Modulation of dual fluorescence modes and emissions of 2-(1,4-dioxo-1,4-dihydro-naphthalen-2-yl-amino)benzoic acid

Article abstract of DOI:10.1016/j.molstruc.2017.08.004

Intra-molecularly hydrogen bonded compound 2-(1,4-dioxo-1,4-dihydronaphthalen-2-yl-amino)benzoic acid (ANQ) shows emission at single wavelength upon excitation in UV-region whereas it shows dual fluorescence emissions on excitation in visible region. Such

Full text of DOI:10.1016/j.molstruc.2017.08.004

Accepted Manuscript
Modulation of dual fluorescence modes and emissions of 2-(1,4-dioxo-1,4-dihydro-
naphthalen-2-yl-amino)benzoic acid
Munendra Pal Singh, Jubaraj B. Baruah
PII:
S0022-2860(17)31058-X
10.1016/j.molstruc.2017.08.004
MOLSTR 24141
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 14 July 2017
Revised Date: 1 August 2017
Accepted Date: 2 August 2017
Please cite this article as: M.P. Singh, J.B. Baruah, Modulation of dual fluorescence modes and
emissions of 2-(1,4-dioxo-1,4-dihydro-naphthalen-2-yl-amino)benzoic acid, Journal of Molecular
Structure (2017), doi: 10.1016/j.molstruc.2017.08.004.
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Graphical abstract
Modulation of dual fluorescence modes and emissions of 2-(1,4-dioxo-1,4-dihydro-
napthalen-2-yl-amino)benzoic acid
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Excitation of solutions of 2-(1,4-dioxo-1,4-dihydronapthalen-2-yl-amino)benzoic acid in UV-
region shows single emission whereas excitation in visible region shows dual emission
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Munendra Pal Singh and Jubaraj B. Baruah
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Graphical TOC
Modulation of dual fluorescence modes and emissions of 2-(1,4-dioxo-1,4-dihydro-
naphthalen-2-yl-amino)benzoic acid
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Excitation of solutions of 2-(1,4-dioxo-1,4-dihydronaphthalen-2-yl-amino)benzoic acid in
UV-region shows single emission whereas excitation in visible region shows dual emission
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Munendra Pal Singh and Jubaraj B. Baruah
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Modulation of dual fluorescence modes and emissions of 2-(1,4-dioxo-1,4-dihydro-
naphthalen-2-yl-amino)benzoic acid
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Munendra Pal Singh and Jubaraj B. Baruah*
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039
Assam, India; email: juba@iitg.ernet.in, phone +91-361-2311; Fax +91-361-2690762
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Abstract:
Intra-molecularly hydrogen bonded compound 2-(1,4-dioxo-1,4-dihydronaphthalen-2-yl-
amino)benzoic acid (ANQ) shows emission at single wavelength upon excitation in UV-
region whereas it shows dual fluorescence emissions on excitation in visible region. Such
emissions depend on solvent, concentration and pH. Solvent dependent structural changes of
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ANQ are reflected in solution studies carried out by HNMR, UV-visible and fluorescence
spectroscopy. DLS study has showed that in addition to monomers, self-assemblies with 415
nm average sized particles are formed in DMF solution. HOMO-LUMO of three tautomeric
forms of the compound was calculated by DFT using CAMB3LYP/6-31+G(d,p) as basis set
shows that naphthoquinone form has about 86.30 KJ/mol higher stability over an imino-
quinone form. From various studies relating concentration dependence, pH, life-time, study
on dual-emissions caused by visible excitation has established the observed emissions to
originate from aggregation contributing as charge-transfer and deprotonated species. Whereas
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the single emission caused by UV-excitation occurs through excited state intra-molecular
proton transfer.
Keywords: Dual-modes; Dual-emissions; Amino-naphthoquinone; Intra-molecular hydrogen
bond; Fluorescence modulation.
Introduction
Fluorescence emissions of small-molecules are extensively used as biological probes [1].
