Tellurium Analogues of Rosamine and Rhodamine Dyes:  Synthesis, Structure,¹²⁵Te NMR, and Heteroatom Contributions to Excitation Energies

The first tellurium-containing analogues of the rosamine and rhodamine dyes were prepared by the addition of Grignard or organolithium reagents to telluroxanthone precursors. The 125 Te spectra of both the dyes and the telluroxanthone precursors are reported. One derivative was characterized by X-ray crystallography and was found to have a nearly planar telluroxanthylium core with a 9-aryl substituent held nearly orthogonal to the core. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were performed to analyze the trends for the excitation wavelengths observed experimentally. The computations indicated that participation of the heteroatom lone pair orbitals is responsible for this trend.