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

Tunneling dynamics dictated by the multidimensional conical intersection seam in the πσ*‐mediated photochemistry of heteroaromatic molecules

“…In other words, when ϕ ≈ 0° (planar), the probability of the reactive flux to funnel through the conical intersection is expected to be high whereas it should be significantly reduced as the molecule becomes nonplanar with the increase of the torsional angle ( ϕ ≠ 0°). Utilizing this fact, the nonadiabatic transition probability could be largely controlled by the manipulation of the conformational structure of thiophenol by the chemical substitution 11,80,86,93–96 . Namely, for instance, the product branching ratio of ~0.75 at the 243 nm excitation estimated for thiophenol‐d 1 is much reduced to ~0.10 for para ‐methoxythiophenol‐d 1 11 .…”

“…The SH(D) bond dissociation of the S 1 (or S 2 ) thiophenol is ultrafast (<<100 fs), as described in Section 3.1, and thus the investigation of the vibronic mode dependence of the tunneling rate is intrinsically nontrivial as the lifetimes of all accessible S 1 modes of thiophenol are much shorter than 100 fs which is beyond the experimental limit. In order to study the slowed‐down tunneling process, dynamics of three ortho ‐substituted thiophenols have recently been investigated 93–97 . These include 2‐methoxythiophenol (2‐MTP; 2‐CH 3 O‐C 6 H 4 SH), 2‐fluorothiophenol (2‐FTP; 2‐F‐C 6 H 4 SH), and 2‐chlorothiophenol (2‐CTP; 2‐Cl‐C 6 H 4 SH).…”

“…The πσ*‐mediated photochemistry, in this aspect, is quite unique as it gives the rational explanation for the ultrafast nonradiative transitions of many heteroaromatic molecular systems upon the electronic excitations, which are often relevant to the photochemical or photobiological activities 6,19–38 . As a matter of fact, spectroscopic and dynamic studies on heteroaromatic systems including phenols, 9,12,39–75 thiophenols, 10–12,76–97 anisoles, 98–102 thioanisoles, 5,103–115 or many others are vast in terms of their diversities and detailed dynamic features. Herein, we focus on the H‐atom detachment reactions of phenols and thiophenols especially in terms of the state‐specific tunneling rate constants.…”

“…The nonadiabatic transition probability at the S 0 /S 2 conical intersection could be then precisely estimated experimentally by measuring the branching ratio of these pathways from translational energy distribution of the H‐fragment, 10–12,76–78,83 providing the great opportunity to understand and control the conical‐intersection dynamics in the state and structure specific ways 11,77,80–83,86,93–95 . The rate of the whole reaction though is governed by that of the tunneling process through the barrier which is dynamically shaped by the S 1 /S 2 conical intersection 54,96,116–121 . It should be emphasized that the tunneling dynamics of phenols (or thiophenols) is multidimensional in nature although the tunneling process could be regarded to occur along the one‐dimensional OH (or SH) bond extension coordinate.…”

“…This is because that the tunneling barrier is dynamically shaped along the branching plane as well as (3 N ‐8) dimensional seam coordinates of the conical intersection 58,61,64,88–90,122 . Multidimensional aspects of the tunneling could be clearly manifested in the mode‐dependent tunneling rates measured for phenols and thiophenols 69,73,85,96,117,121 …”