Active Role of Proton in Excited State Intramolecular Proton Transfer Reaction

Proton transfer is one of the most important elementary reactions in chemistry and biology. The role of proton in the course of proton transfer, whether it is active or passive, has been the subject of intense investigations. Here we demonstrate the active role of proton in the excited state intramolecular proton transfer (ESIPT) of 10-hydroxybenzo­[<i>h</i>]­quinoline (HBQ). The ESIPT of HBQ proceeds in 12 ± 6 fs, and the rate is slowed down to 25 ± 5 fs for DBQ where the reactive hydrogen is replaced by deuterium. The results are consistent with the ballistic proton wave packet transfer within the experimental uncertainty. This ultrafast proton transfer leads to the coherent excitation of the vibrational modes of the product state. In contrast, ESIPT of 2-(2′-hydroxyphenyl)­benzothiazole (HBT) is much slower at 62 fs and shows no isotope dependence implying complete passive role of the proton

Multifaceted Ultrafast Intramolecular Charge Transfer Dynamics of 4‑(Dimethylamino)benzonitrile (DMABN)

Intramolecular charge transfer (ICT) of DMABN has been the subject of extensive investigations. Through the measurements of highly time-resolved fluorescence spectra (TRFS) over the whole emission region, we have examined the ICT dynamics of DMABN in acetonitrile free from the solvation dynamics and vibronic relaxation. The ICT dynamics was found to be characterized by a broad range of time scales; nearly instantaneous (<30 fs), 160 fs, and 3.3 ps. TRFS revealed that an ICT state with partially twisted geometry, ICT­(P), is formed within a few hundred femtoseconds either directly from the initial photoexcited state or via the locally excited (LE) state. The ICT­(P) state undergoes further relaxation along the intramolecular nuclear coordinate to reach the twisted ICT (TICT) state with the time constant of 4.8 ps. A conformational diversity along the rotation of the dimethylamino group was speculated to account for the observed diffusive dynamics

Excitation Energy Transfer within Covalent Tetrahedral Perylenediimide Tetramers and Their Intermolecular Aggregates

Perylenediimides (PDIs) offer a number of attractive characteristics as alternatives to fullerenes in organic photovoltaics (OPVs), including favorable orbital energetics, high extinction coefficients in the visible spectral region, photostability, and the capacity to self-assemble into ordered nanostructures. However, energy transfer followed by charge separation in PDI assemblies must kinetically out-compete excimer formation that limits OPV performance. We report on the excitation energy transfer (EET) rate in a covalently linked PDI tetramer in which the PDI chromophores are arranged in a tetrahedral geometry about a tetraphenyladamantane core. Transient absorption spectroscopy of the tetramer in CH₂Cl₂ reveals a laser intensity-dependent fast absorption decay component indicative of singlet–singlet annihilation resulting from intramolecular EET. Femtosecond fluorescence anisotropy measurements show that the EET time constant τ = 6 ps, which is similar to that predicted for a through-space Förster EET mechanism. Concentration-dependent steady-state spectroscopic studies reveal the formation of intermolecular aggregates of the tetramers in toluene. The aggregates are formed by cofacial π-stacking interactions between PDIs of neighboring tetramers. Transient absorption spectra of the aggregated tetramers in toluene solution demonstrate long-lived excited-state decay dynamics (τ ∼ 30 ns) in agreement with previous observations of PDI excimers

Coherent Nuclear Wave Packets Generated by Ultrafast Intramolecular Charge-Transfer Reaction

Intramolecular charge-transfer (ICT) dynamics, including reaction coordinates, structural changes, and reaction rate, has been noted experimentally and theoretically. Here we report the ICT dynamics of laurdan investigated by time-resolved fluorescence at extreme time resolution of 30 fs. A single high-frequency coherent nuclear wave-packet motion on the product potential surface is observed through the modulation of the fluorescence intensity in time. Theory and experiment show that this vibrational mode involves large displacement of the carbon atoms in the naphthalene backbone, which indicates that the naphthalene backbone coordinates are strongly coupled to the ICT reaction of laurdan, not the twisting or planarization of the dimethylamino group

Investigation of Interface Characteristics and Physisorption Mechanism in Quantum Dots/TiO₂ Composite for Efficient and Sustainable Photoinduced Interfacial Electron Transfer

