Carbazole-based donor-acceptor compounds: Highly fluorescent organic nanoparticles

Article abstract of DOI:10.1021/ol702666g

(Figure Presented) Carbazole-based donor-acceptor compounds 1,2-dicyano-trans-1,2-bis-4-(carbazolyl)phenylethylene (1) and 1,2-dicyano-trans-1,2-bis-4-(3,6-di-tert-butylcarbazolyl)phenylethylene (2) were synthesized. 1 and 2 show negative solvatochromic absorption behavior, but show both positive and negative solvatochromic behavior in the fluorescence spectra. In a water/THF mixture, 1 as well as 2 aggregate into 50-150 nm nanoparticles. The emission of nanoparticles of the new types of fluorescent organic nanoparticles (FONs) 1 and 2 is much higher than that of either 1 or 2 in solution.

Full text of DOI:10.1021/ol702666g

ORGANIC
LETTERS
2008
Vol. 10, No. 2
281-284
Carbazole-Based Donor−Acceptor
Compounds: Highly Fluorescent
Organic Nanoparticles
Sujeewa S. Palayangoda, Xichen Cai, Ravi M. Adhikari, and Douglas C. Neckers*
Center for Photochemical Sciences,1 Bowling Green State UniVersity,
Bowling Green, Ohio 43403
neckers@photo.bgsu.edu
Received November 5, 2007
ABSTRACT
Carbazole-based donor
tert-butylcarbazolyl)phenylethylene (2) were synthesized. 1 and 2 show negative solvatochromic absorption behavior, but show both positive
and negative solvatochromic behavior in the fluorescence spectra. In a water/THF mixture, 1 as well as 2 aggregate into 50 150 nm nanoparticles.
−acceptor compounds 1,2-dicyano-trans-1,2-bis-4-(carbazolyl)phenylethylene (1) and 1,2-dicyano-trans-1,2-bis-4-(3,6-di-
−
The emission of nanoparticles of the new types of fluorescent organic nanoparticles (FONs) 1 and 2 is much higher than that of either 1 or
2 in solution
With the wide application of organic light emitting diodes
(OLEDs), development of highly efficient emitters is im-
portant. Stability and emission in the solid state are important
features of organic compounds to be used as OLEDs.2
However, the substantial emission of compounds in solution
usually becomes weak in the solid state due to both
intermolecular energy and electron transfer.3 Therefore, the
preparation of compounds that emit efficiently in the solid
state is a continuing objective. Donor-acceptor compounds
containing an aromatic fumaronitrile core have attracted
significant attention as candidates in electroluminescent (EL)
devices because of their strong emissions in the solid state.4,5
Strong emission from 1-cyano-trans-1,2-bis-(4′-methylbi-
phenyl)ethylene (CN-MBE),3 2,5-diphenyl-1,4-distyrylben-
zene,6 and 1,4-bis(1-cyano-2-(4-(N-4-methoxyphenyl-N-
phenylamino)phenyl)vinyl)benzene (D-CN)7 compounds in
the solid state has been reported. Either intramolecular
planarization or a specific aggregation in the solid state is
assumed to be important to the emission in the solid state.3,8
Such aggregation-induced emissive materials are promising
as emitters for the fabrication of highly efficient OLEDs.9
Interestingly recently discovered fluorescent organic nano-
particles (FONs) are expected to play roles in a wide variety
of applications such as nano-sized OLEDs due to the
flexibility of synthetic approaches to such compounds.3 FONs
have received a lot of attention,10,11 and those from perylene
and pyrazoline have been reported.3,12 The emission is related
(6) Xie, Z.; Yang, B.; Li, F.; Cheng, G.; Liu, L.; Yang, G.; Xu, H.; Ye,
L.; Hanif, M.; Liu, S.; Ma, D.; Ma, Y. J. Am. Chem. Soc. 2005, 127, 14152.
(7) Kim, D. U.; Paik, S. H.; Kim, S.-H.; Tsutsui, T. Synth. Met. 2001,
123, 43.
(8) OelKrug, D.; Tompert, A.; Gierschner, J.; Egelhaaf, H.-J.; Hanack,
M.; Hohloch, M.; SteinHuber, E. J. Phys. Chem. B 1998, 102, 1902.
(9) Chen, J.; Xu, B.; Ouyang, X.; Tang, B. Z.; Cao, Y. J. Phys. Chem.
A 2004, 108, 7522.
(10) Kasai, H.; Kamatani, H.; Okada, S.; Oikawa, H.; Matsuda, H.;
Nakanishi, H. Jpn. J. Appl. Phys. 1996, 4, L221.
(1) Contribution No. 668 from Center for Photochemical Sciences.
(2) Mondal, R.; Shah, B. K.; Neckers, D. C. J. Org. Chem. 2006, 71,
4085.
(3) An, B.-K.; Kwon, S.-K.; Jung, S.-D.; Park, S. Y. J. Am. Chem. Soc.
2002, 124, 14410.
(4) Yeh, H.-C.; Yeh, S.-J.; Chen, C.-T. Chem. Commun. 2003, 2632.
(5) Chen, C.-T. Chem. Mater. 2004, 16, 4389.
(11) Kasai, H.; Kamatani, H.; Yoshikawa, Y.; Okada, S.; Oikawa, H.;
Watanabe, A.; Itoh, O.; Nakanishi, H. Chem. Lett. 1997, 1181.
10.1021/ol702666g CCC: $40.75
© 2008 American Chemical Society
Published on Web 12/20/2007

