Photophysics of 2-(4′-amino-2′-hydroxyphenyl)-1H-imidazo-[4,5- c ]pyridine and its analogues: Intramolecular proton transfer versus intramolecular charge transfer

Article abstract of DOI:10.1021/jp5064808

Photophysical characteristics of 2-(4′-amino-2′-hydroxyphenyl)-1H-imidazo-[4,5-c]pyridine (AHPIP-c) have been studied in various aprotic and protic solvents using UV-visible, steady state fluorescence and time-resolved fluorescence spectroscopic technique

Full text of DOI:10.1021/jp5064808

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Article
Photophysics of 2-(4'-amino-2'-hydroxyphenyl)-1H-
imidazo-[4,5-c]Pyridine and Its Analogues: Intramolecular
Proton Transfer Versus Intramolecular Charge Transfer
Santosh Kumar Behera, Ananda Karak, and Govindarajan Krishnamoorthy
J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp5064808 • Publication Date (Web): 22 Oct 2014
Downloaded from http://pubs.acs.org on October 28, 2014
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Photophysics of 2ꢀ(4'ꢀAminoꢀ2'ꢀhydroxyphenyl)ꢀ1Hꢀimidazoꢀ[4,5ꢀc]pyridine and its
analogues: Intramolecular Proton Transfer versus Intramolecular Charge Transfer
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Santosh Kumar Behera, Ananda Karak and G. Krishnamoorthy*
Department of Chemistry
Indian Institute of Technology Guwahati
Guwahati 781039, India
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Telephone: +91ꢀ361ꢀ2582315
Eꢀmail: gkrishna@iitg.ernet.in
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Abstract
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Photophysical characteristics of 2ꢀ(4'ꢀaminoꢀ2'ꢀhydroxyphenyl)ꢀ1Hꢀimidazoꢀ[4,5ꢀc]pyridine
(AHPIPꢀc) have been studied in various aprotic and protic solvents using UVꢀVisible, steady
state fluorescence and timeꢀresolved fluorescence spectroscopic techniques. To comprehend
the competition between the intramolecular charge transfer (ICT) and the excited state
intramolecular proton transfer (ESIPT) processes, the photophysical properties of methoxy
derivative 2ꢀ(4'ꢀaminoꢀ2'ꢀmethoxyphenyl)ꢀ1Hꢀimidazoꢀ[4,5ꢀc]pyridine (AMPIPꢀc) and 2ꢀ(4'ꢀ
aminophenyl)ꢀ1Hꢀimidazoꢀ[4,5ꢀc]pyridine (APIPꢀc) were also investigated. Though, APIPꢀc
displays twisted ICT (TICT) emission in protic solvents AHPIPꢀc exhibits normal and
tautomer emissions in aprotic as well as in protic solvents due to ESIPT. However, the
methoxy derivative, AMPIPꢀc emits weak TICT fluorescence in methanol.
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Keywords: TICT, ESIPT, proton coupled charge transfer, dual emission, hydrogen bond
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1. Introduction:
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Proton transfer and electron transfer are some of the important excited state
phenomena those lead to dual emission.1ꢀ11 These phenomena also play vital role in several
photochemical and photobiological processes. In excited state intramolecular proton transfer
(ESIPT) process, the photoinduced proton transfer from a protic acid group to a basic group
occurs through an intramolecular hydrogen bond.1ꢀ7 This results in a phototautomer in the
excited state. On the other hand, in intramolecular charge transfer (ICT) process the charge is
transfer from the charge donor to acceptor in the excited state that leads to formation of an
ICT state.8ꢀ11 Both ESIPT and ICT have diverse applications in numerous fields.4,10ꢀ24 In
addition, the coupled proton and charge transfer process is also observed in several
systems.25ꢀ31
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When groups responsible for ESIPT and ICT are combined in the same fluorophores
then their photophysical properties become more interesting. Kasha et al. illustrated that the
emission can occur from all three states of pꢀdimethylaminosalicylate, i.e. the locally excited
state, the ICT state and the tautomer excited state and they compete with each other.32ꢀ34
Conversely, in 4′ꢀ(N,Nꢀdiethylamino)ꢀ3ꢀhydroxyflavone and its analogues emissions were
observed only from the ICT and phototautomer states, and an equilibrium was established
between the two states.35ꢀ37 7ꢀN,NꢀDiethylaminoꢀ3ꢀhydroxyflavone also emits only dual
fluorescence and were observed from ICT and phototautomer states.38,39 But there was a
precursor–successor relationship between the two states. Recently, Guchhait et al. reported
that the proton transfer supresses the ICT process in 4ꢀ(N,Nꢀdiethylamino)ꢀ2ꢀ
hydroxybenzaldehyde.40 In contrast, they illustrated that the proton transfer assists the charge
transfer in some Schiff bases.41 Rodríguez et al. revealed that 2ꢀ(4′ꢀN,Nꢀdiethylaminoꢀ2′ꢀ
hydroxyphenyl)benzimidazoles emit normal and tautomer emissions and no ICT emission.42
However, they proposed ICT from the deprotonated dialkylaminophenol to the protonated
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benzimidazole in the tautomeric form, but such an ICT process resulted in nonꢀfluorescent
tautomer. In flufenamic acid and mefenamic acid ESIPT occurs in nonpolar solvent and ICT
fluorescence occurs in polar solvents.43 A derivative of 9ꢀaminoacridine exhibits a complex
emission consist of normal emission, ICT emission and emission from the enol–imine
tautomeric form.44 pꢀN,NꢀDimethylaminobenzoic acid emits only normal emission in
nonpolar solvents but Yoon et al. found that pꢀN,Nꢀdimethylaminosalicylic acid emits ICT
emission in nonpolar solvents also.45,46 Therefore, they proposed that the ESIPT through
hydrogen bonding favoured the ICT process. Further they have demonstrated that when the
intermolecular hydrogen bonding was supressed by complexation, the ICT emission
enhanced due to intramolecular hydrogen bond that led to ESIPT. Similarly, though no ICT
emission was detected from 4ꢀaminobenzoic acid and ICT emission was reported from 4ꢀ
aminosalicylic acid.
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On the other hand, in 2ꢀ(4'ꢀaminophenyl)ꢀ1Hꢀimidazo[4,5ꢀc]pyridine (APIPꢀc, Chart
1) intermolecular hydrogen bond promotes the formation of ICT emission.47 Fasani et al.
were the first one to observe ICT emission from APIPꢀc in protic solvent.47 But they
proposed that the hydrogen bonding of protic solvents with >NH and nitrogen of imdazole
twist the acceptor (imidazopyridine) which results in a twisted ICT (TICT) state. Later
studies from our group revealed that the hydrogen bonding of protic solvents with pyridyl
nitrogen also plays a very crucial role in the formation of TICT state in 2ꢀ(4'ꢀ
aminophenyl)imidazopyridines.11,48ꢀ50 We also demonstrated that it is the dimethylamino
group not the imidazopyridine ring that is twisted to form the TICT state.11,48ꢀ50 The
corresponding 2'ꢀhydroxy analogue, 2ꢀ(2'ꢀhydroxyphenyl)ꢀ1Hꢀimidazo[4,5ꢀc]pyridine (HPIPꢀ
c, Chart 1) also emits dual emission depending on the nature of the solvents.51 But its’ dual
emission is due to formation of phototautomer by ESIPT process. The intensity ratio of
tautomer emission to normal emission decreases with protic nature of the environment. Here,
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we reported our investigation on 2ꢀ(4'ꢀaminoꢀ2'ꢀhydroxyphenyl)ꢀ1Hꢀimidazoꢀ[4,5ꢀc]pyridine
(AHPIPꢀc, Chart 1). So far, in the combined ESIPTꢀICT system those reported in the
literature, the intermolecular hydrogen bonding is not absolutely essential for ICT molecules
to emit ICT emission.32ꢀ46 AHPIPꢀc is the first molecule of this kind. The main objectives of
the studies are to ascertain the roles of intermolecular hydrogen bond, intramolecular
hydrogen bond and proton transfer in the ICT process and to find whether intramolecular
hydrogen bond or proton transfer is competent to promote ICT. Further the studies in protic
solvents will be more interesting to determine the dominating process (ICT or ESIPT) in
protic solvent. For better understanding, we also studied the spectral characteristics of 2ꢀ(4'ꢀ
aminoꢀ2'ꢀmethoxyphenyl)ꢀ1Hꢀimidazoꢀ[4,5ꢀc]pyridine (AMPIPꢀc, Chart 1), APIPꢀc (Chart 1)
and 2ꢀphenylꢀ1Hꢀimidazoꢀ[4,5ꢀc]pyridine (PIPꢀc, Chart 1).
