Femtosecond dynamics of dioxygen - picket-fence cobalt porphyrins: Ultrafast release of O2 and the nature of dative bonding

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phenyl)porphyrin that only about 0.8% of the cobalt centers
would have O2 as a sixth ligand in toluene solutions saturated
Femtosecond Dynamics of Dioxygen ± Picket-
Fence Cobalt Porphyrins: Ultrafast Release of
O2 and the Nature of Dative Bonding**
with air.[6] Although more elaborate types of porphyrins
[1, 7]
exhibit higher affinities for O2
we were attracted to the
Beat Steiger, J. Spencer Baskin, Fred C. Anson, and
Ahmed H. Zewail*
original picket-fence cobalt porphyrin (Figures 1 and 2)
because its structure is relatively close to our previously
examined cobalt tetraphenylporphyrins, it binds O2 with high
affinity at room temperature, and the binding is reversible.
The steady-state absorption spectra of the oxygenated and
deoxygenated picket-fence cobalt porphyrin (CoP) are differ-
The natural systems of hemoglobin and myoglobin form
reversible complexes with O2, which are involved in the
transport of O2 in biological systems. The protein environ-
ment surrounding the metal center in these molecules controls
the rate of the binding of O2 and discriminates against
poisonous carbon monoxide. Synthetic models of these
systems have been designed to optimize the binding of O2
and include ªpicket-fenceº, ªpocketº, ªbasket-handleº, ªpic-
nic-basketº, and ªcappedº porphyrins.[1] These models are
attractive systems for study because of their high affinities for
O2 at room temperature. Among the issues that can be
addressed with these model systems are: 1) the nature of the
elementary steps involved in the reactions of O2 complexes,
2) the influence of hydrogen bonding on the rates of O2
binding and release, and 3) the geometry of the O2 ± metal
complex on the time scale of the dynamics.
Herein we report our first study of the femtosecond
dynamics of a) a four-coordinate picket-fence cobalt(ii) por-
phyrin in benzene at room temperature, b) its five-coordinate
1-methylimidazole (1-MeIm) adduct (used to enhance the
binding of O2), and c) the six-coordinate O2 complex of the
1-MeIm adduct (Figure 1). The transient behavior of the two
O2-free porphyrins is very similar to that for cobalt(ii)
tetraphenylporphyrin. However, a dramatic change in the
transient absorption is observed when the porphyrin is
oxygenated. These observations indicate the release of the
bound O2 from the active binding site with a total reaction
time of 1.9 Æ 0.4 ps. We relate such behavior to the nature of
the dative cobalt ± O2 bond and the charge redistribution
triggered by the direct femtosecond excitation of the complex
to the transition state for O2 release.
Figure 1. Structural model of the six-coordinate picket-fence cobalt(ii)
porphyrin examined in this study.
In a recent publication[2] we reported a correlation between
the electrocatalytic activity of tetraruthenated cobalt por-
[3, 4]
phyrins toward the reduction of O2
and the femtosecond
dynamics of their charge transfer states. We have also
investigated the femtosecond dynamics of cobalt(ii) tetraphe-
nylporphyrin (CoIITPP).[5] We were drawn to investigate the
dynamics of oxygenated cobalt porphyrins since the cobalt
center plays a key role in the catalyzed reduction of O2. The
coordination of O2 to most four-coordinate cobalt(ii) porphyr-
ins or five-coordinate amine adducts of cobalt(ii) porphyrins is
not favorable at room temperature. It has been estimated for
the 1-methylimidazole adduct of cobalt(ii) tetra(p-methoxy-
[*] Prof. A. H. Zewail, Dr. B. Steiger, Dr. J. S. Baskin, Prof. F. C. Anson
Laboratory for Molecular Sciences
Arthur Amos Noyes Laboratory of Chemical Physics
California Institute of Technology
Pasadena, CA 91125 (USA)
Fax : (‡ 1)626-796-8315
Figure 2. Top: Molecular structure of the model in Figure 1. Bottom:
Steady-state absorption spectra of (1-MeIm) ± CoIITpivPP in benzene at
room temperature: Solutions were equilibrated with 1 atm of argon
(dashed curve) or O2 (solid curve).
