Excited-State Decay

The minimum prerequisite for generation of upconversion luminescence by any material is the presence of at least two metastable excited states. In order for upconversion to be efficient, these states must have lifetimes sufficiently long for ions to participate in either luminescence or other photophysical processes with reasonably high probabilities, as opposed to relaxing through nonradiative multiphonon pathways. The observed decay of an excited state in the simplest case scenario, as probed for example by monitoring its luminescence intensity I, behaves as an exponential  

p = AEq o/ eff is the dimensionless energy gap between the upper state and the closest lower-energy state in units of the effective vibrational energy, Veff (cm ). C is the electronic factor, and S is the Huang-Rhys dimensionless excited-state distortion parameter in units of vibrational quanta v ff. As shown in Eq. (2), /c ,p is strongly dependent onp. Additionally, for a given reduced energy gap p, the introduction of even small excited-state distortions, S, can rapidly enhance the radiationless multiphonon relaxation rate such that this dominates the total 0 K relaxation. This model is easily extended to elevated temperatures, where substantial increases in may be observed [7,8]. 

For the case of only one radiative decay channel, the radiative decay rate constant in Eq. (2) is described by Eq. (4) [9], where n is the index of refraction, the constant a = 1.5 x 10 s/m, and g denotes the degeneracy of the ground (a) or excited (b) state involved in the transition  

The radiative rate constant is thus directly proportional to the oscillator strength, /, of the transition, and is influenced by both degeneracies and the transition wavelength. 

---

Rate constants" for the excited state decay of 7-azaindole in H/D solvent mixtures... 

Application 9.4. Excited-state decay following light absorption... 

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.)... 

So far, the solid state type I reaction has been reliable only when followed by the irreversible loss of CO to yield alkyl-alkyl radical species (RP-B or BR-B) in a net de-carbonylation process. The type 11 reaction relies on the presence of a y-hydrogen that can be transferred to the carbonyl oxygen to generate the 1,4-hydroxy-biradical (BR C). The type-1 and type-11 reactions are generally favored in the excited triplet state and they often compete with each other and with other excited state decay pathways. While the radical species generated in these reactions generate complex product mixtures in solution, they tend to be highly selective in the crystalline state. 

Blancafort L (2007) Energetics of cytosine singlet excited-state decay paths — A difficult case for CASSCF and CASPT2. Photochem Photobiol 83 603-610... 

Blancafort L, Migani A (2007) Water effect on the excited-state decay paths of singlet excited cytosine. J Photochem Photobiol A 190 283-289... 

AE the internal energy change accompanying excited state decay. 

Studies like those mentioned here on the osmium complexes are more difficult for related complexes of ruthenium because of the intervention of a lowlying, thermally populable d-d excited state. However, it is possible to separate the two contributions to excited state decay by temperature dependent measurements. In the case of Ru(bpy>32+, temperature dependent lifetime studies have been carried out in a series of solvent, and the results obtained for the variation of knr with Eem are in agreement with those obtained for the Os complexes (19). 

The discussion in this section has been oriented toward the use of intramolecular reactions and excited state decay to test... 

For chemical systems of interest, photolysis produces intermediates, such as radicals or biradicals, whose energetics relative to the reactants are unknown. The energetics of the intermediate can be established by comparison of the acoustic wave generated by the non-radiative decay to create the intermediate, producing thermal energy , with that of a reference or calibration compound whose excited-state decay converts the entire photon energy into heat, / (ref). The ratio of acoustic wave amplitudes, a, represents the fraction of the photon energy that is converted into heat. 

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). 

Any d-d states must be well above the emitting level to prevent their thermal excitation, which results in photochemical instability and rapid excited state decay. 

J. V. Caspar and T. J. Meyer, Application ofthe energy gap law to nonradiative, excited state decay, /. Phys. Chem. 87, 952-957 (1983). 

FPA measurements on poly(dA-dT) were also undertaken/146) but the excited-state decay function S(t) contained additional intermediate compo-... 

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. 

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 energy of an electronically excited state may be lost in a variety of ways. A radiative decay is a process in which a molecule discards its excitation energy as a photon. A more common fate is non-radiative decay, in which the excess energy is transferred into the vibration, rotation, and translation of the surrounding molecules. This thermal degradation converts the excitation energy into thermal motion of the environment (i.e., to heat). Two radiative processes are possible spontaneous emission, just like radioactivity, which is a completely random process where the excited state decays ... 

---