Dual Fluorescence and Ultrafast Intramolecular Charge Transfer with 6-N,N-Dialkylaminopurines. A Two-State Model

详细信息    查看全文

• 作者：Attila Demeter ; Sergey I. Druzhinin ; Sergey A. Kovalenko ; Tamara A. Senyushkina ; Klaas A. Zachariasse
• 刊名：Journal of Physical Chemistry A
• 出版年：2011
• 出版时间：March 10, 2011
• 年：2011
• 卷：115
• 期：9
• 页码：1521-1537
• 全文大小：1473K
• 年卷期：v.115,no.9(March 10, 2011)
• ISSN：1520-5215

文摘

6-N,N-Dimethyl-9-methyladenine (DMPURM) and 6-N,N-dimethyladenine (DMPURH) show dual fluorescence from a locally excited (LE) and an intramolecular charge transfer (ICT) state in solvents of different polarity over extended temperature ranges. The fluorescence quantum yields are very small, in particular those of LE. For DMPURM in acetonitrile (MeCN) at 25 掳C, for example, 桅鈥?ICT) = 3.2 脳 10^鈭? and 桅(LE) = 1.6 脳 10^鈭?. The large value of 桅鈥?ICT)/桅(LE) indicates that the forward LE 鈫?ICT reaction is much faster than the back reaction. The data obtained for the intersystem crossing yield 桅(ISC) show that internal conversion (IC) is the dominant deactivation channel from LE directly to the ground state S₀. For DMPURM in MeCN with 桅(ISC) = 0.22, 桅(IC) = 1 鈭?桅(ISC) 鈭?桅鈥?ICT) 鈭?桅(LE) = 0.78, whereas in cyclohexane an even larger 桅(IC) of 0.97 is found. The dipole moment gradually increases upon excitation, from 2.5 D (S₀), via 6 D (LE) to 9 D (ICT) for DMPURM and from 2.3 D (S₀), via 7 D (LE) to 8 D (ICT) for DMPURH. From the temperature dependence of 桅鈥?ICT)/桅(LE), a reaction enthalpy 鈭捨?i>H of 11 kJ/mol is obtained for DMPURM in n-hexane (蔚²⁵ = 1.88), increasing to 17 kJ/mol in the more polar solvent di-n-butyl ether (蔚²⁵ = 3.05). With DMPURM in diethyl ether, an activation energy of 8.3 kJ/mol is determined for the LE 鈫?ICT reaction (k_a). The femtosecond excited state absorption spectra at 22 掳C undergo an ultrafast decay: 1.0 ps in CHX and 0.63 ps in MeCN for DMPURM, still shorter (0.46 ps) for DMPURH in MeCN. With DMPURM in n-hexane, the LE fluorescence decay time 蟿₂ increases upon cooling from 2.6 ps at 鈭?5 掳C to 6.9 ps at 鈭?5 掳C. The decay involves ICT and IC as the two main pathways: 1/蟿₂ k_a + k_IC. As a model compound (no ICT) is not available, its lifetime 蟿₀(LE) 1/k_IC is not known, which prevents a separate determination of k_a. The excited state reactions of DMPURM and DMPURH are treated with a two-state model: S₀ 鈫?LE ICT. With 6-N-methyl-9-methyladenine (MPURM) and 9-methyladenine (PURM), the fluorescence quantum yield is very low (<5 脳 10^鈭?) and dominated by impurities, due to enhanced IC from LE to S₀.