It has been found in recent molecular beam experiments that pyridine molecules photoexcited at 255 nm can abstract hydrogen atoms from hydrogen-bonded water molecules in pyridine–water clusters, resulting in pyridinyl-hydroxyl radical pairs. The reaction could only be detected for clusters containing at least four water molecules. To provide insight into the mechanisms of this reaction, we performed ab initio excited-state trajectory surface-hopping dynamics simulations for two pyridine–water complexes, containing one and four water molecules, respectively, using the second-order algebraic-diagrammatic-construction (ADC(2)) electronic-structure method. A computationally efficient surface-hopping algorithm based on the Landau–Zener formula has been used to evaluate the transition probability between electronic states. The formation of the pyridinyl radical via an electron-driven proton transfer (EDPT) process from water to pyridine is confirmed by the simulations. The analysis of the competing excited-state reaction mechanisms up to 500 fs reveals that adiabatic relaxation to local minima of the S₁(nπ*) potential-energy surface is the dominant channel in both clusters, followed by internal conversion to the electronic ground state via so-called ring-puckering conical intersections. The efficiency of the latter contribution is weakly dependent of the size of the clusters. The EDPT reaction occurs on the fastest time scales (faster than 200 fs) with a branching ratio of several percent. It is found to be four times more efficient in the pyridine–(H₂O)₄ cluster than in the pyridine–H₂O cluster, which is qualitatively consistent with the experimental observations. A detailed understanding of the photoinduced reaction mechanisms in complexes of N-heterocyclic chromophores with water molecules is of relevance for future systematic knowledge-based developments of optimized materials for photocatalytic water splitting with sunlight.

Unidirectional rotation represents a very important functional feature in photochemistry, such as in the design of light-driven molecular rotary motors. Great attention has recently been devoted to the unidirectional preference of the torsional motion of azobenzene and other molecules. Azoheteroarenes offer functional advantages over their more conventional azobenzene counterparts due to the introduction of heteroaromatic rings. In this paper, the Z–E photoisomerization dynamics of two azoheteroarenes, 1,2-bis(1-methyl-1H-imidazol-2-yl)diazene and 1,2-bis(1H-imidazol-2-yl)diazene, are investigated with trajectory surface-hopping molecular dynamics at the semi-empirical OM2/MRCI level. Starting from the S₁ excited state of the M-helical Z-isomer of both azoheteroarenes, more than 99% of the trajectories decay to their ground states through the M-helical conical intersections by twisting about the central NN double bond. This chiral path preference can be well understood by the energy profiles generated by the linear interpolation between the Franck–Condon geometry of the M-helical Z-isomer and the relevant S₁/S₀ conical intersections. The Z–E photoisomerization mechanisms of these two azoheteroarenes display a higher preference for unidirectional rotary dynamics under a chiral path than their counterpart azobenzene.