Chlorine Substitution Effect on the S₁ Relaxation Dynamics of Chlorobenzene and Chlorophenols

The S₁ state relaxation dynamics of chlorobenzene (CB), 3-chlorophenol (3-CP), 3-CP·H₂O, and 2-chlorophenol·H₂O (2-CP·H₂O) have been investigated by means of picosecond time-resolved pump-probe spectroscopy in a state-specific manner. For CB, the S₁ state relaxes via the S₁-S₀ internal conversion in the low internal energy region (<2000 cm^-1), whereas the direct C-Cl bond dissociation channel mediated by the upper-lying repulsive πσ_CCl* state is opened to give the rather sharp increase of the S₁ relaxation rate in the high internal energy region (>2000 cm^-1). A similar dynamic feature has been observed for 3-CP in terms of the lifetime behavior with an increase in the S₁ internal energy, suggesting that the H atom tunneling dissociation reaction from OH might contribute less compared to the internal conversion, although it is not clear at the present time whether or not the sharp increase of the S₁ relaxation rate in the high internal energy region of 3-CP (>1500 cm^-1) is entirely due to that of the internal conversion. The fact that the internal conversion is facilitated by the Cl substitution implies that the energetic location of the S₁/S₀ conical intersection should have been strongly influenced by chlorine substitution on the aromatic ring. The approximate energetic location of the saddle point of the S₁(ππ*)/πσ_CCl* conical intersection along the seam coordinate for CB or 3-CP could be inferred from the energy-dependent S₁ lifetime measurements. It is discussed in comparison with the dynamic role of the S₁(ππ*)/πσ_CCl* conical intersection, which is strongly influenced by the O-H···Cl intramolecular hydrogen bond in the rather complicated yet ultrafast S₁ relaxation dynamics of the cis-2-CP. The S₁ lifetimes of 3-CP·H₂O and 2-CP·H₂O reveal the importance of the conformational structures, especially in terms of the intramolecular hydrogen bonding.