A study of the electronic transitions of a porphyrin dimer

Abstract

Porphyrin Dimer (PD) is a recently developed porphyrin derivative that consists of two conjugated porphyrin units. PD presents favorable photophysical properties to be utilized as a photosensitizer in photodynamic therapy (PDT), such as a high extinction coefficient, good photostability, and shows high intracellular accumulation. Furthermore, these two porphyrin moieties have the unique ability to rotate around its diyne moiety, thus forming a so-called molecular rotor. This physical process (rotation) from planar to non-planar conformation (and vice-versa) is associated with a significant change in its molecular orbitals and presents an easily detectable change in its absorption and fluorescence properties. Importantly, the rings rotation can be robustly hindered by their surroundings, and especially the solvents microviscosity. Therefore, the PD molecular rotor is highly capable of reporting on its local environments viscosity.^To fully utilize PD as a cellular viscometer (rotor) and potentially as a photosensitizer in PDT, this study focuses on exploring the unknown characteristics and photophysical properties of both PDs major conformations. The main goal is to establish a new and robust method to determine the unknown orientation and relative strengths of the transition moments for the planar and twisted conformers of PD. This required first to immobilize and orient PD molecules using solid matrices and then force PDs molecules toward one dominant conformation (planar or twisted). The next step is to utilize methods to study the orientation of transition moments for absorption (linear dichroism - LD) and emission (fluorescence anisotropy) of the dominant conformer and identify the electronic transitions that are most suitable for the PD to be used as a molecular rotor.^In addition, the study also aims to understand the rationals behind the widely used excitation in the 470 nm to 480 nm range in utilizing PD as a molecular rotor. These properties are likely to play a fundamental role in optimizing molecular systems to be used as molecular rotors or as a photosynthesizer for PDT, and the development of the next generation of rotors and photosensitizers.