Novel multi-branched organic compounds with enhanced two-photon absorption benefiting from the strong electronic coupling

Article abstract of DOI:10.1016/j.cplett.2005.08.041

Two novel multi-branched organic compounds, 1,2,4,5-tetrakis[(4′-N,N- diethylamino)styryl]benzene (TDEASB) and 1,2,4,5-tetrakis[(4′-N,N- dibutylamino)styryl]benzene (TDBASB), with strong electronic coupling among the conjugated arms, have been designed and synthesized. Their single-photon and two-photon absorption (TPA) and photoluminescent (PL) properties have been studied carefully. The two-photon absorption cross-sections of the two compounds were estimated by two-photon excited fluorescence technique using 200 fs, 76 MHz, Ti:sapphire laser, which are as high as 810 and 1030 GM for TDEASB and TDBASB, respectively. The strong TPA cross-sections indicate excellent conjugation and electron delocalization properties in these benzene-core based multi-branched compounds.

Full text of DOI:10.1016/j.cplett.2005.08.041

Chemical Physics Letters 414 (2005) 253–258
www.elsevier.com/locate/cplett
Novel multi-branched organic compounds with enhanced two-photon
absorption benefiting from the strong electronic coupling
*
Yan Ren, Qian Xin, Xu-Tang Tao , Lei Wang, Xiao-Qiang Yu,
Jia-Xiang Yang, Min-Hua Jiang
State Key Laboratory of Crystal Materials, Shandong University,
No. 27, Shanda Nan Lu, Jinan 250100, PR China
Received 23 June 2005; in final form 5 August 2005
Available online 16 September 2005
Abstract
Two novel multi-branched organic compounds, 1,2,4,5-tetrakis[(40-N,N-diethylamino)styryl]benzene (TDEASB) and 1,2,4,5-
tetrakis[(40-N,N-dibutylamino)styryl]benzene (TDBASB), with strong electronic coupling among the conjugated arms, have been
designed and synthesized. Their single-photon and two-photon absorption (TPA) and photoluminescent (PL) properties have been
studied carefully. The two-photon absorption cross-sections of the two compounds were estimated by two-photon excited fluores-
cence technique using 200 fs, 76 MHz, Ti:sapphire laser, which are as high as 810 and 1030 GM for TDEASB and TDBASB, respec-
tively. The strong TPA cross-sections indicate excellent conjugation and electron delocalization properties in these benzene-core
based multi-branched compounds.
ꢀ 2005 Elsevier B.V. All rights reserved.
1. Introduction
enhancement attributes to the interchromophore vib-
ronic and electronic coupling.
Recently, special attention has been paid to design
two-photon absorption (TPA) [1–5] materials with mul-
ti-branched structures [6–12] because it is believed that
increasing the molecular dimensionality can further en-
hance the TPA cross-sections of them. In these multi-
branched molecules, several asymmetrically substituted
TPA chromophores are generally linked through a com-
mon group or attached to dendrimer macromolecules by
single bonds. The number density of TPA chromoph-
ores is effectively increased. Such conformation even
caused a nonlinear enhancement of their TPA cross-sec-
tions [6,8], which is much larger than those predicted
simply on the basis of the chromophore number density
increase. The research results show that the nonlinear
In some multi-branched TPA compounds with amino
group as the core, according to the theoretical simula-
tions on them, the electronic coupling is not very
effective between the different arms, whereas the contri-
butions from vibronic coupling play an important role
in the strong enhancement of the two-photon absorp-
tion cross-section. The weak electronic coupling in them
is probably due to use a single amino group as the core,
which breaks the conjugation of the whole molecular
system and make the effective conjugation length of
the multi-branched structures the same as that of the
one-branched structure [13]. However, in another series
of dendrimers with similar amino-branching units,
which possess quasi-planar structure because of the
sp2-hybridization of nitrogen, electronic coupling
between chromophores contributes significantly to the
strong cooperative effect of TPA [8]. It is also found that
the cooperative enhancement may turn over into a linear
*
Corresponding author. Fax: +86 531 88574135.
E-mail address: txt@icm.sdu.edu.cn (X.-T. Tao).
