Synthesis, Photophysical Properties and Computational Studies of beta-Substituted Porphyrin Dyads.

Beta-pyrrole-substituted porphyrin dyads connected by ethynyl linkage to N-butylcarbazole or triphenylamine donors are reported. Donor-π-acceptor type beta-substituted porphyrin dyads and their Zn(II) and Pd(II) complexes were characterized by MALDI-MS, NMR, UV-vis absorption, fluorescence and cyclic voltammetry techniques. The S1 emission dynamics were analyzed by time-resolved spectroscopy (TCSPC); dyads exhibited efficient energy transfer up to 93% from beta-donors (N-butylcarbazole or triphenylamine group) to the porphyrin core. The efficiency of energy transfer for the beta-substituted porphyrin dyads were much higher than those of the corresponding meso-substituted porphyrin dyads, reflecting enhanced communications between the beta-donors and the porphyrin core. The Pd(II) dyads, showed characteristic phosphorescence in the near IR region and very efficient singlet oxygen quantum yields (53-60%); these dyads are promising candidates for photocatalytic oxidations of organic compounds. The donor-acceptor interaction between the porphyrin core and the beta-donors was supported by the DFT studies in the porphyrin dyads.

Water soluble thioglycosylated BODIPYs for mitochondria targeted cytotoxicity.

The facile synthesis of water-soluble mitochondria targeting thioglycosylated BODIPYs is reported. Thioglycosylated BODIPYs were synthesized in 25-26% yields via thioglycosylated dipyrromethanes in four steps. The dipyrromethanes and thioglycosylated BODIPYs were characterized by various techniques including HRMS, NMR spectroscopy and X-ray crystallography. In-vitro cellular investigations in skin keratinocyte (HaCaT) and cervical (HeLa) cancer cells revealed significant cytotoxicities with IC50 values between 23.83 to 48.61 µM. The flow cytometry experiments revealed significant cellular uptake of thioglycosylated BODIPYs into HaCaT cells and thioglucosyl substituted BODIPY (9) showed higher cellular uptake and ROS generation than the rest of the molecules. The highlight of this study is the mitochondrial targeting by the neutral BODIPYs, as judged by the colocalization experiments using confocal microscopy.

Phosphorescent rhenium-dipyrrinates: efficient photosensitizers for singlet oxygen generation.

A series of rhenium(i) dipyrrinato complexes (Re1-Re8) have been prepared and characterized; their crystal structures, phosphorescence and singlet oxygen generation studies are reported. The aromatic substituents, such as thienyl, p-bromophenyl, p-fluorophenyl, m-fluorophenyl, pentaflurophenyl, N-butylcarbazole, N-phenylcarbazole, and N-butylphenothiazine, are linked to the C5 position of Re-dipyrrinates. Varying the electronic nature of the substituents from electron donating (e.g., carbazole) to electron withdrawing (e.g., pentaflurophenyl) allowed the change in the structural, electrochemical, and spectroscopic properties of these complexes. In particular, the rhenium dipyrrinates showed phosphorescence in the near IR region with sufficiently longer triplet state lifetimes (τT = 9-29 µs). Also, a large Stokes shift (Δν = 5682-6957 cm-1) was witnessed for all the rhenium dipyrrinates. Triplet emission was reflected in the efficient singlet oxygen generation yields (ΦΔ âˆ¼ 0.75-0.98) along with the distinct photo-stability. Density functional theory (DFT) calculations revealed that the electron density is spread over the dipyrrin unit in most complexes. Rhenium dipyrrinate having a phenothiazine substituent exhibited the smallest HOMO-LUMO band gap (2.820 eV) among all Re-complexes.

Carbazole Substituted BODIPYs.

Difluoroboron-dipyrromethenes (BODIPYs) are highly popular fluorescent dyes with applications as NIR probes for bioimaging, fluorescent tags/sensors and as photosensitizers in cancer therapy and organic photovoltaics. This review concentrates on the synthesis and spectral properties of BODIPY dyes conjugated with carbazole heterocycle. The carbazole is an electron rich tricyclic compound and due to its excellent electronic properties, it is extensively used in the production of electroluminescent materials and polymers. This review highlights the recent progress made on the series of BODIPY derivatives containing carbazole ring at alpha, beta, and meso-positions of the BODIPY skeleton. Carbazole based hybrid BODIPYs, carbazole linked aza-BODIPYs and carbazole-fused BODIPYs are also discussed.

