Electron spin polarization in the photoexcited triplet state of porphyrins

Electron spin polarization in the photoexcited triplet state of tetraphenyl porphyrin was detected at 100°K using EPR technique. The zero field splitting parameters |D| and |E| the free base porphyrin were found to be 0.0369 ± 0.0005 and 0.0082 ± 0.0005 cm^?1, respectively.     

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.     

Factor group splitting in the lowest triplet state of p-benzoquinone-d4 crystals

Polarized Stark-modulated Zeeman absorption experiments on p-benzoquinone-d₄ single crystals at 2 K show the factor group splitting in the origin of the lowest B_1g (nπ^*) triplet state at 18649 cm^?1 to be 0.62±0.06 cm^?1. The ordering of the crystal states is such that the orbital plus state lies at higher energy. The absence of a measurable factor group solitting in the ³A_u (nπ^*) state at 12.1 cm^?1 from the origin is taken as a further confirmation of the vibronic nature of this state. The ZFS parameter D of this level is found to be ?10±3 GHz.     

ODMR and triplet state kinetics of a carbazolyl substituted diacetylene crystal

Carbazolyl substituted diacetylene (DCH) monomer crystals showing phosphorescence from four traps have been investigated by optically detected magnetic resonance (ODMR) at 1.2 K. These localized triplet states are attributed to the carbazolyl side groups. Their population and depopulation rate constants and zero field splitting parameters (0.097 <|D|< 0.1002 cm^?1; 0.0068 <|E|<0.0105 cm^?1) have been determined. The results suggest that the traps are disturbed substituents. The proposed interaction of the trap states with an exciton band at 23926 cm^?1 is supported by temperature dependent lifetime measurements.     

THE PHOTOEXCITED TRIPLET STATE OF TETRAPHENYL CHLORIN, MAGNESIUM TETRAPHENYL PORPHYRIN AND WHOLE CELLS OF CHLAMYDOMONAS REINHARDI. A LIGHT MODULATION-EPR STUDY

Abstract— The photoexcited triplet states of frozen solutions of tetraphenyl chlorin (TPC), magnesium tetraphenyl porphyrin (MgTPP) and whole cells of Chlamydomonas reinhardi have been studied by light modulation-EPR spectroscopy. The porphyrins were chosen to be studied as model compounds for chlorophyll molecules, From EPR spectra the zero field splitting parameters (ZFS) were calculated. For TPC, |D| = 0.0364 ± 0.0002 cm^-1, |E| = 0.0063 ± 0.0002 cm^-1. For MgTPP, |D| = 0.0310 ± 0.0002 cm^-1. For chloroplasts, |D| = 0.0280 ± 0.0004 cm^-1, |E| = 0.0032 ± 0.0004 cm^-1. In all compounds studied, except MgTPP, electron spin polarization (ESP) was observed. From the analysis of the kinetic curves at each canonical orientation we evaluated the spin lattice relaxation rate W, the depopulation rate constants k_p, and the ratio between the population rate constants, A_p, at zero magnetic field. For TPC in ethanol-toluene (5:1) k_x= (0.70 ± 0.10) × 10³ s^-1, k_y= (0.40 ± 0.07) × 10³ s^-1, k_x= (0.24 ± 0.05) × 10³ s^-1; A_x:A_y:A_z? 1.0:0.6:0.4; W= (2.60 ± 0.40) × 10³ s^-1. For MgTPP, only the total decay rate constant, k_T, was calculated: (1.5 ± 0.2) × 10 s^-1 in n-octane and (4.8 ± 0.8) × 10 s^-1 in ethanol. The results for TPC and MgTPP are compared to those reported previously for chlorophyll. It is concluded that the dynamics of the photoexcited triplet state in chlorophylls are mainly governed by the chlorin macrocycle. From the EPR spectrum and ZFS parameters of chloroplasts, we propose that both chlorophyll a and chlorophyll b are the main constituents of the EPR spectrum. From the analysis of the kinetic curves we obtain separately the kinetic parameters for chlorophylls a and b, k^a_x= (1.30 ± 0.20) × 10³ s^-1, k^a_y;= (0.85 ± 0.15) × 10³ s^-1k^a_x= (0.32 ± 0.05) × 10³ s^-1; A^a_x:A^a_y:A^a_z? 1.0:0.7:0.2; W^a= (1.20 ± 0.20) × 10³ s^-1; k^b_x= (0.56 ± 0.09) × 10³ s^-1, k^b_y= (0.30 ± 0.04) × 10³ s^-1, k^b_z= (0.06 ± 0.01) × 10³ s^-1; A^b_x:A^b_y:A^b_x? 1.0:0.6:0.1; W^b= (5.00 ± 0.80) × 10³ s^-1. These results are very close to those found separately for chlorophyll a and chlorophyll b oligomers in vitro.     

