Nitric oxide deligation from nitrosyl complexes of two transition metal porphyrins: A photokinetic investigation

Article abstract of DOI:10.1021/ja962510s

The results of an investigation of the ultrafast dynamics of photoinduced deligation in two transition metalloporphyrin-nitrosyl complexes, TPPFe(II)NO and TPPCo(II)NO, in conjuction with the results of an energy transfer study lead to the conclusion that the difference in the denitrosylation yields (?(NO) = 0.5 for TPPFe(II)NO and ?(NO) = 1.0 for TPPCo(II)NO is the result of energy partitioning in the upper excited states of the porphyrin. The energy transfer study yielded the energies of the metal centered states, believed to be of CT(π,d(z)²) nature, and of the localized porphyrin triplet states. The CT states in the two complexes were found to lie at similar energies (TPPFe(II)NO 8650 cm^-1 and TPPCo(II)NO 8900 cm^-1); however, the localized porphyrin triplet states were found to be at 16200 cm^-1 in TPPFe(II)NO and 14700 cm^-1 in TPPCo(II)NO. This difference in energies of the respective triplet states facilitates efficient intersystem crossing in the excited state deactivation of TPPFe(II)NO, but does not allow any triplet formation in TPPCo(II)NO. The direct excitation studies revealed that intersystem crossing in TPPFe¹¹NO occurs with a rate constant of 7.3 x 10¹¹ s^-1 to yield a localized porphyrin triplet state that absorbs maximally at 450 nm. This state then relaxes back to the ground state without the loss of NO. Only those excited states that relax via the CT state result in loss of NO. The direct excitation studies yielded no evidence for intersystem crossing in the deactivation of the electronically excited singlet state of TPPCo(II)NO, hence all of the energy deposited in the initial photoexcitation step results in NO loss. The lifetimes and spectral characteristics of the other excited states involved in deactivation of these transition metalloporphyrin-nitrosyl complexes will be discussed.

Full text of DOI:10.1021/ja962510s

11798
J. Am. Chem. Soc. 1996, 118, 11798-11804
Nitric Oxide Deligation from Nitrosyl Complexes of Two
Transition Metal Porphyrins: A Photokinetic Investigation
Elisabeth A. Morlino and Michael A. J. Rodgers*
Contribution from the Center for Photochemical Sciences, Bowling Green State UniVersity,
Bowling Green, Ohio 43403
ReceiVed July 19, 1996X
Abstract: The results of an investigation of the ultrafast dynamics of photoinduced deligation in two transition
metalloporphyrin-nitrosyl complexes, TPPFeIINO and TPPCoIINO, in conjuction with the results of an energy transfer
study lead to the conclusion that the difference in the denitrosylation yields (φNO ) 0.5 for TPPFeIINO and φNO
)
1.0 for TPPCoIINO) is the result of energy partitioning in the upper excited states of the porphyrin. The energy
transfer study yielded the energies of the metal centered states, believed to be of CT(π,dz2) nature, and of the localized
porphyrin triplet states. The CT states in the two complexes were found to lie at similar energies (TPPFeIINO 8650
cm-1 and TPPCoIINO 8900 cm-1); however, the localized porphyrin triplet states were found to be at 16200 cm-1
in TPPFeIINO and 14700 cm-1 in TPPCoIINO. This difference in energies of the respective triplet states facilitates
efficient intersystem crossing in the excited state deactivation of TPPFeIINO, but does not allow any triplet formation
in TPPCoIINO. The direct excitation studies revealed that intersystem crossing in TPPFeIINO occurs with a rate
constant of 7.3 × 1011 s-1 to yield a localized porphyrin triplet state that absorbs maximally at 450 nm. This state
then relaxes back to the ground state without the loss of NO. Only those excited states that relax via the CT state
result in loss of NO. The direct excitation studies yielded no evidence for intersystem crossing in the deactivation
of the electronically excited singlet state of TPPCoIINO, hence all of the energy deposited in the initial photoexcitation
step results in NO loss. The lifetimes and spectral characteristics of the other excited states involved in deactivation
of these transition metalloporphyrin-nitrosyl complexes will be discussed.
