Photocatalytic Decyanation of Arylacetonitriles Enabled by Diphenylboryl Radicals via Spin Delocalization and α-Fragmentation

Organic photovoltaics rely on efficient charge separation and transport, processes facilitated by charge delocalisation across the π-conjugated backbone of donor and acceptor molecules. By probing the interactions between the unpaired electron spin associated with photoinduced charged states and magnetic nuclei in their molecular environment, electron paramagnetic resonance (EPR) spectroscopy enables precise experimental quantification of spin and charge delocalisation. We first characterise the EPR spectral signatures of the positive and negative polarons on polymer donors and non-fullerene acceptors in the PBDB-T:ITIC and PM6:Y6 blends, as well as the corresponding fullerene-based blends PBDB-T:PC61BM and PM6:PC61BM, by multi-frequency EPR spectroscopy. Reliable separation of overlapping donor and acceptor signatures is enabled by EDNMR-induced EPR spectroscopy exploiting unique nuclear hyperfine couplings in the non-fullerene acceptors ITIC and Y6. Then, by combining the measurement of electron-nuclear hyperfine couplings by ENDOR with DFT modelling and a regularised least-squares fitting approach, we quantify the extent of spin and charge delocalisation. The experimental results reveal intramolecular delocalisation of the positive polarons on the PBDB-T and PM6 donors across approximately 6 nm. Delocalisation of the negative polarons, on the other hand, depends on the nature of the acceptor: for ITIC, the electron spin is found to be localised on a single molecule, whereas for Y6, contributions from spins localised on a single molecule, as well as spins delocalised over two adjacent molecules in different π-π stacked configurations, are required to explain the experimental ENDOR and HYSCORE data. Validation of computational predictions by experimental results is shown to be crucial for the accurate estimation of charge delocalisation, and therefore conclusions on its relevance in determining device efficiency in organic photovoltaics.

The Source Function (SF) tool was applied to the analysis of the theoretical spin density in azido CuII dinuclear complexes, where the azido group, acting as a coupler between the CuII cations, is linked to the metal centres either in an end-on or in an end-end fashion. Results for only the former structural arrangement are reported in the present paper. The SF highlights to which extent the magnetic centres contribute to determine the local spin delocalization and polarization at any point in the dimetallic complex and whether an atom or group of atoms of the ligands act in favour or against a given local spin delocalization/polarization. Ball-and-stick atomic SF percentage representations allow for a visualization of the magnetic pathways and of the specific role played by each atom along these paths, at given reference points. Decomposition of SF contributions in terms of a magnetic and of a relaxation component provides further insight. Reconstruction of partial spin densities by means of the Source Function has for the first time been introduced. At variance with the standard SF percentage representations, such reconstructions offer a simultaneous view of the sources originating from specific subsets of contributing atoms, in a selected molecular plane or in the whole space, and are therefore particularly informative. The SF tool is also used to evaluate the accuracy of the analysed spin densities. It is found that those obtained at the unrestricted B3LYP DFT level, relative to those computed at the CASSCF(6,6) level, greatly overestimate spin delocalization to the ligands, but comparatively underestimate magnetic connection (spin transmission) among atoms, along the magnetic pathways. As a consequence of its excessive spin delocalization, the UB3LYP method also overestimates spin polarization mechanisms between the paramagnetic centres and the ligands. Spin delocalization measures derived from the refinement of Polarized Neutron Diffraction data seem in general superior to those obtained through the DFT UB3LYP approach and closer to the far more accurate CASSCF results. It is also shown that a visual agreement on the spin-resolved electron densities ρα and ρβ derived from different approaches does not warrant a corresponding agreement between their associated spin densities.

