2D Fourier Transform Spectroscopy Setup and Ultrafast Dynamics of Porphyrins

The first section of the thesis discusses the development and implementation of a two dimensional Fourier transform spectroscopy setup, that permits the recording of electronic 2D spectra in the visible regime. A hollow core fiber was used to generate broadband pulses, compressed down to sub-15 fs, allowing high spectral and temporal resolution measurements. The design relies on basic optical components (no diffractive optics or liquid crystal display) with pair-wise manipulation of the beams, which lead to reduction of the signal modulation to the difference between the transition frequency and the laser frequency. The required timing precision as well as the mechanical stability is reduced due to the used geometry. For testing the integrity of the setup, stability measurements over several hours are presented as well as measurements on rhodamine 101 dye and Pentacene thin films. The photophysics of porphyrins has been widely investigated because of their importance in biology and their relevance in a wide range of applications such as light harvesting applications, molecular electronics, and gas sensors. The energy of the photon absorbed by these systems is converted into electronic and vibrational energy, eventually triggering photochemical processes such as charge transfer, energy transfer, and bond breaking. Deep understanding of the specific early relaxation mechanism and nuclear wavepacket motion occurring in this family of molecules is therefore fundamental for further comprehension of the role of porphyrins in nature and future application using porphyrin-based molecular devices. In the third chapter we discuss the impulsive heating of free-base porphyrins and the electronic and vibrational relaxation dynamics upon photo-excitation. Transient absorption and fluoresce up-conversion spectroscopy techniques with high temporal resolution were utilized to investigate the relaxation processes, and the heating/cooling dynamics. Wavepacket dynamics of free-base porphyrins are discussed in the fourth chapter, where the phase and spectral intensity of the vibrational modes were extracted using Fourier transform spectroscopy. Based on the mode phase and spectral dependent intensity each mode is assigned to being in-plane or out-of-plane mode and to whether it exists in the excited or ground state. In the last chapter, excited state dynamics of triply fused diporphyrins in toluene is investigated using transient absorption spectroscopy. These highly π-conjugated organic systems are receiving considerable attention because they incorporate ease of synthesis with tunable chemical structure that can be tailored for various applications, such as molecular wires, mimic light harvesting complexes, and saturable absorbers dyes. These porphyrin tapes can chelate different metal centers, changing their optical and electronic properties. In this chapter a comparison between free-base, Zn, and Pt based triply fused diporphyrins is done in order to understand the role of the metal in the relaxation dynamics of these systems. By further comparing these systems with earlier studies, the peripheral substituents effects are also discussed. In the end, we show that the high ISC yield, long triplet lifetime, and high cross-section of Pt-3DP makes it a perfect candidate to be used as NIR sensitizer for photochemical up-conversion via triplet-triplet annihilation.

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