Longer-Wavelength-Absorbing, Extended Chalcogenorhodamine Dyes
This research was supported in part by the National Science Foundation (Grant CHE-1151379) and by a contract from the Wake Forest Medical Center to M.R.D. and M.W.K.
Extended rhodamines were prepared by inserting an additional fused benzene ring into the rhodamine xanthylium core. The synthesis of “bent” dyes 4-E (E = S, Se, Te) began with regioselective lithiation of the 1-position of N,N-diisopropyl 6-dimethylamino-2-naphthamide (11b) with n-BuLi/TMEDA (≥25:1 1- vs 3-lithiation) followed by addition of a dichalcogenide electrophile. The synthesis of “linear” dyes 5-E (E = S, Se, Te) began with regioselective lithiation of the 3-position of N,N-diethyl 6-dimethylamino-2-naphthamide (11a) with lithium tetramethylpiperidide (≥50:1 3- vs 1-lithiation) followed by addition of a dichalcogenide electrophile. Dyes 4-E and 5-E have absorption maxima in the 633–700 nm range. Dyes 4-E generate singlet oxygen upon irradiation while dyes 4-S and 5-S are highly fluorescent, with quantum yields for fluorescence of 0.47 and 0.18, respectively. DFT calculations were performed on the 4-E and 5-E chromophores. For the dyes 4-E, the lowest energy excitation is due solely to the HOMO–LUMO transition. For dyes 5-E, the lowest energy excitation is a combination of two excitations, both having contributions from the HOMO to LUMO and HOMO-1 to LUMO.
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Kryman, M. W., McCormick, T. M., & Detty, M. R. (2016). Longer-Wavelength-Absorbing, Extended Chalcogenorhodamine Dyes. Organometallics, 35(11), 1944-1955.