Carbazole donor-carbazole linker-based compounds: Preparation, photophysical properties, and formation of fluorescent nanoparticles

Article abstract of DOI:10.1021/jp912286u

A new class of highly soluble and stable compounds (1-4) has been synthesized and characterized. Compounds 1, 2, and 3 have 3,6-disubstituted carbazole as electron-donor-linked through the 2,7-positions of N-substituted carbazole with variably substituted phenyl acetylenes as electron acceptors. Compound 4 has two identical donors linked through the 2,7-position of N-substituted carbazole. These compounds absorb from UV to visible region and emit intensely from blue to green. The effect on the photophysical properties of these compounds by changing acceptors while keeping the donor and linker constant has been studied. A study of solvent effects on their photophysical properties has shown that an increase in polarity of the solvent causes a reduction of fluorescence quantum yields. With a decrease in temperature, these compounds showed increased emission intensity accompanied by a red shift of emission. Edge excitation red shift showed as much as 128 nm for compound 2 in isopropanol. Solid-state fluorescence quantum yields vary from 0.27 to 0.68 for these compounds. The fluorescence lifetimes of these compounds are solvent dependent. Compound 2 showed remarkable change in emission with concentration. Compounds 2 and 3 form highly stable fluorescent organic nanoparticles (FONs) in tetrahydrofuran/water mixtures. The general and specific solvent effect on the emission properties of these compounds are investigated by Lipert-Mataga plots. The potentiality of these compounds for use in dye-sensitized solar cells and in organic light-emitting diodes is under study in our laboratories.

Full text of DOI:10.1021/jp912286u

4550
J. Phys. Chem. A 2010, 114, 4550–4557
Carbazole Donor-Carbazole Linker-Based Compounds: Preparation, Photophysical
Properties, and Formation of Fluorescent Nanoparticles
Krishna Panthi,† Ravi M. Adhikari,‡ and Thomas H. Kinstle*,†
Department of Chemistry, Center for Photochemical Sciences, Bowling Green State UniVersity, Bowling Green,
Ohio 43403, and Angstrom Technologies, Inc., Florence, Kentucky 41042
ReceiVed: December 31, 2009; ReVised Manuscript ReceiVed: February 26, 2010
A new class of highly soluble and stable compounds (1-4) has been synthesized and characterized. Compounds
1, 2, and 3 have 3,6-disubstituted carbazole as electron-donor-linked through the 2,7-positions of N-substituted
carbazole with variably substituted phenyl acetylenes as electron acceptors. Compound 4 has two identical
donors linked through the 2,7-position of N-substituted carbazole. These compounds absorb from UV to
visible region and emit intensely from blue to green. The effect on the photophysical properties of these
compounds by changing acceptors while keeping the donor and linker constant has been studied. A study of
solvent effects on their photophysical properties has shown that an increase in polarity of the solvent causes
a reduction of fluorescence quantum yields. With a decrease in temperature, these compounds showed increased
emission intensity accompanied by a red shift of emission. Edge excitation red shift showed as much as 128
nm for compound 2 in isopropanol. Solid-state fluorescence quantum yields vary from 0.27 to 0.68 for these
compounds. The fluorescence lifetimes of these compounds are solvent dependent. Compound 2 showed
remarkable change in emission with concentration. Compounds 2 and 3 form highly stable fluorescent organic
nanoparticles (FONs) in tetrahydrofuran/water mixtures. The general and specific solvent effect on the emission
properties of these compounds are investigated by Lipert-Mataga plots. The potentiality of these compounds
for use in dye-sensitized solar cells and in organic light-emitting diodes is under study in our laboratories.