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H
O
N
N
N
N
NH2
N
N
H
H
APIPꢀc
HPIPꢀc
H3C
O
H
O
N
N
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N
NH2
NH2
N
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H
AMPIPꢀc
AHPIPꢀc
N
N
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H
PIPꢀc
Chart 1. Structures of APIPꢀc, HPIPꢀc, AHPIPꢀc, AMPIPꢀc and PIPꢀc.
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2. Materials and Methods
2.1 Synthesis
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APIPꢀc, AHPIPꢀc and AMPIPꢀc were synthesized by refluxing 3,4ꢀdiaminopyridine
and the corresponding 4ꢀaminobenzoic acid in POCl3 at 110°C by following the procedure
for the synthesis of similar compounds.52 After 8 hours the reaction mixtures were cooled to
room temperature and poured into ice cold water. The solution was neutralized by
concentrated sodium hydroxide solution. The precipitates were filtered and collected. The
compounds were purified by column chromatography. The identities of the compounds were
confirmed by NMR and HRMS mass data.
APIPꢀc
1H NMR (400 MHz, DMSOꢀd6): δ 12.83 (s, 1H), 8.76 (s, 1H), 8.20 (d, 1H), 7.95 (d, 1H),
7.87 (d, 2H), 6.67 (m, 2H), 5.72 (s, 2H).
13C NMR (150 MHz, DMSOꢀd6) δ 109.80, 112.45, 113.48, 116.17, 128.31, 129.55, 140.73,
151.30, 152.11, 171.95
ESI m/z [M + H+] = 211.097
AHPIPꢀc
1H NMR (400 MHz, DMSOꢀd6): δ 8.36 (s, 1H), 7.80 (s, 1H), 7.39 (t, 1H), 7.06 (d, 1H), 6.38
(d, 1H), 5.86 d, 1H), 4.9(br, 3H).
13C NMR (150 MHz, DMSOꢀd6) δ 99.44, 106.16, 106.77, 109.78, 111.49, 120.56, 130.72,
132.53, 137.97, 148.83, 155.20, 163.05
ESI m/z [M + H+] : 227.095
AMPIPꢀc
1H NMR (400 MHz, DMSOꢀd6): δ 12.04 (s, 1H), 8.79 (s, 1H), 8.22 (d, 1H), 8.03 (d, 1H),
7.48 (d, 1H), 6.30 (m, 2H), 5.86 (s, 2H), 3.39 (s,3H)
13C NMR (150 MHz, DMSOꢀd6) δ 55.86, 96.64, 106.99, 108.32,112.30, 130.19, 140.50,
140.90, 143.04, 151.48, 153.13, 154.25, 158.86
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ESI m/z [M + H+] = 241.108
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PIPꢀc is synthesized by heating 3,4ꢀdiaminopyridine and benzoic acid in
polyphosphoric acid heated at 190°C for 5 h. The reaction mixture was cooled to room
temperature and poured to ice cold water. The mixture was neutralized by KOH solution. The
solid product thus obtained was dried in a desiccator. The dried solid product was
recrystallized twice in methanol. The identity of the product was confirmed by NMR and
HRMS mass data.
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1H NMR (600 MHz, CDCl3): δ 8.86 (1H, s), 8.22 (1H, d), 8.08 (2H, dd), 7.36 (4H, m).
13C NMR (150 MHz, CDCl3): δ141.28, 130.52, 129.54, 128.81, 127.1
ESI m/z [M+H+] 196.087
2.2. Spectral Measurements
All the spectral measurements were carried out with ~ 1 ꢀM of solute in order to
avoid aggregation and selfꢀquenching. The absorption spectra were recorded by using Perkin
Elmer Lambda 25 UVꢀVisible spectrophotometer. Steady state emission spectra were
recorded on a HORIBA Jobin Yvon Fluorologꢀ3 fluorimeter. Fluorescence lifetimes were
measured using time correlated singleꢀphoton counting (TCSPC) method on an Edinburgh
Instrument LifeꢀSpec II instrument. 308 nm LED or 375 nm laser diodes was used as the
excitation source. The fluorescence decays were analysed by reconvolution method using the
FAST software provided by Edinburgh Instruments. The goodness of fit was determined by
the reduced χ2 values and weighted residuals which were between the ranges of ± 4.
3. Results and discussion
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3.1. 2ꢀ(4'ꢀaminophenyl)ꢀ1Hꢀimidazo[4,5ꢀc]pyridine (APIPꢀc)
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Fasani et al. studied only the steady state spectral characteristics of APIPꢀc in few
selected solvents.47 For a better comparative study we measured the steady state spectral
characteristics of APIPꢀc in more solvents. The data are presented in Table 1 and the
fluorescence spectra in few solvents are depicted in Figure 1. APIPꢀc exhibits only normal
emission in aprotic solvents. In methanol TICT emission is observed at longer wavelength.
Our results are in good agreement with the spectral maxima reported by Fasani et al. We
measured the fluorescence spectra of APIPꢀc in more protic solvents. In all these solvents
though no clear band appears (as in methanol), but long tailings are observed. We also carried
out time resolved fluorescence measurements (Table 2). The fluorescence decay in propanol
is shown as representative plot (Figure 2). Single exponential decays are detected when
monitored at 380 nm, in all aprotic as well as protic solvents except in methanol. However,
when monitored at 460 nm, biexponential decay was observed in all the protic solvents. The
short lifetimes match with the lifetimes obtained at shorter wavelength. Therefore they can be
assigned to normal emission. The long lifetime components are due to TICT emission and
their relative amplitude increases with protic nature of the solvents. Thus, not only methanol
but also other protic solvents induce the TICT emission in APIPꢀc. The absence of dual
emission in aprotic solvent and its’ presence in protic solvents may be due to enhancement in
the electron withdrawing capacity of the acceptor by hydrogen bonding with protic solvents.
Herbich et al. demonstrated that such an increase in the electron affinity by hydrogen
bonding lowers the energy of the TICT state and the barrier for its’ formation in 4ꢀ(N,Nꢀ
dimethylamino)pyrimidines.53
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The electronic transition dipole moment (Mflu) for the charge transfer state can be
determined using following expression, 54
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64π 4
3h
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kf =
(nν fTluICT )3 M flu
(1)
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where kf is the radiative rate constant, h is the Planck’s constant and n is the refractive index.
