Evgeny O. Danilov

Negative polaron and triplet exciton diffusion in organometallic "molecular wires".

The dynamics of negative polaron and triplet exciton transport within a series of monodisperse platinum (Pt) acetylide oligomers is reported. The oligomers consist of Pt-acetylide repeats, [PtL(2)-C≡C-Ph-C≡C-](n) (where L = PBu(3) and Ph = 1,4-phenylene, n = 2, 3, 6, and 10), capped with naphthalene diimide (NDI) end groups. The Pt-acetylide segments are electro- and photoactive, and they serve as conduits for transport of electrons (negative polaron) and triplet excitons. The NDI end groups are relatively strong acceptors, serving as traps for the carriers. Negative polaron transport is studied by using pulse radiolysis/transient absorption at the Brookhaven National Laboratory Laser-Electron Accelerator Facility (LEAF). Electrons are rapidly attached to the oligomers, with some fraction initially residing upon the Pt-acetylide chains. The dynamics of transport are resolved by monitoring the spectral changes associated with transfer of electrons from the chain to the NDI end group. Triplet exciton transport is studied by femtosecond-picosecond transient absorption spectroscopy. Near-UV excitation leads to rapid production of triplet excitons localized on the Pt-acetylide chains. The excitons transport to the chain ends, where they are annihilated by charge separation with the NDI end group. The dynamics of triplet transport are resolved by transient absorption spectroscopy, taking advantage of the changes in spectra associated with decay of the triplet exciton and rise of the charge-separated state. The results indicate that negative polarons and excitons are transported rapidly, on average moving distances of ~3 nm in less than 200 ps. Analysis of the dynamics suggests diffusive transport by a site-to-site hopping mechanism with hopping times of ~27 ps for triplets and <10 ps for electrons.

Ultrafast energy transfer in oligofluorene-aluminum bis(8-hydroxyquinoline)acetylacetone coordination polymers.

Understanding the excited-state dynamics in conjugated systems can lead to their better utilization in optical sensors, organic photovoltaics (OPVs), and organic light-emitting diodes (OLEDs). We present the synthesis of self-assembled coordination polymers comprising two types of fluorescent moieties: discrete fluorene oligomers of a well-defined length (n = 1-9) connected via aluminum(III) bis(8-quinolinolate)acetylacetone joints. Due to their well-defined structure, these materials allowed for a detailed study of energy migration processes within the materials. Thus, femtosecond transient spectroscopy was used to study the ultrafast energy transfer from the oligofluorene to the quinolinolate moieties, which was found to proceed at a rate of 10(11) s(-1). The experimental results were found to be in agreement with the behavior predicted according to the Beljonnes improved Forster model of energy transfer. In addition, the solid-state and semiconductor properties of these coordination polymers allowed for the fabrication of OLEDs. Preliminary experiments with simple two- and three-layer devices fabricated by spin-coating yield bright yellow electroluminescence with maximum brightness of 6000 cd/m(2), with a turn-on voltage of approximately 6 V and a maximum external quantum efficiency of up to 1.2%, suggesting their potential for use in PLED applications.

TokenRNA: a new type of sequence-specific, label-free fluorescent biosensor for folded RNA molecules.

aptamers; biosensors; fluorescence; paranemic; RNA recognition; tokenRNAFluorescent reporters are highly sensitive, nonperturbing, and convenient probes for biologicalstudies. Current techniques for recognition of specific nucleic acid sequences usually requirecomplementary hybridization to chemically modified probes.[1] Optimal fluorescentbiosensors should rapidly signal the presence of a specific analyte with high selectivity andhigh contrast. Recently, a number of label-free, fluorescent bio-indicators have been reportedincluding aptamer-based riboswitches as sensors for cofactors,[2] aptamer-based proteinsensors,[3] and chimeric aptamers, in which the recognition domain binding the target wascoupled to an aptamer that binds a fluorophore in such a way that the presence of the targetincreases the affinity of the bioindicator for the fluorophore.[ 4,5] When the emission yields ofthe fluorophore in its bound and free states differ, this provides a binary sensor withnonperturbing optical recognition.It is desirable to detect macromolecular analytes (for example, structured RNA molecules) intheir native environment without having to denature or unfold them. In this work, wedemonstrate a technique for sensitive, label-free, real-time sequence-specific recognition ofprefolded RNA sequences. We use RNA constructs designed to form an aptameric pocket forthe fluorophore upon programmable paranemic binding to a specific prefolded analyte RNAsequence. We call these constructs paranemic “token RNAs”.We chose the triphenylmethane dye, Malachite Green (MG), as the fluorescent reporter becausein its unbound state in water solution it exhibits extremely low fluorescence quantum yieldfrom the S1 excited state because of efficient internal conversion.[ 6,7] The emission of the dyeincreases substantially when the nonradiative relaxation channels from S1 are shut down.Whereas the detailed underlying mechanisms of this phenomenon are still being debated,[7–9] related studies show that “rigidifying” the dye by placing it in a highly viscous environmentor in a binding cage increases its emission dramatically.[ 7] For instance, it was reported recentlythat the emission of MG increases by several orders of magnitude upon binding to an RNAaptamer obtained by in vitro selection (SELEX).[8,10] This aptamer has also been used as areporter for ATP recognition[4] and to perform real-time fluorescent monitoring of single-stranded DNA molecules.[11] The DNA detection was based on the separation of the MGaptamer into two strands, each of which is linked to a nucleic acid arm complementary to one

Transient absorption dynamics of sterically congested Cu(I) MLCT excited states.