E-mail: zewail@its.caltech.edu
[**] This work was supported by the US National Science Foundation
(Laboratory for Molecular Sciences).
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ent in that the oxygenated form shows a definite red shift at
room temperature (Figure 2). A series of transient absorption
measurements using a pump wavelength of 545 nm and a
probe wavelength of 600 nm is shown in Figure 3. The upper
We performed similar experiments probing at 471 nm. In
the absence of O2 the longest component (with a 10 to 20 ps
lifetime) was relatively weak compared to that at 600 nm,
which matched the trend observed for CoIITPP.[5] In the
presence of O2 the 471 nm transient was dominated by a
200 fs decay component followed by a long-lived negative
plateau similar to that at 600 nm. However, the response was
complicated by at least three additional relaxation compo-
nents. The measurements at 600 nm are unique because they
probe only the tail of the Q-band absorption (see Figure 2).
Hoshino concluded from the transient absorption spectrum
of CoTPP-O2 in O2-saturated 2-methyltetrahydrofuran at low
temperature (140 ± 200 K)[10] that the product detected on the
nanosecond time scale (following photolysis at 532 nm) was
ground-state CoIITPP, which subsequently recombined (non-
geminately) with O2 on the microsecond time scale. Similarly,
in our experiments B-CoP is produced at long times in its
ground state. A negative change in the transient absorption at
600 nm, as seen in the transient shown in Figure 3B, is then
predicted because the B-CoP complex absorbs only weakly at
600 nm (Figure 2). Lending further support to this assignment
of the product is the fact that the spectrum and transient
behavior observed prior to the addition of O2 were fully
recovered when the same sample was again placed under
argon, which demonstrated that no significant oxidation or
other degradation of the porphyrin sample had occurred
during the experiments under O2 saturation.
Figure 3. Femtosecond transient absorption at 600 nm of picket-fence
cobalt porphyrin in benzene with excitation at 545 nm. A) Transients
measured under argon, with and without complexation by (4 mm) 1-MeIm
(base); B) Transient measured in 4 mm 1-MeIm under 1 atm of O2, and the
transient for pure benzene.
The fact that the 600 nm absorbance does not recover
appreciably on the nanosecond time scale shows that the
extent of geminate recombination of O2 and B-CoP is small.
This behavior contrasts with the 43% recombination effi-
ciency reported for O2 and picket-fence iron porphyrin, B-FeP
(B ˆ 1-MeIm), in toluene with excitation at 314 nm.[11] It is
interesting to note that the photoinduced release of CO from
myoglobin is characterized by an absence of geminate
recombination.[12] In that case the direct monitoring of the
CO has shown that it is released from the metal coordination
site and trapped in a pocket of the heme protein within 500 fs.
The protein environment surrounding the trapping site then
undergoes relaxation with a time constant of 1.6 ps.[13] How-
ever, it is not necessary to invoke such a trapping mechanism
in all cases of low geminate recombination since ligand ±
porphyrin association rates depend strongly on the specific
ligand and metal center. The contrast in the geminate
recombination of O2 with B-CoP and B-FeP is consistent
with the smaller equilibrium binding of O2 with B-CoP.[14, 15]
The nature of the ultrafast dynamics of the O2 release can
now be addressed. The first steps parallel those previously
deduced for CoTPP.[5, 16] Relaxation within the porphyrin
system (200 fs, prominent only when probing at 471 nm)
precedes porphyrin-to-metal electron-transfer, but the latter
occurs at an enhanced rate (500 fs as opposed to 1 ± 2 ps)
because of the dative bonding of cobalt and O2, as discussed
below. (The absence of a clear 200 fs component at 600 nm is
understandable because the 600 nm probe is expected to be
trace in Figure 3A is the transient absorption of CoP in
benzene at room temperature under 1 atm of argon. The
temporal evolution is very similar to that reported previously
for CoIITPP.[5] It consists of three exponential decay compo-
nents (having relative amplitudes of 57%, 34%, and 9%,
respectively) with lifetimes indicated in the figure. The
measurement was repeated in the presence of 4 mm 1-MeIm
(referred to as base, B), which converts more than 98% of the
porphyrin into five-coordinate B-CoP,[8] and resulted in the
generation of the second trace in Figure 3A. Although the
lifetimes are somewhat shortened, and the relative amplitudes
change, the three decay components are still present.
