Structural, electronic and charge transfer studies of highly sensitive fluorescent probe 2-((E)-2-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)vinyl) phenol: Quantum chemical investigations

Article abstract of DOI:10.1007/s10895-013-1302-y

This article presents a facile synthesis of novel class of bluish-green fluorescent 2-((E)-2-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)vinyl)phenol [PPIVP] and their optical, electrochemical and thermal properties. Detailed photophysical and quantum chemical studies have been performed to elucidate the origin of the dual emission shifts. PPIVP undergo excited state intramolecular proton transfer (ESIPT) reaction leading a large Stoke's shifted fluorescence emission from the phototautomer. The results of quantum chemical investigations not only confirmed the intramolecular charge transfer characteristics of the ESIPT tautomers but also provided a rational for the observed high fluorescence quantum efficiency in the solid state. The high photoluminescence quantum yield in the solid state is ascribed to twisted chromophores due to phenyl substituents at 1,2-position of the phenanthroimidazole ring which restricted intramolecular motion, leading to an optically allowed lowest optical transition without self quenching.

Full text of DOI:10.1007/s10895-013-1302-y

J Fluoresc (2014) 24:377–387
DOI 10.1007/s10895-013-1302-y
ORIGINAL PAPER
Structural, Electronic and Charge Transfer
Studies of Highly Sensitive Fluorescent
Probe 2-((E)-2-(1-phenyl-1H-phenanthro[9,10-d]
imidazol-2-yl)vinyl)phenol: Quantum Chemical Investigations
V. Thanikachalam & J. Jayabharathi & A. Arunpandiyan &
P. Ramanathan
Received: 4 August 2013 /Accepted: 11 September 2013 /Published online: 6 October 2013
#
Springer Science+Business Media New York 2013
Abstract This article presents a facile synthesis of novel class
of bluish-green fluorescent 2-((E)-2-(1-phenyl-1H-
phenanthro[9,10-d]imidazol-2-yl)vinyl)phenol [PPIVP] and
their optical, electrochemical and thermal properties.
Detailed photophysical and quantum chemical studies have
been performed to elucidate the origin of the dual emission
shifts. PPIVP undergo excited state intramolecular proton
transfer (ESIPT) reaction leading a large Stoke’s shifted fluo-
rescence emission from the phototautomer. The results of
quantum chemical investigations not only confirmed the in-
tramolecular charge transfer characteristics of the ESIPT tau-
tomers but also provided a rational for the observed high
fluorescence quantum efficiency in the solid state. The high
photoluminescence quantum yield in the solid state is ascribed
to twisted chromophores due to phenyl substituents at 1,2-
position of the phenanthroimidazole ring which restricted
intramolecular motion, leading to an optically allowed lowest
optical transition without self quenching.
organic molecules exhibiting excited-state intramolecular pro-
ton transfer (ESIPT) have attracted considerable research in-
terest due to their optoelectronic applications such as UV
photostabilizers [4], photoswitches [5], fluorescent probes
[6, 7], and organic light-emitting diodes (OLEDs) [8, 9].
Hydroxyphenylbenzazole derivatives exhibits ESIPT
[10–18] and are used as fluorescent probes [19] (for the
labelling of proteins and DNA) and model base pairs [20]
(for studying the environments within the major and minor
grooves of DNA duplexes).
The ESIPT molecules exhibit low fluorescence quantum
efficiency in the solid state is due to concentration quenching.
Aiming at the high performance OLEDs as well as the solid-
state lasers, a high value of solid-state fluorescence quantum
efficiency is in demand to contribute to the improvement of
their technological applications [21, 22]. Conjugated organic
small molecules that showed high quantum yield in the solid
state without concentration quenching problem [23–29]. In
solution, the conjugated backbone of these molecules is sig-
nificantly twisted by steric hindrance [24] and the radiative
decay pathway of the resulting twisted chromophores [25] is
generally suppressed as they dissipate energy for molecular
rotation. In the solid state, the twisted conjugated backbone
enhances the emissive property of the molecules because the
molecular rotation is hindered by adjacent molecules upon
aggregation [24–29].
.
.
.
.
Keywords PEC NBO ESIPT HOMO-LUMO ICT
Introduction
Proton- and charge-transfer reactions are most fundamental
processes involved in chemical reactions as well as in living
systems [1–3]. Among the various studies of proton transfer,
In the present article, we report our precise investigation on
ESIPT, based on the spectral and photophysical properties of
newly synthesized 2-((E)-2-(1-phenyl-1H-phenanthro[9,10-
d]imidazol-2-yl)vinyl)phenol [PPIVP]. Steady state absorp-
tion and emission techniques are exploited for the purpose.
The spectroscopic observations have been correlated with the
theoretical results. Density functional theory (DFT) and time
:
:
:
V. Thanikachalam (*) J. Jayabharathi A. Arunpandiyan
P. Ramanathan
Department of Chemistry, Annamalai University,
Annamalainagar 608 002, Tamilnadu, India
e-mail: vtchalam2005@yahoo.com

