Octupolar polycyclic aromatic hydrocarbons as new two-photon absorption chromophores: Synthesis and application for optical power limiting

Article abstract of DOI:10.1002/chem.201003235

Bigger is better! Large-size octupolar polycyclic aromatic hydrocarbons can be used as new two-photon absorption (TPA) chromophores with large TPA cross sections for optical power limiting (see graphic).

Full text of DOI:10.1002/chem.201003235

COMMUNICATION
DOI: 10.1002/chem.201003235
Octupolar Polycyclic Aromatic Hydrocarbons as New Two-Photon
Absorption Chromophores: Synthesis and Application for Optical Power
Limiting
Zebing Zeng, Zhenping Guan, Qing-Hua Xu,* and Jishan Wu*[a]
Two-photon absorption (TPA)[1–2] materials have recently
attracted intensive interest due to potential applications for
3D microfabrication,[3–4] multiphoton microscopy,[5–6] 3D op-
tical data storage,[7–8] photodynamic therapy,[9–10] bioimag-
ing,[11–12] and optical power limiting.[13–14] It is important to
design materials with a large TPA cross section (dmax) or a
large TPA cross section per molecular weight
(dmax MWÀ1).[15]
A variety of donor–bridge–acceptor (D–p–A) dipoles,
donor–bridge–donor (D–p–D) quadrupoles, paracyclo-
phanes, and porphyrin derivatives have been developed to
achieve large TPA cross sections.[15] However, only limited
works have dealt with octupolar TPA chromophores. Octu-
polar molecules, such as trisACTHNUTRGENUGN(styryl)benzene derivatives
(Scheme 1a), with significant TPA cross sections was first
reported by Cho et al.[16–17] The three-arm octupolar struc-
ture showed significant improvement of the TPA response
in comparison with the singly or doubly branched counter-
parts.[18–19] Other octupolar chromophores, derived from a
triphenylamine core, exhibited similar large TPA cross sec-
tions.[20–21] The important role of a p-conjugated core on the
TPA properties was investigated and it was revealed that
TPA cross section increased with increased planarity and ri-
gidity of the p-conjugated core, the length of the conjuga-
tion system, and the strength of donor–acceptor interactions.
On the basis of these important studies, we intend to use a
largely delocalized and rigid polycyclic aromatic hydrocar-
bon (PAH) molecule as a core to build up new octupolar
TPA chromophores. Among various PAH molecules, hexa-
peri-hexabenzocoronene (HBC, Scheme 1b) is a very attrac-
tive candidate because it possesses a large number of delo-
calized p electrons and rigid planar geometry.[22] The HBC
molecule also has a D6h symmetry similar to a benzene ring
and can be regarded as a “superbenzene”. Thus selective at-
tachment of certain electron-donating groups (D) and elec-
tron-withdrawing groups (A) onto the peripheries of the
HBC core would result in three-fold symmetric “push–pull”
Scheme 1. a) Representative octupolar TPA molecules based on the ben-
zene core. b) Structure of HBC. c) Octupolar HBC chromophores with a
“push–pull” structure.
type octupolar chromophores (Scheme 1c), which are likely
to show an interesting TPA response. However, up to now,
there was no report on TPA materials with HBC as building
block, mainly due to synthetic challenges.
As shown in Scheme 2, three octupolar HBC chromo-
À
À
À
phores, HBC NO2, HBC CN, and HBC CF3, with butoxy
À
À
units as electron donating groups and the NO2, CN and
À
CF3 as electron-withdrawing groups, have been synthe-
sized. The synthesis of this type of HBC chromophores is
challenging because: 1) synthesis of HBC molecules re-
quired oxidative cyclodehydrogenation from the hexaphe-
nylbenzene (HPB) precursors by using oxidant and Lewis
acid (e.g. FeCl3 or CuACHTUNRGTNEUNG(OTf)2–AlCl3, OTf=triflate) and such
a cyclization reaction usually does not work if electron
donors (e.g. arylamine, monoalkoxy) or electron acceptors
[a] Z. Zeng, Z. Guan, Prof. Q.-H. Xu, Prof. J. Wu
Department of Chemistry, National University of Singapore
3 Science Drive 3, Singapore 117543
Fax: (65)67791691
À
À
(e.g. CN or COOR) are directly attached to the HPB pre-
cursors;[22] and 2) separation of C3-symmetric HPB precur-
sors from other isomers is not trivial.[23] Our new strategy is
to first synthesize the HBC core 8 followed by further func-
tionalizations. The 3’,4’,5’-tris(butan-1-yloxy)-4-methyl ethy-
E-mail: chmwuj@nus.edu.sg
chmxqh@nus.edu.sg
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201003235.
