Double proton transfer induced twisted intramolecular charge transfer emission in 2-(4a?2- N, N -Dimethylaminophenyl)imidazo[4,5-b]pyridine

Article abstract of DOI:10.1021/jp404472b

The spectral characteristics of N,N-dimethyl-4-(4-methyl-4H-imidazo[4,5-b] pyridin-2-yl)benzenamine (PyN-Me), 1-methyl-2-(4a?2-(N,N- dimethylaminophenyl)imidazo[4,5-b]pyridine (ImNH-Me), and 2-phenylimidazo[4,5-b] pyridine (PIP) are investigated to unders

Full text of DOI:10.1021/jp404472b

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Article
Double Proton Transfer Induced Twisted Intramolecular Charge Transfer
Emission in 2-(4'-N,N-Dimethylaminophenyl)imidazo[4,5-b]pyridine
Anasuya Mishra, Saugata Sahu, Nihar Dash, Santosh Kumar Behera, and Govindarajan Krishnamoorthy
J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp404472b • Publication Date (Web): 17 Jul 2013
Downloaded from http://pubs.acs.org on August 1, 2013
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Double Proton Transfer Induced Twisted Intramolecular Charge Transfer Emission
in 2ꢀ(4'ꢀN,NꢀDimethylaminophenyl)imidazo[4,5ꢀb]pyridine#
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Anasuya Mishra, Saugata Sahu, Nihar Dash, Santosh Kumar Behera and G.
Krishnamoorthy*
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Department of Chemistry
Indian Institute of Technology Guwahati
Guwahati 781039, Assam
India
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Ph: +91ꢀ3612582315
Fax +91ꢀ3612582349
Email: gkrishna@iitg.ernet.in
#Dedicated to Professor Robert S. H. Liu on his 75th birth anniversary
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Abstract
The spectral characteristics of N,Nꢀdimethylꢀ4ꢀ(4ꢀmethylꢀ4Hꢀimidazo[4,5ꢀ
b]pyridinꢀ2ꢀyl)benzenamine (PyNꢀMe), 1ꢀmethylꢀ2ꢀ(4’ꢀ(N,Nꢀ
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dimetylaminophenyl)imidazo[4,5ꢀb] pyridine (ImNHꢀMe) and 2ꢀphenylimidazo[4,5ꢀ
b]pyridine (PIP) are investigated to understand the mechanism of protic solvent induced
dual fluorescence of 2ꢀ(4’ꢀN,Nꢀdimethylaminophenyl)imidazo[4,5ꢀb]pyridine (DMAPIPꢀ
b). No dual emission is observed from PyNꢀMe where pyridyl nitrogen is blocked from
hydrogen bonding with protic solvents confirms the importance of hydrogen bonding of
protic solvents with the pyridyl nitrogen in dual emission of DMAPIPꢀb. Like DMAPIPꢀ
b, ImNHꢀMe also exhibits weak emission and has shorter fluorescence lifetime in
methanol. However single emission is observed from ImNHꢀMe in all solvents including
protic solvents. This suggests that the imidazole hydrogen also plays a role in the dual
emission process. The longer wavelength emission of DMAPIPꢀb in water increases with
increase in pH of the solution owing to deprotonation of imidazole >NH group. Based on
these results the mechanism for the dual emission of DMAPIPꢀb is proposed.
Keywords: dual fluorescence, ICT, hydrogen bond, proton coupled charge transfer
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1. Introduction
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Intramolecular charge transfer (ICT) has been a topic of growing interest both in
photochemistry and photobiology as it is a possible mechanism for several biological
processes and chemical energy conversion. In the donor acceptor system, upon excitation,
transfer of charge takes place from the donor to the acceptor moiety which results in the
formation of a highly dipolar state namely ICT state and emission takes place from both
locally excited state and ICT state. Among various types of ICT process, the twisted ICT
(TICT) process is a great area of interest.1,2 In TICT model, the twisted geometry is
favored where the donor moiety remains out of plane with respect to the acceptor moiety.
