RESUMEN
The absorption properties of Temoporfin, a second-generation photosensitizer employed in photodynamic therapy, are calculated with an electrostatic-embedding quantum mechanics/molecular mechanics (QM/MM) scheme in methanol. The suitability of several ensembles of geometries generated by different sampling techniques, namely classical-molecular-dynamics (MD) and QM/MM-MD thermal sampling, Wigner quantum sampling and a hybrid protocol, which combines the thermal and quantum approaches, is assessed. It is found that a QM description of the chromophore during the sampling is needed in order to achieve a good agreement with respect to the experimental spectrum. Such a good agreement is obtained with both QM/MM-MD and Wigner samplings, demonstrating that a proper description of the anharmonic motions of the chromophore is not relevant in the computation of the absorption properties. In addition, it is also found that solvent organization is a rather fast process and a long sampling is not required. Finally, it is also demonstrated that the same exchange-correlation functional should be employed in the sampling and in the computation of the excited states properties to avoid unphysical triplet states with relative energies close or below 0 eV.
Asunto(s)
Mesoporfirinas/química , Teoría Cuántica , Análisis Espectral , Simulación de Dinámica Molecular , Análisis Espectral/métodosRESUMEN
Supramolecular assembly of urea-tethered benzophenone molecules results in the formation of remarkably persistent triplet radical pairs upon UV irradiation at room temperature, whereas no radicals were observed in solution. The factors that lead to emergent organic radicals are correlated with the microenvironment around the benzophenone carbonyl, types of proximal hydrogens, and the rigid supramolecular network. The absorption spectra of the linear analogues were rationalized using time-dependent density functional theory calculations on the crystal structure and in dimethyl sulfoxide, employing an implicit solvation model to describe structural and electronic solvent effects. Inspection of the natural transition orbitals for the more important excitation bands of the absorption spectra indicates that crystallization of the benzophenone-containing molecules should present a stark contrast in photophysical properties versus that in solution, which was indeed reflected by their quantum efficiencies upon solid-state assembly. Persistent organic radicals have prospective applications ranging from organic light-emitting diode technology to NMR polarizing agents.
RESUMEN
The inclusion of solvent effects in the calculation of excited states is vital to obtain reliable absorption spectra and density of states of solvated chromophores. Here we analyze the performance of three classical approaches to describe aqueous solvent in the calculation of the absorption spectra and density of states of pyridine, tropone, and tropothione. Specifically, we compare the results obtained from quantum mechanics/polarizable continuum model (QM/PCM) versus quantum mechanics/molecular mechanics (QM/MM) in its electrostatic-embedding (QM/MMee) and polarizable-embedding (QM/MMpol) fashions, against full-QM computations, in which the solvent is described at the same level of theory as the chromophore. We show that QM/PCM provides very accurate results describing the excitation energies of ππ* and nπ* transitions, the last ones dominated by strong hydrogen-bonding effects, for the three chromophores. The QM/MMee approach also performs very well for both types of electronic transitions, although the description of the ππ* ones is slightly worse than that obtained from QM/PCM. The QM/MMpol approach performs as well as QM/PCM for describing the energy of ππ* states, but it is not able to provide a satisfactory description of hydrogen-bonding effects on the nπ* states of pyridine and tropone. The relative intensity of the absorption bands is better accounted for by the explicit-solvent models than by the continuum-solvent approach.