Chem. Eur. J. 2011, 17, 3837 – 3841
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3837

yield. The target octupolar
À
HBC chromophores HBC
À
À
NO2, HBC CN, and HBC CF3
were then synthesized in good
yields by three-fold Wittig–
Horner reactions between
9
and corresponding benzalde-
hyde derivatives 10. Our syn-
thetic strategy overcame the
difficulty of the cyclodehydro-
genation of precursors carrying
donor/acceptors and resolved
the separation problem.
The steady-state absorption
and emission spectra of these
new HBC chromophores were
recorded in different solvents
(Figure S1 in the Supporting In-
formation). In CH2Cl2, an in-
tense absorption band with ab-
sorption maximums at 443, 412,
and 404 nm was observed for
À
À
HBC NO2, HBC CN, and
À
HBC CF3, respectively, indicat-
À
ing that HBC NO2 has the
largest p-conjugated system.
The absorption maximum of
À
HBC NO2 in CH2Cl2 was red-
shifted with about 10 nm com-
pared with that in nonpolar tol-
uene. Such a solvatochromism
effect is likely due to a partially
charge-separated state in more
polar solvents. In contrast, the
À
absorption spectra of HBC CN
À
and HBC CF3 did not show a
clear solvent dependence, in-
dicting a relatively weaker in-
tramolecular charge transfer in
these two molecules. The pho-
Scheme 2. Conditions: a) Pd
(4, 25% yield; 5, 53% yield); c) LiAlH4, THF, 92%; d) SOCl2, pyridine, CH2Cl2, 88%; e) FeCl3, CH3NO2,
CH2Cl2, 80%; f) P(OEt)3, reflux, 24 h, 81%; g) KOtBu, DMF.
ACTHGNUNERTN(NUG PPh3)2Cl2, Et3N, CuI, THF, 608C, 24 h, 80%; b) Co2(CO)8, dioxane, reflux, 24 h
ACHTUNGTRENNUNG
toluminescence
spectra
of
À
HBC NO2 also showed a clear
solvatochromic shift (Figure S2 in the Supporting Informa-
tion). The emission band in THF is broader and redshifted
with about 59 nm compared with that in toluene. This can
be explained by the classical photoinduced intramolecular
electron transfer mechanism in which the molecules on the
excited electronic state have larger dipole moment than that
in the ground state. Such intramolecular charge transfers
were favored in more polar solvents and would result in en-
nylbenzoate (3) was first prepared by Hagihara–Sonogashira
coupling between 1,2,3-tributoxy-5-iodobenzene (1)[24] and
methyl 4-ethynylbenzoate (2)[25] in 80% yield. Cyclotrimeri-
zation of 3 in the presence of the Co2(CO)8 catalyst provid-
ed two HPB isomers, 4 and 5 in 25 and 53% yield, respec-
tively. The presence of three polar ester groups is essential
to provide sufficient difference in polarity between these
two isomers so that they can be separated by column chro-
matography. The C3-symmetric HPB derivative 4 was then
reduced by LiAlH4 to generate alcohol 6, which was subse-
quently converted into 7 by chlorination with thionylchlor-
ide. The oxidative cyclodehydrogenation of 7 was successful-
ly accomplished by treatment with FeCl3 in CH2Cl2 to afford
8 in 80% yield. The methylene chloride functional groups in
8 were then converted into triphosphate to give 9 in 81%
hanced TPA responses.[26,27] Compounds HBC CN and
HBC CF3 exhibited well-resolved emission bands. However,
there was no clear spectral shift in different solvents (Fig-
ure S2 in the Supporting Information).
TPA cross sections of these HBC chromophores were
measured in different solvents and at various concentrations
by using the two-photon excitation fluorescence (TPEF)
À
À
3838
www.chemeurj.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 3837 – 3841

New Two-Photon Absorption Chromophores
COMMUNICATION
technique with fluorescein as reference. The output intensity
of TPEF was found to be proportional to the square of the
input laser intensity, thereby confirming the occurrence of
the TPA process (Figure S3 in the Supporting Information).
