Aggregation of photosensitizers: the role of dispersion and solvation on dimer formation energetics

• Published in: Theoretical chemistry accounts 2015-12, Vol.134 (12), Article 152
• Main Authors: Bettanin, Fernanda, de Carvalho Fontinelles, Tancredo Augusto, Maciel, Cleiton Domingos, Dias, Luís Gustavo, Coutinho-Neto, Mauricio Domingues, Homem-de-Mello, Paula
• Format: Article
• Language: English
• Subjects: Atomic/Molecular Structure and Spectra; Chemistry; Chemistry and Materials Science; Inorganic Chemistry; Organic Chemistry; Physical Chemistry; Regular Article; Theoretical and Computational Chemistry
• ISSN: 1432-881X; 1432-2234
• DOI: 10.1007/s00214-015-1732-5

A detailed account for the dimer formation energetics is computed for two commonly used photosensitizers: metal-free phthalocyanine (Pc) and methylene blue (MB), a cationic phenothiazine. Gas-phase calculations were performed using the density functional theory augmented with dispersion corrections using both the dispersion-correcting atom-centred potentials and the D2/D3 methods. SCS-MP2/CBS results were also provided for comparison. Hydration free energies were determined using continuum methodologies: namely the conductor-like polarizable continuum model, the solvation model density and a reparametrized version of the polarizable continuum model. For methylene blue, the Langevin dipole model was also used. Our results showed that the nature of aggregation is remarkably different for these two molecules, being dispersion driven for Pc while solvent driven for MB. For water and ethanol our results are in good agreement with the experimental data available for MB. We also provide quantitative estimates to the popular explanation put out by Mukerjee and Ghosh more than 40 years ago that aggregation arises from solvent entropic and inter-species enthalpic contributions.