Photoenolization via excited state proton transfer and ion sensing studies of hydroxy imidazole derivatives

Article abstract of DOI:10.1016/j.jphotochem.2016.12.003

A light stimulated photo-enolization in hydroxy imidazole systems (o-hydroxynaphthyl phenanthroimidazole, 3 and o-hydroxyphenyl phenanthroimidazole, 4) by excited state proton transfer induces “turn Off” fluorescence and ratiometric changes in the absorption spectrum. Benzyl phenanthroimidazole (5) does not exhibit the photo-enolization due to absence of o-hydroxyl group. Solvatochromism, time resolved photoluminescence measurement and ionic interactions studies of 3–5 are examined in semi-aqueous medium through UV–vis and fluorescence spectroscopy. Fluorescent probe 3 exhibits fluorescence “turn Off” and “turn On” responses with acetate ions in semi-aqueous medium by tuning the excitation wavelength.

Full text of DOI:10.1016/j.jphotochem.2016.12.003

Journal of Photochemistry and Photobiology A: Chemistry 335 (2017) 190–199
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Invited feature article
Photoenolization via excited state proton transfer and ion sensing
studies of hydroxy imidazole derivatives
Mohammad Shahida,b, , Arvind Misra
*
b,**
a
Department of Chemistry, Indian Institute of Technology Delhi, HauzKhas, New Delhi, India, India
Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi 221005, U.P., India, India
b
A R T I C L E I N F O
A B S T R A C T
Article history:
A light stimulated photo-enolization in hydroxy imidazole systems (o-hydroxynaphthyl phenanthroi-
midazole, 3 and o-hydroxyphenyl phenanthroimidazole, 4) by excited state proton transfer induces “turn
Off” fluorescence and ratiometric changes in the absorption spectrum. Benzyl phenanthroimidazole (5)
does not exhibit the photo-enolization due to absence of o-hydroxyl group. Solvatochromism, time
resolved photoluminescence measurement and ionic interactions studies of 3–5 are examined in semi-
Received 29 October 2016
Received in revised form 2 December 2016
Accepted 3 December 2016
Available online 6 December 2016
Keywords:
ESIPT
Solvatochromism
Photo-enolization
Ionic recognition
Acetate ion
aqueous medium through UV–vis and fluorescence spectroscopy. Fluorescent probe
3 exhibits
fluorescence “turn Off” and “turn On” responses with acetate ions in semi-aqueous medium by tuning
the excitation wavelength.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
level) from the excited enol form. After decaying to the ground
state, the excited keto form reverts to the original enol form via
The excited state intramolecular proton transfer (ESIPT) and
photoinduced proton transfer processes have attracted great
attention due to its wide applications in the design and
development of luminescent materials, photopatterning, chemo-
sensors, proton transfer laser, photostabilizers, molecular logic
gates, molecular probes, metal ion sensors, radiation hard-
scintillator counters, organic light emitting devices (OLEDs) and
optical devices etc [1,2]. The ESIPT chromophores ideally exhibits
remarkable Stokes Shift in comparison to the normal fluorophores
such as fluorescein, rhodamine or boron-dipyrromethene (BOD-
IPY) [3].
In the ESIPT molecule, large Stokes’ shift occurs due to the
typical four-level photophysical scheme consisting of the ground
(GS) and excited (ES) states of two different tautomers. In the GS,
the typical ESIPT molecules prefer to adopt enol form due to
thermodynamic stabilization by the intramolecular hydrogen-
bonding. However, upon photo-excitation excited keto tautomer
generate due to the fast proton transfer reaction (picoseconds
reverse proton transfer. Thus, in an ESIPT cycle the absorbing and
emitting molecular species leads to the total exclusion of self-
absorption and the large Stokes’ shifted keto emission [4].
Therefore, fluorescence due to ESIPT process is an effective tool
in fluorescence based applications and large Stokes’ shift is a
desired feature for fluorophores to minimize the self-absorption or
the inner filter effect [5]. Moreover, the ESIPT process is much
faster than the fluorescence process (radiative decay), and the
observed fluorescence by the ESIPT chromophores is primarily due
to the keto tautomer with some exceptions [6]. Thus, the ESIPT
process is accompanied by a remarkable geometry relaxation
between keto and enol tautomers. The ESIPT molecules have some
limitations such as, short excitation wavelength despite of their
long Stokes’ shift, sensitive to environment, and in protic medium
the emission of ESIPT sensitive chromophores get perturbed [5,7].
