Electronic structure of photochromic 2,4,6-triaryl-1-(spiro[2H-1-benzopyran-2,2′-indoline]-6-yl)pyridinium salts

The nature of chemical bonding of nitrogen in spiro(benzopyran-indoline)pyridinium salts is investigated comparing the near edge X-ray absorption fine structure (NEXAFS) and X-ray emission spectra (XES) of nitrogen with that of reference compounds (thiourea, indoliumiodide and indoliumperchlorate). The X-ray absorption fine structure shows two π* peaks arising from 1 s−1 2pπ* excitations. Photoelectron spectra (XPS) and NEXAFS details point at two differently bound nitrogen atoms (>N and >N+). Resonant N K-emission spectra reveal a strong dependence on the excitation energies.