Polymorph-dependent solid-state fluorescence and selective metal-ion-sensor properties of 2-(2-hydroxyphenyl)-4(3H)-quinazolinone

Article abstract of DOI:10.1002/asia.201100832

2-(2-Hydroxy-phenyl)-4(3H)-quinazolinone (HPQ), an organic fluorescent material that exhibits fluorescence by the excited-state intramolecular proton-transfer (ESIPT) mechanism, forms two different polymorphs in tetrahydrofuran. The conformational twist b

Full text of DOI:10.1002/asia.201100832

FULL PAPERS
DOI: 10.1002/asia.201100832
Polymorph-Dependent Solid-State Fluorescence and Selective Metal-Ion-
Sensor Properties of 2-(2-Hydroxyphenyl)-4(3H)-quinazolinone
Savarimuthu Philip Anthony*[a]
Abstract: 2-(2-Hydroxy-phenyl)-4(3H)-
quinazolinone (HPQ), an organic fluo-
rescent material that exhibits fluores-
cence by the excited-state intramolecu-
lar proton-transfer (ESIPT) mecha-
nism, forms two different polymorphs
in tetrahydrofuran. The conformational
twist between the phenyl and quinazo-
linone rings of HPQ leads to different
molecular packing in the solid state,
giving structures that show solid-state
fluorescence at 497 and 511 nm. HPQ
also shows intense fluorescence in di-
methyl formamide (DMF) solution and
selectively detects Zn2+ and Cd2+ ions
at micromolar concentrations in DMF.
Importantly, HPQ not only detects
Zn2+ and Cd2+ ions selectively, but it
also distinguishes between the metal
ions with a fluorescence lmax that is
blue-shifted from 497 to 420 and
426 nm for Zn2+ and Cd2+ ions, respec-
tively. Hence, tunable solid-state fluo-
rescence and selective metal-ion-sensor
properties were demonstrated in
single organic material.
a
Keywords: fluorescence
solid-state fluorescence
morphism · sensors
·
organic
poly-
·
Introduction
Similarly, finding fluorescent organic molecular probes
that selectively bind metal ions, particularly Zn2+ and Cd2+
ions, has also currently been of interest due to the biological
role of these ions.[9] Such molecular sensors are important,
because although both metal ions often induce similar spec-
tral changes while coordinated to fluorescent sensors, they
play totally different roles in biochemical processes. Zn2+ is
the second-most-abundant transition-metal ion in the
human body and is involved in many important biological
activities.[10] On the contrary, Cd2+ is known as a toxic metal
ion and can cause serious diseases, such as renal dysfunction,
calcium metabolism disorders, prostate cancer, and so
forth.[11] However, fluorescent sensors that distinguish be-
tween Zn2+ and Cd2+ metal ions are rarely reported.[12]
2-(2-Hydroxy-phenyl)-4(3H)-quinazolinone (HPQ), a typ-
ical organic compound that exhibits solid-state fluorescence
by excited-state intramolecular proton-transfer (ESIPT) re-
action (Scheme 1), has been reported to show good photo-
physical properties, such as intense fluorescence, and can be
used as a fluorescent ink and precipitating substrate for vari-
ous enzymes.[13] However, crystal-structure investigation has
never been explored in HPQ. The presence of the hydroxy
pyridyl metal chelating group and intense solid-state fluores-
cence indicates HPQ might be an interesting material to ex-
plore polymorphism and metal-ion-sensor properties. HPQ
derivatives are scarcely used as chemical sensors in analyti-
cal chemistry.[14] Structurally similar 2-(2’-hydroxyphenyl)
Organic fluorescent molecules have long been attracting sci-
entistsꢀ interest owing to their applications in electronics,
optics, and sensors.[1] In particular, developing molecules
with strong fluorescence in both the solid state as well as in
solution with structural features that enable guest interac-
tion would be interesting, because that might allow both op-
toelectronic and sensor applications to realized in a single
materials. The emergence of photonic and optoelectronic
technology such as organic light-emitting diodes (OLEDs),
solid lasers, and fluorescence sensors in recent years have
spurred demand for strong solid-state-fluorescent mole-
cules.[2] However, synthesizing highly efficient solid-state-flu-
orescent organic molecules is a challenging task, because
the molecular aggregation of chromophores, which is inher-
ent in the solid state, typically results in the concentration
quenching of fluorescence.[3] Furthermore, obtaining differ-
ent crystalline phases (polymorphs) in such fluorescent mol-
ecule provides the best opportunity to investigate the rela-
tionship of molecular packing with optical properties as well
as to obtain tunable solid-state-fluorescent materials without
doing further synthesis.[4–6] Solid-state fluorescence of molec-
ular materials has also been tuned either by modification of
substitutions on a single molecule[7] or by supramolecular
chemistry.[8]
imidazoACHTNUTRGNE[NUG 1,2-a]pyridine, which exhibits solid-state fluores-
[a] S. P. Anthony
cence by ESIPT shows polymorphism and tunable fluores-
cence.[6] Hence, we expected that HPQ also might form
polymorphic structures in suitable solvent conditions.
