Controlling the Diradical Character of Thiele Like Compounds

Adopting density functional theory and a hybrid exchange-correlation functional, the relationship between the second hyperpolarizability (gamma) and the diradical character has been investigated for diphenalenyl-based compounds containing different heterocyclic five-membered central rings (C(4)H(4)X, where X = NH, PH, O, S, CH(2), SiH(2), BH, GaH, C=O, C=S, and C=Se) or substituted by donor (NH(2))/acceptor(NO(2)) groups. It turns out that these structural modifications can tune the diradical character from 0.0 to 0.968 and lead to variations of gamma over more than 1 order of magnitude, demonstrating the controllability of gamma in this family of compounds. In particular, when the central ring is strongly aromatic, the diradical character is larger than 0.7, which is associated with pretty large gamma values except for almost the pure diradical case (y approximately 1). On the other hand, when the aromaticity decreases--or the antiaromaticity increases--the diradical character and the second hyperpolarizability get smaller. These relationships are correlated to structural (bond length alternation) and charge distribution (charge transfer between the phenalenyl rings and the central ring) properties, which account for the relative importance of the resonance diradical, zwitterionic, and quinoid forms. Therefore, the diradical character and the second hyperpolarizability can be controlled by the aromaticity of the ring while the paradigm of the enhancement of gamma for intermediate diradical character is globally verified. Then, upon introducing donor groups, the zwitterionic character increases, leading to closed-shell species and small second hyperpolarizabilities. In the case of substitution by acceptor groups, the charge transfer is reduced but the diradical character and the second hyperpolarizability hardly changes.

Variation in spin contamination and diradical character with distance between a singlet biradical molecule and surface

Arylene Diimide Derivatives as Anolyte Materials with Two-Electron Storage for Ultrastable Neutral Aqueous Organic Redox Flow Batteries

Recently, a derivative of the heptazine (tris-triazine) molecule, trianisole-heptazine (TAHz), was synthesized and was shown to catalyze the oxidation of water to hydroxyl radicals under 365 nm LED light in a homogeneous reaction (E. J. Rabe et al., J. Phys. Chem. Lett. 2018, 9, 6257-6261). The possibility of water photo-oxidation with a precisely defined molecular catalyst in neat solvents opens new perspectives for clarifying the fundamental reaction mechanisms involved in water oxidation photocatalysis. In the present work, the effects of chemical substituents on the three CH positions of Hz on the photocatalytic reactivity were explored with wave function-based ab initio electronic-structure calculations for hydrogen-bonded complexes of Hz and three selected Hz derivatives (TAHz, trichloro-Hz, and tricyano-Hz) with a water molecule. While anisole is an electron-donating substituent, Cl is a weakly electron-withdrawing substituent and CN is a strongly electron-withdrawing substituent. It is shown that the barrier for the photoinduced abstraction of an H atom from the water molecule is raised (lowered) by electron-donating (electron-withdrawing) substituents. The highly mobile and reactive hydroxyl radicals generated by water oxidation can recombine with the reduced chromophore radicals to yield photohydrates. The effect of substituents on the thermodynamics of the photohydration reaction was computed. Among the four chromophores studied, TAHz stands out on account of the metastability of its photohydrate, which suggests self-healing of the photocatalyst after oxidation of TAHzH radicals by OH radicals. In addition, the effect of substituents on the H atom photodetachment reaction from the reduced chromophores, which closes the catalytic cycle, has been investigated. The energy of the repulsive 2πσ* state, which drives the photodetachment reaction is lowered (raised) by electron-donating (electron withdrawing) substituents. All four chromophores exhibit inverted S1/T1 gaps. This feature eliminates long-lived triplet states and thus avoids the activation of molecular oxygen to highly reactive singlet oxygen.

Quinoidal compounds with proaromatic structures possess differing degrees of diradical character, where the open-shell diradical resonance form has restored aromaticity throughout the compound. Methods to tune the diradical character of these compounds have traditionally focused on altering the length and the molecular composition of the π-conjugated backbones. However, other molecular design strategies to tune the singlet–triplet gap of π-conjugated quinoidal molecules have not been extensively explored. We previously reported a strikingly small energy gap between the quinoidal and diradical states of a quinoidal small molecule containing methano[10]annulene (TMTQ) that was dictated in large part by the unusual aromaticity of the central annulene ring. Here, we report on two alkylated derivatives of TMTQ that present substantially different torsional biases to the planarity of the TMTQ π-system. Using a combination of electronic and vibrational spectroscopies, magnetic measurements, and quantum chemical calculations, we demonstrate here how a steric effect rather than π-electron compositional molecular engineering can dramatically narrow the singlet–triplet gap of a quinoidal compound to as small as −0.52 kcal/mol, determined experimentally. This study offers important insight for the continued development of open-shell diradical molecules that need not rely exclusively on the design of synthesis of new and complex π-conjugated systems.