Introduction
bound metal is one of the most important features in determining
the photoinduced deligation yields,8 although little is known
about the factors controlling denitrosylation in these compounds
because of (i) the very short time scale on which this process
takes place and (ii) the spectroscopic inaccessibility of the
internal metal states suspected to be involved. The electronic
excited states in these systems are inherently short-lived because
the transition metals have empty d-orbitals which can couple
to the porphyrin π-orbitals to form states of intermediate energy,
thereby facilitating rapid deactivation of the excited porphyrin
(π,π*) states.16,17 A recent communication from this laboratory
indicated that the deligation in a protein-free nitrosylmetallo-
porphyrin occurs on the sub-picosecond time scale from an
internal metal charge transfer state.18
Nitric oxide (NO) is a diatomic free radical that recently has
been found to be a significant biological entity in that it regulates
blood pressure, acts as a neurotransmitter, and aids in the
immune system’s ability to kill intracellular parasites.1-4 Like
the other diatomics O2 and CO, NO can act as an axial ligand
in transition metalloporphyrins (MP). A recent report shows
that nitric oxide binds to the heme center in mouse hemoglobin,
and the suggestion has been made that this also may be the
case in humans.5 Another report has shown that nitric oxide
binds to the ferric-heme prosthetic groups in nitric oxide
synthase (NOS), the dimeric enzyme responsible for NO
production in humans.6 This binding, in turn, deactivates the
enzyme, thus providing an unusual example of a self-regulating
enzyme whose products deactivate itself. The nitrosyl adducts
of metalloporphyrins are known to be light sensitive, losing nitric
oxide with quantum yields that vary from virtually zero to unity
when photons are absorbed by the porphyrin macrocyclic
π-system.7-15 It has been suggested that the identity of the
An intriguing question that generated our interest in this area
of photophysics concerns the way in which the incorporated
transition metal alters and controls the deactivation dynamics
and the deligation quantum efficiencies of the nitrosylmetallo-
porphyrin (NOMP) system. We have elected to avoid protein-
based MPs simply because protein dynamics have been cited19-24
* Author to whom correspondence should be addressed.
X Abstract published in AdVance ACS Abstracts, November 15, 1996.
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S0002-7863(96)02510-3 CCC: $12.00 © 1996 American Chemical Society

Nitric Oxide Deligation from Nitrosyl Complexes
J. Am. Chem. Soc., Vol. 118, No. 47, 1996 11799
Q-band maxima before each experiment was performed. Since TPPFeII-
NO appeared to be more air sensitive than the Co(II) analogue, all
solutions employing this compound were prepared under a nitrogen
atmosphere.
as causing complications in elucidating the core factors that
control deligation as well as determining the absolute deligation
yields. Except for the previously cited communication from
this laboratory, all the previous work done on the protein-free
nitrosylmetalloporphyrins has focused on longer time scale
studies where the recombination of NO and the free metal-
loporphyrin has been the issue of concern. However, our recent
study on the ultrafast dynamics of the TPPCoIINO system
demonstrated that these systems show a richness of photophysi-
cal dynamics which deserves investigation since understanding
them could well shed light on the processes leading to
deligation.18
Moreover, in order to gain access to the internal metal states
which are thought to be responsible for deligation in nitrosyl-
metalloporphyrins, an energy transfer study has been employed.
Stanford and Hoffman used such a study to examine the nature
and energies of the electronic states involved in photodeligation
from a carbonylmetalloporphyrin.25 They showed that the
decarbonylation occurs from a state with higher than singlet
multiplicity residing no higher than 14300 cm-1 in energy. In
the present study, the energies of the internal states of the
nitrosylmetalloporphyrin system as well as some information
concerning their identity are revealed through measurements
of energy transfer rate constants with a series of donors of
varying triplet energy. An analysis based on the Sandros
approach allows energy levels to be determined.26
The results presented in this paper were obtained in nitrogen- or
argon-saturated benzene (Fluka > 99.5% g.c.) solutions. All the
sensitizers employed in this study were either employed in prior studies
in this laboratory31 or used as received from the commercial suppliers.
Benzophenone was purchased from Fisher Scientific; phenanthrene,
pyrene, acridine, anthracene, 2,2′:5′,2′′-terthiophene, perylene, 1,8-
diphenyl-1,3,5,7-octatetraene (DPOT), 2,3-benzanthracene (tetracene),
pentacene, trans-â-carotene, and rubrene were all obtained from Aldrich
Chemical Co. SiPC ((Hex3SiO)2Si-phthalocyanine), GePC ((OSiEt3)2-
Ge-(OBu)8-phthalocyanine), SiNC ([SiO(i-C4H9)2n-C18H37]2Si-naph-
thalocyanine), and SnNC ([SiO(n-C6H13)3]2Sn-naphthalocyanine) were
all obtained from Professor Malcolm E. Kenney at Case Western
Reserve University.
Flash Photolysis. Nanosecond flash photolysis studies were
performed using a kinetic spectrophotometric detection system, previ-
ously described.32 Excitation pulses (7 ns) of light were generated:
(a) at 355 nm, the third harmonic of a Q-switched Nd:YAG laser
(Continuum YAG660, Continuum Surelite I); (b) in the range 420 to
680 nm, from an OPOTEK Magic Prism optical paramagnetic oscillator
(OPO) pumped by the third harmonic of the Nd:YAG laser; and (c) at
683 nm, the output of a home-built Raman shifter filled with 100 psi
of hydrogen gas and pumped by the second harmonic (532 nm) of a
Nd:YAG laser. The combination of these three systems allowed
optimal excitation of the sensitizer or of the porphyrin. Typical
excitation pulse amplitudes were a few millijoules per pulse. Transients
produced were followed temporally and spectrally by a computer-
controlled kinetic spectrophotometer, previously reported.32
Experimental Section
Materials. Nitrosylcobalt(II) meso-tetraphenylporphyrinate was
prepared by introduction of NO gas (Liquid Carbonic, CP grade),
previously scrubbed of higher oxides by passage through a KOH
column, into a nitrogen-saturated solution of TPPCoII (Midcentury
Chemicals) in benzene as described in previous publications.8,18 After
the mixture was bubbled for ∼20 min, NO-saturated MeOH was added
until precipitation of the adduct was complete. The reaction vessel
was evacuated and purged with N2. The resulting precipitate was
collected under N2, dried, and stored under vacuum. The identity of
the product was verified by comparison of its UV-visible absorption
spectrum to previously published spectra.8 The new band observed at
1694 cm-1 in the infrared absorption spectrum, taken in a KBr pellet,
corresponded to the bound NO stretching frequency.27-29
Nitrosyliron(II) meso-tetraphenylporphyrinate was prepared by the
reductive nitrosylation of TPPFeIIICl under conditions previously
reported.30 This involved introduction of NO gas into a nitrogen-
saturated chloroform/pyridine solution of TPPFeIIICl (Midcentury
Chemicals). After the mixture was bubbled with NO for ∼20 min, a
NO-saturated MeOH solution was added until precipitation was
complete. After the reaction flask was purged with N2, the resulting
precipitate was collected under N2, dried, and stored under vacuum.
