Departement für Chemie und Biochemie, Universität Bern, Freiestrasse 3, CH-3012 Bern, Switzerland.
J Chem Phys. 2009 Nov 28;131(20):204308. doi: 10.1063/1.3266937.
We analyze the vibronic band structure of the excitonically coupled S(1)<--S(0)/S(2)<--S(0) excitations of the 2-aminopyridine (2AP) self-dimer (2AP)(2), using a linear vibronic coupling model [R. Fulton and M. Gouterman, J. Chem. Phys. 41, 2280 (1964)]. The vibronic spectra of supersonically cooled (2AP)(2) and its (13)C-isotopomer were measured by two-color resonant two-photon ionization and UV/UV-depletion spectroscopies. In the C(2)-symmetric form of (2AP)(2), the S(1)<--S(0) ((1)A<--(1)A) transition is very weak, while the close-lying S(2)<--S(0) ((1)B<--(1)A) transition is fully allowed. A single (12)C/(13)C isotopic substitution breaks the symmetry of the dimer so that the (2AP)(2)-(13)C isotopologue exhibits both S(1) and S(2) electronic origins, which are split by 11 cm(-1). In Fulton-Gouterman-type treatments, the linear vibronic coupling is mediated by intramolecular vibrational modes and couplings to intermolecular vibrations are not considered. For (2AP)(2), a major vibronic coupling contribution arises from the intramolecular 6a(') vibration. However, the low-energy part of the spectrum is dominated by intermolecular shear (chi(')) and stretching (sigma(')) vibrational excitations that also exhibit excitonic splittings; we apply a linear vibronic coupling analysis for these also. The respective excitation transfer integrals V(AB) are 50%-80% of that of the intramolecular 6a(') vibration, highlighting the role of intermolecular vibrations in mediating electronic energy exchange. The S(1)/S(2) electronic energy gap calculated by the approximate second-order coupled-cluster method is approximately 340 cm(-1). This purely electronic exciton splitting is quenched by a factor of 40 by the vibronic couplings to the Franck-Condon active intramolecular vibrations.
我们利用线性振子耦合模型 [R. Fulton 和 M. Gouterman, J. Chem. Phys. 41, 2280 (1964)] 分析了 2-氨基吡啶（2AP）二聚体（2AP）（2）的激子耦合 S（1）<--S（0）/S（2）<--S（0）激发的振子能带结构。采用双色共振双光子电离和 UV/UV 耗散光谱法测量了超音速冷却的（2AP）（2）及其（13）C 同位素的振子光谱。在（2AP）（2）的 C2 对称形式中，S（1）<--S（0）（（1）A<--（1）A）跃迁非常弱，而近邻的 S（2）<--S（0）（（1）B<--（1）A）跃迁是完全允许的。单个（12）C/（13）C 同位素取代打破了二聚体的对称性，使得（2AP）（2）-（13）C 同位素变体同时显示 S（1）和 S（2）电子起源，它们被分裂 11 cm(-1)。在 Fulton-Gouterman 型处理中，线性振子耦合是通过分子内振动模式介导的，并且不考虑分子间振动的耦合。对于（2AP）（2），主要的振子耦合贡献来自于分子内 6a'（'）振动。然而，光谱的低能部分主要由分子间剪切（chi'）和伸缩（sigma'）振动激发控制，它们也表现出激子分裂；我们也对此进行了线性振子耦合分析。相应的激发转移积分 V（AB）为分子内 6a'（'）振动的 50%-80%，突出了分子间振动在介导电子能量交换中的作用。通过近似二阶耦合簇方法计算的 S（1）/S（2）电子能隙约为 340 cm(-1)。这种纯电子激子分裂被与 Franck-Condon 活性分子内振动的振子耦合猝灭了 40 倍。
