Curcumin is a yellow-orange compound derived from the root of Curcuma longa (Zingiberaceae family), that has been used as a medicine, spice and coloring agent. Curcumin has proved nontoxic in a number of cell culture and whole animal studies. Curcumin has, however, been reported to have bactericidal effects at very high concentrations. When illuminated, curcumin exerted potent phototoxic effects in micromolar amounts. Gram-negative bacteria displayed greater resistance to curcumin phototoxicity relative to Gram-positive bacteria. Oxygen was required for curcumin phototoxicity. Curcumin binding to cells was not required for photokilling; the reactive intermediate therefore must be relatively long-lived. The mechanism(s) of curcumin phototoxicity may involve hydrogen peroxide production. Singlet excited oxygen was not detected.

Temporal behaviors of singlet excited-state absorption and fluorescence of all-trans-1,6-diphenyl-1,3,5-hexatriene in solution are reported in the picosecond time domain. Contrary to an earlier study, the absorbance ratio at the two λ max (∼460 and ∼650 nm) is time (∼10 ps resolution) and, except for small shifts, temperature (238-298 K) independent. Similarly, in contrast to an earlier report, identical time evolution of fluorescence intensity is found at short (405 ( 10 nm) and long (g500 nm) λ. We conclude that the initially formed 1 1 B u state completely relaxes to a 1 1 B u /2 1 A g equilibrium mixture within our time resolution (∼10 ps). Since the latter is favored overwhelmingly, all transient absorption is assigned to n 1 B u r 2 1 A g transitions, consistent also with small red shifts in λ max with increased medium polarizability. We discern no spectral manifestations of phenyl-vinyl torsional motions, or any associated stabilization of the 1 1 B u state, at times longer than 10 ps.

Electronic absorption and fluorescence spectroscopies were used to study the picosecond time scale competition between intersystem crossing and carbon-halogen bond homolysis resulting from 266-nm excitation of 9-chloro-and 9-bromofluorene in the nonpolar solvent cyclohexane. From steady-state fluorescence spectroscopy, fluorescence quantum yields (&) relative to fluorene (relative q?y = 1 .O) of 0.036, 0.046, and 0.034 were measured for 2-bromofluorene, 9-chlorofluorene, and 9-bromofluorene, respectively. Picosecond-resolved absorption spectroscopy permitted the detection of s,, -SI absorption of fluorene near 700 nm which decayed with a time constant of 5.1 ns. Within experimental error, the decay time of this transient absorption band agreed with an observed fluorescence decay time of 5.8 ns for fluorene, thereby providing support for the assignment of this absorption to St of fluorene. From transient absorption measurements, intersystem crossing of 2-bromofluorene occurred with a time constant of -40 ps (k,,, 2.5 x 1O' O s-'). Excitation of 9-chlorofluorene and 9-bromofluorene resulted in the appearance of absorptions assigned to the 9-fluorenyl radical and to T I of the 9-halofluorene. Additional evidence supporting the assignments of absorptions to TI of the 9-halofluorenes was provided from transient absorption experiments in which triplet energy transfer was used to populate TI of fluorene, 9-chlorofluorene, and 9-bromofluorene. Carbon-halogen bond homolysis does not occur from T I for times up to 20 ns postexcitation for these 9-halofluorenes.For 9-chloro-and 9-bromofluorene, intersystem crossing and bond homolysis depopulate SI. Intersystem crossing in 9-bromofluorene occurs faster than in 9-chlorofluorene [t,,, I 20 ps (k,,, 2 5.0 x 1O'O s-l) and 40 ps (k,,, = 2.5 x 1Olo s-l), respectively], resulting in less 9-fluorenyl radical produced via bond homolysis in SI from the bromide than from the chloride although the C-Br bond is weaker than the C-C1 bond. Introductionsuitably polar solvation condition^;^^^ and (4) ion pairs formed

No abstract

The new generation of high energy accelerators - LHC, CEBAF, RHIC - have encouraged the development of more robust plastic scintillators. Monitors and detectors for these machines will require plastic scintillator with greater radiation tolerance. Although the major cause of radiation damage in plastic scintillator is the creation of color centers in the base plastic, the most successful approach to date has been to utilize fluors which circumvent, rather than solve, the radiation damage problem. Two techniques have been found to be useful. First, increase the concentration of fluors so that the optical density of the fluors for absorption of the scintillation photons remains much higher than the optical density of the radiation induced color centers. Second, use fluors which emit at longer wavelengths than traditional fluors, thus avoiding radiation induced color centers. The present work attempts to meet these requirements by modifying the structure of the 3-HF molecule.