Department of Chemistry
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Item Enhanced TNT vapor detection via a donor–acceptor-based imine cross-conjugated aggregation-induced enhanced emission active porous polymer(ACS, 2025-09) Laskar, Inamur RahamanThe development of sensitive and selective probes for the detection of nitro explosives is critical for ensuring public safety and environmental monitoring. Among various detection strategies, porous materials offer significant advantages for vapor-phase detection due to their high surface area and analyte-trapping capability. In this study, we report the design and synthesis of an electron-rich system with a donor–acceptor (D–A)-based organic porous polymer (P1), incorporating triphenylamine as the electron-donating unit and imine-conjugated sulfone (SO2) functionalities as electron acceptors. The resulting aggregation-induced enhanced emissive (AIEE) porous network exhibits selective fluorescence quenching in the presence of nitro explosives, particularly picric acid (PA) and 2,4,6-trinitrotoluene (TNT) in aqueous media. Notably, in the vapor phase, P1 demonstrates a strong and selective response to TNT vapors with a detection limit of 50 ppb, attributed to its higher vapor pressure compared to PA. Experimental and density functional theory (DFT) mechanistic investigations revealed distinct sensing pathways: Förster resonance energy transfer governs PA detection, while photoinduced electron transfer is responsible for TNT sensing. The high porosity of the polymer, confirmed through FESEM imaging and BET surface area analysis, facilitates efficient analyte capture, contributing to its superior vapor-phase sensitivity.Item Photophysics of donor-naphthalimide systems: hidden charge transfer states and emissive pathways governed by vibronic coupling(2025-10) Laskar, Inamur RahamanAlkyl-substituted 1,8-naphthalimide (NI) derivatives are promising luminophores for organic light-emitting diodes (OLEDs), photopolymerization, photoinitiation, and thermally activated delayed fluorescence (TADF). In solution, these compounds exhibit a dominant absorption band at 320-375 nm attributed to a locally excited (LE) state, with no absorption seen beyond 400 nm. This CT absorption beyond 400 nm has been debated, with conflicting claims of its presence and absence by authors without any definitive proof. We demonstrate the presence of a CT absorption beyond 400 nm, which, however, remains “quasi-dark” in solution, even at concentrations up to 10-4 M. This hidden state becomes bright during emission by gaining oscillator strength via molecular planarization and intensity borrowings from the neighboring state. DFT ground-state and coupled-cluster (CC2) excited-state calculations confirm the vanishing oscillator strength of the CT transition in absorption and emission brightening via excited-state geometry changes. Notably, decay kinetics of ultrafast transient absorption with 35-fs excitation reveal coherent Rabi oscillations assigned to vibronic coupling between LE and CT states. Time-resolved emission shows nanosecond prompt fluorescence and microsecond delayed fluorescence from triplet-triplet annihilation (TTA), with one compound exhibiting TADF due to a small ΔEST (0.16 eV) in the solid state. These findings clarify the excited-state dynamics of NI derivatives and highlight the critical role of structural relaxations and LE/CT vibronic coupling in absorption/emission of these molecules, providing design principles for more efficient photocatalysts, photoinitiators, and OLEDs.Item Utilizing inherent couplings in thermally activated delayed fluorescence to mitigate its trade-off with absorption and realizing universal dual delayed fluorescence(2025-11) Laskar, Inamur RahamanCharge-transfer (CT) states govern the performance of optoelectronic and biomedical materials but suffer from an intrinsic dilemma: enhancing CT absorption through orbital overlap inevitably widens the singlet–triplet gap (ΔEST), destroying thermally activated delayed fluorescence (TADF) and crippling emission efficiency. This long-standing absorption-TADF trade-off restricts the brightness of CT-based fluorophores and the efficiency of organic light-emitting diodes (OLEDs), while no molecular class has yet achieved dual TTA–TADF functionality across the entire visible spectrum. Here, a new paradigm is proposed by elevating vibronic coupling (VC) from a secondary phenomenon to a central molecular design principle. Through the Herzberg–Teller mechanism, weak CT transitions borrow oscillator strength from