Porous porphyrin frameworks for sustainable energy and environmental applications

Abstract

Increase in consumption of fossil fuels resulted in global warming and lack of energy sources. To overcome this energy crisis, development of renewable energy is needed which include collection of $CO_2$ and convert it by solar energy. Natural system have solution for this and porphyrins are involved in both selective gas separation (heme) and light harvesting (chlorophyll). To utilize in industrial applications, stable porphyrin structure was synthesized and examined their ability for light harvesting and gas separation. The light-harvesting complex (LHC) in natural systems uses photon-harvesting antenna pigments composed of porphyrin units in a well-ordered J-aggregation, leading to the coherent migration of excitons to the photosynthetic reaction centre via excitonic coupling among porphyrin units. To mimic this strategy of nature, a synthetic light-harvesting unit should be stacked in an ordered J-aggregate giving the lower excitonic state than that of its single unit, thus permitting excitons to funnel through the ordered aggregate. Herein, we demonstrate that two-dimensional porphyrin porous organic network (PON) can be exist in a single sheet and aggregated in ordered J-stacking of the porphyrin units that enable long-range exciton and charge carrier transport. Combination of experiments and molecular dynamics (MD) simulations demonstrate that this J-stacking is composed mainly of two J/J and J/H configurations. The photophysical studies further proves that J-stacking gives red shifted broadband light absorption via strong excitonic coupling between each porphyrin sheets. Also, $H_2 P$ PON enables long-range exciton migration to the reaction centre with an approximately 20-fold enhancement in collection of photons, in a similar manner to the LHC of a natural system, where the J/J and J/H configurations act as an “exciton spreader” and “exciton speeder”, with (6.99 ps)$^{-1}$ and (1.08 ps)$^{-1}$ transfer rate respectively which is similar value of natural LHC. Furthermore, $H_2 P$ PON showed highest exciton diffusion length among porphyrin based materials, and 10 times higher photoconductivity in porous organic polymer materials with ultrastability. Furthermore, synthesized porphyrin array contain micro-pores with high surface area, so that can be used for gas storage and separation. Because heme in red blood cell selectively capture oxygen molecules by center iron atom, selectivity in gas separation can be increase with central atom change. Central atoms are varied with cobalt, nickel and copper which are common divalent transition metal in same row in periodic table. Metallo-porphyrin PONs shows similar structure and surface area except central metal species. $CO_2 /N_2$ and $CO_2 /Ar$ selectivity was calculated from initial slopes of adsorption plots of each gases. Both selectivity increased in series, cobalt, nickel and cooper. Because of difference in effective nuclear charge in three metals may increase the affinity to polar CO2. Therefore, selectivity can be controlled in porphyrin PONs by simple metal change. It have been synthesized new water coordiated metal-organic frameowrks (MOFs) with unsaturated open metal sites which playing as active adsorption sites for $CO_2$, which were demonstrated using the tetra(4-carboxypheny)porphyrin as a linker and the Europium(III) nitrate pentahydrate for a node metal in a secondary building unit and subsequently the coordinated water molecules in the MOF have been removed to give unsaturated coordination on its metal sites via the evacuation process. MOF without water doesn’t possess any open metal sites while MOF which synthesized with water contain open metal sites after removing coordinated water in vacuum. 30% of more $CO_2$ can be captured in MOF with open metal sites than without one due to its higher affinity to $CO_2$ within same surface area.