Quadratic nonlinearity of one- and two-electron oxidized metalloporphyrins and their switching in solution

Article abstract of DOI:10.1021/jp076909m

We report the quadratic nonlinearity of one- and two-electron oxidation products of the first series of transition metal complexes of meso-tetraphenylporphyrin (TPP). Among many MTPP complexes, only CuTPP and ZnTPP show reversible oxidation/reduction cycles as seen from cyclic voltammetry experiments. While centtosymmetric neutral metalloporphyrins have zero first hyperpolarizability, ??, as expected, the cation radicals and dications of CuTPP and ZnTPP have very high ??values. The one- and two-electron oxidation of the MTPPs leads to symmetry-breaking of the metal-porphyrin core, resulting in a large ??value that is perhaps aided in part by contributions from the two-photon resonance enhancement. The calculated static first hyperpolarizabilities, ??₀, which are evaluated in the framework of density functional theory by a coupled perturbed Hartree-Fock method, support the experimental trend. The switching of optical nonlinearity has been achieved between the neutral and the one-electron oxidation products but not between the one- and the two-electron oxidation products since dications that are electrochemically reversible are unstable due to the formation of stable isoporphyrins in the presence of nucleophiles such as halides. ? 2008 American Chemical Society.

Full text of DOI:10.1021/jp076909m

2842
J. Phys. Chem. B 2008, 112, 2842-2847
Quadratic Nonlinearity of One- and Two-Electron Oxidized Metalloporphyrins and Their
Switching in Solution
Abdul Wahab, Mily Bhattacharya, Sampa Ghosh, A. G. Samuelson, and Puspendu K. Das*
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
ReceiVed: August 29, 2007; In Final Form: December 13, 2007
We report the quadratic nonlinearity of one- and two-electron oxidation products of the first series of transition
metal complexes of meso-tetraphenylporphyrin (TPP). Among many MTPP complexes, only CuTPP and
ZnTPP show reversible oxidation/reduction cycles as seen from cyclic voltammetry experiments. While
centrosymmetric neutral metalloporphyrins have zero first hyperpolarizability, â, as expected, the cation radicals
and dications of CuTPP and ZnTPP have very high â values. The one- and two-electron oxidation of the
MTPPs leads to symmetry-breaking of the metal-porphyrin core, resulting in a large â value that is perhaps
aided in part by contributions from the two-photon resonance enhancement. The calculated static first
hyperpolarizabilities, â0, which are evaluated in the framework of density functional theory by a coupled
perturbed Hartree-Fock method, support the experimental trend. The switching of optical nonlinearity has
been achieved between the neutral and the one-electron oxidation products but not between the one- and the
two-electron oxidation products since dications that are electrochemically reversible are unstable due to the
formation of stable isoporphyrins in the presence of nucleophiles such as halides.
Introduction
Coe et al.4 first demonstrated completely reversible chemical
switching in ruthenium amine complexes, although because of
a weak signal, they could not measure the â value of the
oxidized products. Loucif-Saibi et al.5 showed that photoisomer-
ization of the dye, disperse red one (DR-1), dissolved in a
PMMA matrix induced by 488 nm light acted as an external
stimuli and led to the realization of bulk second harmonic
switching. The first electrochemical reversible redox switching
of â in a series of mono-, bi-, and trinuclear organometallic
Fe(II) and/or Fe(III) complexes was reported by Weyland et
al.6 They investigated the effect of oxidation of the metal center,
which was integrated in the donor part of the complex on the
â value. This is known as type I switching, where the donor
capacity of the donor has been lowered by oxidation. In their
case, the stepwise one- or two-electron oxidation of metal
centers led to a change in the â value of the complex. In
complexes where an increase in â upon one-electron oxidation
was observed, the â value of the oxidized product could be as
high as twice that of the neutral complex. However, in those
examples, subsequent oxidation by withdrawing another electron
from the complex invariably led to a decrease in the â value.