nearby locally excited (LE) states, redistributing intensity without increasing HOMO-LUMO overlap. This approach decouples absorption enhancement from geometric constraints, enabling simultaneous strong absorption and a small ΔEST, which is essential for efficient TADF. By directionally aligning the transition dipole moments (TDMs) of the LE and CT states, intensity borrowing enhances both excitation and radiative emission, thereby retaining orthogonal donor-acceptor geometries that are compatible with triplet harvesting. This strategy is further extended to dual TTA-TADF systems, providing a unified framework for achieving broadband fluorescence with inherent triplet management. Harnessing built-in VC allows computational pre-screening of promising structures before synthesis, unlocking universal design flexibility for OLEDs and biolabels. The proposed approach transforms an unavoidable photophysical compromise into a tunable parameter, providing a pathway toward high-efficiency, low-phototoxic materials for next-generation optoelectronics and biomedical imaging.Item Shedding light on photo-redox catalysis of NIR iridium (iii) complex for high photocytotoxicity against Cis-platin resistant ovarian cancer in 3d tumor spheroids(RSC, 2025-12) Laskar, Inamur Rahaman; Roy, AniruddhaThe clinical translation of photodynamic therapy (PDT) is often hindered by the lack of photosensitizers (PSs) that achieve potent cytotoxicity at clinically relevant concentrations. This limitation stems from an incomplete understanding of photochemical mechanisms underlying PDT efficacy. Here, we report RM2, a near-infrared Ir(III)-based PS with a high molar absorption coefficient (17,700 M-1 cm-1 at the excitation wavelength), a key feature for attaining low IC50 values. Upon irradiation, RM2 undergoes two distinct single-electron transfer (SET) pathways: catalytic oxidation of NADH with a turnover frequency (TOF) of 690 h-1, and Type-I ROS generation (O₂•⁻ and H2O2), confirmed by EPR spectroscopy and H2O2 detection assays. In addition, RM2 displays a triplet state energy of 37.68 kcal mol-1 (1.63 eV, 762 nm) with a 5 µs lifetime, enabling efficient energy transfer to 3O2 and a singlet oxygen quantum yield (ϕΔ) of 0.75 in cell-free media. Encapsulation of RM2 in DSPE-mPEG2000 nanoparticles further amplified its activity, enhancing photocytotoxicity by 8.3-fold and achieving an IC50 of 60 nM against ovarian cancer cells. Remarkably, RM2 NPs retained this potency in cisplatin-resistant ID8 cells (IC50 = 60 nM), whereas cisplatin itself showed drastically reduced efficacy (IC50 = 10.46 µM in wild-type and 30.41 µM in resistant cells). Additionally, RM2 NPs exhibited pronounced efficacy against 3D ovarian tumor spheroid models, underscoring their translational potential. These results establish RM2 as a multifunctional photosensitizer, in which the synergy of long-lived triplet energy and favorable redox potentials enables diverse photochemical mechanisms, encompassing NADH oxidation and both Type-I and Type-II ROS generation. This multifaceted mechanism of action offers a powerful strategy to overcome hypoxia and drug resistance, significantly advancing the potential of PDT for effective cancer therapy.Item Pyrene-based AIEE-active vertically grown luminescent material for selective and sensitive detection of TNT vapor(ACS, 2024-10) Laskar, Inamur RahamanPyrene-based molecules often suffer from the “aggregation-caused quenching” (ACQ) effect because of their rigid planar structure having several π–π stacking interactions, which limit their applications as solid-state luminescent materials. From this perspective, it has been strategized to develop two compounds: 2-(pyren-1-yl)-4,6-bis(4-vinylphenyl)-1,3,5-triazine (VinTr) and 4-chloro-N,N-diphenyl-6-(pyren-1-yl)-1,3,5-triazin-2-amine (PyTrDA) in such a way that pyrene triazine frameworks are transformed into “aggregation-induced enhanced emission” (AIEE)-active molecules. All of the compounds showed positive responses to the quenching of trinitrotoluene (TNT). Within these compounds, PyTrDA showed excellent results on sensing TNT with a high level of sensitivity (limit of detection = 216 pM in solution and ∼7.0 ppb in the vapor phase) and selectivity, extending the results from the solution to the vapor phase. The quenching process is due to the photoinduced electron transfer (PET) from the probe (PyTrDA) to the analyte (TNT), which was confirmed by transient absorption spectroscopy. In addition to the relatively large quantum yield of PyTrDA, the morphology transformation from a planar