The identity of this compound was again verified by comparison of its
UV-visible absorption spectrum in benzene to that of the previously
published spectrum8 and the observation of the NO stretching band at
1698 cm-1 in an infrared absorption spectrum taken in a KBr pellet.30
The identity of the product was checked by verifying the Soret and
Picosecond flash photolysis experiments were performed employing
either 355- or 532-nm pulses (30 ps) of a few millijoules per pulse
from an active-passive mode-locked laser system (Continuum
YAG571C). Typical pump-probe experiments were carried out using
a dual-diode-array detection system (Princeton Instruments ST120),
previously described,33
to monitor the ensuing transients. Kinetic
profiles were assembled by recording transient spectra taken at different
time delays between the pump and probe paths. All solutions employed
in these experiments were flowed to ensure the presentation of a fresh
sample to the excitation beam at each shot. The path lengths of the
cells varied from 1 cm to 2 mm in order to obtain optimal signals and
the solutions were prepared to have an absorbance of 0.7-1.0 at the
excitation wavelength in their respective cells.
The femtosecond experiments reported herein were performed at
the Chemistry Department at the University of Michigan (Ann Arbor).
These experiments employed excitation pulses of 400 or 390 nm light
generated by a regeneratively amplified Ti:sapphire laser system, details
of which can be found elsewhere.34 The kinetic profiles of the resulting
transients were followed by using an interference filter to select a
wavelength of interest from a white light continuum generated by
passing the fundamental wavelength through a 1 cm path length cell
of flowing ethylene glycol. Data were recorded using a pair of
photodiodes to monitor the difference in the absorption changes as a
function of delay between the pump and probe beams. The instrument
response of the system was measured at 200 fs optimally. All of the
kinetic profiles show the instrument response function at the time the
data was recorded. The solutions studied were flowed to ensure
presentation of a fresh sample to the excitation beam. The samples
were prepared to have an absorbance of 1.0 at the excitation wavelength
in a 0.5 mm path length cell.
(20) Morris, R. J.; Gibson, Q. H. J. Biol. Chem. 1980, 255, 8050.
(21) Cornelius, P. A.; Hochstrasser, R. M.; Steele, A. W. J. Mol. Biol.
1983, 163, 119.
(22) Petrich, J. W.; Martin, J. L. Chem. Phys. 1989, 131, 31.
(23) Petrich, J. W.; Lambry, J. C.; Kuczera, K.; Darplus, M.; Poyart,
C.; Martin, J. L. Biochemistry 1991, 30, 3975.
(24) Petrich, J. W.; Martin, J. L.; Houde, D.; Poyart, C.; Orszag, A.
Biochemistry 1987, 26, 7914.
(25) Stanford, M. A.; Hoffman, B. M. J. Am. Chem. Soc. 1981, 103,
4104.
(26) Sandros, K. Acta. Chem. Scand. 1964, 18, 2355.
(27) Kozuka, M.; Nakamoto, K. J. Am. Chem. Soc. 1981, 103, 2162.
(28) Scheidt, W. R.; Hoard, J. R. J. Am. Chem. Soc. 1973, 95, 8281.
(29) Wayland, B. B.; Minkiewicz, J. V.; Abd-Elmageed, M. E. J. Am.
Chem. Soc. 1974, 96, 2795.
(31) (a) Firey, P. A.; Ford, W. E.; Sounik, J. R.; Kenney, M. E.; Rodgers,
M. A. J. J. Am. Chem. Soc. 1988, 110, 7627. (b) Rihter, B. D.; Kenney, M.
E.; Ford, W. E.; Rodgers, M. A. J. J. Am. Chem. Soc. 1990, 112, 8064. (c)
Rihter, B. D.; Kenney, M. E.; Ford, W. E.; Rodgers, M. A. J. J. Am. Chem.
Soc. 1993, 115, 8146.
(32) Ford, W. E.; Rodgers, M. A. J. J. Phys. Chem. 1994, 98, 3822.
(33) Logunov, S. L.; Rodgers, M. A. J. J. Phys. Chem. 1992, 96, 2915.
(34) Squier, J.; Salin, F.; Mourou, G.; Harter, D. Opt. Lett. 1991, 16,
324.
