Aromatic fumaronitrile core-based donor-linker-acceptor-linker-donor (D-π-A-π-D) compounds: Synthesis and hotophysical properties

Article abstract of DOI:10.1021/jp9115589

A new class of aromatic fumaronitrile core-based compounds with different donors and linkers has been synthesized and well characterized. Compounds 1 and 2 have indole and 2-phenylindole groups as electron donors, respectively. Compounds 3 and 4 have a diphenylamino group as the electron donor, and compound 5 has a 3,6-di-tert-butylcarbazole group as an electron donor. These compounds absorb in the blue-to-green region and emit in the blue-to-red region depending on the electron donor, linker, and solvents. The quantum yields of fluorescence of these compounds in solution are measured and found to be moderate, but in solid states, they are high. These compounds display strong emission solvatochromism that is reflected by a large shift in their fluorescence emission maxima on changing the solvents. This change is accompanied by a successive decrease in fluorescence intensity. The fluorescence lifetimes of these compounds are measured in different solvent and found to vary from <1 to 7 ns. Optical switching of these compounds with solvents, concentration, and excitation energy have been studied. The correlation between the functional group and optical properties has been established to some extent. The ability of these compounds to function as colorimetric and luminescence pH sensors is demonstrated with color changes and luminescence switching upon the addition of trifluoroacetic acid. The potentiality of these compounds for application in optoelectronics has been optically assessed.

Full text of DOI:10.1021/jp9115589

4542
J. Phys. Chem. A 2010, 114, 4542–4549
Aromatic Fumaronitrile Core-Based Donor-Linker-Acceptor-Linker-Donor (D-π-A-π-D)
Compounds: Synthesis and Photophysical Properties
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 5, 2009; ReVised Manuscript ReceiVed: February 18, 2010
A new class of aromatic fumaronitrile core-based compounds with different donors and linkers has been
synthesized and well characterized. Compounds 1 and 2 have indole and 2-phenylindole groups as electron
donors, respectively. Compounds 3 and 4 have a diphenylamino group as the electron donor, and compound
5 has a 3,6-di-tert-butylcarbazole group as an electron donor. These compounds absorb in the blue-to-green
region and emit in the blue-to-red region depending on the electron donor, linker, and solvents. The quantum
yields of fluorescence of these compounds in solution are measured and found to be moderate, but in solid
states, they are high. These compounds display strong emission solvatochromism that is reflected by a large
shift in their fluorescence emission maxima on changing the solvents. This change is accompanied by a
successive decrease in fluorescence intensity. The fluorescence lifetimes of these compounds are measured in
different solvent and found to vary from <1 to 7 ns. Optical switching of these compounds with solvents,
concentration, and excitation energy have been studied. The correlation between the functional group and
optical properties has been established to some extent. The ability of these compounds to function as colorimetric
and luminescence pH sensors is demonstrated with color changes and luminescence switching upon the addition
of trifluoroacetic acid. The potentiality of these compounds for application in optoelectronics has been optically
assessed.
Introduction
application for use in EL devices, we chose to synthesize and
study the optical properties of electron donor-acceptor com-
pounds containing an aromatic fumaronitrile core showing
higher ΦF in the solid state than in solution. Fumaronitrile core-
based compounds have already attracted significant attention
as candidates in EL devices.9,10
Organic compounds with good photoluminescence responses
are promising candidates for a variety of optoelectronic ap-
plications. Conjugated organic compounds exhibit a variety of
interesting optical, electrical, and photoelectric properties in the
solid state.1 The advantages of organic compounds over
inorganic ones are their cost and the ease of processing.
