EPR Spectra, Luminescence Data, and Radiationless Decay Processes of Copper(II) Porphyrins

Article abstract of DOI:10.1021/ic960604x

For a series of copper(II) porphyrins, we report EPR data from solid solutions as well as E₀ values for the first ring oxidation, emission spectra, and luminescence lifetimes in methylene chloride. Although the EPR parameters are fairly constant, the potentials vary by almost 700 mV, and the room-temperature lifetimes range from 300 ns for Cu(TC12PP) to 15 ns for Cu(TMeOPP), where TC12PP denotes 5,10,15,20-tetra(2′,6′-dichlorophenyl)porphyrin and TMeOPP denotes 5,10,15,20-tetra(4′-methoxyphenyl)porphyrin. The data show that the variation in the lifetime of the emitting π-π* state is not due to the thermal population of another excited state of either d-d or charge-transfer parentage. However, the results are consistent with a model originally introduced by Asano et al. who proposed that an important vibronic distortion occurs in the emitting trip-doublet and trip-quartet states when the excitation involves the a_2u orbital of the porphyrin (Asano, M.; Kaizu, Y.; Kobayashi, H. J. Chem. Phys. 1988, 89, 6567-6576). In view of the fact that the distortion is unique to the copper systems, we suggest that it involves movement toward a sitting-atop structure, consistent with the role the d¹⁰ configuration is likely to have in the excited-state wave function.

Full text of DOI:10.1021/ic960604x

6
08
Inorg. Chem. 1997, 36, 608-613
EPR Spectra, Luminescence Data, and Radiationless Decay Processes of Copper(II)
Porphyrins
Kurstan L. Cunningham, Kristina M. McNett, Rodney A. Pierce, Keith A. Davis,
Holden H. Harris, David M. Falck, and David R. McMillin*
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-1393
X
ReceiVed May 22, 1996
For a series of copper(II) porphyrins, we report EPR data from solid solutions as well as E0 values for the first
ring oxidation, emission spectra, and luminescence lifetimes in methylene chloride. Although the EPR parameters
are fairly constant, the potentials vary by almost 700 mV, and the room-temperature lifetimes range from 300 ns
for Cu(TCl2PP) to 15 ns for Cu(TMeOPP), where TCl2PP denotes 5,10,15,20-tetra(2′,6′-dichlorophenyl)porphyrin
and TMeOPP denotes 5,10,15,20-tetra(4′-methoxyphenyl)porphyrin. The data show that the variation in the
*
lifetime of the emitting π-π state is not due to the thermal population of another excited state of either d-d or
charge-transfer parentage. However, the results are consistent with a model originally introduced by Asano et al.
who proposed that an important vibronic distortion occurs in the emitting trip-doublet and trip-quartet states
when the excitation involves the a2u orbital of the porphyrin (Asano, M.; Kaizu, Y.; Kobayashi, H. J. Chem.
Phys. 1988, 89, 6567-6576). In view of the fact that the distortion is unique to the copper systems, we suggest
1
0
that it involves movement toward a sitting-atop structure, consistent with the role the d configuration is likely
to have in the excited-state wave function.
7
Introduction
As shown by Yan and Holten, however, the temperature
dependence of the lifetime suggests a more complicated energy
The photophysics of copper(II) porphyrins have been of great
interest for a number of reasons including the multiplet
level scheme that includes some type of deactivating state. For
example, in the case of Cu(OEP), where OEP denotes doubly
deprotonated 2,3,7,8,12,13,17,18-octaethylporphyrin, the lifetime
increases from 270 ns to 10 µs in methylcyclohexane as the
temperature decreases from 298 to 150 K. On the basis of the
Arrhenius plot, Yan and Holten invoked a radiationless decay
process that occurred via a short-lived, thermally accessible
1-3
structure and the sensitivity of the lifetime toward the solvent
4
environment. In addition, the luminescence from a water-
soluble copper(II) porphyrin has also proven useful in character-
izing DNA-binding interactions.5 Gouterman and co-workers
have shown that, at least in non-coordinating solvents, the
photoluminescence from copper(II) porphyrins such as the 5,-
2
7
excited state, designated for now as X. At the same time,
they showed that the lifetime of Cu(TPP), where TPP denotes
doubly deprotonated 5,10,15,20-tetraphenylporphyrin, remained
roughly constant at ca. 35 ns over the same temperature range.
On the other hand, in rigid matrices at 77 K the TPP complex
can exhibit a lifetime of hundreds of microseconds.1 In view
1
0,15,20-substituted derivative shown as follows originates from
3
1,2
a triplet π-π* ( π-π*) state of the ligand.
,8
2
of these observations, Yan and Holten proposed that the X
4
quenching state lies very near the T emitting state of Cu(TPP).
2
In neither case did the energy of the X state correspond to the
known energy of the trip-doublet state. They assigned the
quenching state as a ligand-to-metal charge transfer (CT) state,
in light of published excited-state energy calculations.9
Moreover, Gouterman and co-workers had previously proposed
that the broad band shape of the emission from Cu(TPP) was
,10
However, due to interactions with the unpaired electron in the
2
2
metal d(x -y ) orbital, there is a multiplet structure that entails
2
two emissive states, the so-called trip-doublet ( T) and trip-
11
due to the influence of a nearby CT state. The other virtues
4
1,2
quartet ( T) states. Here, the prefix “trip” denotes the intrinsic
of a low-lying CT state were that it could account for the solvent
dependence of the lifetime of Cu(OEP) and explain the lack of
emission in coordinating solvents. The assumption was that
3
π-π* state character, and the suffix designates the overall spin
multiplicity. After pulsed excitation into a higher energy
absorbing state, time-resolved studies indicate that the system
4
the CT state drops below the emitting T state when a fifth
2
first relaxes to the T state. Under most conditions, this state
4,11
ligand binds to the metal.
