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Investigation of the Photophysical Properties of pi-Conjugated Polymers. A Study by Non-Linear, Time-Resolved, and Single-Molecule Spectroscopy
(2007)
- In the past few years conjugated polymers have attracted considerable interest owing to their high potential for applications in optoelectronic devices. Despite the progress in the field of device fabrication, a full understanding of the dynamics of electronic excitations and the intrinsic electronic and optical properties of these functional materials is still not complete. To gain further insight into these issues a model conjugated polymer (methyl-substituted ladder-type poly(para-phenylene), MeLPPP) was investigated in this thesis by two-photon (2P) fluorescence excitation spectroscopy and time-resolved spectroscopy on thin MeLPPP-films as well as by fluorescence spectroscopy on individual MeLPPP-chains. Employing continuous-wave 2P fluorescence-excitation spectroscopy on MeLPPP the symmetry properties of the vibronic wave functions of the lowest electronically excited singlet states were studied. It was found that the symmetry selection rules for optical transitions are rather strictly fulfilled, demonstrating the high intra-chain order in MeLPPP. Additionally, these data gave evidence that the energies of the predominant vibrational modes, the inter-ring stretching vibration and the aromatic ring CC stretching mode, are significantly lower in the second excited singlet state with respect to the corresponding energies in the first excited singlet state and in the electronic ground state. This finding reflects a significant change of the equilibrium geometry of the MeLPPP-backbone in the second excited singlet level with respect to the energetically lower lying singlet states. The time-resolved experiments on MeLPPP-films utilising a Streak-camera technique with high spectral resolution revealed the full relaxation dynamics of electronic excitations within the density of electronically excited states prior to the emission process as well as their decay kinetics from the lowest electronically excited state into the electronic ground state. An inverse Laplace transform of the fluorescence decay curves provided strong evidence that the relaxation dynamics can be described by a distribution of excited state lifetimes. At 1.5 K both the decay dynamics and the rising component of the fluorescence transients from the low-energy tail of the distribution of excited states showed a strong dependence on the emission wavelength. In contrast, at room temperature such a dependency was not observed. Finally, the data allowed to determine the radiative lifetime of the lowest electronically excited state to about 800 ps at both 1.5 K and 296 K. Low-temperature fluorescence spectroscopy on individual MeLPPP-chains in combination with statistical pattern recognition techniques for data analysis permitted to retrieve the profile of the electronic spectra of single chromophores in great detail. From these data the Debye-Waller factor, i.e. the electron-phonon coupling strength, was calculated and for MeLPPP a weak electron-phonon coupling was found at 1.5 K. Moreover, there is strong evidence that the low-frequency vibrational modes, which are coupled to the electronic transitions, stem from vibrations of the surrounding host matrix indicating a weak intra-chain electron-phonon coupling. The analysis of the zero phonon line shapes of the purely electronic emission lines revealed that these lines are inhomogeneously broadened by fast unresolved spectral diffusion processes. Finally, from a detailed study of the vibronic emission lines of single MeLPPP-chains the vibrational relaxation time of the inter-ring stretching and aromatic ring CC stretching mode in the electronic ground state is determined to about 230 fs at low temperatures.
