During the past decade the understanding of photo-induced ultrafast dynamics in molecular systems has improved at an unforeseen speed and a wealth of detailed insight into the fundamental processes has been obtained.This review summarizes our present knowledge on ultrafast dynamics in isolated molecules and molecular clusters evolving after excitation with femtosecond pulses as studied by pump–probe analysis in real time. Experimental tools and methods as well as theoretical models are described which have been developed to glean information on primary, ultrafast processes in photophysics, photochemistry and photobiology. The relevant processes are explained by way of example—from wave packet dynamics in systems with a few atoms all the way to internal conversion via conical intersections in bio-chromophores. A systematic overview on characteristic systems follows, starting with diatomic and including larger organic molecules as well as various types of molecular clusters, such as micro-solvated chromophore molecules. For conciseness the focus is on molecular systems which remain unperturbed by the laser pulses—apart from the excitation and detection processes as such. Thus, only some aspects of controlling and manipulating molecular reactions by shaped and/or very intense laser pulses are discussed briefly for particularly instructive examples, illustrating the perspectives of this prospering field.The material presented in this review comprises some prototypical examples from earlier pioneering work but emphasizes studies from recent years and covers the most important and latest developments until January 2006.

https://iopscience.iop.org/article/10.1088/0034-4885/69/6/R06/pdf

Ultrafast dynamics in isolated molecules and molecular clusters

Using basis-set extrapolation schemes for a given data set, we evaluated the binding energies and geometries at the complete basis set (CBS) limit at the levels of the second order Moller–Plesset perturbation theory (MP2) and the coupled cluster theory with singles, doubles, and perturbative triples excitations [CCSD(T)]. The systems include the hydrogen bonding (water dimer), aromatic interaction (benzene dimer), π–H interaction (benzene–water), cation–water, anion–water, π–cation interaction (cation–benzene), and π–anion interaction (anion–triazine). One extrapolation method is to exploit both BSSE-corrected and BSSE-uncorrected binding energies for the aug-cc-pVNZ (N = 2, 3, 4, …) basis set in consideration that both binding energies give the same CBS limit (CBSB). Another CBS limit (CBSC) is to use the commonly known extrapolation approach to exploit that the electron correlation energy is proportional to N−3. Since both methods are complementary, they are useful for estimating the errors and trend of the asymptotic values. There is no significant difference between both methods. Overall, the values of CBSC are found to be robust because of their consistency. However, for small N (in particular, for N = 2, 3), CBS is found to be slightly better for water–water interactions and cation–water and cation–benzene interactions, whereas CBS is found to be more reliable for bezene–water and anion–water interactions. We also note that the MP2 CBS limit value based on N = 2 and 3 combined with the difference between CCSD(T) and MP2 at N = 2 would be exploited to obtain a CCSD(T)/CBS value for aromatic–aromatic interactions and anion–π interactions, but not for cationic complexes. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2008

Complete basis set limit of Ab initio binding energies and geometrical parameters for various typical types of complexes.

In this mini-review we briefly describe intermolecular interactions ranging from hydrogen bonding to ionic interactions to aromatic interactions. Manifestation of these interaction forces is in the design and realization of various ionophores with chemo-sensing capability for biologically important cations and anions. We also explain how the understanding of hydrogen bonding and π-interactions has led to the design of self-assembled organic nanotubes. We further discuss the conformational changes between stacked and edge-to-face conformers in benzoquinone-benzene complexes, which are controlled by alternating electrochemical potential. The resulting flapping motion illustrates a promising pathway toward the design of nanomechanical devices.

Designing Ionophores and Molecular Nanotubes Based on Molecular Recognition

Despite the importance of water photolysis in atmospheric chemistry, its mechanism is not well understood. Two different mechanisms for water photolysis have been proposed. The first mechanism is driven by water photoexcitation, followed by the reaction of the active hydrogen radical with water clusters. The second mechanism is governed by the ionization process. Both photoexcited and photoionized mechanisms are complementary, which is elucidated by using excited-state ab initio molecular dynamics simulations based on complete active space self-consistent field approach and unrestricted Moller-Plesset second-order perturbation theory based Born-Oppenheimer molecular dynamics simulations.

Photoexcitation and photoionization dynamics of water photolysis.

The hydrated structures, dissociation energies, thermodynamic quantities, infrared spectra, and electronic properties of alkali-metal hydroxides (MOH, M = Na and K) hydrated by up to six water molecules [MOH(H2O)n=1-6], are investigated by using the density functional theory and Moller−Plesset second-order perturbation theory. Further accurate analysis based on the coupled cluster theory with singles, doubles, and perturbative triples excitations is more consistent with the MP2 results. NaOH shows a peculiar trend in dissociation:  it begins to form a partially dissociated structure for n = 3, and it dissociates for n = 4 and 6, whereas it is undissociated for n = 5. However, for n = 5, the dissociated structure is nearly isoenergetic to the undissociated structure. For KOH, it begins to show partial dissociation for n = 5, and complete dissociation for n = 6.

Hydration phenomena of sodium and potassium hydroxides by water molecules.

Upon excitation of Cl(-)(H(2)O)(3) and I(-)(H(2)O)(3) clusters, the electron transfers from the anionic precursor to the solvent, and then the excess electron is stabilized by polar solvent molecules. This process has been investigated using ab initio molecular dynamics (AIMD) simulations of excited states of Cl(-)(H(2)O)(3) and I(-)(H(2)O)(3) clusters. The AIMD simulation results of Cl(-)(H(2)O)(3) and I(-)(H(2)O)(3) are compared, and they are found to be similar. Because the role of the halogen atom in the photoexcitation mechanism is controversial, we also carried out AIMD simulations for the ground-state bare excess electron -- water trimer [e(-)(H(2)O)(3)] at 300 K, the results of which are similar to those for the excited state of X(-)(H(2)O)(3) with zero kinetic energy at the initial excitation. This indicates that the rearrangement of the complex is closely related to that of e(-)(H(2)O)(3), whereas the role of the halide anion is not as important.

Michel Dupuis

Papers

Ab Initio Molecular Dynamics Simulations of an Excited State of X-(H2O)3 (X = Cl, I) Complex