Synthesis and characterization of free base and metal porphyrins and their interaction with CdTe QDs

Multifaceted Applications of Luminescent Metalloporphyrin Derivatives: Fluorescence Turn-On Sensing of Nicotine and Antimicrobial Activity

In this study, we designed and synthesized metalloporphyrin derivatives (with Ni and Zn) specifically intended for the fluorescence detection of nicotine in aqueous solutions. Our results showcased a notable selectivity for nicotine over other naturally occurring food toxins, exhibiting an exceptional sensitivity with a limit of detection as low as 7.2 nM. Through mechanistic investigations (1H NMR, FT-IR, etc.), we elucidated the binding mechanism, revealing the specific interaction between the pyridine ring of nicotine and the metal center, while the N atom pyrrolidine unit engaged in the hydrogen bonding with the side chain of the porphyrin ring. Notably, we observed that the nature of the metal center dictated the extent of interaction with nicotine; particularly, Zn-porphyrin demonstrated a superior response compared to Ni-porphyrin. Furthermore, we performed the quantitative estimation of nicotine in commercially available tobacco products. Additionally, we conducted the antibacterial (Staphylococcus aureus and Escherichia coli) and antifungal (Candida albicans) activities of the porphyrin derivatives.

Ethyne Functionalized Meso-Phenothiazinyl-Phenyl-Porphyrins: Synthesis and Optical Properties of Free Base Versus Protonated Species

Synthesis, structural characterization and photophysical properties for a series of new trans-A2B2- and A3B-type ethynyl functionalized meso-phenothiazinyl-phenyl porphyrin derivatives are described. The new compounds displayed the characteristic porphyrin absorption spectra slightly modified by weak auxochromic effects of the substituents and fluorescence emission in the range of 651-659 nm with 11-25% quantum yields. The changes recorded in the UV-vis absorption spectra in the presence of trifluoroacetic acid (TFA) are consistent with the protonation of the two internal nitrogen atoms of the free-base porphyrin (19 nm bathochromic shift of the strong Soret band and one long wave absorption maxima situated in the range of 665-695 nm). Protonation of the phenothiazine substituents required increased amounts of TFA and produced a distinct hypsochromic shift of the long wave absorption maxima. The density functional theory (DFT) calculations of a porphyrin dication pointed out a saddle-distorted porphyrin ring as the ground-state geometry.

Femtosecond insights into direct electron injection in dye anchored ZnO QDs following charge transfer excitation

The role of the charge transfer (CT) state in interfacial electron transfer in dye-sensitized semiconductor nanocrystals is still poorly understood. To address this problem, femtosecond transient absorption (TA) spectroscopy is used as a probe to investigate the electron injection across a newly synthesized coumarin dye (8-hydroxy-2-oxo-4-phenyl-2 benzo[h]chromene-3-carbonitrile, coded BC5) and ZnO quantum dots (QDs). Steady state and time-resolved spectroscopic measurements reveal that BC5 dye interacts strongly with ZnO QDs in the ground state forming a CT complex. The BC5-ZnO QD complex absorbs more towards red compared to only the dye and QDs, and emits fluorescence due to radiative recombination of photogenerated charges. The formation of charges following the excitation of the CT complex has been demonstrated by observing the signature of dye radical cations and electrons in the conduction band (CB) of the QDs in the TA spectra. The TA signals of these charges grow sharply as a result of ultrafast direct electron injection into the QD. We have monitored the complete dynamics of photogenerated charges by measuring the TA signals of the charges up to a couple of nanoseconds. The injected electrons that are free or shallowly trapped recombine with a time constant of 625 fs, whereas deeply trapped electrons disappear slowly (526 ps) via radiative recombination. Furthermore, theoretical studies based on ab initio calculations have been carried out to complement the experimental findings.

Real-time observation of ultrafast electron injection at graphene-Zn porphyrin interfaces

We report on the ultrafast interfacial electron transfer (ET) between zinc(II) porphyrin (ZnTMPyP) and negatively charged graphene carboxylate (GC) using state-of-the-art femtosecond laser spectroscopy with broadband capabilities. The steady-state interaction between GC and ZnTMPyP results in a red-shifted absorption spectrum, providing a clear indication for the binding affinity between ZnTMPyP and GC via electrostatic and π-π stacking interactions. Ultrafast transient absorption (TA) spectra in the absence and presence of three different GC concentrations reveal (i) the ultrafast formation of singlet excited ZnTMPyP*, which partially relaxes into a long-lived triplet state, and (ii) ET from the singlet excited ZnTMPyP* to GC, forming ZnTMPyP˙(+) and GC˙(-), as indicated by a spectral feature at 650-750 nm, which is attributed to a ZnTMPyP radical cation resulting from the ET process.

Molecular-structure Control of Ultrafast Electron Injection at Cationic Porphyrin-CdTe Quantum Dot Interfaces

Charge transfer (CT) at donor (D)/acceptor (A) interfaces is central to the functioning of photovoltaic and light-emitting devices. Understanding and controlling this process on the molecular level has been proven to be crucial for optimizing the performance of many energy-challenge relevant devices. Here, we report the experimental observations of controlled on/off ultrafast electron transfer (ET) at cationic porphyrin-CdTe quantum dot (QD) interfaces using femto- and nanosecond broad-band transient absorption (TA) spectroscopy. The time-resolved data demonstrate how one can turn on/off the electron injection from porphyrin to the CdTe QDs. With careful control of the molecular structure, we are able to tune the electron injection at the porphyrin-CdTe QD interface from zero to very efficient and ultrafast. In addition, our data demonstrate that the ET process occurs within our temporal resolution of 120 fs, which is one of the fastest times recorded for organic photovoltaics.

Quantum confinement-tunable intersystem crossing and the triplet state lifetime of cationic porphyrin–CdTe quantum dot nano-assemblies

We show the possibility of modulating the triplet-state lifetime of cationic porphyrins on the surface of CdTe quantum dots.