Kotni Santhosh

Vacuum Rabi splitting in a plasmonic cavity at the single quantum emitter limit

The strong interaction of individual quantum emitters with resonant cavities is of fundamental interest for understanding light–matter interactions. Plasmonic cavities hold the promise of attaining the strong coupling regime even under ambient conditions and within subdiffraction volumes. Recent experiments revealed strong coupling between individual plasmonic structures and multiple organic molecules; however, strong coupling at the limit of a single quantum emitter has not been reported so far. Here we demonstrate vacuum Rabi splitting, a manifestation of strong coupling, using silver bowtie plasmonic cavities loaded with semiconductor quantum dots (QDs). A transparency dip is observed in the scattering spectra of individual bowties with one to a few QDs, which are directly counted in their gaps. A coupling rate as high as 120 meV is registered even with a single QD, placing the bowtie-QD constructs close to the strong coupling regime. These observations are verified by polarization-dependent experiments and validated by electromagnetic calculations.

Fluorescence Quenching of CdS Quantum Dots by 4‐Azetidinyl‐7‐Nitrobenz‐2‐Oxa‐1,3‐Diazole: A Mechanistic Study

Fluorescence quenching of CdS quantum dots (QDs) by 4-azetidinyl-7-nitrobenz-2-oxa-1,3-diazole (NBD), where the two quenching partners satisfy the spectral overlap criterion necessary for Förster resonance energy transfer (FRET), is studied by steady-state and time-resolved fluorescence techniques. The fluorescence quenching of the QDs is accompanied by an enhancement of the acceptor fluorescence and a reduction of the average fluorescence lifetime of the donor. Even though these observations are suggestive of a dynamic energy transfer process, it is shown that the quenching actually proceeds through a static interaction between the quenching partners and is probably mediated by charge-transfer interactions. The bimolecular quenching rate constant estimated from the Stern-Volmer plot of the fluorescence intensities, is found to be exceptionally high and unrealistic for the dynamic quenching process. Hence, a kinetic model is employed for the estimation of actual quencher/QD ratio dependent exciton quenching rate constants of the fluorescence quenching of CdS by NBD. The present results point to the need for a deeper analysis of the experimental quenching data to avoid erroneous conclusions.

Modulation of the Excited State Intramolecular Electron Transfer Reaction and Dual Fluorescence of Crystal Violet Lactone in Room Temperature Ionic Liquids

The influence of polarity, viscosity, and hydrogen bond donating ability of the medium on the fluorescence behavior of crystal violet lactone (CVL), which undergoes excited state electron transfer reaction and exhibits dual fluorescence from two different electronic states, termed as CT(A) and CT(B), has been studied in six different room temperature ionic liquids (ILs) using steady state and time-resolved emission techniques. It is shown that the excited state CT(A) --> CT(B) transformation and dual fluorescence of CVL can be controlled by appropriate choice of the ILs. While dual fluorescence of CVL is clearly observed in pyrrolidinium IL, the molecule exhibits a single fluorescence band in ammonium IL. While the second emission from the CT(B) state can barely be seen in 1,3-dialkylimidazolium ILs, dual fluorescence is quite prominent in 1-butyl-2,3-dimethylimidazolium IL, [bmMim][Tf(2)N]. These contrasting results have been explained taking into account the hydrogen bonding interactions of the 1,3-dialkylimidazolium ions (mediated through the C(2)-hydrogen) with CVL and the viscosity of the ILs. The excited state CT(A) --> CT(B) reaction kinetics has been studied in IL by monitoring the time-evolution of the CT(B) emission in [bmMim][Tf(2)N]. The solvation dynamics in this IL has been studied by following the dynamic fluorescence Stokes shift of C153, which is used as a probe molecule. A comparison of the excited state reaction time and solvation time suggests that the rate of the CT(A) --> CT(B) reaction in moderately viscous ILs is primarily dictated by the rate of solvation. Very little or negligible excitation wavelength dependence of the emission behavior of CVL can be observed in these ILs.

Spectroscopic and theoretical investigations on effective and selective interaction of fullerenes C60 and C70 with a derivatized Zn-phthalocyanine: stabilization of charge-recombined state by side-on approach of C70.

