Energy transfer, quenching and sensitization

Energy Transfer Quenching and Sensitization series of acceptors with higher and lower triplet energies.111... 

The relatively long lifetimes of the excited states of these complexes have made them particularly attractive in the study of electron and energy-transfer quenching of excited states through organic bridges. In addition, the more positive redox potentials of these ions, compared with their pentaammine counterparts, mean that the mixed-valence ions are not air sensitive, thus facilitating spectroscopic measurements. 

There are two possible excited state interfacial electron transfer processes that can occur from a molecular excited state, S, created at a metal surface (a) the metal accepts an electron from S to form S+ or (b) the metal donates an electron to S to form S . Neither of these processes has been directly observed. The two processes would be competitive and unless there is some preference, no net charge will cross the interface. In order to obtain a steady-state photoelectrochemical response, back interfacial electron transfer reactions of S+ (or S ) to yield ground-state products must also be eliminated. Energy transfer from an excited sensitizer to the metal is thermodynamically favorable and allowed by both Forster and Dexter mechanisms [20, 21]. There exists a theoretical [20] and experimental [21] literature describing energy transfer quenching of molecular excited states by metals. How-... 

Ausloos and Rebbert carried out similar experiments with 2-pentanone at 3130 A. They also observed the sensitized emission and the quenching of primary process II in the presence of biacetyl. Moreover, they have demonstrated that the efficiency of the fluorescence, emitted by 2-pentanone, changed by no more than 2 % when biacetyl was added. This observation proves that, under the experimental conditions studied, only triplet-triplet energy transfer occurred, and that the triplet state of the ketone was the precursor, completely or at least partly, of primary process II. 

Triplet sensitization (Section 2.2.2) can be used to determine 3A v for the quenching of a triplet reactant. A complete reaction scheme for sensitization of a reactant A yielding product B from the triplet state would lead to a rather complex Stern Volmer expression, if all steps are treated explicitly. However, if ISC of the sensitizer is very fast and efficient (< >T= 1) and if the concentration of A is chosen to be much higher than that of the quencher added, then we may assume that triplet energy transfer from the sensitizer to A is fast and efficient at all quencher concentrations, so that a plot of versus cq will... 

In order to examine more closely the processes which determine , time-resolved fluorescence measurements on dyes adsorbed on semiconductor and glass surfaces were carried out in our laboratory. The first set of experiments used a low repetition rate, mode-locked Nd glass laser and streak camera detection (17). For rhodamine B adsorbed from 4 x 10 M aqueous solutions, we obtained Tj = 55 ps on Sn-doped In203 and = 46 ps on glass. Because these experiments were carried out at high, we concluded that the short on both surfaces was determined mostly by efficient energy transfer quenching. The low sensitivity of the experimental system did not permit experiments at low e. 

---