Direct irradiation of 1,1,3,3-tetraphenyl-5-methyl-1,4-hexadiene gives the products 18-A and 18-B shown below. When the reaction is carried out by photosensitization, only 18-A is formed. Suggest...

Direct irradiation of 1,1,3,3-tetraphenyl-5-methyl-1,4-hexadiene gives the products 18-A and 18-B shown below. When the reaction is carried out by photosensitization, only 18-A is formed. Suggest mechanisms for formation of 18-A and 18-B. A minor product 18-C is formed only in the photosensitized reaction. What other products might have been expected? Can you explain the preference for the observed products?