Anu Krishnan

Schiff base linked ferrocenyl complexes for second-order nonlinear optics

Abstract A series of substituted ferrocenyl compounds where one of the cyclopentadienyl rings is linked to an aromatic Schiff base, have been synthesized and analyzed for their second-order nonlinearity (β). Two photon fluorescence corrected β, of these complexes correlates well with the electron withdrawing nature of the substituted benzene ring. The well-known two-state model has been invoked to rationalize the observed values of the first hyperpolarizability, β, of these complexes. The metal to ligand charge transfer (MLCT) transition dominates their second-order response. These compounds form charge transfer (CT) complexes with acceptors such as iodine, p-chloranil (CA), 2, 3-dichloro-5, 6-dicyano-1, 4-benzoquinone (DDQ), tetracyanoethylene (TCNE), and 7, 7, 8, 8-tetracyanoquinodimethane (TCNQ). The CT complexes exhibit much higher second-order response. A series of bisferrocenyl complexes where two ferrocene moieties are linked through the same aromatic Schiff base spacer has also been synthesized and characterized. The β values of the bisferrocenyl complexes and their CT counterparts are much higher than the corresponding monoferrocene complexes. In all these compounds there is a strong resonant contribution to β due to the MLCT transition around 532 nm. The dispersion free hyperpolarizability, β0 of these complexes have also been calculated using the two-state model.

Ferrocenyl donor-organic acceptor complexes for second order nonlinear optics

Abstract First hyperpolarizability ( β ) of a set of donor–acceptor complexes derived by functionalizing two organic acceptors with a donor such as ferrocene or an aromatic ring through a –CN– linkage has been measured by hyper-Rayleigh scattering in solution. Spectral analysis of the second harmonic scattered light reveals that there is a significant two-photon fluorescence (TPF) contribution to the total signal in some compounds. In fact, the TPF contribution was as high as ∼25% in two compounds. The measured β values after correcting for TPF indicate that these molecules possess high second order nonlinearity, much larger than similar organic compounds or charge transfer complexes formed by ferrocenyl donors and organic acceptors. The dispersion free β values of these complexes have been calculated using two different methods depending on the nature of the complex. The conventional nonresonant two-state model which gives a good estimate of the intrinsic second order polarizability away from resonance, has been applied to compounds with no absorption around the second harmonic wavelength. Another model in which a damping parameter related to the inhomogeneous line width of the absorption band near the second harmonic frequency is used for obtaining reliable values of β 0 , has been employed for the other compounds. In conjugation with anthraquinone as acceptor, ferrocene as a donor appears to be a better choice for quadratic nonlinear optics applications than conventional organic donors.

A new class of three dimensional D–π–A trigonal cryptand derivatives for second-order nonlinear optics

Synthesis, crystal structures, linear and nonlinear optical properties of tris D–π–A cryptand derivatives with C3 symmetry are reported. Three fold symmetry inherent in the cryptand molecules has been utilized for designing these molecules. Molecular nonlinearities have been measured by hyper-Rayleigh scattering (HRS) experiments. Among the compounds studied, L1 adopts non-centrosymmetric crystal structure. Compounds L1, L2, L3 and L4 show a measurable SHG powder signal. These molecules are more isotropic and have significantly higher melting points than the classical p-nitroaniline based dipolar NLO compounds, making them useful for further device applications. Besides, different acceptor groups can be attached to the cryptand molecules to modulate their NLO properties.

Second harmonic generation in ferrocene based hydrogen bonded assemblies

Abstract Supramolecular assemblies formed by electron donors and acceptors might possess very large optical nonlinearity, even larger than the sum of the constituents if they form noncentrosymmetric networks. To probe this hypothesis we have functionalized barbituric acid, by condensing it with ferrocene carboxaldehyde to give 1 , a hydrogen bond donor–acceptor–donor (H DAD ). This molecule complements the hydrogen bond donor acceptor sites of 2,6-diaminopyridine ( 2 ) which is a hydrogen bond acceptor–donor–acceptor (H ADA ). 2,6-Diaminopyridine was further functionalized with acetyl ferrocene to give 3 , which forms complexes with 1 . Complex formation was studied using nuclear magnetic resonance spectroscopy. The second order nonlinear optical (NLO) response of these hydrogen bonded assemblies has been probed by hyper Rayleigh scattering (HRS) measurements and was found to be maximum when 1 was mixed with five equivalents of 3 . A plausible explanation for the formation of 1:5 complex was given on the basis of control experiments with 4 , a methylated analog of 1 , and the addition of a strong hydrogen bond acceptor solvent, methanol.

Nonlinear optical properties of linear chain phosphazenes, (PN) x

The correlated Pariser–Parr–Pople model Hamiltonian for interacting π-electrons is employed for calculating frequency dependent linear polarizability as well as first and second hyperpolarizabilities of linear chain phosphazenes (–P=N–)x (x=3–6). The model parameters for phosphorus and nitrogen are obtained by comparing the theoretical excitation energies with experimental spectra of the known phosphazene systems. The optical gap of the phosphazene oligomers extrapolates to 3.7 eV compared to 2.8 eV of their organic analogs, namely, the polyenes. The linear polarizability of the phosphazene systems are comparable to those of the polyenes. However, the third harmonic generation coefficients are smaller at the same excitation energies. The power law exponent for the third harmonic generation coefficient in phosphazenes is also much smaller than that in polyenes. The second harmonic generation coefficients of the phosphazenes are smaller than those of the push–pull polyenes. Introduction of terminal push–pul...

Synthesis and characterization of mono- and bis-D????????A cryptand derivatives for second-order nonlinear optics and its modulation with different metal ion inputsElectronic supplementary information (ESI) available: 400 MHz 1H-NMR and FAB mass spectra of compounds M1???M4 and B1???B4. See http://www.rsc.org/suppdata/jm/b2/b203199j/

Two new classes of mono- and bis-D-π-A cryptand derivatives with a flexible and a rigid cryptand core have been synthesized. The linear and nonlinear optical properties of these molecules are probed. The three dimensional cavity of the cryptand moiety has been utilized to modulate the SHG intensity to different extents in solution with metal ion inputs such as Ni II , Cu II , Zn II , and Cd II . We also report that decomplexation events can be used to reversibly modulate their NLO responses.