Blue-shifts

Figure C2.17.13. A model calculation of the optical absorjDtion of gold nanocrystals. The fonnalism outlined in the text is used to calculate the absorjDtion cross section of bulk gold (solid curve) and of gold nanoparticles of 3 mn (long dashes), 2 mn (short dashes) and 1 mn (dots) radius. The bulk dielectric properties are obtained from a cubic spline fit to the data of [237]. The small blue shift and substantial broadening which result from the mean free path limitation are...

Solvent Influence. Solvent nature has been found to influence absorption spectra, but fluorescence is substantiaHy less sensitive (9,58). Sensitivity to solvent media is one of the main characteristics of unsymmetrical dyes, especiaHy the merocyanines (59). Some dyes manifest positive solvatochromic effects (60) the band maximum is bathochromicaHy shifted as solvent polarity increases. Other dyes, eg, highly unsymmetrical ones, exhibit negative solvatochromicity, and the absorption band is blue-shifted on passing from nonpolar to highly polar solvent (59). In addition, solvents can lead to changes in intensity and shape of spectral bands (58). 

There is no easy understanding of the spectral properties of these compounds in general, which may or may not have a built-in chromophoric system responsible for a long-wavelength absorption like 7,8-dihydropteridin-4-one or a blue-shifted excitation like its 5,6-dihydro isomer. More important than the simple dihydropteridine model substances are the dihydropterins and dihydrolumazines, which are naturally occurring pteridine derivatives and reactive intermediates in redox reactions. 

Other solvatochromic probes have been proposed. Mukerjee et al. used nitrox-ides for this purpose, finding that their transition energies correlate linearly with Z and t (30). Brooker et al. prepared a polar merocyanine that shows a blue shift... 

A xylylene-fc/.v-phosphonium salt 11 gave films of PPV 1 upon clectropolymer-ization. The absorption and emission spectra of the resultant material were blue-shifted with respect to PPV produced by other routes, suggesting that the electro-polymerized material has a shorter effective conjugation length, possibly because of incomplete elimination of phosphonium groups [22]. 

There have been very few examples of PTV derivatives substituted at the vinylene position. One example poly(2,5-thienylene-1,2-dimethoxy-ethenylene) 102 has been documented by Geise and co-workers and its synthesis is outlined in Scheme 1-32 [133]. Thiophene-2,5-dicarboxaldehyde 99 is polymerized using a benzoin condensation the polyacyloin precursor 100 was treated with base to obtain polydianion 101. Subsequent treatment with dimethyl sulfate affords 102, which is soluble in solvents such as chloroform, methanol, and DMF. The molar mass of the polymer obtained is rather low (M = 1010) and its band gap ( ,.=2.13 eV) is substantially blue-shifted relative to PTV itself. Despite the low effective conjugation, the material is reasonably conductive when doped with l2 (cr=0.4 S cm 1). 

Using a variety of transient and CW spectroscopies spanning the time domains from ps to ms, we have identified the dominant intrachain photoexcitations in C )-doped PPV films. These are spin-correlated polaron pairs, which are formed within picoseconds following exciton diffusion and subsequent dissociation at photoinduced PPV+/Cw> defect centers. We found that the higher-energy PA band of polaron pairs is blue-shifted by about 0.4 eV compared to that of isolated polarons in PPV. 

At high excitation fluence (2.4 mJ/cm2) the emission spectrum collapses into a narrow line located at 2.53 eV (see Fig. 8-2 a, dashed line), slightly blue-shifted with respect to the low intensity emission. The process strongly depends on the shape of the excitation area. For a rectangular spot, line narrowing occurs at 55 pj/cm2, i.e. 45 times a lower fluence than for a small circular spot, while the... 

A comparison of the absorption and emission spectra of Ooct-OPV5 with those of the fully conjugated, similarly substituted polymer Ooct-PPV shows that the absorption and luminescence maxima of the five-ring model compound are only slightly blue-shifted relative to those of the polymer (see Fig. 16-11). Hence, the... 

In addition to these possible blue shifts, there is a general rule that the intensities of the d-d spectra of centrosymmetric molecules decrease with cooling while... 

Ozin et al. 107,108) performed matrix, optical experiments that resulted in the identification of the dimers of these first-row, transition metals. For Sc and Ti (4s 3d and 4s 3d, respectively), a facile dimerization process was observed in argon. It was found that, for Sc, the atomic absorptions were blue-shifted 500-1000 cm with respect to gas-phase data, whereas the extinction coefficients for both Sc and Scj were of the same order of magnitude, a feature also deduced for Ti and Ti2. The optical transitions and tentative assignments (based on EHMO calculations) are summarized in Table I. 

The antibonding properties of the geminal a-a interactions has recently led to a theory of electron localization and its successful application to blue-shifting hydrogen bonds [129],... 

---