Abstract
The SOS//QM/MM [Rivalta et al., Int. J. Quant. Chem., 2014, 114, 85] method consists of an arsenal of computational tools allowing accurate simulation of one-dimensional (1D) and bi-dimensional (2D) electronic spectra of monomeric and dimeric systems with unprecedented details and accuracy. Prominent features like doubly excited local and excimer states, accessible in multi-photon processes, as well as charge-transfer states arise naturally through the fully quantum-mechanical description of the aggregates. In this contribution the SOS//QM/MM approach is extended to simulate time-resolved 2D spectra that can be used to characterize ultrafast excited state relaxation dynamics with atomistic details. We demonstrate how critical structures on the excited state potential energy surface, obtained through state-of-the-art quantum chemical computations, can be used as snapshots of the excited state relaxation dynamics to generate spectral fingerprints for different de-excitation channels. The approach is based on high-level multi-configurational wavefunction methods combined with non-linear response theory and incorporates the effects of the solvent/environment through hybrid quantum mechanics/molecular mechanics (QM/MM) techniques. Specifically, the protocol makes use of the second-order Perturbation Theory (CASPT2) on top of Complete Active Space Self Consistent Field (CASSCF) strategy to compute the high-lying excited states that can be accessed in different 2D experimental setups. As an example, the photophysics of the stacked adenine-adenine dimer in a double-stranded DNA is modeled through 2D near-ultraviolet (NUV) spectroscopy.
Highlights
Ultrafast pump–probe spectroscopy experiments backed up by theoretical models at the quantum-mechanical level have achieved a detailed understanding of the photophysics of the DNA building blocks adenine, guanine, thymine and cytosine in gas-phase and solution
In this paper we present an extension of this protocol to simulate coherent excited state dynamics based on static and dynamic computational data
A time-dependent density functional theory (TD-DFT) study by Barone and co-workers concerning different single- and double-stranded poly(dA) multimers embedded in water indicate that the long living component of the excited state population correspond to a dark exciplex produced by a charge transfer between stacked adenines
Summary
Ultrafast pump–probe spectroscopy experiments backed up by theoretical models at the quantum-mechanical level have achieved a detailed understanding of the photophysics of the DNA building blocks adenine, guanine, thymine and cytosine in gas-phase and solution. Population transfer between different channels cannot be tracked because the correlation between pump and probe wavelengths is not resolved This complexity calls for a spectroscopic method with an enhanced spectral resolution. Simulating ultrafast 2D electronic spectroscopy from rst principles is challenging as it requires the incorporation of knowledge about the relaxation dynamics of singly and doubly excited states accessible along the de-excitation pathways into the non-linear spectroscopy simulations. It is the excited state absorption (ESA) and the stimulated emission (SE) whose temporal evolution re ects the geometrical and electronic changes along the potential energy surface. The SOS//QM/MM protocol is the rst method which couples quantummechanics/molecular mechanics (QM/MM) calculations, multi-con gurational excited state calculations with non-linear spectroscopy to generate time-resolved
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