Search

Long-lived emissive materials based on room temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF) are considered as the cornerstone of the development of optical sensors, security systems and solid-state lighting. Nevertheless, molecular systems with these properties are scarce because most of them suffer from aggregation caused quenching emission (ACQ). One approach to address this shortcoming is by inhibiting the molecular motions/vibrations by employing a fixed matrix as afforded by a metal-organic framework (MOF). There, the organic chromophores are confined in a crystalline framework, and the structure-property relationship can be designed to get RTP/TADF. Inspired by this, the present work explores the relation between the linker arrangement and the physicochemical properties of two isochemical MOFs with different crystalline structures. The denser MOF exhibits a long-lived green RTP due to a hyperfine coupling of the linkers. On the other hand, the more porous MOF presents a long-lived temperature-dependent turquoise emission, reflecting the influence of the TADF. Hence, this study provides a huge advance about the potential of MOFs to undergo RTP and TADF emission, and at the same time, demonstrates their potential applicability in a wide range of photonic technologies, including physical and chemical sensing and the first example of a MOF-LED based on RTP-MOFs., Comment: 7 pages, 4 Figures

International audience; The four main stable isotopes of the [ADy(tta)3(L)]·C6H14 (ADy with A = 161–164) Single-Molecule Magnet (SMM) (tta− = 2-thenoyltrifluoroacetylacetonate and L = 2-{[2-methylpyridiyl]-4,5-[4,5-bis(propylthio)-tetrathiafulvalenyl]-1H-benzimidazol-2-yl}pyridine) have been magnetically investigated and structurally characterized by single crystal X-ray diffraction. The two nuclear spin-free 162/164Dy behave the same from a dynamic magnetic point of view and with slower magnetic relaxation than the two pure magnetically active isotopically substituted 161/163Dy. In addition, this paper demonstrates that 161Dy and 163Dy relax differently even if they both carry I = 5/2 nuclear spin. After the release of the dipolar interactions through magnetic dilution, ([161Dy0.05Y0.95(tta)3(L)]·C6H14) (161Dy@Y) relaxes four times slower than 163Dy@Y due to different hyperfine coupling constants.

A [3Fe–3(μ-S)]3+cluster is reported in which each ferric center has a distorted trigonal pyramidal geometry, with anS= 1/2 ground state for the cluster and unusually anisotropic hyperfine coupling constants as determined by variable temperature magnetometry and Mössbauer spectroscopy.

The behaviour of four stable β-phosphorylated aminoxyl radicals of the pyrrolidinoxyl series has been studied by EPR spectroscopy in the presence of dimyristoyl-phosphatidylcholine (DMPC) unilamellar liposomes. The affinity of these compounds for the liposome structure was found to be more or less high depending on their hydrophobicity. In each case, computer simulations of experimental EPR spectra permitted the determination of the hyperfine coupling constants of the radical inserted in the phospholipid bilayer.

The geometries of the bacteriochlorophyll a doublet radical cation and anion have been optimized employing the BLYP density functional and the DZVP double zeta basis set, augmented by diffuse functions for the anion. Isotropic hyperfine coupling constants (hfcs) from single point calculations applying the BLYP, B88-P86, PW91-PW91, B88-PW91, B3LYP, B1LYP and BHANDH functionals in combination with the EPR-II basis set are reported and compared with the experimental results. The influence of minor conformational changes on the calculated hfcs was estimated for the radical cation. Furthermore, hfcs for the radical cation coordinated by two water molecules have been determined. It is concluded that the B3LYP/EPR-II//BLYP/DZVP level of theory is especially well suited for the calculation of hfcs of hydrogens and other magnetic nuclei.

Using the technique of muon spin relaxation (µSR), repolarisation curves are determined following the deposition of positive muons in samples of apoferritin over the temperature range 50 K to 270 K, and ferritin over the temperature range 15 K to 280 K. Evidence is found for a muonium-substituted species common to both apoferritin and ferritin. In apoferritin at 270 K an estimate for A′, the reduced isotropic muon–electron hyperfine coupling constant, is 168±39 MHz. In ferritin at 280 K the value for A′ is 164±37 MHz.

