Synthesis and photophysical properties of carbazole-based blue light-emitting dendrimers

Article abstract of DOI:10.1021/jo0703911

(Graph Presented) A new class of highly fluorescent and stable carbazole-based dendrimers (1-5) that contain the ethynylbenzene and diethynylbenzene cores has been synthesized and characterized. They show very high extinction coefficients of absorption (A_max ≈ 328-353 nm) and high quantum yields of fluorescence (λ_max ≈ 386-437 nm; Φ_F ≈ 0.72-0.89; τ_F ≈ 2.09-3.91 ns) in dichloromethane. The quantum yields of fluorescence of 1-5 in the solid state are equally high (λ_max ≈ 385-422 nm; Φ_F ≈ 0.40-0.85). These data indicate their potential use as blue-emitting materials in organic light-emitting diodes (OLEDs).

Full text of DOI:10.1021/jo0703911

Synthesis and Photophysical Properties of Carbazole-Based Blue
Light-Emitting Dendrimers
Ravi M. Adhikari, Rajib Mondal, Bipin K. Shah,* and Douglas C. Neckers*
1
Center for Photochemical Sciences, Bowling Green State UniVersity, Bowling Green, Ohio 43403
bipin@bgsu.edu; neckers@photo.bgsu.edu
ReceiVed February 26, 2007
A new class of highly fluorescent and stable carbazole-based dendrimers (1-5) that contain the
ethynylbenzene and diethynylbenzene cores has been synthesized and characterized. They show very
high extinction coefficients of absorption (Amax ≈ 328-353 nm) and high quantum yields of fluorescence
(
λ
max ≈ 386-437 nm; Φ
F
≈ 0.72-0.89; τ ≈ 2.09-3.91 ns) in dichloromethane. The quantum yields of
F
fluorescence of 1-5 in the solid state are equally high (λmax ≈ 385-422 nm; Φ ≈ 0.40-0.85). These
F
data indicate their potential use as blue-emitting materials in organic light-emitting diodes (OLEDs).
3
Introduction
levels. But the design and synthesis of blue emitters suitable
for fabrication of stable OLED devices remains a major
Ever since Tang and Vanslyke constructed electroluminescent
EL) devices using organic materials as emitters, development
4
challenge. The performance of blue emitters is not as efficient
as that of red and green emitters.
(
5
of new EL materials has become an active area of research
because of their potential use in displays. A large effort to make
Carbazole-based compounds are known for their intense
luminescence and widely used in OLEDs as blue emitters,
2
6,7
8-12
organic light-emitting diode (OLED)-based technology a pos-
sible alternative to liquid crystal-based displays and to develop
OLED displays for other niche lighting applications has also
been underway. Red, green, and blue emitters are required for
the fabrication of a full-color display. The stability and efficiency
of green and red emitters have approached commercially viable
(
3) Mondal, R.; Shah, B. K.; Neckers, D. C. J. Org. Chem. 2006, 71,
085.
4) Shah, B. K.; Neckers, D. C.; Shi, J.; Forsythe, E. W.; Morton, D. J.
Phys. Chem. A 2005, 109, 7677.
(5) Chan, L.; Lee, R.; Hsieh, C.; Yeh, H.; Chen, C. J. Am. Chem. Soc.
4
(
2
002, 124, 6469.
(
6) Yu, H.; Zain, S. M.; Eigenbrot, I. V.; Phillips, D. Chem. Phys. Lett.
(1) Contribution No. 633 from the Center for Photochemical Sciences.
1993, 202, 141.
(
2) (a) Tang, C. W.; Vanslyke, S. A. Appl. Phys. Lett. 1987, 51, 913.
(7) Howell, R.; Taylor, A. G.; Phillips, D. Chem. Phys. Lett. 1992, 188,
119.
(8) Almeida, K. D.; Bernede, J. C.; Marsillac, S.; Godoy, A.; Diaz, F.
