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| 2. |
- Ernst, Hanna A., et al.
(författare)
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Ultrafast Dynamics of o-Nitrophenol : An Experimental and Theoretical Study
- 2015
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 119:35, s. 9225-9235
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Tidskriftsartikel (refereegranskat)abstract
- The photolysis of o-nitrophenol (o-NP), a typical push pull molecule, is of current interest in atmospheric chemistry as a possible source of nitrous acid (HONO). To characterize the largely unknown photolysis mechanism, the dynamics of the lowest lying excited singlet state (S-1) of o-NP was investigated by means of femtosecond transient absorption spectroscopy in solution, time-resolved photoelectron spectroscopy (TRPES) in the gas phase and quantum chemical calculations. Evidence of the unstable aci-nitro isomer is provided both in the liquid and in the gas phase. Our results indicate that the Si state displays strong charge transfer character, which triggers excited state proton transfer from the OH to the NO2 group as evidenced by a temporal shift of 20 fs of the onset of the photoelectron spectrum. The proton transfer itself is found to be coupled to an out-of-plane rotation of the newly formed HONO group, finally leading to a conical intersection between Si and the ground state So. In solution, return to So within 0.2-0.3 ps was monitored by stimulated emission. As a competitive relaxation channel, ultrafast intersystem crossing to the upper triplet manifold on a subpicosecond time scale occurs both in solution and in the gas phase. Due to the ultrafast singlet dynamics, we conclude that the much discussed HONO split-off is likely to take place in the triplet manifold.
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| 3. |
- Geng, Ting, et al.
(författare)
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Dynamics in higher lying excited states : Valence to Rydberg transitions in the relaxation paths of pyrrole and methylated derivatives
- 2017
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 146:14
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Tidskriftsartikel (refereegranskat)abstract
- The involvement of intermediate Rydberg states in the relaxation dynamics of small organic molecules which, after excitation to the valence manifold, also return to the valence manifold is rarely observed. We report here that such a transiently populated Rydberg state may offer the possibility to modify the outcome of a photochemical reaction. In a time resolved photoelectron study on pyrrole and its methylated derivatives, N-methyl pyrrole and 2,5-dimethyl pyrrole, 6.2 eV photons (200 nm) are used to excite these molecules into a bright pi pi* state. In each case, a pi 3p-Rydberg state, either the B-1(pi 3p(y)) or the A(2)(pi 3p(z)) state, is populated within 20-50 fs after excitation. The wavepacket then proceeds to the lower lying A(2)(pi sigma*) state within a further 20 fs, at which point two competing reaction channels can be accessed: prompt N-H (N-CH3) bond cleavage or return to the ground state via a conical intersection accessed after ring puckering, the latter of which is predicted to require an additional 100-160 fs depending on the molecule.
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| 4. |
- Geng, Ting, et al.
(författare)
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Time-Resolved Photoelectron Spectroscopy Studies of Isoxazole and Oxazole
- 2020
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 124:20, s. 3984-3992
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Tidskriftsartikel (refereegranskat)abstract
- The excited state relaxation pathways of isoxazole and oxazole upon excitation with UV-light were investigated by nonadiabatic ab initio dynamics simulations and time-resolved photoelectron spectroscopy. Excitation of the bright ππ*-state of isoxazole predominantly leads to ring-opening dynamics. Both the initially excited ππ*-state and the dissociative πσ*-state offer a combined barrier-free reaction pathway, such that ring-opening, defined as a distance of more than 2 Å between two neighboring atoms, occurs within 45 fs. For oxazole, in contrast, the excited state dynamics is about twice as slow (85 fs) and the quantum yield for ring-opening is lower. This is caused by a small barrier between the ππ*-state and the πσ*-state along the reaction path, which suppresses direct ring-opening. Theoretical findings are consistent with the measured time-resolved photoelectron spectra, confirming the timescales and the quantum yields for the ring-opening channel. The results indicate that a combination of time-resolved photoelectron spectroscopy and excited state dynamics simulations can explain the dominant reaction pathways for this class of molecules. As a general rule, we suggest that the antibonding σ*-orbital located between the oxygen atom and a neighboring atom of a five-membered heterocyclic system provides a driving force for ring-opening reactions, which is modified by the presence and position of additional nitrogen atoms.
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| 5. |
- Geng, Ting, et al.
(författare)
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Time-resolved photoelectron spectroscopy studies on pyrazole and several methylated derivatives
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We present femtosecond time-resolved photoelectron spectra and ab initio studies on pyrazole andits methylated derivatives 1-, 3-, and 5-methylpyrazole. Excitation at 200 nm populates both the two lowest lying states, a 1ππ* state and a mixed 1πσ*/1π3s Rydberg state, from where three relaxation channels are observed: ring puckering, N-H bond cleavage, and ring opening via N-N bond breaking. The N-N bond breaking channel is the fastest process, occurring within one vibrational cycle of the Franc Condon active ring stretching mode. N-H bond cleavage is observed to be aminor channel, and occurs upon direct excitation to the mixed π3s/πσ* state. Finally, ring puckering occurs after a timescale of a few hundred fs because the molecules need time to find the gradient towards this conical intersection. However, this channel is accessed if the initially triggered processes are not successful. The quantum yields of the different channels were found to be very sensitive of the relative positioning of the excited states. In pyrazole and 5-methylpyrazole, N-Nbond cleavage dominates. In 1-, and 3-methylpyrazole, while the 1ππ* state drops in energy the dissociating 1πσ* valence state does not, and this leads to an increased barrier towards ring cleavage and a decreased the quantum yield for N-N bond cleavage. Upon excitation at 267 nm of 1- and 3-methylpyrazole, ring puckering is the only available pathway.