The muonium and deuterium isotopomers of the formyl radical HCO 2 A′ are investigated using a variation-perturbation approach for the calculation of vibrational effects in these radicals. After inclusion of anharmonicity MuCO has a calculated zero-point vibrational energy of 74.15 kJ mol -1 compared with a value of 36.11 kJ mol -1 for HCO. The effect is discussed of these zero-point energy contributions to the formation of MuCO. The calculated reduced muon–electron hyperfine coupling constant A µ ′ is 437.75±2.86 MHz for the muonated radical in the ground vibrational level. This represents a calculated α-isotope effect A µ ′ /A p of 1.18, where A p is the proton–electron hyperfine coupling constant in the formyl radical.

The isotropic hyperfine coupling constants (IHFC) of hydroquinone and tetramethylhydroquinone cation radicals have been studied with the UB3LYP and UB3PW91 density functional methods. The ability of various basis sets to reproduce the experimental IHFCs was tested. The effects of the zero-point and temperature-dependent torsional oscillations of the hydroxy groups to the IHFCs were calculated by solving the torsional Schrodinger equation. Consideration of torsional zero-point and temperature-dependent oscillations were essential in order to reproduce the hydroxy proton IHFC experimental results with the computational methods. The applied theory predicts a considerable temperature dependence for the hydroxy proton IHFC and a very small temperature dependence for the ring-proton IHFCs in both cis and trans isomers.

The hyperfine coupling constants and g values of 11 α-carbonyl radicals ˙CR1R2(CRO)(1) and ˙CR3(CN)(CRO)(2)(R = H, Me, Ph, α-Naph; R1, R2, R3= H, CO2Et, Ph, 2,4-Me2Ph, Mes) have been determined by EPR and, partially, ENDOR spectroscopy. The g values lie between 2.0025 and 2.0051. Except for ˙CH(CO2Et)(CMeO), the g values are smaller than expected because of the steric hindrance of the substituents R1, R2 and R3. The CRO group is twisted out of the nodal plane of the radical which diminishes the delocalisation of the unpaired electron over the carboxyl group. The proton splittings in the α-aryl substituents differ significantly from those found in ˙CH2Ph because of distortions. Proton splittings in benzoyl groups have been observed for the first time.

The electronic structures of the azetidine radical cation and the neutral azetidin-1-yl radical have been investigated by EPR spectroscopy and theoretical ab initio UHF, MP2 and SDCl calculations. The radical cation was assigned a structure with a planar ring according to the theoretical and experimental results. Ab initio UHF calculations predicted the neutral azetidin-1-yl radical to have a puckered ring structure, with puckering angles 12.0 and 15.4° on the STO-3G and 6-31G* levels, respectively. This result was supported by the observed nonequivalent values of the experimental β-proton hyperfine coupling constants, suggesting that the neutral radical has a puckered structure at low temperature in a solid halocarbon matrix. Calculated SDCl values for the isotropic and anisotropic hyperfine coupling constants of the azetidine cation and the azetidin-1-yl radical were found to be in qualitative agreement with the experimental data.

A variation-perturbation approach is applied to compute isotope-dependent zero-point vibrational corrections to the β-hyperfine coupling constant of the ethyl radical. The corrections increase the coupling by ca. 5, 7 and 23% for the deuterium-, protium- and muonium-substituted radicals, respectively. The large vibrational correction to the muon β-hyperfine coupling constant could explain the ‘residual’ isotope effect observed experimentally.

ENDOR measurements at 173 K have been used to measure the hyperfine coupling tensors of four protons in a free radical trapped in γ-irradiated single crystals of p-nitrophenol. Two of the tensors can clearly be identified as belonging to α-protons in a planar π-radical. The angle between the positive dipolar components of the coupling tensors in these two protons, which are expected to lie along the C—H bond directions, is 137°. This results is interpreted as suggesting that the free radical is based on a five-membered ring.

