Structure determination of the tetracene dimer in helium nanodroplets using femtosecond strong-field ionization.

Constant Schouder,Florent Calvo,Adam S Chatterley,Lars Christiansen,Henrik Stapelfeldt

doi:10.1063/1.5118005

Abstract

Dimers of tetracene molecules are formed inside helium nanodroplets and identified through covariance analysis of the emission directions of kinetic tetracene cations stemming from femtosecond laser-induced Coulomb explosion. Next, the dimers are aligned in either one or three dimensions under field-free conditions by a nonresonant, moderately intense laser pulse. The experimental angular covariance maps of the tetracene ions are compared to calculated covariance maps for seven different dimer conformations and found to be consistent with four of these. Additional measurements of the alignment-dependent strong-field ionization yield of the dimer narrow the possible conformations down to either a slipped-parallel or parallel-slightly rotated structure. According to our quantum chemistry calculations, these are the two most stable gas-phase conformations of the dimer and one of them is favorable for singlet fission.

Highlights

Noncovalent interactions between aromatic molecules are crucial for many areas, such as molecular recognition, structure of macromolecules, and organic solar cells.1–4 At the most fundamental level, the interaction involves two aromatic molecules
Dimers of tetracene molecules are formed inside helium nanodroplets and identified through covariance analysis of the emission directions of kinetic tetracene cations stemming from femtosecond laser-induced Coulomb explosion
We introduced an alternative method for structure determination of dimers created inside He droplets, namely, Coulomb explosion induced by an intense femtosecond laser pulse and recording of the emission direction of the fragment ions including identification of their angular correlations,25,26 implemented through covariance analysis

Summary

Introduction

Noncovalent interactions between aromatic molecules are crucial for many areas, such as molecular recognition, structure of macromolecules, and organic solar cells. At the most fundamental level, the interaction involves two aromatic molecules. Noncovalent interactions between aromatic molecules are crucial for many areas, such as molecular recognition, structure of macromolecules, and organic solar cells.. At the most fundamental level, the interaction involves two aromatic molecules. This has been the subject of a large numbers of studies, often with a particular focus on determining the structure of the dimers. The main technique to form dimers is supersonic expansion of a molecular gas seeded in a carrier gas of rare gas atoms into vacuum. Combining the resulting molecular beams with various types of high-resolution spectroscopy, including microwave, infrared, and UV spectroscopy as well as rotational coherence spectroscopy7,8—a technique based on pairs of femtosecond or picosecond pulses—the rotational constants can be extracted. Upon comparison with results from theoretical modeling, information about the conformations of a range of different dimers have been obtained. Examples include the dimers of benzene, fluorene, benzonitrile, phenol, and anisole.

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Methods

Conclusion