Crystal-packing modes determine the solid-state ESIPT fluorescence in highly dipolar 2′-hydroxychalcones

Abstract

This work describes the systematic study of the structure–luminescence relationship of 15 hydroxy-chalcones directly in the crystal state. Chalcones are easily assembled at the gram scale allowing for efficient variation of their substitution motifs. Our molecule variants combine two modes of fluorescence generation, ESIPT and ICT, both known for their potential to achieve significant quantum yields even with emission in the red to near infrared, a region preferred for technologies as diverse as optoelectronics and chemical sensing. Quantum yields as high as 48% (at 665 nm) and emission wavelengths in the deep red region (710 nm, 5%) were achieved with variants equipped with a strained amino substituent in the donor portion (azetidinyl). Systematic XRD analysis of large monocrystals allowed for the identification of the subtle interplay of several inter- and intra-molecular parameters in achieving such performances, be it intramolecular planarity, non-classical H-bonds, and stacking modes.