Dynamics of solvent controlled ESIPT of π-expanded imidazole derivatives - PH Effect

Article abstract of DOI:10.1007/s10895-013-1336-1

A set of π-expanded imidazole derivatives employing excited state intramolecular proton transfer (ESIPT) was designed and synthesized. The relationship between the structure and photophysical properties were thoroughly elucidated by comparing with the analogue blocked with ESIPT functionality. The compound possessing an acidic NH function as part of an intramolecular hydrogen bond system has much higher fluorescence quantum yield and Stokes shift and the π-expansion strongly influences the optical properties. The occurrence of ESIPT for imidazole tosylamide derivatives were less affected by the hydrogen-bonding ability of the solvents compared to the unprotected amine. The low pKa values for the monocation ? neutral equilibrium indicate the presence of intramolecular hydrogen bonding between the amino proton and tertiary nitrogen atom.

Full text of DOI:10.1007/s10895-013-1336-1

J Fluoresc
DOI 10.1007/s10895-013-1336-1
ORIGINAL PAPER
Dynamics of Solvent Controlled ESIPT
of π-Expanded Imidazole Derivatives - pH Effect
J. Jayabharathi & V. Kalaiarasi & V. Thanikachalam &
K. Jayamoorthy
Received: 28 September 2013 /Accepted: 20 November 2013
#
Springer Science+Business Media New York 2014
Abstract A set of π-expanded imidazole derivatives
employing excited state intramolecular proton transfer
(ESIPT) was designed and synthesized. The relationship be-
tween the structure and photophysical properties were thor-
oughly elucidated by comparing with the analogue blocked
with ESIPT functionality. The compound possessing an acidic
NH function as part of an intramolecular hydrogen bond
system has much higher fluorescence quantum yield and
Stokes shift and the π-expansion strongly influences the opti-
cal properties. The occurrence of ESIPT for imidazole
tosylamide derivatives were less affected by the hydrogen-
bonding ability of the solvents compared to the unprotected
amine. The low pKa values for the monocation ⇌ neutral
equilibrium indicate the presence of intramolecular hydrogen
bonding between the amino proton and tertiary nitrogen atom.
that display ESIPT, such as benzoxazoles [21], flavones
[22], 10-hydroxybenzo[h]quinoline [23] and imidazo pyri-
dines [24], possess a large Stokes shift and hence have found
applications in laser dyes [25], fluorescence recording [26],
ultraviolet stabilisers [27], probes for solvation dynamics [28],
biological environments [29] and recently organic light emit-
ting devices [30]. The π – expanded multisubstituted imidaz-
oles which exhibit ESIPT process will significantly alter their
optical properties like homoaromatic systems [31–34].
Proton- and charge-transfer reactions are most fundamental
processes involved in chemical reactions as well as in living
systems [35, 36]. In continuation of our research [37–44]
herein we present the synthesis and optical properties of π-
expanded imidazole derivatives containing amino functional-
ity combining for the first time a study of ESIPT. Also the aim
of our present study are (i) to observe whether monoprotonic
phototautomer is formed or not, (ii) to find the site of proton-
ation (for the formation of monocations and dications) of the
molecules under study, (iii) to determine the pKa for various
prototropic reactions in S0 and S1 states and (iv) to perform a
quantum chemical calculation to explain the electronic ab-
sorption spectra of the molecules and to calculate the charge
densities at the basic centres.
.
.
.
.
Keywords ESIPT DFT GSIPT TICT Monocation
Introduction
Imidazole derivatives are of continuing to attract attention due
to their intriguing photophysical properties [1–14].
Photoinduced ESIPT is generally observed for organic com-
pounds featuring both a protic acid group (e.g., OH, NH2,
NHSO2R, etc.,) and a basic site (N- , C = O, etc.,) with a
suitable conformation, forms an intramolecular hydrogen
bond within the distance of 2.0 Å. Excited-state intramolecu-
lar proton transfer (ESIPT) phenomenon can be applied to the
design of fluorescent sensors [15–20]. Typical compounds
Experimental
Spectral Measurements
The infrared spectra were recorded with an Avatar 330-
Thermo Nicolet FT-IR spectrometer. The proton spectra at
400 MHz were obtained at room temperature using a Bruker
400 MHz NMR spectrometer. Proton decoupled 13C NMR
spectra were also recorded at room temperature employing a
Bruker 400 MHz NMR spectrometer operating at 100 MHz.
The mass spectra of the samples were obtained using a
:
:
:
J. Jayabharathi (*) V. Kalaiarasi V. Thanikachalam
K. Jayamoorthy
Department of Chemistry, Annamalai University,
Annamalainagar 608002, Tamilnadu, India
e-mail: jtchalam2005@yahoo.co.in