Grabowski et al. suggested that the dual fluorescence observed in
dimethylaminobenzonitrile in polar solvents are from the locally excited state and the
TICT state.1,2 The factors primarily responsible for the formation and stabilization of the
TICT state are polarity, viscosity and hydrogen bonding.2,3 The effects of polarity and
viscosity of the medium on the TICT process are well established.2 However the role of
hydrogen bonding in the formation and stabilization of the TICT state is still not
unambiguously established.3 Studies of hydrogen bonding effects on the TICT process is
a very active area of research.3ꢀ9 Hydrogen bonding plays a very important role in the
proton coupled charge transfer process which often observed in biological assemblies
including photo system II and other photoprocesses.10ꢀ14
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2ꢀ(4’ꢀN,Nꢀdimethylaminophenyl)imidazo[4,5ꢀb]pyridine (DMAPIPꢀb, Chart 1) is a
biologically active molecule and it acts as an inhibitor for AuroraꢀA, AuroraꢀB and
AuroraꢀC kinases.15 It also has interesting photophysical characteristics. Although, it
emits single emission in nonpolar and polar aprotic solvents, it emits dual emission in
polar protic medium.16 Therefore, DMAPIPꢀb was used as a ratiometric probe to study
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various microheterogeneous environments including protein.17ꢀ19 However, despite the
fact that the shorter wavelength emission and the longer wavelength emission are
attributed to normal and TICT emission respectively, the mechanism for the formation of
TICT state in protic environment is not clearly understood.
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Fasani et al. first studied the amino derivative of DMAPIPꢀb, i.e. 2ꢀ(4’ꢀ
aminophenyl)imidazo[4,5ꢀb]pyridine and suggested that the hydrogen bonding of the
protic solvent with imidazole nitrogen and its >NH group twist the imidazopyridine ring
perpendicular to the rest of the molecule to form the TICT state.20 This contradicts the
commonly accepted model that the TICT state is formed by twisting of donor with
respect to rest of the molecule and not by the twisting of the acceptor.1,2 Yoon et al. also
suggested that the hydrogen bonding of the solvent with acceptor makes it planar with
spacer that facilitates the charge flow from the spacer to the acceptor.4,5 In addition in
Fasani et al. mechanism20 the pyridyl nitrogen has no role in the formation of TICT state,
which contradicts our results that the band maxima and the intensity of the longer
wavelength emission are strongly perturbed by the position of the pyridyl nitrogen.16
Therefore, to understand the mechanism of dual emission of DMAPIPꢀb, we have
synthesized and investigated the photophysical characteristics of related molecules (Chart
1) N,Nꢀdimethylꢀ4ꢀ(4ꢀmethylꢀ4Hꢀimidazo[4,5ꢀb]pyridinꢀ2ꢀyl)benzenamine (PyNꢀMe),
N,Nꢀdimethylꢀ4ꢀ(3ꢀmethylꢀ3Hꢀimidazo[4,5ꢀb]pyridinꢀ2ꢀyl)benzenamine or 1ꢀmethylꢀ2ꢀ
(4’ꢀ(N,Nꢀdimetylaminophenyl)imidazo[4,5ꢀb]pyridine
(ImNHꢀMe)
and
2ꢀ
phenylimidazo[4,5ꢀb]pyridine (PIP).
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CH3
CH3
CH3
CH3
N
N
N
N
N
N
H
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N
N
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CH3
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DMAPIPꢀb
PyNꢀMe
CH3
CH3
N
N
N
N
N
H
N
N
CH3
ImNHꢀMe
PIP
Chart 1. Structures of DMAPIPꢀb, PyNꢀMe, ImNHꢀMe and PIP.
2. Materials and methods
The synthetic procedure of DMAPIPꢀb is reported elsewhere,16 and those of other
molecules are given below:
Synthesis of N,Nꢀdimethylꢀ4ꢀ(4ꢀmethylꢀ4Hꢀimidazo[4,5ꢀb]pyridinꢀ2ꢀyl)benzenamine
(PyNꢀMe). PyNꢀMe was synthesized by irradiating the mixture of DMAPIPꢀb and
methyl iodide by microwave (200 W) using the procedure given in the literature21 and
followed by a treatment with ammonia solution.22 The compound was purified by
preparative TLC. The identity of the compound was confirmed using NMR and mass
spectra. 1H NMR (400 MHz, CDCl3, ppm) δ 7.71 (1H, d), δ 7.53 (2H, d), δ 7.26 (3H, m),
δ 6.99 (1H, d), δ 4.19 (3H, s), δ 3.15 (6H, s). From LCMS the molecular mass (M + 1)
was found to be m/z: 253.06.
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Synthesis
of
1ꢀmethylꢀ2ꢀ(4’ꢀ(N,Nꢀdimetylaminophenyl)imidazo[4,5ꢀb]pyridine
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(ImNHꢀMe). DMAPIPꢀb and methyl iodide (4:9 ratio) was dissolved in a solvent
mixture of 3 ml dimethylformamide and 1 ml tetrahydrofuran. Powdered KOH was
added and was heated at 40 °C for 24 hour by the procedure reported earlier for alkyl
substitution of similar compounds.23 The compound was extracted with dichloromethane
and purified by column chromatography. The identity of the compound was confirmed by
NMR and mass spectra. 1H NMR (400 MHz, CDCl3, ppm) δ 8.35 (1H, d), δ 8.02 (2H, d),
δ 7.77 (1H, t), δ 7.22 (2H, d), δ 6.83 (2H, d), δ 4.00 (3H, s), δ 3.07 (6H, s). From LCMS
the molecular mass (M + 1) was found to be m/z: 253.08.
