One-pot protocol for J-aggregated anthraimidazolediones catalyzed by phosphotungstic acid in PEG-400 under aerobic condition

Article abstract of DOI:10.1039/c5ra19190d

A precise and productive one pot green protocol has been developed for the chemoselective synthesis of anthra[1,2-d]imidazole-6,11-diones using phosphotungstic acid (PTA) as a reusable catalyst on polyethylene glycol-400 (PEG-400) support under aerobic condition. It involves a simple synthetic procedure for a wide range of anthraimidazolediones from 1,2-diaminoanthraquinone and various aldehydes possessing π-enriched aromatic/heteroaromatic rings and lipophilic alkyl chains. In inert atmosphere, the reaction followed a different pathway, yielding a unique naphtho-quinoxaline derivative. Synthesized anthraimidazoledione derivatives had shown different self-aggregation morphologies influenced by the substitution pattern. A nice agreement between crystal packing pattern and photophysical properties with J-aggregation morphology was also observed. Representative derivatives exhibited contrasting fluorescence behaviors in transition from solution state to nanostructured solid state.

Full text of DOI:10.1039/c5ra19190d

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One-pot protocol for J-aggregated anthraimidazolediones catalyzed
by phosphotungstic acid in PEG-400 under aerobic condition
a
b
c
c
Bhaswati Bhattacharyya, Arijit Kundu, Aniruddha Das, Kaliprasanna Dhara* and Nikhil
c
Guchhait
a
Department of Chemistry, A. P. C. Roy Govt. College, Darjeelingꢀ 734010, India
b
Department of Chemistry, Maulana Azad College, Kolkataꢀ700013, India
c
Department of Chemistry, University College of Science & Technology, University of Calcutta,
92, A.P.C. Road, Kolkataꢀ700009, India.
Eꢀmail: chemkpd@gmail.com
Abstract:
A precise and productive one pot green protocol have been developed for the chemoselective
synthesis of anthra[1,2ꢀd]imidazoleꢀ6,11ꢀdiones using phosphotungstic acid (PTA) as a reusable
catalyst on polyethylene glycolꢀ400 (PEGꢀ400) support under aerobic condition. It involves a
simple synthetic procedure for a wide range of anthraimidazolediones from 1,2ꢀ
diaminoanthraquinone and various aldehydes possessing πꢀenriched aromatic/heteroaromatic
rings and lipophilic alkyl chains. In inert atmosphere, reaction followed a different pathway
yielding a unique naphthoꢀquinoxaline derivative. Synthesized anthraimidazoledione derivatives
had shown different selfꢀaggregation morphologies influenced by the substitution pattern. A nice
agreement between crystal packing pattern and photophysical properties with Jꢀaggregation
morphology were also observed. Representative derivatives exhibited contrasting fluorescence
behaviors in transition from solution state to nanostructured solid state.
1

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Introduction
In the new era of research, lowꢀdimensional selfꢀaggregated organic materials (LMSOM)
have gained much attention for their broad spectrum of electrical, magnetic and optical
properties.1ꢀ4 Performances of these organo–derived devices are correlated with the associative
patterns present in their nanoarchitectures.5 Therefore, designing of selfꢀassembled structures
with preferred shape and size by tailoring the structural features of the building blocks, i.e.
modulating the πꢀπ stacking, hydrogen bonding and van der Waal’s interactions etc., is the real
challenge towards scientists.6 Literature study revealed that some typical glycalꢀbased chiral
anthraimidazoledione derivatives had exhibited selfꢀassociation properties.7 Motivated by this
observation we herein fabricated anthraimidazoledione scaffolds with a long lipophilic aliphatic
chain or πꢀenriched aromatic/ heteroaromatic substituents at the 2ꢀposition of the imidazole ring
which might find application in the field of organic nanoꢀmaterials.
