Near-Infrared Optical Properties and Charge Transport in Expanded Porphyrins
Authors: Cardenas-Jiron Gloria , Borges-Martinez Merlys , Montenegro-Pohlhammer Nicolas , Yamamoto Yoh , Baruah Tunna , Alvarez Daniel , Menendez Maria Isabel , Lopez Ramon 
Published: July 2021

Expanded porphyrins have been widely studied due to their potential application to optoelectronic devices and sensors. Several modifications in their molecular structures can be made in this type of molecules that lead to the modulation of their properties related to the electronic structure.
In the search to improve the properties of these systems to be used in solar cells or electronic devices, we have investigated various strategies at the molecular and electronic structure, performing quantum mechanical simulations of the optical, magnetic, and charge transport properties.
We investigated the effect of pyrrolic ring expansion in porphyrins fused with sulfur atoms; pentaphyrin, hexaphyrin, heptaphyrin, and octaphyrin (Figure 1), on the photophysical properties. The electronic absorption spectra were calculated with the time-dependent density functional theory (TD-DFT) approach in the solution phase. The expansion of pyrrolic rings produces an important bathochromic shift of the Q bands, where the absorption of the longest wavelength appears in the range of 745-970 nm. The effect of the extension of the π conjugation and the degree of distortion of the planarity of the macrocycle were also studied.
Another strategy used was the formation of dyads composed of dyes, where one of them is a porphyrin or expanded porphyrin (smaragdyrin). We investigated the effect of substituents in the ß position of the porphyrin bound to a squaraine [1], as well as the effect of the bridging structure that connects smaragdyrin with a BODIPY derivative [2] (Figure 2). TD-DFT calculations for the dyads indicate that those with amines as substituents show a red-shift of the Q bands. Charge transfer bands calculated with TD-DFT and the perturbative approximation ΔSCF show a push effect on porphyrin and a pull effect on squaraine. Current-voltage calculations obtained for a molecular junction of the dyad connected to Au (111) nanowires to simulate the electrodes indicate that the charge transfer follows the same trend, in the sense from porphyrin to squaraine and is more favorable with the amine groups.
Bimetallic hexaphyrins (Zn(II), Cu(II)) of the amethyrin type were also investigated, and it was found that the singlet Cu(II) systems show a significant shift towards the near-infrared (900 nm). Conductance and current-voltage profiles were calculated for molecular junctions with Au (111) electrodes. It was found that the effect of the anchoring group on the electrode is more relevant than the metal effect on the conductance. The important difference in the current of the triplet and singlet configuration of Cu(II) for a voltage range of 0.1-0.5 V suggests that this system could be used as a molecular switch.
In summary, different strategies will be shown to modulate the optical properties for obtaining photosensitizers that can be used in dye-sensitized solar cells. Furthermore, the properties of charge transport are evaluated in the ground state to evaluate the ability of the molecular systems to transfer electrons to the electrodes.

References

[1] Merlys Borges-Martinez, Nicolas Montenegro-Pohlhammer, Yoh Yamamoto, Tunna Baruah, Gloria Cárdenas-Jiron, J. Phys. Chem. C , 124 (2020), 12968-12981.
[2] Merlys Borges-Martínez, Daniel Alvarez, Nicolas Montenegro-Pohlhammer, María Isabel Menéndez, Ramón López, Gloria Cárdenas-Jirón, J. Phys. Chem. C , 123 (2019) 19362-19375.

Acknowledgments. We are grateful for the financial support of ANID/CHILE under FONDECYT Project 1171719 (G.C.-J.). Powered@NLHPC. This research was partially supported by the supercomputing infrastructure of the NLHPC (ECM-02) of the Universidad de Chile.

Figure 1