Tuning charge transfer in model bio-inspired porphyrincarotenedyads

We present a computational study on model bio-inspired donor-acceptor (DA) dyads formed by a carotenoid (C) covalently linked to a tetraphenylporphyrin (TPP) at the ortho position of one of the TPP phenyl rings [1]. The mutual orientation of the components and their distance closely resembles the geometry of the dyad chlorophyll-peridinin in PCP [2]. Dyadic systems are intensively studied for potential application in the construction of organic solar cells and development of efficient photocatalytic systems for the solar energy conversion for the unique advantages they offer with regard to synthetic feasibility. The recent progress in computational methodologies, especially the development of DFT rooted methods suitable to describe charge transfer (CT) processes, allow to perform systematic investigations in silico of those molecular features which might be important to design high performance bio-inspired artificial devices. Focussed on CT process, this study aims (i) at better understanding the effect of slight chemical modifications on the absorption spectra, in particular on the lowest CT bands, as well as (ii) at gaining deeper insight on the role of H2O and hystidine (Hys) in the biological system. The coordination of H2O or Hys might occur in two different positions: it can be sandwiched between the carotene and the porphyrin ring or can be coordinated to the metal under the porphyrin plane. The effect of different metals of biological interest is also investigated to rationalize the fine tuning of the CT process.