Single cycloparaphenylene molecule devices: Achieving large conductance modulation via tuning radial π-conjugation

Conjugated macrocycles cycloparaphenylenes (CPPs) have unusual size-dependent electronic properties because of their unique radially π-conjugated structures. Contrary to linearly π-conjugated molecules, their highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap shrinks as the molecular size reduces, and this feature can, in principle, be leveraged to achieve unexpected size-dependent transport properties. Here, we examine charge transport characteristics of [n]CPPs (n = 5 to 12) at the single molecule level using the scanning tunneling microscope–break junction technique. We find that the [n]CPPs have a much higher conductance than their linear oligoparaphenylene counterparts at small ring size and at the same time show a large tunneling attenuation coefficient comparable to saturated alkane series. These results show that the radially π-conjugated molecular systems can offer much larger conductance modulation range than standard linear molecules and can be a new platform for building molecular devices with highly tunable transport behaviors.