  The generation of single-photon emitters in hexagonal boron nitride around 2
eV emission is experimentally well-recognized; however the atomic nature of
these emitters is unknown. In this paper, we use first-principles calculations
to demonstrate that $C_2C_N$ is a possible source of 2 eV single-photon
emitter. We showcase the calculations of a complete set of static and dynamical
properties related to defects, including exciton-defect couplings and
electron-phonon interactions. In particular, we show it is critical to consider
nonradiative processes when comparing with experimental lifetime for known 2 eV
emitters. We find that $C_2C_N$ has several key physical properties matching
the ones of experimentally observed single-photon emitters. These include the
zero-phonon line (2.13eV), Huang-Rhys factor (1.35), photoluminescence lifetime
(2.19 ns), phonon-sideband energy (180meV), and photoluminescence spectrum. The
identification of defect candidates for 2 eV emission paves the way for
controllable single-photon emission generation.
