Installation/modulation of the emission response via click reaction

Article abstract of DOI:10.1021/jo200231k

We have demonstrated the installation of a fluorescence property into a nonfluorescent precursor and modulation of an emission response of a pyrene fluorophore via click reaction. The synthesized fluorophores show different solvatochromicity and/or intramolecular charge transfer (ICT) feature as is revealed from the UV-visible, fluorescence photophysical properties of these fluorophores, and DFT/TDDFT calculation. We observed that some of the synthesized fluorophores showed purely ICT character while emission from some of them arose from the LE state. A structureless and solvent polarity-sensitive dual emission behavior was observed for one of the triazolylpyrene fluorophores that contains an electron-donating -NMe₂ substituent (fluorophore, 7a). Conversely, triazolylpyrene with an electron-withdrawing -CN group (fluorophore, 7b) showed a solvent polarity-independent vibronic emission. The effect of ICT on the photophysical properties of these fluorophores was studied by fluorescence emission spectra and DFT/TDDFT calculations. Fluorescence lifetimes were also measured in different solvents. All of our findings revealed the delicate interplay of structure and emission properties and thus having broader general utility. As the CT to LE intensity ratio can be employed as a sensing index, the dual emissive fluorophore can be utilized in designing the molecular recognition system too. We envisage that our investigation is of importance for the development of new fluorophores with predetermined photophysical properties that may find a wide range of applications in chemistry, biology, and material sciences.

Full text of DOI:10.1021/jo200231k

ARTICLE
pubs.acs.org/joc
Installation/Modulation of the Emission Response via Click Reaction
Subhendu Sekhar Bag* and Rajen Kundu
Bioorganic Chemistry Laboratory, Department of Chemistry, Indian Institute of Technology, Guwahati-781039, India
S Supporting Information
b
ABSTRACT: We have demonstrated the installation of a
fluorescence property into a nonfluorescent precursor and
modulation of an emission response of a pyrene fluorophore
via click reaction. The synthesized fluorophores show different
solvatochromicity and/or intramolecular charge transfer (ICT)
feature as is revealed from the UVꢀvisible, fluorescence photo-
physical properties of these fluorophores, and DFT/TDDFT
calculation. We observed that some of the synthesized fluor-
ophores showed purely ICT character while emission from some of them arose from the LE state. A structureless and solvent
polarity-sensitive dual emission behavior was observed for one of the triazolylpyrene fluorophores that contains an electron-
donating ꢀNMe2 substituent (fluorophore, 7a). Conversely, triazolylpyrene with an electron-withdrawing ꢀCN group
(fluorophore, 7b) showed a solvent polarity-independent vibronic emission. The effect of ICT on the photophysical properties
of these fluorophores was studied by fluorescence emission spectra and DFT/TDDFT calculations. Fluorescence lifetimes were also
measured in different solvents. All of our findings revealed the delicate interplay of structure and emission properties and thus having
broader general utility. As the CT to LE intensity ratio can be employed as a sensing index, the dual emissive fluorophore can be
utilized in designing the molecular recognition system too. We envisage that our investigation is of importance for the development
of new fluorophores with predetermined photophysical properties that may find a wide range of applications in chemistry, biology,
and material sciences.
’ INTRODUCTION
been given limited attention. Here, we want to report our design
toward the installation of an emission property into a nonfluor-
escent precursor on one hand and the modulation/enhancement
of an emission response of a weakly fluorescent molecule via click
chemistry on the other hand (Figure 1). The main importance of
our work is that we utilized readily available nonfluorescent starting
materials of very low cost and installed an emission property via
triazole formation. The synthesized fluorophores are solvofluoro-
chromic and thus can be used for applications in chemistry, material
science, and other related areas of interest.1e,2b,3
Fluorescent organic molecules have been well-known for a
long time for probing biomolecular structure, properties, and
functions.1 However, the photophysical properties of many of
the fluorophores sometimes could not be interrelated with their
structure due to the deficiency of distinct rationale for their design.
Moreover, many of the fluorophores are neither long wavelength
emissive nor solvatochromic and hence, disabling their use in cell or
in sensing the change in microenvironment in the biological world.
Therefore, the design of microenvironment sensitive as well as long
wavelength emissive organic fluorophores with a predictable photo-
physical property in relation to their designed structures via a simple
and versatile method is highly desirable for such biochemical
applications.1 The excellent regioselectivity, simplicity along with
the versatility of the reaction conditions, render “click chemistry”
advantageous for synthesizing small organic fluorescent molecules
and for achieving the selective coupling of small fluorogenic mole-
cules within a complex biological environment, which enhances
greatly the efficiency of biological studies aimed at understanding
the natural biological processes via a signal generation.1,2
’ RESULTS AND DISCUSSION
In our design donor and acceptor units are linked via a triazole
moiety so as to allow a charge transfer process that leads to an
installation of a fluorescence emission onto a nonfluorescent pre-
cursor (Figure 1). We have also focused on the design of a triazolyl-
linked intrinsic fluorophore for modulation of its emission response.
For this purpose we have chosen a versatile fluorophore, pyrene,
which shows featured excimer/monomer emission and long fluor-
escence lifetime. However, the pyrene fluorophore suffers from short
emission wavelength, which makes it difficult to use for applications
such as in vivo fluorescence imaging. Therefore, preparation of
long wavelength emissive and solvatochromic pyrene derivatives is of
immense importance.4ꢀ6 Thus, we have designed pyrene-based
Only recently, have a few numbers of click fluorescent molecules
been reported wherein the intrinsic fluorescence property of
the well-explored fluorophores is tuned.1c,3 However, the research
efforts toward the installation of the fluorescence property into a
nonfluorescencet precursor and/or the modulation of the emission
response of a weakly fluorescent molecule via “click chemistry” and
thereby the synthesis of new solvatochromic click-fluorophores has
Received: February 10, 2011
Published: March 14, 2011
r
2011 American Chemical Society
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Figure 1. (a) Schematic presentation of the fluorescence installation/modulation via click reaction and (b) the chemical structures of the azides and
alkynes used in this study.
