Spectroscopic exploration of conformational heterogeneity in an amorphous polyphenylene-vinylene derivative.

摘要

Photoluminescence and Resonant Raman spectroscopy were performed on the conjugated polymer MDMO-PPV (Poly[2-methoxy-5-(3&feet;,7&feet;-dimethyloctyloxy)-1,4-phenylenevinylene]), both in its pure form and in blends with electron acceptors, in order to understand how conformations are affected by interactions with neighboring polymer chains and small molecules. Thermal and solvent annealing are used to tune these interactions, and the resulting changes in the electronic structure and processes of the polymer are inferred through interpretation of spectroscopic signatures.;Optical absorption and emission spectra were measured for MDMO-PPV in several solvents, in thin films processed under various conditions, and in nanoparticle form. These different environments and processing conditions can encourage the polymer to coil (bad solvent), extend (good solvent), strongly interact with neighboring molecules (nanoparticles), or interact weakly (dilute solution), and feature the polymer in a more glassy (spin-cast) or more ordered (thermally annealed) state. We can then interrogate a defined conformation of the polymer, and correlate the spectroscopic and structural changes. Analysis of the lineshape parameters of emission and absorption spectra reveal a relationship between the intensity and spacing of vibronic sidebands, and planarity of the conjugated backbone. Planarity is extremely important in conjugated polymers, as it defines the extent of electron delocalization and has serious consequences on every electronic process in the polymer. As well, solvatochromic shifts in the emission spectrum reveal information about the relative polarities of the ground and excited states.;Resonant Raman spectroscopy was used to identify the vibrational modes coupled to electronic excitation, and mode-specific displacements were estimated by modeling the photoluminescence spectra using a time-dependent quantum mechanical simulation to reproduce experimental observations. Coupling of electronic processes to vibrational modes in organic photovoltaics both helps and hinders function. It broadens absorption and emission spectra, increasing the ability to absorb light and transport energy, but can provide a structural barrier to some electronic processes. However, here we use it to investigate changes in the planarity of a conjugated polymer. Spectra of polymer/C60 blend films show increased displacement of a symmetry-forbidden out-of-plane vibrational mode with increasing fullerene doping, indicating a loss of planarity. We also use photoluminescence spectroscopy to confirm this, as the electronic origin of emission blue-shifts with fullerene loading which indicates decreased conjugation length and thus molecular planarity.;Ground state charge transfer complexes of MDMO-PPV and several electron acceptors were studied using linear absorption and Resonant Raman spectroscopy to better understand the nuclear rearrangements that the polymer undergoes upon loss of an electron. Any bonds displaced upon the movement of charge will make it more difficult to move and separate charge in the material, processes essential for the function of a solar cell. Thin films of blended MDMO-PPV:DDQ, MDMO-PPV:TCNQ, and MDMO-PPV:PCBM blends show several orders of Raman overtones. We use the intensity of these Resonant Raman overtones and the framework of time-dependant spectroscopic theory, to estimate vibrational-mode displacements and the reorganizational barriers to charge transfer and transport.