We report the synthesis and spectroscopic study of a new molecularly engineered multilayer structure. The chromophore, which is an oligomer of poly@-phenylenevinylene), is analogous to polymeric PPV which has been used in the fabrication of LEDs. This study was initiated specifically to explore if a multilayer geometry would aid the transport properties of the material leading to better semiconductor performance. We describe the synthesis and film formation of these novel materials. Detailed dynamic photophysical measurements on several new multilayer materials have shown that spectroscopically these thin films behave similarly to the polymers. The quantum yields of the films are of the same order of magnitude indicating that the emissive properties in both materials are due to localized molecular interactions. The relevance of the material properties to device applications is discussed.Electroluminescent materials with properties compatible with current VLSI processing are of growing interest for data displays and optical interconnects. For the former, increased resolution,
Zirconium organophosphonates form thermally stable surface multilayers of predictable ellipsometric thickness. Fluorescence and nonlinear optical properties of dyes incorporated in such films indicate organized structures; however, other data place limits on the degree of order obtained. Here, we present evidence gleaned from IR, XPS, and cyclic voltammetry that Zr phosphonate multilayers are less perfect than previously thought and contain defects arising in the initial stages of film growth. We still consider the transition metal phosphonate matrix to be attractive for constructing novel optical materials, but in fabricating active structures it will be necessary to compensate for defects. Approaches to doing so are under evaluation.
The use of weak, intermolecular forces to orchestrate the contruction of multicomponent systems in membranes has significant implications in diverse areas of chemistry, biology, and medicine. We describe here the construction and characterization of multi-heme molecular ensembles in phospholipid vesicles. A trianionic zinc porphyrin was designed to bind cytochrome c at the membrane surface, while being anchored to a membrane spanning manganese porphyrin in the membrane interior via a terminal imidazole. The structure of the construct was probed by fluorescence and UV spectroscopy. Cytochrome c formed a stoichiometric 1:1 complex with the anionic porphyrin with a high binding constant (K a ≈ 5 × 106 M-1). The ligation of the imidazole to the manganese porphyrin was confirmed by UV spectral changes. Large differences in the fluorescence quenching of Zn porphyrins with and without the terminal imidazole were observed upon their insertion into vesicles containing the Mn porphyrin. These spectroscopic observations were consistent with the formation of a ligated, ternary system consisting of the Mn(II) porphyrin, the imidazole-tailed zinc porphyrin acting as a bridge, and the surface associated cytochrome c. The nature of the binding of cytochrome c at the membrane−water interface was investigated by Langmuir−Blodgett (LB) and differential scanning calorimetric (DSC) techniques. The data obtained suggested that the protein was surface bound with minimal penetration into the membrane. LB studies were also used to probe the orientation of the trianionic porphyrin moiety at the membrane surface, and an edge-on orientation was inferred from the data. The formation of a stable vesicular system was confirmed by the formation of well-defined DSC thermograms. Phase separation was observed at high porphyrin:lipid ratios. Electron transfer from the Mn(II) in the membrane interior to the surface bound ferricytochrome c was investigated, as a probe both for spatial definition of the ensemble and for the elucidation of electron transfer mechanism in the genre of weakly coupled systems over large distances. Trianionic Zn porphyrins with varying tether lengths (12, 8, and 4 carbons) were used. The electron transfer rate was found to be first order and independent of the tether length, indicative of medium mediated electron transfer via multiple pathways. Comparison to similar systems in the literature yielded a predicted distance of ∼23 Å between the Mn and Fe centers in DMPC/DPPC vesicles. This distance suggested that the protein was surface bound to the membrane and separated from the Mn porphyrin by the thickness of one leaflet of the phospholipid bilayer. In thinner DLPC vesicles the predicted increase in the electron transfer rate was observed. Additionally, electron transfer was observed to be bimolecular in systems where trianionic porphyrins lacking the imidazole tether were used to recruit the cytochrome c.
Table 1. Relative Reactivities of Homocuban-9-ylidene (1) Compared to That of PhCCI8 rcilctilnt 1 (25 "C) PhCCl (25 "C) ( k , M-'s-') methanol 3.3 3.8 (1.3 X IO9) tetramethylethylene I .9 0.2-0.4 ((0.7-1.2) X IO8) pyridine 1 .o 1.0 (3.4 x 108)Stern-Volmer analysis gives the relative reactivities of 1 toward various quenchers ( Table I). The selectivity of 1 is comparable to that of chlorophenylcarbenes and is rather insensitive to tempera t u re.The relative reactivity of carbene 1 toward C H 3 0 D and tetramethylethylene determined by LFP and product analysis is in good agreement: k,/k,, = 2.2 (chemical analysis) and 1.7 (LFP). The relative rate data obtained by LFP used pyridine and CH30D concentrations (>0.2 M) sufficient to ensure complete oligomerization of the alcohol and were therefore consistent with the conditions used in the product analysis. At [pyridine] = 0.05 M, the observed absolute rate constant for formation of ylide Sa, koh, is given by eq 6.12 kp,,[ pyridine] k,,,[ pyridine] = kobs or K = -1 (6) K + I k O b s The maximum value of K is defined by the maximum possible value of k (<5 X IO9 M-' s-I)l3 and the minimum value of koh pr' 7 ( 1 5 X I O s-) determined by LFP. Thus, at +25 OC, K is bracketed as in eq 7.15 (7) Supplementary Material Available: Derivations for eqs 4-6, preparation of 5. plots of the dependence of the products formed from 1 and 2 on [CH30D] for the decomposition of both 3 and 5, transient spectra of 8a,b, the absolute rate of formation of Sa, and Stern-Volmer plots (14 pages). Ordering information is given on any current masthead page. 0.23 I K 5 4 (14) Values of k,,, for several carbenes are knownt0 and never exceed 5( I 5 ) Any reaction of 2 with pyridine to produce an invisible dipole (9, for example) will raise this value. This is not considered likely as the yield of 9r/9b is comparable to that realized with other carbenes.x 109 M-I s-1 Models of the active-site chemistry of heme-containing monooxygenases such as cytochrome P-450 have attracted sustained attention for over a decade. Comparisons of the enzyme and synthetic metalloporphyrins have established that the catalytic cycle involves substrate binding at the active site, reduction of the unusual iron( 111) thiolate to iron(ll), and binding and reductive cleavage of molecular oxygen to generate a high-valent iron-oxo species responsible for substrate oxidation.' The two electrons required for oxygen activation are provided by NADPH through a coupled electron transfer flavoprotein, cytochrome P-450 re-( I ) (a) McMurry, T.t 1 0 0 200 Time(min) 400 5 00 hoc 7 0 0 Figure I . Visible spectral changes observed in a 0.5-cm cuvette during the anaerobic reduction of Mn"'(ChPC1) (I) (2.4 X M) by a pyruvate (4.2 X IO-* M)/pyruvate oxidase (4.8 X IO-' M) system in the presence of amphiphilic flavin (11) (4.8 X M) vesicles at pH 6 (1.6 mL of 0.05 M potassium phosphate buffer containing 0.01 M MgClz and 0.1 M KCI). Measurements were taken every IO min after the addition of sodium pyruvate. Inset: time course of the reaction mon...
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