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.     

Triplet state properties of pyridinium

Pyridinium phosphorescence originates at 334 nm and has a lifetime of 3.5 s; the quantum yield is roughly 0.04. The ODMR spectrum gives a zero-field splitting of |D| = 0.134 cm^?1, |E| = 0.030 cm^?1 . Implications for the lowest triplet state of pyridine are discussed.     

Zeeman experiments on the 3Eu α 1A1g transition of platinum porphein in an n-alkane single crystal at 4.2

We report absorption spectra from the ground state to the photoexcited triplet state of platinum porphin (PtP) in single crystals of n-octane (C₈) and n-decane (C₁₀) at 4.2 K, with and without a magnetic field. For PtP in C₁₀ the same transition was studied in emission. From the experiments, values are derived of the spin-orbit coupling parameter Z, the crystal field splitting δ and the orbital angular momentum A for PtP in the two hosts: Z = 76 ± 2 cm^?1 (C₈, C₁₀), δ = 71 ± 1 cm^?1 (C₈), 55 ± 1 cm^?1 (C₁₀) and A = 1.6 ± 0.1 (C₈, C₁₀). For the ratio of the in-plane and the z-polarized electric dipole transition moments we obtain ¦Mx,y¦/¦Mz¦=76± 0.3 (C₈).     

Photochemical generation of triplet—triplet nitrene pairs in aromatic diazide crystals

The molecular and crystal structures of 4-amino-2,6-diazido-3,5-dichloropyridine and 6-amino-2,4-diazido-1,3,5-triazine, as well as the paramagnetic photolysis products of their crystals at 77 K, were studied using X-ray diffraction analysis and ESR spectroscopy. Triplet nitrenes generated during the photolysis of diazidopyridine form triplet—triplet nitrene pairs, whose ESR spectrum corresponds to the quintet spin state. The high-spin state (S = 2) results from the exchange interaction between two triplet molecules with the zero-field splitting parameters |D| = 1.0280 cm⁻¹ and |E| = 0.0038 cm⁻¹ and the γ angle between two C—N nitrene bonds equal to 133°. This angle is close to an angle of 136.2° between the C-N bonds of two adjacent molecules in the crystal structure. No formation of the triplet—triplet nitrene pairs is observed during the photolysis of crystalline diazidotriazine, whose molecules lie in the parallel planes. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 513–520, March, 2008.     

Triplet state quenching by ruthenocene

Ruthenocene quenches triplet states of organic molecules with energies greater than 24000 cm^?1 in benzene solution at a diffusion controlled rate , (6 ± 1) × 10⁹ dm³ mol^?1 s^?1. For triplets with energies less than this the efficiency of quenching is dependent on the energy of the triplet state being quenched but drops off less acutely than expected for endothermic energy transfer following the Arrhenius equation. This is in agreement with the lowest triplet state of ruthenocene being geometrically distorted as expected from the previously observed large Stokes shift between absorption to and emission from its lowest triplet state. Similarities to ferrocene quenching of triplet states are discussed. Quenching of the triplet state of benzil by ruthenocene does not fall on the smooth curve which exists between the quenching rate constants k_q and the energy of the triplet state being quenched. Queching of triplet benzil by ruthenocene is therefore attributed to favourable charge-transfer interactions, also in this case the behaviour is analogous to quenching of triplet methylene-blue by ferrocene where at least a proportion of electron transfer following quenching has been previously established.     

