Photoswitching Molecules Functionalized with Optical Cycling Centers Provide a Novel Platform for Studying Chemical Transformations in Ultracold Molecules.

A novel molecular structure that bridges the fields of molecular optical cycling and molecular photoswitching is presented. It is based on a photoswitching molecule azobenzene functionalized with one and two CaO- groups, which can act as optical cycling centers (OCCs). This paper characterizes the electronic structure of the resulting model systems, focusing on three questions: (1) how the electronic states of the photoswitch are impacted by a functionalization with an OCC; (2) how the states of the OCC are impacted by the scaffold of the photoswitch; and (3) whether the OCC can serve as a spectroscopic probe of isomerization.

Elusive Arylalkylcarbenes in Solution and in Crystals: Facile 1,2-R (R = H, Ph) Migrations of 1,2,2-Triphenylethylidene.

Highly reactive arylalkylcarbenes generated in solution by photolysis of their aryldiazoalkane precursors tend to undergo competing inter- and intramolecular reactions to yield a complex mixture of products. Having previously shown the use of crystals to effectively control the reactivity of arylalkylcarbenes to afford high yields of a single product, it was of interest to investigate whether the crystalline environment could also enable spectroscopic detection of these intermediates en route to the photoproduct. Using 1,2,2-triphenyldiazoethane () as a model substrate to probe the effect of alternative reaction trajectories that yield triphenylethylene () by competing 1,2-H shift or 1,2-Ph migration, we report selectivities consistent with reaction from a spin-equilibrated carbene in solution, while reactions in crystals primarily afford alkene via a lattice-controlled 1,2-H shift.

Computational Study of Ground-State Destabilization Effects and Dipole-Dipole Interaction Energies in Amphidynamic Crystals.

Ground-state destabilization is a promising strategy to modulate rotational barriers in amphidynamic crystals. Density functional theory studies of polar phenylenes installed as rotators in pillared paddle-wheel metal organic frameworks were performed to investigate the effects of ground-state destabilization on their rotational dynamics. We found that as the steric size of phenylene substituents increases, the ground-state destabilization effect is also increased.

Kinetics of Photoreactive 4,4'-Dimethylbenzophenone Nanocrystals: Relative Contributions to Triplet Decay from Intermolecular H-Atom Transfer and Reductive Charge Transfer Quenching.

Crystals of 4,4'-dimethylbenzophenone () are known to react by intermolecular H atom transfer, followed by radical pair recombination. To determine the contribution of the H atom transfer reaction to the deactivation of the triplet ketone, transient absorption spectra and kinetics were obtained using aqueous nanocrystalline suspensions. Single-exponential lifetimes of ca.

Quantum chain amplification in nanocrystalline Dewar benzenes by intramolecular sensitization.

Quantum chain reactions are characterized by the formation of several photoproducts per photon absorbed ( > 1) and constitute a promising signal amplification mechanism. The triplet-sensitized isomerization of Dewar benzene is known to undergo quantum chain reactions characterized by an adiabatic valence-bond isomerization to the excited state of Hückel benzene, which is able to transfer its triplet energy to a new ground state Dewar benzene that reacts to continue the chain. Given that diffusion-mediated energy transfer is the chain-limiting event in solution, we demonstrate here that reactions in crystals are significantly more efficient by taking advantage of energy transfer by a presumed exciton delocalization mechanism.

Exceptionally Long Lifetimes of Strongly Entangled Acyl-Trityl Radical Pairs Photochemically Generated in Crystalline Trityl Ketones.

Triplet acyl-alkyl radical pairs generated by pulsed laser excitation within the constraints of their nanocrystalline ketone precursors were recently introduced as a potential platform for the robust and repeated instantiation of spin qubit pairs for applications in quantum information science. Here, we report the transient spectroscopy of a series of nanocrystalline trityl-alkyl and trityl-aryl ketones capable of generating correlated triplet radical pairs with persistent triphenylmethyl radicals forced to remain within bonding distances of highly reactive acyl radicals. Whereas triplet trityl-acyl radical pairs decay by competing product-forming decarbonylation and intersystem crossing, triplet trityl-benzoyl radical pairs have lifetimes of up to ca.

A Green Chemistry Approach toward the Stereospecific Synthesis of Densely Functionalized Cyclopropanes via the Solid-State Photodenitrogenation of Crystalline 1-Pyrazolines.

