Emergence of s-heptazines: from trichloro-s-heptazine building blocks to functional materials

Sunil Kumar , Neha Sharma and Kamalakannan Kailasam *
Institute of Nano Science and Technology, Habitat Center, Phase 10, Mohali 160062, Punjab, India. E-mail: kamal@inst.ac.in

Received 7th June 2018 , Accepted 7th August 2018

First published on 7th August 2018

The barrier in any particular application is the material design and development of effective materials based on it. Currently, the major focus is on the development of material classes for energy conversion, storage and environmental applications. Thus material design and especially porous materials have created interest in researchers to search for a new class of materials. Polymeric carbon nitrides (g-CN, so-called g-C3N4) are such interesting materials which have been explored from the beginning of the 21st century as an active photocatalyst. The basic constituting unit of g-CN, heptazine, has driven scientists to develop various functional materials. Thus the heptazine class of compounds is an indispensable unit due to its rich-properties and demands extensive research for their exploration in many other applications. This review addresses the recent efforts by researchers on various material-designs based on heptazine and their startling applications from molecular compounds to porous frameworks. Furthermore, a timely and concise review on these emerging heptazines would be of interest for researchers as well as the scientific community. In addition, the probable application areas where these materials can have a deep impact on material design are discussed.

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Sunil Kumar

Dr Sunil Kumar currently holds a National Postdoctoral fellowship (NPDF) as a researcher in the Institute of Nano Science and Technology (INST) since March 2017. Here in Dr. Kailasam's research group (https://drkamalsgroup.com/) his research interests are in developing organic materials for organic solar cell and photocatalytic applications. He has completed his PhD degree from Indian Institute of Technology (IIT), Mandi, where he did research on the development of fluorescent organic materials for OLED applications.

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Neha Sharma

Neha Sharma is a PhD candidate in the Institute of Nano Science and Technology (INST) under the direction of Dr Kailasam. She received her M.Sc. degree from S.G.G.S. Khalsa College, Mahilpur (affiliated to Panjab University). Her research focuses on the development and characterization of novel organic porous materials for CO2 sorption and organophotocatalysis.

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Kamalakannan Kailasam

Kamalakannan Kailasam is working as Scientist-E/Associate Professor in the Institute of Nano Science and Technology (INST), Mohali, Punjab, India. He obtained his PhD with the late Prof. Klaus Müller in Universität Stuttgart, Germany in 2008 followed by a postdoctoral stint at Max Planck Institute for Colloids and Interfaces, Potsdam, Germany in the Colloid Chemistry department in 2009–2010. Then he joined Technische Universität Berlin, Germany under Professor Arne Thomas in 2010 and stayed until March 2015. He joined his current position at INST Mohali in April 2015. He is a Materials Chemist and leading the “Advanced Functional Nanomaterials” group working on material development for various energy and environmental applications. Especially his interest lies in photocatalytic conversions which include water splitting, biomass to fine chemicals and CO2 reduction. In addition, his group has broad interest in organic photovoltaics, fuel cells, humidity and VOC sensing and biotechnology applications.

1. Introduction

The research on developing effective materials as a solution to the growing energy demand and environmental remediation is of high importance. Carbon nitride based materials are becoming a hotspot and are being used to replace conventional catalysts and/or photocatalysts which are being applied in wide range of applications from catalysis to antibacterial activity.1–5 The exceptional properties which make carbon nitride a material of high importance are its electronic band structure, physicochemical stability, low cost synthesis and in addition, its scalability perspectives make the large-scale synthesis easy.1,6,7 Moreover, its response to visible light and metal-free nature have made it a first choice material for metal free photocatalysis.6,8

The structural confirmation of polymeric-CN has a long history and some initial structures were reported9–14 but the efforts of Komatsu and Schnick helped to confirm that the structural building motif of graphitic-CN is the tri-s-triazine unit.10,15–17 Thus, the tri-s-triazine nucleus is the constituting basal unit of polymeric-CNs and hence, understood to be the smallest functioning unit responsible for catalytic activity. The fused tricyclic core (Fig. 1a) termed tri-s-triazine is also called the s-heptazine or cyamelurine core and these names have been used in this review alternatively. This s-heptazine system has not been explored much due to its solubility issues in common organic solvents even though it is one of the oldest moieties reported in the 19th century.

