Lingamallu Giribabu

Molecular engineering of sensitizers for dye‐sensitized solar cell applications

Dye-sensitized solar cells (DSSCs) have attracted considerable attention in recent years as they offer the possibility of low-cost conversion of photovoltaic energy. This account focuses on recent advances in molecular design and technological aspects of sensitizers based on metal complexes, metal-free organics and tetrapyrrolic compounds which include porphyrins, phthalocyanines as well as corroles. Special attention has been paid to the design principles of these dyes, and co-sensitization, an emerging technique to extend the absorption range, is also discussed as a way to improve the performance of the device. This account also focuses on recent advances of efficient ruthenium sensitizers as well as other metal complexes and their applications in DSSCs. Recent developments in the area of metal-free organic and tetrapyrrolic sensitizers are also discussed.

Porphyrin-rhodanine dyads for dye sensitized solar cells

A series of four new porphyrin-rhodanine dyads were synthesized having the rhodanine acetic acid group either at the meso- or the pyrrole-β position of a free-base/zinc porphyrin. These dyads were fully characterized by UV-visible, 1 H NMR, MALDI-MS and fluorescence spectros- copies and cyclic voltammetry. Both the Soret and Q-bands were red-shifted in the case of pyrrole- β substituted dyads. The redox potentials of all four dyads were altered in comparison with their individual constituents. The dyads were tested in dye sensitized solar cells and the zinc metalated dyads gave a better performance in comparison with the corresponding free-base dyads. Copyright

Picosecond and femtosecond optical nonlinearities of novel corroles

We present our results from the experimental and modeling studies of picosecond (ps) and femtosecond (fs) nonlinearities of two novel corroles (a) tritolyl corrole (TTC) (b) triphenyl corrole (TPC) using the Z-scan technique. Both open and closed aperture Z-scan curves were recorded with ~2 ps/~40 fs laser pulses at a wavelength of 800 nm and nonlinear optical coefficients were extracted for both studies. Both the molecules possessed negative nonlinear refractive index (n2) as revealed by signature of the closed aperture data in both (ps and fs) time domains. Picosecond nonlinear absorption data of TPC obtained at a concentration of 5 × 10-4 M demonstrated complex behavior with switching from reverse saturable absorption (RSA) within saturable absorption (SA) at lower peak intensities to RSA at higher peak intensities. TTC data recorded at the similar concentration exhibited saturable absorption (SA) type of behavior at lower peak intensities to switching from RSA with in SA at higher peak intensities. At a concentration of 2.5 × 10-4 M, the ps open aperture data at higher peak intensities illustrated effective three-photon absorption (3PA) for both the molecules. We also report the picosecond spectral dependent Z-scan studies performed at 680 nm, 700 nm, and 740 nm. Nonlinear absorption and refraction of both the samples at these three wavelengths were studied in detail. Femtosecond nonlinear absorption data of TPC and TTC demonstrated the behavior of saturable absorption (SA) at a concentration of 1 × 10-3 M. Solvent contribution to the nonlinearity was also identified. We have also evaluated the sign and magnitude of third order nonlinearity. We discuss the nonlinear optical performance of these organic molecules.

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.

High molar extinction coefficient amphiphilic ruthenium sensitizers for efficient and stable mesoscopic dye-sensitized solar cells

Two efficient heteroleptic ruthenium (II) complexes, cis-di(thiocyanato)(4,4′-dicarboxylic acid-2,2′-bipyridine)(4,4′-bis(2-(3,5-di tert-butylphenyl)ethenyl)-2,2′-bipyridine) ruthenium (II) (HRD-1) and cis-di(thiocyanato)(4,4′-dicarboxylic acid-2,2′-bipyridine)(4,4′-bis(2-(2,3,5-trimethylphenyl)ethenyl)-2,2′-bipyridine) ruthenium (II) (HRD-2) were synthesized and characterized, which when anchored onto nanocrystalline TiO2 shows efficiency of 5.77 and 4.87% respectively, using durable redox electrolytes.

