The application of organic small molecules as metal-free
photocatalysts
for light-driven photoreduction of carbon dioxide (CO2)
has seldom been explored. This work developed four naphthalene diimide
(NDI)-derived donor–acceptor–donor small molecules with
different numbers of thiophene units, namely, NDI-2T, NDI-TT, NDI-4T,
and NDI-6T, as metal-free photocatalysts to catalyze the reduction
of CO2 under irradiation with an air mass 1.5G solar simulator
at one-sun intensity. The structure–property relationship was
investigated by exploring the effects of the electron-donating ability
of the donor units on the optical properties, redox potential, electron–hole
distribution, and exciton lifetime. NDI-6T exhibited the most red-shifted
absorption, longest exciton lifetime, and strongest electron–hole
separation. However, the large upshift in oxidation potential because
of the elevated electron-donating ability of the hexathiophene unit
significantly reduced the driving force for catalyst regeneration,
leading to poor catalytic performance. Alternatively, NDI-4T possessed
proper redox potentials, reduced charge-transfer resistance, and excellent
photocurrent intensity; therefore, it effectively converted CO2 to a single product of CO in the presence of water as an
electron donor without a sacrificial reagent or cocatalyst with a
product yield of 168.6 μmol gcat
–1 24 h–1, which was considerably higher than those
of NDI-TT (111.9 μmol gcat
–1 24
h–1), NDI-2T (88.4 μmol gcat
–1 24 h–1), and NDI-6T (40.5 μmol
gcat
–1 24 h–1). This
study provides a practical guideline for the molecular design of conjugated
organic molecules as promising photocatalysts for CO2 photoreduction.