Femtosecond dynamics of isolated phenylcarbenes

Article abstract of DOI:10.1021/ja804133c

Understanding the primary photophysical processes in molecules is essential for interpreting their photochemistry, because molecules rarely react from the initially excited electronic state. In this study the ultrafast excited-state dynamics of chlorophenylcarbene (CPC) and trifluoromethylphenylcarbene (TFPC), two species that are considered as models for carbene dynamics, were investigated by femtosecond time-resolved pump probe spectroscopy in the gas phase. Their dynamics was followed in real time by time-resolved photoionization and photoelectron imaging. CPC was excited at 265 nm into the 3 ¹A′ state, corresponding to excitation from a π-orbital of the aromatic ring into the LUMO. The LUMO contains a contribution of the p-orbital at the carbene center. Three time constants are apparent in the photoelectron images: A fast decay process with τ₁ ≈ 40 fs, a second time constant of τ₂ ≈ 350 fs, and an additional time constant of τ₃ τ 1 ps. The third time constant is only visible in the time-dependence of low kinetic energy electrons. Due to the dense manifold of excited states between 3.9 and 5 eV, known from ab initio calculations, the recorded time-resolved electron images show broad and unstructured bands. A clear population transfer between the states thus can not directly be observed. The fast deactivation process is linked to either a population transfer between the strongly coupled excited states between 3.9 and 5.0 eV or the movement of the produced wave packet out of the Franck-Condon region. Since the third long time constant is only visible for photoelectrons at low kinetic energy, evidence is given that this time constant corresponds to the lifetime of the lowest excited A ¹A′ state. The remaining time constant reflects a deactivation of the manifold of states in the range 3.9-5.0 eV down to the A ¹A′ state. Copyright

Full text of DOI:10.1021/ja804133c

Published on Web 10/17/2008
Femtosecond Dynamics of Isolated Phenylcarbenes
†,‡
†
†
,‡
Bastian Noller, Lionel Poisson, Raman Maksimenka, Ingo Fischer,* and
,
†
Jean-Michel Mestdagh*
Laboratoire Francis Perrin CNRS-URA 2453, CEA, IRAMIS, SPAM, B aˆ t 522 F-91191 Gif-sur-YVette Cedex,
France, and UniVersity of W u¨ rzburg, Institute of Physical Chemistry, Am Hubland, D-97074 W u¨ rzburg, Germany
Received June 2, 2008; E-mail: jean-michel.mestdagh@cea.fr; ingo@phys-chemie.uni-wuerzburg.de
3
The ultrafast dynamics of two phenylcarbenes (R ) Cl, CF )
was investigated by femtosecond time-resolved pump probe spec-
1
troscopy in the gas phase. Carbenes are ideal model systems for
examining nonadiabatic coupling due to their large number of low-
2
lying electronically excited states. Spectroscopic experiments on
3
phenylcarbenes have mainly been performed in rare gas matrices.
Experiments on isolated triatomic halocarbenes showed evidence
4
of predissociation. Time-resolved femtosecond spectroscopy docu-
5
ments the formation of phenylcarbenes by photolysis of diazirines,
but no information is available on the primary photophysical
processes of the phenylcarbenes themselves. This limits the
understanding of the carbene photochemistry since species rarely
6
react from the initially excited electronic state.
The experiments were performed in a standard molecular beam
apparatus. A cold beam of phenylcarbenes was prepared by
supersonic jet flash pyrolysis of the corresponding diazirines, as
shown in Scheme 1. Evidence proving clean carbene generation is
provided as Supporting Information.
Scheme 1. Supersonic Jet Flash Pyrolysis of Diazirines
Figure 1. Ion signal intensity of CPC (R ) Cl, top) and TFPC (R ) CF3,
bottom) as a function of the time delay (pump 265 nm, probe 795 nm).
calculation provides a zeroth-order picture of the 265 nm excitation
of CPC that is consistent with that of ref 9. Excitation takes place
into the fourth excited singlet state (3 A′). This transition holds
Both carbenes show a strong absorption at 265 nm. They were
excited at this wavelength and ionized in a multiphoton process
with 795 nm light. Figure 1 depicts the time dependence of the
1
7
by far the largest oscillator strength in the calculations and is
predominantly described by a transfer of an electron from a π orbital
of the aromatic ring (MO 30) into the LUMO (MO 33) of the
molecule, which contains a contribution of the p-orbital at the
carbene center. Several electron configurations contribute to the
ion signal for chlorophenylcarbene (CPC) in the top trace and
trifluoromethylphenylcarbene (TFPC) in the bottom. Both inter-
mediates show a two step deactivation process after excitation. The
model used in the best fit (solid line) was a transient “I” decaying
to transient “II”, then to transient “III” relaxation convoluted by a
Gaussian (fwhm ) 105 fs for CPC, 95 fs for TFPC) instrument
1
1
levels 2 A′ to 4 A′′. They involve the promotion of an aromatic
8
electron. Note that there is a relatively large energy gap between
response function. For comparison the best fit to a one step
1
1
the A A′′ and the 2 A′ state of 2.4 eV, whereas the states from
deactivation model (transient “I” to “II”) is depicted as a dashed
line. As shown, it does not represent the decay properly. The model
with two time constants indicates a fast decay of the primary
1
1
2
A′ to 4 A′′ are close in energy and presumably strongly coupled.
The A A′′ state has a dominant electronic configuration corre-
1
sponding to the promotion of an electron at the carbene center (MO
populated transient “I” to transient “II” for both carbenes (τ
1
)
3
2 to MO 33).
4
0-60 fs) followed by a slower decay of transient “II” (τ ) 350
2
Additional information on the excited-state dynamics can be
fs for CPC and 500 fs for TFPC). A secure assignment of the two
relaxation steps to precise deactivation processes is uncertain using
the ion signal alone, but the observation excludes a single step
deactivation.
6
obtained from time-resolved photoelectron spectra (TRPES),
recorded by velocity map imaging and processed by the pBASEX
1
1
algorithm. The TRPES for CPC is depicted in Figure 3 as a
contour map. It presents the photoelectron intensity as a function
of kinetic energy (horizontal axis) and time delay (vertical axis).
Most of the signal originates from a broad band of electrons with
a kinetic energy between 0.3 eV and ∼0.9 eV. It shows the same
time dependence as the mass selected delay curve in Figure 1. For
a proper description of the time dependence of electrons with a
kinetic energy below 0.04 eV (see Supporting Information for a
Ab initio calculations are available for CPC, which is known to
9
have a singlet ground state. The corresponding energy diagram
10
has been reproduced by time dependent density functional theory
(
TD-B3P86/6-311++G**) and is shown in Figure 2. The latter
†
Laboratoire Francis Perrin CNRS-URA 2453.
University of W u¨ rzburg.
‡
1
4908 9 J. AM. CHEM. SOC. 2008, 130, 14908–14909
10.1021/ja804133c CCC: $40.75  2008 American Chemical Society

