Cooperative Enhancement of Two-Photon Absorption in Multi-branched Structures

Article abstract of DOI:10.1021/jp992846z

Recent reports of molecular structures with considerably enhanced two-photon absorption cross-section have generated considerable interest in this phenomenon from both fundamental and applications perspectives. In this letter, we report cooperative enhanc

Full text of DOI:10.1021/jp992846z

J. Phys. Chem. B 1999, 103, 10741-10745
10741
Cooperative Enhancement of Two-Photon Absorption in Multi-branched Structures
Sung-Jae Chung, Kyoung-Soo Kim, Tzu-Chau Lin, Guang S. He, Jacek Swiatkiewicz, and
Paras N. Prasad*
Departments of Chemistry and Physics, Photonics Research Laboratory, State UniVersity of New York at
Buffalo, Buffalo, New York 14260-3000
ReceiVed: August 11, 1999; In Final Form: October 1, 1999
Recent reports of molecular structures with considerably enhanced two-photon absorption cross-section have
generated considerable interest in this phenomenon from both fundamental and applications perspectives. In
this letter, we report cooperative enhancement of two-photon absorption in multi-branched structures which
may lead to new design criteria for development of highly efficient two-photon materials. The multi-branched
structures were synthesized using coupling of two and three two-photon active asymmetric donor-acceptor
chromophores linked together by a common amine group. The two-photon cross-sections measured both
with nanosecond and femtosecond pulses show that the value for the trimer is more than six times larger than
that for the monomer, and not just three times larger as expected from the number density increase.
Introduction
the nonlinear transmission method utilizing nanosecond pulses
and the Z-scan technique utilizing femtosecond pulses have been
The two-photon process involving simultaneous absorption
of two photons was theoretically predicted by M.Goppert-Mayer
in 1931,1 and was experimentally observed in the 1960s.2-5
Pioneering work by Rentzepis in data storage6 and by Webb7
in microscopy demonstrated early potential applications of two-
photon processes. However, until recently, most known mo-
lecular two-photon absorptivities were too small to find usage
in many practical applications. Reports of new dyes with
increased two-photon absorption (TPA) cross-sections and large
upconverted fluorescence yields by our group8-11 and other
groups12,13 have generated considerable interest in the develop-
ment of highly efficient two-photon materials and a myriad of
new applications has opened up.14 New applications include two-
photon upconversion lasing,15,16 two-photon optical power
limiting,17-19 three-dimensional optical data storage,20,21 and
two-photon photodynamic therapy.22 Multiphoton microscopy
appears to be of great value as an imaging technique for
numerous biological systems23,24 as well as for nondestructive
evaluation of organic paints and coatings.24
Molecular structures used so far to develop efficient two-
photon chromophores utilize either a symmetrically (terminating
in two electron-donor or two electron-acceptor groups) or
asymmetrically (terminating in a donor and an acceptor)
substituted π-conjugation unit.9-14 The effect of varying the
electron-donating or electron-accepting strength of the end
groups, introducing additional groups in the middle to vary the
charge redistribution, and varying the effective conjugation
length has been studied in order to achieve at design criteria to
produce structures with enhanced two-photon activity.13
In this paper we report the discovery of cooperative enhance-
ment of two-photon absorption in multi-branched structures,
which may lead to new design criteria for development of multi-
branched polymers and dendrimers with very strong two-photon
activities. We report on the synthesis of these new structures,
which involve linkage of asymmetrically substituted two- or
three-chromophore units through a common amino group. Both
used to obtain effective two-photon absorption cross-sections
and to examine the effect of pulse-width on the effective cross-
section. Finally, we discuss some possible mechanisms of this
cooperative enhancement.
