Schiff bases show a wide variety of applications of great importance in medicinal researchdue to their range of biological activities. In this article we describe the electronic structure, optical, redox and antifungal properties of (E)-2-{[(2-aminopyridin-3-yl)imino]-methyl}-4,6-ditert-butyl-phenol (L1) and (E)-2-{[(3-aminopyridin-4-yl)imino]-methyl}-4,6-di-tert-butyl-phenol (L2), two isomers phenol derivatives Schiff bases exhibiting a strong intramolecular hydrogen bond (O-H•••N). These compounds were characterized by their 1 H, HHCOSY, 13 C-NMR, FT-IR spectra, and by cyclic voltammetry. All the experimental results were complemented with theoretical calculations using density functional theory (DFT) and time-dependent DFT (TDDFT). The antimicrobial activity of the compounds described herein was assessed by determining the minimal inhibitory concentration (MIC) and by a modification of the Kirby-Bauer method. We tested Salmonella enterica serovar Typhi (S. Typhi, Gram-negative bacteria), Cryptococcus spp. (yeast), and Candida albicans (yeast). We found that neither L1 nor L2 showed antimicrobial activity against S. Typhi or Candida albicans. On another hand, L2, in contrast to L1, exhibited antifungal activity against a clinical strain of Cryptococcus spp.(MIC: 4.468 µg/mL) even better than ketoconazole antifungal medicaments. We mentioned above that L1 and L2 are isomer species, because the amino groups is in ortho-position in L1 and in para-position in L2, however no significant differences were detectable by UV-vis, FT-IR, oxidation potentials and TDDFT calculations, but importantly, the antifungal activity clearly discriminated between these two isomers.LUMO) and 74% (HOMO-3 → LUMO) for L1 and 74% (HOMO-4 →LUMO) and 26% (HOMO-2 →LUMO) for L2 in all the solvents and in gas phase. The bands assigned like π→π* transitions involve the HOMO→ LUMO for L1 and L2. The comparison between the solvent spectra with the gas phase showed no significant shift in the UV-vis spectra when the solvent polarity changes. Figure 3. Calculated UV-vis absorption spectra for (E)-2-{[(2-aminopyridin-3-yl)imino]-methyl}-4,6-di-tert-butyl-phenol (L1) in different implicit solvents and gas phase.

The fac-[Re(CO) 3 (deeb)L] + complex (C2) where L is the (E)-2-((3-amino-pyridin-4-ylimino)-methyl)-4,6di-tert-butylphenol ancillary ligand, which presents an intramolecular hydrogen bond, has been synthesized and characterized using UV-vis, 1 H-NMR, FT-IR, cyclic voltammetry and DFT calculations.The UV-vis absorption and emission properties have been studied at room temperature and the results were compared with TDDFT calculations including spin-orbit effects. We report an alternative synthesis route for the fac-Re(CO) 3 (deeb)Br (C1) complex where deeb = (4,4 0 -diethanoate)-2,2 0 -bpy. Besides, wehave found that the C1 shows a red shift in the emission spectrum due to the nature of the ancillary electron donating ligand, while the C2 complex shows a blue shift in the emission spectrum suggesting that the ancillary ligand L has electron withdrawing ability and the importance of the intramolecular hydrogen bond. The calculations suggest that an experimental mixed absorption band at 361 nm could be assigned to MLCT and LLCT transitions. The electron withdrawing nature of the ancillary ligand in C2 explains the electrochemical behavior, which shows the oxidation of Re I at 1.83 V and the reduction of deeb at À0.77 V. 1.44 [s, 6H, (-CH 3 )], 4.49 [m, 4H], 5.92 [s, 2H, -NH 2 ], 6.45 [d; J = 5.5 Hz; 1H], 7.28 [d; J = 1.5 Hz; 1H], 7.47 [s, 1H], 7.51 [s, 1H], 7.53 [s, 1H], 8.11 [s, 1H], 8.20 [d; J = 5.5 Hz; 2H], 8.91 [s, 2H], Scheme 1 Structures of deeb and L ligands used in this work.

Research in fluorescence microscopy presents new challenges, especially with respect to the development of new metal-based fluorophores. In this work, the news fac-[Re(CO) 3 (bpy)L]PF 6 (C3) and fac-[Re(CO) 3 (dmb)L]PF 6 (C4) complexes, where L is an ancillary ligand E-2-((3-amino-pyridin-4-ylimino)-methyl)-4,6-diterbutylphenol, both exhibiting an intramolecular hydrogen bond, have been synthesized for its use as preliminary probes for fluorescence microscopy. The complexes were characterized using chemical techniques such as UV-Vis, 1 H-NMR, TOCSY, FT-IR, cyclic voltammetry, mass spectra (EI-MS 752.22 M + for C3 and 780.26 M + for C4) and DFT calculations including spin-orbit effects. The electron withdrawing nature of the ancillary ligand L in C3 and C4 explains their electrochemical behavior, which shows the oxidation of Re I at 1.84 V for C3 and at 1.88 V for C4. The UV-vis absorption and emission properties have been studied at room temperature in acetonitrile solution. The complexes show luminescent emission with a large Stokes shift (λ ex = 366 nm; λ em = 610 nm for C3 and λ ex = 361 nm; λ em = 560 nm for C4). The TDDFT calculations suggest that an experimental mixed absorption band at 360 nm could be assigned to MLCT (d(Re) →π*(dmb))and LLCT (π(L)→π*(dmb)) transitions. We also assessed the cytotoxicity of C3 and C4 in an epithelial cell line (T84). We found that 12.5 µg/ml of C3 or C4 is the minimum concentration needed to kill the 80% of cell population, as determined by neutral red uptake. Finally, the potential of C3 and C4 as biological dyes for use in fluorescent microscopy was assessed in bacteria (Salmonella enterica) and yeasts (Candida albicans and Cryptococcus spp.), and in an ovarian cancer cell line (SKOV-3). We found that, in all cases, both C3 and C4 are suitable compounds to be used as fluorescent dyes for biological purposes. In addition, we present evidence suggesting that these rhenium (I) tricarbonyl complexes may be also useful as differential fluorescent dyes in yeasts (Candida albicans and Cryptococcus spp.), without the need of antibodies.