Ultrafast Photoinduced Electron Transfer between an Incarcerated Donor and a Free Acceptor in Aqueous Solution

Supramolecular photoinduced electron transfer dynamics between coumarin 153 (C153) and 4,4′-dimethyl viologen dichloride (MV²⁺) across the molecular barrier of a host molecule, octa acid (OA), has been investigated with femtosecond time resolution. The ultrafast electron transfer from C153 to MV²⁺ followed excitation with 150 fs laser pulses at a wavelength of 390 nm despite the fact that C153 was incarcerated within an OA₂ capsule. As a result, the photoexcited coumarin did not show any of the typical relaxation dynamics that is usually observed in free solution. Instead, the excited electron was transferred across the molecular wall of the capsuleplex within 20 ps. Likewise, the lifetime of the charge transfer state was short (724 ps), and electron back-transfer reestablished the ground state of the system within 1 ns, showing strong electronic coupling among the excited electron donor, host, and acceptor. When the donor was encapsulated into the host molecule, the electron transfer process showed significantly accelerated dynamics and essentially no solvent relaxation compared with that in free solution. The study was also extended to <i>N</i>-methylpyridinium iodide as the acceptor with similar results

Exploring Ultrafast Electronic Processes of Quasi-Type II Nanocrystals by Two-Dimensional Electronic Spectroscopy

Colloidal CdTe/CdSe heteronanostructures are model systems for quasi-type II nanocrystals (NCs) and have been examined extensively. However, the complex spectra in these heteronanostructures often make it difficult to reveal details of their optical properties by conventional techniques such as transient absorption spectroscopy. In the present study, two-dimensional electronic spectroscopy (2DES) is used to study colloidal CdTe, CdTe/CdSe, and CdTe/ZnS NCs revealing the nature of absorption bands and ultrafast dynamics in a quasi-type II system. We observe the electronic coupling between the lowest two transitions, oscillations in the population time due to the longitudinal optical (LO) phonon mode, and the high-frequency impulsive Raman modes of the solvent. We observed an excited state absorption near at the band edge only in CdTe/CdSe NCs and established that it is related to the quasi-type II features: the redistribution of excitons among the fine-structured states or the biexciton level shift at the ultrafast time scale

Charge Separation and Recombination in CdTe/CdSe Core/Shell Nanocrystals as a Function of Shell Coverage: Probing the Onset of the Quasi Type-II Regime

Femtosecond transient absorption (TA) is used to investigate population dynamics of electronic states in CdTe/CdSe core/shell nanocrystals (NCs) with varying CdSe shell coverage. Upon CdSe shell growth, the CdTe/CdSe NCs show a gradual evolution of surface passivation toward a quasi-type-II charge separation regime. Our results indicate that ultrafast electron transfer (ET) through the interface of CdTe/CdSe NCs becomes measurable as the CdSe shell approaches the quasi-type-II regime which we experimentally determined to be at 0.4 nm (shell) for a 3.4 nm CdTe core. This work reports the first measurement using femtosecond TA to study the electronic states in CdTe/CdSe NCs and spectroscopically observe the evolution of an ultrafast charge transfer (CT) in such a core/shell system. Time-resolved photoluminescence (PL) spectroscopy is used to study the radiative lifetimes of the CdTe/CdSe NCs. One monolayer CdSe coverage leads to longer-lived PL, which is red-shifted by 140 nm compared to the CdTe core PL. It is concluded that this is due to greater electron−hole separation providing a longer-lived CT state

Ultrafast Electron Transfer across a Nanocapsular Wall: Coumarins as Donors, Viologen as Acceptor, and Octa Acid Capsule as the Mediator

Results of our study on ultrafast electron transfer (eT) dynamics from coumarins (coumarin-1, coumarin-480, and coumarin-153) incarcerated within octa acid (OA) capsules as electron donors to methyl viologen dissolved in water as acceptor are presented. Upon photoexcitation, coumarin inside the OA capsule transfers an electron to the acceptor electrostatically attached to the capsule leading to a long-lived radical–ion pair separated by the OA capsular wall. This charge-separated state returns to the neutral ground state via back electron transfer on the nanosecond time scale. This system allows for ultrafast electron transfer processes through a molecular wall from the apolar capsular interior to the highly polar (aqueous) environment on the femtosecond time scale. Employing femtosecond transient absorption spectroscopy, distinct rates of both forward (1–25 ps) and backward eT (700–1200 ps) processes were measured. Further understanding of the energetics is provided using Rehm–Weller analysis for the investigated photoinduced eT reactions. The results provide the rates of the eT across a molecular wall, akin to an isotropic solution, depending on the standard free energy of the reaction. The insights from this work could be utilized in the future design of efficient electron transfer processes across interfaces separating apolar and polar environments

Near-Infrared Emitting AgInS₂/ZnS Nanocrystals

Near-infrared emitting AgInS₂/ZnS nanocrystals were synthesized by carefully controlling the growth conditions in a Ag/In/Zn/S solution with less zinc relative to the other precursors. The role of having a smaller amount of zinc (8 atom %) was systematically investigated in an effort to understand the mechanism of the largely red-shifted emission. The AgInS₂/ZnS nanocrystals can be transferred to aqueous solutions while retaining the emission intensity. The near-infrared emission and solubility in aqueous solutions make AgInS₂/ZnS nanocrystals excellent candidates for bioimaging and photocatalytic applications

