Ultrafast dynamics of liquid poly(ethylene glycol)s and crown ethers studied by femtosecond Raman-induced Kerr effect spectroscopy.

Ultrafast molecular dynamics of liquid poly(ethylene glycol)s, tetra(ethylene glycol), penta(ethylene glycol), and poly(ethylene glycol) with the molecular weight of 600, and crown ethers, 12-crown-4 and 15-crown-5, have been investigated by means of femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy. Picosecond Kerr transients of poly(ethylene glycol)s and crown ethers are characterized by a biexponential function with the time constants of about 2 and 20 ps. Both the faster and slower time constants do not vary much among the five oligo(ethylene oxide)s. Femtosecond dynamics is discussed based on the Kerr (depolarized Raman) spectra obtained by Fourier transform deconvolution analysis of the high time resolution Kerr transients. The broad low-frequency band (0-200 cm(-1)) in the Kerr spectrum is analyzed by two Brownian oscillators. The spectral shapes of linear poly(ethylene glycol) and cyclic crown ether are very different. Both the low- and high-frequency Brownian oscillators for crown ethers show lower frequency and broader spectral features than those for poly(ethylene glycol)s. The comparison of the low-frequency spectra of poly(ethylene glycol)s and crown ethers shows that the low-frequency spectrum of 15-crown-5 is closer to that of poly(ethylene glycol)s than that of 12-crown-4 is. The difference of the low-frequency spectra between poly(ethylene glycol) and crown ether is discussed with the concepts of molecular conformation and liquid density. The features of the observed intramolecular vibrational bands are also correlated with the molecular conformations.