Experimental Section
Synthesis. We have used triphenylamine as the π-electron
donor25 together with 2-phenyl-5-(4-tert-butylphenyl)-1,3,4-
oxadiazole26,27 as the π-electron acceptor. This structural unit
serves as a building block in the monomeric material. The
respective monomeric, dimeric, and trimeric structures synthe-
sized and studied are N-[4-{2-(4-{5-[4-(tert-butyl)phenyl]-1,3,4-
oxadiazol-2-yl}phenyl)-1-ethenyl}phenyl]-N,N-diphenylamine
(abbreviated as PRL-101), N,N-bis[4-{2-(4-{5-[4-(tert-butyl)-
phenyl]-1,3,4-oxadiazol-2-yl}phenyl)-1-ethenyl}phenyl]-N-
phenylamine (abbreviated as PRL-501), and N,N,N-tris[4-{2-
(4-{5-[4-(tert-butyl)phenyl]-1,3,4-oxadiazol-2-yl}phenyl)-1-
ethenyl}phenyl]amine (abbreviated as PRL-701). Their struc-
tures are shown in Figure 1.
The synthetic routes to the chromophores are shown in
Scheme 1. In the final steps, PRL-101 and PRL-501 were
synthesized by Wittig reaction28 of phosphonium bromide of
the oxadiazole compound (8) with the aldehyde-functionalized
triphenylamine compounds (1 and 4) in 63% and 47% yields.
PRL-701 was obtained by reaction of the vinyl oxadiazole
compound (9) with tribromo triphenylamine (3) under pal-
ladium-catalyzed Heck conditions,29 32% yield. All compounds
were thoroughly washed with methanol and then were purified
by column chromatography. The pure chromophores are highly
soluble in a variety of organic solvents such as methylene
chloride, chloroform, THF, 1,1,2,2-tetrachloroethane, etc., and
are fluorescent greenish yellow in the solid state.
Characterization. All optical characterizations were con-
ducted using solutions of compounds in 1,1,2,2-tetrachloroet-
hane. UV-visible absorption and fluorescence studies were
conducted using a Shimadzu UV-3101PC spectrophotometer and
a Shimadzu RF5000U spectrofluorophotometer. TPA cross-
* Corresponding author.
10.1021/jp992846z CCC: $18.00 © 1999 American Chemical Society
Published on Web 11/18/1999

10742 J. Phys. Chem. B, Vol. 103, No. 49, 1999
Letters
Figure 1. Chemical Structures of PRL-101, PRL-501, and PRL-701.
Figure 2. UV-visible absorption spectra of PRL-101, PRL-501, and
PRL-701 solutions in 1,1,2,2-tetrachloroethane (1 × 10-5 mol/L).
Figure 3. One-photon emission spectra of PRL-101, PRL-501, and
PRL-701 solutions in 1,1,2,2-tetrachloroethane (1 × 10-5 mol/L)
obtained at the excitation wavelength of 405 nm.
section values of the three compounds in solution were measured
with both nanosecond (810 nm) and femtosecond (796 nm)
pulses. We have used direct nonlinear transmission measure-
ments19 at ∼810 nm with ∼8 ns pulses from a Nd:YAG pumped
dye laser at the intensity levels of several hundreds of megawatt
per square centimeter. The experimental setup and data process-
ing procedure are basically the same as those described in
previous publications.19,30 Femtosecond Z-scan measurements
were performed using the same solutions as for the nanosecond
nonlinear transmission measurements. The ultrafast laser system
and the Z-scan technique are described elsewhere.31 For these
experiments, the average pulse length was 173 fs and irradiance
at the focal point was 36 GW/cm2 at a wavelength of 796 nm.
The repetition rate was kept low at 32 Hz, to avoid any
cumulative effect from slow photophysical/photochemical pro-
cesses. The irradiance was reduced to a level at which the
nonlinear absorption coefficient was found to be constant, to
reduce contributions from other nonlinear effects.
