The triplet state of magnesiumporphin · ethanol in an n-octane crystal studied by microwave induced changes in the fluorescence intensity

The zero-field splitting of the triplet state of magnesiumporphin solvated by ethanol is represented by D = 0.035 cm^?1 and |E| = 0.010 cm^?1. The decay rates of the upper two spin components both are found to be about 20 s^?1, while that of the bottom component (where the spin lies in the molecular plane) is about 2 s^?1.     

Cationic η3-Triphenylcyclopropenylnickel complexes with tridentate ligands containing N,P, As as the donor atoms. Molecular structure of η3-triphenylcyclopropenyl-1,1,1-tris(diphenylphosphinomethyl)ethanenickel perchlorate

The [(C₃Ph₃)Ni(PPh₃)₂]ClO₄ complex reacts with the tridentate ligands, 1,1,1-tris(dimethylphosphinomethyl)ethane, 1,1,1-tris(diphenylphosphinomethyl)ethane, (bis(2-diphenylphosphino)ethyl)phenylphosphine, (bis(2-diphenylphosphino)ethyl)-n-propylamine, and 1,1,1-tris(diphenylarsinomethyl)ethane to give cationic η³-triphenylcyclopropenyl complexes of formula [(C₃Ph₃)NiL]Y (Y = ClO₄, BPh₄). An uncharged derivative with the formula [(C₃Ph₃)Ni(hb(3,5-me₂Pz)₃)] (hb(3,5-me₂Pz)₃ = hydrotris(3,5-dimethyl-1-pyrazolyl)borate) has also been prepared. The molecular structure of [(C₃Ph₃)Ni(triphos)]ClO₄ has been determined from counter diffraction data. The crystals are monoclinic, space group P2₁/n with cell dimensions: a 17.750(5), b 17.629(5), c 16.509(4) Å; β 92-59(9)°, D_c = 1.359 g cm^?3 for Z = 4. Full matrix least-squares refinement led to the conventional R factor of 0.064 for 2556 observed reflections. The molecular structure consists of [(C₃Ph₃)Ni(triphos)]⁺ cations and ClO₄^? anions. The nickel atom is coordinated to the three phosphorus atoms of the triphos ligand, and to the C₃Ph₃ fragment in a symmetric η³ fashion.     

Lifetimes of triplet state alkylbenzenes in the vapor phase

Rate constants for triplet state decay of C₆H₆, C₆D₆ and 15 alkylbenzenes in the vapor phase have been found, using a flash-sensitized biacetyl phosphorescence technique, to range between 800 s^?1 and ? 18 000 s^?1. Only benzene has a significant positive activation energy for decay. Kinetic and spectroscopic evidence supports a photoisomerization decay channel in t-butylbenzene. Comparison of lifetimes with molecular size shows that increased density of rotational levels does not account for rapid decay of the triplet state. This contradicts a recent suggestion to explain short lifetimes of triplet state aromatic hydrocarbons in the vapor phase, relative to the long lifetimes of the same molecules in low temperat matrices. Evidence suggests that the dominant decay paths for triplet state alkylbenzenes are different in room temperature vapors and low temperature matrices.     

Emission spectroscopic and ODMR studies of the lowest excited triplet state of dichlorophenylborane

The triplet state T₁ of dichlorophenylborane (PhBCl₂) has been investigated by optical emission and ODMR spectroscopic methods in order to study the influence of substituents with mesomeric and inductive effects. The zero-field splitting (ZFS) parameters D and E, the selective kinetic rates of radiative and non-radiative deactivation of the triplet sublevels and the phosphorescence spectrum were measured. From the small value of D = 0.1201 cm^?1 a considerable charge transfer admixture to the ³L_a state of benzene has to be assumed. The ratio of the radiative rates shows a distortion of the molecule. Further a heavy atom effect of the chlorine atoms on the in-plane rates of the deactivation of T₁ can be observed.     

The triplet state of photosynthetic pigments. I. Pheophytins

ZFS constants (in cm^?1) and decay rate constants for the lowest triplet state of pheophytins have been determined by ESR: pheophytin a: D = 341 ± 3,E = 33 ± 3, K_T = 1050 s^?1; pheophytin b: D = 358 ± 8, E = 46 ± 5, K_T = 630 s^?1; bacteriopheophytin: D = 256 ± 4, E = 54 ± 5/37 ± 5, K_T ≈ 4000 s^?1. In addition values for the decay rate constants and relative populating rates of the individual spin levels have been obtained; these numbers turn out to be appreciably different from those for the corresponding chlorophylls. For the series pheophytin a, b and bacteriopheophytin we find parallel behaviour with the corresponding chlorophylls. The effects of side group substitution and pyrrole ring reduction on the ZFS constant D can be understood by including configuration interaction between the excited states using the 4-orbital model. The change of the mean triplet decay constant K_T upon side group substitution and pyrrole ring reduction follows an energy gap law. Substitution of the central Mg-ion by two protons, however, causes K_T to increase; this is attributed to the introduction of an extra promoting mode - of the NH-group - and/or to the presence of low lying nπ^* states in pheophytins.     

