STRUCTURAL AND ELECTRONIC COUPLING OF ORGANIC HETEROAROMATIC MOLECULES ON INORGANIC SURFACES OF OXIDES

摘要

Many novel technologies base their main capabilities on the interface between organic semiconductors and dielectric surfaces. Understanding and manipulating its behaviour becomes then an important task for their development, from production cost to efficiency and better employment qualities. In the present thesis, I dealt the issue of molecular ordering and electronic properties at the organodielectric interfaces of four donor heteroaromatic molecules regarding the rutile-TiO2 (110) surface in an ultra-high vacuum (UHV) environment, combining some of them with a carbon allotrope as acceptor. The interface electronic structure, morphology, conformation and chemical interaction of in situ grown thin film of metal-free tetraphenyl porphyrin (2H-TPP), have been investigated by means of STM and/or electron spectroscopies. The surface sensitivity and chemical selectivity of soft X-ray photoemission (XPS) and near-edge X-ray absorption (NEXAFS) allowed to monitor the chemical state upon absorption and the orientation of their molecular plane and peripheral substituents. Free base tetraphenyl porphyrins adsorb on the oxygen rows, where they can spontaneously capture additional hydrogen atoms at their iminic nitrogens (4H-TPP). Both species aggregate into a commensurate phase at saturation coverage (1M), and upon sample heating a self-metalation reaction sets in at about 100ºC. The Ti atoms are extracted from the substrate by dehydrogenation of pyrrolic nitrogen atoms from the macrocyle, where remain coordinated to two underlying oxygen atoms. The adsorption geometry and molecular arrangement remain invariable across the self-metalation up to 300ºC. The possible manifestation of metal exchange with reactive Ti atoms must be considered when designing porphyrin sensytized solar cells due to the critical self-metalation temperature, close to its normal operating temperature. Thermal stability up to 450ºC is also demonstrated, with a phase symmetry change from oblique-(2x4) to rect-(2x6), keeping chemical states of both the molecular tetrapyrrolic macrocycle and the substrate unchanged, revealing by NEXAFS that the driving mechanism is the rotation of the phenyl terminations towards the substrate (flattening) promoting the partial cyclodehydrogenation of the molecules, and preserving the coordination of the macrocycle central pocket to the oxygen atoms through the self-metalation and the flattening reactions. Porphyrins trapped at the TiO2(110) surface demonstrate a high thermal stability making it a promising system for implementation in photocatalysis applications and photovoltaic devices.Another heteroaromatic molecules have been also investigated: tert-butyl tetraphenyl porphyrin (2h-tbTPP), octaethyl porphyrin (2H-OEP) and phtalocyanine (2H-Pc). The experimental results point to the existence of an unaffected reaction by the substituents, the hydrogenation of the macrocycle as deposited, converting its aza- nitrogen atoms into pyrrolic- nitrogen atoms. The self-metalation (i.e. incorporation of a Ti ion extracted from the substrate) was also demonstrated by X-ray spectroscopy for the four compounds but is accomplished at different temperature, starting at room temperature for the 2H-Pc, and at 100ºC for 2H-TPP and being completed at 200ºC for 2H-tbTPP and 250ºC for 2H-OEP, opening the possibility of a self-metalation of free-metal compounds and trans-metalation for metalated compounds due to the lower temperatures needed depending on the peripheral substituents, during the experimentation and development of devices with this compounds.