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Item Investigation of Aggregation Induced Emission Mechanism of Tetrabenzoheptafulvalene Derivative by Spin-Flip Time-Dependent Density Functional Theory (SF-TDDFT)(Wiley, 2025-02) Roy, Ram KinkarThis study explores the mechanism of aggregation-induced emission (AIE) in the tetrabenzoheptafulvalene derivative, 10,10′,11,11′-tetrahydro-5,5′-bidibenzo[a,d][7]annulenylidene (abbreviated as THBDBA) in tetrahydrofuran (THF) solution. THBDBA is AIE-active because in THF solution, it emits significantly less emission (or almost non-emissive) and the fluorescence quantum yield increases by 230 times in aggregate state. We adopted spin-flip time-dependent density functional theory (SF-TDDFT), widely acknowledged for its ability to locate the conical intersection (CI) in medium to large-sized molecules (due to its balanced and reliable description of both ground and excited states and ability to capture double excitation and multireference characters at low computational cost). The functional used is long-range corrected ωPBEh (i. e., LRC-ωPBEh). The strategies used are the excited state deactivation processes by taking into account the S1/S0 surface crossing, referred to as the ‘minimum energy conical intersection’ (MECI). Reduction of oscillator strength near the minimum energy gap (MEG) structure or CI is also another parameter used to study fluorescence quenching. For the monomer (i. e., in solution), our findings reveal a significant reduction in oscillator strength (f) for de-excitation near the MEG structure and CI, which led us to conclude that in solution, the flapping motion of the phenyl rings plays a vital role to reach the CI. In a smaller scale, a dimer system was chosen to represent the aggregate state. The higher energy gap as well as higher f-value at MEG structure with just the model dimer system indicates that in the actual aggregate (or the crystal) the MECI might be absent. This is because in the aggregate the flapping motion of the phenyl rings will be highly restricted (because of the steric and electrostatic confinements by a large number of monomers from all sides), thereby favoring radiative transitions for energy dissipation. This study consequently elucidates the AIE mechanism of the chosen tetrabenzoheptafulvalene derivative, shedding light on its photophysical properties.Item Aggregation-induced emission mechanism of styrene derivative: a theoretical study(RSC, 2025-05) Roy, Ram KinkarThe aggregation-induced emission (AIE) mechanism of the fluorescent styrene derivative 4-dimethylamino-2-benzylidene malonic acid dimethyl ester (BIM) in methanol solution is theoretically investigated using spin–flip long-range corrected time-dependent density functional theory (SF-LC-TDDFT). The potential energy surfaces (PESs) for the ground (S0) and first singlet excited (S1) states of BIM were calculated along the rotation of the aryl main axis (α angle rotation), consistent with experimental observations. For the monomer, our findings reveal a significant reduction in oscillator strength, approaching zero at the optimized geometry in the S1 state. As this state corresponds to a charge transfer state, it suggests that the BIM monomer operates as a twisted intramolecular charge transfer (TICT) system, undergoing quenching through α angle rotation. The restriction of TICT, and consequently the inhibition of fluorescence quenching in the aggregate state, is also investigated by extracting the coordinates of 13 monomers from the crystal structure of BIM. The α-torsional angle of the central monomer was manually rotated in both clockwise and anti-clockwise directions to assess the intramolecular restrictions within the constrained environment. This analysis reveals that even a 10° rotation of the α-torsional angle, in either direction, causes the atoms of the central monomer to come into close contact with the atoms of the neighboring monomers. These short contacts effectively inhibit the TICT process, thereby leading to aggregation-induced emission.Item Bimolecular quenching rate of electron transfer reactions from aromatic amines to coumarin dyes: a conceptual density functional theory-based approach(ACS, 2025-07) Roy, Ram KinkarIn the current work, a qualitative trend of the quenching rate of a series of bimolecular electron transfer reactions is computed through an alternative and cost-effective approach. The electron transfer reactions from eight aromatic amines in their ground state to a series of six substituted coumarin dyes in their singlet excited states (S1) are chosen as representative quenching processes. The acetonitrile solvent is used as the reaction medium. While the reaction free energy values are evaluated through conceptual density functional theory (CDFT) [or CDASE-scheme, to be more specific]─based stabilization energy, the reorganization energy values are calculated using a conventional method already prescribed in the literature. The reactions, being diffusion-controlled ones, only outer-sphere reorganization energy (or, solvent reorganization energy) is considered, neglecting the intramolecular reorganization energy (as suggested by the corresponding experimental study available in the literature). The generated data demonstrate that as the absolute values of the CDFT-based stabilization energy become closer to the corresponding reorganization energy, the experimental quenching rate constant values of the chosen bimolecular electron transfer reactions increase. This observation exactly correlates with the Marcus theory of electron transfer reaction. Thus, the authors would like to claim that the Marcus theory is validated through an unconventional approach based on conceptual density functional theory (CDFT).Item Unsymmetrical nne (e = s, se) pincer palladium(ii) complexes: syntheses, structure, and catalytic activity in decarboxylative heteroarylation of coumarin-3-carboxylic acids(ACS, 2025-04) Roy, Ram Kinkar; Kumar, AnilThis report describes the