Single-Molecule Spectroscopy of Intramolecular Electron Transfer in Donor-Bridge-Acceptor Systems

Electron Transfer in a Naphthalene Diimide System Studied by Single-Molecule Delayed Fluorescence

Electron transfer (ET) is a key chemical reaction in nature and has been extensively studied in bulk systems, but remains challenging to investigate at the single-molecule level. A previously reported naphthalene diimide (NDI)-based system (Higginbotham et al., Chem. Commun. 2013, 49, 5061–5063) displays delayed fluorescence with good quantum yield (~0.5) and long-lived (nanoseconds) prompt and delayed fluorescence lifetimes, providing an opportunity to interrogate the underlying ET processes in single molecules. Time-resolved single-molecule fluorescence measurements enabled forward and reverse ET rate constants to be calculated for 45 individual molecules embedded in poly(methylmethacrylate) (PMMA) film. Interpretation of the results within the framework of Marcus–Hush theory for ET demonstrates that variation in both the electronic coupling and the driving force for ET is occurring from molecule to molecule within the PMMA film and over time for individual molecules.

Design, Synthesis, and Isomerization Studies of Light-Driven Molecular Motors for Single Molecular Imaging

The design of a multicomponent system that aims at the direct visualization of a synthetic rotary motor at the single molecule level on surfaces is presented. The synthesis of two functional motors enabling photochemical rotation and fluorescent detection is described. The light-driven molecular motor is found to operate in the presence of a fluorescent tag if a rigid long rod (32 Å) is installed between both photoactive moieties. The photochemical isomerization and subsequent thermal helix inversion steps are confirmed by ¹H NMR and UV–vis absorption spectroscopies. In addition, the tetra-acid functioned motor can be successfully grafted onto amine-coated quartz and it is shown that the light responsive rotary motion on surfaces is preserved.

Bis(perylene diimide) with DACH bridge as non-fullerene electron acceptor for organic solar cells

A set of non-fullerene small molecules based on bis(perylene diimide) with chiral DACH bridges were designed, synthesized, and characterized as electron acceptor materials in BHJ inverted organic solar cells.

Self-assembled vesicle and rod-like aggregates of functionalized perylene diimide: reaction-based near-IR intracellular fluorescent probe for selective detection of palladium

Aggregates ofPS-PDIafter Pd⁰based depropargylation show de-aggregation and near-IR, ratiometric absorbance changes in water and live HeLa cells.

Emergence of Coherence through Variation of Intermolecular Distances in a Series of Molecular Dimers

Quantum coherences between electronically excited molecules are a signature of entanglement and play an important role for energy transport in molecular assemblies. Here we monitor and analyze for a homologous series of molecular dimers embedded in a solid host the emergence of coherence with decreasing intermolecular distance by single-molecule spectroscopy and quantum chemistry. Coherent signatures appear as an enhancement of the purely electronic transitions in the dimers which is reflected by changes of fluorescence spectra and lifetimes. Effects that destroy the coherence are the coupling to the surroundings and to vibrational excitations. Complementary information is provided by excitation spectra from which the electronic coupling strengths were extracted and found to be in good agreement with calculated values. By revealing various signatures of intermolecular coherence, our results pave the way for the rational design of molecular systems with entangled states.

1D nanofiber composites of perylene diimides for visible-light-driven hydrogen evolution from water

Self-assembled 1D nanofibers of donor–accepter type perylene diimides have been used for photocatalytic H₂ production from water-splitting under visible-light irradiation.

Photoinduced electron transfer of oxazine 1/TiO2 nanoparticles at single molecule level by using confocal fluorescence microscopy

