Alexander Vdovin

Proton tunnelling in porphycene seeded in a supersonic jet

Abstract Porphycene, a constitutional isomer of porphyrin, was investigated in a supersonic jet expansion using laser-induced fluorescence and hole burning techniques. The lines recorded in the fluorescence excitation spectrum were split into doublets. No splitting was detected if one or both inner hydrogen atoms were replaced by deuterium. The doublet structure also disappeared upon formation of a complex between porphycene and water or alcohol. These findings indicate that the origin of the doublet structure in bare porphycene is the tunneling splitting due to the exchange of two inner protons between the nitrogen atoms. From the spectral response to deuteration, and to a change in cooling conditions, it is deduced that the tunneling splitting is higher in the ground state than in the first excited singlet state.

Mode-selective promotion and isotope effects of concerted double-hydrogen tunneling in porphycene embedded in superfluid helium nanodroplets.

Intramolecular double-hydrogen tunneling in porphycene (see picture) is investigated. Low-temperature conditions are ensured by doping of single molecules into superfluid helium nanodroplets. The investigation of fluorescence excitation and dispersed emission spectra and the highly dissipative environment allows the observation of mode-selective tunneling splitting and reveals a purely concerted tunneling mechanism for all isotopic variants of porphycene.

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.

Excited state proton transfer in jet-cooled 2,5-di-(2-benzoxazolyl)phenol

Abstract The molecule 2,5-di-(2-benzoxazolyl)phenol (DBP) has been studied by laser spectroscopy in a supersonic free jet. In the jet the fluorescence with a Stokes shift of 7000 cm −1 arises exclusively from the proton-transferred species. The fluorescence excitation spectrum of DBP exhibits the broad fluorescence background underlying some weak, sharp features. The first prominent band has been observed at 27830 cm −1 , but from 250 cm −1 above the origin the spectrum becomes congested and broad. OH→OD substitution of DBP produces a blue shift of 103 cm −1 . For the deuterated sample (DBP-d 1 ) resolved line structure in the fluorescence excitation spectrum has been observed up to an excess energy of 400 cm −1 . To identify spectral features corresponding to deuterated DBP and other species double-resonance depletion spectroscopy has been applied. From analysis of the width of the vibronic bands of DBP-d 1 , we estimated an upper limit of the excited state deuteron transfer rate constant, k DT =1.7×10 12 s −1 .

Photochemistry of 3-hydroxyflavone inside superfluid helium nanodroplets

3-Hydroxyflavone is a prototype system for excited state intramolecular proton transfer which is one step of a closed loop photocycle. It was intensively studied for the bare molecule and for the influence of solvents. In the present paper this photocycle is investigated for 3-hydroxyflavone and some hydrated complexes when doped into superfluid helium droplets by the combined measurement of fluorescence excitation spectra and dispersed emission spectra. Significant discrepancies in the proton transfer behavior to gas phase experiments provide evidence for the presence of different complex configurations of the hydrated complexes in helium droplets. Moreover, for bare 3-hydroxyflavone and its hydrated complexes the proton transfer appears to be promoted by the helium environment.

Electronic spectroscopy of molecules in superfluid helium nanodroplets: an excellent sensor for intramolecular charge redistribution.

Electronic spectra of molecules doped into superfluid (4)He nanodroplets reveal important details of the microsolvation in superfluid helium. The vibrational fine structure in the electronic spectra of phthalocyanine derivatives and pyrromethene dye molecules doped into superfluid helium droplets have been investigated. Together with previous studies on anthracene derivatives [J. Chem. Phys.2010, 133, 114505] and 3-hydroxyflavone [J. Chem. Phys.2009, 131, 194307], the line shapes vary between two limiting cases, namely, sharp Lorentzians and nonresolved vibrational fine structure. All different spectral signatures are initiated by the same effect, namely, the change of the electron density distribution initiated by the electronic excitation. This change can be quantified by the difference of the electrostatic moments of the molecule in the electronic ground state and the corresponding Franck-Condon point in the excited state. According to the experimental data, electronic spectroscopy suffers from drastic line broadening when accompanied by significant changes of the charge distribution, in particular, changes of the dipole moment. Vice versa, the vibrational fine structure in electronic spectra of molecules doped into helium droplets is highly sensitive to changes of the electron density distribution.

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.

Optical and mass selective laser spectroscopy of 9-methylanthracene and 9-cyanoanthracene and their molecular microclusters

Abstract Laser induced fluorescence (LIF) excitation and resonance enhanced two-photon ionization (R2PI) time of flight (TOF) spectroscopy was used to study the spectra of neutral solvent clusters of two anthracene derivatives: 9-methylanthracene (MA) and 9-cyanoanthracene (CA).

Microsolvation of molecules in superfluid helium nanodroplets revealed by means of electronic spectroscopy

The empirical model explaining microsolvation of molecules in superfluid helium droplets proposes a non-superfluid helium solvation layer enclosing the dopant molecule. This model warrants an empirical explanation of any helium induced substructure resolved for electronic transitions of molecules in helium droplets. Despite a wealth of such experimental data, quantitative modeling of spectra is still in its infancy. The theoretical treatment of such many-particle systems dissolved into a quantum fluid is a challenge. Moreover, the success of theoretical activities relies also on the accuracy and self-critical communication of experimental data. This will be elucidated by a critical resume of our own experimental work done within the last ten years. We come to the conclusion that spectroscopic data and among others in particular the spectral resolution depend strongly on experimental conditions. Moreover, despite the fact that none of the helium induced fine structure speaks against the empirical model for solvation in helium droplets, in many cases an unequivocal assignment of the spectroscopic details is not possible. This ambiguity needs to be considered and a careful and critical communication of experimental results is essential in order to promote success in quantitatively understanding microsolvation in superfluid helium nanodroplets.