0009-2614/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2005.08.041

254
Y. Ren et al. / Chemical Physics Letters 414 (2005) 253–258
one if the quasi-planar core of the dendrimer is
deformed and the extended p-conjugation over the
whole molecule diminishes. More evidences from the
experiments and theoretical calculations prove that use
of conjugated connecting groups in multi-branched
structures can increase the effective conjugation length
and obtain the delocalization governing cooperative
enhancement of nonlinear optical response [9,14,15].
Here, we report two new conjugated benzene-core
based multi-branched compounds with simple structures
but strongly enhanced TPA properties, 1,2,4,5-tetra-
kis[(40-N,N-diethylamino)styryl]benzene (TDEASB) and
1,2,4,5-tetrakis[(40-N,N-dibutylamino)styryl]benzene
(TDBASB). The chemical structures of the two molecules
are shown in Fig. 1. Since the benzene-core in the new
compounds bears four styryl segments on the 1,2 and
4,5 positions end-caped with strong donor groups, large
conjugations are expected to form not only in two arms
substituted in 1,4 and 2,5 positions but also in the couples
substituted in 1,2 and 4,5 positions. Unlike the benzene-
core chromophore bearing substituents in meta positions,
in which the conjugation is blocked in the branching
center, the effective p conjugation length is actually in-
creased in the new structures. According to the before-
hand theoretical calculation on their corresponding
model compounds, strong electronic coupling was
achieved in the 1,2,4,5-substituted molecule, and its
TPA cross-section calculated is up to five times higher
than that of the meta substituted molecule [16].
stand the photoluminescent (PL) properties of the com-
pounds. Some new PL phenomena observed were
discussed in detail.
2. Experimental
2.1. Materials and synthesis
Sodium hydride (60.3%), N,N-dibutylaniline and
1,2,4,5-tetrakis(bromomethyl)benzene were purchased
from Acros Organics. 1,2-Dimethoxyethane (DME),
potassium tert-butoxide and trimethyl phosphite were
commercially available. Trimethyl phosphite was dis-
tilled carefully before use. The others of above reagents
were used without further purification. 4-(N,N-Diethyla-
mino)benzaldehyde was prepared via literature proce-
dures. All synthesis processes and characterizations
were described in detail in supporting information.
2.2. Instruments and measurements
The element analysis was performed on Vario ELIII
(German) elemental analyzer. 1H NMR spectra were
obtained from Bruker Avance 400 NMR spectrometer.
The electrospray mass spectrograph was determined
on API 4000 LC–MS/MS system (ESI source). The
UV absorption spectra were measured on TU-1800PC
spectrophotometer. A quartz cell of 10 mm thickness
was used in the solution measurement. The PL measure-
ments were carried out on Hitachi F-4500 fluorescence
spectrophotometer.
The excitation light source used in two-photon fluo-
rescence measurement was provided by a Ti:sapphire la-
ser with 200 fs pulse width, 76 MHz repetition rate. The
fluorescence emitted from the samples were collected at
an angle of ꢀ45ꢁ with the incident laser beam. After col-
limate, it was focused into a fiber spectrometer (Ocean
Optics Inc.) to record the corresponding spectra. The la-
In this Letter, we have synthesized these two new
compounds and reported the experimental results on
the TPA properties of them. The two-photon excited
fluorescence was measured under an optical setup with
Ti:sapphire fs pulse as the laser source. The TPA
cross-section was evaluated by two-photon excited fluo-
rescence technique. Their values are comparable to the
highest TPA reported in the references. In addition,
the single-photon absorption, fluorescence quantum
yield and lifetime were studied in order to fully under-
N
N
N
N
N
N
N
N
TDEASB
TDBASB
Fig. 1. Chemical structures of TDEASB and TDBASB.

Y. Ren et al. / Chemical Physics Letters 414 (2005) 253–258
255
O
P
O
P
ser pulse energy used for excitation was ꢀ0.8 W or
H3CO
H3CO
H3CO
H3CO
OCH3
OCH3
BrH2C
BrH2C
CH2Br
CH2Br
lower.
