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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Symmetry breaking charge transfer as a means to study electron transfer with no driving force

Symmetry breaking charge transfer (SBCT) is a process where a symmetrically disposed pair of identical chromophores forms a charge transfer excited state with the hole and electron on different chromophores, i.e. chr–chr + hν → chr + –chr – . Herein we explore this process in two dipyrrin-based bichromophoric systems. One of these bisdipyrrins involved a pair of BODIPY chromophores linked by a single bond at their meso-positions (compound 1) and the other involved two dipyrrin ligands coordinated in a tetrahedral geometry at the Zn 2+ ion (compound 2). Both compounds show rapid SBCT in polar solvents and only dipyrrin based emission in nonpolar solvents, the latter arising from a dipyrrin localized excited sate (LE). By “tuning” the solvent polarity the equilibrium between the LE and SBCT states can be shifted to favor either state. Ultrafast transient absorption spectroscopy (TA) was used to probe the kinetics of the charge transfer for 2 in solvents where the electron transfer is endergonic, exergonic and has a ΔG close to zero. Our TA derived rates were used to predict fluorescence efficiencies in each of the different solvent systems and showed a good correspondence to measured values. Detailed density functional theory (DFT) and time dependent DFT were used to model the ground states as well as the LE and SBCT states of 1 and 2, in both polar and nonpolar media. Lastly, the ground and LE excited states show small dipole moments, while the SBCT states show dipole moments of 16.4 and 20.3 D for 1 and 2, respectively.

14 SOLAR ENERGY↗
DOE OSTI2015

A generalized force-modified potential energy surface for mechanochemical simulations

We describe the modifications that a spatially varying external load produces on a Born-Oppenheimer potential energy surface (PES) by calculating static quantities of interest. The effects of the external loads are exemplified using electronic structure calculations (at the HF/6-31G{sup ∗∗} level) of two different molecules: ethane and hexahydro-1,3,5-trinitro-s-triazine (RDX). The calculated transition states and Hessian matrices of stationary points show that spatially varying external loads shift the stationary points and modify the curvature of the PES, thereby affecting the harmonic transition rates by altering both the energy barrier as well as the prefactor. The harmonic spectra of both molecules are blueshifted with increasing compressive “pressure.” Some stationary points on the RDX-PES disappear under application of the external load, indicating the merging of an energy minimum with a saddle point.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