3-Hydroxyflavone
= 3-Hydroxyflavone
{title2=$\mathrm{C_{15}H_{10}O_3}$}
\b[<3-Hydroxyflavone>] is the unsubstituted parent molecule of the <flavonol> class: <flavone> with a <hydroxy group> at position3. Its neutral <molecular formula> is $\mathrm{C_{15}H_{10}O_3}$. The position-3 hydroxyl lies next to the position-4 <carbonyl group>, allowing an internal <hydrogen bond> and providing a useful system for studying <proton transfer> after optical excitation.<Flavonols in ChEBI>[CHEBI:5078, structure, formula and 3-hydroxyflavone identity]{ref}
<Pradeep K. Sengupta> and <Michael Kasha> investigated why its green <fluorescence> was far to the red of its first absorption band. Their 1979 paper compared absorption and emission in solution with emission from a rigid matrix at $77\,\mathrm K$. The room-temperature green emission, near $520\,\mathrm{nm}$ in their illustrated solvent, contrasted with violet emission from the normal form at low temperature. The apparatus included an absorption <spectrophotometer> and a <fluorometer>.<Excited state proton-transfer spectroscopy of 3-hydroxyflavone and quercetin by Pradeep K. Sengupta and Michael Kasha>[Printed pp.382–383, Figures1–3 and captions]{ref} The interpretation involved an emitting <tautomer> produced by <excited-state intramolecular proton transfer>, rather than a change in elemental composition.
An original branching model illustrates the difference between spectral intensity and molecular abundance. Let a normal <excited state> $N^*$ fluoresce at rate $k_f$, decay without emission at rate $k_{nr}$, or transfer to a tautomeric excited state at rate $k_p$. If each rate is constant, the transfer probability is
$$
P_{N^*\to T^*}=\frac{k_p}{k_f+k_{nr}+k_p},
$$
and the normal-form <fluorescence quantum yield> is $k_f/(k_f+k_{nr}+k_p)$. If the transferred state emits with conditional probability $\eta_T$, its photon yield is
$$
\Phi_T=\frac{k_p\eta_T}{k_f+k_{nr}+k_p}.
$$
With chosen rates in the ratio $k_f:k_{nr}:k_p=1:1:8$ and $\eta_T=1/2$, the normal and tautomer photon yields are 0.10 and 0.40. The ratio of emitted photon counts is therefore 4 under equally efficient detection, although the transfer probability is 0.80. These are illustrative rates, not fits to the historical data.
Different solvent interactions, competing decay and detector response can change an observed band ratio. <Fluorescence spectroscopy> therefore combines spectral positions, excitation conditions and timing or environmental comparisons when testing a molecular mechanism; one bright band is not an unqualified population measurement.