August H. Maki

Phosphorescence transients induced by magnetic resonance fast passage

Abstract A fast-passage optically-detected magnetic resonance (ODMR) method is presented which allows the determination of relative radiative rate constants, steady-state populations, populating rate constants, and the lifetimes of individual triplet spin sublevels in zero field. This method is applied to the principal phosphorescent trap 0,0 = 24098 cm -1 , vac.) in pure benzophenone at ca. 1.30°K.

Phosphorescence-microwave double-resonance (PMDR) spectroscopy

Abstract The effect of saturating the zero-field transitions with microwave radiations on the phosphorescence spectrum is demonstrated at low temperature. The application of this type of double-resonance experiment in determining the most probable intersystem crossing routes, the mechanisms of the phosphorescence radiation of the different vibronic bands, the determination of zero-field splittings and the electronic distributions in the triplet state is indicated.

Direct measurement of spin-lattice relaxation rates between triplet spin sublevels using optical detection of magnetic resonance

A method is described for the direct measurement of spin-lattice relaxation rates between the triplet spin sublevels of phosphorescent molecules. The method also yields the correct values (in the presence of spin-lattice relaxation) for the total sublevel decay rates and can be used in conjunction with standard optically detected magnetic resonance experiments to find correct values for relative populating rate constants, radiative rate constants and steady state populations of the triplet sublevels. The technique is applied herein to the study of indole and tryptophan molecules in several environments.

Spin‐orbit coupling effects on the zero field splitting of (π, π*) triplet states of conjugated enones

The optically detected paramagnetic resonance transitions of five cyclic conjugated enones have been observed in zero field. The |E|/h c parameters are small and nearly invariant for series, but the |D|/ h c parameters range from 0.22 to 0.30 cm−1. Simple theory suggests that both spin‐spin dipolar interactions and spin‐orbit interactions make significant contributions to the ZFS. The large variation in the ZFS of these enones is primarily due to the dependence of the spin‐orbit interaction on the energy gaps between the 3(π, π*) state and the 1(n, π*) and 3(n, π*) states.

Optical detection of phosphrescent triplet state endor in zero field

Abstract The optical detection at zero-field of ENDOR transitions associated with the 14N nuclear levels of 2,3-dchloroquinoxaline in the Tππ* state is demonstrated. The appearance of hyperfine structure in the zero-field optically detected microwave transitions is explained quantitatviley using a spin-Hamiltonian incorporating the 14N hyperfine and quadrupole interactions.

A phenomenological description of ENDOR in solution; example: The tri-t-butyl phenoxyl radical

Abstract The ENDOR spectrum of the tri- t -butyl phenoxyl radical is interpreted and precise values of the coupling constants reported. The relative intensities of the lines are reported under various conditions of microwave power, rf power, temperature, viscosity, and the Zeeman modulation, and a phenomenological theory beginning with the Bloch equations is developed to explain each effect. The optimization of each parameter for solution ENDOR is discussed.

OPTICALLY DETECTED MAGNETIC RESONANCE OF THE TRYPTOPHAN PHOSPHORESCENT STATE IN NATIVE PROTEINS

Abstract— The phosphorescent triplet state of tryptophan has been studied by the method of optically detected magnetic resonance (ODMR) at pumped helium temperatures in zero magnetic field. Only one of the triplet sublevels is found to be significantly radiative; the other two decay radiationlessly. Although the phosphorescence and ODMR decay lifetimes are influenced by spin–lattice relaxation processes at T= 1.3°K, the lifetime of the radiative level can be estimated as approximately 2 s, whereas the lifetimes of the non–radiative levels are in excess of 10 s. Comparison of the ODMR signals and the phosphorescence spectra has been made for tryptophans in native proteins with the following results: the ODMR signals of the two types of tryptophan sites in horse liver alcohol dehydrogenase can be resolved due to a shift in the D and E values of the respective triplet states; binding of the substrate tri‐N‐acetylglucosamine to hen lysozyme leads to a considerable narrowing of the phosphorescence peaks and ODMR signals as well as to a shift in the E value of the triplet state.

The interpretation of optically detected endor in zero field and their relationship to 14N nuclear quadrupole interactions in 3 (ππ*) states of quinoline and quinoxaline

Abstract It is shown that the proper assignment of 14 N ENDOR transitions in azaaromatic π,π * triplet states must take into consideration the contribution of the electron magnetic moment operator to the intensity of a given transition. In quinoline and quinoxaline, the electron magnetic moment operator is found to give the dominant contribution to ENDOR intensities, and to a high degree of accuracy the nuclear moment operator may be neglected. An erroneous ENDOR assignment of quinoxaline is corrected on this basis, and the ENDOR of quinoline is assigned. In particular, we find for these molecules that the dominant ENDOR transitions do not yield the 14 N qudrupole parameters e 2 qQ and η independently, but only the value of 3 e 2 qQ (1-η/3)/4.

ENDOR study of pentaphenylcyclopentadienyl radicals in solution. Lifting of orbital degeneracy by methyl substitution

An electron-nuclear-double-resonance (ENDOR) study has been made of pentaphenylcyclopentadienyl (PPCPD) and a series of para-methylated PPCPD radicals in liquid solution. The hyperfine splitting (hfs) constants in PPCPD and pentamethyl PPCPD (MPPCPD) do not show a significant π-σ spin delocalization effect. This indicates that the phenyl substituents are twisted less than 60° out of the plane of the five-membered ring. A MO calculation based on σ-π separability using the McLachlan approximation gives good agreement between experimental and theoretical hfs constants for a twist angle of 40°. The ring proton hfs constants in PPCPD and MPPCPD are practically identical, indicating a negligible effect of the methyl groups on the spin density distribution. Moreover, the hfs constants in both radicals are not affected noticeably by temperature variations. The ring proton splittings in the partially methylated PPCPD radicals, on the other hand, vary widely and show a marked temperature dependence. The same is tru...

Triplet‐State Zero‐Field Splittings in Some Aromatic Hydrocarbons and Nitrogen Heterocycles

The SCF‐AOs expanded by Lowdin have been used for the calculation of the electron spin dipolar interactions in the triplet‐state zero‐field splittings of some aromatic molecules. Within a framework of two‐center approximation for the spin–spin interaction integrals over atomic orbitals, and adopting the Sklar‐type approximation for the exchange‐type integrals, excellent agreement with experiments has been obtained for benzene and naphthalene without invoking σ–π interactions. The wavefunctions used were obtained by means of the Pariser–Parr–Pople method, within a full single excitation configuration treatment. This approach has been extended to some aromatic nitrogen heterocycles, but the results, although promising, are poorer, possibly due to the neglect of spin–orbit coupling. Uncertainty in the choice of the C–N resonance integral and the neglect of penetration integrals in the core Hamiltonian may also be factors. Eleven molecules have been treated in all. For comparison, the single‐determinant unre...