7 research outputs found
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
Ï€-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 (H<sub>2</sub>TÂ(<i>o</i>,<i>o</i>′-X)ÂPP, H<sub>2</sub>TÂ(<i>o</i>-X)ÂPP,
and H<sub>2</sub>TÂ(<i>p</i>-X)ÂPP, where X = OCH<sub>3</sub>, CH<sub>3</sub>, 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><sup>ox</sup>, i.e., <i>E</i><sub>2</sub><sup>ox</sup> – <i>E</i><sub>1</sub><sup>ox</sup>), is
generally significantly smaller than those of the H<sub>2</sub>TPPs
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>-BisÂ(arylcarbonyl)Âpentiptycenes <b>2</b> (aryl =
4-(trifluoromethyl)Âphenyl) 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<sup>–1</sup>, 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<sup>–1</sup> at 298 K in spite of the bulkier
aryl groups. Electrochemical reduction of the neutral forms led first
to radical anions (<b>2</b><sup>•–</sup> and <b>3</b><sup>•–</sup>) and then to a bisÂ(radical anion)
for <b>2</b><sup>2–</sup> but a dianion for <b>3</b><sup>2–</sup>. The redox operations switch the independent
α and β rotations in <b>2</b> into a geared rotation
in both <b>2</b><sup>•–</sup> and <b>2</b><sup>2–</sup> and result in a slow–fast–stop
rotation mode for <b>2</b>–<b>2</b><sup>•–</sup>–<b>2</b><sup>2–</sup>. The two redox states <b>3</b><sup>•–</sup> and <b>3</b><sup>2–</sup> retain the geared α and β rotations and follow a fast–slow–stop
mode for <b>3</b>–<b>3</b><sup>•–</sup>–<b>3</b><sup>2–</sup>. 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 CO<sub>2</sub> Adsorption Inside Open-Ended Channels of a Zinc(II)–Organic Framework
A unique spatial
arrangement of amide groups for CO<sub>2</sub> adsorption is found
in the open-ended channels of a zincÂ(II)–organic framework
{[Zn<sub>4</sub>(BDC)<sub>4</sub>(BPDA)<sub>4</sub>]·5DMF·3H<sub>2</sub>O}<sub><i>n</i></sub> (<b>1</b>, BDC = 1,4-benzyl
dicarboxylate, BPDA = <i>N,N′</i>-bisÂ(4-pyridinyl)-1,4-benzenedicarboxamide).
Compound <b>1</b> consists of 4<sup>4</sup>-<b>sql</b> [Zn<sub>4</sub>(BDC)<sub>4</sub>] sheets that are further pillared
by a long linker of BPDA and forms a 3D porous framework with an α-Po
4<sup>12</sup>·6<sup>3</sup> topology. Remarkably, the unsheltered
amide groups in <b>1</b> provide a positive cooperative effect
on the adsorption of CO<sub>2</sub> molecules, as shown by the significant
increase in the CO<sub>2</sub> adsorption enthalpy with increasing
CO<sub>2</sub> uptake. At ambient condition, a 1:1 ratio of active
amide sites to CO<sub>2</sub> molecules was observed. In addition,
compound <b>1</b> favors capture of CO<sub>2</sub> over N<sub>2</sub>. DFT calculations provided rationale for the intriguing 1:1
ratio of amide sorption sites to CO<sub>2</sub> molecules and revealed
that the nanochamber of compound <b>1</b> permits the slipped-parallel
arrangement of CO<sub>2</sub> molecules, an arrangement found in crystal
and gas-phase CO<sub>2</sub> dimer
Cooperative Effect of Unsheltered Amide Groups on CO<sub>2</sub> Adsorption Inside Open-Ended Channels of a Zinc(II)–Organic Framework
A unique spatial
arrangement of amide groups for CO<sub>2</sub> adsorption is found
in the open-ended channels of a zincÂ(II)–organic framework
{[Zn<sub>4</sub>(BDC)<sub>4</sub>(BPDA)<sub>4</sub>]·5DMF·3H<sub>2</sub>O}<sub><i>n</i></sub> (<b>1</b>, BDC = 1,4-benzyl
dicarboxylate, BPDA = <i>N,N′</i>-bisÂ(4-pyridinyl)-1,4-benzenedicarboxamide).
Compound <b>1</b> consists of 4<sup>4</sup>-<b>sql</b> [Zn<sub>4</sub>(BDC)<sub>4</sub>] sheets that are further pillared
by a long linker of BPDA and forms a 3D porous framework with an α-Po
4<sup>12</sup>·6<sup>3</sup> topology. Remarkably, the unsheltered
amide groups in <b>1</b> provide a positive cooperative effect
on the adsorption of CO<sub>2</sub> molecules, as shown by the significant
increase in the CO<sub>2</sub> adsorption enthalpy with increasing
CO<sub>2</sub> uptake. At ambient condition, a 1:1 ratio of active
amide sites to CO<sub>2</sub> molecules was observed. In addition,
compound <b>1</b> favors capture of CO<sub>2</sub> over N<sub>2</sub>. DFT calculations provided rationale for the intriguing 1:1
ratio of amide sorption sites to CO<sub>2</sub> molecules and revealed
that the nanochamber of compound <b>1</b> permits the slipped-parallel
arrangement of CO<sub>2</sub> molecules, an arrangement found in crystal
and gas-phase CO<sub>2</sub> dimer