Since clear dual emission is observed only in methanol calculation of quantum yield is more
reliable in methanol. Therefore, the Mflu is estimated in methanol. The quatum yield, radiative
rate constant, nonradiative rate constant and electronic transition dipole moment for the
longer wavelength emission are 0.039, 1.8 X 107 sꢀ1, 4.4 X 108 sꢀ1 and 1.5 D, respectively.
The small value of the electronic transition dipole moment determined for the longer
wavelength emission suggest a small overlap between the donor and the acceptor orbitals.
This supports the TICT model.55ꢀ57 The absence of dual emission from PIPꢀc (the molecule
which do not have electron donating amino group) in protic solvent and its enhancement in
dimethylamino derivative of APIPꢀc further substantiates the TICT model (see later).
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3.2. 2ꢀ(4'ꢀaminoꢀ2'ꢀhydroxyphenyl)ꢀ1Hꢀimidazo[4,5ꢀc] pyridine (AHPIPꢀc)
The absorption spectra of AHPIPꢀc were recorded in different solvents and the data
are compiled in Table 3. Absorption spectra in all the solvents consist of one band at ~ 300
nm and another band at ~ 345 nm. The longer wavelength absorption band maxima of
AHPIPꢀc are red shifted compared to those of APIPꢀc and HPIPꢀc. This is may be due to
greater conjugation in AHPIPꢀc which has both hydroxyl and amino groups. Upon increasing
the polarity and hydrogen bonding capacity of the solvents a small blue shift is observed in
the longer wavelength absorption band maxima. Whereas, such a change produces a red shift
in the absorption spectrum of APIPꢀc (Table 1) and a blue shift in the absorption spectrum of
HPIPꢀc.51
Fluorescence spectral data of AHPIPꢀc in different solvents are compiled in Table 3.
Figure 3 shows the representative spectra of AHPIPꢀc in selected solvents. Unlike APIPꢀc, in
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AHPIPꢀc dual emission is observed in both aprotic and protic solvents. The fluorescence
decays were monitored at both bands and were found to be single exponentials (Table 4). The
fluorescence lifetime of the shorter wavelength emitting species is shorter than that of the
longer wavelength emitting species.
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Dogra et al. established HPIPꢀc exists in both cis and trans enol forms.51 Cis enol
upon excitation undergoes ESIPT to form a keto tautomer and the emission occurs from the
tautomer, whereas the transꢀenol upon excitation emits the normal emission. The shorter and
the longer wavelength emissions were assigned to normal emission (from trans enol) and
tautomer emission (from the tautomer formed by ESIPT from cisꢀenol), respectively. In
APIPꢀc only a single emission is observed in aprotic solvents. But, Yoon et al. hypothesized
that the activation energy for the ICT processes is lowered by the ESIPT through the
intramolecular hydrogen bonding in 4ꢀaminosalicylic acid.45 Park et al. also described a
consecutive ESIPT/ICT process that led to dual emission when different acceptors were
substituted in the ESIPT exhibiting 2ꢀ(2'ꢀhydroxyphenyl)benoxzazole.58,59
ESIPT yields a phototautomer whose dipole moment is lesser than the ground state.
Therefore, upon increasing the polarity of the solvent, the excited state is less stabilized than
the ground state. This lead to increase in the energy gap between the states, thus, a negative
solvatochromism is experienced. On the other hand, ICT in the excited state generates a
highly polar TICT state whose dipole moment is much higher than the ground state.8ꢀ11
Consequently, the excited state is more stabilized with increase in environmental polarity
which decreases the energy gap. Accordingly, it produces a red shift in the fluorescence
spectra upon increasing the polarity of the environment. In AHPIPꢀc the longer wavelength
emission is blue shifted with increase in solvent polarity (Table 3). In other words a negative
solvatochromism is observed (Figure 4). Hence, it can be concluded that the longer
wavelength emission of AHPIPꢀc in aprotic solvents is due to ESIPT. Guchhait et al. found
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that in some Schiff bases the proton transfer assist the TICT process.41 It was also reported
that the ICT emission is at shorter wavelength than the tautomer emission. But the
solvatochromic shift of the shorter wavelength emission is small to assign the shorter
wavelength as TICT band. In addition the shorter wavelength band is blue shifted with
respect to the TICT emission of APIPꢀc (Table 1 and 3). Therefore, the shorter wavelength
and the longer wavelength emission from AHPIPꢀc in aproic solvents can be assigned to
normal and tautomer emissions, respectively. This conclusion is further substantiated from
the single emission observed in aprotic solvents from the methoxy derivative of AHPIPꢀc
(see later).
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TICT process is responsible for the longer wavelength emission of the 4ꢀaminophenyl
derivatives of imidazopyridines in protic solvents.11, 47ꢀ50 But, in protic solvents also the
longer wavelength emission of 2ꢀhydroxyphenyl derivative is due to ESIPT process.51 Protic
solvents break the intramolecular hydrogen bond in cis enol which is perquisite for the ESIPT
process. This decreases the tautomer intensity. In contrast, the intermolecular hydrogen bond
with the charge acceptor favours the formation of ICT state thereby enhances the ICT
emission.11,47ꢀ50 Therefore, the effect of protic solvents is further more exciting. The single
exponential decays of both emissions clearly suggest that in protic solvents also AHPIPꢀc
exhibits only dual fluorescence and not the triple fluorescence. Therefore, it is clear that one
of the processes (TICT or ESIPT) is supressed by the other in AHPIPꢀc. The longer
wavelength fluorescence of AHPIPꢀc may be due to ESIPT or ICT process. The longer
wavelength emission band maxima in protic solvents also show negative solvatochromism
(Figure 4). The longer wavelength band maxima in methanol is blue shifted compared to
nonpolar solvents (Table 3). This indicates that the longer wavelength emission is the
tautomer emission. The intermolecular hydrogen bonding is essential for APIPꢀc to exhibit
the TICT emission. In AHPIPꢀc, the ESIPT process completely prevents the ICT process in
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protic solvents also despite fact that the protic solvents favour the intermolecular hydrogen
bonding. In 4ꢀ(diethylamino)ꢀ2ꢀhydroxybenzaldehyde also the TICT process is supressed by
ESIPT process.40 But the nature of 4ꢀ(diethylamino)benzaldehyde is different unlike in APIPꢀ
c, the protic environment is not essential for 4ꢀ(diethylamino)benzaldehyde to emit TICT
emission. Rodríguez et al. illustrated though non emissive ICT is observed in 2ꢀ(4′ꢀN,Nꢀ
diethylaminoꢀ2′ꢀhydroxyphenyl)benzimidazoles no ICT emission is found in aprotic or protic
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solvents.42
But
unlike
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derivatives,
2ꢀ(4′ꢀN,Nꢀdiethylaminoꢀ2′ꢀ
hydroxyphenyl)benzimidazoles do not have pyridyl nitrogen which plays a crucial role in the
hydrogen bonding induced TICT emission of 2ꢀ(4'ꢀaminophenyl)imidazopyridines.11,47ꢀ50 In
AHPIPꢀc despite the presence of pyridyl nitrogen TICT emission is suppressed by ESIPT.
