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♦ Size-selective Synthesis and Optical Properties of Carbon Nanorings

Cycloparaphenylene (CPP) is a simple conjugated carbon nanoring consisting solely of benzene rings with para linkage. Although CPP has attracted the interest of chemists for a long time, the chemical synthesis of CPP has become feasible only recently. Adding to its structural simplicity and beauty, CPP has a number of interesting properties such as unique cyclic conjugation, photophysical properties, and guest-encapsulating abilities.
Our research has led to (i) a general strategy for the synthesis of CPPs ([9]-[16]CPP) and related carbon nanorings using cyclohexane derivatives as a benzene-convertible L-shaped unit, (ii) a modular, size-selective, and scalable synthesis of [n]CPPs, (iii) the first X-ray crystal structure analysis of CPPs, (iv) the design and synthesis of acene-inserted CPPs as the shortest segment of chiral CNTs (v) the first synthesis of cyclo-1,4-naphthylene, a π-extended CPP, (vi) the first synthesis of carbon nanocage, and (vii) experimental and theoretical investigations on the optical properies of these novel molecules.
We studied the UV-vis absorption and fluorescence in solution/solid states of [n]CPP (n = 9, 12, 14, 15, and 16), and conducted theoretical studies to better understand the experimental results. The representative experimental findings are unusual blue-shift of both absorption and fluorescence peak with increasing the ring size. High fluorescence quantum yields of [n]CPPs in solution, which decrease in the crystalline state were also found. According to TD-DFT calculations, the interesting and counterintuitive optical properties of CPPs could be ascribed mainly to the ring-size effect in frontier molecular orbitals (in particular the increase of the HOMO-LUMO gap as the number of benzene rings increases). On the basis of comparative calculations using hypothetical model geometries, we conclude that the unique behavior of HOMO and LUMO of CPPs is due mainly to their lack of a conjugation length dependence in combination with a significant bending effect.
We believe that this work is a significant landmark to get insight into the electronic and photophisical properties of tube-shaped π-conjugation systems. Chemical total synthesis of CNTs from CPPs as well as application of CPPs for materials science will be our next campaign.