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Synthesis of 2ꢀphenylimidazo[4,5ꢀb]pyridine (PIP). PIP was synthesized by following
the synthetic procedure of DMAPIPꢀb.16 An equimolar mixture of 2,3ꢀdiaminopyridine
(Aldrich) and benzoic acid (Aldrich) in polyphosphoric acid was heated at 230 °C for 3 h.
The reaction mixture cooled to 100 °C and poured to excess ice cold water and
neutralized with base. The compound was extracted with dichloromethane and purified
by column chromatography. 1H NMR (400 MHz, CDCl3, and ppm) δ 8.41 (1H, d), δ 8.26
(2H, d), δ 8.15 (1H, d), 7.58 (3H, m), δ 7.30 (1H, m). From LCMS the molecular mass
(M +1) was found to be m/z: 196.10.
Spectral measurements. Except glycerol (AR grade) all other solvents were HPLC
grade from Rankem or Merck, India. The solvents were transparent in the spectral region
of interest and were used as received. A Varian Cary 100 UVꢀvisible spectrophotometer
and a Horiba Jobin Yvon Spex Fluoromax 4 spectrofluorimeter were used to collect
absorption and steadyꢀstate fluorescence spectra, respectively. For fluorescence
measurements the absorbance of the solutions is kept ~ 0.1 at the absorption band
maxima. The fluorescence quantum yields were measured with respect to quinine sulfate
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in 1 N sulfuric acid (Φf = 0.55).24 Time resolved fluorescence were measured using
Edinburgh life spec II instrument, where the fluorescence lifetimes were determined by
the method of time correlated single photon counting. A picosecond 375 nm laser diode
was used as light source. The data were analyzed by reconvolution method by using the
software provided by Edinburgh instrument.
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Computational calculations. All the molecules are optimized in the ground state using
gradient corrected exchange functional of Becke25 and the correlation functional of Lee,
Yang and Paar (LYP)26 (B3LYP) using density functional theory (DFT) method and 6ꢀ
31G(d,p) as the basis set. Vibrational frequency analyses were performed to confirm the
minimum energy nature of the stationary points. Nowadays time dependent density
functional theory (TDDFT) method becomes popular for electronic structure calculations
in the excited states due to its moderate efficiency and accuracy.27ꢀ30 The configuration
interaction singles (CIS) predicts the geometries as well as molecular properties quite
reasonably and correctly.31ꢀ33 TDDFT calculations over CIS optimized geometries are
efficient approach in predicting energy parameters for various systems.34ꢀ38 We have
performed TDDFT/B3LYP/6ꢀ31G(d,p) calculations on the optimized ground and excited
states geometries to obtain the transition energies. All computational calculations were
performed using Gaussian 03 software.39
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3. Results and discussion
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3.1. Absorption spectra of methyl derivatives
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fl
Table 1. Absorption band maxima (λmaxab, nm ), fluorescence band maxima (λmax
,
nm) of PyNꢀMe, and ImNHꢀMe and DMAPIPꢀb1
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ab
fl
Solvents
λmax
λmax


DMAPIPꢀb
PyNꢀMe
ImNHꢀMe DMAPIPꢀb
PyNꢀMe
ImNHꢀMe
336, 352
340
346
333, 349, 366
346, 362
347, 361
347, 362
343, 358
341, 355
343, 356
343, 355
343, 356
343, 357
343, 356
323,332
324,333
337
339
331
335
336
336
336
359, 379, 398
383
373, 393, 419
401
362, 381, 401
392
Cyclohexane
Dioxane
DMF
DMSO
428
402
414
403
421
Acetonitrile
Methanol
Ethanol
1ꢀPropanol
2ꢀPropanol
Butanol
345
350
350
349
346
348
360
407
395, 430
402
412
428
421
418
414, 506
413, 494
410, 486
408
397
398
395
415
336
343
407, 475
446
399
406
414
441
Glycerol
1From ref. 16
To understand the role of pyridyl nitrogen and imidazole >NH group in the dual
emission of DMAPIPꢀb, the methyl derivatives, PyNꢀMe and ImNHꢀMe (Chart 1) were
synthesized. The absorption spectral data of methylated derivatives in different solvents
are compared with DMAPIPꢀb in Table 1. The absorption spectra of all the three
molecules are structured in nonpolar solvent. But in polar solvent the vibrational structure
in the absorption spectra of ImNHꢀMe are blurred with a bathochromic shift as that of
DMAPIPꢀb. However, in contrast to other two molecules, the absorption spectra of PyNꢀ
Me is more structured also in polar solvents. The absorption spectra of PyNꢀMe is less
sensitive towards the polarity of the environment (7 nm shift). Whereas the
solvatochromic shift in absorption spectra of ImNHꢀMe is comparatively larger (20 nm
shift) than that of PyNꢀMe and comparable to that of DMAPIPꢀb (24 nm shift).
