On the ultrashort lifetime of electronically excited thiophenol

Conformer-selective Photodynamics of TrpH+ -H2 O

The photodynamics of protonated tryptophan and its mono hydrated complex TrpH⁺ -H₂ O has been revisited. A combination of steady-state IR and UV cryogenic ion spectroscopies with picosecond pump-probe photodissociation experiments sheds new lights on the deactivation processes of TrpH⁺ and conformer-selected TrpH⁺ -H₂ O complex, supported by quantum chemistry calculations at the DFT and coupled-cluster levels for the ground and excited states, respectively. TrpH⁺ excited at the band origin exhibits a transient of less than 100 ps, assigned to the lifetime of the excited state proton transfer (ESPT) structure. The two experimentally observed conformers of TrpH⁺ -H₂ O have been assigned. A striking result arises from the conformer-selective photodynamics of TrpH⁺ -H₂ O, in which a single water molecule inserted in between the ammonium and the indole ring hinders the barrierless ESPT reaction responsible for the ultra-fast deactivation process observed in the other conformer and in bare TrpH⁺ .

πσ*-Mediated Nonadiabatic Tunneling Dynamics of Thiophenols in S1: The Semiclassical Approaches

The S-H bond tunneling predissociation dynamics of thiophenol and its ortho-substituted derivatives (2-fluorothiophenol, 2-methoxythiophenol, and 2-chlorothiphenol) in S₁ (ππ*) where the H atom tunneling is mediated by the nearby S₂ (πσ*) state (which is repulsive along the S-H bond extension coordinate) have been investigated in a state-specific way using the picosecond time-resolved pump-probe spectroscopy for the jet-cooled molecules. The effects of the specific vibrational mode excitations and the SH/SD substitutions on the S-H(D) bond rupture tunneling dynamics have been interrogated, giving deep insights into the multidimensional aspects of the S₁/S₂ conical intersection, which also shapes the underlying adiabatic tunneling potential energy surfaces (PESs). The semiclassical tunneling rate calculations based on the Wentzel-Kramers-Brillouin (WKB) approximation or Zhu-Nakamura (ZN) theory have been carried out based on the ab initio PESs calculated in the (one, two, or three) reduced dimensions to be compared with the experiment. Though the quantitative experimental results could not be reproduced satisfactorily by the present calculations, the qualitative trends among different molecules in terms of the behavior of the tunneling rate versus the (adiabatic) barrier height or the number of PES dimensions could be rationalized. Most interestingly, the H/D kinetic isotope effect observed in the tunneling rate could be much better explained by the ZN theory compared to the WKB approximation, indicating that the nonadiabatic coupling matrix elements should be invoked for understanding the tunneling dynamics taking place in the proximity of the conical intersection.

Real-Time Tunneling Dynamics through Adiabatic Potential Energy Surfaces Shaped by a Conical Intersection

Dynamic shaping of the adiabatic tunneling barrier in the S-H bond extension coordinate of several ortho-substituted thiophenols has been found to be mediated by low-frequency out-of-plane vibrational modes, which are parallel to the coupling vector of the branching plane comprising the conical intersection. The S-H predissociation tunneling rate (k) measured when exciting to the S₁ zero-point level of 2-methoxythiophenol (44 ps)^-1 increases abruptly, to k ≈ (22 ps)^-1, at the energy corresponding to excitation of the 152 cm^-1 out-of-plane vibrational mode and then falls back to k ≈ (40 ps)^-1 when the in-plane mode is excited at 282 cm^-1. Similar resonance-like peaks in plots of S₁ tunneling rate versus internal energy are observed when exciting the corresponding low-frequency out-of-plane modes in the S₁ states of 2-fluorothiophenol and 2-chlorothiophenol. This experiment provides clear-cut evidence for dynamical "shaping" of the lower-lying adiabatic potential energy surfaces by the higher-lying conical intersection seam, which dictates the multidimensional tunneling dynamics.