Carbazole Substituted BODIPYs: Synthesis, Computational, Electrochemical and DSSC Studies.

Carbazole and p-anisyl substituted BODIPY dyes with a cyanoacetic acid anchoring group have been prepared and their spectral, electrochemical properties and photosensitizing potential in DSSC have been evaluated. X-ray structure of N-phenylcarbazole substituted BODIPY revealed lower torsion angle between BODIPY plane and carbazole plane, suggesting increased communication between the two units. DFT studies indicated effective electronic interactions between the BODIPY unit and carbazole substituents. The N-butylcarbazole and N-phenylcarbazole substituted BODIPYs showed anodic shifts in their reduction potentials, indicating facile reduction process. The predicted HOMO-LUMO gaps are in agreement with the electrochemical result and the lower band gap was observed for the carbazole substituted BODIPYs.

Tautomerism-Induced Cis-Trans Isomerization of Pyridylethenyl N-Confused Porphyrin.

Pyridylethenyl-substituted N-confused porphyrins (NCPs) were synthesized, and their cis-trans isomerization was studied. Among four possible isomers, trans-3H and cis-2H types of structures, of which aromaticity and absorption/emission properties differ largely, were isolated. The cis-isomer was largely stabilized by the intramolecular hydrogen bonding between the pyrrolic-NH and the pyridinic-N in the vicinity. The thermal cis-trans isomerization proceeded even at 30 °C, which was significantly accelerated by the pyridine added to the system. The kinetic studies revealed that the isomerization reaction was second-order and the activation energy of the thermal isomerization from cis to trans isomer was ΔG0⧧cis→trans = 35.7 kcal/mol at 298 K, which is significantly smaller than that of Ni complex (42.3 kcal/mol). An intermolecular proton transfer induced cis-trans isomerization mechanism was proposed.

Bridged bis-BODIPYs: their synthesis, structures and properties.

A series of bis-BODIPYs 1-6 bridged via thiophene, furan, N-alkylcarbazole, triphenyl-amine, para- and meta-phenylene groups have been synthesized and characterized by various spectroscopic techniques. The change in the spectroscopic properties of bis-BODIPYs upon varying the size of spacers was studied. X-ray crystal structures of three bis-BODIPYs containing triphenylamine, para- and meta-phenylene bridges were solved. Intermolecular C(H)π and ππ stacking interactions were observed in solid state structures of three bis-BODIPYs. The dihedral angles between the spacer unit and two boron-dipyrrin units were lower in all three compounds as compared to their corresponding monomers. This suggests increased interactions between the two boron-dipyrrin units in molecules which are in turn reflected in the anodic shifts in their reduction potentials. DFT studies indicated effective electronic interactions between spacers and two boron dipyrrin units in all the bis-BODIPYs. The calculated HOMO-LUMO gap was found to be lower for bis-BODIPY having bulky carbazole spacers and higher for bis-BODIPY having smaller furan spacers. Changing the spacer size clearly affected the spectroscopic properties of the bis-BODIPYs and red shifted absorption and emission maxima were observed for bis-BODIPYs with furan and thiophene spacers as compared to bis-BODIPYs with phenylene or bulky aromatic spacers.

Carbazole substituted boron dipyrromethenes.

meso-Substituted BODIPY with N-butylcarbazole () was prepared and derivatized. Dibromo BODIPY , α-formyl BODIPY and ß-formyl BODIPY were synthesized. All compounds were characterized by HRMS, NMR, UV-vis absorption, electrochemical and fluorescence techniques. The crystal structures of BODIPY and its dibromo derivative were also solved. In both the X-ray structures, the dihedral angle between the meso-carbazole group and the dipyrrin plane was decreased, suggesting the increased interaction between the two units. meso-Substitution with the N-butylcarbazole group on the BODIPY core rendered huge Stokes shifts (111-168 nm) and higher quantum yields as compared to meso-aryl BODIPY. An efficient energy transfer from the carbazole unit to the BODIPY core was observed by fluorescence spectroscopy for all the compounds . CV studies of compounds showed anodic shifts of the reduction and oxidation potentials, suggesting that the meso-carbazole group is affecting the electronic properties of the BODIPY core and making them easier to reduce.