Game of Frontier Orbitals: A View on the Rational Design of Novel Charge-Transfer Materials

Since the first application of frontier molecular orbitals (FMOs) to rationalize stereospecificity of pericyclic reactions, FMOs have remained at the forefront of chemical theory. Yet, the practical application of FMOs in the rational design and synthesis of novel charge transfer materials remains under-appreciated. In this Perspective, we demonstrate that molecular orbital theory is a powerful and universal tool capable of rationalizing the observed redox/optoelectronic properties of various aromatic hydrocarbons in the context of their application as charge-transfer materials. Importantly, the inspection of FMOs can provide instantaneous insight into the interchromophoric electronic coupling and polaron delocalization in polychromophoric assemblies, and therefore is invaluable for the rational design and synthesis of novel materials with tailored properties

Design, Evolution, and Evaluation of a General Chemistry-Bridging Course

General chemistry I/II courses are important gateway courses for a variety of STEM majors, as poor performance in these courses is often associated with lower rates of student success and retention. Much research has explored preparatory or remedial strategies to improve student outcomes. In this article, we report a different approach, which involved the development of a bridging or intervention course designed to capture students who are not or have not succeeded in general chemistry I and prepare them to retake the course in the following semester or summer. The course was initially developed as an eight-week module in the second half of the fall semester, where students were required to withdraw from general chemistry I prior to enrolling. After three years of this offering modality, with the inception of a winter J-term, the course was redesigned and moved online, where it was taught for another three years. We describe here in detail the design, evolution, and evaluation of this course over the six-year period

Vertical vs. Adiabatic Ionization Energies in Solution and Gas-Phase: Probing Ionization-Induced Reorganization in Conformationally-Mobile Bichromophoric Actuators Using Photoelectron Spectroscopy, Electrochemistry and Theory

Ionization-induced structural and conformational reorganization in various π-stacked dimers and covalently linked bichromophores is relevant to many processes in biological systems and functional materials. In this work, we examine the role of structural, conformational, and solvent reorganization in a set of conformationally mobile bichromophoric donors, using a combination of gas-phase photoelectron spectroscopy, solution-phase electrochemistry, and density functional theory (DFT) calculations. Photoelectron spectral analysis yields both adiabatic and vertical ionization energies (AIE/VIE), which are compared with measured (adiabatic) solution-phase oxidation potentials (Eox). Importantly, we find a strong correlation of Eox with AIE, but not VIE, reflecting variations in the attendant structural/conformational reorganization upon ionization. A careful comparison of the experimental data with the DFT calculations allowed us to probe the extent of charge stabilization in the gas phase and solution and to parse the reorganizational energy into its various components. This study highlights the importance of a synergistic approach of experiment and theory to study ionization-induced structural and conformational reorganization

Spreading Electron Density Thin: Increasing the Chromophore Size in Polyaromatic Wires Decreases Interchromophoric Electronic Coupling

The development of novel polychromophoric materials using extended polycyclic aromatic hydrocarbons as a single large chromophore holds promise for long-range charge-transfer applications in photovoltaic devices and molecular electronics. However, it is not well-understood how the interchromophoric electronic coupling varies with the chromophore size in linearly connected molecular wires. Here, we show with the aid of electrochemistry, electronic spectroscopy, density functional theory calculations, and theoretical modeling that as the number of aromatic moieties in a single chromophore increases, the interchromophoric electronic coupling decreases and may reach negligible values if the chromophore is sufficiently large. The origin of this initially surprising result becomes clear when one considers this problem with the aid of Hückel molecular orbital theory, as at the polymeric limit energies of the molecular orbitals cluster to form bands and thus the energy spacing between orbitals, and thereby the electronic coupling must decrease with the chromophore expansion

Brief report: RRx-001 is a c-Myc inhibitor that targets cancer stem cells.

