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 and density functional theory (DFT) calculations confirmed their Möbius topology and significant molecular strain as characterized by the deviations in bond angles at the carbonyl carbon. The strain was also evidenced by size-dependent 20-50 cm^-1 shifts in the carbonyl IR stretching bands. The CPP units in 4 significantly alter the electronic properties, particularly the relative orbital energies. Due to the elevation of the energy level of the occupied π-orbitals by CPP units, the highest occupied molecular orbital (HOMO) in 4 is the conjugated π-system, in contrast to benzophenone, where the HOMO is the non-bonding n-orbital. Consequently, the lowest excited state of 4 is the singlet state (S₁) with π-π* character rather than the conventional n-π* triplet (T₁) state. Furthermore, 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. Optical and electrochemical properties showed a clear dependence on ring size, with smaller rings exhibiting greater strain, lower oxidation potentials, and red-shifted absorption and emission. These findings highlight the unique interplay between molecular topology, strain, and photophysics in CPP-based ketones and provide a foundation for designing novel π-conjugated materials with tailored excited-state properties.