Analysis of the excited-state absorption spectral bandshape of oligofluorenes

作者：Hayes Sophia C^*; Silva Carlos

来源：Journal of Chemical Physics, 2010, 132(21): 214510.

DOI：10.1063/1.3432602

• conducting polymers
• excited states
• ground states
• organic compounds
• spectral line breadth
• vibrational states
• vibronic states
• visible spectra

We present ultrafast transient absorption spectra of two oligofluorene derivatives in dilute solution. These spectra display a photoinduced absorption band with clear vibronic structure, which we analyze rigorously using a time-dependent formalism of absorption to extract the principal excited-state vibrational normal-mode frequencies that couple to the electronic transition, the configurational displacement of the higher-lying excited state, and the reorganization energies. We can model the excited-state absorption spectrum using two totally symmetric vibrational modes with frequencies 450 (dimer) or 400 cm(-1) (trimer), and 1666 cm(-1). The reorganization energy of the ground-state absorption is rather insensitive to the oligomer length at 230 meV. However, that of the excited-state absorption evolves from 58 to 166 meV between the oligofluorene dimer and trimer. Based on previous theoretical work [A. Shukla , Phys. Rev. B 67, 245203 (2003)], we assign the absorption spectra to a transition from the 1B(u) excited state to a higher-lying mA(g) state, and find that the energy of the excited-state transition with respect to the ground-state transition energy is in excellent agreement with the theoretical predictions for both oligomers studied here. These results and analysis permit profound understanding of the nature of excited-state absorption in pi-conjugated polymers, which are the subject of general interest as organic semiconductors in the solid state.

• 出版日期2010-6-7