Metalloporphyrins for quadratic nonlinear optics

Article abstract of DOI:10.1021/jp962427r

Cu^II- and Zn^II-based donor-acceptor porphyrins have been examined for second-order nonlinear optics using the hyper-Rayleigh scattering technique at 1064 nm. Introduction of the metal ions enhances the first hyperpolarizability (β) of the free-base donor-acceptor porphyrins significantly. The open shell Cu^II (d⁹) has a greater influence on β of these porphyrin systems than the closed shell Zn^II (d¹⁰). The high β values in the Zn^II porphyrins are understood in terms of the change in dipole moments upon excitation within the context of the two-state model as well as two-photon resonance enhancement. In Cu^II porphyrin complexes a large number of excited states seem to contribute to β with, perhaps, a little contribution from two-photon resonance at the excitation wavelength.

Full text of DOI:10.1021/jp962427r

J. Phys. Chem. 1996, 100, 19611-19613
19611
Metalloporphyrins for Quadratic Nonlinear Optics
Avijit Sen, Paresh Chandra Ray, Puspendu Kumar Das,* and Varadachari Krishnan*,†
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India
ReceiVed: August 8, 1996X
CuII- and ZnII-based donor-acceptor porphyrins have been examined for second-order nonlinear optics using
the hyper-Rayleigh scattering technique at 1064 nm. Introduction of the metal ions enhances the first
hyperpolarizability (â) of the free-base donor-acceptor porphyrins significantly. The open shell CuII (d9)
has a greater influence on â of these porphyrin systems than the closed shell ZnII (d10). The high â values
in the ZnII porphyrins are understood in terms of the change in dipole moments upon excitation within the
context of the two-state model as well as two-photon resonance enhancement. In CuII porphyrin complexes
a large number of excited states seem to contribute to â with, perhaps, a little contribution from two-photon
resonance at the excitation wavelength.
Introduction
There has been an increasing interest in the search for new
nonlinear optical (NLO) materials that are transition metal
organometallics and coordination complexes1,2 exhibiting large
microscopic (â) and macroscopic (ø2) second-order nonlineari-
ties. Extensively delocalized cyclic π-systems, such as phtha-
locyanines and porphyrins, are studied as third-order NLO
materials3,4 owing to their symmetric structures in nature.
Second-order NLO properties, which are exploited in telecom-
munication, data storage, and information-processing applica-
tions, arise in molecules that lack a center of symmetry. The
meso-tetraphenylporphyrins (TPP) are highly soluble in a variety
of solvents and exhibit interesting optical characteristics. In
addition, structural modification of TPP can easily be brought
about by varying the central metal ion and/or substitution of
organic donor-acceptor (D-A) moieties, thereby providing a
two-dimensional network for effective intramolecular charge
transfer (ICT). An appropriate tuning of porphyrin structures
may result in a large second harmonic response. The electric-
field-induced second harmonic (EFISH) generation measure-
ments of unsymmetrically substituted aminonitrotetraphenyl
free-base porphyrins reported by Suslick et al.5 have shown that
the first hyperpolarizability of these molecules are lower (â )
(10-30) × 10-30 esu) than what is expected a priori for such
large cyclic π-systems. The lower â values are due to the
cancellation of CT interactions in the parallel and perpendicular
directions of the permanent dipole moment along which the
major component of â is measured in the EFISH experiments.
Unfavorable orientation of the meso aryl groups (bearing the
amino and nitro substituents) with reference to the porphyrin
plane also reduces the CT interactions between the push-pull
groups in the above examples.
Figure 1. Structures of the donor-acceptor porphyrins.
the determination of the average hyperpolarizability ( â ) in
solution.6-9 Semiclassical results on the dynamic first hyper-
polarizability of the free-base porphyrins in the gas phase are
contrasted with the experimental values.
