On the parallel formation of long-lived excited states of dipyridil[3,2-a:2′3′-c]phenazine, dppz: A contrast between the electrochemically and photochemically induced reduction of dppz

Abstract

The photophysical and photochemical properties of dipyridil[3,2-a:2′3′-c]phenazine, dppz, were investigated in homogeneous solution by time-resolved and steady-state photochemical techniques. Irradiation of dppz in CH 3OH, CH 3CN or CH 2Cl 2 induced short-lived fluorescence with a lifetime, τ ≤20 ns. Different emission quantum yields were determined when dppz was irradiated at 350 nm, ϕ LUM ≤2.5 × 10 −4, and when it was irradiated at ∼400 nm, 3.2 × 10 −3 ≤ ϕ LUM ≤ 2 × 10 −2. Reactions of the luminescent excited states with electron donors, TEOA or TEA, and incapacity for an inefficient H atom abstraction from 2-propanol suggested that they were ππ* excited states. Long-lived transient absorption spectra with lifetimes in the microsecond time domain were associated with adducts of the ground and excited states. The dppz radical, product of the excited state electron transfer reactions, and the product of the reduction of dppz by pulse radiolytically generated e sol − have the same UV–vis spectrum. Their spectrum differs, however, with the spectrum of the radicals from the electrochemically reduced dppz. The photophysical and photochemical results are attributed to low-lying ππ* excited states of dppz.