J Fluoresc
Thermo Fischer LC-Mass spectrometer in FAB mode. The
cyclic voltammetry analyses were performed with CHI
630A potentiostap-electrochemical analyzer at scan rate of
100 mV s−1 using 0.1 M tetra-(n-butyl)-ammonium
hexafluorophosphate as supporting electrolyte with Ag/Ag+
(0.01 M AgNO3) as the reference electrode and Pt electrode
as the working electrode under nitrogen atmosphere at room
temperature. The UV-vis absorption and fluorescence spec-
tra were recorded with PerkinElmer Lambda 35 spectropho-
tometer and PerkinElmer LS55 spectrofluorimeter,
respectively.
DMSO):δ 125.71, 127.28, 128.32, 128.34, 128.43, 129.51,
130.09, 132.97, 146.15. MS: m/z. 437[M+].
4-Methyl-N-(2-(1-Naphthalen-1-yl)
-4,5-Diphenyl-1H-Imidazole-2yl)Phenyl)
Benzenensulfonamide(3)
M.p. 262 °C, Anal. calcd. for C38H29N3O2S: C, 77.13; H,
4.94; N, 7.10; O,5.41; S, 5.42;. Found: C, 77.08; H, 4.89; N,
7.03; O,5.38; S, 5.40;. 1H NMR (400 MHz, DMSO): δ, 2.34
(s, 3 H), 6.10 (bs, 2 H), 7.93 (d, J= 3.2 Hz, 3 H), 7.56 (d, J=
6 Hz, 6 H), 7.43 (t, J= 8.8 Hz, 4 H), 7.27–7.37 (m, 10 H),
12.03 (s, 1 H). 13C NMR (100 MHz, DMSO):δ 21.31, 125.72,
127.18, 128.31, 128.39, 128.42, 128.51, 130.09, 132.96,
136.32, 145.21, 146.23. MS: m/z. 591[M+].
Computational Details
The quantum chemical calculations were performed using the
Gaussian 03 [45] package. Computations of the vertical exci-
tations, difference density plots and optimization of the
ground and excited states were performed using density func-
tional theory (DFT) and time-dependent DFT (TD-DFT)
using B3LYP/6-31G (d,p) basis set, respectively. The ground
and excited states HOMO and LUMO frontier orbital’s of
imidazole derivatives were calculated by both DFT and TD-
DFT methods at the B3LYP/ 6-31(d,p) level.
Result and Discussion
Photophysical studies of π- expanded 2- naphthylimidazoles
bearing NH group as a donor in a intramolecular hydrogen
bond have been carried out. The ESIPTof amino imidazoles 2
and 3 will be discussed relative to non- ESIPT compound 1.
The luminescence spectra of 1 is structured and points to a
mixing with a low energy charge transfer (CT) state. The
presence of such a low energy state is compatible with the
presence of the electron rich amino function in 2 and 3 which
might act as an electron donor toward the imidazole moiety.
The observed luminescence in 2 and 3 is due to electron rich
NH unit which destabilizes the CT state. However, the lumi-
nescence generated is from a state with a sizeable CT compo-
nent. Accordingly, the fluorescence band shifts to a lower
energy in more polar solvent which stabilizes CT. The fluo-
rescence spectra of 2 and 3 in dioxane display a dual emission
observed around 330 nm and another in bluish-green region
around 370 nm (Fig. 1). This dual emission is explained as
follows: the most stable form of the ESIPT molecules in the
ground state is in equilibrium between different conformers
(Fig. 2) arising from tautomerism and rotamerism [46]. The
normal planar syn form (Nsyn) features an intramolecular
hydrogen bond between amino function and the basic nitrogen
atom of the imidazole ring. The Nsyn conformer can undergo
proton transfer to form its tautomer (Tsyn). Upon excitation of
the normal form Nsyn to its first excited singlet state (N*syn),
undergoes an excited state intramolecular proton transfer to
yield the planer tautomer (T*syn) accompanied with large
Stokes’ shifted fluorescence emission. The syn-conformers
can rotamerize to form their non-planar anti forms (Nanti and
General Procedure for the Facile and Rapid Synthesis
of 2-Naphthyl Imidazoles by InF3
A mixture of Benzil (10 mmol), α-Napthylamine (10 mmol),
corresponding aldehyde (10 mmol), ammonium acetate
(10 mmol) and InF3 (1 mol %) in ethanol was refluxed at
80 °C. The reaction was monitored by thin layer chromatog-
raphy (TLC) and the product was purified by column chro-
matography using benzene: ethyl acetate (9:1) as the eluent
(Scheme 1)
1-(Naphthalene-1-yl)-2,4,5-Triphenyl-1H-Imidazole (1)
M.p. 251 °C, Anal. calcd. for C31H22N2: C, 88.12; H, 5.25; N,
6.63; . Found: C, 86.89; H, 4.02; N, 5.35;. 1H NMR
(400 MHz, CDCl3): δ, 7.92 (d, J= 6.8 Hz, 3 H), 7.56 (d, J=
6.4 Hz, 6 H), 7.43 (t, J= 7.6 Hz, 3 H), 7.26–7.37 (m, 10 H).
13C NMR (100 MHz, CDCl3 and DMSO):δ 125.68, 127.10,
128.11, 128.27, 128.33, 128.48, 130.01, 132.92, 146.13. MS:
m/z. 422[M+].
2-(1-(Naphthalene-1-yl)-4,5-Diphenyl-1H-Imidazole-2-yl)
Aniline (2)
M.p. 254 °C, Anal. calcd. for C31H23N3: C, 85.10; H, 5.30; N,
9.60; . Found: C, 85.09; H, 5.25; N, 9.56;. 1H NMR
(400 MHz, DMSO): δ, 7.92 (d, J= 5.2 Hz, 3 H), 7.56 (d,
J= 3.2 Hz, 6 H), 7.43 (t, J= 7.2 Hz, 3 H), 7.26–7.37 (m,
10 H), 11.62 (s, 1 H). 13C NMR (100 MHz, CDCl3 and
Tanti, respectively). In a protic solvent, the Nanti conformer can
form intermolecular hydrogen bonds with solvent molecules
but it cannot undergo ESIPT. Therefore, dual emission is
observed in non-polar solvents. In addition to the fluorescence
from the T*syn and N*anti form can also exhibit normal Stokes’