Advantages from small molecules that show environment sensitive fluorescence are taken to
use them as biological probes to understand micro-environmental changes due to protein
conformational changes [2-3]. There are also situations in which a molecule may not induce
enough environmental changes to cause a noticeable difference to fluorescence probe [4];
hence fluorescence properties associated with a potential probe requires thorough attention.
Interactions of small molecules [5-9] with proteins and DNA are routinely explored. In our
recent study we found wide variations in binding abilities of 2-(1,4-dioxo-1,4-
dihydronaphthalen-2-yl-amino)benzoic acid causing fluorescence emission changes to human
serum albumin in comparison to the positional isomer 4-(1,4-dioxo-1,4-dihydronaphthalen-
2-yl-amino)benzoic acid [10]. Among small fluorescent molecules quinone derivatives are
ubiquitous in electron transfer [11-17], ion-recognition [18-25] and in electrochemistry [26-
29]. Many quinone derivatives are used as fluorescence probes [30-38] and some of them
show non-linear optical property [39]. Fluorescent quinone derivatives are useful in ion
detection and to modulate fluorescence by ions [40-41]. Quinone linked photo-active peptides
have interesting photophysical properties [42]. Certain quinones show fluorescence emission
at two different wavelengths when excited by single wavelength [43]. It is interesting to note
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Fig. 1: Some probable forms of ANQ in neutral and basic conditions.
that certain compounds possessing keto-imine forms show dual fluorescence [44-46]. Keto-
enol tautomers of hydroxyquinones [47] and keto-imine forms of aminoquinones [48] are
well known but less explored for fluorescence study. Recently dual-emissions were observed
by us while exciting at UV- or visible region in inter-molecular hydrogen-bonded 4-
aminobenzoic acid tethered 1,4-naphthoquinone [49]. It may be noted that 2-(1,4-dioxo-1,4-
dihydronaphthalen-2-yl-amino)benzoic acid (ANQ) showed relatively lower binding towards
bovine serum albumin protein in comparison to a series of analogous naphthoquinone
derivatives. This is an interesting point to note, as weakly bound fluorescence probe will be
easy to manipulate in biological conditions. It is worth noting that diethylamino-2-
hydroxybenzaldehyde shows dual emission through suppression of intramolecular charge-
transfer by excited state proton transfer process [50]. Thus, ANQ is chosen to explore
interesting emission features from different intramolecular hydrogen bonded structural forms
shown in Fig. 1 (I-III). Depending on pH anionic species of ANQ such as IV-VI shown in
Fig. 1 are also possible, which may in turn contribute to emission process. Different
tautomeric forms of ANQ in solution should prefer single emission path or combined
emission paths under different conditions. Accordingly understanding of fluorescence
properties of ANQ under different conditions is of interest. With this background we
explored fluorescence emission properties of ANQ to understand independent or collective
contribution of forms I-V to fluorescence emissions under different conditions.
Experimental:
Infrared spectra were obtained using Perkin-Elmer FT-IR spectrophotometer (KBr, 4000-400
cm-1). UV-Vis-spectra were recorded using Perkin-Elmer Lambda 750 UV-Visible
spectrophotometer. Fluorescence emissions were measured in a Perkin-Elmer LS-55
spectrofluorimeter by taking appropriate amount of solutions of the compound (3mL in each
case) in different conditions. NMR spectra were recorded in Varian-AS400 (oxford) 400MHz
and Bruker 600MHz NMR spectrometer. ESI-mass spectra were recorded on a liquid
chromatography mass spectrometer using Water Q-TOF premier mass spectrometer attached
to aquity HPLC. Life-time decay profiles were measured on an Eddinburg Instrument,
Model: FSP920. Dynamic light scattering (DLS) experiments were performed with a Malvern
Zetasizer Nano ZS instrument equipped with a 4.0 mW He-Ne laser operating at a
wavelength of 633 nm at room temperature. The energy of molecules and energy gaps are
calculated by DFT using Gaussian 09 software was with CAMB3LYP/6-31+G (d,p) as basis
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set. For measurements of UV-visible absorptions and fluorescence emissions appropriate 3
ml of each solution in specific solvents in quartz cuvette were measured.
