Absorption and Photoluminescence

7 and 4.7 eV (II and III), and a strong broad band centered at 5.9 eV (peak IV). Recent works by multiple groups [19, 20] have emphasized the importance of fitting the entire absorption spectrum of PPV. Band I is due to the main delocalized Ti-Ti transition and is most probably the result of an inhomogeneously broadened transition followed by several phonon side-bands. The pho- 

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The absorption and photoluminescence (PL) spectra of a-6T measured at 80 K are shown in Figure 7-24. There is an onset of absorption at 2.1 eV, with several sharp peaks and a maximum at 2.8 eV. A second absorption band is seen at 4.5 eV and is due to direct excitation of the thiophene ring. We take the first peak as the 0-0 transition and calculate an optical gap j,=2.4eV. The PL spec-... 

For copolymers of structure I, for both types of side-chains, there is a striking similarity with the optical properties of the corresponding models the absorption and photoluminescence maxima of the polymers arc only 0.08-0.09 eV red-shifted relative to those of the models, as shown in Figure 16-9 (left) for the octyloxy-substituted compounds. The small shift can be readily explained by the fact that in the copolymers the chromophorcs are actually substituted by silylene units, which have a weakly electron-donating character. The shifts between absorption and luminescence maxima are exactly the same for polymers and models and the width of the emission bands is almost identical. The quantum yields are only slightly reduced in the polymers. These results confirm that the active chro-mophores are the PPV-type blocks and that the silylene unit is an efficient re-conjugation interrupter. 

Fig. 4. Absorption and photoluminescence (inlet) spectra of PVA-capped CdSe nanoparticles at (a) 1 hr, (b) 3 hrs, (c) 5 hrs and (d) 24 hrs reaction times.

Fig. 1. Absorption and photoluminescence dashed line) spectra of a thin film of LPPP 26 and electroluminescence solid line) spectrum of an ITO / LPPP 26 (60 nm) / A1 device (from [50])...

Figure 17.2 (A) Absorption and photoluminescence spectra of CdSe quantum dots prepared from CdO, CdC03, and Cd(AcO)2 in the presence of different ligands. (B) Increase in the optical density (at 400 nm) of a CdSe quantum dot reaction mixture with time under reaction at 75 "C. Color pictures in the inset of B represent CdSe (a,b) andCdSe-ZnS (c) quantum...

In principle, optical chemosensors make use of optical techniques to provide analytical information. The most extensively exploited techniques in this regard are optical absorption and photoluminescence. Moreover, sensors based on surface plasmon resonance (SPR) and surface enhanced Raman scattering (SERS) have recently been devised. 

UV/Vis absorption and photoluminescence spectra of semiconductors 368-373 were measured in solution to assess the effect of carbonyl functionalization on the optical absorption/emission spectra and resultant optical HOMO-LUMO energy gaps (Egap) (07CM4864). 

Figure 5.1 Room temperature absorption and photoluminescence spectra of CdSe nanocrystals in the size range of 3 nm (a) to 7 nm (g).16 (Reprinted with permission from D. V. Talapin et al., Nano Lett. 2001,1, 207-211. Copyright 2001 American Chemical Society.)...

FIG. 3.5. Optical absorption and photoluminescence (PL) of a thin layer of Ooct-OPV5 [30]. 

Next we show a number of representative absorption and photoluminescence spectra of CPs and the corresponding oligomers (hexamers). Figure 12a shows the absorption spectra of a trans-PA produced via the precursor route [113] and of the corresponding hexamer, dodecahexaene [114]. Two differences are immediately apparent in the absorption spectra ... 

The situation is somewhat more complicated in two-dimensional polysilanes, which have intermediate properties between the one-dimensional chain-like polysilanes and three-dimensional bulk silicon. The gap is of a quasi-direct nature as the indirect gap is only slightly smaller than the direct one [11]. However, the excitons strongly bind to the lattice which results in a large Stokes shift of the PL [26]. The observed blue shift of the absorption and photoluminescence with decreasing size of the polysilanes is considered to be due to confinement effects of the excitons [12,26]. The strong coupling of the exciton to the lattice decreases somewhat the blue shifts as compared with the linear chains, and it results in a stronger localization of the exciton over a smaller number of Si atoms [12,26]. 