C O M M U N I C A T I O N S
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reaction channel. We do not observe any fragment signal to grow
in at long delays and expect internal conversion to be fast. Therefore
1
we suggest that τ
3
corresponds to the relaxation time of the A A′′
state to the ground state, forming vibrationally hot species.
Figure 3. TRPES of CPC excited with 265 and ionized with 800 nm by
multiphoton ionization, showing only the unpolarized part of the photo-
electron distribution. Slow photoelectrons (left) show an offset at long delay
times indicating the presence of a third time constant with small amplitude
Figure 2. Vertical excited states of CPC (TD-B3P86/6-311++G**).
Irradiation of the carbene around 265 nm excites the molecule into the fourth
excited singlet state.
(
see Supporting Information for an enlarged version).
3
In contrast to CPC, TFPC is known to have a triplet ground state.
comparison with high kinetic energy electrons), an additional time
Nevertheless the results of Figure 1 suggest that its initial dynamics
follows a similar scenario as the one found for CPC.
In conclusion we observe a first very rapid decay of the initially
prepared transient. A three step deactivation process takes place.
constant τ
3
≈ 1 ps is required. The individual contribution of each
of the three components to the overall decay is presented on the
left-hand side of Figure. 3.
Since the close-lying coupled excited states between 3.9 and 5.0
eV are not expected to result in separated bands in the TRPES,
1
The A A′′ state presumably deactivates within ∼1 ps to the hot
ground state, which holds enough internal energy for further
reactions. TDDFT calculations show that promotions of ring
electrons are involved in the initially prepared transition. Lower
excited states correspond to promotions of electrons from the
carbene center to the ring.
they presumably yield the observed broad feature at ∼0.5 eV in a
7
[
1 + 3′] ionization process. The TRPES is consistent with the
assumption that CPC deactivates in a three step process. With regard
to the IEvert of the carbene and the low electronic energy of the
1
A-state, multiphoton ionization out of the A A′′ state will produce
electrons with low kinetic energy. Thus ionization out of the A
A′′ state will produce electrons with low kinetic energy. The 1 ps
Acknowledgment. We thank the Deutsche Forschungsge-
meinschaft and the DAAD for support. B.N. acknowledges a
fellowship by the FCI and thanks Prof. Liu for discussions on
diazirine syntheses.
1
decay, which is observed for the low energy electrons only, can be
explained by the decay of this level. The signal will be weak due
to missing resonances. This is consistent with the observation in
Figure 1 that the ion signal apparently decays to zero after a long
period of time. Since autoionization from the higher electronic states
can also produce low kinetic electrons, all three time constants are
visible in this energy range. The other two time constants, 40 and
Supporting Information Available: Experimental setup, mass
spectra, TRPES, and computational details. This material is available
free of charge via the Internet at http://pubs.acs.org.
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3
50 fs, have to be assigned to the dynamics involving higher lying
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since the states are very close in energy, the pump pulse will
produce a wave packet superposing electronic as well as vibrational
states. A movement of the wave packet, incorporating geometric
deformations, out of the Franck-Condon region, also delivers an
explanation for the fast 40 fs decrease of the ion signal intensity.
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2
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JA804133C
J. AM. CHEM. SOC. 9 VOL. 130, NO. 45, 2008 14909