In another example, type II switching where the acceptor
capacity of the acceptor fragment based on the metal center
has been lowered upon reduction has been reported.7
All examples studied so far for redox switching of quadratic
nonlinearity have exploited the redox properties of the Fe(II)/
Fe(III) and Ru(II)/Ru(III) metal centers in the so-called one-
dimensional systems. However, much scope exists in realizing
switchable optical nonlinearities in multidimensional systems.8
Here, we have chosen metalloporphyrins (Figure 1), which are
two-dimensional NLO chromophores where the porphyrin core
acts as the donor and the central metal ion as the acceptor. They
undergo facile oxidation in one-electron transfer steps to yield
π cation radicals (hereafter, π cation radicals are referred to as
cation radicals) and dications.9,10 The X-ray crystal structure
of the radical perchloratotetraphenylporphynatozinc(II) was
Switching bulk or molecular quadratic nonlinearity by an
external stimuli such as a light beam, a potential gradient, etc.
has been put forward as a part of a strategy to generate molecules
or materials with large optical nonlinearities that are useful in
optoelectronic and all-optical technologies. Quadratic nonlin-
earity or second-order molecular polarizability of molecules has
been altered by oxidation, reduction, or chemical modifications.1
In a prototypical donor-acceptor compound of the type
D-spacer-A, where the donor and the acceptor are separated by
a backbone, this was achieved by reducing the donating power
of the donor by oxidizing the donor, reduction of the accepting
power of the acceptor by reducing the acceptor, or by structural
or other chemical modifications of the linker. If the two forms,
the native form and the one modified by any of the means
described previously, are stable, then the system D-spacer-A
can serve as a second-order NLO switch based on one of the
forms being the on form and the other being the off form. The
switching of quadratic nonlinearity was first demonstrated by
Sakaguchi et al.2 in a mixed Langmuir-Blodgett film of (N,N′-
dioctadecyl-4,4′-dicarboxamide-2,2′-bipyridine)-bis(2,2′-bipy-
ridine)ruthenium(II) perchlorate and dioctadecyldimethylam-
monium bromide in a two-laser experiment. When the film was
exposed to pulsed laser irradiation (photochemical stimulation)
at either 355 or 460 nm, the intensity of the second harmonic
light at 532 nm, which was very strong when excited by a lone
1064 nm pulse, was remarkably suppressed. It was also shown
that upon excitation at 378 nm, the photoinduced switching time
of SHG was less than 2 ps and that the SHG intensity decreased
to 70% of its initial value within that time.3 The recovery time
for the SHG signal to its original value was several hundred
picoseconds. The time taken by the Ru complex to relax was
attributed to the time taken for recovery of the SHG signal.
* Corresponding author. E-mail: pkdas@ipc.iisc.ernet.in; fax: 91-80-
23601552.
10.1021/jp076909m CCC: $40.75 © 2008 American Chemical Society
Published on Web 02/14/2008

One- and Two-Electron Oxidized Metalloporphyrins
J. Phys. Chem. B, Vol. 112, No. 10, 2008 2843
porphyrin solutions were of 10-3 M concentrations containing
0.1 M TBAP. For quantitative oxidation for in situ HRS
measurements, controlled-potential electrolysis was employed
in a cylindrical glass cell with an Ag wire as the reference
electrode. The total volume of the cell was 25 cm3. During
measurements, solutions were stirred continuously for uniform
mixing. The UV-vis absorption spectra of the oxidized por-
phyrins were recorded in situ by employing a miniature SD2000
fiber optic spectrometer system (Ocean Optics).
For a 2-3 × 10-5 M CuTPP or ZnTPP solution, oxidation
from the neutral form to the cation radical typically took ∼30
min and from the cation radical to the dication ∼50 min in our
cell. Reduction of dications to neutral metalloporphyrins via
cation radicals took ∼90 min at -0.3 V. Only cation radicals
took ∼35 min for complete reduction at -0.3 V. By reducing
the concentration and changing the dimensions of the electrodes,
one can change the oxidation/reduction time.
