Letter
Absorption and fluorescence properties of fluorescein

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Abstract

We have characterized the protolytic equilibria of fluorescein and determined the spectroscopic properties of its protolytic forms. The protolytic constants relating the chemical activities (which at low ionic strength equal concentrations) of the cation, neutral form, anion and dianion are pK1 = 2.08, pK2 = 4.31, and pK3 = 6.43. All forms have rather high molar absorptivities being ε437FH31 = 53 000, ε434FH2 = 11 000, ε453FH− = 29 000 (ε472FH = 29 000) and ε490F2 = 76 900 M−1 cm−1 for the cation, neutral form, anion and dianion, respectively. The dianion has the most intense fluorescence with a quantum yield of 0.93 but also the anion shows considerable fluorescence with a quantum yield of 0.37. The neutral and cationic species are upon excitation converted into the anion and fluoresce with quantum yields of about 0.30 and 0.18, respectively.

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    These authors have contributed equally to this work.

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