Nanoscale localization of electromagnetic fields using metallic nanostructures can catalyze chemical reactions in their immediate vicinity. Local optical field confinement and enhancement is also exploited to attain single-molecule detection sensitivity in surface-and tip-enhanced Raman (TER) spectroscopy. In this work, we observe and investigate the sporadic formation of 4-nitrobenzenethiolate upon TER imaging of a 4-nitrobenzenethiol (4NBT) monolayer on Au(111). Density functional theory (DFT), finite-difference timedomain (FDTD), and finite element method (FEM) calculations together confirm that this chemical reaction does not occur as a result of thermal desorption of the molecule, which requires temperatures in excess of 2100 K at the tip−sample junction. Our combined experimental and theoretical analyses strongly suggest that the chemical transformations observed throughout the course of TERS mapping is not driven by plasmonic photothermal heating, but rather by plasmon-induced hot carriers.

Photochemical reactions are widely used by academia and industry to construct complex molecular architectures via mechanisms that are often inaccessible by other means.

Raman and infrared (IR) are two complementary vibrational spectroscopy techniques that enable labelfree, noninvasive, and nondestructive structural characterization of analyzed specimens. IR spectroscopy is broadly utilized in various research areas ranging from food chemistry and agriculture to geology and medicine. Recently, Raman spectroscopy (RS) has attracted the interest of researchers from these fields because of its minimal interference from water and significant reduction of equipment costs. In this study, we evaluated the complementarity of RS and IR for structural characterization of epicuticular waxes, sophisticated chemical mixtures of long fatty acids, and their derivatives excreted by plants. We show that IR can sense R−O−H vibrations, which are characteristic for alcohols and sugars, as well as carbonyl groups, which can be assigned to aldehydes, ketones, acids, and esters in the chloroform-extracted waxes of Sorghum bicolor. At the same time, RS can detect only C−C, C−H, and CH 2 vibrations of these molecules in the same wax extracts. Using theoretical calculations, we were able to elucidate the origin of this phenomenon. It was found that an increase in the aliphatic chain length in carboxylic acids resulted in a quadratic and linear increase in the intensity of aliphatic vibrations in the corresponding Raman and IR spectra, respectively. Thus, the complementary of RS and IR that holds for small molecules may not be always observed for large biological molecules.

We report a novel reductive desulfurization reaction involving π-acidic naphthalene diimides (NDI) 1 using thionating agents such as Lawesson's reagent. Along with the expected thionated NDI derivatives 2-6, new heterocyclic naphtho-p-quinodimethane compounds 7 depicting broken/reduced symmetry were successfully isolated and fully characterized. Empirical studies and theoretical modeling suggest that 7 was formed via a six-membered ring oxathiaphosphenine intermediate rather than the usual four-membered ring oxathiaphosphetane of 2-6. Aside from the reduced symmetry in 7 as confirmed by single-crystal XRD analysis, we established that the ground state UV-vis absorption of 7 is red-shifted in comparison to the parent NDI 1. This result was expected in the case of thionated polycyclic diimides. However, unusual low energy transitions originate from Baird 4nπ aromaticity of compounds 7 in lieu of the intrinsic Hückel (4n + 2)π aromaticity as encountered in NDI 1. Moreover, complementary theoretical modeling results also corroborate this change in aromaticity of 7. Consequently, photophysical investigations show that, compared to parent NDI 1, 7 can easily access and emit from its T state with a phosphorescence (7a)* lifetime of τ = 395 μs at 77 K indicative of the formation of the corresponding "aromatic triplet" species according to the Baird's rule of aromaticity.

Optical imaging in the shortwave infrared region (SWIR, 1000−2000 nm) provides high-resolution images in complex systems. Here we explore substituent placement on dimethylamino flavylium polymethine dyes, a class of SWIR fluorophores. We find that the position of the substituent significantly affects the λ max and fluorescence quantum yield. Quantum-mechanical calculations suggest that steric clashes control the extent of π-conjugation. These insights provide design principles for the development of fluorophores for enhanced SWIR imaging.

