34
D. Ray et al. / Journal of Photochemistry and Photobiology A: Chemistry 274 (2014) 33–42
approach reveals a high energy barrier (∼2 eV) for lactim to lac-
tam conversion in the ground state [23,24]. Hence, the reaction is
intermolecular hydrogen bonding which might lead to reduction
of energy barrier compared to the monomer. A large volume of
theoretical and experimental works have been done in order to
investigate the role of various proton donor/acceptor solvents for
the tautomerization reaction in 2HP system [25–28]. The ground
was also explained by considering tunneling process as light hydro-
gen atom is involved in it. Borst et al. experimentally showed the
contribution of tunneling process in the first excited singlet state
of 2HP system [29]. Latter, Tautermann et al. have developed a reli-
able theoretical method for the determination of the ground state
tunneling splitting in 2HP and generalized the method to molecular
neling process has been successfully applied to various systems to
explain the intrinsic intramolecular proton transfer behavior [5,31].
Double proton transfer model has also been proposed by many
researchers to explain the tautomerization process of 2HP system
[32]. Although DFT//B3LYP method fails to explain the experimen-
tal results of 2HP-2PY system, but this method has been quite
appreciably used by several researchers to generate the poten-
tial energy surfaces (PES), barrier height etc. with commendable
efficiency.
over silica gel (petroleum ether/ethyl acetate, 70/30, v/v) to give
our desired compound 1-(2-hydroxy-5methyl-phenyl)-3,5-dioxo-
1H-imidazo-[3,4-b] isoindole. The compound was characterized
by both 1H and 13C NMR spectroscopy and was crystallized from
acetone. 1H NMR (300 MHz, DMSO-d6): ı 10.64 (bs, NH), 7.80
(d, J = 7.8 Hz, 1H), 7.65 (t, J = 7.6 Hz, 1H), 7.47 (d, J = 7.6 Hz, 1H),
7.41–7.32 (m, 3H), 7.05 (d, J = 8.2 Hz, 1H), 6.96 (t, J = 7.5 Hz, 1H);
13C NMR (75 MHz, DMSO-d6): ı 157.4, 152.3, 144.5, 131.3, 129.1,
128.3, 127.8, 126.6, 124.2, 122.4, 119.0, 117.6, 116.4, 113.8, 113.4,
111.8.
2.2. Materials
Spectroscopic grade solvents such as acetonitrile (ACN), dioxane
(DOX), methanol (MeOH), chloroform (CHCl3) and methyl cyclo-
hexane (MCH) were purchased from Spectrochem (India) and were
used after proper distillation whenever required. Triply distilled
water was used for the preparation of solutions. Trifluoroacetic
acid (TFA) and triethyl amine (TEA) from Spectrochem were used
as supplied. Sulfuric acid (H2SO4) and sodium hydroxide (NaOH)
were obtained from E-Merck and were used as received.
2.3. Instrumentation and procedure
2.3.1. Steady-state spectral measurements
In the present work we have carried out detailed photophysical
study of a synthesized heterocycles of isoindole fused imid-
azole bearing phenolic subunit, namely 1-(2-hydroxy-5methyl-
to understand its dual emissive behavior using simple spectro-
compound is widely used as templates to design a variety of bio-
molecules play an important role in various biochemical processes
too [34–36]. It is important to note that the present compound
ADII can be used successfully to stain human squamous epithe-
lium cells particularly the nuclei [37]. Structurally the molecule
having a four member intramolecular hydrogen bonding unit may
be responsible for intramolecular proton transfer behavior. The
spectral study of ADII is also interesting from the fundamental
aspect of lactam–lactim tautomerisation process. Along with spec-
troscopic technique, quantum chemical calculations have also been
performed in order to get a knowledge related to the stability of dif-
ferent conformers of ADII and viability of ground and excited state
proton transfer process. The ground state structures of different
conformers of ADII have been optimized using Density Functional
Theory (DFT) and Hartree–Fock (HF) levels of theory and the excited
state optimization has been performed only at Hartree–Fock (HF)
level of theory. The ground and excited state potential energy curve
(PEC) along the PT coordinate have been constructed to follow the
possibility of the IPT process.
The absorption and emission measurements were performed on
a Hitachi UV-Vis U-3501 spectrophotometer and Perkin-Elmer LS-
55 fluorimeter, respectively. All the collected spectra were with
appropriate background correction. Only freshly prepared solu-
tions were used for spectroscopic study and all experiments were
carried out at room temperature (300 K).
Fluorescence quantum yield (˚f) was determined using the fol-
lowing equation where -naphthol (˚R = 0.23) in MCH is used as
the secondary standard.
AS · ODR · n2S
˚
S
= ˚R
·
(1)
AR · ODS · n2R
Where, ˚S and ˚R are the quantum yields, AS and AR are the inte-
grated fluorescence areas, ODS and ODR are the absorbance values
and nS and nR are the refractive indices of sample and reference
molecule, respectively.
2.3.2. Time-resolved fluorescence decay
Fluorescence lifetimes were obtained by the method of Time
Correlated Single-Photon counting (TCSPC) on FluoroCube-01-NL
spectrometer (Horiba Jobin Yovon) using light source of nano LED
at 336 nm, 291 nm and laser source at 450 nm. The signals were
о
collected at the magic angle of 54.7 to eliminate any considerable
contribution from fluorescence anisotropy decay. The decays were
deconvoluted using DAS-6 decay analysis software and the accept-
ability of the fits was judged by ꢀ2 criteria and visual inspection of
the residuals of the fitted function to the data. The time-resolved
fluorescence decay (I(t)) is described by the following expression:
ꢀ
2. Experimental
2.1. Synthesis of ADII
I(t) =
˛iꢁi
(2)
i
A 1:3 mixture of ninhydrin (1.4 mmol) and p-cresol (4.2 mmol)
was refluxed in AcOH until the adduct 2-hydroxy-2-(2¢-hydroxy-
aryl)-1,3-indanediones (where aryl = 2-hydroxy-5-methyl-phenyl)
was completely formed. The complete formation of the desired
adduct was checked by TLC. Then urea (16.6 mmol) was added to
the above reaction mixture and the mixture was refluxed for fur-
ther 2.5 h. The reaction mixture turned into red color. Then the
cold reaction mixture was poured into ice-cold water. A yellow
solid product was filtered and purified by column chromatography
and the mean (average) fluorescence lifetimes are calculated using
the following equation [38]:
ꢁ
i˛iꢁi2
ꢁ
ꢀꢁi0ꢁ =
(3)
i˛iꢁi
in which ˛i is the pre-exponential factor corresponding to the ith
decay time constant, ꢁi.