Supersensitizer, mechanism

FIG. 11 General mechanism for the heterogeneous photoreduction of a species Q located in the organic phase by the water-soluble sensitizer S. The electron-transfer step is in competition with the decay of the excited state, while a second competition involved the separation of the geminate ion-pair and back electron transfer. The latter process can be further affected by the presence of a redox couple able to regenerate the initial ground of the dye. This process is commonly referred to as supersensitization. (Reprinted with permission from Ref. 166. Copyright 1999 American Chemical Society.)... 

A more quantitative description of the photocurrent responses, taking into account the contributions from back electron transfer and RuQi attenuation, was achieved by IMPS measurements [83]. Considering the mechanism in Fig. 11, excluding the supersensitization step, and the equivalent circuit in Fig. 18, the frequency-dependent photocurrent for a perturbation as in Eq. (42) is given by... 

A person who responds to an unusually low dose of a drug is called h y per reactive. Supersensitivity refers to increased responses to low doses only after denervation of an organ. At least three mechanisms are responsible for supersensitivity (1) increased receptors, (2) reduction in tonic neuronal activity, and (3) decreased neurotransmitter uptake mechanisms. 

Dilsaver SC, Hariharan M Amitriptyline-induced supersensitivity of a central muscarinic mechanism lithium blocks amitriptyhne-induced supersensitivity. Psychiatry Res 25 181-186, 1988... 

Serra G, Forgioni A, D Aquila PS, et al Possible mechanism of antidepressant effect of L-sulpiride. Chn Neuropharmacol 13 (suppl l 576-583, 1990a Serra G, CoUn M, D Aquila PS, et al Possible role of dopamine D1 receptor in the behavioral supersensitivity to dopamine agonists induced by chronic treatment with antidepressants. Brain Res 527 234-243, 1990b Sershen H, Hashim, A, Lajtha A Behavioral and biochemical effects of nicotine in an MPTP-induced mouse model of Parkinson s disease. Pharmacol Biochem Behav 28 299-303, 1987... 

On balance, these actions could support a decrease rather than an increase in the functional state of CNS NE transmission, because depression can be conceptualized as a state of supersensitive catecholamine receptors secondary to decreased NE availability. This reasoning is consistent with the original hypothesis of diminished NE functioning, with antidepressants returning receptors to a more normal state of sensitivity. Siever and Davis ( 41) further elaborated on this concept by suggesting the possibility of dysregulation in the homeostatic mechanisms of one or more neurotransmitter systems, culminating in an unstable or erratic output. 

There is another possible mechanism which really deserves the name supersensitization because the charge injection becomes either only just possible or is largely increased by the interaction between dye molecule and supersensitizer. This is the case if the excited dye molecule reacts at first with the supersensitizer and reaches in this way a state where it can inject an electric charge even in the ground state. The "term scheme for this mechanism is shown in Fig. 20. The assumption here is that the excited state of the molecule as a donor cannot inject an electron into the conduction band or can do this only very ineffectively. It can, however, act as an electron acceptor against the supersensitizer. 

The electron energy term of the reduced dye may then be so energetic that direct electron injection is possible as indicated in Fig. 20a. The energy conditions for the inverse mechanism with hole injection by the oxidized dye (which reacts in the excited state primarily with an oxidizing supersensitizer) are shown in Fig. 20b. 

It is characteristic for the above mechanism that the photocurrent spectrum is not changed by the presence of the supersensitizer. A change in the spectrum is only to be expected if the interaction between the dye molecule and the supersensitizer has already occurred in the ground state. Though this possibility has... 

Fig. 20. Energy correlations for a mechanism of supersensitization by photoreduction (a) or photooxidation (b) of the excited dye prior to electron transfer...

Olesen, J., Iversen, H. K., Thomsen, L. L. Nitric oxide supersensitivity. A possible mechanism of migraine pain, Neuroreport 1993, 4, 1027-1030. 

In photography, a technique called supersensitization with supersensitizer (SS) is used to improve the quantum yield of spectral sensitization. The SS dye molecules co-adsorbed on the AgBr grains in much smaller concentration than the sensitizing dye improve the sensitization. The shortening of the fluorescence lifetimes by addition of SS is observed and this phenomenon is explained by the hole-trapping mechanism [10,11]. 

Two main mechanisms were proposed for the supersensitization effect. One is the hole-trapping mechanism in which the electron from SS fills the hole in the highest occupied molecular orbital (HOMO) of the excited sensitizing dye, since the HUMO level of SS is chosen to be higher than that of the sensitizer (Fig. 5) [2,10,11]. The resultant ionic state gives up an electron to the conduction band of silver halide with much higher quantum yield. 

The other is the aggregate-partitioning mechanism [2,15], where it is considered that the SS molecules work as J-aggregate partitioners, which decrease the size of the aggregates. This causes an increase in the fluorescence lifetime and/or an increase in the electron injection rate, and hence an increase in the quantum yield of the electron injection [2], In the present work the sensitization and supersensitization processes are described below by the hole-trapping supersensitization. 

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