Structure and dynamics processes in free-base chlorins controlled by chemical modifications of macroring and aryl groups in meso-positions

Abstract

In this work we present the synthesis, detailed structural characterization and analysis of molecular motion for unsymmetrical pyrrolidine-fused chlorins employing NMR, UV spectroscopy and DFT theoretical calculations. In samples, the meso-rings were modified by substitution of hydrogen by fluorine in ortho 1 and meta positions 2. The sample with perfluorinated ring 3 and phenyl derivative 4 were used as reference species. The assignment of signals was performed employing 2D NMR techniques. The rotational dynamics was examined by means of ¹H and ¹⁹F variable-temperature (VT) 1D NMR spectroscopy and 2D EXSY experiments. The synergism of steric effect between pyrrolidine ring and meso-rings is unambiguously proved. Models 1 and 3 behave very similar, aromatic rings are rigid in temperature range 233–373 K. For sample 2 and 4 the distinct molecular dynamics was revealed. The barrier of rotation depends on localization of ring in the chlorin structure. Those which are bonded in the neighborhood of pyrrolidine ring are more rigid compared to those localized on the opposite site. The temperature is a trigger which sequentially releases the rotation of aromatic group in the desired localization. Introduction of fluorine in labeled position has influence on static geometry defined by saddling angles.