Selective resonance Raman enhancement of large amplitude inter-ring vibrations of [34](1,2,4,5)cyclophane radical cation; a model of π-stacked dimer radical ions

Abstract

The compound [3₄](1,2,4,5)cyclophane, which consists of π-stacked two benzene rings, is one of the simplest dimeric molecules with a transannular π–π interaction. Its radical cation showed a characteristic near-infrared electronic absorption due to a transition from “bonding” to “anti-bonding” orbitals between the two benzene moieties. The Raman spectrum was measured in resonance with this electronic transition, and selective enhancement of low-frequency vibrational bands was observed. The strongest band at 241 cm⁻¹ was assigned to one of the inter-ring vibrations that changed the distance between the two benzene rings. The Raman cross-section of this band was estimated to be 9.1 × 10⁻²⁵ cm⁻², which was comparable to typical resonance Raman cross-sections and 100 times stronger than those of the local ring-breathing vibrations (1230 and 1262 cm⁻¹) of the radical cation. To understand this resonance Raman enhancement, the polarized Raman spectra were measured. The depolarization ratios of the cation bands were almost 1/3, which indicates an A-term rigorous resonance Raman effect with a contribution of a single electronic excited state. To analyze this A-term resonance Raman effect, the optimized geometry was calculated for the electronic excited S₂ state of the radical cation responsible for its near-infrared electronic absorption. The bond lengths of the excited S₂ state were almost the same as those of the ground state, while the distance between the two benzene rings extended by 7%. Large Franck–Condon factors were, thus, expected for the inter-ring vibrations, and resulted in the selective resonance Raman enhancement of these large amplitude vibrations.