J Fluoresc
Scheme 1 Synthetic route of
2-naphthyl imidazoles
shifted weak fluorescence [47, 48]. Moreover, the typical
ESIPT produces the tautomeric form can be coupled with
the intramolecular charge transfer (ICT) state with a rotation
around interannular bond to produce a non-planar configura-
tion (TICT*) between the two rings. The TICT* state can be
deactivated back to its ground state via radiationless
relaxation.
The existence of intramolecular hydrogen bond in 2 and 3
is confirmed by the presence of the singlets at 11.62 and
1
12.03 ppm respectively in the H NMR spectra which is a
typical signal for hydrogen bonded hydrogen atom of the
(=N.....H-N≡). The parent compound 1-(naphthalene-1-yl)-
2,4,5-triphenyl-1H-imidazole exhibits emission peak only at
332 nm. Absence of additional peak at longer wavelength
Fig. 1 The fluorescence spectra of dioxane display a dual emission

J Fluoresc
Fig. 2 Potential energy profile of
Nsyn and Nanti form of 2 & 3
confirms absence of intramolecular hydrogen bond in parent
compound 1. It is further evident that intramolecular hydro-
gen bonding is the driving force for ESIPT and the dual
fluorescence emission.
Imidazoles 1–3 displayed a very brilliant solid film
photoluminescence in the bluish-green region of the visible
spectrum at room temperature. Besides electronic effect, there
is probably an interesting steric effect of the multiple phenyl
substituents on molecular arrangements in solid state, and is
most probably interacts in an offset face-to-face fashion with
another imidazolyl ring of a contiguous moiety. The relative
small intermolecular π–π overlapping areas of these
In non-polar solvents, Tsyn form can be detected it disappears
completely in alcoholic solvent, where the emission is only from
the amino/CT state. Proton coupled electron transfer (PCET)
reactions associated with electronically excited states is an inter-
esting class of reactions [49] and the present case also belong to
this class. Because of the introduction of the electron-
withdrawing tosyl group in 3 the NH group become less elec-
tron rich due to the inductive effect of the substituent. Therefore
a low energy CT excited state involving the imidazole unit is no
longer possible. The difference in peak wavelength between
absorption and emission is as large as 15000 cm−1 unambigu-
ously supports the occurrence of ESIPT, forming a proton-
transfer tautomer adiabatically in the excited state.
Quantum Chemical Calculations and Photophysical
Properties
Owing to their rigid and non-coplanar structures is expected to
decrease the intermolecular stacking efficiency decreases due the
steric crowding. Therefore, it is expected to exhibit good solubil-
ity without loss of their excellent thermal properties. The π-
expanded imidazole exhibited high melting point (Tm) around
302 °C. In addition, distinct glass transition temperature (Tg)
could be observed at around 152 °C (Fig. 3). The observed Tg
and Tm values suggest that the imidazoles are amorphous nature
in condensed state, which may greatly benefit the formation of
homogeneous and amorphous films through thermal evaporation.
The solid state fluorescence quantum efficiency of 1–3 is
around 0.48 whereas the same in solution is around 0.35.
Fig. 3 TG-DTA diagram of 1–3

