Photophysics of Thiacarbocyanine Dyes: Relaxation Dynamics in a Homologous Series of Thiacarbocyanines

Identification of Nonradiative Decay Pathways in Cy3

Photoexcited fluorescent markers are extensively used in spectroscopy, imaging, and analysis of biological systems. The performance of fluorescent markers depends on high levels of emission, which are limited by competing nonradiative decay pathways. Small-molecule fluorescent dyes have been increasingly used as markers due to their high and stable emission. Despite their prevalence, the nonradiative decay pathways of these dyes have not been determined. Here, we investigate these pathways for a widely used indocarbocyanine dye, Cy3, using transient grating spectroscopy. We identify a nonradiative decay pathway via a previously unknown dark state formed within ∼1 ps of photoexcitation. Our experiments, in combination with electronic structure calculations, suggest that the generation of the dark state is mediated by picosecond vibrational mode coupling, likely via a conical intersection. We further identify the vibrational modes, and thus structural elements, responsible for the formation and dynamics of the dark state, providing insight into suppressing nonradiative decay pathways in fluorescent markers such as Cy3.

Hetero-biicosahedral [Au24Pd(PPh3)10(SC2H4Ph)5Cl2]+ nanocluster: selective synthesis and optical and electrochemical properties

A recent study implied that a hetero-biicosahedral 25-atom cluster composed of two kinds of icosahedral 13-atom clusters could serve as a molecular rectifier and dipole material. However, no hetero-biicosahedral 25-atom clusters containing three types of ligands, in this case, phosphines, halogens, and thiolates, have been reported. In this study, we selectively synthesized [Au24Pd(PPh3)10(SC2H4Ph)5Cl2]Cl (Au = gold, Pd = palladium, PPh3 = triphenylphosphine, SC2H4Ph = phenylethanethiolate, Cl = chloride), in which one Au was replaced with a Pd. The single-crystal X-ray structural analysis demonstrated that [Au24Pd(PPh3)10(SC2H4Ph)5Cl2]Cl was a hetero-biicosahedral 25-atom cluster in which the central atom of one icosahedral Au13 core was replaced by a Pd atom. Optical absorption spectroscopy suggested that the electronic structure of each individual icosahedral 13-atom core in [Au24Pd(PPh3)10(SC2H4Ph)5Cl2]+ was reasonably well maintained, similar to the case of [Au25(PPh3)10(SC2H4Ph)5Cl2]2+. Density functional theory calculation revealed that the peak splitting in the region below 2.2 eV of the optical absorption spectrum of [Au24Pd(PPh3)10(SC2H4Ph)5Cl2]+ is due to the splitting of HOMOs and also suggested that this cluster has dipole moment. Electrochemical measurements showed that [Au24Pd(PPh3)10(SC2H4Ph)5Cl2]+ was relatively stable to reduction. These results are expected to contribute to the development of molecular rectifiers and dipole materials based on hetero-biicosahedral 25-atom clusters.

π-Stacked and unstacked aggregate formation of 3,3′-diethylthiatricarbocyanine iodide, a near-infrared dye

3,3′-Diethylthiatricarbocyanine iodide (DTCI) emits distinctive monomer and dimer emissions whose intensity ratio varied depending on concentration in methanol and water.

Enhancing near IR luminescence of thiolate Au nanoclusters by thermo treatments and heterogeneous subcellular distributions

A five-to-ten fold enhancement, up to ca. 5-10% quantum efficiency, of near IR luminescence from monothiolate protected gold nanoclusters was achieved by heating in the presence of excess ligand thiols. An emission maximum in the 700-900 nm range makes these Au nanoclusters superior for bioimaging applications over other emissions centered below 650 nm due to reduced background interference, albeit visible emissions could have higher quantum efficiency. The heating procedure is shown to be effective to improve the luminescence of Au nanoclusters synthesized under a variety of conditions using two types of monothiols: mercaptosuccinic acid and tiopronin. Therefore, this heating method is believed to be a generalizable approach to improve the near IR luminescence of aqueous soluble Au nanoclusters, which enables better bioimaging applications. The high quantum yield is found relatively stable over a wide pH range. PEGylation of the Au nanoclusters reduces their quantum efficiency but improves their permeation into the cytoplasm. Interestingly, z-stack confocal analysis clearly reveals the presence of Au nanoclusters inside the cell nucleus in single cell imaging. The finding addresses controversial literature reports and demonstrates the internalization and heterogeneous subcellular distributions, particularly inside the nucleus. The high luminescence intensity, small overall dimension, cell and nuclear distribution, chemical stability and low-to-non toxicity make these Au nanoclusters promising probes for broad cell dynamics and imaging applications.

