The role of the intermolecular π···π interactions in the luminescence behavior of novel coumarin-based pyrazoline materials

Novel nine coumarin-based pyrazolines have been synthesized and fully characterized in search of new fluorescent compounds for design stimuli-responsive chromic materials. All reported compounds behave as highly fluorescent materials, both in solution and in the solid state, the intermolecular interactions playing a key role in their photophysical properties. In particular, compound 1a exhibits non-reversible thermo-, mechano-, solvato- and vapochromic behaviors as a result of the establishment and/or the rupture of intermolecular π···π interactions, and additionally, ionochromic properties due to the complexation upon the addition of several metal salts. Powder XRD studies have been performed to explain the noted stimuli-responsive effects. The compound 1a molecules have π···π interactions proved by short distance of 3.61 (2) Å between the centroids of six-membered cycles and weak intermolecular C–H…O hydrogen bonds, which link the π···π dimers into columns. According to quantum chemical calculations by TD DFT method with the B3LYP functional, in the def2-TZVP basis, intermolecular π···π interactions lead to strong red-shift (around 63 nm) of the electron absorption (EA) of the 1a in solid state. On the contrary, in the 2a structure, there are no any hint for the π···π stacking and no changes in EA spectra from solution to solid state. Results show clear evidences of the usefulness of this compound in the field of sensors.