Investigating the (Poly)Radicaloid Nature of Real-World Organic Compounds with DFT-Based Methods

Abstract

Recent advances in the synthesis of stable organic (open-shell) polyradicaloids have opened their application as active compounds for emerging technologies. These systems typically exhibit small energy differences between states with different spin multiplicities, which are intrinsically difficult to calculate by theoretical methods. We thus apply here some DFT-based variants (FT-DFT, SF-DFT, and SF-TDDFT) on a test set of large and real-world molecules, as test systems for which such energy differences are experimentally available, also comparing systematically with RAS-SF results to infer if shortcomings of previous DFT applications are corrected. Additionally, we explore the spin-spin contribution to the ZFS tensor, of high interest for EPR spectroscopy, and derive the spatial extent of the corresponding (photoexcited) triplet state.

Singlet‐Triplet Excited‐State Inversion in Heptazine and Related Molecules: Assessment of TD‐DFT and ab initio Methods

“…The SC‐NEVPT2 correction significantly decreases the SA‐CASSCF results for and excitation energies of 2T‐4N and 2T‐7N, provinding in all cases very consistent results. Looking at the radicaloid nature of the ground‐state ( S 0 ) of these systems, as given by the integrated number of electrons ( N U ) housed on fractionally occupied orbitals, [68–70] see details as Supplementary Material, we can see a transition from moderately (mono)radicaloid ( for 2T‐N) to negligibly radicaloid ( character for 2T‐7N), passing through an intermediate value of character for 2T‐4N.…”

Addressing the Elusive Polaronic Nature of Multiple Redox States in a π‐Conjugated Ladder‐Type Polymer

“…Contrary to electron bipolarons, hole B+(S) presents a BS solution at the monomer level (BBL1) already, with Δ E = 0.5 and 1 eV for cis and trans , respectively. This is a remarkable aspect, because it highlights the multireference (MR) and polyradicaloid character [ 28 ] of the double charged (bipolaron) hole‐wavefunction.…”

“…In general, the presence of low energy BS‐UDFT solutions, regardless the nature of the charge, highlights a complex electronic structure scenario in which static electron correlation (SEC) and MR effects play relevant role. [ 29 ] Despite BS‐UDFT is based on a single‐determinant approach, [ 30 ] it has been demonstrated that it can qualitatively describe situations in which SEC [ 24b‐g,31 ] and MR or polyradical characters are predominant, [ 28,32 ] providing reasonable results in comparison to MR methods. [ 24b‐g,33 ] Unquestionably, high‐level quantum chemical methods, such as quantum Monte Carlo (QMC) techniques combined with the resonating‐valence‐bond (RVB) theory, [ 34 ] would provide a better description of the electronic structure and correlation effects as compared to BS‐UDFT or wavefunction MR techniques, however such approaches are still too demanding to be applied for polymeric materials.…”

“…The high N FODs for the neutral state (Table S1, Figures S1 and S2, Supporting Information) already indicate a pronounced polyradical character of BBL. [ 28 ] Charged states, P±(S) and B±(D) show higher N FODs than the neutral species, with values for holes larger than electrons. The FOD analysis justifies the presence of BS solutions, [ 38 ] for BBL charged species (Figure 1), pointing out the role of SEC/MR effects in determining the electronic wavefunction of charged states.…”

Near‐Infrared Lasing in Four‐Zigzag Edged Nanographenes by 1D versus 2D Electronic π‐Conjugation

“…Quantitatively speaking, we define the (poly)radicaloid character of any compound by the index (y 0 , y 1 ) arising from the fractional occupation (y i ) of the LUMO and LUMO+1 orbitals, respectively. [ 33 ] A pure closed‐shell, diradical, or tetraradical system will thus exhibit ideal (y 0 , y 1 ) values of (0,0), (1,0), and (1,1), respectively, with more moderate values meaning diradicaloid or tetraradicaloid character. In this case, we obtain (0.19,0.02), (0.35,0.04), and (0.52,0.08) values for AA‐Ar, TT‐Ar, and PP‐Ar, respectively, which indicates a progressive increase of their radicaloid character with system size.…”