Moreover, the use of functional groups has endowed the
molecular materials with unique and interesting optoelectronic
properties.2-4
Many conjugated organic light emitting compounds are highly
emissive in their dilute solutions but become weakly luminescent
when fabricated into devices.5 In the solid state, the molecules
aggregate to form less emissive species, such as excimers,
leading to a reduction in their luminescence efficiency.6 The
conjugated materials are practically utilized in the solid state
commonly as thin films. Many groups have attempted to
diminish aggregate formation through elaborate chemical,
physical, and engineering approaches. Swager and coworkers
found 3.5 times higher quantum yield of fluorescence (ΦF) for
poly(p-phenyleneethynylene) film than that for its solution.7,8
Donor-acceptor compounds having an aromatic fumaronitrile
core have attracted significant attention as candidates in elec-
troluminescent (EL) devices because they emit efficiently in the
solid state.9-11 It is assumed that the enhanced emission in the
solid state is due to the intramolecular planarization or a specific
aggregation (H- or J-aggregation) in such compounds. Such
aggregation-enhanced emissive materials are promising as
emitters in highly efficient OLEDs.8,12 Motivated by their likely
We describe here the synthesis and characterization of a novel
series of aromatic fumaronitrile core-based electron donor-
acceptor compounds 2,3-bis(4-(1H-indol-1-yl)phenyl)fumaroni-
trile (1), 2,3-bis(4-(2-phenyl-1H-indol-1-yl)phenyl)fumaronitrile
(2), 2,3-bis(4-(diphenylamino)phenyl)fumaronitrile (3), 2,3-
bis(4-(2-(4-(diphenylamino)phenyl)ethynyl)phenyl)fumaroni-
trile (4), and 2,3-bis(4-(2-(4-(3,6-ditert-butyl-9H-carbazol-9-
yl)phenyl)ethynyl) phenyl)fumaronitrile (5) (Figure 1). We chose
the carbazolyl, indolyl, and diphenylaminyl donors with and
without a π system extending phenylethynyl moiety to compare
the optical effect of phenylethynyl on the same donor and
acceptor. Not only do these compounds exhibit intense lumi-
nescence in solution and in solid state but they also show
excellent stability. In the pursuit of stable and efficient organic
compounds for optoelectronic application, it is important to
establish some correlation between the fluorescence quantum
yield and the π structure of fluorophores containing donor and
acceptor.13 So, we also tried to find such a direct correlation
between the functional group, π conjugation length, and donor/
acceptor strength versus ΦF.
Because diphenylfumaronitrile was used as the core, con-
centration-related fluorescence quenching in the solid state
should be greatly reduced because of the interaction of anti-
parallel dipoles.14 Carbazole derivatives show high emission
quantum yields in the solid state, and the compounds can easily
be modified at the 3, 6, and 9H positions to tune the optical
properties.15,16 Likewise, the diphenylamino group shows high
* Corresponding author. E-mail: tkinstl@bgsu.edu.
† Bowling Green State University.
‡ Angstrom Technologies, Inc.
10.1021/jp9115589  2010 American Chemical Society
Published on Web 03/17/2010

Aromatic Fumaronitrile Core-Based Compounds
J. Phys. Chem. A, Vol. 114, No. 13, 2010 4543
Results and Discussion
Compounds 6 (indole), 7 (2-phenylindole), and 8 (diphenyl-
amine) were purchased from Sigma-Aldrich and used without
purification. Compound 11 (N-(4-ethynylphenyl)-N-phenylben-
zenamine) was prepared from commercially available 9 (4-
bromotriphenylamine) by Sonogashira coupling to 3-methyl
butyn-3-ol, followed by reverse addition (Scheme 1). Similarly,
compound 16 (3,6-ditert-butyl-9-(4-ethynyl-phenyl)-9H-carba-
zole) was synthesized from commercially available 12 (carba-
zole) in four steps involving alkylation, aromatic substitution,
Sonogashira coupling, and deprotection (Scheme 1).15 Com-
pound 18 (bis(4-bromophenyl)fumaronitrile) was synthesized
from compound 17 (4-bromophenylacetonitrile) in one step
(Scheme 1) following literature procedure.9 Compounds 6, 7,
8, 11, and 16 were then coupled to compound 18 under similar
conditions to obtain compounds 1-5 (Scheme 2 and the
Supporting Information).
