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

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journal contribution
posted on 10.03.2011, 00:00 by Attila Demeter, Sergey I. Druzhinin, Sergey A. Kovalenko, Tamara A. Senyushkina, Klaas A. Zachariasse
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 °C, for example, Φ′(ICT) = 3.2 × 10−3 and Φ(LE) = 1.6 × 10−4. The large value of Φ′(ICT)/Φ(LE) indicates that the forward LE → ICT reaction is much faster than the back reaction. The data obtained for the intersystem crossing yield Φ(ISC) show that internal conversion (IC) is the dominant deactivation channel from LE directly to the ground state S0. For DMPURM in MeCN with Φ(ISC) = 0.22, Φ(IC) = 1 − Φ(ISC) − Φ′(ICT) − Φ(LE) = 0.78, whereas in cyclohexane an even larger Φ(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 Φ′(ICT)/Φ(LE), a reaction enthalpy −ΔH of 11 kJ/mol is obtained for DMPURM in n-hexane (ε25 = 1.88), increasing to 17 kJ/mol in the more polar solvent di-n-butyl ether (ε25 = 3.05). With DMPURM in diethyl ether, an activation energy of 8.3 kJ/mol is determined for the LE → ICT reaction (ka). The femtosecond excited state absorption spectra at 22 °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 τ2 increases upon cooling from 2.6 ps at −45 °C to 6.9 ps at −95 °C. The decay involves ICT and IC as the two main pathways: 1/τ2ka + kIC. As a model compound (no ICT) is not available, its lifetime τ0(LE) ∼ 1/kIC is not known, which prevents a separate determination of ka. The excited state reactions of DMPURM and DMPURH are treated with a two-state model: S0 → LE ⇄ ICT. With 6-N-methyl-9-methyladenine (MPURM) and 9-methyladenine (PURM), the fluorescence quantum yield is very low (<5 × 10−5) and dominated by impurities, due to enhanced IC from LE to S0.