Distant ultrafast energy transfer in a trimetallic {Ru-Ru-Cr} complex facilitated by hole delocalization.

Multi-metallic complexes based on {Ru-Cr}, {Ru-Ru} and {Ru-Ru-Cr} fragments are investigated for their light-harvesting and long-range energy transfer properties. We report the synthesis and characterization of [Ru(tpy)(bpy)(µ-CN)Ru(py)4Cl]2+ and [Ru(tpy)(bpy)(µ-CN)Ru(py)4(µ-NC)Cr(CN)5]. The intercalation of {RuII(py)4} linked by cyanide bridges between {Ru(tpy)(bpy)} and {Cr(CN)5} results in efficient, distant energy transfer followed by emission from the Cr moiety. Characterization of the energy transfer process based on photophysical and ultrafast time-resolved absorption suggests the delocalization of holes in the excited state, providing a pathway for energy transfer between the end moieties. The proposed mechanism opens the door to utilize this family of complexes as an appealing platform for the design of antenna compounds as the properties of the fragments could be tuned independently.

Dynamics of Energy Transfer in a Conjugated Dendrimer Driven by Ultrafast Localization of Excitations.

Solar energy conversion starts with the harvest of light, and its efficacy depends on the spatial transfer of the light energy to where it can be transduced into other forms of energy. Harnessing solar power as a clean energy source requires the continuous development of new synthetic materials that can harvest photon energy and transport it without significant losses. With chemically-controlled branched architectures, dendrimers are ideally suited for these initial steps, since they consist of arrays of chromophores with relative positioning and orientations to create energy gradients and to spatially focus excitation energies. The spatial localization of the energy delimits its efficacy and has been a point of intense research for synthetic light harvesters. We present the results of a combined theoretical experimental study elucidating ultrafast, unidirectional, electronic energy transfer on a complex molecule designed to spatially focus the initial excitation onto an energy sink. The study explores the complex interplay between atomic motions, excited-state populations, and localization/delocalization of excitations. Our findings show that the electronic energy-transfer mechanism involves the ultrafast collapse of the photoexcited wave function due to nonadiabatic electronic transitions. The localization of the wave function is driven by the efficient coupling to high-frequency vibrational modes leading to ultrafast excited-state dynamics and unidirectional efficient energy funneling. This work provides a long-awaited consistent experiment-theoretical description of excited-state dynamics in organic conjugated dendrimers with atomistic resolution, a phenomenon expected to universally appear in a variety of synthetic conjugated materials.

Conjugated polyelectrolyte dendrimers: aggregation, photophysics, and amplified quenching.

Conjugated polyelectrolyte dendrimers (CPDs) are monodisperse macromolecules that feature a fully π-conjugated dendrimer core surrounded on the periphery by ionic solubilizing groups. CPDs are soluble in water and polar organic solvents, and they exhibit photophysics characteristic of the π-conjugated chromophores comprising the dendrimer core. Here we describe the synthesis and photophysical characterization of series of three generations of CPDs based on a phenylene ethynylene repeat unit structure that is surrounded by an array of anionic sodium carboxylate groups. Molecular dynamics simulations indicate that the first-generation CPD is flat while the second- and third-generation CPDs adopt oblate structures. Photophysical studies, including absorption, fluorescence spectroscopy, and lifetimes, show that the ester protected precursor dendrimers exhibit highly efficient blue fluorescence in THF solution emanating from the phenylene ethynylene chromophore that is in the dendrimer core. By contrast, the water-soluble CPDs have much lower fluorescence quantum yields and the absorption and fluorescence spectra exhibit features of strong chromophore-chromophore interactions. The results are interpreted as suggesting that the CPDs exist as dimer or multimer aggregates, even in very dilute solution. Fluorescence quenching of the anionic CPDs with the dication electron acceptor N,N'-dimethylviologen (MV(2+)) is very efficient, with Stern-Volmer quenching constants (K(SV)) increasing with generation number. The third-generation CPD exhibits highly efficient amplified quenching, with K(SV) ∼ 5 × 10(6) M(-1).

Shishiodoshi unidirectional energy transfer mechanism in phenylene ethynylene dendrimers.

Non-adiabatic excited-state molecular dynamics is used to study the ultrafast intramolecular energy transfer between two-, three-, and four-ring linear polyphenylene ethynylene chromophore units linked through meta-substitutions. Twenty excited-state electronic energies, with their corresponding gradients and nonadiabatic coupling vectors were included in the simulations. The initial laser excitation creates an exciton delocalized between the different absorbing two-ring linear PPE units. Thereafter, we observe an ultrafast directional change in the spatial localization of the transient electronic transition density. The analysis of the intramolecular flux of the transition density shows a sequential through-bond two-ring→three-ring→four-ring transfer as well as an effective through-space direct two-to-four ring transfer. The vibrational excitations of C≡C stretching motions change according to that. Finally, a mechanism of unidirectional energy transfer is presented based on the variation of the energy gaps between consecutive electronic excited states in response to the intramolecular flux of the transition density. The mechanism resembles a Shishiodoshi Japanese bamboo water fountain where, once the electronic population has been transferred to the state directly below in energy, the two states decouple thereby preventing energy transfer in the opposite direction.