Herein, we demonstrate concomitant polymorph formation
of HPQ from THF solvent and polymorph-dependent solid-
state fluorescence. HPQ forms polymorphic structures
School of Chemical & Biotechnology, SASTRA University
Thanjavur-613 403, Tamil Nadu (India)
Fax : (+91)4362264120
E-mail: philip@biotech.sastra.edu
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100832.
374
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Chem. Asian J. 2012, 7, 374 – 379

phology. Excitation of needle-morphology crystals at 370 nm
gives blue-green fluorescence, and plate-morphology crystals
show blue fluorescence (Figure 1). Blue and blue-green
Figure 1. Schematic diagram of HPQ solid state fluorescence tuning via
polymorphism.
emitting crystals are hereafter referred to as HPQ-B and
HPQ-BG, respectively. Single-crystal X-ray analysis was per-
formed to get an insight into the HPQ molecular assembly
in the solid state. Single-crystal analysis revealed different
structural arrangements of HPQ molecules in the crystal lat-
tices of HPQ-B and HPQ-BG (Figure 2). Both HPQ-B and
HPQ-BG formed strong amide···amide intermolecular and
OH···N intramolecular H-bonding interactions in the crystal
lattice.[16] The amide···amide intermolecular interactions lead
to the formation of HPQ dimers in both crystal lattices (Fig-
ure 2c,d). However, both the amide···amide intermolecular
and OH···N intramolecular H-bonding distances of HPQ-
BG are slightly shorter than in the HPQ-B polymorph.
HPQ dimers of HPQ-B and HPQ-BG are further inter-
connected through p···p interactions in the crystal lattices
Scheme 1. Basic mechanism for ESIPT reaction of HPQ.
owing to the difference in the conformational twist between
the phenyl and quinazolinone rings. Interestingly, HPQ that
exhibits strong fluorescence in dimethyl formamide (DMF;
F=0.21) selectively detects and differentiates Zn2+ and
Cd2+ metal ions at micromolar concentrations. Thus, both
the tunable solid-state fluorescence and sensor phenomena
have been demonstrated in a single fluorescent organic ma-
terial, HPQ.
À
(Figure 3). In addition, polymorph HPQ-BG shows O···H C
intermolecular interaction between hydroxy oxygen atoms
and quinazolinone aromatic hydrogen atoms and carbonyl···-
carbonyl dipolar (C=OdÀ···Cd+=O) interactions in the crystal
Results and Discussion
lattice (Figure 3b).[17] The O···H C intermolecular interac-
À
HPQ, a quinazolinone derivative, was obtained by an unpre-
cedented yet simple synthetic method. The intended hydrol-
ysis of 2-(2’-cyanophenoxy)benzonitrile (O,O-CN) in etha-
nol/water mixture (80:20) in the presence of NaOH to
obtain the corresponding acid, 2,2’-oxybis(benzoic acid),
lead to the formation of HPQ (98%) exclusively as a single
product. Surprisingly, the expected corresponding carboxylic
acid did not form at all. Use of methanol instead of ethanol
also gives HPQ exclusively. However, hydrolysis of O,O-CN
in water alone with NaOH produces the corresponding acid,
2,2’-oxybis(benzoic acid) in 85% yield and only 10% HPQ.
The reaction mechanism and the role of alcohol were not
clear at this stage. Several synthetic approaches have been
reported for the synthesis of HPQ.[15]
tions of HPQ-BG lead to the formation of two-dimensional
sheets in the crystal lattice (Figure 3c). The most notable
structural difference of the HPQ molecule in the two crystal
lattices is the conformational difference of the N9-C10-C11-
C12 dihedral angle q between the phenyl and quinazolinone
rings (Figure 1). While the two aromatic rings are less twist-
ed (q=3.688) in polymorph HPQ-B, they are more twisted
in HPQ-BG (q=9.978). The conformational differences of
phenyl and quinazolinone rings lead to different molecular
packing in the solid state (Figure 2). The formation of slight-
À
ly stronger H-bond interactions, additional O···H C, and C=
O
dÀ···Cd+=O dipolar intermolecular interactions might be
the stabilizing force in the more twisted conformation of
HPQ in HPQ-BG.
