Synthesis, Structure, and Photophysical Properties of m‑Phenylene-Embedded Cycloparaphenylene Nanorings

Five m-phenylene-embedded cycloparaphenylenes m3[9]­CPP 1–5 were synthesized by the platinum-mediated cyclooligomerization strategy with high overall yields. The structures of m3[9]­CPP 1–3 were determined by X-ray diffraction analysis. Compared to [9]­CPP, m3[9]­CPP 1 caused a significant blueshift in the UV–vis absorption and fluorescence spectra. This result shows that the radial π-conjugation is distorted and partially interrupted. The photophysical properties of m3[9]­CPP 1 were further tuned by the introduction of various substituents for m3[9]­CPP 2–5. Methoxy group substitution at m-phenylene did not change the photophysical properties significantly. Replacement of m-phenylene by tetrafluoro-m-phenylene achieved a significant blueshift. When the carboxyl group was embedded at m-phenylene or the methoxy group was embedded at p-phenylene, significant redshifts were observed with blue color emission. Theoretical calculations revealed that the decrease in the HOMO–LUMO gap in m3[9]­CPP 4 and 5 is favorable for the redshift of the fluorescence spectrum.