Linda A. Peteanu

Mechanism of Photoinduced Metal-Free Atom Transfer Radical Polymerization: Experimental and Computational Studies

Photoinduced metal-free atom transfer radical polymerization (ATRP) of methyl methacrylate was investigated using several phenothiazine derivatives and other related compounds as photoredox catalysts. The experiments show that all selected catalysts can be involved in the activation step, but not all of them participated efficiently in the deactivation step. The redox properties and the stability of radical cations derived from the catalysts were evaluated by cyclic voltammetry. Laser flash photolysis (LFP) was used to determine the lifetime and activity of photoexcited catalysts. Kinetic analysis of the activation reaction according to dissociative electron-transfer (DET) theory suggests that the activation occurs only with an excited state of catalyst. Density functional theory (DFT) calculations revealed the structures and stabilities of the radical cation intermediates as well as the reaction energy profiles of deactivation pathways with different photoredox catalysts. Both experiments and calculations suggest that the activation process undergoes a DET mechanism, while an associative electron transfer involving a termolecular encounter (the exact reverse of DET pathway) is favored in the deactivation process. This detailed study provides a deeper understanding of the chemical processes of metal-free ATRP that can aid the design of better catalytic systems. Additionally, this work elucidates several important common pathways involved in synthetically useful organic reactions catalyzed by photoredox catalysts.

Electroabsorption measurements and ab initio calculations of the dipolar properties of 2-(2′-hydroxyphenyl)-benzothiazole and -benzoxazole: two photostabilizers that undergo excited-state proton transfer

Electroabsorption is used to determine the difference dipole moment, N D m N , and the difference polarizability, D a ,o f . HBT and HBO, two molecules that undergo excited-state intramolecular proton transfer ESIPT . These measurements are ) . . compared to ab initio 6-31G and semi-empirical INDO1rs calculations. Good agreement is obtained between ab initio and experimental N D m N values. However, the negative D a values obtained experimentally for both molecules are in disagreement with the calculated D a values which are positive. The absorption maxima, measured as a function of solvent polarity, support the electroabsorption results. Agreement between the observed and calulated shift requires that D a for both molecules be negative. q 1998 Elsevier Science B.V. All rights reserved.

Fluorescent DNA Nanotags Featuring Covalently Attached Intercalating Dyes: Synthesis, Antibody Conjugation and Intracellular Imaging

We have synthesized fluorescent DNA duplexes featuring multiple thiazole orange (TO) intercalating dyes covalently attached to the DNA via a triazole linkage. The intercalating dyes stabilize the duplex against thermal denaturation and show bright fluorescence in the green region of the spectrum. The emission color can be changed to orange or red by addition of energy-accepting Cy3 or Cy5 dyes attached covalently to the DNA duplex. The dye-modified DNA duplexes were then attached to a secondary antibody for intracellular fluorescence imaging of centrosomes in Drosophila embryos. Bright fluorescent foci were observed at the centrosomes in both the donor (TO) and acceptor (Cy5) channels, because the energy transfer efficiency is moderate. Monitoring the Cy5 emission channel significantly minimized the background signal because of the large shift in emission wavelength allowed by energy transfer.

Metal-to-ligand charge transfer absorption in a rhenium(I) complex that contains a π-conjugated bipyridine acceptor ligand

Abstract This Letter describes a study of the absorption and electroabsorption spectroscopy of a complex consisting of the –Re I (CO) 3 Cl chromophore coordinated to the 4,4 ′ -{bis-[2,5-(dimethoxyphenyl)ethynyl]}-2,2 ′ -bipyridine ligand. The objective of the study is to explore whether Re → bipyridine metal-to-ligand charge transfer (MLCT) excitation leads to electron delocalization into the π-conjugated 2,5-(dimethoxyphenyl)ethynylene moieties. The results indicate that the two low-energy absorption bands observed for the complex have a strong degree of charge transfer character; however, the bands are dominated by intraligand (IL) π,π * transitions. The spectroscopic data are consistent with possible configuration mixing between the lowest IL π,π * and MLCT transitions.

Electronic properties of the conducting form of polyaniline from electroabsorption measurements

The polaron band at1.4 eV of d,l-camphorsulfonic acid-doped polyaniline (()-HCSA-PANI) in a poly(methyl methacrylate) (PMMA) matrix was studied using electroabsorption (Stark-effect) spectroscopy at 298 K. A very small change in dipole moment on excitation (jDmj) in the order of 1:4 0:2 D was measured as well as an average change in polarizability (Da) of onlyˇ1:3 0: 4A ˚ 3 . The electroabsorption signal of ()-HCSA-PANI in this region showed evidence for heterogeneity in the absorption band. # 2001 Elsevier Science B.V. All rights reserved.

