sciencemag.org SCIENCEBy Wolfgang Domcke^1 and
Andrzej L. Sobolewski^2E
xcitation of polyatomic molecules with
visible or ultraviolet (UV) light to a
higher-energy electronic state results
in a complex competition between
radiative and radiationless electronic
decay processes and photochemical re-
actions. Although the time evolution of the
population probability of excited electronic
states has been extensively explored with
time-resolved laser spectroscopy in recent
decades, the accompanying nuclear
motion could so far not be resolved at
the fastest (femtosecond) time scales.
On page 885 of this issue, Yang et al.
( 1 ) report the simultaneous experi-
mental detection of the excited-state
decay and the associated deformation
of the nuclear frame with subpicosec-
ond time resolution and subnanome-
ter structural resolution for the exam-
ple of the pyridine molecule.
The simplest framework for de-
scribing the motion of nuclei and
electrons in a molecule is the Born-
Oppenheimer (BO) approximation,
which assumes that the much faster
electronic motions can be calculated
separately from the much slower
nuclear motions. Traditionally, radia-
tionless transitions in polyatomic mol-
ecules were described in a theoretical
framework that assumes weak devia-
tions of the nuclear motion from the
BO approximation. This concept was
appropriate for decay time scales on
the order of nanoseconds, but femto-
second time-resolved laser spectros-
copy provided ample evidence that
radiationless transitions can occur
on ultrafast time scales ( just tens of
femtoseconds) that approach the periods of
high-frequency vibrations. This evidence re-
quired a profound conceptual revision of the
description of radiationless transitions.
The current understanding of ultra-
fast radiationless transitions is that they
are driven by so-called conical intersec-
tions (CIs), which are manifolds of exactdegeneracy of electronic potential-energy
surfaces at which divergent non-BO cou-
plings cause a complete breakdown of the
BO approximation ( 2 ). For example, the
first excited state and the ground state of
a molecule have the same energy at a CI
of these states, but the nuclear motion will
be subject to different forces. Ultrafast ex-
cited-state deactivation through CIs plays
an essential role in the protection of fun-
damental biological molecules (such as
DNA and proteins) from photodamage by
UV radiation ( 3 ).The photochemistry of benzene and of
aza-arenes such as pyridine played a para-
digmatic role for the understanding of
radiationless transitions through the dis-
covery, in the early 1970s, of the so-called
“channel-three” phenomenon, which is a
sudden increase in the radiationless decay
rate at a certain excess energy in the lowest
excited singlet (S 1 ) state ( 4 ). Extensive spec-
troscopic studies attributed the channel-
three effect to an abrupt onset of intrastate
vibrational relaxation (IVR) and a sudden
shortening of the lifetime of vibrationallevels ( 5 ). The generic mechanism behind
the channel-three effect was revealed by
early ab initio calculations for benzene (6,
7 ). It involves a low-barrier reaction path
to a biradical structure which was termed
“prefulvene” ( 8 ) because it is geometrically
related to the valence isomer fulvene. Along
the reaction path to prefulvene, a low-lying
CI exists at which ultrafast decay from the
S 1 energy surface to the energy surface of
the electronic ground state (S 0 ) can occur.
In pyridine, the lowest singlet excited
state is of np* character, where n denotes a
“nonbonding” orbital mainly localized
on the nitrogen atom and p* denotes
the lowest unoccupied orbital that is
delocalized over the six-membered
ring. Qualitative potential-energy pro-
files along the reaction path to the pre-
fulvenic form of pyridine are displayed
in the figure. The energy profile of the(^1) np state is crossed by the energy pro-
file of the^1 pp state at a CI marked CI 1.
Beyond a plateau, the^1 pp energy in
turn crosses the energy profile of the S 0
state at the CI marked CI 2. This specific
model proposed in the 1990s estab-
lished a direct relation between a pho-
tophysical phenomenon (radiationless
decay) and a photochemical reaction
(photoisomerization to fulvene) ( 9 ).
Although this general scenario of
ultrafast radiationless decay is now
widely accepted, it has not been con-
firmed so far by direct experimental
observation. Time-resolved popula-
tion probabilities of electronic states
can now be measured routinely by
femtosecond laser spectroscopy with
a variety of detection schemes. Time
scales of ultrafast radiationless tran-
sitions have been established for nu-
merous molecular systems, but these
measurements do not provide information
on the nuclear motion driving the elec-
tronic transition. Molecular structure can
be determined with diffraction methods,
and electron diffraction (ED) can be applied
to gas-phase samples. In the 1990s, Zewail
and co-workers pioneered the development
of nonstationary (time-resolved) ED ( 10 ).
A femtosecond UV pump pulse excites the
molecular sample, and diffraction of a time-
delayed electron pulse provides structural
information. However, the time resolution
was limited to ~10 ps.
CHEMICAL PHYSICS
Tracking both ultrafast electrons and nuclei
Electron diffraction correlates the excited-state decay of pyridine with its ring distortion
INSIGHTS | PERSPECTIVES
GRAPHIC: A. KITTERMAN/
SCIENCE
FROM A. SOBOLEWSKI
(^1) Department of Chemistry of the Technical University of
Munich, Munich, Germany.^2 Institute of Physics of the
Polish Academy of Sciences in Warsaw, Warsaw, Poland.
Email: [email protected]
Excitation and decay pathway
Pyridine in its electronic ground state (S 0 ) is photoexcited into
the np (red) excited state. Its relaxation pathway bypasses conical
intersection 1 (CI 1 ) to the pp (blue) excited state toward the
prefulvenic structure. At CI 2 , the transition back to the S 0 state occurs.
p
n
Wave
function
signs
5 .0 p
4.0
3.0
2.0
1.0
0.0
Prefulvenic reaction coordinate
Energy (eV)
(^1) np
(^1) pp
S 0
CI 1
CI 2
Photon
Photophysics of pyridine
Ya n g et al. used ultrafast electron diffraction to reveal both the
structural and electronic changes that occur when photoexcited
pyridine relaxes back to the ground state. The excited state has a
distorted (prefulvenic) structure.820 22 MAY 2020 • VOL 368 ISSUE 6493
Published by AAAS