively, at 298 K. In contrast, the α and β rotations in <b>3</b> are correlated (geared) in a 2-fold cogwheel pathway between the aryl and the pentiptycene groups with a much lower rotation barrier of 6.5 kcal mol^–1 at 298 K in spite of the bulkier aryl groups. Electrochemical reduction of the neutral forms led first to radical anions (<b>2</b>^•– and <b>3</b>^•–) and then to a bis­(radical anion) for <b>2</b>^2– but a dianion for <b>3</b>^2–. The redox operations switch the independent α and β rotations in <b>2</b> into a geared rotation in both <b>2</b>^•– and <b>2</b>^2– and result in a slow–fast–stop rotation mode for <b>2</b>–<b>2</b>^•––<b>2</b>^2–. The two redox states <b>3</b>^•– and <b>3</b>^2– retain the geared α and β rotations and follow a fast–slow–stop mode for <b>3</b>–<b>3</b>^•––<b>3</b>^2–. Both molecular systems mimic tristable molecular brakes and display 8–9 orders of magnitude difference in rotation rate through the redox switching

Cooperative Effect of Unsheltered Amide Groups on CO₂ Adsorption Inside Open-Ended Channels of a Zinc(II)–Organic Framework

A unique spatial arrangement of amide groups for CO₂ adsorption is found in the open-ended channels of a zinc­(II)–organic framework {[Zn₄(BDC)₄(BPDA)₄]·5DMF·3H₂O}_<i>n</i> (<b>1</b>, BDC = 1,4-benzyl dicarboxylate, BPDA = <i>N,N′</i>-bis­(4-pyridinyl)-1,4-benzenedicarboxamide). Compound <b>1</b> consists of 4⁴-<b>sql</b> [Zn₄(BDC)₄] sheets that are further pillared by a long linker of BPDA and forms a 3D porous framework with an α-Po 4¹²·6³ topology. Remarkably, the unsheltered amide groups in <b>1</b> provide a positive cooperative effect on the adsorption of CO₂ molecules, as shown by the significant increase in the CO₂ adsorption enthalpy with increasing CO₂ uptake. At ambient condition, a 1:1 ratio of active amide sites to CO₂ molecules was observed. In addition, compound <b>1</b> favors capture of CO₂ over N₂. DFT calculations provided rationale for the intriguing 1:1 ratio of amide sorption sites to CO₂ molecules and revealed that the nanochamber of compound <b>1</b> permits the slipped-parallel arrangement of CO₂ molecules, an arrangement found in crystal and gas-phase CO₂ dimer

Cooperative Effect of Unsheltered Amide Groups on CO₂ Adsorption Inside Open-Ended Channels of a Zinc(II)–Organic Framework

A unique spatial arrangement of amide groups for CO₂ adsorption is found in the open-ended channels of a zinc­(II)–organic framework {[Zn₄(BDC)₄(BPDA)₄]·5DMF·3H₂O}_<i>n</i> (<b>1</b>, BDC = 1,4-benzyl dicarboxylate, BPDA = <i>N,N′</i>-bis­(4-pyridinyl)-1,4-benzenedicarboxamide). Compound <b>1</b> consists of 4⁴-<b>sql</b> [Zn₄(BDC)₄] sheets that are further pillared by a long linker of BPDA and forms a 3D porous framework with an α-Po 4¹²·6³ topology. Remarkably, the unsheltered amide groups in <b>1</b> provide a positive cooperative effect on the adsorption of CO₂ molecules, as shown by the significant increase in the CO₂ adsorption enthalpy with increasing CO₂ uptake. At ambient condition, a 1:1 ratio of active amide sites to CO₂ molecules was observed. In addition, compound <b>1</b> favors capture of CO₂ over N₂. DFT calculations provided rationale for the intriguing 1:1 ratio of amide sorption sites to CO₂ molecules and revealed that the nanochamber of compound <b>1</b> permits the slipped-parallel arrangement of CO₂ molecules, an arrangement found in crystal and gas-phase CO₂ dimer