Photoexcitation Dynamics of Thymine in Acetonitrile and an Ionic Liquid Probed by Time-resolved Infrared Spectroscopy Arpan Manna, Seongchul Park, Taegon Lee, and Manho Lim * Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 46241, Korea. *E-mail: mhlim@pusan.ac.kr Received April 20, 2016, Accepted May 4, 2016 Femtosecond transient IR absorption spectroscopy was used to probe the decay mechanism of electroni- cally excited thymine (a naturally occurring pyrimidine base in DNA) dissolved in an ionic liquid ([Bmim][BF 4 ]) or CD 3 CN after the absorption of UV light (267 nm). In both solvents, an absorption band grew on a picosecond timescale, along with decaying bleach and evolving red-shifted absorption signals. A population analysis of the observed kinetic data suggested that most of the photoexcited thymine under- went a sub-picosecond non-radiative relaxation to the vibrationally hot ground electronic state. About 4% (16%) of the excited thymine in the ionic liquid (CD 3 CN) relaxed to an intermediate electronic state, which relaxed into a low-lying triplet state by intersystem crossing (ISC) (ISC did not relax to the ground electronic state within the experimental period (1 ns)). The low ISC yield for thymine in an ionic liquid was correlated with molecular properties of the solvent. This observation is significant because the ISC to triplet state transition for excited thymine has been considered as a precursor to cyclobutane–pyrimidine dimer formation, which led to functional damage of the base after UV absorption. This finding may shed light on the photostability of DNA in ionic liquids. Keywords: Femtosecond infrared spectroscopy, Thermal relaxation, Cyclobutane–pyrimidine dimer, Ionic liquid, Photostability of thymine DNA is a prime biological molecule because it contains genetic instructions for the growth and function of the cells that constitute all living organisms. 1,2 Alterations to the genetic integrity affect normal life processes. 2 In addition to its biological significance, DNA is increasingly used as a powerful nanotechnology tool due to its conformational polymorphism (e.g., as a hybrid catalyst in the synthesis of highly enantioselective and asymmetric molecules). 3–6 DNA structure and stability preservation is hindered by a lack of appropriate media. 7,8 Aqueous solutions are consid- ered an important DNA preserver for short- and long-term applications 8,9 ; however, the low solubility of organic reac- tants in water hinders further efficient use of aqueous solutions. In this context, ionic liquids (ILs) have been established as a unique non-aqueous solvent to preserve DNA for long- term use at ambient temperatures. 10,11 ILs contain an organic cation and an inorganic or organic anion with mini- mal symmetry; they remain liquid below 373 K. 12–15 ILs possess unique green solvent characteristics, such as a neg- ligible vapor pressure, low flammability, wide solubility range, chemical inertness, and wide electrochemical win- dow. 13,16,17 Electrostatic interactions among the IL cation and DNA (i.e., hydrophobic interactions between the hydrocarbon chains of the IL and the bases of DNA) and intermolecular hydrogen bonding between the anion of the IL and the bases of DNA have been established as the main causes for the high stability of DNA in ILs. 11,18–21 So far, examinations of the interactions between DNA and ILs have focused on the ground state of DNA. The preservation of DNA in a molecular solvent and water requires a dark storage environment because harmful UV radiation from sunlight causes DNA structural damage and hinders its use in further applications. 3,22–24 In this con- text, it would be convenient if ILs, which have been identi- fied as a long-term preserver at ambient temperatures, could also be proven as a good solvent to offer photo sta- bility to DNA following its interactions with sunlight or other radiation (especially UVB radiation). 22 Cyclobutane–pyrimidine dimers (CPDs) are major lesions of UV-induced DNA. CPDs refer to the formation of a four-membered ring structure involving the C5 and C6 of two neighboring pyrimidine (thymine or cytosine in DNA) bases (see Figure 1). 25–27 Thymine is more prone to the formation of CPDs than cytosine. 28 Understanding the mechanism of relaxation pathway and the nature of the excited electronic states of the UV-excited thymine base is necessary to interpret the de-excitation mechanism of the whole DNA molecule. 29 The pyrimidine- type bases show strong π ! π* transitions, and this transi- tion is responsible for a strong absorption band in the UV– Vis region. Moreover, the presence of a lone pair on the heteroatom (i.e., the O of the C O bonds in thymine) also contributes to the n ! π* excitations, but this transition is Article DOI: 10.1002/bkcs.10825 A. Manna et al. BULLETIN OF THE KOREAN CHEMICAL SOCIETY Bull. Korean Chem. Soc. 2016 © 2016 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Wiley Online Library 1