Electron spin resonance studies of the excited triplet states of DL-5-hydroxytryptophan, 5-hydroxyindole, 6-hydroxynicotinic acid, indole and hippuric acid

Abstract— The electron spin resonances of the metastable triplet states of DL-5-hydroxy-tryptophan, 6-hydroxynicotinic acid, and other related compounds have been observed in various glasses at 77°K. These compounds are biologically interesting in connection with photochemical processes. The Δm= 2 transitions were observed and the spin-spin interaction parameter (D²+ 3E²1/2 was determined. However, for indole, it was also able to observe the Δm= 1 transitions from which D and E were determined separately. From the values of (D²+ 3E²)1/2, D and E whenever possible and from the decay times of the triplet states, it was possible to characterize the lowest triplets as π—π* states in all cases. Studies of the pH dependence of DL-5-hydroxytryptophan, 5-hydroxyindole, and 6-hydroxynicotinic acid indicate that the unionized molecule exhibits a resonance different from that of the related ion with a proton removed from the phenolic hydroxyl group. The zero-field splitting parameters D* and the lifetime of the triplet states have also been measured in various glasses and in Lucite. Both temperature and the specific nature of the host media affected the values of these parameters.     

The electronic structure of 5-methylhexa-1,2,4-triene-1,3-diyl, the first representative of highly delocalized triplet ethynylvinylcarbenes, from ESR spectroscopy data and quantum chemical calculations

The ESR spectrum of the first representative of highly conjugated triplet ethynylvinylcarbenes, 5-methylhexa-1,2,4-triene-1,3-diyl (1), was recorded in solid argon matrix. The zero-field splitting (ZFS) parameters of carbene 1 (D = 0.5054±0.0006 cm^?1 and E = 0.0045±0.0002 cm^?1) determined from the experimental ESR spectrum are in between the corresponding parameters of ethynylcarbene C3H2 (2) and vinylcarbene C₃H₄ (3): D(3) < D(1) < D(2) and E(2) < E(1) < E(3). Quantum chemical calculations of the ZFS parameters of 1, 2, and 3 have been carried out for the first time using two DFT-based approaches, RODFT and UDFT. An analysis of the experimental and theoretical ZFS parameters shows that carbene 1 is characterized by a greater extent of delocalization of the spin density of unpaired electrons than carbenes 2 and 3. The characteristic structural fragments of carbene 1 possess the principal features of the electronic structure of both ethynylcarbene (2) and vinylcarbene (3), respectively. Magnetic spin-spin interactions are identical in carbenes 1 and 2. The dominant contribution to D in 1 and 2 results from the one-center spin-spin interactions on carbon atoms in the propynylidene group, which are subjected to strong spin polarization.     

An unusual oxidative addition-ligand elimination reaction. Preparation and structure of RhCl2(PMe2Ph)2(C3Ph3)

The sym-triphenylcyclopropenium cation (C₃Ph₃⁺) stabilized as the Cl^? or PF₆^? salt, undergoes facile reactions at room temperature with trans-Rh(CO)Cl(PMe₂Ph)₂ to produce complexes which result from the oxidative cleavage of the ring and decarbonylation of the organometallic reactant. The product of the C₃Ph₃⁺Cl^? reaction has been fully characterized by X-ray analysis and is shown to be RhCl₂(PMe₂Ph)₂(C₃Ph₃).     

Cyclopentadienylcobalt Complexes Containing Two Phosphine Ligands

The ionic complexes [C₅H₅Co(L)(Ph₂PMe)I]⁺ [I]^- (L = Ph₃P and Ph₂PMe) were prepared by the reactions of cyclopentadienyl(triphenylphosphine)cobalt and cyclopentadienyl(methyldiphenylphosphine)cobalt diiodides with methyldiphenylphosphine. The treatment of these complexes with sodium tetraphenylborate results in the formation of [C₅H₅Co(L)(Ph₂PMe)I]⁺[BPh₄]^- compounds.     

The relative signs of the zero-field parameters from phosphorescence-microwave double resonance (PMDR) spectroscopy: 1, 2, 4, 5 tetrachlorobenzene

It is shown that the relative signs of D and E as well as the assignment of the symmetry of the electronic spatial wavefunction of the lowest emitting triplet state can be made at very low temperatures by determining the polarized amplitude modulated phosphorescence-microwave double resonance (AM-PMDR) spectrum, the lifetimes of the individual zero-field (zf) levels, and the polarization of the crucial magnetic zf transition. The technique is applied to 1, 2, 4, 5 tetrachlorobenzene. D and E are found to be of the same sign, and the lowest triplet state is found to be of ³B_1u symmetry. In going from benzene to its tetrachloro-derivative, the absolute value of D decreases by only a few percent, while that for E increases by ≈400%.     