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Fig. 1 (a) Heptazine nucleus, (b) melem, (c) cyameluric acid, (d) cyameluric chloride, (e) Pauling's mystery molecule and (f) tri-s-triazine.

The extensive review article by Kroke et al., documented the journey of the s-heptazine nucleus, irrespective of the heptazine precursor used, in both molecular and polymeric materials.18 Since then, complex molecular and polymeric derivatives containing similar basic units to that of carbon nitride have been reported. Until today trichloro-tri-s-triazine (also generally called cyameluric chloride or heptazine chloride or trichloro-s-heptazine), a soluble member of the heptazine family, has been extensively used to incorporate peripheral functionality using nucleophilic displacement reactions in organic solvents. Thus, the aim of this review is to describe the key advances in the materials field where trichloro-tri-s-triazine was used to develop effective materials (both molecular and polymeric compounds) which were used in various applications. We structured our article in such a way that we will first discuss in brief about the emergence of heptazine structures from melon followed by a discussion on the molecular derivatives of heptazine as well as porous polymeric materials. In addition, the detailed applications of these heptazine based derivatives are discussed along with probable areas where these materials can be applied in the near future.

2. From the heptazine nucleus to trichloro-tri-s-triazine

The ignition of mercury(II) thiocyanate (Hg(SCN)2) was first studied by Berzelius and then by Liebig and both reported the observation of formation of a yellow residue in 1835.19 Later Liebig and Gmelin predicted the composition of the yellow residue to be (C2N3H)n and named the polymer melon.19,20 Eventually, melem, cyameluric acid and cyameluric chloride (Fig. 1a–c) were also reported in the period 1830–1844.21–23 These early compounds had solubility issues and thus, the characterization of these derivatives was based on the elemental analysis only.

Although melon was known to the scientific community since the 1830s, the structure of the polymer was unsolved for more than a century until Pauling and Sturdivant (in 1937) reported the structure as a fused tricyclic 1,3,5-triazine unit which is today known as tri-s-triazine or s-heptazine as the basal structural unit of the polymer.24 Since then, Pauling remained interested in this class of compounds as a molecular structure, 2-azido-5,8-dihydroxy-1,3,4,5,7,9,9b-hepta-azaphenalene, having a heptazine core retained on his chalkboard (Fig. 1e). When asked in an interview about the molecule, Pauling responded “similar substance with only one ring can be made into certain derivatives that have anti-cancer activity. And I thought that this substance with only three rings might well operate in the same way and that we should study it”.25 After Pauling's death, this molecule came to the limelight in the year 2000 with the name “mystery molecule” when Chemical and Engineering News asked its readers to give their views about the mystery molecule.26,27 It is believed that he intended to use the compound as a potential spectroscopic label for binding to DNA for anticancer activity but no such studies have been carried out till now.

Following the report of Pauling and Sturdivant, Redemann and Lucas (1940) successfully accounted for the structure of cyameluric acid and even proposed structures for Berzelius' melon with the empirical formula C6H3N9.28,29 The structural analogy between tri-s-triazine and s-triazine ring systems (cyameluric acid and cyanuric acid, respectively) may have initiated the attempts for fabricating the trichloro-derivative of cyameluric acid. The first synthesis of trichloro-s-heptazine was reported by Redemann and Lucas (1940) through a solid phase reaction by reacting K-melem/cyameluric acid with PCl5 in a bomb tube.29 But the first comprehensive synthesis along with extensive characterization data was published in 2002 by Kroke et al.30