Figure 3B shows the response obtained after the B-CoP ‡
4 mm 1-MeIm solution was saturated with O2, which converts
approximately 85% of the porphyrin into B-CoP-O2.[9] Since
the absorbance of B-CoP-O2 at 545 nm is higher than that of
B-CoP (see Figure 2), we estimate that about 90% of the
photoexcited porphyrin contains coordinated O2. The pres-
ence of O2 causes a dramatic change in the transient behavior:
following the absorbance increase at t ˆ 0, there is a sub-
stantial bleaching, or decrease in absorbance, at longer times.
The long-lived plateau of the bleaching signal is reached
within 4 to 5 ps. This plateau does not decay appreciably over
the maximum range of our delay time, which indicates a
recovery lifetime of at least 20 ns. (A slight (<10%) decrease
in this plateau level appears to occur on a time scale of tens of
picoseconds).
less sensitive to the initial state than is the 471 nm probe.[5]
)
The strong O2 bonding in the adduct ground state (DH ˆ
1
12.2 kcalmol )[14] has significant CoIII-O2 character and
is made possible only through the influence of the base on the
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polarization and power of both beams were adjusted as required (pump
power 2 mJ, probe power 50 nJ, magic angle probing), lightly focused (spot
size ca. 0.5 mm), and crossed at a small angle (ca. 28). The probe power was
detected after the cell by a photodiode and the gated and integrated signal
was normalized to a similarly processed signal from a photodiode sampling
the probe power before the cell. The change in absorbance was obtained
from the signals with and without pump excitation as determined by the
chopper. The total temporal response of the experiments was determined
to be 300 ± 350 fs full width at half maximum height.
metal center; the behavior of the transient profile seen in
Figure 3A shows that the perturbation of the porphyrin
system by the base alone is not severe.
The dative bonding of O2 to cobalt depends critically on the
interaction of the dz2 orbital on Co with the p* orbital on O2
(through a s-type interaction) and on the back-bonding from
the metal d-p orbitals to O2. The noncolinear configuration
enhances the s-bonding, and, because the dz2 electron of Co is
engaged in bonding, the oxidation state of Co approaches
‡3.[17, 18, 19] Following femtosecond excitation the charge
transfer from the porphyrin-based p system to the base-
metal-O2 system restores an electron to the cobalt dz2 orbital
and weakens the metal-O2 bond. Reverse charge transfer and
release of the O2 can then occur in concert to produce the free
porphyrin in its ground state in 2 ps. That this process occurs
faster than the 10 ps decay of the charge-transfer state in the
absence of oxygen (Figure 3A) may be related to the different
natures of the charge-transfer state with and without O2. The
porphyrin-to-metal charge-transfer state of B-CoP has a
measurable absorption at 600 nm as evidenced by the 10 ps
component in Figure 3A. The fact that no 10 ps decay
component appears in the transient of B-CoP-O2 means that
B-CoP was not formed in the excited state, and thus confirms
that release of O2 directly produces the B-CoP complex in its
ground state.[20]
The picket-fence free base porphyrin, 5,10,15,20-tetrakis(a,a,a,a-2-pival-
amido-phenyl)porphyrin (a,a,a,a-H2TpivPP), was synthesized by first
preparing the 5,10,15,20-tetrakis(2-nitrophenyl)porphyrin in 11% yield
by condensing 2-nitrobenzaldehyde with pyrrole in boiling glacial acetic
acid according to the procedure of Collman et al.[25] The crude product was
used as obtained in the further reaction in which the nitro groups were
converted into the amines by reduction with excess SnCl2 in concentrated
HCl at 65 ± 708C.[25] The crystalline product, 5,10,15,20-tetrakis(2-amino-
phenyl)porphyrin (H2TamPP), isolated in 92% yield, was a mixture of the
four atropisomers in statistical abundance (a,b,a,b-:a,a,b,b-:a,a,a,b-:
a,a,a,a-atropisomerˆ 1:2:4:1), as indicated by thin layer chromatography
on silica gel with benzene:diethyl ether (1:1). The a,a,a,a-atropisomer was
obtained in 60% yield by conversion of the random mixture of atropisom-
ers by the silica gel/benzene reflux method,[26] followed by purification by
column chromatography and crystallization from CHCl3/MeOH mixtures.
