Guda Ramakrishna

Quantum-sized gold clusters as efficient two-photon absorbers

The two-photon absorption properties of Au25 cluster has been investigated with the aid of two-photon excited fluorescence in the communication wavelength region with a cross-section of 2700 GM at 1290 nm. Additional visible fluorescence has been discovered for small gold clusters which is two-photon allowed (after excitation at 800 nm), and the absolute cross-section has been determined for gold clusters with number of gold atoms varying from 25 to all the way up to 2406 using one and two-photon excited time-resolved fluorescence upconversion measurements. Record high TPA cross-sections have been measured for quantum sized clusters making them suitable for two-photon imaging as well as other applications such as optical power limiting and lithography.

Critical Size for the Observation of Quantum Confinement in Optically Excited Gold Clusters

We present a systematic study of optical properties of a series of hexanethiolate-capped Au clusters of varying sizes using femtosecond transient absorption, time-resolved fluorescence, and two-photon absorption cross-sectional measurements. An abrupt change in optical properties and their trends has been found at the 2.2 nm size. Displacively excited vibrations with a period of 450 fs have been detected in the transient absorption signal for smaller clusters < or = 2.2 nm. These results strongly suggest an emerging optical gap between the highest occupied and lowest unoccupied orbitals in the narrow size range at 2.2 nm.

Single- and Multiphoton Turn-On Fluorescent Fe3+ Sensors Based on Bis(rhodamine)

Selective and sensitive turn-on fluorescent Fe(3+) sensors based on novel bis(rhodamine) dye molecules are reported. The compounds are synthesized with very high yields and characterized with NMR, ESI mass spectrometry, and elemental analysis. Single- and two-photon fluorescence enhancement is observed for both molecules in the presence of Fe(3+). High selectivity and sensitivity is observed over other metal ions and is shown to be due mainly to the spirolactam ring-opening power of Fe(3+). All measurements are made in buffer environments simulating biological conditions to facilitate single- and multiphoton fluorescence imaging of Fe(3+) in vivo and in vitro. Larger enhancement of fluorescence for both one- and two-photon excitation makes them suitable candidates for fluorescent labeling of biological systems. Two photon cross-section and time-resolved fluorescence measurements are utilized to understand the selectivity of the present sensors for Fe(3+)-sensing.

Ultrabright Luminescence from Gold Nanoclusters: Rigidifying the Au(I)–Thiolate Shell

Luminescent nanomaterials have captured the imagination of scientists for a long time and offer great promise for applications in organic/inorganic light-emitting displays, optoelectronics, optical sensors, biomedical imaging, and diagnostics. Atomically precise gold clusters with well-defined core-shell structures present bright prospects to achieve high photoluminescence efficiencies. In this study, gold clusters with a luminescence quantum yield greater than 60% were synthesized based on the Au22(SG)18 cluster, where SG is glutathione, by rigidifying its gold shell with tetraoctylammonium (TOA) cations. Time-resolved and temperature-dependent optical measurements on Au22(SG)18 have shown the presence of high quantum yield visible luminescence below freezing, indicating that shell rigidity enhances the luminescence quantum efficiency. To achieve high rigidity of the gold shell, Au22(SG)18 was bound to bulky TOA that resulted in greater than 60% quantum yield luminescence at room temperature. Optical measurements have confirmed that the rigidity of gold shell was responsible for the luminescence enhancement. This work presents an effective strategy to enhance the photoluminescence efficiencies of gold clusters by rigidifying the Au(I)-thiolate shell.

Giant thienylene-acetylene-ethylene macrocycles with large two-photon absorption cross section and semishape-persistence.

Giant π-conjugated macrocycles composed of thiophene, acetylene, and ethylene subunits arranged as 72π, 108π, 144π, and 180π electron systems were synthesized and analyzed for their one- and two-photon absorbance properties. Increasing π-conjugation showed an increase in the two-photon absorption cross section with magnitudes as high as 100 000 GM in the visible spectral region.

