Question

which compound shows the longest lamda max in the U.V. spectrum​

Answer 1

The first one (

I

) has the longest

λ

max

because it has the longest conjugated system made out of 4 double bonds (two double bonds from the substituent and two in the ring).

A conjugated system is characterized by the alternation of double-single bonds. Every two double bonds are separated by one single bond.

The longer the conjugated system in molecules the higher their capability of interacting with light and absorbing light.