Structureless absorption band in polar solvent and larger solvatochromic shift suggest
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charge transfer in ImNHꢀMe. On the other hand, small solvatochromic shift and
structured spectra indicate less charge transfer in PyNꢀMe.
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Since, the multiparamateric approach of Kamlet et al. separates the different
solvent parameters, it gives information about specific contribution of each solvent
parameter.40 Therefore, the absorption and the fluorescence spectral data are analyzed by
multiple linear regression analysis to identify the different modes of solvation by
employing the following equation.
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E = E0 + sπ* + aα +bβ
Where E is the absorption/fluorescence energy in cmꢀ1, E0 is the value independent of
solvent, π* is the polarity/polarizability parameter of the solvents, α is the index of
hydrogen bond donating ability of the solvent and β is the index of hydrogen bond
accepting capacity of the solvent and s, a and b coefficients are the measure of
sensitivities to each individual contributing parameter. The positive sign indicates the
destabilization and negative sign indicates the stabilization of the system. The multiple
linear regression analysis using the absorption spectral data result in following equations
E (DMAPIPꢀb) = 29860 ꢀ 966 π* ꢀ 900 α + 84 β (r = 0.98)
E (ImNHꢀMe) = 31150 ꢀ 731 π* ꢀ 418 α ꢀ 802 β (r = 0.96)
E (PyNꢀMe) = 28721 + 474 π* + 540 α ꢀ 271 β (r = 0.96)
A good linear correlation is obtained for all the three molecules (regression ≥ 0.96). The
value of E0 obtained from this approach is also very good agreement with that of the
experimental value in nonpolar cyclohexane. As expected due to charge transfer character
the polarity/polarizability parameter stabilizes DMAPIPꢀb and ImNHꢀMe. In addition the
hydrogen bond donating capacity of the solvent also stabilizes both DMAPIPꢀb and
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ImNHꢀMe. On the other hand, in PyNꢀMe where the pyridine nitrogen is blocked and not
available for hydrogen bonding, the coefficient of α becomes positive. Therefore, it is
clear that the hydrogen bonding of the protic solvent with pyridine nitrogen stabilizes the
molecule more than that with the imidazole nitrogen. Large negative values of ‘s’ and ‘a’
for DMAPIPꢀb indicate that the dipolar interaction and hydrogen bond donating ability of
the solvent contribute more for the stabilization of DMAPIPꢀb than ImNHꢀMe.
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3.2. Fluorescence spectra of methyl derivatives
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0.9
0.6
0.3
0
360
420
480
540
Wavelength (nm)
Figure 1. Normalised emission spectra of PyNꢀMe in different solvents: (1) cyclohexane,
(2) 2ꢀpropanol, (3) 1ꢀpropanol, (4) butanol, (5) dioxane, (6) ethanol, (7) DMF, (8)
methanol, (9) DMSO and (10) glycerol.
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Wavelength (nm)
Figure 2. Normalised emission spectra of ImNꢀMe in different solvents: (1) cyclohexane,
(2) dioxane, (3) 2ꢀpropanol, (4) DMF, (5) ethylene glycol, (6) nꢀpropanol, (7) ethanol, (8)
DMSO, (9) methanol and (10) glycerol.
Like DMAPIPꢀb, in methyl derivatives also the fluorescence spectra are
structured in nonpolar solvents and upon increasing the polarity of the medium it
becomes structureless with a red shift (Figure 1 and 2). The spectral shift of ImNHꢀMe is
higher (60 nm) and is comparable to that of shorter wavelength emission of DMAPIPꢀb
(67 nm). From this, it can be inferred that like in DMAPIPꢀb the locally excited state of
ImNHꢀMe also has charge transfer character. On the other hand, the bathochromic shift is
small in the fluorescence spectra of PyNꢀMe (13 nm) which indicates that PyNꢀMe is less
sensitive towards the polarity of the medium.
The fluorescence spectral data are also analyzed by using SCM approach which
result in the following equations.
