Joy E. Rogers

Effect of platinum on the photophysical properties of a series of phenyl-ethynyl oligomers

In this work we detail the photophysical properties of a series of butadiynes having the formula H-(C6H4-C[triple bond]C)n-(C[triple bond]C-C6H4)n-H, n=1-3 and ligands H-(C6H4-C[triple bond]C)n-H, n=1-3 and compare these to previous work done on a complimentary series of platinum-containing complexes having the formula trans-Pt[(PC4H9)3]2[(C[triple bond]-C6H4)n-H]2, n=1-3. We are interested in understanding the role of the platinum in the photophysical properties. We found that there is conjugation through the platinum in the singlet states, but the triplet states show more complex behavior. The T1 exciton, having metal-to-ligand charge-transfer character, is most likely confined to one ligand but the Tn exciton appears to have ligand-to-metal charge-transfer character. The platinum effect was largest when n=1. When n=2-3, the S0-S1,S1-S0,T1-S0, and T1-Tn spectral properties of the platinum complex are less influenced by the metal, becoming equivalent to those of the corresponding butadiynes. When n=1, platinum decreases the triplet state lifetime, but its effect diminishes as n increases to 2.

Mechanisms of Photoinitiated Cleavage of DNA by 1,8-Naphthalimide Derivatives¶

Abstract Using water-soluble 1,8-naphthalimide derivatives, the mechanisms of photosensitized DNA damage have been elucidated. Specifically, a comparison of rate constants for the photoinduced relaxation of supercoiled to circular DNA, as a function of dissolved halide, oxygen and naphthalimide concentration, has been carried out. The singlet excited states of the naphthalimide derivatives were quenched by chloride, bromide and iodide. In all cases the quenching products were naphthalimide triplet states, produced by induced intersystem crossing within the collision complex. Similarly, the halides were found to quench the triplet excited state of the 1,8-naphthalimide derivatives by an electron transfer mechanism. Bimolecular rate constants were <105 M−1 s−1 for quenching by bromide and chloride. As expected from thermodynamic considerations quenching by iodide was 6.7 × 109 and 8.8 × 109 M−1 s−1 for the two 1,8-naphthalimide derivatives employed. At sufficiently high ground-state concentration self-quenching of the naphthalimide triplet excited state also occurs. The photosensitized conversion of supercoiled to circular DNA is fastest when self-quenching reactions are favored. The results suggest that, in the case of 1,8-naphthalimide derivatives, radicals derived from quenching of the triplet state by ground-state chromophores are more effective in cleaving DNA than reactive oxygen species or radicals derived from halogen atoms.

Molecular structure-spectroscopic property relationships in a series of transition metal-containing phenylacetylene oligomers

Abstract To develop novel nonlinear dyes for photonic applications, we synthesized a series of transition metal-containing phenylacetylene oligomers. The optical properties of these compounds were measured by UV/Vis, fluorescence, long-pathlength UV/Vis and nanosecond flash photolysis experiments. The sensitivity of the state energies to molecular size was a measure of their delocalization. The S 0 and T 1 states were found to be more localized than the S 1 and T n states. The results were consistent with the S 0 →S 1 and T 1 →T n transitions having charge transfer character, while the T 1 →S 0 transition was from a confined state to another confined state.

Nucleotide Oxidation Mediated by Naphthalimide Excited States with Covalently Attached Viologen Cosensitizers

The ground‐ and excited‐state interactions of polymethylene‐linked 1,8‐naphthalimide–viologen dyads with calf‐thymus DNA have been investigated. By virtue of the covalently attached viologen, the compounds represent the first example of linked chromophore/cosensitizer systems in the photooxidation of duplex DNA. The compounds associate strongly with DNA. Analysis of ground‐state spectral changes yield binding constants of 0.7–2.5 × 106M−1. Upon 355 nm pulsed irradiation of the compounds in the presence of calf‐thymus DNA, reduced viologen is observed within the laser pulse. Photoproducts are not observed on this time scale in the absence of DNA. Since ground‐state bleaching of the naphthalimide was not observed, the results suggest that DNA nucleobases are the species being oxidized. The quantum efficiency of radical production increases with the extent of binding to DNA. Under conditions where the compounds are bound predominantly to DNA, the quantum efficiencies were found to range from 0.02 to 0.03. Although small, the values represent a substantial increase in charge‐separation yield compared to 1,8‐naphthalimide compounds that lack the covalently attached viologen. The mechanism of radical production and effect of number of intervening methylenes are discussed.