HPQ powder obtained from the reaction solution shows
blue solid-state fluorescence. Recrystallization of HPQ from
ethyl acetate, ethanol, methanol, acetone, DMF, DMSO,
and chloroform gave crystals with plate morphology that ex-
hibit blue solid-state fluorescence (lmax =497 nm, excitation
wavelength 370 nm). However, crystallization of HPQ from
THF produced crystals with needle as well as plate mor-
The crystalline solids of HPQ-B and HPQ-BG were
ground to powders and examined by powder X-ray diffrac-
tion. In each case, the observed PXRD pattern closely
matched the theoretical PXRD pattern calculated from the
corresponding single-crystal structure, thus indicating that
these bulk powders had the same overall structure as their
crystalline solids (Figure 4). The polymorphic structure of
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S. P. Anthony
FULL PAPERS
exhibit fluorescence at shorter
wavelengths than their non-
twisted counterparts, since the
latter can form closer molecular
packing in the solid state that
could facilitate excited energy
transfer between molecules
more easily.[6] But in HPQ,
more twisted HPQ-BG exhibits
fluorescence at a longer wave-
length than HPQ-B. This be-
havior is because more pro-
nounced twisting of HPQ in
HPQ-BG leads to stronger H-
bonds and other intermolecular
interactions, which result in
closer packing of HPQ mole-
cules in the crystal lattice
(Figure 2 and Figure 3).
In spite of its good photo-
physical
properties[10]
and
metal-chelating hydroxy pyridyl
functional group that has the
potential to display chelation-
dependent fluorescence through
coordination with metal ions,
HPQ solution photophysics and
Figure 2. Molecular packing and selected H-bond interactions in the crystal lattice of a,c) HPQ-B and
b,d) HPQ-BG. Only H atoms involved in H-bond interactions are shown. H-bonds are given as dashed lines.
Corresponding distances (dHA (A=H-bond acceptor), ꢂ) are marked.
Figure 3. a) p···p interactions in HPQ-B and b,c) p···p, C=O···C=O dipo-
À
lar and OH···H C interactions in HPQ-BG crystal lattices. In (c), only H
Figure 4. PXRD patterns of HPQ-B and HPQ-BG. Patterns represent
calculations from single-crystal data ((c) and (g)) and experimental
data ((a) and (d)).
atoms involved in H-bond interactions are shown. Weak interactions are
given as broken lines; distances (ꢂ) of H-bonds (dHA) and other interac-
tion are marked.
HPQ leads to the formation of tunable organic solid-state
fluorescence materials. HPQ-B fluoresces at 497 nm while
HPQ-BG fluoresces at 511 nm (Figure 5). The fluorescence
intensity of HPQ-B and HPQ-BG were compared by keep-
ing the optical density around 0.5 in KBr pellets of the re-
spective sample. HPQ-B shows slightly higher fluorescence
intensity than HPQ-BG (Figure S1 in the Supporting Infor-
mation). In general, compounds with twisted conformations
its chemical-sensor properties have never been explored in
detail. HPQ shows weak to strong fluorescence in the visible
region in dilute solution with different fluorescence lmax due
to solvatochromism. In ethyl acetate, HPQ showed weak
fluorescence at lmax =507 nm (quantum yield F=0.005 on
comparison with quinine sulfate), and in DMF it showed
strong fluorescence lmax at 457 nm (F=0.21). It also showed
similar fluorescence intensity in DMSO. The absorption
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Chem. Asian J. 2012, 7, 374 – 379

Fluorescence of 2-(2-Hydroxyphenyl)-4(3H)-quinazolinone
trile), the intramolecular charge-transfer band blue-shifted
to 340–346 nm as small shoulder, and there was a clear n–p*
absorption band at 331 nm. The addition of Zn2+ or Cd2+
metal ions to the HPQ DMF solution completely quench
the intramolecular charge-transfer absorption band without
altering the n–p* absorption band (Figure 6b).
