John Olmsted

Photochemical determination of the solubility of oxygen in various media

A photochemical method for determining the oxygen concentration in air-saturated non-aqueous solvents has been developed. Solutions containing a sensitizer (Rose Bengal or Methylene Blue) and 1,3-diphenylisobenzofuran (DPIBF) as an oxygen acceptor are irradiated at 546 or 633 nm and the absorbance at 404 nm is monitored. The dissolved oxygen content is found from the change in absorbance and the known 1:1 stoichiometry of addition of singlet oxygen to DPIBF. The solubilities found, accurate to +/- 6%, for oxygen in air-equilibrated solvents, are (mM): acetone, 2.37; acetonitrile, 2.42; dimethylsulfoxide, 0.33; ethanol, 1.94; N-methylformamide, 1.31. Measurements on mixed acetone-N-methylformamide solvents showed that the solubility of oxygen does not vary with solvent composition in a predictable manner.

Photochemical storage potential of azobenzenes

Abstract The potential for storage of solar energy using photochemical trans-cis isomerizations of substituted azobenzenes has been studied. Polar substituents are found to res-shift the π-π ∗ absorption spectrum of these compounds without destroying the endothermicity of the photoisomerization. Highly red-shifted compounds such as azonaphthalenes revert thermally to the trans isomer within seconds. Methyl orange has the most favorable energy storage potential of the compounds studied and has a storage efficiency of 16 per cent for 436 nm irradiation and 2 per cent for AM-1 isolation. Because of limited solubilities and rapid thermal reversion rates in polar solvents, it is concluded that azobenzenes are not favorable compounds for photochemical solar energy storage.

On the validity of Shannon-information calculations for molecular biological sequences*

The usefulness of information-theoretic measures of the Shannon-Weaver type, when applied to molecular biological systems such as DNA or protein sequences, has been critically evaluated. It is shown that entropy can be re-expressed in dimensionless terms, thereby making it commensurate with information. Further, we have identified processes in which entropy S and information H change in opposite directions. These processes of opposing signs for delta S and delta H demonstrate that while the Second Law of Thermodynamics mandates that entropy always increases, it places no such restrictions on changes in information. Additionally, we have developed equations permitting information calculations, incorporating conditional occurrence probabilities, on DNA and protein sequences. When the results of such calculations are compared for sequences of various general types, there are no informational content patterns. We conclude that information-theoretic calculations of the present level of sophistication do not provide any useful insights into molecular biological sequences.

Effect of a steric spacer on chromophoric interactions of ruthenium complexes containing covalently bound anthracene

Abstract To study interchromophore interactions in a ruthenium complex linked covalently to an anthryl group, we have synthesized 4-(9-anthrylethyl),4′-methyl-2,2′-bipyridine (dmb-an) and complexed it with ruthenium in two asymmetric complexes, [Ru(L) 2 (dmb-an)] 2+ , where L4,4′-dimethyl, 2,2′-bipyridine (dmb) or 4,4′-di- tert -butyl, 2,2′-bipyridine (dtb). In both complexes, the two independent chromophores rapidly undergo singlet—singlet and triplet—triplet energy transfer which nearly quantitatively quenches luminescence: Φ F =3.5 × 10 −4 from the anthryl group and Φ −5 from the metal—ligand charge transfer state of the complex. Both complexes also readily undergo photoinduced oxygenation of the anthryl group. The anthryl group is charge transfer quenched by methyl viologen, with a cage escape yield that differs substantially for the two complexes. When Ldmb, the cage escape yield is 0.26, in common with other ruthenium tris-imines and much less than free anthracene. However, when Ldtb, the cage escape yield rises to 0.55±0.05. We interpret the improved yield as arising from the bulky tert -butyl substituents, which prevent the anthryl moiety from close approach to the metal center and reduce metal-mediated enhancement of reverse charge transfer.

Intramolecular triplet-triplet energy transfer from xanthene dyes to an anthryl substituent

Abstract The 9-methylanthryl esters of three xanthene dyes, erythrosine, phloxine, and rose bengal, have been synthesized. Flash photolysis and transient absorption measurements at 600 nm (xanthene triplet state) and 435 nm (anthryl triplet state) have been used to determine that intramolecular triplet-triplet energy transfer is highly efficient in two of these esters but is only partial in the ester of rose bengal. These results demonstrate that the triplet states of erythrosine and phloxine lie higher in energy, while the triplet state of rose bengal is close to but lower in energy, than that of the anthryl moiety.

Photon flux measurements using calorimetry

A simple photometric calorimeter has been constructed, consisting of a vacuum-jacketed cell equipped with flat quartz windows to admit incident light, a thermistor probe to monitor temperature change, and a resistance heater for calibration. An optically dense, photochemically inert solution inside the cell absorbs incident radiation, converting it quantitatively into heat. The calorimeter gives photon flux measurements in agreement with ferrioxalate actinometry at 366 and 436 nm. It has a sensitivity of 2x10(15) photons/s and is useful throughout the optical spectral range.