Pritchina Elena A, Gritsan Nina P, Burdzinski Gotard T, Platz Matthew S
Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, USA.
J Phys Chem A. 2007 Oct 25;111(42):10483-9. doi: 10.1021/jp073707h. Epub 2007 Sep 26.
The primary photophysical and photochemical processes in the photochemistry of 1-acetoxy-2-methoxyanthraquinone (1a) were studied using femtosecond transient absorption spectroscopy. Excitation of 1a at 270 nm results in the population of a set of highly excited singlet states. Internal conversion to the lowest singlet npi* excited state, followed by an intramolecular vibrational energy redistribution (IVR) process, proceeds with a time constant of 150 +/- 90 fs. The 1npi* excited state undergoes very fast intersystem crossing (ISC, 11 +/- 1 ps) to form the lowest triplet pipi* excited state which contains excess vibrational energy. The vibrational cooling occurs somewhat faster (4 +/- 1 ps) than ISC. The primary photochemical process, migration of acetoxy group, proceeds on the triplet potential energy surface with a time constant of 220 +/- 30 ps. The transient absorption spectra of the lowest singlet and triplet excited states of 1a, as well as the triplet excited state of the product, 9-acetoxy-2-methoxy-1,10-anthraquinone (2a), were detected. The assignments of the transient absorption spectra were supported by time-dependent DFT calculations of the UV-vis spectra of the proposed intermediates. All of the stationary points for acyl group migration on the triplet and ground state singlet potential energy surfaces were localized, and the influence of the acyl group substitution on the rate constants of the photochemical and thermal processes was analyzed.
利用飞秒瞬态吸收光谱研究了1-乙酰氧基-2-甲氧基蒽醌（1a）光化学中的主要光物理和光化学过程。在270 nm处激发1a会导致一组高激发单重态的布居。内部转换至最低单重态nπ激发态，随后是分子内振动能量重新分布（IVR）过程，其时间常数为150±90 fs。1nπ激发态经历非常快速的系间窜越（ISC，11±1 ps）以形成包含过量振动能量的最低三重态ππ*激发态。振动冷却比ISC稍快（4±1 ps）。主要光化学过程，即乙酰氧基的迁移，在三重态势能面上进行，时间常数为220±30 ps。检测到了1a的最低单重态和三重态激发态以及产物9-乙酰氧基-2-甲氧基-1,10-蒽醌（2a）的三重态激发态的瞬态吸收光谱。瞬态吸收光谱的归属得到了所提出中间体的紫外可见光谱的含时密度泛函理论计算的支持。确定了三重态和基态单重态势能面上酰基迁移的所有驻点，并分析了酰基取代对光化学和热过程速率常数的影响。