The regular synthetic methodology for imidazole derivatives is cyclocondensationꢀcumꢀ
oxidation (CCO) reaction between 1,2ꢀdiamino compounds with aldehydes.8 However, 1,2ꢀ
diaminoanthraquinone is exceptionally unreactive owing to the presence of an intramolecular
hydrogen bond between NꢀH proton of the –NH2 group and the neighboring quinone C9ꢀcarbonyl
oxygen. Reported methodologies for the syntheses of anthraimidazolediones involved harsh
reaction conditions viz. refluxing in ethanol in presence of CF3CO2H,9 or Na2S2O5 at 130°C,10
boiling in glacial CH3CO2H with sodium acetate,11a or with two equivalents of cupric acetate,11b
in nitrobenzene at 120ꢀ140°C for 8ꢀ24 hours,12 in PEGꢀ400 at 120°C for 18 hours13 and a two
step process which required refluxing overnight with formic acid in ethanol followed by in situ
oxidation with lead tetraacetate in acetic acid.14 Huang and Lin et. al.15 and Maity et. al.7 had
achieved syntheses of certain types of anthraimidazoledione derivatives at room temperature
2

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using concentrated sulphuric acid in DMF and a combo catalyst VO(acac)2 ꢀ Ti(OBu)4 ꢀ CeCl3
with molecular O2 as oxidant, respectively. It is pertinent to mention here that several of these
methodologies were explored with some selected substrates only. So the search for an easier
general protocol with environmentally benign catalytic system for the preparation of
anthraimidazoledione derivatives was the major thrust of our work.
Commercially available heteropolyacids (HPAs) is very much significant for their “value
adding properties”.16 HPAs with their Keggin structure, manifest very high Brönsted acidic
character comparable to superꢀacids and show unique redox properties under aerobic condition.16
Of them, phosphotungstic acid (PTA) and phosphomolybdic acid (PMA) are already in use for
several types of organic transformations.17ꢀ21 In this methodology preference was given to PTA
over PMA for its higher acidity and greater thermal stability.22
The difficulties with solid HPAs as heterogeneous catalysts are their inherent nature to
absorb polar and especially basic compounds by forming a “pseudoliquid” phase. 23 The extent of
absorption of those molecules depend on their molecular sizes.23 Additionally the compounds
bearing alcoholic functionality are easily desorbed from HPAs whereas, for the basic amino
compounds, elevated temperature is required for desorption.23 So, to maintain the virtues of
hydroxylic environment and to avoid the “pseudoliquid” formation, poly(ethylene glycol) (PEGꢀ
400) was chosen as a medium because of its high thermal stability (up to 150ꢀ250°C) towards
oxidative environment of HPAs.24 PEGs are also known to act as phaseꢀtransfer catalyst since
they possess both hydrophobic crown ether framework along with polar hydroxyl groups. These
substances are cheap, easily available, biodegradable and may stand in comparison to other
recently much publicized media like micellar systems or costly ionic liquids.25
3

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Considering the above facts we have used PTA as solid heterogeneous catalyst on PEGꢀ400
support to prepare anthra[1,2ꢀd]imidazoleꢀ6,11ꢀdione derivatives in presence of air. A
comparative study was performed with the crystal structure, morphology and photoꢀphysical
property to correlate the substituent effect of the anthraimidazoledione derivatives.
Results and discussion
In the hunt for optimum reaction condition we carried out extensive screening tests
employing a representative reaction between 1,2ꢀdiaminoanthraquinone (1) (1 mmol) and pꢀ
chlorobenzaldehyde (2c) (1 mmol) to furnish the product 3c (Scheme 1) in presence of different
oxidants and acid catalysts. The other parameters such as solvent, temperature and reaction time
etc. were also varied and the results have been summarized in Table 1. It is evident from Table 1
that the oxidant ammonium persulfate [ (NH4)2S2O8 ] in ethanol solvent, used successfully for the
synthesis of benzimidazoles,26 had negligible efficacy in this specific case even after 10 hours
(Table 1, entry 1). Nꢀiodosuccinimide (NIS) in acetonitrile (Table 1, entries 2 and 3), which was
initially employed in our laboratory for simple CCO type reaction at room temperature with good
yield,27 led to moderate yield of 3c even at 60°C. Both Lꢀproline, as an organocatalyst in ethanol
and polyphosphoric acid in dichloromethane with its dual nature as acid and dehydrating agent
(Table 1, entries 4 and 5) failed to prove as suitable reagents after six and ten hours respectively.