HRS Measurements. The detailed experimental setup and
measurement procedures of the HRS technique have been
described elsewhere.21 Briefly, a 1064 nm fundamental of a
Q-switched Nd:YAG laser was focused using a plano-convex
lens of 20 cm focal length on the electrochemical cell. The laser
energy was kept within 18-21 mJ/pulse. The scattered second
harmonic (SH) photons (I2ω) were collected at a 90° angle using
a monochromator and a PMT. The signal from the PMT was
amplified 5 times and averaged over 512 laser shots in a storage
oscilloscope.
Figure 1. Structure of porphyrins used in this work.
determined, and it was found that the cation radical crystallizes
in the monoclinic space group P21/c. The perchlorate group is
covalently coordinated to the zinc, and the structure is asym-
metrically distorted from the planar neutral state.11 A large
number of asymmetrically substituted neutral metalloporphyrins
has been investigated for their nonlinear optical properties, and
a large molecular hyperpolarizability has been reported in
them.12-16 However, their oxidized or reduced products have
not been investigated. Resonance Raman spectra of metallopor-
phyrin cation radicals in CH2Cl2 show that the removal of one
electron causes distortion (ruffling) of the porphyrin core and
weakens the bonding in the macrocycle.17,18 It is expected that
core distortions such as S4 ruffling will affect the electronic states
of the cation radicals. Since porphyrins are highly symmetric
core molecules and do not show much second-order nonlinearity
in their planar form, any distortion should be conducive to
induce large second-order nonlinearity in them.
The first hyperpolarizability, â, of a molecule dissolved in a
solvent is related to the scattered SH light intensity (I2ω) by
In this study, the quadratic nonlinearity of electrochemical
oxidation products of TPP and its first-row transition metal
derivatives were investigated, in solution, by the hyper-Rayleigh
scattering (HRS) technique. We carried out DFT calculations
of static hyperpolarizability, â0, of the oxidized metalloporphy-
rins using the Jaguar 5.5 program in the Schrodinger suite.19
Facile electrochemical switching of a quadratic NLO response
in copper- and zinc-metalloporphyrins between their neutral
and their one-electron oxidation states also was demonstrated.
I
2ω ) G(Nsolventâ2solvent + Nsoluteâ2solute)I2
(1)
ω
where Nsolvent and Nsolute are the number densities of the solvent
and solute molecules, respectively, G is an instrument factor,
and Iω is the intensity of the incident beam. Since all our solute
molecules that are porphyrins absorb significantly at the SH
wavelength (532 nm), eq 1 has to be corrected for self-
absorption as22
solutel
I
2ω ) G(Nsolventâ2solvent + Nsoluteâ2solute)I2ω × 10-ꢀN
Experimental Procedures
(2)
Chemicals. TPP and its metallo complexes (MnClTPP,
FeClTPP, CoTPP, NiTPP, CuTPP, and ZnTPP) except for
CrClTPP were obtained from Sigma-Aldrich (99+%) and used
without further purification. CrClTPP was synthesized and
purified using a literature procedure.20 p-Nitroaniline was
obtained from Aldrich Chemical Co., recrystallized twice from
methanol, and dried in vacuum over P2O5. Anhydrous 1,4-
dioxane was prepared from AR grade 1,4-dioxane obtained
locally by distilling over Na metal, and AR grade CH2Cl2 was
distilled over P2O5. Tetra-n-butylammonium perchlorate (TBAP)
was synthesized from tetra-n-butylammonium bromide and
perchloric acid by dissolving them in the minimum volume of
water at molar equivalent. The white crystalline precipitate of
TBAP was filtered off, washed with water, recrystallized twice
from ethyl acetate, and dried in vacuum over P2O5.
where ꢀ is the extinction coefficient of the solute at 532 nm,
and l is the effective path length or half the cell diameter (1.25
cm). p-Nitroaniline (pNA) was used as an external reference
compound (âpNA ) 16.9 × 10-30 esu in 1,4-dioxane, and we
determined that âpNA ) 19.3 × 10-30 esu in CH2Cl2). Dilute
solutions (10-5 to 10-6 M) were used to ensure linear
2
dependence of I2ω/Iω on solute concentration. The quadratic
dependence of the SH signal on Iω was checked (see Supporting
Information). All the measurements were performed in CH2Cl2
at ∼20 °C. Contribution from two-photon fluorescence to the
SH scattering signal was checked by dispersing the signal
through the SH wavelength. The HRS signal was fitted to a
Lorentzian function and analyzed.21 No background fluorescence
was seen (see data in the Supporting Information).