Interestingly, addition of Zn2+ and Cd2+ ions to HPQ not
only blue-shifted the fluorescence lmax of HPQ but also led
to different lmax for both metal ions. Addition of Zn2+ blue-
shifts the lmax from 497 to 420 nm, whereas Cd2+ addition
blue-shifts HPQ fluorescence lmax to 426 nm (Figure 7). The
reason for the differences in lmax is not clear, since single
crystals of HPQ-Zn and HPQ-Cd could not be obtained to
study the structural organization in the solid state. It is well
known that the hydroxy pyridyl ligand generally forms 2:1
ligand/metal complexes.[18] Fluorescence studies of HPQ
with different concentrations of metal ions also indicate that
HPQ might be forming a 2:1 HPQ-Zn2+/Cd2+ complex
(Figure 7). Addition of 0.1 equivalents of Zn2+ to HPQ
(10 mm) clearly changes the fluorescence spectrum profile,
Figure 5. Normalized solid-state fluorescence of HPQ-B and HPQ-BG
(excitation lmax =370 nm).
spectrum of free HPQ was examined using solvents of dif-
ferent polarity (Figure 6a). In DMF and DMSO, HPQ
showed two absorption bands: an intramolecular charge-
transfer band at 390–400 nm and an n–p* band at 335–
339 nm. In other solvents (MeOH, acetone, and acetoni-
Figure 6. Absorbance spectra of HPQ a) in different solvents and b) with
Zn2+ and Cd2+ metal ions in DMF.
Figure 7. Fluorescence spectra of HPQ (10 mm) upon addition of a) Zn2+
and b) Cd2+ metal ions in DMF (excitation lmax =345 nm).
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S. P. Anthony
FULL PAPERS
blue-shifting the lmax and reducing the intensity. Blue shift
of lmax continued for addition of up to 0.5 equivalents Zn2+
and remained stable on further addition. Although the first
addition of Cd2+ (0.1 equivalents) only reduced the fluores-
cence intensity, the subsequent additions led to blue shift of
lmax with intensity reduction. Similar to Zn2+, fluorescence
lmax remained the same for addition of Cd2+ beyond
0.5 equivalents.
formational twist between phenyl and quinazolinone rings
of HPQ molecules in HPQ-BG than HPQ-B leads to the
À
formation of additional C H···O intermolecular interactions
and C=OdÀ···Cd+=O dipolar interactions in the crystal lat-
tice. Importantly, HPQ not only selectively senses Zn2+ and
Cd2+ metal ions in DMF, but it also distinguishes between
them. Tunable solid-state fluorescence and selective Zn2+
and Cd2+ metal-ion-sensing properties of HPQ makes it an
interesting material for optoelectronic as well as biological
sensor applications.
The selectivity of HPQ for Zn2+ and Cd2+ ions was con-
firmed by monitoring fluorescence spectrum with addition
of other metal ions into the HPQ solution (Figure 8). Alka-
Experimental Section
Chemicals: 2-Hydroxybenzonitrile, 2-fluorobenzonitrile, anhydrous
K2CO3, dimethylsulfoxide (DMSO), NaOH, methanol, absolute ethanol,
and metal salts were obtained from Aldrich and used as received. O,O-
CN was synthesized according to the literature procedure.[19]
Synthesis of 2-(2-hydroxyphenyl)-4(3H)-quinazolinone (HPQ): O,O-di-
phenyl cyano ether (0.5 g, 2.2 mmol) and sodium hydroxide (0.8 g,
20 mmol) was dissolved in 80 mL of water/ethanol mixture (60:40), and
the reaction mixture was heated at reflux overnight. After the reaction
mixture was cooled to room temperature, the solution was acidified using
dilute hydrochloric acid. A white precipitate formed near pH 4. The pre-
cipitate was filtered, washed with distilled water, and dried under
vacuum. Yield: 0.53 g (98%). Mp: 2988C. NMR ([D6]DMSO) 1H: d=
8.21–8.12 (2H, m), 7.84–7.73 (2H, m), 7.54–7.47 (2H, m), 6.99–6.92 ppm
(2H, m); 13C: d=161.9, 160.5, 154.2, 146.5, 135.5, 134.7, 128.1, 127.4,
126.5, 121.1, 119.3, 118.3, 114.2 ppm. MS: m/z=238.
Crystallization of HPQ from ethyl acetate, acetone, DMF, DMSO, and
methanol produced exclusively blue-green fluorescent crystals (HPQ-B),
whereas crystallization from THF produced both blue and blue-green flu-
orescent crystals (HPQ-B and HPQ-BG).
Figure 8. Fluorescence spectra of HPQ (10 mm) upon addition of metal
salts (10.0 equivalents) in DMF (excitation lmax =345 nm).
