Narra Vamsi Krishna

Sterically demanding zinc(II) phthalocyanines: synthesis, optical, electrochemical, nonlinear optical, excited state dynamics studies

Two novel sterically demanding zinc phthalocyanines were synthesized and characterized. Their optical and electrochemical properties were investigated in detail. The emission spectra were recorded in different solvents and the fluorescence yields obtained were in the range of 0.2–0.3 while time-resolved fluorescence data revealed radiative lifetimes of typically few ns. Nonlinear optical (NLO) properties were also evaluated at wavelengths of 600, 640, 680, and 800 nm using picosecond (∼1.5 ps) pulses and further studies with femtosecond (∼140 fs) pulses at 800 and 780 nm were also performed. Two-photon absorption (2PA) and saturable absorption (SA) were the dominant nonlinear absorption mechanisms observed with ps/fs excitation at different wavelengths in the visible spectral region. NLO coefficients were extracted from ps/fs closed and open aperture Z-scan measurements. Large two-photon absorption cross-sections of ∼14 000 GM and n2 values in the range of 1–7 × 10−16 cm2 W−1 were retrieved for these molecules from the ps Z-scan data. The excited state decay dynamics were investigated using degenerate pump-probe experiments with ∼70 fs pulses near 600 nm. Double exponential fits of the pump-probe data suggested two decay times for both molecules investigated.

Optical, electrochemical, third-order nonlinear optical, and excited state dynamics studies of thio-zinc phthalocyanine

Zinc phthalocyanine with S-aryl groups at α-positions have been synthesized and its optical, emission, electrochemical and third-order nonlinear optical properties were investigated. Both the Soret and Q-bands were red-shifted and obeyed Beer–Lamberts law. Electrochemical properties indicated that both oxidation and reduction processes were ring centered. Emission spectra were recorded in different solvents and the fluorescence yields obtained were in the range of 0.02 while time-resolved fluorescence data revealed lifetimes of typically few ns. Excited state dynamics in this novel thio-zinc phthalocyanine molecule has been investigated using femtosecond (fs) degenerate pump-probe spectroscopy. Nonlinear optical properties of this molecule have been examined using the Z-scan technique with picosecond (ps) and fs pulses. Both open and closed aperture Z-scan curves were recorded with ~2 ps/~150 fs laser pulses at a wavelength of 800 nm and nonlinear optical coefficients were extracted from both the studies. Degenerate pump-probe data performed at 600 nm suggested a single long lifetime of ~300 ps, possibly originating from the non-radiative decay of S1 state.

Optical, electrochemical, third order nonlinear optical, and excited state dynamics studies of bis(3,5-trifluoromethyl)phenyl-zinc phthalocyanine†

A zinc phthalocyanine with bis(3,5-trifluoromethyl)phenyl groups at peripheral positions has been synthesized and its optical, emission, electrochemical and third-order nonlinear optical properties were investigated. Both the Soret and Q bands were red-shifted and obeyed Beer–Lamberts law. Electrochemical properties indicated that both oxidation and reduction processes were ring centred. Emission spectra were recorded in different solvents and the fluorescence yields obtained were in the range of 0.29 to 0.10, while the time-resolved fluorescence data revealed lifetimes of typically a few ns. Nonlinear optical coefficients were retrieved from the Z-scan measurements using picosecond and femtosecond pulses. Excited state lifetime was deduced from the femtosecond degenerate pump–probe measurements. Our data suggests that this compound has potential for applications in the field of photonics.

Optical, electrochemical and third-order nonlinear optical studies of triphenylamine substituted zinc phthalocyanine

Zinc phthalocyanine possessing triphenylamine at its peripheral position has been synthesized and its optical, emission, electrochemical and third-order nonlinear optical (NLO) properties were investigated. Soret band was broadened due to the presence of triphenylamine moiety. Electrochemical properties indicated that both oxidation and reduction processes were ring centered. Emission spectra were recorded in different solvents and the fluorescence yields obtained were in the range of 0.02–0.17 while the time-resolved fluorescence data revealed radiative lifetimes of typically few ns. Third-order NLO properties of this molecule have been examined using the Z-scan technique with picosecond (ps) and femtoseocnd (fs) pulses. Closed and open aperture Z-scan data were recorded with 2 ps/1 50 fs laser pulses at a wavelength of 800 nm and NLO coefficients were extracted from both the data. Our data clearly suggests the potential of this molecule for photonics applications.