R. Synth. Met. 2001, 122, 127.
(
2
b) Shinar, J. Organic Light-Emitting DeVices; Springer-Verlag: New York,
003. (c) M u¨ llen, K.; Scherf, U. Organic Light-Emitting DeVices. Synthesis,
Properties and Applications; Wiley-VCH Verlag GmbH & Co. KGaA:
Weinheim, Germany, 2006.
(9) Lee, J.; Woo, H.; Kim, T.; Park, W. Opt. Mater. 2002, 21, 225.
1
0.1021/jo0703911 CCC: $37.00 © 2007 American Chemical Society
Published on Web 06/02/2007
J. Org. Chem. 2007, 72, 4727-4732
4727

Adhikari et al.
CHART 1
SCHEME 1a
a
Reagents and conditions: (a) tert-butyl chloride, zinc chloride, ni-
tromethane, 40-50 °C, 5 h; (b) Cu, potassium carbonate, 18-crown-6,
o-dichlorobenzene, reflux, 12 h.
SCHEME 2a
white emitters,13 green emitters14 and red emitters.15 They also
undergo reversible oxidation processes, making them suitable
hole carriers.16 We have synthesized a new class of carbazole-
based stable dendrimers that contain the ethynylphenyl core (1-
5
, Chart 1) in order to develop stable blue emitters which also
have hole transporting properties. While 9-(4-(2-phenylethynyl)-
phenyl)-9H-carbazole (1) and 3,6-di-tert-butyl-9-(4-(2-phenyl-
ethynyl)phenyl)-9H-carbazole (2) are monodenderons, 9-(4-(2-
(
3-(2-(4-(9H-carbazol-9-yl)phenyl)ethynyl)phenyl)ethyny) phenyl)-
H-carbazole (3), 3,6-di-tert-butyl-9-(4-(2-(3-(2-(4-(3,6-di-tert-
butyl-9H-carbazol-9-yl)phenyl)ethynyl)phenyl)ethynyl)phenyl)-
H-carbazole (4), and 3,6-di-tert-butyl-9-(4-(2-(4-(2-(4-(3,6-di-
9
9
tert-butyl-9H-carbazol-9-yl)phenyl)ethynyl)phenyl)ethy-
nyl)phenyl)-9H-carbazole (5) are meta- and para-substituted
didendrons.
a
Reagents and conditions: (a) CuI, Pd(PPh3)2Cl2, PPh3, triethylamine,
tetrahydrofuran, reflux, 12 h (for 1 and 2), 72 h (for 3 and 4), and 7 days
for 5).
In designing these dendrimers, the ethynylphenyl group was
preferred as the core because of its linearity, generally high
fluorescence quantum yield, emission in the violet-blue region,
(
easier to modify the 3- and 6-positions and tune the molecular
properties. In this report, we present an easy synthesis and
17
19
and synthetic advantage. A number of compounds containing
the ethynylphenyl group as a conventional core have been
synthesized and studied for their applications in electrooptical
devices.1 The carbazole group has another advantage: it is
discuss the interesting photophysical properties of 1-5. Not only
do they exhibit intense luminescence both in solution and in
the solid state, but they also show excellent stability. The
solvatochromism exhibited by these dendrimers is also dis-
cussed.
8
(
10) Ding, J.; Gao, J.; Cheng, Y.; Xie, Z.; Wang, L.; Ma, D.; Jing, X.;
Wang, F. AdV. Funct. Mater. 2006, 16, 575.
11) Morin, J.; Boudreault, P.; Leclerc, M. Macromol. Rapid Commun.
002, 23, 1032.
12) Grigalevicius, S.; Ma, L.; Xie, Z.; Scheri, U. J. Polym. Sci., Part
A: Polym. Chem. 2006, 44, 5987.
13) Liu, Y.; Nishiura, M.; Wang, Y.; Hou, Z. J. Am. Chem. Soc. 2006,
28, 5592.
(
2
Results and Discussions
(
Synthesis. Following literature13 methods, carbazole (6a) and
,6-di-tert-butyl-9H-carbazole (6b) were converted into 9-(4-
(
3
1
bromophenyl)carbazole (7a) and 9-(4-bromophenyl)-3,6-di-tert-
butylcarbazole (7b), respectively (Scheme 1). The latter com-
pounds are the main intermediates in the synthesis of 1-5.