The goal of anticancer therapy is to selectively eradicate all malignant cells. Unfortunately for the majority of patients with metastatic disease, this goal is consistently thwarted by the nearly inevitable development of therapeutic resistance; the main driver of therapeutic resistance is a minority subpopulation of cancer cells called cancer stem cells (CSCs) whose mitotic quiescence essentially renders them non-eradicable. The Wnt signaling pathway has been widely implicated as a regulator of CSCs and, therefore, its inhibition is thought to result in a reversal of therapeutic resistance via loss of stem cell properties. RRx-001 is a minimally toxic redox-active epi-immunotherapeutic anticancer agent in Phase III clinical trials that sensitizes tumors to radiation and cytotoxic chemotherapies. In this article, as a potential mechanism for its radio- and chemosensitizing activity, we report that RRx-001 targets CD133 + /CD44 + cancer stem cells from three colon cancer cell-lines, HT-29, Caco-2, and HCT116, and inhibits Wnt pathway signalling with downregulation of c-Myc

Cofacially Arrayed Polyfluorenes: Spontaneous Formation of π-Stacked Assemblies in the Gas Phase

Understanding geometrical and size dependencies of through-space charge delocalization in multichromophoric systems is critical to model electron transfer and transport in materials and biomolecules. In this work, we examine the size evolution of hole delocalization in van der Waals clusters of fluorene (i.e., (F)n), where a range of geometries are possible, reflecting both π-stacking and C–H/π interactions. Using mass-selected two-color resonant two-photon ionization spectroscopy (2CR2PI), we measure electronic spectra and vertical ionization potentials (IPs) in the gas phase. Results are compared with model covalently linked assemblies (denoted Fn), exhibiting a sterically enforced cofacial (i.e., π-stacked) orientation of chromophores. For both systems, an inverse size dependence (i.e., 1/n) of IP vs cluster size is found. Surprisingly, the values for the two sets fall on the same line! This trend is examined via theory, which emphasizes the important role of π-stacking, and its geometrical dependencies, in the process of hole delocalization in multichromophoric assemblies

Spectroscopy and Dynamics of the Predissociated, Quasi-linear S2 State of Chlorocarbene

In this work, we report on the spectroscopy and dynamics of the quasi-linear S2 state of chlorocarbene, CHCl, and its deuterated isotopologue using optical-optical double resonance (OODR) spectroscopy through selected rovibronic levels of the S1 state. This study, which represents the first observation of the S2 state in CHCl, builds upon our recent examination of the corresponding state in CHF, where pronounced mode specificity was observed in the dynamics, with predissociation rates larger for levels containing bending excitation. In the present work, a total of 14 S2 state vibrational levels with angular momentum ℓ = 1 were observed for CHCl, and 34 levels for CDCl. The range of ℓ in this case was restricted by the pronounced Renner-Teller effect in the low-lying S1 levels, which severely reduces the fluorescence lifetime for levels with Ka \u3e 0. Nonetheless, by exploiting different intermediate S1 levels, we observed progressions involving all three fundamental vibrations. For levels with long predissociation lifetimes, rotational constants were determined by measuring spectra through different intermediate J levels of the S1 state. Plots of the predissociation linewidth (lifetime) vs. energy for various S2 levels show an abrupt onset, which lies near the calculated threshold for elimination to form C(3P) + HCl on the triplet surface. Our experimental results are compared with a series of high level ab initio calculations, which included the use of a dynamically weighted full-valence CASSCF procedure, focusing maximum weight on the state of interest (the singlet and triplet states were computed separately). This was used as the reference for subsequent Davidson-corrected MRCI(+Q) calculations. These calculations reveal the presence of multiple conical intersections in the singlet manifold

Strength of π-Stacking, from Neutral to Cation: Precision Measurement of Binding Energies in an Isolated π-Stacked Dimer

π-Stacking interactions are ubiquitious across chemistry and biochemistry, impacting areas from organic materials and photovoltaics to biochemistry and DNA. However, experimental data is lacking regarding the strength of π-stacking forces—an issue not settled even for the simplest model system, the isolated benzene dimer. Here, we use two-color appearance potential measurements to determine the binding energies of the isolated, π-stacked dimer of fluorene (C13H10) in ground, excited, and ionic states. Our measurements provide the first precise values for π-stacking interaction energies in these states, which are key benchmarks for theory. Indeed, theoretical predictions using ab initio and carefully benchmarked DFT methods are in excellent agreement with experiment