E (DMAPIPꢀb) = 26900 ꢀ 2628 π* ꢀ 836 α ꢀ 593 β (r = 0.93)
E (ImNHꢀMe) = 26453 ꢀ 2547 π* ꢀ 1922 α + 377 β (r = 0.98)
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E (PyNꢀMe) = 25210 ꢀ 318 π* + 561 α ꢀ 476 β (r = 0.67)
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The fluorescence spectral analysis also shows linear correlation in all the molecules and
the values of E0 also match well with the values in nonpolar solvent. As expected the
contribution from π* is also high for ImNHꢀMe and comparable to that of DMAPIPꢀb.
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6600
5200
3800
2400
1000
0
0.1
0.2
0.3
ꢀf
Figure 3. LippertꢀMataga plot for ImNHꢀMe (•) and PyNꢀMe (■).
Table 2. DFT optimized parameters of DMAPIPꢀb, ImNHꢀMe, PyNꢀMe and PIP
Excitation energya
(eV)
Emission
b
b
Molecules
Ground
state
ꢁg (D)
ϕ1
ϕ2
energya (eV)
Energy (eV)
ꢀ20707.0821
DMAPIPꢀb
ImNHꢀMe
PyNꢀMe
PIP
3.82 (3.69)
3.88 (3.84)
3.16 (3.56)
4.23 (4.04)
3.57 (3.27)
3.53 (3.25)
3.09 (3.16)
3.83 (3.65)
5.07
4.77
2.64
1.74
6.77 (3.23) 2.54 (0.11)
7.70 (4.32) 29.3 (0.53)
0.04 (0.04) 0.00 (0.01)
2.28 (0.02)
ꢀ21776.7682
ꢀ21776.5134
ꢀ17061.3840
avalues in parenthesis are experimental value in cyclohexane. bvalues in parenthesis are
those of excited state. ϕ1 ꢀ the torsional rotation of the dimethylamino group, ϕ2 ꢀ
torsional rotation of the dimethylanilino ring.
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The LippertꢀMataga plot41 was constructed for ImNHꢀMe, and PyNꢀMe (Figure 3)
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8
by using the relation
νSS = [2( e ꢀ
hereνSS is the Stokes shift, the superscript “o” indicates the absence of solvent, ꢁg and
e are dipole moments in the ground state and the excited state respectively, a is Onsager
°
ꢁ
ꢁ
g)/hca3] ꢀf + νSS
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ꢁ
cavity radius. The ꢀf , the orientation polarizability is defined as
ꢀf = [(ε ꢀ 1) / (2ε + 1)] – [(n2 – 1) / (2n2 + 1)]
here ε and n are dielectric constant and refractive index of the solvent, respectively. The
excited state dipole moments are estimated from the slope and the ground state dipole
moment calculated by DFT method (Table 2). The excited state dipole moments thus
obtained for ImNHꢀMe and PyNꢀMe are 11.24 D and 5.99 D, respectively. The excited
state dipole moment of ImNHꢀMe is comparable to that of DMAPIPꢀb (12.20 D, small
difference in excited state dipole moment with earlier value16 due the small difference in
the ground state dipole moment predicted by ab initio and DFT calculations).
Table 3. Fluorescence quantum yield (Φf) and life time (τf, ns) of methyl derivatives
Solvent
τf
Φf


PyNꢀMe
1.22
1.27
1.28
1.41
1.05
1.42
1.78
1.67
1.62
1.58
1.60
ImNHꢀMe
1.17
1.42
1.31
1.61
1.63
1.74
0.57
1.87
1.81
1.87
ꢀ
PyNꢀMe
ImNHꢀMe
0.82
Cyclohexane
Dioxane
Ethylacetate
DMF
0.39
0.34
0.45
0.28
0.37
0.47
0.45
0.47
0.43
0.48
0.89
0.29
0.93
0.73
0.88
0.22
0.81
0.92
DMSO
Acetonitrile
Methanol
Ethanol
1ꢀPropanol
2ꢀPropanol
Butanol
0.93
0.98
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30
38
46
54
ET(30) kcal/mol
Figure 4. Plot of quantum yield versus ET(30) parameters for ImNHꢀMe (•) and PyNꢀMe
(♦).
Figure 5. (a) The instrument response function and the fluorescence decays of (b) ImNHꢀ
Me and (c) PyNꢀMe in methanol along with fitted curves and residue plot.
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The fluorescence quantum yields and the lifetime of methylated molecules are
complied in Table 3 and the variation of quantum yield with ET(30) parameter is shown
in Figure 4. The fluorescence quantum yield of PyNꢀMe varies very little with the nature
of the environment. Whereas, the fluorescence quantum yields of ImNHꢀMe reduced in
protic solvents and lowest in methanol. The fluorescence lifetime also attenuated in
methanol (Figure 5 and Table 3). Same behavior is also observed for DMAPIPꢀb, whose
shorter wavelength emission decay time is very low in methanol (0.31 ns) compared to
other solvents (1.50 ns in acetonitrile).16 But the fluorescence lifetime of PyNꢀMe in
methanol is also comparable to those in other solvents. From this it can be inferred that
the hydrogen bonding of the solvent with pyridyl nitrogen leads to fluorescence
quenching.
