Arylthio- and Arylseleno-Substituted s -Heptazines

Experimental and computational studies of the optical properties of 2,5,8-tris(phenylthiolato)heptazine with an inverted singlet-triplet gap

Photophysical properties of the three-fold symmetric 2,5,8-tris(phenylthiolato)heptazine molecule (1) are studied from combined experimental and computational viewpoints. The intense blue photoemission of 1 in the solid state and in toluene solution is proposed to have a fluorescent origin on the basis of a relatively short emission lifetime and no detectable triplet decay. Calculations at correlated ab initio levels of theory also show that 1 has a large inverted singlet-triplet (IST) gap, a non-vanishing spin-orbit coupling matrix element between the first excited singlet and triplet states, and a fast intersystem crossing rate constant that leads to singlet population from the higher-lying triplet state. The IST gap implies that the first excited singlet state is the lowest excited one, agreeing with the measured fluorescent behaviour of 1. IST gaps are also obtained for the oxygen-containing (2) and selenium-containing (3) analogues of 1 at the ADC(2) level of theory, but not for the tellurium one (4). Calculations of the magnetically induced current density demonstrate that the heptazine core of 1 is globally non-aromatic due to the alternation of carbon and nitrogen atoms along its external rim.

State of the Art in the Preparation and Properties of Molecular Monomeric s-Heptazines: Syntheses, Characteristics, and Functional Applications

This review gives an account on the fast expanding field of monomeric (or molecular) heptazines, at the exclusion of their various polymeric forms, often referred to as carbon nitrides. While examples of monomeric heptazines were extremely limited until the beginning of this century, the field has started expanding quickly since then, as has the number of reports on polymeric materials, though previous reviews did not separate these fields. We provide here a detailed report on the synthetic procedures for molecular heptazines. We also extensively report on the different achievements realized from these new molecules, in the fields of physical chemistry, spectroscopy, materials preparation, (photo)catalysis, and devices. After a comprehensive summary and discussion on heptazines syntheses and characteristics, we show that starting from well-defined molecules allows a versatility of approaches and a wide tunability of the expected properties. It comes out that the field of monomeric heptazines is now emerging and possibly heading toward maturity, while diverging from the one of polymeric carbon nitrides. It is likely that this area of research will quickly surge to the forefront of the search for active organic molecules, with special attention to the domains of catalysis and organic-based functional materials and devices.

A straightforward synthesis of a new family of molecules: 2,5,8-trialkoxyheptazines. Application to photoredox catalyzed transformations

We have prepared several 2,5,8-trialkoxyheptazines starting from the soluble precursor 2,5,8-tris(3,5-diethylpyrazolyl)-heptazine. We present their synthesis along with their promising spectroscopic and electrochemical properties, which demonstrate large band gaps and high reduction potentials altogether. Subsequently, we provide a short assessment of the promising ability of one of these molecules to perform catalytic oxidation test-reactions.

Synthesis of Thiocyameluric Acid C6 N7 S3 H3 , Its Reaction to Alkali Metal Thiocyamelurates and Organic Tris(dithio)cyamelurates

Thiocyameluric acid C6 N7 S3 H3 , the tri-thio analogue of cyameluric acid, is a key compound for the synthesis of new s-heptazine (tri-s-triazine) derivatives. Here, two different routes for the synthesis of thiocyameluric acid and its reaction to tris(aryldithio)- and tris(alkyldithio)cyamelurates C6 N7 (SSR)3 are reported as well as transformation to alkali metal thiocyamelurates M3 [C6 N7 S3 ], M=Na, K. These compounds were characterised by FTIR, Raman, solution 13 C and 1 H NMR spectroscopies, thermal gravimetric analysis (TGA) and elemental analysis. The three (de)protonation steps of thiocyameluric acid were investigated by acid-base titration followed via UV/Vis absorption spectroscopy. While it was not possible to determine the three pKa values, it could be postulated that the acid strength probably increases in the following order: cyanuric acid (C3 N3 O3 H3 ) < thiocyanuric acid (C3 N3 S3 H3 ) < cyameluric acid (C6 N7 O3 H3 ) < thiocyameluric acid (C6 N7 S3 H3 ). Single crystals of Na3 [C6 N7 S3 ]⋅10 H2 O and K3 [C6 N7 S3 ]⋅6 H2 O were obtained and the structures analyzed by single crystal X-ray diffraction. Additionally, quantum chemical calculations were performed to get insights into the electronic structure of thiocyameluric acid and to clarify the thiol-thione tautomerism. Based on a comparison of calculated and measured vibrational spectra it can be concluded that thiocyameluric acid and the di- and mono-protonated anions exist in the thione form.

Renewing accessible heptazine chemistry: 2,5,8-tris(3,5-diethyl-pyrazolyl)-heptazine, a new highly soluble heptazine derivative with exchangeable groups, and examples of newly derived heptazines and their physical chemistry

We have prepared 2,5,8-tris(3,5-diethyl-pyrazolyl)-heptazine, the first highly soluble heptazine derivative possessing easily exchangeable leaving groups. We present its original synthesis employing mechanochemistry, along with a few examples of its versatile reactivity. It is, in particular, demonstrated that the pyrazolyl leaving groups can be replaced by several secondary or primary amino substituents or by three aryl- or benzyl-thiol substituents. In addition to being a synthetic platform, 2,5,8-tris(3,5-diethyl-pyrazolyl)-heptazine is fluorescent and electroactive, and its attractive properties, as well as those of the derived heptazines, are briefly presented.