An investigation of the triplet state dynamics of zinc chlorophyll b by microwave-induced changes in the intensity of fluorescence and singlet-singlet absorption

Optical detection of magnetic resonance experiments on the triplet state of zinc-substitution chlorophyll b has provided the zero-field splitting and depopulation rate constants for the individual triplet spin sublevels. The zero field triplet state EPR transitions could be observed at 890 MHz and 1085 MHz as either microwave-induced changes in the fluorescence intensity or in the intensity of S₀ → S_n absorption. The dynamics experiments show that intersystem crossing from the Zn chlorophyll b triplet state into the ground state occurs primarily through the out-of-plane (lowest energy) spin sublevel.     

An EPR study of the triplet state of pentacene by electron spin-echo techniques and laser flash excitation

By combining the electron spin-echo technique with pulsed-laser excitation we have been able to perform an EPR study of the very short-lived (≈ 30 μs) triplet state of pentacene in naphthalene. We have obtained the orientation of the pentacene molecule in the naphthalene host crystal, the zero-field splitting parameters, and the kinetic properties of the triplet spin levels.     

Optically detected zero-field triplet state magnetic resonance in photosynthetic bacteria

Triplet state transitions of the photosynthetic bacteria Rhodospirillum Rubrum, Rhodopseudomonas Spheroides and Chromatium Vinosum in chemically reduced preparations have been observed by zero-field optical detection of magnetic resonance at 2 K. For each bacterial preparation two sharp, structureless, zero-field EPR transitions were observed as microwave-induced decreases in the fluorescence intensity of the frozen cellular preparations. The depopulating rate constants for the spin sublevels of the triplet states observed in R Rubrum and R Spheroides were also measured. The similarities of the triplet state frequencies, spectral features and intersystem crossing rates suggest a common structure for the reaction centers in the photosynthetic bacteria.     

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.     

Detection and measurement of noncoincidence between the principal axes of the g-matrix and zero-field splitting tensor using multifrequency powder EPR spectroscopy: application to cis-[(NH(3))(2)Pt(1-MeU)(2)Cu(H(2)O)(2)](SO(4)) x 4.5H(2)O (1-MeU = monoanion of 1-methyluracil)

Multifrequency continuous wave EPR spectra (4-34 GHz) on a powder of the title compound are consistent with a spin-triplet state. This arises from interaction between centrosymmetrically related pairs of copper(II) ions in the solid. The spectra at all frequencies have been simulated with a single set of spin-Hamiltonian parameters. The results show that there is noncoincidence between the principal axes of the g-matrices on each copper center and those of the zero-field splitting (D) tensor. This noncoincidence is a single rotation of 33 degrees +/- 2 degrees. The parameters from the powder spectra have been verified by a subsequent single-crystal EPR study which yielded the spin-Hamiltonian parameters g(XX) = 2.074, g(YY) = 2.093, g(ZZ) = 2.385, D(XX) = +/-0.0228 cm(-1), D(YY) = +/-0.0211 cm(-1), D(ZZ) = -/+0.0439 cm(-1) with Euler angles of alpha = 179 degrees, chi = 33.4 degrees, and gamma = 328 degrees. Analysis of the zero-field splitting tensor in terms of exchange indicates that the interaction between the pairs of copper(II) ions is almost entirely dipolar in origin. This study shows that multifrequency EPR spectroscopy on powders, coupled with spectrum simulation, can detect and measure noncoincidence between the principal axes of the g-matrix and zero-field splitting tensor, and does not necessarily require the presence of metal hyperfine interactions.     

Triplet state properties of the benzo[α]pyrene-DNA complex as determined by optically detected zero-field magnetic resonance

The zero-field transitions in the photoexcited triplet state of benzo[α]pyrene bound to DNA have been observed by optical detection of magnetic resonance of 2 K. The transition frequencies individual spin sublevel intersystem crossing rates were measured by monitoring the microwave-induced intensity changes of the triplettriplet absorption at 465.8 nm. The triplet state zero-field splitting and dynamics for the benzo[α]pyrene DNA complex are compared with these properties measured for benzo(α)pyrene in other solvent systems.     

EPR study of electron spin polarization in the photoexcited triplet state of chlorophyll a and b

The photoexcited triplet states of chlorophyll à and b are studied by the EPR method at ≈85 K using modulated light excitation. Both compounds show anomalous EPR line intensities and transient kinetics, indicating electron spin polarization (ESP) in the photoexcited triplet state. EPR studies, using Mg-tetraphenyl porphyrin (MgTPP) dissolved in n-octane show that ESP occurs also in that solvent. It is shown that the zero field splitting (ZFS) parameters of MgTPP depend strongly on the solvent. From the analysis of the data for chlorophyll a and b we evaluate: (1) the population rate constants (k_p); (2) the ratio between the population rate constants (A_p) (p = x, y, z) and, (3) the spin lattice relaxation rate W. In both chlorophylls the in-plane component, x, is predominantly populated and depopulated. The ZFS parameters have been also determined for the above compounds.     

