Photophysical study of blue, green, and orange-red light-emitting carbazoles

Article abstract of DOI:10.1021/jo9002757

Simple synthetic procedures have been developed to prepare suitably substituted stable carbazoles B1 - B3, G1-G3, and R1-R3. These compounds emit blue, green, and orange-red light, respectively, and show a red-shifted emission in the solid state relative to that in solution. The extent of the shift is highly dependent on the nature and the positions of the substituents. A red-shift as high as 120 nm can be achieved by a suitable substitution, especially by N-substitution of carbazole. The presence of a carbaldehyde or malononitrile group on the carbazole moiety is found to quench fluorescence severely in solution and in the solid state, as indicated by low fluorescence quantum yields of Bl (φ_F ～0.03), B3 (φ_F ～0.04), and Gl-G3 (φ_F ～0.04-0.15). However, the effect is not the same for the fluorescence lifetime (τ_F ～1- 5.69 ns). The rate constants of radiative and nonradiative deactivation of Bl-R3 have been found to be in the range of 6.40 × 10⁶ to 9.50 × 10⁸ and 1.38 x 108 to 9.84 × 10⁸, respectively. Lowering the temperature from 25 to -10 °C causes a small but distinct red-shift in the emissions and a systematic increase in the φ_F values of the blue and green emitters. Solvatochromism and concentration- dependent emissions of the compounds are also discussed.

Full text of DOI:10.1021/jo9002757

Photophysical Study of Blue, Green, and Orange-Red
Light-Emitting Carbazoles1
Ravi M. Adhikari and Douglas C. Neckers*
Center for Photochemical Sciences, Bowling Green State UniVersity, Bowling Green, Ohio 43402
Bipin K. Shah*
Department of Chemistry, Pittsburg State UniVersity, Pittsburg, Kansas 66762
bshah@pittstate.edu; neckers@photo.bgsu.edu
ReceiVed February 6, 2009
Simple synthetic procedures have been developed to prepare suitably substituted stable carbazoles B1-B3,
G1-G3, and R1-R3. These compounds emit blue, green, and orange-red light, respectively, and show
a red-shifted emission in the solid state relative to that in solution. The extent of the shift is highly
dependent on the nature and the positions of the substituents. A red-shift as high as 120 nm can be
achieved by a suitable substitution, especially by N-substitution of carbazole. The presence of a
carbaldehyde or malononitrile group on the carbazole moiety is found to quench fluorescence severely
in solution and in the solid state, as indicated by low fluorescence quantum yields of B1 (φF ∼0.03), B3
(φF ∼0.04), and G1-G3 (φF ∼0.04-0.15). However, the effect is not the same for the fluorescence
lifetime (τF ∼1-5.69 ns). The rate constants of radiative and nonradiative deactivation of B1-R3 have
been found to be in the range of 6.40 × 106 to 9.50 × 108 and 1.38 × 108 to 9.84 × 108, respectively.
Lowering the temperature from 25 to -10 °C causes a small but distinct red-shift in the emissions and
a systematic increase in the φF values of the blue and green emitters. Solvatochromism and concentration-
dependent emissions of the compounds are also discussed.
Introduction
They also undergo reversible oxidation processes that makes
them suitable hole carriers.20-22 The molecular and optical
properties of carbazole can be tuned by structurally modifying
its 2, 3, 6, 7, and 9H positions.23-25 In fact, the light-emitting
property combined with the hole-transporting capability of
The application of organic electroluminescent materials in
flat-panel displays has led to an intensive search for stable
organic materials that can emit light with high efficiency and
brightness.2-7 A set of pure red, green, and blue emitters is
required for full-color displays. Carbazoles are known for intense
luminescence and are widely used in organic light-emitting
diodes (OLEDs) as blue, green, red, and white emitters.8-19
(3) Shinar, J. Organic Light-Emitting DeVices: Springer-Verlag: New York,
2003.
(4) Organic Light-Emitting DeVices. Synthesis Properties and Applications;
Mullen, K., Scherf, U., Eds.;Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim,
Germany, 2006.
(1) Contribution no. 739 from the Center for Photochemical Sciences.
(2) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913.
(5) Yu, M.-X.; Chang, L.-C.; Lin, C.-H.; Duan, J.-P.; Wu, F.-I.; Chen, I.-C.;
Cheng, C.-H. AdV. Funct. Mater. 2007, 17, 369.
10.1021/jo9002757 CCC: $40.75
Published on Web 03/30/2009
2009 American Chemical Society
J. Org. Chem. 2009, 74, 3341–3349 3341

Adhikari et al.
carbazoles make them one of the important classes of com-
pounds that have been actively pursued for applications in
OLEDs and other optoelectronic devices.
Although studies on carbazoles have appeared in the literature
over the past few years, they are mostly focused on their use as
materials and applications in devices.8-22 We have developed
a new series of carbazoles (Figure 1, B1-B3, G1-G3, and
R1-R3) that exhibit stable blue, green, and orange-red emission
in solution and in the solid state. This prompted a systematic
study of B1-B3, G1-G3, and R1-R3 in an attempt to
understand the photophysical properties and structure-property
relationships in these compounds. While B1 and B3 each possess
mild electron-withdrawing (EW) carbaldehyde groups, B2 lacks
such a group. Compounds G1-G3 and R2-R3 are substituted
with strong EW malononitrile groups at various positions. It
was observed that the effect of the extension of conjugation on
the emission is more profound when an appropriate substituent
is at the N-position than at the 3 and 6 positions. We also report
on the solvatochromic properties, concentration-dependent
fluorescence switching, and the temperature-dependent fluores-
cent behavior of these new systematically substituted carbazoles.
Compounds B1-B3 emit intense blue light both in solution
and in the solid state, while compounds containing the mal-
ononitrile group attached to the carbazole moiety (G1-G3)
show green emission. It is interesting that R1, which is the
immediate synthetic precursor of R2, shows an orange emission
in solution but a bluish-green emission in the solid state, whereas
R2 emits orange in solution and almost red in the solid state.
On the other hand, R3 shows a green emission in dichlo-
romethane and orange-red in the thin film. Moreover, R2 and
R3 are rather different from B1-B3, G1-G3, and R1 in that
the former compounds readily form nanoparticles under our now
well-defined tetrahydrofuran(THF)/water system and their emis-
sion can be switched on and off by manipulating the THF/water
ratio.26 However, the latter compounds show no such behavior.
FIGURE 1. Structure of blue-, green-, and red-emitting carbazoles.
Results and Discussion
Synthesis. Compounds B1-B3were synthesized starting from
carbazole (1) (Scheme 1). The first step involved N-substitution
of 1by the naphthyl and phenanthrynyl groups resulting in 2
and 3, respectively. Bromination and iodination of 2 and 3 at
the 3 and 6 positions provided 4-6. Compound 4 was treated
with n-butyllithium and dimethylformamide resulting in B1.
Sonogashira coupling of 5 with phenylacetylene proved to be a
difficult reaction. Thus, iodo-substituted 6 was synthesized, and
this easily yielded B2. The reaction of 1 with 4,4′-dibromo-
bibenzene (7) in the presence of potassium carbonate yielded
8. Formylation of the latter introduces the aldehydic function-
alities at the 3 and 3′ positions of carbazole, forming B3.