378
J Fluoresc (2014) 24:377–387
dependent density functional theory (TDDFT) are used to
simulate the ground state and the excited state potential energy
curves (PECs) respectively. The simulated PECs, mulliken
charge and natural bond orbital (NBO) analyse reveal that
the intramolecular proton transfer reaction is feasible in the
excited S1 state but not in the ground state S0.
Computational Details
The quantum chemical calculations were performed using the
Gaussian-03 [30] 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
PPIVP were calculated by both DFT and TD-DFT methods
at the B3LYP/6-31G(d,p) level.
Experimental
Spectral Measurements
Hyperpolarizability
The proton spectrum at 400 MHz was obtained at room
temperature using a Bruker 400 MHz NMR spectrometer.
Proton decoupled 13C NMR spectrum was also recorded at
room temperature employing a Bruker 400 MHz NMR spec-
trometer operating at 100 MHz. The mass spectrum of the
sample was obtained using a Thermo Fischer LC-Mass spec-
trometer in fast atom bombardment (FAB) mode. The UV–vis
absorption and fluorescence spectra were recorded with
PerkinElmer Lambda 35 spectrophotometer and PerkinElmer
LS55 spectrofluorimeter, respectively.
The density functional theory has been used to calculate the
dipole moment (μ), mean polarizability (α) and the total first
static hyperpolarizability (β) [31] for PPIVP in terms of x, y, z
components and is given by following equations.
ꢀ
ꢁ
1=2
μ ¼ μ2x þ μ2y þ μ2z
ð1Þ
ð2Þ
À
Á
α ¼ 1=3 αxx þ αyy þ αzz
ꢀ
ꢁ
1=2
βtot
¼
β2x þ β2y þ β2z
ðorÞ
ð3Þ
h
i
1=2
À
Á
À
Á
À
Á
2
2
2
βtot
¼
βxxx þ βxyy þ βxzz þ βyyy þ βyzz þ βyxx þ βzzz þ βzxx þ βzyy
The β components of Gaussian output are reported in
atomic units and therefore the calculated values are converted
into e.s.u. units (1 a.u.=8.3693Χ10−33 e.s.u.).
Synthesis of 2-phenylimidazol2-((E)
-2-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)vinyl)
phenol (PPVIP) by InF3
A mixture of 2-hydroxycinnamaldehyde (1 mmol),
phenanthroquinone (1 mmol), aniline (1 mmol), ammonium
acetate (1 mmol) and indium trifluoride (InF3) (1 mol %) were
stirred at 80 °C. The progress of the reaction was monitored by
TLC (Scheme 1). After completion of the reaction, the mixture
was cooled, dissolved in acetone and filtered. The product was
purified by column chromatography using benzene: ethyl acetate
(9:1) as the eluent. M.p. 200 °C., Anal. calcd. for C29H20N2O: C,
84.44; H, 4.89; N, 6.79; O, 3.88. Found: C, 84.42; H, 4.87; N,
6.77; O, 3.86. 1H NMR (400 MHz, DMSO): δ, 13.99 (s, 1H),
6.69 (d, J =16Hz, 1H), 7.07 (d, J =7.6Hz, 1H), 7.361–7.333 (m,
6H), 7.537–7.479 (m, 3H), 7.682–7.870 (m, 8H), 8.21 (d, J =
7.6Hz, 1H), 8.87 (q, J =8.0Hz, 2H), 13C NMR (100 MHz,
DMSO):δ113.82, 120.17, 122.12, 123.65, 124.46, 125.21,
125.81, 126.46, 126.66, 126.84, 127.22, 127.42, 127.79,
128.29, 128.32, 128.66, 128.79, 128.92, 130.38, 130.53,
134.38, 135.69, 136.73, 136.93, 149.24. MS: m/z. 412[M+].
Natural Bond Orbital (NBO) Analysis
The second order Fock matrix was carried out to evaluate the
donor-acceptor interactions in PPIVP by NBO analysis [32].
For each donor (i) and acceptor (j), the stabilization energy
E(2) associated with the delocalization i→j is estimated as,
2
Fði; jÞ
Eð2Þ ¼ ΔEij ¼ qi
ð4Þ
εj − εi
Where qi is the donor orbital occupancy, εi and εj are
diagonal elements and F (i, j) is the off diagonal NBO
Fock matrix element. The larger the E(2) value, the more
intensive is the interaction between electron donors and elec-
tron acceptors.