The fluorescence excitation spectra of AHPIPꢀc monitored at both emission maxima
are different (Figure 5). This suggests that the ground state precursors for both the emissions
are different. As mentioned earlier, HPIPꢀc and related molecules exist as cisꢀenol and transꢀ
enol conformers. Accordingly, in AHPIPꢀc also the two different ground state species can be
assigned cisꢀ and transꢀ enols (Chart 2). Subsequently, two different lifetimes obtained for
the normal and the tautomer emissions can be assigned to the lifetime of the excited transꢀ
enol and the tautomer, respectively. The fluorescence lifetimes of both species of AHPIPꢀc
are longer than those of respective forms of HPIPꢀc.51 Such enhancements in radiative
lifetimes are observed in ESIPT molecules also upon increasing conjugation.41,51 The increase
in polarity and hydrogen bonding capacity of the environment shifts the cisꢀenolꢀtransꢀenol
equilibrium towards transꢀenol. Therefore, the intensity ratio of the tautomer emission to
normal emission decreases with increase in polarity and hydrogen bonding capacity of the
solvent (Figure 6). The ratio is less in protic solvents than in aprotic solvents and, it decreases
with the protic nature of the solvent. As the intermolecular hydrogen bond breaks the
intramolecular hydrogen bond in cisꢀenol, the competition between the intramolecular and
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intermolecular hydrogen bond decreases the relative population of cisꢀenol in protic
solvents.51 In other words, the ESIPT process is hindered by intermolecular hydrogen
bonding and it is evident from Figure 6.
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3.3. 2ꢀ(4′ꢀaminoꢀ2′ꢀmethoxyphenyl)ꢀ1Hꢀimidazo[4,5ꢀc]pyridine (AMPIPꢀc)
To understand the role of ESIPT in supressing the ICT process of AHPIPꢀc, we
further investigated the spectral properties of methoxy derivative AMPIPꢀc (Chart 1). Upon
enhancing the polarity and the hydrogen bonding capacity of the solvents, though, little
smaller than APIPꢀc a bathochromic shift is observed in the absorption spectrum of AMPIPꢀc
(Table 5). This behaviour of AMPIPꢀc is different from that of methoxy derivative of HPIPꢀc.
Dogra et al. reported that the absorption maxima of 2ꢀ(2'ꢀmethoxyphenyl)ꢀ1Hꢀimidazo[4,5ꢀ
c]pyridine (MPIPꢀc) are nearly insensitive to nature of the solvents.60 Therefore, it is clear
that the substitution of electron donating amino group make it sensitive to environment and
characteristics of AMPIPꢀc are more closer to APIPꢀc than those of MPIPꢀc.
The fluorescence maxima of AMPIPꢀc are also more sensitive to the solvent polarity
(Figure 7 and Table 5). Same as in APIPꢀc a positive solvatochromism is observed. But the
charge transfer is less in AMPIPꢀc than in APIPꢀc. This is substantiated by the lower dipole
moment of AMPIPꢀc than APIPꢀc (see later). The TICT emission of AMPIPꢀc is not as
prominent as in APIPꢀc only long tail is found in methanol (Figure 1 and Figure 7). The
excited decays of AMPIPꢀc measured in all solvents except methanol are single exponentials
(Table 6). Attempt to fit the single exponential decay to the fluorescence decay in methanol
resulted in poor fit with high chi square (χ2). But the biexponential fit yields good results
with two different lifetimes which further substantiate the presence of two different emitting
species (Figure 8 and Table 6). By analogy with APIPꢀc they may be assigned to normal and
TICT emitting species. In other protic solvents even when monitored at 465 nm single
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exponential decays are found (Supporting Information, Table S1). The fluorescence lifetime
also matches with the fluorescence lifetime obtained by monitoring at 385 nm. This shows
that except methanol other alcohols do not induce TICT emission in AMPIPꢀc. Therefore, it
is clear that the methoxy substitution at ortho position weaken the TICT emission of AMPIPꢀ
c. Like AHPIPꢀc, 2ꢀ(4′ꢀN,Nꢀdiethylaminoꢀ2′ꢀhydroxyphenyl)benzimidazoles also undergo
ESIPT and no ICT emission was observed. But unlike AMPIPꢀc, the methoxyderivative
derivative of 2ꢀ(4′ꢀN,Nꢀdiethylaminoꢀ2′ꢀhydroxyphenyl)benzimidazoles did not exhibit even
weak TICT emission.
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3.4 Solvatochromismic approach
LippertꢀMataga analysis61 is employed to estimate the excited state dipole moments of
the molecules to verify the charge transfer characteristics in the excited state.


2 

2(ꢀe
−
ꢀg
)
v =
ꢁf + Constant
(2)
ss
hca3




Where νss is the Stokes shift, ꢁe and ꢁg are the excited state and ground state dipole moments
of the molecule and a denotes the Onsagar cavity radius. The orientation polarizability is
defend by
2


ε
2ε
−1 n −1
ꢁf =
−
(3)




+1 2n +1
Where ε and n are the dielectric constant and refractive index of the solvents respectively,
The LippertꢀMataga plot is constructed for normal emissions of APIPꢀc, AMPIPꢀc and
AHPIPꢀc (Figure 9). Using the Onsager radius and the ground state dipole moments obtained
from the DFT calculations and the slope obtained from Lippert plot and, the excited state
dipole moments are calculated. The excited state dipole moment, thus determined for APIPꢀc,
AMPIPꢀc and AHPIPꢀc (transꢀenol) are 11.9 D, 10.8 D and 10.8 D, respectively. The ground
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state dipole moments of APIPꢀc, AMPIPꢀc and AHPIPꢀc (transꢀenol) are 7.1 D, 7.8 D and 9.6
D, respectively. Thus, it can be interpreted that the charge transfer enhanced in the excited
state. The enhancement in excited state charge transfer is more in APIPꢀc than in AMPIPꢀc,
which is higher than that in AHPIPꢀc.
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The dipole moments for the normal emissions suggest appreciable charge transfer
from the donor to acceptor in all the molecules. Unfortunately, the longer wavelength
emission is observed from AMPIPꢀc only in methanol. Even the longer wavelength emission
of APIPꢀc is buried under the normal emission in other protic solvents expect methanol.
Therefore estimating the dipole moment of the longer wavelength emitting state by
solvatochromic approach is not viable. Hence, to substantiate that the longer wavelength
emission is a TICT emission we synthesized the fluorophore without the charge donor (amino
group). The spectral characteristics of PIPꢀc, thus synthesized are summarized in Table 7 and
the fluorescence spectra are presented in Figure 10. Unlike in amino derivatives no dual
emission is observed in PIPꢀc in not only in aprotic solvents but also in protic solvents. On
the other hand, when the electron donating ability of the donor is higher the longer
wavelength emission induced by protic solvents increases (in methanol the intensity ratio of
longer wavelength to shorter wavelength is 0.18 for APIPꢀc and 0.30 for the dimethyl
derivative of APIPꢀc, 2ꢀ(4'ꢀdimethylaminophenyl)imidazo[4,5ꢀc]pyridine).48 The dependence
of the longer wavelength emission with the capacity of charge donor is consistent with our
assignment that the longer wavelength emission is the TICT emission. In APIPꢀc analogue, 2ꢀ
(4'ꢀdimethylaminophenyl)imidazo[4,5ꢀb]pyridine also dual emission was observed only in
protic solvents. Since 2ꢀ(4'ꢀdimethylaminophenyl)imidazo[4,5ꢀb]pyridine emits clear dual
emission in other protic solvents also the dipole moment was estimated using longer
wavelength emission by solvatochromic approach. The estimated dipole moments (ꢀe = 12.0
D (normal) 24.6 D (TICT)) substantiated the assignment of the TICT mechanism.48,62
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Kamlet Taft solvatochromic method is widely used to understand the interaction
between the solute and solvent molecules.63 This multiꢀparametric approach separates the
effects of polarity and hydrogen bonding, and provides useful information. According to
multiple linear regression analysis approach the spectral band energy (in cmꢀ1)
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ν = ν 0 + sπ* + aα +bβ
(4)
where π*, α and β are the polarity/polarizability, hydrogen bond donating ability and
hydrogen bond accepting capacity of the solvents, respectively. The sensitivity to each
individual parameter were given as s, a and b coefficients. The fits obtained using the
absorption spectral data are presented below:
ν
ν
ν
(APIPꢀc) = 32589 – 863π* ꢀ 750α ꢀ 24β (r = 0.89 )
(AHPIPꢀc) = 28364 + 880π* + 306α ꢀ 57β (r = 0.93)
(AMPIPꢀc) = 31205 – 1167π* ꢀ 713α ꢀ 260 β (r = 0.89)
(5)
(6)
(7)
The ν0 values obtained in all the molecules are in good agreement with the values in
nonpolar solvent. The negative and positive values of coefficients indicate the stabilization
and destabilization of the system by individual parameter. Not only the polarity parameter but
also the hydrogen bond donating ability of the solvent has positive coefficient for the
hydroxyl derivative. The hydroxyl derivative has strong intramolecular hydrogen bond which
stabilizes the system. Therefore, any intermolecular hydrogen bonding through solvents
destabilizes it. On the other hand, when there was no intramolecular hydrogen bond the
intermolecular hydrogen bond stabilizes the molecules. Therefore, the intermolecular
hydrogen bonding stabilizes APIPꢀc and AMPIPꢀc. In other words, when the intramolecular
hydrogen bonding that leads to ESIPT is prevented the intermolecular hydrogen bonding
dominates and it allows TICT to prevail.
