DOI: 10.1002/asia.201100472 The Role of the np* 1 A u State in the Photoabsorption and Relaxation of Pyrazine Chih-Kai Lin,* [a, b] Yingli Niu, [a] Chaoyuan Zhu, [a] Zhigang Shuai, [c, d] and Sheng Hsien Lin [a, b] Introduction Pyrazine is a benchmark system in photochemistry that has been widely studied for more than half a century. Being a diazine molecule, this aromatic compound differs from ben- zene by substituting a pair of nitrogen atoms for carbon ones in the para position. The lone-pair electrons reside in the nonbonding orbitals of the nitrogen atoms and thus gen- erate n–p* transitions in addition to p–p* ones upon excita- tion in the valence shell. In terms of symmetry, the ground state is designated as S 0 1 A g of the D 2h point group, and the lowest singlet np* and pp* excited states have conventional- ly been assigned as S 1 1 B 3u and S 2 1 B 2u , respectively. The details in the ultraviolet absorption spectra of pyra- zine were first resolved in solution in 1950s, [1] and the data from vapors were obtained in the following decades. [2–7] People found that the 325 nm absorption band, that is, the np* 1 B 3u state, has a lot of sharp peaks that have been as- signed as n 6a (a g ), n 9a (a g ), n 10a (b 1g ) fundamentals and their overtones as well as combination bands. [3, 4, 6, 7] The 265 nm band, that is, the pp* 1 B 2u state, presents a distinctly differ- ent feature, which is quite intense and broad with few struc- tures, though the crests could be assigned with the aid of the resonant Raman spectrum. [3] The significant difference in in- tensities between the two absorption bands indicates that the 1 B 2u ! 1 A g transition is strongly dipole-allowed, whereas the 1 B 3u ! 1 A g one is just weakly allowed, and the appear- ance of odd-vibrational-quanta peaks of the non-totally- symmetric mode (n 10a b 1g ) in the 1 B 3u band reveals the “in- tensity borrowing” absorption through vibronic coupling. Moreover, the broad feature implies a quite short lifetime of the 1 B 2u state. The lifetimes of 1 B 2u and 1 B 3u states have been recorded as approximately 20 fs and approximately 22 ps, respectively, by using photoelectron spectra. [8–10] Suc- ceeding researchers proposed an ultrafast 1 B 2u ! 1 B 3u relaxa- tion mechanism through conical intersection. Domcke and co-workers firstly carried out ab initio calculations combined with mathematical models to describe this internal conver- Abstract: The geometric, energetic, and spectroscopic properties of the ground state and the lowest four singlet excited states of pyrazine have been studied by using DFT/TD-DFT, CASSCF, CASPT2, and related quan- tum chemical calculations. The second singlet np* state, 1 A u , which is conven- tionally regarded dark due to the dipole-forbidden 1 A u ! 1 A g transition, has been investigated in detail. Our new simulation has shown that the state could be visible in the absorption spectrum by intensity borrowing from neighboring np* 1 B 3u and pp* 1 B 2u states through vibronic coupling. The scans on potential-energy surfaces fur- ther indicated that the 1 A u state inter- sects with the 1 B 2u states near the equi- librium of the latter, thus implying its participation in the ultrafast relaxation process. Keywords: absorption · pyrazines · quantum chemistry · excited states · vibronic coupling [a] Dr. C.-K. Lin, Dr. Y. Niu, Prof. C. Zhu, Prof. S.H. Lin Department of Applied Chemistry National Chiao Tung University Hsinchu 30010 (Taiwan) Fax: (+ 886) 3-5723764 E-mail: ethene@gate.sinica.edu.tw [b] Dr. C.-K. Lin, Prof. S. H. Lin Institute of Atomic and Molecular Sciences Academia Sinica Taipei 10617 (Taiwan) [c] Prof. Z. Shuai Key Laboratory of Organic Optoelectronics and Molecular Engineering Department of Chemistry Tsinghua University Beijing 100084 (P.R. China) [d] Prof. Z. Shuai Key Laboratory of Organic Solids Beijing National Laboratory for Molecular Science Institute of Chemistry Chinese Academy of Sciences Beijing 100190 (P.R. China) Chem. Asian J. 2011, 6, 2977 – 2985 # 2011 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 2977