Effects of Aromatic Trifluoromethylation, Fluorination, and Methylation on Intermolecular π–π Interactions

Marcus theory states that the rate of charge transfer is directly proportional to the amount of intermolecular orbital overlap. Theoretically optimizing the electronic coupling through the orientation and distance which both can increase the frontier orbital overlap between molecules is an attractive route to potentially provide theoretical insight for discovering new high performance semiconductor materials. To investigate how these parameters qualitatively affect charge transfer of model systems, unconstrained dimer optimizations with MP2 and dispersion-corrected DFT methods were used to probe the π–π interactions of methylated, fluorinated, and trifluoromethylated benzene, pyridine, and bipyridine dimers. These systems can serve as simplified models representing weak noncovalent interactions in organic semiconductor materials. Enhanced intermolecular interaction energies, reduced π–π distances, and more favorable cofacial orientations were found with the trifluoromethylated dimers compared to fluorinated and methylated dimers studied. Similar effects were found with donor–acceptor pairs that represent organic p-n heterojunction systems. These enhanced π–π interactions are likely caused by increased molecular quadrupole moment and dispersion interaction associated with trifluoromethylation. This computational study illustrates the strong potential of trifluoromethylation and, possibly perfluoroalkylation of acenes and heteroacenes, leading qualitatively to enhanced electron transfer through better π–π stacked structures, making them viable candidates for use as n-type organic semiconductor materials. The findings also provide insight for fundamental interactions between drug molecules that include fluorinated and trimethylfluorinated aromatics binding to protein receptors

Steering Power of Perfluoroalkyl Substituents in Crystal Engineering: Tuning the π–π Distance While Maintaining the Lamellar Packing Motif for Aromatics with Various Sizes of π‑Conjugation

Previously, we reported that introducing perfluoroalkyl substituents onto aromatics promotes the formation of lamellar π–π stacked crystalline materials with short interplanar distances. In this report, we developed a new synthetic route that effectively prepares perfluoroalkylated N-containing aromatics by eliminating a side perfluoroalkylation reaction occurring on nonsubstituted C_sp2–H sites of the corresponding bromoaromatics without regioselectivity. This results in a significant improvement of the yield of target perfluoroalkylated aromatics and facilitates the purification and scale-up processes. X-ray single crystal structural analyses show that lamellar π–π stacked structures with tunable interplanar distances are achieved with fused N-containing aromatics with varying sizes of π-conjugation. Both crystal structures and theoretical calculations demonstrated that the interplanar distance can be fine-tuned with the size of π-conjugation, with larger π-conjugation favoring shorter interplanar distances while still maintaining a lamellar π–π stacked packing motif. Compared to our previous results, we find that simply changing the perfluoroalkyl substituent positions and patterns can change molecular topology to exclusively form lamellar π–π stacked packing motifs through prioritization of specific steric effects. Electrochemical results and absorption spectra confirm that the band gap is reduced due to increasing π-conjugation, and the first reduction potential exhibits a significant positive shift due to both increasing π-conjugation and perfluoroalkylation

Rational Design of Lamellar π–π Stacked Organic Crystalline Materials with Short Interplanar Distance

Organic crystalline materials having a lamellar π–π stacked structural motif with short interplanar distance are significant for many applications. By asymmetrically introducing perfluoroalkyl substituents onto and polarizable sulfur atoms into N-containing heteroaromatics, we successfully synthesized a novel type of aromatic material that preferentially forms lamellar π–π stacked crystalline materials with a interplanar π–π distance of 3.247 Å, more than 0.1 Å shorter than that of highly oriented pyrolytic graphite (HOPG) where interplanar distance ranges from 3.35 to 3.39 Å

Rational Design of Lamellar π–π Stacked Organic Crystalline Materials with Short Interplanar Distance

Organic crystalline materials having a lamellar π–π stacked structural motif with short interplanar distance are significant for many applications. By asymmetrically introducing perfluoroalkyl substituents onto and polarizable sulfur atoms into N-containing heteroaromatics, we successfully synthesized a novel type of aromatic material that preferentially forms lamellar π–π stacked crystalline materials with a interplanar π–π distance of 3.247 Å, more than 0.1 Å shorter than that of highly oriented pyrolytic graphite (HOPG) where interplanar distance ranges from 3.35 to 3.39 Å

Molecular Origin of Isomerization Effects on Solid State Structures and Optoelectronic Properties: A Comparative Case Study of Isomerically Pure Dicyanomethylene Substituted Fused Dithiophenes

