Photoinduced Electron Transfer in CdSe/ZnS Quantum Dot–Fullerene Hybrids

Photoinduced electron transfer (ET) in CdSe/ZnS core–shell quantum dot (QD) – fullerene (COOH–C₆₀) hybrids was studied by the means of time-resolved emission and absorption spectroscopy techniques. A series of four QDs with emission in the range 540–630 nm was employed to investigate the dependence of the electron transfer rate on the QD size. Emission of the QDs is quenched upon hybrid formation, and the quenching mechanism is identified as photoinduced electron transfer from the QD to the fullerene moiety due to the fullerene anion signature observed in transient absorption. In order to obtain quantitative information on the ET reaction, several kinetic data analysis techniques were used, including a conventional multiexponential fitting and a maximum entropy method for emission decay analysis, as well as a distributed decay model based on the Poisson distribution of fullerenes in the hybrids. The latter gradually simplifies the interpretation of the transient absorption spectra and indicates that the spectra of QD cations are essentially similar to those of neutral QDs, differing only by a minor decrease in the intensity and broadening. Furthermore, only a minor decrease in the ET rate with the increasing QD size was observed, the time constants being in the range 100–200 ps for all studied QDs. The charge recombination is extended to 10 ns or longer for all hybrids

Effect of Hole Transporting Material on Charge Transfer Processes in Zinc Phthalocyanine Sensitized ZnO Nanorods

The photoinduced electron transfer processes were studied for hybrid systems consisting of self-assembled monolayer of zinc phthalocyanine (ZnPc) assembled on ZnO nanorods and a film of organic hole transporting material (HTM) atop. Polythiophene (P3HT) or Spiro-OMeTAD were used as HTM. The study was carried out by ultrafast transient absorption spectroscopy technique with selective excitation of ZnPc at 680 nm or P3HT at 500 nm. Data analysis revealed that photoexcitation of ZnPc in the structure ZnO|ZnPc|P3HT results in a fast (1.8 ps) electron transfer from ZnPc to ZnO, which is followed by a hole transfer from the ZnPc cation to P3HT roughly in 30 ps. However, in the case of ZnO|ZnPc|Spiro-OMeTAD structure, the primary reaction upon excitation of ZnPc is a fast (0.5 ps) hole transfer from ZnPc to Spiro-OMeTAD, and the second step is electron injection from the ZnPc anion to ZnO in roughly 120 ps. Thus, we demonstrate two structurally very similar hybrid architectures that implement two different mechanisms for photoinduced charge separation found in dye-sensitized or in organic solar cells

Long-Range Observation of Exciplex Formation and Decay Mediated by One-Dimensional Bridges

We report herein unprecedented long-range observation of both formation and decay of the exciplex state in donor (D)–bridge (B)–acceptor (A) linked systems. Zinc porphyrins (ZnP) as a donor were tethered to single-walled carbon nanotube (SWNT) as an acceptor through oligo­(<i>p</i>-phenylene)­s (ZnP–ph_<i>n</i>–SWNT) or oligo­(<i>p</i>-xylene)­s (ZnP–xy_<i>n</i>–1–ph₁–SWNT) with systematically varied lengths (<i>n</i> = 1–5) to address the issue. Exponential dependencies of rate constants for the exciplex formation (<i>k</i>_FEX) and decay (<i>k</i>_DEX) on the edge-to-edge separation distance between ZnP and SWNT through the bridges were unambiguously derived from time-resolved spectroscopies. Distance dependencies (i.e., attenuation factor, β) of <i>k</i>_FEX and <i>k</i>_DEX in ZnP–ph_<i>n</i>–SWNT were found to be considerably small (β = 0.10 for <i>k</i>_FEX and 0.12 Å^–1 for <i>k</i>_DEX) compared to those for charge separation and recombination (0.2–0.8 Å^–1) in D–B–A systems with the same oligo­(<i>p</i>-phenylene) bridges. The small β values may be associated with the exciplex state with mixed characters of charge-transfer and excited states. In parallel, the substantially nonconjugated bridge of oligo­(<i>p</i>-xylene)­s exhibited larger attenuation values (β = 0.12 for <i>k</i>_FEX and 0.14 Å^–1 for <i>k</i>_DEX). These results provide deep insight into the unique photodynamics of electronically strongly coupled D–B–A systems involving exciplex

Photophysical Study of a Self-Assembled Donor–Acceptor Two-Layer Film on TiO₂

The self-assembled monolayer (SAM) technique was employed to fabricate a two-layer donor–acceptor film on the surface of TiO₂. The approach is based on using donor and acceptor compounds with anchoring groups of different lengths. The acceptor, a fullerene derivative, has a carboxyl anchor attached to the fullerene moiety via a short linker that places the fullerene close to the surface. The donor, a porphyrin derivative, is equipped with a long linker that can penetrate between the fullerenes and keep porphyrin on top of the fullerene layer. The two-layer fullerene–porphyrin structures were deposited on a mesoporous film of TiO₂ nanoparticles by immersing the TiO₂ film sequentially into fullerene and porphyrin solutions. Transient absorption spectroscopy studies of the samples revealed that after the selective photoexcitation of porphyrin a fast (<5 ps) intermolecular electron transfer (ET) takes place from porphyrin to the fullerene layer, which confirms the formation of the interlayer donor–acceptor interface. Furthermore, in the second step of ET the fullerene anions donate electrons to the TiO₂ nanoparticles. The latter reaction is relatively slow with an average time constant of 230 ps. It involves roughly half of the primary generated charges, and the second half relaxes by the interlayer charge recombination. The resulting state with a porphyrin cation and electron in TiO₂ has an extremely long lifetime and recombines with an average time constant of 23 ms

