Molar extinction coefficient transitions

The electronic adsorption spectra for the complexes [Ir(OH)6]", where n = 0-2, have been resolved and peak maxima locations, molar extinction coefficients, oscillator strengths, and band half-widths calculated.44 Bands have been assigned in the main part to be one-electron MLCT transitions. Spectrophotometrically determined rate constants for the OH reduction of the IrVI and Irv complexes at 25 °C in 3M NaOH are (2.59 0.09) x 10—3 s—1 and (1.53 0.05) x 10 4 s 1 respectively. The activation energy for the reduction, Irv—>IrIV, is nAkcalmoC1. Cyclic voltammetry and potentiostatic coulometry of [Ir(OEI )r,]2 in 3M NaOH on a Pt electrode show that during the electro-oxidation compounds of Irv and IrVI are formed.45... 

Normal incidence transmission IRLD measurements are used to study thin films (typically 100 pm thickness and less, depending on the molar extinction coefficient of the bands) with in-plane uniaxial orientation. Two spectra are recorded sequentially with the radiation polarized parallel (p) and perpendicular (s) to the principal (machine) direction of the sample. The order parameter of the transition moment of the studied vibration is calculated from either the dichroic ratio (R — Ap/As) or the dichroic difference (AA = Ap—As) as ... 

The resulting extinction coefficient remained constant within the accuracy of the measurements. In the transition from monoazo to disazo pigment, however, the value of the maximum molar extinction coefficient is more than doubled, because the effect of the two azo linkages is enhanced by additional interaction via the diphenyl moiety. This, however, does not improve the conjugation the shade does not shift remarkably ... 

The large molar extinction coefficients of the spin-allowed CT transitions make them much easier to pump optically. The intense 454-nm visible band of Ru(bpy)32+is an MLCT transition. 

Table I. Absorption Cross Sections (cJin) and Molar Extinction Coefficients (e) of Transition of ZnTPP...

For our purpose of discussing thermoluminescence, the main problem is whether the observation that the strong, narrow bands in an overwhelming majority of cases are due to pseudoquadrupolar transitions can be explained by their molar extinction coefficient s being particularly high in the mixed oxides, or whether additional effects operate in favour of hypersensitive luminescence. If we neglect reflection of fight from the sample. 

Solutions of isolated dioxiranes, characteristically dimethyldioxirane (DMD) in acetone, possess a pale yellow color, which serves as a convenient analytical index for monitoring the dioxirane consumption in oxidation reactions and kinetic studies. For DMD, the absorption maximum (n-jt transition ) centers at ca 325 nm, with a molar extinction coefficient (e) of 12.5 0.5 M cm in acetone. The alternative and more rigorous analytical method for dioxirane quantification utilizes iodometry (Kl/starch). 

Figure 3. IS Correlation between oscillator strength /, molar extinction coefficient and lifetime t for different transition types.

A totally allowed transition has oscillator strength /=1 and molar extinction coefficient 105. Different factors may reduce the / values to different extents. Oscillator strengths / are related to integrated absorption intensities by the expression... 

AE, transition energy /, oscillator strength c, molar extinction coefficient. b Slope of the absorption bond. 

Gaussian curves (normal distribution functions) can sometimes be used to describe the shape of the overall envelope of the many vibrationally induced subbands that make up one electronic absorption band, e.g., for the absorption spectrum of the copper-containing blue protein of Pseudomonas (Fig. 23-8) Gaussian bands are appropriate. They permit resolution of the spectrum into components representing individual electronic transitions. Each transition is described by a peak position, height (molar extinction coefficient), and width (as measured at the halfheight, in cm-1). However, most absorption bands of organic compounds are not symmetric but are skewed... 

The electronic spectra of square pyramidal chromophores are characterized by a band in the near IR region from 4000 to 9000cm-1 (3BX- 3E) with molar extinction coefficient M near 10-20, a more intense transition at 12 000-18 000 cm-1 (3BX —> 3E, M 20-100) with a shoulder on the low frequency side due to 3B1- 3B2 transitions, a weak band at 17000-25 000cm-1 (3BX—>3A2 (P)) and the most intense transition at 19000-29000cm-1 (3BX— 3E (P), eM 100-800). 

At this point it is suitable to summarize the discussion by tabulating the intensities found for bands of different types. This is done in Table 4. In principle, the molar extinction coefficient, e, is not a good measure of band intensity. However, for transition metal compounds at any rate, the band widths for spin-allowed bands at ambient temperatures are mostly of the order of 2000 cm-1. With this fact in mind, it has become the custom to use s as a rough measure of band intensity, and to facilitate comparisons of that type the values of e associated with the varying types of transition are included in Table 4. 

Table 4 Representative Values for the Oscillator Strengths and Molar Extinction Coefficients Found in Transition Metal Complexes...

For organic materials, ultraviolet absorption spectra are substantially determined by the presence of functional groups. Identical functional groups in different molecules may not absorb at precisely the same wavelength due to different structural environments which modify the local electric field. The magnitude of the molar extinction coefficient ( e ) for a particular absorption is directly proportional to the probability of occurrence of the particular electronic transition. Spectral features of some isolated chromophoric groups are presented in Table 2... 

The molar extinction coefficient in ND8 is about 10% larger than that previously reported for NH8 solutions (9, 10). Perhaps the density of the solutions could account for the discrepancy since the densities of NH8 and ND8 at —33.7° C. are 0.68 and 0.80 g./cc., respectively (12). Since the solutions under consideration are very dilute, their densities are approximately that of the solvent itself. However, the molar volume of the solvent is a more significant parameter since the solvation of the electron would be expected to be the same in each solvent. The molar volume of NH8 (—33.7 ° C.) is 24.3 cc./mole and that of ND (—33.7 ° C.) is 25.0 cc./mole (12). These quantities differ by only 3% and are in the wrong direction to account for the observed difference in intensity. It is possible that the transition responsible for the absorption is less forbidden in liquid ND8 than in liquid NH8, but the most likely explanation is that some decomposition to amide had already occurred when the measurements in NH8 were made. 

Spectroscopy A spectrum of PtPcCP4, which is typical of these phthalocyanines is shown in Fig. 2. The most intense band is the Q band which occurs between 640 nm and 680 nm for the different metal phthalocyanines. It is the lowest allowed n - n transition of the phthalocyanine ring. In dilute solution, the Q band of the monomer typically had a molar extinction coefficient of 2 x 105 //mole-cm in agreement with previous reports. (2) Additional bands, which have been assigned to phthalocyanine aggregates (2)(3), were observed on the short wavelength side of the Q band in the relatively concentrated solutions used for the nonlinear optical studies. 

A sensitizer is of paramount importance to photovoltaic performance. The sensitizer is attached to the surface of a mesoporous wide band-gap semiconductor serving as electron transporter. While the trivial ultraviolet absorption for 375 with a molar extinction coefficient (s) of 50.0 x 103 M [ cm-1 peaks at 372 nm, the s value of its low-energy band at 525 nm (mainly stemming from the intramolecular charge transfer transition) is 44.8 x 103 IVT1 cm-1 (08JA9202). 

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