Synthesis and electrochemical and spectroscopic characterization of biicosahedral Au25 clusters

The synthesis and electrochemical and spectroscopic characterization of biicosahedral Au(25) clusters with a composition of [Au(25)(PPh(3))(10)(thiolate)(5)Cl(2)](2+) are described. The biicosahedral Au(25) clusters protected with various types of thiol ligands including alkanethiols, 2-phenylethanethiol, 11-mercaptoundecanoic acid, and 11-mercapto-1-undecanol were synthesized in high yields using a one-step, one-phase procedure in which Au(PPh(3))Cl is reduced with tert-butylamine-borane in the presence of the thiol ligand in a 3:1 v/v chloroform/ethanol solution. All biicosahedral Au(25) clusters prepared exhibit characteristic optical absorption and photoluminescence properties. The emission energy is found to be substantially smaller than the optical absorption energy gap of 1.82 eV, indicating a subgap energy luminescence. The electrochemical HOMO-LUMO gap (~1.54 eV) of the clusters is also substantially smaller than the optical absorption energy gap but rather similar to the emission energy. These electrochemical and optical properties of the biicosahedral Au(25) clusters are distinctly different from those of the Au(25)(thiolate)(18) clusters.

Effect of Gold Nanoparticles on Fluorescence Properties of Hyperbranched Poly(amido amine)s

Hyperbranched poly (amino amine) s (HPAMAMs), synthesized by Michael addition of 1-(2-aminoethyl) piperazine (AEPZ) and methyl acrylate (MA), can emit blue fluorescence under excitation wavelength. However, the relatively weak fluorescence of HPAMAMs is still an obstacle for its practical applications. Gold is one of the most frequently used metals for fluorescence enhancement. In this research, the influence of gold nanoparticles on fluorescence property of HPAMAMs was investigated by fluorometry. It was found that gold nanoparticles (GNPs) with smaller size (< 5 nm) presented greatly enhanced emission. The GNPs with larger size (~10 nm in diameter) and definite surface plasmon absorption can quench the fluorescence of HPAMAMs.

NIR Luminescence Intensities Increase Linearly with Proportion of Polar Thiolate Ligands in Protecting Monolayers of Au38and Au140Quantum Dots

The near-infrared photoluminescence of monolayer-protected Au38 and Au140 clusters (MPCs) is intensified with exchange of nonpolar ligands by more polar thiolate ligands. The effect is general and includes as more polar in-coming ligands: thiophenolates with a variety of p-substituents; alkanethiolates omega-terminated by alcohol, acid, or quaternary ammonium groups; and thio-amino acids. Remarkably, place exchanges of the initial phenylethanethiolates on Au38 MPCs by p-substituted thiophenolates and thio-amino acids and of hexanethiolates on Au140 MPCs by omega-quaternary ammonium terminated undecylthiolates result in increases in the near-infrared (NIR) luminescence intensities that are linear with the number of new polar ligands. The increased intensities are systematically larger for thiophenolate ligands having more electron-withdrawing substituents. Analogous effects on intensities are observed in the NIR emission of Au140 MPCs upon place exchange of alkanethiolates with thiolates having short connecting alkanethiolate chains to quaternary ammonium and to omega-carboxylic acid termini, and with oxidative charging of the Au cores. The observations are consistent with sensitivity of the luminescence mechanism to any factor that enhances the electronic polarization of the bonds between the Au core atoms and their thiolate ligands. The luminescence is discussed in terms of a surface electronic excitation, as opposed to a core volume excitation.

