Modulation of the photophysics of 2-(4′-N,N-dimethylaminophenyl) imidazo[4,5-b]pyridine by long chain N-alkylations

Article abstract of DOI:10.1039/c0pp00278j

The N-alkylated products of 2-(4′-N,N-dimethylaminophenyl)imidazo[4, 5-b]pyridine (DMAPIP-b) were synthesized to study the effect of long alkyl chains on the photophysics of the fluorophore. Two different compounds were synthesized: in molecule 1, the alkyl chain was attached to the pyridine nitrogen of DMAPIP-b and in molecule 2 the alkyl chain was substituted on the imidazole nitrogen of DMAPIP-b. Absorption, steady state and time-resolved fluorescence techniques were employed to investigate the spectral properties in different solvents. Density functional theory (DFT) and restricted configuration by singles (CIS) combined with time-dependent DFT (TDDFT) calculations were performed on the ground state and the excited state respectively to calculate the geometrical and the electronic properties of the molecules. Unlike DMAPIP-b, where dual emission was observed in protic solvents, which originates from both a locally excited state and an intramolecular charge transfer (ICT) state, the pyridine nitrogen alkylated product (1) emits a single emission from the ICT state and the imizadole nitrogen alkylated molecule (2) emits dual emission only in methanol and water.

Full text of DOI:10.1039/c0pp00278j

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PAPER
Modulation of the photophysics of 2-(4¢-N,N-dimethylaminophenyl)imidazo-
[4,5-b]pyridine by long chain N-alkylations†
Nihar Dash and G. Krishnamoorthy*
Received 11th September 2010, Accepted 25th January 2011
DOI: 10.1039/c0pp00278j
The N-alkylated products of 2-(4¢-N,N-dimethylaminophenyl)imidazo[4,5-b]pyridine (DMAPIP-b)
were synthesized to study the effect of long alkyl chains on the photophysics of the fluorophore. Two
different compounds were synthesized: in molecule 1, the alkyl chain was attached to the pyridine
nitrogen of DMAPIP-b and in molecule 2 the alkyl chain was substituted on the imidazole nitrogen of
DMAPIP-b. Absorption, steady state and time-resolved fluorescence techniques were employed to
investigate the spectral properties in different solvents. Density functional theory (DFT) and restricted
configuration by singles (CIS) combined with time-dependent DFT (TDDFT) calculations were
performed on the ground state and the excited state respectively to calculate the geometrical and the
electronic properties of the molecules. Unlike DMAPIP-b, where dual emission was observed in protic
solvents, which originates from both a locally excited state and an intramolecular charge transfer (ICT)
state, the pyridine nitrogen alkylated product (1) emits a single emission from the ICT state and the
imizadole nitrogen alkylated molecule (2) emits dual emission only in methanol and water.
occur extremely rapidly. In this case, only long wavelength emission
from the charge transfer state may be observable. Though the
1. Introduction
Research on luminescent organic materials has become an attrac-
TICT model is widely accepted, Zachariasse et al. proposed the
tive field, owing to their wide variety of practical applications
planar intramolecular charge transfer (PICT) model, where the
emitting state is proposed to have a planar structure.7–9
in different fields. On this account, better understanding and
control of the photophysical properties will not only contribute
to the improvement of their technological applications, but is also
expected to encourage novel and innovative application concepts.
