Carbazole Isomerism in Helical Radical Cations: Spin Delocalization and SOMO–HOMO Level Inversion in the Diradical State

Abstract

We report a new molecular design to afford persistent chiral organic open-shell systems with configurational stability and an inversion in energy of the singly occupied molecular orbital (SOMO) and the highest doubly occupied molecular orbital (HOMO) for both mono- and diradical states. The unpaired electron delocalization within the designed extended helical π-conjugated systems is a crucial factor to reach chemical stabilities, which is not obtained using the classical steric protection approach. The unique features of the obtained helical monoradicals allow an exploration of the chiral intramolecular electron transfer (IET) process in solvents of different polarity by means of optical and chiroptical spectroscopies, resulting in an unprecedented electronic circular dichroism (ECD) sign inversion for the radical transitions. We also characterized the corresponding helical diradicals, which show near-infrared electronic circular dichroism at wavelengths up to 1100 nm and an antiferromagnetic coupling between the spins, with an estimated singlet–triplet gap (ΔE ST) of about −1.2 kcal mol–1. The study also revealed an intriguing double SOMO–HOMO inversion (SHI) electronic configuration for these diradicals, providing new insight regarding the peculiar energetic ordering of radical orbitals and the impact on the corresponding (chiral) optoelectronic properties.

Synthesis of π-Conjugated Chiral Organoborane Macrocycles with Blue to Near-Infrared Emissions and the Diradical Character of Cations

“…This fully oxidized fragment highly resembled CMC1 ·+ SbF 6 – in the EPR spectrum with a slightly stronger magnetic response. The broad one-line EPR signal without any hyperfine coupling indicates the formation of diradical dication CMC1 2·2+ 2SbF 6 – with a presumably weak electronic coupling of the two radicals …”

Thermally Ultrarobust S = 1/2 Tetrazolinyl Radicals: Synthesis, Electronic Structure, Magnetism, and Nanoneedle Assemblies on Silicon Surface

“…(For comparison in 2 , the lowest vibrational frequency of 22 cm –1 is computed.) Both radicals follow the typical Aufbau rule, with electrons in their singly occupied molecular orbital (SOMOs) at higher energy, compared to electrons in the corresponding highest occupied molecular orbitals (HOMOs), in contrast to the SOMO/HOMO inversions found in selected nitrogen-centered radicals. − …”

Negative solvatochromism and sign inversion of circularly polarized luminescence in chiral exciplexes as a function of solvent polarity

“…In line with the unpolarized luminescence in dichloromethane, the two emitters show mirror-image CPL signals corresponding to those recorded for the enantiopure helicol (ref. 32 and 49) with identical negative g lum factors of –2.0 × 10 −3 for P - 3 and P - 4 . The CPL response of the investigated emitters differs significantly in methylcyclohexane, with a sign inversion for both enantiomers of P - 3 and P - 4 .…”