(30) Scheidt, W. R.; Frisse, M. E. J. Am. Chem. Soc. 1975, 97, 17.

11800 J. Am. Chem. Soc., Vol. 118, No. 47, 1996
Morlino and Rodgers
Figure 2. Transient absorption difference spectra of TPPFeIINO taken
at (a) 10 and (b) 40 ps. The negative-going signals centered at 532 nm
were due to scattered excitation light (532 nm).
Figure 1. Transient absorption difference spectra of TPPFeIINO taken
at (a) 1.8, (b) 21, and (c) 79.8 µs. (Inset) The kinetic profile of
photoexcited (7-ns pulse) TPPFeIINO monitored at (inset) 443 nm.
Results
Direct Excitation Experiments. Earlier we reported a flash
photolysis study on TPPCoIINO in benzene at room temperature
on time scales from femtoseconds to milliseconds.18 We
deduced therefrom that NO was dissociated from its ligand site
within 10-12 s of photon absorption. Here we describe similar
direct excitation studies on the related species TPPFeIINO.
Argon-saturated solutions of TPPFeIINO in benzene (A355nm
∼
0.4) at room temperature were exposed to 7-ns pulses at 355
nm with an energy of a few millijoules per pulse. The resulting
transient difference spectrum, appearing immediately after the
pulse, is shown in Figure 1. Clearly seen are negative
absorptions (bleachings) centered at 400 and 469 nm and
positive absorption peaks at 425 and 443 nm. This spectrum
corresponds to the difference ground state spectrum between
TPPFeIINO and TPPFeII, indicating loss of NO.8 The kinetic
profile, monitored at 443 nm (Figure 1 inset), showed second-
order kinetic decay to reform the starting material with a
bimolecular rate constant of (5.2 ( 0.05) × 109 M-1 s-1. These
data indicate that NO is ejected from the metal center on time
scales shorter than our ca. 10 ns time scale, which essentially
confirms the conclusions of Hoshino and Kogure.8 The overall
light-induced process can be represented by the following
scheme:
Figure 3. Kinetic profiles of photoexcited (30 ps pulse) TPPFeIINO
at (a) 450 and (b) 470 nm.
examination of the transient spectra in Figure 2 shows that there
are spectral shifts from 455 to 450 nm and from 478 to 470 nm
in the interval between the 10- and 40-ps spectra. The kinetic
profiles monitored at 450 and 470 nm reveal interesting
differences (Figure 3). The kinetic profile at 450 nm shows an
instantaneous rise and a subsequent fast decay, both of which
follow the rise and fall of the laser pulse. The decay tends to
a nonzero baseline. At 470 nm there is an instantaneous
bleaching, which remains for longer than 1400 ps.
In order to examine the early processes involved in deni-
trosylation, a pump-probe experiment employing a 200 fs
excitation pulse at 400 nm was employed. Figure 4 shows the
kinetic profile obtained at 450 nm and represents the detail of
the first 50 ps of curve (a) in Figure 3. Subsequent to the pulse,
there is a rise in absorption (Figure 4, inset) followed by a decay
to a nonzero baseline. The total time profile from t ) 0 to 50
ps shown in Figure 4 could be fitted by a combination of three
exponentials. The rise in absorption followed a single expo-
nential with a rate constant of 7.3 ( 0.4 × 1011 s-1; the two
exponentials in the decaying region had rate constants of 5.0 (
0.3 × 1011 and 1.05 ( 0.05 × 1011 s-1. The kinetics at 470
nm were significantly different (Figure 5). There a negative
absorption was formed post-pulse with a rate constant of 1.0 (
0.05 × 1012 s-1. As can be seen, this was preceded by a
positive-going signal that decayed with the pulse (inset Figure
5). It may be that more information can be gained using shorter
excitation pulses. The rate constant for the growth of the bleach
at 470 nm (1.0 × 1012 s-1) is larger than that for the fast decay
(5.0 × 1011 s-1) monitored at 450 nm, but not too different
from that measured at 450 nm for the growth of the positive
signal (7.3 × 1011 s-1).
TPPFeIINO
9
hν8 TPPFeII + NO
(1)
(2)
TPPFeIINO f TPPFeIINO
In order to pursue the details of reaction 1, shorter time scale
instrumentation was employed. The TPPFeIINO-benzene
system was investigated using a 30-ps pulse of 532-nm light
coupled with the diode array spectrograph. Figure 2 shows
transient absorption spectra recorded at 10 (nominally, i.e.,
during the excitation pulse) and 40 ps (nominally, i.e., at the
conclusion of the excitation pulse). At 40 ps the maximum at
450 nm and the minimum at 470 nm correspond to those spectral
features observed at 1.8 µs (Figure 1). Note that there is no
information below 440 nm because of minimal light transmis-
sion by the highly absorbing sample in this region. Thus, the
transient seen to be present at the end of the pulse in the
nanosecond studies develops within the risetime of our pico-
second system also, showing that denitrosylation occurs at least
within a few picoseconds of the light absorption event. Close

Nitric Oxide Deligation from Nitrosyl Complexes
J. Am. Chem. Soc., Vol. 118, No. 47, 1996 11801
Figure 6. Energy transfer competition plot for TPPFeIINO quenching
SiNC triplet in benzene solution. The bimolecular rate constant for
energy transfer was determined to be 3.3 × 107 M-1 s-1. (Inset) Kinetic
profiles, monitored at 595 nm, of SiNC triplet decay (a) in the absence
of quencher and (b) in the presence of 198 µM of TPPFeIINO.