Experimental Section
The pentafluorophenyl porphyrin has been synthesized through
the conventional route.10 The nitration of one of the pyrrole
carbons has been accomplished by nitration of Cua (see Figure
1) with Cu(NO3)2 in CHCl3 followed by a demetallation
reaction. The amination at the para position of the meso-
fluorophenyl rings of the unsubstituted porphyrin was achieved
by refluxing corresponding porphyrins with dimethylamine
hydrochloride in DMF. A modified procedure was followed
for the amination of the phenyl rings of the nitro-substituted
porphyrins.11 The corresponding metal derivatives were pre-
pared using metal(II) acetates as metal carriers. Compounds
In this paper, we describe a new class of functionalized
fluoroaryl porphyrins bearing nitro- (situated at the 2-position
in a pyrrole ring) and N,N-dimethylamino groups (the para
position of each of the aryl group) in the vicinity of the cyclic
π-system (Figure 1), which exhibits moderate second-order
nonlinearity. To understand the effect of metal ions on the NLO
response, zinc(II) and copper(II) derivatives of the synthesized
porphyrins were also investigated in this paper. We have
employed the hyper-Rayleigh scattering (HRS) technique for
1
were characterized by UV-visible, H-NMR, 19F-NMR, and
FAB-MS spectroscopies.
The experimental setup for the hyper-Rayleigh scattering
technique has been described in detail elsewhere.9,12 Briefly,
the fundamental from a Nd:YAG laser (Spectra Physics, 8ns,
† Also at Jawaharlal Nehru Centre for Advanced Scientific Research,
Jakkur, Bangalore 560 064, India.
X Abstract published in AdVance ACS Abstracts, November 1, 1996.
S0022-3654(96)02427-6 CCC: $12.00 © 1996 American Chemical Society

19612 J. Phys. Chem., Vol. 100, No. 50, 1996
Sen et al.
TABLE 1: Experimental â’s (×1030) in esu,a Calculated
Dipole Moments (µg) in Debye (D), Calculated â’s (×1030) in
esu, and Extinction Coefficients, E (×10-3) in mol-1 cm-1 at
532 nm for All the Compounds
compounds
âHRS
1.90
7.20
10.1
54.0
2.80
11.2
11.8
92.0
â
µg
ꢀ
a
b
c
d
Zna
Znb
Znc
Znd
Cua
Cub
Cuc
Cud
1.20
3.20
4.60
1.33
1.27
6.12
9.78
2.37
4.79
6.25
12.7
11.3
11.0
3.42
5.82
15.9
18.3
8.61
9.54
32.0
3.90
19.2
19.6
118
a The error is within (7%.
Figure 2. Quadratic coefficient I2ω/Iω2 vs number density of compound
d in dichloroethane solution.
of the neutral species. All the absorption spectra were recorded
in a Hitachi (U-3400) spectrophotometer at room temperature.
<10 mJ/pulse) is focused tightly on a glass cell containing the
solution of the compound. The two-photon Rayleigh scattering
signal is imaged efficiently in the perpendicular direction onto
a visible sensitive photomultiplier tube. A 3 nm bandwidth 532
nm interference filter is used on the signal path to isolate the
second-harmonic light. Finally, the signal is averaged and
recorded. The laser power is kept low to avoid stimulated
Raman and Brillouin scattering and other nonlinear processes.
The theory of the harmonic light scattering has been developed
by several authors.6,7,13 For a mixture of two different species,
e.g., for a solute and a solvent, the resulting second-harmonic
intensity I2ω is related to the incident laser intensity, Iω in the
following way:
Results and Discussions
First hyperpolarizabilities of the metalloporphyrins and the
corresponding free-base porphyrins obtained by the HRS
technique at 1064 nm in dichloroethane (â ) 0.42 × 10-30 esu)
are listed in Table 1. Gas phase first hyperpolarizabilities (â)
of the free-base porphyrins are computed using the intermediate
neglect of differential overlap package of Zerner employing the
correction vector (ZINDO/CV)15 technique. We employed
molecular mechanics (MMX) optimized structures as input
geometries and an excitation energy of 1.17 eV, same as the
experimental photon energy at 1064 nm, for â calculations. In
the ZINDO/CV calculations we have retained all singly and
doubly excited configurations generated from the ground state
Slater determinant by considering 10 HOMOs and 10 LUMOs.