The parent molecule of the cyamelurine class is a H-substituted tri-s-triazine core (C6N7H3, Fig. 1f) which was first discussed in theoretical studies carried out by J. Wirz (1980)24,31 and successfully synthesized by Leonard and coworkers (1982) along with comprehensive structural analysis.32,33 From the small outlook presented above it is clear that the heptazine class of compounds were known for two centuries but were the least explored organic moiety because of difficulty in their synthesis protocols and their insolubility in the majority of organic solvents. More precisely, synthesis of three fused aromatic rings of heptazine and their further derivatization to useful molecular compounds would require a starting heptazine core which could be functionalized through known organic synthesis protocols. This is where synthesis of trichloro-s-heptazine (TCH) made a phenomenal breakthrough which has made synthesis of several compounds and polymer networks feasible now. This is possible because the analogous nature of trichloro-s-heptazine with acid chlorides makes it more susceptible towards nucleophilic substitution. This also reminds us of the article published in 2004 titled “Old Molecule, New Chemistry” where the author described that the long-mysterious heptazines are finding use in synthesizing carbon nitride materials, especially g-C3N4 which is commonly known and has revolutionized the field of material science with a variety of applications.34 Polymeric graphitic carbon nitrides, g-C3N4, are not the point of discussion in this review as a lot of reviews are available in this regard.2,3,35

3. Molecular compounds from trichloro-s-heptazine (TCH)

Initial investigations on trichloro-s-heptazine were based on the equal reactivity of the three chloro-groups and thus symmetrically substituted derivatives were reported initially.36 Symmetrical substitutions of all three chlorine atoms were reported while mono- and di-substituted heptazines were found to be difficult to synthesize.36–38 The easy synthesis of trichloro-s-heptazine (λmax,abs = 310 nm, λmax,PL = 466 nm) by Kroke's group in 2002 might have attracted researchers toward the cyamelurine class of materials. Later, Kroke and his co-workers synthesized numerous heptazine based compounds.18,30 Meanwhile, Gillan and co-workers (2004) reported the synthesis of 2,5,8 triazido-s-heptazine (Fig. 2a) by heating trichloroheptazine with neat trimethylsilylazide and the corresponding crystal structure of the product was also reported.39 The high nitrogen content of the molecule was supposed to exhibit explosive/propellant properties. Further, they utilized this molecular heptazine derivative for synthesis of nitrogen rich carbon nitride (CNx) like polymer networks.40 Later in 2004, a diethylamino-substituted heptazine derivative, 2,5,8-tris(diethylamino)-1,3,4,6,7,9,9b-heptaazaphenalene (Fig. 2c), was introduced by Oeser and Wortmann as a novel non-linear optical chromophore.41 The second order polarizability tensor of the diethylamino-substituted heptazine derivative was comparable to pNA (4-nitroaniline, used as the external standard) despite its short conjugation length and short wavelength absorption. Due to the D3h symmetry, the compound had no resultant dipole moment so it was proposed that asymmetrically substituted heptazines with C2v symmetry may have improved NLO properties. In spite of this prediction, further reports exploiting NLO properties have not been reported.
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Fig. 2 (a) Molecular compound synthesized by Gillan and co-workers, (b) Zhou group, (c) Oeser, Wortmann and co-workers.

In 2006, Zhou's group reported the Friedel–Crafts reaction of cyameluric chloride with toluene followed by selective oxidation of the methyl group into carboxylate to give a noble ligand [2,5,8-tris(p-benzoate)-1,3,4,6,7,9,9b-heptaazaphenalenic acid] (Fig. 2b). The Friedel–Crafts reaction on cyameluric chloride is a useful addition to the chemistry of cyamelurines.42

Kroke et al. (2006) hoped to obtain crystalline porous covalent frameworks using the exchange reaction of dichlorodimethylsilane with silylester of cyameluric acid, but instead they obtained gel-type materials.43 The synthesis through a sol–gel approach for gel compounds was irreversible in nature implying that the gel products were comprised of oligomeric species containing an alternating arrangement of heptazine units and organometallic Si–O motifs.

The chemical structure of trichloro-s-heptazine has three equally reactive chlorine atoms and thus asymmetric substitutions are difficult to carry out. Also, asymmetrically substituted heptazines were not reported until 2010 when Kroke et al. reported di-substituted heptazine leaving one chloro-group available for further functionalization.37 It was also observed that successive substitutions with diphenylamine units decreased the reactivity of the last chloro-atom on the heptazine core. The di-substituted product was obtained as an inclusion compound with p-xylene and crystals were grown from the same. But, to date asymmetrically substituted heptazines have been rarely synthesized which make it a promising area for synthetic chemists to design new materials.

