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Conical intersections control excited state reactivity, and thus, elucidating and predicting their geometric and energetic characteristics are crucial for understanding photochemistry. Locating these intersections requires accurate and efficient electronic structure methods. Unfortunately, the most accurate methods (e.g., multireference perturbation theories such as XMS-CASPT2) are computationally challenging for large molecules. The state-interaction state-averaged restricted ensemble referenced Kohn–Sham (SI-SA-REKS) method is a computationally efficient alternative. The application of SI-SA-REKS to photochemistry was previously hampered by a lack of analytical nuclear gradients and nonadiabatic coupling matrix elements. We have recently derived analytical energy derivatives for the SI-SA-REKS method and implemented the method effectively on graphical processing units. We demonstrate that our implementation gives the correct conical intersection topography and energetics for several examples. Furthermore, our implementation of SI-SA-REKS is computationally efficient, with observed sub-quadratic scaling as a function of molecular size. This demonstrates the promise of SI-SA-REKS for excited state dynamics of large molecular systems. [ABSTRACT FROM AUTHOR]

Molecular emitters simultaneously generating light at different wavelengths have wide applications. With a small molecule, however, it is challenging to realize two independent radiative pathways. We invented the first examples of dual‐emissive single‐benzene fluorophores (SBFs). Two emissive tautomers are generated by synthetic modulation of the hydrogen bond acidity, which opens up pathways for excited‐state proton transfer. White light is produced by a delicate balance between the energy and intensity of the emission from each tautomer. We show that the excited‐state antiaromaticity of the benzene core itself dictates the proton movements driving the tautomer equilibrium. Using this simple benzene platform, a fluorinated SBF was synthesized with a record high solubility in perfluorocarbon solvents. White light‐emitting devices and multicolor imaging of perfluorocarbon nanodroplets in live cells demonstrate the practical utility of these molecules. [ABSTRACT FROM AUTHOR]

Quantum-chemical computational methods are benchmarked for their ability to describe conical intersections in a series of organic molecules and models of biological chromophores. Reference results for the geometries, relative energies, and branching planes of conical intersections are obtained using ab initio multireference configuration interaction with single and double excitations (MRCISD). They are compared with the results from more approximate methods, namely, the state-interaction state-averaged restricted ensemble-referenced Kohn-Sham method, spin-flip time-dependent density functional theory, and a semiempirical MRCISD approach using an orthogonalization-corrected model. It is demonstrated that these approximate methods reproduce the ab initio reference data very well, with root-mean-square deviations in the optimized geometries of the order of 0.1 À or less and with reasonable agreement in the computed relative energies. A detailed analysis of the branching plane vectors shows that all currently applied methods yield similar nuclear displacements for escaping the strong non-adiabatic coupling region near the conical intersections. Our comparisons support the use of the tested quantum-chemical methods for modeling the photochemistry of large organic and biological systems. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ehrenfest Force F (r) applied to the two C-O ring-opening photo-reactions of oxirane. • Ehrenfest Force trajectories and hybrid stress tensor trajectory calculated with the F (r) BCP s. • Torsion of the CH 2 group facilitated the photo-reaction pathway towards the CI. • Torsion of CH 2 group induced symmetry breaking evident from S 0 and S 1 F (r) trajectories. • Agreement found between the Ehrenfest Force and hybrid stress tensor trajectories. Ehrenfest Force F (r) trajectories were constructed for the C-O ring-opening photo-reactions of oxirane. The F (r) trajectories were constructed in an eigenvector-space corresponding to bond-flexing, bond-twist and bond-anharmonicity associated with the least and most preferred directions of charge density accumulation and bond critical point (BCP) sliding respectively. The presence of the torsion of a CH 2 group for one of the photo-reactions led to greater symmetry breaking and greater reaction pathway preference. Consistency was found from hybrid Ehrenfest Force F (r) trajectories that were constructed using the Ehrenfest Force F (r) BCP s and the stress tensor eigenvectors. [ABSTRACT FROM AUTHOR]

The conical intersections corresponding to the C─O and C─C ring opening were optimized and the reaction paths traversing these intersections were obtained. Investigation of the C─O ring opening revealed that when traversing the lowest energy conical intersection, the reaction path returns to the closed ring geometry. The C─O path traversing the intersection featuring torsion of terminal CH2 group however, led to a ring‐opened geometry, an H‐shift and the formation of acetaldehyde that can undergo further dissociation. The observation of different reaction paths was explained by the 3‐D paths from quantum theory of atoms in molecules (QTAIM) that defined the most preferred direction of electronic motion that precisely tracked the mechanisms of bond breaking and formation throughout the photo‐reactions. The size, orientation, and location of these most preferred 3‐D paths indicated the extent and direction of motion of atoms, bonds, and the degree of torsion or planarity of a bond indicating a predictive ability. [ABSTRACT FROM AUTHOR]

Graphical abstract Top (photoreaction pathway leading back to the ring-closed reactant): QTAIM bond-path framework sets for the S 0 state; B 0 = { p 0 , q 0 , r 0 } (left panel), the S 1 state; B 1 = { p 1 , q 1 , r 1 } (middle panel) corresponding a to nudged elastic band (NEB) path length = 13.18 Å. Variation of the local total energy density H (r b) for the S 0 and S 1 states with the NEB of the fissile bond (right panel). Bottom: The corresponding results for the photoreaction pathway leading forward to the ring-opened product. Highlights • QTAIM provided an explanation of the direction of the ring-opening photoreactions. • Bond-path framework sets B compared for MEPs of ring-opening photoreactions in DHCL. • Less efficient MEP explained by "sticky" ring-opening bond resisting ring-opening. • No oscillation in chemical character of ring-opening bond of MEP that led to reactant. • Stress tensor stiffness confirmed explanation of ring-opening pathways. Abstract The factors underlying two possible pathways for the Dihydrocostunolide (DHCL) photochemical ring-opening reaction were investigated; the first pathway returned to the ring-closed conformation of the reactant and the second pathway progressed to the ring-opened product. High-level multi-reference DFT methods were used to optimize the two pathways and the density was analyzed using QTAIM and the stress tensor. Oscillations in the chemical character of the fissile bond were found for the first pathway before and after the conical intersection that steered the reaction back to reactant. Conversely, this behavior was absent for the second pathway that led forward to the product. [ABSTRACT FROM AUTHOR]