Compounds 1 and 2 were synthesized by Sonogashira coupling
of 1-ethynylbenzene with 7a and 7b, respectively (Scheme 2).
Similar Sonogashira coupling of 7a and 7b with 1,3-diethynyl-
benzene yielded 3 and 4, respectively. Finally, 5 was prepared
by coupling 1,4-diethynylbenzene with 7b (Scheme 2). The
coupling of 7a with 1,4-diethynylbenzene was not successful
probably due to lower solubility of the final product (dendrimer
similar to 5 without tert-butyl groups) in common solvents.
(
14) Thomas, K. R.; Lin, J. T.; Tao, Y. T.; Ko, C. J. Am. Chem. Soc.
2
001, 123, 9404.
(
15) Guan, M.; Chen, Z.; Bian, Z.; Liu, Z.; Gong, G.; Baik, W.; Lee,
H.; Huang, C. Org. Electron. 2006, 7, 330.
16) Loiseau, F.; Campagna, S.; Hameurlaine, A.; Dehaen, W. J. Am.
Chem. Soc. 2005, 127, 11352 and references therein.
17) (a) Rodr ´ı guez, J. G.; Esquivias, J.; Lafuente, A.; D ´ı az, C. J. Org.
(
(
Chem. 2003, 68, 8120. (b) Yamaguchi, Y.; Kobayashi, S.; Wakamiya, T.;
Matsubara, Y.; Yoshida, Z. Angew. Chem., Int. Ed. 2005, 44, 7040. (c)
Yamaguchi, Y.; Ochi, T.; Miyamura, S.; Tanaka, T.; Kobayashi, S.;
Wakamiya, T.; Matsubara, Y.; Yoshida, Z. J. Am. Chem. Soc. 2006, 128,
4
504.
18) (a) Schumm, J. S.; Pearson, D. L.; Tour, J. M. Angew. Chem., Int.
(
Ed. Engl. 1994, 33, 1360. (b) Meier, H.; Ickenroth, D.; Stalmach, U.;
Koynov, K.; Bahtiar, A.; Bubeck, C. Eur. J. Org. Chem. 2001, 4431. (c)
Anderson, S. Chem. Eur. J. 2001, 7, 4706.
(19) Joule, J. A. AdV. Heterocycl. Chem. 1984, 35, 83.
4728 J. Org. Chem., Vol. 72, No. 13, 2007

Carbazole-Based Blue Light-Emitting Dendrimers
a
TABLE 1. Photophysical Data of 1-5 Recorded in
Dichloromethane
3
-
o
o
Amax ꢀ (dm cm
λmax
τF
k R
(s
k NR
-
1
-1
-1
compd (nm)
mol
)
(nm) ΦF (ns)
)
(s )
8
8
7
7
7
7
1
2
3
4
5
326
334
329
337
353
30125
32808
54712
48525
66750
390 0.72 2.53 2.80 × 10 1.15 × 10
8
410 0.80 3.91 2.04 × 10 5.10 × 10
8
394 0.80 2.44 3.27 × 10 8.28 × 10
8
411 0.83 3.62 2.29 × 10 4.70 × 10
8
432 0.89 2.09 4.26 × 10 5.24 × 10
a
Excitation wavelength ) 340 nm for ΦF and τF. TheΦF values ((0.01-
.03) are relative to that of diphenyanthracene (0.90 in cyclohexane). The
0
τF values were measured from argon-saturated solutions and decay was
monitored at the corresponding λmax.
FIGURE 1. Normalized absorption and emission spectra of 1-5
recorded in dichloromethane.
Photoluminescence in Solution. Absorption spectra of 1-5
recorded in dichloromethane show a π-π* band at ∼325-353
nm (Figure 1). The absorption maxima of 3 (329 nm) and 4
(337 nm) are not significantly red-shifted from that of 1 (326
nm) and 2 (334 nm), respectively. This indicates that there is
only a slight conjugation between the carbazole arms when they
2
0
are linked at the meta-positions of the central phenyl ring.