Introduction
of nanoparticles differ from those of bulk materials because they
are structurally distinct and show confinement effects caused
by their finite size.29,30
Electron donor-acceptor compounds separated by π conjuga-
tion (D-π-A) have attracted much research interest during the
past few years because of their applications as electroactive and
photoactive materials in molecular electronics areas such as
fluorescencetechnology,1chemoluminescence,2andphotovoltaics.3,4
The strong emission and absorption properties of carbazole and
their derivatives5,6 have been exploited as electroluminescent
and hole transporting layers of OLEDs7,8 as wide band gap
energy transfer materials to lower band gap emitters9,10 and in
photovoltaics.11–13 The molecular and optical properties of
carbazole can be tuned by modifying its 2-, 3-, 6-, 7-, and
9-positions.14–16
There have been many studies of carbazole-based compounds
in devices7–16 but fewer of their photophysical properties in
fluids17 and the solid state. So, we focused our study on the
fundamental photophysical properties of some carbazole-based
D-π-A in fluid and solid state. We also focused on a study of
the formation of fluorescent organic nanoparticles (FONs)18–21
and their optical properties. Edge excitation red shifts (EERS)
have been observed in various excited-state reactions leading
to photoisomerization,22 electron transfer,23 and proton transfer.24
The application of EERS in viscous and glass-forming fluids,25
binary solvent mixtures of different polarity proteins,26 and
polymers27 has been demonstrated. Studies of luminescence as
a function of temperature allow estimation of the activation
energy for the deactivation of the excited states.28 FONs have
received less attention, even though they allow wider variability
and flexibility as materials. The electronic and optical properties
In studies using carbazole compounds, the molecular design
has been mostly focused on networking through the 3- and
6-positions,31–34 and only a few examples have the conjugation
pattern through the 2-, 7-, and 9-positions.35–38 There are in fact
no reported D-π-A compounds in which the carbazole donor is
linked through its 9-position to the 2- and 7-linked carbazole.
In view of this, we studied three D-π-A molecules (1-3) that
have N-substituted carbazole bonded through its 2- and 7-posi-
tions to the 9-position of another carbazole and an acceptor
moiety, respectively. Also, we have synthesized one more
compound 4 by bonding N-substituted carbazole through its 2-
and 7-positions to the 9-position of two other carbazoles.
Compounds 1, 2, 3, and 4 differ only in one group, that is, they
have aldehyde in 1, malononitrile in 2, cyanoacetic acid in 3,
and 3,6-di-tert-butyl-9-(4-ethynyl-phenyl)-9H-carbazolyl group
in 4. In this article, we report the synthesis of 4-(2-(7-(2-(4-
(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)ethynyl)-9-(4-(trifluo-
romethyl)phenyl)-9H-carbazol-2-yl)ethynyl)benzaldehyde (1),
2-((4-(2-(7-(2-(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl-
)ethynyl)-9-(4-(trifluoromethyl)phenyl)-9H-carbazol-2-yl)ethy-
nyl)phenyl)methylene)malononitrile (2), 2-cyano-3-(4-(2-(7-(2-
(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)ethynyl)-9-(4-
( t r i fl u o r o m e t h y l ) p h e n y l ) - 9 H - c a r b a z o l - 2 -
yl)ethynyl)phenyl)acrylic acid (3), and 3,6-di-tert-butyl-9-(4-
(2-(7-(2-(4-(3,6-ditert-butyl-9H-carbazol-9-yl)phenyl)ethynyl)-
9-(4-(trifluoromethyl)phenyl)-9H-carbazol-2-yl)ethynyl)phenyl)-
9H-carbazole (4) compounds (Figure 1).
We also report the solvatochromic properties, fluorescence
switching with concentration, temperature-dependent fluores-
cence behavior, and solid-state photoluminescence of these
* Corresponding author. E-mail: tkinstl@bgsu.edu.
† Bowling Green State University.
‡ Angstrom Technologies, Inc.
10.1021/jp912286u  2010 American Chemical Society
Published on Web 03/16/2010

Carbazole Donor-Carbazole Linker-Based Compounds
J. Phys. Chem. A, Vol. 114, No. 13, 2010 4551
decay profiles of the argon-degassed samples were measured
using a single photon-counting spectrofluorimeter. Decays were
monitored at the corresponding emission maximum of the
samples. In-built software allowed the fitting of the decay spectra
(ꢀ ) 1 to 1.5) and produced the fluorescence lifetimes.
