• Energy Research

Matching matters when building supramolecular n/p-heterojunction photosystems on solid supports that excel with efficient photocurrent generation, important critical thickness, smooth surfaces, and flawless responsiveness to functional probes for the existence of operational intra- and interlayer recognition motifs.

Quartz crystal microbalance (QCM) and surface plasmon resonance (SPR) were used to study zipper and layer-by-layer multilayer assemblies of artificial photosystems based on naphthalenediimides (NDIs) attached to an oligophenylethynyl (OPE-NDI) or p-oligophenyl (POP-NDI) backbone in dry and wet state. For the most interesting OPE-NDI zipper, one obtains for the dry film a monolayer thickness of 1.85 nm and a density of 1.58 g/cm(3), while the wet film has a larger monolayer thickness of 3.6 nm with a water content of 36%. The dry thickness of a monolayer in OPE-NDI zippers corresponds to about one-half of the length of the OPE scaffold in agreement with the proposed structure of the zipper. The low water content of the OPE-NDI films confirms their compact structure. The dry monolayer thickness of the POP-NDI films of 1.45 nm is smaller than that for the OPE-NDI films, which is probably related to a tilt of the POP scaffolds within the adsorbed layer. The POP-NDI films swell in water much more substantially, suggesting a much more open structure. These features are in excellent agreement with the better photophysical performance of the OPE-NDI assemblies when compared to the POP-NDI films.

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The objective of this study was to evaluate the possibility of photoinduced stack/rod electron transfer in surface "zipper" architectures composed of stacks of blue (B) naphthalenediimides (NDIs) along strings of oligophenylethynyl (OPE) rods. The synthesis and characterization of anionic and cationic multichromophoric OPE-B systems are reported. Absorption spectra suggest that in OPE-B systems, planarity and thus absorption and conductivity of the OPE can possibly be modulated by intramolecular stacking of the surrounding NDIs, although interfering contributions from aggregation remain to be differentiated. Among surface architectures constructed with OPE-B and POP-B systems by zipper and layer-by-layer (LBL) assembly, photocurrents generated by OPE-B zippers exhibit the best critical thickness and fill factors. These findings confirm the existence and functional relevance of topologically matching zipper architectures. In OPE-B zippers, OPEs generate much more photocurrent than the blue NDIs. Ultrafast electron transfer from OPEs to NDIs accounts for these photocurrents, providing wavelength-controlled access to rod-stack charge separation, and thus to formal supramolecular n/p-heterojunctions (SHJs). NDI excitation is not followed by the complementary hole transfer to the OPE rod. Scaffolds with higher HOMOs will be needed to integrate blue NDIs into SHJ photosystems.