  The chemical reservoir within protoplanetary disks has a direct impact on
planetary compositions and the potential for life. A long-lived carbon-and
nitrogen-rich chemistry at cold temperatures (<=50K) is observed within cold
and evolved planet-forming disks. This is evidenced by bright emission from
small organic radicals in 1-10 Myr aged systems that would otherwise have
frozen out onto grains within 1 Myr. We explain how the chemistry of a
planet-forming disk evolves from a cosmic-ray/X-ray-dominated regime to an
ultraviolet-dominated chemical equilibrium. This, in turn, will bring about a
temporal transition in the chemical reservoir from which planets will accrete.
This photochemical dominated gas phase chemistry develops as dust evolves via
growth, settling and drift, and the small grain population is depleted from the
disk atmosphere. A higher gas-to-dust mass ratio allows for deeper penetration
of ultraviolet photons is coupled with a carbon-rich gas (C/O > 1) to form
carbon-bearing radicals and ions. This further results in gas phase formation
of organic molecules, which then would be accreted by any actively forming
planets present in the evolved disk.
