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.photoreactive processes | ultrafast spectroscopy | 2D spectroscopy | vibrational coherence A basic objective of physical chemistry is to determine the mechanisms underlying chemical reactions. Fundamental insights into these are provided by femtosecond time-resolved spectroscopy, even below the timescale of molecular vibrations (1). The major challenge of ultrafast photochemistry is to identify isolated signatures of reactants, intermediates, and products, whose spectral bands often overlap. Here we show that such ambiguities are unraveled by coherent multidimensional electronic spectroscopy, e.g., 2D and 3D spectroscopy, where the correlation of a system's excitation and emission frequencies is measured separating features that otherwise superpose. We detect photoreactivity directly via cross-peaks in multidimensional spectra. This opens the broad field of femtochemistry (1) to the multidimensional concept.Two-dimensional electronic spectroscopy (2-5) was primarily implemented for studying photophysical phenomena such as energy transfer in multichromophore light-harvesting systems or other excitonically coupled systems (6-11). Recently, charge transfer has also been investigated (12, 13). Here we are interested in photochemical processes leading to ground-state product species with a molecular configuration that is different from the initial reactant. As a general extension of 2D spectroscopy, 3D representations provide an even more detailed picture. Coherent 3D spectroscopy has been introduced to infrared spectroscopy (14, 15), and recently to electronic spectroscopy for isolating excitonic coherences (16, 17), for liquid-and gas-phase model systems (18)(19)(20), and for analyzing photosynthetic lightharvesting (21,22). Regarding these approaches, one has to differentiate between fifth-order experiments (14, 15, 19) for effects of higher nonlinearity (e.g., three-point frequency-fluctuation correlation functions) (14) and third-order techniques (20)(21)(22)(23)(24), as demonstrated here to unravel photochemical reactions.The principal idea of multidimensional spectrosc...

Structure–Property Relationships for Exciton and Charge Reorganization Energies of Dipolar Organic Semiconductors: A Combined Valence Bond Self-Consistent Field and Time-Dependent Hartree-Fock and DFT Study of Merocyanine Dyes

“…Such tortional motion of photoexcited merocyanine dyes has also been verified by various experiments and identified as a major fluorescence quenching pathway, but also utilized for photoreactions by light-induced electrocyclization into spiro compounds. [60][61][62][63][64][65][66] The twisting motion predicted by our calculations is exemplified by a superposition of the planar groundstate structure and the twisted excited-state structure of MD353 in Figure 9. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 The exciton reorganization energies resulting from this induced twist motion are indeed by a factor of 10 higher than the effects discussed so far.…”

Molecular Photofragmentation Dynamics in the Gas and Condensed Phases

“…In the context of this review, 2-D electronic spectroscopy has opened up new opportunities for studying photoisomerisation reaction dynamics with unprecedented detail. 71 The remainder of this Review is devoted to comparing selected molecular photodissociation processes in the gas and condensed phases. 57,72,73 Two themes have been chosen, to illustrate (i) the extents to which dynamical insights from gas-phase (isolated molecule) studies can inform our understanding of the corresponding solution-phase photochemistry and (ii) how, in the specific case of photoinduced ring opening reactions, solution-phase studies can provide dynamical insights complementary to (and in some cases more clearly than) those revealed by the corresponding gas-phase study.…”

Quantum Control of Population Transfer and Vibrational States via Chirped Pulses in Four Level Density Matrix Equations

“…A central aspect in both approaches, CC and QCS, is the control of population transfer between the ground and excited states, as well as the generation of vibrational coherence in both potential surfaces. By controlling the population transfer or by suppressing specific molecular vibrational coherences, a photochemical reaction channel can be selectively chosen [27,29,32,[37][38][39][40][41][42][43][44][45][46][47][48][49], or a certain mode in a multidimensional time-resolved signal can be suppressed [37,[50][51][52][53][54][55].…”