More recently, we have proposed
4
then equilibrates with the T state such that both states decay
that the five-coordinate form deactivates through a d-d excited
with the same lifetime.3
,6
5,12
state instead.
X
Abstract published in AdVance ACS Abstracts, February 1, 1997.
(
(
1) Ake, R. L.; Gouterman, M. Theor. Chim. Acta 1969, 15, 20-42.
2) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic: New
York, 1978; Vol. iii, Part A, pp 1-165.
(6) Asano, M.; Kaizu, Y.; Kobayashi, H. J. Chem. Phys. 1988, 89, 6567-
6576.
(7) Yan, X.; Holten, D. J. Phys. Chem. 1988, 92, 5982-5986.
(8) Eastwood, D.; Gouterman, M. J. Mol. Spectrosc. 1969, 30, 437-458.
(9) Case, D. A.; Karplus, M. J. Am. Chem. Soc. 1977, 99, 6182-6194.
(10) Shelnutt, J. A.; Straub, K. D.; Rentzepis, P. M.; Gouterman, M.;
Davidson, E. R. Biochemistry 1984, 23, 3946-3954.
(11) Antipas, A.; Dolphin, D.; Gouterman, M.; Johnson, E. C. J. Am. Chem.
Soc. 1978, 100, 7705-7709.
(
(
(
3) Kobayashi, T.; Huppert, D.; Straub, K. D.; Rentzepis, P. M. J. Chem.
Phys. 1979, 70, 1720-1726.
4) Kim, D.; Holten, D.; Gouterman, M. J. Am. Chem. Soc. 1984, 106,
2793-2798.
5) Hudson, B. P.; Sou, J.; Berger, D. J.; McMillin, D. R. J. Am. Chem.
Soc. 1992, 114, 8997-9002.
S0020-1669(96)00604-0 CCC: $14.00 © 1997 American Chemical Society

Copper(II) Porphyrin EPR, Luminescence, and Decay
Inorganic Chemistry, Vol. 36, No. 4, 1997 609
Table 1. Ligand Information and Analytical Data
analytical data
observed
calculated
%H
complex
R substituenta
%C
%N
5.89
%C
%H
%N
5.82
1
2
. Cu(TCl2PP)
Cl
55.52
2.12
56.05
1.98
Cl
. Cu(TMP)
3
H C
79.66
6.16
6.64
5.41
79.29
6.18
6.77
5.21
CH3
H3C
3
4
5
6
. Cu(TF5PP)
. Cu(OEP)
C6F5
51.01
72.50
78.14
74.20
0.78
7.45
4.17
3.12
50.91
72.44
77.81
73.83
0.70
7.42
3.95
2.82
CH2CH3b
. Cu(TPP)
H
8.29
7.94
. Cu(TCNPP)
14.43
14.72
CN
7
. Cu(TOMePP)c
67.98
4.29
6.65(6.21)d
68.33
4.26
6.51(6.59)d
OCH3
a
See structure in text. b The eight ethyl groups reside at the â-carbon of the pyrrole rings. c Analyzed as the hemisolvate containing chloroform.
d
Chlorine analysis.
2
Several groups have attempted to detect the X quenching
coordinate Cu(TPP)-like systems, we have measured electrode
potentials and EPR spectra for a series of copper porphyrins in
conjunction with their luminescence lifetimes. The results show
that neither type of excited state plays a direct role in the
radiationless decay process.
1
3-15
state by time-resolved resonance Raman techniques,
but
there is as yet no real consensus about the interpretation of the
results, even for the five-coordinate systems. All workers seem
to agree that the oxygen-donor tetrahydrofuran forms an adduct
with an excited state of Cu(TPP). In the Raman spectrum of
this transient, the core marker vibrations ν2 and ν4 of the
porphyrin both appear about 20 cm lower in energy than they
do in the ground state spectrum of Cu(TPP). Jeong et al. and
de Paula et al. assign the transient as a CT excited state of
Cu(TPP), but Kruglik et al. regard it as a d-d excited state.
Complications involved in these experiments include spectral
overlap with solvent bands as well as bands of the normal
ground-state porphyrin and the possible participation of a
transient, five-coordinate ground-state photoproduct.
In a series of papers, Kaizu and co-workers have offered an
alternative explanation for the different types of photophysics
observed for the four-coordinate Cu(OEP) and Cu(TPP) sys-
Experimental Section
-1
Materials. Mid-Century Chemical Co. supplied the 5,10,15,20-
substituted porphyrins with the substituents indicated in Table 1 as well
as the Cu(OEP). The copper(II) acetate monohydrate came from
Aldrich Chemical Co., while Fisher Chemical Co. supplied neutral
alumina and various reagent grade solvents. The methylene chloride
used for the luminescence measurements was Burdick and Jackson
distilled-in-glass grade solvent from Scientific Products. The tetrabut-
ylammonium hexafluorophosphate (TBAH) electrolyte came from
Aldrich, as did the 2,3-dichloro-5,6-dicyano-benzoquinone (DDQ).