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CH3
CH3
CH3
CH3
N
N
N
N
N
H
N
H
N
H
N
H
O
O
CH3
CH3
Scheme 1. Solvent assisted proton transfer.
Interestingly both methyl derivatives exhibit single emission not only in aprotic
solvents but also in protic solvents. However, DMAPIPꢀb emits dual emission in protic
solvents.16 In PyNꢀMe, the pyridyl nitrogen is prevented from hydrogen bonding. On the
other hand, in ImNHꢀMe, the imidazole >NH group is substituted by >NꢀMe and
therefore >NH group is not available for hydrogen bonding. The absence of dual
emission in PyNꢀMe and ImNHꢀMe show that both pyridyl nitrogen and >NH group are
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vital for the dual emission of DMAPIPꢀb in protic solvents. One such possibility is
transfer of proton from >NH group to pyridyl nitrogen to form tautomer (Scheme 1).
Such solvent assisted inter molecular proton transfer in the excited state occurs in several
molecules.42ꢀ44
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3.3. Spectral characteristics of 2ꢀphenylimidazo[4,5ꢀb]pyridine
Table 4. Absorption band maxima (λmaxab, nm ), fluorescence band maxima (λmax
,
fl
nm) of PIP
ab
fl
Solvent
Cyclohexane 307, 321
Dioxane 308, 321
Ethylacetate 306, 320
λmax
λmax
324, 340, 358, 374
328, 344, 360, 377
326, 341, 357, 378
324, 340, 357, 374
328, 343, 358, 377
328, 343, 359, 378
328, 343, 358, 378
328, 343, 359, 378
327, 341, 359,378
328, 343, 358, 377
332, 348, 362, 382
Ether
DMF
298, 307, 320
308, 321
Acetonitrile 307, 319
Methanol
1ꢀPropanol
2ꢀPropanol
Butanol
307, 320
306 (309) 320
307, 320
307 (309) 321
312, 322
Glycerol
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0.9
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315
355
395
435
475
Wavelength (nm)
Figure 6. Normalised emission spectra of PIP in different solvents: (1) cyclohexane, (2)
dioxane, (3) ether, (4) ethyl acetate, (5) DMF, (6) methanol, (7) 1ꢀpropanol, (8) 2ꢀ
propanol, (9) butanol and (10) glycerol.
To verify the proton transfer hypothesis the spectral characteristics of PIP (Chart
1), the molecule without charge donor dimethylamino group, are investigated (Table 4).
The absorption and the fluorescence band maxima are blue shifted compared to
DMAPIPꢀb and the methylated derivatives due to the absence of charge donating
dimethylamino group. The solvatochromic shift is also very small compared to other
molecules and the fluorescence spectra also posses well resolved vibrational structure
(Figure 6). Markedly no dual emission is observed in any solvent including protic
solvents such as methanol. Despite the fact both pyridyl nitrogen and >NH group are
freely available for hydrogen bonding, PIP exhibits single emission. Therefore, it can be
concluded that the electron donating group is absolutely necessary for the dual emission
in protic solvents and the longer wavelength emission originates from the charge transfer
state. Further, strong positive solvatochromic of the longer wavelength emission of
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DMAPIPꢀb and the higher dipole moment of it’s emitting state (section 3.5) also
substantiate the conclusion that the state is a charge transfer state.
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3.4. Effect of deprotonation
13.57
pH
0.18
9.20
9.50
10.17
10.91
11.20
11.64
12.14
12.57
12.94
13.17
13.41
13.57
9.2
0.12
0.06
0
280
330
380
430
Wavelength (nm)
Figure 7. Absorption spectra of DMAPIPꢀb for neutralꢀanion equilibrium.
13.57
8.93
0.9
0.6
0.3
0
13.57
8.93
360
460
560
660
Wavelength (nm)
Figure 8. Normalised emission spectra (λexc = 349 nm) of DMAPIPꢀb for neutralꢀ
monoanion equilibrium.
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To get further insight about the role of >NH proton in the dual emission, the
neutralꢀanion equilibrium of DMAPIPꢀb was investigated. At pH 9.0 the molecule is in
neutral form.16 Upon increasing the pH the absorption spectrum is blue shifted with
hyperchromic effect (Figure 7). DMAPIPꢀb has only one acidic proton (>NH). The
spectral changes are consistent with the deprotonation of >NH group to form an anion.