J Fluoresc (2014) 24:377–387
379
Scheme 1 Possible mechanism
for catalytic synthesis of PPIVP
Results and Discussion
hydroxyl group 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 to its
first excited singlet state (Nsyn∗), undergoes an excited state
intramolecular proton transfer to yield the planer tautomer T
ESIPT Process
The fluorescence spectra of PPIVP in dioxane display a dual
emission, observed at 361.78 nm and another in bluish-green
region at 419.99 nm (Fig. 1). This dual emission of PPIVP is
explained as follows: the most stable form of the ESIPT
molecules in the ground state is in equilibrium between sev-
eral different conformers (Fig. 2) arising from tautomerism
∗ accompanied with large Stokes’ shifted fluorescence emis-
syn
sion (419.99 nm). The syn-conformers can rotamerize to form
their non-planar anti forms (Nanti and Tanti, respectively). In a
protic solvent, the Nanti conformer can form intermolecular
hydrogen bonds with solvent molecules (Fig. 2, I) but it cannot
undergo ESIPT. Therefore, dual emission is observed in non-
polar solvents. In addition to the fluorescence from the T
and rotamerism [10]. The normal planar syn form (Nsyn
features an intramolecular hydrogen bond between acidic
)

380
J Fluoresc (2014) 24:377–387
Fig. 1 Competition of intramolecular hydrogen bonding of the PPIVP
and PSPI
Fig. 3 Differential scanning calorimetry of PPIVP
configuration (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 PPIVP is
confirmed by the presence of the singlet at 13.99 ppm in the
1H NMR spectrum which is a typical signal for hydrogen
∗
∗
form, the N
form can also exhibit normal Stokes’
anti
syn
shifted weak fluorescence at 361.78 nm [18, 33]. Moreover,
the typical ESIPT process 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
Fig. 2 Structures of various
isomers of PPIVP in the ESIPT
and ICT processes

