Abstract
The excited state structural relaxation of 4‐dimethylaminobenzenes with various para‐acceptor substituents having double‐band emission, local excited (LE) and charge transfer (CT), has been investigated. Fluorescence measurements at different temperatures and in different solvents have confirmed the existence of viscosity‐dependent, temperature, and polarity‐activated relaxation. The kinetics analysis has shown that the radiative deactivation rate constants of the individual LE and CT states differ by 7–112‐fold. The dipole moment changes at the excitation for CT states are significantly larger than those for LE states. The spectral‐kinetics behavior of compounds studied agrees with the models A → A∗ → B∗ or A → A∗↔B∗, where A∗ is the local excited planar and B∗ is the relaxed twisted state of the molecule. The rate constants of the twisted state formation have been calculated in the temperature range 293–77 K. The activation energies of forward process for compounds studied have been estimated.
Highlights
At the present time, the behavior of 4-(dimethylamino)benzonitrile (DMABN) and DMABN-relative compounds at the excitation has been studied extensively
Most of the researchers share the judgement about the dominating contribution of twisted intramolecular charge transfer (TICT) deactivation mechanism for them
Since I–V have the identical donor and the different acceptors, it is reasonable to compare the variations in the areas under longwavelength (SCT) and shortwavelength (SLE) bands at the temperature decrease with the corresponding values of the first ionization potential (IP) of the acceptors
Summary
The behavior of 4-(dimethylamino)benzonitrile (DMABN) and DMABN-relative compounds at the excitation has been studied extensively. The full charge separation in excited DMABN is confirmed by the transient absorption spectra, which match the absorption spectra of acceptor radical anion [1]. All DMABN-related rigid molecules show no CT fluorescence band and ICT does not occur in crystalline DMABN [4]. There are evidences of strong viscosity [6] and polarity [7] influence on the rate of the excited state ICT reaction. It is interesting to investigate the effect of para-acceptor substituent on the excited state structural relaxation (ESSR). The focus of the present work is on the quantitative characterization of the dynamics of excited state intramolecular structural relaxation of DMABN and its related compounds with para-acceptor substituents
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