Hydration effects on the triplet exciplex between 2,3-dihalo-1,4-naphthoquinone and furan studied by steady-state and laser flash photolyses
作者:Minoru Yamaji、Masanori Kurumi、Hiroko Kimura、Haruo Shizuka
DOI:10.1039/a809483g
日期:——
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 (Φpro) of 0.12 and 0.05, respectively, whereas in aqueous ACN, 2,3-dibromo- and 2,3-dichloro-1,4-dihydroxynaphthalenes Φpro=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 (kq) 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 (εani) 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 (αet) 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.
三重态 2,3-二溴-和 2,3-二氯-1,4-萘醌(DBNQ 和 DCNQ)与呋喃在乙腈 (ACN) 以及 ACN 和水的混合物 (4:1 v/v) 中的光化学相互作用通过产物分析、稳态和纳秒激光闪光光解进行研究。在乙腈中存在呋喃的情况下,DBNQ 和 DCNQ 的光产物分别是 2-溴-和 2-氯-3-(2-呋喃基)-1,4-萘醌,生产量子产率 (Φpro) 分别为 0.12 和 0.05,分别,而在乙腈水溶液中,2,3-二溴-和2,3-二氯-1,4-二羟基萘分别Φpro=0.12和0.17。通过355 nm纳秒激光光解,发现三线态DBNQ和DCNQ被呋喃猝灭,速率常数(kq)分别为2.0×109和3.0×109 d mol-1 s-1,在ACN和6.1×109和6.4×分别在乙腈水溶液中为 109 d mol s-1。三线态 DBNQ 和 DCNQ 耗尽后,ACN 中在 360–600 nm 区域没有观察到瞬态吸收,而相应的阴离子自由基在 400 nm 处的摩尔吸收系数 (μani) 分别为 7700 和 7900 d mol-1 cm-1 ,在乙腈水溶液中形成。发现三线态 DBNQ 和 DCNQ 与呋喃在乙腈水溶液中的初始相互作用是电子转移,效率 (αet) 分别为 0.22 和 0.23,而在乙腈中则推测主要是诱导猝灭。从电子转移自由能变化(ΔG)的角度讨论了呋喃对三线态 DBNQ 和 DCNQ 的失活机制。这表明具有弱电荷转移特性的三线态激基复合物在ΔG项中受溶剂化能控制发挥了重要作用。