Figure 4. The kinetic profile of photoexcited (200 fs pulse) TPPFeIINO
at 450 nm. The dashed signal is the instrument response function (300
fs fwhm). (Inset) The first seven picoseconds of the kinetic profile at
450 nm in the main figure.
Table 1.
quenching constant
triplet energy
(cm-1
for
for
sensitizer
)
TPPCoIINOr
TPPFeIINOr
benzophenone
phenanthrene
pyrene
24200g
21800g
16930g
15940g
14870g
13935h
12536i
11050j
10250g
8050k
8.8 × 109
1.4 × 1010
7.9 × 109
9.9 × 109
9.6 × 109
6.4 × 109
4.5 × 109
6.7 × 109
5.1 × 109
1.6 × 109
4.1 × 108
3.1 × 108
1.2 × 1010
1.2 × 1010
1.2 × 1010
5.0 × 109
3.6 × 109
4.4 × 109
3.2 × 109
2.2 × 109
3.3 × 109
4.9 × 108
4.1 × 108
2.0 × 108
5.8 × 107
3.3 × 107
4.5 × 108
acridine
anthracene
terthiophenea
perylene
DPOTb
tetracene
pentacene
â-carotene
rubrene
8050l
8000m
8000n
SiPCc
GePCd
7827o
3.8 × 107
SiNCe
7520p
1.0 × 107
SnNCf
7230q
Figure 5. The kinetic profile of photoexcited (200 fs pulse) TPPFeIINO
at 470 nm. The dashed signal is the instrument response function (300
fs fwhm). (Inset) The first eight picoseconds of the kinetic profile at
470 nm in the main figure.
a
b
2,2′:5′,2′′-Terthiophene. 1,8-Diphenyl-1,3,5,7-octatetraene.
c
d
(Hex3SiO)2Si-phthalocyanine. (OSiEt3)2Ge-(OBu)8-phthalocya-
nine. e [SiO(i-C4H9)2n-C18H37]2Si-naphthalocyanine. [SiO(n-C6H13)3]2Sn-
naphthalocyanine. g See ref 47. h See ref 48. i See ref 49. j See ref 50.
k See ref 51. l Personal communication from Professor Anthony A.
Gorman. m See ref 52. n See ref 53. o See ref 54. p See ref 55. q See ref
56. r M-1 s-1 ( 5%.
f
Energy Transfer Experiments. Direct excitation by photon
absorption in the porphyrin Soret band initially generates π*
states localized on the porphyrin macrocycle which are of the
same multiplicity as the starting ground state. An alternative
approach for excitation of the nitrosyl metalloporphyrin is by
energy transfer from the triplet states of photoexcited donor
molecules (sensitizers). The scheme employed is as follows:
reaction partners (Figure 6). The same experimental series was
repeated for all the chosen sensitizers. The obtained rate
constants and the sensitizer triplet energies are collected in Table
1.
Scheme 1
Discussion
The earlier communication from this laboratory focused on
the processes involved in the deactivation of the photoexcited
TPPCoIINO compound.18 In that work Scheme 2 was presented
In this study a variety of triplet sensitizers with well-character-
ized energies ranging from 7000 to 25000 cm-1 were employed.
The sensitizer triplets were produced by exciting the sensitizer
in argon-saturated benzene solutions with 7 ns pulses of optimal
wavelength light. The bimolecular rate constants for energy
transfer were determined by monitoring the decay of the triplet
of the chosen sensitizer (inset Figure 6) in the presence of a
range of acceptor (iron(II) or cobalt(II) nitrosylporphyrin)
concentrations. The slope of a plot of observed rate constant
for triplet decay versus acceptor concentration yielded the
bimolecular rate constant for energy transfer for those particular
Scheme 2
as defining the reactions pursuant to light absorption in
TPPCoIINO in benzene solution: where the double dagger
indicates a vibrationally excited ground state and the CT state
refers to a metal-centered charge transfer state. The deactivation

11802 J. Am. Chem. Soc., Vol. 118, No. 47, 1996
Morlino and Rodgers
of S2 to S1 was identified as being very rapid, τ < 500 fs. The
S1 state decayed to the CT state with a rate constant of 4.7 (
0.2 × 1011 s-1; the formation of this CT state was deduced to
be the rate-limiting step in the denitrosylation sequence. The
NO-free metalloporphyrin was formed in a vibrationally excited
ground state which relaxed to V ) 0 with a rate constant of 9.0
k ) 7.3 × 1011 s-1 describing the quartet state formation rate
is actually a composite rate constant involving the rate param-
eters for reactions 2a and 2b (V.i.).
