Structure-property correlation of solid-emissive boron-fluorine derivatives

Article abstract of DOI:10.1016/j.jorganchem.2012.07.043

A series of solid-emissive BOPIM (boron 2-(2-pyridyl)imidazole complex) dyes, bearing electron-donating or -withdrawing substituents (methoxy, hydrogen, or nitro), are facilely synthesized. The compounds were characterized by ¹H NMR, ¹³C NMR and MS. X-ray single crystal diffractions of BOPIM 1 and 2 indicate that nonplanar rigid structures were formed through intermolecular non-covalent interactions. Due to these non-covalent interactions, these dyes exhibit intense fluorescence in solution and also in solid state. According to ¹H NMR analysis, electronic effect of the substitutes plays an important role in contribution to these BOPIMs' electronic states. The photophysical measurements reveal that electron-donating groups (methoxy) lead to significant bathochromism of the absorption and emission. In contrast, electron-withdrawing moieties (nitro) play the reverse role. However, BOPIM 3 bearing nitro groups emits long wavelength fluorescence in solid state, probably due to the formation of intermolecular hydrogen bond. This work elucidates the spectroscopic structure-property relationship of solid-emissive BOPIMs, which would be valuable for design of solid-emissive dyes.

Full text of DOI:10.1016/j.jorganchem.2012.07.043

Journal of Organometallic Chemistry 717 (2012) 147e151
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Structureeproperty correlation of solid-emissive boronefluorine derivatives
*
Qiong Cao, Shuzhang Xiao , Miaofu Mao, Xiaohong Chen, Sa Wang, Ling Li, Kun Zou
Hubei Key Laboratory of Natural Products Research and Development, College of Chemistry and Life Science, China Three Gorges University, Hubei Yichang 443002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of solid-emissive BOPIM (boron 2-(2-pyridyl)imidazole complex) dyes, bearing electron-
donating or -withdrawing substituents (methoxy, hydrogen, or nitro), are facilely synthesized. The
compounds were characterized by 1H NMR, 13C NMR and MS. X-ray single crystal diffractions of BOPIM 1
and 2 indicate that nonplanar rigid structures were formed through intermolecular non-covalent
interactions. Due to these non-covalent interactions, these dyes exhibit intense fluorescence in solu-
tion and also in solid state. According to 1H NMR analysis, electronic effect of the substitutes plays an
important role in contribution to these BOPIMs’ electronic states. The photophysical measurements
reveal that electron-donating groups (methoxy) lead to significant bathochromism of the absorption and
emission. In contrast, electron-withdrawing moieties (nitro) play the reverse role. However, BOPIM 3
bearing nitro groups emits long wavelength fluorescence in solid state, probably due to the formation of
intermolecular hydrogen bond. This work elucidates the spectroscopic structureeproperty relationship
of solid-emissive BOPIMs, which would be valuable for design of solid-emissive dyes.
Received 24 May 2012
Received in revised form
21 July 2012
Accepted 26 July 2012
Keywords:
BOPIM
Fluorescence
Solid-emission
Supramolecular interaction
Structureeproperty relationship
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
can emit fluorescence in solid state with large Stokes shift, even
though these molecules have neither bulky groups nor rigid side
Fluorescent dyes have been under wide research because of
their applications in various research fields, especially for these
probes sensitive to environment and solvent polarity [1e5]. Among
all these chromophores reported till now, organic boronefluorine
complexes (known as BODIPY) attract great interest, due to their
large number of advantages over other dyes. They usually have high
fluorescence quantum yield and absorption coefficient, excellent
chemical- and photochemical-stability, plus a large number of
BODIPYs exhibit environment-sensitive properties. However,
typical BODIPYs are non-emissive in solid state, due to their high
planarity which results in concentration-induced quenching
[6e10]. And they usually have small Stokes shift, inducing reab-
sorption of the emission. To keep BODIPYs’ emission in solid state
for practical applications in light-emitting devices, it’s essential to
chains [16e21]. And the large Stokes shift is the result of efficient
intramolecular charge transfer (ICT) from electron-rich groups to
electron-accepting BOPIM skeleton. In these molecules, intermo-
lecular non-covalent interactions provide a comparatively rigid
structure, which exhibit non-planar packing mode. Boron 2-(20-
pyridyl)imidazole complex (BOPIM) is one class of these samples,
which exhibit no aggregate in concentrated solution according to
concentration-dependent fluorescence measurement [22]. And
they emit intense fluorescence both in solutions and also in solid
state with large Stokes shift [23e25]. The synthesis is facile, and the
chromophore can be easily functionalized, which make it ideal
candidate for construction of functional materials. However, the
introduction of electron donating and withdrawing substituents to
these conjugated groups can significantly change the electronic
environment of the chromophore core, resulting in shift of the
emission wavelength [3,26]. In this way, emission of BOPIMs may
be tuned by attaching different electron-donor or acceptor. So it’s
necessary to investigate qualitatively the effect of chemical struc-
ture and substituent groups on fluorescence characteristics, con-
cerning fluorescence intensity, emission wavelength, Stokes shift,
etc. In this report, three solid-emissive BOPIM dyes are synthesized,
bearing different electron-donating or -withdrawing groups
(Scheme 1). Then we study their photophysical properties to
elucidate the relationship between the chemical structure and the
photophysical properties.