Kinetics of photoinduced electron transfer (ET) from oxazine 1 dye to TiO(2) nanoparticles (NPs) surface is studied at a single molecule level by using confocal fluorescence microscopy. Upon irradiation with a pulsed laser at 630 nm, the fluorescence lifetimes sampled among 100 different dye molecules are determined to yield an average lifetime of 2.9 +/- 0.3 ns, which is close to the value of 3.0 +/- 0.6 ns measured on the bare coverslip. The lifetime proximity suggests that most interfacial electron transfer (IFET) processes for the current system are inefficient, probably caused by physisorption between dye and the TiO(2) film. However, there might exist some molecules which are quenched before fluorescing and fail to be detected. With the aid of autocorrelation analysis under a three-level energy system, the IFET kinetics of single dye molecules in the conduction band of TiO(2) NPs is evaluated to be (1.0 +/- 0.1) x 10(4) s(-1) averaged over 100 single molecules and the back ET rate constant is 4.7 +/- 0.9 s(-1). When a thicker TiO(2) film is substituted, the resultant kinetic data do not make a significant difference. The trend of IFET efficacy agrees with the method of fluorescence lifetime measurements. The obtained forward ET rate constants are about ten times smaller than the photovoltage response measured in an assembled dye-sensitized solar cell. The discrepancy is discussed. The inhomogeneous and fluctuation characters for the IFET process are attributed to microenvironment variation for each single molecule. The obtained ET rates are much slower than the fluorescence relaxation. Such a small ET quantum yield is yet feasibly detectable at a single molecule level.

MALDI-Fourier transform mass spectrometric and theoretical studies of donor-acceptor and donor-bridge-acceptor fullerenes

Fourier-transform ion cyclotron resonance (FTICR) mass spectrometric studies have been performed on donor-acceptor and donor-bridge acceptor fullerene-based systems. Matrix-assisted laser desorption/ionization (MALDI) was used for ion production; both the positive and negative ion modes were utilized. In addition, collision-induced dissociation (CID) experiments were carried out to study the movement of the charge (electron or hole) upon fragmentation. The experiments are complemented by ab initio theoretical calculations yielding both molecular orbital energies and electron density distributions. It was found that the theoretical electron density map predicted the experimentally observed fragmentation correctly in every case. Both the calculations and the MS experiments may be useful in studying these and related donor-acceptor systems in view of their use for charge separation and eventually, solar energy production.

Short-time power-law blinking statistics of single quantum dots and a test of the diffusion-controlled electron transfer model

In this work we analyzed the blinking statistics data of single CdSe/ZnS quantum dots at very short times to test some predictions of the diffusion-controlled electron transfer (DCET) model. Using autocorrelation function (ACF) approach we could extract the exponent of the inverse power-law blinking statistics down to 1 micros. Such an approach also minimizes human subjectivity in choosing a bin time and an on-off threshold. We showed that the observed stretched exponential decay in the ACF and its relationship to the blinking statistics are consistent with the DCET model, and we set an upper bound for the characteristic time constant t(c).

Strategy to control the chromism and fluorescence emission of a perylene dye in composite organogel phases

Composite organogels based on 1,3,5-tris(4-dodecyloxybenzoylamino)phenylbenzene (DBAPB), a known gelator, and N,N'-di(octadecyl)-perylene-3,4,9,10-tetracarboxylic diimide (C18PTCDI), a nongelator dye, have been achieved, leading to controllable color and emitting color changes. SEM images and XRD patterns revealed that the packing of the DBAPB-based gelator could almost be maintained in the composite gels. The temperature-dependent UV-vis absorption and temperature-dependent fluorescence emission spectra illustrated that the color and emitting color of the composite gels could be controlled by the content of C18PTCDI as well as the temperature in the gel phases. When the content of C18PTCDI was 1 mol %, C18PTCDI could be isolated as unimolecules in the composite gel, which was yellow and gave bright greenish-yellow emission under 365 nm light. For the mixed systems containing 2-10 mol % C18PTCDI, the fresh gels, which were obtained after cooling the hot solutions for a short time, were yellow and produced greenish-yellow emission under 365 nm illumination. However, the corresponding stable composite gels, which were obtained via prolonging the cooling time, were red and emitted weak red emission excited by UV light as a result of the formation of C18PTCDI aggregates. The reversible color and emitting color changes could be realized in the gel phases over a narrow temperature range. Moreover, the excitation energy of DBAPB could be transferred to C18PTCDI in the composite gels, leading to obvious emission quenching of the former.