Laser Science supplied the dye Coumarin 540A.
1
5
1
3
1
4
Methods. The method of Adler et al.18 or that of Barnett et al.19
proved adequate for the preparation of most of the copper(II) porphyrins.
A dry-column chromatography procedure removed a green-brown
tems.6
,16
They argue that the emitting T and T states of the
2
4
two systems do not have the same orbital parentage and that
the shapes of the respective potential energy surfaces differ as
well. According to this model, thermal population of excited
vibrational levels within the emitting electronic state of
Cu(TPP) promotes deactivation. More specifically, they propose
that a low-frequency ring-doming mode may explain the broad
line shape and the strong quenching observed with Cu(TPP) at
relatively low temperatures.16 However, this model does not
carry over to systems like Pd(TPP) and Pt(TPP) that exhibit
20
impurity, where chloroform was the mobile phase. In the case of
Cu(TCl2PP) the initial product was impure and appeared to contain a
chlorin derivative that gave an absorbance band at 624 nm. Elution
from neutral alumina after treatment with a couple of equivalents of
DDQ in a mixture of methylene chloride and benzene yielded the pure
compound. For Cu(TF5PP) we developed a different synthetic method
that incorporated mercuric acetate as a catalyst.21 With this method,
2
we heated the free ligand (H TF5PP) with 5 equiv of copper(II) acetate
monohydrate in a 30:20 by volume mixture of chloroform and methanol
for 15 min in the presence of 0.2 equiv mercury acetate. After a series
of extractions with water, we evaporated the organic layer to dryness
and isolated pure Cu(TF5PP) from a neutral alumina column by elution
with methylene chloride. Analytical data for the various complexes
appear in Table 1.
17
relatively long emission lifetimes.
In order to shed more light on the possible participation of a
CT state or a d-d state in the excited-state deactivation of four-
(
(
(
(
(
(
12) Liu, F.; Cunningham, K. L.; Uphues, W.; Fink, G. W.; Schmolt, J.;
The solvent for the cyclic voltammetry studies was methylene
chloride containing 0.1 M TBAH as the supporting electrolyte. The
scan rate at the working electrode, which was a gold disk, was typically
McMillin, D. R. Inorg. Chem. 1995, 34, 2015-2018.
13) de Paula, J. C.; Walters, V. A.; Jackson, B. A.; Cardozo, K. J. Phys.
Chem. 1995, 99, 4373-4379.
14) Kruglik, S. G.; Apanasevich, P. A.; Chirvony, V. S.; Kvach, V. V.;
Orlovich, V. A. J. Phys. Chem. 1995, 99, 2978-2995.
(18) Adler, A. D.; Longo, F. R.; V a´ radi, V. Inorg. Synthesis 1976, 16, 213-
220.
15) Jeoung, S. C.; Kim, D.; Cho, D. W.; Yoon, M. J. Phys. Chem. 1995,
9
9, 5826-5833.
16) Asano-Someda, M.; Kaizu, Y. J. Photochem. Photobiol. A (Chem.)
995, 87, 23-29.
17) Callis, J. B.; Knowles, J. M.; Gouterman, M. J. Phys. Chem. 1973,
7, 154-157.
(19) Barnett, G. H.; Hudson, M. F.; Smith, K. M. J. Chem. Soc., Perkin
Trans. 1 1975, 1401-1403.
1
(20) Adler, A. D.; Longo. F. R.; Kampas, F.; Kim, J. J. Inorg. Nucl. Chem.
1970, 32, 2443-2445.
7
(21) Tabata, M.; Miyata, W. Chem. Lett. 1991, 785-788.

610 Inorganic Chemistry, Vol. 36, No. 4, 1997
Cunningham et al.
Table 2. Physical Data Measured in Methylene Chloride at Room
Temperature
0
E , V vs
Cu(TPP)
compound
λ
max, nm (ꢀ, mM-1 cm-1
a
)
+/0
τ, ns
b
Cu(TCl2PP)
Cu(TMP)
Cu(TF5PP)
Cu(OEP)
415 (560), 541 (24.5), 576
415 (436), 540 (23.6), 575
407 (490), 534 (19.2), 570 (9.7)
397 (225), 524 (8.7), 560 (16.9)
0.03
-0.02
0.44
300
220
155
110
-0.24
-
0.20c
0.00
0.11
Cu(TPP)
Cu(TCNPP)
Cu(TOMePP) 418 (367), 540 (16.9), 578
414 (463), 538 (19.2), 574 (sh)
416 (386), 539 (18.4), 573 (sh)
40
25
15
-0.17
a
b
Absorption maxima. Lifetime in deoxygenated solution, error
c
(
10%. Reference 11.
5
0 mV s-1, and the auxiliary electrode was a platinum wire. In the
Figure 1. Uncorrected emission spectra from absorbance-matched
solutions excited at 540 nm in methylene chloride at room temperature.
The bandpass settings were 10 nm for the excitation and emission
beams.
actual experiments, the reference electrode was a AgCl/Ag electrode
in contact with 3 M KCl, but for reporting purposes we relate the
potentials to the Cu(TPP)+/0 system which serves, in effect, as an
internal reference. For the sake of comparison, it had a potential of
0
.66 V versus ferrocene and 1.19 V versus the AgCl/Ag electrode.
of several simultaneous electronic and vibrational excitations
involving different enabling modes. Uncorrected emission
During the electrochemical measurements, the solution was under an
atmosphere of dinitrogen.
