Probing Two-Dimensional vs Three-Dimensional Molecular Aggregation in Metal-Free Tetraphenylporphyrin Thin Films by Optical Anisotropy

Porphyrins represent molecular building blocks for the development of a large variety of functional materials. The control of subtle intramolecular and intermolecular processes in solid porphyrin aggregates is promising for the establishment of organic nanotechnology, which requires the knowledge of the properties of nanometric aggregates down to single atoms. Within this framework, the optical anisotropy in thin and ultrathin metal-free tetraphenylporphyrin (H2TPP) films deposited on graphite carries interesting information related to their molecular aggregation. In this work, we compare the surface morphology and the optical anisotropy of H2TPP 2D layers, obtained following a recently developed experimental strategy, with thin films where both 2D and 3D aggregates are present. After reaching a fine control of the relative amount of the two phases present in films of different thicknesses, we propose a precise physical model for the optical response of the pure 2D phase, therefore correlating its optical anisotropy with the electronic properties of single H2TPP molecules, the occurrence of intramolecular chemical reactions, and the molecular arrangement. The response of thicker films, characterized by the coexistence of the 2D and 3D phases, is described by a phenomenological model that considers the main spectroscopic features.