For instance, in protic solvents, the ESIPT emission due to keto form
(ca. 500 nm) usually diminished, and the emission due to the enol
form dominates at low wavelengths [5,7].
The fundamental photoinduced electron and proton transfer
processes are generally prevalent in keto-enol and azo–phenol
systems due to possibility of intramolecular hydrogen bonding
between hydrogen-bond donors (acidic proton) such as ꢀꢀOH,
ꢀꢀNH and hydrogen-bond acceptors (basic moiety) such as ꢀꢀN:
* Corresponding author at: Department of Chemistry, Indian Institute of
Technology Delhi, HauzKhas, New Delhi, India.
** Corresponding author. Tel.: +91 542 2307321x104/6702503;
fax:+91 542 2368127/2368175.
E-mail addresses: shahidchem@gmail.com (M. Shahid),
arvindmisra2003@yahoo.com, amisra@bhu.ac.in (A. Misra).
and ꢀꢀC
¼
O functions present in an aligned geometry [8]. Excitation
to first excited state leads to a change in charge density, increase in
http://dx.doi.org/10.1016/j.jphotochem.2016.12.003
1010-6030/© 2016 Elsevier B.V. All rights reserved.

M. Shahid, A. Misra / Journal of Photochemistry and Photobiology A: Chemistry 335 (2017) 190–199
191
acidity and basicity of two complementary centers due to which
ultrafast migration of proton through hydrogen bond
a
a
coordinate is possible to develop photo-tautomers each having
different and specific electronic distribution and optical properties
accordingly.
Furthermore, the most of anion recognition studies involve
hydrogen bond interaction between the anion and receptor
molecule. Also, the sensing ability of a specific receptor system
depends on the affinity of interaction with a guest species,
geometry of the molecule/complex, physico-organic nature of
analytes and microenvironment. Therefore, harnessing the ESIPT
process for development of new anion binding signaling systems
seems promising [9]. In view of the fact that anions like,
carboxylate, fluoride and phosphate play significant roles in
chemical, environmental and biochemical processes [10,11].
Particularly, carboxylates are critical components for numerous
metabolic processes [11c] and exhibit specific biochemical
behavior in enzymes and antibodies. Therefore, the design and
synthesis of effective small organic molecular scaffolds having a
suitable receptor site to recognize specific analyte sensitively
through good output optical signal are in great demand and
interesting in the area of supramolecular chemistry [10].
In this communication, to study the ESIPT mechanism we have
developed phenanthroimidazole based molecular probes 3 and 4
that are capable to show strong intramolecular H-bonding,
solvatochromism and photo-enolization processes. The potential
probe 3 is conjugated with naphthalene moiety for extending
Scheme 1. Synthesis of 3–5. (i) Na2Cr2O7/AcOH/D, (ii) AcONH4/aldehyde/EtOH/D.
on excitation at ꢁ365/362 nm respectively. While 5 exhibited
emission maxima at 421 nm with a lesser Stokes’ shift on excitation
at 360 nm.
2.2. Solvatochromism
p
–conjugation to gain large Stokes’ shift (useful to avoid self-
absorption), better electronic transition from keto tautomer with
enhanced energy gap between ground and excited state, and
quantum yield and sensitive to detect specific anion. The chemical
design of compounds 3 and 4 are based on hydroxyl imidazole
system to promote intramolecular H-bonding between ꢀꢀOH and
2.2.1. Steady-state UV–vis absorption and emission spectra
The solvent dependent analysis was monitored for 3, 4 and 5 in
different non-polar, polar aprotic and polar protic solvents such as
hexane, DCM, MeCN, DMSO, MeCN/H2O and MeOH which has been
summarized in Table 1. The UV–vis absorption spectra of 3 and 4
exhibits hypsochromic shift (blue shift) on increasing solvent
polarity i.e. hexane to MeOH (Fig. S13). Absorption maxima of 3
appeared at 373 and 374 nm in hexane and DCM, respectively. In
MeCN and DMSO absorption maxima obtained at 370 and 368 nm
while in MeCN/H2O and MeOH, it was found at 365 and 360 nm. For
4 and 5, similar trends were observed in absorption spectra and
absorption maxima appeared at 337-332 and 314–308 nm from
non-polar to polar protic solvents respectively (Fig. S14, 15).
Fluorescent behavior of molecular receptors 3-5 were evaluated
in solvents of different polarities (Fig. S16-20). In non-polar/less
polar solvents, the emissions maxima of 3 appeared at ꢁ524 nm (in
DCM) and at ꢁ495 nm (in hexane). In MeCN and DMSO, the
emission band appeared at 518 and 507 nm respectively whereas,
in polar protic solvent (MeOH), it appeared at 482 nm (Fig. S16).
