Changing color upon external mechanical stimuli is one promising direction for designing smart functional materials. Organic charge-transfer (CT) complexes are well known for their unique absorption in the visible light range. Our goal is to explore their applications in designing mechanochromic materials. Two supramolecular CT complexes of naphthalene diimide (NDI) and pyrene (PYR) were utilized to investigate whether the CT interaction geometry would influence its characteristic absorption band. Our strategy was to tune the CT interaction geometry by switching on/off an auxiliary H-bonding interaction. Naphthalene diimide acceptor analogues (NDI1 and NDI2) with high structural similarity were thus designed to accommodate and suppress the H-bonding, respectively. Mixing the equal molar PYR with NDI1 and NDI2, respectively, gave two solid powders with distinctly different colors. Crystallographic studies on quality co-crystals of these two CT complexes (NDI2/PYR and NDI1/PYR) revealed differences in the CT interaction geometry as well as the stoichiometry between the donor and acceptor. The stoichiometric factor, however, was not considered to cause the observed color variation by comparing NDI1/PYR with a control model with a similar interaction geometry but different stoichiometry. In-solution study revealed no difference between the two supramolecular CT systems, highlighting the importance of the solid-state confined geometry. The observed geometric influence over color in CT systems points to a new way for designing stimulus responsive materials and easy-to-read sensing systems.

Can we tune the color of organic charge transfer systems by varying their donor-acceptor interaction geometry?

249th National Meeting (2015)

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