Szymon Smolarek

IR Spectroscopy of Molecular Ions by Nonthermal Ion Ejection from Helium Nanodroplets

Infrared spectroscopy provides a means to determine the intrinsic geometrical structures of molecules. Here we present a novel spectroscopic method that uses superfluid helium nanodroplets to record IR spectra of cold molecular ions, in this particular case aniline cations. The method is based on the detection of ions that are ejected from the helium droplets following vibrational excitation of these ions. We find that spectra can be recorded with a high sensitivity and that they exhibit only a small matrix shift. The widths of the individual transitions depend on the excited vibrational level and are thought to be related to the interaction of the ion with the surrounding helium solvent shells.

Fast photodynamics of azobenzene probed by scanning excited-state potential energy surfaces using slow spectroscopy

Azobenzene, a versatile and polymorphic molecule, has been extensively and successfully used for photoswitching applications. The debate over its photoisomerization mechanism leveraged on the computational scrutiny with ever-increasing levels of theory. However, the most resolved absorption spectrum for the transition to S1(nπ*) has not followed the computational advances and is more than half a century old. Here, using jet-cooled molecular beam and multiphoton ionization techniques we report the first high-resolution spectra of S1(nπ*) and S2(ππ*). The photophysical characterization reveals directly the structural changes upon excitation and the timescales of dynamical processes. For S1(nπ*), we find that changes in the hybridization of the nitrogen atoms are the driving force that triggers isomerization. In combination with quantum chemical calculations we conclude that photoisomerization occurs along an inversion-assisted torsional pathway with a barrier of ~2 kcal mol−1. This methodology can be extended to photoresponsive molecular systems so far deemed non-accessible to high-resolution spectroscopy.

Conformational Flexibility of a Rotaxane Thread Probed by Electronic Spectroscopy in Helium Nanodroplets

Ultrahigh-resolution spectroscopic studies have been performed to elucidate the conformational landscape of the succinamide-based thread 1 that is frequently employed in mechanically interlocked molecular assemblies. We show how dissolving single molecules into a helium nanodroplet enables us to resolve the broad absorption spectrum--which is normally observed--into the separate contributions of individual conformers that are populated under the employed experimental conditions. Excellent agreement is obtained with the results of molecular dynamics calculations. The absorption spectrum of each conformer reveals a splitting of the zero-phonon resonance that is different for each conformer and could thus serve as a spectral signature.

Conformational Heterogeneity of Methyl 4-Hydroxycinnamate: A Gas-Phase UV-IR Spectroscopic Study

UV excitation and IR absorption spectroscopy on jet-cooled molecules is used to study the conformational heterogeneity of methyl 4-hydroxycinnamate, a model chromophore of the Photoactive Yellow Protein (PYP), and to determine the spectroscopic properties of the various conformers. UV-UV depletion spectroscopy identifies four different species with distinct electronic excitation spectra. Quantum chemical calculations argue that these species are associated with different conformers involving the s-cis/s-trans configuration of the ester with respect to the propenyl C-C single bond and the syn/anti orientation of the phenolic OH group. IR-UV hole-burning spectroscopy is used to record their IR absorption spectra in the fingerprint region. Comparison with IR absorption spectra predicted by quantum chemical calculations provides vibrational markers for each of the conformers, on the basis of which each of the species observed with UV-UV depletion spectroscopy is assigned. Although both DFT and wave function methods reproduce experimental frequencies, we find that calculations at the MP2 level are necessary to obtain agreement with experimentally observed intensities. To elucidate the role of the environment, we compare the IR spectra of the isolated conformers with IR spectra of methyl 4-hydroxycinnamate-water clusters, and with IR spectra of methyl 4-hydroxycinnamate in solution.

Spectroscopy and dynamics of methyl-4-hydroxycinnamate: the influence of isotopic substitution and water complexation

High-resolution Resonance Enhanced MultiPhoton Ionization (REMPI) and Laser Induced Fluorescence (LIF) excitation spectra of jet-cooled methyl-4-hydroxycinnamate, methyl-4-OD-cinnamate, and of their water clusters have been recorded. Whereas water complexation leads to significant linewidth narrowing, isotopic substitution does for all practical purposes not influence the excited-state dynamics. In this light, we evaluate two previously proposed decay channels of the photoexcited ππ* state involving the dissociative πσ* state (analogous to phenol) and involving the optically dark nπ* state (as concluded for para-coumaric acid). To come to an unambiguous interpretation of the REMPI studies, it has been necessary to determine ionization thresholds. For methyl-4-hydroxycinnamate and its water cluster values of 8.078 and 7.636 eV have been found. Apart from the electronic excitation studies, IR absorption studies have been performed as well. These studies provide important vibrational markers for the assignment of the various conformations that are present under molecular beam conditions, and offer a direct measure of the influence of hydrogen bonding on the properties of the hydroxyl group.

Absorption spectroscopy of adenine, 9-methyladenine, and 2-aminopurine in helium nanodroplets

High-resolution absorption spectra of adenine, 9-methyladenine and 2-aminopurine in helium nanodroplets have been recorded. In contrast to molecular beam experiments, large variations in linewidths are observed for adenine and 9-methyladenine. At the same time, the spectrum of 2-aminopurine remains sharp upon solvation in helium droplets. The line broadening observed for adenine and 9-methyladenine is attributed to a significant decrease of the lifetime of the (1)L(b)(ππ*) state and of (1)nπ* levels vibronically coupled to this state. The origin of the lifetime reduction is argued to be related to the increased accessibility of the (1)nπ*/(1)L(b)(ππ*) conical intersection upon solvation of these molecules in liquid helium.

Vibrational and Electronic Spectroscopy of the 4-Hydroxystyrene−CO2 Cluster and Its Hydrate: A para-Coumaric Acid Impostor

We report on the results of high-resolution spec-troscopic studies on the 4-hydroxystyrene-CO(2) cluster. We show that these clusters are generated upon heating of para-coumaric acid, the chromophore of the photoactive yellow protein (PYP), as the result of a thermal decarboxylation process. Since the mass of the cluster and the starting material are the same, standard mass-resolved multiphoton ionization spectroscopic methods do not suffice to distinguish these clusters from para-coumaric acid. Instead, more advanced methods that include various UV and IR depletion methods need to be applied. These methods, in combination with quantum chemical calculations, enable us to unravel the structural and spectroscopic properties of 4-hydroxystyrene-CO(2) as well as of its hydrate, 4-hydroxystyrene-CO(2)-H(2)O.