EPR, ENDOR and TRIPLE resonance spectra are recorded for 9, 10-anthraquinol-1-sodium sulphonate, 9,10-anthraquinol-2-sodium sulphonate, 9,10-anthraquinol-1,5-disodium sulphonate and 9,10-anthraquinol-2,6-disodium sulphonate cation radicals. The splittings of the 9,10-anthraquinol cation radical have been assigned by deuteration. The spectra of the deuterated anthraquinol cation radicals and the assignment of hyperfine coupling constants are discussed.

Chemically Induced Dynamic Nuclear Polarization (CIDNP) of the diamagnetic products of radical reactions is exploited for the purpose of determination of the hyperfine coupling constants (HFCCs) of the radical intermediates. A simple proportionality relation between geminate CIDNP of a nucleus and its HFCC at the radical stage is established. The applicability range of this relation is determined: the relation is fulfilled in the case of a large difference in g-factor between the radicals involved and for the situation where the number of magnetic nuclei in the system is sufficiently large. The validity of the relation was confirmed by CIDNP experiments on radical pairs with precisely known HFCCs. Using the proportionality relation we were able to measure the HFCCs in various short-lived radicals of the amino acids histidine and tryptophan and of the S-N-centered cyclic radical of methionine derived from the methionine-glycine dipeptide in aqueous solution.

The field of ESR (or EPR) continues apace, predominantly as further applications are discovered for the method. As in all aspects of modern life, some of the greatest advances in the area of ESR and free-radical (paramagnetic) chemistry have arisen through the immense developments made in computing, and so we may witness not only supplements in instrumentation, e.g. pulsed ESR and ENDOR/TRIPLE methods, but also in theoretical techniques, most strikingly through the agency of Density Functional Theory (DFT), which, reminded of my early adventures with semi-empirical methods such as INDO, now appear to accurately reproduce experimental hyperfine coupling constants (always a challenging task for “open-shell” systems), and the energetics of paramagnetic molecular configurations.

Addition of pyridine to the four-coordinate cobalt(II) complex, [Co II (oebOx)]I 3 , which shows an EPR spectrum with well resolved Co hyperfine coupling, converts the complex into one with Co III /ligand radical structure and an EPR spectrum with g z = 1.98, and g xy = 2.02 and no cobalt hyperfine coupling.

The formation of silver–ethylene complexes in dehydrated Ag-SAPO-11 molecular sieve have been observed by electron paramagnetic resonance spectroscopy (EPR) after γ-irradiation at 77 K. Such reactive intermediate can play an important role in catalytic conversion of ethylene on silver loaded molecular sieves. The Ag(C2H4)2 stabilized inside relatively small channels of SAPO-11 molecular sieve it is observed even at 290 K. The ‘gas-phase’ geometries of Ag(C2H4) and Ag(C2H4)2 complexes, and respective hyperfine coupling constants were calculated applying DFT quantum chemical methods. The hyperfine coupling constants calculated for Ag(C2H4)2 complex of D2h symmetry are in very good agreement with those obtained experimentally.

Oxidation of 2,7-di-tert-butyl-1-hydroxypyrene 1 yields the exceptionally persistent and oxygen-insensitive 2,7-di-tert-butylpyren-1-oxyl radical 2 whose EPR and ENDOR spectra give the following proton hyperfine coupling (hfc) constants: 0.526, 0.445, 0.426, 0.386, 0.157, 0.107, 0.088 and 0.0054 mT (g= 2.0037). The assignments of the protons are accomplished by measuring the EPR and ENDOR spectra of partly deuteriated 2,7-di-tert-butyl(4,5,9,10-2H4)pyren-1-oxyl radical. Radical 2 is isolated as a dimer which is in equilibrium with 2 in solution, even at low temperatures. The thermodynamic parameters for the equilibrium are determined to be 7.2 ± 2.0 kJ mol–1(ΔH) and –35 ± 15 J mol–1 K–1(ΔS), respectively. The very low ΔH and negative ΔS values are briefly discussed.

