Structural identification of a novel axially chiral binaphthyl fluorene based salen ligand in solution using electronic circular dichroism: A theoretical-experimental analysis

A novel axially chiral binaphthyl fluorene based salen ligand, AFX-155 [2,2′-(1E,1′E)-(R)-1,1′-binaphthyl-2,2′-diylbis(azan-1- yl-1-ylidene)bis(methan-1-yl-1-ylidene)bis(4-((7-(diphenylamino)-9, 9-dihexyl-9H-fluoren-2-l)ethynyl)phenol)], with potential applications in homogeneous catalysis, biophotonics, and sensing was synthesized. A full comparative theoretical-experimental analysis of the UV-vis and electronic circular dichroism (ECD) spectra of the 10 primary isomers, comprising stereoisomers and optical isomers, revealed the presence of the unique structure in tetrahydrofuran (THF) solution, the trans-R-intra//trans-R-extra. A proposed route of attack of the (R)-(+)-2,2′-diamino-1,1′-binapthalene onto a salicaldehyde 5-(2-(2-(diphenylamino)-9,9-dihexyl-9H-fluoren-7-yl)ethynyl)-2- hydroxybenzaldehyde followed by a consecutive attack of the resulting species onto another salicaldehyde, both via Burgi:Dunitz trajectory, validates the unambiguous formation of the established isomer. Steric hindrances seem to be the determinant factor that defines the 3D structural conformation of this particular stereoisomer of AFX-155 with triple axial chirality.The determination of every optimal structure and the dominant conformers of AFX-155 were calculated evaluating, in CONFLEX, their steric energies using force fields at MMFF94S (2006-11-24HGTEMP) level in gas phase. The geometry of the conformers was optimized in THF (using PCM) using Gaussian 09 at the DFT/B3LYP level of theory and 6-31G* basis set. The first 100 electronic excited states were calculated using the same level of theory and basis set.