Vincent M. Miskowski

The effect of linear chain structure on the electronic structure of pt(II) diimine complexes

Abstract In square planar d8 complexes, the crystalline state can have a pronounced effect upon the electronic structure and photophysical parameters of the monomer due to electronic interactions between nearest neighbors. In this article, we discuss the luminescence behavior of Pt(II) diimine complexes in dilute monomeric environments and in linear chain solid state structures. In these complexes, triplet excited states of ligand field, metal-to-diimine charge transfer, and diimine (π-π*) type lie at similar energies. Their relative ordering is influenced not only by intramolecular factors such as ligand field strength and diimine substituents, but also by the intermolecular structure of the crystal packing. The observed luminescence is a sensitive probe of these factors in the solid state.

Optical Limiting In Solutions Of Metallo-Phthalocyanines And Naphthalocyanines

Optical limiting measurements have been made on solutions of several metal containing phthalocyanines and naphthalocyanines. Measurements at 532nm using nanosecond pulses from a Q-switched Nd:YAG laser show limiting throughputs of 1-10 millijoules with mild focussing in alcohol solutions with nominal transmissions of 30-70%. Measurements on chloro-aluminum-phthalocyanine solutions utilizing individual 30 psec pulses or trains (spanning about 100nsec) of modelocked pulses have shown even lower limiting throughputs. Thus, the dynamic range of the limiting behavior has been shown to cover at least three orders of magnitude. Prompt limiting is attributed to strong singlet-singlet (S1-Sn) absorption, whereas the longer time limiting behavior is postulated to result from strong triplet-triplet (T1-Tn) absorption. In addition to these studies, efforts have been underway to identify materials with reduced limiting throughput and improved optical transmission characteristics.

δ→δ* Revisited: What the energies and intensities mean

Abstract Attempts to extract estimates of the δ-bond strength of quadruply metal-metal-bonded molecules from their δ→δ* electronic transitions have been hindered in the past by limited understanding of the origins of the energies and intensities of these transitions. We show that the energies of the δ→δ* transitions of a wide variety of these molecules are adequately interpreted in terms of a simple zero-differential-overlap model that yields one-electron δ-δ* splittings of 5000–10,000 cm−1 and two-electron exchange terms [K(δ,δ*)] of 5000–8000 cm−1. Because of the magnitude of K, singlet-triplet δ→δ* splittings are very large, and configuration interaction is important for the correct description of the ground state. The intrinsic intensity of 1(δ→δ*) is estimated to be quite low. The considerable intensities observed in many cases do not correlate with δ-bond strength, but instead reflect intensity stealing from charge-transfer excited states as a result of δ,δ*-orbital mixing with ligand orbitals. The intrinsic δ-bond stabilization is estimated to be on the order of 10 kcal mol−1.

Internal conversion in poly(1-vinylnaphthalene)—IV. Photochemical processes in polymeric systems☆☆☆

Abstract Quantum yields of all major photoprocesses have been measured on poly(1-vinylnaphthalene) in fluid solution. These measurements indicate that approx. 84% of the excitation energy is deactivated through internal conversion processes ( S 0 ↞ S 1 ). Time resolved emission and absorbance measurements have been used to characterize the excited states in this system. It appears that several different types of singlet and triplet traps may be distinguished, one of which is the monomeric excited singlet. Existence of two different singlet excimers are postulated in order to interpret the time resolved emission data.

