Ultrafast photofragment ion spectroscopy of the Wolff rearrangement in 5-diazo Meldrum's acid

An ultraviolet and electric field activated photopolymer-ferroelectric nanoparticle composite for the performance enhancement of triboelectric nanogenerators

For the development of high performance triboelectric generators (TENGs), it is required to have facile methods to adjust the triboelectric properties of the friction surfaces. In this work, we present the surface charge density modulation of the photopolymer-ferroelectric nanoparticle composite surface by applying ultraviolet (UV) and electric field. By using the photopolymer, the triboelectric surface property was modulated by exposure to UV. In addition, lithographic surface patterning can be easily adopted to enlarge the frictional surface area as well. Furthermore, the use of the PP allows a facile integration of ferroelectric nanoparticles (NPs) in the form of a nanocomposite structure, which can effectively increase the surface charge density by spontaneous dipole coupling of NPs embedded in the PP layer. As a result, approximately 4-fold higher output power has been achieved by applying this approach. The developed TENGs have also demonstrated superior mechanical durability, generating consistent outputs during 10⁴ cyclic frictional contacts. The approach proposed here is a simple and reliable way to enhance the output performance of TENGs.

An excited-state Wolff rearrangement reaction of 5-diazo Meldrum's acid: an ab initio on-the-fly nonadiabatic dynamics simulation

A global switching on-the-fly trajectory surface hopping dynamics simulation at the 3SA-CASSCF(12,11)/6-31G* quantum level has been employed to probe the photo-induced Wolff rearrangement (WR) reaction of 5-diazo Meldrum's acid (DMA) within three low-lying electronic excited states. The present simulation predicted that the branching ratios for relaxing back to the ground state, isomerizing to diazirine, and reaction to ketene I via carbene I are 69% ± 0.1, 3% ± 0.4, and 28% ± 0.1, which are in excellent agreement with those obtained by the femtosecond spectroscopy experiment, 67%, 3% and 30%, respectively. In particular, the present simulation revealed that the major WR reaction to ketene I pathway is stepwise via the excited-state to carbene I (17.8% ± 0.2) and via the ground-state to carbene I (8.7% ± 0.2), and the minor pathway is concerted synchronous (1.5% ± 0.6). The photo-induced WR reaction of DMA has been quantitatively interpreted in terms of the distribution of extended seam surfaces as a function of CN dissociation bonds for two important conical intersections within three low-lying electronic excited states. Ultrafast dynamic time constants have been estimated to be about 500 fs ± 120 fs and 180 fs ± 80 fs for the stepwise and the concerted WR reaction to ketene I which are also in good agreement with those determined by the experiment. Therefore, the photo-induced excited-state WR reaction mechanism has been quantitatively revealed by the present real-time dynamics simulation.

Competitive solvent-molecule interactions govern primary processes of diphenylcarbene in solvent mixtures

Photochemical reactions in solution often proceed via competing reaction pathways comprising intermediates that capture a solvent molecule. A disclosure of the underlying reaction mechanisms is challenging due to the rapid nature of these processes and the intricate identification of how many solvent molecules are involved. Here combining broadband femtosecond transient absorption and quantum mechanics/molecular mechanics simulations, we show for one of the most reactive species, diphenylcarbene, that the decision-maker is not the nearest solvent molecule but its neighbour. The hydrogen bonding dynamics determine which reaction channels are accessible in binary solvent mixtures at room temperature. In-depth analysis of the amount of nascent intermediates corroborates the importance of a hydrogen-bonded complex with a protic solvent molecule, in striking analogy to complexes found at cryogenic temperatures. Our results show that adjacent solvent molecules take the role of key abettors rather than bystanders for the fate of the reactive intermediate.

Photochemistry in solution often involves coexisting reaction channels that may comprise intermediates capturing a solvent molecule. Here, the authors show for one of the most reactive species, diphenylcarbene, that the decision-maker is not the nearest solvent molecule but its neighbour.

Early events in the photochemistry of 5-diazo Meldrum's acid: formation of a product manifold in C–N bound and pre-dissociated intersection seam regions

Photolysis of 5-diazo Meldrum's acid yields three different primary products in a sub-ps scale. High-level computations show that this is due to the presence of an extended seam of intersection associated to different reaction paths.

Decomposition of diazomeldrum's acid: a threshold photoelectron spectroscopy study

Derivatives of meldrum's acid are known precursors for a number of reactive intermediates. Therefore, we investigate diazomeldrum's acid (DMA) and its pyrolysis products by photoionization using vacuum ultraviolet (VUV) synchrotron radiation. The threshold photoelectron spectrum of DMA yields an ionization energy (IE) of 9.68 eV. Several channels for dissociative photoionization are observed. The first one is associated with loss of CH3, leading to a daughter ion with m/z = 155. Its appearance energy AE0K was determined to be 10.65 eV by fitting the experimental data using statistical theory. A second parallel channel leads to m/z = 69, corresponding to N2CHCO, with an AE0K of 10.72 eV. Several other channels open up at higher energy, among them the formation of acetone cation, a channel expected to be the result of a Wolff-rearrangement (WR) in the cation. When diazomeldrum's acid is heated in a pyrolysis reactor, three thermal decomposition pathways are observed. The major one is well-known and yields acetone, N2 and CO as consequence of the WR. However, two further channels were identified: The formation of 2-diazoethenone, NNCCO, together with acetone and CO2 as the second channel and E-formylketene (OCCHCHCO), propyne, N2 and O2 as a third one. 2-Diazoethenone and E-formylketene were identified based on their threshold photoelectron spectra and accurate ionization energies could be determined. Ionization energies for several isomers of both molecules were also computed. One of the key findings of this study is that acetone is observed upon decomposition of DMA in the neutral as well as in the ion and both point to a Wolff rearrangement to occur. However, the ion is subject to other decomposition channels favored at lower internal energies.