High-Spin Diradical Dication of Chiral π-Conjugated Double Helical Molecule

Abstract

We report an air-stable diradical dication of chiral D₂-symmetric conjoined bis[5]diazahelicene with an unprecedented high-spin (triplet) ground state, singlet triplet energy gap, ΔE_ST = 0.3 kcal mol^-1. The diradical dication possesses closed-shell (Kekulé) resonance forms with 16 π-electron perimeters. The diradical dication is monomeric in dibutyl phthalate (DBP) matrix at low temperatures, and it has a half-life of more than 2 weeks at ambient conditions in the presence of excess oxidant. A barrier of ∼35 kcal mol^-1 has been experimentally determined for inversion of configuration in the neutral conjoined bis[5]diazahelicene, while the inversion barriers in its radical cation and diradical dication were predicted by the DFT computations to be within a few kcal mol^-1 of that in the neutral species. Chiral HPLC resolution provides the chiral D₂-symmetric conjoined bis[5]diazahelicene, enriched in (P,P)- or (M,M)-enantiomers. The enantiomerically enriched triplet diradical dication is configurationally stable for 48 h at room temperature, thus providing the lower limit for inversion barrier of configuration of 27 kcal mol^-1. The enantiomers of conjoined bis[5]diazahelicene and its diradical dication show strong chirooptical properties that are comparable to [6]helicene or carbon-sulfur [7]helicene, as determined by the anisotropy factors, |g| = |Δε|/ε = 0.007 at 348 nm (neutral) and |g| = 0.005 at 385 nm (diradical dication). DFT computations of the radical cation suggest that SOMO and HOMO energy levels are near-degenerate.

Figure 1.

S = ½ open-shell helical molecules.

Figure 2.

A) Chiral conjoined bis[5]diazahelicene 1-D₂, its corresponding radical cation and diradical dication. B) Dihydrazine 1a and selected resonance forms for diradical dication 1a^2•2+, in which one of the four Kekulé resonance forms with 16 π-electron perimeters is shown.

Figure 3.

Cyclic voltammogram of ~0.7 mM 1-D₂ in 0.1 M [n-Bu₄N]⁺[PF₆]⁻ in DCM at a scan rate of 100 mV s⁻¹. Further details are reported in Figs S1–S3, SI.

Figure 4.

In situ EPR (ν = 9.6356 GHz) and UV-vis-NIR spectra for 0.58 mM radical cation 1^•+ BF₄⁻ in DCM at ambient temperature, obtained during oxidation of 1-D₂ with [Ag]⁺[BF₄]⁻: EPR spectral simulations correspond to the following parameters: A(¹⁴N) = 10.0 MHz, g = 2.0030, and line-width = 0.28 mT. Bands at λ_max = 322, 538, 577, 743, and 882 nm have the following extinction coefficients: ε₃₂₂ = 1.16 × 10⁴, ε₅₃₈ = 7.09 × 10³, ε₅₇₇ = 6.83 × 10³, ε₇₄₃ = 3.25 × 10³, and ε₈₈₂ = 3.27 × 10³ L mol⁻¹ cm⁻¹.

Figure 5.

In situ EPR (117 K, ν = 9.4387 GHz) and UV-vis-NIR (294 K) spectra for 0.82 mM diradical dication 1^2•2+ 2SbF₆⁻, obtained during oxidation of 1-D₂ with [NO]⁺[SbF₆]⁻ in dibutyl phthalate (DBP). EPR spin counting at T = 117 K showed χT ≈ 0.92 emu mol⁻¹ K. EPR spectrum of 1^2•2+ 2SbF₆⁻ in DBP at 117 K (inset: the |Δm_s| = 2 transition) with spectral simulation of the |Δm_s| = 1 region: |D/hc| = 1.11 × 10⁻² cm⁻¹, |E/hc| = 1.56 × 10⁻³ cm⁻¹, |A_zz/hc|/2 = 7.3 × 10⁻⁴ cm⁻¹, g_xx = 2.0036, g_yy = 2.0035, g_zz = 2.0025. UV-vis-NIR absorption spectrum of 1^2•2+ 2SbF₆⁻ in DBP, bands at λ_max = 332, 476, 652, and 929 nm have the following extinction coefficients: ε₃₃₂ = 2.39 × 10⁴, ε₄₇₆ = 1.14 × 10⁴, ε₆₅₂ = 1.14 × 10⁴, and ε₉₂₉ = 8.17 × 10³ L mol⁻¹ cm⁻¹. Complete sets of EPR simulation parameters may be found in Figs. S14, S21, S24, and S26, SI.

Figure 6.

Quantitative EPR spectroscopy of 0.82 mM diradical dication 1^2•2+ 2SbF₆⁻ in DBP: experimental values of χT (mean ± SE, n = 3), the product of paramagnetic susceptibility (χ) and T in the T = 105 – 330 K range and numerical one-parameter fit with the variable parameter, 2J/k = 145 ± 4 K (mean ± SE). Further details are reported in the SI, Fig. S12, S16, and S26.

Fig. 7.

Orbital maps for the D₂-symmetric structure of model radical cation 1a^•+-D₂ in the gas phase at the UB3LYP/6–31G(d,p) level. Positive (red) and negative (green) contributions are shown at the isodensity level of 0.02 electron/Bohr³. The singly-occupied α orbital is matched in a nodal pattern (b₂-symmetry) to the lowest unoccupied β orbital to provide the SOMO energy level; doubly occupied molecular orbitals are identified by matching nodal pattern (a-symmetry for HOMO) and energies (in Hartrees) of the α and β occupied orbitals.

Figure 8.

Potential energy surface for racemization of triplet state of diradical dication 1^2•2+-D₂ at the UB3LYP/6–31G(d)+ZPVE level of theory. Values in parentheses are for simplified 1b^2•2+ at the ROMP2/6–31G(d)//UB3LYP/6–31G(d) level of theory

Figure 9.

Orbital maps for the D₂-symmetric structure of radical cation 1^•+-D₂ at the UB3LYP/6–31G(d) level. Positive (red) and negative (green) contributions are shown at the isodensity level of 0.02 electron/Bohr³. The singly-occupied α orbital is matched in a nodal pattern (b₃-symmetry) to the lowest unoccupied β orbital to provide the SOMO energy level; doubly occupied molecular orbitals are identified by matching nodal pattern (a-symmetry for HOMO and b₁-symmetry for SHOMO) and energies (in Hartrees) of the α and β occupied orbitals.

Figure 10.

Spin density maps for triplet ground states of diradical dication and aza-m-xylylene diradical at the UB3LYP/6–31G(d)//UB3LYP/6–31G(d)+ZPVE level. Positive (blue) and negative (green) spin densities are shown at the isodensity level of 0.006 electron/Bohr³.

Figure 11.

ECD spectra at ambient temperature. Top: 1-D₂ and TD-DFT computed spectrum for P,P-1-D₂ (B3LYP/6–31G(d)-optimized geometry) at the CAM-B3LYP/6–31G(d) level in IEF-PCM-UFF-modelled DCM. Bottom: M,M-1^2•2+ 2SbF₆⁻; three overlapped spectra at time 0, 24, and 48 h. Experimental ECD spectra intensities are scaled by the ee. Further details are reported in the SI, Table S4, Figs. S29–S36.

Scheme 1.

Synthesis of 1-D₂.

Scheme 2.