Preparation of Nanoparticles. Samples of predetermined
concentrations of 2 and 3 in THF were made. Appropriate
volumes of these solutions were taken in different vials, and
the required amount of water was rapidly injected into those
vials to maintain the final concentrations of the solutions. Four
different THF/water ratios, that is, 1:0, 1:5, 1:8, and 1:12, for
compound 2 were made, but 3 did not form any nanoparticles
in THF/water 1:12. In all samples, the formation of nanoparticles
could be observed under a 365 nm UV lamp. In some samples,
the formation of nanoparticles could be seen by the naked eye
because of change of color of the solution, although the solution
is transparent.
SEM Images of Nanoparticles. Scanning electron micro-
scope (SEM) images were recorded on a FEI-FP2031/11
microscope at 15 eV using a INCA Penta FETX3 detector.
Samples for SEM were prepared by placing a few drops of
nanoparticle suspension onto a glass coverslip placed on an
aluminum stub. The samples were allowed to dry at room
temperature before observing under the scanning electron
microscope. To enhance the contrast and quality of the SEM
images, the samples were sputter-coated with gold/palladium.
Fluorescence Microscope (FM) Images of Nanoparticles.
Fluorescence microscope images were recorded on a Qimaging
Epi fluorescence microscope using a QICAM camera. The data
were collected using Metaphor software. We prepared the
samples by putting a droplet of nanoparticles suspension on a
glass slide and covering with a coverslip.
Confocal Microscope Images of the Nanoparticles. Con-
focal microscope images were performed using an Olympus
FluoView 1000 microscope. The same method employed to
prepare the samples for FM images was used to prepare the
samples for confocal microscopy.
The excitation source was a diode blue laser (405 nm), and
fluorescence was detected using standard three confocal channels
(three photomultiplier detectors). The emission filter (BA535-
565) at 405 nm laser light was used for the image recording.
Figure 1. Structures of compounds 1-4.
compounds. The specific and general solvent effects on optical
properties of these compounds are discussed with the help of
Lippert-Mataga plots. By varying the acceptors, the absorption
is tuned from 353 to 386 nm, and emission is tuned from 422
to 454 nm in dichloromethane (DCM). Compound 2 with its
electron-withdrawing (EW) group showed red-shifted emission
in DCM with a decrease in temperature, whereas 4, which has
no EW group, showed a small change. Only compounds having
strong EW groups (2 and 3) formed FONs in tetrahydrofuran/
water (THF/water) mixtures. Emission has been red-shifted by
126 nm for 2 and by 120 nm for 3 after the formation of
nanoparticles, which shifts the color from blue to perfectly green
region.
Experimental Section
Results and Discussion
Materials. The synthesis of compounds 1-4, 7, and 8 is
described in the Supporting Information. Synthesis of the
remaining precursors is described elsewhere.39,40 Hexanes,
toluene, DCM, acetonitrile (ACN), THF, and isopropanol are
HPLC grade.
Synthesis. Following literature methods,39 carbazole was
converted to 3,6-di-tert-butyl-9-(4-ethynyl-phenyl)-9H-carba-
zole) (5). The preparation of 2,7-functionalized carbazoles is
not straightforward because both the 2- and 7-positions are in
the meta position of the amino group of the carbazole unit and
cannot be directly functionalized by standard electrophilic
aromatic substitution. Successful synthetic strategies usually
require functionalized 4,4′-biphenyl precursors with an ad-
ditional reactive group at the 2-position used for a subsequent
ring-closure reaction. In this way, 4,4′-biphenyl was converted
to 2,7-dibromocarbazole (6) following a literature procedure.40
N-Arylation of 6 was followed by the Sonogashira coupling to
4-ethynylbenzaldehyde to obtain 8 (Scheme 1). Sonogashira
coupling of 8 to 5 produced 1, which further reacts with
malononitrile and basic aluminum oxide in toluene to produce
2. Treatment of 1 with cyanoacetic acid, ammonium acetate,
and acetic acid produced 3. Sonogashira coupling of 5 to 7
produced 4 (Scheme 2).
Measurements. Fluorescence Quantum Yields (ΦF). The
ΦF values in solution were measured following a general method
using 9,10-diphenylanthracene (ΦF ) 0.9 in cyclohexane) as
the standard.41,42 Dilute solutions of 1-4 in DCM were used
for recording the fluorescence spectra. Sample solutions were
taken in quartz cuvettes and degassed for ∼15 min. The
fluorescence spectra of each were recorded three times, and an
average value of integrated areas of fluorescence was used for
the calculation. The ΦF values in the solid state were measured
following a literature method.41 A concentrated DCM solution
of samples was cast as thin films on quartz plates and then
allowed to dry. The plates were inserted into an integrating
sphere, and the spectra were recorded. The samples were excited
at their Amax
.