(6) Shah, B. K.; Neckers, D. C.; Shi, J.; Forsythe, E. W.; Morton, D. J. Phys.
Chem. A 2005, 109, 7677.
(7) Thomas, K. R.; Lin, J. T.; Tao, Y. T.; Ko, C. J. Am. Chem. Soc. 2001,
123, 9404.
(8) Yu, H.; Zain, S. M.; Eigenbrot, I. V.; Phillips, D. Chem. Phys. Lett. 1993,
202, 141.
(9) Howell, R.; Taylor, A. G.; Phillips, D. Chem. Phys. Lett. 1992, 188, 119.
(10) Almeida, K. D.; Bernede, J. C.; Marsillac, S.; Godoy, A.; Diaz, F. R.
Synth. Met. 2001, 122, 127.
(11) Lee, J.; Woo, H.; Kim, T.; Park, W. Opt. Mater. 2002, 21, 225.
(12) Ding, J.; Gao, J.; Cheng, Y.; Xie, Z.; Wang, L.; Ma, D.; Jing, X.; Wang,
F. AdV. Funct. Mater. 2006, 16, 575.
(13) Morin, J.; Boudreault, P.; Leclerc, M. Macromol. Rapid Commun. 2002,
23, 1032.
(14) Grigalevicius, S.; Ma, L.; Xie, Z.; Scheri, U. J. Polym. Sci., Part A:
Polym. Chem. 2006, 44, 598713.
(15) Liu, Y.; Nishiura, M.; Wang, Y.; Hou, Z. J. Am. Chem. Soc. 2006, 128,
5592.
(16) Guan, M.; Chen, Z.; Bian, Z.; Liu, Z.; Gong, G.; Baik, W.; Lee, H.;
Huang, C. Org. Electron. 2006, 7, 330.
(17) Adhikari, R. M.; Mondal, R.; Shah, B. K.; Neckers, D. C. J. Org. Chem.
2007, 72, 4727.
(18) Yu, M.-X.; Chang, L.-C.; Lin, C.-H.; Duan, J.-P.; Wu, F.-I.; Chen, I.-
C.; Cheng, C.-H. AdV. Funct. Mater. 2007, 17, 369.
(19) Zhao, Z.; Li, J.-H.; Chen, X.; Wang, X.; Lu, P.; Yang, Y. J. Org. Chem.
2009, 74, 383.
(20) Loiseau, F.; Campagna, S.; Hameurlaine, A.; Dehaen, W. J. Am. Chem.
Soc. 2005, 127, 11352.
(21) Jiang, X.; Register, R. A.; Richard, A.; Killeen, K. A.; Thompson, M. E.;
Pschenitzka, F.; Sturm, J. C. Chem. Mater. 2000, 12, 2542.
(22) Tao, Y.; Wang, Q.; Yang, C.; Wang, Q.; Zhang, Z.; Zou, T.; Qin, J.;
Ma, D. Angew. Chem., Int. Ed. 2008, 47, 8104.
Reaction of either of 4 and 5 with BuLi and DMF in ether
(formylation) resulted in a mixture of mono- and diformylated
products (Scheme 2) that were separated using column chro-
matography. Treatment of monoformylated products (9 and B1)
with malononitrile and basic aluminum oxide in toluene
produced G1 and G2, respectively. A similar reaction of 3,6-
diformylated N-naphthyl product (10) yielded G3. The syntheses
of R1-R3 are described elsewhere.26
Absorption and Emission Spectra in Solution. Representa-
tive examples of the absorption and emission spectra recorded
in dichloromethane are shown in Figures 2 and 3, respectively.
(25) Zhu, Z.; Moore, J. S. Macromolecules 2000, 33, 801.
(26) Adhikari, R. M.; Shah, B. K.; Palayangoda, S. S.; Neckers, D. C.
Langmuir 2009, 25, 2402.
(23) Joule, J. A. AdV. Heterocycl. Chem. 1984, 35, 83.
(24) Zhu, Z.; Moore, J. S. J. Org. Chem. 2000, 65, 116.
3342 J. Org. Chem. Vol. 74, No. 9, 2009

Light-Emitting Carbazoles
SCHEME 1. Synthesis of B1-B3a
FIGURE 3. Normalized emission spectra of B1, B3, G1, G3, R1, and
R2 recorded in dichloromethane.
B1-B3 and R1. The low energy absorption band in G1-G3
and R1-R3 underwent a small but gradual red-shift when
solvents of increasing polarity were used (see Supporting
Information). This further supports the assignment of the low
energy band to the ICT transition in these molecules. An ICT
state is expected to be more stable in polar solvents. On the
other hand, the high energy absorption bands showed no
dependency on the solvent polarity. The extinction coefficients
(ε) of absorption at the peak corresponding to the π-π*
a Reactions and conditions: (a) 1-bromonaphthalene or 9-bromophenan-
threne, K2CO3, nitrobenzene, reflux 48 h; (b) N-bromosuccinimide, dichlo-
romethane; (c) n-BuLi, ether, DMF, -70 °C; (d) N-iodosuccinimide,
dichloromethane; (e) phenylacetylene, Pd(PPh3)2Cl2, CuI, PPh3, triethy-
lamine, reflux 12 h; (f) 4-4′-dibromobiphenyl (7), K2CO3, nitrobenzene,
reflux 3 days; (g) POCl3, DMF, 1,2-dichloroethane, reflux 12 h.
SCHEME 2. Synthesis of G1-G3a
transition were in the range of 12 500-48 000 L mol-1 cm-1
,
except for G2 and G3. The ε values of the latter compounds
(293 000 and 110 000 L mol-1 cm-1, respectively) were much
higher.
The emission spectra of B1-B3, G1-G3, and R1-R3
recorded in dichloromethane extend from the violet-blue to
orange-red regions. The samples were excited at wavelengths
corresponding to the π-π* transition to make sure that the
emission originated from the singlet excited state. One would
assume that the presence of the ethynylphenyl groups para to
nitrogen in carbazole should increase the effective conjugation
length in the molecule. Nevertheless, the emission maxima of
B1-B3 (λmax ) 392-405 nm) were similar, indicating that
substitution of carbazole with either a smaller carbaldehyde
group or conjugation-extending ethynylphenyl groups at the 3
and 6 positions causes no significant red-shift. It may be possible
that excitation to the S1 state is principally located at the nitrogen
atom in the carbazole moiety in these molecules. In other words,
the electronic density concentrated around nitrogen contributes
significantly more to the HOMO and LUMO orbitals than that
around the carbon atoms at 3 and 6 positions.
a Reactions and conditions: (a) n-BuLi, DMF, ether, -70 °C; (b)
malononitrile, toluene, basic Al2O3, reflux 6 h.
The geometry optimized structures of B1 and B2 show the
phenynthrynyl group almost orthogonal to the carbazole moiety
(Supporting Information). Interestingly, even though the ethy-
nylphenyl group and the carbazole moiety are coplanar in B2,
its emission (λmax ) 392 nm) is not red-shifted significantly
relative to that of B1 (λmax ) 402 nm). This reinforces that
extension of conjugation due to the ethynylphenyl groups at
the 3 and 6 positions imparts a negligible effect on the emission.
