Photochemistry of framework-supported m(diimine)(CO)₃x complexes in three-dimensional lithium carboxylate metal-organic frameworks: Monitoring the efect of framework cations

The structures and photochemical behaviour of two new metal-organic frameworks (MOFs) are reported. Reaction of Re(2,2'-bipy-5,5'-dicarboxylic acid)(CO)₃ Cl or Mn(2,2'-bipy-5,5'-dicarboxylic acid)(CO)₃Br with LiCl or LiBr, respectively, produces single crystals of {Li₂(DMF)₂ [(2,2'-bipy-5,5'-dicarboxylate) Re(CO)₃Cl]}, (ReLi) or {Li₂(DMF)₂[(2,2/-bipy-5,5/-dicarboxylate)Mn(CO)₃Br]}_n (MnLi). The structures formed by the two MOFs comprise one-dimensional chains of carboxylate-bridged Li(I) cations that are cross-linked by units of Re(2,2'-bipy-5,5'-dicarboxylate)(CO)₃Cl (ReLi) or Mn(2,2'-bipy-5,5'-dicarboxylate)(CO)₃Br (MnLi). The photophysical and photochemical behaviour of both ReLi and MnLi are probed. The rhenium-containing MOF, ReLi, exhibits luminescence and the excited state behaviour, as established by time-resolved infrared measurements, is closer in behaviour to that of unsubstituted [Re(bipy)(CO)₃Cl] rather than a related MOF where the Li(I) cations are replaced by Mn(II) cations. These observations are further supported by density functional theory calculations. Upon excitation MnLi forms a dicarbonyl species which rapidly recombines with the dissociated CO, in a fashion consistent with the majority of the photoejected CO not escaping the MOF channels. This article is part of the themed issue 'Coordination polymers and metal-organic frameworks: materials by design'.