Control of intramolecular orbital alignment in the photodissociation of thiophenol: Conformational manipulation by chemical substitution

Article abstract of DOI:10.1002/anie.200705358

(Graph Presented) Intramolecular orbital alignment can be controlled by conformational tuning of the initial wavepacket location on the two-dimensional potential-energy surfaces of thiophenol (see picture; CI = conical intersection). Chemical substitution induces conformational preference, leading to a dramatic change of the branching ratio between X and A states of the phenylthiyl radical.

Full text of DOI:10.1002/anie.200705358

Angewandte
Chemie
DOI: 10.1002/anie.200705358
Chemical Dynamics
Control of Intramolecular Orbital Alignment in the Photodissociation
of Thiophenol: Conformational Manipulation by Chemical
Substitution**
Jeong Sik Lim, Yoon Sup Lee, and Sang Kyu Kim*
As scientists understand chemical reactions in atomic detail,
many successful examples of reaction control have been
reported in recent decades. These include active control using
pulse shaping in terms of chirps or phases[1–5,20] or passive
control, such as vibration-mediated photodissociation.[6–13]
Apart from these studies, an interesting structure-based
approach relying on conformational specificity has recently
been introduced as an alternative tool for reaction control. In
this approach, a particular conformational isomer is exclu-
sively chosen, and its unique chemical reactivity is thoroughly
examined. For instance, in the photodissociation of the cation
of 1-iodopropane, a gauche or trans conformational isomer
was selectively prepared by the mass-analyzed threshold
ionization (MATI) technique, and it was found that different
conformational isomers give rise to totally different reaction
products.[14] Another example of conformer-specific reaction
control has been demonstrated by Suits and co-workers in the
photodissociation of the gauche or cis conformational isomers
of the cation of 1-propanal. In the latter case, one of two
reaction channels proceeding in the ground state is preferen-
tially controlled by conformational selection in the excited
state.[15] More recently, alanine and b-alanine, which are
biological building blocks, were also found to be quite
conformer-specific in their decarboxylation reactions upon
ionization.[16] The relationship between structure and chem-
ical reactivity is essential for the understanding and control of
chemical reactions, and therefore the study of the conforma-
tional specificity at the atomic level seems to be invaluable at
the present time.
useful tool in stereochemistry. Accordingly, control of the
branching ratio naturally means that the intramolecular
orbital alignment in the reactive phenylthiyl radical could
be controlled by manipulation of the photodissociation
dynamics.
[D]Thiophenol (C6H5SD) is excited to the S2 state via the
1(np,s*) transition at a pump energy of 243 nm, with
ꢀ
subsequent prompt S D dissociation, the time scale of
which is less than 100 fs.[18] Since the repulsive S2 state
belongs to A’’ whereas the ground S0 state belongs to A’, the
diabatic potential-energy surfaces of S2 and S0 cross along the
ꢀ
S D elongation coordinate, as S2 and S0 diabatically correlate
to the X and ffi states of phenylthiyl (C6H5S), respectively.
[18]
˜
This dissociation may be qualitatively described as the
dynamics of a simple wavepacket on potential-energy surfa-
ces (PES) of reduced dimensionality. In this picture, the
˜
bifurcation of the wavepacket into two different ffi and X
states of phenylthiyl takes place at the conical intersection
(CI), which is formed at the planar geometry for which the
plane of symmetry is maintained along the reaction coordi-
nate (Figure 1).
Herein, we use the conformational change upon chemical
substitution to control the branching ratio of two reaction
channels in the photodissocation of thiophenol and its
ꢀ
derivatives. It has been found that, as a result of S H(D)
dissociation, the phenylthiyl radical is produced with its
reactive singly occupied molecular orbital (SOMO) either
[17]
˜
perpendicular (X) or parallel (ffi) to the molecular plane.
Figure 1. Diabatic potential-energy surfaces of thiophenol along the
This so-called “intramolecular orbital alignment” is a novel
concept in stereodynamics that could be developed into a
ꢀ
S H bond elongation coordinate at planar geometry. At 243 nm, the
wavepacket is exclusively prepared on S2.
