Resonance Raman investigation of β-cyclodextrin-encapsulated π-conjugated polymers

Abstract

Resonance Raman (RR) spectroscopy is used to investigate the effect of β-cyclodextrin encapsulation on the structural and photophysical properties of poly(4,4′-diphenylenevinylene). We especially focus on the thermal stability of the polymer. We find that within the range of 10-55 C the uninsulated polymer exhibits decreased Raman intensity in all the vibrational bands with temperature, along with changes in the relative intensity of the C-C inter-ring stretch mode at 1270 cm-1 with respect to the ring C-H in-plane symmetric bend at 1187 cm-1, which provides evidence for conformational changes as a function of temperature. No changes are observed in the intensity of the in-phase CH out-of-plane wag of the vinylene group at 968 cm-1. Therefore, the conformational changes involve mainly dihedral angle modification between the adjacent phenyl rings toward planarization, and little or no change in the planarity of the trans-vinylene group. The decrease of the optical absorption at 55 C with respect to that at room temperature and the appearance of a new absorption band at lower energies explain the decrease in the RR intensities and the wavelength dependence of the relative intensities of the Raman band. We note that the conformational change into a more planar geometry, which affects a significant portion of the polymer population, is irreversible and consistent with thermally induced aggregation. Such a planarization is unexpected in view of the usually observed thermochromic behavior of conjugated polymers, which leads to an increase of the energy gap for increasing temperature, as the average dihedral angles are increased due to excitation of a larger number of vibrational modes. Interestingly, the higher threading ratio polymers are resistant to any conformational changes within this temperature range, as reflected by their unchanged RR spectra, due to the rotaxane's ability to suppress intermolecular interactions and aggregation. Interestingly, the conformation of the uninsulated polymer at room temperature appears to be the same as that in the threaded analogues, which suggests that the cyclodextrin cavity hosts the polymer without physically interacting with it. © 2013 American Chemical Society.