Hard-ligand, high-potential copper sites have been characterized in double mutants of Pseudomonas aeruginosa azurin (C112D/M121X (X = L, F, I)). These sites feature a small A(zz)(Cu) splitting in the EPR spectrum together with enhanced electron transfer activity. Due to these unique properties, these constructs have been called "type zero" copper sites. In contrast, the single mutant, C112D, features a large A(zz)(Cu) value characteristic of the typical type 2 Cu(II). In general, A(zz)(Cu) comprises contributions from Fermi contact, spin dipolar, and orbital dipolar terms. In order to understand the origin of the low A(zz)(Cu) value of type zero Cu(II), we explored in detail its degree of covalency, as manifested by spin delocalization over its ligands, which affects A(zz)(Cu) through the Fermi contact and spin dipolar contributions. This was achieved by the application of several complementary EPR hyperfine spectroscopic techniques at X- and W-band (∼9.5 and 95 GHz, respectively) frequencies to map the ligand hyperfine couplings. Our results show that spin delocalization over the ligands in type zero Cu(II) is different from that of type 2 Cu(II) in the single C112D mutant. The (14)N hyperfine couplings of the coordinated histidine nitrogens are smaller by about 25-40%, whereas that of the (13)C carboxylate of D112 is about 50% larger. From this comparison, we concluded that the spin delocalization of type zero copper over its ligands is not dramatically larger than in type 2 C112D. Therefore, the reduced A(zz)(Cu) value of type zero Cu(II) is largely attributable to an increased orbital dipolar contribution that is related to its larger g(zz) value, as a consequence of the distorted tetrahedral geometry. The increased spin delocalization over the D112 carboxylate in type zero mutants compared to type 2 C112D suggests that electron transfer paths involving this residue are enhanced.

Pulsed EPR spectroscopic techniques, including ESEEM (electron spin echo envelope modulation) and pulsed ENDOR (electron-nuclear double resonance), are extremely useful for determining the magnitudes of the hyperfine couplings of macrocycle and axial ligand nuclei to the unpaired electron(s) on the metal as a function of magnetic field orientation relative to the complex. These data can frequently be used to determine the orientation of the g-tensor and the distribution of spin density over the macrocycle, and to determine the metal orbital(s) containing unpaired electrons and the macrocycle orbital(s) involved in spin delocalization. However, these studies cannot be carried out on metal complexes that do not have resolved EPR signals, as in the case of paramagnetic even-electron metal complexes. In addition, the signs of the hyperfine couplings, which are not determined directly in either ESEEM or pulsed ENDOR experiments, are often needed in order to translate hyperfine couplings into spin densities. In these cases, NMR isotropic (hyperfine) shifts are extremely useful in determining the amount and sign of the spin density at each nucleus probed. For metal complexes of aromatic macrocycles such as porphyrins, chlorins, or corroles, simple rules allow prediction of whether spin delocalization occurs through sigma or pi bonds, and whether spin density on the ligands is of the same or opposite sign as that on the metal. In cases where the amount of spin density on the macrocycle and axial ligands is found to be too large for simple metal-ligand spin delocalization, a macrocycle radical may be suspected. Large spin density on the macrocycle that is of the same sign as that on the metal provides clear evidence of either no coupling or weak ferromagnetic coupling of a macrocycle radical to the unpaired electron(s) on the metal, while large spin density on the macrocycle that is of opposite sign to that on the metal provides clear evidence of antiferromagnetic coupling. The latter is found in a few iron porphyrinates and in most iron corrolates that have been reported thus far. It is now clear that iron corrolates are remarkably noninnocent complexes, with both negative and positive spin density on the macrocycle: for all chloroiron corrolates reported thus far, the balance of positive and negative spin density yields -0.65 to -0.79 spin on the macrocycle. On the other hand, for phenyliron corrolates, the balance of spin density on the macrocycle is zero, to within the accuracy of the calculations (Zakharieva, O.; Schünemann, V.; Gerdan, M.; Licoccia, S.; Cai, S.; Walker, F. A.; Trautwein, A. X. J. Am. Chem. Soc. 2002, 124, 6636-6648), although both negative and positive spin densities are found on the individual atoms. DFT calculations are invaluable in providing calculated spin densities at positions that can be probed by (1)H NMR spectroscopy, and the good agreement between calculated spin densities and measured hyperfine shifts at these positions leads to increased confidence in the calculated spin densities at positions that cannot be directly probed by (1)H NMR spectroscopy. (13)C NMR spectroscopic investigations of these complexes should be carried out to probe experimentally the nonprotonated carbon spin densities.

