Absorption Spectrum of a Conjugated Dye

Solid-State Lasers Radiative Properties of Ruby Crystals Spectroscopic Properties of CdSe Nanocrystals 

As an example, consider a dilute solution of 1,1 -diethyl-4,4 -carbocyanine iodide (cryptocyanine)  

Let us denote the number of carbon atoms in a polymethine chain by p then N= p + 3. Kuhn assumed that L was the length of the chain between nitrogen atoms plus one bond distance on each side thus L = p+ 3)1, where 1 is the bond length between atoms along the chain. Therefore 

Putting 7 = 1.39 A = 0.139 nm (the bond length in benzene, a molecule with similar bonding) and expressing A in nanometers, we find 

If there are easily polarizable groups at the ends of the chain (such as benzene rings), the potential energy of the tt electrons in the chain does not rise so sharply at the ends. In effect this lengthens the path L, and we can write 

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