J Fluoresc
imidazoles is probably a key factor that accounts for their high
fluorescence quantum yield in the solid state and in the thin film.
The corresponding intramolecular motions and relative free
rotation of end phenyl groups in solution may be the origin of
weak fluorescence. In the solid state, the free-rotation occurring
in solution will be depressed due to the intermolecular interac-
tion, which is beneficial to enhance the fluorescent efficiency.
To study further about the relationship between their mo-
lecular structures and optical properties, the geometrical pa-
rameters of Nsyn–Tsyn tautomers and their energy-minimized,
preferred conformations were calculated by DFT using
B3LYP/6-31G(d,p) [46]. The geometry optimization in the
S0 and S1 of Nsyn, Tsyn and their transition state (TS) of the
imidazoles were performed (Table 1). The distance of N26 · · ·
S0 and S1 of Tsyn form. Generally, the Nsyn → Tsyn
tautomerization brought a variation of the chemical bond
length of amino phenyl ring. The maximum difference be-
tween the bond lengths of N55-H56 in the Nsyn form is 0.99Ǻ,
while in the Tsyn form increased to 2.36Ǻ, implying that
aromaticity of this ring in the Tsyn form was lost because of
the internal proton transfer (ESIPT) process. Again the stable
ground state geometry of Nsyn had twisted bond <N17-H52-
C16-C36 by 18.1°. Dihedral angle was decreased to 2.8 °
during Nsyn → Tsyn tautomerization, reflecting that the extent
of molecular distortion was reduced (Fig. 4). In a system with
a great delocalization of π-electron, the torsion angle decided
whether the molecule could be well-conjugated.
Consequently, the small twisted configuration in the excited
state allowed the extension of the conjugated π-electron in the
entire molecular framework and the energy barrier in the
ESIPT process caused by the ground state twisting was re-
duced. It was favourable to undergo ESIPT in 2 and 3.
H56 for Nsyn form of 2 in S0 and S1 was about 1.78 Å. It is well
known that H-bond is short distance force and hence the H-
bond force between H and N should be operating in the Nsyn
form. The distance of N26 · · · H56 was too short (1.00 Å) in the
The plots of the potential curves of internal proton transfer
in S0 and S1 states as a function of N55-H56 distance of
compound 3 were performed and presented in (Fig. 5), which
shows clearly that the Nsyn form is most stable in the S0 state
whereas the Tsyn form is most stable in the S1 state. ΔE (ENsyn
- E Tsyn) reveals that Tsyn form has a higher energy than Nsyn
form in S0 state as the aromatic ring is broken by proton
transfer. The activation barrier for Nsyn → Tsyn in S0 electronic
state is 27.01 kcal mol−1, large enough to make the ground
state intramolecular proton transfer (GSIPT) unviable under
thermal conditions, whereas upon photoexcitation, much
smaller interconversion barrier (Nsyn* → Tsyn*) of
4.01 kcal mol−1 in S1 state preferably allows ESIPT to give
the large Stokes’ shifted fluorescence emission. After
decaying to the ground state, the phototautomer Tsyn* reverts
to the original Nsyn via reverse proton transfer barrier (ΔEr#)
of 8.25 kcal mol−1. This implied that although the occurrence
of GSIPT from Nsyn → Tsyn should be very difficult, the
reverse proton transfer process from Tsyn → Nsyn could take
place easily, which was much favourable for ESIPT occur-
rence. Finally, the proton transfer from Tsyn to Nsyn in thermal
processes in S0 states to finish the cyclic four-level
Table 1 Selected optimized geometry parameters for the Nsyn and
Tsyn of 2
Geometry parameters
Nsyn
Tsyn
Relative energy/kcal mol−1S0(S1)
Dipole moment (μ)D
Bond length (Å)
0.00(2.02)
3.56
27.01(0.00)
8.72
N26-H56
1.78
0.99
1.35
1.41
1.46
1.46
1.77
1.43
1.66
1.48
1.00
2.36
1.44
1.42
1.03
−
1.42
1.32
1.47
1.32
N55···H56
N26-C3
C28-C30
C30-N55
N25-C1
N55- S59
C3-C28 (interannular bond length)
S59-C60
C3-N27
Bond angle (°)
<C1-C26-C3
105.6
123.7
123.7
120.2
11.9
−
<N26-C3-C28
125.29
126.57
124.43
123.54
118.81
<N27-C3-C28
photophysical scheme (Nsyn → Nsyn* → Tsyn* → Tsyn
→
Nsyn) immediately after photoexcitation of the intramolecular
<C3-C28-O30
H-bonded molecules. Therefore, an large Stokes shift without
self-absorption is detected, providing an ideal scheme for UV-
photostabilizers or proton transfer lasers.
<C28-C30-N55
<C30-N55-S59
Dihedral angle (°)
<N10-N17-C16-C37
<N17-H52-C16-C36
<N26-C3-C28-C30
<C1-N26-C3-C28
<C3-C28-C30-N55
Excitation energy/nm
Oscillator strength (f)
112.6
The electron distribution of the frontier orbitals reflects the
electron transition characteristics (Fig. 6). The HOMO and
LUMO of substituted imidazoles exhibited π-type symmetry.
From HOMO to LUMO, the electron density distribution
displayed the transfer from amino ring to imidazole moiety.
The calculated long wavelength absorption peak around
310 nm, oscillator strength (f)=0.29 (Fig. 2) is due to a
mixture of different transitions, e.g., HOMO → LUMO,
83.9
18.1
0.17
165.7
0.18
310
88.2
2.8
0.35
123
2.5
402
0.16
0.29