decrease the aggregate formation due to the
pep stacking of the
chromophore. Some strategies have proved to be effective, such as
attaching bulky groups on BODIPY core structure to inhibit inter-
molecular planar
pep interaction [11e14], introducing rigid side
chain on the chromophore core to enlarge the Stokes shift [15].
Recently it’s found that boronefluorine derivatives with intermo-
lecular interactions (such as CeH/FeB, CeH/O, C/C and C/N)
* Corresponding author. Tel./fax: þ86 717 6397478.
E-mail address: shuzhangxiao@gmail.com (S. Xiao).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.07.043

148
Q. Cao et al. / Journal of Organometallic Chemistry 717 (2012) 147e151
2.3.2. BOPIM 2
Yellow solid (32%). 1H NMR (CDCl3, 300 MHz)
d
8.43 (d,
J ¼ 5.40 Hz, 1H), 8.16 (m, 1H), 8.09 (d, J ¼ 7.80 Hz, 1H), 7.60 (m, 4H),
7.46 (m, 1H), 7.36 (m, 6H). 13C NMR (CDCl3, 75 MHz)
144.68,
d
141.26, 134.56, 130.66, 128.68, 128.59, 128.34, 128.14, 127.31, 122.78,
117.73. MS [M þ H]þ: 346.1 calcd. 346.1.
2.3.3. BOPIM 3
Yellow solid (20%). 1H NMR (CDCl3, 400 MHz)
d
8.59 (m,1H), 8.31
(m, 2H), 8.29 (m, 1H), 8.24 (m, 2H), 7.85 (m, 3H), 7.68 (m, 2H), 7.36
(m, 1H). 13C NMR (CDCl3, 100 MHz)
d 149.03, 147.40, 147.25, 137.52,
128.41, 124.62, 124.23, 124.05, 120.50, 29.70. MS [Mþ2H]þ: 437.2
Scheme 1. Synthesis of BOPIM dyes.
calcd. 437.1.
3. Results and discussion
2. Experimental
3.1. X-ray crystal structure
2.1. Materials
Crystals suitable for X-ray measurements were obtained by slow
evaporation of mixed organic solutions. In the crystal structures of
BOPIM 2 [25], intermolecular interactions (such as CeH/N,
The corresponding ligands for synthesis of BOPIM dyes are
prepared according to literature procedures [27]. BF3$OEt2 is
purchased from Aldrich. All other starting materials are obtained
commercially as analytical-grade and without further purification.
Moisture sensitive reactions were performed under an atmosphere
of nitrogen. Anhydrous dichloromethane was obtained by distilla-
tion of commercial analytical-grade dichloromethane after treated
with molecular sieve.
CeH/B, etc.) provide a rigid structure to inhibit planar
pep
stacking in crystal state, which is also found for BOPIM 1 (Fig. 1).
