title: Structure-mechanical property correlations in mechanochromic luminescent crystals of boron difluoride dibenzoylmethane derivatives.
abstract: The structure and mechanical properties of crystalline materials of three boron difluoride dibenzoylmethane (BF2dbm) derivatives were investigated to examine the correlation, if any, among mechanochromic luminescence (ML) behaviour, solid-state structure, and the mechanical behaviour of single crystals. Qualitative mechanical deformation tests show that the crystals of BF2dbm( (t) Bu)2 can be bent permanently, whereas those of BF2dbm(OMe)2 exhibit an inhomogeneous shearing mode of deformation, and finally BF2dbmOMe crystals are brittle. Quantitative mechanical analysis by nano-indentation on the major facets of the crystals shows that BF2dbm( (t) Bu)2 is soft and compliant with low values of elastic modulus, E, and hardness, H, confirming its superior suceptibility for plastic deformation, which is attributed to the presence of a multitude of slip systems in the crystal structure. In contrast, both BF2dbm(OMe)2 and BF2dbmOMe are considerably stiffer and harder with comparable E and H, which are rationalized through analysis of the structural attributes such as the intermolecular interactions, slip systems and their relative orientation with respect to the indentation direction. As expected from the qualitative mechanical behaviour, prominent ML was observed in BF2dbm( (t) Bu)2, whereas BF2dbm(OMe)2 exhibits only a moderate ML and BF2dbmOMe shows no detectable ML, all examined under identical conditions. These results confirm that the extent of ML in crystalline organic solid-state fluorophore materials can be correlated positively with the extent of plasticity (low recovery). In turn, they offer opportunities to design new and improved efficient ML materials using crystal engineering principles.
authors.affiliation: Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata , Mohanpur Campus, Mohanpur 741252, India.
authors.forename: Gamidi Rama
authors.initials: GR
authors.lastname: Krishna
authors.affiliation: Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata , Mohanpur Campus, Mohanpur 741252, India.
authors.forename: Ramesh
authors.initials: R
authors.lastname: Devarapalli
authors.affiliation: Department of Materials Engineering, Indian Institute of Science , Bangalore 560012, India.
authors.forename: Rajesh
authors.initials: R
authors.lastname: Prusty
authors.affiliation: Department of Chemistry, University of Virginia , Charlottesville, Virginia 22904, USA.
authors.forename: Tiandong
authors.initials: T
authors.lastname: Liu
authors.affiliation: Department of Chemistry, University of Virginia , Charlottesville, Virginia 22904, USA.
authors.forename: Cassandra L
authors.initials: CL
authors.lastname: Fraser
authors.affiliation: Department of Materials Engineering, Indian Institute of Science , Bangalore 560012, India ; Center of Excellence for Advanced Materials Research, King Abdulaziz University , Jeddah 21589, Saudi Arabia.
authors.forename: Upadrasta
authors.initials: U
authors.lastname: Ramamurty
authors.affiliation: Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata , Mohanpur Campus, Mohanpur 741252, India.
authors.forename: Chilla Malla
authors.initials: CM
authors.lastname: Reddy
keywords: crystal engineering; hydrogen bonding; intermolecular interactions; mechanical properties; mechanochromism; nanoindentation; organic solid-state reactions