Dynamic studies of charge-separated-state enhancement of the optical nonlinearity of a porphyrin heterodimer

Article abstract of DOI:10.1063/1.117448

Light-induced enhancement of the third-order nonlinear optical susceptibility of a heterodimer consisting of two different porphyrin monomers, a zinctetra(4-sulfonatophenyl)porphyrin and a zinctetra(4-N-methyl-pyridyl)porphyrin, was observed when the sample was optically pumped by a 30 ps, 532 nm pump pulse. The temporal behavior of the enhancement was studied. It was found that the enhancement was due predominantly to the population of the charge-separated state with a large electronic third-order nonlinear optical susceptibility

Full text of DOI:10.1063/1.117448

Dynamic studies of chargeseparatedstate enhancement of the optical
nonlinearity of a porphyrin heterodimer
Jinhai Si, Yougui Wang, Qiguang Yang, Peixian Ye, Hongjian Tian et al.
Citation: Appl. Phys. Lett. 69, 1832 (1996); doi: 10.1063/1.117448
View online: http://dx.doi.org/10.1063/1.117448
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Dynamic studies of charge-separated-state enhancement of the optical
nonlinearity of a porphyrin heterodimer
Jinhai Si, Yougui Wang, Qiguang Yang, and Peixian Ye
Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, People’s Republic
of China
Hongjian Tian, Qingfu Zhou, and Huijun Xu
Institute of Photographic Chemistry, Chinese Academy of Sciences, Beijing, People’s Republic of China
͑Received 6 March 1996; accepted for publication 15 July 1996͒
Light-induced enhancement of the third-order nonlinear optical susceptibility of a heterodimer
consisting of two different porphyrin monomers, a zinctetra͑4-sulfonatophenyl͒porphyrin and a
zinctetra͑4-N-methyl-pyridyl͒porphyrin, was observed when the sample was optically pumped by a
30 ps, 532 nm pump pulse. The temporal behavior of the enhancement was studied. It was found
that the enhancement was due predominantly to the population of the charge-separated state with a
large electronic third-order nonlinear optical susceptibility © 1996 American Institute of Physics.
͓S0003-6951͑96͒03439-0͔
Nonlinear optics is expected to play a major role in the
technology of photonics. Recent progress in the field of high-
speed optoelectronics and information technology has en-
couraged the search for materials with a large, fast nonlinear
optical ͑NLO͒ response and for new ways to enhance the
NLO response. Studies on light-induced enhancement of the
dynamic behavior of charge-separated-state NLO process has
not been done before.
In this letter, we report our studies on light-induced
enhancement of the optical nonlinearity of a heterodimer
͑here we refer to it as ZnTSPP-ZnTMPyP͒ consisting of
two different porphyrin monomers,
a
zinctetra͑4-
(3)
sulfonatophyenhyl͒porphyrin ͑ZnTSPP͒ and a zinctetra͑4-N-
methyl-pyridyl͒porphyrin ͑ZnTMPyP͒. The temporal behav-
third-order nonlinear optical susceptibility ␹ of materials
have been attracting much attention. Rogovin reported a
stark-enhanced phase conjugation in shaped-microparticle
suspensions.1 Garito and colleagues found a new way to
(3)
ior of the enhancement of ␹ for the porphyrin dimer was
measured by using degenerate-four-wave-mixing ͑DFWM͒
with an additional pump beam. For comparison, the mea-
surement was performed also on the two porphyrin mono-
mers. It was verified by comparing the time evolution of the
enhanced NLO process that the enhancement for ZnTSPP-
ZnTMPyP is due predominantly to the population of the
charge-separated state, whereas the enhancements for
ZnTSPP and ZnTMPP are due to the population of their first
excited singlet state.
ZnTSPP-ZnTMPyP was formed in the liquid phase by
pairing electrostatically ZnTSPP with ZnTMPyP bearing op-
positely charged substitutents. It was prepared by mixing
ZnTSPP with ZnTMPyP in a molar ratio of 1:1 in
achieve large ␹ through excited-state population,2 and the
(3)
enhanced NLO process related to optically pumped elec-
tronic excited states was observed in some conjugated or-
ganic materials.3,4
It has been shown that the stacked and bridged
porphyrin5 or phthalocyanine6 assemblies can exhibit un-
usual optical, electron-transfer and conductivity properties
owing to the strong ␲–␲ interactions between the two mac-
rocycles and the formation of exciton coupling in the sand-
wich complexes. Depending on the nature of the central
metal and peripheral substitutents in porphyrins and phthalo-
cyanines, a wide range of redox properties and photophysical
properties can be obtained. Thus, their association should
lead to a peculiarly wide range of donor-accepted complexes.
