Ultrafast Two-Electron Transfer in a CdS Quantum Dot-Extended-Viologen Cyclophane Complex.

Time-resolved optical spectroscopies reveal multielectron transfer from the biexcitonic state of a CdS quantum dot to an adsorbed tetracationic compound cyclobis(4,4'-(1,4-phenylene) bipyridin-1-ium-1,4-phenylene-bis(methylene)) (ExBox(4+)) to form both the ExBox(3+•) and the doubly reduced ExBox(2(+•)) states from a single laser pulse. Electron transfer in the single-exciton regime occurs in 1 ps. At higher excitation powers the second electron transfer takes ∼5 ps, which leads to a mixture of redox states of the acceptor ligand.

Photoinduced electron transfer within a zinc porphyrin-cyclobis(paraquat-p-phenylene) donor-acceptor dyad.

Understanding the mechanism of efficient photoinduced electron-transfer processes is essential for developing molecular systems for artificial photosynthesis. Towards this goal, we describe the synthesis of a donor-acceptor dyad comprising a zinc porphyrin donor and a tetracationic cyclobis(paraquat-p-phenylene) (CBPQT(4+) ) acceptor. The X-ray crystal structure of the dyad reveals the formation of a dimeric motif through the intermolecular coordination between the triazole nitrogen and the central Zn metal of two adjacent units of the dyad.

Direct measurement of lattice dynamics and optical phonon excitation in semiconductor nanocrystals using femtosecond stimulated Raman spectroscopy.

We report femtosecond stimulated Raman spectroscopy measurements of lattice dynamics in semiconductor nanocrystals and characterize longitudinal optical (LO) phonon production during confinement-enhanced, ultrafast intraband relaxation. Stimulated Raman signals from unexcited CdSe nanocrystals produce a spectral shape similar to spontaneous Raman signals. Upon photoexcitation, stimulated Raman amplitude decreases owing to experimentally resolved ultrafast phonon generation rates within the lattice.

Electron transfer and multi-electron accumulation in ExBox⁴⁺.

Molecules capable of accepting and storing multiple electrons are crucial components of artificial photosynthetic systems designed to drive catalysts, such as those used to reduce protons to hydrogen. ExBox(4+), a boxlike cyclophane comprising two π-electron-poor extended viologen units tethered at both ends by two p-xylylene linkers, has been shown previously to accept an electron through space from a photoexcited guest. Herein is an investigation of an alternate, through-bond intramolecular electron-transfer pathway involving ExBox(4+) using a combination of transient absorption and femtosecond stimulated Raman spectroscopy (FSRS).

Relative unidirectional translation in an artificial molecular assembly fueled by light.

Motor molecules present in nature convert energy inputs, such as a chemical fuel or incident photons of light, into directed motion and force biochemical systems away from thermal equilibrium. The ability not only to control relative movements of components in molecules but also to drive their components preferentially in one direction relative to each other using versatile stimuli is one of the keys to future technological applications. Herein, we describe a wholly synthetic small-molecule system that, under the influence of chemical reagents, electrical potential, or visible light, undergoes unidirectional relative translational motion.

Ultrafast conformational dynamics of electron transfer in ExBox4+⊂perylene.

Multielectron acceptors are essential components for artificial photosynthetic systems that must deliver multiple electrons to catalysts for solar fuels applications. The recently developed boxlike cyclophane incorporating two extended viologen units joined end-to-end by two p-phenylene linkers-namely, ExBox(4+)-has a potential to be integrated into light-driven systems on account of its ability to complex with π-electron-rich guests such as perylene, which has been utilized to great extent in many light-harvesting applications. Photodriven electron transfer to ExBox(4+) has not previously been investigated, however, and so its properties, following photoreduction, are largely unknown.

Lasing action in strongly coupled plasmonic nanocavity arrays.

Periodic dielectric structures are typically integrated with a planar waveguide to create photonic band-edge modes for feedback in one-dimensional distributed feedback lasers and two-dimensional photonic-crystal lasers. Although photonic band-edge lasers are widely used in optics and biological applications, drawbacks include low modulation speeds and diffraction-limited mode confinement. In contrast, plasmonic nanolasers can support ultrafast dynamics and ultrasmall mode volumes.

Self-Assembly-Induced Ultrafast Photodriven Charge Separation in Perylene-3,4-dicarboximide-Based Hydrogen-Bonded Foldamers.

