Flash photolysis of naphthalenedicarboxylic anhydrides

Abstract

The naphthalenedicarboxylic anhydrides (1,2-, 2,3- and 1,8-) do not decompose upon irradiation in a polymer-matrix. Their triplet-states are observed in emission and absorption. The half-lives of the triplet-states are 0.4–1.3 s in a polymer-matrix and 1 to 20 µs in the gas phase.

The anhydrides decompose in gas phase upon u.-v. irradiation (λ < 300 nm) primarily to the corresponding dehydronaphthalenes (DHNs). The u.-v. spectra of the DHNs exhibit broad absorptions which are assigned to π—π^* transitions. 2,3-DHN dimerizes to the extent of 45 %. The rate constant of the dimerization is k_300°C=(4 ± 2)× 10¹⁰ l. mol^–1 s^–1 and is larger than the rate constant of the dimerization of dehydrobenzene (DHB). 1,2-DHN and 1,8-DHN dimerize only to a small extent (< 5 %). This is explained by the thermal instability of the dimers under the given conditions or by the preference for polymerization respectively. 55 % of the 1,8-DHN can be trapped by acetylene. In the reaction of 2,3-DHN with DHB the expected products are formed in approximately statistical proportions whereas in the reactions of 1,8-DHN with DHB or with 2,3-DHN the products are not formed in statistical proportion.