Scheme 1. (a) General Procedure for the Synthesis of Aromatic Azides and Click Fluorophores and (b) Chemical Structures of
the Synthesized Fluorophores
fluorophores keeping in mind that the electron-donating property of
the triazolyl ring may effectively modulate the emission response of
an electronically coupled pyrene (Figure 1).
nonfluorescent precursor amines or alkynes, compounds 7a,b
(Figure 5, 6) have been synthesized to show the modulation of
an emission response of pyrene (Figure 1 and Scheme 1).
Studies of Photophysical Properties. Having all the pro-
ducts in hand, we have studied, first, their photophysical proper-
ties in different solvents of varying dielectric constants. The aim
of this study is to explore the effect of solvent polarity on the
fluorophores’ photophysical properties and to correlate these
effects to their structure.
Whereas solvent polarity has only a minor influence on the
absorptive properties of almost all the fluorophores, it drastically
affects their emission properties. We have observed that all the
products showed an increase in fluorescence intensity over their
parent amine or the alkyne precursors. It has also been found that the
responses of some of the products are highly sensitive toward a
solvent’s dielectric constant leading to a red-shifted emission. In
general, the fluorophore of the type D(donor)-T(triazole)-A-
(acceptor) wherein the precursor amine and alkyne contain a
donor ꢀOMe group and an acceptor ꢀCN group, respectively, as
in the case of 4b, showed a structureless emission with a red shift
(Figure 3b). Also, the quantum yield is increased as the solvent
Synthesis. The synthesis was accomplished (Scheme 1) via a
modified click reaction protocol2 and the products obtained in
moderate to high yields were characterized well by NMR, mass,
and in some cases single crystal X-ray analysis. Thus, to a 1:1
dichloromethane:ethanol solution of aryl azides (AꢀD, Figure 1;
synthesized via a diazotization reaction of the corresponding aryl
amines and subsequent treatment with sodium azide), 1.5 equiv
of alkyne (E and F, Figure 1) was added. After 5 min of degassing
with N2 and stirring, a solution of sodium ascorbate (6 mol %)
followed by a solution of CuSO4 in water (1 mol %) was added to
the reaction mixture. After completion of the reaction monitored
by TLC, the reaction mixture was evaporated to dryness, partitioned
between water and ethyl acetate. The organic layer was washed
with water and brine, then dried over Na2SO4. The final product
was obtained in purity by column chromatography and character-
ized. While compounds 4a, b (Figures 2, 3), 5a, b (see SI, Figure S9,
S10), and 6a, b (see SI, Figure S11, and Figure 4, respectively) are
designed to show an installation of a fluorescence response into
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polarity is increased (Table 1). These results infer an intramolecular
charge transfer (ICT) feature of the lowest-lying excited state,7 which
is supported by a solvatochromicity test (Figure 3c).
We have observed structureless, broad, and strong solvent
polarity dependent dominant emission from the fluorophore 4a
(a D-T-D system) (Figure 2b). Thus, 4a showed a 122 nm red-
shifted emission when compared between the solvent hexane
(almost zero intensity) and MeOH. Upon titration of a dioxane
solution of 4a by water a red shift of 129 nm was observed in 50%
water (Figure 2c). These features indicate an emission from an
ICT state.7,8 Fluorescence quantum yields are also found to
be increased with the increase in solvent polarity (Table 1). This
clearly indicates a highly polar emissive state. However, it is
decreased in MeOH and ethanol, which may be due to a protic
solventꢀsolute interaction.7a,9
Table 1. Fluorescence Photophysical Properties of the
Fluorophoresa
τ1
τ2
Æτæ
kf
knr
[108 sꢀ1
]
entry solvents
Φf
[ns] [ns]
[ns] [108 sꢀ1
]
4a
dioxane
DMSO
EtOH
0.052 0.78 2.71 0.847
0.79
7.5
11.01
17
0.308 0.41
0.093 7.08
0.41
7.08
8.26
0.13
0.11
1.63
2.02
2.6
1.28
1.88
165.0
1.24
3.08
1.09
8.31
3.23
2.83
2.44
3.40
1.1
Fluorophore 6b showed a comparatively less solvent polarity
sensitive emission that is centered at around 406 nm. A spectral
shift of only 23 nm is observed with a decrease in intensity as we
move from hexane to MeOH. This result is clearly an indication
of an ICT emission (Figure 4).
15% H2O 0.094 8.26
4b dioxane
DMSO
0.010 0.04 3.06 0.06
0.619 3.06
0.46 1.75
3.06
1.75
3.45
MeOH
50% H2O 0.623 3.45
1.8
Compounds 5a,b and 6a showed only faint emission under the
condition used for others. However, at large slit width, the
following behaviors were observed where quantum yields could
not be calculated. Thus, fluorophore 5b (A-T-A system) showed
a solvent polarity independent structured emission (see SI,
Figure S10). A highly solvent polarity dependent emission was
exhibited by fluorophore 5a, an A-T-D system. Thus, as the
solvent polarity increased, a shift of 56 nm from cyclohexane to
MeOH with decreased intensity was observed, which is char-
acteristic of ICT emission (see the SI, Figure S9). Fluorophore 6a
exhibited a dual behavior similar to that of 7a in low polar
solvents, but a single emission centered at 357 nm appeared in
high polar solvents like ACN, DMSO, and MeOH. In low polar
solvents like hexane, cyclohexane, ether, 1,4-dioxane, toluene,
chloroform, and ethyl acetate, the second band at 399 nm in hexane
shifted gradually with a decrease in intensity as the polarity increases
and reaches 537 nm in ethyl acetate, which again inferred an emis-
sion from ICT state (see the SI, Figure S11).