The ESR and UV/VIS spectra have been studied for solutions of [N,N′-bis(pyridine-2-carbonyl)-o-phenylenediamido]nitridochromium(V) having a square-pyramidal (CrN5) chromophore. The g and 53Cr hyperfine coupling tensors (A) are axially symmetric. An analysis of the A tensor indicated a 3dxy ground-state Kramers doublet. Fairly high dπ–pπ interaction in the CrN formally triple bond was also revealed. The striking stability of this complex can be attributed to the remarkable dπ–pπ covalency of the CrN bond. None of the d–d* transitions has been observed in the region of λ 600 nm at room temperature. Bands at 25 000 and 29 000 cm–1 have been tentatively assigned to charge-transfer transitions. The considerable difference between the A⊥(53Cr), a(53Cr), g⊥ and giso values of nitrido-and those of oxo-chromium(V) complexes have been explained in terms of the different extents of covalency in the CrN and CrO multiple bonds.

Reaction of nitric oxide with 2,3-dimethyl-1,3-butadiene gave an unstable nitroxide radical characterised by hyperfine coupling to four equivalent β-protons in addition to the normal 14N triplet: however reaction with the tetramethyl derivative (2,5-dimethyl-2,4-hexadiene) gave an extremely stable nitroxide, which may be of use in pollution studies and especially in studies of nitric oxide in mammalian cells.

In [L(O)VOV(O)L]–, the one-electron reduced form of structurally characterised [L(O)VOV(O)L](L = azobenzene-2,2′-diolate), metal valence is delocalised at 300 K but is localised at one centre at 77 K with average hyperfine coupling constants of ≈50 and ≈100 G respectively; the estimated rate constant is ≈4 × 1010 s–1(300 K) and the activation energy is ≈3 kcal mol–1.

We report the observation of muonium-substituted radicals formed in 3-methyl-2-butenal. The reduced hyperfine coupling constants (A′µ) at 273 K are 59.96 MHz for addition to the C—C bond and 2.89 MHz for muonium attachment to CO.

Single crystals of fluorene, γ-irradiated at room temperature, have been studied using e.s.r. spectroscopy and the product of irradiation identified as the fluoren-9-yl radical. The components of the hyperfine coupling tensor have been determined, and the radical has been found to retain the same position as the parent molecule in the crystal lattice.

The e.s.r. spectra of the trigonal-prismatic title complexes have been recorded. The spectra show axial symmetry and are characterised by g∥= 2.00 and g⊥= 1.956 and g∥= 2.00 and g⊥= 1.971 respectively. Analysis of the values of the hyperfine coupling to vanadium, namely A∥= 22 G and A⊥= 138 G, indicates that the unpaired electron is mainly (92 and 95%) in a metal dz2 orbital. On addition of acids the complexes decompose to [VO]2+ions.

Theoretical calculations of hyperfine coupling constants of 1,2-dihydro-3H-3-oxoindole 1-oxyl and 1,2-dihydroquinoline 1-oxyl have been carried out using the semi-epirical INDO–SCF–MO method. The molecular geometries have been determined by X-ray analysis of 1,2-dihydro-3- oxo-2,2-diphenyl-3H-indole 1-oxyl and 1,2-dihydro-2,2- diphenylquinoline 1-oxyl. The application of the semi-empirical INDO–SCF–MO method is discussed.

Calculated values for the anisotropic hyperfine coupling tensors for the ethyl radical by the ab initio UHF method with spin annihilation are in good agreement with experiment. The results are critically compared with those from semi-empirical calculations.