Similarly, the emissions spectra of 4 showed dual emission bands
at ꢁ420 and ꢁ530 nm due to intramolecular charge transfer (ICT)
and ESIPT processes, respectively. The emission spectra in hexane
shows maxima at 515 nm whereas, in DCM, MeCN and DMSO it
appeared at 533, 523 and 533 nm, respectively (Fig. S17). Thus in
hexane, DCM, MeCN, DMSO and MeOH solutions, the observed
blue emission band is possibly for enol tautomer while, the longer
wavelength emission is attributed to a keto tautomer as a result of
ESIPT mechanism [1a,8b,c].The emission band in MeOH blue
shifted to appear at 514 nm and is similar to the data obtained from
already reported probes demonstrating ESIPT [13c,14]. The
observed photophysical changes may be rationalized to the
intermolecular hydrogen bonding with methanol molecule leading
to the stabilization of solvated isomer making the ESIPT reaction of
3 and 4 slower via a proton-relay (Scheme 2) [8b].
¼Nꢀꢀ functions through a stable six membered cyclic ring and
stability in host-guest interaction. The synthesized model com-
pound 5 due to the absence of ꢀꢀOH function is unable to exhibit
photo-enolization. The probes 3 and 4 are able to recognize acetate
ions selectively through a change in color, UV–vis, fluorescence and
1H NMR spectra. Probe 3 on interaction with AcOꢀ induces a
fluorescence “turn-Off” and “turn-On” behavior when excited at
365 and 411 nm respectively with a high association constant and
favorable response time.
2. Results and discussion
2.1. Synthesis
The synthesis (Scheme 1) of o-hydroxynaphthyl phenanthroi-
midazole (3), o-hydroxyphenyl phenanthroimidazole (4) and
benzyl phenanthroimidazole (5) have been carried by reacting
9,10-phenanthroquinone, 2 in ethanol with corresponding alde-
hyde (2-hydroxy-1-naphthaldehyde, salicylaldehyde and benzal-
dehyde respectively) in presence of ammonium acetate (AcONH4)
and iodine (as catalyst). Compound 2 was obtained in good yield by
the oxidation of phenanthrene, 1 with Na2Cr2O7 in acetic acid. All
the derivatives were characterized by different spectroscopic data
analysis (Fig. S1-12).
The UV–vis absorption spectrum of 3, 4 and 5 (1
aqueous-MeCN (20%, v/v) shows absorption bands at lmax 365, 362
and 359 nm (
= 23066, 30370, 13911 Mꢀ1 cmꢀ1) along with high
energy bands at 318, 331 and 311 nm, respectively. The low energy,
n ! * electronic transition band appeared at lmax ꢁ 365 nm is
ascribed to charge transfer (CT) while the high energy band
observed may be attributed to * electronic transitions [12].
mM) in
e
p
Moreover, the observed colorimetric changes for probe 3 and 4
under UV light is probably due to variable ratio between keto and
enol emissions in different solutions (Fig. S19). Probe 5 shows the
p
!
p
The emission spectra of 3 and 4 illustrated emission maxima at
lem ꢁ 464 nm with Stokes’ shift of 1.01 ꢂ105 and 1.05 ꢂ105 cmꢀ1

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M. Shahid, A. Misra / Journal of Photochemistry and Photobiology A: Chemistry 335 (2017) 190–199
Table 1
Spectroscopic parameters for probes 3-5.
s.n.
Solvents
3
4
5
lmax
(nm)
lem (nm) Dy (nm)
e
F
lmax lem
(nm)
Dy (nm)
e
F
lmax
(nm)
lem
(nm)
Dy (nm)
e
F
(nm)
1
2
3
4
5
6
Hexane
DCM
MeCN
DMSO
MeCN/H2O
MeOH
373
374
370
495
524
518
130
159
153
142
133
117
2.90
1.53
3.36 0.256
0.97
2.30
1.03
0.329
0.634
337
336
332
335
515/434
533/424
523/424 188
533/428 198
180
198
1.53
2.76
2.35
2.34
3.03
1.96
0.662
0.345
0.382
0.493
0.295
0.469
314, 403
314, 417
312, 406
315, 426
311, 421
308, 400
403
417
406
426
421
400
83
97
86
106
101
80
3.21
3.10
2.72 0.109
2.94 0.101
3.21
2.94 0.102
0.121
0.091
368/394 507
365
360
0.832
0.504 331
0.643 332
498
482
–
–
0.090
514/411
179
Note:
e F is the quantum yield with respect to Quninine sulphate and Dy is the Stokes Shift (nm).
is the molar extinction coefficient (in 105 cmꢀ1Mꢀ1);
(50%, v/v) displayed absorption band at 365 nm with a shoulder at
high energy band at 310 nm. Probe 3 displayed a dual emission
band at 410 and 498 nm with a large Stokes’ shift (133 nm) (Fig. 1).