It’s interesting that the unit cell of BOPIM 1 contains two molecules
and they exhibit different intermolecular non-covalent interac-
tions. In the crystal structure of BOPIM 1, F1 atom forms BeF/HeC
contact with the methyl protons on the neighboring donor group
ꢀ
(2.621 A). However, F2 atom associates with phenyl proton H16
2.2. Measurements
ꢀ
ꢀ
(2.465 A), and also H31 on pyridine ring (2.376 A). The protons on
pyridine ring are also active, easily to form hydrogen bond with
neighboring atoms, especially with B, F atoms. Thus triangle
structure was formed between H1A on pyridine ring with B, and F
1H NMR and 13C NMR were recorded on Bruker 400 NMR or
Varian 300 Mercury spectrometer. Chemical shifts are reported
in parts per million with CDCl3 as reference (7.26 ppm for 1H
NMR, and 77.0 ppm for 13C NMR). MS data were recorded on
a Waters Quattro Micro API LC-MS spectrometer (Waters, USA) or
Applied Biosystems Voyager-DE STR mass spectrometer. UVevis
and fluorescent spectra were obtained on Hitachi U-3010 and
F-4500, respectively. The fluorescent quantum yield is calculated
using quinine bisulfate as reference. Single crystals suitable for
X-ray measurements were obtained by slow evaporation of
mixed organic solutions, and single-crystal X-ray diffraction data
ꢀ
atoms on neighboring molecule (distance: H1AeF4 2.38 A, H1AeF3
ꢀ
ꢀ
2.653 A, H1AeB2 3.009 A). Although the chromophore containing
imidazole and pyridine fused with B was almost planar, there is
almost no overlap between neighboring chromophore cores, indi-
cating that planar
pep stacking of the chromophores is inhibited
by non-covalent interactions and the steric hindrance from twisted
phenyl rings on the end of the chromophore. The solubility of
BOPIM 3 in conventional organic solvents is poor, and the efforts to
grow single crystal of 3 failed.
were collected on a Bruker APEX II CCD diffractometer with
ꢀ
graphite monochromated Mo K
298 K.
a
radiation (
l
¼ 0.71073 A) at
3.2. 1H NMR analysis
To investigate the electronic effect of terminally electron donor
(methoxy) and acceptor (nitro) on the chromophore, 1H NMR
analysis was carried out as shown in Fig. 2. To make a comparison,
BOPIM 2 bearing terminal H atoms is chosen as reference. With
electron donors attached (BOPIM 1), significant upfield shift was
observed for all the protons on phenyl rings with the largest
shift ꢁ0.46 ppm for H6, indicating that the existence of electron
donor (methoxy group) enriches the electronic density of the
phenyl rings, leading to a higher shielding effect on these protons.
The protons on the chromophore core were only slightly affected,
with upfield shift of resonance signal of H4 0.03 ppm, and H1
0.01 ppm. In contrast, the introduction of nitro group (BOPIM 3)
2.3. Synthesis
The typical synthetic procedure of BOPIMs was described as
following: in a stirred mixture of ligand (8 mmol) and Et3N (5 ml)
in anhydrous CH2Cl2 (25 mL), BF3$OEt2 (5.6 mL, 44 mmol) was
added dropwise at 0 ꢀC. After the addition of BF3$OEt2, the
reaction mixture was allowed to warm to room temperature and
stir at room temperature overnight. The organic phase was
washed with water several times, dried on Na2SO4, and evapo-
rated in vacuo. Then the obtained crude residue was subjected to
column chromatography on a silica gel column to provide prod-
ucts as solid.
lowers the electronic density of the whole
p system on a large scale,
and shifts most resonance signals to down-field pronouncedly,
especially for these protons on phenyl rings. And the boron center is
also highly electron deficient, resulting in the different electronic
states of H5 with H50, and also H6 with H60. These results indicate
that the electronic density of the chromophore core can be tuned
by the terminally attached electron donors and acceptors. And it
needs to note that all the protons on BOPIM 3 exhibit multiple
resonances (H1 for example), suggesting that nitro moieties on
2.3.1. BOPIM 1
Orange solid (30%). 1H NMR (CDCl3, 400 MHz):
d 8.42 (d,
J ¼ 5.60 Hz, 1H), 8.16 (t, J ¼ 7.60 Hz, 1H), 8.06 (d, J ¼ 8.00 Hz, 1H),
7.55 (m, 4H), 7.46 (t, J ¼ 5.60 Hz, 1H), 6.92 (m, 4H), 3.84 (s, 3H), 3.82
(s, 3H). 13C NMR (CDCl3, 100 MHz):
d 159.41, 158.85, 145.81, 144.85,
144.53, 141.18, 133.76, 129.98, 129.30, 127.42, 123.30, 122.35, 117.47,
114.06, 113.77, 55.20. MS [M þ H]þ: 406.2 calcd. 406.1.