Molecular materials using trichloroheptazine have found its worth for OLED applications when Adachi et al. in 2013 reported a thermally activated delayed fluorescence (TADF) active compound (Fig. 3).44 The electron deficient heptazine ring was decorated with substituted triphenylamine (TPA, an electron donor unit) using Friedel–Crafts arylation procedure. Theoretical studies revealed that the HOMO is mainly distributed over the TPA unit whereas the LUMO is localized over the heptazine core which resulted in small ΔEst (energy gap between the lowest singlet state (S1) and the lowest triplet (T1) state) due to the minimum overlap between HOMO and LUMO charge densities. The small ΔEst is necessary to induce the delayed fluorescence process. The synthesized compound, HAP-3TPA, exhibited a high luminescence efficiency of 95% (89.1%, prompt component and 6%, delayed component for toluene solution). The OLED devices fabricated using this material as the active layer possessed a turn-on voltage of 4.4 V and maximum efficiency of 17.5 + 1.3% in spite of weak delayed fluorescence in the solution state. They also proved that the solution state weak delayed fluorescence process was enhanced upon device fabrication and under electrical excitation. The delayed fluorescence occurred due to reduced energy gap between the excited singlet and triplet energy states (ΔEst), which was a result of twisting around the donor–acceptor conjugation bond. In another report in the following year, the same group used a blended hetero-junction containing heptazine derivative (2,5,8-tris(4-fluoro-3-methylphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (HAP-3MF)) in mCP (1,3-di(9H-carbazol-9-yl)benzene) as the emitter layer in an orange emitting OLED.45 HOMO and LUMO energies were estimated to be −6.0 eV and −3.4 eV, respectively, using ultraviolet photoelectron spectroscopy. The utilized exciplex emitter system showed emission at 550 nm with a quantum efficiency of 55.7% and was more efficient than only the HAP-3MF fluorophore having 12.7% quantum efficiency with emission at 526 nm.

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Fig. 3 (a) Molecular structure of HAP-3TPA, (b) theoretically calculated distribution of electron density in the HOMO and LUMO and (c) normalized EL spectrum and normalized PL spectrum of 6 ± 1 wt% HAP-3TPA:26mCPy (d) Current density–voltage-luminance characteristics. Reprinted from ref. 44 with permission from Wiley.

Not only organic molecules, but metal–organic complexes with small molecules are also reported using heptazine as a core. In a modified approach, Coca and Ruiz (2015) reported microwave assisted synthesis of a symmetric tri-substituted heptazine ligand with more than 70% yield in short time (15 min).46 This ligand with a di-2-pyridylamino group on the exterior side of the heptazine core was used to develop a trinuclear Cu-complex which was soluble and thus, was characterized easily. Although the Cu-complex might be useful in the supramolecular field as well as the development of new photocatalysts, studies exploring it were not reported.

One of the problems associated with the structural characterization of carbon nitrides is their insolubility and the structural characterization based on solid state characterization techniques is inefficient and not always conclusive. An easy solution to this problem was reported by Gambarelli and Dubois by synthesizing soluble molecular and oligomeric species containing heptazine.47 Diisobutylamine was selected as the peripheral substituent and the corresponding trisubstituted heptazine derivative (termed monomer) was synthesized. Two more molecular compounds containing two (dimer) and three heptazine rings (trimer) conjugated through N(butyl)-bridges were also synthesized. These three compounds were relatively soluble in common organic solvents. As the number of conjugated heptazine units increased from the monomer to the trimer a bathochromic shift in low energy absorption transitions was observed and the same was supported by theoretical studies. The electrochemical analysis revealed that while the oxidation voltage was almost unaffected, the first reduction potential was decreased on going from the monomer to the trimer. Moreover, due to the soluble nature of the monomer and oligomers, they were considered as suitable models to elucidate structural information on polymeric carbon nitrides. Moreover, their efforts to synthesize asymmetrically substituted heptazine monomers could not be neglected as asymmetrically functionalized heptazine are highly scarce.