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Less sensitivity of the fluorescence maxima toward the solvent results in a poor
correlation for AHPIPꢀc. However, reasonably good correlations were obtained for APIPꢀc
and AMPIPꢀc and are shown below:
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ν
ν
(APIPꢀc) = 27514 – 1203π* ꢀ 863α ꢀ 243β (r = 0.82 )
(AMPIPꢀc) = 27982 – 1507π* ꢀ 825α ꢀ552 β (r = 0.91)
(8)
(9)
Like the ground state, the excited state also stabilized by both polarity and hydrogen
bonding parameters. The solvatochromic approach clearly shows that the intermolecular
hydrogen bonding plays an important role in the stabilization of APIPꢀc and AMPIPꢀc. The
ESIPT prevents the TICT processes of AHPIPꢀc in protic solvents also and blocking of
ESIPT in AMPIPꢀc opens the TICT channel due to strong intermolecular hydrogen bonding.
3.5 Theoretical calculations
All the calculations were performed using Gaussian 09 program.64 The ground state
geometries were optimized DFT method. The first excited state geometries were optimized
using ab initio configuration interaction singles (CIS) approach. The excitation and the
emission energies were computed by TDDFT method from the optimized geometries as input
(vertical transitions). Moderate to good correlations were obtained between the computed and
the experimental transition energies (Table 8, 9 and 10). B3LYP function and 6ꢀ31G(d,p)
basis sets were employed in the calculations. The integral equation formalismꢀpolarizable
continuum (IEFꢀPCM) model was used to include the solvent stabilization effects. The data
computed for all the molecules in acetonitrile are compiled in Table 8 and the frontier
molecular orbitals are given in the Supporting Information. Both cisꢀ and transꢀ enol of
AHPIPꢀc are included for the excitation energies comparison. Since in the excited state cisꢀ
enol undergoes ESIPT to form keto tautomer, the structure of the keto form is optimized in
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the first excited state. The emission energy obtained for keto is compiled instead of cisꢀenol.
The lowest energy transitions in all the molecule are single excitations from the highest
occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO).
These transitions are strongly allowed as indicated by the higher oscillator strength and the
first excited states are ππ* state. However, the S0ꢀS2 and S0ꢀS3 transitions involve multiple
excitations and have smaller oscillator strength.
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APIPꢀc and AMPIPꢀc emit both normal and TICT emissions in methanol. Therefore,
we also computed the transitions for APIPꢀc and AMPIPꢀc in methanol (as solvent) for the
methanol complexes of APIPꢀc and AMPIPꢀc. The methanol complex comprise of several
methanol molecules forming hydrogen bond at different sites. Since, the hydrogen bonding at
pyridyl nitrogen is more crucial for the formation of TICT state,11,48ꢀ50 we have performed the
calculations by adding single methanol at pyridyl nitrogen to save the computational time.
We considered the twisting of both phenyl ring and amino group. The transitions of normal,
phenyl twisted and amino twisted forms of APIPꢀc and AMPIPꢀc are presented in Table 9
and 10. The transition energies computed for the phenyl twisted forms are higher than the
corresponding transition energies of the normal forms. Even the emission energies of the S0ꢀ
S1 transition of the phenyl twisted rotamers are higher than the emission energies of the S0ꢀS1
transition of the normal forms. Therefore, it is clear that TICT emission of APIPꢀc and
AMPIPꢀc are not from the phenyl twisted forms. However, transition energies predicted for
the amino twisted forms are less than the corresponding transition energies of the normal
forms. The S0ꢀS1 transitions are defined by HOMOꢀLUMO single excitation and the
oscillator strengths of S0ꢀS1 for the normal forms are higher which indicate that these
transitions are allowed. In amino twisted form the first excited states are defined by multiple
excitation. In both molecules the first excited states are formed by the excitations from
HOMO to higher orbitals. The contribution of HOMO → LUMO and HOMO → LUMO + 3
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are 0.68849 (87.2 %) and 0.10095 (12.8 %) respectively in APIPꢀc. In AMPIPꢀc, the
contribution HOMO → LUMO, HOMO → LUMO + 1 and HOMO → LUMO + 3 are
0.67742 (71.7 %), 0.13649 (14.5 %) and 0.13077 (13.8 %) respectively. The charge is
localized on the donor amino group in the HOMO and in the higher orbital it is localized on
the rest of the molecule (Figure 11). Therefore, the transition from HOMO involves the
charge transfer from amino groups to other part of the molecule and there is a minimum
overlap between them. Further, negligible oscillator strengths for the transitions of amino
twisted corroborate this. Thus, the result can be interpreted by TICT model. In LUMO + 1
and LUMO + 3 orbitals the charge density on the imidazole ring is less. The contributions of
the excitations to those molecular orbitals are more in AMPIPꢀc than APIPꢀc. Incidentally,
AMPIPꢀc, which emits relatively weaker TICT emission, the oscillator strength for the TICT
emission though it is very little, it has nonꢀzero value. However, the calculation predicts the
oscillator strength for the TICT emission in APIPꢀc as zero.
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The calculation predicted that the amino group is out of plane from the phenyl ring by
nearly same angle (22
̊
imdazopydridine ring and the phenyl ring are out of plane by 4
̊
̊
c and AMPIPꢀc complexes. In APIPꢀc and AMPIPꢀc the TICT state is formed by the twisting
of the amino group. The twisting of phenyl ring is not a favoured process for formation of
TICT state in AMPIPꢀc also. Therefore, we speculate that the nonplanar geometry between
the phenyl ring and imdazopydridine ring in methoxy molecule may be responsible for its’
weak TICT emission. Yoon et al. proposed that the planarization of the benzene ring with
carboxylic acid group favours the TICT processes in dialkylamino benzoic acids owing to
large charge flow from the phenyl ring to carboxylic acid.65,66 The twisting of phenyl ring
might have reduced the charge flow from the phenyl ring to imidazopyridine ring in AMPIPꢀ
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c. Other spectral characteristics also suggest that the intramolecular charge transfer is less in
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APIPꢀc exhibits dual emission only in protic solvents. Dual fluorescence is observed
from AHPIPꢀc in all solvents. The shorter wavelength emission is the normal emission from
the excited transꢀenol. Unlike in APIPꢀc the longer wavelength emission is the tautomer
emission. Like in HPIPꢀc, the radiative lifetime of the tautomer emission is longer than
normal emission. But the lifetime of both normal and tautomer emissions of AHPIPꢀc are
longer than the respective lifetimes of HPIPꢀc. The intensity ratio of the tautomer emission to
the normal emission is sensitive to nature of the solvent. Not only in aprotic solvents but also
in protic solvents the ESIPT process supresses the TICT process. When the intramolecular
hydrogen bond is blocked, the methoxy derivative of AHPIPꢀc, AMPIPꢀc emits normal
emission in most of the solvent. But it exhibits the TICT emission in methanol. The TICT
emission of AMPIPꢀc is weaker than the TICT emission of APIPꢀc. The substitution of bulky
methoxy group twisted the imidazopyridine out of plane from the benzene ring and thereby
reduced the ICT from the benzene ring to imidazopyridine ring. The amino group twist to
form the emitting TICT state in both APIPꢀc and AMPIPꢀc. In the TICT geometry there is a
minimum overlap between the amino group and rest of the molecule.