In general, each chromophore exhibited large TPA cross
À
sections (d, Table 1). In particular, the dmax values of HBC
Table 1. Photophysical data of the octupolar HBC chromophores.
[b]
Compound Solvent
c
lmax F[a] dmax
dmax MWÀ1 Fdmax MWÀ1
ACHTUNGTRENNUNG
[GM ][c]
[m] [nm]
À
HBC CF3 THF
10À4 740 0.05 1149
10À5 740 0.09 955
0.88
0.73
1.32
0.83
1.71
1.00
1.68
1.16
0.04
0.06
0.04
0.08
0.07
0.09
0.07
0.09
0.41
0.26
0.29
0.31
À
HBC CF3 THF
HBC CF3 Toluene 10À4 740 0.03 1722
À
HBC CF3 Toluene 10À5 740 0.10 1083
À
À
HBC CN THF
10À4 750 0.04 2005
10À5 750 0.09 1169
À
HBC CN THF
HBC CN Toluene 10À4 750 0.04 1968
À
HBC CN Toluene 10À5 750 0.08 1366
À
À
HBC NO2 THF
10À4 760 0.03 16718 13.55
À
HBC NO2 THF
10À5 750 0.03 10494 8.51
HBC NO2 Toluene 10À4 760 0.05 7036
5.70
3.93
À
HBC NO2 Toluene 10À5 750 0.08 4845
À
[a] Fluorescence quantum yield, error: Æ10%. [b] Two-photon absorptivi-
ty in 10À50 cm4 s per photon (GM), error: Æ15%. [c] The molecular
weight (MW) was calculated by assuming that OR=OMe.[25]
À
À
NO2 were clearly larger than that of HBC CN and HBC
CF3 under the same measuring conditions. For example,
À
HBC NO2 has shown bright two-photon excited emission
with an extremely high dmax value of 16718 GM (at 760 nm)
in THF at a concentration of 1ꢁ10À4 m, which is about eight
À
and fifteen times that of HBC CN (2005 GM at 750 nm)
À
and HBC CF3 (1149 GM at 740 nm), respectively (Fig-
ure 1a). There was only a very limited number of TPA chro-
mophores (e.g. porphyrin tapes and extended squaraine
dyes)[28] showing such high TPA cross sections. Such excel-
lent TPA properties must be associated to the rigid and
largely delocalized HBC core and strong intramolecular
À
À
Figure 1. Two-photon excitation spectra for HBC NO2, HBC CN, and
HBC CF3: (a,b) at a concentration of 10À4 m in THF and toluene, respec-
À
À
charge transfer in HBC NO2. The solvent effects on the
À
tively; c) concentration dependence of the TPA cross section for HBC
NO2 in THF.
TPA response were also investigated. The two-photon exci-
À
À
tation spectra of HBC CN and HBC CF3 recorded in tolu-
ene (Figure 1b) did not show a clear difference from those
À
recorded in THF (Figure 1a). However, the HBC NO2 ex-
dependence should be related to the aggregation of HBC
chromophores at higher concentrations. Therefore, concen-
tration-dependent 1H NMR spectra of these HBC com-
pounds were recorded in CDCl3 (Figure S5 in the Support-
ing Information). With the increase of the concentration,
the resonance for the protons on the HBC core gradually
shifted to high field due to shielding effect. The redshift of
the absorption and emission spectra at higher concentrations
also confirm the formation of aggregates by intermolecular
interactions (Figure S6 in the Supporting Information). Fur-
thermore, the fluorescence lifetimes of all the HBC chromo-
phores in THF also showed concentration dependence. As
the concentration increases from 1ꢁ10 to 1ꢁ10À4 m, the
fluorescence lifetime decreases due to the formation of mo-
lecular aggregates in the solutions (Figure S7–S9 in the Sup-
hibited relatively smaller TPA-cross-section values in nonpo-
lar solvents, such as toluene (Figure 1b) than that in polar
THF. The results suggest that the solvent polarity has a posi-
À
tive influence on the TPA response of HBC NO2, presuma-
bly due to the large intramolecular charge-transfer charac-
ter. Moreover, we also observed a clear concentration de-
pendence of the two-photon excitation spectra for all of
these HBC-based TPA chromophores. For example, with in-
creased concentration, the TPA-cross-section values of
À
HBC NO2 in THF displayed a significant increase (Fig-
ure 1c). The same tendency was also observed for two other
chromophores (Figure S4 in the Supporting Information).