Palsson, L.O., Pal, R., Murray, B.S., etal. (2007) Two photon absorption and photoluminescence of europium based emissive probes for bioactive systems. Dalton Transactions, 5726-5734. 

The skeleton-side-chain interaction is reflected in optical properties. The absorption and photoluminescence spectra of poly(methylpropylsilane) and poly(methylphenylsilane) are shown in Figure 14. For poly-(methylpropylsilane), the spectrum profiles are explained by the simple band model just discussed. However, for poly(methylphenylpolysilane), the spectrum profiles are very different. The absorption peak at 3.7 eV corresponds to a a-a transition, and the second peak at 4.5 eV corresponds to a tt-tt transition in phenyl side chains. The sharp photoluminescence peak originates from the a -CT transition. 

Figure 19 shows the typical photoluminescencc spectrum of the anchored vanadium oxide catalyst prepared by photo-CVD methods (a), its corresponding excitation spectrum (b), and the UV absorption spectrum of the catalyst (c) (56,115,116). These absorption and photoluminescence spectra (phosphorescence) are attributed to the following charge-transfer processes on the surface vanadyl group (V=0) of the tetrahedrally coordinated VO4 species involving an electron transfer from to V and a reverse radia-... 

Normally, these supports exhibit neither absorption nor photolumincs-cence spectra in the wavelength regions 230-600 nm. However, the supports sometimes exhibit abnormal absorption and photoluminescence bands. 

Both silica and Vycor glass degassed at temperatures >673 K exhibit such abnormal absorption bands at about 230-270 nm and photoluminescence spectra at about 380-580 nm (14-16,207). The intensities of the absorption and photoluminescence spectra depend on the evacuation temperature. 

As shown in Fig. 19, vanadium oxide supported on Vycor glass exhibits a photoluminescence spectrum at about 400-600 nm upon excitation of the absorption band at about 320 nm (33, 34, 63, 69,115,116). The absorption and photoluminescence spectra are represented by Eq. (12). The addition of 62, CO, N2O, C2H4, CsHg, or QHjj to the catalyst led to the quenching of the photoluminescence with differing efficiencies but without any changes in the shape of the spectrum. 

As shown in Fig. 31, the Cu(I)ZSM-5 catalyst exhibits a photoluminescence spectrum at about 400-600 nm upon excitation at about 280-300 nm, attributed to the radiative deactivation pathway from the excited state of the isolated Cu" monomeric species to its ground state. With Cu(l)ZSM-5 catalysts having high copper loadings, another absorption band near 300-320 nm and a weak photoluminescence band near 500-600 nm were observed. These additional absorption and photoluminescence bands are attributed to the presence of the (Cu -Cu ) dimeric species, i.e., to the (3dcr - 3dcr) electronic excitation and its reverse radiative deactivation (3do-- 3dcr ), respectively (29, 181). 

Degassing ZrOa at high temperatures leads to the appearance of an abnormal absorption and photoluminescence spectra which could be attributed to the formation of surface sites in low coordination or coordinatively unsaturated surface sites (see Section lV.A.2.b) (101-104). Moreover, Zr02... 

As shown in Fig. 65, titanium-silicon binary oxides prepared by the sol-gel method exhibit a characteristic photoluminescence spectrum near 480 nm upon excitation at 280 nm. The absorption and photoluminescence spectra are attributed to the charge-transfer processes on the highly dispersed tetrahedral titanium oxide species embedded in the Si02 matrices (168, 200, 201). When the titanium content of the oxides was decreased, the intensity of the photoluminescence spectrum increased, and its peak wavelength shifted to shorter wavelengths. 

UV absorption and photoluminescence emission maxima of compounds 5 - 8 are listed in Table 2 together with the corresponding data for dodecamethylcyclohexasilane. The first maxima appearing at the low-energy side of the absorption spectra are shifted bathochromically with increasing number of siloxy groups because of enhanced a(Si-Si)—>n(0) conjugation. 

For these purposes different spectral methods have been widely used, among them the UV-visible absorption and emission spectroscopy proved to be very informative [2, 3], The investigation of the ground-state absorption and photoluminescence emission spectra of adsorbed probe molecules have been successfiil in the study of the electron-and charge-transfer reactions within zeolites. 

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