[*] J. S. Lim, Prof. Y. S. Lee, Prof. S. K. Kim
Department of Chemistry and School of Molecular Science (BK21)
Korea Advanced Institute of Science and Technology (KAIST)
Daejeon 305-701 (Republic of Korea)
In recent theoretical studies on the photodissociation
dynamics of phenol, the out-of-plane torsional motion of OH
with respect to the benzene moiety is found to be the most
critical mode in the bifurcation dynamics at the conical
intersection.[19,20] The PES of thiophenol involved in optical
excitation and bond dissociation are intrinsically very similar
to those of phenol, except for detailed shapes and energetics,
especially at the asymptotic limit (Figure 1). Therefore, the
Fax: (+82)-42-869-2810
E-mail: sangkyukim@kaist.ac.kr
[**] We would like to acknowledge the support from KOSEF (R01-2007-
000-10766-0, R11-2007-012-01002-0), Echotechnopia 21 project of
KIEST (102-071-606), and KISTI Supercomputing Center (KSC-2007-
S00-1027).
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theoretical result obtained for phenol can naturally be
two-dimensional PES with respect to the C-C-S-D dihedral
angle. As the wavepacket moves on the repulsive surface, it
adopted to explain the bifurcation dynamics on the CI in
thiophenol dissociation. Namely, if the initial wavepacket
position on the potential-energy surface along the C-C-S-D
angle can be chosen arbitrarily, then dynamics on the CI may
be manipulated to give the control over two different reaction
ꢀ
spreads out along the S D bond elongation axis, but it
remains localized around the 08 geometry, since little torque is
exerted on the torsional angle in such an ultrafast wavepacket
motion, as depicted in Figure 3. Since the CI exists at the
˜
channels, in this case the X and ffi states of phenylthiyl. It is
noteworthy that optimal control as an alternative approach
has been theoretically simulated for the photodissociation of
phenol.[20] At the 243-nm excitation of thiophenol and its
derivatives, the wavepacket is prepared on the repulsive S2
state with subsequent diabatic passage through the S2/S1
˜
conical intersection. The X/ffi state bifurcation takes place
at the S2/S0 CI, whereas the total yield might depend on the S2/
S1 crossing dynamics.
[D]Thiophenol in the ground state has a planar geometry
in which the SD moiety lies in the plane of symmetry. When
the molecule is excited to the S2 state, the potential-energy
ꢀ
surface is repulsive in nature along the S D elongation axis.
However, the potential-energy surface becomes more
strongly bound along the C-C-S-D angle in the S2 state than
in S0, according to our time-dependent density functional
theory (TDDFT) calculations (Figure 2).[21] The minimum-
energy geometry remains at 08 in the S2 state, where 08 is
defined as the C-C-S-D dihedral angle at the planar geometry.
Therefore, according to the Franck–Condon principle, the
wavepacket is initially prepared at the bottom of the valley in
Figure 3. A simple wavepacket propagation model on two-dimensional
PES. The S2/S0 conical intersection is formed at the planar geometry.
As the initial location of the optically prepared wavepacket is changed,
the bifurcation dynamics on the CI are expected to be strongly
dependent on the two-dimensional wavepacket spreads.
