Excited state deactivation

Sobolewski AL, Domcke W (2006) Role of electron-driven proton-transfer processes in the excited-state deactivation adenine-thymine base pair. J Phys Chem A 110 9031-9038... 

Miannay F-A, Banyasz A, Gustavsson T, Markovitsi D (2007) Ultrafast excited-state deactivation and energy transfer in guanine-cytosine DNA double helices. J Am Chem Soc 129 14574-14575... 

The potential for additional sensitivity is clear upon analysis of the effect of FQ on excited state lifetime (r), which is merely a representation of all the excited state deactivation processes, as shown in the following equation ... 

Electron transfer rates are naturally subject to molecular-scale electric fields. Therefore, ion binding to a molecule is an effective way of controlling PET within it. Since PET is an excited state deactivation pathway, the competing radiative route, i.e. luminescence, also becomes exposed to ionic manipulation. Under favorable conditions, PET rates can be much faster (lO s" ) than luminescence (10 -10 ° s ). At the other extreme, conditions can be arranged under which PET is effectively non-existent. Therefore, luminescence can be ionically switched between off and on states representing digital action. ... 

Most PET or excited state deactivation studies have been performed in solution, although... 

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 deactivation of PYP was previously reported to be associated to the formation of distinct precursors of Ii absorbing in the 480-500-nm spectral region [4,6,7], The kinetics observed in different spectral regions (Fig. lb) are indeed complex, likely due to the overlap of several bands at the selected wavelengths. 

The transient absorption spectroscopy of the denatured PYP is found to be similar to that of the native protein in the early stages of the excited-state deactivation. This behavior seems to indicate that the intrinsic properties of the chromophore play a determining role in the complex primary photoprocesses of the native PYP. 

Interrelationships of Excited-State Decay Routes. The iLV curves conveniently display the competitive nature of photocurrent and luminescence intensity as excited-state deactivation pathways. Our analysis is limited in the sense that we have obtained absolute numbers for X but have had to content ourselves with relative < > r measurements. We lack measures of nonradiative recombination efficiency (4>nr) although they now appear to be... 

The rate of diffusive separation, k, was determined from separate experimental measurements of iodine radical diffusion rates in the high pressure diffusion limited regime (19). The rate of excited state deactivation, k i, was calculated from the measured quantum yields at high densities where G> = kd/k i (18). It was assumed that k i is proportional to the inverse diffusion coefficient, D 1 (19,23) as both properties are related to the collision frequency. 

The photophysical properties of compounds 31—34 are presented in Table 7. Compared to the Pd-based materials, the Pt-based ones exhibit longer emission lifetimes. This is rationalized by the more stable Pt-Pt and Pt-L bonds compared to the more photolabile Pd-Pd and Pd-L ones. Therefore, the energy-wasting photo-induced cleavage (here bond cleavage process) does not occur or at least in a much less efficiency for the Pt-materials, thus reducing efficient nonradiative excited state deactivation. 

In contrast to the reference compounds 4a-c, the singlet excited state deactivates in the dyads and triads much faster than in the oFL references with rate constants typically on the order of 1012 s-1 (Fig. 8.13). These values match the quantitative quenching of the oFL fluorescence in 5 and 6. A comparison between the decay rates of the conjugates carrying one C60 or two C60 moieties, reveals a 2-fold acceleration of the o/igo(fluorene) deactivation in the latter. Conclusively, placing two Ceos instead of one onto the oFL backbone significantly accelerates the excited state decay. 

By means of time-resolved fluorescence studies we were able to determine the C60 fluorescence deactivation rates, as 2.1 x 1010 s-1 in 9a, 6.6 x 109 s 1 in 9b and 1.3 x 109 s-1 in 9c. Importantly, the indulging trend resembles the relationship between the quantum yields of the conjugates (9a-d) and reference (1). In short, an intensified excited-state deactivation emerges with decreasing bridge length. However, no measurable decay rates were found for the trimer 9d. Conclusively, the indirect or direct population of Cgo possibly leads to an exothermic electron-transfer reaction, resulting in the radical-ion-pair state ... 

In contrast to the references, 23a,b, the singlet-excited state deactivation in the Fc-oFL -C60 conjugates, 24a,b, occurs with rate constants of around 1010 s-1 (Fig. 9.64). The values are in good agreement with the observed quantitative quenching of the oFL fluorescence (Fig. 9.62a). At the conclusion of the singlet-excited state decay, two important transient maxima resemble the successful formation of the radical ion pair state, namely a weak shoulder of the transient... 

Jan-Frederik Gnichwitz, Mateusz Wielopolski, Kristine Hartnagel, Uwe Hartnagel, Dirk M. Guldi, Andreas Hirsch. Cooperativity and Tunable Excited State Deactivation Modular Self-Assembly of Depsipeptide Dendrons on a Hamilton Receptor Modified Porphyrin Platform. J. Am. Chem. Soc. 2008, 130, 8491. 

For photochemical purposes only absorption and luminescence are of importance absorption is the main method of excited state generation, whereas luminescence belongs to photophysical processes, which compete with the photoreactions in the excited state deactivation. 

Figure 4.1 Excitation by light absorption ( w) and main pathways of the excited state deactivation R and R, the reactant molecule in its ground and excited state, respectively P, photochemical reaction product Q, quencher molecule...

EXCITED-STATE DEACTIVATION PATHS AND DYNAMICS OF SINGLE tt-BONDS... 

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