Near-IR Luminescence of Monolayer-Protected Metal Clusters

Visible-near-IR luminescence spectra of gold MPCs that are similar, irrespective of the number of core atoms (all <2 nm diameter) and different monolayers, are reported. The luminescence can be quantitatively invoked by introducing polar ligands into nonpolar MPC monolayers and by galvanic exchange of metal atoms on the MPC core surface with different metals. The observed emissions are believed to result from surface-localized states that depend on both the core metal of the nanoparticle and the ligands attached to the metal surface.

Multiple relaxation pathways in push-pull polyenes

Subpicosecond absorption and gain spectroscopy are used to investigate the excited-state behavior of push-pull polyenes made of a diethylthiobarbituric acid electron-acceptor group and a dibutylaniline electron-donor group linked by a pi-conjugated chain. Four polyenes of increasing length, ranging from n = 2 to 5 double bonds, are compared. The relaxation path and relaxation kinetics are studied in dioxane and in cyclohexane, a polar and a nonpolar solvent, respectively. In dioxane, the results provide evidence for the formation of an emissive transient state on an ultrashort time scale (2-3 ps) attributed to a charge transfer (CT) state. The regular shift of the gain peak of this transient state with increase in the chain length (ca. 100 nm per added double bond) indicates that its structure is similar to that of a cyanine, i.e. with a fully conjugated polyenic chain. Its lifetime ranges from a few tens to a few hundreds of picoseconds depending on the chain length. When the number of double bonds increases from n = 2 to 3, the lifetime increases, then decreases continuously for longer chains. In cyclohexane, where the transient CT state is not formed, the decay of the initial excited state follows the same trend when the chain length increases but the lifetimes are shorter than that of the CT state in dioxane. In both solvents, the characterization of long-lived photoproducts by synchronizing two low repetition-rate subpicosecond laser systems demonstrates a change in the relaxation route as the chain length increases. Isomerization occurs for n = 2, whereas intersystem crossing to the triplet state occurs for n = 4. The change in the relaxation channel is observed for n = 3 in both solvents with however a solvent-dependent behavior. In dioxane, relaxation to the triplet state is already observed for n = 3, while an intermediate regime with a relaxation directly to the ground state is observed in cyclohexane. The photophysics of the studied push-pull polyenes is tentatively compared to that of polymethine cyanines and substituted carotenoids.

Nonradiative deactivation of the electronic excitation energy in cyanine dyes: influence of binding to DNA

The processes of nonradiative deactivation of electronic excitation energy in cyanine dyes determine their quantum yield. Because of that, the study of the influence of cyanines binding to DNA on these processes can provide information on the causes leading to the cyanines fluorescence intensity enhancement in the presence of DNA. In the presented paper, the activation energies of nonradiative degradation of electronic excitation, quantum yields and rate constants of nonradiative transitions of several cyanines in free state and in the presence of DNA were established and compared. The mechanisms of nonradiative deactivation of dye excitation energy were discussed.

Investigation of DNA binding modes for a symmetrical cyanine dye trication: effect of DNA sequence and structure

The DNA binding behavior of a tricationic cyanine dye (DiSC3+(5)) was studied using the [Poly(dA-dT)]2, [Poly(dI-dC)]2 and Poly(dA) x Poly(dT) duplex sequences and the Poly(dA) x 2Poly(dT) triplex. Optical spectroscopy and viscometry results indicate that the dye binds to the triplex structure by intercalation, to the nonalternating Poly(dA) x Poly(dT) duplex through minor groove binding and to the alternating [Poly(dA-dT)]2 duplex by a combination of two binding modes: intercalation at low concentration and dimerization within the minor groove at higher concentration. Dimerization occurs at lower dye concentrations for the [Poly(dI-dC)]2 sequence, consistent with our previous investigations on an analogous monocationic cyanine dye. [Seifert, J.L., et al. (1999) J. Am. Chem. Soc. 121, 2987-2995] These studies illustrate the diversity of DNA binding modes that are available to a given ligand structure.