On changing the pH of the medium from 7 to 1, the absorption
spectra showed a blue shift of 15 nm along with a marginal
decrease in molar absorption while under alkaline condition (pH
12), the absorption band centred at 365 nm diminished with a new
red shifted absorption maxima at 409 nm. Moreover, the emission
spectra of 3 in the acidic medium (pH 1) showed emission band at
445 nm with a hypsochromic shift of ꢁ48 nm (with a Stokes’ shift
of 80 nm). However, in basic medium (pH 12), emission band
appeared at 508 nm with a red shift of 10 nm (with a Stokes’ shift of
143 nm). Probe 3 in basic medium (pH 12) forms an anionic species
resulting in a red-shifted absorption and emission band due to
increase in conjugation or delocalization. In acidic medium, 3 was
protonated (hydroxyl or amino group) which reduces the
conjugation/delocalization resulting blue shift in the spectrum
(Scheme 3). Thus, in the acidic and basic pH conditions ESIPT
reaction was inhibited [13]. The pH dependent studies of probe 4
was reported by Plass et al., where deprotonation of ꢀꢀNH/ꢀꢀOH
unit possibly occurred in basic condition [14a]. In the alkaline
medium where deprotonation of the molecule took place, o-
hydroxyl imidazole gave lower pKa values (pKa ꢁ 11.00) than the
non-hydroxyl one, 5 (pKa ꢁ 13.50). Therefore, the observed anionic
states in alkaline media for the compounds bearing hydroxyl
groups (3 and 4) are due to the phenolic proton removal (Scheme 3)
while the anions of non-hydroxyl analogues (5) resulted from
imidazole ꢀꢀNH deprotonation [14a].
Scheme 2. Possible molecular structure of 3/4 and 3: MeOH/4: MeOH complex
which restrict the ESIPT process.
emission maxima between ꢁ403 and 426 nm for hexane to DMSO
and in protic solvent at 400 nm. It was unable to show the high
Stokes’ shift emission band (ESIPT band, >480 nm) (Fig. S20). The
change in solvent polarity shifted the spectra toward shorter
wavelength region probably due to H-bonding with the lone pair
electron of the probes and polar solvent, in the ground state. While,
in the excited state, hydrogen bonding involves only one electron
of the lone pair (the other have been promoted to an upper energy
state). Thus, due to relatively more stabilized ground state than the
excited state, absorption shifted to the short wavelength region.
2.3. Spectral behavior of 3 at different pHs
Probes 3 and 4 having intramolecular hydrogen bonds between
Since ESIPT molecules have amphoteric properties due to the
ꢀꢀOH and ꢀꢀC¼Nꢀꢀ group, so there could be existence of ESIPT
presence of both Lewis acidic ꢀꢀNHꢀꢀ/ꢀꢀOH and basic
¼Nꢀꢀ sites in
phenomenon in the systems. The most stable form of ESIPT
molecules in the ground state is in equilibrium between different
conformers arising from tautomerism and rotamerism as reported
by Nayak [14b]. The normal planar form of the probes features an
intramolecular hydrogen bond between protic acidic group i.e.
the imidazole ring. Probe 3 has both the proton donor and acceptor
group that can be protonated or deprotonated depending on pH of
the solution [1a].Therefore, we intended to examine the photo-
physical behaviour of 3 at different pHs in aqueous acetonitrile. The
UV–vis absorption spectrum of 3 (1 mM) in aqueous-acetonitrile
Fig. 1. Spectral behavior of 3 at different pHs in aqueous-acetonitrile (50%, v/v).

M. Shahid, A. Misra / Journal of Photochemistry and Photobiology A: Chemistry 335 (2017) 190–199
193
Scheme 3. Prototropic equilibria of 3.