Compound 5 having arms at the para-positions of the central
phenyl ring expectedly showed a larger red-shifted absorption
relative to that of 1-4, indicating a significant conjugation
in 5.
FIGURE 2. Fluorescence decay of 1 monitored at λmax ) 390 nm in
When excited at 340 nm, dilute solutions of 1-5 in
dichloromethane showed violet to blue emission (Figure 1). It
is interesting that the dendrimers that have tert-butyl groups (2
and 4) show significantly red-shifted fluorescence relative to
those that have no tert-butyl groups (1 and 3). On the other
hand, the emission maxima of 3 (394 nm) and 4 (411 nm) are
similar to those of 1 (390 nm) and 2 (410 nm), respectively.
This indicates that the density of chromophores in a molecule
can be enhanced through a meta linkage without significantly
perturbing the purity of the emission. That there is a larger
extension of conjugation in 5 than in 3 and 4 is reflected in its
emission; the fluorescence spectrum of 5 (λmax ) 437 nm) was
the most red-shifted among all the compounds studied.
The fluorescence quantum yields (ΦF) and lifetimes (τF) of
2
dichloromethane (τ ) 2.53 ns, ø ) 1.08); excitation wavelength )
F
340 nm.
TABLE 2. Photophysical Data of 4 and 5 Measured in Different
Solventsa
4
5
Amax
λmax
τF
Amax
λmax
τF
solvent
pentane
toluene
chloroform
(nm) (nm) ΦF (ns) (nm) (nm) ΦF (ns)
340 359 0.76 1.38 356 385 0.89 0.72
345 379 0.82 1.65 359 400 0.86 0.85
344 400 0.84 2.51 356 419 0.69 1.34
dichloromethane 345 416 0.88 3.62 355 437 0.89 2.09
methanol
acetonitrile
338 426 0.81 4.88 354 453 0.41 3.21
340 435 0.87 5.51 353 470 0.67 4.22
a
Excitation wavelength ) 340 nm for measuring ΦF and τF. The ΦF
values ((0.01-0.03) are relative to that of diphenyanthracene (0.90 in
cyclohexane). The τF values were measured from argon-saturated solutions
and decay was monitored at the corresponding λmax.
1-5 recorded in dichloromethane are presented in Table 1 along
with their absorption (Amax) and emission (λmax) maxima. The
ΦF values of 1-5 were found in the range of 0.72-0.89 relative
2
1
to that of 9,10-diphenylanthracene (0.90 in cyclohexane).
those of 5 were comparatively lower in polar solvents than in
nonpolar solvents. For example, the ΦF value of 5 in methanol
Fluorescence decays of 1-5 were fitted with monoexponential
functions indicating emission from the singlet excited state in
each case. The nonradiative decay of the singlet state was
calculated to be about 2 to 4 times lower than the radiative decay
for 1-5.
ΦF values of 4 and 5 measured in various solvents are
presented in Table 2. The ΦF values of 4 were found in the
range of 0.76-0.88 irrespective of the solvent polarity, while
(0.41) was almost half that observed in pentane (0.89) or toluene
(0.86). In each solvent, the fluorescence decay of 4 and 5 could
be fit monoexponentially, indicating the singlet states are formed
exclusively. Interestingly, the τF values of both 4 and 5 showed
strong solvent dependence and gradually increased with an
increase in the solvent polarity. The τF value of 4, for example,
was lowest in pentane (0.72 ns) and highest in acetonitrile (4.22
ns). Similarly, the τF value of 5 gradually increases from 1.38
ns in pentane to as high as 5.51 ns in acetonitrile.
Emission spectra of 1 were recorded at different concentra-
tions in dichloromethane (see the Supporting Information) to
asses the extent of possible dimer formation. We could see sharp
(
20) Yamaguchi, Y.; Ochi, T.; Miyamura, S.; Tanaka, T.; Kobayashi,
S.; Wakamiya, T.; Matsubara, Y.; Yoshida, Z. J. Am. Chem. Soc. 2006,
28, 4504.
21) Sciano, J. C. Handbook of Organic Photochemistry; CRC Press:
Boca Raton, FL, 1989; Vol. 1, p 231.