Kinetics of Dye Regeneration in Liquid Electrolyte Unveils Efficiency of 10.5% in Dye-Sensitized Solar Cells

Due to their fabulous performance, porphyrins are considered to be emerging materials for dye-sensitized solar cell applications (DSSCs). Herein, we have designed and synthesised four novel porphyrin sensitizers tethered with a triphenylimidazole group as the donor, a porphyrin macrocycle as the π-spacer and 4-ethynyl phenyl (LG15), 5-ethynylthiophene (LG16), 2,1,3-benzothiadiazole (BTD)-phenyl (LG17), and BTD-thiophene (LG18) as auxiliary acceptors and either a carboxylic acid or cyanoacrylic acid anchoring group, which resembles the structure of a donor–π-bridge–acceptor (D–π–A). The absorption spectra of LG17 and LG18 are red-shifted, when compared to LG15 and LG16, due to the presence of extended π-conjugation of the BDT group in its molecular structure. The red-shifted absorption spectra of LG17 and LG18 result in highest efficiency of 10.11% and 10.51%, respectively. Optical and electrochemical properties, and Density Functional Theory calculations of LG15–LG18, suggested that LG18 exhibits strong absorption and optimized lowest unoccupied molecular orbital (LUMO) aids the injection of electrons very effectively from the excited state of the dye into the TiO2 conduction band. Both LG17 and LG18 exhibit broad incident to photon conversion efficiency (IPCE) and the onset extends up to 950 nm, with >80% at 700 nm. Current density–voltage (J–V) curves of LG15–LG18 revealed that LG17 and LG18 exhibit highest short-circuit current density of 20.64 and 21.43 mA cm−2 resulting in power conversion efficiency (PCE) of 10.11% and 10.51%, respectively, in a liquid I−/I3− redox couple electrolyte. Bulky donor groups, π-spacer length and anchoring groups have significantly contributed to achieving high efficiency in a liquid redox shuttle that can be helpful to make efficient DSSCs for future generations. We have adopted intensity modulated photovoltage spectroscopy and nanosecond laser flash photolysis spectroscopy to explain the PCE and molecular structure relationship of the LG15–LG18 sensitizers.

Novel Amphiphillic G-quadruplex Binding Synthetic Derivative of TMPyP4 and its Effect on Cancer Cell Proliferation and Apoptosis Induction

Porphyrins are well-known anticancer agents because of their high binding affinity for G-quadruplex DNA and excellent photophysical properties. Several studies carried out using TMPyP4 established it as an efficient chemotherapeutic and a photodynamic therapeutic (PDT) agent, but its use as a lead molecule has been restricted because of its high level of binding to double-stranded DNA (dsDNA), which may have side effects on normal cells and tissues. To minimize its interaction with dsDNA and to enhance internalization into cells, an analogue of TMPyP4 (5Me) was synthesized. Its selectivity for G-quadruplex DNA over dsDNA was evaluated by spectroscopic methods, and its role in stabilizing G-quadruplex DNA was assessed by fluorescence lifetime and thermal melting experiments. Biophysical studies indicated that 5Me interacts well with G-quadruplex DNA. In vitro cytotoxicity experiments with tumor cell lines (PANC-1, B16F10, and MDA MB 231) have revealed that 5Me can inhibit the growth of cancer cells comparable to TMPyP4. MTT and apoptotic assays demonstrated the ability of 5Me to specifically affect cancer cells over normal cells. Cell cycle analysis showed that 5Me, like TMPyP4, induces G2/M phase cell cycle arrest. In addition, 5Me is more effectively taken up by both cancer and normal cells than TMPyP4. In addition, we have noticed that 5Me is more efficient than TMPyP4 in inhibiting the growth of the cancer cells after irradiation with light (600-720 nm, 20 J/cm2, 50 mW/cm2). By and large, these experimental results indicate that 5Me can be an efficient chemotherapeutic as well as a PDT agent.