planar geometry, the diabatic passage of the wavepacket
through the CI is expected to be significant. Experimental
findings are consistent with this prediction. Namely, as
˜
reported earlier, the yield of the X state resulting from the
diabatic passage through the CI is measured to be 0.43,
whereas that of the ffi state is 0.57 in the photodissociation of
thiophenol at 243 nm.[18] Even though vibrational modes
ꢀ
other than C-C-S-D torsion and S D stretching may also
affect dissociation dynamics in general, the bifurcation
dynamics in the vicinity of the CI are not expected to be
significantly influenced by those, as pointed out by Domcke
and co-workers.[19]
For the control, we can then imagine that the manipu-
lation of the initial location of the wavepacket on 2D PES as
the change of the initial location of the wavepacket along the
C-C-S-D angle should result in different bifurcation dynamics
at the CI. In this case, we have utilized the fact that the C-C-
S-D dihedral angle in the ground state is changed as the
chemical group at the para position of thiophenol is sub-
stituted. It has been reported that when an electron-donating
group is substituted at the para position, the SH moiety of
thiophenol becomes nonplanar in the ground state.[22] For
instance, for [D]4-methoxythiophenol, in which the para
position is substituted with the methoxy (OCH3) group, the
C-C-S-D dihedral angle becomes 738, according to our DFT
calculations[21] and reference [22]. This finding means that the
SD moiety becomes almost perpendicular to the molecular
plane in the ground state of 4-methoxythiophenol. In the S2
state, however, the minimum-energy structure adopts the
planar geometry. This result suggests that the overall shape of
the upper PES remains same as that of thiophenol, whereas
the initial location of the wavepacket is changed to the edge
*
Figure 2. Internal energy distributions ( ) of XC6H4Sand the deconvo-
˜
luted X (a) and ffi states (g) deduced from corresponding D
images taken at 243 nm for a) X=H, b) X=F, and c) X=OCH3. On
the right, potential-energy curves along the C-C-S-D dihedral angle are
shown for the ground ( ) and S2 ( ) states. Franck–Condon regions
*
*
are shaded.
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Angewandte
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near the top of the downhill potential along the C-C-S-D
angle for [D]4-methoxythiophenol (Figure 2). Accordingly,
there will be a strong torque exerted on the C-C-S-D dihedral
sample to possibly excite low-frequency modes. In Figure 4,
the translational energy distributions from the photodissoci-
ation of [D]4-fluorothiophenol at 243 nm are compared for
ꢀ
angle as the wavepacket moves out along the repulsive S D
coordinate. Therefore, as the reaction proceeds, the wave-
ꢀ
packet will be spread out not only along the S D bond axis
but also along the C-C-S-D angle. The wide spread of the
wavepacket at the S2/S0 conical intersection is expected to
diminish the probability of the diabatic passage through the
CI significantly. Consistently, the experiment shows a dra-
˜
matic decrease of the X state population, giving a yield of 0.09
˜
for X (0.91 for ffi) in the photodissociation of 4-methoxy-
thiophenol (Figure 2 and Table 1). In other words, the
reaction follows the adiabatic surface with a 91% probability,
whereas the diabatic passage through the CI occurs with a
probability of only 9%.
Figure 4. Total translational energy distribution from the photodissoci-
Table 1: Yields of the XC6H4S(X =H, F, OCH3) radical.
*
ation of [D]4-fluorothiophenol at a backing pressure of 3 atm ( ) and
a(CCSD)
08 (X=H)
318 (X=F)
738 (X=OCH3)
*
0.5 atm ( ). The distributions are normalized for the comparison. A
long tail at the high translational energy region in the distribution at
0.5 atm indicates that the molecule in the jet is vibrationally hot.
57%
74%
91%
43%
26%
9%
two different jet conditions. As the helium backing pressure
goes down from 3 atm to 0.5 atm, the temperature of the jet
increases because of the increase of the seeding ratio. The
Further exploration has been carried out for 4-fluoro-
thiophenol. According to the TDDFT calculations, the most
stable structure of [D]4-fulorothiophenol adopts a geometry
in which the C-C-S-D angle is 318 in the ground state, whereas
it has the minimum energy at the planar geometry in the S2
state (Figure 2). In this case, the initial wavepacket will be
located on the middle of the downhill potential along the C-C-
S-D angle. Therefore, the diabatic passage through the CI will
be less significant than for thiophenol but more probable than
for 4-methoxythiophenol, because the extent of the wave-
packet spread along the dihedral angle in the vicinity of the CI
is likely to be between those of the two other cases.