Photochemical [2+2]-photocycloadditions are used to efficiently access strained organic molecular architectures, storing solar energy in chemical bonds. Functionalized [3]-ladderenes have been shown to undergo [2+2]-photocycloadditions to afford cubanes, an energy-dense class of organic molecules. The substituents (e.g., methyl, trifluoromethyl, and cyclopropyl) affect the overall reactivities of these cubane precursors leading to a yield from 1% to 48%. We now integrate single and multireference calculations and our machine-learning-accelerated nonadiabatic molecular dynamics (ML-NAMD) to understand how substituents affect the mechanistic photodynamics of [2+2]-photocycloadditions. Our calculations show that steric clashes destabilize the 4π-electrocyclic ring-opening pathway and minimum energy conical intersections by 0.72-1.15 eV and reaction energies by 0.68-2.34 eV. In contrast, favorable dispersive interactions stabilize the [2+2]-photocycloaddition pathway, lower the conical intersection energies by 0.31-0.85 eV and cubane reaction energies by 0.03-0.82 eV. The 2 ps ML-NAMD trajectories reveal that closed-shell repulsions block a 6π-conrotatory electrocyclic ring-opening pathway with increasing steric bulk. 57% of the methyl-substituted [3]-ladderene trajectories proceed through the 6π-conrotatory electrocyclic ring-opening, whereas the trifluoromethyl-and cyclopropylsubstituted 3-ladderenes chemoselectively proceed through [2+2]-photocycloaddition pathways. The predicted cubane yields (H: 0.4% < CH3: 1% < CF3: 14% < cPr: 20%) match the experimental trend; these substituents pre-distort the reactants to resemble the conical intersection leading to cubane.

The electronic structure, reduction limits, and coordination abilities of a bowl-shaped polycyclic aromatic hydrocarbon, indacenopicene (C 26 H 12 , 1), have been investigated for the first time using a combination of theoretical and experimental tools. A direct comparison with the prototypical corannulene bowl (C 20 H 10 , 2) revealed the effects of carbon framework topology and symmetry change on the electronic properties and aromaticity of indacenopicene. The accessibility of two reduction steps for 1 was predicted theoretically and then confirmed experimentally. Two reversible one-electron reduction processes with the formal reduction potentials at −1.92 and −2.29 V vs Fc +/0 were detected by cyclic voltammetry measurements, demonstrating the stability of the corresponding mono-and dianionic states of 1. The products of the doubly reduced indacenopicene have been isolated as rubidium and cesium salts and fully characterized. Their X-ray diffraction study revealed the formation of tetranuclear organometallic building blocks with the [M 2 (18-crown-6)] 2+ (M = Rb (3) and Cs (4)) cations occupying the concave cavities of two C 26 H 12 2− anions. The coordination of two outside exo-bound rubidium ions is terminated by crown ether molecules in 3 to form the discrete [Rb + 4 (18-crown-6) 3 (C 26 H 12 2− ) 2 ] tetramer. In contrast, the larger cesium ions allow the 1D polymeric chain propagation in 4 to afford [Cs + 2 (18-crown-6) 2 (THF)(C 26 H 12 2−)] ∞ .

Metrics & MoreArticle Recommendations * sı Supporting Information CONSPECTUS: Photochemical reactions are of great importance in chemistry, biology, and materials science because they take advantage of a renewable energy source, mild reaction conditions, and high atom economy. Light absorption can excite molecules to a higher energy electronic state of the same spin multiplicity. The following nonadiabatic processes induce molecular transformations that afford exotic molecular architectures and high-energy-isomers that are inaccessible by thermal means. Computational simulations now complement time-resolved instrumentation to reveal ultrafast excited-state mechanistic information for photochemical reactions that is essential in disentangling elusive spectroscopic features, excitedstate lifetimes, and excited-state mechanistic critical points. Nonadiabatic molecular dynamics (NAMD), powered by surface hopping techniques, is among the most widely applied techniques to model the photochemical reactions of medium-sized molecules. However, the computational efficiency is limited because of the requisite thousands of multiconfigurational quantum-chemical calculations multiplied by hundreds of trajectories. Machine learning (ML) has emerged as a revolutionary force in computational chemistry to predict the outcome of the resource-intensive multiconfigurational calculations on the fly. An ML potential trained with a substantial set of quantum-chemical calculations can predict the energies and forces with errors under chemical accuracy at a negligible cost. The integration of ML potentials in NAMD dramatically extends the maximum simulation time scale by ∼10 000fold to the nanosecond regime.In this Account, we present a comprehensive demonstration of ML photodynamics simulations and summarize our most recent applications in resolving complex photochemical reactions. First, we address three fundamental components of ML techniques for photodynamics simulations: the quantum-chemical data set, the ML potential, and NAMD. Second, we describe best practices in building training data and our procedure toward training the ML photodynamics model with our recent literature contributions. We introduce a convenient training data generation scheme combining Wigner sampling and geometrical interpolation. It trains reliable and effective ML potentials suitable for subsequent active learning to detect undersampled data. We demonstrate how active learning automatically discovers new mechanistic pathways and reproduces experimental results. We point out that atomic permutation is an essential data augmentation approach to improve the learnability of distance-based molecular descriptors for highly symmetric molecules. Third, we demonstrate the utility of ML-photodynamics by showing the results of ML photodynamics simulations of (1) photo-torquoselective 4π disrotatory electrocyclic ring closing of norbornyl cyclohexadiene, which reveals a thermal conversion from experimentally unobserved intermediates to the reactant in 1 ns; (2) [2 + 2] ph...