2
spectra from room-temperature methylene chloride solutions
appear in Figure 1. It was not possible to obtain corrected
spectra because of the limitations of the phototube response at
such long wavelengths. Once again, the Cu(OEP) and
Cu(TF5PP) systems are distinctive, in this case because of the
well-resolved vibrational structure in their emission spectra.
Vestiges of vibrational structure are also evident in the emission
from Cu(TCl2PP). For the other copper porphyrins in the series,
the emission spectra were broad and essentially devoid of
structure. The emission lifetimes listed in Table 2 are the values
obtained from room-temperature, deoxygenated methylene
chloride solutions. A reviewer suggested that methylene
chloride is a weakly coordinating solvent that could partially
quench the emissions. Actually, the emission lifetimes are
generally longer in methylene chloride than they are in toluene.
Reported emission lifetimes in deoxygenated toluene include
To minimize complications from magnetic interactions, we measured
the EPR spectra of powders that contained about 10% copper(II)
porphyrin by mass. The addition of ethanol to a chloroform solution
of excess ligand and copper(II) porphyrin yielded the desired powder.
For the spectral measurements the microwave frequency fell in the range
of 9.443 to 9.453 GHz, the modulation amplitude was 1 mT, the
microwave power was 20 mW, and the cavity temperature was 110 K.
A literature algorithm yielded spectral simulations. The calculations
assumed axial symmetry, a common axis system for the g tensor and
the hyperfine tensor, and an isotopic coupling interaction with the ligand
nitrogens. The simulations also took no account of the isotopic
composition of copper.
2
2
For the luminescence lifetime studies, we deoxygenated the sample
via a sequence of freeze-pump-thaw cycles. During the luminescence
decay measurements, a 650 or a 700 nm long wave-pass filter protected
the phototube from spurious light. A standard analysis12 yielded the
lifetimes from the decay data, and in each case the residual plots
justified the assumption of a single-exponential decay.
Instruments. Perkin-Elmer Lambda 4C and SLM-Aminco SPF
the following: Cu(TMP), 100 ns; Cu(TF5PP), 69 ns;6
1
2
1
2
6
Cu(TPP), 30 ns; Cu(OEP), 105 ns. A reviewer questioned
whether the hexafluorophosphate ion of the electrolyte might
coordinate to the copper center of the porphyrin in solution. As
a test, we added electrolyte to a solution of Cu(TF5PP) in
methylene chloride. However, the emission spectra with and
without electrolyte were superimposable.
5
00C spectrophotometers provided absorbance and steady-state lumi-
nescence spectra, respectively. A description of the system used to
monitor the luminescence decay appears in the literature.12 The
excitation source was either the nitrogen-pumped dye laser described
12
previously or a Laser Science 337ND-S nitrogen laser with a DLMS-
20 dye laser attachment. The cyclic voltammograph was a Model
2
Electrode Potentials. Table 2 also contains the potentials
for the first oxidation of the copper complexes. As expected
in light of previous work involving unmetalated porphyrins,2
we find that the introduction of substituents on the meso phenyl
groups has a strong influence on the potential. In particular,
the potential shifts in the negative (cathodic) direction with the
introduction of electron-donating groups such as the methoxy
or the methyl group. Contrariwise, the electron-withdrawing
cyano group induces a shift in the positive direction. Like the
lifetimes, the potentials range widely, but there is no correlation
between the two variables (Figure 2).
CV-27 from Bioanalytical Systems Inc. The EPR spectrometer was a
Bruker ESP300E spectrometer equipped with an ER 4121VT temper-
ature controller.
3,24
Results
Absorption and Emission Data. The data in Table 2 reveal
that the wavelength maxima of the Soret bands of most of the
copper porphyrins studied fall in the narrow range of 416 ( 2
nm. The exceptions are Cu(TF5PP) and Cu(OEP) which each
exhibit a Soret maximum at shorter wavelength. At longer
wavelengths each derivative also exhibits a visible absorbance
that typically involves two components, the Q(O,O) and Q(1,O)
EPR Spectra. For many of the copper(II) porphyrins
investigated, the character of the EPR spectrum varied markedly
with the method of sampling. Figure 3 shows the spectrum
obtained from Cu(OEP) under different conditions. As is often
the case with simple copper complexes, the pure solid exhibited
g| and g⊥ values, but no hyperfine structure due to exchange
*
transitions. Both relate to the lowest-energy, spin allowed π-π
excitation of the porphyrin. The vibronically allowed Q(1,O)
transition involves one quantum of vibrational excitation and
is often the more intense of the two absorptions. The reason is
the excited-state wave function is an admixture of two electronic
2
5,26
1
,2
interactions Figure 3B.
The fact that a frozen solution of
configurations with oppositely directed transition moments,
such that Q(O,O), the pure electronic transition, often carries
little dipole strength. In reality, the Q(1,O) band is the envelope
(
23) Kadish, K. M. Prog. Inorg. Chem. 1986, 34, 435-605.
(24) Ransdell, R. A.; Wamser, C. C. J. Phys. Chem. 1992, 96, 10572-
10575.
