Red-shifted emission

Che and coworkers have studied the effect of halide ions on the emission. Initial addition of a small amount of [NBu4]Y to [Au2(dcpm)2](CF3S03)2 in acetonitrile led to enhancement of the emission intensity and red-shifted emission Xmax, from 508 to 510 (Cl ), 514 (Br ) and 530 (F) nm. Further addition of [NBu4]Y resulted in a decrease in emission intensity. It is reasonable to assign the absorption band... 

The anomalous dual fluorescence emission of p-A V-dimethylamino benzoni-trile (DMABN) in polar solvents was first reported by Ernst Lippert in 1962. Emission spectra of DMABN in solvents of different polarity show a dual emission, where the red-shifted emission is stronger relative to the primary emission when the solvent polarity increases. Furthermore, it can be observed that overall emission intensity is reduced in more polar solvents, but higher solvent viscosity increases the emission intensity. Spectra of DMABN in different solvents are shown in the chapter of Tomin in this book [1]. 

Replacing the phenyl group by a coumarin motif produces an important subclass of molecular rotors, represented by 17, which have found several applications in viscosity and polarity measurements [48]. On the other hand, compound 19 has a red-shifted emission as compared to 18 due to the effective delocalization of the 71-electrons in the presence of the thiophene chromophore [49]. 

A. Weller and K. Zachariasse 157-160) thoroughly investigated this radical-ion reaction, starting from the observation that the fluorescence of aromatic hydrocarbons is quenched very efficiently by electron donors such as N,N diethylaniline which results in a new, red-shifted emission in nonpolar solvents This emission was ascribed to an excited charge-transfer complex 1(ArDD(H )), designated heteroexcimer, with a dipole moment of 10D. In polar solvents, however, quenching of aromatic hydrocarbon fluorescence by diethylaniline is not accompanied by hetero-excimer emission in this case the free radical anions Ar<7> and cations D were formed. 

SCHEME 3.26 Chemical structures of Alq3 derivatives with blue- and red-shifted emission. 

In polar solvents such as chloroform, dichloromethane, acetone, and acetonitrile, the fluorescence quantum yields of 97a-d decrease by varying degrees (see Table 19). Moreover, in the case of the phenyl and acetyl derivatives 97c and 97d, the rather drastic decrease of the structured fluorescence from the locally excited anthracene is associated with the appearance of a structureless, red-shifted emission which is attributable to an intramolecular exciplex. For 97d, in which the electron acceptor properties of the aromatic carbonyl moiety are enhanced by p-acetyl substitution, exciplex emission is dominant even in toluene solution (see Figure 22). 

Figure 159 EL spectra (a) and EL transients (b) for SL ITO/ Polymer/Metal LEDs (1) ITO/MEH-PPV/A1 (2) ITO/MEH-PPV/ MEH-PPV/(Mg/Ag) (3) ITO/M3EH-PPV/(Mg/Ag). EL spectra are recorded at different voltage pulse amplitudes. Characteristic cathode metal emission lines are indicated along with the position of broad band-red shifted emission maxima from the polymers. For chemical meaning of MEH-PPV and of M3EH-PPV, see Figs. 106 and 108. The transient current under a lps pulse (4) and transient response of EL polymer and cathode metal emission spectral regions at two different pulse amplitudes, 4.3 MV/cm (5) and 6.6MV/cm (6) have been measured on ITO/MEH-PPV/A1 device. EL transient are normalized to the maximum intensity of the polymer emission. After Ref. [472]. Copyright 2000 American Institute of Physics.

The fluorescence intensity resolved by wavelength constitutes the fluorescence spectrum. The wavelengths of fluorescence photons contain information about the environment of the fluorophore and the sample heterogeneity. Por example, as described above, buried Trp residues tend to have blue-shifted emission bands (Xmax < 330 mn), whereas Trp residues partially or fully exposed to water have red-shifted emission bands ( max > 340 mn). Therefore, protein conformational changes or unfolding may be accompanied by shifts in the native fluorescence spectra. Pluorescence spectra can be measured on a standard fluorometer, which is available from many manufacturers. 

Miscellaneous.—The Schuler band absorption spectrum of ND4, generated photochemically, has been reported by Whittaker e( The response of the signal to pressure and modulation suggests that the ND4 is formed from a FlgND exciplex. The collision-induced relaxation dynamics of UF following excitation in the first electronic absorption band in the near-u.v. has been examined via the resulting red-shifted emission by Castiglione et A... 

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