Calculated dipole moments and â’s for the free-base porphyrins
are given in Table 1. The trend in the experimental â values is
reproduced satisfactorily in the calculation. Nonzero dipole
moment and â for symmetrically substituted porphyrin such as
a indicate that the phenyl rings are not in the same plane with
the porphyrin ring. The â values are found to be highly sensitive
to the nature and position of the substituents. In the free-base
porphyrins, this value increases 3.5 times with the amino
substituent on each of the meso-phenyl rings (total of four donor
groups), while it increases by a factor of 5 in the nitro-substituted
porphyrin. In the amino porphyrins, four amino groups are
positioned at the para position of each meso aryl ring disposed
about 70° to the mean porphyrin plane.16 This would enforce
steric constraints for direct interaction of the phenyl rings with
macrocyclic π-pathways, which hinders effective charge transfer
resulting in a â value less than expected for such a large system.
In this regard, the nitro group at one of the pyrrole carbons in
c is in the conjugative pathway and â is higher than b in spite
of the combination of four strong donor N,N-dimethylamino
groups in the latter. The effectiveness of the CT interaction
between the nitro (acceptor) group and the porphyrin (donor)
moiety in c is also evident from a calculated high ground state
dipole moment as well as high â of this molecule. The highest
value of â was obtained when both N,N dimethylamino and
nitro groups are present in the porphyrin moiety as in d. This
is understandable, since their presence within the porphyrin
periphery reinforces ICT as revealed from the results of ground
state dipole moment calculations. The ground state of d is more
polarized than the other free-base derivatives. It may be noted
here that although the porphyrin ring is highly conjugated, the
measured â values are lower than those reported for the same
I
2ω ) G(Nsolventâ2solvent + Nsoluteâ2solute)Iω2
(1)
where G is an instrument factor, Ni is the number density of
the ith species in solution, and âi is the first hyperpolarizability.
Terhune et al.6 calibrated carbon tetrachloride with respect to
quartz, using the same HRS technique, and we have calibrated
dichloroethane by a method similar to that described by Zyss
et al.8 and obtained âdichloroethane ) 0.42 × 10-30 esu. Equation
1 is adequate for the solutes that do not absorb significantly at
ω or 2ω. However, compounds investigated here absorb in the
green (2ω), and corrections as suggested by Clays et al.7 are
necessary in these cases. The modified equation, which can
take into account the absorption at the second-harmonic
frequency, can be expressed as
I
2ω ) (Nsolventâs2olvent + Nsoluteâs2olute)Iω2 10-ꢀ(2ω)cl
(2)
where ꢀ(2ω) is the absorption cross section (in cm2) of the solute
at the second-harmonic frequency, and l (in cm) is the effective
path length. Figure 2 shows the I2ω/Iω2 vs number density plot
for compound d. The nonlinear nature of Figure 2 at high
concentrations of the solute is due to the self-absorption of the
porphyrin at 532 nm. The slope and intercept of the plot at
low concentrations (the linear part of Figure 2) permit evaluation
of â values. Since protonation and deprotonation of the TPP
free base is possible, we checked for the presence of charged
species, which, in principle, can affect the â-values in solu-
tion.9,14 We found that (data not shown) at neutal pH, that is,
under the conditions of the experiments, the extent of proto-
nation or deprotonation is negligible (<0.1%). Therefore, we
assume that the molecules exist as neutral species in solution
and the measured hyperpolarizability value is attributed to that

Metalloporphyrins
J. Phys. Chem., Vol. 100, No. 50, 1996 19613
D-A acyclic trienes (only three double bonds between the donor
and the acceptor).17,18 A possible reason for this lies in the
cyclic π-conjugation that normally reduces the effective path
length for π-electron delocalization. For example, cyclic tri-
and tetraene systems always have lower â values compared to
their acyclic analogues with the same donor-acceptor substitu-
tions. Charge transfer interaction between the donor and
acceptor through a π-electron backbone is most favorable when
the double bonds are in an all-trans configuration.19
Conclusion
In this paper, we report a series of metalloporphyrins
specifically tailored for quadratic NLO properties. The mo-
lecular hyperpolarizabilities of these porphyrins are high, but
the cyclic nature of the π-backbone and dihedral twist of the
phenyl rings with respect to the porphyrin base limit highly
efficient coupling of donor acceptor moieties across the por-
phyrin periphery. The â values of ZnII porphyrins can be
adequately accounted for by invoking the two-state model. For
the CuII porphyrin derivatives, however, the origin of nonlin-
earity is very much different and the contribution to â from
other electronic states seems to be responsible for large â values.