(
22) Toy, A. D.; Chaston, S. H. H.; Pilbrow, J. R.; Smith, T. D. Inorg.
(25) Hathaway, B. J.; Billing, D. E. Coord. Chem. ReV. 1970, 5, 143-
Chem. 1971, 10, 2219-2225.
207.

Copper(II) Porphyrin EPR, Luminescence, and Decay
Inorganic Chemistry, Vol. 36, No. 4, 1997 611
Table 3. EPR Parameters from Powder Containing Free Porphyrin
porphyrin
g
|
g
⊥
A
|
(Cu)a
A
⊥
(Cu)a
A(N)a
Cu(TCl2PP)
Cu(TMP)
Cu(TF5PP)
Cu(OEP)
2.190 2.036
2.185 2.031
2.189 2.034
2.190 2.035
2.189 2.035
206
208
207
210
204
203
202
206
208
24
26
26
24
31
16
16
16
16
16
Cu(TPP)
b
c
2
2
.187 2.035
.193 2.071
14.4-15.2
14.5-16.1
16
29
29
24
Cu(TCNPP)
Cu(TMeOPP) 2.183 2.031
2.180 2.034
16
a
Hyperfine coupling constants in units of 10-4 cm-1. b In CHCl
3
.
3
c
Alston, K.; Storm, C. B. Biochemistry 1979, 20, 4292-4298. In CHCl
containing H
2
TPP.29
|
Figure 2. A 3D bar plot of the emission lifetimes versus g and the
reduction potential for the first oxidation of the copper(II) porphyrin.
See Table 1 for the key to the numbering system.
between g| and the lifetime, Figure 2 shows that a complex
with a larger g| value tends to exhibit a longer lifetime.
Discussion
2
Emitting States. As Gouterman has shown, an intraligand
π-π* excited state is ultimately responsible for the emission
3
from copper(II) porphyrins. Most treatments classify the orbitals
involved in terms of D4h point symmetry, i.e., without regard
for any symmetry reduction introduced by the peripheral
substituents. In the usual coordinate system the x and y axes
fall along the metal-nitrogen bonds. In this view the lowest
unoccupied energy level is doubly degenerate with eg symmetry,
and the highest occupied level has either a1u or a2u symmetry,
Figure 3. EPR spectra of Cu(OEP) samples at 100 K: (A) coprecipi-
31,32
depending on the substituents.
In the makeup of the a2u
2
tated with the free porphyrin H (OEP); (B) simulated spectrum; (C)
molecular orbital, the pπ atomic orbitals of the pyrrole nitrogens
play a significant role as do the pπ atomic orbitals of the meso
pure solid; (D) in frozen 2-methyltetrahydrofuran; (E) in frozen CHCl
3
solution. The sharp line signal at 338 mT, most evident in E, is a cavity
artifact.
2
carbon atoms (C5, C10, C15, and C20). This orbital is
important in the luminescence processes of Cu(TPP) and most
3
of its derivatives as they have a lowest energy π-π* state that
Cu(OEP) in chloroform gave a similar result (Figure 3E)
indicates that aggregation and microcrystallite formation occur
during the freezing process. Previous authors have also reported
6
,16
derives from a2u f eg excitation.
However, when there are
sufficiently electron-withdrawing substituents on the phenyl
6
31,32
2
7-29
groups, as in Cu(T5FPP) and Zn(T5FPP),
the a2u orbital
problems with aggregation in frozen solution.
Following
2
9
falls below the a1u orbital. The a1u orbital, which has zero
amplitude at the meso carbon positions, then becomes the
highest occupied orbital. This is also true of protoporphyrin
derivatives such as Cu(OEP). As the pπ atomic orbitals of the
CR and Câ carbons of the pyrrole rings contribute most of the
Assour, we prepared a magnetically dilute sample by doping
the copper derivative in a matrix of the free porphyrin and
obtained the well-resolved spectrum in Figure 3A. In this case
we also obtained high-quality data with 2-methyltetrahydrofuran
as solvent, but some of the other porphyrins did not dissolve in
this medium.
2
amplitude for the a1u orbital, this is easily the highest occupied
ligand orbital in Cu(OEP) due to the influence of the electron-
If the energies of low-lying d-d excited states varied in a
systematic way with the lifetimes, the EPR parameters in Table
31,33
donating ethyl substituents.
With either a1u or a2u excitation,
further energy states arise as a result of repulsive interactions
3
, would reflect the trend because the g and the A values depend
2
2
29,30
with the unpaired electron in the d(x - y ) orbital of the metal.
Because of the participation of the nitrogen pπ orbitals in the
a2u molecular orbital, the splitting is larger in systems like
on the energy differences between d orbitals.
Thus, in the
simplest approximation, which ignores the influence of covalent
mixing, the following equations describe the g values:
1
2
4
Cu(TPP). More specifically, the T/ T splitting is around 200
-
1
-1
6,7,34
cm for Cu(OEP) and around 700 cm for Cu(TPP).
For
8
∆|
λ
g| ) 2.0023 -
(1)
(2)
completeness, we note that a Jahn-Teller effect and/or spin-
3
5
orbit coupling may induce further splittings.