Given that the large and planar HBC molecules have a high
tendency to aggregate in solution,[29] such a concentration
Chem. Eur. J. 2011, 17, 3837 – 3841
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3839

J. Wu, Q.-H. Xu et al.
porting Information). Such a strong aggregation can be
driven by strong intermolecular p–p interaction as well as
dipole–dipole interactions in the octupolar molecules. Large
enhancements of nonlinear optical responses in molecular
aggregates have been theoretically predicted by McRae and
Kasha[30] as well as Spano and Mukamel.[31] Later on, Pani-
nelli et al. predicted that the large TPA cross sections of
quadrupolar chromophores could be further amplified by
orders of magnitude as a result of aggregation.[32] Such en-
hancements can be explained by the formation of bound
biexciton states in aggregates of centrosymmetric molecules
as a consequence of attractive exciton–exciton interactions.
Some chromophores, such as porphyrins and isocyanines,
have exhibited aggregation-enhanced TPA response in the
J-aggregate states; this can be explained by a similar
theory.[33] The observed increase of TPA cross sections in
the octupolar HBC chromophores at higher concentrations
can be ascribed to the strong intermolecular interactions in
solutions.
The dmax MWÀ1 values measured for HBC NO2, HBC
À
À
CN, and HBC CF3 in 1ꢁ10À4 m THF solution are 13.55,
À
1.71, and 0.88, respectively (Table 1), which are remarkable
among all known TPA chromophores. The large TPA-cross-
sections dmax and dmax MWÀ1 values, good solubility, and high
stability of these new TPA chromophores indicate that they
could be ideal candidates for optical-power-limiting applica-
tions. The optical-limiting response of these TPA chromo-
phores in THF to femtosecond laser pulses at 800 nm was
then investigated by the open-aperture Z-scan method and
the curves are shown in Figure 2a. The best fitting gave pa-
rameters of the TPA coefficient of b=0.764, 0.543, and
Figure 2. a) Normalized open-aperture Z-scan transmittance curves ob-
tained by using femtosecond laser pulses at
a
pulse energy of
21.3 GWcmÀ2 in THF (5 mm). b) The optical-limiting response of HBC
À
À
À
NO2, HBC CN, and HBC CF3 in THF measured with a 150 fs pulse at a
wavelength of 800 nm.
Experimental Section
0.268 cmGWÀ1 for HBC NO2, HBC CN, and HBC CF3,
respectively. The TPA cross sections of 6304, 4470, and
À
À
À
À
À
À
Synthesis of HBC NO2, HBC CN, and HBC CF3: Compound
(108 mg, 0.067 mmol) was dissolved in dry DMF (10 mL) and the solu-
9
À
tion was cooled to 08C. KOtBu (25 mg, 0.220 mmol) was added and after
2210 GM were then calculated for compounds HBC NO2,
30 min
a solution of corresponding benzaldehyde derivatives 10
À
À
HBC CN, and HBC CF3, respectively. The optical-limiting
behavior of these new octupolar chromophores in solutions
is shown in Figure 2b. At low-input energies (below
1 GMcmÀ2), more than 95% of the light was transmitted,
but as the input energy increased, the transmittance deviat-
ed from the linear behavior and decreased dramatically, sug-
gesting the occurrence of optical-limiting activity. The fem-
tosecond limiting thresholds of 22.8, 55.0, and
(0.020 mmol) in DMF (2 mL) was added dropwise. The mixture was
stirred overnight at room temperature under a nitrogen atmosphere and
the crude product precipitated out from the mixture. The precipitate was
filtered and washed with distilled water and methanol several times and
was further purified by column chromatography (silica gel, toluene) to
afford the desired compounds. Detailed characterization data are shown
in the Supporting Information.
