There is mounting evidence that long-range charge separation determines the efficiency of organic photovoltaic cells, yet different mechanisms remain under debate. One class of proposed mechanism is ultrafast coherent long-range charge separation, and another is a slower process whereby charges incoherently hop apart with a transiently enhanced mobility due to morphology and disorder. Here, we use transient absorption spectroscopy to probe incoherent charge separation dynamics in two different ways. First, we use a family of polymers whose backbone structures allows us to compare 2- phase donor-acceptor morphologies with 3-phase morphologies that feature an intermixed region. In the 3-phase system, we see pronounced spectral signatures associated with charges (holes) occupying the disordered intermixed region, and we track separation via biased charge diffusion to more ordered neat regions on the timescale of hundreds of picoseconds. Secondly, by resolving bimolecular charge recombination at high excitation density, we show that charge mobilities must be substantially enhanced on early timescales, which may be sufficient for separation to occur. Together, these measurements provide support for models of incoherent and relatively slow charge separation.
Proc. SPIE. 9523, International Conference on Nano-Bio Sensing, Imaging, and Spectroscopy 2015
KEYWORDS: Energy efficiency, Polymers, Luminescence, Molecules, Energy transfer, Resonance energy transfer, Picosecond phenomena, Time resolved spectroscopy, Fluorescence resonance energy transfer, Absorption
We have investigated that organic solvent (DMSO, dimethyl sulfoxide) modifies energy transfer efficiency between conjugated polymers (donors) and fluorescein-labeled single stranded DNAs (acceptors). In a mixture of buffer and organic solvent, fluorescence of the acceptors is significantly enhanced compared to that of pure water solution. This result can be attributed to change of the donor-acceptor environment such as decreased hydrophobicity of polymers, screening effect of organic solvent molecules, resulting in an enhanced energy transfer efficiency. Time-resolved fluorescence decay of the donors and the acceptors was modelled by considering the competition between the energy harvesting Foerster resonance energy transfer and the energy-wasting quenching. This enables to quantity that the Foerster distance (R<sub>0</sub> = 43.3 Å) and resonance energy transfer efficiency (E<sub>FRET</sub> = 58.7 %) of pure buffer solution become R<sub>0</sub> = 38.6 Å and E<sub>FRET</sub> = 48.0 % when 80% DMSO/buffer mixture is added.
Solvent effects were studied in fluorescence resonance energy transfer (FRET) from a cationic polyfluorene copolymer
(FHQ, FPQ) to a fluorescein (Fl)-labeled oligonucleotide (ssDNA-Fl). Upon addition of dimethyl sulfoxide (DMSO),
optical properties of the polymers and the probe dye were substantially modified. And the FRET-induced Fl emission
was measured by directly exciting the polymer within the complex, polymer/ssDNA-Fl. The FRET signal was
successfully modulated with changing the DMSO content. In the case of FHQ, the FRET-induced Fl emission was
seriously quenched in phosphate buffer solution (PBS), while a salient FRET signal was observed in a 80 vol%
DMSO/PBS mixture (36.8 time higher than that in PBS). The FPQ-sensitized FRET signal was also 3.8-fold amplified
by the presence of DMSO. That result is from the decrease of hydrophobic interactions between the polymer and
ssDNA-Fl, which induces the weaker polymer/ssDNA-Fl complexation with longer intermolecular separation. The
gradual decrease in Fl PL quenching with increasing the DMSO content was investigated by measuring the Stern-
Volmer quenching constants (3.3-4.2 × 10<sup>6</sup> M<sup>-1</sup> in PBS, 0.56-1.1 x 10<sup>6</sup> M<sup>-1</sup> in 80 vol% DMSO) in PBS/DMSO mixtures.
The substantially reduced PL quenching would amplify the resulting FRET Fl signal. This approach suggests a simple
way of modifying the fine-structure of polymer/ssDNA-Fl and improving the detection sensitivity in conjugated
polymer-based FRET bioassays.
A new fully functionalized photorefractive polyimine was prepared by the condensation polymerization between a photoconducting carbazole derivative, 9-(2- ethylhexyl)carbazole-3,6-dicarbaldehyde, and a nonlinear optical stilbene chromophore, 4-[N,N-bis(2- aminoethyl)amino]-4'-nitrostilbene. It showed excellent solubility in common organic solvents such as chloroform, cyclohexanone, tetrahydrofuran, etc. and high optical quality films were obtained by free standing film casting. Very high second order optical nonlinearity with d<SUB>33</SUB> equals 120 pm/V was observed by second harmonic generation at the fundamental 1064 nm wavelength. In demonstrated good thermal stability of the aligned dipoles by electric field up to ca. 125 degrees C. The diffraction grating was formed by the interference of two coherent Ar-ion laser beam at the wavelength of 488 nm. A holographic diffraction efficiency of about 15 percent has been achieve din a 10 micrometers -thick film. Storage state of our film shows remarkably long stability at room temperature.