Polymeric
carbon nitrides (pCN) have garnered immense attention, ranging from super-hard
materials to artificial photosynthesis, due to their exceptional chemical and optoelectronic
properties. The most studied C<sub>3</sub>N<sub>4</sub> along with other
stoichiometric pCN, such as C<sub>3</sub>N, C<sub>2</sub>N and C<sub>3</sub>N<sub>5</sub>,
commonly employed a six-membered ring as the basic units; while the
five-membered rings are also popular in a myriad of natural and artificial
molecules with a more polarized framework and intriguing functionalities. Here,
we report a facile synthesis of C<sub>3</sub>N<sub>2</sub> with a topological
structure of five-membered rings, endowing by far the narrowest the first
electronic transition energy (0.81 eV) in pCN family. The basic imidazole unit with
dangling bonds, resulting in an unusual electronic band of p-π conjugation and split
molecular orbitals, was
revealed in C<sub>3</sub>N<sub>2</sub> by both experiments and density
functional theory calculations. Moreover, a NIR-responsive photoelectrochemical
(PEC) biosensor for non-transparent biosamples was constructed for the first
time using C<sub>3</sub>N<sub>2</sub> with outstanding performance. This work would
not only open a new vista of pCN with different topological structures but also
broaden the horizon of their application, such as prospective <i>in vivo</i> PEC
bioassay.