TPEF measurements: The excitation source for the TPEF measurement
was a Spectra Physics femtosecond Ti:sapphire oscillator system (Tsu-
nami). The output laser pulses with a central wavelength of 740–800 nm
were used as the two-photon excitation source. The average output
energy was 200 mW. The samples were excited by directing a tightly fo-
cused laser beam onto the sample. The emission from the sample was col-
lected at a 908 angle by a pair of lenses and an optical fiber and directed
100.6 GWcmÀ2 were obtained for HBC NO2, HBC CN,
À
À
À
and HBC CF3, respectively. This performance is superior to
that of many TPA chromophores and comparable to that of
three-photon- and multiphoton-absorption-based optical
limiting.[15b]
to the spectrometer, which was
a monochromator (Acton, Spectra
In summary, a series of novel TPA chromophores based
on octupolar HBC molecules were prepared and they
showed large TPA response. The TPA properties were de-
pendent on the electron-acceptor strength, the solvent polar-
ity, and the concentration. These new TPA chromophores
also exhibited very good optical-power-limiting performance
with low limiting thresholds. Our results suggest that large
polycyclic aromatic hydrocarbons could be good candidates
for the design of new TPA materials in the future.
Pro 2300i) coupled CCD (Princeton Instruments, Pixis 100B) system. A
short pass filter with a cut-off wavelength at 700 nm was placed in front
of the spectrometer to minimize the intensity of pumping light. A fluo-
rescein water solution (pH 11) was used as reference.
Z-scan and optical-limiting experiments: Nonlinear optical properties
were characterized by using a femtosecond open-aperture Z-scan experi-
ment. The excitation source was a Spectral Physics Ti:sapphire amplifier
laser system. The laser output has a central wavelength of 800 nm and
pulse duration of ꢀ150 fs at a repetition rate of 1 kHz. The laser pulses
were focused onto the sample contained in a 1 mm path-length quartz
cuvette by using a lens with a focal length of 15 cm, which give a focal
3840
www.chemeurj.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 3837 – 3841

New Two-Photon Absorption Chromophores
COMMUNICATION
spot size of ꢀ30 mm radius. In a Z-scan measurement, the transmittance
of the sample was measured as the sample moved toward and away from
the beam focus. The Z-scan experimental setup was tested by measuring
the nonlinear absorption coefficient of a solution of Rhodamine B in
methanol.
Pawlicki, H. A. Collins, R. G. Denning, H. L. Anderson, Angew.
Chem. 2009, 121, 3292; Angew. Chem. Int. Ed. 2009, 48, 3244.
[16] B. R. Cho, K. H. Son, S. H. Lee, Y. S. Song, Y. K. Lee, S. J. Jeon,
J. H. Choi, H. Lee, M. Cho, J. Am. Chem. Soc. 2001, 123, 10039.
[17] H. M. Kim, M. S. Seo, S. J. Jeon, B. R. Cho, Chem. Commun. 2009,
7422.
[18] F. Xu, Z. W. Wang, Q. H. Gong, Opt. Mater. 2007, 29, 723.
[19] D. Beljonne, W. Wenseleers, E. Zojer, Z.-G. Shuai, H. Vogel, S .
J. K. Pond, J. W. Perry, S . R. Marder, J. L. Brꢂdas, Adv. Funct.
Mater. 2002, 12, 631.
Acknowledgements
[20] L. Porrꢃs, O. Mongin, C. Katan, M. Charlot, T. Pons, J. Mertz, M. B.
Desce, Org. Lett. 2004, 6, 47.
This work was financially supported by Singapore NRF Competitive Re-
search Program (R-143-000-360-281), NUS Young Investigator Award
(R-143-000-356-101), and DSTA Singapore (R-143-000-432-422).
[21] J. C. Collings, S. Y. Poon, D. C. Le, M. Charlot, C. Katan, L. O. Pals-
son, A. Beeby, J. A. Mosely, H. M. Kaiser, D. Kaufmann, W. Y.
Wong, B. D. Mireille, T. B. Marder, Chem. Eur. J. 2009, 15, 198.
[22] J. Wu, W. Pisula, K. Mꢄllen, Chem. Rev. 2007, 107, 718.
[23] J. Wu, M. Baumgarten, M. G. Debije, J. M. Warman, K. Mꢄllen,
Angew. Chem. 2004, 116, 5445; Angew. Chem. Int. Ed. 2004, 43,
5331.
Keywords: aromaticity · chromophores · optical limiting ·
polycycles · two-photon absorption
[24] T. Cardolaccia, Y. Li, K. S. Schanze, J. Am. Chem. Soc. 2006, 128,
16365.
[25] G. Z. Liu, J. J. Gooding, Langmuir 2006, 22, 7421.
[26] N. Sarkar, K. Das, D. N. Nath, K. Bhattacharyya, Langmuir 1994,
10, 326.
[1] M. Gçppert-Mayer, Ann. Phys. 1931, 401, 273.
