Equilibrium and mid-infrared driven vibrational dynamics of artificial hydrogen-bonded networks

Abstract

Stereo-selectively synthesized 1,3-poly-alcohols are introduced as low-dimensional spectroscopic model systems for molecular dynamics in hydrogen-bonded networks. The molecular and vibrational structures of the artificial networks are studied by means of density functional theory. The flexibility of the networks and the time scales associated with hydrogen-bond breakage and formation are investigated through Langevin dynamics simulations. Experimentally, the dynamics of the polyols are explored by femtosecond mid-infrared spectroscopy in the OH-stretching spectral region. Polyols with their hydroxyl groups distributed along the hydrocarbon backbone in an all-syn configuration are highly rigid and form an extended quasi 1-dimensional hydrogen-bond wire that is stable for tens of picoseconds. The mid-infrared pump–probe data on these rigid networks exhibit biexponential kinetics. This finding supports a mechanism for vibrational energy relaxation in all-synpolyols that is mediated by hydrogen-bond dissociation within 850 fs. The hydrogen-bond wire is subsequently re-established on a time scale of about 14 ps. In contrast, poly-alcohols with their OH groups in an all-anti configuration are highly flexible and display hydrogen-bond breakage–formation on a 100 fs time scale already at thermal equilibrium. As a result the pump–probe data are mono-exponential and can be understood in terms of pure intramolecular vibrational relaxation occurring with a time constant of 1.3 ps.