  Metal organic frameworks (MOFs) are promising photocatalytic materials due to
their high surface area and tuneability of their electronic structure. We
discuss here how to engineer the band structures and optical properties of a
family of two-dimensional (2D) porphyrin-based MOFs, consisting of M tetrakis(4
carboxyphenyl)porphyrin structures (M TCPP, where M = Zn2+ or Co2+) and metal
(Co2+, Ni2+, Cu2+ or Zn2+) paddlewheel clusters, with the aim of optimising
their photocatalytic behaviour in solar fuel synthesis reactions (water
splitting and/or CO2 reduction). Based on density functional theory (DFT) and
time-dependent DFT simulations with a hybrid functional, we studied three types
of composition/structural modifications: a) varying the metal centre at the
paddlewheel or at the porphyrin centre to modify the band alignment; b)
partially reducing the porphyrin unit to chlorin, which leads to stronger
absorption of visible light; and c) substituting the benzene bridging between
the porphyrin and paddlewheel, by ethyne or butadiyne bridges, with the aim of
modifying the linker to metal charge transfer behaviour. Our work offers new
insights on how to improve the photocatalytic behaviour of porphyrin- and
paddlewheel-based MOFs.
