Abstract Photoinduced electron transfer (ET) between a magnesium-porphyrin and benzoquinone in a model molecular complex is investigated employing ab initio multi-configuration electronic-structure calculations combined with quantum dynamical methods. The microscopic parameters controlling the electron-transfer process are obtained using a first-principles diabatization procedure. A model Hamiltonian which includes both linear and quadratic vibronic couplings of all nuclear degrees of freedom of the system is constructed. Quantum dynamical simulations of the ET process are performed employing the multi-layer multi-configuration time-dependent Hartree method. A detailed analysis of the ET dynamics for models of increasing complexity reveals that the dynamics is strongly influenced by resonances associated with vibronically active nuclear modes, leading to significant deviations from the results of classical ET theory. The comparison with results obtained with the simplified spin-boson model reveals the effects related to the Duschinsky rotation of normal modes. Keywords: electron transferquantum dynamicsML-MCTDHporphyrin–quinone complex Acknowledgements This work is dedicated to Bill Miller on the occasion of his 70th birthday. MT and HW are deeply indebted to Bill Miller for many insightful discussions on quantum and semi-classical dynamics. This work has been supported by the Deutsche Forschungsgemeinschaft through a research grant (MT), the National Science Foundation CHE-1012479 (HW), and the cluster of excellence ‘Munich Center for Advanced Photonics’ (WD and MT) and has used resources of the computing centres in Munich (LRZ) and Jülich (JSC), which is gratefully acknowledged.
The inversion of the energies of the lowest singlet (S1) and triplet (T1) excited states in violation of Hund’s multiplicity rule is a rare phenomenon in stable organic molecules. S1-T1 inversion has significant consequences for the photophysics and photochemistry of organic chromophores. In this work, ab initio computational methods were employed to explore the possibility of S1-T1 inversion in hexagonal polycyclic aromatic and heteroaromatic compounds. Although the singlet-triplet energy gap ΔST = ES1 – ET1 decreases with increasing size of hexagonal polycyclic aromatics, it remains positive up to kekulene (19 rings). However, symmetric substitution of C-C pairs by B-N groups in the interior, keeping the conjugation of the outer rim intact, results in compounds with robustly negative ΔST. The non-overlapping pattern of the densities of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is identified as the decisive criterion for S1-T1 inversion. These findings establish the existence of a new family of boron carbon nitrides with inverted singlet-triplet gaps.
We demonstrate chemical tuning and laser-driven control of intermolecular H atom abstraction from protic solvent molecules. Using multipulse ultrafast pump-push-probe transient absorption (TA) spectroscopy, we monitor hydrogen abstraction by a functionalized heptazine (Hz) from substituted phenols in condensed-phase hydrogen-bonded complexes. Hz is the monomer unit of the ubiquitous organic polymeric photocatalyst graphitic carbon nitride (g-C₃N₄). Previously, we reported that the Hz derivative 2,5,8-tris(4-methoxyphenyl)-1,3,5,6,7,9,9b-heptaazaphenalene (TAHz) can photochemically abstract H atoms from water, in addition to exhibiting photocatalytic activity for H₂ evolution matching that of g-C₃N₄ in aqueous suspensions. In the present work, we combine ultrafast multipulse TA spectroscopy with predictive wave function-based ab initio electronic-structure calculations to explore the role of mixed nπ*/ππ* upper excited states in directing H atom abstraction from hydroxylic compounds. We use an ultraviolet (365 nm) laser pulse to photoexcite TAHz to a bright upper excited state, and, after a relaxation period of roughly 6 ps, we use a near-infrared (NIR) (1150 nm) pulse to “push” the chromophore from the long-lived S₁ state to a higher-lying excited state. When phenol is present, the NIR push induces a persistent decrease (ΔΔOD) in the S₁ TA signal magnitude, indicating an impulsively driven change in photochemical branching ratios. In the presence of substituted phenols with electron-donating moieties, the magnitude of ΔΔOD diminishes markedly due to the increased excited-state reactivity of these complexes that accompanies the cathodic shift in phenol oxidation potential. In the latter case, H atom abstraction proceeds unaided by additional energy from the push pulse. These results reveal new insight into branching mechanisms among unreactive locally excited states and reactive intermolecular charge-transfer states. They also suggest molecular design strategies for functionalizing aza-aromatics to drive important photoreactions, such as H atom abstraction from water. More generally, this study demonstrates an avidly desired achievement in the field of photochemistry, rationally redirecting excited-state reactivity with light.
