Theoretical Study of Novel Porphyrins Bearing Electron Donor–Acceptor Groups Carolina Caicedo, Ana Martı ´nez* and Ernesto Rivera This research project is focused on molecules that comprise a series of asymmetrically A 3 B-type meso-substituted free-base porphyrins and their related Zn-metalloporphyrins. A and B were taken as electron-donor and electron–acceptor groups. Full geometry optimizations without symmetry constrains were performed with B3LYP/6-31G(d,P) methodology. Time- dependent density functional theory calculations of the optimized structures indicate that there is a good agreement with the available experimental results. The highest occupied molecular orbital–lowest occupied molecular orbital (LUMO) gaps (ranging between 2.62 and 2.80 eV) are similar to those reported before for other porphyrins (2.29 eV). Also, the LUMO is situated close to the conduction band of titanium oxide, increasing the possibility of a charge transfer process. As porphyrins may act as electron transfer systems, the electron donor–acceptor capacity of these systems is characterized using two parameters; electrodonating (v) and electroaccepting (vþ) electronegativity. The main goal of this investigation is to analyze the electronic structure and the donor–acceptor properties of these porphyrins to see if these compounds could be useful for further applications related to the design of solar cells. V C 2012 Wiley Periodicals, Inc. DOI: 10.1002/qua.24316 Introduction Porphyrins are part of a very important family of fluorophores, which have been widely studied in the context of macromo- lecular and material sciences. [1–4] These chromophores are highly delocalized p-systems, considered as a unique category of ionic scavengers, whose defined characteristics arise from the heteroatoms present in their structure. [5] Because of their efficient light absorption, porphyrins have been the subject of intense research for the purpose of solar energy transfer and electron transfer systems. [6–9] The incorporation of porphyrins into polymers permits easy handling, recycling, and adaptation of this important set of complexant agents for ongoing proc- esses. Porphyrins have also been used in the synthesis of push-pull p-conjugated systems bearing electron-donor and electron-acceptor groups and also in the design of dendritic molecules able to act as molecular antennae for photovoltaic applications. [6,7] Moreover, several electro- and photoactive units have been incorporated into porphyrins to tune their electronic and photophysical properties. The electron donor– acceptor character of porphyrins can also be modified, depending on their coordination state and their photoactive units which are linked together. [10–12] Thus, the preparation, electronic, and optical properties of several porphyrin deriva- tives, linked to electro- and photoactive units such as fullerene C 60 , [10] anthracene, [11] pyrene, [12] and functionalized porphyr- ins [13] have been described in the literature. In the context of theoretical studies, certain authors use density functional theory (DFT) calculations, [14–24] to study the geometry and electronic structure of porphyrins with varying substituents for the purpose of designing an efficient material with the poten- tial applications of an organic-based dye for solar cells. Usually, the absorption spectra are interpreted qualitatively in terms of the Gouterman’s four-orbital model [25–27] which only considers transition in the cases of the two highest occupied and the two lowest unoccupied molecular orbitals. In previous works, it has been established that the porphy- rin synthesizers for dye-sensitized solar cells must absorb most of the radiation from the solar light in the near-IR and visible regions and for this purpose, the energy difference between the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO) should be approximately 2 eV, since this value corresponds to the maximum in the solar radiation energy spectrum. For this reason, the HOMO–LUMO energy gap is a good parameter to analyze the potential effi- ciency of these materials. Likewise, it is important that the LUMO to be situated above and close to the conduction band of titanium oxide, to augment the charge transfer character. Using these two parameters: the HOMO–LUMO gap and the value of the LUMO energy when compared to the conduction band of titanium oxide, in a previous work, [17] the authors ana- lyzed Zn meso-tetraphenylporphyrin (ZnTPP) complexes bear- ing different substituents and found that inserting thiophene units produces materials that improve these properties than porphyrins with other substituents. Another report [18] indi- cated that the asymmetric substitution of the porphyrin rings will result in a strong mixing of configurations, which contrib- utes to the red shift of the absorption spectra. In all these studies, the absorption spectra were analyzed with reference to Gouterman’s four orbital model. C. Caicedo, A. Martı´nez, E. Rivera Instituto de Investigaciones en Materiales, Universidad Nacional Aut  onoma de M exico. Ciudad Universitaria, C.P. 04510 M exico D.F. M exico E-mail: martina@iim.unam.mx V C 2012 Wiley Periodicals, Inc. International Journal of Quantum Chemistry 2012, DOI: 10.1002/qua.24316 1 FULL PAPER WWW.Q-CHEM.ORG