Tunable Porosity and Conjugation in Ferrocene Polymers of Intrinsic Microporosity

Ferrocene (Fc) metallopolymers of intrinsic microporosity (MPIMs) have recently been reported as soluble, porous, non-network polymers, with evidence of electron delocalization along the polymer backbone. The combination of these properties makes Fc-MPIMs ideal candidate materials for optoelectronic devices, and the ability to tune these properties would broaden the impact of these materials. In this work, density functional theory (DFT) calculations at the CAM-B3LYP/def2SVP level were carried out on Fc MPIM fragments to examine the effect of pendant functional groups on conformational stability and electron delocalization in these systems. The conformational stability of the Fc MPIMs can affect the porosity, and the electronic delocalization is related to the conjugation in the material. The Fc MPIM fragments are most stable when the dihedral angle between Fc cyclopentadienyl (Cp) rings is 11.5°. Pendant functional groups are found to affect the stability of the local minimum at 144°, with alkyl chains increasing the stability, and bulky tert-butyl and trifluoromethyl groups decreasing stability. It is also possible to tune the electron delocalization of the HOMO and LUMO across the molecule. The Fe center of the Fc moiety contributes to the frontier orbitals, which is expected to enhance electronic communication in the parent polymer. Time-dependent density functional theory calculations indicate the π→π^* transition is slightly affected by the orientation of the dihedral angle between Cp rings, but primarily depends on the electronic nature of the pendant group. This work shows that the conformational stability and orbital delocalization of a model Fc MPIM can be tuned by functionalization with different pendant groups.