Intramolecular CH/π interaction of Poly(9,9-dialkylfluorene)s in solutions: Interplay of the fluorene ring and alkyl side chains revealed by 2D 1H-1H NOESY NMR and 1D 1H-NMR experiments

Abstract

Herein, we demonstrate for the first time the existence of intramolecular CH/π interactions within three chiral and one achiral poly(9,9- dialkylfluorene)s (S2MO, S37DMO, R37DMO and PF8) as well as a 9,9-di-n-octylfluorene molecule (F8) in chloroform-d1 at 20 °C. The interaction is experimentally demonstrated by (1) intense cross-peaks between the ring and alkyl groups in two-dimensional 1H-1H nuclear overhauser enhancement (NOE) correlated spectroscopy (NOESY) nuclear magnetic resonance (2D 1H-1H NOESY NMR) and by (2) a marked upfield shift of several alkyl proton signals in one-dimensional 1H nuclear magnetic resonance (1D 1H-NMR) spectra. To support this idea theoretically, possible folded conformations were searched using Conflex7 (MMFF94s force field) for simple molecular models of S2MO, S37DMO and PF8, followed by optimizing the most probable conformation by second-order Møller-Plesett perturbation calculations (Gaussian 03, 6-311 G(d) basis set). This calculation revealed that the distances between the fluorene ring and the (S)-2-methyl and β-methylene protons of the alkyl groups (that is, ≈2.53-2.64 Å) was shorter than the closest van der Waals C-H/ring contact (2.90 Å). S2MO showed intense circular dichroism bands in the π-π* region because of a stronger CH/π interaction, whereas S37DMO and R37DMO exhibited no such detectable circular dichroism bands because of a weaker CH/π interaction. © 2013 The Society of Polymer Science, Japan (SPSJ).