Nevertheless, N-substitution by the ethynylphenyl group that
contains an aldehydic or malononitrile group causes a large red-
shift in the emission (vide infra). This suggests that the extent
of electron delocalization through the 9 position in carbazoles
is significantly higher than that through the 3 or 6 positions.
This is similar to the observations that electron transfer is more
FIGURE 2. Absorption spectra of B1, B3, G1, G3, R1, and R2
recorded in dichloromethane.
The absorptions of compounds that have a weak EW group (B1,
B3, and R1) or no EW group (B2) are similar to that of
carbazole, each comprising two absorption maxima (λmax ∼290
nm and λmax ∼320-350 nm). The latter absorption can be
assigned to the π-π* transition. The absorptions of G1-G3
and R2-R3 are different in that each of them shows an
additional absorption maximum. The low energy absorption
(λmax ∼380-420 nm) of G1-G3 and R2-R3 can be assigned
to an intramolecular charge-transfer (ICT) transition.18,26,27 In
other words, formation of the ICT state was not observed in
(27) Nikolaev, A. E.; Myszkiewicz, G.; Berden, G.; Meerts, W. L.; Pfanstiel,
J. F.; Pratt, D. W. J. Chem. Phys. 2005, 122, 84309.
J. Org. Chem. Vol. 74, No. 9, 2009 3343

Adhikari et al.
TABLE 1. Photophysical Properties of B1-B3, G1-G3, and R1-R3 Recorded in Dichloromethane and in Thin Filmsa
solution
solid
0
0
compound
Amax (nm)
λmax (nm)
τF (ns)
φF
kR 10-7 (s-1
)
kNR 10-8 (s-1
)
λmax (nm)
φF
B1
B2
B3
G1
G2
G3
R1
R2
R3
290, 321
299, 330
292, 329
294, 323, 406
292, 321, 404
316, 356, 418
293, 354
296, 350, 410
350, 382
402
392
405
480
473
453
528
534
474
4.68
4.27
∼1
0.03 ( 0.01
0.32 ( 0.03
0.04 ( 0.01
0.04 ( 0.01
0.05 ( 0.01
0.07 ( 0.01
0.10 ( 0.01
0.40 ( 0.03
0.30 ( 0.02
0.64
10.5
4.10
0.85
5.00
7.00
1.78
9.50
8.10
2.06
2.23
9.84
2.04
9.50
9.30
1.60
1.42
1.80
418
400
418
515
502
525
445
600
594
0.04 ( 0.01
0.45 ( 0.06
0.06 ( 0.01
0.11 ( 0.02
0.12 ( 0.02
0.15 ( 0.02
0.21 ( 0.03
0.34 ( 0.05
0.25 ( 0.04
4.68
<1
<1
5.69
4.20
3.70
a Compounds were excited at the corresponding Amax representing the π-π* transition for τF and φF. The φF and τF values were measured from
argon-saturated solutions, and decay was monitored at the corresponding λmax. The optical density of the samples for τF and φF measurements were less
than 0.13 and 0.09, respectively. The φF values for B1-B3 and G1-G3 are relative to that of diphenylanthracene (0.90 in cyclohexane). The φF values
for R1-R3 are relative to that of riboflavin (0.3 in ethanol).
efficient when an electron-accepting group is linked to nitrogen
(9 position) in carbazoles.28,29
causing a blue-shifted emission from G3. It is also likely that
the order of the emission wavelength (G1 > G2 > G3) is related
with the propensity of the ICT complex of these molecules.
For example, since G1 has a better electron-donating group
(phenanthryl) than G2 and G3 (naphthyl), the propensity of the
ICT complex may be higher in G1 than in G2. Although the
phenanthryl and naphthyl groups are almost orthogonal to the
carbazole moiety, these groups do contribute to the electronic
density of the carbazole nucleus.
The malononitrile group also causes a significant decrease
in the fluorescence quantum efficiencies similar to the carbal-
dehyde group. The φF values of G1-G3 (0.04-0.07) were
found to be much lower than that of carbazole (0.42).32 Thus,
these compounds exhibit a behavior similar to that of other
systems where quenching of fluorescence has been observed to
be caused by EW groups such as the cyano group.33,34 The effect
of the malononitrile group on the fluorescence lifetime (τF) was,
however, not similar. Compounds G2 and G3 exhibited
abnormally short lifetime τF (<1 ns), while that of G1 was ∼4.68
ns.
Significant differences were observed in the fluorescence
quantum yields (φF) of B1-B3 (Table 1). The φF values of B1
(0.03) and B3 (0.04) were smaller by an order of magnitude
than that of B2 (0.32). Carbaldehyde substitution significantly
decreases the fluorescence quantum yield. Since there is no ICT
complex forming in B1-B3, the quenching of the fluorescence
cannot be attributed to a charge transfer process. This may be
related with intramolecular electron transfer or free bond rotation
around C-CHO causing excitation diffusion and deactivation.30
In other words, rotational deactivation increases the chance of
nonradiative relaxation of the excited singlet state, resulting in
the decrease in the φF value. The effect of the carbaldehyde
group on the fluorescence lifetime (τF) was not the same.
Although the τF value of B3 (1 ns) was abnormally short, the
τF value of B1 (4.68 ns) was found to be similar to that of B2
(4.27 ns).
The difference in the structure of B1 and G1 is that the latter
contains a malononitrile group instead of a carbaldehyde group
(Figure 1). However, the emission of G1 (λmax ) 480 nm) was
red-shifted by about 80 nm relative to that of B1 (λmax ) 402
nm). As pointed out above, the additional conjugation provided
by the malononitrile groups in G1 does not account for such a
significant red-shift in the emission. If this were the case, the
emission of G3 would have been red-shifted in comparison to
that of G2 because the former contains one malononitrile group,
whereas the latter has two such groups. Since both malononitrile
groups are in conjugation with nitrogen of carbazole, the
electronic propensity of the carbazole nucleus should have
extended more in G3 than in G2. In contrast, the emission of
G3 (λmax ) 454 nm) is actually blue-shifted relative to that of
G2 (λmax ) 473 nm).
We have earlier shown that N-phenylethynylphenyl-substi-
tuted carbazoles exhibit an intense blue emission in dichlo-
romethane.17 However, the presence of a carbaldehyde or
malononitrile substitutent on the N-ethynylphenyl group com-
pletely changes the emission behavior, as exhibited by the large
red-shifted emission of R1 and R2. For example, R1 (λmax
)
528 nm) and R2 (λmax ) 534 nm) emit orange light in
dichloromethane, whereas 3,6-di-tert-butyl-9-(4-(2-phenylethy-
nyl)phenyl)-9H-carbazole, an analogue of R1 and R2 without
the carbaldehyde or the maloninitrile group, emits blue (λmax
)
410 nm).17 The emission behavior of R3 is more interesting. It
emits bluish-green light (λmax ∼474 nm) in dichloromethane,
although it contains an additional ethynylphenyl group relative
to R1 and R2. The emission maximum of R3 is blue-shifted
about 55-65 nm relative to those of R1 and R2. The emission
of R2 originating from the ICT state and that of R3 originating
from the singlet excited state does not appear to be the reason.