Results and Discussion
Figure 2 compares the UV-visible absorption spectra of the
chromophores in solution. It shows that there are two strong
absorption bands attributed to the solute molecules. The
absorption at about 300-320 nm is localized in the phenylene
rings, whereas the longer wavelength region absorption (about
400 nm) has considerable charge-transfer character.32,33 PRL-
101 shows a peak of its one-photon absorption at 399 nm,
whereas for PRL-501 and PRL-701 the peaks are shifted to
417 and to 426 nm, respectively. One-photon emission spectra
of the chromophores, obtained at the excitation wavelength of
405 nm, are compared in Figure 3. As one can see, the λmax

Letters
J. Phys. Chem. B, Vol. 103, No. 49, 1999 10743
SCHEME 1. Synthetic Routes for PRL-101, PRL-501, and PRL-701
values of emission also shift from 503 nm for PRL-101 to 510
nm for PRL-501 and 516 nm for PRL-701, respectively. That
is, the same trend is observed in the UV-visible absorption
and the emission spectra of chromophores as one increases the
content of the chromophore moiety per molecule. This is a clear
indication of some interactions between chromophore moieties
in the molecule, resulting in charge redistribution and extended
delocalization. No absorption in the wavelength range from 550
to 1000 nm is observed in the single-photon spectra. The two-
photon energy of ∼810 nm radiation falls within the strong
linear absorption band of all these materials in solutions. Very
strong frequency-upconverted fluorescence emission is easily
observed from solutions excited with a Q-switched Nd:YAG
pumped dye laser at 810 nm. This observation indicates that a
two-photon absorption process is occurring within the samples.
The values of the two-photon absorption cross-section calculated
for the materials by fitting both the nonlinear transmission data
for the nanosecond experiment and the Z-scan data for the
femtosecond experiment are listed in Table 1. The values
obtained in experiments with nanosecond pulses are more than
two orders of magnitude larger than those evaluated from the
corresponding experiments with femtosecond laser pulses. Such
large differences have also been observed in other materials31
and can be attributed to the possible contribution of excited state
absorption during excitation. Nevertheless, both experiments
clearly indicate a relative increase of the effective nonlinear

10744 J. Phys. Chem. B, Vol. 103, No. 49, 1999
Letters
predicts a 1/∆E6 dependence of ø(3). Using the experimentally
observed peaks in the UV-visible absorption spectra for the
estimates of band gap, this model predicts relative values for
PRL-101, PRL-501, and PRL-701 to be 1:2.6:4.5, taking into
account also the number density increase. Even these values
are smaller than what we observe. Therefore, delocalization
effect itself can not explain the observed enhancement. We
therefore suggest that because the same electron donor group
is being shared by all of the electron acceptors, the charge
redistribution also influences the dipole terms (∆µ for the first
term and/or the transition dipole moment coupling the inter-
mediate states for the second term). The molar extinction
coefficients, ꢀ, for the three compounds also do not follow the
TABLE 1: Optical Properties of PRL-101, PRL-501, and
PRL-701 in 1,1,2,2-Tetrachloroethanea
compound
PRL-101
PRL-501
PRL-701
solvent
(mol/L)
λmax
linear abs. nm
CH2Cl2CH2Cl2 CH2Cl2CH2Cl2 CH2Cl2CH2Cl2
(1 × 10-2
)
(1.4 × 10-2
)
(9 × 10-3
)
399
417
426
(upconv. em. nm) (503)
(510)
208
(1.1)
(516)
587
(2.4)
σ
60
(×10-20 cm4/GW) (0.35)
a Experimental σ values determined with nanosecond pulses at 810
nm in ∼8 ns range and femtosecond pulses at 796 nm in ∼ 173 fs
range (given in parentheses) are reported; the uncertainties in the
experimental σ values are estimated to be (15.
number of density ratios [ꢀPRL-701/ꢀPRL-101 ) 2.2, ꢀPRL-501
/
ꢀPRL-101 ) 1.4], indicating a change in transition dipole. Higher-
order multipolar terms may also make contribution. Further
theoretical work needs to be done to establish the mechanism
of cooperative enhancement in these multi-branched structures.
absorption cross-section as the number of chromophore moieties
increases, which is not linear (the relative values are 1:3.5:9.8
with nanosecond pulses and 1:3.1:6.8 with femtosecond pulses).
The two-photon absorption cross-section σ2 is related to the
imaginary part of the molecular third-order nonlinear polariz-
ability (second hyperpolarizability, γ). In the formalism of the
sum-over-states it can be written as34
Conclusion
In conclusion, we have synthesized, using Wittig and Heck
reactions, new multi-branched chromophores with different
numbers of two-photon moieties per molecule. All of our
chromophores exhibit very large two-photon absorption cross-
section as determined by the use of nanosecond and femtosecond
pulses. We have discovered cooperative enhancement of two-
photon absorption in these multi-branched structures.
