Photochemistry of indeon[2,1-a]indene

Abstract

The photochemistry of indeno (2,1-a) indene, a trans-stilbene analogue, was studied. The fluorescence quantum yield and singlet lifetime of this compound were 0.85 and 1.99 nsec respectively. The Stern-Volmer constants ($k_q\cdot\tau$) of fluorescence quenching increased with increasing olefin electron affinity. The plot of log $k_q$ vs. ionization potential gave near linearity. Irradiation of benzene solution of indeno (2,1-a) indene gave two $c_4$-cyclodimers, head-to-tail and head-to-head dimer in a 2:1 ratio. The concentration effect on quantum yield of photodimerization, triplet sensitization and external heavy atom effect were investigated. From these investigations, it was concluded that $C_4$-photocyclodimerization of indeno (2,1-a) indene occurs via a singlet excimer intermediate. Photoreaction of indeno (2,1-a) indene with electron rich olefins gave only two cyclodimers without giving cross addition product in contrast to that of transstilbene. Irradiation of benzene solution of indeno (2,1-a) indene and moderate electron defficient olefins gave $C_4$-cycloaddicts and other stereochemistry was influenced by secondary $\pi$-orbital overlap. The photocycloaddition of indeno (2,1-a) indene with methyl crotonate was most efficient among the tested olefins. In photoreaction of indeno (2,1-a) indene with dimethyl fumarate, ene-reaction product was obtained as major product and that of indeno (2,1-a) indene with fumaronitrile gave no product. The linear relationship between $\Phi_a^{-1}$ and $(A)^{-1}$, quantum yields of photocycloaddition reaction and stereochemistry of photocycloadducts were consistent with the singlet exciplex mechanism and the reaction rate of photocycloaddition was found to be controlled by the stability of these intermediates.