Noncovalently bound heterodimers consisting of two differ-
ent porphyrin monomers, one of which is equipped with four
positively charged sidegroups and the other with four nega-
tively charged sidegroups, are attractive assembly models
because an effective charge transfer can occur in these mol-
ecules through photoexcitation of the sensitizer component.7
Subpicosecond time-resolved photoinduced absorption mea-
surements for porphyrin dimers show that charge transfer
occurs within 300 fs after photoexciton, and the charge-
recombination time is 50–500 ps.8 Therefore, we believe that
it is more meaningful to study the charge-separated-state
NLO process of supramolecules such as porphyrin dimers.
Enhanced nonlinear absorption related to the absorption of
charge-separated state in a polymer/methanofullerene film
was reported recently,9 but to our knowledge, study on the
H O/CH OH 4:1 / , in which more than 90% of the por-
v v
2
3
phyrin monomers were dimerized. The concentration of the
sample has 1.18ϫ10Ϫ4 M. The porphyrin dimer has a
slipped ‘‘face-to-face’’ structure with a plane-to-plane dis-
tance of 30–35 nm, and two pyrroles rings of each monomer
are mutually overlapping. The Soret band ͑410–430 nm͒ of
ZnTSPP exhibits a blue shift on addition of ZnTMPyP,
whereas the Q band ͑500–650 nm͒ shifts to the red.7 The two
monomers fluoresce strongly, while in the dimer the fluores-
cence is quenched, indicating the existence of a very efficient
charge transfer from the electron-donating ZnTSPP to the
electron-accepting ZnTMPyP. The Q band ͑500–650 nm͒ of
the dimer is attributed to the transition involving delocalized
molecular orbital due to the strong ␲–␲ coupling between
the two macrocycles, to give a new singlet excited state
1
*
͑␲,␲ ͒. Excitation of the dimer with a 532 nm pump pulse,
1
*
lead to the population of the (␲,␲ ) excited state. They
deactivate rapidly within a few hundred fs to the low-
1832
Appl. Phys. Lett. 69 (13), 23 September 1996
0003-6951/96/69(13)/1832/3/$10.00
© 1996 American Institute of Physics
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(3)
FIG. 1. Time evolution of ⌬␹ of ZnTSPP-ZnTMPyP on the action of the
532 nm pump pulse of energy of 0.6 mJ. The solid curve is a fit with an
exponent of 340 ps.
lying charge-separated state through the charge transfer from
ZnTSPP to ZnTMPyP.
In our experiments, a forward DFWM geometry with an
additional variable-time-delay pump beam was used,4 where
the light source was a Nd:YAG laser system with a 30 ps,
1064 nm single pulse output. The 1064 nm pulse from the
laser was doubled by a KDP crystal to produce the second-
harmonic pulse at 532 nm. The 532 nm beam then served as
the pump beam. The remaining 1064 nm beam was split into
two beams which served as the DFWM probes. To eliminate
thermal grating contribution the polarizations of the two
probe beams were chosen to be orthogonal. The two probe
beams with wave-vectors k1 and k2 were focused in coinci-
dence on the sample at a small angle of 1.5 degree. The
pump beam was also focused on the interaction region in the
sample cell whose thickness was 2 mm. The diameter of the
pump beam was ϳ0.6 mm, whereas those of the probe
beams were ϳ0.4 mm to ensure their overlap. The excited-
state DFWM experiment began with the arrival of the pump
pulse which excited sample’s molecules to the excited states.
After excitation, and before relaxation to the ground state,
the two probe pulses arrived, producing a diffracted signal in
the 2k2 –k2 direction which was proportional to the square
(3)
FIG. 2. Time evolution of ⌬␹ of ZnTSPP ͑a͒ and ZnTMPyP ͑b͒ on the
action of the 532 nm pump pulse of energy of 0.6 mJ. The solid curves are
a fit with an exponent.