The blue shift is also observed in absorption spectra of similar molecules when the
imidazole >NH group is deprotonated.45,46 The effect of pH on the fluorescence spectra is
shown in Figure 8. The fluorescence spectrum of DMAPIPꢀb has emission band in water
at 451 nm with long tail. With increasing pH of the solution, the normal emission band is
blue shifted. But the most interesting change is the emergence of longer wavelength
emission upon deprotonation. At higher pH, two different lifetimes are observed for
DMAPIPꢀb when monitored at 425 nm and 545 nm (Table 5). Compared to shorter
wavelength when monitored at longer wavelength, the relative amplitude of the short
lifetime species decreases while that of the long lifetime species increases. This indicates
that the longer wavelength emitting state has longer lifetime and the shorter wavelength
emitting species has shorter lifetime. This is consistent with the lifetimes of the neutral
form of DMAPIPꢀb.16 The observation of clear dual emission in anion suggests that the
deprotonation may be one of the important step in the formation of longer wavelength
emitting state in DMAPIPꢀb.
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Table 5. Fluorescence lifetime (τ, ns) of the monoanionic form of DMAPIPꢀb and the
values in parenthesis are relative amplitude
χ2
Monitoring
wavelength (nm)
425
τ1
τ2
0.37 (30.44)
0.39 (20.14)
2.79 (69.56)
2.81 (79.86)
1.05
0.95
545
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3.5. Double Proton transfer induced TICT emission
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The studies suggest that both pyridyl nitrogen and imidazole >NH group play
important role in the dual emission of DMAPIPꢀb in protic solvents. The charge donor is
absolutely essential and the longer wavelength emission is from charge transfer state. The
dipole moment of the longer wavelength fluorescence emitting state was estimated by
both solvatochromic (24.6 D) and thermochromic (25.5 D) methods.47 The value is much
higher than that of shorter wavelength emitting state (12.2 D). Based on the theoretical
calculations, it was reported in the literature that the dipole moment of the TICT state is
much higher than the planar ICT state.48 Much higher dipole moment of the longer
wavelength emitting state of DMAPIPꢀb clearly suggests that the ICT state is a TICT
state. The enhanced dual emission in the anion reveals that the formation of TICT state in
protic solvent involves the deprotonation of imidazole >NH proton. Based on these
results we hypothesize that the formation of the TICT state is induced by solvent assisted
(double) proton transfer from imidazole >NH to pyridyl nitrogen. The dissociation of
proton from >NH group will produce a negative charge on the pyridyl nitrogen by
resonance (Scheme 2), abstraction of proton by pyridyl nitrogen results in proton transfer.
It is reported that the excited state intramolecular proton transfer is accompanied by a
torsional rotation of the tautomer leads to formation of twisted charge transfer state in
number of molecules.38,49ꢀ52 However, in contrast to DMAPIPꢀb, the formation of
tautomer in those systems is an intramolecular process and torsional motion of the
tautomer often directs to nonemissive charge transfer state.
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CH3
CH3
N
N
CH3
N
N
N
N
CH3
N
7
N
8
9
Scheme 2. Resonance stabilization of the anion form of DMAPIPꢀb.
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S1
3.6282
X
3.5363
3.3670
3.3529
S0
0.8355
0.6919
0.2445
0
Twisted
(ϕ1 = 900)
Twisted
(ϕ2 = 900)
Planar
(ϕ1 = 00)
Planar
CH3
CH3
CH3
CH3
N
CH3
N
N
N
N
N
ϕ2
N
H
N
ϕ1
N
H
CH3
N
H
N
H
N
H
H
O
O
O
CH3
CH3
CH3
Scheme 3. Energy level diagram of different tautomer of DMAPIPꢀb.
To verify the hypothesis the structures of methanolꢀDMAPIPꢀb complexes are
optimized and the energy level diagram was constructed (Scheme 3). The energy level
diagram also shows that the excited state energy of the planar tautomer is less than that of
normal form. The TICT state can be formed by rotation of either dimethylamino group or
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dimethylaminophenyl ring. The calculations indicate that the energy of the
dimethylaminophenyl ring twisted state is higher than that of the planar state and the
energy of the twisted state is even higher than that of the locally excited state. Therefore,
the formation of the TICT state by twisting of the dimethylaminophenyl ring is not a
favored process. On the other hand, the energy of the dimethylamino group twisted state
of the tautomer is lower than that of the planar state. This suggests that the TICT state is
formed by the twisting of the dimethylamino group rather than the dimethylaminophenyl
ring. The experimental emission energy of the longer wavelength fluorescence (2.46
eV)16 is in better agreement with the emission energy predicted from the twisted state
(2.66 eV) than the predicted energy from the planar state (3.12 eV).16 This supports the
proposed mechanism. Yoon et al. reported that the excited state proton transfer promotes
the formation of TICT state in 4ꢀN,Nꢀdimethylaminosalylic acid and 4ꢀaminosalylic acid.