It is well known that organic molecules containing both donor and
withdrawing groups in close proximity with a suitable geometry
may undergo an intramolecular charge transfer from the donor
group to the acceptor group. These types of organic molecules
have also received considerable attention as possible models for
a number of photochemical and photobiological processes.1–3
When such a molecule is excited, electron transfer takes place
from the donor moiety to the acceptor. This is accompanied
by the rotational relaxation to a twisted conformation of the
donor relative to the acceptor, forming the so-called twisted
intramolecular charge transfer (TICT) state.4–6 According to the
TICT model, the fluorescence originates from the locally excited
state as well as from the charge transfer (CT) state, accessible only
by an adiabatic photoreaction from the locally excited state, which
includes rotational motion around the bond linking the donor and
the acceptor moieties. If there is no energy barrier separating the
precursor and the product species, the excited-state relaxation can
It is also well demonstrated that the polarity and the viscosity
play key roles in the excited state relaxation process of the
ICT molecules.5,6 Later it was understood that specific hydrogen
bonding of the solvent with the electron donor or acceptor groups
of the ICT molecule might also play an important role in the
formation and the stabilization of the ICT state in numerous
systems.10–13 These properties make the ICT molecules potential
probes for a variety of systems such as cyclodextrins, micelles,
microemulsion, polymers, membranes, proteins etc.14–21
2-(4¢-N,N-Dimethylaminophenyl)imidazo[4,5-b]pyridine
(DMAPIP-b) is an interesting molecule. It emits a single emission
in aprotic solvents, but dual emission in protic solvents. The
longer wavelength emission of DMAPIP-b in protic solvents is
due to the formation of the ICT state. Hydrogen bonding of
the solvent with the pyridine nitrogen of DMAPIP-b induces
the formation of a TICT state.22 Here we report the effect of
attaching long alkyl chain on the optical properties of the system,
in particular on its dual emission.
Periasamy et al. found that the spectral characteristics of
the aminostyryl pyridinium chromophore are not significantly
perturbed by the length of the alkyl chain attached to the
pyridinium nitrogen.23 Thus we have synthesized two N-alkylated
products with the same chain length, but attached at different
positions. The molecules are namely 2-[4¢-(N,N-dimethylamino-
phenyl)imidazo[4,5-b]hexadecylpyridinylium bromide (1), where
Department of Chemistry, Indian Institute of Technology Guwahati, Guwa-
hati, 781039, India. E-mail: gkrishna@iitg.ernet.in; Fax: +91-3612582349;
Tel: +91-3612582315
† Dedicated to Prof. S. K. Dogra on the occasion of his 69th birthday.
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a
hexadecyl chain is attached at the pyridine nitrogen
were prepared in 0.5% methanol. Millipore water was used for
preparing aqueous solutions. The pH of the aqueous solutions
was adjusted by addition of sodium hydroxide and phosphoric
acid. Absorption and fluorescence spectra were recorded on Cary
50 and Edinburgh Instruments FLSP920 spectrophotometers
respectively. Fluorescence lifetimes were measured with the use of
Edinburgh Instruments Life Spec II single photon counting system
that employs Hamamatsu microchannel plate photomultiplier as
detector. Edinburgh 375 nm laser diode was used as the excitation
source. The fluorescence decay was analyzed by tail-fitting method
using the software provided by Edinburgh Instruments. All
experiments were performed at 298 K.
and 1-hexadecyl-2-[4¢-(N,N-dimethylaminophenyl)imidazo[4,5-
b]pyridine (2), where the chain is substituted in the imidazole
nitrogen, >N–H (Chart 1).
DFT27 calculations are popular for the ground state calculation.
Calculating molecular parameters for an electronic excited com-
plex molecular system is difficult. Nowadays, the CIS,28 TDDFT29
and complete active space self-consistent field (CASSCF)30,31
methods are very popular computational methods for excited
state electronic structures. Though the CASSCF method can have
more accuracy, it is computationally expensive for large systems.
TDDFT method is good for electronic structure calculations in
the excited states for large molecular systems due to its moderate
efficiency and accuracy.32–36 But optimization by TDDFT method
gives an incorrect ordering of energies in a few cases.37 The
geometries as well as molecular properties obtained at the CIS level
are quite reasonable and correct, at least as a first approximation
for a variety of molecules.38,39 However it overestimates the energies
in many cases. The TDDFT calculations over CIS optimized
geometries have been proven to be an efficient approach in
predicting energy parameters for various system.40,41 We also found
that the combined approach of CIS with TDDFT is successful for
the parent compound, DMAPIP-b; therefore, we implemented
the same approach for the alkyated molecules. Since ZINDO
calculations were found to be very good in predicting vertical
transition energies and therefore we have also used the method
to calculate the transition energies. All the calculations were out
using Gaussian 03W software.42
Chart 1 Structures of DMAPIP-b and its alkylated products.