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2-(1,4-dioxo-1,4-dihydronaphthalen-2yl-amino)benzoic acid (ANQ) was prepared by
reacting 1,4-naphthoquinone with 2-aminobenzoic acid by following procedure: To a well
stirred solution of 1,4-naphthoquinone (0.32 g, 2 mmol) in methanol (20 ml) a solution of 2-
aminobenzoic acid (0.27 g, 2 mmol) in methanol (10 ml) was drop-wise added. Reaction
mixture was stirred overnight and kept at room temperature to slowly evaporate solvent. A
dark red precipitate was formed which was filtered and further purified by crystallization
from a solution made in N,N'-dimethylacetamide. After five days red crystals were observed
Yield : 65%. IR (KBr, cm-1): 3370 (m), 3236 (m), 3060 (w), 2917(w), 1673 (s), 1611 (s),
1584 (s), 1572 (s), 1528 (s), 1453 (m), 1412 (w), 1357 (m), 1296 (s), 1262 (s), 1215 (m),
1151 (m), 1124 (w), 1083 (w), 991 (w), 753 (w), 719 (w). 1H-NMR (600 MHz, CD3OD):
10.84 (s, 1H), 8.15 (m, 2H), 8.06 (d, J = 6 Hz, 1H), 7.84 (t, J = 12 Hz 1H), 7.77 (t, J = 12 Hz ,
1H), 7.69 (m, 2H), 7.23 (m, 1H), 6.65 (s, 1H). 13C NMR (100MHz, DMSO-d6): 181.9, 178.4,
169.4, 144.3, 139.4, 135.7, 134.9, 133.7, 132.7, 131.8 127.0, 126.5, 126.0, 123.9, 121.2,
117.0, 105.6. Observed HRMS mass (m/e) 294.0792 [M+1]; calculated mass [M+1] for
C17H11NO4, 294.0722.
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Results and discussion:
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The compound ANQ was characterized by recording HNMR, 13CNMR, IR, HRMS and
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determining crystal structure. Compound shows HNMR signals of aromatic protons in the
region between 7.15 ppm to 8.25 ppm. Assignments of aromatic protons of ANQ recorded in
methanol-d4 are marked in the 2-D HOMO-COSY 1HNMR spectra illustrated in the Fig. 2a.
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It shows that it remains in methanol as form I as shown in Fig 1. Comparison of HNMR
spectra recorded in different deuterated solvents showed that chemical shift positions of
proton Ha and Ha' and Hb, Hb' (inset of Fig. 2a) are shifted by each solvent to different
extents. Signals of Hd and Hf appear as a triplet and a doublet. These signals are less affected
by solvents [Figs. 2b(i).-(iv)]. This suggests that solvent influences the resonance structure by
affecting intra-molecular hydrogen bond.
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Fig. 2: (a) 2D-HOMO-COSY H-NMR spectra of ANQ in methanol-d4 in the range 7.00
ppm-8.30 ppm. (b) 1H-NMR spectra (region showing 7.15-8.25 ppm) of ANQ in (i) DMSO-
d6, (ii) Methanol-d4, (iii) Acetone-d6 and (iv) Mixture of methanol-d4 and DMSO-d6 (2:1 v/v).
Depending on solvent signal for N-H proton appears in the region 10.79-10.87 ppm and
carboxylic OH is in offset region hence not observed (supporting Fig. 1S). 1HNMR spectra of
the compound in methanol-d4 has two superimposed doublets for Ha' and Hc. These signals
are relatively shielded and split into two distinct doublets in DMSO-d6. Whereas signal for
Ha' was slightly deshielded in acetone-d6 or in mixture of methanol-d4 and DMSO-d6 with
respect to peak observed for it in methanol-d4. On the other hand, signals for Hb and Hb' are
deshielded in DMSO-d6 than corresponding signals in methanol-d4. The peak for proton Hg
appeared as singlet at 6.65 ppm and this signal is insignificantly affected by solvents. Hence,
signals of protons attached to naphthoquinone rings are more affected by solvents and
splitting patterns of signals of protons on carboxyphenyl unit changes with solvents. The
changes in chemical shifts and splitting patterns are attributed to geometrical changes in the
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molecule due to stabilization or disruption patters are attributed to the changes caused by
solvents of intra-molecular hydrogen bond between the carboxylic acid with the N-H of
amine. Accordingly extent of conjugation of N-H group with the naphthoquinone ring as well
as the environment of the hydrogen of the carboxyphenyl unit changes with solvents.
