(Invited) Synthesis and Photophysical Properties of Photofunctional Porphyrin Supramolecular Assemblies
Authors: Hasobe Taku , Sakai Hayato 
Published: February 2020

The self-assembled formation of dye molecules to form ordered nanoarchitectures is currently receiving great interest as a means of constructing well-defined superstructures. The possibility for controlling macroscopic structures through a proper choice of the molecular components paves a way to design and synthesize materials with specific properties and functions. One of the main challenges is the consturction of highly organized supramolecular multicomponent nanostructures in a well-controlled manner, which is very related to the molecular systems of natural photosynthesis. Therefore, utilization of electron donor (D) and acceptor (A) molecules such as porphyrins and polycyclic aromatic hydrocarbon derivatives (PAHs) for controlled assembly has stimulated the related research fields such as molecular electronics and solar energy conversion.1-4

In this work, one-dimensional supramolecular hexagonal columnar phases composed of porphyrins (electron donor: D) and benzo[ghi]perylenetriimides (electron acceptor: A) through triple hydrogen bonds have been successfully constructed to perform sequential light-harvesting and electron-transfer processes. In the steady-state spectroscopic measurements, the spectra of benzo[ghi]perylenetriimides became significantly red-shifted and broadened, whereas the successive positive shifts of first reduction potentials were observed with increasing the number of electron-deficient substituents. These are consistent with the results of DFT calculations. Then, formation of the 1 : 1 supramolecular complex between zinc porphyrin and benzo[ghi]perylenetriimide derivatives through triple hydrogen bonds was confirmed by job’s plot of 1H NMR titration. The large formation constant (~104 M–1) measured by the 1H NMR titration agreed with the values estimated from the UV-vis and fluorescence titration. Then, the one-dimensional supramolecular nanoarrays were successfully prepared in a mixed solvent of benzonitrile and methylcyclohexane. X-ray diffraction (XRD) measurement, polarization optical microscopy (POM) and differential scanning calorimetry (DSC) suggested that these nanoarrays contained one-dimensional hexagonal columnar phases composed of stacked donor and acceptor layers. Finally, femtosecond transient absorption and electron spin resonance (ESR) measurements clearly indicated that photoinduced electron transfer occurred via the singlet excited states in the supramolecular columns.

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