Integrated experimental and computational investigations of crystal structure, and photophysical properties of 2,7-dibromo-9-(4-halobenzylidene)-9H-fluorene frameworks

Abstract

2,7-dibromo-9-(4-halobenzylidene)-9H-fluorene derivatives (1–4) were synthesized via 1,8-diazabicyclo[5.4.0]undec‑7-ene (DBU) mediated condensation reaction between 2,7-dibromofluorene, and 4-fluoro/chloro/bromobenzaldehyde. Compounds 1–4 were characterized by FT-IR, UV–Vis, ¹H and ¹³CNMR spectral techniques. 2,7-Dibromo-9-(4-chlorobenzyli dene)-9H-fluorene (3) was characterized by single crystal X-ray diffraction analysis. Crystal packing of 3 stabilized by the intermolecular C–H···Br/Cl and π-stacking and the Br···Br interaction. The energetics of intermolecular dimers in the crystal packing of 3 was analyzed by CLP-PIXEL energy and density functional theory (DFT) calculations. The nature of Br···Br bond (Type II halogen bond) was characterized by molecular electrostatic surface potential and deformation electron density map. Different noncovalent interactions in various molecular dimers were characterized using theoretical charge density analysis. Also, the photophysical properties of 1–4 was experimentally analyzed and compared with DFT results are reported. The λ_max of 1–4 was red shifted compared with 2,7-dibromofluorene (DBF). Which might be due to the extended conjugation of molecule which arises from the benzylidene unit in 1–4. The Frontier Molecular Orbitals (FMO's), density of states (DOS), electronic properties and global reactivity descriptors are calculated, and experimental band gap values obtained from the electrochemical method, are reported. Experimental band gap value indicates that it in the following order: 1∼2 > 3 > 4 (i.e. H ≈ F>Cl>Br) and it shows the influence halogen substitution in 1–4.