Microwave and far-IR spectra,conformational stability,molecular structure,barriers to internal rotation and ab initio calculations for the anticlinal conformer of trans-1-fluoro-2-butene

The microwave spectrum of trans-1-fluoro-2-butene, trans-(CH₃)HCCH(CH₂F), has been recorded in the region of 18.0–39.0 GHz. Both a-type R- and b-type Q-branch assignments have been made for the ground and first two vibrationally excited states of the asymmetric torsion for the gauche (anticlinal) conformer. The ground state rotational constants for this conformer are found to have the following values: A = 19,938.33±0.48, B = 2071.37±0.01, C = 2022.17±0.01 MHz. From an analysis of the internal rotational splittings of the Q-branches, the three-fold rotational barrier for the methyl group is determined to be 596±7 cm⁻¹ (1.70±0.02 kcal/mol). From the Stark effect the dipole moment components for the gauche conformer were determined to be |μ_a| = 1.86±0.01, |μ_b| = 1.16±0.01, |μ_c| = 0.31±0.05, and |μ_t = 2.21±0.01 D. The fundamental asymmetric torsion for the cis (synclinal) conformer has been observed in the far-IR spectrum of the vapor at 123.95 cm⁻¹ whereas that for the gauche conformer has been determined to occur at 82.8±5 cm⁻¹ based on relative intensity measurements obtained from the microwave spectrum. From these data the potential function which governs the internal rotation of the asymmetric top has been determined and the following potential constants have been evaluated: V₁ = −191±10, V₂ = 598±10, V₃ = 786±13, V₄ = 59±5, and V₆ = 79±5 cm⁻¹. These data are consistent with the more stable conformer having the fluorine atom cis (synclinal) to the double bond and lying 300±33 cm⁻¹ (858±94 cal/mol) lower in energy than the gauche rotamer. Utilizing ab initio calculations with the MP2/6-31G* basis set and the three rotational constants, r₀ structural parameters are estimated. Also, the barriers to conformer interconversion have been calculated with the RHF/3-21G, RHF/6-31G*, and MP2/6-31G* basis sets. All of these results have been compared to the similar quantities of some corresponding molecules.     

Experimental study of triplet exciton diffusivity in crystalline pyrene at 300 K

Preliminary measurements of the triplet diffusivity tensor in crystalline pyrene at 300 K are reported. Maximum diffusivity occurs along the b axis and D_ab = (1.25 ± 0.3) × 10^?4 cm² s^?1. Diffusion in the ac′ plane is nearly isotropic with D ≈ (0.3 ± 0.1) × 10^?4 cm² s^?1. These results are tentatively interpreted as diffusion dominated by nonlocal scattering.     

The structure of low temperature triplet energy traps in organic solids from pmdr spectroscopy 11. 1,3,5-Trichloro and hexachlorobenzenes