Morphology-controlled self-assembled nanostructures of 5,15-di[4-(5-acetylsulfanylpentyloxy)phenyl]porphyrin derivatives. Effect of metal-ligand coordination bonding on tuning the intermolecular interaction

Novel metal-free 5,15-di[4-(5-acetylsulfanylpentyloxy)phenyl]porphyrin H2[DP(CH3COSC5H10O)2P] (1) and its zinc congener Zn[DP(CH3COSC5H10O)2P] (2) were designed and synthesized. Single-crystal X-ray diffraction (XRD) analysis confirmed the tetrapyrrole nature of these two compounds, revealing the existence of metal-ligand coordination bond between the carbonyl oxygen in the aryloxy side chain of meso-attached phenyl group in the porphyrin molecule with the zinc center of neighboring porphyrin molecule in the crystal structure of 2. This intermolecular Zn-O coordination bond induces the formation of a supramolecular chain structure in which the porphyrinato zinc moieties are arranged in a "head-to-tail" mode (J-aggregate), which is in contrast to a "face-to-face" stacking mode (H-aggregate) in the supramolecular structure formed depending on the C-H...pi interaction in the crystal of 1. Their self-assembling properties in MeOH and n-hexane were comparatively investigated by scanning electronic microscopy and XRD technique. Intermolecular pi-pi interaction of metal-free porphyrin 1 leads to the formation of hollow nanospheres and nanoribbons in MeOH and n-hexane, respectively. In contrast, introduction of additional Zn-O coordination bond for porphyrinato zinc complex 2 induces competition with intermolecular pi-pi interaction, resulting in nanostructures with nanorod and hollow nanosphere morphology in MeOH and n-hexane. The IR and XRD results clearly reveal the presence and absence of such metal-ligand coordination bond in the nanostructures formed from porphyrinato zinc complex 2 and metal-free porphyrin 1, respectively, which is further unambiguously confirmed by the single-crystal XRD analysis result for both compounds. Electronic absorption spectroscopic data on the self-assembled nanostructures reveal the H-aggregate nature in the hollow nanospheres and nanoribbons formed from metal-free porphyrin 1 due to the pi-pi intermolecular interaction between porphyrin molecules and J-aggregate nature in the nanorods and hollow nanospheres of 2 depending on the dominant metal-ligand coordination bonding interaction among the porphyrinato zinc molecules. The present result appears to represent the first effort toward controlling and tuning the morphology of self-assembled nanostructures of porphyrin derivatives via molecular design and synthesis through introduction of metal-ligand coordination bonding interaction. Nevertheless, availability of single crystal and molecular structure revealed by XRD analysis for both porphyrin derivatives renders it possible to investigate the formation mechanism as well as the molecular packing conformation of self-assembled nanostructures of these typical organic building blocks with large conjugated system in a more confirmed manner.

Clay nanoparticle-supported single-molecule fluorescence spectroelectrochemistry

Here we report that clay nanoparticles allow formation of a modified transparent electrode, spontaneous adsorption of fluorescent redox molecules on the clay layer, and thus the subsequent observation of single-molecule fluorescence spectroelectrochemistry. We can trace single-molecule fluorescence spectroelectrochemistry by probing the fluorescence intensity change of individually immobilized single redox molecules modulated via cyclic voltammetric potential scanning. This work opens a new approach to explore interfacial electron transfer mechanisms of redox reactions.

The effects of anomalous diffusion on power-law blinking statistics of CdSe nanorods

In this study of fluorescence blinking statistics for nanorods, we present a diffusion-controlled reaction model that leads to a more general formula: t(-m) exp[-(Gammat)(n)]. This formula describes a short-time power law with a crossover to a stretched exponential decay at later times. Based on quantum Brownian motion for a coupled central harmonic oscillator coupled to heat bath oscillators, we show that the position distribution follows anomalous diffusion with time-dependent diffusion coefficient and drift coefficient. The first and the second moments of the energy fluctuations are shown to be related to the exponent m and n for the blinking statistics.

Fluorescence intermittency of silicon nanocrystals and other quantum dots: a unified two-dimensional diffusion-controlled reaction model

We present a unified model involving two-dimensional diffusion-controlled reactions of both slow and fast reaction coordinates to elucidate the dynamic origin of fluorescence intermittency observed not just in quantum dots but also in organic chromphores and biomolecules. This improved model also solves the puzzling behavior of Si nanocrystals which display an unusually large m (exceeding 2) for the power-law decay of t(-m) and provides remedies for the deficiencies in existing models.

