15min:

YASUYUKI KOWAKA, NOLITAKA ASHIZAWA, MASAAKI BABA, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan.

Pyrene is one of the prototypical compact polycyclic aromatic hydrocarbons (PAHs), and It is important to investigate its molecular structure precisely, because it does not conform to Hückel's 4n+2 rule. We analyzed high-resolution and ultrahigh-resolution spectra of jet-cooled pyrene and elucidated the vibrational and rotational structures in the S₀ ¹A_g and S₁ ¹B_3u states. \footnoteM. Baba, Y. Kowaka et al. , J. Chem. Phys. , 131, 224318 (2009) We conclude that the molecule is planar with D_2h symmetry. The rotational constants and vibrational normal energies are very similar for the S₀ and S₁ states, indicating that its geometrical structure and potential energy curves are not changed much upon electronic excitation. This small change is common to large PAH molecules because the changes of bond orders by one electron excitation is diluted with a large number of electrons. The rates of Radiationless transitions in the S₁ state are closely related with the molecular structure and the potential energy curves. Intersystem crossing (ISC) to the triplet state is expected to be very slow in planar PAHs. Internal conversion (IC) to the S₀ state does not occur, if the molecular structure and potential energy curves are identical for the S₀ and S₁ states. In perylene, the fluorescence lifetime is 1400 ns, and the fluorescence quantum yield is considerably high. These properties are attributed to its small changes in molecular structure and potential energy curves upon S₁ S₀ excitation.