All compounds 1-5 are solid and soluble in common organic
solvents such as toluene, DCM, ACN, and THF. The visible
color of the compounds 1-5 are yellow, deep orange, red, dark
red, and orange, respectively. These materials are perfectly stable
in the solid state and could be stored without the need for any
special precautions.
Figure 1. Structures of compounds 1-5.
Absorption and Emission Spectra in Solution. The optical
properties of the synthesized D-π-A-π-D compounds were
investigated by UV/vis absorption and photoluminescence (PL)
spectroscopy using DCM solutions at room temperature.
Representative examples of the absorption and emission spectra
recorded in DCM are shown in Figures 2 and 3, respectively,
and the data obtained are summarized in Table 1. The absorption
maxima of these compounds range from 300 to 484 nm. The
peak around 290-350 nm is due to absorption by carbazole or
indole or biphenyl moiety. Similarly, peaks at 390-425 nm are
absorption by the intramolecular charge transfer complex (ICT)
formed by some compounds even in the ground state.22 The
presence of a strong electron donating (ED) group, an electron
withdrawing (EW) group, or both in 3 and 4 induces the
formation of strong ICT. Compound 5 forms the twisted
intramolecular charge transfer state (TICT) in the presence of
polar solvents.23,24 So, a weak band in the region of 425 nm is
due to the absorption by TICT for 5. Interestingly, the absorption
of compound 4 is blue-shifted from that of compound 3 even
though the former has two π-extending phenylethynyl moieties.
This indicates that the ethynyl groups allow free rotation in the
molecule to conformations in which electrons cannot be
delocalized easily from the nitrogen atom of the donors to the
acceptors vide infra.
emission efficiency as a dopant in electroluminescence studies
when attached to a cyano acceptor.17 Finally, indole-containing
compounds have been used for the preparation of luminescent
chelate compounds and can be used as effective red EL
materials.18
Experimental Section
Materials. The synthesis of compounds 1-5 is described in
the Supporting Information. Hexanes, toluene, dichloromethane
(DCM), acetonitrile (ACN), and tetrahydrofuran (THF) are
HPLC grade.
Measurements. Fluorescence quantum yields in solutions
were measured following a general method using riboflavin (0.3
in ethanol) and anthracene (0.27 in ethanol) as the standard.19,20
Dilute solutions of these compounds in appropriate solvents were
used for recording the fluorescence. Sample solutions of these
compounds in quartz cuvettes were degassed for ∼15 min. The
degassed solutions had an absorbance of 0.06 to 0.09 at
absorbance maxima. The fluorescence spectra of each of the
sample solutions were recorded three to four times and an
average value of integrated areas of fluorescence used for the
calculation of ΦF. The refractive indices of solvents at the
sodium D line were used.
Values of ΦF in the solid state were measured following a
literature method.21 A DCM solution of sample was cast as a
thin film on a spherical quartz plate and then allowed to dry.
The plate was then inserted into an integrating sphere, and the
required spectra were recorded. The samples were excited at
their absorption maxima in DCM. 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 because of
absorption of the emitted light.
To measure the fluorescence lifetimes (τF), we put the samples
in different solvents in quartz cuvettes. Fluorescence decay
profiles of the argon-degassed samples were measured using a
single photon-counting spectrofluorimeter. Decays were moni-
tored 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.
When excited at their Amax, dilute solutions of 1-5 in DCM
showed violet to red emission (Figure 3). Despite the similar
linker length and acceptor, the emission maxima vary from 360
to 614 nm. This indicates that the emission is mostly dependent
on the linkers and donors of these compounds vide infra.
Compound 1 has an indole moiety as ED and has the most red-
shifted emission of all. It forms strong ICT upon excitation.
