Surface architectures built around perylenediimide stacks

Simple stacks of perylenediimides (PDIs) grown directly on solid surfaces are an intriguing starting point for the construction of multicomponent architectures because their intrinsic activity is already very high. The ability of PDI stacks to efficiently generate photocurrent originates from the strong absorption of visible light and the efficient transport of both electrons and holes after generation with light. The objective of this study was to explore whether or not the excellent performance of these remarkably simple single-channel photosystems could be further improved in more sophisticated multicomponent architectures. We report that the directional construction of strings of anions or cations along the PDI stacks does not significantly improve their activity; that is, the intrinsic activity of PDI stacks is too high to yield ion-gated photosystems. The directional construction of electron- and hole-transporting stacks of naphthalenediimides (NDIs) and oligothiophenes along the central PDI stack did not improve photocurrent generation under standard conditions either. However, the activity of double-channel photosystems increased with increasing thickness, whereas increasing charge recombination with single-channel PDI stacks resulted in decreasing activity with increasing length. Most efficient long-distance charge transport was found with double-channel photosystems composed of PDIs and NDIs. This finding suggests that over long distances, PDI stacks transport holes better than electrons, at least under the present conditions. Triple-channel photosystems built around PDI stacks with oligothiophenes and triphenylamines were less active, presumably because hole mobility in the added channels was inferior to that in the original PDI stacks, thus promoting charge recombination.

Ion-gated synthetic photosystems

Herein, molecular strings of ions built along charge-transporting channels are shown to dramatically increase photocurrents and enable charge transport over long distances, thus confirming the existence and significance of ion-gated photosystems. For their synthesis, ordered and oriented stacks of naphthalenediimides were grown on indium tin oxide by ring-opening disulfide-exchange polymerization. To these charge-transporting channels, coaxial strings of anions or cations-fixed, mobile, complete, partial, pure, or mixed-were added by orthogonal hydrazone exchange. The presence of partially protonated carboxylates was found to most significantly increase activity, implying that they both attract holes and repel electrons, that is, facilitate photoinduced charge separation and hinder charge recombination at the same time. As a result of this quite remarkable situation, photocurrents increased rather than decreased with increasing charge stabilization on their "stepping stones." The presence of mobile anions facilitated long-distance charge transport through thick films. Turned off by inhibited anion mobility, that is, proton hopping, hole/proton antiport is identified to account for long-distance charge transport in ion-gated photosystems.

Recent progress with functional biosupramolecular systems

The objective of this account is to summarize our recent progress with functional biosupramolecular systems concisely. The functions covered are artificial photosynthesis, anion transport, and sensing in lipid bilayer membranes. With artificial photosynthesis, the current emphasis is on the construction of ordered and oriented architectures on solid surfaces. Recent examples include the zipper assembly of photosystems with supramolecular n/p-heterojunctions and oriented antiparallel redox gradients. Current transport systems in lipid bilayers reveal new interactions at work. Examples include anion-macrodipole or anion-π interactions. Current attention with membrane-based sensing systems shifts from biosensor approaches with enzymatic signal generation to aptamers (i.e., the DNA version of immunosensing) and differential sensing with dynamic polyion-counterion transporters. The functional diversity accessible with biosupramolecular systems is highlighted, as is the critical importance of cross-fertilization at intertopical convergence zones.