Im γ(-ω;ω,-ω,ω) ) Im P
M2ge ∆µ2ge
D
(Ege - pω - iΓge)(Ege - 2pω - iΓge)(Ege - pω - iΓge)
M2ge M2ee′
T
+
∑
(Ege - pω - iΓge)(Ege - 2pω - iΓge′)(Ege - pω - iΓge)
e′
M4ge
[
]
Acknowledgment. This work was supported in part by the
Air Force Office of Scientific Research Directoriate of Chem-
istry and Life Science through contract number F4962093C0017
and in part by the Army through a subcontract from Laser
Photonics Technology Inc.
N
(Ege - pω - iΓge)(Ege + pω + iΓge)(Ege - pω - iΓge)
In this expression P corresponds to a permutation operator over
the optical frequencies; Mge is the transition dipole moment
between the ground state, g, and the first-lying charge-transfer
excited state; Ege denotes the transition energy; Γge is the
associated damping factor, and ∆µeg is the difference in the
dipole moments. For asymmetric dipolar structures, as used here,
both the first (D) and second (T) terms contribute. The third
(N) term is the negative term for which only one-photon
resonances occur and which therefore makes no contribution
to the TPA sum. If each arm of the multi-branched structure is
taken as a linear conjugated structure, one can assume that the
largest tensor component of γ will be γ1111 along the length.
Using a simple tensor component addition,35 one would expect
the orientationally averaged second hyperpolarizability to follow
a 1:2:3 ratio, regardless of the value of the bond angle in the
amino group as long as this angle does not change in going
from the monomer to the trimer. Accordingly, one would expect
the relative two-photon cross-section for the dimer and trimer
to be σdimer/σmonomer ) 2 and σtrimer/σmonomer ) 3, provided that
the assumption of the constant bond angle is valid and that the
multi-branched molecule consists of noninteracting units. Clearly
these relative values are much smaller than what we observe
under femtosecond conditions (which provide a more accurate
assessment of the true two-photon absorption cross-section).
Therefore, the individual chromophore units are definitely
interacting and delocalization is extending through the various
arms. This delocalization effect is also evident from a reduction
in the excitation energy as seen in the UV-visible absorption
and emission spectra discussed above.
References and Notes
(1) Goppert-Mayer. M. Ann. Phys. 1931, 9, 273.
(2) Rentzepis, P. M.; Pao, Y. H. Appl. Phys. Lett. 1964, 156, 964.
(3) Peticolas, W. L. Annu. ReV. Phys. Chem. 1967, 18, 233.
(4) McClain, W. M. Acc. Chem. Res. 1974, 7, 129.
(5) Birge, R. R.; Pierce, B. M. J. Chem. Phys. 1979, 70, 165.
(6) Parthenopoulos, D. A.; Rentzepis, P. M. Science 1989, 245, 843.
(7) Denk, W.; Strickler, J. H.; Webb, W. W. Science 1990, 248, 73.
(8) He, G. S.; Gvishi, R.; Prasad, P. N.; Reinhardt, B. A. Opt. Commun.
1995, 117, 133.
(9) He, G. S.; Bhawalkar, J. D.; Zhao, C. F.; Prasad, P. N. Appl. Phys.
Lett. 1995, 67, 2433.
(10) He, G. S.; Yuan, L.; Cheng, N.; Bhawalkar, J. D.; Prasad, P. N.;
Brott, L. L.; Clarson, S. J.; Reinhardt, B. A. J. Opt. Soc. Am. B 1997, B14,
1079.
(11) Reinhardt, B. A.; Brott, L. L.; Clarson, S. J.; Dillard, A. G.; Bhatt,
J. C.; Kannan, R.; Yuan, L.; He, G. S.; Prasad, P. N. Chem. Mater. 1998,
10, 1863.
(12) Ehrlich, J. B.; Wu, X. L.; Lee, I.-Y. S.; Hu, Z.-Y.; Rockel, H.;
Marder, S. R.; Perry, J. W. Opt. Lett. 1997, 22 (24), 1843.
