The conversion of photoexcitations into charge carriers in organic solar cells is facilitated by the dissociation of excitons at the donor/acceptor interface. The ultrafast timescale of charge separation demands sophisticated theoretical models and raises questions about the role of coherence in the charge-transfer mechanism. Here, we apply two-dimensional electronic spectroscopy to study the electron transfer process in poly(3-hexylthiophene)/PCBM (P3HT/PCBM) blends. We report dynamics maps showing the pathways of charge transfer that clearly expose the significance of hot electron transfer. During this ultrafast electron transfer, vibrational coherence is directly transferred from the P3HT exciton to the P3HT hole polaron in the crystalline domain. This result reveals that the exciton converts to a hole with a similar spatial extent on a timescale far exceeding other photophysical dynamics including vibrational relaxation.

We present strong evidence for the oxidation of conjugated polymers in the formation of conjugated polymer dots (CPdots) using Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Although recent studies show that folding of the polymer chain into a compact 3D structure is involved in the formation of these nanoparticles, the process by which these intrinsically hydrophobic nanoscale particles circumvent aggregation in water is still not well understood. Zeta potential results show that these dots have a negatively charged surface at neutral pH, with a zeta potential and surface charge density of approximately -40 mV and (1.39 - 1.70) × 10(-2) C/m(2), respectively. In addition, quantitative elemental analysis of CPdots indicates that oxygen composes 7-13% of these nanoparticles. The overall results support the presence of chemical defects in forming a hydrophilic surface of CPdots. As a consequence, the charged surface contributes to inhibiting the aggregation of CPdots in water, leading to colloidal stability.

The dynamics of charge separation in aqueous suspensions of regioregular P3HT nanoparticles containing PCBM were investigated for the first time using femtosecond transient absorption spectroscopy. This investigation is supported by the recently reported use of regioregular P3HT/PCBM nanoparticles as charge trapping and storage devices. In this study, the presence of excited-state and charge-separated species, including singlet excitons, polymer polarons and free charges, generated in rr-P3HT/PCBM nanoparticles was identified through visible pump and visible/near-infrared probe femtosecond transient absorption spectroscopy at a range of electron acceptor concentrations. The decrease of the singlet exciton lifetime by charge transfer to PCBM is well described by a one-dimensional diffusion model with a P3HT domain size of approximately 5 nm for 5-50 wt % PCBM. This model also indicates that bimolecular recombination is the dominant charge recombination mechanism at 20 wt % PCBM and above.

The morphology dependence of exciton transport in the widely used conjugated polymer poly(3-hexylthiophene) (P3HT) is elucidated by combining an accurate mesoscale coarse-grained molecular dynamics simulation model of P3HT structure with a Frenkel−Holstein exciton model. This model provides a more realistic representation than previously achieved of the molecular-level details of exciton transport on large length scales relevant to electronic applications. One hundred 300-monomer regioregular P3HT chains are simulated at room temperature for microseconds in two implicit solvents of differing solvent quality in which the polymer chains adopt contrasting morphologies: nanofiber-like aggregates or well-separated extended conformations. The model gives reasonable quantitative agreement with steady-state absorption and fluorescence and time-resolved fluorescence experiments, and provides valuable insight into the mechanism of exciton transport in conjugated polymers. In particular, exciton transfer in nanofiber aggregates is shown to occur mainly through interchain hops from chromophores on the aggregate surface toward the aggregate core, a behavior with important implications for organic electronic applications. Furthermore, the counterbalancing effects of greater orientational order and faster exciton transport in nanofiber aggregates than in extended chains are found to explain the puzzling observation of similar fluorescence anisotropy decay rates in nanofibers and free chains.

The use of fluorescent polymers has been reported in studies of scintillators, 1 luminescent solar concentrators, 2 laser-resistant materials, 3 fiber-optic sensors, 4 and laser dyes. 5 Such polymers usually consist of a backbone substituted with fluorophores such as the naphthyl, 6À9 pyrenyl, 7,10À14 and dansyl 8,15À19 entities. The fluorescence intensities and lifetimes of such fluorophores are sensitive to local environment such that steady-state and time-dependent fluorescence studies have provided considerable insight into polymer interactions. 6À19 As part of an exploration of substituent interactions of randomly substituted poly(acrylate)s, PAA, 20À25 we report a UVÀvis, steady-state, and time-resolved fluorescence and 2D 1 H NOESY NMR study of the variation of dansyl substituent spectroscopic response to environmental change at the molecular level in dilute aqueous solution. These observations are used to interpret macroscopic observations gained through rheological studies of variations in the viscosities in terms of network formation in more concentrated solutions. The poly(acrylate)s studied are 3% randomly substituted by N-(2-aminoethyl)-, N-(6-aminohexyl)-, and N-(12-aminododecyl)-5-dansylsulfonamide, PAADSen, PAADShn, and PAADSddn, in which the dansyl substituent tether length progressively increases (Scheme 1). This facilitates studies of the effect of tether length on dansyl substituent, aggregation and complexation by β-cyclodextrin, βCD, and the linked dimer N,N 0 -bis(6 A -deoxy-6 A -β-cyclodextrin)urea, 66βCD 2 ur, in dilute aqueous solution where interactions occur dominantly within individual substituted poly(acrylate) strands.In more concentrated solutions aggregation of dansyl substituents between substituted poly(acrylate) strands occur to form networks which may either be disrupted through βCD forming