Vibronic coupling in quantum wires: Applications to polydiacetylene

Abstract

A theory describing vibronic coupling in direct band gap, one-dimensional semiconductors is developed to account for the photophysical properties of isolated, defect-free conjugated polymers. A Holstein-like Hamiltonian represented in a multi-particle basis set is used to evaluate absorption and emission due to Wannier-Mott excitons. The photophysical properties of such quantum wires are shown to strongly resemble those of Frenkel exciton J-aggregates. The 1(1)B(u) exciton coherence length and effective mass are readily determined from the ratio of the 0-0 and 0-1 line strengths, I(0 - 0)/I(0 - 1), in the photoluminescence spectrum. I(0 - 0)/I(0 - 1) is shown to follow a T(-1/2) dependence, in an excellent agreement with experiments on the red-phase of polydiacteylene.

Two-dimensional spatial coherence of excitons in semicrystalline polymeric semiconductors: Effect of molecular weight

“…In Ref. 31 it was also shown that the 0-0/0-1 line strength ratio in the PL spectrum of the single chain increases with intrachain exciton bandwidth -as expected for a J-aggregate -while the 0-2/0-1 ratio decreases. The behavior can be crudely described as a decrease in the effective single chain Huang-Rhys factor with increasing exciton coupling, although the relative intensities are generally not Poissonian.…”

“…Hence, within a π-stack of such chains, the exciton is delocalized over two dimensions; along the polymer backbone as well as along the stacking axis. Because the Wannier-Mott excitons in one dimension lead to J-aggregate photophysics, 31 the aggregate can be understood as J-like along the backbone dimension and H-like along the stacking direction, allowing for interesting hybrid H-J photophysical properties. If disorder is absent, as was the case treated in Ref.…”

“…A compelling example of this limit is provided by isolated, extended chains of red-phase polydiacetylene derivatives, 27 which show ultra narrow photoluminescence linewidths and superradiance [28][29][30] characteristic of J-aggregates at low temperature. 31 The work by Niles et al 1 therefore demonstrates that in regioregular P3HT films, there is a competition between J-like (intrachain) and H-like (interchain) excitonic coupling. 32 This interplay between through-bond and through-space excitonic coupling of adjacent chromophores must be strongly influenced by energetic disorder and hence the macromolecular conformation and packing in the solid state.…”

Using coherence to enhance function in chemical and biophysical systems

“…As well as modifying spectral band progressions, vibronic coupling can localize excitation or charge. The manifestation of vibronic coupling in spectra of molecular aggregates can therefore be used to measure exciton delocalization by relating the mean delocalization length of excitons to the ratio of the electronic photoluminescence band intensity (I 0-0 ) and the first vibronic band (I 0-1 ) 50,51 . Analysis of the photoluminescence ratio reveals the extraordinary coherent delocalization of the exciton along disorder-free polydiacetyelene chains 50,52 (Fig.…”

Determining the spatial coherence of excitons from the photoluminescence spectrum in charge-transfer J-aggregates

“…demonstrated the idea experimentally by utilizing time-resolved PL to monitor subpicosecond exciton localization in perylene diimide aggregates [33]. The PL ratio has also been used to justify the ultra-large coherence lengths measured in practically defect-free polyacetylene chains, also referred to as quantum wires [34,35]; to determine N coh in polyacenes crystals [31,36] and dinaptho-thieno-thiophene (DNTT) crystals [37]; and to monitor coherence in a phosphorescent Pt-containing polymer. [38] In this work we extend our analysis of the PL ratio to include aggregates with significant intermolecular charge transfer.…”