1
(
J. Org. Chem, Vol. 72, No. 13, 2007 4729

Adhikari et al.
FIGURE 4. Mataga-Lippert plot of 1 (red) and 2 (blue) (solvents:
toluene, chloroform, methanol, and acetonitrile in the order of increasing
Stokes shifts).
FIGURE 3. Normalized emission spectra of 2 recorded in different
solvents.
concentration quenching at very high concentration only but
no red shift in its emission spectra indicating no aggregation
up to concentrations as high as 10 mmol.
Each dendrimer shows significant solvatochromism. The
emission spectra exhibit a more significant red shift in polar
solvents than in nonpolar solvents (see the Supporting Informa-
tion). For example, the emission maximum of 2 in acetonitrile
(434 nm) is about 75 nm red-shifted from that in pentane (359
nm) (Figure 3). Similarly the emission maxima of 5 gradually
increases going from pentane (385 nm) to acetonitrile (470 nm).
This solvatochromic effect can be attributed to a decrease in
the energy of the singlet excited states as a function of an
increase in the polarity of the solvents.
Typically, a fluorophore has a larger dipole moment in the
excited state than in the ground state. Following excitation,
solvent dipoles can reorient or relax lowering the energy of the
21
excited state. The solvent dependency of emission of 1-5 can
be described by plotting the Stokes shift against orientation
polarizability (∆×c4).22 Mataga-Lippert plots of 1 and 2 are
shown in Figure 4. The slope gives the variation of dipole
moment upon excitation. The linear relationship suggests the
presence of just one excited state. This was also revealed from
the monoexponential decay of the florescence of the dendrim-
FIGURE 5. Normalized solid-state fluorescence spectra of 1-5
recorded in thin films of poly(methyl methacrylate).
16
ers. The solvatochromic effect indicates the dendrimers possess
intramolecular charge transfer from the donor (carbazole) to the
acceptor (ethynyl) units.23 Compounds 3 and 4 have higher
solvatochromic displacements than 2 because the former
compounds have an additional donor acceptor unit. On the other
hand, although 5 also has an additional donor acceptor unit, it
shows less solvatochromic shift. This may be due to 5 being
more symmetrical than 3 and 4, which results in lower excited
24
state dipole of 5 getting less stabilization by the solvent dipole.
Solid-State Photoluminiscence. Solid-state emissions of 1-5
as recorded in poly(methyl methacrylate) (PMMA) matrix
(Figure 5) are structured and similar to those obtained in
solution. However, the emission spectra recorded from thin films
of the dendrimers (Figure 6) are less structured and broader.
FIGURE 6. Normalized solid-state fluorescence spectra of the thin
films of 1-5.
For example, thin films of 1 and 2 show emission from ∼350
nm to more than 550 nm. Interestingly, these broad solid-state
emissions were observed mainly from 1 and 2. Thin films of
(
22) Lakowich, J. R. Principles of Fluorescence Spectroscopy; Kluwer
Academic/Plenum: New York, 1999; p 187.
23) Fu, H.; Wu, H.; Hou, X.; Xiao, F.; Shao, B. Synth. Met. 2006, 156,
09.
24) Thomas, K. R.; Lin, J. T.; Velusamy, M.; Tao, Y.; Chuen, C. AdV.
Funct. Mater. 2004, 14, 83.
(
8
3
-5 showed more structured and narrower emissions like that
(
observed in the PMMA matrix. A head-to-tail compact packing
4730 J. Org. Chem., Vol. 72, No. 13, 2007

Carbazole-Based Blue Light-Emitting Dendrimers
TABLE 3. Emission Maxima (λmax) and Quantum Yields of
Fluorescence (ΦF) of 1-5 Recorded in the Solid State
Conclusions
Carbazole-based monomeric and dimeric dedrimers (1-5)
were synthesized. Photophysical studies of 1-5 indicate that
chromophoric density may be increased in these dendrimers
through a meta-linkage of the dendrons to the central ethnylphe-
nyl core without perturbing the purity of the emission. However,
a para-linkage of dendrons results in a significant red shift in
the emission. Compounds 1-5 showed intense luminescence
both in solution (ΦF ≈ 0.72-0.89) and in the solid state (ΦF ≈
compd
λmax (nm)a
λmax (nm)b
ΦFb
1
2
3
4
5
392
393
392
395
422
406, 431
434
402
392
426
0.40
0.65
0.85
0.62
0.68
a
Thin films of PMMA were used as matrices. b Data obtained from thin
films of 1-5; thin films were prepared by the spin casting method, excitation
wavelength ) 340 nm; the ΦF values were measured by using an integrating
sphere (errors within 15% range).