J Fluoresc (2014) 24:377–387
381
Fig. 4 HOMO – LUMO surface
diagram in Nsyn and Nanti form of
PPIVP
bonded hydrogen atom. The parent compound 1-phenyl-2-
styryl-1H-phenanthro[9,10-d]imidazole (PSPI) exhibits emis-
sion peak only at 382 nm. Absence of additional peak at
longer wavelength confirms absence of intramolecular hydro-
gen bond in parent compound (Fig. 1). It is further evident that
intramolecular hydrogen bonding is the driving force for
ESIPT and the dual fluorescence emission.
due the steric crowding. Therefore, it is expected to exhibit
good solubility without loss of their excellent thermal proper-
ties. The diaryl substituted phenanthrimidazoles PPIVP
exhibited high melting point (Tm) around 200 °C. In addition,
distinct glass transition temperature (Tg) could be observed at
78.3 °C (Fig. 3). The observed Tg and Tm values suggest that
the PPIVP has amorphous nature in condensed state, which
may greatly benefit the formation of homogeneous and amor-
phous films through thermal evaporation.
PPIVP exhibits the absorption peaks around 318, 262, 311
and 345 nm (10−4 M) in chloroform, acetonitrile, 1-propanol
and methanol at room temperature, which can be considered to
be absorptions belonging to π–π* transitions on the basis of
Quantum Chemical Calculations and Photophysical
Properties
Owing to their rigid and non-coplanar structures, PPIVP is
expected to decrease the intermolecular stacking efficiency

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Table 1 Selected optimized geometry parameters for the enol and keto-
forms of PPIVP
extinction coefficients and oscillator strengths (f) from molecu-
lar orbital (MOs) calculation. The molar absorption coefficients
range between 12,500 and 38,100 cm−1 M−1, is the typical
range for allowed π–π* transitions. The distinct vibrational
structured, spectra of 2-phenyl- 9,10-phenanthroimidazole
(PhI) [34] is due to the conjugation of the π system extended
by the electronic substituents on the 1,2 positions of the imid-
azole ring leading to the prominence of a π–π* nature of
transition to the excited states rather than n–π*. In the solid thin
films, the chromophores maintained similar absorption spectral
positions to those of the solution. The optical bandgap (Eg) of
PPIVP is lower than PhI (Eg=3.522 eV in CH3CN) [35]
because of the lower HOMO energy level as shown in Fig. 4.
This may be due to the presence of charge transporting substit-
uents on the phenanthroimidazole moiety.
Geometry parameters
enol (Nsyn
)
Keto (Tsyn)
Relative energy/kcal mol−1S0(S1)
Dipole moment (μ)D
Bond length (Å)
0.00(3.12)
4.41
28.96(0.00)
7.06
O51–H52
0.96
2.66
1.35
1.38
1.39
–
1.45
–
N17···H52
N17–C16
O51–C43
N17–C10
N17–H52
1.43
1.25
1.42
1.03
–
N17···H52
1.66
1.34
1.46
1.45
1.41
1.46
C39–C37
1.42
1.37
1.37
1.47
1.32
C16–C35 (interannular bond length)
C41–C30
The solid state fluorescence quantum efficiency of PPIVP is
0.63 whereas the same in solution is 0.51. PPIVP 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 moi-
ety in PPIVP. Possibly, the relative small intermolecular π–π
overlapping areas of these diaryl phenanthrimidazoles com-
pared with that of mono-aryl phenanthrimidazoles 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 interaction, which is bene-
ficial to enhanced fluorescent efficiency the substituents in the
ESIPT molecules that favour the ICT from the proton donor to
the proton acceptor moiety will increase the nonradiative decay
and, consequently, decrease the fluorescence quantum yield.
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) [36]. The geometry optimization in the
S0 and S1 of Nsyn, Tsyn and transition state (TS) of the
phenanthrimidazoles was performed and respective parame-
ters are listed in Table 1. As shown in Table 1, in the S0 and S1
for Nsyn form, the distance of N17···H52 in PPIVP was about
1.66Å. It is well known that H-bond is short distance force in
essence; hence the H-bond force between H and N should be
operating in the Nsyn form. On the other hand, the distance of
N17···H52 was too short (1.03Å) in the S0 and S1 for the Tsyn
form. Generally, the Nsyn→Tsyn tautomerization brought a
variation of the chemical bond length of phenolic ring. The
maximum of the difference between the bond lengths of
phenolic O51-H52 in the Nsyn form of PPIVP is 0.96 Ǻ, while
C43–C41
C40–C38
Bond angle (°)
<H52-O51-C43
108.15
123.66
126.08
108.17
–
–
<N17-C16-C31
125.29
126.57
124.43
123.54
118.81
<C37-C39-C41
<C41-C43-O51
<O51-C43-C41
<N17-C16-H52
–
Dihedral angle (°)
<C10-N17-C16-C37
<N17-H52-C16-C36
Excitation energy/nm
Oscillator strength (f)
83.9°
13.2°
320
88.2°
3.1°
410
0.38
0.16
the value in the Tsyn form increased to 1.45Å, 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 of PPIVP had twisted bond ϕ2
(2′-ph) by 13.2°. Dihedral angle was decreased to 3.1° during
Nsyn→Tsyn tautomerization, reflecting that the extent of mo-
lecular distortion was reduced Fig. 5. In a system with a great
delocalization of π-electron, the torsion angle decided wheth-
er 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 reduced. It was favourable to
undergo ESIPT for PPIVP.
The plots of the potential curves of internal proton transfer
in S0 and S1 states of PPIVP as a function of O51-H52 distance
were performed and presented in Fig. 6, 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 (energy difference
between Nsyn and Tsyn) again reveals that Tsyn form has a