Vibronic structure and predissociation dynamics of 2-methoxythiophenol (S1): The effect of intramolecular hydrogen bonding on nonadiabatic dynamics

Vibronic spectroscopy and the S-H bond predissociation dynamics of 2-methoxythiophenol (2-MTP) in the S₁ (ππ^*) state have been investigated for the first time. Resonant two-photon ionization and slow-electron velocity map imaging (SEVI) spectroscopies have revealed that the S₁-S₀ transition of 2-MTP is accompanied with the planar to the pseudoplanar structural change along the out-of-plane ring distortion and the tilt of the methoxy moiety. The S₁ vibronic bands up to their internal energy of ∼1000 cm^-1 are assigned from the SEVI spectra taken via various S₁ vibronic intermediate states with the aid of ab initio calculations. Intriguingly, Fermi resonances have been identified for some vibronic bands. The S-H bond breakage of 2-MTP occurs via tunneling through an adiabatic barrier under the S₁/S₂ conical intersection seam, and it is followed by the bifurcation into either the adiabatic or nonadiabatic channel at the S₀/S₂ conical intersection where the diabatic S₂ state (πσ^*) is unbound with respect to the S-H bond elongation coordinate, giving the excited (Ã) or ground (X̃) state of the 2-methoxythiophenoxy radical, respectively. Surprisingly, the nonadiabatic transition probability at the S₀/S₂ conical intersection, estimated from the velocity map ion images of the nascent D fragment from 2-MTP-d₁ (2-CH₃O-C₆H₄SD) at the S₁ zero-point energy level, is found to be exceptionally high to give the X̃/Ã product branching ratio of 2.03 ± 0.20, which is much higher than the value of ∼0.8 estimated for the bare thiophenol at the S₁ origin. It even increases to 2.33 ± 0.17 at the ν₄₅ ² mode (101 cm^-1) before it rapidly decays to 0.69 ± 0.05 at the S₁ internal energy of about 2200 cm^-1. This suggests that the strong intramolecular hydrogen bonding of S⋯D⋯OCH₃ in 2-MTP at least in the low S₁ internal energy region should play a significant role in localizing the reactive flux onto the conical intersection seam. The minimum energy pathway calculations (second-order coupled-cluster resolution of the identity or time-dependent-density functional theory) of the adiabatic S₁ state suggest that the intimate dynamic interplay between the S-H bond cleavage and intramolecular hydrogen bonding could be crucial in the nonadiabatic surface hopping dynamics taking place at the conical intersection.

Experimental observation of nonadiabatic bifurcation dynamics at resonances in the continuum

The surface crossing of bound and unbound electronic states in multidimensional space often gives rise to resonances in the continuum. This situation happens in the πσ*-mediated photodissociation reaction of 2-fluorothioanisole; optically-bright bound S1 (ππ*) vibrational states of 2-fluorothioanisole are strongly coupled to the optically-dark S2 (πσ*) state, which is repulsive along the S-CH3 elongation coordinate. It is revealed here that the reactive flux prepared at such resonances in the continuum bifurcates into two distinct reaction pathways with totally different dynamics in terms of energy disposal and nonadiabatic transition probability. This indicates that the reactive flux in the Franck-Condon region may either undergo nonadiabatic transition funneling through the conical intersection from the upper adiabat, or follow a low-lying adiabatic path, along which multiple dynamic saddle points may be located. Since 2-fluorothioanisole adopts a nonplanar geometry in the S1 minimum energy, the quasi-degenerate S1/S2 crossing seam in the nonplanar geometry, which lies well below the planar S1/S2 conical intersection, is likely responsible for the efficient vibronic coupling, especially in the low S1 internal energy region. As the excitation energy increases, bound-to-continuum coupling is facilitated with the aid of intramolecular vibrational redistribution, along many degrees of freedom spanning the large structural volume. This leads to the rapid domination of the continuum character of the reactive flux. This work reports direct and robust experimental observations of the nonadiabatic bifurcation dynamics of the reactive flux occurring at resonances in the continuum of polyatomic molecules.

Electronic spectrum and characterization of diabatic potential energy surfaces for thiophenol

We present an accurate simulation of the UV spectrum and a diabatization of three singlet potential surfaces along four coordinates.