At this stage we turned our attention towards HPAs as more proficient catalytic system. We had
chosen phosphotungstic acid (PTA) in dichloromethane at room temperature which increased the
yield of 3c (35%) a bit though the reaction was slow and incomplete even after 20 h (Table 1,
entry 6). But, employing PMA as catalyst in similar solvent system diminished the yield of 3c to
4

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20% (Table 1, entry 7). It may be mentioned here that the general limitations of HPAs as solid
heterogeneous catalysts are their low specific surface area (approx. 5 m2 gꢀ1).23 To overcome this
predicament, we had dispersed PTA on solid surface such as silica gel to obtain better surface
catalytic activity (Table 1, entries 8 and 9) but no significant change in the yield of the product
was obtained. This phenomenon could be interpreted as the loss of Keggin structure due to
interaction between the surface silanol groups of silica and surface P atoms of PTA.23 Since
HPAs absorb polar molecules which affect the yields of the corresponding products, we had
explored the effects of polar protic and polar aprotic solvents to raise the yield of the products. It
provided yields of the product in the order 55%, 60%, 63% and 70% in ethanol, acetonitrile,
dimethylformamide and PEGꢀ400 respectively (Table 1, entries 10, 11, 12 and 13). So PEGꢀ400
was favored for its better activity as supportive medium. Henceforth, additional screening tests
were performed with PTA in PEGꢀ400 in order to find out the best reaction condition by varying
temperature and time (Table 1, entries 14, 15, 16, 17 and 18). Finally the optimum condition was
achieved with the reaction being carried out at 80°C for 2 hours by using PTA (10 mole %) in
PEGꢀ400 (3 ml) when 85% yield of the desired product (Table 1, entry 16) was obtained.
Scheme 1: Chemoselective CCO protocol for the synthesis of anthraimidazoledione derivatives
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Table 1. Optimization study using 1 with aldehyde 2ca
Oxidant/catalyst
(NH4)2S2O8 (1.1 mmol)
Conditions Time
Temp
r. t.
Yieldb (%)
Sl. No.
1
EtOH
ACN
10 h
4 h
0
2
NIS (1.2 mmol)
r. t.
22
32
15
10
35
20
10
20
55
60
63
70
75
80
85
85
85
3
NIS (1.2 mmol)
ACN
6 h
60°C
60°C
r.t.
4
LꢀProline (30 mol %)
EtOH
6 h
5
Polyphosphoric acid (10 mol %)
Phosphotungstic acid (10 mol %)
Phosphomolybdic acid (10 mol %)
Phosphotungstic acidꢀSiO2
Phosphotungstic acidꢀSiO2
Phosphotungstic acid (10 mol %)
Phosphotungstic acid (10 mol %)
Phosphotungstic acid (10 mol %)
Phosphotungstic acid (10 mol %)
Phosphotungstic acid (20 mol %)
Phosphotungstic acid (10 mol %)
Phosphotungstic acid (10 mol %)
Phosphotungstic acid (10 mol %)
Phosphotungstic acid (10 mol %)
CH2Cl2
CH2Cl2
CH2Cl2
ACN
10 h
20 h
20 h
10 h
6 h
6
r.t.
7
r.t.
8
r.t.
9
ACN
reflux
reflux
reflux
60°C
50°C
50°C
70°C
80°C
100°C
80°C
10
11
12
13
14
15
16
17
18
EtOH
6 h
ACN
6 h
DMF
6 h
PEGꢀ400
PEGꢀ400
PEGꢀ400
PEG-400
PEGꢀ400
PEGꢀ400
6 h
6 h
3 h
2 h
2 h
3 h
a Reaction: aldehyde (1 mmol), 1,2ꢀdiaminoanthraquinone (1 mmol)
bIsolated and optimised yield, r.t.= room temperature
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To study the extent and limitations of this CCO protocol we explored the new
methodology with variety of aldehydes possessing πꢀenriched aromatic/heteroaromatic rings and
lipophilic alkyl chains reacting with 1,2ꢀdiaminoanthraquinone 1 (Scheme 1, Table 2). In all
cases reaction progressed smoothly with good to excellent yields of the products. The reactions
were consistently executed at the 1 mmol scale and no change of the product yield was observed
when scaled up to the 10 mmol. High chemoselectivity was a rewarding aspect of this CCO
protocol as no undesired 1,2ꢀdisubstituted derivative was obtained. It was pertinent to mention
here that among the three types of aldehydes used, in terms of reaction times and isolated yields,
aliphatic aldehydes with a chain length of C4ꢀC16 (2mꢀ2q, Table 2) showed highest reactivity
followed by heteroaromatic aldehydes (2sꢀ2w, Table 2) and then aromatic aldehydes (2aꢀ2l,
Table 2). Metallocenealdehyde like ferrocene carboxaldehyde, 2r was equally reactive and
yielded 85% of 3r (Table 2) within 2 hours. All the new products were well characterized by
1
13
spectral (IR, H and C NMR) and elemental analyses. The known products were identified by
comparing the literature reports with their respective 1HꢀNMR spectra and melting points.
