Isolation of a chiral anthracene cation radical: X-ray crystallography and computational interrogation of its racemization

Abstract

Chiral cation-radical salts hold significant promise as charge-transfer materials, chiroptical switches, and electron-transfer catalysts for enantioselective synthesis. Herein we demonstrate that the readily-available chiral 9,10-diphenyleanthracene derivative (i.e. SANT) forms a robust cation radical, whose structure was elucidated by X-ray crystallography and DFT calculations. While SANT was observed to racemize on a timescale (t1/2 ) of 1.1 hours, a computational conformational search and kinetic analysis of the racemization pathway led us to identify a simple methyl substituted SANT derivative, which does not racemize (racemization t1/2 ≃ 1013–1017 years).

Ask Not How Many, But Where They Are: Substituents Control Energetic Ordering of Frontier Orbitals/Electronic Structures in Isomeric Methoxy-Substituted Dibenzochrysenes

“…It was shown that 40% of the HF exchange provides a balanced description of the electronic structure of the poly- p -phenylene cation radicals similarly to that shown by Kaupp and co-workers for a reliable description of the another class of organic mixed-valence systems. Indeed, modified B1LYP-40/6-31G­(d) functional performed exceptionally well in reproducing the experimental redox/optoelectronic properties of a variety of poly- p -phenylene- , and polyfluorene-based wires and other PAHs. , …”

“…The importance of this finding is recognized in light of prior works that have shown, through X-ray structural analysis of PAH cation radicals, that contraction/elongation of the C−C bonds upon 1-e − oxidation track in accordance with the nodal structure of HOMO (i.e., bonds with bonding HOMO lobes undergo elongations, whereas the bonds with antibonding HOMO lobes undergo contractions). 28,29,38 Moreover, the experimental bond length changes in various PAH cation radicals can be fully reproduced by DFT calculations and the calculated spin/charge distributions in these cation radicals closely resemble the HOMO density distributions. 28,29,38 Indeed, the calculated spin-density distributions (Figure 7A) correspond to phenanthrene-like orbital in DBC +• , m-DBC +• and m/p-DBC +• and biphenyl-like orbital in p-DBC +• , in accordance with the nodal arrangement of HOMOs displayed in Figure 5.…”

“…28,29,38 Moreover, the experimental bond length changes in various PAH cation radicals can be fully reproduced by DFT calculations and the calculated spin/charge distributions in these cation radicals closely resemble the HOMO density distributions. 28,29,38 Indeed, the calculated spin-density distributions (Figure 7A) correspond to phenanthrene-like orbital in DBC +• , m-DBC +• and m/p-DBC +• and biphenyl-like orbital in p-DBC +• , in accordance with the nodal arrangement of HOMOs displayed in Figure 5.…”

“…Indeed, modified B1LYP-40/6-31G(d) functional performed exceptionally well in reproducing the experimental redox/optoelectronic properties of a variety of poly-p-phenylene- 10,26 and polyfluorene-based 27 wires and other PAHs. 28,29 Accordingly, in this manuscript, we performed electronic structure calculations of neutral DBCs and their cation radicals using B1LYP-40/6-31G(d) level of theory. Due to the important role of the solvation in stabilization of charged molecules and the effect on the nature of the excited states, 21,30,31 we also performed (vide inf ra) a comparative analysis of the electronic structure of DBC cation radicals in solvents with varied polarity using polarizable continuum model (PCM) .…”

Probing Charge Delocalization in Solid State Polychromophoric Cation Radicals Using X-ray Crystallography and DFT Calculations

“…In this context, we introduced a customized version of B1LYP functional with 40% of HF exchange term (i.e., B1LYP-40) where the amount of HF was empirically adjusted to accurately reproduce oxidation potentials and the cation radical excitation energies of poly­( p -phenylene)­s. Remarkably, it was shown in multiple studies that B1LYP-40/6-31G­(d) performs exceptionally well in reproducing the experimental redox/optoelectronic properties of a variety of poly­( p -phenylene)-based wires, ,, other PAHs, , and π-stacked assemblies − that were not included in the original training set. In a recent study, we also showed on several selected PAHs that time-dependent DFT paired with B1LYP-40 functional provides excitation energies and orbitals involved in the excitation that are consistent with those obtained using a highly reliable EOM-IP-CCSD method.…”