Kinetics of nucleophilic attack on coordinated organic moieties : XII. Addition of triphenylphosphine to [(C8H11)Co(C5H5)]BF4

A stopped-flow investigation of the reversible addition of Ph₃P to [(C₈H₁₁)Co(C₅H₅)]⁺ indicates the rate law, k_obs = k₁[Ph₃P] + k_?1. The low Δ2 of 21.0 ± 1.2 kJ mol^?1 and the negative ΔS2 of ?114 ± 5 J K^?1 mol^?1 are consistent with rapid addition to the enyl ligand. The higher Δ2 of 86.2 ± 5.1 kJ mol^?1 and the positive ΔS2 of +60 ± 17 J K^?1 mol^?1are as expected for the reverse dissociation. Preliminary studies show that the related complex [(C₇H₉)Co(C₅H₅)]⁺ is at least 65 times more electrophilic towards Ph₃P.     

MODELS FOR BACTERIAL PHOTOSYNTHESIS: ELECTRON TRANSFER FROM PHOTOEXCITED SINGLET BACTERIOPHEOPHYTIN TO METHYL VIOLOGEN AND m-DINITROBENZENE

Abstract. As a model for the primary reactions of photosynthesis, we studied photochemical electron transfer from bacteriopheophytin (BPh) to methyl viologen (MVC1₂) and to m-dinitrobenzene (m-DNB) in solution. Both MVC1₂ and m-DNB cause reductions in the lifetime of the first excited singlet state of BPh (BPh*), in the fluorescence quantum yield, and in the quantum yield of the triplet state, BPh ⁺. The quenching of BPh* probably results from electron transfer, which generates short-lived radical pairs involving the BPh radical cation (BPh⁺) and the reduced form of the quencher. Electron transfer from BPh* is thermodynamically favorable, but that from BPh^T is not. From the magnitude of the quenching, we calculate rate constants for electron transfer in collision complexes formed between BPh* and MVC1₂ or m-DNB. Measurements of the quantum yield of the free BPh⁺ radical indicate that about 3/4 of the [BPh⁺ MV⁺] radical pairs decay by reverse electron transfer, rather than dissociating to give the free radicals. Essentially all of the [BPh⁺m-DNB ⁺] radical pairs must decay by reverse electron transfer, because free BPh⁺ cannot be detected in this case. From these data, we estimate the rate constants for the reverse electron transfer reactions. The higher probability of dissociation in the [BPh⁺ MV⁺] radical pair can be explained by coulombic repulsion. The rate of the primary electron transfer reaction in photosynthetic bacteria is comparable to that of forward electron transfer in the BPh* collision complexes. Reverse electron transfer, however, is at least 10³-times slower in the radical pair formed in the bacterial reaction center than it is in [BPh⁺m-DNB^?], and more than 10⁴-times slower than in [BPh⁺ MV⁺]. The explanation for this dramatic and crucially important difference remains unclear, but several possibilities are discussed.     

A novel reaction producing the rhodium(I) complexes with π-coordinated tetraphenylborate anion, (π-PhBPh3). X-ray study of [Rh(PPh3)2(π-PhBPh3)]

Treating the complexes [Rh(TFA)(PPh₃)₂], [Rh(HFA)(PPh₃)₂], and [Rh(TFA)(Cod)] (TFA - trifluoroacetylacetonate, HFA - hexafluoroacetylacetonate, Cod - 1,5 cyclooctadiene) with an excess of NaBPh₄ in acetonitrile yields the rhodium(I) complexes with coordinated [BPh₄]⁻ anion, [Rh(PPh₃)₂(π-PhBPh₃)] · 2MeCN (I) and [Rh(Cod)(π-PhBPh₃)] (II). The reactions present a new example of β-diketonate ligand replacement. The ¹H, ³¹P, and ¹¹B NMR spectra of I and II are discussed. [Rh(PPh₃)₂(π-PhBPh₃)] has been characterized by single crystal X-ray analysis.     

Time-resolved resonance raman spectra and decay kinetics of triplet anthracene in fluid media

Reported here are the time-resolved resonance Raman spectra and decay kinetics of the lowest triplet state (³B_2u⁺) of anthracene-h₁₀ and anthracene-d₁₀ molecules in fluid media at room temperature. The triplet population (≈3 × 10^?5 M) is observed to decay at microsecond times by triplet—triplet annihilation. Vibrational assignments for the observed Raman bands are proposed.     

