doi:10.1016/S0009-2614(03)00419-6 
Copyright © 2003 Elsevier Science B.V. All rights reserved.
Electron-vibrational relaxation of photoexcited polyfluorenes in the presence of chemical defects: A theoretical study
Ignacio Francoa, b and Sergei Tretiak
, 
, a
a Theoretical Division and Center for Nonlinear Studies, Los Alamos National Laboratory, Mail stop B268, Los Alamos, NM 87545, USA
b Abdus Salam International Centre for Theoretical Physics, Trieste 34014, Italy
Received 26 February 2003.
Available online 1 April 2003.
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Abstract
A quantum chemical semiempirical investigation of electron-vibrational dynamics of photoexcited conjugated polyfluorenes shows that delocalized electronic excitations dominate the absorption, whereas chemical defects (if present) dramatically impact the emission by trapping the photogenerated exciton into a localized state and acting as guest emitters at recombination. These results offer theoretical insight into the effect of non-quenching defect sites in conjugated polymers and explain the origin of a controversial low energy emission band frequently observed in bulk polyfluorene samples.
Fig. 1. (a) Structure of polyfluorene. (b) Schematic representation of photoexcitation dynamics. ΩA and ΩF are vertical absorption and fluorescence transition energies, respectively; Eg(q) and Ee(q) represent the energy as a function of nuclear coordinates q for the ground and excited states, respectively. (c) Excimer formation due to aggregation (upper scheme) and chemical keto defects (lower scheme) are proposed hypotheses for the origin of the yellow emission in PF.
Fig. 2. (a) Structures of undoped (O1) and keto-doped (O2) PF oligomers. Variation of (b) the dihedral angle and (c) bond length alternation parameter (defined as d2−(d1+d3)/2) along the oligomer chain for the ground and excited state relaxed geometries in O1 and O2. X-axis labels consecutive monomers (half-integers correspond to positions between monomers) as shown in (a).
Fig. 3. Variation of vertical absorption and fluorescence transition frequencies (left panel) and their corresponding oscillator strengths (right panel) with the number of repeat units in pristine PF oligomers.
Fig. 4. Contour plots of transition density matrices of O1 and O2. (a)–(c) represent transitions from the ground state (equilibrium geometry) to the lowest excited state (third excited state for (c)) in O1 and O2 and correspond to vertical absorption. (a′), (b′) represent the same quantities as (a) and (b) but computed at the excited state equilibrium geometries, and correspond to vertical fluorescence. The axis labels represent the individual atoms in the numbering sequence shown in Fig. 2a. The color code is given in the lower right corner. Each plot depicts probabilities of an electron moving from one molecular position (horizontal axis) to another (vertical axis) upon electronic excitation.
Table 1. Computed vertical absorption ΩA and fluorescence ΩF transition energies for O1 and O2
The corresponding oscillator strengths are given in parentheses. Experimental values correspond to the maximum of the absorption/emission profiles reported in [6].
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