2. Materials and methods
DMAPIP-b was synthesized by heating an equimolar mixture of
2,3-diaminopyridine (Aldrich) and 4-dimethylaminobenzoic acid
(Aldrich) in polyphosphoric acid as the described in our previous
work.22
Synthesis of 2-[4¢-(N,N-dimethylaminophenyl)imidazo[4,5-b]-
hexadecylpyridinylium bromide (1)
DMAPIP-b (2 mmol) and 1-bromohexadecane (5 mmol) were
completely mixed using an ultrasonic bath. The mixture was
irradiated intermittently (4 min irradiation with 30 s mixing) in
a household MW oven at 200 W for 20 min as reported for the
synthesis of similar compounds.24 After irradiation the mixture
was cooled, washed with ether and dried. The compound was
purified by column chromatography. Mass 463.34 [M - Br]+
Synthesis of 1-hexadecyl-2-[4¢-(N,N-dimethylaminophenyl)-
imidazo[4,5-b]pyridine (2)
DFT calculations have been performed by using the hybrid
Becke343 and the Lee–Yang–Parr functional (B3LYP)44 in conjunc-
tion with 6-31G(d,p) basis set, using Gaussian 03W software42 to
obtain the molecular parameters. The choice of the B3LYP func-
tional is based on the studies of Grimme et al.45 To substantiate
this further, the calculations have been preformed using different
functionals available in Gaussian 03W software for the parent
molecules and the excitation energies obtained along with the
experimental values are compiled in Table 1. The value obtained
by using B3LYP is in better agreement with the experimental value.
BVP8643,46 and B3PW9143,47 functional are used along with B3LYP
for the alkylated molecules (Table 2). Though for 2, B3PW91
results are in close agreement with the experimental values over
B3LYP, the B3LYP results are also in reasonable agreement with
the experimental values, but B3PW91 significantly underestimates
excitation energy of 1.
DMAPIP-b (4 mmol) and 1-bromohexadecane (9 mmol) were
dissolved in a solvent mixture of 30 ml dimethylformamide and
10 ml tetrahydrofuran. To the above mixture powered KOH (100
mmol) was added and heated at 100 ◦C for 24 h by the procedure
reported earlier for the alkyl substitution of similar compounds.25
After 24 h the compound was extracted with dichloromethane and
it was further purified by column chromatography. Mass 463.33
[M + H]+
Spectral measurements
Dioxane, 1-propanol and glycol were AR grade and all other
solvents were HPLC grade, except for ethanol that was ACS
grade (Merck). The solvents were purchased from Rankem, India.
All solvents were shown to be free of spurious fluorescence in
the region of the fluorescence measurements. The fluorescence
quantum yields were determined for the solutions having ab-
3. Results and discussions
max
sorbances of ~0.1 at lab
with reference to quinine sulfate in
aqueous 1 N sulfuric acid as the standard (U = 0.546).26 The solute
concentrations are on the order of micromolar and concentration
studies revealed the absence of molecular aggregation at those
concentrations. Due to poor solubility, aqueous solutions of 2
3.1 Absorption spectra
Absorption spectra of 1 and 2 are recorded in different solvents
and the observed long-wavelength band maxima are compiled in
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0.04◦ and 0.07◦, respectively. Bearing the positive on the pyridine
ring, the imidazopyridinium ring acts as a better acceptor that
increases the conjugation which shifts the spectrum towards red.
The substitution of the alkyl chain in the imidazole nitrogen
makes the molecule nonplanar by twisting the imidazopyridine
moiety by 41.4◦ out of the phenyl plane, the dihedral angles of
the dimethylamino group is also increased to 8.9◦, thus reducing
the conjugation. This is consistent with the blue shift in the
spectral maxima of 2. It is useful to examine the frontier molecular
orbitals (FMOs) as it provides a reasonable qualitative indication
of the excitation properties. The isosurface plots of FMOs are
shown in Fig. 1. The effect of protic solvent (methanol) was
determined by using the integral equation formalism of the
polarizable continuum model (IEFPCM).48 The calculations were
performed by adding two methanol molecules with fluorophore in
IEFPCM. The frontier orbitals thus obtained are also depicted in
Fig. 1.
