EXCITATION OF POLYENES AND PORPHYRINS 91 



and lifetimes are shown in Fig. 2-8. The n-g transitions are weak, with 

 €max between 10 (forbidden) and 1000 (allowed). In the allowed tran- 

 sitions the polarization is predicted to be perpendicular to the molecu- 

 lar plane. The general weakness of the transitions is due to the rela- 

 tively small overlap between the localized n-orbitals and the spread-out 

 ^-orbitals. 



The result of this weakness, as shown in Fig. 2-7, is a long time con- 

 stant for fluorescence when the first excited singlet state is ^U, so that the 

 radiationless transition to the lowest triplet is more probable than radia- 

 tion to ground. At room temperatures the energy goes into the triplet 

 state, which is cpenched, and no luminescence is observed. At low tem- 

 peratures, phosphorescence only is seen. For molecules large enough so 

 that ^B comes below ^U, the behavior reverts to the normal hydrocarbon 

 pattern (Kasha, 1950). [It is also possible for ^U to be the lowest singlet 

 and ^B the lowest triplet. The fluorescence behavior in such cases can 

 be predicted from Figs. 2-7 and 8. These cases are important in the light 

 of Reid's recent demonstration (1953) that the ^U-^U and ^ir-^TI^ sepa- 

 rations are theoretically and experimentally very small, of the order of 

 100 cm-i.] 



The ^Ll and ^W states have still another identifying characteristic. 

 This is their behavior in polar and acid solvents, which lower the energy 

 of the exposed n-orbitals, producing strong blue shifts of the n-transitions, 

 just opposite to the shifts of the 7r-transitions, which move to the red 

 with increasing refractive index (McConnell, 1952). The n-g transitions 

 are also distinguished by their narrow "atomic-like" vibrational struc- 

 ture in vapor phase, presumably indicating a very small coupling between 

 the n-electrons and the molecular vibrations, and, conversely, by their 

 almost complete absence of structure in solution, indicating their strong 

 interaction with the solvent molecules (Kasha, 1950). 



Where there are two or more conjugated hetero atoms in the same 

 system, their n-orbitals interact, producing red shifts of the n-g transitions 

 (Piatt, 1951a) and frequently changing the lowest transition from a W 

 type to a ^TF type, as in going from pyridine to the diazines (Halverson 

 and Hirt, 1951). 



The sequence of the highest n-orbital energies for different hetero 

 groups is shown in Table 2-2, as observed and estimated from the tabu- 

 lated positions of typical ^A-^U and ^A-^V transitions in a number of 

 compounds (McConnell, 1952). 



Hetero Shifts in Odd-atom Systems. In certain cases, hetero replace- 

 ment of carbon produces large shifts in the tt-tt transitions. Kuhn (1950) 

 and Dewar (1950) accounted for a large class of such cases, where sym- 

 metrical anilinium ions have a central CH replaced by N. Such an aza 

 substitution on Michler's Hydrol Blue, shown in Figs. 2-2, 6, and 13, 

 converts it to Bindschedeler's Green, with a shift of the ^A-^B absorption 