Electrochemical Measurements. Cyclic voltammetry (CV),
HRS measurements, and UV-vis spectral measurements were
carried out using a conventional three-electrode cell on a
potentiostat (CH Instruments Electrochemical Analyzer, 617A).
Platinum plates (1 cm × 2 cm) were used as both working and
counter electrodes, while an Ag/AgCl electrode was used as a
reference electrode. All solutions were purged by N2 gas prior
to the electrochemical measurements. CV data were obtained
at different scan rates varied from 30 to 800 mV s-1. All the
Computational Details. DFT calculations were performed
using the Jaguar 5.5 program in the Schrodinger suite. Structures
of neutral metalloporphyrins and their oxidized products were
fully optimized in the ground state using the B3LYP functional
with a LACVP** basis set (which uses the effective core
potential (ECP) basis set for the transition metal atoms and
6-31G** for other atoms). Known X-ray crystal structures11,23
were used as input geometries for optimization. Molecular static
first hyperpolarizabilities, â0, were calculated by the coupled

2844 J. Phys. Chem. B, Vol. 112, No. 10, 2008
Wahab et al.
TABLE 2: Linear and Nonlinear Optical Properties of
MTPP and Their Oxidized Species in CH2Cl2
max (nm)
Q band
λ
B
ꢀ at 532 nm â × 1030 âo × 1030
compda band
(Qâ and QR)
(M-1 cm-1
)
(esu)b
(esu)c
TPP
413 515, 550, 591, 647
435 555, 605, 656
438 510, 609, 657
414 539, 617
6021
2423
0
234
393
0
351
371
0
0.1
10.4
28.7
0.1
16.8
43.2
0.1
TPP+
TPP++
CuTPP
CuTPP+
15228
15177
10369
10919
11329
11928
6014
416 592, 656
CuTPP++ 417 662
ZnTPP
418 548, 587
411 603, 656
ZnTPP+
407
606
18.1
50.1
ZnTPP++ 441 655
a MTPP+: cation radicals and MTPP++: dications. b Measured â
values with an estimated error of (13%. c Calculated â0 values.
ratio of the redox peak currents of Oxd1 and Oxd3 deviate from
unity, and peak currents are also not proportional to the square
root of the scan rate. These led us to infer that oxidation steps
in CoTPP are quasi-reversible except for the second step (Oxd2),
which is reversible.
Figure 2. Cyclic voltammograms of TPP and its transition metal
complexes in CH2Cl2 containing 0.1 M TBAP at a scan rate of
0.1 V s-1
.
For in situ electrochemical oxidation of the metalloporphyrins,
we employed controlled-potential electrolysis in the electro-
chemical cell. Potentials slightly higher than the oxidation
potentials obtained from the CV measurements were applied
for complete conversion of the metalloporphyrins to the
corresponding cation radicals and dications. First hyperpolar-
izabilities of metalloporphyrins and their cation radicals and
dications were measured in CH2Cl2 by HRS and are shown in
Table 2. TBAP was used as a supporting electrolyte, which was
checked to have a very low HRS response. In the irreversible
(TPP, CrClTPP, MnClTPP, and FeClTPP) and quasi-reversible
(CoTPP and NiTPP) porphyrin systems, complete redox conver-
sion was not possible within a few hours as monitored by the
change in absorption spectrum of the porphyrin compounds.
