Related Experiment Video
Updated: Jun 22, 2026

Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals
Published on: May 29, 2018
Excited-state backbone twisting of polyfluorene as detected from photothermal after-effects
1Department of Physics, National Taiwan University, Taipei 106, Taiwan.
Abstract:
By means of time-resolved photoluminescence and photothermal techniques, after-effects from excited-state dynamics, energy migration, and conformational rearrangement of poly(9,9-di-n-octyl-2,7-fluorene) (PFO) and its homologues has been examined and interpreted with rotational potential maps from quantum mechanical calculations. Steady-state photoluminescence spectral changes and time-resolved photoluminescence measurements of oligofluorenes and PFO diluted in toluene suggest excited state ring torsion occurring within 30 ps of photoexitation. With all effects from internal conversion/intersystem crossing processes properly accounted for, we show that the conformational changes associated with this twisting motion can be quantitatively probed by means of photothermal methods. Results suggest mean torsion between neighboring fluorene units by ca. 40 degrees upon excitation, in agreement with the shift of rotational potential minimum from +/-40 degrees (and +/-140 degrees) in the ground state to +/-20 degrees (and +/-160 degrees) in the first excited singlet state according to results of quantum mechanical calculations.
More Related Videos
Related Concept Videos
Thermal and Photochemical Electrocyclic Reactions: Overview
Photochemical Electrocyclic Reactions: Stereochemistry
Selection Rules: Photochemical Activation
Thermal Electrocyclic Reactions: Stereochemistry
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
UV–Vis Spectroscopy: Molecular Electronic Transitions
¹H NMR of Conformationally Flexible Molecules: Temporal Resolution
π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