The effect of the malononitrile group in causing a blue-shift
in the emission is probably associated with its ability to influence
the polarity of the excited singlet state. This is also supported
by the strong solvatochromic effect observed in these com-
pounds (vide infra). The singlet states of G1 and G2 are less
polar than that of G3 due to the presence of two EW
malononitrile groups in the latter.31 A highly polar singlet state
would be less stabilized in relatively nonpolar dichloromethane,
(31) The free rotation of the C-C single bond between the phenyl and the
two malononitrile groups in G3 allows the latter to alternately come close together
and go far away from each other. Such a rotation may lead to a higher resultant
dipole in the ground state of G3 than in the ground state of G2, which has only
one malononitrile group. The order of the dipole in the ground state may be
retained in the singlet excited states of these componds.
(32) Handbook of Organic Photochemistry; Scaiano, J. C., Ed.; CRC Press:
Boca Raton, FL, 1989; Vol. I, p 22.
(33) Michel, B.; Jean-Francois, M.; Mario, L.; Gilles, D. J. Phys. Chem. A
2005, 109, 6953.
(34) Li, C.-L.; Shieh, S.-J.; Lin, S.-C.; Liu, R.-S. Org. Lett. 2003, 5, 1131.
(28) Zotti, G.; Schiavon, G.; Zecchin, S.; Morin, J.-F.; Leclerc, M. Macro-
moelcules 2002, 35, 2122.
(29) Kapturkeiwicz, A.; Herbich, J.; Karpiuk, J.; Npwacki, J. J. Phys. Chem.
A 1997, 101, 2332.
(30) Lakowicz, J. R. Principles of Fluorescence Spectroscopy: Springer: New
York, NY, 2006; p 278.
3344 J. Org. Chem. Vol. 74, No. 9, 2009

Light-Emitting Carbazoles
red-shift in solid-state emission results from interaction of
neighboring polar substituents since the compounds are more
polar in the singlet excited states than in the ground states (vide
infra). Intermolecular interaction of neighboring polar substit-
uents lowers the energy level of the excited states. This also
explains why the extent of the red-shift was smaller for B1-B3
(8-16 nm), which have either weak or no EW substituents.
Compounds G1-G3 contain more polar malononitrile groups
and exhibit a moderate red-shift (∼30 nm). The red-shift was
large in the cases of R2 (66 nm) and R3 (120 nm). On the
other hand, R1 is strikingly different in that its emission showed
a blue-shift of ∼85 nm, even though the structures of R1 and
R2 are similar. The replacement of the carbaldehyde group in
R1 (λmax ) 445 nm) with a malononitrile group R2 (λmax
)
600 nm) does not electronically account for the emission
difference in the solid state. However, it is possible that these
groups may induce formation of different types of aggregates
in the solid state. Since R2 is more polar than R1, the different
aggregate formation is likely due to their different dipole
moments. Compound R1 probably forms H-aggregates in the
solid state resulting in a blue-shift, whereas R2 undergoes
J-aggregation that accounts for the red-shift.35,36
FIGURE 4. Normalized emission spectra of thin films of B1, B3, G1,
G3, and R1-R3.
If this were the case, the emission of R1 would not have been
highly red-shifted because there is no formation of the ICT state
in R1.
The additional ethynylphenyl group in R3 should increase
its rotational flexibility. However, the geometry optimized
structure of R3 shows that the long 1,4-diethynylphenylbenzene
side chain is flat in this molecule. Thus, it may be that the overall
molecular rigidity increases in R3 compared to that in R1 or
R2 due to the additional ethynylphenyl group, which may be
the cause of the blue-shifted emission of the former. Nonethe-
less, less communication between the carbazole moiety and the
diethynylphenylbenzene side chain and both behaving as
separate chromophoric units in R3 appears to be a more
reasonable explanation. The blue-greenish emission of R3 is
broader than that of R1 and R2 and may have contribution from
both chromophoric units. The observance of two distinct
fluorescence lifetimes in the case of R3 further supports this
explanation (vide infra). It is also noted that the absorption of
R3 is structureless and much broader than those of B1-B3,
G1-G3, or R1-R2 (see Supporting Information).
The solid-state emission behavior of R3 seems interestingly
different and needs special mention. There is no apparent
difference in the structure of R2 and R3, except that the later
contains an additional ethynylphenyl group, but the red-shift
observed in the case of R3 (120 nm) is almost double that
observed in the case of R2 (66 nm). The huge red-shift in the
solid-state emission of R3 is partly due to the fact that the
solution absorption of R3 is significantly blue-shifted compared
to that of R2. Nonetheless, it is clear that emissions in solution
and in the solid state are from different excited states of R3. It
appears that R3 in solution emits from the π-π* state of the
discrete carbazole moiety and the phenylethynylphenyl side
chain (vide supra), but from the ICT state in the solid state. It
is also possible that, while both R2 and R3 probably form
J-aggregates in the solid state, the latter may be more closely
stacked. The presence of the additional ethynyl group and the
longer flat portion of the molecule provide a possibility of
greater interamolecular π-interaction in the molecules of R3,
resulting in highly red-shifted emission in the solid state. It is
unfortunate that we could not obtain crystal structures of these
compounds because of their amorphous nature and cannot
comment on the exact nature of the molecular stacking in the
solid state.
All of these compounds, except for G3, exhibit an excimer
emission. Very weak excimer emission was observed in the case
of G3 (630 nm). The φF values of B1-R3 in the solid state
were found similar to the corresponding values observed in
dichloromethane (Table 1). The φF values of B2 (0.45) and
R1-R3 (0.21-0.34) were reasonable, but B1, B3, and G1-G3
showed very low φF values (<15%), indicating that the presence
of the carbaldehyde or the malonitrile group on the carbazole
moiety severely quenches the fluorescence. However, the φF
values of R1-R3 (0.21-0.34) suggest that the presence of these
groups on the side ethynylphenyl function fails to quench
fluorescence in the solid state.
The φF values of R1 (∼0.10), R2 (∼0.40), and R3 (∼0.30)
were found to be larger than those of G1-G3 (∼0.04-0.07).
Compound R1 showed a longer τF (5.69 ns) compared to those
of R2 (τF ∼4.20 ns) and R3 (τF ∼3.70 ns). In fact, the
fluorescence decays of R1-R3 were biexponential, whereas
those of B1-B3 or G1-G3 were monoexponential. However,
the lifetimes of the shorter components of R1 and R2 were
observed to be too low (<1 ns) and close to the instrument
response time to have any physical significance. The two distinct
lifetimes (1.80 and 3.70 ns) observed in the case of R3 are most
probably due to two separate singlet states associated with the
carbazole and 1,4-diethynylphenylbenzene moieties. It should
be noted that there is no systematic difference in the rate
0
constants of radiative deactivation (kR ) of the three classes of
0
compounds. For example, the kR values of B1-B3 (∼6.40 ×
106 to 1.49 × 108 s-1), G1-G3 (∼8.54 × 106 to 7.00 × 107
s-1), and R1-R3 (∼1.78 × 106 to 9.50 × 108) are in the same
range, although the difference between individual compounds
is as high as 2 orders of magnitude.