Q. Cao et al. / Journal of Organometallic Chemistry 717 (2012) 147e151
149
Fig. 1. Crystal structure and packing diagram of BOPIM 1.
neighboring molecules may associate with these protons to form
intermolecular non-covalent bonds.
Especially for BOPIM 1, the maximum absorptive band was
observed at 452 nm in hexane and 400 nm in methanol, mani-
festing an efficient intramolecular charge transfer (ICT)
characteristics.
3.3. Photophysical study
The fluorescent character of BOPIM 2 is complicated, which
varies with solvents. The longest emission was observed at 537 nm
in THF with fluorescence quantum yield 0.17. With electron-rich
methoxy moieties attached, there appears a significant bath-
ochromic shift. The emission in THF is centered at 560 nm, with
23 nm shift to the longer wavelength compared to 2. In contrast, the
effect of electron-withdrawing groups (nitro) blue-shifts the
emission significantly. All these three dyes exhibit weaker fluo-
rescence in polar solvents, due to the strong intramolecular charge
transfer (ICT) characteristics. On the other hand, efficient ICT
process can helps to increase Stokes shift, as presented in Table 1.
Surprisingly, the emission of BOPIM 3 is much more compli-
cated than those of 1 and 2. There appear dual emission bands in
mid-polar organic solvents. The emission around 420 nm should be
ascribed to the monomer of BOPIM 3, just as exhibited in hexane
and methanol. To ascribe the emission peak around 540 nm,
concentration-dependent fluorescent analysis was conducted
(Fig. 3F). In dilute chloroform solution such as 10ꢁ6 M, the main
fluorescence was observed at 424 nm with minimal emission
around 540 nm. With concentration increasing, the fluorescent
intensity at 424 nm decreases, meanwhile the intensity around
540 nm increases. When the concentration was beyond 10ꢁ4 M, the
blue emission was suppressed completely, and there appears only
one intense peak at 542 nm. For comparison, concentration-
dependent fluorescent spectra for BOPIM 1 (Fig. 3E) and 2 [25]
only show one emissive peak with no shift at all. It indicates that
BOPIM 3 may form other emissive species through intermolecular
interactions of nitro groups [28e31], especially in concentrated
solution. This result is also consistent with 1H NMR measurement,
from which only multiple resonance peaks were observed, indi-
cating that all the protons on pyridyl and phenyl rings associates
with intermolecular species, which is not observed for BOPIM 1
and 2.
The photophysical properties of these three BOPIM dyes were
studied in various solvents with different polarity. It’s found that
the electron-donating/withdrawing character of the terminal
attached groups has a significant effect on the absorption. As
reference, BOPIM 2 exhibits a broad absorptive band centered at
427 nm in hexane. With methoxy group as electron donor, the
maximum absorption band shifts bathochromically to 452 nm. The
25 nm red-shift reflects the electron donating property of methoxy
groups. In contrast, the absorption of BOPIM 3 bearing nitro groups
blue-shifts to 363 nm. Compared to 2, the absorptive difference
reaches 64 nm, further proving that the electronic effect of nitro
groups is greater than methoxy moieties, which is consistent with
1H NMR results. Interestingly, all these three dyes are environ-
mental sensitive and exhibit solvent-dependent absorptive bands.
The most interesting property of BOPIM dyes is their solid-
emissive character. In solid state, all these dyes exhibit intense
fluorescent emission, mainly due to that planar
pep stacking is
highly inhibited by the intermolecular non-covalent interactions, as
proved by X-ray single crystal analysis. As a result, there is no sign
of aggregate formation in solid state, and the absorption band even
Fig. 2. 1H NMR of synthesized BOPIMs bearing different electron donors and acceptors.

150
Q. Cao et al. / Journal of Organometallic Chemistry 717 (2012) 147e151
Table 1
Absorption and fluorescent maxima (nm) and the Stokes shift (n) of BOPIM dyes.
