Articles you may be interested inPhotodissociation of (SO2XH) Van der Waals complexes and clusters (XH = C2H2, C2H4, C2H6) excited at 32 040-32090 cm−1 with formation of HSO2 and X van der Waals complexes Na•••XCH 3 ͑XϭF, Cl, and Br͒ have been generated by crossing a beam of sodium with the expansion region of a supersonic jet of the appropriate halide, seeded with a rare gas. The identity of these complexes was determined by photoionization time-offlight mass spectrometry. The primary route for photodepletion of these complexes is thought to be the excitation of the Na chromophore followed by a charge-transfer dissociation:Measurement of the photodepletion cross section as a function of the excitation wavelength provided an approach to the study of these harpooning reactions starting in selected transition states. The action spectra for the three complexes consisted of up to four broad peaks. An assignment, made by ab initio calculations, was based on the electronically excited states of Na* perturbed by the halide molecule in the complex. Peaks, ranging from the red ͑ϳ700 nm͒ to the blue ͑ϳ400 nm͒, were assigned to a superposition of Na* states 3 2 P x,y , and to successively higher excited states 3 2 P z , 4 2 S, and 3 2 D. The transition probabilities computed for the various Na•••XCH 3 →͓Na*•••XCH 3 ͔ ‡ transitions were generally in qualitative accord with experiment. Vibrational progressions of the covalent excited states 3 2 P x,y,z were observed and analyzed.

The contradictory behaviors in light harvesting and non-photochemical quenching make xanthophyll lutein the most attractive functional molecule in photosynthesis. Despite several theoretical simulations on the spectral properties and excited-state dynamics, the atomic-level photophysical mechanisms need to be further studied and established, especially for an accurate description of geometric and electronic structures of conical intersections for the lowest several electronic states of lutein. In the present work, semiempirical OM2/MRCI and multi-configurational restricted active space self-consistent field methods were performed to optimize the minima and conical intersections in and between the 1Ag − , 2Ag − , 1Bu + , and 1Bu − states. Meanwhile, the relative energies were refined by MS-CASPT2(10,8)/6-31G*, which can reproduce correct electronic state properties as those in the spectroscopic experiments. Based on the above calculation results, we proposed a possible excitedstate relaxation mechanism for lutein from its initially populated 1Bu + state. Once excited to the optically bright 1Bu + state, the system will propagate along the key reaction coordinate, i.e., the stretching vibration of the conjugated carbon chain. During this period of time, the 1Bu − state will participate in and forms a resonance state between the 1Bu − and 1Bu + states. Later, the system will rapidly hop to the 2Ag − state via the 1Bu + /2Ag − conical intersection. Finally, the lutein molecule will survive in the 2Ag − state for a relatively long time before it internally converts to the ground state directly or via a twisted S 1 /S 0 conical intersection. Notably, though the photophysical picture may be very different in solvents and proteins, the current theoretical study proposed a promising calculation protocol and also provided many valuable mechanistic insights for lutein and similar carotenoids.

Previously, the MS‐CASPT2 method was performed to study the static and qualitative photophysics of tellurium‐substituted cytosine (TeC). To get quantitative information, we used our recently developed QTMF‐FSSH dynamics method to simulate the excited‐state decay of TeC. The CASSCF method was adopted to reduce the calculation costs, which was confirmed to provide reliable structures and energies as those of MS‐CASPT2. A detailed structural analysis showed that only 5% trajectories will hop to the lower triplet or singlet state via the twisted (S2/S1/T2)T intersection, while 67% trajectories will choose the planar intersections of (S2/S1/T3/T2/T1)P and (S2/S1/T2/T1)P but subsequently become twisted in other electronic states. By contrast, ~28% trajectories will maintain in a plane throughout dynamics. Electronic population revealed that the S2 population will ultrafast transfer to the lower triplet or singlet state. Later, the TeC system will populate in the spin‐mixed electronic states composed of S1, T1 and T2. At the end of 300 fs, most trajectories (~74%) will decay to the ground state and only 17.4% will survive in the triplet states. Our dynamics simulation verified that tellurium substitution will enhance the intersystem crossings, but the very short triplet lifetime (ca. 125 fs) will make TeC a less effective photosensitizer.