Synthesis and 119mSn Mössbauer studies of cationic tributyltin complexes

A series of cationic tributyltin complexes with a tetraphenylborate counter anion, [Bu₃SnL₂]⁺BPh₄,^?, where L = DMSO, Ph₃PO, Ph₃,AsO, pyO or DMF, and L₂ = diphosO₂, has been prepared and their ^119mSn Mössbauer spectra investigated. These indicate that the triorganotin cation occupies a pentacoordinate trigonal bipyramidal geometry with equatorial organic groups when L = monodentate donor, whilst the bidentate diphosO₂ ligand produces a mer-R₃SnX₂ stereochemistry about tin.     

Relativistic molecular orbital theory of zero-field splittings in triplets

A method of calculating matrix elements of 1/r₁₂ in a basis of Dirac scattered-wave (DSW) orbitals is outlined. In the limit c → ∞, this method reduces to that described by Cook and Karplus for non-relativistic orbitals. For triplet states that can be described by a single configuration with two unpaired electrons, the relativistic exchange integrals give not only the singlet—triplet splittings (as in non-relativistic theory), but also the spin—orbit contributions to the triplet zero-field splittings. Results are reported for the ³ (n → π*) excited state of γ-thiopyrone (4H-pyran-4-thione), which has a very large value of D (calc. ?31 cm^?1, exp. ?24 to ?28 cm^?1).     

Spin polarization in the lowest triplet state of chlorophyll

Chlorophyll-b in glassy solution has a spin-polarized lowest triplet state at and above 77 K. The magnitude of the effect is different for MTHF and ethanol as solvents, in contrast to what is found for the porphin free base. Chlorophyll-a does not exhibit spin-polarization under identical conditions as for chlorophyll-b. Zero-field parameters are found to be:chlorophyll-a (MTHF) D = (281 ± 6) × 10^?4 cm^?1; E = (39 ± 3) × 10^?4 cm^?1;chlorophyll-b (MTHF) D = (289 ± 4) × 10^?4 cm^?1; E = (49 ± 3) × 10^?4 cm^?1,From ESR signal kinetics it follows that for chlorophyll-b, population and depopulation mainly involve the spin level y?, describing a spin moving in a plane perpendicular to the molecular plane:P_y ? P_x ? P_z; k_x = 240 ± 40 s^?1; k_y = 600 ± 120 s^?1; k_z ? 75 s^?1,where P_i and k_i denote populating and decay rates. Thus, the kinetic scheme for the chlorophyll triplet is different from that of porphyrins with heavier metal ions, but very similar to that of the porphin free base. The spin-lattice relaxation time is found to be anisotropic and shorter than the decay rates of individual spin levels. Nevertheless, spin polarization can be observed, essentially because the ESR signal amplitude depends on population differences.     

Distortion of the triplet state of benzene in a borazole host. A study by microwave induced delayed phosphorescence at 1.2 K

The zero-field splitting parameters of C₆H₆ and C₆D₆ in a borazole crystal host have been measured. The absolute value of the parameter E is three times as large as for C₆H₆ in C₆D₆. The relative rate constants for radiative decay in some vibronic bands have been obtained by MIDP methods. In the OO band of the phsophorescence spectrum the radiation stems from the upper zf level indicating a quinoid electronic structure for benzene in borazole, i.e. opposite to that for C₆H₆ in C₆D₆.     

Equilibrium and kinetics of reversible capture of solvated electron-sodium+ pairs by 1,1-diphenylethylene in tetrahydrofuran

Solvated electron-Na⁺ pairs, e^?,Na⁺, and 1,1-diphenylethylene reversibly recombine in THF, the capture constant = 3 × 10⁷ M^?1 s^?1 and the detachment constant = 46 s^?1; the e^?,Na⁺, formed by flash-photolysis of Na⁺,$\text{C}\text{?}$(Ph)₂CH₂CH₂$\text{C}\text{?}$(Ph₂, Na⁺ survive for 0.1 s in this solvent at ambient temperature without any detectable decay.     

Low-temperature photolysis of azidostyrylquinolines

Low-temperature photolysis of 2-and 4-(4′-azidostyryl)quinolines and azidohemicyanine dye, 1-methyl-4-(4′-azidostyryl)quinolinium iodide, was studied in an ether-ethanol matrix at 77 K and a methyltetrahydrofuran matrix at 5 K by means of electronic absorption spectroscopy and ESR technique. The formation of corresponding triplet nitrenes with absorption bands at 380–440 nm and zero-field splitting parameters of |D/h _cl = 0.781–0.790 cm^?1 and E = 0 was detected. It was found that the introduction of the positive charge into the azidostyrylquinoline molecule resulted in a bathochromic shift of the nitrene absorption band by ～40 nm and a decrease in the D by 0.005 cm^?1 due to charge transfer from the nitrene center to the quinoline moiety.