1
2
3
l abs nm (log
3 )
lem nm (fF
)
n
nm
92
l abs nm (log
3 )
lem nm (fF
)
n
nm
88
l
abs nm (log
3 )
lem nm (fF
)
n nm
Hexane
THF
Chloroform
Dioxane
Methanol
Solid
452 (4.04)
420 (4.06)
416 (4.17)
425 (4.10)
400 (4.18)
420
544 (0.10)
560 (0.25)
556 (0.10)
557 (0.28)
557 (0.03)
572 (0.37)
427 (4.37)
404 (4.24)
388 (4.55)
387 (4.70)
391 (4.30)
402
515 (0.28)
537 (0.18)
528 (0.30)
510 (0.23)
534 (0.08)
524 (0.18)
363 (3.92)
331 (4.52)
352 (4.46)
345 (4.55)
343 (4.41)
364
416 (0.02)
53
89
72
75
74
e
140
140
132
157
152
133
140
123
143
122
420, 521 (0.08)
424, 542 (0.04)
420, 513 (0.08)
417 (0.01)
537 (0.02)
Fig. 3. (A) Absorption of BOPIMs in solution (hexane, 1.0 ꢂ 10ꢁ5 M); (B) fluorescent spectra of BOPIMs in solution (hexane, 1.0 ꢂ 10ꢁ5 M); (C) absorption of BOPIMs in solid state; (D)
fluorescent spectra of BOPIMs in solid state; (E) concentration-dependent fluorescent spectra of BOPIM 1 in CHCl3; (F) concentration-dependent fluorescent spectra of BOPIM 3 in
CHCl3 (Ex: 365 nm).
turns narrower compared to those in hexane solutions, which is
quite unique and interesting. The fluorescent quantum yield of
BOPIM 1 in solid state is measured to be 0.37 by integrating sphere
method, which is much higher than 2. It may be the result of more
dilute solution, even though they may have formed dimer or trimer
through intermolecular interactions.
4. Conclusion
effective inhibition of planar
pep stacking, since methoxy groups
participate in the formation of intermolecular non-covalent bonds.
As nitro group is a well-known quenching group of fluorescence
[32], BOPIM 3 has rather low quantum yield (0.02) in solid as in
Solid-emissive BOPIM dyes bearing different electron donating/
withdrawing groups were synthesized. They emit fluorescence
with large Stokes shift in solution and also in solid state. Electron

Q. Cao et al. / Journal of Organometallic Chemistry 717 (2012) 147e151
151
[10] G.Q. Zhang, S.E. Kooi, J.N. Demas, C.L. Fraser, Adv. Mater. 20 (2008)
2099e2104.
[11] T. Ozdemir, S. Atilgan, I. Kutuk, L.T. Yildirim, A. Tulek, M. Bayindir, E.U. Akkaya,
Org. Lett. 11 (2009) 2105e2107.
[12] T.T. Vu, S. Badre, C. Dumas-Verdes, J.J. Vachon, C. Julien, P. Audebert,
E.Y. Senotrusova, E.Y. Schmidt, B.A. Trofimov, R.B. Pansu, G. Clavier, R. Meallet-
Renault, J. Phys. Chem. C 113 (2009) 11844e11855.
[13] K. Ono, K. Yoshikawa, Y. Tsuji, H. Yamaguchi, R. Uozumi, M. Tomura, K. Taga,
K. Saito, Tetrahedron 63 (2007) 9354e9358.
donor leads to red-shift of the absorption and emission with higher
fluorescent quantum yield, and electron acceptor plays the reverse
role. However, BOPIM 3 bearing nitro moieties also emits fluores-
cence at long wavelength in concentrated solutions and in solid
state, probably due to the formation of dimer or trimer through
intermolecular interactions.
[14] Y. Kubota, J. Uehara, K. Funabiki, M. Ebihara, M. Matsui, Tetrahedron Lett. 51
(2010) 6195e6198.
Acknowledgments
[15] D.K. Zhang, Y.G. Wen, Y. Xiao, G. Yu, Y.Q. Liu, X.H. Qian, Chem. Commun.
(2008) 4777e4779.
[16] Y. Zhou, Y. Xiao, D. Li, M.Y. Fu, X.H. Qian, J. Org. Chem. 73 (2008) 1571e1574.
