Quantum-Classical Protocol for Efficient Characterization of Absorption Lineshape and Fluorescence Quenching upon Aggregation: The Case of Zinc Phthalocyanine Dyes

A quantum-classical protocol that incorporates Jahn-Teller vibronic coupling effects and cluster analysis of molecular dynamics simulations is reported, providing a tool for simulations of absorption spectra and ultrafast nonadiabatic dynamics in large molecular photosystems undergoing aggregation in solution. Employing zinc phthalocyanine dyes as target systems, we demonstrated that the proposed protocol provided fundamental information on vibronic, electronic couplings and thermal dynamical effects that mostly contribute to the absorption spectra lineshape and the fluorescence quenching processes upon dye aggregation. Decomposing the various effects arising upon dimer formation, the structure-property relations associated with their optical responses have been deciphered at atomistic resolution.

Probing the high performance of photoinduced birefringence in V-shaped azo/PMMA guest-host films

Optical birefringence in polymeric films containing azo-chromophores is an important feature related to the development of several technologies such as electro-optic modulators, optical switching, and optical gates, to cite a few. Therefore, it is essential to understand the main underlying mechanisms describing dynamic switching. In this context, we have investigated the optical birefringence performance of a guest-host film produced from a poly(methyl methacrylate) (PMMA) matrix containing a V-shaped azo-chromophore, which exhibited a larger optical response in comparison to the linear chromophores. The optical birefringence was induced by a linearly polarized diode laser (532 nm, writing laser), while a low-intensity HeNe (632.8 nm) laser and a tungsten-halogen lamp are employed, respectively, to monitor the optical storage and the absorption change during the photoinduced birefringence. Our results pointed out that the guest-host film presents maximum residual optical memory at around 50% and local optical birefringence at around 3.3 × 10^-4 in the low concentration and intensity regimes. The high optical birefringence obtained in guest-host films was attributed to the considerable photoisomerization quantum yield in the solid-state (0.15 ± 0.02 for 532 nm). Besides, we have shown that the switching mechanism is driven by angular hole-burning during the first seconds after excitation, and, subsequently, molecular reorientation quickly rises, dominating the photochemical process. The latter mechanism is highly efficient in converting cis to trans molecules (100%), which is responsible for the high residual optical memory obtained. In order to better understand the isomerization mechanism of the azo-chromophore/PMMA film, we performed quantum chemical calculations within the DFT framework. The electronic transitions of the azo-chromophore isomers were determined using the TD-DFT method and potential energy curves (PECs) were constructed to investigate the possibility of the thermal-isomerization process of the V-shaped azo-chromophore through both rotation and inversion mechanisms. For both mechanisms, the amplitude of the energy barrier and activation energy for thermal isomerization are determined and the results are discussed.

Revealing the Electronic and Molecular Structure of Randomly Oriented Molecules by Polarized Two-Photon Spectroscopy

In this Letter, we explored the use of polarized two-photon absorption (2PA) spectroscopy, which brings additional information when compared to methods that do not use polarization control, to investigate the electronic and molecular structure of two chromophores (FD43 and FD48) based on phenylacetylene moieties. The results were analyzed using quantum chemical calculations of the two-photon transition strengths for circularly and linearly polarized light, provided by the response function formalism. On the basis of these data, it was possible to distinguish and identify the excited electronic states responsible for the lowest-energy 2PA-allowed band in both chromophores. By modeling the 2PA circular-linear dichroism, within the sum-over-essential states approach, we obtained the relative orientation between the dipole moments that are associated with the molecular structure of the chromophores in solution. This result allowed to correlate the V-shape structure of the FD48 chromophore and the quantum-interference-modulated 2PA strength.

Polarization effect on the two-photon absorption of a chiral compound

In this report, we investigate the polarization effect (linear, elliptical and circular) on the two-photon absorption (2PA) properties of a chiral compound based in azoaromatic moieties using the femtosecond Z-scan technique with low repetition rate and low pulse energy. We observed a strong 2PA modulation between 800 nm and 960 nm as a function the polarization changes from linear through elliptical to circular. Such results were interpreted employing the sum-over-essential states approach, which allowed us to model the 2PA circular-linear dichroism effect and to identifier the overlapping of the excited electronic states responsible by the 2PA allowed band.

Chemoselective signalling of selected phospho-anions using lanthanide luminescence

Selectivity in the binding of phosphorylated tyrosine residues to aqua-lanthanide complexes is signalled by changes in spectral form by luminescence emission and 1H NMR spectroscopy.

Design, synthesis and evaluation of ratiometric probes for hydrogencarbonate based on europium emission

A series of cationic, zwitterionic and anionic macrocyclic europium complexes has been prepared incorporating a N or C- linked acridone chromophore that allows sensitisation of Eu emission following excitation at 390-410 nm. Each of these complexes selectively binds bicarbonate at physiological pH and reversible binding is signalled by a change in the form and relative intensity of the Eu emission spectrum. Affinity for bicarbonate is regulated by overall complex charge and falls within the range required for intracellular or extracellular analyses. Monitoring the ratio of the intensity of Eu emission at up to three wavelengths, e.g. 618/588 or 618/702 nm allows the solution concentration of bicarbonate to be deduced in a background of competing anions such as lactate, citrate and phosphate. Preliminary screens reveal the complexes to be non-toxic to NIH-3T3 cells and to be taken inside the cell, encouraging further study.

Protection against radiation-induced degradation of DNA bases by polyamines

Polyamines have been reported to protect DNA against the formation of radiation-induced strand breaks and crosslinks to proteins. The present study was aimed at investigating the protective effect of spermine, spermidine and putrescine against the degradation of DNA bases upon exposure to gamma rays in aerated aqueous solution. The yield of 8-oxo-7,8-dihydroguanine and 5-hydroxycytosine was found to decrease for concentrations of spermine and spermidine greater than 0.1 mM. A protection factor of 10 was observed for a concentration of 1 mM of the latter two polyamines. Putrescine afforded a lower protection. In addition, the formation yield of a series of radiation-induced degradation products of the purine and pyrimidine bases was determined within DNA in the presence or absence of spermine. The protection factor was within the same range for all the lesions measured. The latter observation ruled out the possibility of degradation of DNA by radiation-induced polyamine peroxyl radicals. This was confirmed by studies involving radiolysis of DMSO and decomposition of 2,2'-azobis(2-methyl-propionamidine) as sources of alkylperoxyl radicals. Therefore, it is likely that the polyamine-mediated protection against the radiation-induced degradation of DNA bases is due to the compaction of the DNA structure and the reduction in the accessibility of DNA to .OH rather than by scavenging .OH in the bulk solution or in the vicinity of the DNA.