Quenching States. Yan and Holten have argued that there
is another low-energy state and that it is responsible for the
strong temperature dependence of the emission lifetimes of four-
2
∆⊥
λ
g⊥ ) 2.0023 -
7
coordinate copper(II) porphyrins. Figure 4A illustrates an
where λ (inherently negative) is the spin-orbit coupling constant
expanded state diagram. This diagram is highly schematic and
only shows the relative state energies for a single molecular
2
+
2
2
2
of the free Cu ion, ∆| ) E(x -y ) - E(xy), and ∆⊥ ) E(x -
2
30
y ) - E(xz,yz). To the extent that there is any correlation
2
geometry. To be effective at quenching, the X state would
probably have to have a different equilibrium geometry than
(
26) Pake, G. E.; Estle, T. L. The Physical Principles of Electron
Paramagnetic Resonance, 2nd ed.; W. A. Benjamin, Inc.: Reading,
MA, 1973; pp 162-172.
(31) Spellane, P. J.; Gouterman, M.; Antipas, A.; Kim, S.; Liu, Y. C. Inorg.
Chem. 1980, 19, 386-391.
(27) MacCragh, A.; Storm, C. B.; Koski, W. S. J. Am. Chem. Soc. 1965,
87, 1470-1476.
(32) Hodge, J.; Hill, M. G.; Gray, H. B. Inorg. Chem. 1995, 34, 809-812.
(33) Binstead, R. A.; Crossley, M. J.; Hush, N. S. Inorg. Chem. 1991, 30,
1259-1264.
(28) Blumberg, W. E.; Peisach, J. J. Biol. Chem. 1965, 240, 870-876.
(29) Assour, J. M. J. Chem. Phys. 1965, 43, 2477-2489.
(30) Wertz, J. E.; Bolton, J. R. Electron Spin Resonance; Chapman and
Hall: New York, 1986; Chapter 11, pp 258-312.
(34) Smith, B. E.; Gouterman, M. Chem. Phys. Lett. 1968, 2, 517-519.
(35) Gouterman, M. Ann. N. Y. Acad. Sci. 1973, 206, 70-82.

612 Inorganic Chemistry, Vol. 36, No. 4, 1997
Cunningham et al.
Vibronic Mixing. The other possibility is that the nature of
4
the T potential energy surface itself determines the emission
lifetime and the emission band shape. In fact, Kaizu and co-
workers have pointed out that the emission from systems with
a relatively temperature-independent lifetime and a broad
4
emission band shape generally stems from a T state that
involves excitation from the a2u molecular orbital, i.e., a
4
T(a2u) state. They have gone on to propose that in these
Figure 4. Excited states of a copper(II) porphyrin: (A) state energies
for a fixed nuclear configuration, where GS denotes the ground state
systems a vibronic interaction broadens the potential energy
surface and facilitates radiationless conversion to the ground
(see the text for an explanation of the other labels; energies not to scale);
6,16
state.
Figure 4B depicts representative forms for the potential
(
B) slices through the potential energy surfaces of the ground state
4
4
energy surfaces envisioned for the T(a1u) and T(a2u) states.
Other workers studying free porphyrins with broad fluorescence
spectra have invoked a similar structural distortion.4
According to the band shape criterion, the broad, red-shifted
emission spectra from the Cu(TMP), Cu(TCNPP), and Cu-
(TMeOPP) systems (Figure 1) are all consistent with emission
*
and two π-π excited states. The double minimum represents the
4
T(a2u) state.
1,42
4
either the T state or the ground state in order to have Franck-
Condon factors that are favorable for radiationless decay. In
2
the model of Yan and Holten, the X state is analogous to the
4
from a T(a2u) excited state as is that of Cu(TPP). By the same
CT state that they invoked to explain emission quenching in
the five-coordinate forms of copper(II) porphyrins.4 Recently,
however, Kruglik et al. have argued that the vibrational energies
and the Raman excitation profile of the five-coordinate excited
token, the emissions from Cu(TF5PP) and Cu(OEP) are of the
,11
4
6
T(a1u) type. The emission from the Cu(TCl2PP) system is
unique in that the spectrum is rather broad, but the onset of the
emission occurs at a wavelength comparable to that of
Cu(TF5PP). Moreover, the emission shows vestiges of vibronic
1
4,36
state are more consistent with a d-d assignment.
In
contrast, for the four-coordinate porphyrin even the assignment
4
4
structure. It is possible that the T(a1u) and T(a2u) states have
similar energies in this derivative and that the system exhibits
multiple emissions. However, in view of the long lifetime it
seems more likely that the emission originates from a state with
a mixed orbital parentage. The possibility of populating a higher
*
36,37
of the vibrational modes of the π-π state
has proven to
be difficult, although Jeong et al. claim to have identified a CT
excited state of four-coordinate Cu(TPP).15 Nevertheless, our
data are quite inconsistent with the existence of a thermally
accessible excited state of either CT or d-d character. Consider
first the possibility of a CT state. There is a wealth of evidence
4
energy T(a2u) state could, on the other hand, account for the
temperature dependence of the lifetime of Cu(OEP).
that the electrochemical potentials of the centers involved govern
the energy of a CT excited state.3
8,39
One possible explanation for the double-minimum profile of
Accordingly, for ring-
4
the T(a2u) state in Figure 4B is a Jahn-Teller effect. In the
to-metal excitation, the more easily oxidized porphyrins should
exhibit the lower energy CT states. But the data in Figure 2
reveal that systems with relatively low reduction potentials, e.g.,
Cu(OEP) and Cu(TMP), exhibit some of the longer lifetimes
in this series. There is no way to reconcile these data with the
notion that a thermally accessible CT state promotes deactiva-
tion.