Compound ANQ crystallizes as dimethylacetamide solvate. Crystal structure of solvated
ANQ shows that it has naphthoquinone backbone tethered to 2-aminocarboxylic acid (Form I
of Fig. 1). The C10-O3 and C17-O2 carbon-oxygen double bond distances are 1.23(3) Å and
1.22(3) Å respectively. The C-N bond distances of the compound are 1.41 (4) Å for C7-N1
and 1.37(4) Å for C8-N1 bond. The bond difference between the bond distances of two C-N
bonds linking the naphthoquinone and phenyl carboxylic acid indicate that N-H group has
delocalization with carboxyphenyl unit. This is also reflected in the cyclic geometry adopted
by intra-molecular hydrogen bond between N1-H and O1 as illustrated in figure 3.
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Fig. 3: Structure of dimethylacetamide solvate of ANQ (drawn with 50% thermal ellipsoids)
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Figure 4: (a) UV-Visible spectra of ANQ (10-4 M) in different solvents. (b) HOMO and
LUMO and their engegy gaps determined by DFT calculation of diferent forms of ANQ
calculated by DFT using CAMB3LYP/6-31+G (d,p) as basis set.
UV-visible spectra of ANQ in methanol shows two equal intensity absorptions at 243 and
279 nm due to π→ π* transitions. Ratio of intensity of these two absorption peaks changes
from one solvent to another solvent. This suggests that they originate from contribution from
two independent forms (Fig. 4a). In all solvents ANQ consistently showed absorption at 330
nm. In DMSO there were two major absorption peaks at 277 nm and 330 nm due to π→ π*
and n→ π* transitions. Wavelength of absorptions along with extinction coefficients of ANQ
from spectra recorded in different solvents are listed in Table S1. Each solution showed an
absorption in the range of 457 nm to 496 nm. We have also carried out DLS study and found
self-aggregation of average particle size 415 nm dimension. Thus, absorption is from the
aggregates formed by AQN molecules. Observed peak from charge-transfer in the aggregate
is relatively weak in DMSO than methanol. Relative energies of the three forms I-III (Figure
1) are calculated by DFT using CAMB3LYP/6-31+G (d,p) as basis set. It is found that the
form I and II has equal energy, whereas naphthoquinone form I has 86.30 KJ/mol lower
energy than imino-quinone form III. Energy gaps between HOMO-LUMO of the form I and
II are very similar (Fig. 4b). HOMO and LUMO gap of III is smaller than respective energy
gaps of other two forms. Quantitatively, the difference in energies of the gaps of I and III is
0.1138 eV (10.98 KJ/mol) whereas difference between such gaps of II and III is 0.1143 eV
(11.03 KJ/mol). Symmetry and comparable energy permits interaction of LUMO of III with
HOMO of I forming transient charge-transfer species; which provides the basis for CT
transition.
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Fig. 5: (a) Excitation and emission spectra of ANQ in DMF (10-4 M, 3 mL) upon excitation at
277 nm. Fluorescence emission of ANQ (10-4 M, 3 mL) in (b) MeOH and (c) DMSO at
different pH (λex in each case 300 nm) in each case (i) pH = 3, (ii) pH = 7, and (iii) pH = 9
Excitation spectra obtained by exciting of a solution of ANQ dissolved in DMF by 277 nm
radiation had overlapped excitation peaks due to S0 to S1 transitions with vibrational
contributions (Fig. 5a). The shapes of such excitation spectra of ANQ recorded in different
solvents are distorted in each case. This is due to the vibrational contributions that depend on
the solvent polarity and solvation (Supporting Figs. 3S-14S). The probable involvement of
excitation from S0 to S1+1 state is ruled out as the energy-gap between such states are high
(Refer to DFT energy Fig. 22S).