J Fluoresc
Fig. 4 Molecular modeling of
compound 3
HOMO → LUMO + 1, HOMO → LUMO + 2, etc. The
LUMO electron density is delocalized through the acceptor
and π-bridges. This change in the electron density acts as a
driving force for the very fast intramolecular proton transfer in
these compounds upon excitation to the S1 state. Thus, the
aminophenyl ring had smaller electron density in the excited
state, which was the driving force for proton transfer in the
excited state. Hence, the HOMO → LUMO transition could
be ascribed to π–π* excitation with internal charge transfer
character [50].
The Mulliken atomic charge population of the key atoms
(N26…H56 and N55-H56) are shown in Fig. 7. On excitation
the hydrogen atom had high positive charge, which indicated
that it was a real proton transfer if ESIPT took place.
Interestingly, the positive charge of hydrogen experienced
increasing and then decreasing during intramolecular proton
transfer both in ground and excited states, reaching the
maximum at transition state because of largest hydrogen-
bond strength. While Nsyn was excited to Nsyn*, the part of
negative charge was transferred from the NH to tertiary
nitrogen, which resulted in the change of the force balance
of hydrogen bond. Consequently the negative charge of the
proton donor decreased (more acidic) and the negative
charge of the proton acceptor increased (more basic) which
enhanced the change of geometry in the excited state and
prompted the ESIPT occurrence for photoreactive hydrogen
bonded chromophore. The ground-state dipole moment of
the Tsyn form is remarkably higher (8.72 D) than that of the
Nsyn (3.56 D). Such enlargement of dipole moment for the
Tsyn-geometry provided us the impetus to extend the calcu-
lations further so as to be able to delineate the influence of
solvent reaction field on the relative stabilities of various
conformers, and optimized geometry parameters. The dipole
moments centralized on the Y-dimension and the electronic
transition led to the changes in the charge distribution, which
was accompanied by the increase of the molecular dipole
moments in the excited state.
Fig. 5 Potential energy curves (PECs) of 2 & 3

J Fluoresc
Fig. 6 Frontier molecular orbitals of 1–3
MEP surface diagram is further schematic DFT evidence
for the electron densities of the atoms (Fig. 8). The negative
regions can be regarded as nucleophilic centres, whereas the
positive regions are potential electrophilic sites. The MEP
map suggests that the nitrogen atom represent the most neg-
ative potential region and the hydrogen atoms bear the max-
imum brunt of positive charge. The predominance of green
region in the MEP surfaces corresponds to a potential halfway
between the two extremes red and dark blue colour.
Restricted intramolecular/intermolecular motion [51–59]
has been suggested as one of the most significant mechanisms
of aggregation-induced emission enhancement phenomenon.
In dilute solutions of molecular dispersed imidazole, two end-
substituted units of the molecule could rotate freely around the
single bonds and the radiative decay would be effectively
restricted by intramolecular torsion. While in aggregate state,
the intramolecular rotation and torsion were greatly impeded
and therefore the non-radiative decay channel was effectively
restricted and resulted in a great increase of fluorescence.
Hence, it was easy to understand why the molecularly dis-
persed dilute solutions of imidazoles were so weakly
luminescent while their solid particles and aggregates were
highly emissive. All these results in agreement with the pro-
posed mechanism for the intramolecular H-bonded com-
pounds 2 and 3 in dilute solution undergo an excited state
charge transfer coupled proton transfer. Upon excitation of
Nsyn, an ESIPT process takes place to give Tsyn* fluorescent
species, after which the excited tautomer undergoes a confor-
mational change associated with a charge migration from
deprotonated amino moiety to the protanated imidazole moi-
ety, yielding the non-fluorescent charge transfer intermediate
TICT*, which probably deactivates very fast [60]. The elec-
tronic and geometric structure of TICT* in Fig. 2 is hypothet-
ical, not only about the nature of the conformational change
experienced by Tsyn* but also in relation to the charge distri-
bution. Accordingly, the fluorescence enhancement in the
solid state is due to the prevented TICT by kinetic constraint
which blocks the twisting motion about the interannular C-C
bond along the ESIPT reaction coordinate.
The results of NBO treatment have been briefly summa-
rized in Table 2, which reflects prominent interactions found
in the Nsyn and Tsyn of 2 and 3 and the results for the IMHB
Fig. 7 Mulliken atomic charge
population of Nsyn, Nsyn* and Tsyn
forms of 3