P(OCH3)3
OCH3
P
P
OCH3
O
O
1
3. Results and discussion
2
The target compounds were synthesized by Wittig–
Horner–Emmons reaction using 1,2,4,5-tetrakis
(bromomethyl)benzene as the precursor. In considering
the possible large steric hindrance in ortho position of
the target molecules, trimethyl phosphite instead of tri-
phenyl phosphite in Wittig reaction was used to form
carbanion in an alkaline environment. 4-Dibutylamino-
benzaldehyde is in a liquid state, which is favorable to
the corresponding Wittig–Horner–Emmons reaction in
a solventless system. In our synthesis, the condensation
of [1,2,4,5-tetrakis(dimethoxyphosphorylmethyl)benzyl]
phosphonic acid dimethyl ester with 4-dibutylamino-
benzaldehyde and C(CH3)3OK placed in a dry mortar
milled manually for ten minutes afforded 1,2,4,5-tetra-
kis[(40-N,N-dibutylamino)styryl]benzene with a high
yield at room temperature. The solventless synthesis of
TDBASB is shown in Scheme 1. The reaction for syn-
thesis of TDEASB was performed in a normal liquid
phase, and a high yield was also obtained [17].
POCl3
DMF
N
4
CHO
N
3
N
N
The UV–Vis absorption and single-photon fluores-
cence spectra of the new compounds with the concentra-
tion of 1 · 10ꢁ5 mol/L in different solvents were
measured prior to the two-photon optical properties
measurements in order to ascertain the possible TPA
excitation wavelengths, which are about two times long-
er than that of the single-photon absorption according
to the definition of TPA. The corresponding photophys-
ical data were listed in Table 1. The two compounds
show similar absorption behaviors in certain solvent
due to their similar molecular structures. Additionally,
the single-photon fluorescence spectra of the two new
compounds were shown in Fig. 2a and b. It is interesting
that in low polar solvents of n-hexane and toluene, the
single-photon fluorescence spectra of the samples are
characterized by dual fluorescent peaks. However, it is
noted that one peak is disappeared in the high polarity
solvents; the other shows a solvatochromic red-shifted
and the PL intensity decreased with the increase of the
solvent polarity.
Table 1 also gives the values of the PL quantum yield
and lifetime. It is known that the PL quantum yield and
lifetime are perhaps the most important characteristics
of light-emitting materials. The quantum yield is the ra-
tio of photons absorbed to photons emitted through
fluorescence, which gives the probability of the excited
state being deactivated by fluorescence rather than by
nonradiative mechanism. The lifetime can be available
from its emission, which determines the time available
for the fluorophore to interact with its environment.
Therefore, we measured the quantum yield of the sam-
N
N
5
Scheme 1. Solventless synthesis of TDBASB.
ples by using the comparative method [18]. Coumarin
307/ethanol solution was used as the standard because
its absorption and emission spectra are similar to those
of the new compounds under investigation [19]. The
fluorescence lifetime was calculated based on the follow-
ing known Strickler–Berg formula [20], which is given
by:
Z
ꢀ
ꢁꢁ1
1
9.61 ꢂ 10ꢁ20n2
¼
mꢁ3
ed ln m
sr
c2
and
R
ꢀ
ꢁꢁ1
IðmÞdm
mꢁ3
¼
;
R
mꢁ3IðmÞdm
where e is the molar extinction coefficient, c is the speed
of light in vacuum, n is the refractive index of the med-
ium, m is the frequency and I is the intensity of the fluo-
rescence spectrum. The multiplicities of the lower
(ground) and upper (emitting) states are ignored. The
PL lifetime s is given by s = Usr, where U is the quantum
yield.

256
Y. Ren et al. / Chemical Physics Letters 414 (2005) 253–258
Table 1
The UV–Vis absorption maximum kabs (nm), single-photon excited fluorescence maximum kspf (nm), quantum yield U, calculated fluorescence
lifetime s (ns) and two-photon excited fluorescence maximum ktpf (nm) of TDEASB and TDBASB in different polarity solvents (l, dipole moment in
Debye units D in the gas phase)
Solventsa
l
TDEASB
kabs
TDBASB
kabs
kspf
Ub
s
ktpf
kspf
Ub
s
ktpf
n-Hexane
0.00
0.36
377
461
476
477
492
492
493
507
510
0.240
0.341
379
463
480
478
495
490
503
509
510
0.225
0.233
Toluene
390
0.182
0.158
510
515
528
391
0.267
0.273
511
518
530
THF
1.63
1.69
3.82
3.92
392
389
399
395
0.159
0.166
0.098
0.082
0.151
0.176
0.098
0.099
390
388
397
393
0.258
0.178
0.162
0.133
0.330
0.231
0.203
0.195
Ethanol
DMF
Acetonitrile
a
Abbreviations: THF, tetrahydrofuran; DMF, dimethylformamide.