Strong Visible Absorption and Broad Time Scale Excited-State Relaxation in (Ga_1–<i>x</i>Zn_<i>x</i>)(N_1–<i>x</i>O_<i>x</i>) Nanocrystals

(Ga_1–<i>x</i>Zn_<i>x</i>)­(N_1–<i>x</i>O_<i>x</i>) is a visible absorber of interest for solar fuel generation. We present a first report of soluble (Ga_1–<i>x</i>Zn_<i>x</i>)­(N_1–<i>x</i>O_<i>x</i>) nanocrystals (NCs) and their excited-state dynamics over the time window of 10^–13–10^–4 s. Using transient absorption spectroscopy, we find that excited-state decay in (Ga_0.27Zn_0.73)­(N_0.27O_0.73) NCs has both a short (<100 ps) and a long-lived component, with a long overall average lifetime of ∼30 μs. We also find that the strength of the visible absorption is comparable to that of direct band gap semiconductors such as GaAs. We discuss how these results may relate to the origin of visible absorption in (Ga_1–<i>x</i>Zn_<i>x</i>)­(N_1–<i>x</i>O_<i>x</i>) and its use in solar fuel generation

Study of the Partial Ag-to-Zn Cation Exchange in AgInS₂/ZnS Nanocrystals

AgInS₂–ZnS (AIZS) nanocrystals (NCs) were synthesized using a simple one-step approach by heating a Ag/In/Zn/S solution to 210 °C providing highly tunable photoluminescence (PL). The incorporation of Zn even at low temperatures (∼150 °C) and the increased cation exchange of silver by zinc at higher temperatures strongly influence the optical properties of the resulting NCs. The correlation between synthesis parameters and resulting optical properties provided insights on the growth and stability of ternary and quaternary semiconductors. Systematic investigation with time-resolved spectroscopy showed distinguishable PL behaviors between developing and fully grown AIZS NCs. Attempts to coat as-prepared AgInS₂ NCs resulted in the same PL behavior as the one-step reaction product indicating that Zn readily exchanges with Ag ions even when not directly incorporated in the initial reaction mixture. Even with a low amount of zinc, the fully grown AIZS NCs showed improved PL QYs and single exponential decay behavior with long PL lifetimes. Control of the optical properties of these NCs makes them potentially useful for applications in photovoltaics and bioimaging particularly in light of their nontoxicity

Synthesis and Photoelectrochemical Properties of (Cu₂Sn)_<i>x</i>Zn_{3(1–<i>x</i>)}S₃ Nanocrystal Films

This work provides new routes for developing efficient photoelectrodes for photoelectrochemical (PEC) water splitting using a low-cost electrophoretic film preparation method. A series of (Cu₂Sn)_<i>x</i>Zn_{3(1–<i>x</i>)}S₃ (0 ≤ <i>x</i> ≤ 0.75) quaternary nanocrystals (NCs) with tunable optical band gaps are synthesized. Morphologies including particles, rods, and wires are obtained by tuning the composition of the NCs. (Cu₂Sn)_0.75Zn_0.75S₃ (Cu₂ZnSnS₄) has a pure kesterite structure, but an increase in the Zn content results in a kesterite–wurtzite polytypism. (Cu₂Sn)_<i>x</i>Zn_{3(1–<i>x</i>)}S₃ films are fabricated from their colloidal solutions via electrophoretic deposition, and the PEC properties of these films with p-type character have been examined under water-splitting conditions. It is shown that the photocurrent varies as a function of film thickness as well as chemical composition. The produced (Cu₂Sn)_0.45Zn_1.65S₃ (<i>x</i> = 0.45) film has the highest photocurrent, and the incident photon to current conversion efficiency is improved compared with previously reported results of Cu₂ZnSnS₄ photocathodes

Synthesis and Optical Properties of Linker-Free TiO₂/CdSe Nanorods

Linker-free TiO₂/CdSe hybrid nanorods (NRs) were prepared by growing CdSe QDs in the presence of TiO₂ NR seeds using seeded–growth type colloidal injection approach. TEM studies revealed the abundant formation of anatase TiO₂ NRs with different lengths. We found that the as-prepared TiO₂ seeds determined the morphology of TiO₂/CdSe NRs. The photophysics of these materials were studied by photoluminescence (PL) and femtosecond transient absorption spectroscopy. While we observed the efficient PL quenching in TiO₂/CdSe NRs, the bleach dynamics of TiO₂/CdSe NRs is similar to that of CdSe NRs. It suggests that while surface traps that arise from the lattice mismatch between CdSe and TiO₂ are mainly observed in transient absorption measurements the ultrafast exciton dissociation over the experimental time resolution occurs in TiO₂/CdSe NRs

What Is the Optoelectronic Effect of the Capsule on the Guest Molecule in Aqueous Host/Guest Complexes? A Combined Computational and Spectroscopic Perspective

Encapsulation of dye molecules is used as a means to achieve charge separation across different dielectric environments. We analyze the absorption and emission spectra of several coumarin molecules that are encapsulated within an octa-acid dimer forming a molecular capsule. The water-solvated capsule effect on the coumarin’s electronic structure and absorption spectra can be understood as due to an effective dielectric constant where the capsule partially shields electrostatically the dielectric solvent environment. Blue-shifted emission spectra are explained as resulting from a partial intermolecular charge transfer where the capsule is the acceptor, and which reduces the coumarin relaxation in the excited state