The new technique of polarized microwave PMDR spectroscopy is used to determine the structure of the low temperature (1.6 – 4.2°K) triplet energy traps. The structure of 1.3,5-trichlorobenzene (Tri-CB) and hexachlorobenzene (HCB) molecules in hexamethylbenzene (HMB) host as well as in their own neat crystals (present as x-traps) is determined from the linear polarization characteristics of their optically detected microwave zerofield (zf) transitions as well as from the analysis of their phosphorescence emission. The former technique gives the direction of the principal magnetic axes of these trap molecules in the crystal as well as the relative order of their zf levels.ln HMB host, deviation from trigonal symmetry is found to be only slight for HCB and absent for Tri-CB. ln the neat crystals, large deviations from trigonal symmetry are observed for the traps of both HCB and Tri-CB. In HCB x- traps, the HCB molecule is found to be slightly contracted along the CCl axis near parallel to the c′ crystal axes. Deviation From planarity is also strongly suggested by the large value of the zf parameter D. ln addition, the principal magnetic axis of the 2|E| moment for HCB x-traps is found to lie only 5° off the molecular N axis. The |D| + |E| and |D| — |E| moments, however, are 15° away from the A and B molecular axis, respectively.In Tri-CB neat crystal. two traps are observed optically with their phosphorescence origins 10 cm^?1 apart. The zf parameter E is found to have a non-zero value and is opposite in sign for the two traps. If the distortion is to be blamed on pseudo-Jahn-Teller forces, the results lead to the conclusion that the x-(shallow) trap is contracted while the y- (deep) trap is expanded along an in-plane axis going through the CCl bonds near parallel to the a axis of the Tri-CB crystal. The plane containing the |D| + |E| and |D| — |E| moments of the x-trap suffers a rotation around the N-molecular axis, which is almost parallel to the 2|E| moment. Th |D| + |E| moment is 10° off the A axis and the |D| — |E| moment is 10° off the B axis. The 2|E| transition moment of the y-trap lies off the molecular N axis and the plane containing the |D| + |E| and |D| — |E| moments moves upward from theThe results of these and other studies suggest that low temperature trapping in neat crystals of this type results from crystal induced geometrical and orientational changes in the molecules at point defects. The observed traps are those molecules for which the crystal field induced deformation leads to a lowering in their singlet-triplet transition energy as compared with that for the host lattice.     

Electron paramagnetic resonanace of the photoexcited triplet state of electron—donor—acceptor complex crystals: Naphthalene—tetrachlorophthalic anhydrides

The EPR spectra and kinetics of the photoexcited triplet state of naphthalene—tetrachlorophthalic anhydride (N—TCPA) complex crystals are reported. The phosphorescent state of N—TCPA is naphthalene-like with 10% charge-transfer character at 4.2 K. Detailed temperature and orientational studies show that donors and acceptors reorientate along the stack axis at very low temperatures (20 K and below) to gain a maximum overlapping (X-trap). The activation energy of the detrapping process is 60 ± 10 cm^?1. At higher temperatures (50 K and above), the state of the system is best described as an excitation jumping between a localized state and a thermally accessible higher delocalized state (exciton). The activation energy of the excitation jumping is 150 ± 10 cm^?1. The upper limit of the average time spent in the exciton state is 8 × 10^?10 s. The transient studies yield the triplet population and decay rate constants of N—TCPA complexes which differ from those of uncomplexed naphthalenes. The difference is attributed to the excitation to CT singlet state followed by intersystem crossing in different pathways.     

Absorption and emission spectra of the lowest triplet state in hexachloroacetone

T₁ ← S₀ absorption and T₁ → S₀ phosphorescence spectra of neat cystalline hexachloroacetone have been analyzed at 4.2°K. From the lifetime and energy the upper state is assigned as ³nπ^*. The spectra are sharp compared to other aliphatic ketones, with the 0-0 band at 26 248 ± 2 cm ^?1. The phosphorescence shows two strong progressions; one involving the CO stretching mode at 1784 cm^?1 (x), the other a long progression of at least 8 bands involving a mode at 143 cm^t-1 (a). The 143 cm^?1 progression forming mode can best be asigned to the CO out-of-plane wagging vibration. The absorption shows the same two strong progressions, reduced in frequency to 1270 cm^t-1 and 123 cm^?1, respectively, but with the progression in mode a broadened with increasing n. The broadening is interpreted as arising from inversion doublets; the close harmonicity up to n = 5 allowing the potential barrier to inversion to be estimated as > 700 cm^?1. A feature of the spectra is the absence of low frequency torsional modes suggesting lack of pseudo Jahn-Teller distortion of the triplet state potential surface. For comparison, the phosphorescence of crystalline hexafluoroacetone was also studied at 4.2°K. The spectrum exhibits broad bandedness with a 00 band tentatively assigned at 26 870 ± 20 cm^?1.     