Ultrafast optical excitations in supramolecular metallacycles with charge transfer properties

New organometallic materials such as two-dimensional metallacycles and three-dimensional metallacages are important for the development of novel optical, electronic, and energy related applications. In this article, the ultrafast dynamics of two different platinum-containing metallacycles have been investigated by femtosecond fluorescence upconversion and transient absorption. These measurements were carried out in an effort to probe the charge transfer dynamics and the rate of intersystem crossing in metallacycles of different geometries and dimensions. The processes of ultrafast intersystem crossing and charge transfer vary between the two different classes of metallacyclic systems studied. For rectangular anthracene-containing metallacycles, the electronic coupling between adjacent ligands was relatively weak, whereas for the triangular phenanthrene-containing structures, there was a clear interaction between the conjugated ligand and the metal complex center. The transient lifetimes increased with increasing conjugation in that case. The results show that differences in the dimensionality and structure of metallacycles result in different optical properties, which may be utilized in the design of nonlinear optical materials and potential new, longer-lived excited state materials for further electronic applications.

Optically Excited Acoustic Vibrations in Quantum-Sized Monolayer-Protected Gold Clusters

We report a systematic investigation of the optically excited vibrations in monolayer-protected gold clusters capped with hexane thiolate as a function of the particle size in the range of 1.1-4 nm. The vibrations were excited and monitored in transient absorption experiments involving 50 fs light pulses. For small quantum-sized clusters (< or =2.2 nm), the frequency of these vibrations has been found to be independent of cluster size, while for larger clusters (3 and 4 nm), we did not observe detectable optically excited vibrations in this regime. Possible mechanisms of excitation and detection of the vibrations in nanoclusters in the course of the transient absorption are discussed. The results of the current investigation support a displacive excitation mechanism associated with the presence of finite optical energy gap in the quantum-sized nanoclusters. Observed vibrations provide a new valuable diagnostic tool for the investigations of quantum size effects and structural studies in metal nanoclusters.

Two-photon absorption properties of chromophores in micelles: electrostatic interactions.

Two-photon absorption (2PA) cross sections of neutral Coumarin 485 (C485) and anionic Coumarin 519 (C519(-)) solubilized in Triton X-100 (Tx-100, neutral), sodium dodecyl sulfate (SDS, anionic), and cetyltrimethylammonium bromide (CTAB, cationic) are reported. The objective of the study is to probe the influence of local electrostatic fields in micelles on the 2PA properties of chromophores. The 2PA measurements have shown that the cross sections of neutral C485 are unchanged in different micellar environments, although the local micropolarities of chromophores are different. On the other hand, the 2PA cross sections of C519(-) are unchanged or slightly decreased in Tx-100 and SDS micelles when compared to water while 100% increase in 2PA cross sections was observed for C519(-) in CTAB micelles. The enhancement in 2PA cross section is attributed to the electrostatic fields arising in the Stern layer of CTAB, where C519(-) is solubilized. The titration measurements have shown that the 2PA enhancement is due to the organized medium only and not because of the simple association of C519(-) and the quaternary ammonium group of CTAB. From the analysis, local electric field of 0.7 ± 0.3 MV/cm is estimated for the Stern layer of CTAB.

Single-color pseudorotaxane-based temperature sensing

Colored pseudorotaxane solutions can be used to assess temperature changes over large temperature windows. The color intensity of our novel pseudorotaxane systems decreases gradually from −50 to +50 °C with no shift in absorption maximum, making these and similar pseudorotaxanes attractive candidates for single-wavelength colorimetric temperature sensors.

Nonlinear optical properties of quantum sized gold clusters

Gold clusters with the sizes close to the Fermi wavelength of electron shows interesting quantum size effects. Linear and nonlinear optical properties show dramatic trends when the sizes of clusters are in the range of quantum confinement. We have investigated the size dependence of the non-linear optical property of two-photon absorption (TPA) cross-sections of the gold clusters. Absolute TPA cross-sections are measured by a combination of one and two-photon excited fluorescence upconversion measurements. Large cross-sections and abrupt changes in the trend of cross-sections are observed in the size-dependence when size is reduced from nanoparticles to cluster. The results can be attributed to the appearance of quantum confinement in these monolayer protected gold clusters.