Emission in the Solid State. The emissions of all compounds
except R1 in the solid state were found to be red-shifted
compared to the corresponding emission in solution (Figure 4).
This is most probably due to molecular stacking in the solid
state. Solid-state packing has, however, little effect on the shape
of the emission because the thin film emissions of each were
similar to the corresponding emissions in dichloromethane. The
Solvatochromism. A solvatochromic effect was observed for
those compounds (B1, B3, and G1-G3) that contain EW groups
(35) Palayangoda, S. S.; Cai, C.; Adhikari, R. M.; Neckers, D. C. Org. Lett.
2008, 10, 281.
(36) Oelkrug, D.; Tompert, A.; Gierschner, J.; Egelhaaf, H.-J.; Hanack, M.;
Hohloch, M.; Steinhuber, E. J. Phys. Chem. B 1998, 102, 1902.
J. Org. Chem. Vol. 74, No. 9, 2009 3345

Adhikari et al.
TABLE 2. Absorption Maxima (Amax), Emission Maxima (λmax), and Fluorescence Quantum Yields (OF) of B1, G1, and R2 Measured in
Different Solventsa
B1
G1
λmax (nm)
R2
λmax (nm)
solvent
Amax (nm)
λmax (nm)
φF
Amax (nm)
φF
Amax (nm)
φF
pentane
toluene
dichloromethane
acetonitrile
313
316
320
314
376
380
402
416
0.025
0.022
0.020
0.015
393
402
405
395
451
445
481
503
0.16
0.16
0.11
0.09
345
348
350
346
448
554
535
490
0.50
0.48
0.40
0.33
a Excitation wavelengths: B1, λex ) 320 nm; G1, λex ) 320 nm; and R2, λex ) 350 nm.
FIGURE 7. Emission spectra of R3 recorded in dichloromethane at
different concentrations (λex ) 380 nm). Inset: enlarged spectrum for
the concentration of 2.6 × 10-4 M, which appears as a straight line in
the main figure.
FIGURE 5. Normalized emission spectra of B1 recorded in different
solvents.
(Table 2). Nonetheless, the presence of the EW groups does
account for the solvatochromic effect shown by these com-
pounds. Compound B2 with no EW group shows no significant
solvatochromism (see Supporting Information). The emission
maximum of B2 in pentane (λmax ) 386 nm), for example, is
similar to that observed in acetonitrile (λmax ) 390 nm). The
cases of R1-R3 were different, and a systematic solvatochromic
effect was not observed for these compounds. Interestingly, R2
shows a negative solvatochromic effect from toluene to dichlo-
romethane (λmax ) 535 nm) and acetonitrile (λmax ) 490 nm),
although it shows a large red-shift from pentane (λmax ) 448
nm) to toluene (λmax ) 554 nm). Such a behavior of R1-R3 is
probably due to formation of a twisted ICT in their excited
singlet states.18,27 N-Substituted carbazoles that have an elon-
gated structure similar to that of R1-R3 are known to form
twisted ICT excited states that behave differently in different
solvents.18,27
The φF values of B1, G1, and R2, three representative
examples, are included in Table 2. The φF values decrease
slightly going from a nonpolar to a polar solvent. The φF value
of B1 in pentane (0.025), for example, was slightly higher than
that in acetonitrile (0.015). Similarly, the φF value of R2
decreased from 0.50 in pentane to 0.33 in acetonitrile.
Concentration-Dependent Fluorescence Switching. A pro-
nounced concentration-dependent fluorescence switching was
observed in R3 (Figure 7). Such a striking behavior was not
shown by other compounds studied. A dilute solution of R3
(4.10 × 10-9 M in dichloromethane) emits blue (λmax ∼430
nm). However, the emission shows a red-shift and becomes
structureless at higher concentrations. The latter emission can
be assigned to the excimer state. The emission becomes green
and the intensity keeps increasing in the concentration range of
FIGURE 6. Normalized emission spectra of G1 recorded in different
solvents.
on the carbazole. However, the effect was moderate, again
suggesting that the EW groups at the 3 and 6 positions are not
significantly involved in intermolecular charge distribution in
the excited state. Representative examples of normalized
fluorescence of B1 and G1 recorded in hexanes, toluene,
dichloromethane, and acetonitrile are shown in Figures 5 and
6. The emission maxima were gradually red-shifted when the
compounds were dissolved in the nonpolar pentane to the polar
acetonitrile. The ground states of these compounds are polar
due to varying degrees of charge separation in these molecules.
It is clear that the singlet excited states are more polar than the
ground states. The increased polarity of the singlet states stems
from further charge separation following excitation. This
solvatochromic effect is attributed to the decrease of the singlet
excited-state energy with the increases of the solvent polarity.
It is noted that the extent of the red-shift from pentane to
acetonitrile is almost similar for B1 (40 nm) and G1 (52 nm),
although the latter contains a more polar group than the former
3346 J. Org. Chem. Vol. 74, No. 9, 2009

Light-Emitting Carbazoles
FIGURE 8. Emission spectra recorded from thin films of R2: (a)
pristine, (b) after exposing the film for more than 4 weeks at ambient
condition, and (c) after heating at 150 °C for 24 h and cooling to room
temperature.
FIGURE 9. Emission spectra of B1 recorded at different temperatures.
TABLE 3. Emission maxima (λmax) and Fluorescence Quantum
Yields (OF) of B1 and G3 Recorded at Different Temperaturesa
10-6 to 10-5 M. In fact, the emission was green (λmax ∼475
nm) at a concentration as high as 2.60 × 10-4 M, but severe
quenching of fluorescence was observed when the concentration
was further increased. For example, the emission intensity was
negligible at a concentration of 5.30 × 10-3 M. This indicates
that the excimer emission is facilitated by an increase in
concentration to a certain extent. Since the solution emission
of R3 originates from the π-π* state, a large increase in
concentration quenches such an excited state.
Interestingly, the fluorescence of this compound was not
quenched in the solid state, even though the molecules are closer
than in solution. R3 contains an elongated, flat side chain
comprised of multiple ethynylphenyl groups. As its concentra-
tion is increased, molecules of R3 come in close proximity
forming stacked structures even in solution. This explains why
the compound exhibits significantly red-shifted emission in
concentrated solution and in the solid state (120 nm). Although
fluorescence was severely quenched at higher concentrations
in solution, the φF measured in the solid state (0.25) was similar
to that recorded in dichloromethane (φF ) 0.30) at a lower
concentration. This again indicates that the solid-state emission
of R3 originates from the ICT state. An emission from the ICT
state should not be quenched by the stacking of the molecules.
A π-π interaction would rather facilitate the formation of the
ICT state.
