Hydration effects on the triplet exciplex between 2,3-dihalo-1,4-naphthoquinone and furan studied by steady-state and laser flash photolyses

Abstract

Photochemical interactions of triplet 2,3-dibromo- and 2,3-dichloro-1,4-naphthoquinones (DBNQ and DCNQ) with furan in acetonitrile (ACN) and a mixture of ACN and water (4:1 v/v) were investigated by means of product analysis, steady-state and nanosecond laser flash photolysis. The photoproducts of DBNQ and DCNQ in the presence of furan in ACN were 2-bromo- and 2-chloro-3-(2-furyl)-1,4-naphthoquinones with the quantum yields for production (Φ_pro) of 0.12 and 0.05, respectively, whereas in aqueous ACN, 2,3-dibromo- and 2,3-dichloro-1,4-dihydroxynaphthalenes Φ_pro=0.12 and 0.17, respectively. By nanosecond laser photolysis at 355 nm, it was found that triplet DBNQ and DCNQ were quenched by furan with rate constants (k_q) of 2.0×10⁹ and 3.0×10⁹ d mol^-1 s^-1 in ACN and 6.1×10⁹ and 6.4×10⁹ d mol s^-1 in aqueous ACN, respectively. After depletion of triplet DBNQ and DCNQ, no transient absorption in the region 360–600 nm was observed in ACN while the corresponding anion radicals having molar absorption coefficients (ε_ani) of 7700 and 7900 d mol^-1 cm^-1 at 400 nm, respectively, were formed in aqueous ACN. The initial interaction of triplet DBNQ and DCNQ with furan in aqueous ACN was found to be electron transfer with efficiencies (α_et) of 0.22 and 0.23, respectively, while that in ACN was presumed to be dominated by induced quenching. The deactivation mechanism of triplet DBNQ and DCNQ by furan was discussed from the viewpoint of the free energy changes (ΔG) for electron transfer. It was suggested that the triplet exciplex with weak charge-transfer character played an important role being controlled by the solvation energy in the ΔG term.