A combination of theory and experiment is used to identify a novel variable excitonic coupling in a series of building blocks for small phenylacetylene dendrons. Systematic changes in the experimental emission spectra, radiative lifetimes, and polarization anisotropies as the number of meta-conjugated branches increases provide evidence for a qualitative change in the electronic structure in the relaxed excited state. The excited state electronic structure is investigated theoretically using ab initio CASSCF and CASPT2 calculations, which indicate the presence of large electronic coupling in the emitting geometry that is not seen for the absorbing geometry of the same molecules. The changes in electronic structure that occur upon excited-state relaxation can be understood in terms of a variable excitonic coupling between the phenylactylene branches, which takes these molecules from the weak coupling to the strong coupling regime as they relax on the excited state. The origin of this geometry-dependent coupling is investigated through the interpretation of ab initio calculations in terms of Fo ¨rster, Dexter, and through-bond charge-transfer interactions. We find that the change in the coupling arises primarily from an increase in the through-bond or charge-transfer component of the coupling, despite the absence of large changes in charge distribution. A theoretical comparison of metaversus para-substituted phenylacetylenes clarifies why this effect is so pronounced in the meta-substituted molecules.
Energy transfer in the conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) is investigated using femtosecond degenerate pump-probe experiments at 298 and 4 K. The polarization anisotropy decays are of the form exp [-(t) 1/2 /T pol ], as predicted by theories of energy transfer in dilute chromophoric systems. At 4 K, these decays depend on the excitation wavelength, with T pol ) 26 fs -1/2 at the peak of the absorption (520 nm) and T pol ) 78 fs -1/2 at the low-energy side of the absorption (580 nm). This wavelength dependence becomes less pronounced at higher temperatures but is always present. We find that models for Förster energy transfer in dilute chromophore solutions cannot describe our data using a single energy-transfer rate calculated from the Förster overlap of the steady-state absorption and emission spectra. The Förster radius R 0 obtained from fitting the experimental anisotropy decays does not agree with that obtained from the steady-state absorption and fluorescence spectra. This fact, along with the wavelength dependence of the anisotropy decays, indicates that the steady-state spectral properties alone are insufficient to explain the energy-transfer properties of MEH-PPV. By use of a simple model to account for inhomogeneous broadening, vibrational line shape, and the intramolecular Stokes shift, we obtain semiquantitative agreement with the experimental results. The key quantity in this modeling is the ratio of inhomogeneous to homogeneous broadening. As the temperature increases, this ratio decreases, leading to less wavelength dependence in the anisotropy decays. The agreement between our modeling and the data suggests that models developed to describe incoherent energy transfer in dilute solutions may be useful for predicting the energy-transport properties of amorphous conjugated polymers, as long as they are modified to take factors such as vibrational structure and inhomogeneous broadening into account.
Traditional pictures of optical properties in phenylacetylene dendrimers view the molecule as a collection of independent chromophores, linked by meta-substitution at the central phenyl ring. While this picture is reasonable for explaining the observed absorption trends, it breaks down in describing the emission behavior. We utilize a combination of ab initio theory and experiment to demonstrate that differences in the absorbing and emitting states can be described using an exciton model with very weak chromophore coupling for the absorption geometry and strong coupling for the emission geometry. This result may have significant implications for the design of energy-funneling dendrimeric molecules.
The problem of electronic energy transfer in a network of two-level systems coupled to a single trapping site is investigated using a simple Haken-Strobl model with diagonal disorder. The goal is to illustrate how the trapping time T(trap), coherence time T(d), and molecular topology all affect the overall efficiency of a light-harvesting network. Several issues are identified that need to be considered in the design of an optimal energy transfer network, including the dephasing-induced decoupling the trap from the rest of the network, the nonlinear dependence of trapping rate on the coherence time, and the role of network size and connectivity in determining the effect of the coherence time on efficiency. There are two main conclusions from this work. First, there exists an optimum combination of trapping time and coherence time, which will give the most rapid population transfer to the trap. These values are not in general the shortest trapping time and the longest coherence time, as would be expected based on rate equation models and/or simple considerations from previous analytical results derived for the Haken-Strobl model in an infinite system. Second, in the coherent regime, where T(d) is longer than the other relevant timescales, population trapping in a finite system can be suppressed by quantum interference effects, whose magnitude is sensitive to the molecular geometry. Suggestions for possible methods of observing such effects are discussed. These results provide a qualitative framework for quantum coherence and molecular topology into account for the design of covalent light-harvesting networks with high energy transfer efficiencies.
A femtosecond pump−dump−probe anisotropy experiment is used to study the time-dependent motion of singlet excitons in the conjugated polymer poly[2-methoxy-5-((2-ethylhexyl)oxy)-1,4-phenylenevinylene] (MEH−PPV). The pump pulse prepares an unpolarized population of excitons at time t = 0, which is then depleted by a linearly polarized dump pulse at t = T 12. The anisotropy decay of the remaining population is then monitored as a function of the probe delay, T 23. At both room temperature and 4 K, the T 23 anisotropy decay is observed to slow as T 12 increases. Although interpretation of our results is complicated by the possible presence of excited-state absorption at 600 nm, the wavelength of the dump and probe pulses, the T 12 dependence is consistent with a slowing down of the diffusion of the luminescent exciton during its lifetime in the material. This experiment provides a way to directly probe anomalous diffusion of the exciton at different points during its lifetime, which is what ultimately determines the distance it can travel in the polymer.
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