Lobsiger Simon, Etinski Mihajlo, Blaser Susan, Frey Hans-Martin, Marian Christel, Leutwyler Samuel
Departement für Chemie und Biochemie, Universität Bern, Freiestrasse 3, CH-3012 Bern, Switzerland.
Fakultet za Fizičku Hemiju, Univerzitet u Beogradu, Studentski Trg 12-16, SRB-11000 Beograd, Serbia.
J Chem Phys. 2015 Dec 21;143(23):234301. doi: 10.1063/1.4937375.
The amino-keto tautomer of supersonic jet-cooled cytosine undergoes intersystem crossing (ISC) from the v = 0 and low-lying vibronic levels of its S1((1)ππ(∗)) state. We investigate these ISC rates experimentally and theoretically as a function of S1 state vibrational excess energy Eexc. The S1 vibronic levels are pumped with a ∼5 ns UV laser, the S1 and triplet state ion signals are separated by prompt or delayed ionization with a second UV laser pulse. After correcting the raw ISC yields for the relative S1 and T1 ionization cross sections, we obtain energy dependent ISC quantum yields QISC (corr)=1%-5%. These are combined with previously measured vibronic state-specific decay rates, giving ISC rates kISC = 0.4-1.5 ⋅ 10(9) s(-1), the corresponding S1⇝S0 internal conversion (IC) rates are 30-100 times larger. Theoretical ISC rates are computed using SCS-CC2 methods, which predict rapid ISC from the S1; v = 0 state with kISC = 3 ⋅ 10(9) s(-1) to the T1((3)ππ(∗)) triplet state. The surprisingly high rate of this El Sayed-forbidden transition is caused by a substantial admixture of (1)nOπ(∗) character into the S1((1)ππ(∗)) wave function at its non-planar minimum geometry. The combination of experiment and theory implies that (1) below Eexc = 550 cm(-1) in the S1 state, S1⇝S0 internal conversion dominates the nonradiative decay with kIC ≥ 2 ⋅ 10(10) s(-1), (2) the calculated S1⇝T1 ((1)ππ(∗)⇝(3)ππ(∗)) ISC rate is in good agreement with experiment, (3) being El-Sayed forbidden, the S1⇝T1 ISC is moderately fast (kISC = 3 ⋅ 10(9) s(-1)), and not ultrafast, as claimed by other calculations, and (4) at Eexc ∼ 550 cm(-1) the IC rate increases by ∼50 times, probably by accessing the lowest conical intersection (the C5-twist CI) and thereby effectively switching off the ISC decay channels.
超声速喷射冷却的胞嘧啶的氨基 - 酮互变异构体在其S1((1)ππ(∗))态的v = 0和低振动电子能级发生系间窜越(ISC)。我们通过实验和理论研究这些ISC速率与S1态振动过剩能量Eexc的函数关系。用一个约5 ns的紫外激光泵浦S1振动电子能级,用第二个紫外激光脉冲通过即时或延迟电离来分离S1和三重态离子信号。在对相对S1和T1电离截面的原始ISC产率进行校正后,我们得到了能量依赖的ISC量子产率QISC(校正)= 1% - 5%。将这些与先前测量的振动电子态特异性衰变率相结合,得到ISC速率kISC = 0.4 - 1.5 ⋅ 10(9)s(-1),相应的S1⇝S0内转换(IC)速率要大30 - 100倍。使用SCS - CC2方法计算理论ISC速率,该方法预测从S1;v = 0态到T1((3)ππ(∗))三重态有快速ISC,kISC = 3 ⋅ 10(9)s(-1)。这种El Sayed禁戒跃迁的惊人高速率是由于在其非平面最小几何构型下,(1)nOπ(∗)特征大量混入S1((1)ππ(∗))波函数中。实验和理论的结合表明:(1)在S1态中Eexc = 550 cm(-1)以下,S1⇝S0内转换以kIC≥2 ⋅ 10(10)s(-1)主导非辐射衰变;(2)计算得到的S1⇝T1((1)ππ(∗)⇝(3)ππ(∗))ISC速率与实验结果吻合良好;(3)作为El - Sayed禁戒的,S1⇝T1 ISC适度快速(kISC = 3 ⋅ 10(9)s(-1)),并非如其他计算所声称的超快;(4)在Eexc ∼ 550 cm(-1)时,IC速率增加约50倍,可能是通过进入最低的锥形交叉点(C5 - 扭曲CI),从而有效地关闭了ISC衰变通道。