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Table 2. PTA in PEGꢀ400 catalyzed synthesis of anthra[1,2ꢀd]imidazoleꢀ6,11ꢀdionesa
3c, 2h, 85%15
3b, 2h, 83%11b
3f, 5h, 60%15
3j, 3h, 80%44
3a, 2h, 87%15
3d, 3h, 75%15
3g, 3h, 77%11b
3h, 3h, 75%15
3e, 4h, 82%11b
3l, 4h,80%10
3i, 4h, 70%13
3k, 3h,82%
3m, 1.5h, 96%15
3n, 1.5h, 92%15
3o, 1h, 97%
3p, 1.5h, 90%
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3t, 3h, 92%14d
3q, 2h, 93%
3r, 2h, 85%9
3s, 3h, 88%14d
3w, 3h, 82%14c
3u, 4h, 86%
3v, 3h, 85%
a Reactions: aldehyde (1 mmol), 1,2ꢀdiaminoanthraquinone (1 mmol); PTA (10 mol %); Solvent PEG 400; 80°C.
In order to comprehend the reaction pathway for the formation of anthraimidazoledione
derivatives involving PTA and PEGꢀ400, an experimental strategy was developed with aldehyde
2o since 3o was formed within a short period of time. A mixture of 2o and 1 was stirred in PEGꢀ
400 at 80°C for 45 minutes in absence of PTA and then quenched with water and worked up.
Compound 1 was returned back in almost quantitative amount without isolation of imine or
cycloꢀcondensed product. In a successive experiment, PTA (10 mol %) was added to the reaction
mixture after 45 minutes and the reaction was complete in next 1 hour with the formation of the
product, 3o. This result demonstrated the importance of both PTA and PEGꢀ400 for
cyclocondensation step which involved the aldehydic functional group of 2o with amino groups
of 1. PEGꢀ400 formed strong interꢀmolecular Hꢀbonds with its free hydroxyl groups and C9ꢀ
carbonyl oxygen of 1 thus reducing the strength of intraꢀmolecular Hꢀbond existing in compound
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1 as discussed earlier. Both of the amino groups would now be available on the catalyst surface
to react in tandem with aldehyde group in presence of PTA. At this stage it became imperative to
understand the oxidation step of this CCO reaction which was carried out under aerobic
condition. To investigate the role of aerial oxygen, if any, we repeated the experiment under dry
nitrogen atmosphere when an altogether different reaction pathway was identified after the
isolation of a novel compound, 2,3ꢀdipentylnaphtho[2,3ꢀf]quinoxalineꢀ7,12(1H,4H)ꢀdione, 3x
along with unreacted 1,2ꢀdiaminoanthraquinone 1 approximately in the ratio of 1:2 (Scheme 2).
Scheme 2. 1,2ꢀdiaminoanthraquinone 1 reacts with hexanal 2o in presence of PTA under
nitrogen atmosphere forming 3x
A plausible mechanistic pathway was presented (Scheme 3) to explain the formation of
3x under oxygenꢀfree condition. Initially 1,2ꢀdiaminoanthraquinone 1 would form the diimine
derivative 4 with hexanal 2o followed by a cascade pathway involving 6π electrocyclisation and
two consecutive 1,5ꢀ and 1,3ꢀprototropic shifts to produce 3x. Aromatization of the compound 3x
did not occur in absence of oxidative environment probably due to the formation of intraꢀ
molecular hydrogen bond between NꢀH proton and the Cꢀ11 carbonyl group of the quinone ring
1
which was reflected in the HꢀNMR spectrum of 3x. A broad singlet at δ 11.04 was assigned to
Hꢀbonded ꢀNHꢀ whereas the singlet at δ 6.89 was attributed to free –NHꢀ function. This type of
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6π electrocyclisation of diimine derived from aromatic 1,2ꢀdiamine and aldehyde in inert
atmosphere was not reported earlier in the literature.
Scheme 3. A plausible mechanistic pathway to the formation of 3x
Therefore from the above study it may be concluded that the aerial oxygen was required
for oxidative dehydrogenation of the dihydroimidazole intermediate leading to the formation of
heteroaromatic imidazole moiety and/or regeneration of PTA as oxidant from low valence states
of tungsten.16e
A study regarding recycling of PTA was also performed and it was successfully reused
four times without any pretreatment. To verify the property of reused PTA, field emission
scanning electron microscopy (FESEM) images of fresh and reused PTA (after first cycle) was
taken which indicated no obvious change in the morphology of the catalyst (see ESI, Fig. S1).