Coumarin 307/ethanol solution was used as the quantum-yield standard, the value of which is 0.56 [19].
b
3500
3000
2500
2000
1500
1000
500
in high polar acetonitrile, the value decreases to only
1/3 of the former; the lifetime of TDEASB/n-hexane
solution is 0.341 ns, while in acetonitrile, the lifetime is
as little as 99 ps. Such short excited state lifetime hints
some very efficient nonradiative excited state relaxation
of TDEASB.
a
n-hexane
toluene
THF
ethanol
DMF
acetonitrile
The emission behaviors of dual fluorescence in less
polar solvent and single, strongly solvent-polarity
dependent emission in more polar solvent are frequently
reported for the conjugated donor–acceptor com-
pounds. Such phenomena are interpreted by the pres-
ently wide accepted TICT (twisted internal charge
transfer) model suggested by Grabowski and coworkers
[21]. According to the model, after excitation into a lo-
cally excited state, the dialkylamino group rotates from
the planar conformation into a lower-energy perpendic-
ular conformation, and the structure isomerizes into a
twisted internal charge transfer state. The TICT state
possesses larger dipole moment compared with the
ground state, so it prefers to stabilize in more polar sol-
vents. On the other hand, since the TICT state may rap-
idly nonradiatively transit to triplet state, it results in a
marked decrease in the fluorescence quantum yield with
a rise of solvent polarity. Obviously, the PL behaviors of
the compounds in this Letter with donor–p–donor struc-
ture symmetry are very similar to the donor–acceptor
compounds reported. However, for symmetrical com-
pounds, only small dipole moments are expected for
both ground state and excited state. In principle, their
PL could hardly follow the same rules as that of the do-
nor–acceptor molecules in polar solvents. Although
some authors have ever reported that they observed
fairly large excited state dipole moment in some sym-
metrical molecule and speculated that they may also
populate a TICT state [22], no further evidences support
their judgment. The preliminary calculations about the
planar and twisted conformation of the two new com-
pounds showed that they only possess very small dipole
moments for both ground and excited states. We think
that there may be some other specific solvent–solute
0
400
450
500
550
600
650
wavelength (nm)
3000
2500
2000
1500
1000
500
b
n-hexane
toluene
THF
ethanol
DMF
acetonitrile
0
400
450
500
550
600
650
wavelength (nm)
Fig. 2. Single-photon excited fluorescence spectra of TDEASB (a) and
TDBASB (b) in different polar solvents.
The results from the quantum yield measurement and
lifetime calculation of the two compounds can clearly
demonstrate the sensitivity of their fluorescence behav-
iors to the solvents. From Table 1, one can see that both
TDEASB and TDBASB have the relatively high PL
quantum yield and lifetime in low polarity solvents,
and the quantum yield and lifetime decrease with the
increasing of solvent polarity. For example, the PL
quantum yield of TDEASB in n-hexane is 0.240, while

Y. Ren et al. / Chemical Physics Letters 414 (2005) 253–258
257
interactions, which lead to the dual fluorescence and the
3000
2500
2000
1500
1000
500
solvatochromic red-shift of one of the emission bands.
The detailed theoretical studies of the dual fluorescence
and the solvent effects on the PL are still on the way.
The experimental results about the two-photon fluo-
rescence measurement are also listed in Table 1. During
measurement, all two-photon excited fluorescence spectra
were recorded at exactly the same experimental condi-
tions. As seen in Table 1, the two-photon excited fluores-
cence of TDEASB and TDBASB show a similar
solvatochromic property as their single-photon excited
fluorescence does. For example, their two-photon excited
fluorescence peak at ꢀ510 nm in toluene, and exhibits
about 20 nm red-shift in high polar solvent of DMF.
The TPA cross-section of TDEASB and TDBASB
was measured by using the same two-photon excited
fluorescence technique as described in [23] based on
the following equation,
Coumarin 307
TDEASB
TDBASB
0
350 400 450 500 550 600 650 700
wavelength (nm)
Fig. 3. Two-photon excited fluorescence spectra of Coumarin 307/
methanol (solid line), TDEASB (dash line) and TDBASB/toluene (dot
line) solutions with same concentration of 1 · 10ꢁ3 mol/L excited by
772 nm, 200 fs, 76 MHz Ti:sapphire laser.
