Dong-Qing Lin

Progress in fluorene-based wide-bandgap steric semiconductors

Molecular bulks are favorable for the thermal and morphological stability in organic wide-bandgap semiconducting polymers with potential applications in both information and energy electronics. In this review, we present our progress in the design of fluorene-based bulky semiconductors with a fractal four-element pattern. Firstly, we established one-pot methods to spirofluorenes, especially spiro[fluorene-9,9′-xanthene] (SFX) serving as the next-generation spiro-based semiconductors. Secondly, we observed the supramolecular forces at the bulky groups and discovered the supramolecular steric hindrance (SSH) effect on polymorphisms, nanocrystals as well as device performance. Thus, a synergistically molecular attractor-repulsor theory (SMART) was proposed for the control of nanocrystal morphology, thin film phase and morphology. Thirdly, the third possible type of defects has been identified to generate green band (g-band) emission in wide-bandgap semiconductors by the introduction of molecular strain design of cyclofluorene. Finally, the first bulky polydiarylfluorene with highly crystalline and β conformation was achieved by an attractor-repulsor design of tadpole-shape monomer, which offered an effective platform to fabricate stable wide-bandgap semiconducting devices. All the discoveries offer the solid basis to break through bottlenecks of organic/polymer wide-bandgap semiconductors by the improvements of overall performances.

Label-Free Dynamic Detection of Single-Molecule Nucleophilic-Substitution Reactions

The mechanisms of chemical reactions, including the transformation pathways of the electronic and geometric structures of molecules, are crucial for comprehending the essence and developing new chemistry. However, it is extremely difficult to realize at the single-molecule level. Here, we report a single-molecule approach capable of electrically probing stochastic fluctuations under equilibrium conditions and elucidating time trajectories of single species in non-equilibrated systems. Through molecular engineering, a single molecular wire containing a functional center of 9-phenyl-9-fluorenol was covalently wired into nanogapped graphene electrodes to form stable single-molecule junctions. Both experimental and theoretical studies consistently demonstrate and interpret the direct measurement of the formation dynamics of individual carbocation intermediates with a strong solvent dependence in a nucleophilic-substitution reaction. We also show the kinetic process of competitive transitions between acetate and bromide species, which is inevitable through a carbocation intermediate, confirming the classical mechanism. This unique method creates plenty of opportunities for carrying out single-molecule dynamics or biophysics investigations in broad fields beyond reaction chemistry through molecular design and engineering.