Longer-Wavelength-Absorbing, Extended Chalcogenorhodamine Dyes

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.