The rigid, planar heptazine nucleus with C3 symmetry is ideal to achieve columnar packing and to get discotic liquid crystals which are ideal candidates in the organic optoelectronic field. A detailed study by Pal and Kailasam et al. highlighted the effect of alkoxy chain length on the self-assembly behavior of trialkoxy-substituted heptazine (Fig. 4).48 The shorter alkoxy chains showed a smectic phase whereas the long chain derivative exhibited the hexagonal columnar mesophase. The solid state blue emission of the compounds attracted the authors as the solutions of the compounds were non-fluorescent. The self-assembly property of the 3,4,5-tris(dodecyloxy)-aniline functionalized heptazine core was also reported by Yang et al.49 A room temperature discotic liquid crystal (DLC) with hexagonal assembly having an improved band gap of 1.63 eV was reported. A similar report exploring the DLC behavior of the heptazine core from Kumar et al. highlighted that the electron deficient nature of heptazine in unification with electron donor units is a useful material design for n-type organic semiconductors.50 These reports on DLCs using TCH added a new electron acceptor core in the liquid crystal application and also show a promising future for such materials in the optoelectronic field (Fig. 4).

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Fig. 4 (a) Molecular structure of compound 3 (3,4,5-trialkoxyphenylamine-substituted heptazine derivatives), (b) compound 3b in the Sm phase; (i and ii), hydrogen-bonded, ribbon-like aggregates of the heptazine ring; (iii) assembled structures forming layers in the Sm phase. (c) Compound 3c in the Colh phase. (i and ii) Molecules modeled as a disc amassed into the column; (iii) assembled structures forming the columnar hexagons; (d) columnar textures of compound 3c at 80 °C, observed on cooling from the isotropic liquid; (e) normalized PL spectrum and (f) photo showing strong blue-light emission under UV light in the solid state for 3c. Reprinted from ref. 48 with permission from Wiley.

Pal and co-workers recently reported the solid state emission and AIE property of the heptazine based discotic liquid crystal, Hpz-3C12 having an optical band gap of 2.91 eV exhibiting sky-blue emission around 475 nm (Fig. 5).51 They also explored the electrochemical stability and carried out OLED device fabrication. The electrochemical band gap was found to be 0.91 eV less than the optical band gap and the oxidation potential for the molecule was less than 1.0 V. DFT studies showed localization of HOMO and LUMO electron densities over the peripheral tri-alkoxy benzene group and heptazine core, respectively. The fabricated OLED devices showed blue light electroluminescence when used as guest emitters in various host matrices. The device exhibited blue emission with an EQE of 1.6% in a CBP (4,4′-bis(N-carbazolyl)-1,1′-biphenyl) host. In another report on LC behavior from the Pal and Kailasam group, synthesized heptazine derivatives, Hpz-Cm (m = 6, 10) showed columnar liquid crystalline assemblies upon complexation with tri-alkoxy benzoic acid.52 The fluorescence properties of these H-bond mediated supramolecular assemblies Hpz-Cm/An (m = 6, 10; n = 10, 12, 16) were also reported. Interestingly, the emission of supramolecular assemblies of the heptazine derivative, Hpz-Cm/An (m = 6, 10; n = 10, 12, 16), in the solid state was blue shifted from the emission of the pure heptazine derivative, Hpz-Cm (m = 6, 10), in the solution state, whereas in the solid state, emission of supramolecular assemblies shifted to longer wavelengths compared to the solid state emission of heptazine based fluorophores.

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Fig. 5 (a) Chemical structure of Hpz-3C12, (b) fluorescence spectra of Hpz-3C12 in the solid state and in the solution state (chloroform), (c) doping concentration effects on the EL spectra of the solution-processed blue OLED devices using a CBP host with 1, 3, and 5 wt% dopant concentrations at brightness 100 cd m−2, (d) CIE coordinates obtained for the device at 3 wt% dye concentration with CBP, spiro-2CBP, mCP and TCTA hosts. Inset shows the image of the fabricated device. Reprinted from ref. 51 with permission from Wiley.

