Aaron A. Rachford

Thermochromic Absorption and Photoluminescence in [Pt(ppy)(μ-Ph2pz)]2

The temperature effects on the spectral properties and photophysics of a new d(8)-d(8) dinuclear Pt(II) chromophore, [Pt(ppy)(mu-Ph(2)pz)](2) (ppy is 2-phenylpyridine and Ph(2)pz is 3,5-diphenylpyrazolate), have been investigated. The thermochromic shifts are tentatively ascribed to intramolecular sigma interactions between the two pseudocofacial d(z(2)) orbitals. Substantial emission profile changes occur in the solid state, solution, and doped polymer films.

Triplet Excited State Distortions in a Pyrazolate Bridged Platinum Dimer Measured by X-ray Transient Absorption Spectroscopy.

The excited-state structure of a dinuclear platinum(II) complex with tert-butyl substituted pyrazolate bridging units, [Pt(ppy)(μ-(t)Bu(2)pz)](2) (ppy = 2-phenylpyridine; (t)Bu(2)pz = 3,5-di-tert-butylpyrazolate) is studied by X-ray transient absorption (XTA) spectroscopy to reveal the transient electronic and nuclear geometry. DFT calculations predict that the lowest energy triplet excited state, assigned to a metal-metal-to-ligand charge transfer (MMLCT) transition, has a contraction in the Pt-Pt distance. The Pt-Pt bond length and other structural parameters extracted from fitting the experimental XTA difference spectra from full multiple scattering (FMS) and multidimensional interpolation calculations indicates a metal-metal distance decrease by approximately 0.2 Å in the triplet excited state. The advantages and challenges of this approach in resolving dynamic transient structures of nonbonding or weak-bonding dinuclear metal complexes in solution are discussed.

Coherence in metal-metal-to-ligand-charge-transfer excited states of a dimetallic complex investigated by ultrafast transient absorption anisotropy.

Coherence in the metal-metal-to-ligand-charge transfer (MMLCT) excited state of diplatinum molecule [Pt(ppy)(μ-(t)Bu(2)pz)](2) has been investigated through the observed oscillatory features and their corresponding frequencies as well as polarization dependence in the single-wavelength transient absorption (TA) anisotropy signals. Anticorrelated parallel and perpendicular TA signals with respect to the excitation polarization direction were captured, while minimal oscillatory features were observed in the magic angle TA signal. The combined analysis of the experimental results coupled with those previous calculated in the literature maps out a plausible excited state trajectory on the potential energy surface, suggesting that (1) the two energetically close MMLCT excited states due to the symmetry of the molecule may be electronically and coherently coupled with the charge density shifting back and forth between the two phenylpyridine (ppy) ligands, (2) the electronic coupling strength in the (1)MMLCT and (3)MMLCT states may be extracted from the oscillation frequencies of the TA signals to be 160 and 55 cm(-1), respectively, (3) a stepwise intersystem crossing cascades follows (1)MMLCT → (3)MMLCT (T(1b)) → (3)MMLCT (T(1a)), and (4) a possible electronic coherence can be modulated via the Pt-Pt σ-interactions over a picosecond and survive the first step of intersystem crossing. Future experiments are in progress to further investigate the origin of the oscillatory features. These experimental observations may have general implications in design of multimetal center complexes for photoactivated reactions where coherence in the excited states may facilitate directional charge or energy transfer along a certain direction between different parts of a molecule.

Generation of Powerful Tungsten Reductants by Visible Light Excitation

The homoleptic arylisocyanide tungsten complexes, W(CNXy)6 and W(CNIph)6 (Xy = 2,6-dimethylphenyl, Iph = 2,6-diisopropylphenyl), display intense metal to ligand charge transfer (MLCT) absorptions in the visible region (400-550 nm). MLCT emission (λ(max) ≈ 580 nm) in tetrahydrofuran (THF) solution at rt is observed for W(CNXy)6 and W(CNIph)6 with lifetimes of 17 and 73 ns, respectively. Diffusion-controlled energy transfer from electronically excited W(CNIph)6 (*W) to the lowest energy triplet excited state of anthracene (anth) is the dominant quenching pathway in THF solution. Introduction of tetrabutylammonium hexafluorophosphate, [Bu(n)4N][PF6], to the THF solution promotes formation of electron transfer (ET) quenching products, [W(CNIph)6](+) and [anth](•-). ET from *W to benzophenone and cobalticenium also is observed in [Bu(n)4N][PF6]/THF solutions. The estimated reduction potential for the [W(CNIph)6](+)/*W couple is -2.8 V vs Cp2Fe(+/0), establishing W(CNIph)6 as one of the most powerful photoreductants that has been generated with visible light.