7a
hexane
toluene
dioxane
EtOAc
CHCl3
MeOH
0.180 0.81 2.50 0.986
0.202 1.59 3.26 2.463
0.076 2.95 6.57 3.256
1.82
0.82
0.23
0.13
0.40
0.003
0.09
0.10
0.23
0.08
0.022 3.87
0.059 2.62
3.87
2.62
0.003 1.87 14.30 8.988
10% H2O 0.019 1.70 5.47 2.285
4.28
1.39
0.37
0.58
7b hexane
DMSO
0.071 1.40 8.71 6.655
0.39 16.49
0.13 14.99
16.49
14.99
MeOH
a Excitation at λex nm and recorded at λmax. Æτæ, kf, and knr are weighted
means from the biexponential fits: Æτæ = 1/(R1/τ1 þ R2/τ2), kf = Φf/Æτæ,
and knr = (1 ꢀ Φf)/Æτæ.7c,g
Table 2. The Dipole Moment and Excitation Energy Values
of 4a, 4b ,and 7a
We have observed an interesting fluorescence photophysical
property exhibited by the fluorophore 7a (an A-T-D system), a
triazolyl modified pyrene. Thus, the fluorophore 7a showed a
strong structureless emission centered at 415 nm in hexane and
cyclohexane. However, in ether, dioxane, toluene, CHCl3, and
EtOAc, a dual emission behavior has been observed with a strong
red-shifted pattern of the ICT band as the solvent polarity is in-
creased from toluene (466 nm) to EtOAc (544 nm) (Figure 5b).
Δμ in debye
4aa
4ba
7aa
Δμb = (μe ꢀ μg)
excitation energy (calcd) in eV
24
21
21
4.55
4.27
4.14
4.35
3.54
3.61
excitation energy (exptl) in eV
a Onsager radii for 4a, 4b, and 7a are 6.6, 7.2, and 6.3 Å, respectively
(obtained from B3LYP/6-31G* optimized geometry). b Calculated from
eq 2 and slope from the plots of Δν~ values against Δf.
Figure 2. UVꢀvisible (a) and fluorescence spectra (b) of 4a in different solvents and fluorescence in dioxaneꢀwater solvent system (c) (10 μM, rt; λex
= λmax of each solvent).
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Figure 3. UVꢀvisible (a) and fluorescence spectra (b) of 4b in different solvents and fluorescence in dioxaneꢀwater solvent system (c) (10 μM, rt; λex
= λmax of each solvent).
Figure 4. UVꢀvisible (a) and fluorescence spectra (b) of 6b in different solvents (10 μM, rt; λex = λmax of each solvent).
In highly polar solvents like ACN and MeOH, a structured band
characteristic of a pyrene emission with a very low intensity has
been observed that is also supported by a solvatochromicity test
(Figure 5c). Thus, in pure dioxane, the ICT emission is highly
dominated over the LE emission and as the percent of H2O is
increased, the intensity of the LE emission is also going to
increase. However, in 50% H2O in dioxane, we have observed a
pure LE emission (Figure 5c). Compared to pyrene-amine, 7a
showed about 100 nm of red-shifted emission in polar solvents.
On the other hand, in the case of the fluorophore 7b (D-T-A
system), which contains an electron-withdrawing ꢀCN substi-
tuent, the emissioniscenteredat400nm, andthe vibronicstructure
of a pyreneemission is maintained, i.e., the emission originates from
a purely LE state (Figure 6b).7c,10 The fluorescence quantum yield
of 7b is increased as the solvent polarity is increased with no
solvatochromicity (see the SI, Table S5) while it is lower in a polar
solvent than that in a nonpolar solvent in the case of 7a (Table 1,
andseeTableS5 intheSI). These resultsinferanICT feature ofthe
lowest-lying excited state in 7a (with the pyrene as an electron
acceptor) but not in 7b.7,8,10
these fluorophores show a direct trend between the fluorescence
intensities and solvent dielectric constants and thus have a broader
general utility (see the SI, Figures S5ꢀ12). Examination of the
band-shape and fluorescence quantum yields of the fluorophores
reveals the delicate interplay of structure and emissive states. Thus,
compounds 4a, 4b, and 6b showed a highly emissive, pure ICT state
as is indicated by a high quantum yield, broad band-shape, and
red-shifted pattern as the solvent polarity is increased. The solvent
polarity independent LE emission of 7b suggests the emitting state as
a nonpolar 1πꢀπ* state. The dual emission exhibited by 7a indicates
the presence of a mixed LE and CT state. Switching between these
two states depends on the structure and the solvent polarity.8,10e
To get an insight into the different solvatochromic behaviors
of 4a,b and 7a,b, the spectral dependency on solvent polarity was
studied on the basis of the LipertꢀMataga model.11 The Lip-
pertꢀMataga polarity parameter (Δf) has been considered as the
measure of the polarity of different solvents and solvent mixtures
used and was calculated by using the following eq 1.11ꢀ14
ε ꢀ 1
n2 ꢀ 1
Δf ¼
ꢀ
ð1Þ
2ε þ 1 2n2 þ 1
Therefore, our study clearly shows that a “click” reaction between
an azide and an alkyne sufficiently installs a fluorescence emission
property into a nonfluorescent precursor and modulates the emis-
sion response of a pyrene fluorophore. It is important to note that
Thus, the absorption (λambasx) and fluorescence (λflmax) maxima
of the more intense fluorophores 4a,b and 7a were measured in
different solvents and solvent mixtures along with the polarity
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Figure 5. UVꢀvisible (a) and fluorescence spectra (b) of 7a in different solvents and fluorescence in dioxaneꢀwater solvent system (c) (10 μM, rt;
λex = λmax of each solvent) showing a dual behavior.
