The optical properties of 3 + 3 macrocyclic Schiff base thin material obtained by the Molecular Beam Epitaxy method

Abstract

3 + 3 optically active macrocyclic Schiff bases were synthesized in the reaction between 4-tert-butyl-2,6-diformylphenol with (1R,2R)-(+)-1,2-diphenylethylenediamine (S1) or (1S,2S)-(−)-1,2-diphenylethylenediamine (S1a). The new compounds were spectroscopically characterised by NMR, IR, X-ray (S1a), UV–Vis and fluorescence spectroscopy. The S1a molecule creates channels with distances between oxygen atoms ranging from 5.8-6.3 Å and sufficiently large to host acetonitrile molecule. Both compounds exhibit green-yellow emission in solution and solid state. Thin layers of the S1 compound obtained via Molecular Beam Epitaxy (MBE) were characterised by scanning electron microscopy with energy-dispersive X-ray spectroscopy SEM/EDS and atomic force microscopy (AFM). The optical properties of the S1/Si thin material were analysed using spectroscopic ellipsometry (SE), fluorescence spectroscopy and synchrotron radiation (SR). The time constant for the decay investigated under SR, denoted by τ1, was determined to be approximately 1.02 ns, suggesting a fast deactivation process of the excited electronic state in the S1/Si material. The ellipsometric analysis of the S1/Si layer showed semiconducting behaviour with pronounced absorption features in the UV range, attributed to π → π* and n → π* transitions, characteristic of Schiff bases. The band-gap energy, determined using the Tauc method, is 3.46 ± 0.01 eV. These analyses highlight the material’s potential in applications requiring precise control of optical properties. In the emission spectrum of S1a, a significant emission peak of approximately 561 nm indicates the presence of a prominent emissive process within this wavelength. The S1a compound is emissive in the yellow-green region of the spectrum and has a longer decay time, which suggests that it can be used in sensing optical technologies.