So, in this compound, an excited ICT state is responsible for
the emission in the red region with large Stokes shift. Compound
2 has a weaker phenyl-substituted indole as an ED group
because nitrogen lone pair of electrons is delocalized by the
phenyl group. Additionally, the presence of the bulky phenyl
group causes planarity distortion of the phenylindole with the
rest of the molecule, thereby lowering the conjugation. The
formation of the ICT excited state is thus impeded. On keeping
EA and linker constant, ΦF in solution increases with the
stronger ED (Table 2). But in the solid state, ΦF follows a

4544 J. Phys. Chem. A, Vol. 114, No. 13, 2010
Panthi et al.
SCHEME 1: Synthesis of Compounds 11, 16, and 18a
a Reagents and conditions (a) 2-Methyl-3-butyn-2-ol, CuI, Pd(PPh3)2Cl2; (b) KOH; (c) t-BuCl, ZnCl2; (d) 1,4-dibromobenzene, K2CO3, Cu,
18-crown-6; (e) 2-methyl-3-butyn-2-ol, CuI, Pd(PPh3)2Cl2; (f) KOH.
SCHEME 2: Synthesis of Compounds 1-5
reverse order. A similar trend is observed in 3, which has a
stronger ED than 1 or 2. In compounds 4 and 5, which have
same EA and linker, ΦF is higher for the compound with weaker
ED both in solution and in solid state. Red-shifted emission
was expected for compounds 4 and 5 as compared with others
because of the presence of the extra phenylacetylene moiety in
the linker. However, we observed a blue-shifted emission for
them. The increased length of linkers distorts the planar
structure, and hence conjugation is impeded. Similar behavior
Figure 2. Normalized absorption spectra of 1-5 in DCM.
of linkers has been reported in similar compounds.25 Moreover,
their strange emissive behavior could also be due to the different
kind of hybridization of the bond. The triple bond is sp
hybridized, which is of linear configuration and substituents can
rotate freely in solution because of small energy barrier, thus
leading to a more twisted structure for 4 and 5 than similar
double-bonded compounds.26
The magnitudes of the Stokes shifts indicate large (vibrational,
electronic, geometric) differences between the excited state
reached immediately after absorption and the excited state from
which the emission starts. Charge-transfer processes should be
fairly effective because of the presence of a π-donor and a
π-acceptor group on these compounds.27 This effect also leads
to a very low degree of self-absorption of emitted light.

Aromatic Fumaronitrile Core-Based Compounds
J. Phys. Chem. A, Vol. 114, No. 13, 2010 4545
TABLE 2: Photophysical Data of π-Conjugated Compounds
1-5 in Different Solventsa
compd solvents ΦF τF (ns)
Kr (s-1
)
Knr (s-1
)
Aπ (Å)
1
2
3
4
5
hexanes
toluene
DCM
0.15
0.43
0.1
0.88
6.25
1.34
3.51
1.63
0.01
1.01
1.42
1.00
1.42
0.35
0.07
2.24
3.20
7.65
2.09
2.33
3.42
3.40
3.42
1.70 × 108 9.65 × 108
6.88 × 107 9.12 × 107
7.46 × 107 6.71 × 108
5.69 × 106 4.99 × 108
7.36 × 107 5.39 × 108
-1.73
-0.28
-2.20
-2.43
-1.99
ACN
0.02
0.12
0.10
0.01
<0.01
0.16
0.10
0.01
<0.01
0.49
0.80
0.74
0.01
0.43
0.77
0.12
0.08
hexanes
toluene
DCM
1.0 × 1010 9.10 × 1010 -2.20
9.90 × 106 9.80 × 108
-4.59
-4.74
-1.65
ACN
7.042 × 106 6.97 × 108
hexanes
toluene
DCM
1.6 × 108
8.4 × 108
Figure 3. Normalized emission spectra of 1-5 in DCM. (Compounds
were excited at the corresponding Amax. Compounds 1-5 were excited
at 418, 306, 450, 440, and 347 nm, respectively.)