The photophysical aspects of noncovalently linked fullerenes C(60) and C(70) with a designed metallophthalocyanine, namely, zinc-2,3,9,10,16,17,23,24-octakis-(octyloxy)-29H,31H-phthalocyanine (1) have been investigated employing various spectroscopic tools such as UV-vis absorption spectrophotometry, steady state and time-resolved fluorescence, along with solution state IR measurements in toluene medium. The ground state interaction between fullerenes and 1 is first evidenced from UV-vis measurements. Binding constants (K) for the complexes of C(60) and C(70) with 1 are determined to be 13,235 and 27,670 dm(3) x mol(-1), respectively. Steady state fluorescence experiment reveals efficient quenching of the excited singlet state of 1 in presence of both C(60) and C(70). Emission studies reveal K values of 8945 and 19,175 dm(3) x mol(-1) for C(60)/1 and C(70)/1 complexes, respectively. The magnitude of K suggests that 1 preferentially binds C(70) in comparison to C(60) as average selectivity in K of C(70) over C(60) is enumerated to be 2.0. Time resolved emission measurements establish that C(70)/1 complex is stabilized much more in comparison to C(60)/1 complex in terms of charge recombination. Liquid IR studies provide very good support in favor of ground state complexation between fullerenes and 1. Molecular mechanics calculations at force field model and semiempirical calculations employing third parametric method substantiate the strong binding between C(70) and 1, and at the same time, determine the orientation of bound guest (here C(70)) within the cavity of 1.

Photophysical and theoretical insights on fullerene/zincphthalocyanine supramolecular interaction in solution.

The present article reports photophysical studies on supramolecular interaction of a zinc phthalocyanine derivative, namely, zinc-2,9,16,23-tetra-tert-butyl phthalocyanine (1) with C(60) and C(70) in solvents having varying polarity, i.e., toluene and 1,2-dichlorobenzene (DCB). The interesting feature of the present work is the observation of charge transfer (CT) absorption bands of the fullerene/1 complexes in DCB. Utilizing the CT transition energy, many important physicochemical parameters like vertical ionization potential of 1, degrees of CT, oscillator strength, transition dipole moment, and resonance energy of interaction have been determined in the present case. The influences of 1 on the UV-vis spectral characteristics of C(60) and C(70) have been explained using a theoretical model that takes into account the interaction between electronic subsystems of 1 with fullerenes. Steady state fluorescence experiment elicits efficient quenching of the fluorescence intensity of 1 in the presence of both C(60) and C(70). The average binding constants of the C(60) and C(70) complexes of 1 (estimated by UV-vis and steady state fluorescence measurements) are determined to be 18,330 dm(3)·mol(-1) (12,595 dm(3)·mol(-1)) and 19,160 dm(3)·mol(-1) (15,292 dm(3)·mol(-1)) in toluene (DCB), respectively. Lifetime experiment yields a larger magnitude of charge separated rate constant for the C(70)/1 species. The faster charge recombination of the fullerene/1 systems observed in more polar solvent results from solvent reorganization energies. Quantum chemical calculations by the ab initio method explore the geometry and electronic structure of the supramolecules and testify the significant redistribution of charge between fullerenes and 1 during fullerene/1 interaction. A variable temperature (13)C NMR study nicely demonstrates that the end-on orientation of C(70) is very much responsible for the low selectivity in binding between C(60)/1 and C(70)/1 systems. Free energy of charge recombination and free energy of radical ion-pair formation signify that electron transfer from the excited 1 to C(60) and C(70) in the C(60)/1 and C(70)/1 complexes, respectively, is an unlikely process. Finally, transient absorption measurements in the visible region establish that energy transfer from (T)C(60*) (and (T)C(70*)) to 1 occurs predominantly in both toluene and DCB, which is subsequently confirmed by the consecutive appearance of the triplet state of 1.

New Photophysical Insights in Noncovalent Interaction between Fulleropyrrolidine and a Series of Zincphthalocyanines

The present article reports, for the first time, the photophysical aspects of noncovalent interaction of a fullerene derivative, namely, C(60) pyrrolidine tris-acid ethyl ester (PyC(60)) with a series of zincphthalocyanines, for example, underivatized zincphthalocyanine (1), zinc-1,4,8,11,15,18,22,25-octabutoxy-29H,31H-phthalocyanine (2), and zinc-2,3,9,10,16,17,23,24-octakis-(octyloxy)-29H,31H-phthalocyanine (3) in toluene. Ground state electronic interaction of PyC(60) with 1, 2 and 3 has been evidenced from the observation of well-defined charge transfer (CT) absorption bands in the visible region. Utilizing the CT transition energy, vertical electron affinity (E(A)(v)) of PyC(60) is determined. Steady state fluorescence experiment enables us to determine the value of binding constant (K) in the magnitude of 2.60 × 10(4) dm(3)·mol(-1), 2.20 × 10(4) dm(3)·mol(-1), and 1.27 × 10(4) dm(3)·mol(-1) for the noncovalent complexes of PyC(60) with 1, 2, and 3, respectively. K values of PyC(60)-ZnPc complexes suggest that PyC(60) is incapable of discriminating between 1, 2, and 3 in solution. Lifetime experiment signifies the importance of static quenching phenomenon for our presently investigated supramolecules and it yields larger magnitude of charge separated rate constant for the PyC(60)-1 species in toluene. Photoinduced energy transfer between PyC(60) and ZnPc derivatives, namely, 1, 2, and 3, in toluene, has been evidenced with nanosecond laser photolysis method by observing the transient absorption bands in the visible region; transient absorption studies establish that energy transfer from (T)PyC(60)* to the ZnPc occurs predominantly, as confirmed by the consecutive appearance of the triplet states of PyC(60). Theoretical calculations at semiempirical level (PM3) evoke the single projection geometric structures for the PyC(60)-ZnPc systems in vacuo, which also proves that interaction between PyC(60) and ZnPc is governed by the electrostatic mechanism rather than dispersive forces associated with π-π interaction.

