Record Dissymmetry in Vibrational Optical Activity: Combining Vibronic Coupling and Aggregation in Cobalt(II)
Gianmarco Papi1, Massimo Guelfi1, Stefano Zacchini2
1Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Pisa, Italy.
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A family of cobalt(II) bis-trifluoroacetylcamphorate complexes (1-3), along with the diamagnetic zinc(II) analog 4, has been synthesized and structurally characterized. The cobalt complexes exhibit solvent-driven interconversion between cis- and trans-octahedral geometries, coupled to labile solvent coordination. Their chiroptical response spans the UV, visible, NIR, and IR ranges and is highly sensitive to coordination geometry and solvent. In dilute solutions, vibrational circular dichroism (VCD) dissymmetry factors up to |g| ≈4 × 10- 3 reflect vibronic coupling with low-lying electronic states, a hallmark of high-spin CoII centers. At concentrations above 30 mM in acetonitrile, spontaneous microcrystallization yields suspensions with an exceptional g value of approximately 0.2 at 1600 cm- 1, that is the largest dissymmetry factor ever reported by VCD. This record value arises from the unprecedented concurrence of vibronic and aggregation-induced enhancement in the same species, with aggregation providing the dominant contribution, as supported by crystallographic, spectroscopic, and computational evidence.
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