The Electronic Spectra of μ-Peroxodicobalt(III) Complexes

Abstract Some of the problems encountered in the determination of the electronic spectra of μ-peroxodicobalt(III) complexes are discussed. After account is taken of the common formation of mixtures of different μ-peroxocomplexes upon oxygenation of Co(II)-ligand solutions, it can be concluded that three classes of spectra exist. Planar single-bridged complexes exhibit a strong (∊ ≃ 12,500) π∗h→dσ-∗ peroxide LMCT band at 300–310 nm, with a weak (∊ ≃ 600) π∗v →d∊∗ band at 425–430 nm. Nonplanar (dihedral angle near 145°) single-bridged complexes exhibit two strong (∊ ≃ 6000) π∗(02- 2) → dσ∗ bands near 320 and 390 nm. Dibridged μ.-(OH-,02- 2) complexes, with a dihedral angle near 60°, exhibit one broad peroxide LMCT band at ≃ 355 nm (∊ ≃ 6000). The latter compounds have an additional band near 280 nm which is not due to peroxide. All of the peroxide LMCT spectra are shown to be consistent with a simple model that assumes a sinusoidal dependence of π∗(02 2) energies and σ-overlaps upon the dihedral angle.

Mechanistic Aspects of Solar Energy Storage Reactions Involving Polynuclear Rhodium Isocyanide Complexes. Preparation of New Binuclear Isocyanide Complexes of Iridium, Cobalt, Nickel, and Ruthenium

We have found1 that 546 nm irradiation of solutions of the binuclear Rh(I) complex Rh2(bridge)4 2+ (bridge = 1, 3-diisocyano- propane), or Rh2 2+, in 12 M HCl(aq) results in clean conversion to H2 and a Rh(II) complex. The Rh(II) complex can also be prepared by Cl2 oxidation and has been structurally characterized.2 Recent work in our laboratory has shown that a thermal reaction generates the blue photoactive species, Rh4C12 4+, as follows: Open image in new window Notably, the photochemical step is uphill, as H2 reacts slowly with Rh2C12 2+ to regenerate Rh4C12 4+.

Cyro-vibrational spectroscopy of blue copper proteins

Abstract Resonance Raman (RR) and Fourier transform infrared (FTIR) spectra at 12K have been obtained for Pseudomonas aeruginosa azurin, spinach plastocyanin, stellacyanin, and tree laccase. The temperature dependence of the azurin, plastocyanin, and stellacyanin spectra have been recorded as have the RR excitation profiles at 12 K. Room temperature RR spectra have been obtained for azurins from Alcaligenes fecalis, Alcaligenes sp., Bortadella pretussis and Bortadella bronchiseptica; bean plastocyanin; fungal laccase, human ceruloplasmin; and zucchini squash ascorbate squash ascorbate oxidase. Isotope studies employing 63 Cu/ 65 Cu and H/D substitution have been performed on the azurins from Ps. aeruginosa, Alc. fecalis , and Alc. sp . Principal conclusion include the following; The intense RR modes near 400 cm −1 include internal ligand deformations and the Cu-S(cys) stretch, rather than the Cu-S(cys) stretch and Cu-N(his-Im) stretches as previously supposed. The Cu-N(his-Im) stretches are assignable to the ubiquitous feature near 265 cm −1 ,consistent with the frequencies of similar motions in other proteins and in model complexes. Spinach plastocyanin exhibits a frequency shift of 14 cm −1 in its cysteine CS stretching frequency ( ca. 750 cm −1 ) upon freezing of the protein solution, suggesting that extra-protein forces (e.g.,solvent structure, crystallization, or substrate binding.) can influence the conformation of the active site. Above the freezing point of the solvent the plastocyanin RR modes are unusually broad, suggesting either extremely facile due to thermally accessible conformations in the high-temperature form of the active site. No selective enhancement of either the strong or weak RR modes is observed in the S(cys)→ Cu charge transfer excitation profiles of azurin, plastocyanin, or stellacyanin at 12 K. The azurin species other than Ps. aeruginosa exhibit an ‘extra» strong RR peak near 400 cm −1 which is, however, seen to be related to an unresolved shoulder in the 12 K Ps. aeruginosa spectrum. It is therefore unnecessary to invoke higher coordination numbers than four for copper to explain the RR spectra of the azurins other than Ps. aeruginosa . RR peaks appear in the azurin spectrum below 200 K which may be due to methionine C-S stretching modes and Cu-S(met) stretch or methionine C-S-C angle bend. If these low-temperature features are indeed due to such motions,methionine must be closely coordinated to copper in azurin at low temperature. The RR spectra are consistent with a monotonic relationship between the force constant of the Cu-S(cys)bond and the energy of the ligand-field transitions of the various proteins. The isotope studies lead to tentative identification of the RR modes which contain significant contributions from M-L stretches and internal histidine motions. The results provide an initial basis for reliable structural interpretation of the RR spectra of the blue copper proteins.