0
.40-0.85). The solvatochromic effects observed in 1-5 can
be explained on the basis of charge-transfer complex formation
in the excited singlet state. These dendrimers were found to
retain their color purity even when they were exposed to 150 °C
for more than 24 h. The assessment of OLED device perfor-
mance of some dendrimers is underway.
Experimental Section
Instrumentation. Mass spectra were recorded on a Shimadzu
GCMS-QP5050A instrument equipped with a direct probe (ioniza-
tion 70 eV). Matrix assisted laser desorption ionization (MALDI)
spectra were obtained on a Bruker Daltonic Omniflex instrument
2
(N laser, 337 nm). Melting points are uncorrected. A Bruker
1
spectrometer (working frequency 300.0 MHz for H) was used to
record the NMR spectra. CDCl was the solvent for NMR, and
3
chemical shifts relative to tetramethylsilane at 0.00 ppm are reported
in parts per million (ppm) on the δ-scale. Absorption and
fluorescence spectra were recorded on a Shimadzu UV-2401
spectrophotometer and a Fluorolog-3 spectrometer, respectively.
All measurements were carried out at room temperature unless
otherwise specified.
Synthesis. 9-(4-Bromophenyl)carbazole (7a) and 9-(4-bromophe-
nyl)-3,6-di-tert-butylcarbazole (7b) were synthesized following a
literature method13 starting from carbazole (6a).
FIGURE 7. Fluorescence spectra recorded from thin films of PMMA
containing 1: (a) pristine, (b) after exposing the film for more than 15
days at ambient conditions, and (c) after heating at 150 °C for 24 h
and cooling to room temperature.
may be responsible for excimer formation in the case of 1 and
9
-(4-(2-Phenylethynyl)phenyl)-9H-carbazole (1). Compound 7a
2.405 g, 7.5 mmol), trans-dichlorobis(triphenylphosphine)palladium-
II) (0.265 g, 0.375 mmol), triphenylphosphine (0.0975 g, 0.375
2
. Thus, in these cases, the emission can be considered to be
(
(
the combination of the monomeric and excimer emission.
Prohibition of compact packing in the case of 3-5 due to two
bulkier carbazole groups may be the reason why these com-
pounds did not show broader emission.
The quantum yields of fluorescence (ΦF) in the solid state
were measured by using an integrating sphere25 and are
presented in Table 3 along with the emission maxima (λmax).
The solid-state emission maxima of 1 and 2 were substantially
red-shifted compared to those measured in the matrix PMMA,
while such a shift was not observed for 3-5. The solid-state
ΦF values of 1-5 ranged from 0.40 to 0.85 with 1 showing the
lowest and 3 showing the highest.
Effect of Aging and Annealing on the Solid-State Emis-
sion. To evaluate the effect of aging and annealing on the solid-
state emission of 1-5, thin films of PMMA containing the
dendrimers were exposed to ambient light for several weeks as
well as heated to 150 °C for 24 h. The emission spectra of 1
recorded immediately after forming the PMMA thin film,
exposing the film to ambient conditions for two weeks, and
annealing the film at 150 °C for more than 24 h are shown in
Figure 7. There was no change in the emission spectra before
and after such treatment. This indicates that 1 maintains its color
purity under the conditions we used for aging and annealing.