Two reasons present themselves which make it unlikely that
4
NO is lost from the T state. The first involves a multiplicity
argument. To conserve the spin in the system, for a quartet
state to unimolecularly dissociate to yield a doublet (NO), the
resulting TPPFeII must be formed in an (excited) triplet state,
yet there was no evidence for the existence of such a state in
this system. The other reason concerns the differing evolutions
of the kinetic profiles at 450 (Figure 4) and 470 nm (Figure 5).
The growth of the bleach at 470 nm shows development of the
NO-free porphyrin from photoexcited TPPFeIINO with k ) 1.0
( 0.5 × 1010 s-1
. The ultrafast experiments showed no
indication of excited states of the nitroso complex formed by a
parallel process to the S1 f CT transition. Thus, each absorbed
photon leads to NO loss. Eventually the metalloporphyrin and
NO which were generated in the argon-purged solution recom-
bined according to second-order kinetics with a bimolecular rate
constant of 7.9 ( 0.4 × 109 M-1 s-1
.
4
× 1012 s-1. This is more rapid than the decay of the T state
The results presented in this work taken in conjunction with
those presented in the earlier communication18 demonstrate that
denitrosylation of both TPPFeIINO and TPPCoIINO occurs on
time scales shorter than the time resolution of our picosecond
instrumentation. The nanosecond studies indicate that the
recombination of the NO and the NO-free metalloporphyrins
occurs in a bimolecular, second-order process, under the
experimental conditions, on µs to ms time scales (Figure 1) in
agreement with the earlier work of Hoshino et al.8,14
as monitored at 450 nm (5.0 × 1011 s-1) and indicates that NO
loss does not arise from the decay of the quartet state. In fact,
the similarity of the rate constants of 1 × 1012 s-1 for the bleach
induced at 470 nm and 7.3 × 1011 s-1 for the growth of
absorption at 450 nm suggests that processes 2a and 2b are
together controlling the NO release. Thus, the 4T state and the
NO-producing CT state arise as competing processes from the
decay of a common precursor, the 2T state. These considerations
provide a hint to the explanation of why Φ(ΝÃ) values differ in
the Fe and Co porphyrins, of which more later.
The results of the ultrafast studies obtained here for the iron
analogue can best be discussed with the aid of Scheme 3. Given
that the species observed at 50 ps (Figure 4) in the femtosecond
study is identical to that seen at post 30 ps (Figure 3) in the
picosecond experiments, Viz., the thermally relaxed, NO-free
porphyrin, and that the lowest-valued rate constant extracted
from the kinetic profile at 450 nm (1.05 × 1011 s-1) corresponds
to vibrational cooling of the released metalloporphyrin,35-37 then
the other two rate constants evaluated must account for the four
processes labeled 1 through 4 in Scheme 3. The deligation
The energy transfer experiments were undertaken to gain
insight into the nature of excited states of the nitrosyl metallo-
porphyrins that could not be revealed by the direct excitation
method. The approach employed borrows from that originated
by Sandros, who demonstrated that the energy level of an
energy-accepting state could be evaluated from a series of
measurements of bimolecular rate constants for energy transfer
from a set of donors of known energy (ED).26 The required
state energy, EA, could be obtained from a fit of the equation:
Scheme 3
exp(-∆E/kT)
1 + exp(-∆E/kT)
kET ) kd
(3)
[
]
in which ∆E ) EA - ED, and kd represents the value of the
diffusion-limited rate constant. An extended version of the
Sandros equation was employed by Wilkinson et al. in a study
of energy transfer involving a series of chromium complexes
with a number of accepting states of different multiplicity.39-43
Following Wilkinson et al., we used the following equation for
the Fe(II) complex:
process itself (3), by analogy with the cobalt(II) porphyrin case,
is probably very rapid and rate-limited by the formation of the
CT state, process 2b. The indication of processes occurring
within the pulse in the 470 nm profile (Figure 5 inset) probably
indicates that the deactivation of the initially formed upper
excited doublet state, Q, to yield the lower doublet state, T,
process 1, happens on time scales shorter than 200 fs. The
remaining two rate constants must then describe the remaining
two processes: the partitioning of the 2T state into the CT and
4T states (7.3 × 1011 s-1), and the slower deactivation of the
4T state to the ground state (5.0 × 1011 s-1). Thus, in Figure
4 (inset) the species that reaches maximal absorbance near 5 ps
is assigned to the 4T(π,π*) state of TPPFeIINO. This is
supported by a report from Cornelius et al., who observed a
transient absorbing at 450 nm and decaying with a lifetime of
50 ps in the photoexcited TPPFeIIICl system which they ascribed
to the triplet state.38 Worth noting here is that the rate parameter
4 exp(-∆EIV/kT)
2 exp(-∆EII/kT)
kd
kET
)
+
(4)
IV
II
[
]
6
1 + exp(-∆E /kT) 1 + exp(-∆E /kT)
2
2
In this equation the first exponential term refers to the energy
IV
transfer to the quartet (trip-doublet) state and ∆EIV ) EA
-
ED; the second term relates to the doublet CT state and ∆EII )
EAII - ED. The pre-exponential multipliers of 4/6 and 2/6 apply
because of the combination of the spin-angular momentum terms
of a triplet state donor with a doublet ground state acceptor.