An analysis is made of pi-electron spin delocalization into the methyl-group hydrogen pseudo pi orbital of an ethyl radical. Configuration-interaction molecular orbital theory with semiempirical integral parameters is employed to achieve a separation of the exchange polarization and the electron-transfer contributions. A set of calculations for the allyl-type radical (Ċ3–C2=C1), which serves as a simpler model for the ethyl radical (Ċ3–C2=H31), shows that essentially equivalent results are obtained from treatments with and without overlap. In the ethyl calculation without overlap, as well as in the allyl studies, it is found that both pi-electron exchange polarization and electron transfer are important, with the former contributing ∼60% and the latter ∼40% to the spin delocalization. A perturbation treatment is introduced to evaluate the dependence of the results on values of the integral parameters; it is shown that the relative contributions of exchange polarization and electron transfer are rather insensitive to the parameter values, although the two types of electron-transfer terms (into and out of the methyl group) are individually dependent on the electronegativity difference between the carbon pi orbital and the methyl group. The results demonstrate that neither a simple valence bond treatment (which is based on an exchange polarization mechanism) nor a simple Hückel calculation (which is constructed from electron-transfer terms) provides by itself a completely satisfactory description of spin delocalization in these systems.

Spin delocalization into the alkyl chains of nickel(II) bis(alkyl xanthate) bipyridyl complexes was measured by {sup 1}H and {sup 13}NMR. The trends in the spin delocalization are consistent with the previously reported observation that nickel-nitroxyl interaction in spin-labeled Ni(II) xanthates decreased rapidly as the number of carbons between the xanthate and nitroxyl rings, increased. In a series of alkylamine derivatives of pyridine-2-carboxaldehyde coordinated to nickel bis(ethyl xanthate), the spin delocalization onto the first carbon of the N-alkyl group was larger than that for the first carbon of the N-alkyl xanthates. This is consistent with the observation of stronger nickel-nitroxyl interaction for a spin-labeled pyridine-2-carboxaldimine than for the spin-labeled xanthates. The {sup 1}H NMR spectrum of a spin-labeled nickel(II) xanthate indicated that the Ni(II) provided an efficient relaxation mechanism for the nitroxyl protons. The contributions to the isotropic shifts of protons in the nitroxyl ring from the Ni(II) and nitroxyl unpaired electrons were approximately additive. 34 refs., 3 tabs.

Three major forms of gaseous radical-cationic amino acids (RCAAs), keto (COOH), enolic (C(OH)OH), and zwitterionic (COO(-)), as well as their tautomers, are examined for aliphatic Ala(.+), Pro(.+), and Ser(.+), sulfur-containing Cys(.+), aromatic Trp(.+), Tyr(.+), and Phe(.+), and basic His(.+). The hybrid B3LYP exchange-correlation functional with various basis sets along with the highly correlated CCSD(T) method is used. For all RCAAs considered, the main stabilizing factor is spin delocalization; for His(.+), protonation of the basic side chain is equally important. Minor stabilizing factors are hydrogen bonding and 3e-2c interactions. An efficient spin delocalization along the N-C(alpha)-C(O-)O moiety occurs upon H-transfer from C(alpha) to the carboxylic group to yield the captodative enolic form, which is the lowest-energy isomer for Ala(.+), Pro(.+), Ser(.+), Cys(.+), Tyr(.+), and Phe(.+). This H-transfer occurs in a single step as a 1,3-shift through the sigma-system. For His(.+), the lowest-energy isomer is formed upon H-transfer from C(alpha) to the basic side chain, which results in a keto form, with spin delocalized along the N-C(alpha)-C=O fragment. Trp(.+) is the only RCAA that favors spin delocalization over an aromatic system given the low ionization energy of indole. The lowest-energy isomer of Trp(.+) is a keto form, with no H-transfer.