Figure 6. UVꢀvisible (a) and fluorescence spectra (b) of 7b in different solvents (10 μM, rt; λex = λmax of each solvent).
parameter Δf of the different solvent systems calculated with eq 1
(see the SI, Table S3). As is revealed from the UVꢀvisible and
fluorescence spectra (Figures 2 and 3 for 4a,b and Figures 5 and 6
for 7a,b, respectively), the λflmax values gradually undergo a
the fluorophore 4a,b and 7a, respectively, with the solvent
polarity function Δf (Figure 7aꢀc). Thus, from Figure 7b and
c it is seen that for the fluorophore 4b, and 7a, the ν~abs values
apparently correlate linearly with Δf, suggesting that the ground
state of these fluorophores is moderately polar in nature. For the
fluorophore 4a, a smaller correlation is observed for the ν~abs vs Δf
plot (Figure 7a). However, following the UVꢀvisible spectra
(Figurea 2a and 3a for 4a and 4b, respectively), which is quite
similar to that of the 4b, we expect that the ground state of the 4a
is also of polarity similar to that of the 4b.
However, as is shown in Figure 7aꢀc, the ~νfl values for all the
fluorophores show a very good linear correlation with Δf for
the whole range of the solvent polarity tested. This observation,
therefore, suggests that for all the fluorophores the nature of
the fluorescent states remains essentially unchanged in all the solvents
used. However, the spectral feature changes significantly due to the
solvent-induced modulation. The reasonably high slopes of the ~νfl vs
Δf plots for all the fluorophores further suggest that the fluorescent
states of these fluorophores are highly polar in nature, most possibly
of intramolecular charge transfer (ICT) character.7,8,10,15ꢀ17
red shift when the solvent polarity increases for all the florophores.
max
abs
However, the red shift of λmfl ax is much larger than the shift in λ
values, suggesting the fluorescent states of all the fluorophores to be
more polar in nature than their ground states.7c,10g,11b,11c,14b,15ꢀ17
In particular, fluorophore 4a,b showed a slight red shift with Δf in
the lower solvent polarity region (Δf < 0.265) but at the higher
abs
max
solvent polarity region the λ values showed a small blue shift (see
the SI, Table S3). However, as the solutions of 4a,b in dioxane were
titrated with water, a normal red shift was observed for 4a while a
blue shift was the result for fluorophore 4b. The λflmax values gradually
undergo a more red shift when the solvent polarity increases for 4a,b.
abs
max
In the case of fluorophore 7a, the λ values showed a very small red
shift in low polar solvents and a blue shift in high polar solvents;
abs
max
however, in the dioxaneꢀwater solvent system the λ remains
almost unchanged. A similar behavior was observed for fluorophore
7b. However, the λflmax of the ICT band shifted much more to the red
for 7a while an unchanged but increased intensity of the λflmax was
observed for 7b as the solvent polarity is increased.
In different solvents, the absorption and fluorescence spectra
involve the electronic transitions between the same two ground
and excited electronic states, and the Stokes’ shift (Δ~ν) is expected
to follow a linear relation with Δf, as suggested by the Lippert and
Next, we examined the correlations of the absorption and
fluorescence maxima (ν~ and ν~flmax, respectively, in cmꢀ1) of
abs
max
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Figure 7. Plots of ν~fl and ν~abs values against Δf for fluorophore 4a (a), 4b (b), and 7a (c) (ν~fl(ICT) and ν~abs values) in different solvents.
are fully allowed for all the fluorophores (Figure 9, and in the SI
Figures S16ꢀ23, and Table S6). This means that reverse tran-
sition, i.e., S0 r S1, is also fully allowed indicating the potential
fluorophoric nature of the molecules. The emissive state of 7b
and 5b is basically a locally excited (LE) state as is suggested
by the HOMOꢀLUMO distributions. For 4b (Figure 9b), 5a,
and 6b, however, the emissive state is characterized with more
significant electron redistribution, i.e., ICT feature. Calculated
results supported our above explanation.19 Calculated excitation
energies for the transition of S0 f S1 of 7b and 5b are 355 and
358 nm (in vacuum), respectively. These values are close to the
experimental result of 342 and 278 nm (in hexane), respectively.
The excitation energies for the same transition of 4b, 5a, and 6b
are 299, 547, and 318 nm (in vacuum), respectively. These values
are close to the experimental result of 293 (in toluene), 290 (in
dioxane), and 300 nm (in dioxane), respectively. These calcula-
tions rationalized the explanation of ICT origin of the solvent
polarity dependency of the fluorophores’ emission (Figures S17,
S18, and S21, respectively, in the SI). Fluorophore 7a and 6a
showed dual emission, which again depends on solvent polarity,
thus showing an emission from both LE and CT states. The
excitation energies for the transition of S0 f S1 of 4a, 7a, and 6a
are 330, 447, and 419, respectively, which are closer to the
experimental results (experimental: 294, 340, and 292 nm in
hexane, respectively). Fluorophore 4a showed a strong ICT
emission (Figure 9a) and 7a showed dual emission, which again
depends on solvent polarity, thus showing an emission from both
LE and CT states. For fluorophore 7a, the S0 f S1 transition is
excited at a much longer wavelength of 447 nm (exptl in hexane
340 nm) with an f value of 0.10. The transition S0 f S2 (349 nm,
in vacuum) is, however, closer to the experimental value with
higher f value (f = 0.39) indicating that the S2 state is populated
well (Figure 9c and in the SI, Figure S22).
