Fluorescence detection of electron spin echoes in the triplet state of nonphosphorescent molecules and a photosynthetic bacterium

We have observed electron spin echo signals in zero magnetic field in the triplet state of the porphin free base and the photo-induced triplet state of the photosynthetic bacterium Rhodopseudomonas spheroides (wild type) via changes in the intensity of the fluorescence. With the help of the echo signals we have been able to determine the spin memory times of the triplet spins.     

Room-temperature kinetics of the photoexcited triplet state of acridine in fluorene crystals as obtained from electron spin echo studies

It is shown that electron spin echo results allow an analysis of the complete kinetics of excited triplet spin states at room temperature, not only with respect to all relevant kinetic parameters of the triplet decay and spin relaxation but also the spin selectivity of the triplet population and decay processes.     

Energetics of dioxygen binding into graphene patches with various sizes and shapes

Density functional theory (DFT) calculations were employed to investigate the electronic properties of an H-atom terminated graphene patch (hydrographene) smaller than a rhombic C96H26 structure with zigzag edges. Depending on shapes and sizes of hydrographenes, some hydrographenes have the triplet ground state where unpaired electrons are localized on their zigzag edges. The stability of the triplet spin state is diminished, decreasing the hydrographene sizes. The existence of the localized spin densities allows triplet dioxgen to bind into a hydrographene. According to the DFT calculations, the energetics of the dioxygen bindings is negatively influenced by downsizing hydrographenes, as well as depends on their shapes. The size-and shape-dependences of the dioxygen bindings reflect from the stability of the triplet state of a hydrographene, because its localized unpaired electrons can be utilized to be attached to an unpaired electron of triplet dioxygen.     

An electron spin echo and laser photolysis study of chemically induced electron spin orientation in the triplet ground state of diphenylmethylene

The electron spin in the triplet ground state of diphenylmethylene is found to be oriented predominantly in the plane perpendicular to the long molecular axis after the photo-dissociation reaction of diphenyldiazomethane. The spin selectivity of the intersystem crossing which occurs when the methylene relaxes to its ground state is suggested as the orientation mechanism. The combined spin echo and laser photolysis are shown to be very suitable for this kind of investigation.     

Electron Spin Relaxation Rates for High-Spin Fe(III) in Iron Transferrin Carbonate and Iron Transferrin Oxalate

To optimize simulations of CW EPR spectra for high-spin Fe(III) with zero-field splitting comparable to the EPR quantum, information is needed on the factors that contribute to the line shapes and line widths. Continuous wave electron paramagnetic resonance (EPR) spectra obtained for iron transferrin carbonate from 4 to 150 K and for iron transferrin oxalate from 4 to 100 K did not exhibit significant temperature dependence of the line shape, which suggested that the line shapes were not relaxation determined. To obtain direct information concerning the electron spin relaxation rates, electron spin echo and inversion recovery EPR were used to measure T(1) and T(m) for the high-spin Fe(III) in iron transferrin carbonate and iron transferrin oxalate between 5 and 20-30 K. For comparison with the data for the transferrin complexes, relaxation times were obtained for tris(oxalato)ferrate(III). The relaxation rates are similar for the three complexes and do not exhibit a strong dependence on position in the spectrum. Extrapolation of the observed temperature dependence of the relaxation rates to higher temperatures gives values consistent with the conclusion that the CW line shapes are not relaxation determined up to 150 K.     

Novel Pulsed Electron Paramagnetic Resonance Techniques for the Studies of Structure and Dynamics of Photo‐excited Triplet State of Organic Molecules: A Professional Journey

We present a few novel pulsed electron paramagnetic resonance techniques developed in our laboratory for the studies of structure and dynamics of the photo‐excited triplet state of organic molecules. We discuss many aspects of these new techniques and the significances of these measurements: (1) enhancing NMR signal intensity by dynamic nuclear polarization ‐ integrated solid effect, (2) performing magnetic resonance in zero‐field and low‐field by pulsed microwave, (3) mapping molecular motion of organic crystals by pulsed zero‐field and low‐field experiments, (4) probing spin dynamics at level anti‐crossing by fast field switching, (5) measuring hyperfine interaction by electron spin echo envelop modulation and spin‐echo electron nuclear double resonance and (6) detecting spin dynamics, nuclear quantum oscillation, entanglements and new avenues for quantum computer. We have employed the highly electron spin polarized pentacene triplet state as the model system in all of our pulsed EPR experiments. We performed most of our experiments at room temperature. The goals of our studies are aiming to improve spin detectability, to probe molecular dynamics, to determine electronic structures, to measure molecular interaction and motion, and to examine quantum coherence and oscillation which may yield new avenues in the applications of pulsed EPR techniques to quantum computer.     

