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Femtosecond torsional relaxation

Abstract

Molecular conformational reorganization following photon absorption is a fundamental process driving reactions such as the cis–trans isomerization at the heart of the primary step of vision and can be exploited for switching in artificial systems using photochromics. In general, conformational change occurs on a timescale defined by the energy of the main vibrational mode and the rate of energy dissipation. Typically, for a conformational change such as a twist around the backbone of a conjugated molecule, this occurs on the tens of picoseconds timescale. However, here we demonstrate experimentally that in certain circumstances the molecule, in this case an oligofluorene, can change conformation over two orders of magnitude faster (that is sub-100 fs) in a manner analogous to inertial solvent reorganization demonstrated in the 1990s. Theoretical simulations demonstrate that non-adiabatic transitions during internal conversion can efficiently convert electronic potential energy into torsional kinetic energy, providing the ‘kick’ that prompts sub-100 fs torsional reorganization.

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Figure 1: Excited states of oligofluorenes in solution.
Figure 2: Time-resolved torsional relaxation.
Figure 3: Experimental observation of ultrafast planarization.
Figure 4: Nature of Snand reorganization dynamics from Sm.
Figure 5: Photoinduced dynamics simulations for S0, S1, Sn and Sm states, averaged over 500 trajectories to obtain statistical averages.

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Acknowledgements

We thank S. Fernandez-Alberti for stimulating discussions and for help with the code for the non-adiabatic simulations. J.C. acknowledges the Royal Society for a Dorothy Hodgkin Fellowship. We also acknowledge support of the Center for Integrated Nanotechnology (CINT), the Center for Nonlinear Studies (CNLS) and the LDRD programme at Los Alamos National Laboratory, operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the US Department of Energy under contract DE-AC52-06NA25396, as well as the European Union for financial support through FP6-026365.

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J.C. and G.L. devised the experiments, J.C. and G.C. carried out the experiments and J.C. and G.L. analysed the data. S.T. and T.N. carried out the calculations that were devised by S.T. J.C., G.L., S.T. and T.N. wrote the paper.

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Correspondence to J. Clark.

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Clark, J., Nelson, T., Tretiak, S. et al. Femtosecond torsional relaxation. Nature Phys 8, 225–231 (2012). https://doi.org/10.1038/nphys2210

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