1
The 500 MHz H NMR and UV/Vis spectra of a,a,a,a-H2TamPP were in
agreement with those previously reported.[26]
a,a,a,a-H2TamPP was reacted with pivaloyl chloride and the product was
purified by column chromatography on silica gel followed by recrystalliza-
tion from a CHCl3/EtOH/heptane mixture to give pure a,a,a,a-H2TpivPP,
as confirmed by the 500 MHz 1H NMR and UV/Vis spectra. The picket-
fence cobalt(ii) porphyrin, [5,10,15,20-tetrakis(a,a,a,a-2-pivalamidophe-
The total reaction time is 2 ps. In general, the release of O2
involves the breakage of the bond and the reorganization
around the active site, as discussed for myoglobin ± CO.[13] It
should be noted that the dative character of the bond could
lead to two different channels: either homolytic cleavage to
release O2 or heterolytic cleavage to form superoxide. Our
results, as in the case of ref. [10], support the homolytic
nyl)porphyrinato]cobalt(ii) (CoIITpivPP), was prepared by heating
a
solution of a,a,a,a-H2TpivPP in THFat 508C for 2 h with excess anhydrous
CoCl2 and 2,6-lutidine.[14] The product was purified by chromatography on
alumina followed by crystallization from benzene. The 500 MHz 1H NMR
and UV/Vis spectra of a,a,a,a-CoIITpivPP were in agreement with those
previously reported.[14, 27]. The photo- and thermal isomerization of several
picket-fence systems have been studied.[28] The thermal process is
negligible at room temperature and the photochemical process occurs
only in long-lived triplet states with very low yields; unlike these systems,
CoIITpivPP has an ultrashort excited state lifetime.
1
channel: the ground state of B-CoP is dz2 , similar to the CT
state of B-CoP-O2, while the ground state of the latter is
essentially dz20. Our findings may be relevant to other recent
studies, including a report of the photoejection of O2 in less
than 2 ps from an oxygenated CoIIporphyrin ± AlIIIphthalo-
cyanine aggregate[21] and studies on oxyhemoproteins (oxy-
hemoglobin, oxymyoglobin, etc.) in fast spectroscopic experi-
Typically, we performed
a sequence of four femtosecond transient
absorption measurements. We first measured the transient absorption of
CoIITpivPP in benzene, freshly passed through a column of alumina, at
room temperature and under 1 atm of argon in a 1 mm path-length glass
cell (with reservoir, hi-vac valve, and a 24/40 joint adapter closed with
septum); the concentration of the porphyrin was adjusted to yield a static
absorbance of about 0.2 at 550 nm. 1-Methylimidazole (redistilled) was
subsequently added, still under 1 atm of argon, to produce a 4 mm solution.
The visible absorption spectrum showed bands at 508 and 530 nm. The
transient absorption measurement was then repeated. For the next
experiments, O2 at 1 atm was bubbled through the solution at room
temperature until the absorption band at 548 nm (B-CoP-O2) showed no
further change. Preparation of the sample for the final run involved
bubbling argon through the solution until the visible absorption spectrum
for B-CoP was recovered.
ments by Petrich et al.,[22] and others, as summarized by
[23]
Ï
Sima.
In conclusion, excitation of the O2 adduct of the picket-
fence porphyrin within the Q-band leads to ultrafast (2 ps)
ejection of O2, which is a reflection of the influence of charge
redistribution within the complex on the O2-Co dative bond.
The entry to the transition state of these systems allows us to
study the direct evolution to products and to dissect the
elementary steps, similar to what has been achieved in
elementary charge-transfer systems.[24] There is a host of
additional experiments to be carried out on these novel
models.
Received: September 23, 1999 [Z14060]
[1] J. P. Collman, L. Fu, Acc. Chem. Res. 1999, 32, 455 ± 463, and
references therein.