Thus, in the encounter complex there are a total of six spin
sub-states, four quartet and two doublet. Figure 7 shows a plot
of the data in Table 1 for TPPFeIINO presented in semiloga-
rithmic form of eq 4 (Sandros type). Two clearly separated
plateaus are seen; the upper one (E > 16200 cm-1) corresponds
to rate constants of 1.2 × 1010 M-1 s-1, which equals the
(35) Alden, R. G.; Chaez, M. D.; Ondrias, M. R.; Courtney, S. H.;
Friedman, J. M. J. Am. Chem. Soc. 1990, 112, 3241.
(36) Lingle, R., Jr.; Xu, X.; Zhu, H.; Yu, S. C.; Hopkins, J. B.; Straub,
K. D. J. Am. Chem. Soc. 1991, 113, 3992.
(39) Wilkinson, F.; Tsiamis, C. J. Am. Chem. Soc. 1983, 105, 767.
(40) Wilkinson, F.; Tsiamis, C. J. Chem. Soc., Faraday Trans. 1981,
77, 1681.
(37) Henry, E. R.; Eaton, W. A.; Hochstrasser, R. M. Proc. Natl. Acad.
Sci. U. S. A. 1986, 83, 8982.
(38) Cornelius, P. A.; Steele, A. W.; Chernoff, D. A.; Hochstrasser, R.
M. Chem. Phys. Lett. 1981, 82, 9.
(41) Wilkinson, F. Pure Appl. Chem. 1975, 41, 661.
(42) Wilkinson, F.; Farmilo, A. J. Chem. Soc., Faraday Trans. 1976,
72, 604.
(43) Wilkinson, F.; Tsiamis, C. J. Phys. Chem. 1981, 85, 4153.

Nitric Oxide Deligation from Nitrosyl Complexes
J. Am. Chem. Soc., Vol. 118, No. 47, 1996 11803
from quenching of the excited state triplets. This state is thought
to be a charge transfer state of (π, dz2) nature. Since it is a
charge transfer state involving the internal orbitals of the
incorporated metal, it is not unreasonable to assume that access
to this buried orbital by the triplet sensitizer is sterically hindered
and therefore the rate constant for quenching should be less
than diffusion controlled. The magnitude of the lower plateau
should be some indication as to the geometric constraints
required in the encounter complex to facilitate energy transfer.
1
A similar slight negative deviation from the /3 diffusion limit
can be seen in the Sandros plot (Figure 7) for TPPFeIINO,
reflecting the steric inhibition of the triplet donor in that system.
The energy transfer kinetic studies and the modified Sandros
treatment described above lead to the significant conclusion that
both nitrosyl porphyrins studied possess triplet (or trip-doublet)
and CT states that can be effectively populated by energy
transfer and the energies of these states are revealed by the
treatment. This allows the construction of the energy-level
diagrams presented in Figure 9 for the two nitrosyl complexes.
Significant observations emerge from these diagrams: one is
that the two CT states have energies that are different by only
Figure 7. Modified Sandros plot for TPPFeIINO (see text). Fit shows
energies of excited states of TPPFeIINO are EIV ) 16200 cm-1 and EII
) 8650 cm-1
.
250 cm-1
. Thus the large differences in the photoinduced
deligation yields (ΦΝÃ ) 1.0 for TPPCoIINO and ΦΝÃ ) 0.5
for TPPFeIINO) in benzene reported by Hoshino and Kogure8
and confirmed here would appear not to be a reflection of the
energy of the state responsible for deligation. On the other hand,
there is a large difference in the energy gaps that govern
intersystem crossing in the two complexes. In the Fe(II)
complex this 2T f 4T gap is 1200 cm-1, whereas the 1S f 3T
gap in the Co(II) complex is 3900 cm-1. This presumably
imparts an energy-gap-law type of dependence on the rate
constants which enables the intersystem crossing in the Fe(II)
system to effectively compete with internal conversion to the
CT state. In the Co(II) system the energy gap is much larger,
and the intersystem crossing rate constant is too small to effect
any serious competition to the internal conversion process.
Another possible influence on the intersystem crossing efficiency
resides in the fact that the Fe(II) system may be considered as
a combination of a localized singlet on the porphyrin and a
localized doublet on the FeII-NO, in which case it is not
unreasonable to infer that the unpaired electron in the region
of the porphyrin provides an inhomogeneous magnetic field that
induces intersystem crossing much as in bimolecular reactions
where paramagnetic entities efficiently quench molecular excited
states.44-46 Nitric oxide itself has been shown to be particularly
effective in this regard.44,45
Figure 8. Modified Sandros plot for TPPCoIINO (see text). Fit shows
energies of excited states of TPPCoIINO are EIII ) 14700 cm-1 and EI
) 8900 cm-1
.
diffusion limit. Here exothermic energy transfer is occurring
to the nitrosylporphyrin from donor triplet states having ED in
excess of that of all possible accepting states of the iron
complex, whatever their spin nature. The lower plateau (8650
cm-1 < E < 16200 cm-1) is near k ) 4 × 10 M-9 s-1, i.e., 1/3
of kd. This corresponds to a diffusion and multiplicity-limited
reaction between a triplet donor of intermediate energy and a
doublet acceptor (1/3 of all collisions are effective). Along this
plateau, quenching to form the doublet state is exothermic and
quenching to form the quartet state is endothermic. The fit of
These considerations lead to the conviction that it is a
difference in the partitioning of energy in the upper excited states
and not a difference in the absolute magnitude of the energy of
IV
eq 4 to the data in Table 1 yields values of EA ) 16200 (
800 cm-1 and EAII ) 8650 ( 450 cm-1 for the Fe(II) complex.
