Energy Efficiency Factors Quantum yields

Direct Photolysis. Direct photochemical reactions are due to absorption of electromagnetic energy by a pollutant. In this "primary" photochemical process, absorption of a photon promotes a molecule from its ground state to an electronically excited state. The excited molecule then either reacts to yield a photoproduct or decays (via fluorescence, phosphorescence, etc.) to its ground state. The efficiency of each of these energy conversion processes is called its "quantum yield" the law of conservation of energy requires that the primary quantum efficiencies sum to 1.0. Photochemical reactivity is thus composed of two factors the absorption spectrum, and the quantum efficiency for photochemical transformations. 

The choice of new complexes was guided by some simple considerations. The overall eel efficiency of any compound is the product of the photoluminescence quantum yield and the efficiency of excited state formation. This latter parameter is difficult to evaluate. It may be very small depending on many factors. An irreversible decomposition of the primary redox pair can compete with back electron transfer. This back electron transfer could favor the formation of ground state products even if excited state formation is energy sufficient (13,14,38,39). Taking into account these possibilities we selected complexes which show an intense photoluminescence (0 > 0.01) in order to increase the probability for detection of eel. In addition, the choice of suitable complexes was also based on the expectation that reduction and oxidation would occur in an appropriate potential range. 

Transient absorption studies demonstrated that the yield of the final C+-P-Q T state for 10 is 0.30 in dichloromethane. This increase by a factor of 7 over that found for 4 must be due to a higher efficiency for step 4, as was hoped for. Indeed, the efficiency of this step has been increased from 0.04 in 4 to 0.73 in 10. Thus, fine-tuning of the energetics of the various electron transfer steps in the C-P-Q triads is indeed a powerful tool for controlling quantum yield without necessarily affecting the amount of energy stored in the final charge separated state (this is essentially the same for both 4 and 10). 

When porphyrins with much higher triplet energies such as palladium octaethylporphyrin (17 Et = 44.8 kcal mol" ) were used as sensitizers, even the cis trans isomerization of stilbene took place as a quantum chain process = 1-6) [95]. The high quantum efficiencies were explained by a quantum chain process in which the metalloporphyrin serves as both an energy donor and an acceptor. Since the quantum yield of cis trans isomerization of 1,2-diphenylpropene = 0-37) remained as a normal value under the same experimental conditions as those of stilbene, the potential energy surface of the triplet state is an important factor for occurrence of the quantum chain cis-trans isomerization. That is, in 1,2-diphenylpropene the triplet state exists exclusively as a perpendicular conformation, where the triplet state and the ground state lay very close in energy and the deactivation can only take place thermally. 

Energy efficiencies have been evaluated using factors such as the Electrical Energy per Order (EE/0) (Bolton and Cater, 1994 Notarfonzo and McPhee, 1994), the quantum yield and the appai ent quantum yield (Fox and Dulay, 1993 Nimlos et al., 1993 Sczechowski et al., 1995 Valladares and Bolton, 1993 Zhang et al., 1994a). More recently, SeiTano and de Lasa (1997) proposed a photo-catalytic themiodynamic efficiency factor (PTEF) based on thermodynamic considerations. 

The ODMR technique has a distinct advantage in sensitivity over conventional EPR spectroscopy for measurements made on biological molecules. To begin with, in ODMR, the absorption or stimulated emission of microwave quanta is not detected directly, as is the case with EPR. Rather, the microwave quanta are converted to optical photons which are the detected entities. The quantum up-conversion by a factor of about 10 in energy results in greatly increased sensitivity over conventional EPR the actual attainable sensitivity depends on various factors such as phosphorescence quantum yield, the light collection efficiency, the decay characteristics of the triplet state, and other factors discussed later. 

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