Acknowledgement
The authors thank Department of Science and Technology (DST), India for the
financial support (SR/S1/PC/0046/2010) and the Central Instruments Facility (CIF), Indian
Institute of Technology Guwahati for the LifeSpec II instrument and NMR spectrometers.
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Supporting Information:
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The fluorescence lifetime data of AMPIPꢀc in protic solvent (obtained by monitoring at 465
nm), full authors list for reference 64 and the computed frontier molecular orbitals. This
information is available free of charge via the Internet at http://pubs.acs.org
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References
1. Kasha, M. Proton –Transfer Spectroscopy – Perturbation of the Tautomerization
Potential. J. Chem. Soc., Faraday Trans. 2. 1986, 82, 2379ꢀ2392.
2. Douhal, A.; Kim, S. K.; Zewail, A. H., Femtosecond MolecularꢀDynamics of
Tautomerization in Model BaseꢀPairs. Nature. 1995, 378, 260ꢀ263.
3. Kwon, J. E.; Park, S. Y. Advanced Organic Optoelectronic Materials: Harnessing
ExcitedꢀState Intramolecular Proton Transfer (ESIPT) Process. Adv. Mater. 2011, 23,
3615ꢀ3642.
4. Zhao, J.; Ji, S.; Chen, Y.; Guo, H.; Yang, P. Excited State Intramolecular Proton
Transfer (ESIPT): From Principal Photophysics to the Development of New
Chromophores and Applications in Fluorescent Molecular Probes and Luminescent
Materials. Phys. Chem. Chem. Phys. 2012, 14, 8803–8817.
5. Iyer, E. S. S.; Khorwal, V.; Sundararajan M.; Datta, A. Intense Phototautomer
Emission of 2ꢀ(3'ꢀpyridyl)benzimidazole Encapsulated in Nafion Membrane. RSC.
Adv. 2013, 3, 1145ꢀ1150.
6. Vetokhina, V.; Dobek, K.; Kijak, M.; Kaminska, I. I. ; Muller, K. ; Thiel, W. R. ;
Waluk, J.; Herbich, J. Three Modes of Proton Transfer in One Chromophore:
Photoinduced Tautomerization in 2ꢀ(1HꢀPyrazolꢀ5ꢀyl)Pyridines, Their Dimers and
Alcohol Complexes. Chemphyschem, 2012, 13, 3661ꢀ3671.
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7. Basaric, N.; Doslic, N.; Ivkovic, J. ; Wang, Y. –H. ; Veljkovic, J.; MlinaricꢀMajerski,
K.; Wan, P. Excited State Intramolecular Proton Transfer (ESIPT) from Phenol to
Carbon in Selected Phenylnaphthols and Naphthylphenols. J. Org. Chem. 2013, 78,
1811ꢀ1823.
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23
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25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
8. Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Structural Changes Accompanying
Intramolecular Electron Transfer: Focus on Twisted Intramolecular ChargeꢀTransfer
States and Structures. Chem. Rev. 2003, 103, 3899ꢀ4031.
9. Rettig, W. Charge Separation in Excited States of Decoupled SystemsꢀTICT
Compounds and Implications Regarding the Development of New Laser Dyes and the
Primary Process of Visionand Photosynthesis. Angew. Chem., Int. Ed. Engl.1986, 25,
971ꢀ988.
10. Herbich, J.; Kapturkiewicz, A. Electronic Structure and Molecular Conformation in
the Excited Charge Transfer SingletꢀStates of 9ꢀAcridyl and Other Aryl Derivatives of
Aromatic Amines. J. Am. Chem. Soc. 1998, 120, 1014ꢀ1029.
11. Chipem, F. A. S.; Mishra, A.; Krishnamoorthy, G. The Role of Hydrogen Bonding in
Excited State Intramolecular Charge Transfer. Phys. Chem. Chem. Phys. 2012, 14,
8775ꢀ8790.
12. Sakai, K.; Tsuzuki, T.; Itoh, Y.; Ichikawa, M.; Taniguchi, Y. Using ProtonꢀTransfer
Laser Dyes for Organic Laser Diodes. Appl. Phys. Lett. 2005, 86, 081103.
13. Fluegge, A. P.; Waiblinger, F.; Stein, M.; Keck, J.; Kramer, H. E.A.; Fischer, P.;
Wood, M. G.; DeBellis, A. D.; Ravichandran, R.;Leppard, D. Probing the
Intramolecular Hydrogen Bond of 2ꢀ(2ꢀhydroxyphenyl)benzotriazoles in Polar
Environment: A Photophysical Study of UV Absorber Efficiency. J. Phys. Chem. A
2007, 111, 9733−9744.
ACS Paragon Plus Environment

Page 23 of 45
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
14. Wu, J.; Liu, W.; Ge, J.; Zhang, H.; Wang, P. New Sensing Mechanisms for Design of
Fluorescent Chemosensors Emerging in Recent Years. Chem. Soc. Rev. 2011, 40,
3483−3495.
9
15. Chipem, F. A. S.; Behera, S. K.; Krishnamoorthy, G. Ratiometric Fluorescence
Sensing Ability of 2ꢀ(2′ꢀhydroxyphenyl)benzimidazole and Its Nitrogen
Substituted Analogues Towards Metal Ions. Sens. Actuators B: Chem.2014, 191,
727ꢀ733.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
16. Lim, S. J.; Seo, J.; Park, S. Y. J. Photochromic Switching of Excitedꢀstate
Intramolecular ProtonꢀTransfer (ESIPT) Fluorescence: A Unique Route to Highꢀ
contrast Memory Switching and Nondestructive readout. J. Am. Chem. Soc. 2006
,
128, 14542−14547.
17. Mutai, T.; Tomoda, H.; Ohkawa, T.; Yabe, Y.; Araki, K. Switching of Polymorphꢀ
Dependent ESIPT Luminescence of an Imidazo[1,2ꢀa]pyridine Derivative.
Angew.Chem., Int. Ed. 2008, 47, 9522−9524.
18. Lapinski, L.; Nowak, M. J.; Nowacki, J.; Rode, M. F.;Sobolewski, A. L. A Bistable
Molecular Switch Driven by Photoinduced HydrogenꢀAtom Transfer.
ChemPhysChem. 2009, 10, 2290−2295.
19. Park, S.; Seo, J.; Kim, S. H.; Park, S. Y. Tetraphenylimidazoleꢀbased Excitedꢀstate
Intramolecular
Protonꢀtransfer
Molecules
for
Highly
Efficient
Blue
Electroluminescence. Adv. Funct. Mater. 2008, 18, 726−731.
