Nonradiative decays

Modem electron transfer tlieory has its conceptual origins in activated complex tlieory, and in tlieories of nonradiative decay. The analysis by Marcus in tire 1950s provided quantitative connections between the solvent characteristics and tire key parameters controlling tire rate of ET. The Marcus tlieory predicts an adiabatic bimolecular ET rate as... 

The first study was made on the benzene molecule [79], The S ISi photochemistry of benzene involves a conical intersection, as the fluorescence vanishes if the molecule is excited with an excess of 3000 crn of energy over the excitation energy, indicating that a pathway is opened with efficient nonradiative decay to the ground state. After irradiation, most of the molecules return to benzene. A low yield of benzvalene, which can lead further to fulvene, is, however, also obtained. 

Nonradiative Decay. To have technical importance, a luminescent material should have a high efficiency for conversion of the excitation to visible light. Photoluminescent phosphors for use in fluorescent lamps usually have a quantum efficiency of greater than 0.75. AH the exciting quanta would be reemitted as visible light if there were no nonradiative losses. 

The occurrence of nonradiative losses is classically illustrated in Figure 3. At sufficiently high temperature the emitting state relaxes to the ground state by the crossover at B of the two curves. In fact, for many broad-band emitting phosphors the temperature dependence of the nonradiative decay rate P is given bv equation 1 ... 

Fig. 3. A configurational—coordinate diagram showing mechanisms of radiationless decay to the ground state. Nonradiative decay to the ground-state...

Nonradiative energy transfer is induced by an interaction between the state of the system, in which the sensitizer is in the excited state and the activator in the ground state, and the state in which the activator is in the excited and the sensitizer in the ground state. In the presence of radiative decay, nonradiative decay, and energy transfer the emission of radiation from a single sensitizer ion decays exponentially with time, /. 

Finally, nonradiative decay can occur. This name is given to the process by which the energy of the excited state is transferred to the surrounding molecules as vibrational (thermal) energy without light emission. The proeesses that can occur after photochemical excitation are summarized in Fig. 13.1. 

The interpretation of our CPG data is complicated by the presence of comparatively fast radiative and nonradiative decay channels for the singlet exciton, which compete with the field-induced dissociation. In order to provide a clear picture of the observed mechanism and disentangle it from the singlet exciton decay dynamics, we define the following phenomenological time-dependent parameter ... 

Starting C" cells (yellow) = 0 (So for the nonradiative decay from state Ti) The transition probability matrix is ... 

All the nucleic acid bases absorb UV radiation, as seen in Tables 11-1, 11-2, 11-3, 11-4, and 11-5, making them vulnerable to the UV radiation of sunlight, since the energy of the photons absorbed could lead to photochemical reactions. As already mentioned above, the excited state lifetimes of the natural nucleobases, and their nucleotides, and nucleosides are very short, indicating that ultrafast radiationless decay to the ground state takes place [6], The mechanism for nonradiative decay in all the nucleobases has been investigated with quantum mechanical methods. Below we summarize these studies for each base and make an effort to find common mechanisms if they exist. 

Cohen B, Hare P, Kohler B (2003) Ultrafast excited-state dynamics of adenine and monomethy-lated adenines in solution implications for the nonradiative decay mechanism. J Am Chem Soc 125 13594... 

Sobolewski AL, Domcke W (2002) On the mechanism of nonradiative decay of DNA bases ab initio and TDDFT results for the excited states of 9H-adenine. Eur Phys J D 20 369... 

Sobolewski AL, Domcke W, Dedonder-Lardeux C, Jouvet C (2002) Excited-state hydrogen detachment and hydrogen transfer driven by repulsive (l)pi sigma states a new paradigm for nonradiative decay in aromatic biomolecules. J Phys Chem Chem Phys 4 1093—1100... 

Langer H, Doltsinis NL (2004) Nonradiative decay of photoexcited methylated guanine. Phys Chem Chem Phys 6 2742-2748... 

In the absence of triplet quenchers two processes compete for triplet deactivation radiative (phosphorescence) and nonradiative decay of the triplet to the ground state ... 

The third process sensitive to heavy-atom perturbation is the radiative decay from the triplet to the ground state (phosphorescence). Since phosphorescence is commonly not observed in fluid solution at room temperature, the rate of phosphorescence in the presence of heavy-atom perturbation relative to the rate of intersystem crossing and nonradiative decay need not be considered. At low temperatures in a rigid glass, however, phosphorescence... 

A typical ligand capable of generating a dendritic structure is 1,4,5,8,9,12-hexaazatriphenylene (HAT). Photophysical studies of trinuclear species based on HAT have been reported [14 a, 49]. Representative example of complexes of this type are 26, 27, and 28. For some of these complexes, the luminescence, originating from MLCT levels involving the central HAT ligand, was found to decay with multiexponential kinetics. Furthermore, the vibrational modes responsible for the nonradiative decay of the luminescent MLCT states are different in the polynuclear dendritic edifices with respect to the mononuclear [M(L)2(HAT)]2+ compounds [14a]. 

Substitution of the exponential term from (6) into (7) replaces the free-volume term by the solvent viscosity and provides an equation for the viscosity-dependent nonradiative decay rate, from which the quantum yield emerges (8) ... 

The lifetime, therefore, depends not only on the intrinsic properties of the fluorophore but also the characteristics of the environment. For example, any agent that removes energy from the excited state (i.e., dynamic quenching by oxygen) shortens the lifetime of the fluorophore. This general process of increasing the nonradiative decay rates is referred to as quenching. 

Luminescence spectroscopy provides simple access to the splitting of the ground multiplet but this technique is not always accessible due to nonradiative decay and strong ligand absorptions as encountered, for example, in the [Ln(Pc)]-/0 systems. For these reasons, alternative spectroscopic tools should be available for magnetochemists. The use of INS as a spectroscopic probe for molecular magnetic systems has recently been reviewed by Guidi [36], Amoretti et al. [37]... 

In the limit that Huty >> kgT, the rate constant for nonradiative decay is simply the product of the square of the vibrational overlap integral and an electronic term for the... 

Figure 4. Schematic energy level diagram for a catenane based on charge-transfer (CT) interactions and for its separated components. The wavy lines indicate nonradiative decay paths of the electronic excited states.

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