Effect of Aging and Annealing. Thin films of B1-R3 were
exposed to ambient light for several weeks, as well as being
heated to 150 °C for 24 h. There was no change in the emission
spectra before and after either treatment. For example, the
fluorescence spectra of thin films of R2 recorded under different
conditions (pristine, after exposing the film for several weeks,
and after heating the film at 150 °C for 24 h) are essentially the
same (Figure 8). Compounds B1-R3 maintain their color purity
and are reasonably stable under the conditions used for aging
and annealing.
Temperature-Dependent Emission. We also studied the
effect of temperature on the absorption and emission spectra of
B1, B3, G2, and G3. There is no change in the absorption
spectra, but these compounds show a small red-shift in the
emission with decreasing temperature (Figure 9). For example,
the emission of B1 recorded at -10 °C (λmax ) 420 nm) is
about 14 nm red-shifted from that at 25 °C (λmax ) 406 nm)
(Table 3) likely because alignment of the solvent dipole around
the molecule in the excited state is prevented at low temperature.
A change in the dielectric constant of the solvent upon cooling
B1
G3
temp (°C)
λmax (nm)
φF (nm)
λmax (nm)
φF (nm)
25
10
5
0
-5
-10
406
412
414
414
416
420
0.05
0.10
0.12
0.15
0.18
0.20
453
454
456
456
456
458
0.07
0.15
0.17
0.19
0.21
0.25
a Excitation wavelength for B1 (λex ) 330 nm) and G3 (λex ) 360
nm); error in the φF values ) (0.01.
may be important. The φF values increase nominally but
systematically as the temperature is decreased (Table 3). The
φF values of B1 (0.05) and G3 (0.07) at 25 °C, for example,
are lower than those measured at -10 °C (0.20 and 0.25,
respectively). The rotational deactivations of the singlet excited
states are minimized at lower temperatures, resulting in an
increase in the fluorescence quantum yield.
Conclusions
A photophysical study of stable blue, green, and orange-red
light-emitting carbazoles (B1-B3, G1-G3, and R1-R3) was
carried out. The solid-state emission was red-shifted from the
solution (dichloromethane) emission for each compound, except
for R1 which showed an unusual blue-shift. The substitution
pattern on the carbazole significantly affected the relative red-
shift of the corresponding solution spectra and that observed in
the solid state. For example, the red-shifts in the solid state of
B1-B3 and G1-G3 are in the range of ∼15 and ∼30 nm,
respectively, but as high as 120 nm for R3. The φF values of
B2 and R1-R3 were found to be decent both in solution
(0.30-0.52 in dichloromethane) and in the solid state (0.21-0.45).
However, the φF values of B1, B3, G1-G3, and R1-R3 were
extremely low (0.03-0.15), indicating that the EW groups such
as the carbaldehyde and malononitrile groups effectively quench
the fluorescence. The τF values, on the other hand, were not
found to be affected by the substitution pattern. The singlet states
of the compounds having EW groups are more polar than their
corresponding ground states. This results in B1, B3, and G1-G3
exhibiting positive solvatochromism and B2 showing negligible
solvatochromism. Such a systematic solvatochromism was not
exhibited by R1-R3 because of the twisted ICT nature of their
singlet states. Lowering the temperature from 25 to -10 °C
J. Org. Chem. Vol. 74, No. 9, 2009 3347

Adhikari et al.
(300 MHz, CDCl3): δ 6.88 (d, 2H), 7.15 (d, 1H), 7.35 (d, 1H),
7.44 (d, 2H), 7.60 (m, 2H), 7.68 (d, 1H), 8.08 (m, 2H), 8.30 (s,
2H). 13C NMR (300 MHz, CDCl3): δ 112.5, 113.3, 123.0, 123.5,
124.0, 126.0, 126.5, 127.0, 127.5, 128.5, 129.5, 130.5, 133.0, 135.0,
and 141.5. MS (EI) calculated for C22H13Br2N 448.9415, measured
448.9449.
caused a small but distinct red-shift in the emissions and a slight
increase in the φF values of B1, B3, G2, and G3. When the
thin films containing B1-R3 were exposed to ambient condi-
tions for at least 4 weeks or heated at 150 °C for 24 h, the
emission purity was retained, indicating that these are robust
compounds.
3,6-Dibromo-9-(phenanthren-9-yl)-9H-carbazole (5). Com-
pound 3 (20 mmol) was mixed with acetic acid in a round-bottom
flask and subjected to sonication followed by stirring for 30 min.
Into this mixture was slowly dropped bromine (10 mmol) in 30
mL of glacial acetic acid (30 min) with stirring. After 2 h of
additional stirring, ice-cold water was poured in. The mixture was
subjected to suction filtration. The residue was washed with water
several times to obtain 5 in the form of a yellowish white compound,
which was dried under vacuum (yield 95%). 1H NMR (300 MHz,
CDCl3): δ 6.91 (d, 2H), 7.11 (d, 1H), 7.41 (m, 3H), 7.70 (d, 2H),
7.79 (d, 1H), 8.29 (s, 2H), 8.80 (s, 2H). 13CNMR (300 MHz,
CDCl3): δ 112.0, 113.0, 122.8, 123.3, 123.7, 123.9, 127.4, 127.8,
127.9, 128.0, 129.0, 129.1, 129.5, 130.8, 131.4, 131.6, 131.8, and
141.0. MS (EI) calculated for C26H15Br2N 498.9571, measured
498.9606.
Experimental Section
Fluorescence Quantum Yields (OF). The φF values in solution
were measured following a general method with 9,10-diphenylan-
thracene (φF ) 0.9 in cyclohexane) and riboflavin (0.3 in ethanol)
as the standard.32 Dilute solutions of these compounds in appropriate
solvents were used. Sample solutions were in quartz cuvettes were
degassed for ∼15 min. The degassed solutions had absorbances of
0.05-0.09 at absorbance maxima. The fluorescence spectra of each
were recorded 3-4 times, and an average value of integrated areas
of fluorescence was used for the calculation of φF in solution. The
refractive indices of solvents at the sodium D line were used. The
φF values in the solid state were measured following a litera-
ture method.37-39 A concentrated dichloromethane solution of
sample was cast as a thin film on a quartz plate and then allowed
to dry. The plate was inserted into an integrating sphere, and the
required spectra were recorded. The samples were excited at
absorption maxima in dichloromethane. 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 of the emitted
light. A method employed earlier was used to minimize the impact
of this on the calculation of the φF.37
3,6-Diiodo-9-(phenanthren-9-yl)-9H-carbazole (6). Compound
3 (5 mmol) was dissolved in dichloromethane (100 mL). A solution
of N-iodosuccinimide (11 mmol) in dichloromethane (10 mL) was
slowly added to it. The whole mixture was stirred in the dark for
6 h at room temperature. The solvent was evaporated to get 6 (yield
1
90%). H NMR (300 MHz, CDCl3): δ 6.81 (d, 2H), 7.11 (d, 1H),
7.40 (m, 1H), 7.58 (d, 2H), 7.69 (m, 2H), 7.78 (m, 1H), 7.84 (s,
2H), 8.45 (s, 2H), 8.81 (s, 2H). 13C NMR (300 MHz, CDCl3): δ
82.0, 112.0, 122.8, 123.2, 123.5, 124.2, 127.3, 127.38, 127.6, 127.7,
128.0, 129.0, 129.1, 129.6, 130.6, 131.3, 131.5, 131.8, 135.0, and
141.0. MS (EI) calculated for C26H15I2N 594.9294, measured
594.9302.
