Fluorescence spectra characteristics, oxidation

Bril and Wanmaker (149) examined the fluorescent properties of some europium-activated phosphors. In particular, they studied gadolinium oxide, gadolinium borate, gadolinium phosphate, lanthanum borate, and europium phosphate. The phosphors all show an emission spectrum characteristic of europium. From the excitation spectra one can deduce that, in the gadolinium containing host energy absorbed by the gadolinium, ions can be transferred to the europium ions. 

The UV-visible absorption and fluorescence spectra of NKX-2311 in solution are measured. The absorption and fluorescence maxima are located at 490 and 555 nm, respectively. The fluorescence spectrum is shown in Fig. 1. This figure also shows the transient absorption spectrum of NKX-2311 in deuterated methanol (CD3OD) solution measured by the nanosecond laser system. The spectrum is ascribed to the excited singlet state (dye ). The absorption spectrum of the oxidized form of NKX-2311 (dye+) was measured by nanosecond transient absorption spectroscopy after adding electron acceptor, 1,4-benzoquinone, in NKX-2311 solution of CD3OD. As shown in Fig. 1, there are two characteristic peaks at around 875 and 1010 nm. 

In the presence of an activator, naturally, the spectrum, yield, and lifetime are characteristics of the activator molecule. Indeed, the luminescence spectrum of the oxidized ethylbenzene was found to be identical to that of activator fluorescence [221]. 9,10-Dibromanthracene, 9,7-dipropylanthracene, and derivatives of oxazole were used as activators [221,223]. 

The absorption band at 384 nm is composed of contributions of the radical species and the second chromophore, whereas the fluorescence spectra with excitation maxima at 398 nm and emission maxima at 470-480 nm are attributed to the pterin alone (146, 155). The 7,8-dihydropterin cofactor, Xmax = 360 nm when free in solution and 390 nm when protein bound, is labile at neutral pH, readily decomposing upon denaturation to form products without significant visible absorption maxima. The photoreduction described above also reduces the second cofactor but in an irreversible manner with complete loss of its fluorescence and visible absorption characteristics (157). Reduction of the blue semiquinone FAD cofactor to the fully reduced form has no effect on the absorption spectrum of the pterin, suggesting that the absorption spectrum of the second cofactor must be independent of the oxidation state of the flavin and that the two cofactors are electronically isolated from each other (157). However, reduction of the flavin radical results in an increase in the fluorescence of the second cofactor, possibly indicating that the flavin radical acts as a potent quencher of fluorescence of the 7,8-dihydropterin. 

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