Intramolecular charge transfer of 4-(dimethylamino)benzonitrile probedby time-resolved fluorescence and transient absorption: No evidencefor two ICT states and a π~* reaction intermediate

摘要

For the double exponential fluorescence decays of the locally excited (LE) and intramolecularcharge transfer (ICT) states of 4-(dimethylamino)benzonitrile (DMABN) in acetonitrile (MeCN) the same times τ_1and τ_2are observed. This means that the reversible LEICT reaction, starting fromthe initially excited LE state, can be adequately described by a two state mechanism. The most important factor responsible for the sometimes experimentally observed differences in thenanosecond decay time, with τ_1(LE) < τ_1(ICT), is photoproduct formation. By employing a global analysis of the LE and ICT fluorescence response functions with a time resolution of 0.5 ps/channelin 1200 channels reliable kinetic and thermodynamic data can be obtained. The arguments presented in the literature in favor of a πσ~* state with a bent CN group as an intermediate in the ICT reactionof DMABN are discussed. From the appearance of an excited state absorption (ESA) band in the spectral region between 700 and 800 nm in MeCN for N,N-dimethylanilines with CN, Br, F, CF_3, and C(=O)OC_2H_2p-substituents, it is concluded that this ESA band cannot be attributed to a πσ~*state, as only the C–C≡N group can undergo the required 120° bending.