Introduction of a strong electron-withdrawing dicyanomethylene (−CH–(CN)₂) group onto a fused bithiophene frame is a useful strategy to convert fused bithiophene derivatives from p-type organic semiconductor materials into n-type materials. Here, through systematic studies of isomerically pure 7-dicyanomethylene-7<i>H</i>-cyclopenta-[1,2-<i>b</i>:4,3-<i>b</i>′]­dithiophene (<b>1</b>), 4-dicyanomethylene-4<i>H</i>-cyclopenta­[2,1-<i>b</i>:3,4-<i>b</i>′]­dithiophene (<b>2</b>), and 7-dicyanomethylene-7<i>H</i>-cyclopenta­[1,2-<i>b</i>:3,4-<i>b</i>′]­dithiophene (<b>3</b>) as well as their oligomers and polymers, we report that isomerization has the potential to fine-tune the optoelectronic properties of these materials including band gap (<i>E</i>_g), electron affinities (EAs), ionization potentials (IPs), electrochemical polymerization behaviors, and the solid state molecular packing, all of which are important for the performance of semiconductor devices. The monomers of these isomers exhibit noticeable difference in maximum absorption energies; and the oligomers and polymers composed of these monomers exhibit increased band gap difference as predicted by DFT calculation. Furthermore, the isomer <b>2</b> exhibits better electrochemical polymerization behavior as well as profound electrochromic switching in the near to middle infrared region. X-ray diffraction and quantum mechanical calculations reveal that the difference of dipole and quadrupole moments in these isomers is likely responsible for the difference in the solid state packing and subsequent polymer assembly

Molecular Origin of Isomerization Effects on Solid State Structures and Optoelectronic Properties: A Comparative Case Study of Isomerically Pure Dicyanomethylene Substituted Fused Dithiophenes

Introduction of a strong electron-withdrawing dicyanomethylene (−CH–(CN)₂) group onto a fused bithiophene frame is a useful strategy to convert fused bithiophene derivatives from p-type organic semiconductor materials into n-type materials. Here, through systematic studies of isomerically pure 7-dicyanomethylene-7<i>H</i>-cyclopenta-[1,2-<i>b</i>:4,3-<i>b</i>′]­dithiophene (<b>1</b>), 4-dicyanomethylene-4<i>H</i>-cyclopenta­[2,1-<i>b</i>:3,4-<i>b</i>′]­dithiophene (<b>2</b>), and 7-dicyanomethylene-7<i>H</i>-cyclopenta­[1,2-<i>b</i>:3,4-<i>b</i>′]­dithiophene (<b>3</b>) as well as their oligomers and polymers, we report that isomerization has the potential to fine-tune the optoelectronic properties of these materials including band gap (<i>E</i>_g), electron affinities (EAs), ionization potentials (IPs), electrochemical polymerization behaviors, and the solid state molecular packing, all of which are important for the performance of semiconductor devices. The monomers of these isomers exhibit noticeable difference in maximum absorption energies; and the oligomers and polymers composed of these monomers exhibit increased band gap difference as predicted by DFT calculation. Furthermore, the isomer <b>2</b> exhibits better electrochemical polymerization behavior as well as profound electrochromic switching in the near to middle infrared region. X-ray diffraction and quantum mechanical calculations reveal that the difference of dipole and quadrupole moments in these isomers is likely responsible for the difference in the solid state packing and subsequent polymer assembly

Elucidating the role of non-radiative processes in charge transfer of core–shell Si–SiO₂ nanoparticles

<div><p>Crystalline silicon is the most commonly used material in photovoltaics but has limitations due to its high cost and non-tunable band gap. A new approach of using inexpensive, non-toxic materials with layers that have different band gaps which absorb a wide range of the solar spectrum has the potential to dramatically increase the efficiencies and lower the costs. Core–shell Si–SiO₂ nanoparticles are ideally suited for the photovoltaic application and have been synthesised by different groups in an array of sizes allowing for absorption in a wide spectral range. A theoretical investigation of fundamental charge transfer processes in these systems can potentially lead to improved devices. Calculations on a model core–shell interface with the formula Si₂₆₄O₁₆₀ which features a silicon layer sandwiched between two SiO₂ layers were performed using the Vienna <i>ab initio</i> software package. The Perdew–Burke–Ernzerhof functional in the basis of plane waves was used along with pseudopotentials to simulate electronic structure. The nuclear motion was considered using <i>ab initio</i> molecular dynamics. The density of states, absorption spectrum, partial charge densities, and radiative recombination lifetimes have been calculated. This interface shows quantum confinement behaviour similar to a particle in a box. The role of non-radiative recombination was also determined by relaxation dynamics.</p></div

Strengthening π–π Interactions While Suppressing C_sp2–H···π (T-Shaped) Interactions via Perfluoroalkylation: A Crystallographic and Computational Study That Supports the Beneficial Formation of 1‑D π–π Stacked Aromatic Materials

The design and synthesis of aromatic crystalline materials with controllable crystal structure packing is of particular interest in organic semiconductor and optoelectronic devices, where 1-D π–π stacked structures that enhance charge mobility are the most beneficial. We report here that the π–π interactions between aromatic molecules can be strengthened and the C_sp2–H···π (T–shape) interaction can be suppressed by perfluoroalkylation of corresponding aromatics. Both crystal structure data and ab initio calculations show that the π–π interaction is strengthened due to the electronic effects of perfluoroalkyl substituents, and the C_sp2–H···π interaction is suppressed by the steric effects of the perfluoroalkyl substituents. The C_sp3–F···F–C_sp3 attractive interactions between perfluoroalkyl chains further stabilize the crystal structures. We also found that C_sp3–F···π interaction can be eliminated if an optimal electron deficiency of the π system is tuned by adjusting the number of perfluoroalkyl substituents. The insight gained from this study is of particular interest in organic semiconductor research as well as the fields of molecular recognition, sensing, and design of enzyme inhibitors where π–π interactions are also important