As revealed from the overlapping of HOMOꢀLUMO as well
as the transition oscillator strength for all the fluorophores except
for 6a all the electronic transitions of S0 f S1are allowed. This
means that reverse transition, i.e., S0 r S1, is also fully allowed
indicating the potential fluorophoric nature of the molecules. For
molecule 6a, however, the S0 f S1 transition is excited at a much
longer wavelength of 419 nm with an f value of 0.0651(see the SI,
Figure S20). This small f value indicated the very weakly fluo-
rescent properties of 6a, which was true as we did not see any
fluorescence for it under the same condition as for the other fluo-
rophores. For 5a, however, the S0 f S1 transition is excited at a
much longer wavelength of 547 nm (2.27 eV, exptl in dioxane
290 nm) with an f value of 0.119 (see the SI, Figure S18).
The emissive state of 7b is basically a locally excited (LE) state
as is suggested by the HOMOꢀLUMO distributions. For 4b
and 6b, however, the emissive state is characterized with more
Figure 8. (a) A plot of Δν~ vs Δf of 4a, 4b, and 7a, and (b) proposed
photophysical process for the dual emission.
Mataga eq 212ꢀ14
2
2ðμe ꢀ μgÞ
~
~
Δν¼ Δν0 þ
Δf
ð2Þ
hcr2
where μe and μg are the excited (fluorescent) state and the ground
state dipole moments of the fluorophore, h is Planck’s constant, c is
the velocity of light, and r is the Onsager radius of the dipoleꢀsol-
vent interaction sphere. The Δ~ν values for 4a, 4b, and 7a (for the
long wavelength emissive band) in different solvents were calcu-
lated (in cmꢀ1) (see the SI, Table S2) and the Δ~ν was ploted
against the solvent polarity Δf following eq 2. Thus, it is seen from
the plot that the Δ~ν (Figure 8a) values correlate linearly well with
the Δf values for all the solvents studied. Linear correlation with
large slopes of the Δ~ν vs Δf plots suggests that the fluorescence
states are highly polar in nature for all the fluorophores. Consider-
ing the Onsager radius r values for the fluorophores given in the
footnote of Table 2 (obtained from geometry optimized structures
at the B3LYP/6-31G* level18), the (μe ꢀ μg) values for 4a,b and 7a
are estimated by using the slopes of the respective Δ~ν vs Δf plots
and are found to be about 24.0, 21.0, and 21.0 D, respectively.
Substantially high values of (μe ꢀ μg) thus obtained from the plot
(Figure 8a and Table 2) suggest that the fluorescence states of all
these fluorophores are of strong ICT character, which was inferred
earlier. The (μe ꢀ μg) value for 4a is somewhat higher than that of
the 4b and 7a, which indicates a higher ICT character of 4a
compared to others. Comparing the spectral behaviors and all the
experimental results, it is clear that the ICT state is developed from
the LE state in the case of 7a (Figure 8b).
Theoretical Calculation. To investigate the relation between
the polarity-dependent emission and the ICT feature, we have
carried out TDDFT calculations using the Gaussian 03 program
package18 (see the SI, Figures S16ꢀ23 and Table S6). Thus, as
revealed from the overlapping of HOMOꢀLUMO as well as the
transition oscillator strength, the electronic transitions of S0 f S1
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Figure 9. HOMO and LUMO of the representative fluorophores (a) 4a, (b) 4b, and (c) 7a.
Table 3. Fluorescence Lifetime Dependence on Wavelength of 7a
entry
solvents
dioxane
Φf
λ [nm]
τ1 [ns]
τ2 [ns]
Æτæ [ns]
kf [108 sꢀ1
]
knr [108 sꢀ1
]
7a
0.001
0.076
0.030
0.022
0.058
0.059
0.003
0.019
406
496
385
548
384
498
401
577
3.78 (29.0%)
2.95 (82.7%)
3.27 (5.9%)
3.87 (100%)
7.20 (100%)
2.62 (100%)
4.27 (11.7%)
1.70 (63.0%)
2.00 (71.0%)
6.57 (17.3%)
1.27 (94.1%)
2.317
3.256
1.305
3.87
0.33
0.23
0.39
0.13
0.14
0.40
0.08
0.09
3.98
2.83
7.26
2.44
1.24
3.40
3.64
4.28
EtOAc
CHCl3
7.20
2.62
10% H2O in dioxane
2.56 (88.3%)
5.47 (37.0%)
2.685
2.285
significant electron redistribution, i.e., ICT feature. Calculated
results support our above explanation (see the SI, Table S6).19
Fluorophore 4a showed a strong ICT emission and 7a showed a
dual emission, which again depends on solvent polarity. There-
fore, an emission from both the LE and ICT states has been
observed in the case of 7a (see the SI, Table S6).
from 3.78 to 4.27 ns (monitoring wavelength is the LE band). This
observation correlates with the knr value that is decreased from 3.98 to
3.64 ns. When the monitoring emission wavelength is the ICT band,
we have observed that with decrease in intensity, the lifetime is also
decreased from 2.95 ns to 1.70 ns, which again correlates with the
increased knr value of 4.28 ns from 2.83 ns.
In conclusion, we have been able to install the fluorescence
response into a nonfluorescent precursor and modulate the
emission response of a pyrene fluorophore via click reaction.