case of Zn(TPP) Walters et al. have used a pump-probe method
to show that b1g and b2g stretching modes of the porphyrin
3
become active in the resonance Raman spectrum of the Eu
4
3
excited state. The modes are active because they represent
3
totally symmetric vibrations when the Eu state is unstable with
respect to distortion to a lower symmetry like D2h. However,
Jeoung et al. have measured the time-resolved resonance Raman
It is just as evident that deactivation via a d-d excited state
4
spectrum of Cu(TPP) and have argued that its T excited state
is not feasible. Thus, Yan and Holten have suggested that the
does not undergo the same type of Jahn-Teller distortion.15 In
any event, a distortion based on a low-amplitude, high-frequency
vibration involving mainly CdC stretching motions is unlikely
to account for the way in which the medium influences the
photophysics of Cu(TPP). Thus, Yan and Holten showed that
the emission lifetime remained practically constant between 30
and 40 ns in solution in the temperature range from 150 to 300
2
energy difference between X, the quenching state, and the
emitting 2 T states is about 1000 cm greater in four-coordinate
,4
-1
7
Cu(OEP) than it is in Cu(TPP). According to the EPR data,
the energies of the d-d states are practically constant throughout
the series and are too high to mediate excited-state relaxation.
Indeed, indications are that the d-d states of copper(II)
-
1 29,40
porphyrins have energies on the order of 20 000 cm .
If,
7
K. But, Ake and Gouterman reported that emission from
-1
in fact, the d-d energies varied by 1000 cm , we would expect
on the order of a 5% variation in the g values listed in Table 3.
The actual variations are much smaller. Moreover, some of
the systems with relatively long-lived emissions exhibit large
g values. These results are incompatible with the idea that low-
energy d-d states are responsible for the differences in the
lifetimes. The same conclusion follows from the fact that
Cu(TPP) and Cu(TMP) exhibit lifetimes near the opposite
extremes found in Table 2 despite the fact that their d-d states
must occur at similar energies.
Cu(TPP) had a relatively long lifetime and showed multiexpo-
nential decay in a low-temperature poly(methyl methacrylate)
matrix. More specifically, at 77 K one component had a
lifetime of 610 µs, and there was a 1.14 ms component at 25
K. The obvious way to reconcile these results is to assume
that a rigid matrix inhibits the vibronic distortion that facilitates
the decay in fluid solution. A low-frequency, out-of-plane
deformation would be more compatible with this type of rigid
matrix effect.
1
Indeed, Asano-Someda and Kaizu have proposed that the
4
particular form of distortion that the T(a2u) states of Cu(TPP)
(
36) Kruglik, S. G.; Galievsky, V. A.; Chirvony, V. S.; Apanasevich, P.
A.; Ermolenkov, V. V.; Orlovich, V. A.; Chinsky, L.; Turpin, P. Y.
J. Phys. Chem. 1995, 99, 5732-5741.
and related systems undergo may involve a doming or a ruffling
(
(
(
(
37) Asano-Someda, M.; Sato, S.; Aoyagi, K.; Kitagawa, T. J. Phys. Chem.
(41) Gentemann, S.; Medforth, C. J.; Forsyth, T. P.; Nurco, D. J.; Smith,
K. M.; Fajer, J.; Holten, D. J. Am. Chem. Soc. 1994, 116, 7363-
7368.
(42) Vergeldt, F. J.; Koehorst, P. B. M.; van Hoek, A.; Schaafsma, T. J. J.
Phys. Chem. 1995, 99, 4397-4405.
(43) Walters, V. A.; dePaula, J. C.; Babcock, G. T.; Leroi, G. E. J. Am.
Chem. Soc. 1989, 111, 8300-8302.
1
995, 99, 13800-13807.
38) Curtis, J. C.; Sullivan, B. P.; Meyer, T. J. Inorg. Chem. 1983, 22,
224-236.
39) Vlc eˇ k, A. A.; Dodsworth, E. S.; Pietro, W. J.; Lever, A. B. P. Inorg.
Chem. 1995, 34, 1906-1913.
40) Sontum, S. F.; Case, D. A. J. Phys. Chem. 1982, 86, 1596-1606.

