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

Photochemical interactions of triplet 2,3-dibromo- and 2,3-dichloro-1,4-nap hthoquinones (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 analysi s, 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 (Phi( pro)) of 0.12 and 0.05, respectively, whereas in aqueous ACN, 2,3-dibromo- and 2,3-dichloro-1,4-dihydroxynaphthalenes Phi(pro) = 0.12 and 0.17, respec tively. 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 x 1 0(9) and 3.0 x 10(9) dm(3) mol(-1) s(-1) in ACN and 6.1 x 10(9) and 6.4 x 1 0(9) dm(3) mol s(-1) in aqueous ACN, respectively. After depletion of tripl et DBNQ and DCNQ, no transient absorption in the region 360-600 nm was obse rved in ACN while the corresponding anion radicals having molar absorption coefficients (epsilon(ani)) of 7700 and 7900 dm(3) mol(-1) cm(-1) at 400 nm , respectively, were formed in aqueous ACN. The initial interaction of trip let DBNQ and DCNQ with furan in aqueous ACN was found to be electron transf er with efficiencies (alpha(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 viewpoi nt of the free energy changes (Delta G) for electron transfer. It was sugge sted that the triplet exciplex with weak charge-transfer character played a n important role being controlled by the solvation energy in the Delta G te rm.