The lowest electronic transition of the parent and the alkylated
molecules are described mainly by the promotion of an electron
from the highest occupied molecular orbital (HOMO) to the
lowest unoccupied molecular orbital (LUMO) which corresponds
to a pp* excited singlet state. The HOMO and the virtual LUMO
distribution patterns show that the LUMO is relatively more con-
centrated on the imidazopyridine ring than on the dimethylamino
group in all three molecules and thus the first electronic transitions
should possess some charge transfer nature in all the molecules
in general. However in 1 the entire dimethylaminophenyl ring
has only a minor contribution and the major contribution is
from imidazopyridine ring. The HOMOs of DMAPIP-b and its
imidazole alkylated derivative 2 are more or less delocalized on
Table 1 Excitation energy of DMAPIP-b calculated by different func-
tionals along with the experimental value
Functionala
Excitation energy/eV
LSDA54
3.32
3.28
3.82
3.83
3.29
3.94
3.93
3.36
3.36
3.39
3.80a
BVP8643,54,46
B3LYP43,44
B3PW9143,47
PBEPBE55
PBE1PBE55
MPW1PW9147
HCTH56
THCTH56
TPSSTPSS57
Experimental
a From ref. 22.
Table 3. The absorption spectra of 2 are blue shifted compared to
those of DMAPIP-b.22 On the other hand, the absorption spectra
of 1 are strongly red shifted relative to those of parent molecule.22
However on increasing the polarity and the hydrogen bonding
capacity of the medium, the same as DMAPIP-b, the absorption
spectra of the alkylated molecules undergo a red shift with loss of
vibrational structures.
The optimized geometrical parameters are presented in Table 4.
DMAPIP-b is nearly planar in the ground state, the imidazopyri-
dine ring and the dimethylamino groups are out of the phenyl
plane only by 0.0◦ and 2.5◦, respectively. Attaching an alkyl
chain to the pyridine nitrogen causes the molecule to become
completely planar, the dihedral angles of the imidazopyridine ring
and the dimethylamino group with the phenyl ring reduced to
Table 2 Comparison of vertical excitation energies calculated (DEcal) and the experimental values (DEexp) and the error DEexp - DEexp obtained by
different functional (in eV)
DEcal
Error DEexp - DEexp
Molecule
BVP86
B3LYP
B3PW91
DEexp
BVP86
B3LYP
B3PW91
DMAPIP-b
1
2
3.29
3.25
2.06
3.82
3.94
2.85
3.83
3.90
2.59
3.80a
3.85
3.12
0.51
0.61
1.06
-0.02
-0.09
0.27
-0.03
-0.05
0.53
a From ref. 22.
Table 3 Absorption band maxima (lmaxab /nm), log emax, fluorescence band maxima (lmaxfl), quantum yield (Uf) and lifetimesa (t/ns) of alkylated products
(1 and 2) in different solvents
Molecule 1
Molecule 2
ab
fl
ab
fl
a
Solvent
lmax
lmax (Uf)
t
lmax
lmax (Uf)
t
Cyclohexane
Diethyl ether
Ethyl acetate
Acetonitrile
Dimethylformamide
Butanol
2-Propanol
1-Propanol
Ethanol
380, 397(sh)
386
378
383
387
388
389
389
390
410, 432
1.93
2.29
2.49
3.15
3.25
3.29
3.30
3.24
3.15
2.10
2.15
322
323
327
328
312
330
327
329
330
335
331
347
361,380 (0.58)
375 (0.69)
390 (0.70)
407 (0.73)
422 (0.69)
412 (0.642)
410 (0.74)
415 (0.72)
415 (0.65)
431 (0.57)
424 (0.19)
407 (0.07)
444 (0.89)
465 (0.52)
497 (0.72)
503 (0.74)
496 (0.66)
496 (0.75)
500 (0.70)
505 (0.66)
525 (0.39)
515 (0.34)
1.41
1.69
1.70
1.76
1.78
1.84
1.84
0.76
Glycol
Methanol
Water
395
390
0.64 (97.4), 1.73 (2.6)
0.24 (59.0), 0.83 (41.0)
a Values in parenthesis are relative amplitudes.
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Table 4 Theoretical parameters obtained from the optimized geometry of DMAPIP-b and its alkylated products (1 and 2) along with experimental
excitation and emission energy
S0 state
DMAPIP-ba
Molecule 1
Molecule 2
Energy (a.u.)
