Photochemistry of 1,4,5,8-tetraazaphenanthrene

Abstract

The photophysical and photochemical properties and the photocycloaddition reaction of 1,4,5,8-tetraazaphenanthrene(TAP) which has both (n,$π^{\ast}$) and ($π, π^{\ast}$) excited states were investigated. Due to the close-lying (n,$π^{\ast}$) and ($π, π^{\ast}$) singlet states, TAP shows very weak fluorescence ($Φ_f$=0.001) and the laser flash photolysis study indicated the efficient intersystem crossing ($Φ_{isc}$=0.95). The low pH and alkaline salts enhance the fluorescence by reversing the energy level of the lowest excited state from (n,$π^{\ast}$) state to the fluorescing ($π,π^{\ast}$) state. Because of the mixing between the lowest excited $^1(π,π^{\ast})$ and $^1(n,π^{\ast})$, TAP shows the heavy atom effect which is generally not observed in the pure, $^1(n,π^{\ast})$ state. As phosphorescence lifetime is relatively long and phosphorescence polarization is negatively polarized, it is considered that the lowest triplet state has the ($π, π^{\ast}$) configuration. Irradiation of a chloroform solution of TAP and trans-1,2-dichloroethylene with 300 nm UV light yielded only one 1:1 cycloadduct in quantitative yield. Short wavelength irradiation of cycloadduct gives rise to the starting material, TAP. This suggests the product to be a [2+2] cycloadduct. The NMR spectra of cycloadduct exhibits a symmetrical AA``XX`` splitting pattern for the cyclobutane ring protons indicating the $\mbox{\underline{cis}}$ stereochemistry for the two carbon-chlorine bonds. The chloroform solutions of TAP in the presence of excess vinlyidene chloride, trichloroethylene, and tetrachloroethylene were irradiated with 300 nm UV light to obtain cycloadducts in 45, 32, and 17% yields, respectively. Irradiation of a chloroform solution of TAP in the presence of excess vinyl bromide and 2-chloroacrylonitrile with 300 nm UV light led to the formation of single cycloadduct in quantitative yields, while two isomeric cycloadducts were obtained with acrylonitrile. LC analysis and NMR spectr...