The overlapping nature of the redox processes, such as in
NiTPP, complicated the electrolysis process further. Since the
oxidation process was not completed, the concentration of the
oxidized product could not be determined with accuracy, and
hence, â for the oxidized species of these complexes could not
be measured. However, for the reversible CuTPP and ZnTPP
systems, the â values of cation radicals and dications were easily
determined (Table 2) since the quantitative transformation to
the oxidized species in a short time span (30 min) could be
monitored accurately by a change in the UV-vis spectra and
coulometric current variation. Figure 3a,b presents the absorption
spectra of CuTPP and ZnTPP, respectively, in CH2Cl2 during
the progress of the electrochemical oxidation. Important absorp-
tion parameters are listed in Table 2. Spectral data of the one-
and two-electron oxidized products show good agreement with
those reported in the literature. For comparison, â of the TPP
ligand and its oxidation products were also measured. Since
TPP oxidation steps are irreversible, we waited for a long time
for the oxidation to reach >80% completion (as judged from
the decrease in intensity of the absorption spectrum), and the
concentration of the oxidized products was assumed to be 100%
of that of the initial neutral compound. Therefore, the â values
for the radical cation and dication of TPP shown in Table 2
have large errors (∼20%) and should only be considered to be
approximate. In fact, the incompletely oxidized metalloporphy-
rins also showed a large SH signal upon successive oxidation,
but due to the lack of knowledge regarding the correct
concentrations of the oxidized products, their â values were not
determined. The first one-electron oxidized product of CoTPP
exhibited a very weak SH signal in comparison to other MTPPs,
TABLE 1: Half-Wave Redox Potentials (V vs Ag/AgCl) and
Peak Separation Potentials (∆E, mV) of TPP and Its Metallo
Complexes in CH2Cl2 Containing 0.1 M TBAP
compd
Oxd1
∆E
Oxd2
∆E
Oxd3
∆E
TPP
1.07
1.15
1.16
1.19
0.81
1.10
1.05
0.82
55
110
24
64
62
51
54
55
1.30
1.39
1.52
1.45
1.03
1.18
1.29
1.12
50
CrClTPP
MnClTPP
FeClTPP
CoTPP
NiTPP
CuTPP
63
79
54
52
53
54
1.18
42
ZnTPP
perturbed Hartree-Fock (CPHF) method. For comparison with
experimental values, the calculated â0 values were multiplied
by a factor of 2 (B convention) as suggested by Willetts et al.24
Results and Discussion
Figure 2 displays the cyclic voltammograms of all the
metalloporphyrins in CH2Cl2 investigated by us at a 0.1 V s-1
scan rate. The half-wave redox potentials and redox peak
separation potentials (∆E) versus Ag/AgCl are summarized in
Table 1. The oxidation processes are labeled as Oxdn (n ) 1,
2, or 3). The measured half-wave potentials for one- and two-
electron oxidations agree well with literature values.9,25,26 From
Figure 2, Table 1, and other peak current and peak separation
characteristics,27 we infer that most of the metalloporphyrins
including TPP show irreversible or quasi-reversible oxidation
steps except CuTPP and ZnTPP. Both CuTPP and ZnTPP show
two facile, well-resolved, and reversible stepwise one-electron
oxidation processes as reported earlier.26 On the basis of a series
of measurements such as EPR, magnetic susceptibility, magnetic
circular dichroism, and linear spectroscopy, it has been estab-
lished that the oxidations in CuTPP and ZnTPP are centered
on the porphyrin ligands.9,10,28 In most of the metalloporphyrins,
the electrons are removed from the porphyrin ring in succession
upon oxidation except in CoTPP, where the first oxidation
occurs at the metal center and the other two one-electron
oxidations take place in the porphyrin ring. Although the
oxidation steps in CoTPP were suggested to be reversible29 in
CH2Cl2 containing 0.1 M TBAP in earlier studies, we find that
the peak position corresponding to Oxd1 varies with the voltage
scan rate and that ∆E is more than the expected 59 mV. The

One- and Two-Electron Oxidized Metalloporphyrins
J. Phys. Chem. B, Vol. 112, No. 10, 2008 2845
not known in the literature except for CoTPP and MgTPP.
Further oxidation of both CuTPP+ and ZnTPP+ makes the Q
band vanish and the B bands less intense. In addition, ZnTPP++
shows another broad absorption band at 725-900 nm.
For the neutral porphyrins, a scattered SH signal could not
be detected. This is not surprising since â is expected to be
zero in perfect D4h (MTPP) or D2h (TPP) symmetries. Upon
successive oxidation, large â values were obtained for the cation
radicals and dications of CuTPP and ZnTPP. The hyperpolar-
izability increased from neutral to cation radical to dication,
and the stepwise increment was more pronounced in ZnTPP.