Similar results were obtained for 2-5. This suggests that they
are reasonably stable compounds and suitable for use in OLED
devices.
mmol), CuI (0.07 g, 0.375 mmol), and anhydrous triethylamine
were mixed in a degassed round-bottom flask. The mixture was
heated to reflux. A solution of phenylacetylene (0.765 g, 7.6 mmol)
in 10 mL of tetrahydrofuran (THF) was added to the above mixture.
The mixture was refluxed overnight with stirring. After the reaction
mixture was allowed to cool to room temperature, the solvent was
evaporated under vacuum. The residue was mixed with dichlo-
romethane and washed with water and then with brine. The organic
4
layer was collected and dried under MgSO . After filtration, the
solvent was removed under vacuum. The crude product was purified
by chromatography (silica gel, 80% hexane in dichloromethane)
to obtain pure 1 (0.75 g, 31%) as a yellowish white shining solid.
1
H NMR (300 MHz, CDCl
3
) δ 7.30 (m, 2 H), 7.35-7.38 (m, 3 H),
7
1
.41-7.45 (m, 4H), 7.54-7.6 (m, 4H), 7.75 (m, 2H), 8.14 (d, J )
13
3
0 Hz, 2H); C NMR (300 MHz, CDCl ) δ 89, 91, 110, 120, 122,
122.5, 123, 124, 124, 126, 128, 132, 137, 140; mass spectrum (DIP-
MS) m/z M 343 (100%); HRMS (EI+) m/z 343.1362, calcd m/z
343.1361; mp 134-136 °C.
+
3
,6-Di-tert-butyl-9-(4-(2-phenylethynyl)phenyl)-9H-carba-
zole (2). This compound was obtained in 25% yield as a white
shiny solid, following the same procedure described for the synthesis
1
of 1 with 7b instead of 7a. H NMR (300 MHz, CDCl
3
) δ 1.45 (s,
1
7
3
1
8H), 7.3-7.39 (m, 4H), 7.44-7.5 (m, 1H), 7.52-7.6 (m, 6H),
13
.73-7.78 (m, 2H), 8.15 (s, 2H); C NMR (300 MHz, CDCl
3
) δ
2, 35, 88.5, 90,109, 116.5, 122, 123.5, 124, 126.5, 128, 129, 132,
+
32.5, 133.5, 138, 139, 143.5; mass spectrum (DIP) m/e M 455
(60%), 440 (80%), 185 (100%); HRMS (EI+) m/z 455.2621, calcd
m/z 455.2613; mp 217-220 °C.
9
-(4-(2-(3-(2-(4-(9H-Carbazol-9-yl)phenyl)ethynyl)phenyl)-
(
30.
25) De Mello, J. C.; Wittman, J. C.; Friend, R. H. AdV. Mater. 1997, 9,
2
ethyny) phenyl)-9H-carbazole (3). Compound 3 was obtained
J. Org. Chem, Vol. 72, No. 13, 2007 4731

Adhikari et al.
following the same procedure described for the synthesis of 1 with
,3-diethynylbenzene instead of phenylacetylene in a half molar
ratio. 1,3-Diethynylbenzene was added to 7a, and the mixture was
the fluorescence spectra. Sample solutions were taken in quartz
cuvettes and degassed for ∼15 min. The degassed solutions had
absorbances of 0.05-0.09 at 340 nm. The fluorescence spectra of
each were recorded 3-4 times and an average value of integrated
1
refluxed for 3 days. This gave pure 3 in 20% yield as a white shiny
1
solid. H NMR (300 MHz, CDCl
9
3
) δ 7.3 (m, 4H), 7.4-7.5 (m,
areas of fluorescence used for the calculation of Φ
indices of solvents at the sodium D line were used. The Φ
in the solid state were measured following a literature method. A
F
. The refractive
13
H), 7.6 (m, 6H), 7.8 (m, 5H), 8.15 (d, J ) 10 Hz, 4H); C NMR
300 MHz, CDCl ) δ 89, 119.5, 120, 122, 123.5, 126, 127, 129,
32, 133.5, 135, 138, 141; mass spectrum (MALDI-TOF) m/z M
08; HRMS (FAB+) m/z 608.225, calcd m/z 608.2252; mp 217-
19 °C.