Subpicosecond through supra-nanosecond transient absorption dynamics of the homoleptic Cu(I) metal-to-ligand charge transfer (MLCT) photosensitizers including the benchmark [Cu(dmp)2](+) (dmp =2,9-dimethyl-1,10-phenanthroline) chromophore, as well as [Cu(dsbp)2](+) (dsbp =2,9-di(sec-butyl)-1,10-phenanthroline and [Cu(dsbtmp)2](+) (dsbtmp =2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline) were investigated in dichloromethane and tetrahydrofuran solutions. Visible and near-IR spectroelectrochemical measurements of the singly reduced [Cu(dsbp)2](+) and [Cu(dsbtmp)2](+) species were determined in tetrahydrofuran, allowing for the identification of redox-specific phenanthroline-based radical anion spectroscopic signatures prevalent in the respective transient absorption experiments. This study utilized four different excitation wavelengths (418, 470, 500, and 530 nm) to elucidate dynamics on ultrafast times scales spanning probe wavelengths ranging from the UV to the near-IR (350 to 1450 nm). With the current time resolution of ∼150 fs, initial excited state decay in all three compounds was found to be independent of excitation wavelength. Not surprisingly, there was little to no observed influence of solvent in the initial stages of excited state decay in any of these molecules including [Cu(dmp)2](+), consistent with results from previous investigators. The combined experimental data revealed two ranges of time constants observed on short time scales in all three MLCT chromophores and both components lengthen as a function of structure in the following manner: [Cu(dsbtmp)2](+) < [Cu(dsbp)2](+) < [Cu(dmp)2](+). The molecule with the most inhibited potential for distortion, [Cu(dsbtmp)2](+), possessed the fastest ultrafast dynamics as well as the longest excited state lifetimes in both solvents. These results are consistent with a small degree of excited state distortion, rapid intersystem crossing, and weak vibronic coupling to the ground state. The concomitant systematic variation in both initial time constants, assigned to pseudo-Jahn-Teller distortion and intersystem crossing, suggest that both processes are intimately coupled in all molecules in the series. The variability in these time scales illustrate that strongly impeded structural distortion in Cu(I) MLCT excited state enables more rapid surface crossings in the initial deactivation dynamics.

Sensing of 2,4,6‐Trinitrotoluene (TNT) and 2,4‐Dinitrotoluene (2,4‐DNT) in the Solid State with Photoluminescent RuII and IrIII Complexes

A series of metal-organic chromophores containing Ru(II) or Ir(III) were studied for the luminometric detection of nitroaromatic compounds, including trinitrotoluene (TNT). These complexes display long-lived, intense photoluminescence in the visible region and are demonstrated to serve as luminescent sensors for nitroaromatics. The solution-based behavior of these photoluminescent molecules has been studied in detail in order to identify the mechanism responsible for metal-to-ligand charge-transfer (MLCT) excited state quenching upon addition of TNT and 2,4-dinitrotoluene (2,4-DNT). A combination of static and dynamic spectroscopic measurements unequivocally confirmed that the quenching was due to a photoinduced electron transfer (PET) process. Ultrafast transient absorption experiments confirmed the formation of the TNT radical anion product following excited state electron transfer from these metal complexes. Reported for the first time, photoluminescence quenching realized through ink-jet printing and solid-state titrations was used for the solid-state detection of TNT; achieving a limit-of-quantitation (LOQ) as low as 5.6 ng cm(-2). The combined effect of a long-lived excited state and an energetically favorable driving force for the PET process makes the Ru(II) and Ir(III) MLCT complexes discussed here particularly appealing for the detection of nitroaromatic volatiles and related high-energy compounds.

Charge Generation Dynamics in Efficient All-Polymer Solar Cells: Influence of Polymer Packing and Morphology.

All-polymer solar cells exhibit rapid progress in power conversion efficiency (PCE) from 2 to 7.7% over the past few years. While this improvement is primarily attributed to efficient charge transport and balanced mobility between the carriers, not much is known about the charge generation dynamics in these systems. Here we measured exciton relaxation and charge separation dynamics using ultrafast spectroscopy in polymer/polymer blends with different molecular packing and morphology. These measurements indicate that preferential face-on configuration with intermixed nanomorphology increases the charge generation efficiency. In fact, there is a direct quantitative correlation between the free charge population in the ultrafast time scales and the external quantum efficiency, suggesting not only the transport but also charge generation is key for the design of high performance all polymer solar cells.

Commercial quantities of ultrasmall fluorescent nanodiamonds containing color centers

Optically active nanodiamond particles remain one of the most popular research topics due to the photoluminescent properties of crystallographic defects in the diamond lattice, referred to as color centers. A number of groups are currently undertaking efforts to commercialize this material. Recently, our group succeeded in large-scale production of fluorescent diamond particles containing nitrogen-vacancy (NV) color centers in hundred-gram per batch scales using irradiation with 2-3 MeV electrons. Production of ND-NV fractions with median sizes ranging between 10 nm and 100 nm was achieved. While 100 nm fluorescent nanodiamonds (FNDs) are ~10x brighter than a conventional dye (Atto 532), the brightness of FNDs drops with decreasing particle size. Because of this, significant efforts must be undertaken to elucidate the size/brightness compromise and identify relevant application niches for FND in bioimaging and biolabeling. In order for a new material to be considered for applications in the overcrowded optical reagent market, the reagent must be convenient to use by an end user from the biomedical community, be validated both in vitro and in vivo, and offer measurable and significant (rather than incremental) benefit to end users in specific applications. This paper reports on the characteristics of the ultrasmall (10-40nm) and larger fluorescent nanodiamonds as well as our efforts toward their adaptation for use in the biological science community.