-1351.1095
0.07
0.04
2.85 (2.97)
3.12
20.4
-1390.0239
8.9
41.4
3.94 (3.70)
3.85
4.7
Phenyl ring/NMe2 dihedral angle (◦)
Phenyl/PI dihedral angle (◦)
Theoretical excitation energyb/eV
Experimental excitation energy/eV
mg/D
2.5
0.0
3.82
3.80
7.2
S1 state
Phenyl ring/NMe2 dihedral angle(◦)
Phenyl/PI dihedral angle
Theoretical emission energyb/eV
Experimental emission energy/eV
0.0
0.0
3.56 (3.11)
3.45
0.02
0.00
2.66 (2.79)
2.87
5.5
5.0
4.01 (3.09)
3.44
a Except theoretical emission energy, all other data from ref. 31. b The values are obtained by TDDFT and those obtained by ZINDO are included within
the parenthesis.
the experimental excitation energies and the theoretical calculated
values from both TDDFT and ZINDO methods (Table 4).
3.2 Fluorescence spectra
The fluorescence spectra of 1 and 2 in a few selected solvents
are shown in Fig. 2 and 3 and the spectral data in all the
solvents are tabulated in Table 3. The fluorescence spectra of
both molecules have well-resolved vibrational structures in the
nonpolar solvents. Upon increasing the polarity of the solvent the
spectrum undergoes a bathochromic shift with loss of vibrational
structure. The red shift is more pronounced by 1 than by 2. The
fluorescence decays of both 1 and 2 were measured to obtain the
fluorescence lifetime. The fluorescence decay of 2 in water is shown
as a representative plot (Fig. 4) and the fitted data are compiled
in Table 3. The fluorescence lifetime of 1 is longer than that of 2,
and the emission lifetimes of 1 and 2 are comparable to that of
the ICT and the normal emission of DMAPIP-b, respectively.
The fluorescence of both 1 and 2 are strongly quenched in
methanol. Low fluorescence quantum yield in methanol may be
due to the strong intermolecular hydrogen bonding between the
fluorophore and the solvent molecules which results in an increase
in the nonradiative decay rate. The attenuation of the fluorescence
lifetime supports this. The fluorescence of DMAPIP-b was also
found to be quenched by protic solvents.22 Since no hydrogen
bonding is possible between the pyridium nitrogen of 1 and the
solvent molecules, it is clear that the hydrogen bonding of the
solvent with the imidazole nitrogen contribute substantially to the
fluorescence quenching in all the three molecules. The fluorescence
of DMAPIP-b was also strongly quenched when the imidazole
nitrogen of the molecule is protonated.22
Fig. 1 The isosurface plots of frontier molecular orbitals, HOMO
(bottom) and LUMO (top) of DMAPIP-b, 1 and 2. (a) Isolated molecules
in vacuum; (b) with 2 methanol molecule in methanol medium.
the dimethylamino, the phenyl and the imidazo pyridine moieties
with a nodal plane on the pyridine nitrogen. But the localization
on the dimethylamino group is more in the case of the alkylated
derivative (2).
The solvatochromic effect can be studied by using following
Lippert–Mataga equation:49,50
Upon absorption of photons, the molecules are excited to
the corresponding Franck–Condon regions, which should have
the ground state geometries. The vertical excitation energies for
the molecules were calculated by performing energy calculation
on the DFT-optimized structure by using the ZINDO and
TDDFT methods. A small discrepancy is observed between the
experimental and the theoretically predicted excitation energies for
1 that has been obtained by the TDDFT method, but the ZINDO
value is in reasonable agreement with the experimental value.
However in all other cases a good agreement is found between
nSS = [2(me - mg)2/hca3]Df + constant
(1)
¯
¯
where nSS is the Stokes shift, mg and me are dipole moments in
ground state and excited state respectively, a is Onsager cavity
radius, Df is the orientation polarizability defined as:
Df = [(e - 1)/(2e + 1)] - [(n2 - 1)/(2n2 + 1)
(2)
where e and n are solvent dielectric constant and refractive
index respectively. Since the Lippert–Mataga equation only takes
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Fig. 2 Normalized fluorescence spectra of 1 in (1) cyclohexane, (2) ether, (3) ethyl acetate, (4) 2-propanol, (5), 2-propanol, (6) ethanol, (7) methanol and
(8) glycol. The inserts show the Lippert–Mataga plot.