[2] H. Z. Yu, J. S. Baskin, B. Steiger, F. C. Anson, A. H. Zewail, J. Am.
Chem Soc. 1999, 121, 484 ± 485.
Experimental Section
[3] B. Steiger, F. C. Anson, Inorg. Chem. 1997, 36, 4138 ± 4140.
[4] F. C. Anson, C. Shi, B. Steiger, Acc. Chem. Res. 1997, 30, 437 ± 444.
[5] H. Z. Yu, J. S. Baskin, B. Steiger, C. Z. Wan, F. C. Anson, A. H.
Zewail, Chem. Phys. Lett. 1998, 293, 1 ± 8.
Our laser system has been described in detail elsewhere.[5] Basically, an
amplified 790 nm pulse train (1.8 W, 1 kHz) from a Ti/Sapphire laser was
used to pump two optical parametric amplifiers to produce independently
tunable pump and probe beams. The pump beam was directed through a
variable delay line and a beam chopper and overlapped with the probe
beam in a 1 mm path-length glass cell containing the sample solution. The
[6] F. A. Walker, J. Am. Chem Soc. 1973, 95, 1154 ± 1159.
Â
[7] C. K. Chang, Y. Liang, G. Aviles, J. Am. Chem Soc. 1995, 117, 4191 ±
4192.
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[8] Calculated from the thermodynamic data in ref. [14].
[9] Calculated from the thermodynamic data in ref. [14]. The solubility of
O2 in benzene was taken to be 9.1 Â 10 3 m atm 1, see ref. [29].
[10] M. Hoshino, Chem. Phys. Lett. 1985, 120, 50 ± 52.
[11] T. G. Grogan, N. Bag, T. G. Traylor, D. Magde, J. Phys. Chem. 1994, 98,
13791 ± 13796.
Femtosecond Dynamics of Norrish Type-II
Reactions: Nonconcerted Hydrogen-Transfer
and Diradical Intermediacy**
Steven De Feyter, Eric W.-G. Diau, and
Ahmed H. Zewail*
[12] T. A. Jackson, M. Lim, P. A. Anfinrud, Chem. Phys. 1994, 180, 131 ±
140.
[13] M. Lim, T. A. Jackson, P. A. Anfinrud, Nat. Struct. Biol. 1997, 4, 209 ±
214.
[14] J. P. Collman, J. I. Brauman, K. M. Doxsee, T. R. Halbert, S. E. Hayes,
K. S. Suslick, J. Am. Chem. Soc. 1978, 100, 2761 ± 2766.
[15] J. P. Collman, J. I. Brauman, K. S. Suslick, J. Am. Chem. Soc. 1975, 97,
7185 ± 7186.
[16] C. D. Tait, D. Holten, M. Gouterman, Chem. Phys. Lett. 1983, 100,
268 ± 272.
[17] B. B. Wayland, J. V. Minkiewicz, M. E. Abd-Elmageed, J. Am. Chem
Soc. 1974, 96, 2795 ± 2801.
[18] A. Dedieu, M.-M. Rohmer, A. Viellard, J. Am. Chem. Soc. 1976, 98,
5789 ± 5800.
[19] I. Bytheway, M. B. Hall, Chem. Rev. 1994, 94, 639 ± 658, and references
therein.
[20] The B-CoP ground state is relatively transparent at 600 nm, but has a
large absorbance at 471 nm (see Figure 2), so processes such as
vibrational relaxation and solvation after release of the O2 may cause
a further evolution of the 471 nm absorbance, in addition to the
constant background (beyond 5 ps) arising from photo-bleaching of
B-Co-O2. For example, the observed 471 nm dynamics may reflect
relaxation of the picket structure surrounding the binding site or a
secondary interaction between that structure and the dissociated O2
that are undetected in the 600 nm data.
Herein we report our first real-time study of the dynamics
of Norrish type-II reactions: the intramolecular hydrogen
transfer and the reaction of the intermediate, diradical
species. The nuclear motions associated with the breakage
of C H and C C bonds and formation of O H and C C
bonds are studied for the series 2-pentanone, 2-hexanone, and
5-methyl-2-hexanone, using femtosecond time-resolved mass
spectrometry. The time scale for the ultrafast hydrogen atom
transfer (70 ± 90 fs) and diradical closure and cleavage (400 ±
700 fs) are obtained from the time evolution of the mass-
gated speciesÐthe observed and vastly different reaction
times indicate the nonconcerted nature of the two steps.
