doi:10.1016/j.cplett.2004.10.155 
Copyright © 2004 Elsevier B.V. All rights reserved.
A joint theoretical and experimental study of phenylene–acetylene molecular wires
R.J. Magyara, S. Tretiaka, 
, 
, Y. Gaob, H.-L. Wangb and A.P. Shreveb
aTheoretical Division and Center for Nonlinear Studies, Los Alamos National Laboratory, Mail Stop B268, Los Alamos, NM 87545, USA
bBioscience Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
Received 2 October 2004;
revised 27 October 2004.
Available online 30 November 2004.
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Abstract
The excited state electronic structure of π conjugated phenylene–acetylene oligomers is calculated using time-dependent density functional theory approaches. The theoretical fluorescence spectra are analyzed in terms of Frank–Condon active nuclear normal modes and shown to compare well with experiment. Theoretical and experimental results for the optical absorption and emission spectra of these molecules indicate that the conjugation length can be significantly reduced by conformational rotations about the triple-bonded carbon links. This has serious implications on the electronic functionalities of polyphenylene–acetylene based molecular wires and their possible use as charge/energy conduits in nano-assemblies.
Fig. 1. Top: molecular structure of phenylene–acetylene and schematic structures of the planar and alternating geometries, respectively. Middle: scaling of the energy of the singlet excited state which dominates UV–Visible absorption as a function of the number of polymer repeat units. Theoretical values correspond to the vertical excitation and emission, and experimental values are taken for the absorption and emission maxima. Bottom: same as above but for fluorescence.
Fig. 2. Absorption and fluorescence line shapes for various length phenylene–acetylene oligomers; experiment vs. theory. The top two panels are the experimental absorption and fluorescence. The bottom two panels are the theoretical emission profiles calculated with broad (0.2 eV) and narrow (0.02 eV) spectral line width. Frank–Condon active vibrational normal modes (II–IV) are shown in Fig. 3.
Fig. 3. The dominant normal modes contributing to the vibrational structure of the fluorescence spectrum for phenylene–acetylene. The top diagram is a low energy stretching mode contributing to the spectral broadening. The next two diagrams show vibrations contributing to the II–III peak, which becomes resolved as the polymer length increases; these are oscillations in the length of the bonds within the benzene rings. The final mode is a high energy oscillation in the length of the triple bond, which contributes to the IV peak.
Fig. 4. Top: typical electronic structure of optically active states in singlet and triplet manifolds for conjugated polymers. Bottom: size-scaling of calculated excitation energies of T1, S1, Tn, and Sn states, as a function of the reciprocal conjugation length for phenylene–acetylene oligomers.
Fig. 5. Selected transition orbitals for 2, 4, and 10-unit phenylene–acetylene chains in the planar and alternating ground state geometries. These are calculated at the B3LYP/6-31G level. The NH2 end is to the right.
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