Spectroscopic and structural insights on molecular assembly consisting high potential zinc phthalocyanine photosensitizer attached to PyC60 through non-covalent interaction

Efforts to improve the ease of self-assembly formation through non-covalent interaction has led to the development of zinc phthalocyanine (zinc-2,9,16,23-tetra-tert-butyl phthalocyanine, i.e., ZnPc) as a high potential photosensitizer molecule towards C60 pyrrolidine tris-acid ethyl ester (PyC60) in toluene and 1,2-dichlorobenzene (DCB). Steady state fluorescence experiment elicits efficient quenching of the fluorescence intensity of both H2- and ZnPc in presence of PyC60. The average value of binding constant for PyC60/H2-Pc and PyC60/ZnPc systems in toluene (DCB) are determined to be 9910 (13,460) and 12,710 (24,060) dm(3) mol(-1), respectively. Lifetime experiment yields ∼3 times larger magnitude of charge separated rate constant for the PyC60/ZnPc system compared to PyC60/H2-Pc in toluene. Photoinduced energy transfer between PyC60 and H2- (/ZnPc) has been evidenced with nanosecond laser photolysis method; transient absorption studies establish that energy transfer from (T)PyC60(∗) to H2- and ZnPc occurs predominantly, as confirmed by the consecutive appearance of the triplet states of PyC60.

Diffusion of organic dyes in bovine serum albumin solution studied by fluorescence correlation spectroscopy

The understanding of the transport of drugs and naturally occurring molecules in living cells and tissues requires a thorough knowledge of the diffusion behaviour of the molecular systems in these media. In this work, we studied the translational diffusion of three fluorescent molecules, electrically neutral coumarin 102 (C102), cationic rhodamine 6G (R6G) and anionic fluorescein (FL) in phosphate-buffered (pH 7) aqueous solutions of bovine serum albumin (BSA) protein in the absence and presence of common salt and urea using fluorescence correlation spectroscopy (FCS) by monitoring the fluorescence intensity fluctuations in a small confocal observation volume. The diffusion due to both free and BSA-bound molecules is observed in the case of the C102-BSA system. While no exchange between the bound and free states of the molecule is observed in this case, a rapid exchange between the two states is observed in the case of electrically charged hydrophilic dyes R6G and FL. This molecular picture, which is the first of its kind, is a reflection of a weaker binding of R6G and FL compared to C102 with the protein molecule. The binding sites of the probe molecules in BSA were identified based on the urea-induced change of diffusion of the probes in BSA.

Absorption spectrophotometric, fluorescence, transient absorption and quantum chemical investigations on fullerene/phthalocyanine supramolecular complexes.

The present paper reports the photophysical investigations on supramolecular interaction of a phthalocyanine derivative, namely, 2,9,16,23-tetra-tert-butyl-29H,31H-Pc (1) with C(60) and C(70) in toluene. The binding constants of the C(60) and C(70) complexes of 1 are estimated to be 27,360 and 25,205 dm(3), respectively. Transient absorption measurements in the visible region establishes that energy transfer from C60*T (and C70*T) to 1 occurs predominantly in toluene which is subsequently confirmed by the consecutive appearance of the triplet states of 1. Quantum chemical calculations at DFT level of theory explore the geometry and electronic structure of the supramolecules and testify the significant redistribution of charge between fullerenes and 1.

Role of charge transfer interaction and the chemical physics behind effective fulleropyrrolidine/porphyrin non-covalent interaction in solution

The present paper reports the photophysical insights on supramolecular interaction of a monoporphyrin derivative, namely, 1, with C60 pyrrolidine tris-acid ethyl ester (PyC60) in toluene and benzonitrile. The ground state interaction between PyC60 and 1 is facilitated through charge transfer interaction. Both UV-Vis and steady state measurements elicit almost similar magnitude of binding constant for the PyC60/1 complex in toluene and benzonitrile, viz., 6825 and 6540 dm(3 )mol(-1), respectively. Life time measurement evokes that rate of charge separation is fast in benzonitrile. Both hybrid-DFT and DFT calculations provide very good support in favor of electronic charge-separation in PyC60/1 system in vacuo.