Photophysics of 2- and 3-fluoropyridine vapors

Studies of the First Electronically Excited State of 3-Fluoropyridine and Its Ionic Structure by Means of REMPI, Two-Photon MATI, One-Photon VUV-MATI Spectroscopy and Franck-Condon Analysis

3-Fluoropyridine (3-FP) has been investigated by means of two-photon resonance-enhanced multi photon ionization (REMPI), mass-analyzed threshold ionization (MATI) and one-photon vacuum-ultraviolet (VUV) MATI spectroscopy. The aim was the determination of the effect of m-fluorine substitution on the vibronic structure of the first electronically excited and ionic ground state. The S₁ excitation energy has been determined to be 35 064 ± 2 cm^-1 (4.3474 ± 0.0002 eV). Strong evidence of a distinct vibronic coupling via ν_16b and ν_{[Wag.out.,16a]} to one or both of the lowest ¹ππ* states has been found, which results in a warped S₁ minimum structure with C₁ symmetry. The adiabatic ionization energy of the ionic ground state (14a', n_N-LP orbital) has been determined to be 76 579 ± 6 cm^-1 (9.4946 ± 0.0007 eV), which is the first value reported for this state. The origin of the D₁ state (4a'', π-orbital) is located close by at 77 129 cm^-1 (9.5628 eV). As a result of the D₀-D₁ vicinity, the ionic ground state is coupled to the D₁ state via ν_{[Wag.out.,16a]} and ν_10a, which induces a twisted D₀ geometry with C₁ symmetry. Furthermore, for the first time two-photon and one-photon MATI spectra are presented together, which yield a much better understanding of the ionic vibronic structure in comparison to either of these experiments alone.

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

The cluster structures of hydrated aminopyrazines, APz-(H₂O)_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-(H₂O)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-(H₂O)₂. On the other hand, APz-(H₂O)_n=3,4 has a linear hydrogen-bond network ending at the 4-position one (N4), which resembles 3-aminopyridine-(H₂O)_n=3,4. The hydrogen-bond network switching from the N1 type to the N4 one provides the accompanying red shifts of the S₁-S₀ 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 nπ* to ππ*, resulting in the decreasing spin-orbit coupling to T₁ (ππ*).