7.04 × 1010
7.0 × 1011 -2.29
2.85 × 107 2.82 × 109
-4.59
ACN
1.42 × 108 8.00 × 1010 -6.33
hexanes
toluene
DCM
2.18 × 108 2.28 × 108
2.50 × 108 6.12 × 107
9.64 × 107 3.36 × 107
4.78 × 108 4.74 × 108
1.84 × 108 2.45 × 108
-0.20
1.40
1.05
-4.59
-0.28
1.21
Therefore, these compounds are predicted to perform well in
organic light-emitting diodes and as laser dyes.
ACN
hexanes
toluene
DCM
Solvatochromism: Optical Switching With Solvents. In an
effort to gain further insight into the photophysical processes
in these compounds, we investigated their absorption and
emission behaviors in different solvents. The results of these
investigations are summarized in Table 1. The absorption spectra
are nearly independent of solvent polarity except for a slight
insignificant shift that indicates a negligible intramolecular
interaction between donor and acceptor groups in the ground
state. In contrast, the emission spectra exhibit distinct solvent
dependence. Broad structureless emission and larger Stokes
shifts were observed for compounds 1-3 on increasing the
solvent polarity along with a successive decrease in the
fluorescence intensity. Compounds 4 and 5 showed the reverse
behavior. This kind of behavior has been explained fully for
various chromophores containing donor-acceptor units.14 In
general, the absorption and emission maxima are gradually red-
shifted when the compounds are dissolved in the nonpolar
hexanes to polar ACN. Surprisingly, both absorption and
emission are either the same or blue-shifted in ACN with respect
to medium polar solvents like toluene or DCM for compounds
4 and 5 vide infra.
2.25 × 108
3.5 × 107
6.7 × 107
2.5 × 108
-1.96
-2.44
ACN
2.33 × 107 2.68 × 108
a Compounds were excited at the corresponding Amax representing
the π-π* transition for τF and ΦF. The τF and ΦF were measured
from argon-saturated solutions, and decay was monitored at the
corresponding λmax. The ΦF values for 1, 3, and 4 are relative to
that of riboflavin (0.30 in ethanol). The ΦF values for 2 and 5 are
relative to that of anthracene (0.27 in ethanol).
Also, the Stokes shift in all solvents for this compound is much
smaller (50 nm) in comparison with other compounds for the
reason described above. The compounds 1 and 3 show positive
solvatochromism. The cases of compounds 4 and 5 are different
because a systematic solvatochromic effect is not observed
among solvents like in hexanes, toluene, and DCM (Figures 5
and 6 and the Supporting Information). Compound 4 shows
positive solvatochromic effect from hexanes to toluene and
negative solvatochromism from toluene to ACN (Figures 5 and
6). In toluene a red-shifted emission is observed, which is
probably due to the stabilization of excited state by aromatic
electrons of toluene. The excitation spectra obtained by monitor-
ing the emission at the emission maximum of 4 (Supporting
Compound 2 shows a much smaller solvatochromic effect
for absorption and emission. (See the Supporting Information.)
TABLE 1: Absorption Maximum (Amax), Emission Maximum (λmax), Molar Absorptivity (ε), Stokes Shifts, and Solid-State
Quantum Yields of 1-5 Recorded in Four Different Solventsa
absorption
(Amax) (nm)
molar ext. coefficient
emission (λmax
)
Stokes shifts
(nm)
compound
solvent
ε (dm3 cm-1 mol-1
)
(nm)
solid state ΦF
1
hexanes
toluene
DCM
391
414
418
400
304
306
306
305
456
479
450
480
440
450
440
420
346
346
347
345
516
550
631
552
351
360
360
360
560
603
637
700
566
630
614
528
477
513
446
446
125
136
213
152
47
54
54
54
104
122
177
220
126
180
174
108
131
177
99
0.80
19 147
ACN
2
3
4
5
hexanes
toluene
DCM
38 933
46 058
29 953
62 183
0.49
0.38
0.55
0.64
ACN
hexanes
toluene
DCM
25 177
18 693
20 881
47 178
43 060
73 266
69 523
16 496
25 237
29 812
41 966
ACN
hexanes
toluene
DCM
ACN
hexanes
toluene
DCM
ACN
101
a Excitation is at Amax for each compound in corresponding solutions. Solid-state ΦF values were measured by using an integrating sphere
(errors within 15% range).