to the size of FONs with a red-shifted emission resulting
from increasing the nanoparticle size.12
Scheme 1. Synthesis of 4 and 6
Herein, we report the synthesis of the carbazole-based
donor-acceptor (D-A) compounds, 1,2-dicyano-trans-1,2-
bis-4-(carbazolyl)phenylethylene (1) and 1,2-dicyano-trans-
1,2-bis-4-(3,6-di-tert-butylcarbazolyl)phenylethylene (2). These
contain the diphenylfumaronitrile core, Figure 1. Formation
Scheme 2. Synthesis of 1 and 2
Figure 1. The structures of 1,2-dicyano-trans-1,2-bis-4-(carba-
zolyl)phenylethylene (1) and 1,2-dicyano-trans-1,2-bis-4-(3,6-di-
tert-butylcarbazolyl)phenylethylene (2).
tion band from 450 to 415 nm for 1 and from 470 to 440
nm for 2, Figure 2. It is known that solvatochromism depends
of 1 and 2 FONs with diameters of 50-150 nm in a THF/
water mixture was affected. The quantum yield of emission
of 1 and 2 in the solid state is substantially higher than that
observed in the solution.
Diphenylfumaronitrile was utilized as the core because,
in the trans form, it consists of antiparallel dipoles. Thus,
concentration-related fluorescence quenching in the solid
state should be considerably reduced due to dipole-dipole
interaction.
Carbazole deriviatives show high emission quantum yields
in the solid state, and the compounds can easily be modified
at the 3- and 6-positions of the carbazole to tune the optical
properties.13,14 Carbazole (3) was converted into 3,6-di-tert-
butyl-9H-carbazole (4)14 and 4-bromophenylacetonitrile (5)
was converted into bis(4-bromophenyl)fumaronitrile (6)15 in
good yield following literature methods, Scheme 1.
Following a modified literature procedure (Scheme 2) 1
and 2 were synthesized (see the Supporting Information).
While compound 1 is red, compound 2 is yellow in the solid
state. Both 1 and 2 are stable and dissolve in common organic
solvents such as carbon tetrachloride (CCl4), dichloromethane
(DCM), tetrahydrofuran (THF), and N,N-dimethylformamide
(DMF).
Figure 2. Normalized absorption spectra of 1 and 2 in (a) carbon
tetrachloride, (b) dichloromethane, (c) tetrahydrofuran, and (d) N,N-
dimethylformamide.
A change in solvent from nonpolar CCl4 to polar DMF
causes a negative solvatochromic shift in the π-π* absorp-
(12) Fu, H.-B.; Yao, J.-N. J. Am. Chem. Soc. 2001, 123, 1434.
(13) Adhikari, R. M.; Mondal, R.; Shah, B. K.; Neckers, D. C. J. Org.
Chem. 2007, 72, 4727.
(14) Liu, Y.; Nishiura, M.; Wang, Y.; Hou, Z. J. Am. Chem. Soc. 2006,
128, 5592.
on molecular structure, the nature of the chromophore, as
well as the solvents.16 Reichardt’s pyridinium N-phenoxide
(15) Yeh, H.-C.; Wu, W.-C.; Wen, Y.-S.; Dai, D.-C.; Wang, J.-K.; Chen,
C.-T. J. Org. Chem. 2004, 69, 6455.
(16) Reichardt, C. Chem. ReV. 1994, 94, 2319.
Org. Lett., Vol. 10, No. 2, 2008
282