20. Klymchenko, A. S.; Demchenko, A. P. Electrochromic Modulation of Excitedꢀstate
Intramolecular Proton Transfer: The New Principle in Design of Fluorescence
Sensors. J. Am. Chem. Soc. 2002, 124, 12372ꢀ12379.
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 24 of 45
1
2
3
4
5
6
7
8
21. Pivovarenko, V. G.; Vadzyuk, O. B.; Kosterin, S. O. Fluorometric Detection of
Adenosine Triphosphate with 3ꢀHydroxyꢀ4′ꢀ(dimethylamino)flavone in Aqueous
Solutions. J. Fluoresc. 2006, 16, 9ꢀ15.
9
22. Henary, M. M.; Wu, Y.; Fahrni, C. J. Zinc(II)ꢀselective Ratiometric Fluorescent
Sensors Based on Inhibition of ExcitedꢀState Intramolecular Proton Transfer. Chem.
Eur. J. 2004, 10, 3015ꢀ3025.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
23. Lin, Q.; Liu, X.; Wei, T. –B.; Zhang, Y.–M. TICTꢀICT State Change Mechanism
Based Acetate Fluorescent Sensor Act as an "OffꢀOnꢀOff" Switch and Logic Gate.
Sens. Actuators B: Chem. 2014, 190, 459ꢀ463.
24. Mahato, P. ; Saha, S. ; Das, A. Rare Example of TICT Based Optical Responses for
the Specific Recognition of Cr3+ by a 2,2':6 ',2 ''ꢀTerpyridine Derivative and
Demonstration of Multiple Logic Operations. J. Phys. Chem. C 2012, 116, 17448ꢀ
17457.
25. Chou, P.ꢀT.; Yu, W.ꢀS.; Cheng, Y.ꢀM.; Pu, S.ꢀC.; Yu, Y.ꢀC.; Lin,Y.ꢀC.; Huang; Chen,
C.ꢀT. SolventꢀPolarity Tuning ExcitedꢀState Charge Coupled ProtonꢀTransfer
Reaction in pꢀN,NꢀDitolylaminosalicylaldehydes. J. Phys. Chem. A 2004, 108,
6487−6498.
26. Zgierski, M. Z.; Fujiwara, T.; Lim, E. C. Coupled Electron and Proton Transfer
Processes in 4ꢀDimethylaminoꢀ2ꢀhydroxyꢀbenzaldehyde. J. Phys. Chem. A 2011, 115,
10009−10017.
27. Brenlla, A.; RodríguezꢀPrieto, F.; Mosquera, M.; Ríos, M. A.; Ríos Rodríguez, M. C.
SolventꢀModulated GroundꢀState Rotamerism and Tautomerism and ExcitedꢀState
ProtonꢀTransfer Processes in oꢀHydroxynaphthylbenzimidazoles. J. Phys. Chem. A
2008, 113, 56−6.
ACS Paragon Plus Environment

Page 25 of 45
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
28. Hsieh, C. C.; Jiang, C. M.; Chou, P. T. Recent Experimental Advances on Excitedꢀ
State Intramolecular Proton Coupled Electron Transfer Reaction. Acc. Chem. Res.
2010, 43, 1364−1374.
9
29. HammesꢀSchiffer, S.; Stuchebrukhov, A. A. Theory of Coupled Electron and Proton
Transfer Reactions. Chem. Rev. 2010, 110, 6939−6960.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
30. Weinberg, D. R.; Gagliardi, C. J.; Hull, J. F.; Murphy, C. F.;Kent, C. A.; Westlake, B.
C.; Paul, A.; Ess, D. H.; McCafferty, D. G.; Meyer, T. J. ProtonꢀCoupled Electron
Transfer. Chem. Rev. 2012, 112, 4016−4093.
31. Ghosh, R.; Palit, D. K. Dynamics of Solvent Controlled Excited State Intramolecular
Proton Transfer Coupled Charge Transfer Reactions. Photochem. Photobiol. Sci.,
2013, 12, 987ꢀ995.
32. Gormin, D.; Kasha, M. Triple Fluorescence in AminosalicylatesꢀModulation of
Normal, ProtonꢀTransfer, and Twisted Intramolecular Charge – Transfer (TICT)
Fluorescence by Physical and Chemical Perturbation. Chem. Phys. Lett. 1988, 153,
574–576.
33. Gormin D.; Kasha, M. A Comparative Picosecond Spectroscopic Study of the
Competitive Triple Fluorescence of Aminosalicylates and Benzanilides. Chem. Phys.
1989, 136, 321–334.
34. Gormin, D.; Heldt, J.; Kasha, M. Molecular Phosphorescence Enhancement via
Tuning Through Proton Transfer Potentials. J. Phys. Chem. 1990, 94, 1185–1189.
35. Chou, P. T.; Martinez, M. L.; Clements, J. H. J. Reversal of Excitation Behaviour of
ProtonꢀTransfer Vs ChargeꢀTransfer by Dielectric Perturbation of Electronic
Manifolds. J. Phys. Chem. 1993, 97, 2618–2622.
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 26 of 45
1
2
3
4
5
6
7
8
36. Chou, P. T.; Martinez, M. L.; Clements, J. H. The Observation of Solvent –Dependent
Proton –Transfer Charge Transfer Lasers From 4′ꢀDiethylaminoꢀ3ꢀhydroxyflavon.
Chem. Phys. Lett. 1993, 204, 395–399.
9
37. Parsapour, F.; Kelley, D. F. Torsional and Proton Transfer Dynamics in Substituted 3ꢀ
Hydroxyflavones. J. Phys. Chem. 1996, 100, 2791–2798.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
38. Chou, P.ꢀT.; Huang, C.; Pu, S.ꢀC.; Cheng, Y.ꢀM.; Liu, Y.ꢀH.; Wang, Y.; Chen, C.ꢀT.
Tuning ExcitedꢀState Charge/Proton Transfer Coupled Reaction via the Dipolar
Functionality. J. Phys. Chem. A. 2004, 108, 6452–6454.
39. Cheng, Y.ꢀM. ; Pu, S.ꢀC. ; Yu, Y.ꢀC. ; Chou, P.ꢀT. ; Huang, C.ꢀH. ; Chen, C.ꢀT. ; Li,
T.ꢀH. ; Hu, W.ꢀP. Spectroscopy and Dynamics of 7ꢀN,Nꢀdiethylaminoꢀ3ꢀ
hydroxyflavone. The Correlation of Dipole Moments Among Various States to
Rationalize the ExcitedꢀState Proton Transfer Reaction. J. Phys. Chem. A. 2005, 109,
11696–11706.
40. Jana, S.; Dalapati, S.; Guchhait, N. Excited State Intramolecular Charge Transfer
Suppressed Proton Transfer Process in 4‑(Diethylamino)ꢀ2ꢀhydroxybenzaldehyde. J.
Phys. Chem. A 2013, 117, 4367−4376.
41. Jana, S.; Dalapati, S.; Guchhait, N. Proton Transfer Assisted Charge Transfer
Phenomena in Photochromic Schiff Bases and Effect of –NEt2 Groups to the Anil
Schiff Bases. J. Phys. Chem. A 2012, 116, 10948−10958.
42. Ríos Va´zquez, S.; Ríos Rodríguez, M. C.; Mosquera, M.; RodríguezꢀPrieto, F.
Rotamerism, Tautomerism, and ExcitedꢀState Intramolecular Proton Transfer in 2ꢀ(4′ꢀ
N,NꢀDiethylaminoꢀ2′ꢀhydroxyphenyl)benzimidazoles:
Novel
Benzimidazoles
Undergoing ExcitedꢀState Intramolecular Coupled Proton and Charge Transfer. J.