The European Young Chemist Award 2012

Substituent effects and photo-physical properties in polythiophene and its derivatives: A PBC-DFT study

In this work, theoretical methods of quantum chemistry are employed to estimate the effects that the structural modification of 1,5- and 9,10-anthraquinone molecules might produce in their electronic structure, in the pursuit of a common strategy to improve the electrochemical and singlet fission features of conjugated quinones. The proposed modifications are the following: (i) substitution of the carbonyl oxygen atom, (ii) insertion of heteroatoms in the carbon backbone, and (iii) introduction of electron-withdrawing and electron-donating substituents in different positions of the rings. This work shows how specific modifications of the electronic structure can be used to tune the electrochemistry and photophysics of the quinones, improving their suitability to be singlet fission sensitizers and cathode materials. As it was previously known, intermediate diradical characters favor the accomplishment of the singlet fission process. This can be achieved for 1,5-anthraquinone by introduction of B and Si atoms and electron-donating groups in low spin density positions of the carbon backbone or N atoms in high spin density positions. The introduction of heteroatoms or substituents in 9,10-anthraquinone hardly facilitates the singlet fission process. Regarding the electrochemistry, it is observed that the reduction potentials greatly depend on the nature of the modification rather than on the diradical character.

13C NMR chemical shifts delta(C)(C=N) were measured in CDCl3 for a wide set of mesogenic molecule model compounds, viz. the substituted benzylidene anilines p-X-C6H4CH=NC6H4-p-Y (X = NO2, CN, CF3, F, Cl, H, Me, MeO, or NMe2; Y = NO2, CN, F, Cl, H, Me, MeO, or NMe2). The substituent dependence of delta(C)(C=N) was used as a tool to study electronic substituent effects on the azomethine unit. The benzylidene substituents X have a reverse effect on delta(C)(C=N): electron-withdrawing substituents cause shielding, while electron-donating ones behave oppositely, the inductive effects clearly predominating over the resonance effects. In contrast, the aniline substituents Y exert normal effects: electron-withdrawing substituents cause deshielding, while electron-donating ones cause shielding of the C=N carbon, the strengths of the inductive and resonance effects being closely similar. Additionally, the presence of a specific cross-interaction between X and Y could be verified. The electronic effects of the neighboring aromatic ring substituents systematically modify the sensitivity of the C=N group to the electronic effects of the benzylidene or aniline ring substituents. Electron-withdrawing substituents on the aniline ring decrease the sensitivity of delta(C)(C=N) to the substitution on the benzylidine ring, while electron-donating substituents have the opposite effect. In contrast, electron-withdrawing substituents on the benzylidene ring increase the sensitivity of delta(C)(C=N) to the substituent on the aniline ring, while electron-donating substituents act in the opposite way. These results can be rationalized in terms of the substituent-sensitive balance of the electron delocalization (mesomeric effects). The present NMR characteristics are discussed as regards the computational literature data. Valuable information has been obtained on the effects of the substituents on the molecular core of the mesogenic model compounds.

The improvement of solar cell performance in the near-infrared (near-IR) region is an important challenge to increase power conversion efficiency under one-sun illumination. PbS quantum-dot (QD)-based heterojunction solar cells with high efficiency in the near-IR region were constructed by combining ZnO nanowire arrays with PbS QDs, which give a first exciton absorption band centering at wavelengths longer than 1 μm. The morphology of ZnO nanowire arrays was systematically investigated to achieve high light-harvesting efficiency as well as efficient carrier collection. The solar cells with the PbS QD/ZnO nanowire structures made up of densely grown thin ZnO nanowires about 1.2 μm long yielded a maximum incident-photon-to-current conversion efficiency (IPCE) of 58% in the near-IR region (@1020 nm) and over 80% in the visible region (shorter than 670 nm). The power conversion efficiency obtained on the solar cell reached about 6.0% under simulated one-sun illumination.

A series of bis[N,N‐di‐(4‐methoxylphenyl)amino]arene dications 12+–32+ have been synthesized and characterized. Their electronic structures were investigated by various experiments assisted by theoretical calculations. It was found that they are singlets in the ground state and that their diradical character is dependent on the bridging moiety. 32+ has a smaller singlet–triplet energy gap and its excited triplet state is thermally readily accessible. The work provides a nitrogen analogue of Thiele’s hydrocarbon with considerable diradical character.

A series of compounds of the general formula 3- and 4-FC6H4CH2MLmL′n where M is a transition metal have been synthesised and characterised. Ground state substituent effects of the metallomethyl groups CH2MLmL′n have been investigated by examination of the 19F substituent chemical shifts (SCS). The results indicate that whilst these metallomethyl groups produce net electron donation to the aromatic ring, the magnitude of the effect is strongly dependent on the metal and its attached ligands. The substituent effects in these ground states are markedly smaller than those previously detected in transition states and in studies of equilibria, but the order of electron-donating ability within the series of metallomethyl groups is the same for the ground states and for the transition states and equilibria. The results show that several neutral (carbonyl)metallomethyl substituents are as electron donating as the most effective main group metallomethyl substituents, but the electron-donating character of several neutral cobalt(III)-containing substituents is little different from that of the methyl group. The analogous rhodium(III) substituents are markedly more electron donating.