J Fluoresc
Fig. 8 MEP surface diagram of
1–3
interaction (N26 H56). The delocalization interactions are
found to be especially sizeable for the π system and for the
lone pairs of primary and tertiary nitrogen atoms. Table 2
dictates that in Nsyn form the charge transfer interactions from
the lone pair of electron donor (N26) are directed mainly to the
antibonding orbitals on the remote part of the molecule. In
showing the overlap between the N26 lone pair orbital and the
σ*( N55–H56) orbital (Fig. 9b). The presence of a finite,
nonzero overlap between the orbitals obviously manifests
Tsyn-form also, similar interactions are present to advocate for
the IMHB interaction leading to the formation of six-member
quasicycle [61]. Figure 9a depicts that the hyperconjugative
stabilization interaction [(ΔE(2)ij )] between N26 lone pair
and the σ*(N55–H56) orbital is sharply diminished with elon-
gation of the N26.....H56 IMHB, justifying the presence of
IMHB in 3. This is a gain substantiated from the contour plot
Table 2 Second order
φi
φj
Nsyn
Tsyn
perturbation energies of
some prominent interac-
tions in the Nsyn and Tsyn
of 2 from NBO analysis
LP N26
LP N26
LP N26
LP N26
LP N26
LP N27
LP N27
LP N27
LP N27
LP N55
LP N55
LP N55
LP N55
σ*C1-C2
1.88
1.57
5.97
2.32
29.25
20.67
23.84
3.35
1.17
0.61
5.35
6.27
−
32.43
45.95
21.23
19.56
−
12.23
18.56
8.23
σ*C1-C4
σ*C3-N27
σ*C3-C28
σN55-H56
σ*C1-C2
σ*C3-N26
σ*C38-C39
σ*C41-H46
σ*C1-N26
σ*C28-C30
σ*C30-C33
σ*N26-H56
9.56
5.23
24.56
30.85
24.95
Fig. 9 a Hyperconjugative interaction energy ΔE2 [LPN26 → σ*(N55-
H56)] as a function of N26.......H56 IMHB distance for the E-form of 2
from NBO analysis. b Contour diagram of NBOs interactions between
LP N26 and σ*(N55-H56)
(φi donor NBO; φj ac-
ceptor NBO)

