X-ray emission study of aromaticity in heteroorganic switches

TNA project number: 22002856-ST

Scientific background:

Sulphur is a key element in organic molecules, used as parts in organic electronics. To improve S based molecular devices, an understanding of their electronic structures is of vital importance. This information can be obtain by preforming X-ray emission spectroscopy (XES), which provides detailed information about the occupied electronic states, oxidation numbers and atomic environment. Sulphur strong Kβ emission is caused by the transitions from molecular orbitals, including the S 3p fill, into the S 1s core hole. Valence to core XES offers high chemical sensitivity, and DFT-based calculations have indicated a high degree of responsiveness of emission spectra to the small changes in aromaticity.

X-ray emission spectroscopy of sulphur falls in the "tender energy" X-ray region and the dedicated in-vacuum Johansson spectrometer at Jožef Stefan Institute in Ljubljana is capable of recording high energy resolution proton induced emission spectra in this region.

Measurements performed within TNA project:

The wavelength-dispersive in-vacuum x-ray spectrometer of J. Stefan Institute (Ljubljana, Slovenia) has been used to record high energy resolution S X-ray emission spectra on a series of 11 heteroaromatic compounds and 2 reference materials. The electron structure of samples was tuned by structural variations in the bonding of the system around a thiophene unit. X-ray emission was induced with a 2MeV proton beam and Si(111) crystal analyzer was employed in the spectrometer. The samples were in a form of pressed pellets, each individually mounted on a cooled holder inside a vacuum chamber. Most of Kβ spectra were recorded over approximately 2-3 hours, with few measurements lasting over night. The experimental spectra will be used to identify molecular orbitals and assign them to individual peaks in order to confirm the validity of the calculated spectra.

Data files:

We are sharing the detector files (a series of single exposure 2D raw images) collected by the Andor DX438-BV CCD camera (770 × 1152 pixels with pixel size 22.5×22.5 μm2) after the diffraction on the crystal analyzer (Si(111)). The horizontal axis of the detector corresponds to the dispersion axis and diffracted photons are detected at different horizontal positions according to their wavelength, the vertical axis of the detector serves to accumulate more statistics. The final emission spectra are obtained from the corresponding detector files using the home-written data processing software