Soft X-ray Absorption and X-ray Photoelectron Spectroscopic Study of Tautomerism in Intramolecular Hydrogen Bonds of N-Salicylideneaniline Derivatives

The tautomerism in solid phase was studied by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at the N K - and O K -edges and X-ray photoelectron spectroscopy (XPS) in the N1s and O1s regions for five N -salicylideneaniline derivatives: N, N‘ -disalicylidene-1,6-pyrenediamine (DSPY), N, N‘ -disalicylidene- p -phenylenediamine (BSP), N -(2-hydroxy-1-naphthylidene)-1-pyrenylamine (NPY), N, N‘ -di(2-hydroxy-1-naphthylidene)- p -phenylenediamine (DNP), and the complex between DNP and 7,7,8,8-tetracyanoquinodimethane (DNP-TCNQ). The NEXAFS spectral features were assigned by (1) the comparison with the core excitation spectra of reference compounds and (2) ab initio MO calculations of the core-excited states by the improved virtual orbital method using the relaxed Hartree−Fock orbitals. The tautomeric structures were deduced by the intensity of peaks characteristic of the tautomers in OH- and NH-forms. The XPS spectra gave structures consistent with those by NEXAFS and also allowed quantitative estimation of the relative tautomer populations. The results range from the dominance of the OH-form in DSPY and BSP to a comparable mixture of the OH- and NH-forms in DNP and DNP-TCNQ. These structures are consistent with the previous estimation by X-ray diffraction (XD), except for DNP-TCNQ where XD suggested the NH-form dominance. The good correlation of the population by XPS with the C−O bond length by XD supports the present conclusion for DNP-TCNQ. These results demonstrate the usefulness of NEXAFS as well as XPS for studying the tautomerism in hydrogen bonded systems.