Ž. Ž. Dual fluorescence and fast intramolecular charge transfer ICT is observed with 4-diisopropylamino benzonitrile Ž. Ž. DIABN in alkane solvents. The rate constant k for the reaction from the locally excited LE to the ICT state has a value a of 3.4 = 10 11 s y1 in n-hexane at 258C, with an activation energy E of 6 kJ mol y1. Efficient intersystem crossing with a a Ž. yield of 0.94 takes place from the ICT state. With 4-dimethylamino benzonitrile, in contrast, dual fluorescence is not Ž. observed in alkanes. The charge transfer reaction of DIABN is mainly favoured by its small energy gap D E S ,S , in
The fluorescence quenching of two azoalkanes, the weak acceptor 2,3-diazabicyclo[2.2.2]oct-2-ene
and its less reactive derivative 4-methyl-1-isopropyl-2,3-diazabicyclo[2.2.2]oct-2-ene, by a series of aliphatic
amine donors was examined by time-resolved spectroscopy in benzene. The fluorescence quenching rate
constants ranged from 0.13 × 107 to 25 × 107 M-1 s-1, while the activation energies were found to be 1.3 and
3.0 kcal mol-1 for the best (N-ethyldicyclohexylamine) and the poorest (N,N-diisopropyl-3-pentylamine) donor.
The oxidation potentials of the amines and the reduction potentials of the azoalkanes were measured. Deviations
from the expected dependence of the quenching rate constants on the energetics of electron transfer were
observed. Steric effects on the kinetics of quenching, namely a reduction by up to a factor of 50, became
significant for amines with three secondary alkyl substituents, triisopropylamine, and N,N-diisopropyl-3-pentylamine, while a stereoelectronic effect was found for 1,4-diazabicyclo[2.2.2]octane (reduction by a factor
of 2). The experimental data are interpreted within a general kinetic scheme, which involves both exciplex
formation and electron transfer, by assuming a steady-state kinetics for quenching. The steric effects are attributed
to the hindrance toward exciplex formation, based on the experimental results and semiempirical calculations
(PM3). Factors governing steric hindrance by amines, differences between ketone and azoalkane acceptors,
and the persistence of the corresponding amine radical cations are discussed.
e.g., molecules of biochemical interest, in cooperation with Professor A. A. Bothner-By (Pittsburgh). Attention is also being paid to small molecules to study the correspondence with determinations using microwave spectroscopy.42EFNMR studies are at the moment focused on binary mixtures. The addition of a second component to the solution affects the alignment which thus provides information about intermolecular interactions.43,44 In a recent study,44 the change of the alignment upon dilution is interpreted in possible changes of the quadrupole (42)
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