Singlet Diradical Character from Experiment

Abstract

Understanding the electronic structure of the singlet diradical state tackles the fundamentals of the chemical bond itself. The singlet diradical character is a key factor, which determines the chemical reactivity, the reaction products, as well as the chemical/physical (electronic, optical and magnetic) properties, although this quantity is defined in a purely theoretical manner. In this Letter, on the basis of the valence configuration interaction scheme, we present an explicit connection between the singlet diradical character and measured quantities obtained from one- and two-photon absorption spectra as well as from phosphorescence and electron spin resonance peaks. This enables us to estimate the singlet diradical character experimentally.

Perylene Bisimide Radicals and Biradicals: Synthesis and Molecular Properties

“…S6). [23] This remarkably high value is consistent with the small degree of exchange interactions between the two unpaired electrons in OS-2CC.…”

Study of the Diradicaloid Character in a Prototypical Pancake‐Bonded Dimer: The Stacked Tetracyanoethylene (TCNE) Anion Dimer and the Neutral K2TCNE2 Complex

“…tive optimized geometries (with orbital Set 4) also indicate strongly that this pancake-bonded system has a low singlettriplet splitting similar to other pancake-bonded systems. [44] A low DE ST value is associated with high diradicaloid character [45] and it is also in concordance with weak bonds compared with ordinary covalent bonds. [46] The longitudinal (L) and transversal (T) scans of the interaction energy are given in Figure 5 b. Interestingly, a second local minimum appears in both cases.…”

Recent Progress in Quinoidal Singlet Biradical Molecules

“…89 This formula is alternatively expressed as y ¼ 1 À ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 À E S1u,S1g À E T 1u,S1g E S2g,S1g 2 s ð1Þ where E S2g,S1g , E S1u,S1g , and E T1u,S1g correspond to the excitation energies of the higher singlet state of the g symmetry (twophoton allowed excited state), of the lower singlet state with u symmetry (one-photon allowed excited state), and of the triplet state with u symmetry, respectively. These parameters can be determined experimentally from two-photon absorption (TPA), UVvisNIR, and ESR (or phosphorescence, SQUID) measurements, respectively.…”