Subphthalocyanine as hole transporting material for perovskite solar cells

Non planar 14-π aromatic subphthalocyanine has been introduced for the first time as hole transporting material for organometal halide perovskite solar cells and achieved a power conversion efficiency of 6.6%. Cells stored in the dark under ambient conditions underwent an incubation period of nine days during which, we observed an increase in efficiency followed by slow progressive deterioration. However, Raman spectral analysis of pristine perovskite deposited on titania revealed a much faster degradation thus indicating that the subphthalocyanine layer provides a temporary protection to the underlying perovskite layer.

Photodynamic Therapy: Past, Present and Future

Though we crossed many milestones in the field of medicine and health care in eradicating some deadly diseases over the past decades, cancer remained a challenge taking the lives of millions of people and having adverse effects on the quality of life of survivors. Chemotherapy and radiotherapy, the two existing major treatment modalities, have severe side effects and patients undergoing these treatments experience unbearable pain. Consequently, clinicians and researchers are working for the alternate treatment regimens, which can provide complete cure with minimum or no side effects. To this end, the present review highlights the major advances and future promises of photodynamic therapy, an emerging and promising therapeutic modality for combating cancer. We delve on various important aspects of photodynamic therapy including principle, mechanism of action, brief history and development of photosensitizers from first generation to the existing third generation, delivery strategies, development or suppression of immunity, combination therapy and future prospects.

Recent Advances of Cobalt(II/III) Redox Couples for Dye-Sensitized Solar Cell Applications.

In recent years dye-sensitized solar cells (DSSCs) have emerged as one of the alternatives for the global energy crisis. DSSCs have achieved a certified efficiency of >11% by using the I(-) /I3 (-) redox couple. In order to commercialize the technology almost all components of the device have to be improved. Among the various components of DSSCs, the redox couple that regenerates the oxidized sensitizer plays a crucial role in achieving high efficiency and durability of the cell. However, the I(-) /I3 (-) redox couple has certain limitations such as the absorption of triiodide up to 430 nm and the volatile nature of iodine, which also corrodes the silver-based current collectors. These limitations are obstructing the commercialization of this technology. For this reason, one has to identify alternative redox couples. In this regard, the Co(II/III) redox couple is found to be the best alternative to the existing I(-) /I3 (-) redox couple. Recently, DSSC test cell efficiency has risen up to 13% by using the cobalt redox couple. This review emphasizes the recent development of Co(II/III) redox couples for DSSC applications.

Phosphorus(V)corrole- Porphyrin Based Hetero Trimers: Synthesis, Spectroscopy and Photochemistry

Abstract‘Axial-bonding’-type hetero trimers have been constructed by employing a simple ‘inorganic’ reaction such as axial bond formation of main group element containing phosphorus corrole. The approach is simple and modular in nature. The architecture of these hetero trimers such that, while a phosphorus(V)corrole forms the basal scaffolding unit, either two free-base porphyrins [(H2)2–PCor] or ZnII porphyrins [(Zn)2–PCor] occupy the two axial sites via an aryloxy bridge. Both hetero trimeric species have been completely characterized by mass (FAB), UV/Vis, proton nuclear magnetic resonance spectroscopies and also by the differential pulse voltammetric method. Comparison of their spectroscopic and electrochemical data of these trimers with those of the corresponding reference compounds reveal that there is no apparent ring-to-ring interactions in these ‘vertically’ linked hetero trimers. Reduced fluorescence quantum yields were observed for [(H2)2–PCor] and [(Zn)2–PCor] when compared to corresponding monomeric chromophores. Finally, a comparison is made between the presently reported phosphorus(V)corrole based hetero arrays and the previously reported analogous arrays based on Ge(IV)corrole with regard to their spectroscopic properties and photochemical activities.

Microwave-Assisted, Rapid, Solvent-Free Aza-Michael Reaction by Perchloric Acid Impregnated on Silica Gel

Abstract Highly efficient solvent-free aza-Michael additions of a variety of amines to α,β-unsaturated carbonyl compounds under microwave-irradiation conditions catalyzed by perchloric acid impregnated on silica gel (HClO4/SiO2) is reported. The reactions are completed within 2–7 min in a microwave oven to produce the corresponding adducts in excellent yields, and the catalyst can be recovered and reused.