Does the wavelength dependent photoisomerization process of the p‑coumaric acid come out from the electronic state dependent pathways?

Radiationless mechanism of UV deactivation by cuticle phenolics in plants

Hydroxycinnamic acids present in plant cuticles, the interphase and the main protective barrier between the plant and the environment, exhibit singular photochemical properties that could allow them to act as a UV shield. Here, we employ transient absorption spectroscopy on isolated cuticles and leaf epidermises to study in situ the photodynamics of these molecules in the excited state. Based on quantum chemical calculations on p-coumaric acid, the main phenolic acid present in the cuticle, we propose a model in which cuticle phenolics display a photoprotective mechanism based in an ultrafast and non-radiative excited state deactivation combined with fluorescence emission. As such, the cuticle can be regarded as the first and foremost protective barrier against UV radiation. This photostable and photodynamic mechanism seems to be universal in land plants giving a special role and function to the presence of different aromatic domains in plant cuticles and epidermises.

Phenolics are abundant in plant cuticles. Here, via transient absorption spectroscopy and quantum chemical calculations, the authors propose a model by which cuticle phenolics provide photoprotection due to ultrafast and non-radiative excited state deactivation combined with fluorescence emission.

Position of the Benzene Ring Substituent Regulates the Excited-State Deactivation Process of the Benzyluracil Systems

A systematic theoretical study of the regulating effect of the substituent position on the photoinduced deactivation process of the benzyluracil systems has been performed based on the high-level static electronic structure calculations and on-the-fly full-dimensional excited-state dynamics simulations. Similarities and differences coexist for the two systems by comparative studies on the photoinduced deactivation process of the 5-benzyluracil (5-BU) and 6-benzyluracil (6-BU) systems. They both obey an S₂ → S₁ → S₀ two-step decay pattern, and the decay coordinates of the S₂ → S₁ and S₁ → S₀ processes are mainly driven by the elongation of the bridging bond and the out-of-plane ring deformation motion, respectively. However, the puckering motion occurring at the C2 atom in the uracil fragment dominates the decay pathway of the 5-BU system. On the contrary, the puckering motion at the C5 atom in the benzene fragment mainly drives the decay coordinate of the 6-BU system. Therefore, the substituent position could play significant roles in the deactivation process of the benzyluracil systems. Moreover, the S₁ → S₀ decay process of the 6-BU system consists of five pathways, possessing a more complex deactivation picture than the 5-BU system. The fitted time scale of the puckering motion is compatible with the experimentally observed lifetimes. This work provides a fundamental understanding of the photophysical and photochemical properties of the benzyluracil systems and can give rational suggestions to further design or regulate the bionic molecular systems.

Theoretical studies of the ultrafast deactivation mechanism of 8-oxo-guanine on the S1 and S2 electronic states in gas phase

The 8-oxo-deoxyguanosine is the most abundant specie of the DNA oxidative damage. Despite the deleterious effects such as gene mutation it may cause, the 8-oxodG was also reported to have beneficial effect such as repairing the nearby cyclobutane pyrimidine dimer (CPD) after photoexcitation. Due to its strong biological relevance, the photoinduced excited state dynamics behavior of 8-oxo-deoxyguanosine is of particular interest. In this work, a theoretical investigation by combination of complete active space self-consistent field (CASSCF) ab initio calculations and on-the-fly nonadiabatic dynamics simulations are implemented to provide intrinsic deactivation mechanism of its free base 8-oxoguanine after being excited to the S₁ and S₂ states. Two minimum energy conical intersections (MECIs) characterized by the C3-puckered motion with attractive chiral character are located, which contribute appreciably to the S₁ state deactivation process. When the system is being excited to the S₂ state directly, a S₂ → S₁ → S₀ two-step decay pattern is proposed. A nearly planar S₂/S₁ intersection plays a significant role in the S₂ → S₁ decay process. The subsequent S₁ state relaxation process is also dominated by the C3-puckered deformation motion. One decay time is estimated to be 704 fs, which compares well with the experimental observation of 0.9 ± 0.1 ps in solvents. Particular illustration is the fact that the MECIs configurations we located bear an exceptional resemblance with previous reported thymine, cytosine and guanine, suggesting that the current work could lend support for better understanding of the non-natural nucleobases and derivatives.