Owing to their superior stability compared to those of conventional molecular dyes, as well as their high UV–visible absorption capacity, which can be tuned to cover the majority of the solar spectrum through size adjustment, quantum dot (QD)/TiO2 composites are being actively investigated as photosensitizing components for diverse solar energy conversion systems. However, the conversion efficiencies and durabilities of QD/TiO2-based solar cells and photocatalytic systems are still inferior to those of conventional systems that employ organic/inorganic components as photosensitizers. This is because of the poor adsorption of QDs onto the TiO2 surface, resulting in insufficient interfacial interactions between the two. The mechanism underlying QD adsorption on the TiO2 surface and its relationship to the photosensitization process remain unclear. In this study, we established that the surface characteristics of the TiO2 semiconductor and the QDs (i.e., surface defects of the metal oxide and the surface structure of the QD core) directly affect the QD adsorption capacity by TiO2 and the interfacial interactions between the QDs and TiO2, which relates to the photosensitization process from the photoexcited QDs to TiO2 (QD* → TiO2). The interfacial interaction between the QDs and TiO2 is maximized when the shape/thickness-modulated triangular QDs are composited with defect-rich anatase TiO2. Comprehensive investigations through photodynamic analyses and surface evaluation using X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and photocatalysis experiments collectively validate that tuning the surface properties of QDs and modulating the TiO2 defect concentration can synergistically amplify the interfacial interaction between the QDs and TiO2. This augmentation markedly improved the efficiency of photoinduced electron transfer from the photoexcited QDs to TiO2, resulting in significantly increased photocatalytic activity of the QD/TiO2 composite. This study provides the first in-depth characterization of the physical adhesion of QDs dispersed on a heterogeneous metal-oxide surface. Furthermore, the prepared QD/TiO2 composite exhibits exceptional adsorption stability, resisting QD detachment from the TiO2 surface over a wide pH range (pH = 2–12) in aqueous media as well as in nonaqueous solvents during two months of immersion. These findings can aid the development of practical QD-sensitized solar energy conversion systems that require the long-term stability of the photosensitizing unit

Influence of the π‑Bridge-Fused Ring and Acceptor Unit Extension in D−π–A-Structured Organic Dyes for Highly Efficient Dye-Sensitized Solar Cells

Three new D−π–A-structured organic dyes, coded as SGT-138, SGT-150, and SGT-151, with the expansion of π-conjugation in the π-bridge and acceptor parts have been developed to adjust HOMO/LUMO levels and to expand the light absorption range of organic dyes. Referring to the SGT-137 dye, the π-bridge group was extended from the 4-hexyl-4H-thieno[3,2-b]indole (TI) to the 9-hexyl-9H-thieno[2′,3′:4,5]thieno[3,2-b]indole (TII), and the acceptor group was extended from (E)-3-(4-(benzo[c][1,2,5]thiadiazol-4-yl)phenyl)-2-cyanoacrylic acid (BTCA) to (E)-3-(4-(benzo[c][1,2,5]thiadiazol-4-ylethynyl)phenyl)-2-cyanoacrylic acid (BTECA), where TII was introduced as a π-bridging unit for the first time. It was determined that both extensions are promising strategies to enhance the light-harvesting ability. They present several features, such as (i) efficiently intensifying the extinction coefficient and expanding the absorption bands; (ii) exhibiting enhanced intramolecular charge transfer in comparison with the SGT-137; and (iii) being favorable to photoelectric current generation of dye-sensitized solar cells (DSSCs) with cobalt electrolytes. In particular, the π-spacer extension from TI to TII was useful for modulating the HOMO energy levels, while the acceptor extension from BTCA to BTECA was useful for modulating the LUMO energy levels. These phenomena could be explained with the aid of density functional theory calculations. Finally, the DSSCs based on new SGT-dyes with an HC-A1 co-adsorbent presented good power conversion efficiencies as high as 11.23, 11.30, 11.05, and 10.80% for SGT-137, SGT-138, SGT-150, and SGT-151, respectively. Furthermore, it was determined that the use of the bulky co-adsorbent, HC-A1, can effectively suppress the structural relaxation of dyes in the excited state, thereby enhancing the charge injection rate of SGT-dyes. The observations in time-resolved photoluminescence were indeed consistent with the variation in the PCE, quantitatively

Synergistic Effect of Size-Tailored Structural Engineering and Postinterface Modification for Highly Efficient and Stable Dye-Sensitized Solar Cells