[25–27]. Besides, TDEASB and TDBASB possess simi-
lar structures to some organic two-photon polymeriza-
tion initiators as reported by Cumpston et al. [1],
which stimulates our interests in exploring the potential-
ity of the two multi-branched molecules as the initiators
in two-photon photopolymerization reaction. It is men-
tionable that, in the preliminary experiment, TDBASB
exhibit excellent mutual solubility with many oligomers,
probably because the long alkyl chains of TDBASB im-
part this multi-branched molecule high solubility. This
character is helpful to find suitable oligomer to obtain
smooth film of the initiator and oligomer. Moreover,
the increased initiator concentration in oligomer will de-
crease the required two-photon initiation threshold of
the laser. Progress in these two multi-branched mole-
cules as two-photon photopolymerization initiators for
micro-fabrications will be reported in another paper.
F sUr/rcrn
F rUs/scsns
d ¼
r dr.
Here, d is the TPA cross-section, F is the integral area of
the two-photon excited fluorescence, U is the fluorescence
quantum yield, / is the overall fluorescence collection effi-
ciency of the experimental apparatus, c is the number den-
sity of the molecules in solution, n is the refractive index.
The subscripts, r and s, stand for the reference and sample,
respectively. In the experiment, the Coumarin 307/meth-
anol solution was used as the reference because its absorp-
tion and two-photon excited fluorescence are in the
similar region to the test samples. In [23], 776 nm pulse la-
ser was used as the excited source to determine the abso-
lute two-photon excited cross-section value of
Coumarin 307/methanol solution. In our experiments,
similar pulse laser at wavelength of ꢀ772 nm was used
as excited source to avoid the possible error. Fig. 3 shows
the two-photon excited fluorescence spectra of Coumarin
307/methanol, TDEASB and TDBASB toluene solu-
tions. They emit almost in the same region.
4. Conclusions
We have synthesized two novel multi-branched or-
ganic compounds TDEASB and TDBASB with sym-
metrical donor-p bridge-donor structural motif, which
theoretically predicted possessing strong electronic cou-
pling among the conjugated arms. The strong TPA
cross-sections measured provide strong evidence of
excellent conjugation and electron delocalization
properties in these benzene-core based multi-branched
compounds. These materials with enhanced TPA
cross-sections may be used in two-photon absorption
based micro-structure fabrications.
The TPA cross-section of TDEASB and TDBASB/
toluene solutions obtained by using the above technique
are 810 GM and 1030 GM, respectively. TDBASB
exhibits stronger TPA properties than TDEASB proba-
bly because N,N-dibutylamino group has relatively
strong electron-donating abilities compared with N,N-
diethylamino group. According to Albota et al. report
[24], the terminal substitution with strong electron do-
nors will bring an increase of electron delocalization in
the excited state and result in a substantial increase in
the transition dipole moment, which is the major con-
tributor to the enhanced TPA cross-section.
Obviously, both compounds are among the largest
values of TPA cross-section reported for organic com-
pounds. The high TPA cross-section makes the materi-
als promising candidates for optical power limiting
applications and two-photon excited microscopy
Acknowledgements
The authors acknowledge Professor Chuan-Kui
Wang for useful discussions on electronic and vibronic

258
Y. Ren et al. / Chemical Physics Letters 414 (2005) 253–258
[9] A. Abbotto, L. Beverina, R. Bozio, A. Facchetti, C. Ferrante,
G.A. Pagani, D. Pedron, R. Signorini, Chem. Commun. 17 (2003)
2144.
coupling and two-photon absorption. The work is sup-
ported by a grant for the state National Natural Science
Foundation of China (Grant Nos. 50323006, 50325311)
and 973 Program of China (2004CB6190002).
`
[10] O. Mongin, L. Porres, C. Katan, T. Pons, J. Mertz, M. Blanchard-
Desce, Tetrahedron Lett. 44 (2003) 8121.
`
[11] O. Mongin, J. Brunel, L. Porres, M. Blanchard-Desce, Tetrahe-
dron Lett. 44 (2003) 2813.
Appendix A. Supplementary data
[12] I. Fuks-Janczarek, J.-M. Nunzi, B. Sahraoui, I.V. Kityk, J.
Berdowski, A.M. Caminade, J.-P. Majoral, A.C. Martineau, P.
Frere, J. Roncali, Opt. Commun. 209 (2002) 461.
Supplementary data associated with this article can
be found, in the online version at doi:10.1016/
j.cplett.2005.08.041.
