Fluorescent “rhodamine-like” hemicyanines derived from the 6-(N,N-diethylamino)-1,2,3,4-tetrahydroxanthylium system

• Two new “rhodamine-like” molecules were synthesized.
• Both dyes presents a largest Stokes shift compared with common rhodamines.
• Fluorescence quantum yields and lifetimes were obtained in different solvents.
• The dyes adsorbed onto microcrystalline cellulose exhibit a two component emission.
• Singlet oxygen and intersystem crossing quantum yield were determined.

Two new compounds derivatives of the “rhodamine-like” heterocyclic system 6-(N,N-diethylamino)-1,2,3,4-tetrahydroxanthylium were synthesized, bearing an aniline fragment in the conjugation chain.Their spectral-luminescent properties were investigated; fluorescence quantum yields and lifetimes were obtained for each dye in three different solvents. Dye 3a presents the highest fluorescence quantum yield in dichloromethane (ΦF = 0.60). In contrast with classic rhodamines, a large Stokes shift was detected, compatible with a mechanism of deactivation involving twisted intramolecular charge transfer interactions. These rhodamine-like compounds exhibit very low quantum yield of singlet oxygen formation (ΦΔ = 0.07 for 3a), similar to what is observed for other hemicyanines. Triplet–triplet absorption spectra were also obtained, as well as the quantum yield of intersystem crossing (ΦT = 0.10 for 3a). Photostability was evaluated under visible and laser irradiation and dye 3a proved to be the most stable one, making it a good candidate to be used as imaging agent.

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