Umbelliferone (7-Hydroxycoumarin)

Technical Specifications

Product Description & Scientific Applications

The 7-hydroxy group makes umbelliferone a photoacid: on excitation the phenolic proton becomes far more acidic than in the ground state and is transferred to solvent. In water its excited-state acidity constant pKa* is about 0.4, and excited-state proton transfer (ESPT) to water proceeds at 2 × 1010 s−1. The excited molecule is also a photobase, taking up a proton from weak acids such as acetic acid. ESPT is solvent-gated: fast in water, but in methanol much slower than nonradiative decay of the excited singlet, so its efficiency there is low. Ground-state acidity is milder — the phenol pKa reported for the dye tethered in a single DNA strand is 8.8, rising above 10 on intercalation into a duplex. Protonation quenches the fluorescence and deprotonation to the anion restores it, so emission reports the protonation state, and both absorption and emission are solvatochromic. In nature, umbelliferone is a coumarin that accumulates in plant leaves and stems near sites of microbial infection, where sunlight-driven ESPT is proposed to act as a resistance mechanism against fungal and bacterial attack.

Applications

Umbelliferone is a reporting fluorophore across several sensing formats, and the unsubstituted parent of the 7-hydroxycoumarin fluorophores, whose ESPT and prototropic kinetics are tuned by substitution:

• pH and hybridisation sensing — as a quencher-free molecular beacon, the phenol pKa rises by more than 1.2 units on duplex formation, so that at pH 8.5 the duplex fluoresces about one-fifteenth as strongly as the single strand, and target binding that opens the beacon raises emission roughly tenfold.
• FRET donor for enzyme sensing — as a blue emitter it serves as a Förster resonance energy transfer donor; in a ratiometric two-photon probe for γ-glutamyltranspeptidase the 7-hydroxycoumarin donor emits near 461 nm against a BODIPY acceptor at 610 nm, and enzyme turnover that separates the chromophores restores the donor band.
• FRET donor in a metal–organic framework — as 7-hydroxycoumarin-4-acetic acid, encapsulated in a zirconium MOF as the donor to a fluorescein acceptor in a ratiometric bacterial nanoprobe.
• Hydrogen peroxide sensing — umbelliferone-based probes give large fluorescence turn-on, up to one hundredfold, with good selectivity over other reactive oxygen species; the signal is the liberated umbelliferone once an oxidatively cleaved cage is removed.
• Metal-ion sensing — the 7-hydroxycoumarin moiety is the reporting unit in turn-on fluoroionophores for metal ions such as Mg(II).
• Mitochondrial pH imaging — the substituted derivative 3-benzimidazole-7-hydroxycoumarin is a mitochondria-localised two-photon probe with linear pH response around pKa 4.20 and 7.20 at 480 nm emission.
• Genetically encoded fluorophore — the amino acid l-(7-hydroxycoumarin-4-yl)ethylglycine carries the fluorophore into proteins for FRET-based readout of transcription-factor binding in living bacteria.

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