THE STRUCTURE OF VISUAL PURPLE (rHODOPSIN) 



provided the groups to which the ends of the chain were attached 

 would permit such a rearrangement of valencies. In such cases the 

 molecule could exist in two structural states. Some molecules would 

 be in one state, some in the other. Some (perhaps the majority) 

 would be neither in one nor the other but in some intermediate 

 condition represented by 



H H H 



I I 1 



I I 



H H 



in which the black dots represent unpaired electrons. 



According to modern ideas the electrons which remain (five in the 

 present example), after allocation of a single bond linkage between 

 each adjacent atom, mutually interact to form so-called 7r-orbitals. 

 These are not Hmited to the space separating adjacent atoms but 

 extend over the whole length of the conjugated chain. Several 

 TT-orbitals can occur, much as a violin string can vibrate in several 

 modes to give harmonics in addition to the fundamental note. In 

 the unexcited state the electrons occupy orbitals of lowest energy, 

 but, on absorption of a quantum of suitable magnitude, they enter 

 orbitals of higher energy. By the exclusion principle of Pauli only 

 one electron (of a given spin) can occupy each orbital. As the con- 

 jugated chain increases in length the number of vr-electrons increases 

 also. Consequently more of the ground-state orbitals are filled and 

 less energy (i.e. a quantum of lower frequency) is required to raise an 

 electron to the next higher orbital — the incremental energy of suc- 

 cessive orbitals being progressively smaller. Thus in simple conju- 

 gated systems the energy required is so great that absorption occurs 

 only in the ultra-violet, where the frequency (and hence the quantal 

 energy) is sufficiently large. In systems of greater conjugation, the 

 necessary activation energy is smaller and may be supplied by a 

 quantum in the 'visible' range. Thus in the diphenyl polyenes, 



^[— 1„-0 



the simplest members (n = 1 and 2) absorb in the ultra-violet and are 

 colourless but, as n is increased the spectral location of the absorp- 

 tion band advances towards the visible, and the subsequent members 

 are coloured. Thus when « = 3, the compound is pale yellow; when 

 n = 5, it is orange; when «= 11 it is violet and when n = 15 it is 



101 