J Fluoresc (2014) 24:377–387
383
Fig. 5 Molecular modelling of
PPIVP using Gaussian-03
higher energy than Nsyn form in S0 state as the aromatic ring
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
9.86 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 occurrence.
Finally, the protons transfer from Tsyn to the starting Nsyn in
thermal processes in S0 states to finish the cyclic four-level
is broken by proton transfer. The activation barrier for Nsyn
T
→
syn in S0 electronic state is 28.96 kcal mol−1, large enough to
make the ground state intramolecular proton transfer (GSIPT)
unviable under thermal conditions, whereas upon photoexci-
tation, much smaller interconversion barrier (Nsyn*→Tsyn*)
of 3.12 kcal mol−1 in S1 state preferably allows ESIPT to give
photophysical scheme (1Nsyn→1Nsyn*→1Tsyn*→1Tsyn
→
1Nsyn), immediately after photoexcitation of the intramolecular
H-bonded molecules. Therefore, an abnormally large Stokes
shift without self-absorption is detected, providing an ideal
scheme for UV-photostabilizers or proton transfer lasers.
The electron distribution of the frontier orbitals reflects the
electron transition characteristics Fig. 4. The HOMO and
Fig. 6 Ground (S0) and excited-state (S1) potential energy curves (PEC)
of PPIVP
Fig. 7 Mulliken atomic charge population of ESIPT site of PPIVP

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J Fluoresc (2014) 24:377–387
Fig. 8 MEP surface diagram of
PPIVP
LUMO of substituted phenanthrimidazoles exhibited π-type
symmetry. From HOMO to LUMO, the electron density
distribution displayed the transfer from phenolic ring to
phenanthroimidazole moiety. The calculated long wavelength
absorption peak of PPIVP (320 nm, oscillator strength
(f)=0.38) is due to a mixture of different transitions, e.g.,
HOMO→LUMO, 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
phenolic ring had smaller electron density in the excited state,
which was driving force for the occurrence of proton transfer
for PPIVP in the excited state. Hence, the HOMO→LUMO
transition could be ascribed to π–π* excitation with internal
charge transfer character [37].
The Mulliken atomic charge population of the key atoms
(O51-H52 and N17-H52) of HPPhI and PPIVP are shown in
Fig. 7. Remarkably, as per expectation, hydrogen atom had
high positive charge, which indicated that it was a real proton
transfer if ESIPTor GSIPT 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
hydroxyl to the imino group, 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
keto-form is remarkably higher (7.06D) than that of the enol-
form (4.41D). Such enlargement of dipole moment for the keto-
geometry provided us the impetus to extend the calculations
further so as to be able to delineate the influence of solvent
reaction field on the relative stabilities of various con-
formers, 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 mo-
ments in the excited state.
MEP surface diagram (Fig. 8) is further schematic DFT
evidence for the electron densities of the atoms. The negative
regions can be regarded as nucleophilic centres, whereas the
positive regions are potential electrophilic sites. The MEP
map of PPIVP clearly suggests that the nitrogen and oxygen
atoms represent the most negative potential region. The hy-
drogen atoms bear the maximum brunt of positive charge. The
predominance of green region in the MEP surfaces corre-
sponds to a potential halfway between the two extremes red
and dark blue colour.
Table 2 Second order perturbation energies of some prominent interac-
tions in the enol and keto-forms of PPIVP from NBO analysis
φi
φj
enol-form
keto-form
LP N17
LP N17
LP N17
LP N17
LP N17
LP O51
LP O51
LP O51
LP O51
σ*C7–C10
σ*C9–C10
σ*N15–C16
σ*C16–C37
σ*O51–H52
σ*C37–H38
σ*C41–C43
σ*C43–C46
σ*C42–C45
3.35
1.71
6.17
2.20
2.25
3.05
2.00
2.65
–
32.98
40.15
–
Table 3 Electric dipole moment (μ, D), polarizability (α, 10−30 esu) and
hyperpolarizability (βtot, 10-32 esu) of enol and keto forms of PPIVP
–
S.No.
Parameters
Enol
Keto
–
1
2
3
4
5
Dipole moment
4.41
7.06
2.24
25.76
4.62
25.50
Polarizability
3.27
34.47
5.33
2.13
Hyperpolarizability
r
44.83
269.49
59.60
μβ0
152.01
(φi donor NBO, φj acceptor NBO)

