Stack Exchange Strategies for the Synthesis of Covalent Double-Channel Photosystems by Self-Organizing Surface-Initiated Polymerization

Title: Stack Exchange Strategies for the Synthesis of Covalent Double-Channel Photosystems by Self-Organizing Surface-Initiated Polymerization

Authors:  N. Sakai and S. Matile

Journal: Journal of the American Chemical Society

Affiliation: University of Geneva, Geneva, Switzerland

Take Home Importance: Well-ordered polymeric ‘columns’ of different chromophores in the same polymer were prepared and demonstrated to have higher photocurrents when both electron and hole transport chromophores were present.

Abstract graphic
Figure 1. Molecular details of the polymer.

Summary: Unlike traditional silicon photovoltaics, biological photosystems utilize a precise arrangement of donor and acceptor chromophores to funnel absorbed energy to ‘chemical factories’ where that energy drives the synthesis of chemical fuels. Naturally, we would like to imitate this strategy in artificial light harvesting systems. Synthetic photovoltaics are commonly constructed of a sandwich-type approach where the photoactive material is placed between a conductive electrode (frequently a metal) and a transparent conductive glass substrate which acts a second electrode. In this way light-generated exictons (electron-hole pairs) may be separated, with the electron going to one electrode and the hole migrating to the other. Hooking up wires to connect the transparent glass substrate electrode and conductive metal counter electrode results in a complete circuit and electricity can be captured! However, there exists a real problem in these types of systems: how does one ensure that captured energy doesn’t, say, go right or left instead of up or down to the electrons as shown in cartoon form in Figure 2?

Figure 2. Two potential pathways for the light-induced exciton (e-, h+) pair to take in this simplified solar cell schematic: pathway a) where the exciton successfully diffuses to the correct electrodes and current is produced; and pathway b) the exciton diffuses in random directions and the absorbed energy is lost.

The authors here have a unique solution to this problem. Using a three-step approach detailed in Figure 3 below adjacent π-stacked columns of two different chromophores were arranged parallel to a substrate surface. In the first step, a technique termed “self-organized surface-initiated polymerization” (SOSIP) permitted the growth of a polymer containing the first column of chromophores, here a naphthalene diimide (NDI) derivative, parallel to a transparent conductive substrate (indium tin oxide or ITO on glass in this case). The columns was anchored to the ITO substrate using diphosphonate linkages (filled black boxes in Figure 3).  Why do the columns align in the way they do? Each column is connected to adjacent columns, and so this polymer can be imagined as a series of roughly parallel sheets, each sheet containing chromophores (the yellow boxes in Figure 3) and hydrazone moieties (the gray spheres in Figure 3). The substrate-loving disphosphonate linkages at the bases of the columns then enforce the desired alignment with respect to the substrate. At this state there are adjacent columns of chromophores and hydrazones. In the second step  the hydrozones are removed by addition of hydroxlamine, leaving behind primary amine groups. In the third step condensation with the aldehyde functional group of a second naphthalene derivative (red boxes) resulted in attachment of the new naphthalene and release of water.

Figure 3. Flow scheme for the preparation of adjacent columns of light-absorbing molecules (yellow and red boxes) within this polymer. A) Initial growth of polymer by SOSIP. B) Replacement of hydrozones by amine groups. C) Condensation of a second chromophore (red box) with the amine group to yield a final polymer.

Volia! Two adjacent columns of different chromophores aligned parallel to the substrate! The authors chose two different naphthalene chromophores such that one could act as an electron shuttle pathway (red chromophore) and the other (the yellow chromophore) as a hole shuttle pathway. Irradiation of polymers made from two different types of chromophores had higher photocurrents than control polymers made from only one of the two naphthalene chromophores, confirming the importance of having both electron and hole channels available. The authors additionally note that their precise arrangement of chromophores resulted in highly desirable alignment of a redox gradient across the polymer – this redox gradient provides the driving force for exciton separation and transport to opposing electrodes. Practical applications (photovoltaics?) and exploration of the exchange process are reported to be ongoing.

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