Absorption and Emission Spectra in Solution. Absorption
spectra of 1-4 recorded in DCM are shown in Figure 2. The
absorption around 300 nm is due to the carbazole moiety. The
Fluorescence Lifetime (τ) Measurement. The samples in
different solvents were put in quartz cuvettes. Fluorescence

4552 J. Phys. Chem. A, Vol. 114, No. 13, 2010
Panthi et al.
SCHEME 1: Synthesis of Compounds 7 and 8a
a Reagents and conditions: (a) 1-bromo-4-trifluoromethylbenzene, K2CO3, DMF, reflux 12 h; (b) Pd(PPh3)2Cl2, CuI, PPh3, N,N-diisopropylethylamine,
reflux 10 h.
SCHEME 2: Synthesis of Compounds 1-4a
a Reagents and conditions: (a) Pd(PPh3)2Cl2, CuI, PPh3, N,N-
diisopropylethylamine, reflux 10 h; (b) malononitrile, basic Al2O3,
Figure 3. Normalized emission spectra of compounds 1-4 in DCM
(note: λex ) Amax).
2-methylpropanol, reflux 5 h; (c) cyanoacetic acid, ammonium acetate,
CH3COOH, reflux 5 h; (d) Pd(PPh3)2Cl2, CuI, PPh3, N,N-diisopropy-
lethylamine, reflux 6 h.
because compound 2 has a stronger EW group, and the larger
extension of conjugation in 4 might be reflected in its emission.
The fluorescence spectrum of compound 2 is most red-shifted
among all four compounds. The formation of ICT in 2 is more
prominent than in 1 and 3. The chance of formation of ICT in
4 is nearly zero because it does not have an EW group. Even
though compounds 1, 2, and 3 show ICT absorption transition,
emission resulted mostly from a local excited (LE) state,28,43 as
shown by excitation spectra (Supporting Information, Figures
S9-S11), which suggests that the ICT excited state of some
compounds is destabilized with increasing solvent polarity vide
infra.
Figure 2. Normalized absorption spectra of compounds 1-4 in DCM.
The fluorescence quantum yields (ΦF) and the fluorescence
lifetimes (τF) of 1-4 recorded in DCM are summarized in Table
1 along with their absorption maxima (Amax) emission maxima
(λmax) and molar absorptivity (ε). The quantum yield values of
1-4 are found in the range of 0.12 to 0.72 relative to that of
9,10-diphenylanthracene (0.90 in cyclohexane).42,44 Fluorescence
decays of 1-4 in DCM fitted with monoexponential functions
indicating emission from the singlet excited state in all cases.
Each compound shows significant solvatochromism (Sup-
porting Information, Figures S1-S8) both in absorption and
emission spectra. The emission spectra exhibit a more significant
red shift in polar solvents for compounds 1 and 4 and follow
the exact order of solvent polarity in their red shift. The
excitation spectra of 1 and 4 show that emission in ACN results
from a wide range of absorption from 300 to 450 nm. The
emission in these compounds occurs from variously solvated
chromophores including LE state, ICT excited state, and relaxed
ICT excited state.45 The emission region in solvents with higher
polarity is also broader, and so are the excitation spectra in
compounds 1 and 4. The same is not true for compounds 2 and
3. The solvatochromism of compounds 2 and 3 does not follow
a smooth order with a change in solvent polarity. With a medium
polar solvent toluene, they show the most red-shifted emission
absorption around 325 to 475 nm shows that the EA group
influences the absorption transition property significantly.
Compound 4, which has no EW group, shows absorption
maximum at 350 nm. The addition of an EW group increases
the red shift in absorption transition. The order of red shift on
absorption follows the order of strength of EW group vide infra.