Fig. 2. (a) Normalized absorption (red), emission (lex = 365; blue) and excitation (lem = 498 nm; black) spectra of 3 in aqueous acetonitrile. (b) Schematic representation
shows 4 step ESIPT mechanism. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
hydroxyl and the basic nitrogen atom of the imidazole ring. The
normal planer conformer (3 and 4) can undergo a rapid proton
transfer to form its tautomer (3a and 4a). Upon excitation of the
normal form to its first excited singlet state, undergoes an ultrafast
excited state intramolecular proton transfer to yield planer
tautomer (3a and 4a) accompanied with fluorescence spectra
showing large Stokes shift (Fig. 2b). The derivative 5 did not
possessed o-hydroxy group and therefore could not able to exhibit
the ESIPT phenomenon.
Fig. 2a shows the normalized absorption, excitation and
emission spectra of 3. The normalized electronic transition spectra
of 3 showed a low energy absorption band at 365 nm with a high
energy band at 310 nm due to the presence of phenolic and
quinonoid forms in the medium respectively. Similarly, the
at ꢁ495 nm excitation the observed emission band was almost
structurally identical to the absorption spectra of 3. It clearly
suggested that the emission has originated from the same ground
state species [13]. These observation clearly indicated that due to
the presence of hydroxyl imidazole moiety, the emission of 3
proceed via excited state intramolecular proton transfer (ESIPT) in
the
p
!
p* state during the period of low-frequency vibrational
motions associated with the hydrogen bond [13b]. Thus, it is worth
to mention that the enhanced emission observed is due to the
dominance of quinonoid form which remain in the equilibrium
with H-bond stabilized phenolic form, as well as due to
deprotonation and protonation of 3 in the alkaline and acidic
conditions, respectively.
normalized emission spectrum of 3 due to S0 ! S1
(
p
!
p
*)
2.3.1. Time-Resolved spectroscopic study
transition showed emission at 498 nm with high Stokes shift
(133 nm). However, when the excitation spectra were acquired
Fluorescence excited state lifetimes are very sensitive to the
structure and dynamics of a fluorophore. Fluorescence decay
profiles of 3 and 4 were obtained in aqueous-acetonitrile solution
(Fig. 3). Double exponentially decaying constants with short (t1
)
and long (t2) decay components were observed and summarized
in Table (S1, S2). The lifetime decay profile of probe 3 and 4, showed
two decay constants however, the dominant component was due
to the fast decay constant t1. In case of 3, the numerical values of t1
and t2 were 0.25 and 1.96 ns respectively. The relative contribu-
tions from these components were 6.36% and 93.64% for t1 and t2
,
respectively and the average lifetime (t0) was calculated as 1.85 ns.
While, in case of 4, the numerical values of t1 and t2 were 0.60 ns
and 2.44 ns respectively. The relative contributions from these
components were 4.78% and 95.22% for t1 and t2 respectively and
the average lifetime (t0) was calculated as 2.34 ns. Moderate
lifetime value as well as high quantum yield of 3 and 4 suggested
that the rate of non-radiative decay (knr) is not large in 3 and 4.
This can also be explained by the formation of intramolecular
hydrogen bond between phenolic ꢀOH and imidazole N, which
seems to produce certain amount of rigidity in the system and
hence suppresses the non-radiative decay pathways [15].
Fig. 3. Fluorescence decay profile of 3 and 4 (10
mM) in aqueous acetonitrile
solution, measurement conditions: lex = 377 nm and lem = 470 nm.

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M. Shahid, A. Misra / Journal of Photochemistry and Photobiology A: Chemistry 335 (2017) 190–199
Fig. 4. Change in absorption spectra of (a) 3 and (b) 4 (1 mM) upon photo-irradiation of l 365 nm in tetrahydrofuran.
2.4. Photo-enolization
appeared at 395 nm illustrated the conversion of one form of
molecule to another form. Similarly, when 4 was subjected to
photo-irradiation for approximately 500 s, the strong emission
band centered at 465 nm quenched almost completely without
formation of a new band (Fig. 5) while the 5 could not show any
change on photo-irradiation under similar experimental condition
(Fig. S21).