The photoinduced triplet of flavins and its protonation states

The photogenerated triplet states of riboflavin and flavin mononucleotide (FMN) have been examined by time-resolved electron paramagnetic resonance (EPR) spectroscopy at low temperature (T = 80 K). Because of the high time resolution of the utilized EPR instrumentation, the triplets are for the first time observed in the nonequilibrated electron-spin polarized state and not in their equilibrated forms with the population of the triplet sublevels governed by Boltzmann distribution. The electron-spin polarization pattern directly reflects the anisotropy of the intersystem crossing from the excited singlet-state precursor. Spectral analysis of the resulting enhanced absorptive and emissive EPR signals yields the zero-field splitting parameters, |D| and |E|, and the zero-field populations of the triplet at high accuracy. These parameters are sensitive probes for the protonation state of the flavin's isoalloxazine ring, as becomes evident by a comparison of the spectra recorded at different pH values of the solvent. The three protonation states of the flavins can furthermore be distinguished by the kinetics of the transient EPR signals, which are dominated by spin-lattice relaxation. The fastest decays are observed for the protonated FMN and riboflavin triplets, followed by the deprotonated flavin triplets. Slow decays are measured for the triplet states of neutral FMN and riboflavin. Because proton transfer is found to be slow on the time scale of spin-polarized triplet detection by transient EPR, the pH-dependent spin-relaxation and zero-field splitting parameters offer a novel approach to probe the protonation state of flavins in their singlet ground state through the characterization of their triplet-state properties.     

Triplet EPR of fluorobenzonitriles

The triplet EPR spectra of the three monofluorobenzonitriles in ethanol glasses at ≈90 K have been compared. Ortho- and meta-fluorine merely broaden the ΔM = 2 signals, whereas para-fluorine produces a 54 G splitting. In accord with simple symmetry considerations, the ³La state of benzonitrile is predominately a 1,4-diradical with little free spin density at the ortho and meta positions.     

ESR study of the photoexcited triplet state of substituted 2-acetonaphthones

Electron spin resonance of phosphorescent triplet states of a number of 6-substituted-2-acetonaphthones is studied at 77 K in toluene glass. The life-times of the triplet states and zero-field parameter D* have been measured from Δm_s = ± 2 resonance. The life-times and the zero-field splitting parameters indicate that the lowest triplet states have mainly π, π* character.     

ESR of triplet states in single crystals of photochromic 2,3,4,4-tetrachloro-1-keto-1,4-dihydronaphthalene (β-TKN)

Triplet ESR spectra of irradiated single crystals of β-TKN have been investigated as a function of their orientation with respect to the magnetic field. From the directions of the principal axes of the dipolar coupling tensor and the values of the zero field parameters D = ?96.2 gauss and E = ?2.7 gauss, it follows that the four triplet sites, generated in the crystals, consist of radical pairs. Each pair radicals is formed from two β-TKN molecules, which are nearest neighbours in the lattice. Measurements at the temperatures 4.2 K and 1.2 K show that the triplet state is the ground state of the radical pair.     

Time-resolved ESR study of the lowest excited triplet states of para-quinones in glassy matrices at 77 k

The lowest excited nπ* triplet of 9.10-anthraquinone, 1.4-naphthoquinone and 1,4-benzoquinone were studied in glassy matrices at 77 K using a time-resolved ESR method. The D value of the triplet state of 9,10-anthraquinone varied from ?0.351 cm^?1 in a polar solvent to ?0.318 cm^?1 in a non-polar solvent. Both 1,4-naphthoquinone and 1,4-benzoquinone in polar solvents showed triplet state spectra with a D value of ?0.330 cm^?1. A computer simulation revealed the existence of widely distributed zero-field splitting parameters in the glassy condition. These data are compared with an analysis of CIDEP results of para-quinones.     

Structure and dynamics of the triplet state of oligothiophenes in isotropic and partially oriented matrices

The photoexcited triplet states of 4,4'-dipentoxy-2,2'-dithiophene (4-T2), 3,3'-dipentoxy-2,2'-dithiophene (3-T2), and 4,4'-dipentoxy-2,2':5',2':5',2'-tetrathiophene (4-T4) have been investigated by time-resolved electron paramagnetic resonance in glassy toluene and in a frozen oriented liquid crystal, which provides a partially ordered medium. The preferential orientation of the rod-like 4-T2 and 4-T4 is compared to that of the disk-like 3-T2. The use of an oriented matrix coupled to simple semiempirical calculations allowed us to determine the orientation of the principal axes of the fine interaction with respect to the molecular axes. The motional behavior of the molecules in the isotropic and anisotropic matrices has been studied by comparing the spectral profiles of the Echo detected EPR (Echo-EPR) spectra with those of the continuous wave time resolved EPR (TR-EPR). A model considering the modulation of the zero-field splitting (ZFS) by molecular libration accounts for the Echo-EPR lineshape, on the basis of motions around preferential axes depending on the embedding matrix. The differences in the ZFS parameters of the two isomers 4-T2 and 3-T2 are attributed to a mesomeric effect due to the substituents.     