syntheses of two novel benzoxazolyl-derived unsymmetrical NNE (E = S, Se) pincer ligands and their palladium(II) complexes. Treatment of benzoxazolyl-derived pincer ligands L1 and L2 with PdCl2(CH3CN)2 provided complexes C1 and C2, respectively, in which the NNE ligands coordinated to Pd in a tridentate pincer manner. The ligands and complexes were characterized by 1H and 13C Nuclear Magnetic Resonance (NMR), Fourier-transform infrared spectroscopy (FTIR), high-resolution mass spectrometry (HRMS), UV–visible, X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry techniques. The structure of the ligand and its coordination mode with the palladium center was established using a single-crystal X-ray diffraction technique. The complex C2 showed a pincer coordination mode with a distorted square planar geometry. The catalytic performance of palladium complexes C1 and C2 was investigated for the decarboxylative heteroarylation of coumarin-3-carboxylic acid. Notably, heteroarenes could give up to 80% of heteroarylation products in the presence of 5 mol % of the catalyst. A broad substrate of heteroarenes showed tolerance toward the developed protocol. Among complexes, the selenium ligand coordinated complex (C2) outperformed the sulfur ligand coordinated complex C1, which may be due to stronger σ-donation by the Se atom.Item Validation of Hammett’s Linear Free Energy Relationship Through an Unconventional Approach(ACS, 2020) Roy, Ram KinkarThe present study tries to validate Hammett’s linear free energy relationship through an unconventional approach based on the density functional reactivity theory (DFRT). A kinetic energy component [ΔEB(A)], derived from the DFRT-based comprehensive decomposition analysis of stabilization energy scheme, is used to verify the linear nature of Hammett’s log(kX/kH) versus σ plot. The study shows that the versus σ plot (where −X is the atom or group substituted in place of −H) is linear in nature (with reasonably high correlation coefficient values) for different series of reactions. The slopes of the plots also reveal the electrophilic or nucleophilic nature of the transition states as is obtained from the conventional log(kX/kH) versus σ plot. The study thus establishes that the DFRT-based energy component ΔEB(A) (which is very easy to compute) can be used, instead of k-values, obtained either from the experiment or from computationally intensive conventional thermochemistry calculations to generate reliable Hammett’s plot.Item Hammett constants from density functional calculations: charge transfer and perturbations(Springer, 2022-01) Roy, Ram KinkarThermodynamic and kinetic components of density functional reactivity theory -based stabilization energies between interacting electron acceptors and electron donors are evaluated with and without taking into account perturbative effects on one reactant caused by the other. The values of the two energy components generated through these two approaches are then correlated to the Hammett’s substituent constant through the relation . Here and represent, respectively, energy components (either thermodynamic or kinetic) of the substituted and unsubstituted benzene derivatives. The generated data on six different series of reactions demonstrate that both perturbative and unperturbative approaches are of comparable reliability when either thermodynamic or kinetic energy components are used in the proposed relation, justifying the validity and generality of Hammett’s free energy relation.Item Components of density functional reactivity theory-based stabilization energy: descriptors for thermodynamic and kinetic reactivity(Elsevier, 2023) Roy, Ram KinkarDensity Functional Reactivity Theory (DFRT) based global (i.e., for the whole chemical system) descriptors are used to explain various types of chemical interactions in this chapter. Some formal developments as well as applications of DFRT based CDASE (Comprehensive Decomposition of Stabilization Energy) scheme e.g., (i) Full profiles of the kinetic and thermodynamic energy components of the DFRT based stabilization energy and the role of perturbation on external potential; (ii) Effect of solvent on stabilization energy; (iii) Correlation of equilibrium constant with stabilization energy; (iv) Validation of Hammett's Linear Free Energy Relationship (LFER) through DFRT are discussed in details to gain some insight in chemical reactivity.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 Screening the Band Shape of Molecules by Optimal Tuning of Range-Separated Hybrid Functional with TD-DFT: A Molecular Designing Approach(ACS, 2022-08) Roy, Ram KinkarIn the present article we have demonstrated the effectiveness of optimally tuned range-separated hybrid (RSH) functional to determine the electronic transitions from two fluorophore moieties (blue and yellow/orange) within a single white light emitter (WLE). First, the optimally tuned range separation parameter (ω) is calculated for two white emitting fluorophores (W1 and W2) already reported in the literature. The success of the optimally tuned RSH functional ω*B97XD, used in the TDDFT study, encouraged the authors to design eight new single organic white light emitters with frustrated energy transfer between the two individual fluorophore moieties (blue and yellow/orange). The simulated spectra (the band shapes, to be more specific) generated by TDDFT study and outcomes through natural transition orbital (NTO) and natural bond orbital (NBO) studies clearly demonstrate that all the designed eight organic molecules are potential white light emitters and can be synthesized in future.Item Principle of maximum hardness: an accurate ab initio study(ACS, 1993) Roy, Ram KinkarIn this paper we test the principle of maximum hardness through an accurate quantum chemical calculation. Computations include extensive correlation and relaxation effects for the calculation of ionization potential and electron affinity. The molecule water has been chosen as a primary test case.