Hydroxy imidazole systems are known to illustrate the keto-
enol tautomerism. In order to understand the potentiality for such
kind of phenomena in the present studies, solutions of 3 and 4
were prepared (1 ꢂ10ꢀ6 M) in an aprotic solvent such as
tetrahydrofuran (THF) and were taken in a sealed 1 cm path
length quartz cuvette [8d]. Each vial has given UV exposure of
365 nm (UV lamp, GeNei) for a definite time intervals at room
temperature and the absorption and emission spectra were
acquired. Upon photo-irradiation at 365 nm (Fig. 4), the absorption
Moreover, the photochromic behavior of 3 and 4 were further
examined by 1H NMR spectra before and after the photo-
irradiation at 365 nm UV exposure. Before irradiation, the 1H
NMR spectra of 3 and 4 were sharp and well resolved whereas,
after irradiation 1H NMR spectra displayed additional resonance
signals indicating the presence of a mixture of isomers, possibly
the keto and its newly formed species with broadened resonance
signals (Fig. S22, 23, 23a). Further, HRMS spectra were also
acquired before and after photo-irradiation (Fig. S24-27). The
appearance of same molecular ion peak even after photo-
irradiation, suggested no degeneration of probe, and at same time
suggested about the conformational change in the medium. The
observed photophysical changes upon photo-irradiation and
generation of new band in the absorption, emission and 1H
NMR spectra of 3 and 4 are attributed due to (i) keto–enol
tautomerization by proton transfer from the ꢀꢀOH of phenol to
spectra of
coefficient corresponding to both high
3
was steadily modulated and molar extinction
*) electronic
(p
!
p
transition bands centered at 317 and 368 nm were decreased.
After 650 s of photo-irradiation, the low energy band was
completely disappeared and concomitantly
a shoulder was
appeared at 425 nm with an isosbestic point at 399 nm. The color
of solution was changed from yellow to a light yellow. Similarly, 4
upon photo-irradiation, the absorption bands centered at 335 and
365 nm were reduced ratio metrically and a new broad band at
420 nm generated concomitantly with an isosbestic point at
374 nm. The presence of isosbestic points is in agreement with the
structural transformation of the 3 and 4 between the Keto and Enol
forms [8d].
The photo-irradiation experiment was also examined by
emission spectroscopy. The probes 3 and 4 were subjected to
photo-irradiation for stipulated time intervals as mentioned above
at ꢁ317 nm in THF. The relative fluorescence intensity of 3 at
468 nm decreased gradually and disappeared completely with the
generation of a new band at 376 nm. The isoemissive point
ꢀꢀC¼Nꢀꢀ group of imidazole to generate enol and (ii) syn-anti
isomerization of the ꢀꢀOH function with respect to ꢀꢀC¼Nꢀꢀ[16].
2.5. Quantum chemical calculations
Experimental observations and their possible structure were
analyzed by quantum chemical calculations. The geometry
Fig. 5. Change in emission spectra of (a) 3 and (b) 4 (1 mM) upon photo-irradiation of l 365 nm in THF.

M. Shahid, A. Misra / Journal of Photochemistry and Photobiology A: Chemistry 335 (2017) 190–199
195
Fig. 6. DFT Optimized minimum energy structure of 3 and 4 as well as their HOMO-LUMO diagram.
optimization were performed for probe 3 and 4 using density
functional theory (DFT) method as implemented in Gaussian 03
suits of program [17]. A 6–31G(d,p) basis set was used for all the
atoms. DFT calculation was carried to calculate the energy the
tautomeric form of both probes 3 and 4 (Fig. 6). The short distance
a solution of 3-5, a prominent change in the absorption spectra of 3
and 4 were observed in which the transition band at ꢁ365 nm
disappeared and concomitantly, a new band appeared at 411 and
390 nm respectively (Fig. 7). The observed red-shift of 46 and
32 nm respectively is attributed to the increase in intramolecular
charge transfer from receptor site (ꢀꢀOH/ꢀꢀNH) to phenanthrene
ring. Also, the probe 3 showed considerable spectral change with
fluoride ion in anhydrous acetonitrile solution but could not in
H2O-ACN (Fig. S28). Addition of other tested anions to the solution
of 3 and 4, showed no considerable change in the absorption
spectra due to comparable high solvation energies of anions18a
thereby suggested about the specificity of 3 and 4 for selective
interaction with AcOꢀ in semi-aqueous medium. The model
compound 5 could not show any considerable change with tested
anions in aqueous-MeCN solution (Fig. S29a). Similarly, the
emission band observed at 464 nm for 3 (at lex 365 nm), illustrated
significant fluorescence quenching (68.4%) in presence of AcOꢀ (5
equiv). However 4 and 5 exhibited insignificant change in the
emission spectra (Fig. 8, S29b). The observed dual mode of
emission spectra in absence and presence of AcOꢀ clearly
suggested different mode of interaction of 3 in the medium that
can be accounted by assuming inhibition of ESIPT process due to
formation of anionic species or H-bonded complex with acetate
ions.