Photoinduced reactions—XLVIII : Steric and substituent effects on photoreactions of 2,4,6-trialkylphenyl ketones

<p>This thesis reports the coordination chemistry of phosphinocarbonyl ligands with platinum and describes the influence of phosphine substituents on the mechanism of chelation and the coordination mode of the carbonyl moiety. The ligands synthesised were 2-diphenylphosphinobenzaldehyde (1), 2-diphenylphosphinoacetophenone (2), 2-bis(pentafluorophenyl)phosphinobenzaldehyde (3), and 2-di-tert-butylphosphinobenzaldehyde (4). Compounds 1, 3, and 4 were selected on the basis of their steric bulk and extent to which they donate electron density to the metal. Compound 2 contained the same phosphine substituents to 1, but is the methyl ketone analogue and therefore does not contain the CHO moiety. The cone angle and electronic parameter of compounds 1–4 were compared to the reported values of PPh3, PPh(C6F5)2, and PPhtBu2. Compounds 3 and 4 were similarly bulky, and had larger cone angles than 1. The electron donating capacity of compound 4 was greater than that of 1, and compound 3 was the least electron donating. A new synthetic method for the preparation of 4 is also reported. The coordination chemistry of ligands 1–4 was investigated with platinum(II) and platinum(0) starting materials to assess the influence of the steric and electronic parameters of the phosphine on the chelation of the ligand through the carbonyl to platinum. Coordination of the ligand went through the initial coordination of the phosphine and, depending on the identity of that phosphine, may be followed by chelation of the carbonyl moiety to form a P,C chelate. However, the site of the platinum–carbon bond in the P,C metallacycle depends on the ligand employed. Coordination of the phosphinoaldehyde ligands 1, 3, and 4 produced Pt-C bonds via the C-H activation of the aldehyde CHO group whereas for ketophosphine 2, C-H activation occurred at the α-methyl group. The rate at which C-H activation occurred increased with increasing electron donation from the phosphorus to platinum. Compound 4 chelates to platinum more rapidly than compound 1, while 3 did not undergo chelation at room temperature. Although chelation was only observed to occur via C-H activation, the final products of the coordination reactions of 1–4 with platinum starting materials differed depending on the identity of the ligand. The C-H activation of two molecules of 1 with platinum(II) or platinum(0) produced a platina-β-diketone, cis-[Pt(P,C-2-PPh2C6H4CO)2] (21), which is capable of coordinating to H+, Li+, BF2 +, and [Rh(1,5-cyclooctadiene)]+ between the mutually cis carbonyl groups. One carbonyl moiety of 21 can also undergo condensation with primary amines and ammonia to produce platina-β-ketoimine complexes. The ketone moiety of ligand 2 reacted with platinum(II) starting materials through C-H activation of the terminal methyl group to form the six-membered bis-chelate complex analogous to complex 21. The reaction of 2 with platinum(0) starting materials resulted in the formation of a platinum hydride intermediate which mediated chelation through the partial reduction of the ketone group of one ligand, to form the product, [Pt(P,C-2-PPh2C6H4COCH2)(P,C-2-PPh2C6H4C(OH)CH3)] (48) . The reaction of 3 with [PtMe2(1,5-hexadiene)] at elevated temperatures resulted in the formation of [Pt(P,C-2-PPh2C6H4)(P,C-2-PPh2C6H4CO)] (54) – a decarbonylated and ortho-metallated complex containing a four-membered metallacycle. The platinum-phosphorus bond in the four-membered ring of 54 has a bond distance of 2.385(2) Å – the longest Pt–P bond reported to date. Ligand 4 reacted rapidly with platinum(II) starting materials and produced numerous chelation products. Complexes of ligand 4 were only observed to contain mutually trans phosphines, likely due to the steric bulk of the tert-butyl substituents. Comparison of the coordination chemistry of ligands 1–4 suggests that the propensity toward C-H activation of the ligands is predominantly determined by the electronic character of the phosphine (although steric effects cannot be disregarded), and the more electron-rich the phosphine, the more rapidly chelation occurs.</p>

Fluoreno[2,1-a]fluorene, a molecule comprising fused ortho-quinodimethane units in a 1,5-napthoquinodimethane core, has been prepared and investigated with spectroscopy (UV-Vis-NIR, 1H-NMR and Raman), SQUID magnetometry, spectroelectrochemistry and quantum chemistry. While para-quinodimethanes with a 2,6-substitution pattern exist as closed-shell species and meta-quinodimethanes with 2,7-substitution favour a ground electronic state with very large diradical character, our 1,5-substituted ortho-naphthoquinodimethane-based system exhibits an intermediate degree of diradical character.