Photoswitching molecules like the azoarenes have myriad potential applications, ranging from energy storage to targeted drug delivery. Upon irradiation, azoarenes convert from an E-form to a Z-form. The Z-form can thermally revert to the E-form through a classical-adiabatic or a triplet-assisted rotational mechanism. We show that strategic placements of heteroatoms and substituents can modulate spin delocalization in the triplet state, thereby tuning the S0-T1-S0 crossing points. Increased spin delocalization lowers the crossing point and shortens the thermal half-life of the Z-form. Computed spin densities at the diazo N atoms, along with computed S0-T1-S0 crossing points, activation entropies, and rate constants for 15 azoarenes are compared with available experimental data. There is evidence that some of the studied Z-E isomerizations proceed simultaneously via a classical-adiabatic and a triplet-assisted mechanism. The ratio of the two reaction pathways is temperature-dependent. Triplet spin density at the diazo N atoms is recognized as a useful indicator for predicting the thermal half-lives of these species.

An analysis is made of the spin distribution and hyperfine splittings of the toluene anion and cation radicals. Considered are the importance and magnitude of the splitting of the benzene-ion degeneracy by the interaction between the ring and the methyl group, the effects of spin delocalization into the methyl group by different mechanisms, and the consequences of vibrational and thermal coupling of the near-degenerate levels. The spin delocalization mechanism in the toluene ions is shown to be in agreement with a perturbation model for the Ċ–B = A system (here Ċ corresponds to the aromatic ring). The vibronic calculation is based on an ASMO–CI treatment of the electronic energy levels and their variation with bond lengths and angles; an attempt is made to include all significant vibrational contributions. The previously neglected alteration of the neutral-molecule vibrational functions due to the addition or removal of a pi-electron is found to be of particular importance. Although the exact quantitative results are dependent on the electronic parameters, the lowest anion vibronic state consists of about 90% of the antisymmetric and 10% of the symmetric electronic state. Comparison of the calculated and experimental temperature dependence of the anion hyperfine splittings shows that the vibronic treatment gives reasonable results.

The cyclopentane-1,3-diyl triplet diradicals T and T' with the triplet-bonded acetylene, cyano, and isocyano functionalities at one of the radical sites are readily prepared from the corresponding azoalkanes by photodenitrogenation in a 2-methyltetrahydrofuran (2-MTHF) matrix at 77 K. The EPR-spectral D values of these triplet diradicals show that the spin delocalizing ability of the triple-bonded pi substituent follows the order -Ctbd1;CH > -NC approximately -CN. Good correlations of the D values have been obtained with the hyperfine coupling constants (a(H)) and with the calculated spin densities (PM3/AUHF-CI method) of the corresponding monoradicals M. The propargyl-type mesomeric structure is favored over the allenyl-type contributor for all three triple-bonded functionalities; spin delocalization is less pronounced in the heteropropargyl derivatives due to the electronegativity effect of the nitrogen atom.

We theoretically investigate the charge and spin transport through a binuclear Fe(III)Fe(III) iron complex connected to two metallic electrodes. During the transport process, the Fe(III)Fe(III) dimer undergoes a one-electron reduction (Coulomb blockade transport regime), leading to the reduced mixed-valence Fe(II) Fe(III) species. For such a system, the additional electron may be fully delocalized leading to the stabilization of the highest spin ground state S = 9/2 by the double exchange mechanism, while the original Fe(III)Fe(III) has usually an S = 0 spin ground state due to the antiferromagnetic exchange coupling between the two Fe(III) ions. Intuitively, the spin delocalization within the mixed-valence complex may be thought to favor charge and spin transport through the molecule between the two metallic electrodes. Contrary to such an intuitive concept, we find that the increased delocalization leads in fact to a blocking of the transport, if the exchange coupling interaction within the Fe(III)Fe(III) dimer is antiferromagnetic. This is due to the violation of the spin angular momentum conservation, where a change of half a unit of spin (DeltaS = 1/2) is allowed between two different redox states of the molecule. The result is explained in terms of a double-exchange blockade mechanism, triggered by the interplay between spin delocalization and antiferromagnetic coupling between the magnetic cores. Consequently, ferromagnetically coupled dimers are shown not to be affected by the double-exchange blockade mechanism. The situation is evocative of the onset and removal of giant magnetoresistance in the conductance of diamagnetic layers, as a function of the relative alignment of the magnetization of two weakly antiferromagnetically coupled ferromagnetic contacts. Numerical simulations accounting for the effect of vibronic coupling show that the spin current increases as a function of spin delocalization in Class I and Class II mixed-valence dimers. The signature of vibronic coupling on sequential spin-tunneling processes through Class I and Class II mixed-valence systems is identified and discussed.