J Fluoresc (2014) 24:377–387
385
Restricted intramolecular/intermolecular motion [21–29]
has been suggested as one of the most significant mechanisms
of aggregation-induced emission enhancement phenomenon. In
dilute solutions of molecular dispersed phenanthroimidazole,
two end-substituted phenyl units of the molecule could rotate
freely around the single bonds and the radiative decay would be
effectively restricted by this kind of intramolecular torsion.
While in the aggregate state, the intramolecular rotation and
torsion were greatly impeded and therefore the non-radiative
decay channel was effectively restricted, which in turn popu-
lated the irradiative state of the excited molecules and
resulted in a great increase of fluorescence. Hence, it was
easy to understand why the molecularly dispersed dilute solu-
tions of phenanthrimidazoles were so weakly luminescent
while their solid particles and aggregates were highly emissive.
All these results in agreement with the proposed mechanism
for the intramolecular H-bonded compounds PPIVP in dilute
solution undergo an excited state charge transfer coupled pro-
ton transfer depicted in (Fig. 2). Upon excitation of Nsyn an
optimized structure (N17 H52 IMHB distance=2.66 Å) to
1.21 kcal/mol in a geometry in which the N17 H52 IMHB
distance has been elongated to 2.96 Å. This suggests that the
change is not as drastic as is normally observed for a strong
IMHB [39], suggesting the presence of weak hydrogen bond-
ing interaction PPIVP. This observation supports the presence
of relatively weak IMHB should put its signature on the way
to ascertain an ESIPT in PPIVP with a finite and well-defined
barrier.
Theoretical investigation plays an important role in under-
standing the structure–property relationship, which is able to
assist in designing novel NLO chromophores. The electrostat-
ic first hyperpolarizability (β) and dipole moment (μ) of the
imidazole chromophore have been calculated by using
Gaussian 03 package [30]. From Table 3, it is found that the
phenanthroimidazole chromophore show larger μgβo value,
which is attributed to the positive contribution of their conju-
gation. This chromophore exhibits larger non-linearity and its
λabs is red-shifted when compared with unsubstituted imidaz-
∗
ESIPT process takes place to give Tsyn fluorescent species,
ole. Therefore, it is clear that the hyperpolarizability is a strong
function of the absorption maximum. Since even a small
absorption at the operating wavelength of optic devices can
be detrimental, it is important to make NLO chromophores as
transparent as possible without compromising the molecule’s
non-linearity. The observed positive small ρ2D value (0.14)
reveals that the βiii component cannot be zero and these are
dipolar component. Since most of the practical applications
for second order NLO chromophores are based on their dipo-
lar components, this strategy is more appropriate for designing
highly efficient NLO chromophores.
after which the excited tautomer undergoes a large-amplitude
conformational change associated with a charge migration
from deprotonated phenol moiety to the protanated imidazole
moiety, yielding the non-fluorescent charge transfer interme-
diate TICT∗, which probably deactivates very fast [12]. The
electronic and geometric structure of TICT∗ in Fig. 2 is hypo-
thetical, 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 large amplitude twisting motion about the interannular
C-C bond (torsion angle, ϕ2) along the ESIPT reaction
coordinate.
Conclusions
The results of NBO treatment on PPIVP have been briefly