[17] Y. Kubota, T. Tsuzuki, K. Funabiki, M. Ebihara, M. Matsui, Org. Lett. 12 (2010)
4010e4013.
We are grateful for the financial support from National Natural
Science Foundation of China (21002059), and China Three Gorges
University (KJ2010B035, KJ2011B004).
[18] Y. Kubota, H. Hara, S. Tanaka, K. Funabiki, M. Matsui, Org. Lett. 13 (2011)
6544e6547.
[19] Y. Zhou, Y. Xiao, S.M. Chi, X.H. Qian, Org. Lett. 10 (2008) 633e636.
[20] J.F. Araneda, W.E. Piers, B. Heyne, M. Parvez, R. McDonald, Angew. Chem., Int.
Ed. 50 (2011) 12214e12217.
[21] D. Frath, S. Azizi, G. Ulrich, P. Retailleau, Org. Lett. 13 (2011) 3414e3417.
[22] T.R. Chen, R.H. Chien, M.S. Jan, A. Yeh, J.D. Chen, J. Organomet. Chem. 691
(2006) 799e804.
[23] M.F. Mao, S.Z. Xiao, T. Yi, K. Zou, J. Fluorine Chem. 132 (2011) 612e616.
[24] M.F. Mao, R.H. Zhang, S.Z. Xiao, K. Zou, J. Coord. Chem. 64 (2011)
3303e3310.
Appendix A. Supplementary material
CCDC 870759 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
References
[25] M.F. Mao, S.Z. Xiao, J.F. Li, Y. Zou, R.H. Zhang, J.R. Pan, F.J. Dan, K. Zou, T. Yi,
Tetrahedron 68 (2012) 5037e5041.
[26] G. Ulrich, R. Ziessel, A. Harriman, Angew. Chem., Int. Ed. 47 (2008)
1184e1201.
[1] C. McDonagh, C.S. Burke, B.D. MacCraith, Chem. Rev. 108 (2008) 400e422.
[2] S.M. Borisov, O.S. Wolfbeis, Chem. Rev. 108 (2008) 423e461.
[3] A. Loudet, K. Burgess, Chem. Rev. 107 (2007) 4891e4932.
[4] L. Basabe-Desmonts, D.N. Reinhoudt, M. Crego-Calama, Chem. Soc. Rev. 36
(2007) 993e1017.
[27] X.J. Wu, R. Jiang, X.P. Xu, X.M. Su, W.H. Lu, S.J. Ji, J. Comb. Chem. 12 (2010)
829e835.
[28] R. Bala, N. Kaur, J. Kim, J. Mol. Struct. 1003 (2011) 47e51.
[29] Y.H. Deng, S.Y. Wang, J. Liu, Y.L. Yang, F. Zhang, H.W. Ma, Acta Chim. Sinica. 65
(2007) 809e815.
[30] J.M.A. Robinson, B.M. Kariuki, D. Philp, K.D.M. Harris, Tetrahedron Lett. 38
(1997) 6281e6284.
[5] H. Sunahara, Y. Urano, H. Kojima, T. Nagano, J. Am. Chem. Soc. 129 (2007)
5597e5604.
[6] A. Coskun, E.U. Akkaya, J. Am. Chem. Soc. 127 (2005) 10464e10465.
[7] Y. Gabe, Y. Urano, K. Kikuchi, H. Kojima, T. Nagano, J. Am. Chem. Soc. 126
(2004) 3357e3367.
[31] N.I. Giricheva, G.V. Girichev, Y.S. Medvedeva, S.N. Ivanov, A.V. Bardina,
V.M. Petrov, Struct. Chem. 22 (2011) 373e383.
[8] A. Hepp, G. Ulrich, R. Schmechel, H. von Seggern, R. Ziessel, Synth. Met. 146
(2004) 11e15.
[32] A.J. Cui, X.J. Peng, J.L. Fan, X.Y. Chen, Y.K. Wu, B.C. Guo, J. Photochem. Photo-
biol., A 186 (2007) 85e92.
[9] B. Domercq, C. Grasso, J.L. Maldonado, M. Halik, S. Barlow, S.R. Marder,
B. Kippelen, J. Phys. Chem. B 108 (2004) 8647e8651.