Crystal data: HPQ-B (ccdc-809552): C14H10N2O2, M=238.24, monoclinic,
space group P21/n, a=13.378(3), b=5.136(10), c=16.648(3) ꢂ, b=
101.62(3), V=1120.6(4) ꢂ3, Z=4, T=150 K, 5844 reflections measured,
1947 unique (Rint =0.0224), Final R values: 0.0388, wR: 0.1063; HPQ-BG
(ccdc-809553): C14H10N2O2, M=238.24, monoclinic, space group C2/c, a=
30.496(6), b=4.984(1), c=16.804(3) ꢂ, b=119.53(3), V=2222.3(10) ꢂ3,
Z=8, T=123 K, 7867 reflections measured, 2764 unique (Rint =0.0241),
Final R values: 0.0496, wR: 0.1364.
line-earth-metal ions (Mg2+ and Ca2+) did not have any in-
fluence on HPQ fluorescence. However, addition of transi-
tion and P-block metal ions (Mn2+, Fe3+, Hg2+, and Pb2+
)
have varied influence on the HPQ fluorescence. Fe3+ com-
pletely quenches the fluorescence, whereas Mn2+, Pb2+, and
Hg2+ only reduced the fluorescence intensity without chang-
ing lmax. But Co2+, Ni2+, and Cu2+ reduced the fluorescence
intensity as well as blue-shifting the lmax to 424 nm. No
change in fluorescence wavelength and intensity was ob-
served upon changing of the counteranion of the salt used
to add Zn2+ and Cd2+ cation. Addition of Fe3+ into HPQ-
Zn and HPQ-Cd solution completely quenches the fluores-
cence, and other transition-metal ions except Mn2+ reduce
the fluorescence intensity of HPQ-Zn and HPQ-Cd solu-
tions. This result indicates that those metal ions bind more
Spectroscopic characterization: Absorption and fluorescence spectra
were recorded using Perking Elmer Lambda 1050 and Horiba Jobin
Yvon Fluorolog instruments. Other metal-ion-selectivity measurements
were performed by adding excess amounts of metal salt (10:1) to HPQ
solution. Similarly, HPQ ligand binding with other metal ions compare to
Zn2+ and Cd2+ were performed by adding excess metal salt (10:1) into
HPQ-Zn/Cd solution. Solid-state fluorescence was measured by spread-
ing the powdered samples on a glass plate. To compare the intensity of
the solid-state fluorescence, transparent KBr pellets of HPQ-B and
HPQ-BG were prepared, and the concentration of the compounds in
solid matrix was adjusted to keep the optical density (OD) around 0.5.
KBr pellets of these samples show similar fluorescence lmax as their pure
solid samples.
Structural analysis: PXRD measurements were recorded using Siemens
diffraktometer-D500 at room temperature. For single-crystal structure de-
terminations, crystals were carefully chosen after they were viewed
through a polarizing microscope. The crystals were glued to a thin glass
fiber using an adhesive (cyano acrylate) and mounted on a diffractometer
equipped with an APEX CCD area detector. The data collection was car-
ried out at 150 K and no extraordinary methods were employed, except
that the crystals were smeared in NIH immersion oil to protect them
from ambient laboratory conditions. The intensity data were processed
using Brukerꢀs suite of data-processing programs (SAINT), and absorp-
tion corrections were applied using SADABS.[20] The structure solution
of all the complexes was carried out by direct methods, and refinements
strongly with HPQ than Zn2+ and Cd2+
.
Conclusions
Both tunable solid-state fluorescence as well as selective
metal-ion-sensing properties was demonstrated in a single
organic fluorescent material. Polymorphism of HPQ in THF
leads to the tunable solid-state fluorescence. The larger con-
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Chem. Asian J. 2012, 7, 374 – 379

Fluorescence of 2-(2-Hydroxyphenyl)-4(3H)-quinazolinone
were performed by full-matrix least-squares on F2 using the SHELXTL-
PLUS[21] suite of programs. All the structures converged to good R fac-
tors. All the non-hydrogen atoms were refined anisotropically, and the
hydrogen atoms were fixed on calculated position using appropriate
HFIX options in SHELXTL and were refined isotropically. Intermolecu-
lar interactions were computed using the PLATON program.[22]
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Acknowledgements
The financial support from SASTRA University (TRR fund) is acknowl-
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Received: October 6, 2011
Published online: December 19, 2011
Chem. Asian J. 2012, 7, 374 – 379
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
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