γ-Irradiation of P4O10, P4S10 and P4Se10 produces the following radicals : [P4O10]+, [P4S10]+, and [P4Se10]+ in which the unpaired electron is based on one PX4 unit and thus related to [PO4]2–; [P4O10]– and [P4S10]– related to [PO4]4–; and [P4S9]+ and [P4Se9]+ related to [P03]2–. In the case of [P4S10]–, further hyperfine coupling (21.6 G) to the other three phosphorus atoms is seen. A mechanism for the radiation damage is proposed.

Hyperfine coupling constants for a wide range of radicals of type ·PL3(related to ·PO32–) and ·PL4(related to ·PO4–+) are reviewed, and trends as L is varied are displayed and discussed. From these correlations it is concluded that a radical formed by exposure of rigid solutions of phosphoric acid in sulphuric acid to γ-rays is ·P(OH)3+ rather than ·P(OH)4, whilst that formed from similar solutions of arsenic acid is ·As(OH)4 rather than ·As(OH)3+. Also, two new radicals formed from trimethyl phosphate are shown to be (MeO)3ṖO– and (MeO)2ṖO. Two radicals whose properties appear to be anomalous in terms of these correlations are ·PO44– and ·PH4. Possible reasons for these anomalies are considered.

After exposure of phosphoryl chloride to 60Co γ-rays at 77 K, three radical species were detected. One, having a large 31P hyperfine coupling of about 1350 G is thought to be POCl3–, having two strongly coupled chlorine atoms (Aiso 66 G) and one weakly coupled chlorine atom (Aisoca. 20 G). The second species, thought to be POCl2 gave an axially symmetric spectrum at 77 K, but this became isotropic on warming the sample to 100 K, when Aiso(31P) was 759 G and Aiso(35Cl) was 18 G.The third major species, thought to be PO2Cl2, had Aiso(31P) 43 G and Aiso(35Cl) 3 G. This species was probably formed from HPO2Cl2 impurity.With similar treatment, phosphorus trichloride gave PCl2 and PCl4 radicals whilst the pentachloride gave PCl4 and an unidentified radical containing two chlorine atoms.

The isotropic hyperfine coupling constants to 14N in the radical anions of p-dinitrobenzene and p-nitroanisole in mixed solvents have been measured as a function of solvent composition. The results are interpreted in terms of a simplified model of solvation involving strong specific solvation by one or two solvent molecules and market, unspecific solvation by the remainder. The onset of marked line-width alternation is interpreted in terms of the formation of solvent-separated ion-pairs and some details of their structure are inferred.

The observation of spin–spin coupling between 89Y and 31P nuclei in the complex [Y(S2PR2)4]–, R = OEt, is interpreted as evidence for covalent bonding. The magnitude of the reduced coupling constant 2KY–P is of the same order as 2KRh–P and 2KPt–P in [Rh(S2PR2)3] and [Pt(S2PR2)2] indicating similarity in the covalent contribution to their bonds. This result is extrapolated to the chemically similar lanthanoid complexes. The mechanism of hyperfine coupling in paramagnetic lanthanoid complexes, the nephelauxetic effect and the intensities of observed f–f transitions are discussed with respect to the above model.

Exposure of NaBH4(NaBD4) to 60Co γ-rays at 77 K gave a species having a large proton hyperfine coupling to two equivalent protons (deuterons) and a small coupling to two other protons(deuterons), together with a strongly anisotropic coupling to 11B.

γ-Irradiation of SrCl2,6H2O at 77°K results in the formation of a radical having e.s.r. parameters expected for HO2. The values of the proton hyperfine coupling along the g-tensor axes are Ax=(–)12·7, Ay=(–)6 ± 1, and Az=(–)17·2 gauss. These results are discussed in terms of a model in which the radical is compared with the hydroxyl radical. A reasonable fit is obtained for a bond angle of about 108° and a spin density of about 0·35 on the central oxygen atom.

