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Excited-State Backbone Twisting of Polyfluorene As Detected from Photothermal After-Effects

2009; American Chemical Society; Volume: 113; Issue: 25 Linguagem: Inglês

10.1021/jp901556v

1520-6106

Hsin‐Liang Chen, Yida Huang, Tsong‐Shin Lim, Chun‐Jen Su, P. H. Chen, A. C. Su, Ken‐Tsung Wong, Tzu Cheng Chao, Shu‐Hua Chan, Wunshain Fann,

Advanced Chemical Physics Studies

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° upon excitation, in agreement with the shift of rotational potential minimum from ±40° (and ±140°) in the ground state to ±20° (and ±160°) in the first excited singlet state according to results of quantum mechanical calculations.