Modelling quenching mechanisms of disordered molecular systems in the presence of molecular aggregates

Exciton annihilation and diffusion length in disordered multichromophoric nanoparticles

Efficient exciton transport is the essential property of natural and synthetic light-harvesting (LH) devices. Here we investigate exciton transport properties in LH organic polymer nanoparticles (ONPs) of 40 nm diameter. The ONPs are loaded with a rhodamine B dye derivative and bulky counterion, enabling dye loadings as high as 0.3 M, while preserving fluorescence quantum yields larger than 30%. We use time-resolved fluorescence spectroscopy to monitor exciton-exciton annihilation (EEA) kinetics within the ONPs dispersed in water. We demonstrate that unlike the common practice for photoluminescence investigations of EEA, the non-uniform intensity profile of the excitation light pulse must be taken into account to analyse reliably intensity-dependent population dynamics. Alternatively, a simple confocal detection scheme is demonstrated, which enables (i) retrieving the correct value for the bimolecular EEA rate which would otherwise be underestimated by a typical factor of three, and (ii) revealing minor EEA by-products otherwise unnoticed. Considering the ONPs as homogeneous rigid solutions of weakly interacting dyes, we postulate an incoherent exciton hoping mechanism to infer a diffusion constant exceeding 0.003 cm2 s-1 and a diffusion length as large as 70 nm. This work demonstrates the success of the present ONP design strategy at engineering efficient exciton transport in disordered multichromophoric systems.

Concentration Quenching of Fluorescence Decay Kinetics of Molecular Systems

Fluorescence concentration quenching occurs when increasing molecular concentration of fluorophores results in a decreasing fluorescence quantum yield. Even though this phenomenon has been studied for decades, its mechanisms and signatures are not yet fully understood. The complexity of the problem arises due to energy migration and trapping in huge networks of molecules. Most of the available theoretical work focuses on integral quantities like fluorescence quantum yield and mean excitation lifetime. In this work, we present a numerical study of the fluorescence decay kinetics of three-dimensional and two-dimensional molecular systems. We investigate the differences arising from the variations in models of trap formations. We also analyze the influence of the molecular orientations to the fluorescence decay kinetics. We compare our results to the well-known analytical models and discuss their ranges of validity. Our findings suggest that the analytical models can provide inspiration for different ways of approximating the fluorescence kinetics, yet more detailed analysis of the experimental data should be done by comparison with numerical simulations.

Extraction-less green spectrofluorimetric method for determination of mercury using an Astra Phloxine fluorophore: Comprehensive experimental and theoretical studies

A new sensitive and selective extraction-less method for the spectrofluorimetric determination of Hg2+ has been developed using the polymethine dye Astra Phloxine (AP) with requirements for green analytical chemistry. The principle of this method for analytical use is the formation of an ion associate (IA) in the presence of mercury ions and AP, which causes a change in the electronic structure of the dye cation and its subsequent fluorescence quenching. The IA created during the process was sufficiently stable in aqueous solutions and did not require the use of surfactants or organic solvents, which are typically used for similar analytical systems. The system indicated high selectivity towards Hg2+ in the presence of at least 100-fold higher concentrations of Co2+, Fe2+, Mg2+, Ni2+, Zn2+, Ca2+, Ba2+, K+, Na+. The calibration curve was linear in the range from 0.003 to 0.1 mg L-1. Precision and accuracy were expressed as intra-day and inter-day analysis with RSD values of 2.9 - 4.8 % and recovery of 97.1 - 110.0 %. The proposed method was used for the determination of Hg2+ on the model and real water samples. Theoretical calculations were used to predict the IA structure and to explain the experimentally observed fluorescence spectra. A few theoretical methods were tested for predicting IA emission energies, and the CIS(D) method was found to be the most accurate.