Phys. Chem. A 2008, 112, 376ꢀ387.
ACS Paragon Plus Environment

Page 27 of 45
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
43. ElꢀKemary, M. A. Relaxation Pathways of Photoexcited Nonꢀsteroidal Antiꢀ
inflammatory Drugs: Flufenamic and Mefenamic acid. Chem. Phys. 2003, 295, 1–10.
44. Pereira, R. V.; Ferreira, A. P. G.; Gehlen, M. H. ExcitedꢀState Intramolecular Charge
transfer in 9ꢀAminoacridine Derivative. J. Phys. Chem. A. 2005, 109, 5978–5983.
45. Kim, Y.; Yoon, M.; Kim, D. ExcitedꢀState Intramolecular Proton Transfer Coupledꢀ
Charge Transfer of pꢀN, Nꢀdimethylaminosalicylic Acid in Aqueous betaꢀ
Cyclodextrin Solutions. J. Photochem. Photobiol., A. 2001, 138, 167–175.
46. Kim, Y.; Yoon, M. Intramolecular Hydrogen Bonding Effect on the ExcitedꢀState
Intramolecular Charge Transfer of pꢀAminosalicylic Acid. Bull. Korean Chem. Soc.
1998, 19, 980–985.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
47. Fasani, E.; Albini, A.; Savarino, P.; Viscardi, G.; Barni, E. The Role of Hydrogen
Bonding in Excited State Intramolecular Charge Transfer Hydrogen Bonding,
Protonation and Twisting in the Singlet Excited State of some 2ꢀ(4ꢀ
Aminophenyl)pyridoꢀoxaꢀ, ꢀthiaꢀ, and –imidazoles. J. heterocyclic chem. 1993, 30,
1041ꢀ4044.
48. Krishnamoorthy, G.; Dogra, S. K. Dual Fluorescence of 2ꢀ(4'ꢀN,N-
Dimethylaminophenyl)pyrido[3,4ꢀd]imidazole: Effect of Solvents. Spectrochim. Acta,
A. 1999, 55, 2647ꢀ2658.
49. Dash, N.; Chipem, F. A. S.; Swaminathan, R.; Krishnamoorthy, G. Hydrogen Bond
Induced
Twisted
Intramolecular
Charge
Transfer
in
2ꢀ(4′ꢀN,Nꢀ
dimethylaminophenyl)imidazo[4,5ꢀb]pyridine. Chem. Phys. Lett. 2008, 460, 119ꢀ124.
50. Mishra, A.; Sahu, S.; Dash, N.; Behera, S. K.; Krishnamoorthy, G. Double Proton
Transfer Induced Twisted Intramolecular Charge Transfer Emission in 2‑(4′‑N,N‑
Dimethylaminophenyl)imidazo[4,5ꢀb]pyridine. J. Phys. Chem. B 2013, 117,
9469−9477
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 28 of 45
1
2
3
4
5
6
7
8
51. Balamurali, S. S.; Dogra, S. K. Excited State Intramolecular Proton Transfer in 2ꢀ(2'ꢀ
hydroxyphenyl)ꢀ1Hꢀimidazo[4,5ꢀc]pyridine: Effects of Solvent. J. Photochem.
Photobiol. A: Chem. 2002, 154, 81ꢀ92.
9
52. Mishra, A.; Krishnamoorthy, G. Photophysical Study of 2ꢀ(4′ꢀN,Nꢀ
dimethylaminophenyl)oxazolo[4,5ꢀb]ꢀpyridine in Different Solvents and at Various
pH. Photochem. Photobiol. Sci. 2012, 11, 1356–1367.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
53. Herbich, J.; Grabowski, Z. R.; Wo´jtowicz, H.; Golankiewicz, K. Dual Fluorescence
of 4ꢀ(Dialkylamino)pyrimidines. Twisted Intramolecular Charge Transfer State
Formation Favoredꢀby Hydrogen Bond or by Coordination to the Metal Ion. J. Phys.
Chem., 1989, 93, 3439–3444.
54. Birks, J.B., Photophysics of Aromatic Molecules, Wiley, New York, 1978.
55. Herbich, J.; Waluk, J. Excited Charge Transfer States in 4ꢀAminopyrimidines, 4ꢀ
(Dimetylanilino)pyrimidine and 4ꢀ(Dimethylamino)pyridine. Chem. Phys. 1994, 188,
247–265.
56. Szydłowska, I.; Kyrychenko, A.; Nowacki, J.; Herbich, J. Photoinduced
Intramolecular Electron Transfer in 4ꢀDimethylaminopyridines. Phys. Chem. Chem.
Phys. 2003, 3, 1032–1038.
57. Szydłowska, I.F.; Nowacki, J.; Herbich, J., Structural Changes Accompanying
Excitedꢀstate Electron Transfer in metaꢀ and paraꢀDialkylaminopyridines, J.
Photochem. Photobiol. A 2010, 209, 135–146.
58. Seo, J.; Kim, S.; Park, S. Y. Strong Solvatochromic Fluorescence From the
Intramolecular ChargeꢀTransfer State Created by ExcitedꢀState Intramolecular Proton
Transfer. J. Am. Chem. Soc., 2004, 126, 11154–11155.
ACS Paragon Plus Environment

Page 29 of 45
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
59. Kim, C. H.; Park, J.; Seo, J.; Park, S. Y.; Joo, T. Excited State Intramolecular Proton
Transfer and Charge Transfer Dynamics of a 2ꢀ(2′ꢀHydroxyphenyl)benzoxazole
Derivative in Solution, J. Phys. Chem. A 2010, 114, 5618–5629.
9
60. Balamurali M. M.; Dogra, S. K. Ground and Excited State Prototropism in 2ꢀ(2 'ꢀ
methoxyphenyl)ꢀ1Hꢀimidazo[4,5ꢀc]pyridine. J. Mol. Struct. 2004, 691, 59ꢀ70.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61. Lippert, E. Spektroskopische Bistimmung Des Dipolmomentes Aromatischer
Verbindugen Im Ersten Angeregten Singulettzustand. Z. Electrochem., 1957, 61, 962–
975.
62. Dash, N.; Krishnamoorthy G. Effect of Temperature on the Spectral Characteristics of
2ꢀ(4ꢀN,Nꢀdimethylaminophenyl)imidazo[4,5ꢀb]pyridine. Spectrochim. Acta A 2012
95, 540–546.
,
63. Kamlet, M. J.; Abboud, J. –L.; Abraham, M. H.; Taft, R. W. Linear Solvation Energy
Relationships. 23. A Comprehensive Collection of the Solvatochromic Parameters,
π*, α and β and Some Methods for Simplifying the Generalized Solvatochromic
Equation. J. Org. Chem. 1983, 48, 2877ꢀ2887.
64. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al.
Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford CT, 2013.
65. Kim, Y. H.; Cho, D. W.; Yoon, M.; Kim, D. Observation of HydrogenꢀBonding
Effects on Twisted Intramolecular Charge Transfer of pꢀ(N,NꢀDiethylamino)benzoic
Acid in Aqueous Cyclodextrin Solutions. J. Phys. Chem., 1996, 100, 15670–15676.
66. Kim, Y.; Lee, B. I.; Yoon, M. ExcitedꢀState Intramolecular Charge Transfer of pꢀN,Nꢀ
Dimethylaminobenzoic acid in Y Zeolites: HydrogenꢀBonding Effects. Chem. Phys.
Lett., 1998, 286, 466–472.
ACS Paragon Plus Environment