4546 J. Phys. Chem. A, Vol. 114, No. 13, 2010
Panthi et al.
Figure 7. Solutions of compound 4 in different solvents (A) hexanes,
(B) toluene, (C) DCM, and (D) ACN. The left side of the Figure is
under day light, and the right side is under UV 365 nm.
Figure 4. Normalized absorption and emission spectra of 3 in hexanes,
toluene, DCM, and ACN. (Emission spectra are recorded at the
respective Amax of the samples. They are excited in hexanes at 456 nm,
toluene at 479 nm, DCM at 450 nm, and ACN at 480 nm).
formation of ICT excited state occurs. When exciting DCM
solution of 4 at 340 nm, we observed emission mainly on the
blue region with a shoulder around 575 nm. The emission in
the blue region is due to the LE state, and that around 575 is
from the ICT excited state. When exciting the same solution at
440 nm, we observed emission primarily in the red region, which
is emission from the ICT state only. The same results were not
observed in other solvents because only LE was formed in ACN
and predominantly ICT was formed in toluene and hexanes.
Changes in the emission color of 4 in different solvents on
excitation at 365 nm is illustrated in Figure 7.
Significantly high Stokes shifts of 213 nm for 1, 220 nm for
3, and 180 nm for compound 4 are observed. The absorption
and emission spectra of compound 3 show 104 nm Stokes shifts
in hexanes, 122 nm in toluene, 177 nm in DCM, and 220 nm
in ACN (Figure 4).
Figure 5. Emission spectra of 4 in hexanes, toluene, DCM, and ACN
excited at 340 nm.
Correlation between Quantum Yield and Magnitude Aπ
of the π Conjugation Length. We tried to establish correlations
among ΦF, donor strength, and length of linker in these
compounds. Because no simple correlations could be observed,
a possible correlation between the ΦF and the magnitude (Aπ)
of the π conjugation length in the excited singlet state has been
discussed. We derive Aπ from the radiative (rate constant: kr)
and radiationless process (rate constant: knr).13 The obtained kr
and knr values are shown in Table 2. On absorption of light,
unshared electrons on the nitrogen atom of the donor group
promote to the acceptor. Because the acceptor and donor are
strong, the excited state is a charge transfer state with partial
positive charge on the N atom of donor and partial negative
charge on nitrile moiety. The kr is the rate constant for the return
process of an n-electron from the EA group promoted to the
excited singlet (S1) state to the ED in the ground (S0) state of
a D-π-A-π-D system molecule accompanying fluorescence
emission.
Similarly, knr is the rate constant for the radiationless process
due to internal conversion and intersystem crossing. The
magnitude of conjugation length may be defined as the distance
between dipoles arising from S0 to S1 transition (light absorp-
tion). Thus, knr is considered to be the rate constant of
radiationless decay due to the electron transfer (ET) from the
negative charge to the positive charge. As shown in Table 2,
the kr values increase and the knr values decrease with π
conjugation length of the D-π-A-π-D system, suggesting that
the kr/knr value might be related to π extension in the S1 state
of the D-π-A-π-D molecule. The magnitude of conjugation
length (Aπ) is derived from kr and knr. On plotting ΦF against
Aπ (Figure 8), a line with a positive slope is obtained. From
this graph, we can see that increasing the strength of the ED,
the length of the linker, or both also increases the ΦF.
Fluorescence Switching With Concentration: Concentra-
tion Quenching. A significant fluorescence quenching of
compounds 1-5 has been observed in THF at higher concentra-
tions (Supporting Information). For example, the fluorescence
intensity almost triples when the concentration of compound 5
Figure 6. Emission spectra of 4 in hexanes, toluene, DCM, and ACN
excited at 440 nm.