Nonadiabatic molecular dynamics simulations of the energy transfer between building blocks in a phenylene ethynylene dendrimer.

The ultrafast dynamics of electronic and vibrational energy transfer between two- and three-ring linear poly(phenylene ethynylene) units linked by meta-substitution is studied by nonadiabatic molecular dynamics simulations. The molecular dynamics with quantum transitions (1, 2) method is used including an "on the fly" calculation of the potential energy surfaces and electronic couplings. The results show that during the first 40 fs after a vertical photoexcitation to the S(2) state, the nonadiabatic coupling between S(2) and S(1) states causes a fast transfer of the electronic populations. A rapid decrease of the S(1)-S(2) energy gap is observed, reaching a first conical intersection at approximately 5 fs. Therefore, the first hopping events take place, and the S(2) state starts to depopulate. The analysis of the structural and energetic properties of the molecule during the jumps reveals the main role that the ethynylene triple bond plays in the unidirectional energy transfer process.

Coherent exciton-vibrational dynamics and energy transfer in conjugated organics.

Coherence, signifying concurrent electron-vibrational dynamics in complex natural and man-made systems, is currently a subject of intense study. Understanding this phenomenon is important when designing carrier transport in optoelectronic materials. Here, excited state dynamics simulations reveal a ubiquitous pattern in the evolution of photoexcitations for a broad range of molecular systems. Symmetries of the wavefunctions define a specific form of the non-adiabatic coupling that drives quantum transitions between excited states, leading to a collective asymmetric vibrational excitation coupled to the electronic system. This promotes periodic oscillatory evolution of the wavefunctions, preserving specific phase and amplitude relations across the ensemble of trajectories. The simple model proposed here explains the appearance of coherent exciton-vibrational dynamics due to non-adiabatic transitions, which is universal across multiple molecular systems. The observed relationships between electronic wavefunctions and the resulting functionalities allows us to understand, and potentially manipulate, excited state dynamics and energy transfer in molecular materials.

Trapping intermediate MLCT states in low-symmetry {Ru(bpy)} complexes.

The picosecond excited state dynamics of [Ru(tpm)(bpy)(NCS)]+ (RubNCS+ ) and [Ru(tpm)(bpy)(CN)]+ (RubCN+ ) (tpm = tris(1-pyrazolyl)methane, bpy = 2,2'-bipyridine) have been analyzed by means of transient absorption measurements and spectroelectrochemistry. Emissive 3MLCTs with (GS)HOMO(h+)-(GS)LUMO(e-) configurations are the lowest triplet excited states regardless of whether 387 or 505 nm photoexcitation is used. 387 nm photoexcitation yields, after a few picoseconds, the emissive 3MLCTs. In contrast, 505 nm photoexcitation populates an intermediate excited state that we assign as a 3MLCT state, in which the hole sits in a metal-centered orbital of different symmetry, prior to its conversion to the emissive 3MLCTs. The disparities in terms of electronic configuration between the intermediate and the emissive 3MLCTs have two important consequences. On one hand, both states feature very different fingerprint absorptions in transient absorption measurements. On the other hand, the reconfiguration is impeded by a kinetic barrier. As such, the conversion is followed spectroscopically and kinetically on the 300 ps timescale.

Energy transfer in unsymmetrical phenylene ethynylene dendrimers.

We have applied the fluorescence upconversion technique to explore the electronic excitation energy transfer in unsymmetrical phenylene ethynylene dendrimers. Steady-state emission spectra show that the energy transfer from the dendrons to the core is highly efficient. Ultrafast time-resolved fluorescence measurements are performed at various excitation wavelengths to explore the possibility of assigning absorption band structures to exciton localizations. We propose a kinetic model to describe the time-resolved data. Independent of the excitation wavelength, a typical rise-time value of 500 fs is measured for the fluorescence in the dendrimer without an energy trap, indicating initial delocalized excitation. While absorption is into delocalized exciton states, emission occurs from localized states. When an energy trap such as perylene is introduced on the dendrimer, varying the excitation wavelength yields different energy-transfer rates, and the excitation energy migrates to the trap through two channels. The interaction energy between the dendrimer backbone and the trap is estimated to be 75 cm(-1). This value is small compared to the vibronic bandwidth of the dendrimer, indicating that the monodendrons and the energy trap are weakly coupled.