Emission spectra of ANQ obtained upon excitation in UV-region showed single emission
at higher wavelength than the corresponding excitation wavelength (Fig. 5a). Fluorescence
emission of ANQ in DMF, methanol, DMSO, acetonitrile and THF occur at 389 nm, 405
nm, 403 nm, 390 nm, 388 nm (Fig. 11S) respectively upon excitation at 277 nm. Stoke’s shift
in each solvent is different, thus emission process occurs through excited state intramolecular
proton transfer (ESIPT). Upon change in excitation wavelength within the UV-region,
position of emission wavelengths were changed by 1 nm to 2 nm due to involvement of
vibrational levels. Though each emission spectrum had single prominent emission, positions
of such emissions depend on solvent (Table S1). Relative emission positions were THF <
acetonitrile < DMSO < DMF < MeOH which did not follow a trend in solvent polarity THF
< DMF < DMSO < acetonitrile < MeOH. These emissions are pH dependent and are
quenched in basic condition at pH = 9 (Figs. 5b, 5c). In acidic medium (pH =3) emission in
methanol and DMSO solutions has similar Stoke’s shifts. For example, excitation at 325 nm
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in methanol Stoke’s shift is 70 nm, whereas excitation at 330 nm in DMSO it is 78 nm. 1'-
Hydroxy-2'-acteophenone is a related system which shows small Stoke’s shift and shows
ESIPT [51]. It may be noted that in DMSO there is a very weak emission at 570 nm in acidic
and in neutral conditions [Fig. 5c(ii)]; this is transition from S1 to a transient charge transfer
state (CT0), where transient charge transfer interaction between I and III generates CT0 state.
Fluorescence decay of the excitation caused by exciting at 290 nm in methanol has a single
decay path with 7.48 ns life-time, whereas similar excitation in DMSO has two life-times
1.84 ns (3.9%) and 8.74 ns (96.1%). The relative intensities of emission peaks also differ at
different pH as shown in fig 5b and 5c. In basic medium complete deprotonation of all acidic
protons takes place to cause increase in intensity of emission in this region, this is due to
extended conjugation. Whereas in neutral condition both the labile protons namely NH and
COOH are intact; shows emission from intramolecular hydrogen bonded state. At acidic pH
the NH is protonated which disrupts intramolecular hydrogen bond causing quenching of
fluorescence.
Excitation at visible region (wavelength > 450 nm) of solutions of ANQ resulted dual
emissions in visible region. For example, dual emissions were observed in solvents such as
DMF, DMSO, THF, methanol and acetonitrile respectively upon excitation at 504 nm, 500
nm, 471 nm, 488 nm and 471 nm respectively. Differences in intensities of two emission
peaks varied from solvent to solvent. Representative excitation and emission spectra of ANQ
in DMF upon excitation 500 nm is shown in Figure 6a.
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Fig. 6: (a) Excitation and emission spectra of ANQ in DMF (10-4 M, 3 mL) upon excitation at
500 nm. (b) Fluorescence emission of ANQ in DMSO at different concentration (λex =500
nm). (i) 10-6 M ( ii) 10-5 M (iii) 10-4 M, (3 ml in each case, inset is expansion in 700 to 850
nm)). Fluorescence emission of ANQ (10-4 M, 3 mL) in (c) MeOH and (d) DMSO at
different pH (λex in each case 500 nm) in each case (i) pH = 3, (ii) pH = 7, and (iii) pH =
9.