TRIPLET STATE OF METALLOPORPHYCENES: ZnPCl, PdPC2, PtPC2, and NiPC2

We report on a study of the photoexcited triplet state, at low temperatures in ordered liquid crystals, and at room temperatures in the liquid phase, of some new metalloporphycenes: normal zincporphycene, ZnPCl; Pd(II), and Ni(II) complexes of 2,7,12,17-tetra- n -propylporphycene, PdPC2, PtPC2, and NiPC2, respectively. The triplet state at low temperature is obtained by selective laser excitation and its magnetization response is detected by EPR in the 250 ns time scale. From triplet EPR line shapes and triplet spin polarization directions it is concluded that the zinc cation lies above the molecular plane and the palladium cation fits into the porphycene's cavity. Such a proposed structure implies a strong spin-orbit interaction in PdPC2, thus resulting in an out-of-plane active spin state (z-axis), whereas that in ZnPCl, having a smaller spin-orbit interaction, in-plane ( x,y -axes) are the active spin states. Laser photolysis of the metalloporphycenes gives rise to detectable triplets of ZnPCl, PdPC2 and PtPC2 with triplet lifetimes of 26, 5 and 0.2 µs, respectively. The sensitization experiment, using β-acetonaphthone as a sensitizer to produce the triplet, results in detectable transients of only ZnPCl and PdPC2 with much longer triplet lifetimes of 85 and 20 µs, respectively. The short triplet lifetime of PtPC2 (and probably that of NiPC2) do not allow for triplet detection with the present sensitizer. However, the latter two metalloporphycenes quench the triplet lifetime of β-acetonaphthone from 29 to 10µs.     

Isotropic reorientations of fullerene C70 triplet molecules in solid glassy matrices revealed by light-induced electron paramagnetic resonance

Continuous-wave X-band electron paramagnetic resonance (EPR) of fullerene C(70) molecules excited to a triplet state by continuous light illumination was studied in molecular glasses of o-terphenyl and cis/trans-decaline and in the glassy polymers polymethylmethacrylate (PMMA) and polystyrene (PS). Above ～100 K, a distinct narrowing of EPR lineshape of the triplet was observed, which was very similar for all systems studied. EPR lineshape was simulated reasonably well within a framework of a simple model of random jumps, which implies that the C(70) molecule performs isotropic orientational motion by sudden jumps of arbitrary angles. In simulations, a single correlation time τ(c) was used, varying in the range of 10(-7)-10(-8) s. Near and below 100 K electron spin echo (ESE) signals were also obtained which were found to decay exponentially. Correlation times τ(c) obtained from simulation of the EPR spectra in the slow-motion limit (τ(c) close to 10(-7) s) turned out to be in good agreement with the phase memory times T(M) of the ESE decay, which additionally supports the employed simple model. The observed motional effects provide evidence that the nanostructure of the solid glassy media of different origins is soft enough to allow a large asymmetric C(70) molecule to reorient rapidly. Except for the EPR spectra of the triplet, in the center of the spectra, a small admixture of a narrow line was also observed; its possible nature is briefly discussed.     

Phase locking of molecular two-level quantum systems: Application to ionic solids

Photon locking of optical transitions in molecular systems has been reported recently by Sleva, Xavier, Zewail and Glasbeek. These observations were made on I₂ vapor using phase-coherent multiple pulses and photon echo detection. The same pulse sequence to observe locking in ionic solids using optical detection of triplet state spin coherence is applied here. Agreement between theory and experiment is very satisfactory.     

Ultrafast observation of a solvent dependent spin state equilibrium in CpCo(CO)

We report the observation of a solvent-dependent spin state equilibrium in the 16-electron photoproduct CpCo(CO). Time-resolved infrared spectroscopy has been used to observe the concurrent formation of two distinct solvated monocarbonyl photoproducts, both of which arise from the same triplet CpCo(CO) precursor. Experiments in different solvent environments, combined with electronic structure theory calculations, allow us to assign the two solvated photoproducts to singlet and triplet CpCo(CO)(solvent) complexes. These results add to our previous picture of triplet reactivity for 16-electron organometallic photoproducts, in which triplets were not believed to interact strongly with solvent molecules. In the case of this photoproduct, it appears that spin crossover does not present a significant barrier to reactivity, and relative thermodynamic stabilities determine the spin state of the CpCo(CO) photoproduct in solution on the picosecond time scale. While the existence of transition metal complexes with two thermally accessible spin states is well-known, this is, to our knowledge, the first observation of a transient photoproduct that exhibits an equilibrium between two stable spin states, and also the first observed case in which a solvent has been able to coordinate as a token ligand to two spin states of the same photoproduct.     

