INTERPRETATION OF FLUORESCENCE SPECTRUM 751 



the sake of uniformity, all figures in Table 23.IIB are based on the data of 

 Dhere and Raffy in Table 23.IA.) 



In bacieriochlorophyll the distance between the two fluorescence bands 

 (shown in fig. 24.4) is much larger than in chlorophyll— 2400 cm.-^; and 

 the short-wave (red) band is weaker than the long-wave (infrared) band. 

 This points to two different electronic transitions, rather than two vibra- 

 tional bands in a common band system. The two fluorescence bands may 

 even belong to two different molecular species. Only the stronger of them 

 —that at 810 myu- appears to be correlated with a known absorption band 

 of bacteriochlorophyll, that at 770 m/x (c/. fig. 21.7). (This correlation im- 

 plies that AX, the displacement of the fluorescence band relative to the 

 absorption band, is of the order of 40 mn, as against < 15 mn in ordinary 

 chlorophyll.) 



This may be the place to mention the luminescence that occurs when a chlorophyl 

 solution in tetralin is heated to 125° C. This phenomenon was first described by Rothe- 

 mund (1938) and investigated spectroscopically by Stewart, Knorr and Albers (1942). 

 The maximum of the luminescence band was found at 677.5 m^. After the tetralin 

 solution was heated for five minutes, chlorophyll showed a change— its fluorescence 

 band was shifted from its original position at 688.5 to 671.0 niyu and was reduced to one 

 third its original intensity; the absorption spectrum also had undergone a transforma- 

 tion, especially in the blue-violet region. The origin of this luminescence is as yet un- 

 known, and its interpretation as chemiluminescence, suggested by the investigators, 

 although plausible, requires confirmation. 



2. Yield of Fluorescence and Life-Time of the Excited States 



of Chlorophyll 



The fluorescence yield can be defined either as the proportion of ab- 

 sorbed energy re-emitted in the form of radiation or, more significantly, 

 as the proportion of re-emitted photons. The two figures coincide only in 

 the case of resonance fluorescence; usually (particularly in condensed sys- 

 tems) the emitted light is of a lower frequency than the absorbed light 

 ("Stokes' rule"), and the "energy yield," e/, is therefore smaller than the 

 "quantum yield," (p. 



The relation €f < f holds true for the main fluorescence bands of all 

 derivatives of porphin and chlorin, and the difference becomes particularly 

 large if blue, violet or ultraviolet light is used for excitation. It was stated 

 above (page 748) that only red light is emitted in the fluorescence of these 

 compounds ; this means that the absorbed ultraviolet, violet or blue energy 

 ciuanta are transformed into much smaller red quanta, while up to 50% of 

 the absorbed light energy is dissipated. Since the "theoretical" life- 

 time of the excited state B (upper state of the blue-violet band system) is 

 of the order of 5 X 10 "« sec. {of. page 634), the absence of even 0.01% 

 fluorescence in this system shows that the electronic energy of the state B 

 is dissipated m less than 5 X IQ-^^ sec, i. e., after less than one hundred 

 molecular vibrations. 