Our findings reveal the delicate interplay of structure and emission
properties, thus having a broader general utility. As the ICT to LE
intensity ratio can be employed as a sensing index, the dual emissive
fluorophore can be utilized in designing molecular recognition
systems. Our investigation on the photophysical properties of click
chemistry derived fluorophores is of immense importance for the
development of new fluorophores with predetermined photophy-
sical properties that may find a wide range of applications in chem-
istry, biology, or material sciences. Currently, the exploration of the
dual fluorescent probe as a metal ion sensor is under investigation.
Fluorescence Lifetime Measurement. To interpret the
photophysical properties in a more intuitive manner, the fluor-
escence lifetimes of 4a,b, 6b, and 7a,b were measured in different
solvents. Thus, biexponential decay is observed in nonpolar
dioxane or hexane and the lifetimes of the major components
are less than or closer to 1.0 ns (Table 1). However, as the solvent
polarity is increased, lifetimes are also increased. As for example,
in dioxane, the major component (68%) of fluorophore 4b is
short-lived (τ1 = 0.04 ns) and its relative contribution is increased
to 100% in 1:1 dioxane:water solution (τ1 = 3.45 ns), which is an
indication of a 1πꢀπ* state. In protic polar solvent, like methanol,
compared to DMSO, the lifetime of 4b is decreased, which may be
because of the fluorescence quenching through the nonradiative
pathway via hydrogen bonding.7a The relative contribution of the
longer lifetime component of 7a (τ2 = 2.5 ns) in hexane is increased
from 27% to 100% as the solvent polarity is increased (in CHCl3, τ1
= 2.62 ns). In methanol, the minor component showed a decay time
τ1 of 1.87 ns, for which a weak emission from LE state was observed.
No ICT emission has been observed as the decay time of the major
component (τ1 = 14.30 ns) is very large and the fluorophore foll-
owed a nonradiative pathway.
Lifetime data were also collected at two emission maxima for
7a in solvents displaying biexponential decays (Table 3). Thus, it
has been observed that the values of τ1 and τ2 remain consistent,
but the relative contributions of the two lifetimes vary according
to the observed wavelength. As for example, for 7a, in dioxane,
the relative contribution of the longer lifetime τ1 is increased
from 29% to 83% as the observed wavelength is increased from
406 to 496 nm. Similar behavior was observed in 10% water in
dioxane solvent. As the intensity of the LE band is increased from
dioxane to 10% water in dioxane solution, the lifetime is also increased
’ EXPERIMENTAL SECTION
General Methods. All reactions were carried out under a nitrogen
atmosphere in flame-dried glassware, using a nitrogen filled balloon.
Organic extracts were dried over anhydrous sodium sulfate. Solvents
were removed in a rotary evaporator under reduced pressure. Silica gel
(60ꢀ120 mesh size) was used for the column chromatography. Reac-
tions were monitored by TLC on silica gel 60 F254 (0.25 mm). 1H NMR
spectra were recorded at 400 MHz and 13C NMR spectra were recorded
at 100 MHz. Coupling constants (J value) were reported in hertz. The
chemical shifts were shown in ppm downfield from tetramethylsilane,
1
using residual chloroform (δ = 7.24 in H NMR, δ = 77.23 in 13C
NMR), dimethyl sulfoxide (δ = 2.48 in 1H NMR, δ = 39.5 in 13C NMR),
as an internal standard. Mass spectra were recorded with an HR mass
spectrometer and data analyzed by using the built-in software. IR spectra
were recorded in KBr or neat on a FT-IR spectrometer.
General Procedure for the Preparation of Aryl Azides. An
ice cold solution of sodium nitrite (3 equiv) in water was added dropwise
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ARTICLE
to a cold solution of aryl amine 1 (1 equiv) in water and concentrated
hydrochloric acid at 0 ꢀC over 7ꢀ10 min. The reaction mixture was
slowly stirred for 1ꢀ2 min before an ice cold solution of sodium azide
(6 equiv) in water was added dropwise at 0 ꢀC over 10 min. The mixture
was stirred for 15 min. The resulting mixture was extracted with ether.
The organic layer was washed with water, followed by a brine solution,
dried over anhydrous Na2SO4, and then concentrated to yield pure aryl
azides 2 (average yield 60ꢀ90%). Formation of the product azide was
confirmed from IR study (1-azido-4-methoxybenzene (A): yield 60%;
7.95 (1H, s), 8.33 (2H, d, J = 8.8 Hz); 13C NMR (CDCl3 þ d6-DMSO;
100 MHz) δ 113.2, 117.4, 124.8, 125.3, 129.0, 130.1, 132.6, 134.4, 136.0,
140.3, 147.5; HRMS calcd for C15H9N5O2 ([M þ H]þ) 292.0834, found
292.0840.
Synthesis of 4-(1-(5-Bromonaphthalen-1-yl)-1H-1,2,3-
triazol-4-yl)-N,N-dimethylbenzenamine (6a). Using the general
procedure, starting from 0.045 g of 1-azido-5-bromonaphthalene C and
0.040 g of E, 0.060 g of the title compound 6a was isolated as a pink solid
(85%). IR (KBr) 1617, 1597, 1498, 1352, 802, 780, 532 cmꢀ1; 1H NMR
(CDCl3; 400 MHz) δ 2.99 (6H, s), 6.79 (2H, d, J = 8.8 Hz), 7.34 (1H, t,
J = 8.0 Hz), 7.67 (3H, m), 7.79 (2H, d, J = 9.2 Hz), 7.86 (2H, d, J = 6.8 Hz),
7.98 (1H, s), 8.44 (1H, dd, J = 1.4, 7.6 Hz); 13C NMR (CDCl3; 100 MHz)
δ 40.6, 112.7, 118.2, 121.1, 122.8, 123.3, 124.6, 126.5, 127.0, 128.2, 129.7,
130.2, 131.4, 132.9, 134.4, 148.6, 150.8; HRMS calcd for C20H17N4Br ([M
þ H]þ) 393.0715, found 393.0717.