sheet-type structure (observed in PyTr) to a vertically grown nanorod (in PyTrDA) offers increased surface area. The vertically grown nanostructural morphology of PyTrDA should properly facilitate the diffusion of TNT molecules and provide a confined environment, where one-to-one host–guest interactions between the probe molecule and analytes are possible. To the best of our knowledge, this is the first study that explores the role of nanostructural morphology with an enhanced surface area for improved TNT sensing using small organic moleculesItem Facile, selective and cost-effective detection of creatinine from human urine using a cyclometalated dinuclear iridium(III) complex through creatinine-triggered emission(RSC, 2025-02) Laskar, Inamur RahamanA new cyclometalated oxalyl-bridged dinuclear iridium(III)-based phosphorescent complex (M3) was synthesized, which detected creatinine in the solid phase by increasing the emission intensity with a blue shift. The probe M3 was synthesized in a straightforward synthetic route by forming a dichloro-bridged iridium(III) intermediate. The creatinine detection process was developed on a low-cost filter paper strip impregnated with M3, and it was successfully tested on human urine samples. It was observed that the probe was highly selective towards creatinine, and no response was observed with most of the other components present in human urine. Furthermore, the mechanism of creatinine detection was rationally explored.Item Electronic substitution effect on esipt-driven ph and amine sensing: exploring mechanism(Wiley, 2025-01) Laskar, Inamur RahamanIt is required to have a more straightforward and easier way to check the quality of food to ensure the safety of the public health. The decomposition of meat protein results in ammonia and biogenic amines (BAs). Consequently, to evaluate the safety and quality of meat products throughout the storage, transit, and consumption depends on the sensitive detection of the released BAs. Here, we have designed and synthesized three luminescent-based probe molecules, which originated from 2-(2-hydroxyphenyl) benzothiazole (HBT) derivatives and showed the excited state-induced proton transfer (ESIPT) phenomenon. The two substituents (OMe and NO2) were used rationally at the para position of HBT, and the electronic properties were evaluated using Hammett substituent constants. The proton donating ability of the O−H to the acceptor is largely facilitated by the presence of a strong electron-withdrawing group, which in this case is NO2. The proton transfer rate can be controlled, and in this case, to a slower rate with the influence of the electron donating group OMe. The controllability of proton transfer led us to use it in pH sensing. A prominent and multi-color change with pH variation was observed in the case of the OMe substituted compound. These probes were further employed for amine sensing, and the limit of detection (LOD) was determined to be 28.6 μM and 61.34 nM for ammonia and hydrazine, respectively. In addition, strip-based detection of spoilage of chicken meat was studied for real-world applications via both contact and non-contact modes.Item Development of a Multifunctional Aggregation-Induced Emission-Active White Light-Emissive Organic Sensor: A Combined Theoretical and Experimental Approach(ACS, 2022-07) Laskar, Inamur Rahaman; Roy, Ram KinkarA far-red to near-infrared (NIR) “aggregation-enhanced emission” (AEE)-active donor–acceptor (D–A)-type probe (denoted as IMZ-CN) is designed and synthesized. The probe IMZ-CN is designed rationally using the quantum mechanical gap tuning (highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy gap, i.e., ΔEg) approach. The probe is tethered with two different functionalities, i.e., dicyanovinyl (DCV) and benzimidazole (IMZ), which effectively lower the value of ΔEg and cause emission in the far-red to near-infrared region. Furthermore, it selectively detects cyanide (CN–) and fluoride (F–) ions by turn-on emission in pure water without interference between each other. Apart from CN–/F– sensing, the probe (IMZ-CN) is also sensitive toward the acidic environment due to the presence of potential basic nitrogen on the benzimidazole unit. The binding of CN–/F– induces a blue shift in the electronic spectra of IMZ-CN, whereas an acidic environment (e.g., the change in pH from 7 to 2.3) causes red-shifted emission (∼60 nm). Interestingly, IMZ-CN displays a nearly pure white emission during the course of the aggregation-enhanced emission (AEE) mechanism study in the PEG–water binary mixture (99%) with CIE coordinates (0.30, 0.33). The mechanisms behind the emission of white light and the anion-binding/sensing applications (photophysical properties of the probe) are supported by quantum mechanical calculations [using “frontier molecular orbitals” (FMO), “time-dependent density functional theory” (TD-DFT), and “natural transition orbital” (NTO) calculations]. As this multifunctional probe IMZ-CN interacts with CN–, F–, H+, etc., the reactivity parameters [e.g., global chemical hardness (η), global electrophilicity (ω), etc.] were calculated by applying the concept of “density functional reactivity theory” (DFRT) to validate its reactivity.Item Enhanced TNT vapor sensing through a PMMA-mediated AIPE-active monocyclometalated iridium(III) complex: a leap towards real-time monitoring(RSC, 2024-02) Laskar, Inamur RahamanBased on the explosive nature and harmful effects of nitro-based explosive materials on living beings and the environment, it is extremely important to develop luminescence-based probe molecules for their detection with excellent selectivity and sensitivity. Two AIPE (aggregation-induced phosphorescence emission)-active iridium(III) complexes (M1 and M2) were developed for the sensitive detection of TNT in both contact and non-contact modes. The aggregate solutions of both complexes (M1 and M2 in THF/H2O, 1/9 by volume) detected TNT at the pico-molar (pM) level. These complexes showed greatly enhanced emission intensity while embedded in a PMMA(polymethyl methacrylate) matrix film. The amplified quantum efficiency, improved phosphorescence lifetime, and enhanced porous network of M2-PMMA composite helps to improve the sesitivity of TNT vapor detection. Interestingly, the sensitivity of the detection of TNT by the M2 complex was significantly improved (5-fold) in a PMMA-incorporated complex (CP) with an observed limit of detection (LOD) of 12.8 ppb. From the BET analysis of CP, it was observed that the mesoporous network of CP has an average pore diameter of 8.52 nm and a surface area of 2.03 m2 g−1. The porous network of CP assists in trapping TNT vapor in a polymeric network containing an electron-rich probe (iridium(III) complex, M2), which helps to effectively trap TNT, thus enhancing electronic communication. As a result, significant emission quenching was observedItem Precise molecular design for a twisted pyrene-thiophene based mechanofluorochromic probe with large Stokes shift and feasibility study towards security ink and re-writable papers(RSC, 2024) Laskar, Inamur RahamanThe creation of MFC-active smart molecules by tuning functionality has received considerable attention owing to its versatile applicability. Pyrene-based twisted donor–acceptor (D–A) dyes (PySS and PySP) have been synthesized and characterized. Here, the pyrene is directly connected with thiophene, and this unit is further linked terminally to photoactive species (thiophene/pyridine) via a four-carbon unit conjugated spacer. These molecules show excellent solvatochromic properties, with a substantial shifting of the emission wavelength (PySS- 147 nm and PySP- 130 nm). The lowest transition state contains a significant contribution from ICT characteristics, as evidenced by spectral analysis and computational calculations. Moreover, these are identified as ‘aggregation-induced enhanced emission’ (AIEE) active compounds and exhibit mechanofluorochromism (MFC). By grinding, PySS and PySP display MFC features with 50 nm and 54 nm red shifting, respectively. Interestingly, PySS shows a gradual emission change from green (510 nm) to orange emission (578 nm) by gradually changing the pressure with a hydraulic press (0 to 12.5 tons). The single crystal structure of both compounds was investigated to understand the structure–property relationship for MFC. The crystal packing shows that the twisted molecules (dihedral angle between pyrene and thiophene is 59.36° and 56.93° for PySS and PySP, respectively) are loosely bound with several weak interactions (C–H⋯π, C–π⋯H, H⋯H, C–H⋯O). Interestingly, it was observed that two molecules in a unit cell are arranged in an antiparallel fashion; these molecular pairs are linearly connected to another pair axially, forming a long one-dimensional chain-type arrangement. On applying pressure, these twisted molecular pairs may slowly planarize, leading the molecules to come closer, thus changing the molecular interaction and the emission properties. A feasibility study of the potentiality of using these compounds in data encryption–decryption and security ink has also been demonstrated.