At 300 K, both urea and formamide, on exposure to 60Co γ-rays, gave e.s.r. spectra characteristic of CONH2 radicals. The 13C hyperfine coupling confirmed that this is a σ radical, having a high spin-density on carbon. At 77 K urea gave a spectrum characteristic of the π radical H2NCONH, having a high spin-density on nitrogen. Possible mechanisms for radiation damage are outlined.

Phosphorus-31 hyperfine splittings in tetraphenylporphyrincobalt(II) complexes with Et3P and (EtO)3P and the corresponding dioxygen complexes are used in evaluating the influence of substituents on the nature of the phosphorus σ-donor orbital.

Diaquacobaloxime promotes formation of radicals from formaldehyde; products of trapping of these radicals by nitroso-compounds have unusually small methylene hyperfine coupling constants.

A linear correlation is found between CA π-bond energies in compounds of the type H2CARn(or HCARn–1) and β-proton hyperfine coupling constants for the corresponding radicals H3C–ȦRn(or H2CARn-1): an empirical method is therefore available for either estimating π-bond energies from e.s.r. data or predicting hyperfine coupling constants from bond energy data.

Reasons for supposing that a radical exhibiting well defined hyperfine coupling to 127I, formed in γ-irradiated α-iodoacetamide, is HĊ(I)CONH2 are summarised.

Experimental values of the σ–π spin polarisation parameters for phosphorus have been determined from a comparison of calculated spin densities and observed hyperfine coupling constants in several free radicals and ions containing phosphorus.

The mercuration of various arenes is promoted when they are photolysed in trifluoroacetic acid containing mercury(II) trifluoroacetate, and 199Hg hyperfine coupling can be observed in the e.s.r. spectra of the radical cations which are formed.

Ab initio SCF MO calculations are described for the radicals CH3, CCl3, CF3, SiH3, SiCl3, SiF3, PH+3, PCl+3 and PF+3. The calculated optimized geometries follow the prediction of Pauling using electronegativity arguments, and the calculated hyperfine coupling constants are in moderate agreement with the experimental values.

The finding that 1H coupling constants for Me2S+ and Me2Se+ exceed those for the corresponding dimer radical cation Me2S[graphic omitted]SMe2+ and Me2Se[graphic omitted]SMe2+ by a factor of three in each case is attributed to the anti-bonding character of the σ* semi-occupied molecular orbital in the dimeric species.

A new and totally general method for the determination of the hyperfine coupling tensors from the ENDOR spectra of polycrystalline samples is presented. The tensor for the γ-proton of the complex bis(pentane-2,4-dionato)oxovanadium(text-decoration:overlineIV) is extracted by fitting the experimental variation of the hyperfine splitting to that calculated from theory.

Exposure of K[Ag(CN)2] to 60Co γ-rays at 77 K gives two paramagnetic species, A and B. Species A, having g∥ 2.001, g⊥ 2.006. A∥(109Ag) 60 G, and A⊥(109Ag) 30 G, is identified as an electron-gain centre, [Ag(CN)2]2–, with the extra electron in a σ* orbital with 4dz2+ 5s character on silver and considerable density on the ligands. Species B, having gx 2.091, gy 2.051, gz 2.008. Ax(109Ag) 42 G, Ay 34 G, and Az⩽ 65 G, is thought to be the electron-loss centre, Ag[CN]2 with the hole largely localised in silver (again, 4dz2± 5s); on annealing above 77 K, centre B, acquired axial symmetry (g⊥ 2.078, A⊥ 29 G). Solutions in methanol give a poorly defined centre similar to A at 77 K. On annealing, a new species having Aiso.(109Ag) 155 G and A(14N)(one) 17.3 G with gav. 2.002 is obtained. This is tentatively identified as [Ag(CN)]– and reasons for the large hyperfine coupling to 14N are discussed.

