Geometrical Dynamics in the Excited States of Thermally Activated Delayed Fluorescence Materials

Abstract

To investigate excited states is important for elucidating mechanisms of photo-functional materials. Here, we have focused on the third-generation organic light emitting diodes (OLED) materials, namely, thermally activated delayed fluorescence (TADF) molecules, from a viewpoint of molecular geometry in the electronically excited states. We investigated four carbazole-benzonitrile (Cz-BN) derivatives that possess identical energy difference between S₁ and T₁ but show distinct TADF activities. We systematically compared their geometrical changes upon photoexcitation using time-resolved infrared (TR-IR) vibrational spectroscopy. We found that the most TADF-active molecule, 4CzBN, shows little structural change after photoexcitation, while the TADF-inactive molecules show relatively large deformation upon S₁−T₁ conversion. This implies that the suppression of structural deformation is critical for minimizing the activation energy barrier for reverse intersystem crossing in cases of the Cz-BN derivatives.