Energy transfer in the nanostar: the role of coulombic coupling and dynamics.

We present a theoretical investigation of energy transfer in the phenylene ethynelene dendrimer known as the nanostar. Data from extensive molecular dynamics simulations are used to model the dynamical effects caused by torsional motion of the phenyl groups. We compare rate constants for energy transfer between the two-ring chromophore and the three-ring chromophore obtained via the Förster model, the ideal dipole approximation (IDA), and the transition density cube (TDC) method, which has as its limit an exact representation of the Coulombic coupling. We find that the rate constants obtained with the TDC are extremely sensitive to the phenyl group rotation, whereas the constants computed with the Förster model and the IDA are not. The implications of these results for the interpretation of recent pump-probe experiments on the nanostar are discussed in detail. Finally, we predict the temperature dependence of the rate constant for energy transfer.

Emissive cyanide-bridged bimetallic compounds as building blocks for polymeric antennae.

A series of cyanide-bridged bimetallic compounds of the general formula [Ru(L)(bpy)(µ-NC)(M)](2-/-/2+) (L = tpy, 2,2'-6',2''-terpyridine, or tpm, tris(1-pyrazolyl)methane, bpy = 2,2'-bipyridine, M = Ru(II)(CN)5, Os(III)(CN)5, Os(II)(CN)5, Ru(II)(py)4(CN), py = pyridine) have been synthesized and fully characterized. Most of them present MLCT emission (λ = 690-730 nm, Φ = 10(-3)-10(-4)) and their photophysical properties resemble the ones of the respective mononuclear Ru(L)(bpy) species. The exception is when M is Os(III)(CN)5, where an intramolecular electron transfer quenching mechanism is proposed. The conditions that should be met for avoiding the reductive or oxidative quenching of the excited state are also discussed.

Efficient energy transfer via the cyanide bridge in dinuclear complexes containing Ru(II) polypyridine moieties.

We report the synthesis, structure and properties of the cyanide-bridged dinuclear complex ions [Ru(L)(bpy)(µ-NC)M(CN)(5)](2-/-) (L = tpy, 2,2';6',2''-terpyridine, or tpm, tris(1-pyrazolyl)methane, bpy = 2,2'-bipyridine, M = Fe(II), Fe(III), Cr(III)) and the related monomers [Ru(L)(bpy)X](2+) (X = CN(-) and NCS(-)). All the monomeric compounds are weak MLCT emitters (λ = 650-715 nm, ϕ ≈ 10(-4)). In the Fe(II) and Cr(III) dinuclear systems, the cyanide bridge promotes efficient energy transfer between the Ru-centered MLCT state and a Fe(II)- or Cr(III)-centered d-d state, which results either in a complete quenching of luminescence or in a narrow red emission (λ ≈ 820 nm, ϕ ≈ 10(-3)) respectively. In the case of Fe(III) dinuclear systems, an electron transfer quenching process is also likely to occur.

Energy transfer in extended thienylene-phenylene-ethynylene dendrimers.

We present a new family of dendrimers with all-conjugated, thienylene (Th) containing photoactive backbones and branched end-groups. Steady-state spectroscopy demonstrates a donor-acceptor system, while picosecond time-resolved fluorescence characterizes a vectorial energy transfer from phenylene-ethynylene (PE) units at the periphery to thienylene-containing PE units at the core. Energy transfer rates of 1.5 and 3.5 ps are observed for generation 2 and 3 dendrimers, indicative of a weakly coupled donor-acceptor system, with couplings on the order of 40-60 cm(-1).

Mapping excited-state dynamics by coherent control of a dendrimer's photoemission efficiency.

Adaptive laser pulse shaping has enabled impressive control over photophysical processes in complex molecules. However, the optimal pulse shape that emerges rarely offers straightforward insight into the excited-state properties being manipulated. We have shown that the emission quantum yield of a donor-acceptor macromolecule (a phenylene ethynylene dendrimer tethered to perylene) can be enhanced by 15% through iterative phase modulation of the excitation pulse. Furthermore, by analyzing the pulse optimization process and optimal pulse features, we successfully isolated the dominant elements underlying the control mechanism. We demonstrated that a step function in the spectral phase directs the postexcitation dynamics of the donor moiety, thus characterizing the coherent nature of the donor excited state. An accompanying pump-probe experiment implicates a 2+1 photon control pathway, in which the optimal pulse promotes a delayed excitation to a second excited state through favorable quantum interference.

Synthesis and characterization of colloidal ternary ZnCdSe semiconductor nanorods.

