Direct observation of carbene and diazo formation from aryldiazirines by ultrafast infrared spectroscopy

Article abstract of DOI:10.1021/ja805922b

Ultrafast laser flash photolysis (λ_ex = 270 nm) of phenyldiazirine produces transient infrared absorptions at 2040 and 1582 cm^-1. The first band is assigned to phenyldiazomethane, and the second is assigned to singlet phenylcarbene. This assignment is consistent with DFT calculations. Diazo band integration reveals that photoisomerization from diazirine to diazo occurs within a few picoseconds of the laser pulse. The majority of carbene produced is also formed instantaneously. Copyright

Full text of DOI:10.1021/ja805922b

Published on Web 11/11/2008
Direct Observation of Carbene and Diazo Formation from Aryldiazirines by
Ultrafast Infrared Spectroscopy
Yunlong Zhang,† Gotard Burdzinski,‡ Jacek Kubicki,‡ and Matthew S. Platz*,†
Department of Chemistry, The Ohio State UniVersity, 100 West 18th AVenue, Columbus, Ohio 43210, and Quantum
Electronics Laboratory, Faculty of Physics, Adam Mickiewicz UniVersity, 85 Umultowska, Poznan 61-614, Poland
Received August 7, 2008; E-mail: Platz.1@osu.edu
Diazirines are useful precursors for mechanistic studies of
carbenes.1,2 The photoisomerization of diazirines to diazo com-
pounds and their light induced fragmentation to form carbenes
(Scheme 1) has motivated matrix isolation and nanosecond (ns)
time-resolved spectroscopic studies.3
Scheme 1. Photochemical Reactions of Phenyldiazirine
Ultrafast time-resolved (fs, ps) laser flash photolysis (LFP) studies
with UV-vis and IR detection are needed to understand early events
in the photochemistry of carbene precursors. We have recently
reported the use of ultrafast UV-vis spectroscopic studies of
arylhalodiazirines4 and IR studies of carbonyl diazo compounds
and their related carbenes.5 Herein, the application of ultrafast IR
spectroscopy to the study of the photochemistry of simple aryl-
diazirines and the observation of singlet carbenes is described.
Ultrafast LFP (270 nm) of chlorophenyldiazirine in chloroform
produces the transient IR spectrum shown in Figure 1. Two broad
bands are observed within a few picoseconds of the laser pulse.
Figure 1. Transient IR spectra produced by photolysis of chlorophenyl-
These bands narrow and shift to the blue over 100 ps, then center
at 2040 and 1583 cm-1, and do not decay within the detection
window (3 ns).
diazirine (λex ) 270 nm) in chloroform at selected time delays; (a) The
diazo band in the region 2060-1940 cm-1 and (b) the carbene band in the
region 1600-1550 cm-1
.
These transient absorption bands can be readily assigned to the
diazo band of chlorophenyldiazomethane (PhCN2Cl) and a CdC
vibrational band of singlet chlorophenylcarbene (1PhCCl), re-
spectively, on the basis of matrix IR3 and solution phase ns time-
resolved IR6 spectroscopic studies and computational7 work. The
blue shift and band narrowing are characteristic of vibrational
cooling (VC) of intermediates born with excess vibrational energy.8
The diazo band integration reveals that the majority of diazo
compound is formed instantaneously (<1 ps, Figure S1). The
growth of carbene 1PhCCl can not be accurately determined because
the CdC band of hot carbene 1PhCCl# severely overlaps bleaching
centered at 1568 cm-1. We have reported that ultrafast photolysis
(350 nm) of chlorophenyldiazirine produces a zwitterionic species,
detected by UV-vis spectroscopy, which undergoes biexponential
decay. The fast process has a time constant of 0.9 ps in
acetonitrile.4,9 In the interest of economy we associate the growth
of the diazo compound with the “fast” decay of the zwitterion
(Supporting Information (SI), Scheme S1).
of the laser pulse. It narrows and shifts to the blue at 2064 cm-1
over ∼100 ps. This band does not decay within the detection
window (3 ns) and is confidently assigned to the diazo band of
phenyldiazomethane which is born with excess energy and under-
goes VC.
Another transient band, centered at 1582 cm-1, was observed in
the region 1640-1550 cm-1 in chloroform (Figure 2). This initially