Fluorescence Lifetime (τF) Measurements. Solutions with an
absorbance of 0.1-0.25 at the absorption maxima were placed in
quartz cuvettes. Fluorescence decay profiles of argon-degassed (∼15
min) solutions were recorded with the use of a single photon
counting spectrofluorimeter. Decays were monitored at the corre-
sponding emission maxima of the compounds. In-built software
allowed the fitting of the decay spectra (ꢀ2 ) 1-1.5) and yielded
the fluorescence lifetimes.
9-(Phenanthren-9-yl)-9H-carbazole-3-carbaldehyde (B1). Com-
pound 5 (4 mmol) in dry tetrahydrofuran (60 mL) was stirred under
argon and cooled to -78 °C. n-Butyl lithium (8 mmol in pentane)
was slowly added with vigorous stirring. Stirring was continued
for two hours at the same temperature. Dry dimethylformamide
(25 mmol) was added and the mixture allowed stirring for two more
hours at the same temperature. It was mixed with 2N hydrochloric
acid at room temperature, extracted with ether, and purified by
chromatography (silica gel, dichloromethane/hexanes (1:5)) to get
Synthesis. Compounds B1-B3 and G1-G3 were synthesized
starting from carbazole. The synthesis of R1-R3 is provided
elsewhere.26
1
9-(Phenanthren-9-yl)-9H-carbazole (3). Carbazole (30 mmol),
9-bromophenanthrene (60 mmol), potassium carbonate (200 mmol),
and nitromethane (200 mL) were mixed in a dry round-bottom flask
and refluxed for 2 days. Solvent was distilled off, and the product
was purified by column chromatography (silica gel, dichlo-
romethane/hexanes (1:5)), affording pure 3 as a white solid (yield
70%). 1H NMR (300 MHz, CDCl3): δ 7.16 (d, 2H), 7.39 (m, 6H),
7.71 (m, 1H), 7.80 (m,1H), 7.90 (d,1H), 8.01 (s, 2H), 8.30 (d, 2H),
8.90 (t, 2H). 13C NMR (300 MHz, CDCl3): δ 110.2, 119.7, 120.3,
122.8, 123.2, 124.1, 126.0, 127.2, 127.3, 127.5, 127.6, 127.8, 129.0,
129.5, 130.6, 131.8, 132.6, and 142.5. MS (EI) calculated for
C26H17N 343.1334, measured 343.1364.
3,6-Dibromo-9-(naphthalen-1-yl)-9H-carbazole (4). 9-(Naph-
thalen-1-yl)-9H-carbazole (12 mmol) in 100 mL of glacial acetic
acid was subjected to sonication for 20 min and then stirred for 30
min in a round-bottom flask. Bromine (6 mmol) in 20 mL of glacial
acetic acid was slowly added into the mixture over 20 min while
stirring at room temperature. The mixture was stirred for an
additional 20 min at the same temperature. Ice cold water (100
mL) was then added. The mixture was then subjected to suction
filtration. The residue was washed several times with water and
pure B1 (yield-30%). HMR (300 MHz, CDCl3): δ 7.10 (m, 2H),
7.2 (s, 1H), 7.40 (m, 3H), 7.70 (m, 2H), 7.80-8.02 (m, 4H), 8.32
(m, 1H), 8.75 (s, 1H), 8.84 (m, 2H), 10.15 (s, 1H). 13CNMR (300
MHz, CDCl3): δ 110.5, 111.0, 120.8, 121.3, 122.8, 123.0, 123.3,
123.6, 123.8, 127.0, 127.2, 127.5, 127.8, 128.0, 129.0, 129.5, 130.5,
131.5, 131.8, 131.9, 142.0, 145.0, and 191.0. MS (EI) calculated
for C27H17NO 371.1310, measured 371.1309.
9-(Phenanthren-9-yl)-3,6-bis(2-phenylethynyl)-9H-carbazole
(B2). Compound 6 (3 mmol), trans-dichlorobis(triphenylphos-
phine)palladium(II), diisopropylamine (50 mL), phenylacetylene
(6.2 mmol), benzene (10 mL), copper iodide (0.1 mmol), and
triphenyl phosphine (0.3 mmol) were added in a dry round-bottom
flask. The mixture was refluxed for 12 h. The solvent was
evaporated, and the solid obtained was subjected to column
chromatography (silica gel, dichloromethane/hexanes (1:9)). The
fraction containing B2 was evaporated to obtain pure B2 as white
powder (yield 56%). 1H NMR (300 MHz, CDCl3): δ 7.05 (d, 2H),
7.28 (s, 1H), 7.35-7.45 (m, 6H), 7.47 (d, 1H), 7.54-7.65 (m, 6H),
7.75 (m, 2H), 7.85 (t, 1H), 8.00 (s, 2H), 8.45 (s, 2H), 8.90 (t, 2H).
13C NMR (300 MHz, CDCl3): δ 88.0, 90.0, 111.0, 115.0, 122.8,
123.2, 123.5, 123.8, 124.2, 127.3, 127.5, 127.7, 128.0, 128.4, 129.0,
129.1, 130.0, 130.5, 131.5, and 131.7. MS (EI) calculated for
C42H25N 543.1987, measured 543.1998.
1
was then dried over vacuum to get pure 4 (yield 95%). H NMR
(37) Shah, B. K.; Neckers, D. C.; Shi, J.; Forsythe, E. W.; Morton, D. Chem.
Mater. 2006, 18, 603.
4,4′-Bis((9H-carbazol-9-yl)-3,3′-dicarbaldehyde)biphenyl (B3).
Phosphoryl chloride (0.1 mol) was added dropwise into cooled N,N-
dimethylformamide (DMF, 0.1 mol) in an ice bath. The mixture
was maintained at room temperature for 1 h, and a solution of 8
(38) de Mello, J. C.; Wittmann, H. F.; Friend, R. H. AdV. Mater. 1997, 9,
230.
(39) Palsson, L. O.; Monkman, A. P. AdV. Mater. 2002, 14, 757.
3348 J. Org. Chem. Vol. 74, No. 9, 2009

Light-Emitting Carbazoles
(0.004) in 5 mL of DMF was added. The reaction mixture was
heated at 130 °C with stirring for 24 h and then poured into cracked
ice. After neutralizing with a base, the mixture was extracted with
chloroform. The extract phase was dried with anhydrous magnesium
sulfate, and the solvent was removed by distillation in a vacuum.
