K. George Thomas

Photochemistry of chromophore-functionalized gold nanoparticles

It is generally believed that metal nanoparticles strongly quench the singlet-excited states of chromophores when attached to nanoparticle surfaces, through an energy-transfer mechanism, which limits their application in optoelectronic devices and photonic materials. Recent studies of fluorophore-linked metal nanoparticles reveal that there is a dramatic suppression in the quenching of the singlet-excited state of these molecules and they possess unusual photophysical properties. A summary of our work on the photophysical and excited-state properties of chromophore-functionalized gold nanoparticles is presented in this article. Pyrene-capped gold nanoparticles showed normal fluorescence in nonpolar solvents and an intermolecular excimer formation at higher loadings. The quenching of the emission, observed in pyrene-labeled gold nanoparticles in polar solvents, is attributed to the formation of pyrene radical cation through a photoinduced electron-transfer process. We have also functionalized gold nanoparticles using a thiol derivative of fullerene. The quenching of fluorescence and decreased yields of triplet-excited state, observed in these systems, are attributed to an energy-transfer process.

Photosensitizing properties of squaraine dyes

The present report summarises our results on the photophysical and photochemical investigations of a series of squaraine dyes which exhibit intense and sharp absorption bands in the visible and near infrared regions. The intramolecular charge-transfer transitions arising from the “donor-acceptor-donor” arrangements of these dyes have an interesting effect on their excited state properties. The major nonradiative decay process of squaraines is by rotation about the C-C bonds between the central cyclobutane unit and its neighbouring phenyl groups. Microencaging of one of the dyes by Β-cyclodextrin or poly(4-vinylpyridine) was found to restrict this motion, bringing about up to 90-fold enhancement in its fluorescence yield. These aspects as well as the dynamics of charge transfer from the excited singlet state of some of the squaraine dyes to TiO2 and the recombination of the injected charge with the dye radical cation are discussed.

Interfacial properties of hybrid nanomaterials

A brief summary of our ongoing efforts to understand the surface properties of nanoparticles using fluorophores, namely pyrene alkanethiols, is presented. Excited state interactions were investigated by varying the length of the spacer group and the concentration of fluorophore. The flexible long alkyl chain tethering pyrene inAu-P2/Au-P3 allows free interaction between fluorophores resulting in excimer formation whereas the intermolecular interactions are limited in theAu-P 1 system due to the restriction imposed by the curvature of spherical gold nanoparticle. A gradual increase in the peak intensity ratio of III/I band of the normal fluorescence of pyrene was observed indicating that the surface of nanoparticle is more polar than the bulk solvent (toluene)

Cation binding and photophysical properties of a monoaza-15-crown-5-ether linked cyanine dye

A cyanine chromoionophore1 has been synthesised and its photophysical properties have been investigated. Complexation with alkali metal cations (Li+ and Na+) brings about a significant shift in the absorption and emission spectra to shorter wavelengths, which is accompanied by an enhancement in the fluorescence quantum yield. These effects can be attributed to the suppression of intramolecular charge transfer processes of the cyanine dye, brought about by complexation with the metal cations.

Molecular Assembly of Fullerenes as Nanoclusters and Nanostructured Films

Design of molecular systems, which can spontaneously self-assemble and generate three dimensionally extended structures such as clusters is of interest in “chemical nanotechnology”. Of particular interest is the utilization of fullerene materials for developing next generation nanodevices. Recent studies on molecular systems based on particularly the dyads and triads, have renewed the interest towards building photovoltaic systems and mimicking photosynthesis. Organizing molecules as nanoclusters or binding them to inorganic nanoparticles serve as important building blocks in developing new generation nanodevices. Assembly of such nanostructures in an orderly fashion would assist in designing optoelectronic nanodevices that can perform specific functions such as light induced energy and electron transfer processes. Thus, designing strategies to obtain robust films of carbon nanostructures on desired electrode surfaces with a well-controlled morphology remains a major challenge. Fullerenes and their derivatives readily form self-assembled molecular clusters in mixed solvents. These molecular clusters are optically transparent and thermodynamically stable. Recent studies have demonstrated interesting photochemical and photoelectrochemical properties of fullerene-based cluster systems. Furthermore, one can dope these spherical fullerene assemblies with electron donors such as ferrocene, amines, phenothiazine. The local concentration of electron donors is much higher in these clusters. Formation of long lived electron transfer products, following the photoexcitation of the fullerene cluster and various electron donors were confirmed through flash photolysis studies. The size and shape of these clusters can be altered with structural modifications. These clusters can be readily assembled as 3dimensional arrays on a conducting glass electrode. An efficient method has also been reported to fabricate almost perfect and uniformly shaped nanotubular crystals, which order spontaneously by self-assembly of derivatized fullerenes. These interesting fullerene nanostructures show unusual electrochemical sensing and light energy