A.M. Nishimura

Solvent and substitution effects upon the phosphorescent triplet state of carbonyl pyridines

Abstract The effects of perturbing nitrogen upon localized triplet excitations are observed for several carbonyl pyridines. Conventional and pulse optically detected magnetic resonance (ODMR) techniques are used to characterize the spin-relaxation processes in these aromatic carbonyl molecules.

Spin relaxation in the phosphorescent triplet state of several pyridyl carbonyl compounds

Abstract Spin relaxation is observed in benzophenone, isomers of benzoylpyridines and dipyridyl ketones. Models are used to describe optically detected echo shapes observed by transient techniques in the phosphorescent triplet state. Framed after the density matrix approach, the echo shapes were found to adequately fit the analytical expressions. Hahn spin echo, Carr-Purcell multiple echo, and rotary echo pulse sequences were used to study the nuclear contribution to the electron spin dephasing and the proximity effect of the nitrogen nuclei to the localized triplet excitation in the carbonyl portion of the molecule.

Spin-echo in the phosphorescent triplet state of crystalline 2-indanone

Abstract The relative ordering of the three sublevels of the phosphorescent triplet state of 2-indanone was determined by correlating optically detected magnetic resonance (ODMR) spectra taken with the sample in a Zeeman field with calculated spectra. Spin coherence was observed in the phosphorescent triplet state of 2-indanone in neat and doped single crystals by conventional and real time detection. The Hahn electron spin-echo signal was observed in real time by a pulse sequence which terminates with a continuous-wave low power field (LPF). The Fourier transform of the echo decay as a function of τ yielded a peak at 9.0 MHz which was attributed by isotopic substitution studies to nuclear coupling of the triplet electrons via the α-protons in the molecule. Dephasing of the Hahn and stimulated spin echo in 2-indanone due to energy trapping was measured with chemically mixed crystals of 2-indanone prepared with varying concentrations of quinoxaline. Energy transfer to quinoxaline which served as a deep trap contributed to the dephasing of the echo.

A study of the phosphorescent triplet state of cycloalkanones by pulse optically detected magnetic resonance

Abstract Previous studies of saturated cycloalkanones have characterized the phosphorescent triplet state as n,π*. To elucidate further the characteristics of the excited state of these compounds of photochemical importance, we report the dephasing studies on the spin coherence in the phosphorescent triplet state. For these molecules where the excitation is localized on the carbonyl moiety, spin relaxation is a direct function of the perturbing atoms in the vicinity of the chromophore.

External spin-orbit coupling on the 3n,π* state of several cycloalkanones

Abstract The results of a study on the external spin-orbit coupling on the triplet state of several cyclic aliphatic ketones by optically detected magnetic resonance are reported.

A study of the physical binding of benzanthracene to DNA bases by optically detected magnetic resonance

Abstract Effects on the electronic structure of benzanthracene due to physical binding to bases of nucleic acid were studied by optically pumping the molecular triplet state and monitoring the subsequent decay. Since the triplet electronic states of molecules are sensitive to such perturbations, the observed changes in the total phosphorescence decay rate constants were used as a measure of the extent of molecular interaction. Additionally, changes in the triplet state zero field splitting parameters arising from the anisotropic spin—spin interaction of the triplet electrons were used as another measure of the extent of interaction. Changes in these parameters were observed for base pairs, polynucleotides and methylated adenines.

Localized states in dichloronaphthalene crystals

Abstract Molecular crystals of 1,4-dichloronaphthalene form an excellent model for a one-dimensional exciton. Isotopically mixed crystals have been found to form multiple localized traps of varying depths. The inherently shallow traps generated in isotopically mixed crystals and the possibility of thermal repopulation of the exciton band made possible the estimation of the trap depths. Optically detected magnetic resonance methods have been used tentatively to identify these traps as those arising from aggregation of deuterated dichloronaphthalene. Further confirmation of the trap assignment was found in an analysis of the dephasing times measured by optically detected spin coherence methods.

Electronic energy transfer in benzophenone adlayer.

The extent to which energy transfer occurs in electronically excited organic adlayer films on dielectric surfaces is investigated. Migration and subsequent trapping of the energy in the film are observed by pumping the singlet state of an organic adlayer of benzophenone and by monitoring the phosphorescence and fluorescence lifetimes. To observe the effects of adsorption, benzophenone was chosen as the adlayer because the energies of its well characterizedn,π carbonyl states are remarkably sensitive to solvent interactions. Upon excitation with a nitrogen laser, the perturbation on the electronic states of benzophenone by the substrate caused the emergence of the normally absent fluorescence from the adlayer traps at the interface between the surface of the dielectric substrate and the adlayer. Energy transfer to this interface was observed as a function of film thickness. On the surface of a single crystal of an organic crystal, naphthalene, energy transfer from the adlayer to the substrate was observed, whereas such transfer was not energetically possible with the other dielectric surfaces.

Spin coherence in crystalline complexes. Naphthalene: 2(1,4-diiodotetrafluorobenzene)

Abstract The weak crystalline complex of naphthalene : 2(1,4-diiodotetrafluorobenzene), which exhibits naphthalene phosphorescence with a lifetime in milliseconds, is conveniently investigated by a combination of conventional and pulse techniques of optically detected magnetic resonance. The D + E and D − E resonances, investigated here, reveal no transferred hyperfine interaction, but increased D and E values because of the external spin-orbit coupling which is found to be anisotropic. Measurements are made of T 2 , T 2 *, and T 2 ϱ for this complex. This coherence study finds T 2 to be similar in magnitude to what has been observed for mixed organic crystals. T 2 *, when compared with T 2 , confirms the earlier suggestion that the crystalline complex is highly ordered. T 2ϱ measurement suggests interaction with the nuclear spins of C 6 I 2 F 4 .