Mode‐Selective Promotion and Isotope Effects of Concerted Double‐Hydrogen Tunneling in Porphycene Embedded in Superfluid Helium Nanodroplets

Abstract

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.

Vibrations and hydrogen bonding in porphycene

“…The experiments performed for porphycenes isolated in the jet or embedded in helium droplets revealed tunneling splittings of which the values were strongly mode-dependent. 15,20,31 The present findings clearly indicate the multidimensional character of the internal hydrogens motions. Time-resolved vibrational spectroscopy experiments in the condensed phase are planned, with the goal of obtaining a detailed picture of intramolecular mode coupling.…”

“…The studies of Pc in the regime of molecular isolation, either in supersonic jets 15,31 or in helium nanodroplets, 20 revealed tunneling splittings in electronic absorption and fluorescence, due to delocalization of the internal hydrogens. These splittings are vibrationally mode-selective.…”

Ground and Excited State Double Hydrogen Transfer in Symmetric and Asymmetric Potentials: Comparison of 2,7,12,17‐Tetra‐n‐propylporphycene with 9‐Acetoxy‐2,7,12,17‐tetra‐n‐propylporphycene

“…Due to a much lower barrier for the transfer, the splitting due to coherent double hydrogen tunneling has been observed for porphycenes isolated in supersonic jets [18,19] and in helium nanodroplets. [20] We have recently developed a technique, based on polarized femtosecond pump-probe spectroscopy, which makes it possible to determine the rate constant of tautomerization in a condensed, rigid environment. [21,22] The rate constants for both ground and lowest excited electronic states have been determined for the parent porphycene and several derivatives.…”

“…Experimentally, such behavior has been confirmed by the values of tunneling splittings observed for parent porphycene, much smaller in S 1 than in S 0 . [19,20] In our calculations, only trans tautomeric forms have been explored. Since for 1 and 2 there is no experimental evidence for the presence of cis tautomers, the structures with the inner hydrogen atoms residing on adjacent pyrrole moieties, such forms were not included.…”

Ultrafast Dynamics in Helium Nanodroplets Probed by Femtosecond Time-Resolved EUV Photoelectron Imaging

“…[1][2][3] Today, clusters of 4 He atoms are probably best known for their use as a superfluid host that enables spectroscopic studies on regular and exotic species at temperatures of ~0.4 K. [4][5][6][7][8][9][10][11][12] He droplets exhibit a unique combination of cryogenic temperatures, superfluidity, nanoscale dimensions, excellent dopant pickup capability, high effective heat capacity, and high electronic excitation energies. These properties continue to inspire a range of proven and proposed applications, from providing nano-scale reaction chambers 13,14 to hosting biological samples in ultrafast single shot diffractive imaging experiments at 4th…”