Conformational effects on optical charge transfer in the emeraldine base form of polyaniline from electroabsorption measurements and semiempirical calculations

The electroabsorption (Stark) spectrum of the emeraldine base form of polyaniline (EB) in a polymethyl methacrylate (PMMA) matrix at room temperature is presented. For the lowest-energy (1.98 eV) band, the absolute value of the change in dipole moment on excitation (|Δμ|) is 6.1 D and the trace of the change in polarizability (Tr Δα⇊) is 200 A3. Similar values were obtained for the corresponding electronic transition of a five-ring oligomer of EB that is symmetrically terminated with phenyl rings, also in a PMMA matrix. In order to understand the origin of the substantial value of |Δμ| measured for these nominally symmetric species, a series of calculations were performed in which the geometries of model oligomers were varied in a systematic fashion and resulting effects on the values of |Δμ| were determined. Numerous low-energy conformers were identified by this method having substantial values of |Δμ|, suggesting that the large |Δμ| measured experimentally results from conformational heterogeneity....

Electric-Field-Induced Fluorescence Quenching in Polyfluorene, Ladder-Type Polymers, and MEH-PPV Evidence for Field Effects on Internal Conversion Rates in the Low Concentration Limit

Electric field-induced fluorescence quenching has been measured for a series of conjugated polymers with applications in organic light-emitting diodes. Electrofluorescence measurements on isolated chains in a glassy matrix at 77 K show that the quenching efficiency for poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) is an order of magnitude larger than that for either a ladder-type polymer (MeLPPP) or polyfluorene (PFH). This effect is explained in terms of the relatively high probability of field-enhanced internal conversion deactivation in MEH-PPV relative to either MeLPPP or PFH. These data, obtained under dilute sample conditions such that chain-chain interactions are minimal, are contrasted with the much higher quenching efficiencies observed in the corresponding polymer films, and several explanations for the differences are considered. In addition, the values of the change in dipole moment and change in polarizability on excitation (|Δμ| and tr(Δα), respectively) are reported, and trends in these values as a function of molecular structure and chain length are discussed.

The Effects of Side-Chain-Induced Disorder on the Emission Spectra and Quantum Yields of Oligothiophene Nanoaggregates: A Combined Experimental and MD-TDDFT Study

Oligomeric thiophenes are commonly used components in organic electronics and solar cells. These molecules stack and/or aggregate readily under the processing conditions used to form thin films for these applications, significantly altering their optical and charge-transport properties. To determine how these effects depend on the substitution pattern of the thiophene main chains, nanoaggregates of three sexithiophene oligomers having different alkyl substitution patterns were formed using solvent-poisoning techniques and studied using steady-state and time-resolved emission spectroscopy. The results indicate the substantial role played by the side-chain substituents in determining the emissive properties of these species. Both the measured spectral changes and their dependence on substitution are well-modeled by combined quantum chemistry and molecular dynamics simulations. The simulations connect the side-chain-induced disorder, which determines the favorable chain-packing configurations within the aggregates, with their measured electronic spectra.

Electroabsorption of dimers containing MM (M = Mo, W) quadruply bonded units: insights into the electronic structure of neutral coupled redox centers and their relationship with mixed valence ions.

The electroabsorption spectra for the metal-to-ligand charge transfer transition in complexes containing oxalate and terephthalate bridged MM quadruply bonded units, [(MM)(pivalate)(3)](2)-mu(2)-BR, where M = Mo or W and BR = oxalate or terephthalate, are reported. The measured magnitude of the change in dipole moment (|Deltamu|) and the change in polarizability (Deltaalpha) that accompany this electronic transition are found to be small and not to follow the behavior expected on the basis of the two-state model. In addition, the trend in the value of Deltaalpha for the neutral states is mirrored by the trend in the degree of electronic coupling (H(AB)) for the strongly coupled mixed valence states formed by the same complexes in their singly oxidized states.

Spectroscopic and MD Study of Dynamic and Structural Heterogeneities in Ionic Liquids

The structure of ionic liquids (ILs) surrounding solute dyes and the effects of solvent structure on solute diffusion have been investigated using molecular dynamics (MD) and the experimental tools of confocal and fluorescence correlation spectroscopies. Although confocal microscopy and simulations show that the local environment around solutes in ILs is heterogeneous and that the structural heterogeneity is rather long-lived, the local polarity and the diffusion constant were found to be uncorrelated. Moreover, the complex diffusion observed experimentally is not due to the structural heterogeneity of the IL but rather due to the dynamic heterogeneity arising from the viscous glassy nature of the IL environment. MD simulations show that the degree of dynamic heterogeneity depends on the first nonvanishing electric multipole moment of the solute. The dynamics of a cationic solute are the least heterogeneous, whereas those of a solute without an electric multipole moment are the most heterogeneous. This indicates that the length scale over which the solute-solvent interactions occur, and thus the number of solvent degrees of freedom that couple to the solute, are the key factors governing the dynamic heterogeneity of the solute.