Synthesis and physical properties of [N]Cycloparaphenylene Ketone (N = 6, 7, 8, and 10)

[N]cycloparaphenylene ketone 4, [N]CPP-CO, in which a carbonyl group is inserted into [N]CPP framework, with N = 6, 7, 8, and 10, was synthesized by a coupling reaction between a CPP precursor and a ketone unit, followed by reductive aromatization. Single crystal X-ray diffraction analysis, infrared (IR) measurements, and theoretical calculations revealed the strained nature of the carbonyl group in 4. For example, the C=O stretching vibration bands of 4 observed by IR spectroscopy appeared at wavenumbers 20–50 cm-1 higher than that of benzophenone, with larger shifts observed for smaller ring sizes. Moreover, a Möbius topology of 4a with N = 6 in the solid state was clarified. The CPP units in 4 significantly alter the electronic properties, particularly the relative orbital energies. Due to the elevation of the occupied pi-orbitals of CPP units, the highest occupied molecular orbital (HOMO) in 4 is the conjugated -system, in contrast to benzophenone, where the HOMO is the non-bonding n-orbital. Consequently, the pi-pi* transition becomes the main excited-state transition, and intersystem crossing from the S1 state to the T1 state, which is a characteristic process for typical aromatic ketones, does not occur efficiently. As a result, 4 exhibits unique photophysical properties opposite to those of typical aromatic ketones, such as fluorescing but not phosphorescing. Furthermore, the S1 state of 4 was quenched by molecular oxygen, most likely by the energy exchange mechanism. As for the electrochemical properties, 4 was found to be susceptible to oxidation despite the incorporation of an electron-withdrawing carbonyl group.