Van der Waals complexes of jet-cooled aromatic thiones with noble gases: The phosphorescence excitation spectrum of 4H-pyran-4-thione

The solvent spectral shifts of the S₀ → T₁ origin bands of jet-cooled 4H-pyran-4-thione (PT) and 4H-1-benzopyran-4-thione (BPT) were observed in phosphorescence excitation spectra. Van der Waals complexes (1:1) of seed molecules with the carrier gas were observed in several rare gases. In all cases, the combination of a dispersive red-shift and a dipole-induced dipole blue-shift produced a total red-shift, up to a maximum of δṽ = -21 cm^-1 for PT-Xe and δṽ = -30 cm^-1 for BPT-Xe. Complexes of 1:n (n > 1) were not observable owing to the weakness of the direct S₀ → T₁ transition. The inductive contributions to the shifts were calculated using values of excited-state dipole moments estimated from solution Stokes shifts; the dipole moments of both molecules in T₁ were found to be close to zero, significantly smaller than their ground-state dipole moments. The phosphorescence excitation spectrum of isolated PT is discussed in some detail. The electronic origin of T₁ at 16 844 cm^-1 is assigned to the transition S_0,0 → T_1z,v the transitions S_0,0 ⇌ T_1(xy),0 are too weak to be detected with this method. Vibronic transitions S_0,0 → T_1z,v in PT were analyzed up to excess energies of ∼hc1450 cm^-1.