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Abstract Crystalline pentacene is a model solid-state light-harvesting material because its quantum efficiencies exceed 100% via ultrafast singlet fission. The singlet fission mechanism in pentacene crystals is disputed due to insufficient electronic information in time-resolved experiments and intractable quantum mechanical calculations for simulating realistic crystal dynamics. Here we combine a multiscale multiconfigurational approach and machine learning photodynamics to understand competing singlet fission mechanisms in crystalline pentacene. Our simulations reveal coexisting charge-transfer-mediated and coherent mechanisms via the competing channels in the herringbone and parallel dimers. The predicted singlet fission time constants (61 and 33 fs) are in excellent agreement with experiments (78 and 35 fs). The trajectories highlight the essential role of intermolecular stretching between monomers in generating the multi-exciton state and explain the anisotropic phenomenon. The machine-learning-photodynamics resolved the elusive interplay between electronic structure and vibrational relations, enabling fully atomistic excited-state dynamics with multiconfigurational quantum mechanical quality for crystalline pentacene. 

Abstract Benzene fluorination increases chemoselectivities for Dewar‐benzenes via 4π‐disrotatory electrocyclization. However, the origin of the chemo‐ and regioselectivities of fluorobenzenes remains unexplained because of the experimental limitations in resolving the excited‐state structures on ultrafast timescales. The computational cost of multiconfigurational nonadiabatic molecular dynamics simulations is also currently cost‐prohibitive. We now provide high‐fidelity structural information and reaction outcome predictions with machine‐learning‐accelerated photodynamics simulations of a series of fluorobenzenes, C₆F_6‐nH_n, n=0–3, to study their S₁→S₀decay in 4 ns. We trained neural networks with XMS‐CASPT2(6,7)/aug‐cc‐pVDZ calculations, which reproduced the S₁absorption features with mean absolute errors of 0.04 eV (<2 nm). The predicted nonradiative decay constants for C₆F₄H₂, C₆F₆, C₆F₃H₃, and C₆F₅H are 116, 60, 28, and 12 ps, respectively, in broad qualitative agreement with the experiments. Our calculations show that a pseudo Jahn–Teller distortion of fluorinated benzenes leads to an S₁local‐minimum region that extends the excited‐state lifetimes of fluorobenzenes. The pseudo Jahn–Teller distortions reduce when fluorination decreases. Our analysis of the S₁dynamics shows that the pseudo‐Jahn–Teller distortions promote an excited‐statecis‐transisomerization of a π_C‐Cbond. We characterized the surface hopping points from our NAMD simulations and identified instantaneous nuclear momentum as a factor that promotes the electrocyclizations. 

Abstract Photochemical C−C coupling reactions can be tailored to industrial chemical processes and preparations of pharmaceuticals. Recent approaches in this area are limited to using precious transition metal coordination complexes that facilitate light absorption and redox processes with benchtop chemicals. Herein, we propose a paradigm that involves all‐in‐one organo‐photo‐auxiliaries,thio‐heteroarenes, which exhibit unique photophysical properties. Thesethio‐heteroarenes were employed to prepare several all‐in‐one ionic photo‐salts from commercially available alkyl/benzyl and heterocyclic halides via aromaticity‐mediated nucleophilic substitution reactions. From the library of >30 salts, we performed on‐demand photochemical C−C coupling reactions to isolate numerous symmetrical and unsymmetrical diary/alkyl‐ethanes with yields up to 84% and mass balance as high as 96%. We also investigated the influence of structural features/properties on the outcomes of the photochemical C−C coupling reactions. The current photochemical C−C method was successful in the isolation of >30 photoproducts, including the natural product Brittonin A, a precursor of Imipramine, and derivatives of the bioactive Honokiol Analogues. Furthermore, transient absorption spectroscopy and time‐dependent density functional theory calculations were used to decipher the nature of light‐promoted electronic transitions.