˚
[13] P. Macak, Y. Luo, P. Norman, H. Agren, J. Chem. Phys. 113
(2000) 7055.
[14] D. Beljonne, W. Wenseleers, E. Zojer, Z. Shuai, H. Vogel, S.J.K.
´
Pond, J.W. Perry, S.R. Marder, J.-L. Bredas, Adv. Funct. Mater.
12 (2002) 631.
˚
References
[15] P. Norman, Y. Luo, H. Agren, J. Chem. Phys. 111 (1999)
7758.
[16] Y.H. Sun, K. Zhao, C.K. Wang, Y. Luo, Y. Ren, X.T. Tao, M.H.
Jiang, J. Mol. Struct. (Theochem) 682 (2004) 185.
[17] Q. Xin, X.T. Tao, J. Solid State Chem. (submitted).
[18] J.N. Demas, G.A. Crosby, J. Phys. Chem. 75 (1971) 991.
[19] G.A. Reynolds, K.H. Drexhage, Opt. Commun. 13 (1975) 222.
[20] S.J. Strickler, R.A. Berg, J. Chem. Phys. 37 (1962) 814.
[21] K. Rotkiewicz, K.H. Grellmann, Z.R. Grabowski, Chem. Phys.
Lett. 19 (1973) 31.
[1] B.H. Cumpston, S.P. Ananthavel, S. Barlow, D.L. Dyer, J.E.
Ehrlich, L.L. Erskine, A.A. Heikal, S.M. Kuebler, I.-Y. Sandy
Lee, D. McCord-Maughon, J. Qin, H. Ro¨ckel, M. Rumi, X.L.
Wu, S.R. Marder, J.W. Perry, Nature 398 (1999) 51.
[2] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N.
´
Marks, C. Taliani, D.D.C. Bradley, D.A. dos Santos, J.L. Bredas,
M. Lo¨gdlund, W.R. Salaneck, Nature 397 (1999) 121.
[3] W.R. Dichtel, J.M. Serin, C. Edder, J.M.J. Frechet, M.
Matuszewski, L.-S. Tan, T.Y. Ohulchanskyy, P.N. Prasad, J.
Am. Chem. Soc. 126 (2004) 5380.
´
[22] J.F. Letard, R. Lapouyade, W. Rettig, J. Am. Chem. Soc. 115
(1993) 2441.
[23] C. Xu, W.W. Webb, J. Opt. Soc. Am. B 13 (1996) 481.
[24] M. Albota, D. Beljonne, J.L. Bredas, J.E. Ehrlich, J.Y. Fu, A.A.
Heikal, S.E. Hess, T. Kogej, M.D. Levin, S.R. Marder, D.
McCord-Maughon, J.W. Perry, H. Ro¨ckel, M. Rumi, G.
Subramaniam, W.W. Webb, X.L. Wu, C. Xu, Science 281
(1998) 1653.
[25] C. Xu, W. Zipfel, J.B. Shear, R.M. Williams, W.W. Webb, Proc.
Natl. Acad. Sci. USA 93 (1996) 10763.
[26] W. Denk, J.H. Strickler, W.W. Webb, Science 248 (1990) 73.
[27] L. Ventelon, S. Charier, L. Moreaux, J. Mertz, M. Blanchard-
Desce, Angew. Chem. Int. Ed. 40 (2001) 2098.
[4] G.S. He, L. Yuan, Y. Cui, M. Li, P.N. Prasad, J. Appl. Phys. 81
(1997) 2529.
[5] Y. Ren, X.Q. Yu, D.J. Zhang, D. Wang, M.L. Zhang, G.B. Xu,
X. Zhao, Y.P. Tian, Z.S. Shao, M.H. Jiang, J. Mater. Chem. 12
(2002) 3431.
[6] S.-J. Chung, K.-S. Kim, T.-C. Lin, G.S. He, J. Swiatkiewicz, P.N.
Prasad, J. Phys. Chem. B 103 (1999) 10741.
[7] A. Adronov, J.M.J. Frechet, G.S. He, K.-S. Kim, S.-J. Chung, J.
Swiatkiewicz, P.N. Prasad, Chem. Mater. 12 (2000) 2838.
[8] M. Drobizhev, A. Karotki, Y. Dzenis, A. Rebane, Z. Suo, C.W.
Spangler, J. Phys. Chem. B 107 (2003) 7540.