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

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 degrees C, for example, Phi'(ICT) = 3.2 x 10(-3) and Phi(LE) = 1.6 x 10(-4). The large value of Phi'(ICT)/Phi(LE) indicates that the forward LE -> ICT reaction is much faster than the back reaction. The data obtained for the intersystem crossing yield Phi(ISC) show that internal conversion (IC) is the dominant deactivation channel from LE directly to the ground state S-0. For DMPURM in MeCN with Phi(ISC) = 0.22, Phi(IC) = 1 - Phi(ISC) - Phi'(ICT) - Phi(LE) = 0.78, whereas in cyclohexane an even larger Phi(IC) of 0.97 is found. The dipole moment gradually increases upon excitation, from 2.5 D (S-0), via 6 D (LE) to 9 D (ICT) for DMPURM and from 2.3 D (S-0), via 7 D (LE) to 8 D (ICT) for DMPURH. From the temperature dependence of Phi'(ICT)/Phi(LE), a reaction enthalpy - Delta H of 11 kJ/mol is obtained for DMPURM in n-hexane (epsilon(25) = 1.88), increasing to 17 kJ/mol in the more polar solvent di-n-butyl ether (epsilon(25) = 3.05). With DMPURM in diethyl ether, an activation energy of 8.3 kJ/mol is determined for the LE -> ICT reaction (k(a)). The femtosecond excited state absorption spectra at 22 degrees 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 tau(2) increases upon cooling from 2.6 ps at -45 degrees C to 6.9 ps at -95 degrees C. The decay involves ICT and IC as the two main pathways: 1/tau(2) similar or equal to k(a) + k(IC). As a model compound (no ICT) is not available, its lifetime tau(0)(LE) similar to 1/k(IC) 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: S-0 -> LE reversible arrow ICT. With 6-N-methyl-9-methyladenine (MPURM) and 9-methyladenine (PURM), the fluorescence quantum yield is very low (< 5 x 10(-5)) and dominated by impurities, due to enhanced IC from LE to S-0.