= Excited-state intramolecular proton transfer
{title2=ESIPT}
= ESIPT
{c}
{synonym}
\b[<Excited-state intramolecular proton transfer>], abbreviated \b[<ESIPT>], is transfer of a <proton> between sites within the same <molecule> after electronic excitation. A <hydrogen bond> can connect donor and acceptor sites, while excitation changes the energetic preference along the <proton transfer> coordinate. The resulting excited <tautomer> may emit at a different energy from the initially excited form.
The classic <3-Hydroxyflavone> study by <Pradeep K. Sengupta> and <Michael Kasha> compared absorption, green emission in room-temperature solution, violet emission in a rigid $77\,\mathrm K$ matrix, excitation spectra and deuteration effects.<Excited state proton-transfer spectroscopy of 3-hydroxyflavone and quercetin by Pradeep K. Sengupta and Michael Kasha>[Printed pp.382–384, Figures1–4]{ref} The change of emission did not require a change in elemental composition: it concerned different arrangements and electronic states of the same molecular system.
A large <Stokes shift> is informative but is not a direct measurement of a reaction barrier. For an original example, absorbing a $350\,\mathrm{nm}$ photon and emitting at $520\,\mathrm{nm}$ corresponds to energies near $3.54$ and $2.38\,\mathrm{eV}$, using $hc\simeq1240\,\mathrm{eV\,nm}$. Their difference, about $1.16\,\mathrm{eV}$, includes the overall relaxation and emission energy balance; it is not automatically the activation energy of the transfer step.
An original pulsed-excitation model makes a kinetic prediction. Let $N(t)$ and $T(t)$ denote populations of the normal and transferred excited forms, with $N(0)=N_0$ and $T(0)=0$. Neglect reverse transfer and write
$$
\frac{\mathrm dN}{\mathrm dt}=-aN,\qquad
\frac{\mathrm dT}{\mathrm dt}=k_pN-bT,
$$
where $a$ includes transfer and all other normal-state decay, $b$ is total transferred-state decay and $0\leq k_p\leq a$. For $a\ne b$,
$$
N=N_0e^{-at},\qquad
T=\frac{k_pN_0}{b-a}(e^{-at}-e^{-bt}).
$$
If $a=b$, the limiting expression is $T=k_pN_0t e^{-at}$. The initially absent transferred form rises and then decays; its maximum occurs at $t_{\max}=\log(b/a)/(b-a)$ for positive unequal decay rates.
\Image[/esipt-population-rise.png]
{title=An excited-state tautomer can rise after a light pulse}
{description=Original illustrative model with a=10, b=1 and transfer rate=8 per nanosecond, without reverse transfer. Curves show populations normalized by the initially excited population, not measured fluorescence intensities.}
For chosen $a=10$, $b=1$ and $k_p=8\,\mathrm{ns^{-1}}$, the transferred population peaks at approximately $0.256\,\mathrm{ns}$. These are model values, not rate measurements from the historical experiment. Detected emission instead weights each population by its radiative rate and instrument response. <Fluorescence lifetime> and <fluorescence instrument response> must therefore be considered alongside spectral band ratios when testing a transfer mechanism.
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