MICHAEL KASHA 53 



G. Low Transition Energy 



lii many diftereiU types of heteromolecules it is observed that fre- 

 quently the longest wavelength singlet-singlet transition is of n -» tt* 

 type. In the case of simple aldehydes and ketones the lowest ioniza- 

 tion potential is for an /^-orbital on the oxygen atom; consequently, 

 longest wavelength )i —> tt* transitions might be expected. However, 

 in the case of N-heterocyclics the lowest ionization potential is almost 

 certainly that for a Tr-electron, the //-orbital having an ionization 

 potential which may be 2-3 ev. higher (15, 6) . Nevertheless, as will 

 be seen in the next section, the n -^ tt* transition is almost always 

 lower in energy than the tt -^ tt* transition in N-heterocyclics, al- 

 though frequently very close to the lowest tt ^^ tt* transition. Proba- 

 bly the charge redistribution examined by Orgel (30) for n -^ tt* 

 promotion greatly alters the relative orbital energies, reversing the 

 expectation derived from the ionization potential concerning the 

 relative excitation energy. 



H. Fluorescence Qiienching 



The clue to the presence of lowest singlet-singlet n -> tt* transi- 

 tions in many diverse molecules was the absence of fluorescence in 

 such molecules (17) . The interpretation of the intramolecular 

 quenching depends especially on the greatly enhanced singlet-triplet 

 mixing originally proposed on an empirical basis (17) , but established 

 theoretically by Clemen ti and Kasha (5) ; the calculations of Sidman 

 confirm this idea (33a). 



The detailed interpretation of the intramolecular quenching of 

 liuorescence as a consequence of the existence of n,TT* lowest excited 

 singlet states has been given recently (15). 



/. Characteristic Vibrational Couplitig 



One of the observations made on n -^ tt* transitions is the extremely 

 characteristic vibrational-electronic coupling which occurs (15), per- 

 mitting one to identify the heterogroup from which the n ^ tt* 

 transition originates in many cases. Frequently, the appearance of a 

 particular molecular vibration coupled with the electronic excitation 

 is simply the result of the occurrence of a node betw^een the atoms of 

 a characteristic functional group in the excited state, leading to ex- 

 citation of one to several quanta of a stretching frequency with the 

 electronic transition. 



However, in some cases, as in pyridine and the diazines, a particu- 