betaine dye and N-(o-phosphoniobenzylidene)-4-aminophe-
nolate betaine dyes have been reported to show negative
solvatochromic behavior.17 Generally in order for this to be
observed, the dipole moment of the molecule in the ground
state (µg) is expected to be higher than that of the excited
state (µe).16 The µg of 1 and 2 should be larger than µe,
because negative solvatochromic behavior was observed,
Figure 2.18
Excitation spectra of 1 and 2 in CCl4 are similar to their
UV absorption spectra, indicating that the emission mainly
comes from 1 and 2 in the singlet excited state (S1). New
absorption bands appear between 300 and 450 nm when 1
and 2 were dissolved in DCM. The 350 nm band became
stronger and the 450 nm band became weaker when 1 and
2 were dissolved in DMF.
We submit that intramolecular electron transfer occurs in
1 and 2 caused by a gradual increase in the polarity of the
solvents. The quantum yields of emission of 1 and 2 decrease
with increasing solvent polarity. In DMF, the charge separa-
tion in 1 and 2 is almost complete. Therefore, the emission
yield becomes very low.
Interestingly, the emission spectra of 1 and 2 show both
positive and negative solvatochromism with increasing
solvent polarity, Figure 3. For example, the emission
The markedly enhanced solid-state emision quantum yields
of 1 and 2 were measured on a thin film with use of an
integrating sphere,13 Table 1. Park and co-workers observed
Table 1. Photophysical Properties of 1 and 2a
Amax
λmax (nm)
Φfl
compound 1
CCl4
DCM
THF
DMF
thin film
nanoparticles
445
435
423
415
455
453
530
618
608
520
608
590
0.37
0.07
0.02
<0.01
0.72
0.70
compound 2
CCl4
DCM
THF
DMF
thin film
nanoparticles
470
457
446
440
478
472
568
670
638
518
553
550
0.44
0.01
<0.01
<0.01
0.88
0.81
Figure 3. Normalized fluorescence spectra of 1 and 2 recorded in
(a) carbon tetrachloride, (b) dichloromethane, (c) tetrahydrofuran,
and (d) N,N-dimethylformamide.
a Solution quantum yield was calculated with riboflavin (0.30 in ethanol)
as a reference. Solid-state quantum yield was measured by using an
integrating sphere.
spectrum of 2 was observed with a peak around 580 nm in
CCl4. A broad and red-shifted emission of 2 was observed
(λmax) 680 nm) in the solvent of medium polarity, dichlo-
romethane. The emission spectrum of 2 was blue-shifted to
520 nm in DMF.
Such inverted solvatochromism has not been observed in
fluorescence spectra, although the inverted solvatochromic
behavior of the absorption of 4′-hydroxy-1-methylstilbazo-
lium betaine has been reported.19 A solvent-induced change
in the electronic ground state structure results in various
solvents.16 Accompanying structural changes of 1 and 2 in
the excited state is intramolecular electron transfer between
the carbazole chromophore and the diphenylfumaronitrile
core to give 1 and 2 as a charge-separated state.
similar behavior for 1-cyano-trans-1,2-bis(4′-methylbiphe-
nyl)ethylene (CN-MBE). In the case of CN-MBE, the polar,
bulky cyano group minimizes parallel face-to-face interac-
tion, which induces the H-type aggregation, and this mini-
mizes the fluorescence in the solid state. Since J-type
aggregation enhances the emission,3 1 and 2 may form J-type
aggregates in the solid state.3,20
Nanoparticles of 1 and 2 were prepared following a
literature method.3 The red-shifted absorption of 1 and 2 in
water/THF (9:1, with nanoparticles) compared to that in
water/THF (1:9, without nanoparticles) indicates J-type
aggregation occurs with an increasing amount of water (see
the Supporting Information). The nanoparticle solutions were
transparent and stable. No precipitation was observed on
keeping 1 and 2 at room temperature for several months.
The diameters of nanoparticles from 1 and 2 were in the
range of 50-150 nm as determined by Scanning Electron
Microscopy (SEM), Figure 4 (top). Strong emissions in
The excitation spectra of 1 and 2 obtained at 600 nm show
that the emission comes from different choromophores in
CCl4, DCM, and DMF (see the Supporting Information).
(17) Allen, D. W.; Li, X. J. Chem. Soc., Perkin Trans. 2 1997, 1099.
(18) Soujanya, T.; Fessenden, R. W.; Samanta, A. J. Phys. Chem. 1996,
100, 3507.
(19) Jacques, P. J. Phys. Chem. 1986, 90, 5535.
(20) Yoshika, H.; Nakatsu, K. Chem. Phys. Lett. 1971, 11, 255.
Org. Lett., Vol. 10, No. 2, 2008
283

(0.69) FONs.3 However, no clear change of particle size was
observed on making the particles in solvents differing in the
water/THF ratio. We believe that FONs of 1 and 2 with D-A
character and high emission will have a number of potential
applications in nanosized optoelectronic devices.
In summary, carbazole-based D-A compounds 1 and 2
were successfully synthesized and characterized. The com-
pounds exhibit good stability and solubility in common
organic solvents and aggregate to nanoparticles in solvents
containing a suitable water/THF ratio. A negative solvato-
chromism in the absorption spectra of 1 and 2 was observed.
Both positive and negative solvatochromic behavior in the
emission spectra suggest intramolecular electron transfer in
1 and 2.
Acknowledgment. The authors greatly appreciate the
financial support provided to this work by the Office of Navel
Research ONR00014-06-1-0948. We thank Kelechi
Anyaogu, our group member, for taking SEM images.
Figure 4. SEM images of 1 and 2 nanoparticles in a water/THF
(9:1) mixture (top). The pictures of 1 and 2 taken in a water/THF
(9:1) mixture (a) and THF (b) under 365 nm UV light, respectively
(bottom). The concentration of 1 and 2 was 1 × 10-5 mol L-1
.
Supporting Information Available: Experimental de-
tails. This material is available free of charge via the Internet
at http://pubs.acs.org.
water/THF (9:1) can be clearly seen, Figure 4 (bottom), with
quantum yields higher than those reported for CN-MBE
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Org. Lett., Vol. 10, No. 2, 2008