Multidimensional spectroscopy of photoreactivity

Abstract

Coherent multidimensional electronic spectroscopy is commonly used to investigate photophysical phenomena such as light harvesting in photosynthesis in which the system returns back to its ground state after energy transfer. By contrast, we introduce multidimensional spectroscopy to study ultrafast photochemical processes in which the investigated molecule changes permanently. Exemplarily, the emergence in 2D and 3D spectra of a cross-peak between reactant and product reveals the cis-trans photoisomerization of merocyanine isomers. These compounds have applications in organic photovoltaics and optical data storage. Cross-peak oscillations originate from a vibrational wave packet in the electronically excited state of the photoproduct. This concept isolates the isomerization dynamics along different vibrational coordinates assigned by quantum-chemical calculations, and is applicable to determine chemical dynamics in complex photoreactive networks.

Keywords: 2D spectroscopy; photoreactive processes; ultrafast spectroscopy; vibrational coherence.

Fig. 1.

Principle of coherent multidimensional spectroscopy of photoreactivity. (A) For illustration, we assume a photoreactive network composed of three chemical species connected by different reaction channels. (B) 2D frequency spectra of this network visualize the correlations of reactants and products. (C) Involved reactive modes and associated reaction pathways that are still not resolvable from 2D spectra are exposed by introducing a third frequency dimension.

Fig. 2.

Absorption spectrum (gray) of ring-open 6-nitro-BIPS in acetonitrile, which is the sum of the (estimated) contributions of the two displayed isomers (TTC, red dotted line; TTT, blue dashed line). The pump laser spectrum is shown in green.

Fig. 3.

Long-time evolution of absorptive electronic 2D spectra of 6-nitro-BIPS in acetonitrile for various population times T. All spectra are normalized to the minimum of the spectrum. Negative values (blue) correspond to a decrease of the sample absorption, positive ones (yellow/red) to an increase. Contour lines (dashed for negative, solid for positive values) are drawn in intervals of 4% of the maximum amplitude. Maxima of the TTC contributions are indicated by red dashed lines, those of the TTT isomer by blue dashed lines. A scheme of the employed pulse sequence used to collect 2D/3D spectra in the pump–probe beam geometry is shown in the upper left.

Fig. 4.

Evolution of the 2D signal up to 1.5 ps after photoexcitation. (A) Exemplary 2D spectrum for . The temporal evolution as a function of population time is shown for (B) ESA (circles), (C) GSA (squares), and (D) SE signals (diamonds). Corresponding coordinates are indicated in A by their associated markers. Red markers represent contributions after TTC excitation, blue ones those after mostly TTT excitation. White filling corresponds to excitation and probing of the same isomer, black filling to different isomers, respectively.

Fig. 5.

Third-order 3D spectrum. (A) A 3D isosurface representation is used to visualize the absolute value of the 3D spectrum. Iso values of 0.65%, 0.9%, 1.15%, and 1.40% of the maximum amplitude were chosen. Red dashed lines indicate TTC excitation, blue dashed lines the TTT GSA probe wavenumber. (B) Slices of the 3D spectrum in the plane for (Left) and 360 cm⁻¹ (Right) and the phase for at a vertical cut at the TTC excitation wavenumber (Center).

Fig. 6.

Quantum-chemical analysis of the reaction mechanism and involved vibrational normal modes. (A) The C–C bond around which the cis–trans isomerization occurs (arrow in C) is substantially stretched during the torsional motion both for ground (blue) and first excited state (red). (B) The calculated amplitudes of this C–C stretching for all low-frequency normal modes show strong contributions (red) at the two experimentally observed frequency ranges (gray). (C) The dominant 185 cm⁻¹ mode visualized by overlaying the two reversal point geometries. (D) Simplified picture of signatures arising from a vibrational wave packet in the S₁ state of TTT after TTC excitation. Signal oscillations are most pronounced at the turning points with a phase change of π in between.