Photophysics of organic materials exhibiting strong two-photon and excited state absorption

We report photophysical measurements and application of an effective three-photon absorption model that characterize the two-photon and excited state absorption in organic D-π-A chromophores. The key parameter is an effective three-photon absorption coefficient that depends on the intrinsic molecular two-photon absorption cross section and excited state photophysical properties. We measure all of these molecular parameters independently in a variety of experiments and then compare the model predictions with nanosecond nonlinear absorption measurements. We find excellent agreement with the data using only experimentally measured molecular quantities and no free parameters. We conclude that excited state absorption from both singlet and triplets states is the dominant contribution to the nonlinear transmittance loss in the nanosecond regime, and that the chief role of two-photon absorption in this regime is to populate the excited triplet state.

The Relationship Between Molecular Structure and Spectroscopic Properties of a Series of Transition Metal-Containing Phenylacetylene Oligomers and Polymers

To develop novel nonlinear dyes for photonic applications, we synthesized a series of transition metal-containing phenylacetylene oligomers and polymers. The optical properties of these compounds were measured by UV/Vis, fluorescence, and flash photolysis experiments. As the number of oligomer units increased, the transition energies decreased. A solvatochromism experiment suggested the fluorescing state was different from the absorbing state. As a group, the spectra of the polymeric versions of these complexes were red shifted from the spectra of the oligomers. The polymeric complexes had less clear trends relating the number of oligomer units to transition energies. A comparison of a low molecular weight and a high molecular weight polymer showed the degree of polymerization caused spectroscopic shifts comparable to the number of phenylacetylene units in the monomer unit.

Combined nonlinear effects in two-photon absorption chromophores at high intensities

Large two-photon and excited state absorption have been reported in donor-acceptor-substituted π-conjugated molecules. We have performed detailed nonlinear absorption and photophysical measurements on a system of AFX chromophores and calculate the nonlinear transmission based on an effective three-level model. A numerical model that includes far wing linear absorption has been developed and compared with an analytical three-photon absorption model. The models are in accordance and yield excellent agreement with experimental nonlinear transmission data for 0.02-M AFX solutions up to laser intensities ~ 1-5 GW/cm2. We have extended our modeling efforts to include some new effects that may be anticipated in this regime, such as stimulated scattering, molecular interactions, and saturation. Effects of chirped pulses and linewidth of the pump laser on stimulated scattering are included. Self-focusing and de-focusing are also considered. We report on our experimental observations of various materials and discuss results with respect to our extended theoretical models.

Synthesis and Characterization of Novel Platinum Acetylide Oligomers

As part of an effort to develop a spectroscopic structure-property relationship in platinum acetylide oligomers, we have prepared a series of bidentate Pt(PBu 3 ) 2 L 2 compounds. The ligand was the series o-syd-C 6 H 4 -CΞC-(C 6 H 4 -CΞC) n -H, n = 0,1,2. The terminal oligomer unit consisted of a sydnone group ortho to the acetylene carbon. The compounds were characterized by various methods, including 13 C-NMR, ground state absorption, fluorescence, phosphorescence and laser flash photolysis. The acetylenic 13 C-NMR resonances showed sydnone influences that decreased with increasing number of monomer units. The ground state absorption spectra were slightly red shifted from those of the baseline oligomers not having a sydnone group. The low temperature emission and excitation spectra showed complex dependence on excitation and emission wavelengths, suggesting the chromphores resided in a distribution of solvent environments and conformations. Finally, broad triplet state absorption spectra were observed, with absorption throughout the visible and near infrared regions.

Spectral and kinetic behavior of phenylacetylene oligimers

To develop novel nonlinear dyes for photonic applications, we synthesized a series of transition metal-containing phenylacetylene oligomers. Theoretical properties of these compounds were measured by UV/Vis absorption, photoluminescence, and nanosecond flash photolysis experiments. It was found that as the number of oligomer units increased ,the transition energies decreased without saturation. The low ground state absorption and UV absorption edge gives rise to solutions that are nearly water clear. A very broad triplet state absorption extending from the absorption edge to the limits of our spectrometer is demonstrated to also be intense. These results enhance the understanding of these materials when used for nonlinear absorption applications and enable the prediction properties for materials extending this class of dyes.