Photochemical cycloaddition of two nearly isoenergetic singlet states of 1-naphthonitrile

Irradiation of 1-naphthonitrile in the presence of 1,2-dimethylcyclopentene results in the formation of 2 + 2 cycloadducts with both the nitrile group and aromatic ring. Both types of addition occur via singlet 1-naphthonitrile. The ratio of nitrile/ring addition in ethyl ether solution is dependent upon dimethylcyclopentene concentration, oxygen concentration, and excitation wavelength. These results require kinetically nonequivalent singlet-state precursors for nitrile vs. ring addition. With the aid of a recent fluorescence polarization study by Suzuki, we assign ring adduct formation to the ¹L_a state and nitrile adduct formation to either the ¹L_b state or a hidden state with bent nitrile geometry. This assignment provides the first example of divergent reaction pathways for two nearly isoenergetic singlet states. The chemical behavior of the ¹L_b, vs. ¹L_a states is discussed in terms of the electronic description of these states provided by a molecular orbital (INDO/S) calculation. The results and conclusions of the present investigation are shown to be compatible with the extensive photophysical investigations of the 1-naphthonitrile/1,2-dimethylcyclopentene system by Ware and co-workers.