broad band is also formed within a few picoseconds of the laser
pulse, and it grows and narrows with a blue shift and reaches its
maximum intensity at 50 ps, post laser pulse. This band (monitored
at 1582 cm-1) decays with a lifetime of 414 ps in chloroform (SI,
Figure S3). Three negative bands at 1625, 1608, and 1580 cm-1
were also observed immediately after the laser pulse due to
depletion of the ground-state diazirine. The carrier of the transient
absorption band at 1582 cm-1 can be assigned to singlet phenyl-
carbene (1PhCH). DFT calculations predict that singlet 1PhCH has
a CdC vibration at 1571 cm-1 (SI, Table S1), in good agreement
with experiment. This is also consistent with the observed singlet
chlorophenylcarbene (1PhCCl) CdC vibration at 1583 cm-1 and
Ultrafast photolysis of phenyldiazirine in acetonitrile produces
the transient IR spectra shown in Figure S2 (SI). A broad band in
the region 1900-2100 cm-1 was formed within a few picoseconds
1
singlet NpCCO2CH3 which has a CdC vibrational band at 1584
cm-1.5 This is the first direct spectroscopic observation of singlet
1PhCH, and it provides the first direct measurement of its lifetime.
† The Ohio State University.
‡ Adam Mickiewicz University.
9
16134 J. AM. CHEM. SOC. 2008, 130, 16134–16135
10.1021/ja805922b CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Matrix ESR10 and infrared11 spectroscopy and computational
studies12-14 confirm that PhCH has a triplet ground state. Thus,
the decay of singlet 1PhCH in this solvent will lead to either triplet
3PhCH or a chlorobenzyl radical (PhCHCl) produced by Cl
abstraction from chloroform. DFT calculations predict that triplet
3PhCH has its CdC vibration at 1543 cm-1 and the radical PhCHCl
in chloroform.17 The lifetime of carbene BpCH in chloroform is
1
longer than that in dichloromethane (382 ps, SI, Figure S7b) and
much longer than that in cyclohexane (77 ps).18 Singlet carbene
lifetimes are known to be extended by halogenated solvents due to
complexation of the empty p orbital of the carbene with nonbonding
electron pairs of solvent atoms which accounts for the data obtained
in this study.19 In two highly reactive solvents, methanol-O-d and
cyclohexene, the BpCH carbene lifetimes are reduced to 19 and
38 ps (SI, Figure S7c,d) respectively, consistent with the assignment
of the transient IR band to the singlet carbene.
3
CdC band will be observed at 1550 cm-1. Neither triplet PhCH
1
nor radical PhCHCl was observed in this experiment, presumably
because of their low intensity (calculated to be 0.2 for 3PhCH and
1
PhCHCl), compared with that of singlet PhCH (99.3) (SI, Table
S1).
Biphenyldiazomethane formation was also observed by ultrafast
IR spectroscopy (SI, Figure S8), and the diazo band integration
indicates that the majority of the diazo compound is formed in the
isomerization process within a few picoseconds of the laser pulse,
consistent with the results obtained with chlorophenyldiazirine.
In conclusion, ultrafast IR spectroscopy is a valuable tool for
studying the photochemistry of aryldiazirines and allows study of
the dynamics of its photoisomerization to diazo compounds and
the first direct observation of singlet phenylcarbene. Both the diazo
compound and the majority of carbene are formed within a few
picoseconds of the laser pulse.
Acknowledgment. This work was performed at The Ohio State
University Center for Chemical and Biophysical Dynamics. Support
of this work by the NSF and the Ohio Supercomputer Center is
gratefully acknowledged. G.B. thanks MF EOG and FNP for
”Homing” grant in 2008.
Supporting Information Available: Descriptions of the ultrafast
spectrometer, the zwitterion structure (Scheme S1), details of calcula-
tions (Table S1), and the transient spectra and kinetics (Figures S1–S8).
This material is available free of charge via the Internet at http://
pubs.acs.org.
Figure 2. Transient IR spectra produced by photolysis of phenyldiazirine
(270 nm) in chloroform. (a) Formation of the carbene 1PhCH band, and (b)
1
decay of carbene PhCH band at 1582 cm-1
.
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