Excited-State Decay Kinetics

This behavior is what would be expected for a two-state, ground-state ionization. The pKa obtained from the pH at which the amplitudes associated with the two decay constants are equal compares well with that of alkyl carboxyl groups of similar compounds. 9,40) 

The pK of tyrosine explains the absence of measurable excited-state proton transfer in water. The pK is the negative logarithm of the ratio of the deprotonation and the bimolecular reprotonation rates. Since reprotonation is diffusion-controlled, this rate will be the same for tyrosine and 2-naphthol. The difference of nearly two in their respective pK values means that the excited-state deprotonation rate of tyrosine is nearly two orders of magnitude slower than that of 2-naphthol.(26) This means that the rate of excited-state proton transfer by tyrosine to water is on the order of 105s 1. With a fluorescence lifetime near 3 ns for tyrosine, the combined rates for radiative and nonradiative processes approach 109s-1. Thus, the proton transfer reaction is too slow to compete effectively with the other deactivation pathways. 

The fluorescence decay parameters of tyrosine and several tyrosine analogues at neutral pH are listed in Table 1.2. Tyrosine zwitterion and analogues with an ionized a-carboxyl group exhibit monoexponential decay kinetics. Conversion of the a-carboxyl group to the corresponding amide results in a fluorescence intensity decay that requires at least a double exponential to fit the data. While not shown in Table 1.2, protonation of the carboxyl group also results in complex decay kinetics.(38) 

This rotamer model for the fluorescence decay of an aromatic amino acid also predicts that the amplitudes of the kinetic components should correspond to the ground-state rotamer populations, provided that interconversion 

The phosphorescence decays of phenol, tyrosine, and related compounds, which had been examined extensively during the 1960s, have been reviewed by 

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Selected entries from Methods in Enzymology [vol, page(s)] . Applications, 246, 335 [immunoassay, 246, 343-344 nucleic acids, 246, 344-345 photoreceptors, 246, 341-343 protein conformation, 246, 339-340 protein-membrane interactions, 246, 340-341 two-dimensional imaging, 246, 345] energy level diagram, 246, 336 excited state decay kinetics, 246, 337-338 in-... 

The Photoactive Yellow Protein (PYP) is the blue-light photoreceptor that presumably mediates negative phototaxis of the purple bacterium Halorhodospira halophila [1]. Its chromophore is the deprotonated trans-p-coumaric acid covalently linked, via a thioester bond, to the unique cystein residue of the protein. Like for rhodopsins, the trans to cis isomerization of the chromophore was shown to be the first overall step of the PYP photocycle, but the reaction path that leads to the formation of the cis isomer is not clear yet (for review see [2]). From time-resolved spectroscopy measurements on native PYP in solution, it came out that the excited-state deactivation involves a series of fast events on the subpicosecond and picosecond timescales correlated to the chromophore reconfiguration [3-7]. On the other hand, chromophore H-bonding to the nearest amino acids was shown to play a key role in the trans excited state decay kinetics [3,8]. In an attempt to evaluate further the role of the mesoscopic environment in the photophysics of PYP, we made a comparative study of the native and denatured PYP. The excited-state relaxation path and kinetics were monitored by subpicosecond time-resolved absorption and gain spectroscopy. 

The excited-state decay kinetics of Ru(bpy)2(dcb) + -Ti02 immersed in neat acetonitrile, probed by transient absorption spectroscopy, exhibited nonexponential kinetics. By minimizing the excitation irradiance, near exponential kinetics were observed for excited-state decay. However, at high excitation irradiance, second-order kinetics were found to fit the experimental data well. These observations are consistent with competitive first- and second-order processes attributed to radiative and nonradiative excited-state deactivation, Eq. 21, proceeding in parallel with excited-state annihilation, Eq. 22 ... 

FIG. 12 Simulation of fluorescent decays for dye species located in the aqueous phase following laser pulses in TIR from the water-DCE interface according to Eq. (38). A fast rate constant of excited state decay (10 s ) was assumed in (a). The results showed no difference between infinitely fast or slow kinetics of quenching. On the other hand, a much slower rate of decay can be observed for other sensitizers like Eu and porphyrin species. Under these conditions, heterogeneous quenching associated with the species Q can be readily observed as depicted in (b). (Reprinted with permission from Ref 127. Copyright 1997 American Chemical Society.)... 