Information) shows that the emission is due to radiative
deactivation of both local excited (LE) and ICT excited state
in nonpolar solvents (hexanes and toluene), but in polar solvents
such as ACN and DCM, the emission occurs predominantly
from the LE state. In its ground state, 4 forms ICT state in polar
solvents. On excitation, a red-shifted emission is observed in
nonpolar solvents, but in polar solvents, a blue shift emission,
a property called negative solvatochromism, is observed.28 This
suggests that the dipole moment of 4 decreases on excitation,
and thus the less polar ICT excited state becomes more stable
in nonpolar solvents and less stable in polar solvents. The same
holds true for 5, where TICT forms instead of ICT (Supporting
Information). The nonplanar alignment of the carbazolyl moiety
with 9H-substituted phenyl group is responsible for the forma-
tion of TICT.29 Therefore, this abnormal solvatochromic be-
havior of compounds 4 and 5 is probably due to their different
donors and to the formation of TICT or ICT in their singlet
states.23,24
Compound 4 shows interesting emission properties. When a
DCM solution is excited at 340 nm, emission in the blue region
was observed, and on exciting the same solution at 440 nm, we
observed emission in the red region only (Figures 5 and 6).
DCM is less polar than ACN, so upon excitation, very little

Aromatic Fumaronitrile Core-Based Compounds
J. Phys. Chem. A, Vol. 114, No. 13, 2010 4547
Figure 8. Plot of quantum yield versus π conjugation length in hexanes
and toluene for compounds 1-5.
Figure 9. Emission spectra of 4 recorded in THF at different
concentrations: (C1) 7.3 × 10-4 M, (C2) 7.3 × 10-5 M, (C3) 7.3 ×
10-6 M, and (C4) 7.3 × 10-7 M. (Inset: the enlarged spectrum of 4
recorded at 7.3 × 10-4 M).
is increased from 4.5 × 10-7 to 4.5 × 10-6 M, but it decreases
by almost the same amount when the concentration is increased
to 4.5 × 10-5 M and is almost completely quenched at a
concentration at 4.5 × 10-3 M. Although the fluorescence
intensity is significantly decreased at a higher concentration,
the λmax remains unchanged. A similar type of behavior is shown
by all other compounds (Supporting Information). Although no
considerable quenching behavior of fluorescence was observed
in compounds 4, 3, and 2, in compound 1, quenching was
observed even at 3.1 × 10-5 M concentrations. However, in
these cases, the λmax values were found to be slightly blue-
shifted. This might be due to the aggregation at higher
concentration. In compound 5, there is no shift on emission with
concentration change, which is probably because the tertiary
butyl groups prevent aggregation.
Figure 10. Emission spectra of compound 4 in DCM at different
concentrations. (A) excitation wavelength 340 nm, (B) excitation
wavelength ) 400 nm, and (C) excitation wavelength 440 nm. C1 )
5.55 × 10-4 M, C2 ) 2.22 × 10-4 M, C3 ) 6.93 × 10-5 M, C4 )
2.22 × 10-5 M, and C5 ) 2.22 × 10-6 M concentration.
In Figure 9, the emission spectra of 4 in THF at different
concentrations are plotted. At higher concentration, the emission
appears mainly in the 500-600 nm region and on dilution, the
emission appears mainly in the region of 400-500 nm with an
increase in intensity. We did not observe such significant shift
on emission with the other compounds, although there is
concentration quenching in all of them. To investigate this
interesting property of 4 further, we measured emission spectra
in DCM and found a similar but more significant effect
(Supporting Information).
Figure 11. Emission spectra of DCM solutions of compound 4 in the
presence of TFA and Et3N at various pH. (A) DCM, (B) TFA (pH 4),
(C) TFA (pH 3), (D) TFA (pH 2), (E) TFA (pH 1), (F) Et3N (pH 9),
and (G) Et3N during neutralization of D (pH 6).
