Arunkumar Kathiravan

The analytical performance of fluorescent chemosensors is fundamentally governed by the interplay among intramolecular charge transfer efficiency, coordination-site accessibility, and receptor−analyte binding affinity. However, in compact fluorophore−receptor scaffolds, planar fluorophores and sterically demanding receptor units can impose torsional strain, attenuating donor−acceptor electronic transitions while restricting analyte access to the coordination cavity. To delineate this structure−performance relationship, a sterically congested D−A probe was investigated as a structural counterpart to its phenyl π-spacer-integrated D−π−A analogue. The π-spacer serves as an internal preorganization unit that relieves steric strain, preserves near-global coplanarity, improves receptor accessibility, and promotes ICT characteristics. As a result, the D−π−A probe exhibited stronger affinity toward Cu2+ and Zn2+ ions with superior detection limits of 1.86 nM and 0.85 nM, respectively. Complementing the spacer-mediated internal preorganization, Zn2+ coordination provides an external supramolecular template that organizes the receptor into a binding-competent geometry for sequential Cu2+ recognition. This template effect was more pronounced for the sterically congested D−A probe, increasing its apparent Cu2+ binding constant by nearly 1 order of magnitude (1.59 × 105 M−1) with an improved detection limit of 1.06 nM. Nevertheless, the D−π−A + Zn2+ complex remained the most sensitive system, achieving an enhanced detection limit of 0.72 nM. Furthermore, the practical applicability of the probes was demonstrated by their reliable performance in mixed aqueous media, sensitive recognition of Cu-GHK, spike-recovery studies, and robust performance in both fluorescent paper-strip assays and live-cell imaging across mammalian and plant cells. Overall, this work establishes π-spacer-mediated internal preorganization and Zn2+ template-assisted external preorganization as complementary strategies for designing efficient fluorescent chemosensors.

Elucidating excited-state conformational dynamics in D–π–A systems is central to the development of functional molecules. In particular, the emergence of twisted intramolecular charge transfer (TICT) states offers a powerful handle to regulate electronic relaxation pathways through molecular design. Herein, a systematic investigation of two structurally similar D–π–A chromophores (SV27 and SV31) is engineered to either suppress or facilitate excited-state twisting. The control molecule SV27 exhibits limited conformational flexibility and relaxes predominantly through a locally excited state, whereas the strategic incorporation of a sterically demanding donor fragment in SV31 induces substantial excited state torsional reorganization, leading to the stabilization of a TICT state. A combination of steady-state spectroscopy, time-resolved fluorescence measurements, and quantum chemical calculations reveals a strong coupling between molecular architecture, excited-state potential energy surfaces, and charge transfer dynamics. Solvent-dependent studies further demonstrate that higher polarity of the medium selectively stabilizes the twisted charge separated state, which is identified as a crucial element for the accessibility of TICT. Upon leveraging this sensitivity to the local environment, the TICT-active chromophore SV31 is employed as a highly responsive fluorescent probe for trace water detection in organic solvents, achieving trace level sensitivity (30 ppm). Altogether, this study offers a rational framework by correlating molecular architecture with excited-state twisting and charge separation, providing a rationale for activating TICT and laying the foundation for the development of advanced functional molecular architectures for the detection of water.

