1798 SPECTROSCOPY AND FLUORESCENCE OF PIGMENTS CHAP. 37C 



of chlorophyll and bacteriochlorophyll preparations, combine to make the 

 suggested interpretation of the spectra uncertain. It provides, however, a 

 better starting point for further investigation than the purely empirical 

 suggestion, made on p. 622, that the addition of a pair of hydrogen atoms to 

 the porphin system generates a new low electronic state, and pushes all the 

 old levels upward. According to the present hypothesis, the homologous 

 bands are 621 m^u in protochlorophyll, 660 m^ in chlorophyll, and 775 mju 

 in bacteriochlorophyll; in other words, the shift with increasing hydro- 

 generation is toward the longer waves. 



In Part 1 of Vol. II it was argued — by analogy with polyene chains, or 

 with the benzene-naphthalene-anthracene series — that addition of new 

 links to a conjugated double bond system (which occurs in the series tetra- 

 hydroporphin-dihydroporphin-porphin) should lower, rather than raise, 

 the first excited level (z. e., produce a "red" rather than "blue" shift of the 

 first absorption band). Longuet-Higgins and Piatt (1950) pointed out, 

 however, that a shift of the absorption band toward shorter waves with 

 increasing number of conjugated bonds has been noted also in other ring 

 structures, in which this addition broadened, rather than elongated the 

 conjugated system. 



To make the theoretical treatment of the spectra of poi-phin pigments 

 more precise and reliable, both better calculations and better experimen- 

 tation are needed. In particular, spectroscopic measurements on oriented 

 molecules (in flowing solutions, monolayers, or single crystals) could be 

 helpful, by providing direct evidence of the polarization of the several 

 bands in respect to the symmetry axes of the molecule. Further develop- 

 ment of the above-mentioned studies of fluorescence polarization also is 

 desirable. 



The life-time of the lowest excited state of chlorophyll a and h in solu- 

 tion, first estimated by Prins (p. 633), was re-estimated by Livingston 

 (personal communication) for ethereal solutions and by Jacobs (1952, 

 1954) for acetonic solution. They used a quantum-mechanical equation 

 given by Lewis and Kasha (1945), instead of the classical relation used by 

 Prins, to calculate the "oscillator strength," /, and the mean life time of 

 excitation, r, from the integral under the absorption band. The results 

 of the two calculations do not agree well: 



Peirin (1929) and Stupp and Kuhii (1952) estimated the life time of chlorophyll a 

 excitation in solution from the relative polarization of fluorescence in two solvents, one 

 of these (castor oil) so viscous as to practically suppress Brownian rotational movement. 

 The calculated (actual) excitation time was r^ = 3 X 10"^ sec. (Perrin) and r^ = 1.5 X 