of 1.692 and 1.724 Å between imidazole (
¼Nꢀꢀ) and hydrogen
(ꢀꢀOH) suggested about the possibility of strong intramolecular
ꢀꢀOꢀꢀH—Nꢀꢀ hydrogen bonding, which favors the mobility of the
proton from ꢀꢀOH to Lewis base
¼Nꢀꢀ sites present in the
imidazole center. The bond distance for 3 and 4 between ꢀꢀO and
aromatic carbon is 1.338 and 1.334 Å, and between ꢀꢀO and ꢀꢀH is
0.998 and 0.994 Å respectively. The calculation results revealed
that both the highest occupied molecular orbital (HOMO) of 3 and
4 were mainly distributed over the whole conjugated backbone
whereas, the lowest unoccupied molecular orbital (LUMO)
primarily resided on the imidazole and hydroxyl part. In case of
4, the HOMO and LUMO mainly distributed over whole conjugated
system. The HOMO and LUMO energies gap were calculated for 3
and
4
and was found to be 85.341 and 99.912 kcal molꢀ1
respectively (Fig. 6).
2.6. Ion recognition properties of 3-5 with anions
The anion sensing ability of 3, 4 and 5 were examined through
UV–vis absorption spectroscopy after the addition of different
anions such as, Brꢀ, Clꢀ, Fꢀ, Iꢀ, SCNꢀ, H2PO4ꢀ, HSO4ꢀ, N3ꢀ and AcOꢀ
(as their tetrabutyl ammonium salts) to a solution of 3-5 in
aqueous-acetonitrile (20%, v/v). Upon addition of AcOꢀ (5 equiv) to
In order to understand the binding affinity of 3 with AcOꢀ,
absorption and emission titration experiments were performed.
From Fig. 9, it can be seen that as concentration of AcOꢀ (0–5 equiv)
is increased to a solution of 3, the absorption band at 364 nm
Fig. 7. Change in Absorption spectra of 3 and 4 (1 mM) upon addition of different anions (5 equiv) in aqueous-acetonitrile.

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M. Shahid, A. Misra / Journal of Photochemistry and Photobiology A: Chemistry 335 (2017) 190–199
Fig. 8. Change in emission spectra of 3 and 4 (1 mM) upon addition of different anions (5 equiv) in aqueous-acetonitrile.
Fig. 9. Absorption (I) and emission (II) titration spectra of 3 with AcOꢀ ions (0–5 equiv) in aqueous-acetonitrile.
reduced gradually with concomitant formation of a new band at
411 nm while in emission spectra, the band centered at 464 nm,
quenched gradually with enhancement of a shoulder at 550 nm.
The formation of isosbestic points at 378, 339 nm and isoemissive
point at 537 nm suggested the generation of a new species in the
solution. The observed red shifted low energy absorption band was
probably due to interaction of acetate anion with receptor site
(NꢀꢀH and OꢀꢀH) of 3 that increase the intramolecular charge
transfer (CT) and inhibit ESIPT processes. The naked eye visible
color of the solution changed from colorless to yellow. The Job's
plot analysis revealed a 1:1 stoichiometry between 3 and AcOꢀ
with association constant, calculated by non-linear fitting of
absorption and emission titration data, as Kassoc. = 1.03 ꢂ106 and
3.92 ꢂ106/M respectively (inset of Fig. 9). Additionally, the limit of
detection (LOD) of 3 for AcOꢀ was estimated in aqueous-MeCN
(Fig. S30) by our previously reported method [18]. The estimated
0.025 mM (25 nM) LOD was found comparable to other reported
methods and well in limit, as suggested by EPA.
Further, to explore the ionic recognition property of 3 with
acetate ions, fluorescence titration experiments were examined on
excitation at 380 (isosbestic point) and 411 nm (newly formed
band in absorption spectra). At 380 nm excitation, the emission
band observed at 464 nm exhibited fluorescence quenching while
intensity at ꢁ560 nm enhanced ratiometrically with the formation
of isoemissive point at 514 nm. While on excitation at 411 nm, 3
showed weak emission spectrum however on increasing concen-
tration of AcOꢀ, the fluorescence enhancement observed at 478 nm
due to increase in charge transfer (CT) (Fig. 10). Further, titration of
0.16
300
I
II
0.12
0.08
0.04
0.00
400
200
100
0
300
200
100
0
0.0
0.2
0.4
0.6
0.8
1.0
[ 3 / 3 + AcO- ]
455
520
585
650
390
455
520
585
650
Wavelength (n m)
Wavelength (n m)
Fig. 10. Emission titration spectra of 3 with AcOꢀ ions (0–5 equiv) in aqueous-acetonitrile at excitation (I) 380 and (II) 411 nm. Inset shows the Job's plot.

