Photoinduced Dynamics of 13,13'-Diphenylpropyl-β-carotene

Carotenoids are ubiquitous pigment systems in nature which are relevant to a range of processes, such as photosynthesis, but the detailed influence of substitutions at the polyene backbone on their photophysics is still underexplored. Here, we present a detailed experimental and theoretical investigation of the carotenoid 13,13'-diphenylpropyl-β-carotene using ultrafast transient absorption spectroscopy and steady-state absorption experiments in n-hexane and n-hexadecane, complemented by DFT/TDDFT calculations. In spite of their bulkiness and their potential capability to "fold back" onto the polyene system, which could result in π-stacking effects, the phenylpropyl residues have only a minor impact on the photophysical properties compared with the parent compound β-carotene. Ultrafast spectroscopy finds lifetimes of 200-300 fs for the S₂ state and 8.3-9.5 ps for the S₁ state. Intramolecular vibrational redistribution with time constants in the range 0.6-1.4 ps is observed in terms of a spectral narrowing of the S₁ spectrum over time. We also find clear indications of the presence of vibrationally hot molecules in the ground electronic state (S₀*). The DFT/TDDFT calculations confirm that the propyl spacer electronically decouples the phenyl and polyene π-systems and that the substituents in the 13 and 13' positions point away from the polyene system.

Photophysical properties and biological evaluation of a Zinc(II)-5-methyl-1H-pyrazole Schiff base complex

A new ZnL₂ complex containing two molecules of a tridentate Schiff base derived from 5-methyl-1H-pyrazole (HL) is synthesized and characterized. The photophysical properties of HL and ZnL₂ are disclosed and supported by CAMB3LYP DFT/TDDFT calculations. It is shown that there is keto-tautomer stabilization upon excitation with an energetically accessible triplet state in HL, not present in ZnL₂, this explaining the differences found in the emissions of the compounds. The intrinsic fluorescence of ZnL₂ is used as probe for a detailed study of its binding to human serum albumin. The protein-complex association is thermodynamically favourable and it is shown by fluorescence quenching and time-resolved analysis that the fluorescence quenching involves a mixed mechanism with prevalence of static quenching, which corroborates adduct formation at site I, close to the Trp214 residue. The ability of ZnL₂ to bind DNA was also evaluated, as well as its cytotoxic activity against MCF7 (breast), PC3 (prostate) cancer cells and hamster V79 fibroblasts. ZnL₂ is a moderate DNA intercalator (K_app = 3.9 × 10⁴ M^-1) and depicts a quite low IC₅₀ value at 48 h against MCF7 cells (IC₅₀ = 530 nM), but much higher for PC3 and V79 cells. The relevance of a more careful speciation evaluation of ZnL₂ and other potential metal-based drugs in incubation media used in in vitro tests is highlighted.

Charge transfer optical absorption and fluorescence emission of 4-(9-acridyl)julolidine from long-range-corrected time dependent density functional theory in polarizable continuum approach

A long-range-corrected time-dependent density functional theory (LC-TDDFT) in combination with polarizable continuum model (PCM) have been applied to study charge transfer (CT) optical absorption and fluorescence emission energies basing on parameterized LC-BLYP xc-potential. The molecule of 4-(9-acridyl)julolidine selected for this study represents typical CT donor-acceptor dye with strongly solvent dependent optical absorption and fluorescence emission spectra. The result of calculations are compared with experimental spectra reported in the literature to derive an optimal value of the model screening parameter ω. The first absorption band appears to be quite well predictable within DFT/TDDFT/PCM with the screening parameter ω to be solvent independent (ω ≈ 0.245 Bohr(-1)) whereas the fluorescence emission exhibits a strong dependence on the range separation with ω-value varying on a rising solvent polarity from about 0.225 to 0.151 Bohr(-1). Dipolar properties of the initial state participating in the electronic transition have crucial impact on the effective screening.