30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 43 



As a result of such studies, it has been shown that phosphorescence is due 

 to the transition of an excited molecule to a metastable triplet energy level 

 from which it may return to the ground state by the emission of light 

 (phosphorescence). The triplet state is one in which the molecule possesses 

 two electrons with unpaired spins and hence is a diradical which is quite 

 reactive chemically. When to this is added the fact that it is relatively long- 

 lived, the importance of the triplet state in photochemical reactions may be 

 readily understood. Reid 6 objects to the formulation of the triplet state as a 

 diradical and points out that a diradical is not a triplet, while one may also 

 visualize triplet states which are not diradicals. 



Considerable impetus has been given to the study of the optically excited states 

 of molecules by the development by Porter 8 and others 911 of the flash-photolysis, 

 flash-spectroscopy technique, applicable to molecules in solution at room tempera- 

 ture under conditions identical to those normally used in photochemical studies. 

 The basic principle involved is the very rapid formation of excited molecules followed 

 by the recording of the absorption spectra of these excited molecules. The substance 

 under investigation is submitted to the "flash" from a high-intensity source, 12 lasting 

 several microseconds and intense enough to excite an appreciable portion of the solute 

 molecules. Following a predetermined delay a second short flash is passed through 

 the sample into a spectrograph which records the absorption spectrum of the excited 

 state produced by the first flash. Once the characteristic maxima and extinction co- 

 efficients of the excited state have been obtained in this way, the photographic plate 

 of the spectrograph may be replaced by a photoelectric cell by means of which the 

 lifetime and decay kinetics of the excited state may be examined. 



An excellent illustration of this technique is its application to the study 

 of the various excited species produced in quinones, 13 which include the 

 triplet state as well as the semiquinone radical and radical ion (Fig. 1). 



Figure 2 shows the absorption spectrum of the excited, probably triplet, 

 state of chlorophyll a. 14 While the method has already been applied to stud- 

 ies on the metastable states of proteins and aromatic amino acids, 15 it has 

 not as yet been used on nucleic acids or their derivatives notwithstanding 



4 P. Pringsheim, "Fluorescence and Phosphorescence." Interscience, New York, 

 1949. 



5 M. Kasha, Chem. Revs. 41, 401 (1947). 



6 C. Reid, "Excited States in Chemistry and Biology." Butterworths, London, 1957. 



7 M. C. R. Symonds and M. G. Townsend, J. Chem. Soc. p. 263 (1959). 



8 G. Porter, Proc. Roy. Soc. A200, 284 (1950); Radiation Research, Suppl. 1, 479 

 (1959); Spectrochim. Acta 14, 261 (1959). 



9 Q. H. Gibson, J. Physiol. 134, 112 (1956). 



10 S. Claesson and L. Lindqvist, Arkiv kemi 11, 535 (1957). 



11 R. Livingston and E. Fujimori, ./. Am. Chem. Soc. 80, 5610 (1958). 



12 R. (i. W. Xorrish and G. Porter, Nature 164, 658 (1949). 



13 N. K. Bridge and G. Porter. Proc. Roy. Soc. A244, 276 (1958). 



14 R. Livingston, J. Am. Chem. Soc. 77, 2179 (1955). 



15 L. I. Grossweiner, J. Chem. Phys. 24, 1255 (1956); Radiation Research 9, 124 (1958). 

 160 L. I. Grossweiner and W. A. Mulac, Radiation Research 10, 515 (1959). 



