(Invited) Photoinduced Donor-Acceptor Interaction in Nanocarbon-Based Systems
Authors: Imahori Hiroshi 
Published: May 2020

Nanographenes, characterized by well-defined planar structures of sp2 carbon network with sizes over 1 nm, have attracted tremendous attention, as their extended π-conjugation offers intriguing characteristics in optical and electronic properties, such as large extinction coefficients, high photoluminescence quantum yields, and excellent charge transport abilities. These properties suggest their potential utility as basic components in various electronic devices including light-emitting diodes, solar cells, and field-effect transistors. Along this line, nanographenes with different sizes and edge configurations have been prepared, demonstrating the possibility of fine-tuning their electronic and optoelectronic properties through the structural modulation. Among them, hexa-peri-hexabenzocoronenes (HBCs) consisting of 42 sp2 carbons in the π-conjugated core can be regarded as the minimum unit of nanographenes. In contrast with nanographene molecules, other two-dimensional (2D) nanocarbons such as graphenes, graphene oxides (GOs), chemically-converted graphenes (CCGs), and graphene quantum dots (GQDs) are structurally inhomogeneous at the molecular level, containing different π-conjugation sizes, edge structures, and defect sites. Nanographenes can be regarded as a part of the larger 2D nanocarbons, and therefore, π-extended HBC derivatives with well-defined structures are ideal model units to understand the intrinsic structure–property correlations of the 2D nanocarbons.
Covalent linking of nanographenes and photoactive molecules is an effective strategy to endow them with new functions, as well as to examine their optical and photophysical properties, without changing the nanographene core structures. Various HBC linked systems with photoactive molecules including porphyrins, perylene diimides, and fullerenes were prepared and their basic optical properties such as steady-state absorption and fluorescence spectra were investigated. Nevertheless, so far such covalent functionalization of nanographenes has been limited to HBCs solely, and, to the best of our knowledge, covalently linked systems of photoactive molecule–nanographenes larger than HBC have never been reported. Therefore, the size and shape effects of nanographenes on the optical and photophysical properties of nanographene–photoactive molecule linked systems have yet to be studied systematically.
To this end, we employed two nanographenes as the model: the symmetrical HBC and its π-extended rectangular strip-shaped nanographene consisting of 114 sp2 carbons, which is a short graphene nanoribbon (GNR). They were linked with two zincporphyrin (ZnP) units through a p-phenylene bridge at the periphery positions for the better understanding of their intrinsic properties. The short, rigid phenylene spacer ensures a well-defined geometry between the nanographene and the attached porphyrins. The HBC core was selected as a typical, basic example of nanographenes. Although various linked systems of HBC with photoactive molecules including porphyrins have already been reported, ZnP–HBC linked systems with the p-phenylene spacer have yet to be synthesized. Meanwhile the GNR core was chosen because of its closely related structure to HBC. GNR possesses the same size on the short axis, but the large size on the long axis compared to HBC. Furthermore, GNR and HBC have the same edge structures, which provides an opportunity to focus on the size effect of nanographenes.
In this talk, I will give an overview of photoinduced donor-acceptor interaction in nanocarbon-based systems. For instance, photoexcitation of the porphyrin–HBC linked system led to exclusive energy transfer (EnT) from the first singlet excited state (S1) of the nanographene to the porphyrin, whereas opposite selective EnT occurred from the first and second singlet excited states (S1 and S2) of the porphyrin to the nanographene in the porphyrin–GNR linked system. In particular, ultrafast efficient EnTs from both the S2 and S1 states of the porphyrin to GNR mimic the corresponding ultrafast EnTs from the S2 and S1 states of carotenoids to chlorophylls in light-harvesting systems of natural photosynthesis. Such unique photophysical properties will be useful for the rational design of carbon-based photofunctional nanomaterials in optoelectronics and solar energy conversion devices.
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