Excitation wavelength-dependent emission is a unique photophysical channel with significant implications in analytical and biological applications; however, such phenomena remain rarely explored. To address this, carbon dots (CDs) were synthesized via a microwave-assisted method and systematically characterized using SEM, TEM, FTIR, XPS, UV-visible absorption, and fluorescence spectroscopy. Spectroscopic studies reveal that the emission from CDs strongly depends on the excitation wavelength, confirming the presence of energetically distributed and heterogeneous emissive surface states. These surface states enable a selective population of distinct excited states, thereby modulating the emission characteristics. The global analysis also identified three emissive pathways in the CDs. These pathways have a short lifetime of about 1 ns, an intermediate lifetime that corresponds to the intrinsic citrazinic acid emission (6.3 ns), and a long lifetime that can extend up to 10.8 ns. This provides further support for the presence of multiple surface states that contain rapidly decaying traps and emissive traps that have longer radiative lifetimes compared to free citrazinic acid. On the other hand, the sensing performance was also governed by excitation wavelength, indicating that different excited states possess varying electron transfer quenching efficiencies toward Hg²⁺ ions. The Stern-Volmer analysis and time-resolved fluorescence decay measurements reveal an electron transfer quenching mechanism, where reduced fluorescence lifetimes and negative free energy change values confirm thermodynamically favorable electron transfer from excited CDs to Hg²⁺ ions. Furthermore, the CDs exhibit reversible sensing behavior with l-cysteine, restoring fluorescence via the competitive chelation of Hg²⁺ ions, as supported by steady-state and time-resolved fluorescence measurements. Cytotoxicity and bioimaging studies have been performed on human lung carcinoma (A549) cells that reveal negligible toxicity and enable intracellular imaging of Hg²⁺ quenching and l-cysteine-mediated recovery. Overall, the obtained results on CDs establish a direct correlation between excitation-dependent photophysical properties and sensing mechanisms, providing a rational framework for the design of wavelength-dependent metal-ion detection and live-cell imaging applications.

The persistent trade-off between sensitivity and linear concentration range in reaction-based chemosensors remains a critical limitation for rapid analyte detection with high precision. This significantly limits the practical deployment of these reaction-based chemosensors, especially in the detection of hazardous ions such as cyanide. Addressing this challenge, we rationally developed donor-π-acceptor fluorescent probes, TRMN and TπRMN, featuring triphenylamine donors, conjugated rhodanine acceptors, and differentiated π-spacers. Concurrently, their parent aldehydes were investigated to understand their photophysical baseline, wherein the spacer rigidity suppresses the nonradiative process. Both TRMN and TπRMN exhibit rapid (b]indole spacer. Additionally, the rigid spacer design constrains the conformational freedom upon adduct formation, suppressing the nonradiative decay processes. These factors collectively enable TπRMN to achieve an extended linear range of 8-34 μM with a detection limit of 4.2 nM. On the other hand, the probes exhibit excellent solid-state fluorescence with a pronounced bathochromic shift relative to their solution state and thus have been employed for latent fingerprint imaging. Overall, this work establishes spacer-acceptor synergy engineering as a generalizable strategy to overcome fundamental sensitivity-range trade-offs, providing a comprehensive framework for next-generation reaction-based chemosensors in environmental monitoring.

Donor-acceptor-based fluorescent molecules have garnered considerable attention as excellent tool for trace detection of Cu²⁺ ions, owing to their tunable photophysical properties and higher sensitivity. Concurrently, these molecules may occasionally display undesired fluorescence quenching due to several factors, thereby diminishing selectivity. To address this concern, this work focuses on developing selective and reversible fluorescent sensors for Cu²⁺ ions. To pursue this, novel fluorescent probes were designed by incorporating donor moieties like diphenylamine and dimethylamine into an imidazopyridine unit. The probes were synthesized and characterized using conventional techniques. As synthesized probes exhibit strong fluorescence attributed to intramolecular charge transfer, confirmed by density functional theory calculations. The HOMO is localized on amine moiety, while LUMO resides on imidazopyridine segment. Utilizing an electron-rich pyridine acceptor as a selective binding site, both probes exhibit exceptional specificity toward Cu²⁺ ions over other competing metal ions. The binding of Cu²⁺ induces the ground-state complex formation, evidenced by the appearance of an isosbestic point in the absorption spectrum. This interaction results in a "turn-off" fluorescence response, disrupting the intramolecular charge transfer process that governs donor-acceptor fluorescence. The fluorometric titration method was employed to assess the sensitivity of probes for Cu²⁺ ions, which exhibited excellent linearity at lower concentrations, achieving a detection limit at picomolar levels. Moreover, owing to their low toxicity, validated by the MTT assay, the probes were successfully utilized for detecting Cu²⁺ ions in naturally occurring copper proteins. Consequently, the probe comprising of imidazopyridine unit serves as a viable tool for the detection of Cu²⁺ ions within intricate biological systems.