Binding water to a PEG-linked flexible bichromophore: IR spectra of diphenoxyethane-(H2O)n clusters, n = 2-4

Towards elucidating the photochemistry of the sunscreen filter ethyl ferulate using time-resolved gas-phase spectroscopy

Ultrafast time-resolved ion yield (TR-IY) and velocity map imaging spectroscopies are employed to reveal the relaxation dynamics after photoexcitation in ethyl 4-hydroxy-3-methoxycinnamate (ethyl ferulate, EF), an active ingredient in commercially available sunscreens. In keeping with a bottom-up strategy, the building blocks of EF, 2-methoxy-4-vinylphenol (MVP) and 4-hydroxy-3-methoxycinnamyl alcohol (coniferyl alcohol, ConA), were also studied to assist in our understanding of the dynamics of EF as we build up in molecular complexity. In contrast to the excited state dynamics of MVP and ConA, which are described by a single time constant (>900 ps), the dynamics of EF are described by three time constants (15 ± 4 ps, 148 ± 47 ps, and >900 ps). A mechanism is proposed involving internal conversion (IC) between the initially excited S₁(1¹ππ*) and S₂(1¹nπ*) states followed by intramolecular vibrational redistribution (IVR) on both states, in competition with intersystem crossing onto neighbouring triplet states (15 ± 4 ps). IVR and IC within the triplet manifold then ensues (148 ± 47 ps) to populate a low-lying triplet state (>900 ps). Importantly, the fluorescence spectrum of EF at the S₁ origin, along with the associated lifetime (6.9 ± 0.1 ns), suggests that population is trapped, during initial IVR, on the S₁(1¹ππ*) state. This serves to demonstrate the complex, competing dynamics in this sunscreen filter molecule.

The phenyl vinyl ether-methanol complex: a model system for quantum chemistry benchmarking

The structure of the isolated aggregate of phenyl vinyl ether and methanol is studied by combining a multi-spectroscopic approach and quantum-chemical calculations in order to investigate the delicate interplay of noncovalent interactions. The complementary results of vibrational and rotational spectroscopy applied in molecular beam experiments reveal the preference of a hydrogen bond of the methanol towards the ether oxygen (OH∙∙∙O) over the π-docking motifs via the phenyl and vinyl moieties, with an additional less populated OH∙∙∙P(phenyl)-bound isomer detected only by microwave spectroscopy. The correct prediction of the energetic order of the isomers using quantum-chemical calculations turns out to be challenging and succeeds with a sophisticated local coupled cluster method. The latter also yields a quantification as well as a visualization of London dispersion, which prove to be valuable tools for understanding the role of dispersion on the docking preferences. Beyond the structural analysis of the electronic ground state (S0), the electronically excited (S1) state is analyzed, in which a destabilization of the OH∙∙∙O structure compared to the S0 state is observed experimentally and theoretically.

The Structure of Diphenyl Ether-Methanol in the Electronically Excited and Ionic Ground States: A Combined IR/UV Spectroscopic and Theoretical Study

Diphenyl ether offers competing docking sites for methanol: the ether oxygen acts as a common hydrogen-bond acceptor and the π system of each phenyl ring allows for OH-π interactions driven by electrostatic, induction, and dispersion forces. Based on investigations in the electronic ground state (S₀ ), we present a detailed study of the electronically excited state (S₁ ) and the ionic ground state (D₀ ), in which an impact on the structural preference is expected compared with the S₀ state. Dispersion forces in the electronically excited state were analyzed by comparing the computed binding energies at the coupled-cluster-singles (CCS) and approximate coupled-cluster-singles-doubles levels of theory (CC2 approximation). By applying UV/IR/UV spectroscopy, we found a more strongly bound OH-π structure in the S₁ state compared with the S₀ state, in agreement with spin-component-scaled CC2 calculations. A structural rearrangement into a non-hydrogen-bonded structure takes places upon ionization in the D₀ state, which was revealed by using IR photodissociation spectroscopy and confirmed by theory.

Single Conformation Spectroscopy of Suberoylanilide Hydroxamic Acid: A Molecule Bites Its Tail

Suberoylanilide hydroxamic acid (SAHA) is a histone deacetylase inhibitor that causes growth arrest and differentiation of many tumor types and is an approved drug for the treatment of cancer. The chemical structure of SAHA consists of formanilide "head" and a hydroxamic acid "tail" separated by an n-hexyl chain, C₆H₅NH(C═O)-(CH₂)₆-(C═O)NHOH. The alkyl chain's preference for extended structures is in competition with tail-to-head (T-H) or head-to-tail (H-T) hydrogen bonds between the amide and hydroxamic acid groups. Laser desorption was used to bring SAHA into the gas phase and cool it in a supersonic expansion before interrogation with mass-resolved resonant two-photon ionization spectroscopy. Single conformation UV spectra in the S₀-S₁ region and infrared spectra in the hydride stretch and mid-IR regions were recorded using IR-UV hole-burning and resonant ion-dip infrared spectroscopy, respectively. Three conformers of SAHA were distinguished and spectroscopically characterized. Comparison of the experimental IR spectra with the predictions of density functional theory calculations (DFT, B3LYP D3BJ/6-31+G(d)) leads to assignments for the three conformers, all of which possess tightly folded alkyl chains that enable formation of a T-H (conformer A) or H-T (conformers B and C) hydrogen bonds. A modified version of the generalized Amber force field was developed to more accurately describe the hydroxamic acid OH internal rotor potential, leading to predictions for the relative energies in reasonable agreement with experiment. This force field was used to generate a disconnectivity graph for the low-energy portion of the potential energy landscape of SAHA. This disconnectivity graph contains more than one hundred minima and maps out the lowest-energy pathways between them, which could then be characterized via DFT calculations. This combination of force field and DFT calculations provides insight into the potential energy landscape and how population was funneled into the three observed conformers.

Multi-spectroscopic and theoretical analyses on the diphenyl ether–tert-butyl alcohol complex in the electronic ground and electronically excited state

Multi-spectroscopic and theoretical investigations on the isolated diphenyl ether–tert-butyl alcohol complex – an ideal benchmark system for theory with strongly competing OH–O and OH–π binding motifs.

Aromatic embedding wins over classical hydrogen bonding – a multi-spectroscopic approach for the diphenyl ether–methanol complex

A clear dispersion-enhanced preference for OH–π binding over the competing and more intuitive OH–O binding motif is observed.