The low-energy, charge-transfer excited states of 4-amino-4′- nitrodiphenyl sulfide

Absorption and emission spectra of 4‐amino‐4’‐nitrodiphenyl sulfide in polar and nonpolar solvents were used to characterize and assign the low‐energy excited states of the molecule. Fluorescence‐excitation anisotropy spectra, fluorescence and phosphorescence lifetimes, and fluorescence quantum yields were also used to characterize the photophysics of these states. The lowest‐energy, fluorescent singlet state was determined to be an intramolecular charge transfer (ICT) state involving transfer of a full electron charge from the amino to the nitro group yielding a dipole moment of ∼50 D. A low‐energy, intense absorption band is assigned as a transition to a different ICT state involving a partial electron charge transfer from sulfur to the nitro group.

Photophysics and photosensitization behavior of microcrystalline [Pt(bpy)2]2[Pt2(pop)4]·nH2O

Abstract The insoluble double salt complex [Pt(bpy) 2 ] 2 [Pt 2 (pop) 4 ]· n H 2 O (“PBPop”; bpy=2,2′-bipyridine and pop=H 2 P 2 O 5 2− ) is prepared by mixing an aqueous solution of the monomers. In the solid state, the initial orange complex has ca . 4 waters of hydration and a variable diffuse reflectance maximum in the region of 510–530 nm which imparts an orange color. The complex dehydrates readily and irreversibly to a yellow form with ca . 2 waters of hydration whose diffuse reflectance spectrum shows no maximum at wavelengths greater than 500 nm. Both orange and yellow forms exhibit the same emission spectrum, with a maximum at 615 nm and a halfwidth of 3100 cm −1 at room temperature. The emission of yellow PBPop is extremely intense, with a room temperature lifetime of 45 ns and an estimated radiative rate constant of 70 000 s −1 . The emission maximum does not shift with decreasing temperature, but the halfwidth narrows to 2500 cm −1 at 40 K. In the case of orange PBPop, the emission intensity is very weak. The emission is attributed to either a chromophore or a trap state common to both orange and yellow forms. Powder suspensions of both forms of PBPop are active as photosensitizers for generating H 2 from aqueous solutions containing ethylenediaminetetraacetate (EDTA 2− ) as an electron donor and a colloidal Pt catalyst. Visible light excitation of the yellow double salt (λ ex >400 nm) leads to large turnover numbers ([H 2 ]/[PBPop]>100), indicating that H 2 is evolved catalytically. The photoproduction of H 2 is also observed in the absence of the Pt catalyst; however, the rate is a factor of 15 lower. The yellow form of PBPop is considerably more active than the orange form. In the presence of the latter complex, a relative long induction period accompanies H 2 formation. Both the yield and the rate of H 2 production with PBPop display a marked dependence on the excitation wavelength in the region 400–515 nm. Although PBPop is relatively stable in acidic EDTA 2− solution (pH 4.75), it undergoes dark decomposition in alkaline triethanolamine solution (pH 9.5). Also, during UV illumination (λ ex >345 nm), photodegradation of PBPop particles occurs along with H 2 evolution. The basis of these and other phenomena are discussed.

Franck-Condon analyses on the IBM PC/AT

Abstract A program for the rapid calculation of electronic absorption and emission spectra has been developed. The program is designed to run on the IBM PC series of computers with maximal flexibility. Sample calculations are provided for several spectra to demonstrate the utility of the software. Extensive graphic options are also provided.