Dynamic Role of the Intramolecular Hydrogen Bonding in the S 1 State Relaxation Dynamics Revealed by the Direct Measurement of the Mode-Dependent Internal Conversion Rate of 2-Chlorophenol and 2-Chlorothiophenol.

The dynamic role of the intramolecular hydrogen bond in the S 1 relaxation of cis -2-chlorophenol (2-CP) or cis -2-chlorothiophenol (2-CTP) has been investigated in a state-specific manner. Whereas ultrafast internal conversion is dominant for 2-CP, the H-tunneling competes with internal conversion for 2-CTP even at the S 1 origin. The S 0 -S 1 internal conversion rate of 2-CTP could be directly measured from the S 1 lifetimes of 2-CTP- d 1 (Cl-C 6 H 4 -SD) as the D-tunneling is kinetically blocked, allowing distinct estimations of tunneling and internal conversion rates with increasing the energy. The internal conversion rate of 2-CTP increases by two times at the out-of-plane torsional mode excitation, suggesting that the internal conversion is facilitated at the nonplanar geometry. It then sharply increases at ∼600 cm -1 , indicating that the S 1 /S 0 conical intersection is readily accessible at the extended C-Cl bond length. The strength of the intramolecular hydrogen bond should be responsible for the distinct dynamic behaviors of 2-CP and 2-CTP.