RESUMEN
Computationally inspired design of organic electronic materials requires robust models of not only the ground and excited electronic states but also of transitions between these states. In this work, we introduce a strategy for obtaining electronic transition dipole moments for the lowest-lying singlet-singlet transition in organic chromophores from time-independent excited-state density-functional tight-binding (ΔDFTB) calculations. Through small-molecule benchmarks and applications to larger chromophores, we explore the accuracy, potential, and limitations of this semiempirical strategy. While more accurate methods are recommended for small systems, we find some evidence for the method's potential in high-throughput molecular screening applications and in the analysis of molecular dynamics simulations.
RESUMEN
What we as scientists and educators assess has a tremendous impact on whom we authorize to participate in science careers. Unfortunately, in critical gateway chemistry courses, assessments commonly emphasize and reward recall of disaggregated facts or performance of (often mathematical) skills. Such an emphasis marginalizes students based on their access to pre-college math preparation and misrepresents the intellectual work of chemistry. Here, we explore whether assessing intellectual work more authentic to the practice of chemistry (i.e., mechanistic reasoning) might support more equitable achievement. Mechanistic reasoning involves explaining a phenomenon in terms of interactions between lower scale entities (e.g., atoms and molecules). We collected 352 assessment tasks administered in college-level introductory chemistry courses across two universities. What was required for success on these tasks was rote math skills (165), mechanistic reasoning (36), neither (126), or both (25). Logistic regression models predict that the intellectual work emphasized on in an assessment could impact whether 15-20% of the cohort passes or fails. Whom does assessment emphasis impact most? Predicted pass rates for those often categorized as "at-risk" could be 67 or 93%, depending on whether their success was defined by rote calculation or mechanistic reasoning. Therefore, assessment transformation could provide a path toward advancing the relevance of our courses and educational equity.
RESUMEN
Incorporation of the triad of redox activity, hemilability, and proton responsivity into a single ligand scaffold is reported. Due to this triad, the complexes Fe(PyrrPDI)(CO)2 (3) and Fe(MorPDI)(CO)2 (4) display 40-fold enhancements in the initial rate of NO2- reduction, with respect to Fe(MeOPDI)(CO)2 (7). Utilizing the proper sterics and p Ka of the pendant base(s) to introduce hemilability into our ligand scaffolds, we report unusual {FeNO} x mononitrosyl iron complexes (MNICs) as intermediates in the NO2- reduction reaction. The {FeNO} x species behave spectroscopically and computationally similar to {FeNO}7, an unusual intermediate-spin Fe(III) coupled to triplet NO- and a singly reduced PDI ligand. These {FeNO} x MNICs facilitate enhancements in the initial rate.