Zahid H. Khan

Electronic absorption spectra of amino substituted anthraquinones and their interpretation using the ZINDO/S and AM1 methods.

Electronic absorption spectra of 1,2-diamino-9,10-anthraquinone (12DAAQ), 1,4-diamino-9,10-anthraquinone (14DAAQ), 1,5-diamino-9,10-anthraquinone (15DAAQ), and 2,6-diamino-9,10-anthraquinone (26DAAQ) are investigated. Molecular geometries of the amino anthraquinones in the ground state are optimized using the semiempirical ZINDO/1 and AM1 methods without imposing any symmetry constraints. The ground state geometries of all the molecular systems are found to be planar. For interpretation of the spectra, ZINDO/S-CI and AM1-CI calculations employing singly excited configuration using the completely optimized geometry are carried out. Such calculations on the electronic spectra of amino anthraquinones are carried out for the first time. On the basis of these calculations, the assignment of the spectra are successfully made.

Experimental and theoretical investigations of nonlinear optical properties of 1,4-Diamino-9,10-Anthraquionone.

Nonlinear optical properties of 1,4-Diamino-9,10-Anthraquinone dye in solution at different concentrations are investigated by utilizing single beam Z-scan technique using a low power continuous wave laser (λ=532 nm). The anthraquinone dye is found to exhibit self-defocusing and reverse saturable absorption behavior. Effect of concentration on nonlinear refractive index and nonlinear absorption coefficient are also studied. The nonlinear absorption coefficient (β) and nonlinear refractive index (n2) have been evaluated from the open aperture and closed aperture Z-scan data and are found to increase with increase in concentration. The order of magnitude obtained for nonlinear refractive index and nonlinear absorption coefficient are found to be 10(-6) esu and 10(-4) m/W, respectively. The optical limiting behavior and induced self-diffraction patterns are also observed. To have a theoretical insight of nonlinear optical properties of 1,4-Diamino-9,10-Anthraquinone, first hyperpolarizability (β) is also evaluated by using quantum chemical calculations employing DFT method using 6-311 G basis set. The results obtained confirm the nonlinear optical behavior of 1,4-Diamino-9,10-Anthraquinone dye.

Electronic absorption spectra of benzoquinone and its hydroxy substituents and effect of solvents on their spectra.

The electronic absorption spectra of 1,4-benzoquinone (BQ) and its 2,5-dihydroxy and tetrahydroxy derivatives have been studied in detail. The interpretation of the electronic bands is made on the basis of PPP and CNDO calculations. It is found that the pi --> pi* transitions are well predicted by the PPP method. The predictions of the CNDO method are however superior both in their accuracy as well as ability to predict the n --> pi* transitions. The effect of solvents on the electronic absorption bands have also been investigated in detail. Linear correlations are found between the solvents dielectric constant and wavelength of the absorption bands. The solvent shifts are explained on the basis of the polarities of the solute and solvent molecules as well as due to hydrogen bonding.

Laser radiation effects on optical and structural properties of nanostructure CdSe thin film

Cadmium selenide (CdSe) thin films were deposited on a glass substrate using the thermal evaporation method at room temperature. The changes in the optical properties (optical band gap and absorption coefficient) after irradiation by TEA N2 laser at different energies were measured in the wavelength range 190–800 nm using a spectrophotometer. It was found that the optical band gap is decreased after irradiating the thin films. The samples were characterized using X-ray diffraction (XRD), and the grain size of the CdSe thin film was calculated from XRD data, which was found to be 41.47 nm as-deposited. It was also found that grain size increases with laser exposure. The samples were characterized using a scanning electron microscope and it was found that big clusters were formed after irradiation by TEA N2 laser.

Förster's resonance energy transfer between Fullerene C60 and Coumarin C440

The interaction between Coumarin C440 with Fullerene C60 has been studied by fluorescence and time resolved spectroscopic techniques. The Coumarin C440-Fullerene C60 pair shows Forsters resonance energy transfer (FRET) from Coumarin C440 (donor) to Fullerenes C60 (acceptor). The FRET efficiency of this pair increases with the increase of the acceptor concentration. The critical energy transfer distance (R0) at which transfer efficiency is 50% is found to be 34Ǻ. Stern-Volmer plot indicates static as well as dynamic quenching. However, the FRET studies show highest efficiency at the critical stage of dimer formation.

