We are working on developing new analysis to extract quantitative information in photophysical studies. We also focus on solving long-standing controversies in various fields. We also apply theoretical tools (quantum mechanical calculations and molecular dynamics simulations) to support experimental results. A brief overview of our works is given below.
Site-selective excited-state proton transfer (ESPT) inside surfactant assemblies
We exploit the site-selective location of the photoacid 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) to probe different regions of a surfactant assembly, in particular, micelle and reverse micelles comprising of charged surfactants. Since HPTS is highly anionic (containing three sulphonate groups), it prefers to stay at the core of an anionic reverse micelle, whereas, at the interfacial region in a cationic reverse micelle. Using the concept, we obtained many important findings.
1. We showed that the miscibility of a compound in confined water is different from ordinary water. The water pool of the AOT/n-heptane/water reverse micelle can be a source of water with variable confinement. We found that water-immiscible long-chain alcohol octanol becomes highly soluble in the highly confined water (water confined in a small pool at w0=5). (ChemComm 2015, 51, 14103-14106).
In a cationic reverse micelle, water/ benzylhexadecyldimethylammonium chloride (BHDC)/benzene HPTS shows much slower and less sensitive ESPT dynamics. The non-negligible ESPT indicates that the interface region of reverse micelle remains hydrated (wet) due to significant penetration of water molecules in the interface (Langmuir 2015, 31, 12587-12596). We also showed the presence of substantial hydration at the interface of CTAB quaternary reverse micelle, which was conceived as perfectly dry in a previous study. Furthermore, we showed that the hydration level depends on the nature and amount of cosurfactant used (Langmuir 2016, 32, 10659-10667). The interfacial hydration modulates with the morphology of a reverse micelle. The DDAB/water/cyclohexane reverse micelles showed a remarkable change in the ESPT dynamics during the rod to sphere transition triggered by the increase of w0 above 8 (Langmuir 2016, 32, 6656-6665).
We investigated the interfacial hydration of micelles formed by cationic (DTAB and CTAB) and zwitterionic sulfobetaine (SB12 and SB16) surfactants. Sulfobetaine surfactant contains both positive quaternary ammonium and negative sulfonate groups in their headgroup and hence are essentially electroneutral. We found that both micelles bear common characteristics; the anionic HPTS binds strongly to both the micellar interfaces and display very slow dynamics. The hydration level in the zwitterionic micelle is found to be less hydrated than the cationic micelle (Phys. Chem. Chem. Phys. 2017, 19, 31461-31468). Interaction of the anionic HPTS with cationic and zwitterionic micelle was found to be very different in the premicellar region (J. Photochem. Photobiol. A 2018, 357, 140-148).
However, our best contribution could be the development of a new analysis method for ESPT dynamics from the ratio of time-resolved area-normalized spectra (TRANES). In the prevailing literature, ESPT dynamics were usually interpreted from the fluorescence decays of the protonated or the deprotonated forms by a multi-exponential analysis or by model-specific equations. However, the new method is very convenient, model-free, and highly intuitive. The ratio of steady-state protonated and deprotonated emission intensities are often used to express the favorability of ESPT in a chemical system; however, for the first time, we extend the ratio analysis to the time-resolved ESPT. (J. Phys. Chem. B 2018, 122, 6610-6615). Subsequently, we refined the method and presented a complete analytical expression for the time-dependent ratio from the much-celebrated Eigen-Weller model. We applied the analysis to the ESPT of HPTS in a methanol-water mixture (J. Photochem. Photobiol. A 2019).
​
Effect of Hydrogen bond on photoinduced electron transfer (PET)
Using a combination of experiment and molecular dynamics simulations, we showed that donor-acceptor hydrogen bonding plays an important role in guiding photoinduced electron transfer (PET). The effect of H-bonding is masked in a neat H-bonding donor solvent (e.g. aniline or phenol), but becomes dominant in the presence of an inert co-solvent (e.g. cyclohexane) is used. From the experiment, we found that PET is fastest at an intermediate composition. MD simulation reveals that the number of donor-acceptor H-bonding is highest at the same mole fraction confirming the role of the H-bonding ( Phys. Chem. Chem. Phys. 2015, 17, 32556-32563, J. Phys. Chem. A 2017, 121, 616-622).
Using TD-DFT calculations, we showed that proton-coupled electron transfer is a feasible process in C102-phenol H-bonded system. Thus, PCET may be an addition fluorescence quenching pathways other than the excited state H-bond strengthening proposed by Zhao and Han (J. Phys. Chem. A 2018, 122, 2394-2400).)
Fluorescent metal nanocluster
We also have interest in fluorescent nanolcusters. Nanoclusters are often stabilized inside bulky capping which can be detrimental for catalytic applications and bioconjugation. We synthesis a stabilizer free copper nanocluster in DMF solvent by hydrothermal synthesis and characterized its optical properties and its interaction with a protein, bovine serum albumin (BSA) (J. Photochem. Photobiol. A 2017, 347, 17-25). We further applied these nanolcusters to sensing of Fe(III) proteins (Sensing and Bio-Sensing Research 2019, 22, 100250).
We demonstrate an easy and convenient way to prepare protein-capped silver nanocluster inside a cationic gemini-surfactant reverse micelle using the liquid-liquid extraction routes. Thanks to the coincidence that the basic pH (pH~11) usually employed for the synthesis of protein protected AgNCs, is also critical for the efficient transfer for the protein to the cationic RM phase. The most critical point is that the nanoparticle- nanocluster equilibrium is strongly favoured in the direction of nanocluster in the RM, which allows an efficient conversion of nanoparticle into nanocluster. This expands the landscape of the ever-increasing importance of the protein capped-nanocluster into non-aqueous domain (ChemPhysChem 2018, 19, 2153-2158).48
Nanoparticle-Fluorophore Interaction
The collective coherent oscillation of conductive electrons on the surface of noble metal nanoparticles (NPs), known as surface plasmon resonance (SPR), can induce a large electromagnetic field around NPs upon irradiation. The enhanced fields may interact strongly with the molecular transition of a closely spaced molecular fluorophore. The nearfield interaction of the SPR with the molecular transitions may produce optical properties in the composite, which are entirely different from their constituents. We exploit the natural capacity of a RM to act simultaneously as a template for nanoparticle formation and host the fluorophores. (ACS Omega 2017, 2, 5494-5503).
We characterize a non-aqueous acetonitrile/AOT/n-heptane microemulsion by using photophysical studies using 4-AP as a molecular probe. There was an open debate, whether the microemulsion exists as reverse micelle or a bicontinuous microemulsion (BMC). We found that the system remains a reverse micelle at small acetonitrile content but transformed into BMC after a certain acetonitrile loading (J. Phys. Chem. B 2018, 122, 6966-6974). Subsequently, we applied the system as a template to synthesize silver nanoparticle. Interestingly the synthesized nanoparticle morphologies also show the nature of the transformation from reverse micelle to BMC (Colloids Surf. A 2019, 574, 171-177).
​
Collaborative works
Excited-state and chiroptic behavior of helical coumarins (with Prof. J. N. Moorthy, Department of Chemistry, IIT Kanpur)
Influence of helicity on the excited-state as well as chiroptical properties of two sets of regiohelical coumarins has been investigated both by experiment (by Moorthy group) and TD-DFT computation (our contribution). (Chem. Eur. J. 2017, 23, 14797-14805; J. Org. Chem. submitted)
​
Spectroscopic Studies of t-RNA-synthetase
This is a collaborative work with Dr. Rajat Banerjee, University of Calcutta. (Protein & Peptide Letters 2019, 26, 435-4