Several factors can be ascribed to this: (a) the removal of one
or two porphyrin π electrons causes ruffling of the porphyrin
core, (b) the breaking away from the centrosymmetric geometry
-
is further aided by the participation of the ClO4 ions as an
axial ligand to the metal center, and (c) the metalloporphyrin
cation radicals and dications show moderate absorption at 532
nm, perhaps leading to the two-photon resonance enhancement
of â. The fact that â values of CuTPP and ZnTPP cation radicals
and dications are not derived in part from the contribution of
multiphoton fluorescence to the HRS signal is evidenced from
the spectral pattern of the HRS signal dispersed through the
monochromator before detection. The evidence for the deviation
of the core structure from planarity comes from X-ray crystal-
lographic studies, where a large ruffling of the porphyrin core
in solid ZnTPP11 and CuTPP23 cation radicals containing the
-
ClO4 counterion has been reported. Also, in going from the
neutral to the oxidized form, the direction of charge transfer is
altered. In the neutral form, porphyrin acts as the donor, while
in the oxidized form, it acts as an acceptor. The acceptor strength
of the porphinato cation is further increased by oxidation, which
leads to the enhancement of â.
Molecular static hyperpolarizabilities, â0, of all the oxidized
metalloporphyrins were calculated using the coupled perturbed
Hartree-Fock method under the framework of DFT as imple-
mented in the Jaguar 5.5 program in the Schrodinger suite and
are displayed in Table 2. Although the calculated gas phase â0
values cannot be compared directly with experimental â values
since the dispersion and solvent effects in â are not accounted
for in the calculation, it is helpful in understanding the trend in
hyperpolarizabilities in transition metal porphyrins as a function
of structural change. In fact, the ruffled geometry of the
porphyrin core upon oxidation is clear from the optimized
geometry displayed in Figure 4. The ground state structure of
the oxidized metalloporphyrins belongs to C1 symmetry. It was
found that all 10 isotropic components of â have nonzero values
(data not shown) consistent with C1 symmetry. This also
provides indirect evidence about the distortion of the core
structure upon oxidation. The HRS â for unpolarized measure-
ments in the laboratory frame was calculated from the compo-
nents using a literature procedure.33 The â0 value for pNA was
calculated and found to be 10.2 × 10-30 esu. The calculated
hyperpolarizabilities of all the oxidized species are higher than
that of pNA, which is consistent with the measured â values.
Although dispersion corrected â0 values can be extracted from
the measured hyperpolarizabilities using the two-state model34
for comparison with calculations, we chose not to do that since
multiple electronic transitions in metalloporphyrins around the
SH wavelength in our measurement make the choice of the
excited state for the two-state model very difficult. Nevertheless,
the trends of the measured â values in oxidized porphyrins
match well with those from calculations.
Figure 3. Absorption spectra of (a) CuTPP (2.546 × 10-5 M) and (b)
ZnTPP (2.002 × 10-5 M) during oxidation from neutral to cation
radicals, MTPP+ in CH2Cl2. Insets show corresponding absorption
spectra during oxidation from MTPP+ to MTPP++. Spectra 1, neutral
metalloporphyrin; spectra 2, cation radical; and spectra 3, dication.
suggesting that oxidation of the metal center in CoTPP is not
conducive for the enhancement of â. However, the controlled-
potential electrolysis in CoTPP appeared to be slow.
