Photoemission study on poly(pyridine-2,5-diyl), poly(2,2′-bipyridine-5,5′-diyl), and their K-doped states

Ultraviolet photoelectron spectra were measured using synchrotron radiation for two kinds of π-conjugated polymers, poly(pyridine-2,5-diyl) (PPy) and poly(2,2′-bipyridine-5,5′-diyl) (PBPy) which exhibit n-type electrically conducting properties. The two compounds show similar spectra, and they were analyzed with MO calculations. Upon potassium doping of PBPy, two new states appeared in the originally empty energy gap. This finding indicates that bipolaron bands are formed in K-doped PBPy. While K-doped PPy also shows similar gap states, it requires higher dopant concentration to create bipolaron bands than in the case of K-doped PBPy. The difference of the dependence on dopant concentration between K-doped PPy and K-doped PBPy is discussed based on the conformational difference between these polymers.