Tuning the Visible/Near-Infrared Optical Properties and Charge Transport of Porphyrins and Phthalocyanines

Porphyrins (Por) and phthalocyanines (Pc) have been widely studied because of their photophysical and electrochemical properties. Por and Pc sensitizers, as large π-conjugated systems, have been used in dye-sensitized solar cells (DSSCs) due to their high extinction coefficients, appropriate redox properties for the sensitization of TiO₂ films, and robustness. 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 structures, performing quantum chemistry calculations of the optical and charge transport properties. The former is obtained with the time-dependent density functional theory (TD-DFT) approach and the latter using the Keldysh nonequilibrium Green′s function (NEGF) formalism.

We investigated the effect of thiophene ring expansion in expanded porphyrins fused, pentaphyrin, hexaphyrin, heptaphyrin, and octaphyrin, on the photophysical properties.  The ring expansion produces a significant bathochromic shift of Q bands (745-970 nm). While there is no trend of increasing in current with the π-electron number, the molecular junction for octaphyrin connected to Au (111) nanowires achieves the most significant current of about 1 μA when 1 V is applied. 

Another strategy is the formation of dyads composed of dyes. We studied the effect of the bridging structure that connects smaragdyrin (S) with a BODIPY derivative. It is shown that acetylene bridges favor a red-shift absorption (689 nm) of Q bands in S while acene moieties favor the absorption at the longest wavelength of BODIPY bands (707 nm). The binding of two dyes allows a panchromatic absorption. The current-voltage profiles for the molecular junction system achieve up to 1 pA for 0.6 V.  

Asymmetrical zinc phthalocyanines were also designed to be used as efficient sensitizers for DSSCs, and studied the effect of the donor groups in the β-positions. Both contain one carboxylic acid as anchoring group and bearing bulky tert-butylsulfanyl peripheral substituents (GT6) to prevent the aggregation phenomenon or hexylsulfanyl groups as the electron-donating group (GT4).  It is shown that the hexyl chains favor a red-shift absorption up to 695 nm for Q bands. The lower absorption of GT6 is affected by the steric hindrance induced by the non-planar tert-butyl moieties.

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.