Novel solid-state luminous composites from a layered inorganic–organic monolith containing neutral porphyrins
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We have succeeded in loading a neutral zinc porphyrin derivative (ZnTPPder) into a layered inorganic–organic compound, despite the well-known difficulties associated with incorporation of neutral pigments into the interlayer spaces of layered materials. We have utilized a synthetic alkylammonium–smectite monolith (A-Sm) as the host layered inorganic–organic compounds to facilitate porphyrin loading. Alkylammonium groups are located between the layers of the phyllosilicate-analogue moieties (the inorganic moiety) bonding covalently with the inorganic moiety to form the monolith. The interlayer space of A-Sm is organophilic and swells in organic solvents. ZnTPPder was loaded into A-Sm by mixing A-Sm and solutions of ZnTPPder in organic solvents under different conditions yielding ZnTPPder/A-Sm composite films. UV–VIS measurements revealed several absorption peaks assigned to the characteristic Soret (432 nm) and Q (500–650 nm) bands of ZnTPPder for all the prepared ZnTPPder/A-Sm composite films. Fluorescence emission due to ZnTPPder was observed from all the ZnTPPder/A-Sm composite films, while a reference ZnTPPder/Sm cast film prepared from ZnTPPder and a smectite (Sm) rather than A-Sm was only weakly fluorescent. This emission behaviour suggests that ZnTPPder is intercalated in the interlayer space of A-Sm with suppression of aggregation of ZnTPPder. In contrast, ZnTPPder was hardly intercalated into the interlayer space of Sm. Thus, we have demonstrated a novel and relatively easy route for loading of neutral (uncharged) pigments into a solid-state two-dimensional nanospace, a method which might be applied to prepare novel light-harvesting systems.
We are grateful to Dr. S. Hayashi, National Institute of Advanced Industrial Science and Technology (AIST), for in-depth discussions and kind instruction regarding solid-state NMR analyses of A-Sm. We are grateful to Dr. T. Wakahara, NIMS, for his instruction for SEM observations of the prepared films. We would also like to express our appreciation to Mr. S. Takenouchi, NIMS, for elemental analyses, Mr. K. Kurashima, NIMS for performing TEM observations and Mr. K. Kosuda, NIMS, for performing SEM observation of A-Sm. We would like to acknowledge the Co-op Chemical Co., Ltd (present: Katakura & Co-op Agri Corporation) for their kind donation of a sample of the synthetic smectite SWN. This study was partially supported by JSPS KAKENHI Grant Numbers 26420687 and JP16H06518 (Coordination Asymmetry) and CREST, JST.
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