Experimental findings support this simple model, giving
DFT torsional frequency of 4-fluorothiophenol is 29 cmꢀ1 [23]
,
and it is most likely that a significant portion of the molecules
in the warm jet populate the vibrationally excited state with
several quanta of the C-C-S-D torsional mode. Since the
torsional barrier height is calculated to be very low in the
ground state for 4-fluorothiophenol (Figure 2), the UV
excitation of the molecule in the warm jet would result in
the widening of the initial wavepacket location in the two-
dimensional potential-energy surface along the C-C-S-D
dihedral angle. Therefore, the wavepacket will experience
more significant spread at the CI, giving the lower probability
˜
of diabatic passage to the X state of the fragment. Quite
˜
consistently, the experiment gives a yield of 0.18 for the X
˜
˜
0.26 and 0.74 for yields of the X and ffi states, respectively,
state of FC6H4S in the warm jet, which is much less than the X
for 4-fluorothiophenol (Table 1). It should be noted that the
yield of 0.26 measured for the molecule in the cold jet. This
experimental finding strongly supports the above two-dimen-
sional bifurcation dynamics at the CI.
In conclusion, we have employed the conformational
preference of thiophenol derivatives to manipulate the initial
location of the wavepacket along the C-C-S-D dihedral-angle
˜
Xꢀffi energy gaps of phenylthiyl derivatives (XC6H4SD; X =
H, CH3O, F) are found to be almost identical, thus indicating
that the relative energy of the SOMO localized on sulfur (3p)
ꢀ
is only slightly affected by the chemical substitution. The S D
bond energy is slightly affected by the chemical substitution.
The UV absorption spectra of all thiophenol derivatives
studied herein show very similar features at around 243 nm,
ꢀ
coordinate on two-dimensional PES leading to prompt S D
bond dissociation. The branching ratio between X and ffi
˜
!
where the S2 S0 transition is dominant.
states of the final fragment is controlled by the conforma-
tional preference induced by chemical substitution at the para
position of [D]thiophenol. The experimental trend is success-
fully explained by the simple wavepacket propagation model.
Namely, the wavepacket spread along the C-C-S-D dihedral
angle either through conformational control or vibrational
excitation strongly affects the probability of diabatic passage
through the conical intersection in such a way that the
probability of the diabatic passage at the CI diminishes as the
Another way of manipulating the wavepacket location on
S2 would be the selective excitation of the out-of-plane C-C-
S-D torsional mode in the ground state. However, such an IR–
UV double excitation scheme is experimentally quite
demanding, as the actual torsional mode frequency is too
low (less than 100 cmꢀ1) to be excited by a conventional IR
laser pulse. In this case, the backing pressure of the supersonic
jet is varied to change the vibrational temperature of the
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Communications
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˜
special meaning in the case of the phenylthiyl radical, since it
is now possible to control the intramolecular orbital align-
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Experimental Section
Detailed experimental conditions have been described else-
where.[18,24] Briefly, all samples were heated to 608C, mixed with the
He carrier gas, and expanded into vacuum. The D images were taken
for all velocity components and averaged over 36000 laser shots. The
three-dimensional images were reconstructed from raw images using
the LV-pBASEX[25] or BASEX[26] algorithm. Total translational
energy distributions were corrected by the Jacobian factor.
[17]J. S. Lim, I. S. Lim, K. S. Lee, D. S. Ahn, Y. S. Lee, S. K. Kim,
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[18]I. S. Lim, J. S. Lim, Y. S. Lee, S. K. Kim, J. Chem. Phys. 2007, 126,
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[19]Z. Lan, W. Domcke, V. Vallet, A. L. Sobolewski, S. Mahapatra, J.
Chem. Phys. 2005, 122, 224315.
[20]M. Abe, Y. Ohtsuki, Y. Fujimura, Z. Lan, W. Domcke, J. Chem.
Phys. 2006, 124, 224316.
Received: November 22, 2007
Keywords: conformational isomerism · conical intersections ·
.
photodissociation · reaction mechanisms
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