According to Gouterman’s model,30 the electronic transition
from the π HOMO (a1u and a2u) to the π* LUMO (eg) orbitals
is primarily responsible for the highly intense B or Soret band
(assigned as a1u f eg) around 400 nm and a moderately intense
Q (QR and Qâ) band (assigned as a2u f eg) around 500-660
nm in free base porphyrins. The HOMO and HOMO - 1 states
(a1u and a2u) of free base porphyrins are nearly degenerate, while
the LUMO and LUMO + 1 states (eg) are rigorously degener-
ate.31 The energies of these porphyrin MOs are altered by the
particular metal ion that binds to the core. Absorption spectra
of CuTPP and ZnTPP show a weak QR band at 580-625 nm,
a strong Qâ band at 530-560 nm, and a much stronger B band
at 380-440 nm. After one-electron oxidation, the well-resolved
absorption bands of the neutral metalloporphyrins appear to be
red-shifted with some additional broad features appearing in
the spectrum. The calculated32 and previously reported10,18,28
spectra of ZnTPP+ and CuTPP+ are in agreement with our
spectrum. However, dication spectra of metalloporphyrins are
In CuTPP and ZnTPP, the reverse oxidation of the cation
radicals is facile and leads to the formation of original symmetric

2846 J. Phys. Chem. B, Vol. 112, No. 10, 2008
Wahab et al.
Figure 4. Optimized structures of neutral, cation radical, and dication of ZnTPP.
Figure 5. Electrochemical switching of HRS responses between neutral porphyrins (MTPP) and their cation radicals (MTPP+) in CH2Cl2: (a)
CuTPP (2.724 × 10-5 M) and (b) ZnTPP (2.536 × 10-5 M).
neutral complexes that have zero â. This forms the basis of
on-off or off-on switching of quadratic nonlinearity in these
metalloporphyrins. We followed the second harmonic intensity
(I2ω) during oxidation of neutral CuTPP and ZnTPP to cation
radicals and their conversion back to neutral porphyrins. The
electrochemical on-off switching of â is apparent from the HRS
signal plot in Figure 5a,b. The switching cycle is reproducible
up to several cycles as evidenced from the in situ absorption
spectra (not shown) of the components. We assign the neutral
state of CuTPP or ZnTPP with a zero â value as the off state
and the respective cation radical with a large â value as the on
state. However, the second oxidation step from cation radical
to dication was not quantitatively reversible, and on-off
switching was not recorded. In fact, it is known that MTPP
dications are powerful electrophiles and unstable in the presence
of a variety of nucleophiles such as water, methanol, halides,
etc. to produce isoporphyrins.35 These isoporphyrins are stable
and have been characterized as the perchlorate salts.
Conclusion
Quadratic nonlinearity of the first transition metal series
metalloporphyrins, MTPPs, has been investigated under one-
and two-electron oxidations in CH2Cl2 at 1064 nm. Among the
transition metal porphyrins, only CuTPP and ZnTPP show
reversible two one-electron oxidation steps. Neutral porphyrins
exhibit zero quadratic nonlinearity, but oxidation leads to a
substantial increase in their â values. The experimental trend
in â is also supported by DFT calculations. Electrochemical
switching of nonlinearity between the neutral and the cation
radical states has been achieved. Although the second oxidation
is electrochemically reversible, repeated NLO switching could
only be demonstrated for the first oxidation step. The observa-
tion of a large HRS response in the oxidized metalloporphyrins
paves the way of realizing very large quadratic nonlinearity in
some selected metalloporphyrin systems under oxidation and
on-off switching in those higher dimensional systems for device
applications.

One- and Two-Electron Oxidized Metalloporphyrins
J. Phys. Chem. B, Vol. 112, No. 10, 2008 2847
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Acknowledgment. We are grateful to N. Munichandriah and
K. Sakthivel for helping us with the electrochemical experiments
and to S. Ramakrishnan for lending us the Fiber Optic
spectrometer for in situ absorption measurements. A.W. is
thankful to the Department of Biotechnology for the award of
a Research Associateship. M.B. and S.G. are thankful to the
Council of Scientific and Industrial Research for Senior
Research Fellowships. Generous funding from the Department
of Science and Technology for our research is acknowledged.
Therien, M. J. J. Am. Chem. Soc. 2005, 127, 9710.
Supporting Information Available: Dependence of SH
signal (I2ω) on the incident laser power (Iω) for cation radicals
and dications of CuTPP and ZnTPP (Figures 1S-4S). Typical
wavelength scans of HRS signal over spectral range of 510-
550 nm for cation radicals and dications of CuTPP and ZnTPP
(Figures 5S-8S). This material is available free of charge via
the Internet at http://pubs.acs.org.
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