Excited-state backbone twisting of polyfluorene as detected from photothermal after-effects
Journal
Journal of Physical Chemistry B
Journal Volume
113
Journal Issue
25
Pages
8527-8531
Date Issued
2009
Author(s)
Chen, H. L.
Huang, Y. F.
Lim, T. S.
Su, C. H.
Chen, P. H.
Su, A. C.
Chao, T. C.
Chan, S. I.
Fann, W.
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.
SDGs
Type
journal article