Optimal Control of Photoisomerization

We report on optimal control of the photoisomerization of 3,3-diethyl-2,2-thiacyanine iodide dissolved in methanol. Enhancement and reduction of the relative yield of $cis$ to $trans$ isomers are achieved; i.e., the quantum efficiency of the photoisomerization is controlled with optimally phase and amplitude shaped 400 nm femtosecond laser pulses. Single-parameter control schemes, like chirp or intensity variation, fail to change the ratio of the photoproducts. The successful modification of the molecular structure can be regarded as a first step towards controlled stereoselectivity in photochemistry.

Figure 1

(a) Molecular structure of the two isomer configurations of the cyanine dye NK88. Irradiated by light of the proper wavelength, this molecule can undergo $trans\mathrm{\text{−}}cis$ isomerization. (b) Simplified potential energy surface. The reaction coordinate is the twist angle about the $\mathrm{C}\mathrm{C}$ double bond. (c) The ground-state absorption spectra of the two isomers. While the $trans$ isomer shows a broad ground-state absorption centered at 420 nm, the absorption band of the $cis$ isomer is redshifted with a maximum around 450 nm.

Figure 2

(a) Spectrally resolved pump-probe transients. The $z$ axis ($\Delta T/T$) of the contour plot describes the change in transmitted probe light normalized to the transmission without the pump laser. While red (black) denotes a high transmission through the sample, blue (gray) marks the absorption of the probe beam. (b) Pump-probe absorption transients recorded with a 400 nm pump and 460 nm as well as 400 nm probe wavelengths.

Figure 3

(a) Maximization and (b) minimization of the $cis/trans$ ratio as a function of iterations using an evolutionary algorithm. The filled dots represent results obtained with shaped laser pulses, and the open dots show the reference signal obtained with an unshaped laser pulse. On the $y$ axis of both graphs the achievable ratio changes for the single-parameter control schemes of intensity (dark gray, $2\sigma$ width) and quadratic spectral phase (light gray) variation are also shown.

Figure 4

XFROG traces of the optimal pulse shapes for (a) maximizing and (c) minimizing the ratio between the $cis$ and $trans$ isomers. For comparison the XFROG trace of the unmodulated pulse is shown in (b).