Knowledge Bank

“Many linear and cyclic polyenes which have heteroatoms such as O, N, S. etc conjugated with their $\pi$ electron systems show photochemical activity. Such photochemical reactions usually involve lower excited $n\pi \ast$ singlet or triplet states, either directly or indirectly, and as a consequence this activity is markedly dependent on thy lifetimes and populations of such electronic states. These factors in turn depend on the relative energetic positions of these excited states and on the efficiencies of the radiationless transitions between them. A number of classes of such polyenes show of wide variation in photochemical activity depending on the position and kind of Substituent interacting with the $\pi$ electron system. As has been suggested by Abrahamson and $Panik1$, the effect of introducing such a substituent into a polyene system is to shift the $\pi ,\pi \ast$ and $n\pi \ast$ electronic levels, thereby markedly affecting their actual populations and lifetimes and hence their photochemical activity. A study of the comparative photochemical activity, the absorption spectra and fluorescence and phosphorescence behavior of a number of classes of such compounds---particularly the substituted anthraquinoncs and acetophenones---bears out this general principle.”