Modulation effects in the electron spin echo of trans bis-(2,2,5,5-tetramethyl-3-imidazoline-3-oxide-1-oxyl-4-il) ethylene biradicals

Electron spin echo modulation effects have been investigated in trans bis(2,2,5,5-tetramethyl-3-imidazoline-3-oxide-1-oxyl-4-il) ethylene biradicals involving ¹⁴N or ¹⁵N nuclei in different positions. The modulation frequencies are shown to be the nitrogen nuclear transition frequencies in radical fragments at m_S = 0 of a triplet electronic spin. Quadrupole splitting parameters for these nitrogen nuclei have been determined.     

Optically detected zero field magnetic resonance in the lowest triplet nπ* state of p-benzoquinone

Optically detected magnetic resonance (ODMR) techniques are used to determine the orbital assignment of the phosphorescent triplet state of p-bezonquinone in the condensed phase. The principal spin—orbit coupling routes are discussed in terms of the observed spin—vibronic activity.     

Deuterium solid echo line shape study of poly(ethylene oxide) dynamics in a blend with poly(methyl methacrylate)

Deuterium solid echo line shapes were measured on deuterated poly(ethylene oxide) (d₄PEO) in a blend with protonated poly(methyl methacrylate) to characterize chain dynamics of this component in the blend. Line shapes were observed as a function of temperature from 183 to 243 K and echo delay times from 10 to 100 μs on a blend containing 20 wt % d₄PEO. The line shapes and the associated relative intensities were quantitatively interpreted in terms of segmental motion and libration. The results of the interpretation are compared to an earlier study of deuterium spin‐lattice relaxation times over the temperature range of 313 to 413 K. A combined interpretation of both sets of data is developed based on bimodal distribution of correlation times that are separated by about 2 orders of magnitude in time. The faster mode is 30% of the correlation function with a stretched exponent near one while the slower mode is characterized by an exponent of 0.5. The source of the bimodal character is not revealed by the line shape and relaxation data but is consistent with the presence of two glass transition temperatures in this miscible blend and anomalous translational diffusion of diethyl ether through the blend. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2433–2444, 2005     

Transient electron spin resonance spectroscopy of the carotenoid triplet state in Rhodopseudomonas Sphaeroides wild type

We have determined the triplet state spin sublevel dynamics of spheroidene contained within the photosynthetic bacterium Rhodopseudonionas sphaeroides wild type. The triplet state decay dynamics were measured using direct-detection, rapid-transient, high-field, electron spin resonance spectroscopy on chromatophores isolated from the bacterium. The transient signals were analyzed as a function of microwave power to obtain the individual triplet state spin sublevel decay rate constants and the spin—lattice relaxation time. The individual triplet state spin sublevel decay constants yielded a value of 4.2±0.3 μs for the overall triplet state lifetime, which agrees with the lifetime measured previously by other workers using optical spectroscopic methods.     

Transient esr observation of triplet-soliton pairs in a conjugated polymer single crystal

We have observed directly by pulsed ESR the formation and decay of a triplet state in a polydiacetylene single crystal after photoexcitation at 337 nm. The unusual spin polarization properties of the observed triplet suggest the formation of a tripletsoliton pair.     

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.     

具有强可见光吸收的富勒烯-Bodipy衍生物作为光催化剂在硫醚光催化氧化中的应用

以氟硼吡咯染料（Bodipy）做为光吸收天线,富勒烯作为分子内自旋转换单元,分别利用前者的强吸光以及后者的高效系间窜越的优点,制备了吸收波长灵活可调、无重原子、具有强可见光吸收能力、长寿命三重激发态（92.1 μs）的二元（Dyad）、三元化合物（Triad）做为有机三重态光敏剂;其中Triad具有宽谱带可见光吸收能力,提高了光敏剂参与的分子间能量转移或电子转移过程的效率。与传统的Ru（Ⅱ）配合物等三重态光敏剂相比,光催化硫醚氧化的反应时间大大缩短。通过吉布斯自由能的计算、活性氧物种的捕获实验、电子顺磁共振等方法,证明了在光催化氧化硫醚的反应过程中,同时存在超氧负离子自由基和单线态氧两个活性物种,从而加快了光催化氧化反应速率。该研究结果将对新型有机三重态光敏剂的分子结构设计以及在光催化有机合成反应中的应用起到一定的促进作用。     

AB initio calculation of the spin dipole-dipole interaction in ground-state triplets

Ab initio results are presented for the spin dipole-dipole interaction in the ground-state linear triplet radicals NCN, CNN and CCO. The relative contributions from the first-order spin dipole-dipole interaction and the second-order spin-orbit interaction, to the observed spin coupling constant in these systems, are discussed.     

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.     

Pseudo-jahn-teller distortion of pyridine in its lowest triplet state

We have performed an electron spin echo detected electron paramagnetic resonance study of the nitrogen hyperfine interaction in the lowest triplet state of pyridine. It is concluded that the molecule is non-planar in this state owing to pseudo-Jahn-Teller coupling between the ³B₁(nπ) and the close-lying ³A₁(ππ) states.