The cyclopropane ring features prominently in active pharmaceuticals, and this has spurred the development of synthetic methodologies that effectively incorporate this highly strained motif into such molecules. As such, elegant solutions to prepare densely functionalized cyclopropanes, particularly ones embedded within the core of complex structures, have become increasingly sought-after. Here we report the stereospecific synthesis of a set of cyclopropanes with vicinal quaternary stereocenters via the solvent-free solid-state photodenitrogenation of crystalline 1-pyrazolines.

Evaluation of the photodecarbonylation of crystalline ketones for the installation of reverse prenyl groups on the pyrrolidinoindoline scaffold.

We report synthetic efforts toward the regiocontrolled installation of the prenyl moiety in debromoflustramine A by the regiospecific photodecarbonylation of a prenyl-substituted ketone. Synthetic approaches to access the plausible photodecarbonylation substrates beginning from tryptamine were evaluated. Initial attempts to synthesize a suitable substrate for photodecarbonylation were hampered by a lack of substrate crystallinity (a prerequisite for solid-state photochemistry).

Slip/Stick Viscosity Models of Nanoconfined Liquids: Solvent-Dependent Rotation in Metal-Organic Frameworks.

Artificial molecular machines are expected to operate in environments where viscous forces impact molecules significantly. With that, it is well-known that solvent behaviors dramatically change upon confinement into limited spaces as compared to bulk solvents. In this study, we demonstrate the utility of an amphidynamic metal-organic framework with pillars consisting of H-labeled dialkynyltriptycene and dialkynylphenylene barrierless rotators that operate as NMR sensors for solvent viscosity.

Rotational Dynamics of an Amphidynamic Zirconium Metal-Organic Framework Determined by Dielectric Spectroscopy.

A zirconium metal-organic framework with a difluorophenylene rotator bearing a permanent electric dipole of ∼3.2 D was synthesized, and its rotational motion was analyzed by temperature- and frequency-dependent broadband dielectric spectroscopy. While solid-state NMR confirms fast rotation qualitatively, the dissipation factors measured between 113 and 153 K suggested an activation energy = 2.

Correlated motion and mechanical gearing in amphidynamic crystalline molecular machines.

In this review we highlight the recent efforts towards the development of molecular gears with an emphasis on building molecular gears in the solid state and the role that molecular gearing and correlated motions may play in the function of crystalline molecular machines. We discuss current molecular and crystal engineering strategies, challenges associated with engineering correlated motion in crystals, and outline experimental and theoretical tools to explore gearing dynamics while highlighting key advances made to date.

Strongly Entangled Triplet Acyl-Alkyl Radical Pairs in Crystals of Photostable Diphenylmethyl Adamantyl Ketones.

Radical pairs generated in crystalline solids by bond cleavage reactions of triplet ketones offer the unique opportunity to explore a frontier of spin dynamics where rigid radicals are highly entangled as the result of short inter-radical distances, large singlet-triplet energy gaps (Δ), and limited spin-lattice relaxation mechanisms. Here we report the pulsed laser generation and detection of strongly entangled triplet acyl-alkyl radical pairs generated in nanocrystalline suspensions of 1,1-diphenylmethyl 2-ketones with various 3-admantyl substituents. The sought-after triplet acyl-alkyl radical pairs could be studied for the first time in the solid state by taking advantage of the efficient triplet excited state α-cleavage reactions of 1,1-diphenylmethyl ketones and the slow rate of CO loss from the acyl radicals, which would have to generate highly unstable phenyl and primary alkyl radicals or relatively unstable secondary and tertiary alkyl radicals.

Enhanced Gearing Fidelity Achieved Through Macrocyclization of a Solvated Molecular Spur Gear.

Molecular spur gear dynamics with high gearing fidelity can be achieved through a careful selection of constituent molecular components that favorably position and maintain the two gears in a meshed configuration. Here, we report the synthesis of a new macrocyclic molecular spur gear with a bibenzimidazole stator combined with a second naphthyl bis-gold-phosphine gold complex stator to place two 3-fold symmetric 9,10-diethynyl triptycene cogs at the optimal distance of 8.1 Å for gearing.

Taming Radical Pairs in the Crystalline Solid State: Discovery and Total Synthesis of Psychotriadine.

Solid-state photodecarbonylation is an attractive but underutilized methodology to forge hindered C-C bonds in complex molecules. This study discloses the use of this reaction to assemble the vicinal quaternary stereocenter motif present in bis(cyclotryptamine) alkaloids. Our strategy was enabled by experimental and computational investigations of the role of substrate conformation on the success or failure of the solid-state photodecarbonylation reaction.

Encapsulating -Heterocyclic Carbene Binuclear Transition-Metal Complexes as a New Platform for Molecular Rotation in Crystalline Solid-State.