Solvent-induced configuration mixing and triplet excited-state inversion: insights from transient absorption and transient dc photoconductivity measurements

Solvent-induced excited-state configuration mixing in a Pt(II) diimine chromophore with phenylene ethynylene containing acetylide ligands, [Pt((t)Bu2bpy)(PE3)2] (1), was characterized by nanosecond transient absorption spectroscopy and transient dc photoconductivity (TDCP). The mixing is a result of closely spaced triplet charge transfer (3CT) and intraligand-localized (3IL) triplet energy levels that are finely tuned with solvent polarity as ascertained by their parent model chromophores [Pt((t)Bu2bpy)(PE1)2] (2) and [Pt(P2)(PE3)2] (3), respectively. The absorption difference spectrum of the mixed triplet state is dramatically different from those of the 3CT and 3IL state model chromophores. The 3CT, 3IL and configuration-mixed triplet states led to distinct TDCP signals. The TDCP response is of negative polarity for 3CT excited states but of positive polarity for 3IL excited states. TDCP transients for 1 in mixed solvents are a combination of signals from the 3IL and 3CT states, with the signal magnitude depending on the polarity of solvent composition. The fraction of 3CT state character in the configurationally mixed excited state was quantified by TDCP to be approximately 0.24 in pure benzene, while it decreased to approximately 0.05 in 20 : 80 (v : v) benzene-CH2Cl2. The charge transfer fraction appears to increase slightly to approximately 0.11 in the lower polarity 20 : 80 n-hexane-CH2Cl2 medium. TDCP is shown to be a useful tool for the identification of the lowest excited state in electrically neutral metal-organic chromophores.

Stimulating changes in the elastic modulus of polymer materials by molecular photochromism

Photonastic materials are those that deform or change shape in a pre-determined or non-random fashion when exposed to light. We report here the elastic modulus of a photonastic polymer containing the photoreversible photochrome [Ru(bpy)2(pySO-NB)]2+, where bpy is 2,2′-bipyridine, and pySO-NB is a chelating ligand containing a norbornene monomer attached through an amide group (Scheme 1), both pre- and post-irradiation. Nanoindentation results show that the polymer exhibits an increase in the elastic modulus (E) upon irradiation. We propose a bilayer cantilever model to explain the macroscopic deformation. Our analysis reveals that the concentration of the ruthenium is an important parameter through the optical density of the film, the optical penetration depth and the force generated from irradiation.

Progress in resolution, sensitivity, and critical dimensional uniformity of EUV chemically amplified resists

This paper will discuss further progress obtained at Dow for the improvement of the Resolution, Contact critical dimension uniformity(CDU), and Sensitivity of EUV chemically amplified resists. For resolution, we have employed the use of polymer-bound photoacid generator (PBP) concept to reduce the intrinsic acid diffusion that limits the ultimate resolving capability of CA resists. For CDU, we have focused on intrinsic dissolution contrast and have found that the photo-decomposable base (PDB) concept can be successfully employed. With the use of a PDB, we can reduce CDU variation at a lower exposure energy. For sensitivity, we have focused on more efficient EUV photon capture through increased EUV absorption, as well as more highly efficient PAGs for greater acid generating efficiency. The formulation concepts will be confirmed using Prolith stochastic resist modeling. For the 26nm hp contact holes, we get excellent overall process window with over 280nm depth of focus for a 10% exposure latitude Process window. The 1sigma Critical dimension uniformity [CDU] is 1.1 nm. We also obtain 20nm hp contact resolution in one of our new EUV resists.

Ranking Solvent Interactions and Dielectric Constants with [Pt(mesBIAN)(tda)]: A Cautionary Tale for Polarity Determinations in Ionic Liquids

The solvatochromic properties of [Pt(mesBIAN)(tda)] are studied in traditional molecular solvents and ionic liquids and duly compared along established empirical solvent parameter scales. The charge-transfer absorption band of [Pt(mesBIAN)(tda)] is determined to be primarily dependent upon solvent acidity and dipolarity. Notably, ionic liquids do not obey the same well-behaved trend as molecular solvents, highlighting the complexity and domain (nano)segregation inherent to ionic liquids.

Platinum II Acetylide Photophysics

The photophysics of PtII acetylide chromophores represents an important emergent area of research and development. The present review exclusively deals with photophysical processes in square planar PtII complexes of the general formulae: Pt(N^N)(C≡CR)2, [Pt(N^N^N)(C≡CR)]+, Pt(N^N^C)(C≡CR), trans-Pt(PR3)2(C≡CR)2, and cis-Pt(P^P)(C≡CR)2, where N^N is a bidentate 2,2’-bipyridine, N^N^N and N^N^C are tridentate polypyridines, PR3 is a monodentate phosphine and P^P is a bidentate phosphine ligand. These molecules exhibit a range of photophysical attributes depending upon the nature of the lowest electronic triplet excited state(s) which are either charge-transfer, ligand-localized, or an admixture of the two. Under special circumstances, intermolecular interactions further complicate the electronic structures of the ground and excited states and the resulting spectroscopy. Recent computational approaches emphasizing the successful application of DFT and TD-DFT methods towards understanding the absorption and emission processes of these chromophores are also presented.

Phototriggered sulfoxide isomerization in [Ru(pic)2(dmso)2]

We report the characterization and photochemistry of a simple ruthenium coordination complex containing only picolinate (pic) and dmso, which exhibits a large isomerization quantum yield (Phi(SS-->OO) = 0.50) in various solvents. The picolinate ligands of [Ru(pic)(2)(dmso)(2)] are in a cis arrangement so that the carboxylate oxygen of one pic ligand (O1) is trans to the pyridine of the second picolinate (N2). One dmso ligand (S1) is trans to a pyridine nitrogen (N1), while the second dmso (S2) is trans to a carboxylate oxygen (O3). The cyclic voltammetry, (1)H NMR, IR, and UV-vis spectroscopy data suggest that while both dmso ligands isomerize photochemically, only one dmso ligand isomerizes electrochemically. Isomerization quantum yields for each dmso ligand differ by an order of magnitude (Phi(SS-->SO) = 0.46 and Phi(SO-->OO) = 0.036). In agreement with previous results, the isomerization quantum yield for each dmso is dependent on the ligand that is trans to the dmso.