Theoretical studies concerning the electron density distribution on the tri-s-triazine ring were reported by the LeBreton and Leonard group and it was indicated that the lone pair of N atoms in the tri-s-triazine system exhibits significant delocalization of electron density.53 The detailed valence orbital structure also helped to understand the chemical nature and stability of this tri-s-triazine system as it was reported that the high degree of delocalization of lone pair orbitals was responsible for its weakly basic nature. The Wong and Tian group reported theoretical results on geometric and electronic structures of the parent heptazine system along with its ten symmetrically tri-R-substituted molecular compounds (Fig. 6, R = –NH2, –OH, –N3, –NO2, –F, –Cl, –Br, –CN, –HC[double bond, length as m-dash]CH2, and –C[triple bond, length as m-dash]CH).54 All compounds exhibited planar geometries and the significant conjugation over the parent heptazine ring explained the stability of this system. An increase in C–N bond lengths (both C–N and C[double bond, length as m-dash]N) was observed when the heptazine was substituted with electron donor motifs (–NH2, –OH, –N3, –CN, –HC[double bond, length as m-dash]H2, and –C[triple bond, length as m-dash]CH) and vice versa. Moreover, the substituent effect on the electronic structure, HOMO and LUMO was also discussed. Theoretical calculations indicated that the HOMO of each molecule was localized on the peripheral N-atom while the LUMO was shared by every atom present in the tri-s-triazine system. It was also calculated that the tri-substituted heptazine derivatives would have a comparatively increased band gap and thus a blue shift in the absorption wavelength could be observed compared to unsubstituted heptazine. The same group also reported the theoretical aspects on mono-substituted heptazine system and almost similar results were produced to that of tri-substituted products.55

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Fig. 6 Molecular derivatives of heptazine from theoretical studies reported by Wong and Tian.54

Nucleophilic substitution reactions using primary amines (aromatic and aliphatic) on the trichloroheptazine unit have been successful. But, the scope of other nucleophiles has not been considered. Very recently, Kroke et al. reported the first thio-, seleno-, phosphine and phosphine oxide derivatives of trichloro-s-heptazine.56,57 So, the search for nucleophiles could be an interesting topic for researchers to explore molecular materials. As the molecular compounds of heptazines reported above are finding a place in interesting applications, further synthesis of new molecular derivatives is still an exciting and challenging area for synthetic chemists for finding their potential application.

4. Porous polymeric frameworks

Apart from molecular compounds, development of heptazine based organic/inorganic polymeric networks/materials has been an area of interest for the researchers to explore them in a wide variety of applications. The high reactivity of trichloroheptazine toward nucleophilic attack has been utilized to develop frameworks for potential application in gas storage, heterogeneous catalysis for organic conversions, water splitting, etc., and has been explored recently and discussed in this section.

The first example of polymer frameworks synthesized using a heptazine core as a monomer was reported in 2007 by Gillan and co-workers where they utilized triazido-s-heptazine (TAH) to synthesize nitrogen rich carbon nitride-like networks (Fig. 7).40 Thermal analysis of TAH showed its stability up to 185 °C after which the compound decomposed violently with exothermic heat release.

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Fig. 7 (a) Structural speculation for TAH CNx products resulting from thermal decomposition under inert conditions with minimal acetone in the precursor, (b) followed by air exposure and (c) under inert conditions with acetone present in the precursor. Reprinted from ref. 40 with permission from the American Chemical Society.

Bojdys and Thomas et al. (2010) utilized the interesting heptazine as a useful toolbox and developed expanded polymer frameworks using different interconnecting linkers between heptazine units through strong covalent bonds.58 This approach would provide not only chemically and thermally stable frameworks but also ordered porous and light-weight materials. Although, they did not use the heptazine containing monomer, they synthesized interesting heptazine frameworks by using triazine and carbonitrile functionality containing monomers. They reported 1,4-aryl (HBF-1), 4,4′-aryl (HBF-2) and 1,3-aryl (HBF-3) bridged heptazine networks which were synthesized in LiBr/KBr eutectic melt and preliminary studies comprising water adsorption characteristics were also reported. It was proposed that the polymers could be exfoliated to mono- and oligo-layers and thus further applications could be explored.

