Tracing the Steps of Photoinduced Chemical Reactions in Organic Molecules by Coherent Two-Dimensional Electronic Spectroscopy Using Triggered Exchange

We establish coherent triggered-exchange two-dimensional (TE2D) electronic spectroscopy as an expansion of pump-repump-probe transient absorption spectroscopy and uniquely elucidate the role of higher-lying electronic states in ultrafast photochemistry. As an example, this is demonstrated for a molecular switch present in two ring-open conformations. The formation of a new species—the radical cation—is observed and its precursor state is identified via TE2D.

Figure 1

Left: Common techniques in fs spectroscopy: (a) pump-probe, (b) pump-repump-probe. Right: FT-2D techniques in pump-probe geometry: (c) 2D, (d) TE2D.

Figure 2

(a) Absorption spectra of 6,8-dinitro BIPS for the ring-closed spiropyran (black) and the ring-open merocyanine (green) form. Red (TTC) and blue (TTT) lines indicate the absorption maxima of the displayed merocyanine isomers. (b) Spectrally resolved pump-repump-probe signal $\Delta {\mathrm{OD}}_{\mathrm{diff}}$ as a function of pump-repump delay $t_{PR}$ and for fixed probe delay $T=2000\mathrm{ps}$.

Figure 3

(a) $\Delta {\mathrm{OD}}_{\mathrm{diff}}$ for selected wavelengths with a global fit (black) revealing a decay time of 80 ps, (b) $\Delta {\mathrm{OD}}_{\mathrm{diff}}$ for selected pump-repump delays $t_{PR}$. The absorption of the new product is evident from the curve for $t_{PR}=30\mathrm{ps}$ (orange squares).

Figure 4

Extraction of the TE2D spectrum ($t_{PR}=0.5\mathrm{ps}$, $T=2000\mathrm{ps}$). (a) From the $2{\mathrm{D}}_{PR\mathrm{Pr}}$ data with both pump and repump interacting with the sample, (b) $2{\mathrm{D}}_{P\mathrm{Pr}}$ and (c) $2{\mathrm{D}}_{R\mathrm{Pr}}$ are subtracted, yielding (d) the TE2D spectrum.