A series of rhenium(0) radicals were spin trapped by 9,10-phenanthroquinone (C14H8O2) and its derivatives, [Re(˙O2C14H8 –nXn)(CO)4], [Re(˙O2C14H8 –nXn)(CO)3(PPh3)], and [Re(˙O2C14H8 –nXn)(CO)2(PPh3)2](n= 1, X = H, OMe-3, Cl-3, Br-3, Pri-3, CN-3, NO2-2, or NO2-4; n= 4, X4= Me4-1,3,6,8 or -2,4,5,7), and were studied by e.s.r. spectroscopy. The rhenium hyperfine coupling constants, ARe, for [Re(˙O2C14H7X)(CO)4](X = H, OMe-3, Cl-3, Br-3, Pri-3, CN-3, NO2-2, or NO2-4) correlate very well with the Hammett parameter σp. Values of ARe for [Re(˙O2C14H8 –nXn)(CO)3(PPh3)] and AP for [Re(˙O2C14H8 –nXn)(CO)3(PPh3)] and [Re(˙O2C14H8 –nXn)(CO)2(PPh3)2] also correlate with the ARe values for [Re(˙O2C14H8 –nXn)(CO)4]. However, in the correlation of values of ARe for bis(phosphine) and tetracarbonyl radicals, the data for radicals with a strong electron-withdrawing group attached to 9,10-phenanthroquinone are irregular. It is proposed that the irregularity is due to the stereochemical change of [Re(˙O2C14H8 –nXn)(CO)3(PPh3)2] induced by the electron-withdrawing group.

The 51V hyperfine coupling in vanadyl acetylacetonate has been measured for solutions in mixtures of ethanol and carbon tetrachloride as a function of solvent composition. The results indicate that interaction with at least two solvent molecules determines the hyperfine interaction, and the observations have been characterized quantitatively in terms of two equilibria.

This overview is divided into two sections, one concerned primarily with radicals stabilised by matrix-isolation procedures and the other with the detection of transients. In the former, attention is given to e.s.r. studies of radical cations, with particular emphasis on results for alkane cations and the remarkable range of structures revealed by the spectra.In our consideration of transient radicals major attention is given to the electron-spin requirements of radical combination and the way in which interconversion between the spin states of radical pairs is often controlled by proton hyperfine coupling. This leads us to consider CIDEP and CIDNP effects. In addition, the uses of ODMR techniques in the study of transient radicals is outlined.

The e.s.r. spectrum of the cation radical from 2,3,4,7,8,9-hexahydro-2,2,5,7,7,10-hexamethylbenzo[1,2-b:4,5-b′]dipyran is temperature dependent and shows the alternating line-width effect from interconversion.ENDOR and TRIPLE resonance measurements provided precise values of hyperfine coupling constants and their relative signs at the slow limit. These data enabled us to simulate spectra from the slow to the fast limits, which are observed at ca.–70 and ca. 100 °C, respectively. The activation parameters for the interconversion process were found to be +28(± 4) kJ mol–1, +50(± 3) kJ mol–1 and +71(± 12) J mol–1 K–1 for the free energy, enthalpy and entropy, respectively, at 298 K. The e.s.r. spectrum of the cation radical from 2,3,4,5,6,7-hexahydro-2,2,7,7,9,10-hexamethylbenzo[1,2-b:4,3-b′]dipyran is also temperature dependent, but the slow limit is observed at room temperature and the alternating line-width effect is masked: activation parameters for the interconversion were found to be +42(± 6) kJ mol–1, +29(± 3) kJ mol–1 and –46(± 9) J mol–1 K–1 for the free energy, enthalpy and entropy, respectively, at 298 K. A third tricyclic chroman, namely 2,3,6,7-tetrahydro-2,2,4,6,6,8-hexamethylbenzo[1,2-b:4,5-b′]difuran, gives a cation radical which has a temperature-independent e.s.r. spectrum, owing to interconversion, which is rapid on the e.s.r. time-scale.

We have correlated the observed phosphorus coupling constants of five radical anions obtained from phosphine derivatives with the calculated spin densities, and find that dπ–pπ interactions account satisfactorily for the results whereas pπ–pπ interactions give results very different from those observed.