Molecules that absorb light in the visible region have been applied since prehistoric times as coloring materials. Modern society has relied on dyes not only for color but also for medical, energy, and technological applications such as photodynamic therapy, solar cells (in dye-sensitized solar cells—DSSC), and optical devices. Dye molecules have chromophores containing double bonds, typically present in a rigid or semi-rigid scaffold. Heteroatoms can function as electron donor and acceptor groups, helping to modulate the wavelength and intensity of light absorption. Naturally occurring macrocycles composed of methine (or azamethine) linkages connecting four rings form chromophores present in many naturally occurring dyes, such as in porphyrin and phthalocyanine. Porphyrin electronic absorption spectra are characterized by two bands: a strong absorption in the 400 nm region named the Soret band and a weaker absorption in the visible region named the Q band. In this work we explore the possibility of modifying carbon atoms present in a Cu(II) complexed porphyrin by boron and nitrogen atoms with the intent of modifying its electronic properties. The number of possible derivatives produced by modifying all 20 carbon atoms in porphyrin with boron and nitrogen atoms is very large. Without symmetry considerations, the total number of possible derivatives is 203, or about 3.5 billion structures. When symmetry is considered, the total number of derivative compositions is reduced to about 436 million, a massive number that presents a challenge to standard approaches. Alchemical modifications using Quantum Alchemy (QA), on the other hand, can be used to explore that vast dataset by limiting exploration to isoelectronic changes. In QA, molecular properties such as total energies, dipoles, and orbital energies can be expanded in terms of alchemical derivatives to any arbitrary order. In this work, we present our results based on the entire dataset of 436 million distinct compositions, as well as a reduced dataset that focuses on a chemical space more closely aligned with reference porphyrins. In this reduced dataset, we limit the chemical space to a maximum of 10 modified sites and the difference between the number of nitrogen and boron atoms to a maximum of 2, amounting to circa 13 million different compositions. A QA expansion up to second order was produced for the total energy, dipole moment, and orbital energies in the valence region from a total of 800 reference DFT calculations. Energy stability analysis revealed a rich valley of diverse structures that is modulated by the total charge of the system. To trace orbital changes with composition, a new method to follow orbital changes has been developed. Validation with respect to full DFT results is also provided for a representative set of structures.
Prof. Dr. Mauricio Domingues Coutinho Neto