[2] W. Kaiser, C. G. B. Garret, Phys. Rev. Lett. 1961, 7, 229.
[3] S. Kawata, H. B. Sun, T. Tanaka, K. Takada, Nature 2001, 412, 697.
[4] W. H. Zhou, S. M. Kuebler, K. L. Braun, T. Y. Yu, J. K. Cammack,
C. K. Ober, J. W. Perry, S. R. Marder, Science 2002, 296, 1106.
[5] R. M. Williams, W. R. Zipfel, W. W. Webb, Curr. Opin. Chem. Biol.
2001, 5, 603.
[6] H. M. Kim, C. Jung, B. R. Kim, S.-Y. Jung, J. H. Hong, Y.-G. Ko,
K. J. Lee, B. R. Cho, Angew. Chem. 2007, 119, 3530; Angew. Chem.
Int. Ed. 2007, 46, 3460.
[7] D. A. Parthenopoulos, P. M. Rentzepis, Science 1989, 245, 843.
[8] K. Belfield, K. Schafer, Chem. Mater. 2002, 14, 3656.
[9] W. Dichtel, J. Serin, C. Edder, J. Frꢂchet, M. Matuszewski, L.-S. Tan,
T. Ohulchanskyy, P. Prasad, J. Am. Chem. Soc. 2004, 126, 5380.
[10] S. Kim, T. Ohulchanskyy, H. Pudavar, R. Pandey, P. Prasad, J. Am.
Chem. Soc. 2007, 129, 2669.
[11] R. H. Kohler, J. Cao, W. R. Zipfel, W. W. Webb, M. R. Hanson, Sci-
ence 1997, 276, 2039.
[12] M. J. Miller, S. H. Wei, I. Parker, M. D. Cahalan, Science 2002, 296,
1869.
[13] G. S. He, J. D. Bhawalkar, C. F. Zhao, P. N. Prasad, Appl. Phys. Lett.
1995, 67, 2433.
[14] J. E. Ehrlich, X. L. Wu, L.-Y. Lee, Z.-Y. Hu, H. Roeckel, S. R.
Marder, J. Perry, Opt. Lett. 1997, 22, 1843.
[27] X. M. Wang, D. Wang, G. Y. Zhou, W. T. Yu, Y. F. Zhou, Q. Fang ,
M. H. Jiang, J. Mater. Chem. 2001, 11, 1600.
[28] a) T. K. Ahn, K. S. Kim, D. Y. Kim, S. B. Noh, N. Aratani, C. Ikeda,
A. Osuka, D. Kim, J. Am. Chem. Soc. 2006, 128, 1700; b) S. Chung,
S. Zheng, T. Odani, L. Beverina, J. Fu, L. A. Padilha, A. Biesso,
J. M. Hales, X. Zhan, K. Schmidt, A. Ye, E. Zojer, S. Barlow, D. J.
Hagan, E. W. Van Stryland, Y. Yi, Z. Shuai, G. A. Pagani, J. L.
Brꢂdas, J. W. Perry, S. R. Marder, J. Am. Chem. Soc. 2006, 128,
14444.
[29] J. Wu, A. Fechtenkçtter, J. Gauss, M. D. Watson, M. Kastler, C.
Fechtenkçtter, M. Wagner, K. Mꢄllen, J. Am. Chem. Soc. 2004, 126,
11311.
[30] E. G. McRae, M. J. Kasha, J. Chem. Phys. 1958, 28, 721.
[31] F. C. Spano, S. Mukamel, Phys. Rev. Lett. 1991, 66, 1197.
[32] G. DꢅAvino, F. Terenziani, A. Painelli, J. Phys. Chem. B 2006, 110,
25590.
[33] a) E. Collini, C. Ferrante, R. Bozio, J. Phys. Chem. B 2005, 109, 2;
b) K. D. Belfield, M. V. Bondar, F. E. Hernandez, O. V. Przhonska,
S. Yao, Chem. Phys. 2006, 320, 118.
[15] a) H. M. Kim, B. R. Cho, Chem. Commun. 2009, 153; b) G. S. He,
L. S. Tan, Q. Zheng, P. N. Prasad, Chem. Rev. 2008, 108, 1245; c) M.
Received: November 10, 2010
Published online: February 25, 2011
Chem. Eur. J. 2011, 17, 3837 – 3841
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3841