Electron transfer quenching of singlet and triplet excited states of flavins and lumichrome by aromatic and aliphatic electron donors

Abstract

The quenching of excited singlet and triplet states of riboflavin, lumiflavin and lumichrome was investigated in methanol. The quenchers were aromatic electron donors and aliphatic amines. Bimolecular quenching rate constants were determined from static and dynamic fluorescence measurements. Triplet quenching was studied by laser flash photolysis. Transient absorption spectra showed the presence of semireduced flavins and lumichrome, and the radical cation of quenchers. The results confirm that the quenching rate constants for aliphatic donors are lower than those of aromatic donors of similar oxidation potential. Plots of the quenching rate constants vs. the free energy for the electron transfer reaction, DG , were fitted by the Rehm-Weller model of electron transfer quenching. The aliphatic quenchers needed a higher intrinsic barrier for the fitting that was ascribed to the internal reorganisation of the amines. For the aromatic donors the singlet quenching rate constants reach the diffusional limit at highly negative DG . However, for the triplet quenching the limiting value of the rate constants is lower than the plateau of the singlet quenching. This is explained in terms of the non-adiabaticity of the triplet quenching process.

Contact and Distant Luminescence Quenching in Solutions

“…The quasi-exponential quenching of triplets was actually detected recently in line with the luminescence quenching [63]. Some of the data obtained are shown in Figure 14.…”

“…Almost the same is true when K κ i /k c , where K and k c were specified in (60) and (63). The quasireversible ionization is controlled by RIP recombination which proceeds with the rate constant…”

On the generation and quenching of reactive-oxygen-species by aqueous vitamin B2 and serotonin under visible-light irradiation

“…D ET G 0 = N A {e [E 0 (In-der/In-der Å+ ) À E 0 (Rf/Rf ÅÀ )] + w(Inder Å+ Rf ÅÀ ) À w(In-der Rf)} À DE 0,0 , where e is the elementary charge, N A is the Avogadro constant, E 0 (In-der/In-der Å+ ) is the standard electrode potential of the donor, (0.94 V for Ser) [32], E 0 (Rf/ Rf ÅÀ )) is the standard electrode potential for the acceptor (À0.80 V) [33], the difference w(In-der Å+ Rf ÅÀ )-w(In-der Rf) is the coulombic energy term (À0.06 V) [33] and DE 0,0 is the vibrational zero energy of the excited partner (2.17 eV) [33]. The so-calculated D ET G 0 = À0.93 eV indicates that process 4 may be operative and, consequently, that the species O ÅÀ 2 could be formed through a radical-mediated mechanism from 3 Rf à .…”

Photosensitized degradation in water of the phenolic pesticides bromoxynil and dichlorophen in the presence of riboflavin, as a model of their natural photodecomposition in the environment

“…is the standard electrode potential of the acceptor Rf (−0.80 V), E Rf* is the 3 Rf* energy (2.17 eV), C is the coulombic energy term (−0.06 V) [44] and E 0(PP/PP+) is the standard electrode potential of each pesticide. We employed the experimentally determined E 1/2 values (0.71 V for BXN and 0.62 V for DCP) as the respective E 0(PP/PP+) values [38].…”