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The intrinsic nonꢀcovalent interactions of low molecular mass selfꢀaggregated organic
materials (LMSOM) are identifiable from its stacking pattern in the crystal lattice.4b, 28, 29 We
could obtain a single crystal of the compound 3t (Fig. 2) (more information see ESI).
100
80
60
40
20
0
1
2
3
4
5
6
No. of Runs
Fig 1: Reusability graph of PTA
Fig 2: Xꢀray crystallographic structure of compound 3t
To investigate the effect of the crystal structure on molecular aggregation, we had
performed analysis of the Xꢀray data of 3t using SHELXS 97,30 SHELXL 97,31 PLATON 99,32
ORTEPꢀ3233 and WINGX system verꢀ1.64.34 (Fig. 3a-d). It has been observed from the analysis
that several types of intermolecular hydrogen bonding interactions (Fig. 3a and 3b) are present
in the crystal lattice of 3t (see ESI, Table-S2). Besides these intermolecular Hꢀbonding
interactions, an interꢀmolecular π–π stacking interaction is also present between two phenyl rings
of anthraquinone unit of the two neighboring molecules with a centroid Cg⋯Cg distance of
3.596(4) Å to generate a 1D chain (Fig. 3c) along the crystallographic ‘c’ axis. Both Hꢀbonding
and π–π stacking interactions present in the crystal lattice of 3t formed a 3D network (Fig. 3d)
with a brickwork type arrangement containing a knot of water molecule. A bent and headꢀtoꢀtail
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array of the molecules in the crystal lattice indicated the Jꢀaggregation property of the
compound.4b
Figure 3: (a) Hydrogen bonding pattern between water molecule and anthraimidazoledione
moiety; (b) Hꢀbonding between carbonyl oxygen and the aromatic hydrogen atom of neighboring
molecule; (c) Intermolecular π–π stacking between the two phenyl rings of neighboring
molecule; (d) The 3D network of compound 3t having brickworkꢀlike arrangement with a water
molecule like a knot.
A powder XRD study was performed for 3t by using Xꢀray diffraction (XRD) analysis
applying Sherrer's formula Dp ¼ 0.941l/b cos q; where Xꢀray wave length (l) ¼ 1.5406 °A, q ¼
Bragg's diffraction angle for the planes (010), (110), (111), (200), (211), (220), (311), (222) and
(440) and b as the corresponding full width at half maximum (FWHM) value35 A comparison
between the simulated PXRD derived from single crystal diffraction data of 3t with the
experimental one clearly showed that it was pure and entirely crystalline in the bulk state (see
ESI, Fig. S2). Similar comparative PXRD studies between 3t and 3p (see ESI, Fig. S3) using
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the planes (010), (110), (111), (200), (210) and (310) for 3p also demonstrated the existence of
microꢀcrystallinity of 3p in its bulk state.35, 36
It is relevant to mention here that 3p and 3t varies in their substituent pattern at 2ꢀposition
of the imidazole ring with a thiophene ring for the former and a long aliphatic chain of ten
carbon atoms for the latter. Nonꢀcovalent interactions observed in the crystal lattice of 3t and the
microꢀcrystalline nature of both 3p and 3t as evinced from their respective PXRD data prompted
us to Scanning Electron Microscopy (SEM) study. Compound 3t in toluene formed wellꢀdefined
bundles of needleꢀshaped nanowires (width 129 nm) with a knot (Fig. 4a and 4b), whereas 3p
spontaneously aggregates as ordered hemispherical particle assemblies with several holes on the
surface ranging from 148ꢀ410 nm (Fig. 4c and 4d) in toluene. Presence of several hydrogen
bonding and πꢀπ stacking interactions within the framework of the πꢀextended planar molecule 3t
would be the main driving force for the formation of the aforesaid morphology. On the other
hand, molecule 3p assumes a unique amphiphilic nature bearing a polar head
(anthraimidazoledione part) and an elongated flexible hydrophobic tail (ten carbon alkyl chain)
which might compel the molecule to aggregate as an ordered particle assembly with holes on the
surface.37 Thus in case of 3p, morphology is dominated by the van der Waals interactions
involving the alkyl chains.37, 38
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Figure 4: (a) and (b) SEM images of compound 3t showing bundle of needleꢀshaped nanowire
with a knot; scale bar represent 1ꢁm and 200 nm respectively; (c) and (d) SEM images of
compound 3p showing an ordered hemispherical particle assembly with several holes on surface;
scale bar represents 1ꢁm and 200 nm respectively)
As LMSOMs exhibit unique optical and optoelectronic properties both in their solid and
solution state28 we took UVꢀVis absorption spectra of the representative compounds (1 × 10ꢀ6 M)
in polar aprotic and nonꢀpolar solvents and in the solid state (Fig. 5). In case of 3p, a
π−π∗ transition band of the aromatic ring appears at ~390 nm in toluene (1 × 10ꢀ6 M) and it
shows slight solvent dependency in acetonitrile solvent (λmax = ~380 nm, 1 × 10ꢀ6 M). The
absorption spectrum of 3p in the solid state is found to be comparatively broad and flat at the
peak position (Fig. 5a) may be due to the selfꢀaggregation in the solid state.39 The same
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π−π∗ transition of 3t (421 nm in toluene and 417 nm in acetonitrile, both concentrations are 1 ×
10ꢀ6 M) exhibited red shift due to extended conjugation (Fig. 5b). Here also absorption band in
solid is broad and flat at the peak position (Jꢀaggregation as per Xꢀray study). Systematic
concentration dependant UVꢀVIS spectroscopy experiments for both 3p and 3t revealed that with
increase in concentrations (from 10ꢀ6 to 10ꢀ4 M), the peak saturated at the OD value which were
almost equivalent to the spectra obtained in their solid state.
