Multiple Decay Mechanisms and 2D‐UV Spectroscopic Fingerprints of Singlet Excited Solvated Adenine‐Uracil Monophosphate

Abstract

The decay channels of singlet excited adenine uracil monophosphate (ApU) in water are studied with CASPT2//CASSCF:MM potential energy calculations and simulation of the 2D‐UV spectroscopic fingerprints with the aim of elucidating the role of the different electronic states of the stacked conformer in the excited state dynamics. The adenine 1La state can decay without a barrier to a conical intersection with the ground state. In contrast, the adenine 1Lb and uracil S(U) states have minima that are separated from the intersections by sizeable barriers. Depending on the backbone conformation, the CT state can undergo inter‐base hydrogen transfer and decay to the ground state through a conical intersection, or it can yield a long‐lived minimum stabilized by a hydrogen bond between the two ribose rings. This suggests that the 1Lb, S(U) and CT states of the stacked conformer may all contribute to the experimental lifetimes of 18 and 240 ps. We have also simulated the time evolution of the 2D‐UV spectra and provide the specific fingerprint of each species in a recommended probe window between 25 000 and 38 000 cm−1 in which decongested, clearly distinguishable spectra can be obtained. This is expected to allow the mechanistic scenarios to be discerned in the near future with the help of the corresponding experiments. Our results reveal the complexity of the photophysics of the relatively small ApU system, and the potential of 2D‐UV spectroscopy to disentangle the photophysics of multichromophoric systems.

Assessment of the Potential Energy Hypersurfaces in Thymine within Multiconfigurational Theory: CASSCF vs. CASPT2

“…Figure 1 displays these structures for thymine, the system under study in the present work. They correspond to two different excited state minima, one planar [11] and one partially twisted or "boat-like" found in more correlated computations or solvated environments [24,34,63,64], and an "ethene-like" CI [65,66] connecting the 1 (ππ*) and ground states. The CI is usually employed to rationalize the ultrafast decay of thymine [8] and the extremely low fluorescence quantum yields recorded experimentally [62], and was initially reported in the pioneering study of Perun et al [22].…”

“…The same result holds true after adding extra correlating orbitals to the active space [CASSCF (10,11)] (see Figure S1), thus validating the previous result obtained with the 6-31G* basis set and the CASSCF(10,8) methodology. Active space selection and validation becomes essential as the size of the system under study increases and computations become impractical [64], and has also been documented to affect the outcome in photoinduced dynamics [71]. This illustrates the importance of the electronic correlation in order to give a proper description of the photochemical pathways actually accessible in a given molecular system [72].…”

Ultrafast and radiationless electronic excited state decay of uracil and thymine cations: computing the effects of dynamic electron correlation

“…54,55 Additional computations averaging over the lowest-lying 30 electronic doublet states were carried out on top of the different minima to evaluate the excited state absorption signals. 56,57 Our simulations of these spectral signals assume that excited state absorptions of the individual 2 n + O and 2 π + states are dominated by the electronic structure at their corresponding minima, 58,59 thus neglecting the time-evolution of the system and its lineshape as their simulation requires more sophisticated approaches that are out of the scope of the present study. [60][61][62][63] The CAS state interaction method 64 was employed to evaluate transition dipole moments and oscillator strengths and the energies were corrected with the standard (single-state) CASPT2 formulation with an IPEA shift of 0.0.…”

Two-dimensional electronic spectroscopy as a tool for tracking molecular conformations in DNA/RNA aggregates

“…It is in this context that theoretical chemistry comes in handy, by simulating the spectra we can separate the different peaks for their contributing specific electronic transitions, providing a route map to recognise, understand and interpret the main fingerprints observed. 22,23 In this contribution we introduce a computational strategy that combines ground-state molecular dynamics with QM/MM approaches in order to sample the conformational space of dimeric and multimeric flexible bio-chromophores and separate their different nuclear arrangements to be characterised spectroscopically by 2DES. The methodology is applied to ApA, a model system that contains both unstacked (non-interacting) and πor T-stacked (interacting) adenine moieties, and is shown to display distinct fingerprints for these different conformations, proving to be a promising tool to separate them and understand precisely how they impact the underlying photophysical and photochemical pathways in such complex and extended DNA/RNA systems.…”