IR: 2105 cmꢀ1; 1-azido-4-nitrobenzene (B): yield 62%; IR 2126 cmꢀ1
;
1-azido-5-bromonaphthalene (C): yield 89%; IR 2134 cmꢀ1; 1-azido-
pyrene (D): yield 75%; IR 2133 cm-1) and the produced azides were
then immediately used for the next step without further purification.
General Procedure for the [3 þ 2] Cycloaddition of Azides
and Terminal Alkynes. In a round-bottomed flask fitted with a
septum, azide 2 (1.0 equiv) was dissolved by dry 1:1 dichloromethane
and ethanol and degassed for 5 min with N2. Then, alkyne 3 (1.5 equiv)
was added and both the stirring and degassing were continued for the
next 5 min. After that a 6 mol % sodium ascorbate was added. The
reaction mixture was stirred for 5 min before 1 mol % powdered CuSO4
was added. Degassing was continued for the next 10 min. The reaction
was allowed to proceed to room temperature and monitored by TLC.
After total consumption of the starting azide, the reaction mixture was
evaporated completely to a solid material and partitioned between water
and ethyl acetate. The organic layer was washed with water followed
by brine solution, dried over Na2SO4, and then concentrated. The title
triazolyl compounds were separated by column chromatography and
characterized. The average yields were 50% to 85%.
Synthesis of 4-(1-(5-Bromonaphthalen-1-yl)-1H-1,2,3-tri-
azol-4-yl)benzonitrile (6b). Using the general procedure, starting
from 0.024 g of 1-azido-5-bromonaphthalene C and 0.018 g of F, 0.02 g
of the title compound 6b was isolated as a reddish pink solid (56%). IR
(KBr) 2227, 1602, 1591, 1497, 839, 773, 555 cmꢀ1; 1H NMR (CDCl3;
400 MHz) δ 7.38 (1H, t, J = 8.0 Hz), 7.57 (1H, d, J = 8.8 Hz), 7.68 (1H, t,
J = 6.6 Hz), 7.72 (1H, s), 7.76 (1H, d, J = 8 Hz), 7.89 (1H, d, J = 7.6 Hz),
8.05 (2H, d, J = 8.4 Hz), 8.21 (1H, s), 8.50 (1H, d, J = 8.4 Hz); 13C NMR
(CDCl3; 100 MHz) δ 112.2, 118.9, 122.4, 123.7, 124.8, 126.5, 128.5,
130.1, 130.4, 131.7, 133.1, 134.7, 146.3; HRMS calcd for C19H11N4Br
([M þ H]þ) 375.0245, found 375.0248.
Synthesis of N,N-Dimethyl-4-(1-(pyren-1-yl)-1H-1,2,3-tria-
zol-4-yl)benzenamine (7a). Using the general procedure, starting
from 0.025 g of 1-azidopyrene (D) and 0.022 g of E, 0.024 g of the title
compound 7a was isolated as a light pink solid (68% yield). IR (KBr)
1617, 1567, 1503, 1460, 1351, 819 cmꢀ1; 1H NMR (CDCl3; 400 MHz)
δ 3.03 (6H, s), 6.84 (2H, d, J = 8.0 Hz), 7.87 (2H, d, J = 8.4 Hz), 7.97
(1H, d, J = 9.2 Hz), 8.06ꢀ8.20 (6H, m), 8.24ꢀ8.29 (3H, m); 13C NMR
(CDCl3; 100 MHz) δ 40.6, 112.7, 118.6, 121.4, 123.5, 124.3, 124.9,
125.2, 126.1, 126.5, 126.9, 127.1, 128.9, 129.7, 130.8, 131.3, 132.3, 148.6,
150.8; HRMS calcd for C26H20N4 ([Mþ H]þ) 389.1766, found 389.1768.
Synthesis of 4-(1-(Pyren-1-yl)-1H-1,2,3-triazol-4-yl)benzo-
nitrile (7b). Using the general procedure, starting from 0.010 g of
1-azidopyrene (D) and 0.008 g of F, 0.010 g of the title compound 7b
was isolated as a brown solid (66% yield). IR (KBr) 2225, 1614, 1489,
1461, 845 cmꢀ1; 1H NMR (CDCl3; 400 MHz) δ 7.77 (2H, d, J = 7.6 Hz),
7.88 (1H, d, J = 8.4 Hz), 8.08ꢀ8.29 (10H, m), 8.33 (1H, s); 13C NMR
(CDCl3; 100 MHz) δ 112.0, 118.9, 121.0, 123.5, 124.0, 124.3, 124.9, 125.3,
126.5, 126.9, 127.1, 129.4, 130.2, 130.8, 131.3, 132.7, 133.1, 134.9, 146.3;
HRMS calcd for C25H14N4 ([M þ H]þ) 371.1297, found 371.1302.
UVꢀVisible Measurements. All the UVꢀvisible spectra of the
compounds (10 μM) were measured in different solvents, using a
UVꢀvisible spectrophotometer with a cell of 1 cm path length.
Fluorescence Experiments. All the sample solutions were pre-
pared as described in UV measurement experiments. Fluorescence
spectra were obtained with use of a fluorescence spectrophotometer (for
4a, 4b, and 7a) at 25 ꢀC using a 1 cm path length cell. The fluorescence
quantum yields (Φf) were determined with quinine sulfate as a reference
with the known Φf (0.55) in 0.1 M solution in sulfuric acid.
