Electronic structure of n-cycloparaphenylenes directly observed by photoemission spectroscopy

A series of n-cycloparaphenylenes ([n]CPP, n = 8, 9, and 12) were studied by ultraviolet photoemission, inverse photoemission, ultraviolet-visible absorption, and X-ray photoemission spectroscopy to detect their unique electronic structures. [n]CPP has a cyclic structure in which both ends of n-poly(p-phenylene)s (nP) are connected. The molecular size dependence of the HOMO-LUMO gap of [n]CPP was investigated by direct observation and was found to increase as the molecular size increased. This trend is opposite to that of typical π-conjugated systems. Highly strained molecular structures, especially of small [n]CPPs, significantly impact their electronic structure. Insights into the electronic structure of [n]CPP obtained here will aid the design of electronic functionality of non-planar π-conjugation systems.