Femtosecond torsional relaxation

Journal name:
Nature Physics
Volume:
8,
Pages:
225–231
Year published:
DOI:
doi:10.1038/nphys2210
Received
Accepted
Published online

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-100fs) 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-100fs torsional reorganization.

At a glance

Figures

  1. Excited states of oligofluorenes in solution.
    Figure 1: Excited states of oligofluorenes in solution.

    a, Absorption (black solid), photoluminescence (red dashed) and transient absorption (−ΔT/T, 1ps delay; blue markers) spectra of the fluorene pentamer in toluene solution. Absorption spectra of the heptamer (grey dashed) and trimer (grey dotted) are shown for comparison. Coloured arrows denote approximate energies of the pump (blue, 3.18eV), push (red, 1.59eV) and ultraviolet-pump (cyan, 4.77eV) pulses, as in part c. b, Calculated ground-state (black line) and excited-state (blue markers) absorption of the pentamer. Inset shows the chemical structure of the oligomers with n=3,5 or 7 (R is octyl for n=3, 5 and hexyl for n=7). c, Schematic (not to scale) showing the S0, S1, Sn and Sm states. S1 and Sm are excited through one-photon transitions (blue and turquoise arrows), whereas Sn requires two-photon or sequential (pump+push) excitation, as marked (see text for details); the energies of the pump, ultraviolet pump and push are marked in a. d, Contour two-dimensional plots of transition density matrices of the excited states S1, Sn and Sm for the fluorene pentamer. The axes label the repeat units of the oligomer. Each plot depicts probabilities of an electron moving from one molecular position (horizontal axis) to another (vertical axis) following electronic excitation according to the colour code. Le defines the exciton size (maximal distance between photoexcited electron and hole) on the polymer backbone.

  2. Time-resolved torsional relaxation.
    Figure 2: Time-resolved torsional relaxation.

    Two-dimensional pump–probe spectra (ΔT(τ,λ)/T) of the trimer, pentamer and heptamer of fluorene in toluene solution. The pentamer and heptamer show a dynamic redshift, marked by the black arrow. The trimer shows a much smaller redshift (black arrow) as the excitation wavelength is at the bottom of the density of states (exciting already planar molecules).

  3. Experimental observation of ultrafast planarization.
    Figure 3: Experimental observation of ultrafast planarization.

    a, Pump–probe (red lines) and pump–push–probe (markers) dynamics of the trimer, pentamer and heptamer in solution. The push arrives at 4ps for the trimer and pentamer and at a variety of times for the heptamer (100fs, diamonds; 1.2ps, triangles; 2ps, squares; 3.2ps, circles; 18ps crosses). b, Pump–probe (left) and pump–push–probe (right) spectra at the zero-crossing region for the pentamer. Note the slow redshift in the pentamer pump–probe spectra. The push (arriving at 2.5ps) causes an immediate redshift. c, Pump–push–probe dynamics at 480nm of the heptamer in solution, measured with ≤20fs temporal resolution, showing only the arrival of the push (push arrival arbitrarily set to 0fs) and the subsequent recovery, which has a time constant of ~60fs.

  4. Nature of Snand reorganization dynamics from Sm.
    Figure 4: Nature of Snand reorganization dynamics from Sm.

    a,b, Pump–probe (red lines) and pump–push–probe (black lines with markers) dynamics measured in the heptamer in solution at 480nm (a) and 820nm (b) probe wavelength. c, Pump–probe dynamics exciting at 390nm (3.18eV; directly to S1, red lines) and exciting at 260nm (4.77eV; directly to Sm black lines with markers). Measured on the heptamer in toluene (390nm) or cyclohexanone (266nm) with a probe wavelength of 480nm in both cases.

  5. Photoinduced dynamics simulations for S0, S1, Sn and Sm states, averaged over 500 trajectories to obtain statistical averages.
    Figure 5: Photoinduced dynamics simulations for S0, S1, Sn and Sm states, averaged over 500 trajectories to obtain statistical averages.

    a, An increase of population of the S1 state during non-adiabatic dynamics starting from Sm and Sn states. Population of Sn quickly drops to the S1 state within ~100fs, showing ultrafast non-adiabatic relaxation, whereas the Sm state relaxes on a much slower timescale (>400fs). b, Variation of the minimum torsion angle between fluorene units on each molecule averaged over 500 molecules (snapshots) taken along the trajectory. The curves for S0 and Sm are flat, showing no time dependence; the S1 state shows weak torsional relaxation over the first 400fs, whereas vibrational relaxation following Sn excitation results in a local planarization of the molecule within the first 100fs of dynamics.

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Author information

Affiliations

  1. Cavendish Laboratory, University of Cambridge, J J Thomson Avenue, Cambridge CB3 0HE, UK

    • J. Clark
  2. Theoretical Division, Group T-1/CINT Mail Stop B268, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA

    • T. Nelson &
    • S. Tretiak
  3. Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

    • G. Cirmi
  4. Center for Nano Science and Technology @Polimi, Istituto Italiano di Tecnologia, Via Pascoli 70/3, 20133 Milano, Italy

    • G. Lanzani
  5. Department of Physics Politecnico di Milano, Piazza L. Da Vinci 32, 20133 Milano, Italy

    • G. Lanzani

Contributions

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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The authors declare no competing financial interests.

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