Excited State Intramolecular Proton Transfer in Electron-Rich and Electron-Poor Derivatives of 10-Hydroxybenzo[<i>h</i>]quinoline

Eight previously inaccessible derivatives of 10-hydroxybenzo­[<i>h</i>]­quinoline were prepared via a straightforward strategy comprising formation of the benzo­[<i>h</i>]­quinoline skeleton followed by C–H acetoxylation at position 10. The occurrence of excited state intramolecular proton transfer (ESIPT) was detected in all cases since emission was observed only from the excited keto-tautomer. Studies on derivatives bearing both electron-donating and electron-withdrawing groups adjacent to the pyridine ring allowed us to identify some design patterns giving rise to NIR emission and large Stokes shifts. For a derivative of 10-hydroxybenzo­[<i>c</i>]­acridine, emission at 745 nm was observed, one of the lowest energy fluorescence ever reported for ESIPT system. On the basis of time-resolved measurements, proton transfer was found to be extremely fast with time constants in the range (0.08–0.45 ps)

Effect of Single-Crystal TiO₂/Perovskite Band Alignment on the Kinetics of Electron Extraction

The kinetics of electron extraction at the electron transfer layer/perovskite interface strongly affects the efficiency of a perovskite solar cell. By combining transient absorption and time-resolved photoluminescence spectroscopy, the electron extraction process between FA0.83Cs0.17Pb(I0.83Br0.17)3 and TiO2 single crystals with different orientations of (100), (110), and (111) were probed from subpicosecond to several hundred nanoseconds. It was revealed that the band alignment between the constituents influenced the relative electron extraction process. TiO2(100) showed the fastest overall and hot electron transfer, owing to the largest conduction band and Fermi level offset compared to FA0.83Cs0.17Pb(I0.83Br0.17)3. It was found that an early electron accumulation in these systems can have an influence on the following electron extraction on the several nanosecond time scale. Furthermore, the existence of a potential barrier at the TiO2/perovskite interface was also revealed by performing excitation fluence-dependent measurements

Fluorescent Protein Based FRET Pairs with Improved Dynamic Range for Fluorescence Lifetime Measurements

<div><p>Fluorescence Resonance Energy Transfer (FRET) using fluorescent protein variants is widely used to study biochemical processes in living cells. FRET detection by fluorescence lifetime measurements is the most direct and robust method to measure FRET. The traditional cyan-yellow fluorescent protein based FRET pairs are getting replaced by green-red fluorescent protein variants. The green-red pair enables excitation at a longer wavelength which reduces cellular autofluorescence and phototoxicity while monitoring FRET. Despite the advances in FRET based sensors, the low FRET efficiency and dynamic range still complicates their use in cell biology and high throughput screening. In this paper, we utilized the higher lifetime of NowGFP and screened red fluorescent protein variants to develop FRET pairs with high dynamic range and FRET efficiency. The FRET variations were analyzed by proteolytic activity and detected by steady-state and time-resolved measurements. Based on the results, NowGFP-tdTomato and NowGFP-mRuby2 have shown high potentials as FRET pairs with large fluorescence lifetime dynamic range. The <i>in vitro</i> measurements revealed that the NowGFP-tdTomato has the highest Förster radius for any fluorescent protein based FRET pairs yet used in biological studies. The developed FRET pairs will be useful for designing FRET based sensors and studies employing Fluorescence Lifetime Imaging Microscopy (FLIM).</p></div

Variations in the FRET response of FRET pairs on proteolytic cleavage over time.

<p>The control is NowGFP-mRuby2 FRET pair without thrombin cleavage site (LVPS instead of LVPR). ΔR/R was computed as (R₀-R_F)/R₀ where R is donor:acceptor ratio and R₀ is the donor:acceptor ratio when there is no FRET and R_F is the FRET ratio</p

Emission spectra of the FRET constructs.

<p>Fluorescence emission spectrum (at 480 nm excitation) of the FRET constructs treated with thrombin. The schematics of the FRET constructs are displayed above the spectrum. "LVPR" represents the sequence GGGSLVPRGS. The decrease in the FRET in time as a result of proteolytic cleavage can be observed from the spectrum. </p

Intracellular FLIM of <i>E</i>. <i>coli</i> cells.

<p><i>(A)</i> Fluorescence lifetime image of the cells displaying FRET. The cells are excited at 483 nm and the selective emission from the donor was monitored through band pass filter (510/20 nm). NowGFP is the cells expressing donor alone and the variation in lifetime as a result of FRET can be observed from the cells expressing the FRET pairs. The average lifetime is calculated from approximately 30 cells. Image size is 10 μm × 10 μm. <i>(B)</i> Fluorescence decay curve from the cells showing FRET. The decrease in the fluorescence lifetime due to FRET can be observed from the decay curve.</p