A heptazine based metal–organic framework (MOF) was developed and reported for the first time in 2006. The Zhou group reported an s-heptazine tribenzoate ligand (HTB) considering the pi–pi stacking (dimerization) of planar units, and incorporated the dimers (also nicknamed Piedfort ligand pairs) into the zinc based MOF.42 The resulted Zn based MOF possessed a chiral network and exhibited high solvent accessible volume (84.4%). But the framework was so unstable and collapsed whenever the mother solvent was removed. In another work, an rht-MOF containing trichloroheptazine as the core (rht-MOF-9) was reported by Eddaoudi et al.59 This highly porous rht-MOF-9 exhibited promising CO2 and H2 adsorption properties owing to the affinity of the nitrogen-rich polycyclic core toward CO2. Shi et al. also reported the rht-MOF (2014) which exhibited a high surface area of 2171 m2 g−1 with high gas adsorption capabilities for C2 hydrocarbons and carbon dioxide with excellent selectivity over methane.60 It was also mentioned that such rht-MOFs could find application in the methane purification process but no further studies have been reported till today.

The first report on utilizing the heptazines as building blocks to synthesize microporous polymer networks was reported by Kailasam and Thomas et al. in 2013 (Fig. 8) by utilizing trichloroheptazine as the starting building block.61 They prepared a model framework by using simple aryldiamines with trichloroheptazine to obtain extended networks, HMPs (Heptazine based Microporous Polymeric Networks). The frameworks HMP-1 with a surface area of 185 m2g −1 and HMP-2 with 169 m2 g−1 were reported. The corresponding hydrogen evolution under UV irradiation for HMP-1 and HMP-2 was about 21 (and 2 μmol H2 h−1 for λ > 420 nm) and 18 μmol H2 h−1, respectively. A total of 44 μmol H2 gas was observed over a period of 20 h (λ > 420 nm) in four consecutive photochemical runs without any deactivation of the HMP-1 photocatalyst. Thus, such porous materials could be considered as structural variants of carbon nitride type materials with a high surface area when compared to g-C3N4 polymers facilitating water splitting.

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Fig. 8 (a) Synthetic route to heptazine-based microporous polymer networks (HMP-1) and a benzidine linker to obtain HMP-2, (b) diffuse reflectance UV-vis spectrum of HMP-1 and HMP-2 and (c) hydrogen evolution of HMP-1 over 20 h in the presence of platinum as the co-catalyst and triethanolamine as an electron donor (λ > 420 nm). Reprinted from ref. 61 with permission from Wiley.

Zhang et al. anticipated that the high N-content of heptazine was suitable for CO2 gas capture (2015). With this consideration, a piperazine linked heptazine based porous organic framework (Cy-PIP) was developed having a BET surface area of 115.8 m2 g−1 with a superior CO2 uptake of 103.4 mg g−1 (1 bar/273 K) with good selectivity over N2 (Fig. 9).62 The H2 adsorption studies were also carried out with an uptake of 85 cm3 g−1 (1 bar/77 K). They extended the studies to explore the catalytic activity and successfully carried out Knoevenagel condensation reactions by using this polymeric framework as a heterogeneous catalyst. The developed framework Cy-PIP was stable up to 400 °C with only 9% mass loss up to 230 °C. In another report, Yoon et al. used the already reported porous framework, HBF-2 to immobilize Ir on its surface for CO2 to formate conversion.63 The developed CO2 hydrogenation catalyst exhibited excellent activity with a TON of 6400 and TOF of 1500 h−1.

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Fig. 9 (a) Synthesis of Cy-pip, (b) CO2 and N2 adsorption isotherms of Cy-pip at 273 K and (c) H2 adsorption isotherm of Cy-pip at 77 K. Reprinted from ref. 62 with permission from the American Chemical Society.

