The photocycloaddn. of 1-naphthonitrile (1-NpCN) and 2,3-dimethyl-2-butene (DMB) in cyclohexane or benzene gave cyclobutane I primarily, but that of 2-NpCN gave azetine II as the major and cyclobutane III as the minor product in low conversion photolysis. The photocycloaddn. of 1-NpCN with DMB is retarded by increasing amts. of trifluoroacetic acid (TFA) which is shown to form non-photoreactive complexes with ground state as well as with singlet excited state 1-NpCN. It also quenches the *(1-NpCN-DMB) exciplex. The photocycloaddn. of 2-NpCN with DMB is accelerated by TFA in the low concentration range of <0.1 M but retarded in the higher concentration rangr >0.1 M. These observations were interpreted as follows: the *(2-NpCN-DMB) exciplex reacts with TFA to enhance the yields of II and III but is counter-balanced by a non-photoreactive ground state complex formation as well as quenching of singlet excited 2-NpCN. In nonpolar solvent both singlet excited naphthonitriles form TFA exciplexes that fluoresce in the nearly same region of the parent *NpCN fluorescence.

Key words: BUTENE P, PHOTOCHEMICAL REACTION, TRIFLUOROACETIC ACID, REACTION MECHANISM, ADDITION REACTION, NAPHTHONITRILE, METHYL GROUP