Electron transfer reactions can now be followed at the single-molecule level, but the connection between the microscopic and macroscopic data remains to be understood. By monitoring the conductance of a single molecule, we show that the individual electron transfer reaction events are stochastic and manifested as large conductance fluctuations. The fluctuation probability follows first-order kinetics with potential dependent rate constants described by the Butler–Volmer relation. Ensemble averaging of many individual reaction events leads to a deterministic dependence of the conductance on the external electrochemical potential that follows the Nernst equation. This study discloses a systematic transition from stochastic kinetics of individual reaction events to deterministic thermodynamics of ensemble averages and provides insights into electron transfer processes of small systems, consisting of a single molecule or a small number of molecules.
Cyclopenta[b]thiopyran, isomeric to benzo[b]thiophene
while isoelectronic to azulene, is involved
as a building block to construct soluble organic semiconductors for
field-effect transistors. Two series of angular-shaped heteroarenes
based on cyclopenta[b]thiopyran, that is, C
n
-SS (n = 4, 6, 8, 10) with different linear alkyl groups and C
8
-SS-Cl
m
(m = 2, 3, 4) with chlorides
substituted at different positions, have been straightforward synthesized.
The obtained seven S-heteroarenes exhibit intriguing and similar photophysical
and electrochemical properties, such as near-infrared absorption and
high-energy levels of the highest occupied molecular orbitals. Nevertheless,
the S-heteroarenes with identical π-conjugated skeletons demonstrate
completely different molecular packing structures, which is proofed
to be the key determinate factor for the charge carrier mobilities.
Upon the engineering of the pendant alkyl lengths, the highest hole
mobility in the C
n
-SS series is achieved for C
8
-SS (1.1 cm2 V–1 s–1) with moderate alkyl length. The further incorporation
of chlorides on C
8
-SS results in the shortened intermolecular H···S contacts
and the interplane distances. Most interestingly, when chlorine-containing
chloroform and chlorobenzene are used as crystallization solvents,
single crystals of C
8
-SS-Cl
m
with different packing
structures are produced owing to the intermolecular interactions among
the solute and solvent molecules. Upon further engineering of the
chlorination position and the crystallization solvent, the maximum
hole mobility in the ambient air improves to 2.7 cm2 V–1 s–1 for C
8
-SS-Cl
2
crystallized
from chlorobenzene, suggesting that the introduction of the accessible
chlorides is a feasible pathway to engineering the crystal structures
and controlling the charge transport characteristics.
BN‐embedded oligomers with different pairs of BN units were synthesized by electrophilic borylation. Up to four pairs of BN units were incorporated in the large polycyclic aromatic hydrocarbons (PAHs). Their geometric, photophysical, electrochemical, and Lewis acidic properties were investigated by X‐ray crystallography, optical spectroscopy, and cyclic voltammetry. The B−N bonds show delocalized double‐bond characteristics and the conjugation can be extended through the trans‐orientated aromatic azaborine units. Calculations reveal the relatively lower aromaticity for the inner azaborine rings in the BN‐embedded PAH oligomers. The frontier orbitals of the longer oligomers are delocalized over the inner aromatic rings. Consequently, the inner moieties of the BN‐embedded PAH oligomers are more active than the outer parts. This is confirmed by a simple oxidation reaction, which has significant effects on the aromaticity and the intramolecular charge‐transfer interactions.
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