Understanding the solid-state emission of a trio of 1,4-diaryl-1,3-butadiynes in terms of their molecular structure and crystal packing interactions

A trio of 1,4-diaryl-1,3-butadiynes with methoxy and trifluoromethoxy substituents in the 4- and 4'-positions were prepared in useful yields (42% for one, 95+% for the others), their structures determined and their purity ascertained. Analysis of the single crystal X-ray data for the three compounds revealed that two were mostly planar with small dihedral angles of 6-9 degrees between their aryl rings, while one existed in a twisted conformation with a dihedral angle of approximately 53 degrees between it's aryl rings. The solid-state absorption spectra of all three compounds were similar, with structured absorptions at approximately 340 nm. One of the compounds had absorption bands at 263 and 457 nm. Two of the compounds had similar emission bands with maxima that ranged from 440 to 450 nm, while the other was quite different, with a broad emission band with a maximum at 564 nm. Each of the three compounds had excited state decay traces that were fit to tri-exponential curves which yielded three life-times: a longer one attributed to aggregate emission, a shorter one arising from an intermolecular charge transfer transition, and a medium one that is assigned to a locally excited transition. (C) 2018 Elsevier B.V. All rights reserved.

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