Investigations in the methoxy-iminocoumarin series: Highly efficient photoluminescent dyes and easy preparation of green-emitting crystalline microfibers

• Two N-substituted iminocoumarins exhibited strong fluorescence in the solid-state.
• The first example of crystallization-enhanced emission in the iminocoumarin series was reported.
• One of the dyes spontaneously formed long crystalline microfibers.

The fluorescence properties of four methoxy derivatives of 3-cyano-2-(N-ethoxycarbonyl)iminocoumarin, differing by the position and number of the methoxy substituents, were investigated. The 8-methoxy-iminocoumarin derivative was virtually non-fluorescent. The 7-methoxy-iminocoumarin derivative was only weakly fluorescent both in solution and in the solid state. The 6-methoxy-iminocoumarin derivative was poorly fluorescent in every solvent investigated, but it showed high photoluminescence in the solid state. To our knowledge, this is the first example of crystallization-enhanced emission (CEE) in the iminocoumarin series. The mechanism probably encompasses restricted rotation and favorable molecular packing. Moreover, this compound spontaneously crystallized as long microfibers. The 5,7-dimethoxy-iminocoumarin derivative exhibited good fluorescence in very polar solvents and was intensely luminescent in the crystalline state. The last two substitution patterns, 6-methoxy and 5,7-dimethoxy, could be of high interest for the design of new fluorescent materials.

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