Excitation Energy Dependence Fluorescence; Edge Exci-
tation Red Shift (EERS). Contrary to Kasha’s rule, fluores-
cence of 4 depends on the excitation energy. Compound 4 in
DCM was excited with different energies (340, 400, and 440
nm) at the same concentration. The emission spectra obtained
are plotted in Figure 10A,B,C. The switching of emission with
excitation energy was observed. On excitation at 340 nm,
emission was found mainly on the blue region with lower
intensity in the red region. On excitation of same solution at
400 nm, a perfect white emission is observed, and on excitation
of the same solution with 440 nm, we observed the emission
mainly in the red region. This effect has been referred to as
EERS or red-edge excitation shift.30 This phenomenon can be
described by considering the emission contribution from ad-
ditional 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
Franck-Condon excited-state distribution (F-CESD) that is a
function of excitation energy.30-32 For a polar compound in polar
solvent, the energy required to excite the solvated species is a

4548 J. Phys. Chem. A, Vol. 114, No. 13, 2010
Panthi et al.
Figure 12. Change in color of DCM solutions of compound 4 in the presence of TFA and Et3N at various pH. (A) DCM, (B) TFA (pH 4), (C)
TFA (pH 3), (D) TFA (pH 2), (E) TFA (pH 1), (F) Et3N (pH 9), (G) Et3N during neutralization of D (pH 6), and (H) Et3N during the neutralization
of E (pH 5).
function of solvent orientation. If the excitation energy is
smaller, then only limited configurations of the ground state
may be excited. The excitation with particular energy will excite
only the particular fraction of total fluorophore population, which
is surrounded by solvent dipoles.
Fluorescence Switching With a Change in pH. The nitrogen
atoms of donors are basic centers that can be protonated.
Therefore, the effect of protonation on the optical properties of
DCM solutions of 1-5 was also studied. DCM solutions of
some compounds underwent a significant color change in the
presence of trifluoroacetic acid (TFA). This color change was
found to be reversible. Representative spectra for the change in
optical properties with a change in pH for compound 4 are
illustrated in Figure 11, and the color change under UV light at
365 nm is given in Figure 12.
There is not a remarkable change in absorption spectra on
addition of TFA on DCM solution of 4. However, the emission
completely switches from the white region to the blue region
upon addition of TFA. In DCM, the emission is a consequence
of radiative deactivation from both excited ICT and LE state at
this particular concentration. The ICT state is responsible for
emission in the red region, and the LE state is responsible for
emission in the blue region. When TFA is added, the ICT state
becomes less prominent as H+ from TFA consumes the lone
pair of electrons from N of the donor, forming N+, so no charge-
separated ICT is possible. The protonated species emits in blue
region. Again, on addition of triethyl amine (Et3N), the proton
is removed by it, ICT becomes more prominent, and emission
by both the LE and ICT states occurs in the blue and orange-
red region. This property of 4 could find the application as pH
sensors. Similar but less significant effects have been observed
for compounds 1 and 5. Compounds 2 and 3 did not show a
similar change in optical properties on the addition of the same
amounts of TFA (Supporting Information).
sensor is optically assessed with a change in emission color
with a change in pH. Their optical properties in solution and in
the solid state indicate that these compounds could find
application in optoelectronic device fabrication.
Acknowledgment. This present work has been partially
supported by grant NSF-EXP 40000/10380080. We thank the
donors of these funds. We thank Dr. Sujeewa S. Palayangoda
for suggestions concerning synthesis and Dr. Douglas C.
Neckers and Puran De for instruments and assistance.
Supporting Information Available: Experimental details
1
and H and 13C NMR spectra of compounds 1-5, 10, and 11,
UV-visible spectra in different solvents, excitation spectra,
emission spectra in THF at different concentrations, and
absorption and emission spectra during the addition of TFA and
Et3N in DCM maintaining different pH of compounds 1-5. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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