The solid residue was purified by using silica gel column chroma-
tography (ethyl acetate/hexanes (1:4)) to obtain a whitish-yellow
solid obtained was purified by chromatography (silica, dichlo-
romethane/hexanes (1:4)). The procedure afforded pure G1 as
yellow powder (yield 80%). 1H NMR (300 MHz, CDCl3): δ
7.10-7.20 (m, 3H), 7.40-7.48 (m, 3H), 7.55 (m, 2H), 7.80-7.98
(m, 5H), 8.30 (dd, 1H), 8.85 (m, 3H). 13C NMR (300 MHz, CDCl3):
δ 111.1, 113.3, 114.0, 115.0, 120.8, 121.7, 122.5, 122.7, 123.5,
123.6, 124.3, 124.8, 127.5, 127.5, 127.6, 127.7, 127.7, 127.8, 128.3,
128.8, 129.1, 129.3, 130.8, 131.2, 131.4, 131.9, 143.0, 145.0, and
160.0. MS (EI) calculated for C30H17N3 419.1422, measured
419.1436.
2-((9-(Naphthalen-1-yl)-9H-carbazol-3-yl)methylene)malono-
nitrile (G2). The same synthetic procedure employed for G1 was
used to prepare G2. Compound 12 (4 mmol) was used instead of
B1. The procedure afforded pure G2 as yellow powder (yield 80%).
1H NMR (300 MHz, CDCl3): δ 7.05 (d,1H), 7.20 (d, 1H), 7.40 (m,
3H), 7.55-7.7 (m, 4H), 7.92, (m, 2H), 8.05 (d, 1H), 8.13 (d, 1H),
8.28 (d, 1H), 8.82 (s, 1H). 13C NMR (300 MHz, CDCl3): δ 111.0,
111.2, 121.0, 121.8, 122.6, 123.4, 124.1, 124.8, 126.0, 126.5, 127.1,
127.8, 127.9, 128.8, 129.0, 130.0, 130.3, 132.4, 135.0, 143.0, 145.0,
and 160.0. MS (EI) calculated for C26H15N3 369.1266, measured
369.1266.
2,2′-((9-(Naphthalen-1-yl)-9H-carbazol-3,3′-yl)methylene)ma-
lononitrile (G3). This compound was prepared using the synthetic
procedure also used for the preparation of G1. Compound 13 (4
mmol) was used instead of B1. Double the amount of malononitrile
(8.4 mmol) was used. Compound G3 was obtained as yellow
powder (yield 80%). 1H NMR (300 MHz, CDCl3): δ 7.10 (d, 1H),
7.15 (d, 2H), 7.40 (t, 1H), 7.62 (m, 2H), 7.72 (t, 1H), 7.95 (s, 2H),
8.08 (d, 3H), 8.19 (s, 1H), 8.80 (s, 2H). 13C NMR (300 MHz,
CDCl3): δ 80.0, 112.3, 113.4, 114.3, 122.0, 123.5, 124.7, 125.3,
125.9, 126.5, 127.4, 128.1, 129.0, 129.7, 130.8, 131.2, 135.0, 146.0,
and160.0. MS (EI) calculated for C30H15N5 445.1327, measured
445.1332.
1
solid (yield 50%). H NMR (300 MHz, CDCl3): δ 7.40 (m, 2H),
7.50-7.60 (m, 6H), 7.75 (d, 4H), 8.01 (m, 6H), 8.25 (d, 2H), 8.70
(s, 2H), 10.10 (s, 2H). 13C NMR (300 MHz, CDCl3): δ 110.0, 110.3,
121., 121.5, 123.3, 124.7, 125.0, 127.0, 127.5, 128.0, 129.0, 129.7,
136.5, 140.0, 142.0, 144.5, and 192.0. MS (EI) calculated for
C38H24N2O2 540.1838, measured 540.1853.
9-(Naphthalen-1-yl)-9H-carbazole-3-carbaldehyde (9). This
compound was obtained as a side product while synthesizing 13
(see below). It was obtained in 30% yield as white powder and
dried under vacuum. 1H NMR (300 MHz, CDCl3): δ 7.04 (d, 2H),
7.20 (d, 1H), 7.35-7.47 (m, 3H), 7.55 (d, 1H), 7.65 (m, 2H), 7.90
(d, 1H), 8.10 (dd, 2H), 8.28 (d, 1H), 8.80 (s, 1H), 10.15 (s, 1H).
13C NMR (300 MHz, CDCl3): δ 110.5, 111.0, 120.5, 121.0, 123.0,
123.3, 123.5, 123.8, 125.8, 126.8, 127.3, 127.6, 127.8, 128.5, 129.5,
129.8,, 130.5, 133.0, 135.0, 143.0, 146.0, 192.0. MS (EI) calculated
for C23H15NO 321.1154, measured 321.1136.
9-(Naphthalen-1-yl)-9H-carbazole-3,6-dicarbaldehyde (10).
This compound was synthesized in a different way than the reported
method.40 Compound 4 (10 mmol) was dissolved in dry tetrahy-
drofuran in a dry two-necked round-bottom flask that was charged
with argon and cooled in an acetone-dry ice bath. tert-Butyllithium
in pentane (20 mmol) was added slowly with vigorous stirring. The
mixture was stirred for an additional 2 h at the same temperature.
Dry dimethylformamide (25 mmol) was added, and the mixture
was stirred for an additional 2 h at the same temperature. It was
subsequently mixed with 2 N hydrochloric acid at room temperature,
extracted with ether, and purified by chromatography (silica,
dichloromethane/hexanes (1:5)). Compound 13 was obtained as
white powder (yield 40%). It was dried under vacuum before
characterization. 1H NMR (300 MHz, CDCl3): δ 7.10 (m, 3H), 7.38
(d, 1H), 7.55- 7.70 (m, 3H), 7.95 (d, 2H), 8.05 (d, 1H), 8.15 (d,
1H), 8.80 (s, 2H), 10.20 (s, 2H). 13C NMR (300 MHz, CDCl3): δ
111.0, 122.5, 123.5, 124.0, 126.0, 126.5, 127.3, 127.8, 128.5, 129.0,
130.5, 130.7, 132.0, 135.0, 147.0, and 192.0. MS (EI) calculated
for C24H15NO2 349.1103, measured 349.1106.
Acknowledgment. We thank Dr. T. H. Kinstle for valuable
suggestions concerning synthesis and Dr. Rajib Mondal for
fruitful discussions about photophysical measurements. Financial
support from the Research Corporation for Science Advance-
ment (Cottrell College Science Award to BKS) is highly
appreciated. This work was partly supported by the Endowment
Fund of the Center for Photochemical Sciences. We thank the
donors of these funds.
2-((9-(Phenanthren-9-yl)-9H-carbazol-3-yl)methylene)ma-
lononitrile (G1). Compound B1 (4 mmol), malononitrile (4.2
mmol), basic aluminum oxide (20 mmol), and toluene (100 mL)
were added to a round-bottom flask. The mixture was refluxed for
6 h, cooled to room temperature, and filtered, and the residue was
washed with dichloromethane. The filtrate was evaporated, and the
Supporting Information Available: Absorption and emis-
sion spectra of B2, G2, and R2, fluorescence decay profiles,
geometry optimized structures, and NMR spectra of B1-B3,
G1-G3, and R1-R3. This material is available free of charge
via the Internet at http://pubs.acs.org.
(40) Feng, L.; Zhang, C.; Chen, Z.; Qin, A.; Yuan, M.; Bai, F. J. Appl. Polym.
Sci. 2006, 100, 923.
JO9002757
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