This journal is c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 15725–15733 15725 Controlling the mechanism of fulvene S 1 /S 0 decay: switching off the stepwise population transferw David Mendive-Tapia, a Benjamin Lasorne, b Graham A. Worth, c Michael J. Bearpark* a and Michael A. Robb a Received 8th September 2010, Accepted 29th October 2010 DOI: 10.1039/c0cp01757d Direct quantum dynamics simulations were performed to model the radiationless decay of the first excited state S 1 of fulvene. The full space of thirty normal mode nuclear coordinates was explicitly considered. By default, ultrafast internal conversion takes place centred on the higher-energy planar region of the S 1 /S 0 conical intersection seam, giving the stepwise population transfer characteristic of a sloped surface crossing, and leading back to the ground state reactant. Two possible schemes for controlling whether stepwise population transfer occurs or not—either altering the initial geometry distribution or the initial momentum composition of the photo-excited wavepacket—were explored. In both cases, decay was successfully induced to occur in the lower-energy twisted/peaked region of the crossing seam, switching off the stepwise population transfer. This absence of re-crossing is a direct consequence of the change in the position on the intersection at which decay occurs (our target for control), and its consequences should provide an experimentally observable fingerprint of this system. I. Introduction Selective control of product formation in a particular chemical reaction has traditionally been accomplished at the macroscopic level, through manipulation of external variables such as tempera- ture, pressure or solvent character. Nevertheless, recent theoretical and experimental work has shown that photochemical control of reactions can also be achieved at the microscopic level using ultra- fast light pulses. 1,2 The general approach is known as optical control, which has as a central objective the design of laser pulses capable of driving or forcing a chemical system to a targeted outcome. Nowadays, commercial lasers can be programmed to routinely generate short laser pulses on the scale of a few femtoseconds, whose complex internal structure can be variably shaped in space and time with great precision. Several schemes for laser-controlled manipulation of molecules have been proposed and successfully applied experi- mentally. These procedures, which predict a high degree of control over the position, spread and vibrational mode generation of a photo-excited wavepacket, comprise a vast range of techniques. However, we are not directly concerned with laser pulse shapes and experimental techniques here. Instead, we suggest—using the radiationless decay of fulvene as a prototype system—that understanding shapes of Potential Energy Surfaces (PESs) is a vital initial step in targeting a specific outcome of a photophysical or photochemical process. Rather than indirectly targeting a particular product to do this, we aim for a specific region of the conical intersection/crossing seam, at molecular geometries that correlate with a well-defined experimental signature. This does not control whether radiationless decay occurs or not, but affects how and when decay takes place. Direct quantum dynamics simulations on fulvene were performed with the aim of modelling two main aspects: the natural decay of fulvene centred at the planar region of the intersection seam; and controlled decay at the twisted crossing region. Two possible schemes for controlling the population transfer—either altering the initial geometry distribution or the initial momentum composition of the photo-excited wavepacket—were explored as a proof of concept: the first slows the initial relaxation towards the planar crossing, allowing twisting to develop; while the second drives the system directly towards the twisted crossing. We chose fulvene as a test system for several reasons. First, we have previously mapped its extended S 1 /S 0 conical inter- section seam using a complete second-order description. 3 Second, fulvene is a useful model system in the description of E/Z photo-isomerisation mechanisms (when substituted appropriately) and photo-induced rotation. 4 Finally, fulvene’s radiationless decay has also been studied using surface hopping 5 and quantum dynamics. 6–9 The surface hopping calculations took into account all degrees of freedom, but as with other semi-classical approaches, the nuclear motion was treated using Newton’s equations of motion; only the electron distribution was described quantum mechanically. On the other hand, previous complete quantum dynamics calculations were performed, solving the time-dependent Schro¨dinger equation a Department of Chemistry, Imperial College London, London, SW7 2AZ, United Kingdom. E-mail: m.bearpark@imperial.ac.uk b Institut Charles Gerhardt Montpellier, UMR 5253, CNRS-UM2-ENSCM-UM1, CTMM, Universite´ Montpellier 2, CC 1501, Place Euge `ne Bataillon, 34095 Montpellier, France c School of Chemistry, University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom w Electronic supplementary information (ESI) available: Cartesian coordinates and energies of all optimised geometries; frequency- mass-weighted components of the initial momentum vector giving the directions of the induced geometrical displacements; animations of the trajectory followed by the centre of the wavepacket for: (a) S 1 /S 0 natural planar decay, (b) S 1 /S 0 controlled decay preventing the initial relaxation along the Q s coordinate (full dimensionality including the three selected trajectories extracted from the GBFs) and (c) S 1 /S 0 controlled decay driving the system to the twisted region of the seam. See DOI: 10.1039/c0cp01757d PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics Published on 16 November 2010. Downloaded by Imperial College London Library on 23/06/2013 18:24:20. View Article Online / Journal Homepage / Table of Contents for this issue