Power-Law Blinking in the Fluorescence of Single Organic Molecules

The blinking behavior of perylene diïmide molecules is investigated at the single-molecule level. We observe long-time scale blinking of individual multi-chromophoric complexes embedded in a poly(methylmethacrylate) matrix, as well as for the monomeric dye absorbed on a glass substrate at ambient conditions. In both these different systems, the blinking of single molecules is found to obey analogous power-law statistics for both the on and off periods. The observed range for single-molecular power-law blinking extends over the full experimental time window, covering four orders of magnitude in time and six orders of magnitude in probability density. From molecule to molecule, we observe a large spread in off-time power-law exponents. The distributions of off-exponents in both systems are markedly different whereas both on-exponent distributions appear similar. Our results are consistent with models that ascribe the power-law behavior to charge separation and (environment-dependent) recombination by electron tunneling to a dynamic distribution of charge acceptors. As a consequence of power-law statistics, single molecule properties like the total number of emitted photons display non-ergodicity.

Solvent Mediated Intramolecular Photoinduced Electron Transfer in a Fluorene-Perylene Bisimide Derivative

Photoinduced electron transfer from fluorene to perylene bisimide has been studied for 2,7-bis(N-(1-hexylheptyl)-3,4:9,10-perylene-bisimide-N'-yl))-9,9-didodecylfluorene (PFP) in 11 different organic solvents. The intramolecular charge-separated state in PFP is almost isoenergetic with the locally excited state of the perylene bisimide. As a consequence of the small change in free energy for charge separation, the electron transfer rate strongly depends on subtle changes in the medium. The rate constant k(CS) for the electron transfer from fluorene to perylene bisimide moiety in the excited state varies over more than 2 orders of magnitude ( approximately 10(8)-10(10) s(-1)) with the solvent but does not show the familiar increase with polarity. The widely differing rate constants can be successfully explained by considering (1) the contribution of the polarization energy of the dipole moment in the transition state and by (2) the classical Marcus-Jortner model and assuming a spherical cavity for the charge-separated state. Using the first model, we show that lnk(CS) should vary linearly with Deltaf [Deltaf = (epsilon(r) - 1)/(2epsilon(r) + 1) - (n(2) - 1)/(2n(2) + 1), where epsilon(r) and n represent the static dielectric constant and the refractive index of the solvent, respectively], in accordance with experimental results. The second model, where the reorganization energy scales linearly with Deltaf, provides quantitative agreement with experimental rate constants within a factor of 2.

DNA and microfluidics: Building molecular electronics systems

The development of molecular electronics using DNA molecules as the building blocks and using microfluidics to build nanowire arrays is reviewed. Applications of DNA conductivity to build sensors and nanowire arrays, and DNA conjugation with other nanostructures, offers an exciting opportunity to build extremely small analytical devices that are suitable for single-molecule detection and also target screening.

Single Molecule Blinking and Photobleaching Separated by Wide-Field Fluorescence Microscopy

Single molecule fluorescence detection of Atto590 in poly(vinyl alcohol) was achieved by using a wide-field epifluorescence microscope with CCD-camera detection. Image sequences are obtained from which the time traces of the detected molecules are built. We find a distinctive difference between the time evolution of the fluorescence originating from the molecules detected in the first image of the sequence compared to the time evolution of the fluorescence of the molecules detected in each image of the sequence. Atto590 shows very long blinking times and photobleaching and photoblinking that are both quadratically dependent on the irradiation power density. Our approach allows kinetic separation of photobleaching from blinking. The possibility of choosing different ensembles of molecules is demonstrated and taken advantage of for this aim. Initially dark molecules or low emitting ones that might be overlooked are important to describe the complete ensemble behavior.

Electron Transfer at the Single-Molecule Level in a Triphenylamine-Perylene Imide Molecule

Photoinduced electron transfer (ET) processes in a donor-acceptor system based on triphenylamine and perylene imide have been studied at the single-molecule (SM) and ensemble levels. The system exists as two isomers, one of which undergoes forward and reverse ET in toluene with decay constants of 3.0 and 2.2x10(9) s(-1), respectively, resulting in the dual emission of quenched and delayed fluorescence while the other isomer remains ET-inactive. The fluorescence of both isomers is heavily quenched in the more polar solvent, diethyl ether, by ET. A broad range of ET dynamics is seen at the SM level in polystryene with the two isomers nonresolvable indicating that the local nanoenvironment of the SMs varies considerably throughout the polymer matrix. Both the electronic coupling and the driving force for ET are shown to influence the ET dynamics. Many fluorescence trajectories of SMs show long periods (tens of milliseconds to seconds) where the count rate is attenuated either partly (a "dim" state) or to the background level (an "off-time"). During these periods, the reduction or interruption of emission is attributed to cycles of rapid charge separation followed by charge recombination to the ground state reducing the fluorescence quantum yield of the SM.