Compound 1 with the weakest EW moiety absorbs at 425 nm,
3 with one nitrile and one carboxylate group absorbs up to 450
nm, and 2 with dinitrile group absorbs up to 475 nm. Even
though compound 4 has longer conjugation length, its absorption
transition is blue-shifted relative to the other compounds because
there is no strong EW group. The red-shifted absorptions in
these compounds as compared with 4 are due to the formation
of different kinds of chromophores in DCM solution. Com-
pounds 1, 2, and 3 form intramolecular charge transfer
complexes (intramolecular CT), even in the ground state, so
the absorptions at lower energies are due to the transition from
ICT state to excited state.
When excited at their absorption maximum, dilute solutions
of 1-4 in DCM showed violet-to-green emission (Figure 3). It
is interesting that compounds 2 and 4 have more red-shifted
fluorescence emission than that of compounds 1 and 3. It is

Carbazole Donor-Carbazole Linker-Based Compounds
J. Phys. Chem. A, Vol. 114, No. 13, 2010 4553
TABLE 1: Absorption Maximum (Amax), Molar Absorptivity (ε), Emission Maximum (λmax), Stokes Shift, Quantum Yield (ΦF),
and Lifetime (τF) of Compounds 1-4 in DCMa
Amax
(nm)
λmax
(nm)
Stokes
shift (nm)
quantum
yield (ΦF)
lifetime
(τF) (ns)
compd
ε (dm3 cm-1mol-1
)
1
2
3
4
377
355
386
353
64 813
73 548
54 741
72 938
422
454
426
431
45
99
40
78
0.21
0.12
0.12
0.72
0.84
0.29
1.27
0.1
a Excitation is at Amax for each compound in corresponding solutions.
among all solvents used. The excitation spectra in toluene are
broad-ranging from 300-450 nm. The emission spectrum also
extends from 400 to 600 nm, a perfect white emission. The
emission of compounds 2 and 3 in toluene is red-shifted because
π-electron-rich toluene interacts with π electrons and lone-pair
electrons of these compounds. Compounds 2 and 3 have more
π electrons and lone pair of electrons to interact with π electron
of toluene. Compounds 1 and 4 have fewer π electrons and
lone pair of electrons, so their interaction with π electrons of
toluene is lessened. A similar observation was found in
benzanthrone derivatives.46 Similarly, the red shift of emission
of 1 and 4 follows the exact order of solvent polarity; however,
in compounds 2 and 3, the presence of lone-pair of electrons in
THF interacts with the lone pair of electrons of these com-
pounds. The same is less significant with compounds 1 and 4
because they have less lone pair electron density (Figures 4 and
S13-S15 of the Supporting Information).
Emission spectra of 1-4 were recorded at different concen-
trations in THF to assess the extent of possible aggregate
formation. For example, the fluorescence intensity becomes
maximum for compound 1 when the concentration is 3.1 × 10-6
M (Figure 4), but it decreases by almost triple when the
concentration is decreased to 3.1 × 10-7 M and also decreased
about half when the concentration becomes 3.1 × 10-5 M and
less at a concentration of 3.1 × 10-4 M. We could see sharp
concentration quenching of emission at very high concentration
for 1, 2, and 3 but with no concomitant red shift in their emission
spectra, which indicates that there was no aggregation even at
high concentration. Compound 4 shows aggregation at a
concentration of 1.4 × 10-3 M, as indicated by red-shifted
emission (Supporting Information).
2). For example, λmax of compound 2 in the solid state was 559
nm but only 454 nm in DCM. This remarkable red shift in the
solid state could be caused by twisting of the compound in
solution (because of the presence of phenylethynyl group)
thereby decreasing the conjugation from ED through the EW
group, by intermolecular charge transfer in the solid state, or
both. The ΦF in solid state for compounds 1-3 are found to be
higher than that in DCM. These compounds have both ED and
EW groups, so stabilization of polar excited chromophores by
polar solvents reduces ΦF.47 The ΦF for 4 in solid thin film is
less than that in DCM.
Optical Switching with Concentration. To understand the
number and nature of emissive species of compound 2 further,
we recorded emission spectra at different concentrations in
toluene (Figure 6) and monitored excitation spectra at different
emission maxima (Figure 7 and Supporting Information). When
2 was excited with 350 nm light and spectra collected, the
highest concentration sample (C0, 2.7 × 10-4 M) showed only
one peak at 500 nm with a small hump around 400-450 nm.
