Abstract:We have prepared several 2,5,8-trialkoxyheptazines starting from the soluble precursor 2,5,8-tris(3,5-diethylpyrazolyl)-heptazine. We present their syntheses along with their promising spectroscopic and electrochemical properties, which demonstrate large band gaps and high...
“…Generally, s -heptazines bearing heteroatomic substituents are colorless (white) or very slightly yellow with most of them absorbing above 300 nm, while some molecules display a tiny absorption tail or shoulder in the visible range, likely due to a charge transfer band of a very low intensity. ,,− , These compounds all exhibit fluorescence, stemming from the same low-intensity band, with a broad emission going from violet (amino derivatives) through bluish white (TPDH) to orange-red (thiol derivatives in the solid state). Fluorescence quantum yields (QYs) are rarely given , but are usually low to average (10–20%) while exhibiting long lifetimes, , reminiscent of thermally activated delayed fluorescence (TADF). In some occurrences, the emission color and QYs are different in the solid state than in solution, which suggests that aggregated induced emission (AIE) might also be encountered although no proof has yet been given.…”
Section: Physical
Chemistry Of S-heptazinesmentioning
confidence: 94%
“…This may seem as an unpassable hurdle, but we managed to overcome it, using the new process of mechanochemistry, more precisely ball-milling. , Indeed, crushing the crude, unpurified tris(hydrazino)- s -heptazine for only about 15 min with a moderate excess of the pure diketone and an acidic catalyst yielded TDPH ( 72 ) with a 30–40% yield on a few grams’ scale (Scheme ). Audebert’s group was able in the same publication to demonstrate the exchange of the pyrazolyl groups by mild nucleophiles, such as thiols and amines (Scheme ), and very recently alcohols …”
Section: Synthetic Approaches For Molecular S-heptazinesmentioning
confidence: 99%
“…With its high band gap around 3 V, TPDH ( 72 ) itself has a redox potential of about +1.7 V in the excited state, which already situates this molecule among the strongest purely organic photooxidants known to date. Audebert recently published the tris(trifluorothoxy)- s -heptazine and demonstrated this molecule is capable of oxidizing thioethers . Given that the electron deficiency of s -heptazines can be thoroughly tuned through the substituents’ donor/acceptor character, this value is certainly not going to remain a maximum within this family!…”
Section: Physical
Chemistry Of S-heptazinesmentioning
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.
“…Generally, s -heptazines bearing heteroatomic substituents are colorless (white) or very slightly yellow with most of them absorbing above 300 nm, while some molecules display a tiny absorption tail or shoulder in the visible range, likely due to a charge transfer band of a very low intensity. ,,− , These compounds all exhibit fluorescence, stemming from the same low-intensity band, with a broad emission going from violet (amino derivatives) through bluish white (TPDH) to orange-red (thiol derivatives in the solid state). Fluorescence quantum yields (QYs) are rarely given , but are usually low to average (10–20%) while exhibiting long lifetimes, , reminiscent of thermally activated delayed fluorescence (TADF). In some occurrences, the emission color and QYs are different in the solid state than in solution, which suggests that aggregated induced emission (AIE) might also be encountered although no proof has yet been given.…”
Section: Physical
Chemistry Of S-heptazinesmentioning
confidence: 94%
“…This may seem as an unpassable hurdle, but we managed to overcome it, using the new process of mechanochemistry, more precisely ball-milling. , Indeed, crushing the crude, unpurified tris(hydrazino)- s -heptazine for only about 15 min with a moderate excess of the pure diketone and an acidic catalyst yielded TDPH ( 72 ) with a 30–40% yield on a few grams’ scale (Scheme ). Audebert’s group was able in the same publication to demonstrate the exchange of the pyrazolyl groups by mild nucleophiles, such as thiols and amines (Scheme ), and very recently alcohols …”
Section: Synthetic Approaches For Molecular S-heptazinesmentioning
confidence: 99%
“…With its high band gap around 3 V, TPDH ( 72 ) itself has a redox potential of about +1.7 V in the excited state, which already situates this molecule among the strongest purely organic photooxidants known to date. Audebert recently published the tris(trifluorothoxy)- s -heptazine and demonstrated this molecule is capable of oxidizing thioethers . Given that the electron deficiency of s -heptazines can be thoroughly tuned through the substituents’ donor/acceptor character, this value is certainly not going to remain a maximum within this family!…”
Section: Physical
Chemistry Of S-heptazinesmentioning
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.
“…3 More recently, new derivatives of Hz were synthesized [4][5][6] and their use as emitters in organic light-emitting diodes (OLEDs) [7][8][9] or as photoredox catalysts were explored. 10,11 A comprehensive up-to-date account of the literature on molecular (monomeric) Hz and derivatives thereof can be found in Ref. 12 The polymer melon, also referred to as graphitic carbon nitride (g-C3N4), has found vast attention as a metal-free and photochemically highly stable photocatalyst for hydrogen evolution from water with sacrificial reagents.…”
It has recently been shown that cycl[3.3.3]azine and heptazine (1,3,4,6,7,9,9b-heptaazaphenalene) as well as related azaphenalenes exhibit inverted singlet and triplet states, that is, the energy of the lowest singlet excited...
“…Correlated wave function theories suggested that S 1 of heptazine lies 0.2-0.3 eV below T 1 , though S 1 is a 'dark' state, meaning that the electronic transition to the ground state (S 0 ) is dipole-forbidden and the oscillator strength (f) is zero in the D 3h symmetry point group. Interestingly, the heptazine core is shared by several synthesised molecules that exhibit intense TADF with positive DE ST [21][22][23][24] . We thus hypothesized that appropriate chemical substitutions would lead to heptazine analogues with negative DE ST and su cient f for intense uorescence.…”
Hund’s multiplicity rule states that for a given electronic configuration, a higher spin state has a lower energy. Rephrasing this rule for molecular excited states predicts a positive energy gap between spin-singlet and spin-triplet excited states, which has been consistent with numerous experimental observations over almost a century. Here, we report a fluorescent molecule that disobeys Hund’s rule, possessing a negative singlet–triplet energy gap of –11 meV. The energy inversion of the singlet and triplet excited states results in delayed fluorescence with short time constants of 0.2 μs, which anomalously decrease with decreasing temperature due to the emissive singlet character of the lowest-energy excited state. Organic light-emitting diodes using this molecule exhibited a fast transient electroluminescence decay with a peak external quantum efficiency of 17%, demonstrating potential implications for optoelectronic devices, including displays, lighting, and lasers.
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