Synthesis, characterization, and Pb2+ion sensing application of hexa-armed dansyl end-capped poly(ε-caprolactone) star polymer with phosphazene core

Solid-State Fluorescent Organic Polymers for Visual Detection and Elimination of Heavy Metals in Water

Selective sensing and removal of toxic heavy metals from water are highly essential since their presence poses significant health and environmental hazards. Herein, we designed and synthesized a novel fluorescent nonconjugated organic polymer by strategically incorporating two key functional groups, namely, a dansyl fluorophore and dithiocarbamate (DTC). Different characterization techniques, including 1H nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive X-ray analysis (EDAX), Fourier transform infrared (FTIR), and fluorescence spectroscopy, were performed to understand its structure and material properties. The quantum yield of 4.72% and its solid-state fluorescence indicate that it has potential for various applications in several technological and scientific domains. In this study, we investigated a specific application involving the detection and elimination of heavy metals from water. Interestingly, the presence of dansyl and DTC moieties demonstrated remarkable selectivity toward Cu2+, Co2+, Ni2+, Fe3+, and Fe2+ sensing, displaying distinct color changes specific to each metal. Cu2+ resulted in a yellow color, Co2+ showed a green color, Ni2+ displayed a pale yellowish-green color, and Fe2+/Fe3+ exhibited a brown color. The LOD (limit of detection) for each metal was obtained in the nanomolar range by using a fluorescence spectrometer and the micromolar range from UV-visible spectra: 13.27 nM and 0.518 μM for Cu2+, 8.27 nM and 0.581 μM for Co2+, 14.36 nM and 0.140 μM for Ni2+, 14.95 nM and 0.174 μM for Fe2+, and 15.54 nM and 0.33 μM for Fe3+. Moreover, the DTC functionality on its backbone facilitates effective interaction with the aforementioned heavy metals, subsequently removing them from water (except Fe2+ and Fe3+), validating its dual functionality as both an indicator and a purifier for heavy metals in water. The polymer exhibited a maximum adsorption capacity of 192.30 mg/g for Cu2+, 159.74 mg/g for Co2+, and 181.81 mg/g for Ni2+. Furthermore, this approach exhibits versatility in crafting fluorescent polymers with adjustable attributes that are suitable for a wide range of applications.

Dansyl Emits from a PICT Excited State

Two derivatives of dansyl (1-dimethylamino-5-naphthalenesulfonyl) in which the amino group is constrained in a ring are prepared as neopentyl esters. Their photophysical behavior is compared with that of the dansyl ester. The solvatochromism and quantum yields are similar for all three. Since the two constrained derivatives cannot twist about the amino group, they must emit from a planar intramolecular charge-transfer excited state. The similar photophysical behavior suggests that dansyl also emits from a PICT excited state instead of a twisted intramolecular charge transfer state.

Thermal and solvatochromic effects on the emission properties of a thienyl-based dansyl derivative

Environmental conditions have a profound effect on the photophysical behavior of highly conjugated compounds, which can be exploited in a large variety of applications. In this context, we use a combination of experimental and computational methods to investigate thermal and solvatochromic effects on the fluorescence properties of a dansyl derivative bearing a thienyl substituent, namely 2-(3-thienyl)ethyl dansylglycinate (TEDG). In particular, we analyze how the solvent polarity and temperature affect the ground and excited state energies of TEDG by using time-resolved and steady-state fluorescence techniques. We determine the changes in dipole moment of the TEDG molecule upon photoexcitation, as well as the solvent polarity effects on the excited state lifetime. Besides, we provide theoretical modeling of the HOMO-LUMO orbitals and the vertical absorption and emission energies using time-dependent density functional theory (TDDFT) as well as the polarizable continuum model (PCM) to include the solvent contribution to the absorption and emission energies. Our results show that the emission mechanism of TEDG involves locally excited states derived from hybrid molecular orbitals, accompanied by a moderate variation of the molecular dipole moment upon light excitation. Our findings demonstrate that TEDG exhibits desirable fluorescence properties that make it a promising candidate for use as a photoactive material in electrochromic, optical thermometry, and thermography applications.