DFWM signal intensities from which the corresponding
could be obtained. When the probe pulses preceded the
pump pulse, it encountered only molecules in the ground
state and, therefore, the measured ␹ was contributed only
from the solvent and the ground-state molecules. When the
probe followed the pump, it encountered a mixture of mol-
ecules in the excited and ground states; and the ␹ was
(3)
eff
␹
(3)
eff
(3)
eff
contributed from the solvent, excited-state molecules, and
the ground-state molecules. From the data of ␹ such mea-
(3)
eff
sured, we can get the corresponding susceptibility difference
⌬␹(3)ϭ␹ Ϫ(␹solventϩ␹(3)), where ␹ is the susceptibility
(3)
eff
(3)
(3)
g
(3)
g
of ␹ of the sample, and detected by a photomultiplier.
of ZnTSPP-ZnTMPyP when all molecules are in the ground
state. Then, we obtained the delay-time dependence of
In an optically pumped third-order NLO process the ma-
terial exhibits an effective third-order nonlinear optical sus-
⌬␹ as shown in Fig. 1. One can see that ⌬␹(3), which
(3)
(3)
eff
ceptibility, ␹ , that is related to populations of all the popu-
represents the NLO enhancement, exhibits a rise time that is
longer than the pulse width, and then followed by a single
exponential decay whose decay time is ϳ340 ps, consistent
with the time of the charge recombination of porphyrin
heterodimers.8 Therefore, this enhanced NLO process can be
explained as following: the optical pumping initially popu-
lates the (␲,␲ ) excited state with a very short lifetime ͑a
few hundreds fs͒, then an efficient charge transfer from the
(␲,␲ ) state to the low-lying charge-separated state with a
lated states:
͑3͒
͑3͒
solvent
Sn
␹
eff ϭ␹
ϩF
N ␥ ,
Sn
͚
Sn
(3)
where ␹
is the third-order NLO susceptibility of the
solvent
solvent, and F is the local field factor. NSn and ␥Sn are the
number density and the molecular hyperpolarizability of the
state Sn, respectively.
1
*
1
*
When the ZnTSPP-ZnTMPyP solution was pumped by
the 532 nm pump beam, enhancement of the DFWM signal
was observed. No evident absorption of ZnTSPP-ZnTMPyP
at 1064 nm was observed either the pump beam existed or
not, indicating that the nonlinearities of the ground state and
the excited states at this wavelength are nonresonant. Vary-
ing the delay time of the pump pulse while two probe pulses
arrived simultaneously, we measured the corresponding
very fast rate, and finally the population of the charge-
(3)
TABLE I. Measured values of ␹ with and without the pump beam and the
charge-recombination time of ZnTSPP-ZnTMPyP.
(3)
(3)
␹
͑without pump͒
␹
͑with pump͒
Charge-recombination time
340 ps
1.6ϫ10Ϫ15 esu
1.3ϫ10Ϫ14 esu
Appl. Phys. Lett., Vol. 69, No. 13, 23 September 1996
Si et al.
1833
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separated state recovers to the ground state through the
charge recombination with a much slower rate. Since the
charge-transfer rate is more than 1000 times faster than any
competing process, the quantum efficiency for charge trans-
charge-recombination time of ZnTSPP-ZnTMPyP.
In summary, we investigated dynamic behavior of light-
induced enhancement of the third-order NLO susceptibility
of a heterodimer, ZnTSPP-ZnTMPyP. By measuring the
time evolution of the enhanced NLO process, it was verified
that the enhancement is due predominantly to the population
of the charge-separated state with a large electronic third-
order nonlinear optical susceptibility.
fer approaches unity. It is the population of the charge-
(3)
separated state that produces the enhancement of ␹
be-
1
*
cause the population of the (␲,␲ ) state with a very short
lifetime is much less than that of the charge-separated state.
In order to confirm our explanation, we measured also
This work was supported by the National Natural Sci-
ence Foundation of China.
(3)
the time evolution of the enhancement of ␹ for the two
monomers, ZnTSPP and ZnTMPyP, which are shown in Fig.
2͑a͒ and 2͑b͒, respectively. From Fig. 2, one can find that
they exhibit a similar temporal behavior to that of ZnTSPP-
ZnTMPyP, but the decay times are much longer than that of
ZnTSPP-ZnTMPyP, and determined to be 2.5 and 3.2 ns,
respectively. These times are consistent with the lifetimes of
the first excited singlet state of S1 of ZnTSPP and
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ZnTMPyP.10 Therefore, the enhancement of ␹ for the two
(3)
monomers are attributed to the population of the excited state
S1.
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130, 555 ͑1986͒.
When the pump pulse was set to precede the probe
(3)
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ZnTMPyP under an unsaturated pumping condition ͑energy
of the pump pulse was ϳ0.6 mJ͒, which was enhanced by
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Appl. Phys. Lett. 67, 3850 ͑1995͒.
(3)
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(3)
values of ␹
with and without the pump beam and the
1834
Appl. Phys. Lett., Vol. 69, No. 13, 23 September 1996
Si et al.
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