Fig. 3 Normalized fluorescence spectra of 2 in (1) cyclohexane, (2) ether, (3) ethyl acetate, (4) 2-propanol, (5), 2-propanol, (6) ethanol, (7) methanol and
(8) glycol. The inserts show the Lippert–Mataga plot.
into account dipole–dipole interactions and neglects the specific
interaction, the protic solvents are excluded to fit the Lippert–
Mataga equation (inserts in Fig. 2 and 3). A good linear correlation
was obtained for both 1 (r = 0.98) and 2 (r = 0.98). The slope
obtained for 1 and 2 are 14 230 cm-1 and 8840 cm-1 respectively.
Larger solvatochromic slope for 1 than that for 2 shows that
1 has a higher excited state dipole moment than 2 in the
excited state. This is consistent with the fact that 1 and 2 are
emitting from the ICT and the locally excited state respectively
(see later).
After excitation the molecules should relax to their most stable
geometry in the first excited state before the emission process
can occur that results in Franck–Condon region of the ground
state. Since in this Franck–Condon region, the molecule possesses
the relaxed geometry of the first excited state, the molecules were
optimized in the S1 state with the ab initio restricted CIS approach
incorporated in the Gaussian 03W program. The 6-31G(d,p) basis
set was chosen for the excited state calculations also. As expected
the planarity increases in the excited state. Only minimal reduction
in the angle between the imidazopyridine, the phenyl and the
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Fig. 5 Normalized fluorescence spectra of (a) 1, (b) 2 and (c) DMAPIP-b
in methanol.
Fig. 4 Fluorescence decay of 2 in water along the fitted curve (the residual
plot of double exponential decay fit is also shown).
for the locally excited state are reported for alkylated aminostyryl
pyridinium dyes.23
dimethylamino group occur in the excited molecules (DMAPIP-b
and 1) which are already planar in the ground state. In molecule
2, a small reduction occurs in the dihedral angle between the
phenyl ring and the dimethylamino group, but a drastic decrease
in the angle between the imidazo pyridine ring and the phenyl
ring is observed. The fact that in the first excited state DMAPIP-
b and 1 does not have significant changes in their geometrical
geometries is supported by the small Stokes shifts observed for
those compounds (~1900 cm-1 in cyclohexane). Large geometrical
change in the excited structure of 2 is also corroborated by the
large Stokes shift in fluorescence spectrum of 2 (~3350 cm-1 in
cyclohexane). The emission energies from the relaxed excited
states are obtained by both TDDFT and ZINDO calculations
using the optimized excited state as inputs (Table 4). A reasonable
agreement is found between the theoretically predicted values and
the experimental measurements.
A remarkable aspect of the fluorescence behavior of DMAPIP-
b is its dual emission in protic solvents. However single emission
bands are observed for both 1 and 2 in all the solvents including
most of the protic solvents (Table 3, except in methanol and water
for 2). The fluorescence spectrum of DMAPIP-b in methanol
is compared with that of the alkylated molecules in Fig. 5.
Comparative spectra clearly suggest that 1 and 2 are emitting
from the ICT state and the locally excited state, respectively.
The ICT fluorescence depends on the strength of the donor and
the acceptor.6 DMAPIP-b exhibits ICT emission in the protic
solvents due to hydrogen bonding of the solvent molecule with the
pyridine nitrogen. Such hydrogen bonding increases the electron
accepting nature of the imidazopridine ring which favors the
formation of the ICT state. Thus, the observation of the ICT
emission from 1 is not surprising; addition of the alkyl chain on
the pyridine nitrogen made it positively charged, which should
increase the electron withdrawing potential of the system. It is
further substantiated by the fact that only ICT emission had been
reported from the monocation ions formed by the protonation
of pyridine nitrogen in DMAPIP-b,51,52 as well as in 2-(4¢-N,N-
dimethylaminophenyl)imidazo[4,5-c]pyridine.53 The fluorescence
lifetimes of 1 are also longer than those of normal emission of
DMAPIP-b and are close to those of the ICT emission of the
parent compound. Similar longer lifetimes for the ICT state than
Interestingly, 2 emits only a single emission from the locally
excited state in most of the alcoholic solvents (except methanol).