Work is in progress to further probe the role of the metal ion
electronic configuration on the NLO response of metallopor-
phyrins as well as the effect of solvent polarity on their â values.
In these free-base porphyrins, â increases substantially on
metallation. The presence of metal ions eliminates the distinct
CT interaction arising from the nitro or dimethylamino substitu-
tion (compounds b, c, vs Znb, Znc and Cub, Cuc). The major
effect due to the metal ion is observed in the derivatives bearing
both donor and acceptor moieties. Solvatochromic studies on
both absorption and fluorescence transitions of all the com-
pounds (data not shown) indicate that with increasing solvent
polarity both bands shift to the red region. This suggests that
the change in dipole moment, µ, is positive, and within the
context of the two-state model,20 â is also positive. We have
observed from the solvatochromic measurements that µ is higher
for compound Znd than either amino- or nitro-substituted
porphyrins (Znb and Znc). Also, significant solvatochromic
shifts of the emission bands were observed in the case of Znd
(λex ) 410 nm) compared to that in d, indicating more charge-
transfer nature of the excited state of the zinc(II) derivative than
the corresponding free-base porphyrin. Therefore, the higher
value of â in the ZnII derivatives originates partly in the large
change in dipole moment upon excitation. However, the
absorption shifts in CuII complexes in various solvents are not
significant, implying that the two-state model cannot account
for the high â values in CuII porphyrins. It is of interest to
note that for the same D-A substituents, the open shell metal
ion CuII (d9 system) exhibits (Table 1) a higher â value than
the corresponding closed shell ZnII (d10 system) porphyrin.
Similar observations have been made recently by Di Bella et
al.21 for a series of metallosalophen systems. They have shown
experimentally as well as theoretically by the ZINDO/sum-over-
state (SOS) approach that for the same ligand, open shell metal
ions [CuII and CoII in their case] lead to much higher â values
than the closed shell NiII metal ion. Unlike in the CuII-salophen
complexes, we have not observed any distinct metal-ligand or
ligand-metal charge transfer band (MLCT or LMCT) in the
visible or near-UV region of the absorption spectrum for open
shell CuII porphyrins. Since all the porphyrin molecules studied
here absorb 532 nm light (ꢀ532 g 103 M-1 cm-1), the two-photon
resonance enhancement of â will be significant.22 For similar
substitutions, ꢀ532 values of copper(II) derivatives are higher
than those of the zinc(II) derivatives. Therefore, two-photon
resonance enhancement is partially responsible for the large â
in CuII porphyrins. However, the free-base porphyrin d, which
has the highest ꢀ532 value relative to the corresponding copper-
(II) and zinc(II) derivatives, exhibits the lowest â value among
them. Therefore, we believe that the contribution from reso-
nance enhancement to the first hyperpolarizability cannot be
all that important and the metal d-electrons are more effective
in fine tuning the first hyperpolarizability.
Acknowledgment. The laser used in these experiments was
purchased with a grant from the Department of Science and
Technology (DST), Government of India. One of us (P.C.R.)
thanks the Council for Scientific and Industrial Research,
Government of India for a senior research fellowship.
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JP962427R