In crystalline solids, molecules generally have limited mobility due to their densely packed environment. However, structural information at the molecular level may be used to design amphidynamic crystals with rotating elements linked to rigid, lattice-forming parts, which may lead to molecular rotary motions and changes in conformation that determine the physical properties of the solid-state materials. Here, we report a novel design of emissive crystalline molecular rotors with a central pyrazine rotator connected by implanted transition metals (Cu or Au) to a readily accessible enclosure formed by two -heterocyclic carbenes (NHC) in discrete binuclear complexes.

Scalable Synthesis of Vicinal Quaternary Stereocenters via the Solid-State Photodecarbonylation of a Crystalline Hexasubstituted Ketone.

We report the synthesis and stereospecific solid-state photodecarbonylation of a hexasubstituted ketone featuring six distinct α-substituents. The photoproduct of the solid-state transformation features vicinal all-carbon quaternary stereocenters. While reactions carried out in bulk powders and aqueous crystalline suspensions were complicated by secondary photochemistry of the primary photoproduct, optimal conditions provided good yields and recyclable starting material.

2D Arrays of Organic Qubit Candidates Embedded into a Pillared-Paddlewheel Metal-Organic Framework.

The creation of ordered arrays of qubits that can be interfaced from the macroscopic world is an essential challenge for the development of quantum information science (QIS) currently being explored by chemists and physicists. Recently, porous metal-organic frameworks (MOFs) have arisen as a promising solution to this challenge as they allow for atomic-level spatial control of the molecular subunits that comprise their structures. To date, no organic qubit candidates have been installed in MOFs despite their structural variability and promise for creating systems with adjustable properties.

Discovery and Total Synthesis of a Bis(cyclotryptamine) Alkaloid Bearing the Elusive Piperidinoindoline Scaffold.

Bis(cyclotryptamine) alkaloids have been popular topics of study for many decades. Five possible scaffolds for bis(cyclotryptamine) alkaloids were originally postulated in the 1950s, but only four of these scaffolds have been observed in natural products to date. We describe synthetic access to the elusive fifth scaffold, the piperidinoindoline, through syntheses of compounds now termed "dihydropsychotriadine" and "psychotriadine".

Computational Investigation into Ligand Effects on Correlated Geared Dynamics in Dirhodium Supramolecular Gears-Insights Beyond the NMR Experimental Window.

The rotational dynamics of dirhodium supramolecular gears, formed with four 9-triptycene carboxylates cyclically arranged around a dirhodium core with variable axial ligands as originally designed by Shionoya , provide an excellent opportunity to evaluate the potential of computational methods and expand our understanding of the factors determining geared dynamics. Rotational dynamic rates in these structures depend on the nature of the axial ligand as shown by simulations over timescales that are not accessible experimentally. Molecular dynamics simulations gave information on the gearing mechanism, and the activation barriers to gearing were calculated using density functional theory.

Fluorescence Anisotropy Decay of Molecular Rotors with Acene Rotators in Viscous Solution.

Herein, we report the use of fluorescence anisotropy decay for measuring the rotation of six shape-persistent molecular rotors with central naphthalene (), anthracene (, , and ), tetracene (), and pentacene () rotators axially linked by triple bonds to bulky trialkylsilyl groups of different sizes. Steady-state and time-resolved polarization measurements carried out in mineral oil confirmed that the vibrationally resolved lowest-energy absorption bands are characterized by a transition dipole moment oriented along the short acene axes, in the direction of the alkyne linkers. Fluorescence lifetimes increased significantly with increasing acene size and moderately with a decrease in the size of the trialkylsilyl group.

Molecular Spur Gears with Triptycene Rotators and a Norbornane-Based Stator.

We report efforts to prepare a molecular spur gear utilizing a convenient synthesis of a norbornane stator that positions two interdigitated diyne-linked triptycenes in parallel alignment. While gearing was not observed by F NMR for a -CF-labeled analog at temperatures as low as 213 K, we used molecular dynamics simulation and 2D metadynamics calculations to understand the gearing/slippage energetic profile for various molecular spur gears to guide future designs of these systems.

Kinetic Control in the Synthesis of a Möbius Tris((ethynyl)[5]helicene) Macrocycle Using Alkyne Metathesis.

The synthesis of conjugated Möbius molecules remains elusive since twisted and macrocyclic structures are low-entropy species sporting their own synthetic challenges. Here we report the synthesis of a Möbius macrocycle in 84% yield via alkyne metathesis of 2,13-bis(propynyl)[5]helicene. MALDI-MS, NMR spectroscopy, and X-ray diffraction indicated a trimeric product of twofold symmetry with / configurations in the helicene subunits.