On reduction of concentration, emission intensity increased in
the 400-450 nm region and decreased in the 500 nm regions.
At higher concentrations, molecules of 2 are close enough
together to allow intermolecular charge transfer excited-state
(intermolecular CT*) formation. So, excitation spectra in the
450 nm region correspond to the intermolecular CT*. Similarly,
at higher concentration, carbazole moieties of adjacent molecules
assemble together, and some electronic interaction between them
causes the red shift of absorption. This is reflected by the
Solid-State Photoluminescence. To assess the potential
application of these compounds in the solid state, electrolumi-
nescent emission properties in the thin film were studied.
Emissions of these compounds in thin film are substantially red-
shifted in comparison with emission in DCM. The solid-state
emission spectra of these compounds are broad (Figure 5), and
quantum yields of fluorescence were found to be good (Table
Figure 5. Normalized solid-state emission of the thin solid films of
1-4 (note: λex ) Amax in DCM).
TABLE 2: Emission Maxima (λmax) and Quantum Yields
Fluorescence ΦF of 1-4 Recorded in the Solid State
λmax
(nm) (DCM)
λmax
(nm) (solid)
red shift
(nm)a
ΦF
(solid)
compound
1
2
3
4
422
454
430
431
500
559
451
545
78
105
21
0.27
0.68
0.30
0.51
Figure 4. Emission spectra of 1 recorded in THF at different
concentrations: (C1) 3.1 × 10-4 M, (C2) 3.1 × 10-5 M, (C3) 3.1 ×
10-6 M, and (C4) 3.1 × 10-7 M. (Inset: the enlarged spectrum of 1
recorded at 3.1 × 10-4 M) (note: λex ) Amax).
114
a Red shift of emission is between DCM solution and solid state
of compounds.

4554 J. Phys. Chem. A, Vol. 114, No. 13, 2010
Panthi et al.
Figure 6. Emission of compound 2 on excitation at 350 nm with
dilution in toluene. Concentrations of the solutions are C0 ) 2.7 ×
10-4 M, C1 ) 2.22 × 10-5 M, C2 ) 4.5 × 10-7 M, 9.0 × 10-9 M, C3
)18.0 × 10-11 M, C4 ) 36.0 × 10-13 M, C5 ) 72.0 × 10-15 M, and
C6 ) 144.0 × 10-17 M.
Figure 8. Jablonski diagram showing the possible excitation and de-
excitation pathways for compound 2.
Figure 9. Emission of compound 2 in hexanes (1 × 10-5 M) on
excitation at different wavelengths.
Figure 7. Normalized excitation spectra of compound 2 in toluene by
monitoring emission at 480 nm. Concentrations of the solutions are
C0 ) 2.7 × 10-4 M, C1 ) 2.22 × 10-5 M, C2 ) 4.5 × 10-7 M, 9.0
× 10-9 M, C3 ) 18.0 × 10-11M, C4 ) 36.0 × 10-13 M, C5 ) 72.0
× 10-15 M, and C6 ) 144.0 × 10-17 M.
excitation spectra of higher concentration samples. Conse-
quently, the Franck-Condon excited-state distribution (F-
CESD) of the intermolecular CT* state will increase with an
increase in concentration, leading to a higher distribution of the
intermolecular CT* state in the excited-state equilibrium.
Similarly, on reduction of concentration, the distance between
the molecules of 2 in solution increases, and consequently, the
formation of intermolecular CT* in the excited state decreases.
At lower concentration, intramolecular CT* are formed. So,
excitation spectra in 350-400 nm regions correspond to the
intrarmolecular CT*. Similarly, the peak around 300 nm
corresponds to carbazole moiety. Therefore, at lower concentra-
tion, Franck-Condon excited-state distribution (F-CESD) of the
intramolecular CT* state will increase with decrease in con-
centration leading to a higher distribution of the intramolecular
CT* state in the excited-state equilibrium. Excitation spectra
taken at different concentrations with monitoring at different
emission maxima supports this claim. As seen in Figure 7, the
peak of excitation obtained by monitoring emission at 480 nm
increases around 300 and 450 nm with an increase in concentra-
tion. The peak at 350-425 nm increases on intensity with a
decrease in concentration. The broad spectrum at lower con-
centration is possibly due to the combined LE, intermolecular
CT*, and intramolecular CT*. A Jablonski diagram for the
optical behavior of the LE, intermolecular CT*, and intramo-
lecular CT* states is depicted in Figure 8.
