Robert E. Connors

Phosphorescence investigation of xanthone in n-hexane at 77 and 4.2 K

Abstract The phosphorescence spectrum of xanthone embedded in a polycrystalline matrix of n -hexane has been measured at 77 and 4.2 K. Dramatic differences in spectral features and triplet lifetime are evident at these two temperatures. The emitting species at 4.2 K is assigned as predominantly 3 (ππ*) in character; whereas, the phosphorescence state observed at 77 K is clearly 3 (nπ*).

Zero-field optically detected magnetic resonance study of the lowest triplet state of magnesium tetraphenylchlorin

Abstract The triplet state zero-field splitting parameters and intersystem crossing (ISC) decay constants for the individual triplet spin sublevels of magnesium tetraphenylchlorin and magnesium porphin dissolved in pyridine are presented. Comparison is made with previously published data for other model photosynthetic compounds and chlorophylls.

Temperature and deuterium isotope effects on the intersystem crossing rates for the lowest triplest state of tetraphenylchlorin

Abstract The average rate of intersystem crossing from the lowest triplet state to the ground state increases by a factor of approximately 2.5 between 2 K and 77 K for tetraphenylchlorin and tetraphenylchlorin- d 2 . This observation is discussed in terms of radiationless-transition theory. It is also found that, contrary to the case for aromatic hydrocarbons, the position dependence of the deuterium isotope effect for the several chlorins studied does not correlate with the predicted one-center π-election densities in the highest filled and lowest unfilled molecular orbitals.

Polarized IR and DFT study of the fundamental vibrations of xanthone

Abstract The polarized infrared spectrum of xanthone embedded in a stretched polyethylene film has been measured. Orientation factors obtained from dichroic ratios have been determined for 20 transitions. Symmetry assignments derived from the polarization data are in excellent agreement with the predictions of scaled B3-LYP/6-31G(d) density functional theory calculations. The unpolarized infrared spectrum of xanthone in KBr has also been recorded. A combination of polarized and unpolarized infrared data, published Raman results, and B3-LYP/6-31G(d) predictions have been used to assign the fundamental vibrational frequencies of xanthone. These new results clarify some ambiguities found in the previous studies and have led to the reassignment of several vibrations.

A photosensitive surface capable of inducing electrophysiological changes in NG108-15 neurons

Retinal prostheses promise to be a viable therapy for many forms of blindness. Direct stimulation of neurons using an organic light-sensitive, self-assembled monolayer surface offers a simple alternative to conventional semiconductor technology. For this purpose we have derivatized an indium tin oxide (ITO) substrate with the photosensitive dye, NK5962, using 3-(aminopropyl)trimethoxysilane (APTMS) as cross-linker. The surface was characterized through contact angle goniometry, electrochemical impedance spectroscopy, grazing angle infrared and ultraviolet-visible spectrophotometry. NG108-15 neurons were grown on the ITO-APTMS-NK5962 surface and neural responses from electrical stimulation vs. photostimulation through the ITO-APTMS-NK5962 surface were measured using patch clamp electrophysiology. Under these conditions, photostimulation of depolarized cells caused an approximate 2-fold increase in voltage-gated sodium (Na(+)) current amplitude at a membrane potential of -30mV. Our results demonstrate the feasibility of stimulating neurons, grown on light-sensitive surfaces, with light impulses, which ultimately may facilitate the fabrication of a simple, passive retinal prosthetic.

Temperature dependence of aryl butatriene fluorescence: barrier to twisting on S1 for 1,1,4,4-tetraphenylbutatriene

Abstract The temperature dependence of aryl butatriene fluorescence is assigned to a competition between radiative decay and butatriene chain rotation against a barrier followed by radiationless decay from S 1 . Fluorescence quantum yields were measured as a function of temperature for 1,1,4,4-tetraphenylbutatriene (TPBT) in a series of n -alkane solvents. Application of the medium-enhanced barrier model and transition state theory allow the observed barrier to be expressed in terms of an intrinsic and solvent-induced contribution ( Δ H ‡ obsd = Δ H ‡ t + Δ H ‡ v ). It is found that there is little or no intrinsic barrier to rotation ( Δ H ‡ t = 0.075 ± 0.155 kcal/mol ) on S 1 for TPBT. The solvent-induced component is related to E η , the activation energy for viscous flow, by Δ H ‡ v = aE η with a = 0.86 ± 0.06 for TPBT. This is in contrast to twisting on S 1 for trans -stilbene for which a significant intrinsic barrier has been reported. AM1 molecular orbital calculations for gas phase TPBT show a twist angle of ∼0° on S 0 with a barrier to rotation of 20.1 kcal/mol. Consistent with the experimental results, the calculations show that TPBT undergoes a barrierless relaxation on S 1 from its Franck-Condon structure to a minimum with a twist angle of 85.6°.