J Fluoresc
the presence of a finite, nonzero stabilization interaction
[(ΔE(2)ij )] due to the hyperconjugative charge transfer inter-
action from N26 lone pair to the σ*( N55–H56) orbital resulting
in the formation of the IMHB. Also a good extent of overlap
stands in justification of a strong IMHB in 3, in corroboration
to other findings.
fluorescence bands to the red, whereas the same at the nitro-
gen atom of amino group will shift the band to the blue with
respect to that of the neutral molecule. The large red shift for
the monocation I, can be explained by the resonance interac-
tion of the amino group with the aryl part of the molecule that
leads to structure I’ responsible for the stabilization of the
species. The formation of both types of monocations I and II
indicate the involvement of the lone pair of electrons of amino
group in resonance interaction, thus reducing the charge den-
sity on the nitrogen atom and enhancing the same on the
heterocyclic nitrogen atom [62].
Effect of [H+] on Absorption and Luminescence
The absorption spectra of 1-(naphthalene-1-yl)-2,4,5-
triphenyl-1H-imidazole recorded at pH 0.20 exhibits two
bands, the peak at 292 nm is red shifted and the other at
251 nm is blue shifted with respect to that of the neutral
molecule, 270 nm. The fluorescence spectra at different pH
values are shown in Fig. 10. The fluorescence spectra obtained
by exciting at the respective band maxima of the above
mentioned two absorption bands are also different. Similar
to the absorption spectrum, the peak at 453 nm is red shifted
and the other at 403 nm is blue shifted with respect to that of
the neutral molecule 420 nm. The fluorescence excitation
spectrum corresponding to the blue-shifted fluorescence band
matches exactly with the short wavelength absorption band
while that for the red-shifted fluorescence band resembles the
long wavelength absorption band. This suggests the formation
of two different kinds of monocation species I and II
(Scheme 2). The red-shifted absorption and emission bands
correspond to the monocation I, formed by the protonation of
the heterocyclic nitrogen of the imidazole ring and the blue-
shifted absorption and emission bands is associated with the
monocation II formed as a result of protonation at the amino
nitrogen of the phenyl ring. Since the lowest energy transition
of the molecule is π → π* in nature, the protonation at the
heterocyclic nitrogen atom will shift the absorption and
As the pH of the medium increases pH (1.62) the inten-
sity of the fluorescence band corresponding to species I
decrease whereas that of the species II increases.
Decreasing the concentration of hydrogen ions the possibil-
ity of formation of monocation I is reduced. Above pH 1.62,
the intensity of the monocation II starts decreasing and at the
expense of that a new band starts appearing in the wave-
length 412 nm lying between the above mentioned two
bands; (453 nm and 403 nm), which is almost same as
λfl
of TICT band in water. The appearance of TICT
max
emission band at pH 2.18 is shown in Fig. 10, intensity of
which becomes maximum at pH~3.95. All these observa-
tions supports the equilibrium [monocation II⇌ neutral mol-
ecule III] in the pH range of 1.62–3.95. The excited state
protonation constant Pka of this equilibrium, calculated by
fluorimetric titration method [63–65] is 2.55. The monoca-
tion II shows its maximum accumulation at a pH of 1.62.
With increasing pH the equilibrium concentration of the
neutral species increases in the equilibrium between mono-
cation II⇌ neutral species III. This results an increase in
fluorescence intensity of species III. When intensity of spe-
cies II at pH 3.91 is very low, the intensity of species III
becomes close to the intensity of species II at pH 1.62. The
pK* of this equilibrium is found to be 2.55, whereas the
a
ground state protonation constant (pKa) of the same equilib-
rium is 4.80. The higher value of pKa compared to pK*
a
can be explained by the fact that basicity of N atom of
amino group decreases due to a greater extent of charge
transfer from the same N atom on excitation that results in
the lowering of equilibrium concentration of species II in the
excited state [63, 66]. The appearance of a new band at pH
2.18 suggests that this band is due to the TICT emission,
which is the only predominant emission that can exist in
water. This also confirms that monocation II is formed as a
result of protonation of N atom of amino group, for which
TICT band is not expected due to the engagement of lone
pair of electrons with the H+ ion. Therefore no TICT band
appears at pH 0.20 for monocation I, although it involves
greater charge transfer from N atom of amino group to the
protanated imidazole ring. The protonation of N atom in this
ring facilitates the conjugation of the amino group with the
π-system, which in turn leads to the appearance of a double
Fig. 10 Different pH values for fluorescence spectra of 1–3

J Fluoresc
Scheme 2 Formation of two
different kinds of monocation
species
bond character of the phenyl carbon–nitrogen bond and thus
reducing the rotational relaxation of the amino group in the
excited singlet state.
Quantum Chemical Calculations Vs TICT
To rationalize the experimental findings and also to locate the
TICT state, DFT calculations [67–71] have been carried out to
determine the optimized geometries of the different confor-
mations of 2 and 3 to explain their properties in the ground
and excited states. When the donar-acceptor moieties orbitals
are decoupled due to twisting of amino (2) and tosylamide (3)
groups then full intramolecular charge transfer (ICT) takes
place in the excited state. In the ground state the amino group
shows a pyramidal shape because of the sp3 hybridization of
the nitrogen atom and changes to trigonal planar in the excited
state due to a change in hybridization of the nitrogen atom to
sp2 [72]. DFTcalculation suggests the requirement of twisting
of donor group perpendicular to the plane of the rest of the
molecule for the formation of stable TICT state in polar
medium. In this process the overlapping orbitals in the planar
geometry get decoupled and as a result of that complete charge
transfer takes place.
The TICT fluorescence intensity reaches its maximum
value at pH~3.95 after which the fluorescence intensity starts
decreasing and becomes constant at pH~7.0. Therefore, from
this observation it can be concluded that the H-bonding effect
of water becomes important only when the pH of the solution
is greater than 3.95. Increasing the pH of the solution above
3.95 drives more and more H3O+ ions to lose a proton to form
H2O. As a consequence the efficiency of H-bonding interac-
tions increases, thus decreasing the TICT fluorescence inten-
sity, which attains its minimum constant value at around pH
7.0. This observation indicates the sensing efficiency towards
H-bonding interaction [67]. pKa and pK*a values of monoca-
tion II⇌ neutral species equilibrium have been calculated and
compared. Moreover, the fluorescence quenching of neutral
molecule has been observed above pH 3.95 and the possible
reason behind the same has been explained above.