Figure 10. Emission of compound 2 in isopropanol (1 × 10-5 M) on
excitation at different wavelengths.
TABLE 3: Emission Maxima of Compound 2 in
Isopropanol at Different Excitation Wavelengths
λex (nm)
λmax (nm)
300
412
330
414
350
415
370
416
390
416
410
490
440
540
contribution from additional excited species. In a case when
the reorientation relaxation time is larger than the fluorescence
time, the total fluorescence emission spectrum is a composite
of fluorescence emission from differently solvated species
governed by F-CESD that is a function of excitation energy.48
For a polar compound in a polar solvent, the energy required
to excite the solvated species is a function of solvent orientation.
If the excitation energy is smaller, then only limited configura-
tions of the ground state may be excited. The excitation with
lower energy will excite only the fraction of total fluorophore
population that is surrounded by solvent dipoles to decrease
the energy of fluorophores. Therefore, the resultant total
emission lacks some high-energy components, and this causes
the red-shifted emission.
Compound 2 shifted as much as 128 nm in isopropanol
(Figure 10 and Table 3). Compound 2 shows no EERS effect
in hexanes (Figure 9). This is due to the fact that polar solvents
solvate the fluorophore more strongly than nonpolar solvents
do. Compound 4 did not show any EERS in either solvent,
isopropanol or hexanes (Supporting Information).
Excitation-Energy-Dependent Fluorescence; Edge Excita-
tion Red Shift. In contrast with Kasha’s rule, we observed that
the fluorescence of 2 depends on the excitation energy. This
effect has been referred to as the EERS.48 We observed this
remarkable red shift only when excitation is at the longer
wavelength edge of the lowest energy absorption band. Fletch-
er49 described this phenomenon by considering the emission

Carbazole Donor-Carbazole Linker-Based Compounds
J. Phys. Chem. A, Vol. 114, No. 13, 2010 4555
Figure 12. Lippert-Mataga plot of compounds 1-4.
Figure 11. Emission spectra of compound 2 in DCM on excitation at
350 nm.
TABLE 4: Change in Emission Maxima of Compounds 2
and 4 at Different Temperatures
temperature (°C)
compounds
-60 °C -40 °C -20 °C 0 °C 8 °C 25 °C
2 λex 350 nm
4 λex 350 nm
457
441
452
437
446
433
443
430
440
428
434
426
Temperature-Dependent Emission. With a decrease in
temperature, the emission spectra of 1 and 4 recorded in DCM
show a red-shifted fluorescence emission accompanied by an
increase in emission intensity. All of these nonplanar compounds
have significant degrees of free rotation in solution. With
decreased temperature, the system attains some solid-state
character, and molecular packing restricts intramolecular rota-
tion.50 This explains why ΦF is also higher at lower temperature.
The red-shifted emission spectra of 2 (Figure 11 and Table 4)
with decreasing temperature are in contrast with some literature
reports.45 Compound 2 shows a red shift of 23 nm in its emission
spectra at reduced temperature, whereas 4 shows a red shift of
only 15 nm.
Figure 13. Solutions of nanoparticles of compound 3 under UV 365
nm.
Compound 2 possesses strong EW and ED groups, and thus
an ICT state even in the ground state will be more stable at
reduced temperature. Absorption spectra of these compounds
show two bands vide supra. Because the viscosity of the solvent
increases at lower temperature, the reorientational relaxation of
the solvent molecules is inhibited, and enhanced stabilization
of the ICT* emitting state is expected because reduction of
thermal motion allows better alignment of the solute and solvent
dipoles. Therefore, the ICT excited state dominates over the
LE state at lower temperature. Consequently, the F-CESD of
the ICT excited state will increase with a decrease in temper-
ature. Compound 4 with no strong EW groups shows smaller
red shift of emission with a decrease in temperature from 25 to
-60 °C.