A simple model of electron scattering resonances near electronic excitation thresholds is discussed. The model consists of a single discrete electronic state coupled to several electronic continua. The vibrational dynamics in the resonance state is treated, taking proper account of non-Born-Oppenheimer effects in near-threshold electron-molecule scattering. The effect of long-range potentials is included via the threshold expansion of the partial decay widths of the resonance. The analytic properties of the fixed-nuclei $S$ matrix are analyzed in detail for two special cases ($s$-wave scattering in the absence of long-range potentials and scattering from a strongly polar target molecule). The dipole potential is shown to lead to a qualitatively new behavior of the trajectories of resonance poles near excitation thresholds. The model yields a qualitative description of the measured excitation function of the ${B}^{1}{\ensuremath{\Sigma}}^{+}$ state of the CO molecule, where a strong and narrow threshold peak is observed.
Heptazine is the molecular core of the widely studied photocatalyst carbon nitride. By analyzing the excited-state intermolecular proton-coupled electron-transfer (PCET) reaction between a heptazine derivative and a hydrogen-atom donor substrate, we are able to spectroscopically identify the resultant heptazinyl reactive radical species on a picosecond time scale. We provide detailed spectroscopic characterization of the tri-anisole heptazine:4-methoxyphenol hydrogen-bonded intermolecular complex (TAHz:MeOPhOH), using femtosecond transient absorption spectroscopy and global analysis, to reveal distinct product absorption signatures at ∼520, 1250, and 1600 nm. We assign these product peaks to the hydrogenated TAHz radical (TAHzH•) based on control experiments utilizing 1,4-dimethoxybenzene (DMB), which initiates electron transfer without concomitant proton transfer, i.e., no excited-state PCET. Additional control experiments with radical quenchers, protonation agents, and UV-vis-NIR spectroelectrochemistry also corroborate our product peak assignments. These spectral assignments allowed us to monitor the influence of the local hydrogen-bonding environment on the resulting evolution of photochemical products from excited-state PCET of heptazines. We observe that the preassociation of heptazine with the substrate in solution is extremely sensitive to the hydrogen-bond-accepting character of the solvent. This sensitivity directly influences which product signatures we detect with time-resolved spectroscopy. The spectral signature of the TAHzH• radical assigned in this work will facilitate future in-depth analysis of heptazine and carbon nitride photochemistry. Our results may also be utilized for designing improved PCET-based photochemical systems that will require precise control over local molecular environments. Examples include applications such as preparative synthesis involving organic photoredox catalysis, on-site solar water purification, as well as photocatalytic water splitting and artificial photosynthesis.
A trajectory method of calculating tunneling probabilities from phase integrals along straight line tunneling paths, originally suggested by Makri and Miller [J. Chem. Phys. 91, 4026 (1989)] and recently implemented by Truhlar and co-workers [Chem. Sci. 5, 2091 (2014)], is tested for one- and two-dimensional ab initio based potentials describing hydrogen dissociation in the 1B1 excited electronic state of pyrrole. The primary observables are the tunneling rates in a progression of bending vibrational states lying below the dissociation barrier and their isotope dependences. Several initial ensembles of classical trajectories have been considered, corresponding to the quasiclassical and the quantum mechanical samplings of the initial conditions. It is found that the sampling based on the fixed energy Wigner density gives the best agreement with the quantum mechanical dissociation rates.
Starting from a model Hamiltonian comprising an arbitrary number of discrete electronic configurations and an arbitrary number of electronic ionization continua as well as the vibrational degrees of freedom, a comprehensive theoretical description of near-threshold autoionization structure in molecular photoionization is developed. The discrete–continuum interaction is treated to infinite order and the infinite Rydberg series converging to the ionization thresholds are included as a whole in the treatment of the nuclear dynamics. The equivalence of the Feshbach projection-operator formulation and the multichannel-quantum-defect description is explicitly established in this rather general context. We derive a simplified model from the general formalism which is shown to reproduce naturally some recently observed features of vibronic autoionization via nontotally symmetric modes in polyatomic molecules.
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