Singlet oxygen production by pyrano and furano 1,4-naphthoquinones in non-aqueous medium

The influence of ring size on the photobehaviour of condensed 1,4-naphthoquinone systems, such as pyrano- and furano-derivatives (1 and 2, respectively) has been investigated. The absorption spectra for both families of naphthoquinones reveal clear differences; in the case of 2 they extend to longer wavelengths. A solvatochromic red shift in polar solvents is consistent with the π,π* character of the S(0)→ S(1) electronic transition in all cases. Theoretical (B3LYP) analysis of the HOMO and LUMO Kohn-Sham molecular orbitals of the S(0) state indicates that they are π and π* in nature, consistent with the experimental observation. A systematic study on the efficiency of singlet oxygen generation by these 1,4-naphthoquinones is presented, and values larger than 0.7 were found in every case. In accordance with these results, laser flash photolysis of deoxygenated acetonitrile solutions led to the formation of detectable triplet transient species with absorptions at 390 and 450 nm (1) and at 370 nm (2), with φ(ISC) close to 1. Additionally, the calculated energies for the T(1) states relative to the S(0) states at UB3LYP/6-311++G** are ca. 47 kcal mol(-1) for 1 and 43 kcal mol(-1) for 2. A comparison of the geometrical parameters for the S(0) and T(1) states reveals a marked difference with respect to the arrangement of the exocyclic phenyl ring whilst a comparison of electronic parameters revealed the change from a quinone structure to a di-dehydroquinone diradical structure.