Lippert-Mataga Plot and Its Significance on Specific and
General Solvent Effects. The nonlinearity of Lippert-Mataga
plots for 1-4 (Figure 12) indicates the presence of multiple
excited states that produce multiple emitting species.33 Nonlin-
earity is also evidence of a specific solvent effect on the spectral
shift. The possible excited states might be the locally LE, the
intra or intermolecular charge transfer excited state (ICT*), or
the relaxed intra or intermolecular charge transfer excited
(ICTr*) states.33 LE is the emitting species in nonpolar solvents,
and ICT* and ICTr* are the emitting species in polar solvents.
Each of the compounds show multiple emitting species: one
locally excited (LE or 1S) and the other ICT*, ICTr*.
Compounds 1 and 4 have weaker or no EW groups, respectively.
The number of excited states could be less than that in 2 and 3.
Compounds 1 and 4 make LE state and solvent relaxed excited
state whereas 2 and 3 make LE, relaxed LE, ICT, and relaxed
Figure 14. Emission of compound 3 in THF and THF/H2O in the
ratio of 1:5 and 1:8 (note: λex ) Amax).
ICT states. So, there are more possible emissive entities in 2
and 3, as shown by Lippert-Mataga plots.47
Formation of Nanoparticles. In this study, nanoparticles of
compounds 2 and 3 were prepared by a precipitation technique;21
nanoparticles of compounds 1 and 4 could not be formed.
Identical concentrations of solutions (1 × 10-5 M) were prepared
for all THF/water mixtures. The nanoparticle suspensions
formed were visibly transparent and stable at room temperature
for several months. Suspensions of compound 3 in THF and in
THF/water have the appearances shown in Figure 13 when
placed under a UV light.
The sizes of nanoparticles formed were determined by SEM
and fluorescence microscopy (Figure 15 and Supporting Infor-
mation). Also, we obtained images from confocal microscopy
and obtained only green light, showing that nanoparticles emit
in the green region (Figure 16). Emission spectra of 3 in THF
solution and as nanoparticles suspended in solvents differing
in the THF/water ratio are given in Figure 14, and for 2, the
spectra are in Supporting Information. In both compounds, there
is no red shift of absorption upon adding water in THF.
Interestingly, the emission is red-shifted by 126 nm for
compound 2 and by 120 nm for compound 3. (Figure 14, Table
5 and Supporting Information).

4556 J. Phys. Chem. A, Vol. 114, No. 13, 2010
Panthi et al.
studied. Formations of different emitting species in dilute and
concentrated solution for compound 2 have been suggested on
the basis of their emission and excitation spectra. In polar
solvents, the effect of temperature on emission is higher than
that in nonpolar solvents. Similarly, compounds with EW and
ED groups showed more change in emission properties with
change in temperature than that of a compound without an EW.
EERS effects for D-π-A compounds are more significant than
for the compound without EW. The specific and general solvent
effects on optical properties of these compounds are discussed
with the help of Lippert-Mataga plots. Their higher solid-state
ΦF values suggest their higher potential in fabrication of solid-
state electroluminescent devices.
Acknowledgment. This present work has been partially
supported by grant NSF-EXP 40000/10380080. We thank the
donors of these funds. We thank Dr. Douglas C. Neckers and
Puran De for instruments and assistance.
Supporting Information Available: Synthetic procedure
details and H and 13C NMR spectra of compounds 1-4, 7,
1
Figure 15. SEM image of nanoparticles of compound 3 in THF/H2O
1:5.
and 8, UV-visible spectra in different solvents, excitation
spectra, and absorption and emission spectra of 1-4 during the
formation of nanoparticles as well in solutions of compounds
in THF and THF/water mixtures. This material is available free
of charge via the Internet at http://pubs.acs.org.
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ratio of
solvents
Amax λmax
red shift
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lifetimes
(ns)
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2
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THF/water 1:5
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A new class of carbazole donor-carbazole linker-based
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acceptors can form fluorescent nanoparticles, and these nano-
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formation of nanoparticles. Compounds with EW and ED
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Solvatochromic effects of these compounds have also been

Carbazole Donor-Carbazole Linker-Based Compounds
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