Single-molecule spectroscopy selectively probes donor and acceptor chromophores in the phycobiliprotein allophycocyanin

We report on single-molecule fluorescence measurements performed on the phycobiliprotein allophycocyanin (APC). Our data support the presence of a unidirectional Förster-type energy transfer process involving spectrally different chromophores, alpha84 (donor) and beta84 (acceptor), as well as of energy hopping amongst beta84 chromophores. Single-molecule fluorescence spectra recorded from individual immobilized APC proteins indicate the presence of a red-emitting chromophore with emission peaking at 660 nm, which we connect with beta84, and a species with the emission peak blue shifted at 630 nm, which we attribute to alpha84. Polarization data from single APC trimers point to the presence of three consecutive red emitters, suggesting energy hopping amongst beta84 chromophores. Based on the single-molecule fluorescence spectra and assuming that emission at the ensemble level in solution comes mainly from the acceptor chromophore, we were able to resolve the individual absorption and emission spectra of the alpha84 and beta84 chromophores in APC.

Using Single-Molecule Fluorescence Spectroscopy to Study Electron Transfer

Techniques in single-molecule fluorescence spectroscopy now allow sophisticated studies of photophysical processes in single molecules. As interest grows in the possibilities of molecular electronics, researchers have begun to turn these techniques to the study of electron transfer. Electron-transfer reactions have now been detected and measured at the single-molecule level in a variety of systems and on a variety of timescales by adapting techniques from previous single-molecule fluorescence studies.

Self-Organized Perylene Diimide Nanofibers

A propeller-shaped perylene diimide trimer was synthesized and a simple evaporation method was used for the self-organization of trimer molecules into fluorescent nanofibers. The sizes of these fibers-from 4 to 150 nm in diameter-were measured by atomic force microscopy and can be controlled by adjusting the concentration of the initial solution. The aspect ratios (length/height) are around 500. The plane of the trimer was determined by polarized scanning confocal microscopy to be perpendicular to the axis of the fibers, in agreement with molecular mechanics calculations. UV/vis and NMR spectroscopies were used to monitor concentration-dependent pi-pi stacking in solution. Single-fiber fluorescence imaging and spectroscopy were performed using a total internal reflection fluorescence microscope equipped with a digital color camera and imaging CCD spectrometer. Strongly red-shifted fluorescence from these fibers indicates a high degree of electronic delocalization, and breaking up this delocalization by photobleaching blue-shifts the emission toward that of an isolated noninteracting molecule. The delocalization along these nanofibers and the ability to study the electronic structure using fluorescence make them potentially useful in nanoscale devices, such as field effect transistors and photoconductors.

Single molecule photobleaching probes the exciton wave function in a multichromophoric system

The exciton wave function of a trichromophoric system is investigated by means of single molecule spectroscopy at room temperature. Individual trimers exhibit superradiance and loss of vibronic structure in emission spectrum, features proving exciton delocalization. We identify two distinct photodegradation pathways for single trimers upon sequential photobleaching of the chromophores. The rate of each pathway is a measure for the contribution of the separate dyes to the collective excited state of the system, in this way probing the wave function of the delocalized exciton.

Single Photon Emission from a Dendrimer Containing Eight Perylene Diimide Chromophores

A novel dendrimer containing eight perylene diimide chromophores has been synthesized and studied by ensemble and single-molecule spectroscopic techniques. Photon anti-bunching (coincidence) measurements on single molecules embedded in zeonex polymer films show that the dendrimer behaves as a deterministic (triggered) single photon source with only one fluorescence photon being emitted following pulsed laser excitation, even when more than one chromophore is excited. This behaviour is due to efficient singlet–singlet annihilation being operative in this dendrimer. Preliminary results indicate that the triplet lifetime and yield for this molecule are similar to the values for a molecule containing a single perylene diimide chromophore.