D. Raghunath Reddy

Enhanced optical limiting and nonlinear absorption properties of azoarene-appended phosphorus (V) tetratolylporphyrins

Optical limiting performance, third-order nonlinearity chi(3), and nonlinear absorption properties have been investigated in a new class of azoarene phosphorus (V) porphyrins with charge transfer (CT) states. The introduction of axial azoarene groups into the phosphorus porphyrin structure is found to reduce the limiting threshold by a factor of 2 and lead to a rise in the second hyperpolarizability by 1 order of magnitude in the picosecond time regime and by 2 orders of magnitude in the nanosecond regime. The experimental data show reverse saturation of absorption in the nanosecond time regime and a saturation of the nonlinear absorption above a fluence of 0.5 J/cm2 in the picosecond regime. The presence of the CT state reduces saturation of excited-state absorption (ESA) in the S1 --> Sn transition through the S1 --> CT transition. Faster CT --> T1 transition increases the ESA from T1 --> Tn states in the nanosecond regime. A self-consistent theoretical analysis based on rate equations is used to estimate the high-lying excited-state lifetimes and absorption cross sections from the experimental results.

Third-order nonlinearity and optical limiting studies in phosphorus (V) porphyrins with charge transfer states

Enhancement in the third-order nonlinearity v ð3Þ , nonlinear absorption and optical limiting threshold with the introduction of charge transfer (CT) states are observed in a new class of azoarene phosphorus (V) porphyrins. Introduction of axial azoarene groups into the phosphorus porphyrin structure led to a rise in the measured hci values. Faster intersystem crossing from CT states to triplet and excited state absorption from the triplet states contribute to the enhancement of nonlinear absorption thereby reducing the optical limiting threshold by a factor of 2. 2002 Elsevier Science B.V. All rights reserved.

Bis(aryloxo) derivatives of tin(IV) porphyrins: synthesis, spectroscopy and redox activity

Spectroscopic, electrochemical and fluorescence properties of five octahedral tin(IV) porphyrins ([(TpTP)Sn(O-m,p-C6H3(CH3)2)2] (1), [(TpTP)Sn(O-p-C6H4(CH3))2] (2), [(TpTP)Sn(O-C6H5)2] (3), [(TpTP)Sn(O-p-C6H4O-o,p-C6H3(NO2)2)2] (4) and [(TpTP)Sn(O-p-C6H4(NO2))2] (5)) bearing electron donating/electron withdrawing aryloxo subunits as the axial ligands are investigated in detail. These complexes have been synthesized by the reaction of 5,10,15,20-tetra(4-methylphenyl)porphyrinato tin(IV) dihydroxide ([(TpTP)Sn(OH)2]) and either 3, 4-dimethylphenol, p-cresol, phenol, 4-(2,4-dinitrophenoxy)phenol or p-nitrophenol and were isolated in good-to-moderate yields. Analysis of the spectral data (FAB mass, IR, UV-vis and 1H NMR) of 1 - 5 suggests that the two trans axial aryloxo ligands are strongly bound in a symmetric manner at the tin center in these complexes. Specifically, the general symmetry observed in the 1H NMR spectra and the ring current effect experienced by the protons present on the axial ligands are revealing in this regard. Each porphyrin ring undergoes two successive, reversible/quasi-reversible, one-electron reductions in CH2Cl2, 0.1 M TBAP. The bound nitroaromatic axial ligands of complexes 4 and 5 could also be reduced under the same set of experimental conditions. The wavelengths of maximum emission, the singlet energies and the Stokes’ shifts observed in the fluorescence spectra of 1 - 5 are close to the corresponding parameters of the reference porphyrin, [(TpTP)Sn(OH)2]. On the other hand, fluorescence intensities of complexes 1 - 4 are quenched in comparison with that of [(TpTP)Sn(OH)2] in three different solvents. A detailed analysis of the emission and redox potential data indicates that a photoinduced electron transfer from the axial aromatic subunit to singlet state of the basal tin(IV) porphyrin can, in principle, explain the fluorescence quenching observed in these donor-acceptor systems. The spectroscopic and redox features of these tin(IV) porphyrins are compared with those of the analogous phosphorus(V) porphyrins reported by us earlier.

A molecular photoswitch based on an‘axial-bonding’ type phosphorus(V) porphyrin

Reversible isomerization of the two axial azobenzene subunits leads to modulation of the fluorescence due to the basal tetrapyrrolic chromophore in a new hexa-coordinated phosphorus(V) porphyrin 3, illustrating its utility as a molecular photoswitch.

Study of the excited state dynamics to improve the optical limiting performance of tetra tolyl porphyrins

The last two decades have witnessed an extensive research activity in the nonlinear optical, photophysical and photochemical properties of organic materials in general and metallo-porphyrins/related compounds in particular. The interest in metallo-porphyrins is many-fold. These molecules are found to have strong nonlinearity and fast response times, the desired criteria for making useful photonic devices. Previous theoretical and experimental studies have shown that there is an enhancement in the optical nonlinearity through population of the excited state1. Here, we look at the excited state contribution to nonlinear optical properties of tetra tolyl porphyrins as a function of the input intensity, the metal ions and the axial substituents. The aim of the present study has been to incorporate structural modifications to the porphyrins for application towards optical limiting. Optical limiting devices show linear absorbance at lower intensities and limit to a threshold transmittance at higher intensities due to reverse saturation of absorption. Such a nonlinear absorption can be achieved through excited state absorption (ESA) and two-photon absorption (TPA) in porphyrins. As the porphyrins posses very strong excited state absorption from both the triplet as well as the singlet states, they serve as the one of the best optical limiting materials. Study of the nonlinearity and elucidation of the dynamics associated with excited states of such molecules is therefore important from a fundamental as well as technological point of view.