Vibrational coherence transfer in the ultrafast intersystem crossing of a diplatinum complex in solution

We investigate the ultrafast transient absorption response of tetrakis(μ-pyrophosphito)diplatinate(II), [Pt₂(μ-P₂O₅H₂)₄]^4- [hereafter abbreviated Pt(pop)], in acetonitrile upon excitation of its lowest singlet ¹A_2u state. Compared with previously reported solvents [van der Veen RM, Cannizzo A, van Mourik F, Vlček A, Jr, Chergui M (2011) J Am Chem Soc 133:305-315], a significant shortening of the intersystem crossing (ISC) time (<1 ps) from the lowest singlet to the lowest triplet state is found, allowing for a transfer of vibrational coherence, observed in the course of an ISC in a polyatomic molecule in solution. Density functional theory (DFT) quantum mechanical/molecular mechanical (QM/MM) simulations of Pt(pop) in acetonitrile and ethanol show that high-lying, mostly triplet, states are strongly mixed and shifted to lower energies due to interactions with the solvent, providing an intermediate state (or manifold of states) for the ISC. This suggests that the larger the solvation energies of the intermediate state(s), the shorter the ISC time. Because the latter is smaller than the pure dephasing time of the vibrational wave packet, coherence is conserved during the spin transition. These results underscore the crucial role of the solvent in directing pathways of intramolecular energy flow.

Two-Color Nonlinear Spectroscopy for the Rapid Acquisition of Coherent Dynamics

There has been considerable recent interest in the observation of coherent dynamics in photosynthetic systems by 2D electronic spectroscopy (2DES). In particular, coherences that persist during the "waiting time" in a 2DES experiment have been attributed to electronic, vibrational, and vibronic origins in various systems. The typical method for characterizing these coherent dynamics requires the acquisition of 2DES spectra as a function of waiting time, essentially a 3DES measurement. Such experiments require lengthy data acquisition times that degrade the signal-to-noise of the recorded coherent dynamics. We present a rapid and high signal-to-noise pulse-shaping-based approach for the characterization of coherent dynamics. Using chlorophyll a, we demonstrate that this method retains much of the information content of a 3DES measurement and provides insight into the physical origin of the coherent dynamics, distinguishing between ground and excited state coherences. It also enables high resolution determination of ground and excited state frequencies.

Effects of Different Quantum Coherence on the Pump-Probe Polarization Anisotropy of Photosynthetic Light-Harvesting Complexes: A Computational Study

Observations of oscillatory features in the 2D spectra of several photosynthetic complexes have led to diverged opinions on their origins, including electronic coherence, vibrational coherence, and vibronic coherence. In this work, effects of these different types of quantum coherence on ultrafast pump-probe polarization anisotropy are investigated and distinguished. We first simulate the isotropic pump-probe signal and anisotropy decay of the Fenna-Matthews-Olson (FMO) complex using a model with only electronic coherence at low temperature and obtain the same coherence time as in the previous experiment. Then, three model dimer systems with different prespecified quantum coherence are simulated, and the results show that their different spectral characteristics can be used to determine the type of coherence during the spectral process. Finally, we simulate model systems with different electronic-vibrational couplings and reveal the condition in which long time vibronic coherence can be observed in systems like the FMO complex.

Multidimensional spectroscopy of photoreactivity

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.

Disentangling Electronic and Vibrational Coherence in the Phycocyanin-645 Light-Harvesting Complex

Energy transfer between chromophores in photosynthesis proceeds with near-unity quantum efficiency. Understanding the precise mechanisms of these processes is made difficult by the complexity of the electronic structure and interactions with different vibrational modes. Two-dimensional spectroscopy has helped resolve some of the ambiguities and identified quantum effects that may be important for highly efficient energy transfer. Many questions remain, however, including whether the coherences observed are electronic and/or vibrational in nature and what role they play. We utilize a two-color, four-wave mixing experiment with control of the wavelength and polarization to selectively excite specific coherence pathways. For the light-harvesting complex PC645, from cryptophyte algae, we reveal and identify specific contributions from both electronic and vibrational coherences and determine an excited-state structure based on two strongly coupled electronic states and two vibrational modes. Separation of the coherence pathways also uncovers the complex evolution of these coherences and the states involved.

Broad-Band Impulsive Vibrational Spectroscopy of Excited Electronic States in the Time Domain

We demonstrate that transient absorption spectroscopy performed with an ultrashort pump pulse and a chirped, broad-band probe pulse is capable of recording full vibrational spectra of excited electronic states in the time domain. The resulting spectra do not suffer from the nontrivial baselines and line shapes often encountered in frequency domain techniques and enable optimal and automated subtraction of background signatures. Probing the molecular dynamics continuously over a broad energy bandwidth makes it possible to confidently assign the vibrational coherences to specific electronic states and suggests the existence of mode-specific absorption spectra reminiscent of resonance Raman intensity analysis. The first observation of the nominally forbidden one-photon ground to first excited electronic state transition in β-carotene demonstrates the high sensitivity of our approach. Our results provide a first glimpse of the immense potential of broad-band impulsive vibrational spectroscopy (BB-IVS) to study ultrafast chemical reaction dynamics.

Coherent High-Frequency Vibrational Dynamics in the Excited Electronic State of All-Trans Retinal Derivatives

Coherent vibrational dynamics of retinal in excited electronic states are of primary importance in the understanding of photobiology. Using pump-DFWM, we demonstrate for the first time the existence of coherent double-bond high-frequency modulations (>1300 cm(-1)) in the excited electronic state of different retinal derivatives. All-trans retinal as well as retinal Schiff bases exhibit a partial frequency downshift of the C═C double-bond mode from ∼1580 cm(-1) in the ground state to 1510 cm(-1) in the excited state. In addition, a new vibrational band at ∼1700 cm(-1) assigned to the C═N stretching mode in retinal Schiff bases in the excited state is detected. The newly reported bands are observed only in specific spectral regions of excited-state absorption. Implications regarding the observation of vibrational coherences in naturally occurring retinal protonated Schiff bases in rhodopsins are discussed.