Copper(II) Porphyrin EPR, Luminescence, and Decay
Inorganic Chemistry, Vol. 36, No. 4, 1997 613
of the porphyrin core.16 They reasoned that the T(a2u) state
4
might be especially prone to the distortion because excitation
4
to the T(a2u) state shifts electron density from the donor
nitrogens to CR and Câ carbons. However, if elongation of
the CusN bonds occurs, a ruffling distortion seems improbable
because this motion normally provides for the formation of
shorter metal-nitrogen bonds.44,45 Doming would be more
consistent with CusN bond lengthening,4 but there is also a
problem with this model. If a shift in electron density within
the π system promotes doming, why does the distortion not
occur in the more strained palladium(II) derivative which has a
longer metal-to-nitrogen distance than Cu(TPP)?48,49 Unlike
what we find with Cu(TPP), the phosphorescence spectrum of
the palladium derivative exhibits well-resolved vibrational
structure, even at room temperature.5
6,47
0,51
Whatever the nature of the distortion, it seems to be particular
to the copper derivative, and the hallmark of copper is the
stability of the copper(I) state. We suggest that the distortion
may involve the formation of a sitting-atop structure that is more
typical of copper(I) with its larger ionic radius. In the sitting-
52
atop configuration, the metal moves out of the mean plane of
the porphyrin as do two of the ring nitrogens such that there
are two relatively short Cu-N bonds. This distortion is
consistent with a2u f eg excitation because contour plots show
that excitation to a particular component of the eg level shifts
electron density off of only one pair of nitrogen atoms on
opposite sides of the porphyrin ring. Excitation to the the other
eg component would give rise to a similar structure in which
the other pair of nitrogens follow copper out of the plane of the
porphyrin. It is worth noting that the sitting-atop structure is
achievable via a Jahn-Teller distortion of a porphyrin like
Cu(TPP). Consider the conformation with the phenyl substit-
uents rotated as in Figure 5A. The point group is then D2d,
and it is convenient to redefine the x and y axes such that they
lie along the C2′ axes, bisecting the NsCusN angles. The
Figure 5. Out-of-plane displacements of the copper and/or nitrogen
atoms in D2d symmetry: (A) a totally symmetic (a ) motion; (B) a
1
doming distortion with b symmetry.
2
influences the decay process by helping shape the active
potential energy surface. Indeed, an interaction with a CT state
that has the same symmetry as the excited state in the final
point group (B1 or B2 in C2V symmetry) could enhance the
53
distortion via a second-order Jahn-Teller effect. However,
neither of the two highest-energy occupied porphyrin π orbitals
gives rise to a π f d state with appropriate symmetry.
*
lowest energy π-π excited state has E symmetry in the D2d
point group, and the vibrational modes that support a first-order
Jahn-Teller distortion have the following symmetry:
Conclusions
E X E ) A + A + B + B
2
The lifetimes of copper(II) porphyrins are quite variable in
fluid solution at ambient temperature. Indeed, in the series of
Cu(TPP) derivatives investigated herein, the lifetimes vary by
more than a factor of 10 in methylene chloride. From the E0
data it is clear that the effect is not due to a variable energy CT
state that mediates decay, and EPR data exclude the possibility
of a thermally accessible d-d state. The results are most
consistent with the model of Asano et al. which posits that a
1
2
1
Although the ordinary Jahn-Teller effect cannot give rise
to an out-of-plane distortion of a D4h system, this is possible in
D2d symmetry. In fact, the modes indicated in Figure 5A,B
are both compatible with a Jahn-Teller distortion from D2d
symmetry, and a combination of these deformations gives the
sitting-atop structure.
*
The attractive feature of this model is a recognition of the
vibronic distortion within one of the π-π excited states is
10
6
legitimate role that the d configuration can play in the
photophysics even if the CT states themselves are thermally
inaccessible. Thus, the emitting state can take on an admixture
of a CT excited-state configuration in conjunction with a
vibronic distortion. In this way, the CT state indirectly
responsible for the short lifetimes. There are, however, several
qualifications to keep in mind: (1) The vibronic distortion is
only important when the emitting state involves excitation from
the a2u orbital of the porphyrin. (2) Even in these systems, bulky
groups like the mesityl substituents in Cu(TMP) or the 2,6-
dichlorophenyl groups in Cu(TCl2PP) apparently inhibit the
(
(
44) Scheidt, W. R.; Lee, Y. J. Struct. Bonding (Berlin) 1987, 64, 1-70.
45) Jentzen, W.; Simpson, M. C.; Hobbs, J. D.; Song, X.; Ema, T.; Nelson,
N. Y.; Medforth, C. J.; Smith, K. M.; Veyrat, M.; Mazzanti, M.;
Ramasseul, R.; Marchon, J. -C.; Takeuchi, T.; Goddard, W. A., III;
Shelnut, J. A. J. Am. Chem. Soc. 1995, 117, 11085-11097.
16
quenching pathway. (3) The distortion probably involves out-
of-plane motion and perhaps the formation of a sitting-atop
structure. (4) Finally, CT states may influence the lifetime,
albeit indirectly, via vibronic interactions that help shape the
potential energy surface of the emissive state.
(
(
46) Hoard, J. L. Science 1971, 174, 1295-1302.
47) Ravikanth, M.; Chandrashekar, T. K. Struct. Bonding (Berlin) 1995,
8
2, 105-188.
48) Fleischer, E. B.; Miller, C. K.; Webb, L. E. J. Am. Chem. Soc. 1964,
6, 2342-2347.
49) Scheidt, W. R.; Kastner, M. E.; Hatano, K. Inorg. Chem. 1978, 17,
06-710.
(
(
Acknowledgment. The National Science Foundation sup-
ported this research through Grant No. CHE-9401238.
8
7
IC960604X
(
(
(
50) Sapunov, V. V.; Tsvirko, M. P. Opt. Spectrosc. 1980, 49, 152-156.
51) Harriman, A. J. Chem. Soc., Faraday Trans. 2 1981, 77, 1281-1291.
52) Ohtaki, H.; Inada, Y.; Funahashi, S.; Tabata, M.; Ozutsumi, K.;
Nakajima, K. J. Chem. Soc., Chem. Commun. 1994, 1023-1025.
(53) Douglas, B. E.; Hollingsworth, C. Symmetry in Bonding and Spectra;
Academic Press: New York, 1985; pp 344-347.