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Excitation spectra were recorded by exciting at wavelengths > 450 in different solvents, they
showed similar emission features. They show two emission peaks around 500-600 nm and
there is also an emission at 800 nm. The emission peak at 800 nm is highly dependent on
concentration and is not observed at lower concentration (Fig. 6b). Based on its occurrence at
high wavelength and concentration dependence and absence at low concentration this
emission peak is thus attributed to aggregation induced excitation. For example in methanol it
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had an excitation peak for CT0-CT0 transition at 475 nm (Fig. 6a). There is also excitation
peak at 330 nm (Fig. 6a) which is due to excitation of S0-S1 of keto or enol form. The
emission spectrum under neutral condition in methanol showed two emission peaks at 530
nm and 556 nm. The peak at 530 nm was not observed in basic medium. Thus, the peak at
556 nm is attributed to emission from anion of ANQ. Weak emission around 800 nm was
observed in methanol solution under neutral and acidic condition, but this emission was
absent in basic condition. This peak is attributed to aggregation induced emission as it is
concentration dependent as well as pH dependent. On the other hand, in neutral and acidic pH
dual fluorescence were slightly affected but intensities were changed by pH (Fig. 6b, 6c). In
basic condition single emission occurs, which is suggests that in basic medium anion are
responsible for such emission.
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Fig. 7: Mecahnism of single and dual fluoroscence emission shown by ANQ
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Life-time of fluorescence decay of ANQ upon excitation at 475 nm in methanol
shows two decay paths with life-times 1.69 ns (18.5%) and 4.80 ns (81.5%). On the other
hand, a similar excitation in DMSO has two life-times one at 1.52 (25.5%) and other at 4.78
ns (74.5%). This suggests that decay paths in these solvents are similar but different
percentages of molecules take independent paths. Entire emission process of ANQ involves
contribution of ESIPT and aggregation of ANQ. In case of excitation at short wavelength
ESIPT was operative (E* to E of Fig. 7). While upon excitation in longer wavelength caused
emission involving charge-transfer as well as anions are involved. In basic medium charge-
transfer are reduced hence single emission at visible wavelength is observed. Dual emissions
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take place by CT0 -CT0 and S1' to S0' states of anion. At higher concentration aggregation of
ANQ takes place to show aggregation induced emission at 800 nm. Dual fluorescence and
excited-state proton transfer in 1,5-dihydroxyanthraquinone was reported earlier. In such
example, structured fluorescence pattern at shorter wavelength region, whereas less
structured emission at longer wavelengths were observed [52-53]. On the other hand, a recent
study has revealed dual emission in dihydroxy-9,10-anthraquinone as consequence of proton
transfer [54]. Dual emission in 3-hydroxychromones accompanies proton transfer, where
solvents play a major role [55]. Based on these our observations on dual emission involving
anion generated under ambient condition facilitated by solvents add value of uncommon dual
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fluorescence emission found in ANQ. It also clearly suggests that the substrate dependent
single or dual emission [49] of positional isomers by UV-radiations.
This study shows study that concentrations of ANQ in various solvents modulate
multiple mode emissions. Single emission caused by ESIPT at higher wavelength than the
excitation wavelength and dual emissions due to aggregation leading to charge transfer and
anion formation induced emissions are clear from this study. The modulation of multiple
modes and understanding their modulation with clear distinctions of emissions upon
excitation by visible wavelength source giving detectable color to naked eyes projects such
compounds useful for imaging techniques.
Supporting information: Crystallographic information file is deposited to CCDC and it has
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CCDC No. 1494729. Experimental details, H-NMR spectra of ANQ in different solvents,
excitation and emission spectra, Fluorescence life-time data of ANQ, theoretical data of DFT
are available.
Acknowledgements: The authors thank Ministry of Human Resources and Development,
India for financial support (Grant No. F. No. 5-1/2014-TS.VII).
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Highlights
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Dual fluorescence modes of 2-aminobenzoic acid tethered 1,4-naphthoquinone is
established.
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Emission in UV or visible region independently caused by varying excitation
wavelength.
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Fluorescence emissions are modulated by changing pH, concentration and solvent.
Single emission upon excitation at UV-region is consequence of ESIPT.
Dual emissions at visible region are due to contribution from charge-transfer and
anions.
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