Figure 5: (a) Comparison of UVꢀVis spectra of the compound 3p in solution (1× 10ꢀ6 M) and
solid phase.
(b) Comparison of UVꢀVis spectra of the compound 3t in solution (1× 10ꢀ6 M) and
solid phase.
However 3p and 3t showed contrasting behavior in their emission spectroscopy.
Compound 3t exhibited emission maxima at 527 nm (λex = 421 nm) in toluene solution (1 × 10ꢀ6
M) and showed higher solvent dependency (557 nm in ACN solvent, λex = 417 nm, 1 × 10ꢀ6 M
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Fig. 6a). In solid form, it furnished slightly different spectral pattern with peak at 607 nm
(irradiated at ~502 nm) having higher fluorescence intensity (Fig. 6b). In comparison, compound
3p was found to be nonꢀemissive in acetonitrile (λex = 380 nm) as well as in toluene solution (λex
= 390 nm) (both concentrations are 1 × 10ꢀ6 M), but showed high fluorescence intensity (nearly
400 times) in the aggregated solid state (λem = 530 nm, irradiated at ~439 nm) as indicated in
Fig. 6a. Presence of long tail with high flexibility may be the cause of nonꢀemission property of
3p in solution (1 × 10ꢀ6 M) as flexible bonds usually open up nonꢀradiative channels.
Figure 6:(a) Comparison of fluorescence spectra of the compound 3p in acetonitrile (λex = 380
nm, 1 × 10ꢀ6 M) and toluene solution (λex = 390 nm, 1 × 10ꢀ6 M) and solid phase (λex= 439 nm).
(b) Comparison of fluorescence spectra of the compound 3t in ACN (λex = 417 nm, 1 × 10ꢀ6 M)
and in toluene solution (λex = 421 nm, 1 × 10ꢀ6 M) and solid phase (λex= 502 nm).
Citations of some particular organic entities having enhanced emission in their solid state
are correlated with the intraꢀ and intermolecular forces present in the molecules.39 Intraꢀ
molecular effects like conformational aspects of the chromophore influence the radiation process
in solid state or in solution phase.40 Interꢀmolecular interactions such as molecular aggregation
(Hꢀtype or Jꢀtype) are reported to control the fluorescence efficiency of the molecule.41 Generally
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Jꢀaggregated molecules showed comparatively higher fluorescence intensity due to their bent or
headꢀtoꢀtail arrangement with a red shift in UV absorption maxima.41 But the unusual strong
fluorescence efficiency of the compound 3p in Jꢀaggregated form compared to nonꢀemissive
nature of its acetonitrile or toluene solution can be explained in terms of nonꢀradiative channels
operated through flexible bonds. In 3p, intermolecular forces cause aggregation induced rigidity
resulting in high solidꢀstate emission. But in solution, both in polar or nonꢀpolar medium,
conformational aspects of the long flexible aliphatic chain is dominating, thus favoring energy
dissipative paths and it becomes nonꢀemissive. As a consequence, 3p imparts a fluorescence
“ONꢀOFF” mechanism from solid to solution state. In case of 3t, structural flexibility is less and
it shows emission in the solution phase and enhances its emission intensity in the solid by the
formation of Jꢀaggregated state.