For the synthesis of colloidal ternary ZnCdSe nanorods, CdSe nanorods were first prepared under a mixture of tetradecylphosphonic acid/trioctylphosphine oxide surfactants at 250 degrees C, and then ZnSe shell layer was grown onto CdSe nanorods at 180 degrees C, forming CdSeZnSe core/shell nanorods. Green-yellow emitting ternary ZnCdSe nanorods were obtained by a subsequent alloying process at 270 degrees C for 1-3 h through the diffusion of Zn ions into CdSe nanorods. The photoluminescence quantum yield (QY) of ZnCdSe nanorods was 5%-10%, which is higher than that from pristine CdSe nanorods (0.6%). The QY of these alloy nanorods depends on the alloying time and is discussed in terms of compositional disorders and defects produced by the alloying process. The Raman and time resolved photoluminescence spectroscopies were used to understand the detailed alloying process from CdSeZnSe core/shell to ZnCdSe alloy nanorods.

The role of exciton hopping and direct energy transfer in the efficient quenching of conjugated polyelectrolytes.

The dynamics of fluorescence quenching of a conjugated polyelectrolyte by a cyanine dye are investigated by femtosecond fluorescence up-conversion and polarization resolved transient absorption. The data are analyzed with a model based on the random walk of the exciton within the polymer chain and a long-range direct energy transfer between polymer and dye. We find that rapid intrachain energy migration toward complex sites with the dye leads to the highly efficient energy transfer, whereas the contribution from direct, long-range energy transfer is negligible. We determine the actual density of complexes with the dye along the polymer chain. A clear deviation from calculations based on a constant complex association constant is found and explained by a reduced effective polymer concentration due to aggregation. Altogether, the quenching efficiency is found to be limited by (i) the energetic disorder within the polymer chain and (ii) the formation of loose polymer aggregates.

Amplified quenching of a conjugated polyelectrolyte by cyanine dyes.

The conjugated polyelectrolyte PPESO3 features a poly(phenylene ethynylene) backbone substituted with anionic 3-sulfonatopropyloxy groups. PPESO3 is quenched very efficiently (KSV > 10(6) M(-1)) by cationic energy transfer quenchers in an amplified quenching process. In the present investigation, steady-state and picosecond time-resolved fluorescence spectroscopy are used to examine amplified quenching of PPESO3 by a series of cyanine dyes via singlet-singlet energy transfer. The goal of this work is to understand the mechanism of amplified quenching and to characterize important parameters that govern the amplification process. Steady-state fluorescence quenching of PPESO3 by three cationic oxacarbocyanine dyes in methanol solution shows that the quenching efficiency does not correlate with the Forster radius computed from spectral overlap of the PPESO3 fluorescence with the cyanines' absorption. The quenching efficiency is controlled by the stability of the polymer-dye association complex. When quenching studies are carried out in water where PPESO3 is aggregated, changes observed in the absorption and fluorescence spectra of 1,1',3,3,3',3'-hexamethylindotricarbocyanine iodide (HMIDC) indicate that the polymer templates the formation of a J-aggregate of the dye. The fluorescence dynamics in the PPESO3/HMIDC system were probed by time-resolved upconversion and the results show that PPESO3 to HMIDC energy transfer occurs on two distinctive time scales. At low HMIDC concentration, the dynamics are dominated by an energy transfer pathway with a time scale faster than 4 ps. With increasing HMIDC concentration, an energy pathway with a time scale of 0.1-1 ns is active. The prompt pathway (tau < 4 ps) is attributed to quenching of delocalized PPESO3 excitons created near the HMIDC association site, whereas the slow phase is attributed to intra- and interchain exciton diffusion to the HMIDC.

Optical and photophysical properties of light-harvesting phenylacetylene monodendrons based on unsymmetrical branching.

The optical and photophysical properties of phenylacetylene dendritic macromolecules based on unsymmetrical branching are investigated using steady-state and time-dependent spectroscopy. Monodendrons, up to the fourth generation, are characterized with and without a fluorescent perylene trap at the core. The higher generation monodendrons without the perylene trap exhibit high molar extinction coefficients (>10(5) M(-1) cm(-1)) and high fluorescence quantum yields (65-81%). When a perylene trap is placed at the core, then the monodendrons typically exhibit high energy transfer quantum yields (approximately 90%), as well as subpicosecond time scale excited-state dynamics, as evidenced by ultrafast pump-probe measurements. The photophysical properties of the unsymmetrical monodendrons are compared to those of phenylacetylene monodendrons with symmetrical branching, which have been described recently. The high fluorescence quantum yields and large energy transfer quantum efficiencies exhibited by the unsymmetrical monodendrons suggest they have potential for applications in molecular-based photonics devices.