This is substantiated by the single exponential decay of 2 in all
those solvents. However, although no clear two band system
is observed in methanol and water, the fluorescence decays are
biexponential in these solvents (Table 3), such double exponential
decays indicate that the ICT emission is buried underneath the
normal emission. In methanol the contribution of the second
(ICT) species is negligible (Fig. 5, Table 3). In water the relative
amplitude of the ICT emission increases (Table 3) and the
fluorescence spectrum also has a long tailing (Fig. 6a). Kim
et al. proposed that the ICT emission was enhanced in p-
diethylamino- and p-dimethylaminobenzoic acid b-cyclodextrin
inclusion complexes due to an increase in coplanarity of the
acceptor with the benzene ring by hydrogen bonding.13 According
to their model, the increase in planarity of the acceptor and the
benzene ring increases the charge flow from the benzene ring to the
acceptor, which is supposed to favor the formation of the ICT state.
Unlike in DMAPIP-b and 1, in 2 the acceptor imidazopyridine
ring and the benzene ring are out of the plane by 41.4◦ in the
ground state. This should reduce the charge flow from phenyl ring
to imidazole ring. However, we speculate that the absence of ICT
Fig. 6 Absorption (solid lines) and fluorescence (dotted lines) spectra of
(a) neutral (pH 7) and (b) monocationic (pH 1.5) forms of 2.
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emission is due to difficulty in the formation of a hydrogen bond
with pyridine nitrogen by solvent rather than due to twisting of the
imidazole ring. The following facts strengthened this hypothesis:
(i) Although the alkylation pushes the phenyl ring out of the plane
from the imidazopyrine ring in the ground state, in the excited
state the angle is reduced to 5◦. (ii) A single exponential decay
is observed in ethanol and longer alcohols, but the biexponential
decay is observed in methanol and the relative contribution of the
minor component further increases in the much smaller solvent
water. It may also be noted here that not only the bulkiness of the
solvent decreases, but also the hydrogen bond donating capacity
increases in water compared to methanol. Thus, the combined
factor may be responsible for inducing ICT emission in 2.
To substantiate further we have studied the effect of pH. Upon
decreasing the pH, the enhanced electron withdrawing effect of
the imidazopyridium ion that formed by protonation is expected
to increase the ICT emission. The absorption spectrum of 2
is red shifted on protonation (Fig. 6). This is consistent with
the protonation of the ring nitrogen. The fluorescence spectrum
of DMAPIP-b in water undergoes a small red shift when the
imidazole nitrogen is protonated,22 but highly red-shifted ICT
fluorescence was observed on protonation at pyridine nitrogen.51,52
On protonation, the fluorescence spectrum of 2 is red shifted
nearly by 70 nm (Fig. 6). This suggested that the monocation is
formed by protonation at the pyridine nitrogen of 2 and is emitting
from the ICT state. The theoretical excitation energies obtained
by TDDFT and ZINDO for the monocation are 2.34 eV and 2.95
eV respectively and the corresponding emission energies obtained
are 2.50 eV (TDDFT) and 2.73 eV (ZINDO).
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Conclusion
N-Alkylation strongly perturbed the fluorescence characteristics
of DMAPIP-b. Upon addition of a long alkyl chain at the pyridine
nitrogen in DMAPIP-b, the molecule (1) starts to emit from the
ICT state even in polar aprotic solvents. On the other hand, in the
case of the imidazole nitrogen alkyl-substituted molecule 2, only
normal fluorescence is observed in most of the polar protic solvents
also. Methanol and water are the exception where smaller size and
larger hydrogen bond donating capacity of solvents facilitate dual
emission in 2. The emission life time of 1 and 2 are close to that of
the ICT and the normal emissions of DMAPIP-b.
Acknowledgements
The work is supported by Department of Science and Technology,
New Delhi, by funding the projects SR/S1/PC-19/2006 and
SR/S1/PC0046/2010. The authors thank Dr R. Punekar for
useful suggestions.
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21 M. Novaira, F. Moyano, M. A. Biasutti, J. J. Silber and N. M. Correa,
An example of how to use AOT reverse micelle interfaces to control a
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