Stereochemistry, Stereodynamics, and Redox and Complexation Behaviors of 2,2′‐Diaryl‐1,1′‐Biazulenes

Abstract

2,2′‐Diaryl‐1,1′‐biazulenes were synthesized and electronic communication between the azulene subunits was suggested based on redox measurements. The linkage of azulene at the 1‐position also appeared to increase the HOMO levels. In addition, cyclic voltammetry measurements of 2‐arylazulenes showed a return peak associated with the oxidation, which was not observed for azulene. The stabilization of the single‐electron oxidant may be due to the SOMO‐HOMO energy inversion phenomenon. X‐ray crystallography of the azulene dimers revealed that this species possessed a syn‐type structure in which both aryl groups in the 2‐positions formed π‐stacks. The twisted structure was indicated to be in the (R)‐ or (S)‐configuration for all molecules in the unit cell. Spontaneous resolution was also shown. Furthermore, from the solid circular dichroism (CD) spectral measurements, the relationship between the absolute configuration of the molecules and the CD spectra was determined. A racemization rotational barrier of ca. 27 kcal mol−1 was calculated. Moreover, the pyridylazulene dimer cyclized upon reaction with PdCl2 to form a 3 : 3 complex, in which the biazulene units cyclized to give ratios between the (R)‐ and (S)‐forms of either 2 : 1 or 1 : 2.

Synthesis and Properties of Carbo- and Heterocyclic Benz[a]azulenes

“…In addition, a weak short-axis polarized L b band (ε = 308 M –1 cm –1 at 624 nm, 4aa ) was observed, which is derived from a S1 ← S0 transition. The latter displayed a clear vibrational fine structure and appeared in the range of 540 to 800 nm, similar to parent azulene and other azulene-fused π-conjugated systems. − Moreover, due to their long-wavelength S1 ← S0 transition in the visible region, the compounds display blue or green colors, although the molar absorption coefficients ε are low.…”

“…Azulene exhibits a non-mirror-symmetric frontier orbital geometry with a LUMO largely localized on the even positions and a HOMO with maximum density on the odd positions. This reduces the repulsive interaction between the two unpaired electrons and, thus, results in a small HOMO–LUMO energy gap. − Azulene is known for its anomalous emission from S2, whereas the emission from S1 is negligible (an apparent violation of Kasha’s rule). This unique character of azulene can be explained by a low-lying S1 which endows a relatively large energy gap between S1 and S2, decreasing the rate of the S2 → S1 emission, and thus, the S2 → S0 fluorescence becomes dominant.…”

Singly Occupied Molecular Orbital−Highest Occupied Molecular Orbital (SOMO−HOMO) Conversion

“…13). [32] Another example was reported by Casado, Nakamura, and their co-workers in 2021. [33] They observed that the radical trianion [S1] 3and the radical cation [As1] þ exhibited SHC.…”

Organic radicals with inversion of SOMO and HOMO energies and potential applications in optoelectronics

“…SHI in a dimer molecular design was also theoretically investigated in cationic azulene radical by Mazaki et al in 2019, 141 and more recently by Casado, Nakamura et al in 2021. 23 Interestingly, in the latter report, the authors explored spiro- conjugated systems with the ability to adopt SHI in both anionic and cationic radical forms (Fig.…”

“…Chemical structures of thiophene-based double helix radicals 16 and 17 •+ (TMS = trimethylsilyl). The molecular fragment in red highlights the predominant localization of the SOMO within the molecular structure.SHI in a dimer molecular design was also theoretically investigated in cationic azulene radical by Mazaki et al in 2019,141 and more recently by Casado, Nakamura et al in 2021 23. Interestingly, in the latter report, the authors explored…”