Dynamics of guest molecules in PHTP inclusion compounds as probed by solid-state NMR and fluorescence spectroscopy

Abstract

Partially deuterated 1,4-distyrylbenzene (2PV) is included into the pseudohexagonal nanochannels of perhydrotriphenylene (PHTP). The overall and intramolecular mobility of 2PV is investigated over a wide temperature range by ¹³C, ²H NMR as well as fluorescence spectroscopy. Simulations of the ²H NMR spectral shapes reveal an overall wobble motion of 2PV in the channels with an amplitude of about 4° at T = 220 K and 10° at T = 410 K. Above T = 320 K the wobble motion is superimposed by localized 180° flips of the terminal phenyl rings with a frequency of 10⁶ Hz at T = 340 K. The activation energies of both types of motions are around 40 kJ mol⁻¹ which imply a strong sterical hindrance by the surrounding PHTP channels. The experimental vibrational structure of the fluorescence excitation spectra of 2PV is analyzed in terms of small amplitude ring torsional motions, which provide information about the spatial constraints on 2PV by the surrounding PHTP host matrix. Combining the results from NMR and fluorescence spectroscopy as well as of time-dependent density functional calculations yields the complete potential surfaces of the phenyl ring torsions. These results, which suggest that intramolecular mobility of 2PV is only reduced but not completely suppressed by the matrix, are corroborated by MD simulations. Unrealistically high potential barriers for phenyl ring flips are obtained from MD simulations using rigid PHTP matrices which demonstrate the importance of large amplitude motions of the PHTP host lattice for the mobility of the guest molecules.