The kinetics of three redox processes have been studied for sensitized Ti02 systems where the sensitizers are [Ru(dicarboxy-bpy)2(CN)2], [Ru(dicarboxy-bpy)2(SCN)2], [Os(dicarboxy-bpy)2(CN)2], and [Os(dicar-boxy-bpy)2(SCN)2] (30). The Ru(II) complexes display characteristic excited-state spectra in methanol solution and decay back to the ground state with lifetimes of about 200 ns. For the Os(II) complexes in solution the excited states decay much more rapidly (< 10ns). On the other hand, when these complexes are adsorbed on Ti02 excitation leads to the prompt conversion to the M(III) oxidation state, as indicated by transient visible absorption spectra. These results imply that electron injection from all four of the excited sensitizers into the Ti02 occurs rapidly (< 10 ns). 

A study on mechanistic aspects of di-ir-methane rearrangements has been published recently [72]. The kinetic modeling of temperature-dependent datasets from photoreactions of 1,3-diphenylpropene and several of its 3-substituted derivatives 127a-127d (structures 127 and 128) show that the singlet excited state decays via two inactivated processes, fluorescence and intersystem crossing, and two activated processes, trans-cis isomerization and phenyl-vinyl bridging. The latter activated process yields a biradical intermediate that partitions between forma-... 

The concentration of the iron porphyrins was adjusted to be between 0.2 and 0.3 OD for 2 mm cell at 530 nm. All relaxation times were calculated from the first order kinetic curves of excited state decay or ground state reappearance. This procedure eliminates error in delay times between the excitation and different wavelength probe pulses ("chirp") since constant delay times are subtracted out of the kinetic curves. There may, however, be some error introduced in the shorter decay times because of the excitation pulse and the probe pulse may overlap at the earliest points of the kinetic curve calculations. 

We have developed a model to take into account these evaporation processes (for more details, in particular kinetic equations, see ref 27) that can be both applied to phenol and naphthol, The main idea is the following the excited state decays observed correspond to evaporation of ammonia molecules after excitation of ground state proton transferred naphthol-(NH3) >6 clusters. As in the case of phenolate [31], a strong change in dipole... 

Thus, it is very reasonable that the other factors involved in the excited state decay of [Ru(trpy)2]2+ include dissociation of at least one pyridyl ligator. Kirchhoff et al.258) have used an argument based on a kinetic scheme involving photolysis to rationalize inefficient luminescence in [Ru(trpy)2]2+ and related compounds however, they do not observe extensive photolysis in this system. 

In addition to their effects on emission energy, the diimines and dithiolates also influence the emission lifetime and quantum yield of the Pt(diimine) (dithiolate) chromophore. The complexes display lifetimes ranging from 1 ns to > 1 ps and <3>eill ranging from < 10 5 up to 6.4 x 10-3, indicating such an influence on the kinetics of excited-state decay (see Table II). The tdt complexes have lifetimes that are significantly longer than those measured previously for... 

The triplet state decay kinetics of benzophenone has been monitored in fluorinated surfactants (sodium perfluorooctanoate, SPFO) where the surfactant does not quench the triplet40>. In this case, the excited benzophenone cannot react with the surfactant and the excited molecule escapes into the aqueous phase. When increasing amounts of SDS is added to solution, the observed triplet lifetime (by nanosecond transient absorption techniques) decreases indicating that hydrogen abstraction is occurring from the SDS (Fig. 17). 

The same technique, picosecond optical calorimetry, was employed to study the kinetics of p (twisted zwitterionic singlet excited state) decay in various TPE derivatives [64]. The results obtained indicate that two excited states are involved 1.) the vertically excited fluorescent state and 2.) the twisted zwitterionic excited state (p ). Furthermore, they indicated that these two states are in equilibrium in nonpolar solvents. 

In the simplest case of a single type of fluorescing chromophore being depopulated by various processes, the relationships determining the fluorescence decay kinetics and their relation to steady-state parameters are given by the following equations. The excited state decay is given by a first-order differential equation ... 

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