Bruce H. Robinson

Saturation transfer spectroscopy: signals sensitive to very slow molecular reorientation

Abstract A density matrix-transition rate matrix formalism is employed to compute the eight unique electron paramagnetic resonance signals at the first two harmonics of the applied Zeeman modulation. Calculations are carried out for conditions appropriate for investigating spin-labeled biomolecules with partially saturating microwave fields and Zeeman modulation amplitudes comparable to resonance linewidths. Spectra are computed for rotational correlation times ranging from 10 −7 to 10 −3 s and for modulation frequencies of 50 and 100 kHz. These simulations indicate that of the eight signals the in-phase dispersion signal at the first harmonic of the modulation and the out-of-phase dispersion signal at the first harmonic afford the best sensitivity to molecular motion and the largest signal amplitudes. It is suggested that study of these signals is the method of choice for monitoring slowly tumbling spin labels when signal-to-noise considerations are critical. The conclusions derived from computer simulations are borne out by experimental measurements performed on 10 −3 M solutions of the steroid spin label 17β-hydroxy-4′,4′-dimethylspiro-[5α-androstrance-3,2′-oxazolidin]-3′-oxyl in sec-butylbenzene.

Fast computer calculation of ESR and nonlinear spin response spectra from the fast motion to the rigid lattice limits

Abstract Fast computer simulation of the electron spin resonance and adiabatic rapid passage spectra of spin labels characterized by rotational correlation times ranging from the fast motion to the rigid lattice limits is demonstrated. Calculations are based upon a modification of the stochastic Liouville equation for the density matrix which explicitly includes interaction of the spins with applied radiation and modulation fields. Several mathematical simplifications of previous calculations are demonstrated, permitting computation with core and CPU requirements compatible with small computers.

Rapid computer simulation of E.S.R. spectra

A fast computer algorithm is presented which permits simulation of the effects of rotational diffusion, electron and nuclear relaxation, microwave power, and modulation frequency upon saturation transfer (passage) E.S.R. spectra. Comparison of theoretical and experimental spectra for nitroxide spin-labelled biomolecules suggests that while the dependence of electron spin-lattice relaxation time upon rotational correlation time is weak, the variation of the ratio of the electron to nuclear spin-lattice relaxation times is significant and consideration of strong nuclear relaxation is necessary for the simulation of spectra characterized by correlation times near the reciprocal of the nitrogen nuclear resonance frequency.

Rapid computer simulation of ESR spectra. Conventional ESR of axially symmetric 14N-nitroxide spin labels

Abstract A computer algorithm is presented for the simulation of the effect of molecular tumbling on ESR spectra, and is applied to simulation of the conventional ESR signal (the absorption signal detected at the first harmonic of the modulation frequency and in-phase with the modulation frequency, in the limit of low microwave and modulation power) of axially symmetric 14 N-nitroxide spin labels. The algorithm is extremely fast and is economical in terms of computer memory requirements.

Theory of modulation effects in electron electron double resonance

Abstract The nonlinear response of spin systems to intense radiation fields is quantitatively treated by a modification of the stochastic Liouville equation for the spin density matrix. In particular, applied modulation terms are included in this equation. The resulting formalism provides a general method for calculating nonlinear spin response for dilute systems of radicals in a high magnetic field. In this communication, frequency and field swept absorption and dispersion electron-electron double resonance spectra are calculated and compared with experimental spectra recorded under conditions of sinusoidal magnetic field modulation and phase-sensitive detection. Good reproduction of the detailed lineshapes of experimental spectra is observed in all cases. The dependence of ELDOR reduction factors upon modulation frequency is discussed. A theoretical analysis such as employed in the present communication is shown to be essential if ELDOR reduction factors are to be related to relaxation times and hence to molecular dynamics, and if the design of ELDOR experiments is to be optimized.

Endor induced electron paramagnetic resonance: Application to the resolution of overlapping spectra

Abstract A technique for resolving overlapping EPR spectra is demonstrated in a study of X- and γ-irradiated crystals of 1,1-cyclobutanedicarboxylic acid. Discrimination of overlapping spectra is accomplished by recording the electron paramagnetic resonance desaturation induced by a radiofrequency field which excites nuclear spin transitions of only one of the several paramagnetic species present. This spectrum, referred to as ENDOR induced electron paramagnetic resonance (EI-EPR), is the difference between the ordinary partially saturated EPR spectrum and the EPR spectrum recorded in the presence of a resonant radiofrequency field. An instrumental arrangement for the fast and convenient measurement of EI-EPR at several Zeeman modulation frequencies is described. The technology is employed to resolve the complicated EPR spectrum which arises from three distinct radical species existing in radiation damaged 1,1-cyclobutanedicarboxylic acid. The EI-EPR, electron nuclear double resonance, and electron electron double resonance spectra of the alicyclic and aliphatic radical are recorded as a function of temperature and crystal orientation. EI-EPR is demonstrated to permit resolution of EPR spectra due to different radical species, different molecular conformations of a given species, and radicals in different (magnetically inequivalent) sites in the crystal unit cell. All alicyclic radicals exhibit temperature dependent beta hyperfine interactions although alpha and gamma interactions show little variation with temperature. EI-EPR signal intensities are found to vary with nuclear transition probability, relaxation parameters which characterize the various transitions, and with the applied de magnetic field (i.e., the nuclear Zeeman interaction).

Poling-induced birefringence in OEO materials under nanoscale confinement

Standard models for evaluating the electro-optic (EO) response of organic materials typically assume that the refractive index of the material in the absence of a RF modulation field is isotropic and homogeneous. Such assumptions work very well for low-concentration guest-host materials in bulk devices. However, current generation organic EO materials at high densities and under nanoscale confinement can show sufficient birefringence to affect device performance. We use computer simulations and spectroscopic experiments to characterize and predict changes in the index of refraction under poling. We also demonstrate that poling-induced birefringence can lead to a non-linear relationship between the apparent EO coefficient and poling field strength.

Hybrid electro-optics and chipscale integration of electronics and photonics

Taken together, theory-guided nano-engineering of organic electro-optic materials and hybrid device architectures have permitted dramatic improvement of the performance of electro-optic devices. For example, the voltage-length product has been improved by nearly a factor of 104 , bandwidths have been extended to nearly 200 GHz, device footprints reduced to less than 200 μm2 , and femtojoule energy efficiency achieved. This presentation discusses the utilization of new coarse-grained theoretical methods and advanced quantum mechanical methods to quantitatively simulate the physical properties of new classes of organic electro-optic materials and to evaluate their performance in nanoscopic device architectures, accounting for the effect on chromophore ordering at interfaces in nanoscopic waveguides.

Nano-Engineering of Molecular Interactions in Organic Electro-Optic Materials

Integration of electronic and photonic devices, especially chip-scale integration, is dramatically impacting telecommunication, computing, and sensing technologies (Dalton et. al., 2010; Dalton & Benight, 2011; Benight et. al., 2009). For organic electronics and photonics, control of molecular order is crucial for effective device performance. For organic photonics, formation of molecular aggregates can result in unacceptable optical loss. For organic electronics, molecular aggregates can adversely influence charge mobilities. Control of intermolecular electrostatic interactions can be exploited to control molecular organization including molecular orientation. Such control can lead to optimized material homogeneity and optical transparency and also to control of molecular conductivity. Here we focus on techniques for systematically nano-engineering desired intermolecular electrostatic interactions into organic electroactive materials. While our primary focus will be on electrooptic materials (which require acentric molecular organization), the discussion is also relevant to optimizing the performance of photorefractive, electronic, photovoltaic, and opto-electronic materials.