summarized in (Table 2), which reflects the collection of some
prominent interactions found in the enol and keto-forms of
PPIVP and the results for the IMHB interaction (N17 H52).
The delocalization interactions are found to be especially
sizeable for the π system and for the lone pairs of nitrogen
and oxygen atoms. Table 2 clearly dictates that in enol-form
the charge transfer interactions from the lone pairs of electron
donor (N17) are directed mainly to the antibonding orbitals on
the remote part of the molecule. In case of the keto-form also,
similar interactions are present to advocate for the IMHB
interaction leading to the formation of eight-member
quasicycle [38]. The hyperconjugative stabilization interac-
tion (ΔEij(2)) between N17 lone pair and the σ*(O51–H52)
orbital is sharply diminished with elongation of the N17 H52
IMHB, justifying the presence of IMHB in the eight- member
quasicycle. However, it is noteworthy that inspite of the
reasonable sensitivity of the hyperconjugative stabilization
interaction (ΔEij(2)) between N17 lone pair and the σ*(O51–
This article presents a facile synthesis of novel class of bluish-
green fluorescent 2-((E)-2-(1-phenyl-1H-phenanthro[9,10-
d]imidazol-2-yl)vinyl)phenol [PPIVP] and their optical, elec-
trochemical, and thermal properties. The branched imidaz-
ole are more emissive than their linear 2-phenyl-1H-
phenanthro[9,10-d]imidazole analogs, both in solution and when
aggregated, presumably because of increased π-electron contri-
butions to the electronic structure of the molecule, as well as
restricted nonradiative deactivation resulting from less facile
rotational deactivation via the interannular C-C bond (torsion
angle, ϕ2). The quantum chemical studies reveal that the forma-
tion of non-fluorescent isomers (TICT*) of PPIVP was effectively
suppressed in the solid state due to the bulky substituents (phenyl
and styryl) preventing a large-amplitude conformational change
in the excited-state. The ESIPT dye containing branched groups
showed excellent thermal properties with high Tg (78.3 °C) and
an efficient large Stokes’ shifted emission with very high fluo-
rescence quantum yield in the neat solid films without free from
concentration self-quenching. The molecular design concept
(2)
H52) orbital, ΔEij decreases from 2.25 kcal/mol in the

386
J Fluoresc (2014) 24:377–387
Arylsulfonamidophenyl)benzimidazole derivatives: the effect of do-
nor and acceptor substituents. J Org Chem 72:4784–4797
18. Wu Y, Peng X, Fan J, Gao S, Tian M, Zhao J, Sun S (2007)
Fluorescence sensing of anions based on inhibition of excited-state
intramolecular proton transfer. J Org Chem 72:62–70
established in this study should provide guidelines for fine-
tuning the emission properties of this class of ESIPT fluorophore,
which is beneficial for developing a new class of advanced
optoelectronic applications.
19. Holler MG, Campo LF, Brandelli A, Stefani V (2002) Synthesis and
spectroscopic characterisation of 2-(2′-hydroxyphenyl)benzazole
isothiocyanates as new fluorescent probes for proteins. J Photochem
Photobiol 149:217–225
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Differential solvation and tautomer stability of a model base pair
within the minor and major grooves of DNA. J Am Chem Soc 127:
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Acknowledgments One of the authors Prof. J. Jayabharathi is thankful
to DST [No. SR/S1/IC-73/2010], DRDO (NRB-213/MAT/10-11) and
CSIR (NO 3732/NS-EMRII) for providing funds to this research study.
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