Density functional theory (DFT) calculations are also reported
to elucidate the energetics along the reaction path, and we
address the analogy with the McLafferty rearrangement in ion
chemistry.
Norrish type-I[1] and type-II[2] reactions are of fundamental
importance in photochemistry. The contrast between photo-
chemical and thermal reactions of ketones has been of interest
for more than 50 years. Elsewhere, the femtosecond (fs)
dynamics of Norrish type-I reactions have been reported.[3, 4]
In these reactions the a-cleavage is the pathway for product
formation. In contrast, in Norrish type-II reactions carbonyl
compounds containing g C H bonds undergo a 1,5-hydrogen
shift upon electronic excitation and yield new products by
cleavage and cyclization processes. Figure 1 lists the molec-
ular structures of the systems studied here; three have g C H
bonds and for calibration purposes one does not.
[21] T. Fournier, Z. Liu, T.-H. Tran-Thi, D. Houde, N. Brasseur, C.
La Madeleine, R. Langlois, J. E. van Lier, D. Lexa, J. Phys. Chem. A
1999, 103, 1179 ± 1186.
[22] J. W. Petrich, C. Poyart, J. L. Martin, Biochemistry 1988, 27, 4049 ±
4060.
Ï
[23] J. Sima in Structure and Bonding, Vol. 84 (Ed.: J. W. Buchler),
Springer, Berlin, 1995, pp. 135 ± 193.
[24] D. P. Zhong, A. H. Zewail, Proc. Natl. Acad. Sci USA 1999, 96, 2602 ±
2607, and references therein.
[25] J. P. Collman, R. R. Gagne, C. A. Reed, T. R. Halbert, G. Lang, W. T.
Robinson, J. Am. Chem. Soc. 1975, 97, 1427 ± 1439.
[26] J. Lindsey, J. Org. Chem. 1980, 45, 5215.
The literature is rich with detailed studies in the solution
and gas phases, with primary focus on the photochemistry of
the first excited S1 state.[2] As noted in these studies the
Norrish type-II reaction is deduced, from yield and quenching
experiments, to be on the nanosecond time scale and
competes with vibrational relaxation and intersystem cross-
ing; there is a barrier for excited singlet reactions of about
[27] H. Imai, K. Nakata, A. Nakatsubo, S. Nakagawa, Y. Uemori, E.
Kyuno, Synth. React. Inorg. Met. Org. Chem. 1983, 13, 761 ± 780.
[28] a) R. A. Freitag, J. A. Mercer-Smith, D. G. Whitten, J. Am. Chem. Soc.
1981, 103, 1226 ± 1228; b) R. A. Freitag, D. G. Whitten, J. Phys. Chem.
1983, 87, 3918 ± 3925.
[29] H. L. Clever, R. Battino in Solutions and Solubilities Part 1 in
Techniques of Chemistry, Vol. VIII (Ed.: M. R. J. Dack), Wiley, New
York, 1975, pp. 379 ± 441.
1
4 kcalmol . The studies reported here are at higher energy
(see Figure 2) and are designed to address the primary
ultrafast dynamics without complications from much slower
relaxation processes. However, the Norrish type-II elemen-
tary processes discussed here are the two described in ref. [2]
for the overall mechanism.
[*] Prof. A. H. Zewail, Dr. S. De Feyter, Dr. E. W.-G. Diau
Arthur Amos Noyes Laboratory of Chemical Physics
California Institute of Technology
Pasadena, CA 91125 (USA)
Fax: (‡1)626-792-8456
E-mail: zewail@caltech.edu
[**] This work was supported by the US Airforce Office of Scientific
Research and Office of Naval Research. S.D.F., a postdoctoral fellow
of the Fund for Scientific Research-Flanders, acknowledges a Ful-
bright scholarship and financial support by the Katholieke Universi-
teit Leuven and by Caltech.
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