Electronic spectra of radical cations and their correlation with photoelectron spectra. V: Pyrenes

Abstract Optical absorption spectra of the radical cations of a variety of pyrenes, viz. pyrene, 1.2-benzopyrene, 3.4-benzopyrene, 1.2,3.4-dibenzopyrene, 1.2,4.5-dibenzopyrene, 3.4,9.10-dibenzopyrene, 1.2,4.5,8.9-tribenzopyrene, 1.2,3.4,9.10-tribenzopyrene, 1.2,3.4,6.7,9.10-tetrabenzopyrene, 3.4,9.10-dibenzonaphtho-(2′.3′:1.2)pyrene and pyreno-(1′.2′:1.2)pyrene, are reported. The results are analysed in the light of open-shell SCFMOLCI calculations. For the I -type transitions in radical cations, the correspondence between the optical and the photoelectron spectra is closely examined and it is found that, barring small matrix shifts, the overall agreement between the two types of spectra is excellent. Correlation diagrams for the calculated electronic transitions of radical cations of pyrenes are also sketched which clearly reveal a systematic trend in the movement of different types of transitions from one system to another. We have further established an interesting relation between the first vertical ionization potentials (IP 1 s) of pyrenes and the energies of the so-called A type optical bands ( E A s) in their cation spectra, viz. IP 1 = (5.59±0.13) + (0.65±0.06) E A . From this expression, the first IPs of pyrenes can be reasonably estimated from the knowledge of the optical spectra of their cations.

Study of self-defocusing, reverse saturable absorption and photoluminescence in anthraquinone PMMA nanocomposite film.

The nanocomposite film of 1,5-diamino-9,10-anthraquinone in poly methyl methacrylate has been fabricated by following guest-host method. Intense reverse saturable absorption and self-defocusing effect have been investigated by employing Z-scan technique with low power CW laser at 532 nm with different intensities for the dye-polymer composite film. The estimated values of nonlinear absorption coefficient β, nonlinear refractive index n2 and third order susceptibility χ((3)) of the composite film are of the order of 10(-3) (m/W), 10(-11) (m(2)/W) and 10(-4) (esu), respectively. The dye molecules have been encapsulated uniformly between molecules of polymer as a nanocomposite with average roughness ∼7.96 nm as characterized by AFM technique. The nanocomposite film also exhibited strong photoluminescence emission when excited with 532 nm. The second order hyperpolarizability of composite film has also been estimated. The evaluated figure of merit W having a value greater than 1, and the results obtained suggest that the composite film of 1,5-diamino-9,10-anthraquinone-PMMA has potential applications in nonlinear optical devices.

On the correlation between electronic spectra of alternant aromatic hydrocarbons and their radical cations

On the basis of the spectroscopic data for some 40 alternant aromatic hydrocarbons, a simple correlation is given between the excited singlet state (p band) of such molecules and the excited lowest-energy non-Koopmans state (A band) for their radical cations. It is shown that, with the use of this relation, the energy of the doublet state for a radical cation can be reasonably estimated if the energy of the p band for its neutral precursor is known. We have also computed the doublet state energies of the aromatic hydrocarbons using the “SDT” relations, which involve three spectroscopic quantities, viz., the excited singlet (S) and triplet (T) states of a neutral molecule and the doublet (D) state of its radical cation. The predictions of the proposed singlet-doublet state correlation are found to be far superior than those of the SDT relations.

Electronic spectra of radical cations and their correlation with photoelectron spectra—III. Perylenes and coronenes

Abstract Radical cations of perylene, 1,12-benzoperylene, coronene, 1,2-benzocoronene, and naphtho-(2′,3′:1,2)coronene are produced by photooxidation in boric acid matrix and their electronic absorption spectra are measured. The results are discussed in terms of Longuet-Higgins-Pople and Wasilewski type Open-Shell SCF-MO calculations and the u.v. photoelectron spectra of the parent molecules. The correspondence between optical and photoelectron spectra is found to be fairly good. A correlation diagram for the electronic transitions for some of the molecular ions is presented to demonstrate their movement from one system to another. Finally, an expression showing the relationship between the first ionization potentials of the parent molecules and A -type electronic band energies in the cation spectra is given from which the first IPs of the hydrocarbons may be estimated.

Use of constant wavelength synchronous spectrofluorimetry for identification of polycyclic aromatic hydrocarbons in air particulate samples.

We have developed a simple, rapid, inexpensive method for the identification of fluoranthene (Flan), benz(a)anthracene (BaA), benzo(a)pyrene (BaP), benzo(k)fluoranthene (BkF), pyrene (Pyr), benz(ghi)perylene (BghiP) in suspended particulate matter in an urban environment of Delhi. Suspended particulate matter samples of 24h duration were collected on glass fiber filter papers. Polycyclic aromatic hydrocarbons (PAHs) were extracted from the filter papers using dichloromethane (DCM) and hexane with ultrasonication method. Comparison of the characteristic emission of spectra of PAHs with standard spectra indicated the degree of condensation of aromatic compounds present in investigated mixtures. It was also possible to identify some individual compounds. However, this identification could be more effective with the use of the respective values of Δλ parameter for each particular component of the mixture.