Excited state energy transfer

When one metal ion is used as a donor for sensitizing the emission of a second accepting metal ion, the characteristic lifetimes r of their excited states, which are related to their deactivation rates by r = k l, are affected by the metal-to-metal communication process. This situation can be simply modeled for the special case of an isolated d-f pair, in which the d-block chromophore (M) sensitizes the neighboring lanthanide ion (Ln) thanks to an energy transfer process described by the rate constant k 1 ". In absence of energy transfer, excited states of the two isolated chromophores decay with their intrinsic deactivation rates kxl and kLn, respectively, which provides eqs. (32) and (33) yielding eqs. (34) and (35) after integration ... 

Keywords Chromophores Electron transfer Energy transfer Excited-state chemistry Metal-organic chromophores Photoluminescence Photophysics Platinum... 

Typical application (as demonstrated in chapter) Characterization of quenching, spectral shifts, polarization, energy transfer, excited state reactions... 

Such reactions are commonly found as a result of the decomposition of charge transfer excited states. For example, while excitation of the MC bands of Co(NH3)5X (X = Cl, Br, I) leads to photosolvation and the formation of Co(NH3)5(0112) and Co(NH3)4(OH2)X, shorter wavelengths yield the LMCT state which decomposes into Cc II) ions and halogen atoms. The quantum yield for the reaction is found to depend on the excitation energy, indicating a role for the initially formed radical pair (Eq. 5). This may reform the starting complex (Eq. 6) or decompose to the redox products stabilised by the solvent or some other species (Eq. 7). The Co(II) complexes eventually decomposes (Eq. 8). 

Figure 2. Schematic representation of some relevant ground and excited-state properties of Ru(bpy)j. MLCT and MLCT are the spin-allowed and spin-forbidden metal-to-ligand charge transfer excited states, responsible for the high intensity absorption band with = 450 nm and the luminescence band with = 615 nm, respectively. The other quantities shown are intersystem crossing efficiency energy (E°°) and lifetime (x) of the MLCT state luminescence quantum yield ( ) quantum yield for ligand detachment (O,). The reduction potentials of couples involving the ground and the MLCT excited states are also indicated.

Which iow-energy electronically excited states of the substrate might be on the reaction pathway, and which of these is formed initially (by light absorption or by energy transfer) ... 

Energy Transfer—(Excitation transfer).— As used in photochemistry the term is nonspecific and refers to any transfer of energy from an excited molecule to other species. The energy acceptor may itself be promoted to an excited electronic state or the electronic energy may be donated to a host system as vibrational, rotational or translational energy. The term is used to describe variously overall processes which may involve two or more steps (e.g., internal conversion followed by transfer of vibrational energy) and the individual steps in which the energy passes from one molecule to another (or others). 

The excited-state dynamics of the 2-hydroxypheylbenzotriazole (HPB) photostabilizer copolymerized with polystyrene (51) are reported in Ref. 195. The HPB fluorescence from these copolymer films is observed at 630 nm, characteristics of the proton-transferred excited state of HPB, and it has a rise time of < 10 psec and a decay time of 28 4 psec at room temperature. Measurement of the relative fluorescence quantum yield as a function of temperature gives the activation energy for nonradiative decay of this state as 259 25 cm-1. 

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