α-Bond Dissociation of p-Phenylbenzoyl Derivatives in the Higher Triplet Excited State Studied by Two-Color Two-Laser Flash Photolysis

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• 作者：Minoru Yamaji*^† ; ; Xichen Cai^‡ ; ; Masanori Sakamoto^‡ ; ; Mamoru Fujitsuka^‡ ; ; Tetsuro Majima*^‡ ;
• 刊名：Journal of Physical Chemistry A
• 出版年：2009
• 出版时间：March 5, 2009
• 年：2009
• 卷：113
• 期：9
• 页码：1696-1703
• 全文大小：182K
• 年卷期：v.113,no.9(March 5, 2009)
• ISSN：1520-5215

文摘

Photochemical properties in the lowest singlet and triplet excited states (S₁ and T₁) and in the higher triplet excited states (T_n) of p-phenylbenzoyl derivatives (PB-X) having C−O and C−S bonds were investigated in solution using a stepwise two-color two-laser flash photolysis technique. PB-Xs (X = OPh and SPh as a leaving group) undergo α-bond dissociation in the S₁ state, while the C−O bonds in PB-OH and -OMe were stable upon the 266-nm laser flash photolysis. The T₁(π,π*) states of PB-X were efficiently produced during the 355-nm laser flash photolysis of PB-X in the presence of benzophenone as a triplet sensitizer. The T₁(π,π*) states of PB-Xs deactivate to the ground-state without producing any intermediates. However, when PB-Xs (X = OPh and SPh) in the T₁ states (PB-X(T₁)) were excited upon the 430-nm laser flash photolysis, their disappearance was observed. These observations indicate that PB-X(T₁) (X = OPh and SPh) is excited to PB-Xs in the T_n states which decompose through the C−X bond cleavage. From the transient absorption measurements, quantum yields (Φ_dec) of the disappearance of PB-X(T₁) were determined while bond dissociation energies (BDE) of the C−X bonds of PB-X were calculated by computations. On the basis of the Φ_dec and the BDE values, it was shown that the rates of the decomposition process of PB-X in the T_n states were expressed in an energy gap low form as a function of BDE. Features of the C−X bond cleavage of PB-X in the T_n states were discussed.