J Fluoresc
Table 3 Electric dipole moment (μ, D), polarizability (α, 10−30 esu) and
hyperpolarizability (βtot, 10−32 esu) of Nsyn and Tsyn forms of 3 from
NLO analysis
S.No.
Parameters
Nsyn
Tsyn
1
2
3
4
5
Dipole moment
3.56
8.72
Polarizability
4.82
7.56
Hyperpolarizability
46.8
5.96
r
43.82
168.93
230.82
43.68
μβ0
located on the donor and acceptor parts of the molecule. The
LE
S0 → S3 transition occurs only with a fraction of a full
electron transfer from nitrogen atom of amino function to the
remainder of the molecule, which results in a calculated dipole
moment at S3LE state. At the molecular twist of 90°, π orbitals
of amino group are completely decoupled from the remaining
π orbitals, so that the HOMO → LUMO excitation entails a
full electron transfer from the donor to the acceptor. This leads
to the formation of the S3TICT state with a very high dipole
moment of 17.02 D which on stabilization due to solvation
with a high polar solvent and becomes responsible for the
highly Stokes shifted fluorescence. In the gas phase, the
energies of TICT states are greater than the respective planar
S1 and S2 states, whereas the energy of S3TICT state is lower
Fig. 11 Dipole moments of different singlet states with variation of the
torsion angle
The formation of TICT state can be well examined through
energy evolutions of several excited singlet states as a function
of twist angle (φ) between the donor and acceptor parts of the
molecule. Such an examination allows ascertaining the dual
fluorescence and can be interpreted in terms of TICT state
[71–78]. The remarkable lowering the energy of S3 state has
been observed at twist angle, φ =90°, which is consistent with
increase in dipole moment (17.02 D) of the S3 state (Fig. 11).
The effect of polar solvent results in lowering the energy of
TICT
than the S3 planar state. The dipole moment of only the S3
state among the first four singlet states is greater than the
respective planar state. This dipole moment is large enough
relative to the dipole moments of the remaining states to locate
the TICT state by the dielectric continuum estimation in polar
solvents [81].
The electrostatic first hyperpolarizability (β) and dipole
moment (μ) of the imidazole chromophore have been calcu-
lated by using Gaussian 03 [45] and displayed in Table 3. The
studied imidazole chromophores show larger μgβo value,
which is attributed to the positive contribution of their conju-
gation. The reported chromophores exhibit larger non-
linearity and its λabs is shifted when compared with
TICT
LE
TICT
S3
state even lower than S1 as well as S1
states,
which makes S3TICT state responsible for anomalous fluores-
cence at a critical polarity. These calculations suggest the
presence of TICT state and highly Stokes shifted fluorescence
in polar solvents and also support the experimental observa-
tions [79, 80].
Further formation of TICT can be confirmed by frontier
molecular orbital (FMO) pictures (Fig. 6). In the planar ge-
ometry, the HOMO and LUMO have considerable delocali-
zation over whole π-system. Both orbitals are primarily
Fig. 12 The parallel & anti
parallel alignment of the
molecular dipole moments

J Fluoresc
6. Hayashi T, Maeda K (1962) Bull Chem Soc Jpn 35:2057
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unsubstituted imidazole. Therefore, it is clear that the
hyperpolarizability is a strong function of the absorption max-
imum. The observed positive small ρ2D value (0.18) reveals
that the βiii component cannot be zero and these are dipolar
component. Since most of the practical applications for sec-
ond order NLO chromophores are based on their dipolar
components, this strategy is more appropriate for designing
highly efficient NLO chromophores. The overall polarity of
the synthesized imidazole derivatives was small when their
dipole moment aligned in a parallel fashion [82] (Fig. 12).
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Conclusions
This article presents a facile synthesis of novel bluish-green
fluorescent π- expanded imidazole derivatives and their opti-
cal, electrochemical, and thermal properties. The π- expanded
branched imidazoles are more emissive, both in solution and
when aggregated, presumably because of increased π-electron
contributions to the electronic structure of the molecule, as
well as restricted non-radiative deactivation resulting from
less facile rotational deactivation via the interannular C-C
bond [<N17-H52-C16-C36]. The quantum chemical studies re-
veal that the formation of non-fluorescent isomers (TICT*) was
effectively suppressed in the solid state due to the bulky
substituents preventing a large-amplitude conformational
change in the excited-state. The low pKa, value for the mono-
cation ⇌ neutral equilibrium indicate the presence of intramo-
lecular hydrogen bonding between the amino-proton and ter-
tiary nitrogen atom. The ESIPT amino imidazoles containing
branched groups showed excellent thermal properties with
high Tg (82.3 °C) and an efficient large Stokes’ shifted emis-
sion with very high fluorescence quantum yield. The molec-
ular design concept established in this study provide guide-
lines for fine-tuning the emission properties of this class of
ESIPT fluorophore, which is beneficial for developing a new
class of advanced optoelectronic applications and biological
imaging.
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