Structural Effects Induced by Interchain Interactions Self Solvation

The general trend observed from the MM3 results is a decrease in the interchain separation as larger numbers of oligomer segments are added. This decrease is on the order of 0.3 A for oligomer segments in the center of a particle and indicates some type of chain-chain self-solvation effect. In order to get a better idea of the change in the distance due to self-solvation we performed limited multiscale modeling (QM/MM) simulations. The simulations were set up by modeling the inner folded MEH-PPV molecule with semiempirical quantum mechanics (AMI model) and the outer chains with molecular mechanics (MM3 model). The outer chains were fixed at a distance of d1 = 3.7 A from the center MEH-PPV molecule in accord with the MM3 results and the MEH-PPV molecule was optimized using AMI. As we are only comparing differences here instead of absolute structure, the AM1 model for the molecule should be reasonable. The results of these QM/MM calculations show a significant decrease in the interchain separation of about 0.9 A. The resulting structure of the MEH-PPV molecule were actually very similar to that obtained from the MM3 calculations (interchain separation of d1 ~ 3.5 A and much smaller torsional rotations about the bonds adjacent to the vinyl group) but differed substantially from the AM1 vacuum results. This provides some interesting evidence that interchain separation of the rod-shaped morphologies tends to decrease toward the center of the single-molecule nanoparticle. As we will show below, the electronic structure depends quite strongly on this interchain distance, with enhanced singlet to singlet transitions moment for shorter distances. This might provide some rationalization of the definitive experimental results that show photon antibunching for single molecule z-oriented nanoparticles [27]. These results show that the z-oriented single molecule nano-particles act as single photon emitters but multichromophore absorbers. The self-solvated inner core structure, where the interchain distance becomes closer and there is much higher degree of structural organization, is probably acting as the primary emission site. From our semiempirical results as well as others, we know that the HOMO-LUMO band gaps in molecular systems are strongly dependent on the degree of confinement, generally decreasing with increasing confinement. For the PPV-based systems, we have observed the following HOMO-LUMO gap dependencies:

1. A decreasing HOMO-LUMO gap with increasing numbers of folded oligomer segments for PPV, CN-PPV, and MEH-PPV.

2. For a fixed oligomer segment length between folds, the magnitude of the HOMO-LUMO gap decrease becomes smaller with increasing number of segments.

3. For a fixed number of oligomer segments but a varying number of monomers in each oligomer, there is also a general decrease in the HOMO-LUMO band gap for the PPV systems.

4. The computed HOMO-LUMO gap dependence on self-solvation as determined for a 14 (one tetrahedral defect) monomer MEH-PPV oligomer is a decrease of ~0.1 eVand an increase in the wavelength for the first excited state transition ~23 nm (this is now a redshift from quantum results). The change in the electronic structure comes primarily from a LUMO lowering of ~0.09 eV. The HOMO, which often shows greater sensitivity to confinement, does not change as much as the LUMO, increasing by ~0.02 eV.

The self-solvation effects noted in the present simulations lead to reasonably large changes in the electronic structure and related optical transitions. It seems plausible to consider these single molecule nanoparticles as dielectric core-shell systems, with the emissive core composed of the self-solvated more ordered and tightly packed chains and the shell as less tightly packed chains (larger interchain separation but still close enough to allow orbital delocalization and nonradiative Forster energy transfer processes). This interpretation, which is backed by the self-solvation computational results, would also allow direct implementation of classical electrostatics arguments due to vacuum field interactions attenuating the fluorescence lifetimes, a property which has been observed experimentally.

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