Mutual annihilation of triplet excitons in a nearly one-dimensional system: 1,4-dibromonaphthalene

The analysis of the variation with incident flux of the time dependence of the delayed fluorescence in conjunction with the determination of the absolute ground state-first excited triplet absorption coefficients at room temperature, yields the value of γ_tot = (5.5 ± 2.0) × 10^?12 cm³ s^?1, for the total triplet-triplet annihilation rate constant in 1,4-dibromonaphthalene crystals. The one-dimensional mutual annihilation rate constant for the triplet exciton motion restricted to linear chains along the crystal c axis is γ¹_tot = (1.0 ± 0.4) × 10³ cm s^?1. The results are discussed in terms of recent theories of mutual annihilation of triplets in one-dimensional systems.     

Triplet exciton band structure of crystalline phenazine

The structure of the lowest triplet state of crystalline phenazine was studied by optical and magnetic resonance techniques. The exciton band was characterized by a large translationally equivalent splitting (17 ± 1 cm^?1) which was determined from the resonance pair spectrum. The spin splittings were obtained using the optical detection of magnetic resonance of phosphorescent phenazine crystals at 1.5 K.     

Phosphorescence- and delayed-fluorescence-detected magnetic resonance at zero-field on mobile and localized triplet states in anthracene

Optically-detected magnetic resonance (ODMR)-experiments at zero field were performed on the lowest triplet state of anthracene single crystals monitoring the delayed fluorescence (DF) as well as the phosphorescence (P) at 1.2 K. At low RF-power levels the P-ODMR and DF-ODMR signals are almost identical and can be attributed to at leat two X-traps with the following ZFS-parameters: D₁ = 0.06967(3), |E₁| = 0.00793(3), D₂ = 0.06940(3) and |E₂| = 0.00808(3). At RF-levels exceeding the threshold of the trap signals by about 5 orders of magnitude, DF-ODMR signals from excitons are observed. Their ZFS-parameters show clearly, that the motion is mainly between translationally non-equivalent oriented anthracene molecules [D^* = ?0.004256(1) cm ^?1 and E^* = 0.03311(1) cm ^?1]. ODMR-linewidth analysis for optical excitation directly into one of the two Davydov components of the first triplet exciton band indicate, that radiationless interbranch transitions between both components can populate k ≠ 0 states in the lower band with band energies higher than kT. The thermalization time within this band can be estimated to be about 10 ^?7 s.     

Theoretical study of spin–orbit coupling and kinetics in spin-forbidden reaction between Ta(NH2)3 and N2O

The activation mechanism of the nitrous oxide (N₂O) with the Ta(NH₂)₃ complex on the singlet and triplet potential energy surfaces has been investigated using the hybrid exchange correlation functional B3LYP. The minimum energy crossing point (MECP) is located by using the methods of Harvey et al. The rate-determining step of the N–O activation reaction is the intersystem crossing from ¹ 2 to ³ 2. The reacting system will change its spin multiplicities from the singlet state to the triplet state near MECP-1, which takes place with a spin crossing barrier of 32.5 kcal mol^?1, and then move on the triplet potential energy surface as the reaction proceeds. Analysis of spin–orbit coupling (SOC) using localized orbitals shows that MECP-1 will produce the significant SOC matrix element, the value of SOC is 272.46 cm^?1, due to the electron shift between two perpendicular π orbitals with the same rotation direction and the contribution from heavy atom Ta. The rate coefficients are calculated using Non-adiabatic Rice-Ramsperger-Kassel-Marcus (RRKM). Results indicate that the coefficients, k(E), are exceedingly high, k(E) > 10¹² s^?1, for energies above the intersystem crossing barrier (32.5 kcal mol^?1); however, in the lower temperature range of 200–600 K, the intersystem crossing is very slow, k(T) < 10^?6 s^?1.     

The T3 (π, π) State of acridine

The 00 band maximum of the transition T₃(π, π^*) ← T₁ (π, π^*) of acridine occurs at ≈ 10200 ± 20 cm^?1 in inert (n-hexane, benzene, CCl₄), at 10220 ± 20 cm^?1 in polar (acetonitrile) and at 10170 ± 50 cm^?1 in hydrogen-bonding (methanol, 2-propanol and alkaline water) solvents. Based on the solvent-independent energy of T₁ (π, π^*), the T₃(π, π^*) state of acridine is estimated at 26050 ± 50 cm^?1 in all the solvents.