Koyama Daisuke, Orr-Ewing Andrew J
School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK.
Phys Chem Chem Phys. 2016 Sep 21;18(37):26224-26235. doi: 10.1039/c6cp05110c.
The photochemical dynamics of the thione 2-mercaptobenzothiazole (MBT) initiated by absorption of 330 nm ultraviolet light are investigated by ultrafast transient absorption spectroscopy. The lowest energy triplet state (T) has mixed ππ*/nπ* character and is populated with a quantum yield of 0.58 ± 0.01 from the photo-excited ππ* S state in methanol solution via rapid internal conversion to the nπ* S state (with time constant τ < 150 fs). The spectroscopic evidence points to a mechanism involving intersystem crossing from S to the nπ*/ππ* T state (τ = 400 ± 100 fs) and internal conversion to T (with time constant for growth τ = 6.1 ± 0.4 ps). The remainder of the photoexcited molecules return to the ground state by S → S internal conversion. In methanol solution, the T state is long-lived but when the solvent is changed to styrene, triplet quenching is observed with a time constant of 107 ± 8 ps and assigned to the adduct-mediated energy transfer process MBT (T) + styrene (S) → [MBT-styrene] → MBT (S) + Styrene (T). Transient vibrational absorption spectroscopy observes the [MBT-styrene] biradical intermediate and determines its lifetime to be 700 ± 80 ps. Computational studies identify the mechanistic pathway for triplet quenching, which involves a curve crossing between two triplet states of the MBT-styrene adduct. The quenching process occurs with high efficiency, and no long-lived isomers of the initial adduct are observed.
通过超快瞬态吸收光谱研究了硫酮2-巯基苯并噻唑（MBT）在吸收330 nm紫外光后引发的光化学动力学。最低能量三重态（T）具有混合的ππ*/nπ特征，在甲醇溶液中，通过快速内转换至nπ S态（时间常数τ < 150 fs），从光激发的ππ* S态以0.58 ± 0.01的量子产率填充。光谱证据表明其机制涉及从S到nπ*/ππ* T态的系间窜越（τ = 400 ± 100 fs）以及内转换至T（生长的时间常数τ = 6.1 ± 0.4 ps）。其余光激发分子通过S → S内转换回到基态。在甲醇溶液中，T态寿命较长，但当溶剂换为苯乙烯时，观察到三重态猝灭，时间常数为107 ± 8 ps，归因于加合物介导的能量转移过程MBT（T） + 苯乙烯（S） → [MBT-苯乙烯] → MBT（S） + 苯乙烯（T）。瞬态振动吸收光谱观察到[MBT-苯乙烯]双自由基中间体，并确定其寿命为700 ± 80 ps。计算研究确定了三重态猝灭的机理途径，涉及MBT-苯乙烯加合物两个三重态之间的曲线交叉。猝灭过程高效发生，未观察到初始加合物的长寿命异构体。