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| 6. |
- Glover, William J., et al.
(författare)
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Excited state non-adiabatic dynamics of the smallest polyene, trans 1,3-butadiene. II. Ab initio multiple spawning simulations
- 2018
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 148:16
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Tidskriftsartikel (refereegranskat)abstract
- The excited state non-adiabatic dynamics of the smallest polyene, trans 1,3-butadiene (BD), has long been the subject of controversy due to its strong coupling, ultrafast time scales and the difficulties that theory faces in describing the relevant electronic states in a balanced fashion. Here we apply Ab Initio Multiple Spawning (AIMS) using state-averaged complete active space multistate second order perturbation theory [SA-3-CAS(4/4)-MSPT2] which describes both static and dynamic electron correlation effects, providing a balanced description of both the initially prepared bright 1(1)B(u) (pi pi*) state and non-adiabatically coupled dark 2(1)A(g) state of BD. Importantly, AIMS allows for on-the-fly calculations of experimental observables. We validate our approach by directly simulating the time resolved photoelectron-photoion coincidence spectroscopy results presented in Paper I [A. E. Boguslavskiy et al., J. Chem. Phys. 148, 164302 (2018)], demonstrating excellent agreement with experiment. Our simulations reveal that the initial excitation to the 11Bu state rapidly evolves via wavepacket dynamics that follow both bright-and dark-state pathways as well as mixtures of these. In order to test the sensitivity of the AIMS results to the relative ordering of states, we considered two hypothetical scenarios biased toward either the bright B-1(u) or the dark 2(1)A(g) state. In contrast with AIMS/SA-3-CAS(4/4)-MSPT2 simulations, neither of these scenarios yields favorable agreement with experiment. Thus, we conclude that the excited state non-adiabatic dynamics in BD involves both of these ultrafast pathways.
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| 7. |
- Liang, Yu, et al.
(författare)
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On the origin of high transient anisotropies : An exemplification in a Cd-porphyrin
- 2013
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 139:22, s. 224309-
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Tidskriftsartikel (refereegranskat)abstract
- Transient anisotropy is a widely used spectroscopic method to access the polarization dynamics of a molecular sample. In this contribution, we present results on 5,10,15,20-tetraphenyl-porphyrinato cadmium (II) in tetrahydrofuran which exhibits values exceeding the typical range between 0.4 and -0.2 in dependence of the probe wavelength. These findings are explained by varying contributions from excited state absorption and ground state bleaching/stimulated emission. Model calculations show that time zero values and time decays are complex values that often do not correlate with the underlying physical processes. As a consequence, the interpretation of anisotropy experiments necessitates extreme care.
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| 8. |
- Liang, Yu, et al.
(författare)
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Ultrafast Dynamics of meso-Tetraphenylmetalloporphyrins : The Role of Dark States
- 2013
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Ingår i: ChemPlusChem. - : Wiley. - 2192-6506. ; 78:10, s. 1244-1251
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Tidskriftsartikel (refereegranskat)abstract
- Studying the relaxation pathways of porphyrins and related structures upon light absorption is crucial to understand the fundamental processes of light harvesting in biosystems and many applications. Herein, we show by means of transient absorption studies, following Q- and Soret-band excitation, and abinitio calculations on meso-tetraphenylporphyrinato magnesium(II) (MgTPP) and meso-tetraphenylporphyrinato cadmium(II) (CdTPP) that electronic relaxation following Soret-band excitation of porphyrins with a heavy central atom is mediated by a hitherto disregarded dark state. This accounts for an increased rate of internal conversion. The dark state originates from an orbital localized at the central nitrogen atoms and its energy continuously decreases along the series from magnesium to zinc to cadmium to below 2.75eV for CdTPP dissolved in tetrahydrofuran. Furthermore, we are able to directly trace fast intersystem crossing in the cadmium derivative, which takes place within (110 +/- 20)ps.
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| 9. |
- MacDonell, R. J., et al.
(författare)
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Excited state dynamics of acrylonitrile: Substituent effects at conical intersections interrogated via time-resolved photoelectron spectroscopy and ab initio simulation
- 2016
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 145:11
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Tidskriftsartikel (refereegranskat)abstract
- We report a joint experimental and theoretical study on the photoinitiated ultrafast dynamics of acrylonitrile (AN) and two methylated analogs: crotonitrile (CrN) and methacrylonitrile (MeAN). Time-resolved photoelectron spectroscopy (TRPES) and ab initio simulation are employed to discern the conical intersection mediated vibronic dynamics leading to relaxation to the ground electronic state. Each molecule is pumped with a femtosecond pulse at 200 nm and the ensuing wavepackets are probed by means of one and two photon ionization at 267 nm. The predominant vibrational motions involved in the de-excitation process, determined by ab initio trajectory simulations, are an initial twisting about the C=C axis followed by pyramidalization at a carbon atom. The decay of the time-resolved photoelectron signal for each molecule is characterized by exponential decay lifetimes for the passage back to the ground state of 60 +/- 10, 86 +/- 11, and 97 +/- 9 fs for AN, CrN, and MeAN, respectively. As these results show, the excited state dynamics are sensitive to the choice of methylation site and the explanation for the observed trend may be found in the trajectory simulations. Specifically, since the pyramidalization motion leading to the conical intersection with the ground state is accompanied by the development of a partial negative charge at the central atom of the pyramidal group, the electron donation of the cyano group ensures that this occurs exclusively at the medial carbon atom. In this way, the donated electron density from the cyano group "directs" the wavepacket to a particular region of the intersection seam. The excellent agreement between the experimental and simulated TRPES spectra, the latter determined by employing trajectory simulations, demonstrates that this mechanistic picture is consistent with the spectroscopic results.
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| 10. |
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