Excited-State Dynamics Affected by Switching of a Hydrogen-Bond Network in Hydrated Aminopyrazine Clusters.

The cluster structures of hydrated aminopyrazines, APz-(H ₂ O) _{n =2-4} , in supersonic jets have been investigated measuring the size-selected electronic and vibrational spectra and determined with the aid of quantum chemical calculations. The APz-(H ₂ O) ₂ structure is assigned as a cyclic N1 type where a homodromic hydrogen-bond chain starts from the amino group and ends at the 1-position nitrogen atom of the pyrazine moiety, corresponding to 2-aminopyridine-(H ₂ O) ₂ . On the other hand, APz-(H ₂ O) _{n =3,4} has a linear hydrogen-bond network ending at the 4-position one (N4), which resembles 3-aminopyridine-(H ₂ O) _{n =3,4} . The hydrogen-bond network switching from the N1 type to the N4 one provides the accompanying red shifts of the S ₁ -S ₀ electronic transition that are entirely consistent with those of the corresponding 2-aminopyridine and 3-aminopyridine clusters and also shows the drastically strengthened fluorescence intensity of origin bands in the electronic spectrum. The significant change in the excited-state dynamics is explored by the fluorescence lifetime measurement and the time-dependent density functional theory (TD-DFT) calculation. It is suggested that the drastic elongation of fluorescence lifetimes is due to the change in the electronic structure of the first excited state from n π* to ππ*, resulting in the decreasing spin-orbit coupling to T ₁ (ππ*).