The results obtained from emission spectrum prompted us to carry out DFT calculations
for 3p and 3t. The HOMOꢀLUMO pictures obtained by using B3LYP/6ꢀ311++G as the
functional and basis set showed that HOMOs for both molecules are localized on their respective
substituent arms whereas the LUMOs are contributed by the anthraquinone units (see ESI, Fig.
S4). For this reason, photophysical properties of these molecules would be guided by the nature
of the substituent present at the 2ꢀposition of the imidazole ring as observed in this case.42
Considering all the morphological and photophysical phenomena we may anticipate that
anthraimidazolediones containing a lipophilic long aliphatic chain and polar anthraquinone
moiety could find application in the field of organogelators.37 Furthermore, the holes present in
the SEM structure of 3p are very unusual which might find use as a receptor site or host or a
transporter.43 Both 3p and 3t, due to their remarkable fluorescent nature, are eligible candidates
for application as chemical sensors.
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Conclusions
In conclusion, a straightforward tandem green protocol was developed for the
chemoselective synthesis of anthra[1,2ꢀd]imidazoleꢀ6,11ꢀdione derivatives via CCO reaction
employing PTA as reusable catalyst on PEGꢀ400 support in open air. In inert atmosphere,
methodology adopted a complete different pathway leading to the formation of unique naphthoꢀ
quinoxaline derivative. Wide range of applicability of the reaction procedure was proved by
using an array of aldehydes which furnish good to excellent yields of the anthraimidazoledione
derivatives. Single crystal data of compound 3t reveals distinct crystal packing pattern with πꢀπ
stacking and hydrogen bonding interactions. 3p and 3t, bearing different substituents at 2ꢀ
position of the anthra [1,2ꢀd]imidazoleꢀ6,11ꢀdione moieties, showed microꢀcrystalline nature in
bulk state from PXRD studies and different morphologies in their corresponding SEM images
which justified the effect of substituents. DFT studies of aforesaid compounds revealed
similarities in their HOMOꢀLUMO characteristics. Both molecules show comparable Jꢀ
aggregation behavior and form LMSOMs in their solid state as evidenced from photophysical
studies. Further studies related to the generation of novel nanomaterials based on this scaffold
possessing interesting optoelectronic and gelation properties and also as chemosensing agents are
in progress in our laboratory.
Experimental
General reaction procedure for the formation of anthra [1,2-d]imidazole-6,11-diones (3a-
w):
A mixture of 1,2ꢀdiaminoanthraquinone 1 (1 mmol), aldehydes 2 (1 mmol) were stirred at 80°C
in an oil bath in 3 mL PEGꢀ400 support in open air. To this stirring mixture, phosphotungstic
acid (10 mol %) was added and stirred for stipulated time. After completion (as monitored by
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TLC), the reaction mixture was cooled to 0ꢀ5°C and extracted with ethyl acetate. The reaction
mixture was filtered and the filtered out solid PTA was washed with acetone and dried under
vacuum for reuse. Ethyl acetate part was washed with water thrice to remove PEGꢀ400 and
evaporated under vacuum. Pure product was isolated by column chromatography over silica gel
using different mixtures of petroleum ether: ethyl acetate.
Known products were compared with their m.p. and spectral data as obtained from concerned
literature. Spectral data of representative compounds, 3p and 3t are listed below;
(±)2-(2,6-dimethylhept-5-enyl)-1H-anthra[1,2-d]imidazole-6,11-dione (3p). Yield: 242 mg,
1
90%, yellow crystalline solid; Mp 149ꢀ150°C; H NMR (300 MHz, DMSOꢀd6) δ 12.94 (s, 1H),
8.09ꢀ8.05 (m, 2H), 7.88 (s, 2H), 7.80ꢀ7.77 (m, 2H), ca. 4.96 (m, 1H), 2.85ꢀ2.83 (m, 1H), 2.74ꢀ
2.71 (m, 1H), 2.10ꢀ1.89 (m, 2H), 1.51 (s, 3H), 1.44 (s, 3H), 1.35ꢀ1.23 (m, 1H), 1.20ꢀ1.09 (m,
2H), 0.81(d, J = 6.6 Hz, 3H); 13C NMR (75 MHz, DMSOꢀd6) δ 183.8, 182.9, 162.3, 149.7, 134.8,
134.6, 133,6, 133.4, 132.3, 131.1, 127.6, 127.2, 126.6, 124.9, 124.7, 120.6, 118.3, 36.9, 35.9,
32.6, 25.9, 25.4, 19.8, 17.9; IR υ
max (KBr) cmꢀ1 3737, 3359, 2927, 2365, 1655, 1579, 1509; anal.
calcd for C24H24N2O2: C: 77.39, H: 6.49, N: 7.52 %, found: C: 77.37, H: 6.50, N: 7.50 %.
