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′-(1<i>E</i>,1′<i>E</i>)-(<i>R</i>)-1,1′-binaphthyl-2,2′-diylbis­(azan-1-yl-1-ylidene)­bis­(methan-1-yl-1-ylidene)­bis­(4-((7-(diphenylamino)-9,9-dihexyl-9<i>H</i>-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 <i>trans</i>-<i>R</i>-<i>intra</i>//<i>trans</i>-<i>R</i>-<i>extra</i>. A proposed route of attack of the (<i>R</i>)-(+)-2,2′-diamino-1,1′-binapthalene onto a salicaldehyde 5-(2-(2-(diphenylamino)-9,9-dihexyl-9<i>H</i>-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