F
values
2
5
(
3
+
1
6
2
concentrated dichloromethane solution of sample was cast as thin
films on quartz plates and then was allowed to dry. The plate was
inserted into an integrating sphere and the required spectra were
recorded. The samples were excited at 340 nm. It is well-known
that for compounds showing an overlap of the absorption and the
emission spectra (a small Stokes shift), the use of an integrating
sphere results in a substantial loss of emission due to reabsorption
3,6-Di-tert-butyl-9-(4-(2-(3-(2-(4-(3,6-di-tert-butyl-9H-carba-
zol-9-yl)phenyl)ethynyl)phenyl)ethynyl)phenyl)-9H-carbazole (4).
Compound 4 was obtained following the same procedure described
for the synthesis of 3. 1,3-Diethynylbenzene was treated with 7b,
and the reaction mixture was refluxed for 3 days to obtain pure 4
of the emitted light. A method employed earlier was used to
1
3
in 20% yield as a white shiny solid. H NMR (300 MHz, CDCl
3
)
minimize the impact of this on the calculation of the Φ
F
.
δ 1.45 (s, 36H), 7.38-7.42 (m, 5H), 7.47-7.51 (m, 4H), 7.53-
.62 (m, 7H), 7.75-7.8 (m, 4H), 8.15 (s, 4H); 13C NMR (300 MHz,
CDCl ) δ 32, 35, 89.5, 90, 109, 110, 116.5, 120.5, 121, 124, 126,
26.5, 127, 128, 132, 133.5, 135, 138, 138.5, 143.5; mass spectrum
Fluorescence Lifetime (τ
F
) Measurement. Solutions of 1-5
7
that showed absorbances of 0.1-0.25 at 340 nm were placed in
quartz cuvettes. Fluorescence decay profiles of argon-degassed (∼15
min) solutions were recorded with use of a single photon counting
spectrofluorimeter. Decays were monitored at the corresponding
emission maximum of the compounds. In-built software allowed
3
1
+
(
MALDI-TOF) m/z M 833; HRMS (FAB+) m/z 832.4756, calcd
m/z 832.4754; mp 202-205 °C.
,6-Di-tert-butyl-9-(4-(2-(4-(2-(4-(3,6-di-tert-butyl-9H-carba-
2
3
the fitting of the decay spectra (ø ) 1-1.5) and yielded the
zol-9-yl)phenyl)ethynyl)phenyl)ethynyl)phenyl)-9H-carbazole (5).
Compound 5 was obtained by reaction of 1,3,5-triethynylbenzene
fluorescence lifetimes.
Acknowledgment. We thank Dr. T. H. Kinstle and Dr.
Jianmin Shi for valuable discussions. This work has been
partially supported by ONR N0014-05-1-0372 and the Ohio
Department of Development.
(
in one-third molar ratio) with 7b. The mixture was refluxed for 7
1
days to obtain pure 5 in 18% yield as a white shiny solid. H NMR
(
(
300 MHz, CDCl
m, 8H), 7.72-7.78 (m, 4H), 8.15 (s, 4H); C NMR (300 MHz,
) δ 32, 35, 89.5, 90, 109, 117, 123, 127, 132, 133, 139, 143;
3
) δ 1.45 (s, 36H), 7.38-7.52 (m, 8H), 7.58-7.6
13
CDCl
mass spectrum (MALDI-TOF) m/z M 833, HRMS (FAB+) m/z
32.4756, calcd m/z 832.4754; mp >360 °C.
Fluorescence Quantum Yields (Φ ). The Φ
were measured following a general method with 9,10-diphenylan-
thracene (Φ ) 0.9 in cyclohexane) as the standard. Diluted
solutions of 1-5 in appropriate solvents were used for recording
3
Supporting Information Available: 1H NMR and 13C NMR
spectra of 1-5, UV-vis and fluorescence spectra of 1-5 recorded
in different solvents, and fluorescence decay profiles of 1-5 in
dichloromethane. This material is available free of charge via the
Internet at http://pubs.acs.org.
+
8
F
F
values in solution
F
JO0703911
4732 J. Org. Chem., Vol. 72, No. 13, 2007