At lower donor triplet energy (ca. <8650 cm-1), energy transfer
to form both doublets and quartets is endothermic and the rate
constants drop off exponentially with donor triplet energy.
The TPPCoIINO complex contains a d-7 metal combined with
a doublet NO and is therefore a ground state singlet; yet the
Sandros-type plot (Figure 8) shows the double-plateau feature
(44) Gijzeman, O. L. J.; Kaufman, F.; Porter, G. J. Chem. Soc., Faraday
Trans. 1973, 69, 727.
(45) Kearns, D. R.; Stone, A. J. J. Chem. Phys. 1971, 55, 3383.
(46) Organic Molecular Photophysics; Birks, J. B., Ed.; Wiley: London,
1975; Vol. 2, pp 552-555.
(47) Handbook of Photochemistry; 2nd ed.; Murov, S. L., Carmichael,
I., Hug, G. L., Eds.; Marcel Dekker: New York, 1993; pp 4-97.
(48) Evans, C. H.; Scaiano, J. C. J. Am. Chem. Soc. 1990, 112, 2694.
(49) Monti, S.; Gardini, E.; Bortolus, P.; Amouyal, E. Chem. Phys. Lett.
1981, 77, 115.
(50) Chattopadhyay, S. K.; Das, P. K.; Hug, G. L. J. Am. Chem. Soc.
1982, 104, 4507.
(51) Hall, G. C. Proc. R. Soc. A 1952, 213, 113.
(52) Dreeskamp, H.; Koch, E.; Zander, M. Ber. Bunsenges. Phys. Chem.
1974, 78, 1328.
(53) Wheeler, B. L.; Nagasubramanian, G.; Bard, A. J.; Schechtman, L.
A.; Dininny, D. R.; Kenney, M. E. J. Am. Chem. Soc. 1984, 106, 7404.
(54) Rihter, B. D.; Kenney, M. E.; Ford, W. E.; Rodgers, M. A. J. J.
Am. Chem. Soc. 1990, 112, 8064.
(55) Firey, P. A.; Ford, W. E.; Sounik, J. R.; Kenney, M. E.; Rodgers,
M. A. J. J. Am. Chem. Soc. 1988, 110, 7626.
III
as in the Fe(II) case, and the fit leads to energy values of EA
I
) 14700 ( 735 cm-1 and EA ) 8900 ( 450 cm-1. These
plateaus are a manifestation of two effects. The first concerns
the allowedness of the singlet state formation. Quenching of a
triplet excited state by a singlet ground state quencher to form
a singlet ground state sensitizer and an excited state singlet
quencher is a spin-forbidden reaction. The rate constant for
this type of energy transfer process should be less than diffusion
controlled, hence the existence of the lower plateau. The other
effect concerns the identity of the lower energy state formed

11804 J. Am. Chem. Soc., Vol. 118, No. 47, 1996
Morlino and Rodgers
Figure 9. Proposed state energy diagrams for TPPFeIINO and TPPCoIINO.
the NO-releasing state that is responsible for determining the
quantum efficiency of NO loss from the photoexcited nitrosyl
metalloporphyrins. The other major conclusion from this study
is that the rate at which the CT states release NO is too fast to
measure and is determined by the rate at which the state is
populated from the upper reaches of the manifold. All that can
be said is that the M-NO bond dissociates in less than 10-12
s after the dissociative state is formed, presumably within the
first vibration. Because of the rate-limited property of this
process, this is not a question that can be resolved by employing
rate constant of some 3.6 × 1011 s-1
. It is of interest to
contemplate that even reactions that proceed in a few picosec-
onds or less can find themselves in competition, and that even
the most rapid processes have effects that have significance on
the human time scale.
Acknowledgment. The femtosecond studies reported herein
were performed at the Chemistry Department at the University
of Michigan (Ann Arbor). We would like to thank Roseanne
Sension and Larry Walker II for access and for assistance with
the experiments. E.A.M. would like to thank the Graduate
College of Bowling Green State University and the Helen and
Harold McMaster Foundation for fellowship support throughout
this work. This work was supported in part by the Center for
Photochemical Sciences at Bowling Green State University.
4
even shorter excitation pulses. Finally, since the T state in
the Fe(II) complex is formed with a rate constant of 7.3 × 1011
s-1 and this is the composite of two processes of equal
efficiency, then the intersystem crossing process alone has a
(56) (a) Ford, W. E.; Rihter, B. D.; Rodgers, M. A. J.; Kenney, M. E. J.
Am. Chem. Soc. 1989, 111, 2362. (b) Ford, W. E.; Rodgers, M. A. J.;
Schechtman, L. A.; Sounik, J. R.; Rihter, B. D.; Kenney, M. E. Inorg. Chem.
1992, 31, 3371.
JA962510S