2-(thiophen-2-yl)-1H-anthra[1,2-d]imidazole-6,11-dione (3t). Yield: 280 mg, 92%, brown
crystalline solid; Mp 282ꢀ283°C; 1H NMR (300 MHz, DMSOꢀd6) δ 13.40 (s, 1H), 8.51 (d, J = 3
Hz, 1H), 8.21ꢀ8.16 (m, 2H), 8.03 (s, 2H), 7.91ꢀ7.89 (m, 2H), 7.83ꢀ7.82 (m, 1H), 7.26ꢀ7.25 (m,
1H); 13C NMR (75 MHz, DMSOꢀd6) δ 183.7, 182.8, 153.6, 149.7, 134.9, 134.7, 133.6, 133.2,
132.8, 132.1, 131.5, 130.9, 129.2, 128.4, 127.3, 126.7, 124.9, 121.7, 118.9; IR υmax (KBr) cmꢀ1
3539, 3390, 3075, 2925,1655, 1577, 1560; anal. calcd for C19H10N2O2S: C: 69.08, H: 3.05, N:
8.48 %, found: C: 69.06, H: 3.06, N: 8.45 %.
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General reaction procedure for synthesis of 2,3-dipentylnaphtho[2,3-f]quinoxaline-
7,12(1H,4H)-dione (3x):
A mixture of 1,2ꢀdiaminoanthraquinone 1 (1 mmol), aldehyde 2o (1.2 mmol) were stirred at
80°C in an oil bath in 3 mL PEGꢀ400 support under N2ꢀatmosphere. To this stirring mixture
phosphotungstic acid (10 mol %) was added and stirred for one hour. Then the reaction mixture
was cooled to 0ꢀ5°C and extracted with ethyl acetate. Ethyl acetate part was washed with water
thrice to remove PEGꢀ400 and evaporated under vacuum. Pure product was isolated by column
chromatography over silica gel using different mixtures of petroleum ether:ethyl acetate.
2,3-dipentylnaphtho[2,3-f]quinoxaline-7,12(1H,4H)-dione (3x). Yield: 150 mg, 37%, yellow
crystalline solid; Mp 124ꢀ126°C; 1H NMR (300 MHz, CDCl3) δ 11.04 (br. s, 1H), 8.28ꢀ8.18 (m,
4H), 7.77ꢀ7.45 (m, 2H), 6.89 (br. s, 1H), ca. 2.69 (m, 2H), ca. 2.35 (m, 2H), 1.50ꢀ1.32 (m, 12H),
0.90ꢀ0.86 (m, 6H); 13C NMR (75 MHz, CDCl3) δ 185.1, 182.2, 157.4, 134.8, 134.5, 134.0, 133.9,
133.0, 131.5, 129.1, 127.8, 126.6, 124.3, 122.9, 118.0, 31.7, 31.5, 29.1, 28.7, 27.7, 22.7, 22,5,
13.9; IR υmax (
KBr) cmꢀ1 3326, 2925, 2854, 1660, 1583, 1519; anal. calcd for C26H30N2O2: C:
77.58, H: 7.51, N: 6.96 %, found: C: 77.56, H: 7.50, N: 6.94 %. HRMS (TOF MS ES+): m/z
calcd for (MꢀH)+ C26H30N2O2ꢀ 403.2385; found 403.2339.
Acknowledgements
We gratefully acknowledge the financial support received from the University of Calcutta.
Crystallography was performed at the DSTꢀFIST, Indiaꢀfunded Single Crystal Diffractometer
facility at the Department of Chemistry, University of Calcutta. We would like to thank the
Centre for Research in Nanoscience and Nanotechnology, University of Calcutta for providing
SEM and FESEM facilities. We express our gratitude to Lakshmi Kanta Das and Aniruddha
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Ganguly for their valuable suggestions. We further express our gratefulness to Dr. Dipankar
Chakraborty for providing powder XRD facilities.
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Graphical Abstract
One-pot protocol for structurally tailored J-aggregated anthra[1,2-d]imidazole-6,11-dione was
developed via tandem cyclocondensation cum oxidation, catalyzed by heterogeneous, reusable
phosphotungstic acid in PEG-400 support in presence of air. Photophysical studies confirm their
strong J-aggregation properties while they exhibit contrasting fluorescence behaviors during
transition from solution to nanostructured solid state.