For a photocatalyst, it is desirable to absorb visible light photons to utilize sunlight explicitly, and thus materials with a band gap < 2.7 eV are desirable. In this context, donor–acceptor based material designs, having lower band gaps and enhanced absorption in the visible spectrum, have been explored for photovoltaic/photocatalytic applications. Kailasam and Thomas et al. (2016) introduced alternating donor–acceptor (D–A1–D–A2) design and developed a heptazine based extended network using heptazine and benzothiadiazole as acceptor units with an alternating aniline unit as the donor.64 They were able to improve the band gap to 1.99 eV and enhanced charge carrier separation efficiency for the porous polymer, HMP-3_2:3, which gave H2 evolution of 32 μmol h−1 (λ > 395 nm).

The structural properties of porous polymers have mostly been used for gas storage and heterogeneous catalysis including water splitting, but their optical properties have been exploited very recently. The luminescent properties were utilized for the detection of organic explosives, metal ions and pollutants. Thus, Li and Shi et al. developed a sensing platform using polymer POP-HT (Fig. 10), which was able to selectively detect Fe3+ ions over other metal ions by significant luminescence quenching. The fluorescence emission at 478 nm (λex = 437 nm) quenched when the concentration of Fe3+ ions was increased. Moreover, POP-HT exhibited luminescence enhancement by 124% when dispersed in 1,4-dioxane.65 Thermogravimetric analysis revealed that POP-HT was stable up to 250 °C.

image file: c8ta05430d-f10.tif
Fig. 10 (a) Synthesis of POP-HT, (b) photograph of the POP-HT test film under natural light and a UV lamp (excitation at 365 nm), (c) concentration-dependent luminescence intensity of Fe3+@POP-HT upon addition of different contents of Fe3+ in acidic aqueous solution (excitation at 437 nm) and (d) emission spectra of POP-HT in different solvents (excitation at 437 nm) and photographs of POP-HT in aqueous solution and 1,4-dioxane under the UV lamp (excitation at 365 nm). Reprinted from ref. 65 with permission from the American Chemical Society.

5. Conclusions and future outlook

Although, s-heptazine is known since the 1830s, the low solubility of available molecular systems, melem, cyamelurine and cyameluric acid has limited the number of scientific reports until very recently. In Sections 3 and 4, some of the molecular compounds and polymeric networks containing the heptazine core are presented in detail. Their interesting properties in a few fields such as liquid crystals, OLEDs, gas storage and heterogeneous catalysts including water splitting were described. These developments happened in a short span of past 5 years after heptazine chloride was characterized thoroughly by E. Kroke in 2002. Thus the s-heptazine core provides new opportunities for synthetic chemists to explore improved synthetic protocols, tuning their properties for wide application challenges facing the material chemists.

The materials arising out of heptazine chemistry are still in the infancy stage but amazingly, the materials reported so far have interesting applications in various fields. As its parent polymeric carbon nitride, g-CN (so-called g-C3N4), has been exploding with a wide variety of applications in the past two decades, the development of new exciting materials using this soluble wonder core would add to the demand for new advanced functional materials. But, at the same time one has to solve the synthetic challenges as so far only one of the readily soluble and reactive precursors like trichloroheptazine has been produced in respectable yields. Efforts should be made to find other easily functionalisable s-heptazine precursors to achieve fruitful material design like tribromoheptazine or tricyanoheptazines and so on. Moreover, only nucleophilic attack has been reported to modify trichloro-s-triazine into useful materials which has limited the substrate scope to study structure–activity relationships. To date, in spite of the problems stated above, by virtue of the reported number of materials one could foresee the huge potential of the heptazine based systems for emerging applications. This can be ranging from yet to be explored organic solar cells to biological applications like cancer treatment which Linus Pauling dreamed of and we believe that the research community will achieve this feat in near future.

Conflicts of interest

There are no conflicts to declare.


Dr K. Kailasam thanks the DST-Nano Mission NATDP funded Technology Project, File No. SR/NM/NT-06/2016 and DST-SERB for the Early Career Research Award (ECR), File No. ECR/2016/001469. Dr S. Kumar is thankful to SERB for the NPDF (PDF/2016/002227) and for the financial support. Neha Sharma is thankful to the Institute of Nano Science and Technology for the doctoral fellowship. The authors are thankful to Mr B. V. Rao for his kind support during the preparation of this review.

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