doi:10.1016/S0301-0104(99)00183-4 
Copyright © 1999 Elsevier Science B.V. All rights reserved
Reaction dynamics of photochromic dithienylethene derivatives
J. Erna, A. T. Bensb, H. -D. Martinb, S. Mukamelc, D. Schmida, S. Tretiakc, E. Tsiperc and C. Kryschia, *
a Lehrstuhl für Festkörperspektroskopie (IPkM), Heinrich-Heine-Universität, D-40225 Düsseldorf, Germany
b Institut für Organische Chemie und Makromolekulare Chemie I, Heinrich-Heine-Universität, D-40225 Düsseldorf, Germany
c Department of Chemistry, University of Rochester, Rochester, NY 14627, USA
Received 2 April 1999.
Available online 7 January 2000.
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Abstract
The reaction dynamics of the photochromic ring-opening reaction of 1,2-bis(5-formyl-2-methyl-thien-3-yl)perfluorocyclopentene (CHO-BMTFP) in dichloromethane solution was investigated using femtosecond transient absorption spectroscopy. The data were analyzed in terms of a model potential and single-electron density matrices, which were calculated using the collective electronic oscillator (CEO) approach and the INDO/S semiempirical Hamiltonian. The S0–S1 and S0–S2 transitions of the closed isomer were resonantly excited using 120 fs pump pulses at 610 and 410 nm, respectively. A temporally delayed white light continuum probe pulse monitors the decay of the S1 or S2 state as well as the recovery of the S0 state. Within the first picosecond after excitation, CHO-BMTFP was observed to undergo a fast structural relaxation along the S1 potential energy surface into a minimum constituting a precursor of the ring-opening process. The rather long lifetime of the precursor, τ2=13 ps, was consistent with the calculated potential barrier in front of the conical intersection with the S0 potential energy surface, which may arise from stabilization of the nearly planar closed isomer by an efficiently delocalized π-electron system.
Author Keywords: Photochromism; Dithienylethenes; Reaction kinetics; Femtosecond laser spectroscopy
Fig. 1. Absorption spectra of the open (solid line) and closed (dashed line) isomer of CHO-BMTFP in CH2Cl2.
Fig. 2. Structure formulae of the open (left) and closed (right) isomer of CHO-BMTFP.
Fig. 3. Structure formula of the closed isomer of CHO-BMTFP with the numbering of the ‘heavy' atoms: C, S, F, O.
Fig. 4. Schematic illustration of the S0, S1, and S2 potential energy surfaces (i.e., Eg(q), E1(q), E2(q)) along the reaction coordinate q defined as the distance between the atoms C4 and C24 (see Fig. 3); C* and P* denote the initially excited state and precursor, respectively. The rate parameters kRO, kST, k1 and k2 are described in Section 4.
Fig. 5. 2D-Contour plots of the electron modes of the closed isomer: ξ0, ξ1, ξ2 (left side) and of the open isomer: ξ0′, ξ1′, ξ2′ (right side).
Fig. 6. Variation of the ground -state density matrix element between the atoms C9 and C19 (ρ9,19) along the reaction coordinate q; the value of ρ9,19, which reflects the π conjugation between the blocks L and R of CHO-BMTFP, increases sharply as q increases from 175 to 225 pm. This characterizes the change of the single to double bond (see Fig. 2).
Fig. 7. Variations of the total electronic coherences within (solid lines, ξintra) and between (dashed lines, ξinter) the blocks L (O1–C9) and R (C19–O27) along the reaction coordinate q for the S1 and S2 states. The coherences were calculated by summation of the absolute values of the respective matrix elements in the contour plots of ξ1 and ξ2 displayed in Fig. 4 as explained in the text.
Fig. 8. Temporal evolution of the transient absorption spectra recorded at delay times τD between −0.5 ps and +31 ps. The excitation wavelength is 610 nm.
Fig. 9. Temporal evolution of the transient absorption spectra recorded at delay times τD between −0.6 ps and +61 ps. The excitation wavelength is 410 nm.
Fig. 10. Transients of photoinduced absorption detected at 442 nm (top), 693 nm (bottom) and transient of photoinduced bleaching detected at 544 nm (center). The pump pulse wavelength is 610 nm.
Fig. 11. Transients of photoinduced absorption detected at 464 nm (top), 767 nm (bottom) and transient of photoinduced bleaching detected at 581 nm (center). The pump pulse wavelength is 410 nm.
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