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They suggested that 4ꢀaminosalylic acid which has weak electron donating group
also emits TICT emission due to formation of phototautomer. They also demonstrated
that the TICT emission of 4ꢀN,Nꢀdimethylaminosalylicacid is enhanced inside
cyclodextrin cavity due to enhancement in the intramolecular hydrogen bond which
favors the formation of phototautomer. Recently Guchhait et al. also proposed such
proton transfer assisted charge transfer in some photochromic Schiff bases.55 Nonetheless,
in DMAPIPꢀb the formation of tautomer is a double proton transfer process assisted by
protic solvent molecule not an intramolecular process.
4. Conclusion
Both PyNꢀMe and ImNHꢀMe exhibit single emission not only in aprotic solvents, but
also in protic solvents. The absence of dual emission in PyNꢀMe where pyridyl nitrogen
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is prevented from hydrogen bonding with protic solvents emphasizes that the hydrogen
bonding with pyridine nitrogen is crucial for the dual emission of DMAPIPꢀb in protic
solvents. On the other hand, the absence of dual emission in ImNHꢀMe suggests that the
hydrogen bonding with hydrogen of imidazole >NH group is also playing an important
role in the process. The absence of dual emission in PIP substantiates the conclusion that
the longer wavelength emitting state is a TICT state. The enhancement of longer
wavelength emission at higher pH illustrates that the formation of charge transfer state
involves the deprotonation of imidazole >NH hydrogen. The dual emission of DMAPIPꢀ
b in protic solvents is due to the formation of TICT state that is induced by double proton
transfer. The TICT state is formed by the twisting of the dimethylamino group. The
energy level diagram obtained by theoretical calculation also supports the hypothesis.
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Acknowledgement
The authors acknowledge department of science and technology (DST), India and
council of scientific and industrial research (CSIR), India for the financial support. The
authors also thank the Central Instruments Facility (CIF), Indian Institute of Technology
Guwahati for the Life Spec II and NMR instruments.
Supporting Information:
Full authors list for references 28 and 39. This material is available free of charge via
the Internet at http://pubs.acs.org.
References
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1. Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Structural Changes Accompanying
Intramolecular Electron Transfer: Focus on Twisted Intramolecular Chargeꢀ
Transfer States and Structures. Chem. Rev. 2003, 103, 3899–4031.
2. Rettig, W. Charge Separation in Excited States of Decoupled Systems—TICT
Compounds and Implications Regarding the Development of New Laser Dyes and
the Primary Process of Vision and Photosynthesis. Angew. Chem., Int. Ed. Engl.
1986, 25, 971ꢀ988.
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3. Chipem, F. A. S.; Mishra, A.; Krishnamoorthy, G. Role of Hydrogen Bonding in
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8775–8790.
4. Cho, D. W.; Kim, H. Y.; Kang, S. G.; Yoon, M.; Kim, D. Cyclodextrin Effects on
ExcitedꢀState Geometry Change and Intramolecular Charge Transfer of 4ꢀ
Biphenyicarboxylic Acid. J. Phys. Chem. 1994, 98, 558–562.
5. Kim, Y.; Lee B. I.; Yoon, M. ExcitedꢀState Intramolecular Charge Transfer of pꢀ
N,Nꢀdimethylaminobenzoic Acid in Y Zeolites: HydrogenꢀBonding Effects. Chem.
Phys. Lett. 1998, 286, 466–472.
6. Zakharov, M.; Krauss, O.; Nosenko, Y.; Brutschy B. B.; Dreuw, A. Specific
Microsolvation Triggers DissociationꢀMediated Anomalous RedꢀShifted
Fluorescence in the Gas Phase. J. Am. Chem. Soc. 2009, 131, 461–469.
7. Zhao, G.; Han, K. Timeꢀdependent Density Functional Theory Study on
Hydrogenꢀbonded Intramolecular Chargeꢀtransfer Excited State of 4ꢀ
Dimethylaminoꢀbenzonitrile in Methanol. J. Comput. Chem. 2008, 29, 2010–2017.
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8. CazeauꢀDubroca, C.; Lyazidi, S. A.; Cambou, P.; Peirigua, A.; Cazeau P.;
Pesquer, M. Twisted Internal Charge Transfer Molecules: Already Twisted in the
Ground State. J. Phys. Chem. 1989, 93, 2347–2358.
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