| Summary: | <p>The excited state structural relaxation of <mml:math alttext="$4$"> <mml:mn>4</mml:mn> </mml:math>-dimethylaminobenzenes with various <emph>para</emph>-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 <mml:math alttext="$7$"> <mml:mn>7</mml:mn> </mml:math>–<mml:math alttext="$112$"> <mml:mn>112</mml:mn> </mml:math>-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 <mml:math alttext="$ext{A} ightarrowext{A}^ast ightarrowext{B}^ast$"> <mml:mrow> <mml:mtext>A</mml:mtext><mml:mo>→</mml:mo><mml:msup> <mml:mtext>A</mml:mtext> <mml:mo>∗</mml:mo> </mml:msup> <mml:mo>→</mml:mo><mml:msup> <mml:mtext>B</mml:mtext> <mml:mo>∗</mml:mo> </mml:msup> </mml:mrow> </mml:math> or <mml:math alttext="$ext{A} ightarrowext{A}^astleftrightarrowext{B}^ast$"> <mml:mtext>A</mml:mtext><mml:mo>→</mml:mo><mml:msup> <mml:mtext>A</mml:mtext> <mml:mo>∗</mml:mo> </mml:msup> <mml:mo>↔</mml:mo><mml:msup> <mml:mtext>B</mml:mtext> <mml:mo>∗</mml:mo> </mml:msup> </mml:math>, where <mml:math alttext="$ext{A}^{ast}$"> <mml:msup> <mml:mtext>A</mml:mtext> <mml:mo>∗</mml:mo> </mml:msup> </mml:math> is the local excited planar and <mml:math alttext="$ext{B}^{ast}$"> <mml:msup> <mml:mtext>B</mml:mtext> <mml:mo>∗</mml:mo> </mml:msup> </mml:math> is the relaxed twisted state of the molecule. The rate constants of the twisted state formation have been calculated in the temperature range <mml:math alttext="$293$"> <mml:mn>293</mml:mn> </mml:math>–<mml:math alttext="$77$"> <mml:mn>77</mml:mn> </mml:math> K. The activation energies of forward process for compounds studied have been estimated.</p>
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