Sensitization by energy transfer

The dienes, specially 1,3-pentadiene (piperylene) and hexadiene, quench the triplets of suitable sensitizers by energy transfer with unit efficiency. Hence, they are used widely in mechanistic studies of photochemical reactions, either to count the triplets or to establish the triplet energy of a sensitizer whose Et is not determinable from spectroscopic data (chemical spectroscopy). 

The luminescence arises from dye molecules which are sensitized by energy transfer from the excited carbonyl group. 

Fluorescence Sensitization. The optical pumping efficiency and thereby the prospects for oscillation of several actinides may be improved by codoping with sensitizer ions using schemes similar to those employed for the lanthanides in Table IV. For example, the first excited J states of l)3+, Np3+, and Pu3+ could be sensitized by energy transfer from lanthanide codopants such as Ho3+, Er3+, Tm3+, and Yb3+ as in the "alphabet" Ho3+ scheme (31). Other schemes using actinides include Cf3+-sensitized Bk3+ ( 4- 6), l)3+-sensitized Np3+... 

Figure 14. Absorption spectrum (top) and fluorescence excitation spectrum (middle) of isoquinoline vapor at room temperature, and the excitation spectrum of biacetyl phosphorescence (bottom), sensitized by energy transfer from the triplet isoquinoline. The band positions (in cmT1) are relative to the starred 0+ band at 31,925cm-1. (From ref. [45] with permission.)...

Figure 1. Schematic representation of a photochemical systein including sensitization by energy transfer and quenching...

Luminescence of organic chromophores sensitized by energy transfer from singlet oxygen has also been reported for many years. " Several mechanisms have been suggested for this luminescence. 

Figure 10-22 shows a Stem-Volmer plot of the relationship between the fluorescence intensities of a PBA film at 474 nm, attributed to PBA excimer emission, and the oxygen concentration. At higher PBA concentrations, the emission of the PBA excimer is sensitized by energy transfer from the PBA monomer. 

Sensitization by energy transfer is extremely important both theoretically and practically. Several practical applications will be discussed in other chapters of this book. A mechanistic application can be illustrated using again Cr(CN)g. Besides the quenching experiments described above, one can use sensitization experiments to establish which excited state is responsible for the photochemical reaction. With the lowest excited state of Ru(bpy)3 as an energy donor, one can selectively populate (for thermodynamic reasons, equation 25) the excited state (figure 19a). Under such conditions luminescence but not reaction is obtained, showing that is not responsible for the reaction observed upon direct excitation [44]. 

Other basic applications of sensitization by energy transfer include increasing quantum yield of a reaction [50], increasing range of useful exciting light wavelength [50], and evaluation of association constants [51,52]. 

---