We report the synthesis, self-assembly characteristics, and ultrafast electron transfer dynamics of a perylene-3,4-dicarboximide (PMI) covalently linked to an N,N'-bis(3,4,5-tridodecyloxyphenyl)melamine electron donor (D) via a biphenyl spacer (PMI-Ph2-D). Synchrotron-based small- and wide-angle X-ray scattering (SAXS/WAXS) measurements in methylcyclohexane solution show that PMI-Ph2-D self-assembles into π-π stacked, hydrogen-bonded foldamers consisting of two or three hexameric rings or helices. Ultrafast transient absorption spectroscopy reveals that photoinduced charge separation within these nanostructures occurs by a unique pathway that is emergent in the assembly, whereas electron transfer does not occur in the PMI-Ph2-D monomers in tetrahydrofuran.

Plasmonic bowtie nanolaser arrays.

Plasmonic lasers exploit strong electromagnetic field confinement at dimensions well below the diffraction limit. However, lasing from an electromagnetic hot spot supported by discrete, coupled metal nanoparticles (NPs) has not been explicitly demonstrated to date. We present a new design for a room-temperature nanolaser based on three-dimensional (3D) Au bowtie NPs supported by an organic gain material.

Vibrational Dynamics of a Perylene-Perylenediimide Donor-Acceptor Dyad Probed with Femtosecond Stimulated Raman Spectroscopy.

The ultrafast vibrational dynamics of the photoinduced charge-transfer reaction between perylene (Per) and perylene-3,4:9,10-bis(dicarboximide) (PDI) were investigated using femtosecond stimulated Raman spectroscopy (FSRS). Specifically probing the structural dynamics of PDI following its selective photoexcitation in a covalently linked dyad reveals vibrational modes uniquely characteristic to the PDI lowest excited singlet state and radical anion between 1000 and 1700 cm(-1). A comparison of these vibrations to those of the ground state reveals the appearance of new (1*)PDI and PDI(-•) stretching modes in the dyad at 1593 and 1588 cm(-1), respectively.

Ultrafast photodriven intramolecular electron transfer from an iridium-based water-oxidation catalyst to perylene diimide derivatives.

Photodriving the activity of water-oxidation catalysts is a critical step toward generating fuel from sunlight. The design of a system with optimal energetics and kinetics requires a mechanistic understanding of the single-electron transfer events in catalyst activation. To this end, we report here the synthesis and photophysical characterization of two covalently bound chromophore-catalyst electron transfer dyads, in which the dyes are derivatives of the strong photooxidant perylene-3,4:9,10-bis(dicarboximide) (PDI) and the molecular catalyst is the Cp*Ir(ppy)Cl metal complex, where ppy = 2-phenylpyridine.

Direct observation of nanoparticle-cancer cell nucleus interactions.

We report the direct visualization of interactions between drug-loaded nanoparticles and the cancer cell nucleus. Nanoconstructs composed of nucleolin-specific aptamers and gold nanostars were actively transported to the nucleus and induced major changes to the nuclear phenotype via nuclear envelope invaginations near the site of the construct. The number of local deformations could be increased by ultrafast, light-triggered release of the aptamers from the surface of the gold nanostars.

Exponential distance dependence of photoinitiated stepwise electron transfer in donor-bridge-acceptor molecules: implications for wirelike behavior.

Donor-bridge-acceptor (D-B-A) systems in which a 3,5-dimethyl-4-(9-anthracenyl)julolidine (DMJ-An) chromophore and a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor are linked by oligomeric 2,7-fluorenone (FN(n)) bridges (n = 1-3) have been synthesized. Selective photoexcitation of DMJ-An quantitatively produces DMJ(+•)-An(-•), and An(-•) acts as a high-potential electron donor. Femtosecond transient absorption spectroscopy in the visible and mid-IR regions showed that electron transfer occurs quantitatively in the sequence: DMJ(+•)-An(-•)-FN(n)-NI → DMJ(+•)-An-FN(n)(-•)-NI → DMJ(+•)-An-FN(n)-NI(-•).

Electron transfer within self-assembling cyclic tetramers using chlorophyll-based donor-acceptor building blocks.