Synthesis of 4-(1-(4-Methoxyphenyl)-1H-1,2,3-triazol-4-
yl)-N,N-dimethylbenzenamine (4a). Using the general procedure,
starting from 0.116 g of 1-azido-4-methoxybenzene A and 0.169 g of E,
0.137 g of the title compound 4a was isolated as a white solid (60%). IR
(KBr) 1618, 1593, 1518, 1503, 1354, 1250, 1040, 808 cmꢀ1; 1H NMR
(CDCl3; 400 MHz) δ 3.01 (6H, s), 3.87 (3H, s), 6.81 (2H, d, J = 7.2 Hz),
7.03 (2H, d, J = 7.6 Hz), 7.68 (2H, d, J = 8.0 Hz), 7.77 (2H, d, J = 8.0 Hz),
7.98 (1H, s); 13C NMR (CDCl3; 100 MHz) δ 40.7, 55.8, 112.8, 114.9,
116.5, 118.9, 122.2, 126.9, 130.9, 148.9, 150.6, 159.8; HRMS calcd for
C17H18N4O ([M þ H]þ) 295.1559, found 295.1562.
Synthesis of 4-(1-(4-Methoxyphenyl)-1H-1,2,3-triazol-4-
yl)benzonitrile (4b). Using the general procedure, starting from
0.116 g of 1-azido-4-methoxybenzene A and 0.148 g of F, 0.144 g of the
title compound 4b was isolated as a white solid (67% yield). IR (KBr)
2231, 1611, 1519, 1489, 1258, 1025, 825 cmꢀ1; 1H NMR (CDCl3; 400
MHz) δ 3.87 (3H, s), 7.04 (2H, d, J = 8.8 Hz), 7.66 (2H, d, J = 8.8 Hz),
7.73 (2H, d, J = 8.0 Hz), 8.00 (2H, d, J = 8.0 Hz), 8.18 (1H, s); 13C NMR
(CDCl3; 100 MHz) δ 55.8, 111.8, 115.1, 118.9, 119.2, 122.4, 126.3,
130.3, 132.9, 134.9, 146.5, 160.3; HRMS calcd for C16H12N4O ([M þ
H]þ) 277.1089, found 277.1091.
Synthesis of N,N-Dimethyl-4-(1-(4-nitrophenyl)-1H-1,2,3-
triazol-4-yl)benzenamine (5a). Using the general procedure, start-
ing from 0.067 g of 1-azido-4-nitrobenzene B and 0.090 g of E, 0.050 g of
the title compound 5a was isolated as a orange solid (50%). IR (KBr)
1621, 1595, 1517, 1497, 1368, 1335, 853 cmꢀ1; 1H NMR (CDCl3; 400
MHz) δ 3.00 (6H, s), 6.78 (2H, d, J = 6.8 Hz), 7.08 (2H, d, J = 8.8 Hz),
7.59 (2H, d, J = 8.8 Hz), 7.77 (1H, s), 8.28 (2H, d, J = 8.8 Hz); 13C NMR
(CDCl3; 100 MHz) δ 41.1, 113.5, 125.0, 125.5, 126.2, 129.9, 131.1,
133.6, 138.5, 141.9, 147.6; HRMS calcd for C16H15N5O2 ([M þ H]þ)
310.1304, found 310.1309.
Fluorescence Lifetime Measurements. Fluorescence lifetimes
were measured with the use of a system that employs a microchannel
plate photomultiplier as the detector. Edinburgh 290 nm (for 4b) and
308 nm Pulsed LED (for 4a) and 375 nm (for 6b, 7a, 7b) Laser Diode
were used for excitation. The fluorescence decay was analyzed by
reconvolution (for 4a,b) and tail fitting (for 6b, 7a, 7b) method, using
built-in software. All experiments were performed at 298 K.
Theoretical Calculation. The ground state structures of the
fluorophores were optimized by using density functional theory (DFT)18
with B3LYP functional and 6-31G (d) basis set. The excited state related
Synthesis of 4-(1-(4-Nitrophenyl)-1H-1,2,3-triazol-4-yl)b-
enzonitrile (5b). Using the general procedure, starting from 0.064 g
of 1-azido-4-nitrobenzene B and 0.075 g of F, 0.017 g of the title
compound 5b was isolated as a yellow solid (51%). IR (KBr) 2228, 1612,
1594, 1521, 1490, 1366, 1338, 854 cmꢀ1; 1H NMR (CDCl3; 400 MHz)
δ 7.35 (2H, d, J =8.4 Hz), 7.54 (2H, d, J= 8.8 Hz), 7.70 (2H, d, J = 8.0 Hz),
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The Journal of Organic Chemistry
ARTICLE
calculations were carried out with the time dependent density functional
theory (TD-DFT) with the optimized structure of the ground state (B3L
YP/6-31G(d)). There are no imaginary frequencies in frequency analysis of
all the calculated structures, therefore each calculated structure is a local
energy minimum.
Thiagarajan, V.; Selvaraju, C.; Malar, E. J. P.; Ramamurthy, P. Chem.
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’ ASSOCIATED CONTENT
S
Supporting Information. Full chemical structures, crys-
b
tallographic data, photophysical spectra and lifetime measure-
ment, Cartesian coordinates, TDDFT calculation, and 1H /13C
NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
Corresponding Author
*Fax: (þ)91-361-258-2324. E-mail: ssbag75@iitg.ernet.in.
’ ACKNOWLEDGMENT
The authors thank DST (SR/SI/OC-69/2008) and CSIR
[01(2330)/09/EMR-II], Govt. of India, for financial support. R.
K. thanks IIT Guwahati for a fellowship.
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