We present a complete Raman spectroscopic study in two structurally well-defined diradical species of different lengths incorporating oligo p-phenylene vinylene bridges between two polychlorinated triphenylmethyl radical units, a disposition that allows sizeable conjugation between the two radicals through and with the bridge. The spectroscopic data are interpreted and supported by quantum chemical calculations. We focus the attention on the Raman frequency changes, interpretable in terms of: (i) bridge length (conjugation length); (ii) bridge conformational structure; and (iii) electronic coupling between the terminal radical units with the bridge and through the bridge, which could delineate through-bond spin polarization, or spin delocalization. These items are addressed by using the "oligomer approach" in conjunction with pressure and temperature dependent Raman spectroscopic data. In summary, we have attempted to translate the well-known strategy to study the electron (charge) structure of π-conjugated molecules by Raman spectroscopy to the case of electron (spin) interactions via the spin delocalization mechanism.

Radicals resulting from one-electron reduction of (N-methylpyridinium-4-yl) methyl esters have been reported to yield (N-methylpyridinium-4-yl) methyl radical, or N-methyl-gamma-picoliniumyl for short, by heterolytic cleavage of carboxylate. This new reaction could provide the foundation for a new structural class of bioreductively activated, hypoxia-selective antitumor agents. N-methyl-gamma-picoliniumyl radicals are likely to damage DNA by way of H-abstraction and it is of paramount significance to assess their H-abstraction capabilities. In this context, the benzylic C-H homolyses were studied of toluene (T), gamma-picoline (P, 4-methylpyridine), and N-methyl-gamma-picolinium (1c, 1,4-dimethylpyridinium). With a view to providing capacity for DNA intercalation the properties also were examined of the annulated derivatives 2c (1,4-dimethylquinolinium), 3c (9,10-dimethylacridinium), and 4c (1,4-dimethylbenzo[g]quinolinium). The benzylic C-H homolyses were studied with density functional theory (DFT), perturbation theory (up to MP4SDTQ), and configuration interaction methods (QCISD(T), CCSD(T)). Although there are many similarities between the results obtained here with DFT and CI theory, a number of significant differences occur and these are shown to be caused by methodological differences in the spin density distributions of the radicals. The quality of the wave functions is established by demonstration of internal consistencies and with reference to a number of observable quantities. The analysis of spin polarization emphasizes the need for a clear distinction between "electron delocalization" and "spin delocalization" in annulated radicals. Aside from their relevance for the rational design of new antitumor drugs, the conceptional insights presented here also will inform the understanding of ferromagnetic materials, of spin-based signaling processes, and of spin topologies in metalloenzymes.

Vinyl-ruthenium entities as markers for intramolecular electron transfer processes

We have theoretically investigated the magnetic properties of heteroallene (>C=C=X-) and heterocumulene (>C=C=C=X-) based tert-butyl nitroxide diradicals (X is P/As). Calculation of magnetic exchange coupling constant (J) shows ferromagnetic interaction in heteroallene based diradicals. Whereas, in heterocumulene based diradicals, tuning of J value from antiferro- to ferro-magnetic state is observed from Z- to E- isomer. Delocalization of spin density from radical site to the coupler (in planar arrangement) is observed in spin distribution analysis which is also advocated by molecular orbital analysis. The typical feature of tert-butyl nitroxide radical creates spin delocalization along with spin polarization within the coupler. The J values of all the diradicals strongly depend on the dihedral angle between radical center and coupler. Magneto-structural correlation shows that the change in dihedral angle tunes the magnetic property for both the Z- and E- isomers of heterocumulenes depending on the spin accumulation on two nearby magnetic centers. The extent of spin delocalization and conformation of spin centers on the molecular axis are important for the different J values observed in our designed systems.