Photostability is considered a key asset for photosensitizers (PS) used in medical applications as well as for those used in energy conversion devices. In light-mediated medical treatments, which are based on PS-induced harm to diseased tissues, the photoinduced cycle of singlet oxygen generation has always been considered to correlate with PS efficiency. However, recent evidence points to the fundamental role of contact-dependent reactions, which usually cause PS photobleaching. Therefore, it seems reasonable to challenge the paradigm of photostability versus PS efficiency in medical applications. We have prepared a series of Mg(II) porphyrazines (MgPzs) having similar singlet oxygen quantum yields and side groups with different electron-withdrawing strengths that fine-tune their redox properties. A detailed investigation of the photobleaching mechanism of these porphyrazines revealed that it is independent of singlet oxygen, occurring mainly via photoinduced electron abstraction of surrounding electron rich molecules (solvents or lipids), as revealed by the formation of an air-stable radical anion intermediate. When incorporated into phospholipid membranes, photobleaching of MgPzs correlates with the degree of lipid unsaturation, indicating that it is caused by an electron abstraction from the lipid double bond. Interestingly, upon comparing the efficiency of membrane photodamage between two of these MgPzs (with the highest and the lowest photobleaching efficiencies), we found that the higher the rate of PS photobleaching the faster the leakage induced in the membranes. Our results therefore indicate that photobleaching is a necessary step toward inflicting irreversible biological damage. We propose that the design of more efficient PS for medical applications should contemplate contact-dependent reactions as well as strategies for PS regeneration.

The photophysical and photochemical properties of phthalocyanine and naphthalocyanine with similar structures were studied in solution and with density-functional theory (DFT) computational method. The extended π-conjugated system in naphthalocyanines causes a bathochromic shift in UV-Vis, emission and excitation bands, and promotes lesser generation of singlet oxygen in solution when compared to phthalocyanines. Time dependent DFT (TD-DFT) calculations point out the molecular orbitals involved in Q-band transition, corresponding to highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) transition with a concentration of charge along x-axis, while the transition to LUMO+1 is in y-axis direction. The presence of tert-butyl substituents does not affect the molecular orbitals shape, but affect their energies. Aggregation studies in dimethyl sulfoxide (DMSO):water solutions showed that naphthalocyanines studied have more aggregation tendency than the phthalocyanines. DFT studies indicated that stacked-dimers are preferred to rotated-stacked conformation due the interaction between Zn II and nitrogen atom from different monomers.

Chemical fixation of CO 2 to produce cyclic carbonates can be a green and atomic efficient process. In this work, a series of porphyrazines (Pzs) containing electronwithdrawing groups and central M II ions (where M = Mg, Zn, Cu, and Co) were synthesized and investigated as catalysts for the cycloaddition of CO 2 to epoxides. Then, the efficiency of the Pzs was tested by varying cocatalyst type and concentration, epoxide, temperature, and pressure. Mg II Pz bearing trifluoromethyl groups (1) showed the best conversion, producing, selectively, 78% of propylene cyclic carbonate (PCC), indicating that a harder and stronger Lewis acid is more effective for epoxide activation. Moreover, cocatalyst variation showed a notable effect on the reaction yields. Spectrophotometric titrations, MALDI-TOF mass spectra, and theoretical calculations suggest poisoning of the catalyst when tetrabutylammonium chloride (TBAC) and large amounts of tetrabutylammonium bromide (TBAB) were used in the system. The same was not observed for tetrabutylammonium iodide (TBAI), indicating that the metal−cocatalyst interaction may govern the reaction rate. In addition, two rare examples of crystalline structures were obtained, proving the distorted square pyramidal geometry with water molecule as axial ligand. This is one of the first studies reporting Pzs as catalysts for the chemical fixation of CO 2 , and we believe that the intricate balance between cocatalyst concentration and conversion efficiency shown here may aid future studies in the area.

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Studies on the synthesis, structure, spectroscopy and photodynamics of macrocycles such as porphyrin, phthalocyanine, and chlorin have increased in importance in the last decades. This interest is mainly due to their catalytic, spectrophotometric, magnetic, and electrochemical properties, which enable these compounds to be employed in many different applications. [1][2][3][4][5][6] In combination with experimental approaches, theoretical studies based on quantum mechanics are useful for the comprehension of electronic structure and environmental effects. [2,[7][8][9][10][11][12][13][14][15][16][17][18] There are two main methodologies that aim to increase the efficiency for energy transfer appliances: complexation with different metallic ions and the addition of (a) polar/(un)charged substituents. Both methodologies are able to avoid the aggregation and/or enhance the substrate interaction while maintaining or even improving photochemical properties.[2,7-9,19-33]One of the first ab initio studies for porphyrins [34] was performed through the MO-LCGO-SCF method, described by Roothaan. [35][36][37] Since then, the number of ab initio studies for these macrocycles has grown significantly, particularly concerning the calculations based on density functional theory (DFT), due to its good relation and cost/accuracy ratio. [2,8,9,[17][18][19][20][21][22][23]25,26,28,29,31,[38][39][40][41][42][43][44][45][46][47] DFT studies of most porphyrins and phthalocyanines have employed B3LYP functio nal; [2,8,9,25,28,[38][39][40]48] nevertheless, other functionals such as BLYP, [8,29,49] VWN-B-P, [26] and BHandHLYP [8] have also been used. T. E. Shubina and T. Clark [25] performed a methodological study for porphyrins complexed to Fe(III) and Co(II), which indicated that B3LYP and OLYP functionals are excellent choices for studying these macrocycles. Time-dependent Hartree-Fock, [26,38,[41][42][43] ZINDO/S, [38,42] configuration interaction, [41,42] and CASSCF/CASPT2Phthalocyanines, porphyrins, and chlorins have been widely studied as photosensitizers. Both experimental and computational strategies are employed in order to propose new and more active molecules derived from those macrocycles. In this context, there are two main strategies used: (i) the addition of different substituents and (ii) the complexation of the macrocycle with different metallic ions. In this work, we present selected descriptors based on quantum chemistry calculations for forty macrocycles, including some approved drugs. We have found that density functional theory is a suitable methodology to study the large sets of molecules when applying the B3LYP/LanL2DZ methodology for geometry optimization and TD-OLYP/6-31G(d) for absorption spectrum. The inclusion of solvent effects by means of continuum model is important in order to obtain the accurate electronic data. We have verified that by bonding charged or polar substituents to the macrocycle, it is possible to enhance water solvation as well as to improve spectroscopic properties because molecular orbital...