(13) Albota, M.; Beljonne, D.; Bredas, J.-L.; Ehrlich, J. E.; Fu, J.-Y.;
Heikal, A. A.; Hess, S. E.; Kogej, T.; Levin, M. D.; Marder, S. R.; McCord-
Maughon, D.; Perry, J. W.; Rockel, H.; Rumi, M.; Subramaniam, G.; Webb,
W. W.; Wu, X.-L.; Xu, C. Science 1998, 281, 1653.
(14) Bhawalkar, J. D.; He, G. S.; Prasad, P. N. Rep. Prog. Phys. 1996,
59, 1041.
(15) Bhawalkar, J. D.; He, G. S.; Park, C.-K.; Zhao, C. F.; Ruland, G.;
Prasad, P. N. Opt. Commun. 1996, 124, 33.
(16) Zhao, C. F.; Gvishi, R.; Narang, U.; Ruland, G.; Prasad, P. N. J.
Phys. Chem. 1996, 100, 4526.
(17) He, G. S.; Xu, G. C.; Prasad, P. N.; Reinhardt, B. A.; Bhatt, J. C.;
Dillard, A. G.; McKellar, R. Opt. Lett. 1995, 20, 435.
(18) He, G. S.; Weder, C.; Smith, P.; Prasad P. N. J. Quantum Electron.
1998, 34, 2279.
Various power laws for the dependence of γ on the excitation
energy, ∆E, have been predicted depending on the type of
theoretical formulation used.35,36 A simple electron-free model
(19) Tutt, L. W.; Boggess, T. F. J. Quantum Electron. 1993, 17, 299.

Letters
J. Phys. Chem. B, Vol. 103, No. 49, 1999 10745
(20) Dvormikov, A. S.; Rentzepis, P. M. Opt. Commun. 1997, 136, 1.
(21) Pudavar, H. E.; Joshi, M. P.; Prasad, P. N.; Reianhardt, B. A. Appl.
Phys. Lett. 1999, 74, 1338.
(22) Bhawalkar, J. D.; Kumar, N. D.; Zhao, C. F.; Prasad, P. N. J. Clin.
Med. Surg. 1997, 37, 510.
(23) Gura, T. Science 1997, 276, 1988.
(29) Dieck, H. A.; Heck, R. F. J. Am. Chem. Soc. 1974, 96, 1133.
(30) To compare the TPA cross-section of materials without interference
from solvent polarity effect, the same solvent, 1,1,2,2-tetrachloroethane,
was used.
(31) Swiatkiewicz, J.; Prasad, P. N.; Reinhardt, B. A. Opt. Commun.
1998, 157, 135.
(24) Bhawalkar, J. D.; Swiatkiewicz, J.; Prasad, P. N.; Pan, S. J.; Shin,
A.; Samarabandu, J. K.; Cheng, P. C.; Reinhardt, B. A. Polymer 1997, 38
(17), 4551.
(25) Veatweber, H.; Sander, R.; Greiner, A.; Heitz, W.; Mahrt, R. F.;
Bassler, H. Synth. Met. 1994, 195, 2023.
(32) Gruen, H.; Gaeraner, H. J. Phys. Chem. 1989, 93, 7144.
(33) Beljonne, D.; Bredas, J. L.; Cha, M.; Torruellas, W. E.; Stegeman,
G. I.; Hofstraat, J. W.; Horsthuis, W. H. G.; Mohlmann, G. R. J. Chem.
Phys. 1995, 103, 7834.
(34) Kogej, T.; Beljonne, D.; Meyers, F.; Perry, J. W.; Marder, J. L.;
Bredas. J. L. Chem. Phys. Lett. 1998, 298, 1.
(35) Prasad, P. N.; Williams, D. J. Introduction to Nonlinear Optical
Effects in Molecules and Polymers; Wiley: New York, 1991.
(36) Norman, P.; Luo, Y.; Agren, H. Chem. Phys. Lett. 1998, 296, 8.
(26) Yang, Y.; Pei, Q. J. Appl. Phys. 1995, 77, 4807.
(27) Chung, S.-J.; Kwon, K.-K.; Lee, S.-W.; Jin, J.-I.; Lee, C. H.; Lee,
C. E.; Park, Y. AdV. Mater. 1998, 10, 1112.
(28) Maercker, A. Org. React. 1965, 14, 270.