The synthesis and photoinduced charge transfer properties of a series of Chl-based donor-acceptor triad building blocks that self-assemble into cyclic tetramers are reported. Chlorophyll a was converted into zinc methyl 3-ethylpyrochlorophyllide a (Chl) and then further modified at its 20-position to covalently attach a pyromellitimide (PI) acceptor bearing a pyridine ligand and one or two naphthalene-1,8:4,5-bis(dicarboximide) (NDI) secondary electron acceptors to give Chl-PI-NDI and Chl-PI-NDI(2). The pyridine ligand within each ambident triad enables intermolecular Chl metal-ligand coordination in dry toluene, which results in the formation of cyclic tetramers in solution, as determined using small- and wide-angle X-ray scattering at a synchrotron source.

Intersystem crossing involving strongly spin exchange-coupled radical ion pairs in donor-bridge-acceptor molecules.

Intersystem crossing involving photogenerated strongly spin exchange-coupled radical ion pairs in a series of donor-bridge-acceptor molecules was examined. These molecules have a 3,5-dimethyl-4-(9-anthracenyl)-julolidine (DMJ-An) donor either connected directly or connected by a phenyl bridge (Ph), to pyromellitimide (PI), 1 and 2, respectively, or naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptors, 3 and 4, respectively. Femtosecond transient optical absorption spectroscopy shows that photodriven charge separation produces DMJ(+•)-PI(-•) or DMJ(+•)-NI(-•) quantitatively in 1-4 (τ(CS) ≤ 10 ps), and that charge recombination occurs with τ(CR) = 268 and 158 ps for 1 and 3, respectively, and with τ(CR) = 2.

Ultrafast photodriven intramolecular electron transfer from a zinc porphyrin to a readily reduced diiron hydrogenase model complex.

Diiron complexes modeled on the active site of the [FeFe] hydrogenases having the general formula [Fe(2)(mu-R)(CO)(6-n)(L)(n)], where commonly R = alkyl or aryl dithiolate and L = CO, CN(-), or PR(3), are a promising class of catalysts for use in photodriven H(2) production. However, many of these catalysts are difficult to photoreduce using chromophores that absorb visible light. Here we report the synthesis and spectroscopic characterization of a naphthalene-4,5-dicarboximide-1,8-dithiolate diiron complex [NMI-Fe(2)S(2)(CO)(6), 1] and a covalently linked, fixed-distance zinc 5,10,15-tri-n-pentyl-20-phenylporphyrin-NMI-Fe(2)S(2)(CO)(6) donor-acceptor dyad (2).

Narrow-bandwidth tunable picosecond pulses in the visible produced by noncollinear optical parametric amplification with a chirped blue pump.

Narrow-bandwidth (approximately 27 cm(-1)) tunable picosecond pulses from 480 nm-780 nm were generated from the output of a 1 kHz femtosecond titanium:sapphire laser system using a type I noncollinear optical parametric amplifier (NOPA) with chirped second-harmonic generation (SHG) pumping. Unlike a femtosecond NOPA, this system utilizes a broadband pump beam, the chirped 400 nm SHG of the Ti:sapphire fundamental, to amplify a monochromatic signal beam (spectrally-filtered output of a type II collinear OPA). Optimum geometric conditions for simultaneous phase- and group-velocity matching were calculated in the visible spectrum.

Photophysics and redox properties of rylene imide and diimide dyes alkylated ortho to the imide groups.

Ruthenium-catalyzed C-H bond activation was used to directly attach phenethyl groups derived from styrene to positions ortho to the imide groups in a variety of rylene imides and diimides including naphthalene-1,8-dicarboximide (NMI), naphthalene-1,4:5,8-bis(dicarboximide) (NI), perylene-3,4-dicarboximide (PMI), perylene-3,4:9,10-bis(dicarboximide) (PDI), and terrylene-3,4:11,12-bis(dicarboximide) (TDI). The monoimides were dialkylated, while the diimides were tetraalkylated, with the exception of NI, which could only be dialkylated due to steric hindrance. The absorption, fluorescence, transient absorption spectra, and lowest excited singlet state lifetimes of these chromophores, with the exception of NI, are nearly identical to those of their unsubstituted parent chromophores.

Rotationally resolved absorption cross sections of formaldehyde in the 28100-28500 cm(-1) (351-356 nm) spectral region: implications for in situ LIF measurements.

The rotationally resolved ultraviolet absorption cross sections for the 2(0)(0)4(1)(0) vibrational band of the A(1)A(2)-X(1)A(1) electronic transition of formaldehyde (HCHO) at an apodized resolution of 0.027 cm(-1) (approximately 0.0003 nm at 352 nm) over the spectral range 28100-28500 cm(-1) (351-356 nm) at 298 and 220 K, using Fourier transform spectroscopy, are first reported here.