Triplet xanthone in n-pentane: multiple emission from a single site

The three phosphorescence components of xanthone in n-pentane originate from three states of one solvated species: From the z sublevel of the second triplet state, of ³nπ^* origin, and from two widely split sublevels of the lowest triplet state, of ³ππ_* origin. Its z sublevel is thermally depleted across the spin-orbit mixing induced zero-field sublevel splitting of 15.1 cm^?1.     

Anisotropy of the triplet spin—lattice relaxation in molecular crystals at low temperatures and high magnetic fields

The anisotropy of the triplet relaxation rates in molecular crystals at low temperatures (T = 1.6 K) and very high magnetic fields (B = 5.2–10 T) is explained by a model, which assumes time dependent matrix elements of the electronic dipole tensor and of the electronic g-tensor. The time dependencies may be due, for instance, to librations of the molecule or to changes in the electronic configuration; they arise from a direct process. This fact is used in order to reduce the number of parameters. The relaxation rates are given as functions of the direction cosines of the magnetic field and of eight parameters which are determined in a least-squares fit for the system quinoxaline in perdeuterated naphthalene. Some uncertainties concerning the numerical values of the parameters would be reduced by measurements with the magnetic field directed along the principal axes of the electronic dipole tensor.     

The lowest excited triplet state of triphenylboron and of the isoelectronic triphenylcarbenium ion

Triphenylboron BPh₃ and the triphenylcarbenium salts C⁺Ph₃/SbCl₆^? and C⁺Ph₃/BF₄^? have been investigated by ODMR and emission spectroscopic methods. The zero-field splitting (ZFS) parameters D and E and the decay rate constants of the triplet zero-field levels (ZFL) as well as the phosphorescence spectra were measured. The non-zero E values indicate a symmetry lower than D₃ for the Jahn-Teller unstable triplet state of all compounds. The radiative decay of T₁ shows a strong delocalization of the triplet wavefunction for C⁺Ph₃, but a strong localization on the benzene rings for BPh₃. This is in agreement with MO calculations.     

Triplet exciton zero-field odmr on naphthalene at 1.2 K

Using optical detection via delayed fluorescence we have observed the T_y^*–T_x zero-field transition of the triplet exciton in pure naphthalene at 1.2 K. This finding is compared with the results of a numerical calculation for the resonance frequency and linewidth of this transition.     

Polarization of zero-field transitions of the lowest triplet state of benzophenone

The polarization characteristics of the zero-field (zf) transitions of benzophenone in bis(4-bromophenyl) ether are examined at 1.6 K using optical detection. The results indicate that the transitions are mostly magnetic dipole in nature and that the size of the electric dipole transition moment resulting from spin-orbit perturbation is less than 10^?4 debye. This is being blamed on the small moment of the n, π^? π, π^* triplet-triplet transition that could provide electric allowance in these magnetic transitions. Anomalous magnetic dipole results are obtained on the ab face that can be explained by mixing with host crystal states.     

The lowest triplet state of tetramethyl-1,3-cyclobutanedithione. III. Single-crystal Zeeman spectroscopy

Pulsed, high-field Zeeman spectra of the lowest singlet-to-triplet transition in single-crystal tetramethyl-1,3-cyclobutanedithione (TMCBDT) have been measured. The analysis of the spectra was performed using an extension of the theory of Castro and Hochstrasser which allows for an arbitrary alignment of the magnetic field with respect to the principal crystal axes. From this study we provide evidence that in TMCBDT (1) the lowest triplet is ³A_u (D_2h molecular symmetry). (2) only one spin-orbit coupling route is active. (3) a substantial zero-field splitting (2.5 ± 1.0 cm⁻¹) is present. (4) the magnetic and molecular axes are rotated by 90°. and (5) the spin-orbit contribution to the triplet-state g values is small compared to its effect on the zero-field splitting parameters.     

Zero-field splitting parameters of mono- and di-azaphenanthrenes

Zero-field splitting parameters of phenanthrene and 26 azaanalogues in their lowest triplet state have been evaluated from EPR spectra; triplet lifetimes axe also given The parameters D of phenanthrene and the monoazaanalogues are in good accord with calculated values. The results are discussed in terms of differences in electron distribution and configuration.