M. Shahid, A. Misra / Journal of Photochemistry and Photobiology A: Chemistry 335 (2017) 190–199
197
Fig. 11. 1H NMR titration spectra of 3 with acetate ions (0.0, 0.5, 1.0, 2.0 and 3.0 equiv) in DMSO-d6.
3 with acetate ions was examined through excitation spectra at
3. Conclusions
emission band of 465 nm. The excitation band was found at 365 nm
(similar to their UV spectrum) on addition of different concen-
trations of acetate ions (0–5 equiv), in which the band at 365 nm
decreased and a new band appeared at 410 nm (Fig. S31).
The potential probes 3 and 4 have been synthesized and studied
to understand photo-enolization process through the ESIPT
mechanism. Probes have shown light triggered enolization of a
hydroxy imidazole derivative with fluorescence “turn Off” response
and chromogenic ratiometric changes. The solvatochromic studies
revealed that ESIPT process is checked in protic solvents as well as
in acid-alkaline medium. Fluorescent probes 3 and 4 upon
interaction with different class of anions showed high affinity
for acetate anion in partial aqueous medium in which, intramo-
lecular H-bonding get interrupted and new H-bonding interaction
followed by deprotonation occurred with acetate ion, which
ultimately restricted ESIPT pathway. The model compound 5 could
not show photo-enolization process and supported our hypothesis.
2.7. Sensing mechanism of 3 with AcOꢀ anion
The probable binding mechanism of 3 with AcOꢀ was examined
by 1H NMR titration experiment (Fig. 11, S32). The 1H NMR
spectrum of 3 (2.3 ꢂ10ꢀ2 M) in DMSO-d6 showed resonance for
phenanthrene protons H10, H7 at
H8 protons
naphthalene ring protons resonance appeared at
doublet) while H6 and H2 protons merged to appear at
The resonance appeared at 7.49 (t) is due to H5 proton while a
triplet (overlapped) appeared at 7.39 ppm is attributed H4, H1
d 8.89, 8.53 (d) ppm and for H9,
d
7.75 (t) ppm which was primarily overlapped. The
8.0 ppm (H3,
7.93 ppm.
d
d
d
d
4. Experimental
protons respectively. On addition of AcOꢀ (0.5 and 1.0 equiv) led to
a downfield shift for H6 proton. On increasing the concentration of
AcOꢀ (2.0 equiv) the H9, H8 proton resonances separated and H8,
H1 protons shifted up field (Dd = 0.09, 0.08 ppm respectively).
However, the H6 proton further down field shifted to appear at
9.12 ppm (Dd = 1.19 ppm). Further, increase in acetate ions (3.0
equiv) led to an upfield shift corresponding to H1, H2, H3, H4
protons while H6 resonance shifted downfield. Thus, the respec-
tive upfield and downfield shifts in the resonances of probe clearly
supported about the increase in charge density of the probe
probably due to deprotonation of NH fragment in the presence of
AcO- ions which is also evidence from observed bathochromic shift
in the UV–vis absorption titration spectra with acetate ions
(Fig. 12).
4.1. Synthesis of 1,10-phenanthroquinone 2
To a heated solution of concentrated H2SO4 (10 ml), H2O (30 ml)
and phenathrene (2 gm) at 90–95 ꢃC, K2Cr2O7 (12 gm) was added
portion wise during 1 h and reaction mixture was heated for
30 min. After completion of the reaction (monitored through TLC),
cold water was added to the reaction mixture and filtered the
precipitate followed by washing with water. The precipitate
suspended in ethanol (60 ml) and saturated solution of sodium
metabisulfide (30 ml) with frequent stirring for 15 min. Cold water
(150 ml) was added to the mixture to dissolve the addition product
and filtered. In the filtrate part, Na2CO3 solution (20%, 50 ml) was
added to decompose adduct and allowed to precipitate. The
Fig. 12. Probable complexation mechanism of 3 with AcOꢀ ion.

198
M. Shahid, A. Misra / Journal of Photochemistry and Photobiology A: Chemistry 335 (2017) 190–199
precipitate was filtered followed by washing with water and air
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Acknowledgement
The author (MS) is thankful to SERB unit of Department of
Science and Technology, New Delhi for financial support vide
section no. SB/FT/CS-010/2013. The authors are also thankful to the
Council of Scientific and Industrial Research (CSIR) (02(0199/14/
EMR-II), and Department of Science and Technology (DST-FIST
program) New Delhi for financial support. Thanks to Professor
Sameer Sapra and Mohd. Samim Hasan, Department of Chemistry,
IIT Delhi for their kind support in securing time-resolved
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