Despite significant progress in device performance, dye-sensitized solar cells (DSSCs) continue to fall short of their theoretical potential. Moreover, research in recent years needs to pay more attention to improving the device fabrication process. To achieve the theoretical efficiency limit, it is crucial to optimize the interface between the dye and TiO2 nanoparticles in the entire device stack. Our study indicates that optimizing the structure or size of the coadsorbents and implementing a monolayer adsorption process can be an effective strategy to reduce charge recombination and enhance light-harvesting properties. Our research aims to develop a surface-coating adsorbent plan that controls the TiO2 nanoparticle interface to achieve the radiative limit of power conversion efficiency (PCE). Specifically, we utilized 2-thiophenecarboxylic acid (THCA) or chenodeoxycholic acid (CDCA) as postinterfacial surface-coating adsorbents. Our results demonstrate that this approach effectively achieves the desired PCE limit. Combined with the coadsorbent structure engineering and interface optimization, the device increased the packing area on the TiO2 nanoparticles’ surface, reaching an improved PCE of over 13.17% under simulated sunlight (1.5G), which is the highest efficiency of a porphyrin single dye-based DSSC. In particular, this practical approach was also applied to a large-area DSSC with an area of 3 cm2, yielding a remarkable PCE of 9.04%. Furthermore, when applied to a polymer gel electrolyte, this novel approach recorded the highest PCE of 11.16% with a long-term operational stability of up to 1000 h for the quasi-solid-state DSSCs. Our research findings provide a promising avenue for achieving high-performance DSSCs with ease of access and demonstrate practical applications as alternatives to conventional power sources

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 <i>a</i> was converted into zinc methyl 3-ethylpyrochlorophyllide <i>a</i> (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–NDI₂. 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. Femtosecond and nanosecond transient absorption spectroscopy of the monomers in toluene–1% pyridine and the cyclic tetramers in toluene shows that the selective photoexcitation of Chl results in intramolecular electron transfer from ^1*Chl to PI to form Chl^+•–PI^–•–NDI and Chl^+•–PI^–•–NDI₂. This initial charge separation is followed by a rapid charge shift from PI^–• to NDI and subsequent charge recombination of Chl^+•–PI–NDI^–• and Chl^+•–PI–(NDI)­NDI^–• on a 5–30 ns time scale. Charge recombination in the Chl–PI–NDI₂ cyclic tetramer (τ_CR = 30 ± 1 ns in toluene) is slower by a factor of 3 relative to the monomeric building blocks (τ_CR = 10 ± 1 ns in toluene–1% pyridine). This indicates that the self-assembly of these building blocks into the cyclic tetramers alters their structures in a way that lengthens their charge separation lifetimes, which is an advantageous strategy for artificial photosynthetic systems

Extraordinary Nonlinear Absorption in 3D Bowtie Nanoantennas

This paper reports that arrays of three-dimensional (3D), bowtie-shaped Au nanoparticle dimers can exhibit extremely high nonlinear absorption. Near-field interactions across the gap of the 3D bowties at the localized surface plasmon resonance wavelengths resulted in an increase of more than 4 orders of magnitude in local field intensity. The imaginary part of the third-order nonlinear susceptibility (Im χ⁽³⁾) for the 3D bowtie arrays embedded in a dielectric material was measured to be 10^–4 esu, more than 2 orders of magnitude higher than reported for other metal nanoparticle-dielectric composites. Moreover, 3D dimers with increased nanoscale structure (such as folding) exhibited increased optical nonlinearity. These 3D nanoantennas can be used as critical elements for nanoscale nonlinear optical devices

Functional domain mapping of dTULP for ciliary localization of Iav and NompC.

<p>(A) Schematic diagram showing different domains of dTULP as well as dTULP mutant forms (mutA, mutB, and mutAB). Location and identity of each mutation are marked. IFT, intraflagellar transport; NLS, nuclear localization signal; PIP, phosphoinositide. (B–D) Confocal imaging of the second antennal segment in the <i>dTulp</i> knockout flies expressing dTULP wild-type (dTULP_wt), dTULP_mutA, dTULP_mutB, and dTULP_mutAB. (B) Confocal imaging of Iav-GFP counterstained with 22C10 which stains neuronal cells except for the cilia located in the outer segment. (C) Immunostaining of NompC counterstained with phalloidin that specifically stains actin-rich scolopales. (D) Immunostaining of dTULP. Arrows indicate the junction between inner and outer segment. (E–F) Quantification of Iav-GFP and dTULP expression levels in the proximal cilia. The number of images analyzed is shown in parentheses. (E) Quantification of Iav-GFP expression level in the proximal cilia. *<i>p</i><0.05 and **<i>p</i><0.01 compared to dTULP_wt-expressing <i>dTulp</i> mutant. (F) Quantification of dTULP expression level in the proximal cilia. **<i>p</i><0.01 compared to dTULP_wt-expressing <i>dTulp</i> mutant. (G) Representative traces of sound-evoked potentials recorded from the antennal nerve of dTULP_wt, dTULP_mutA, dTULP_mutB, and dTULP_mutAB-expressing <i>dTulp¹</i> flies. (H) Quantification of sound-evoked potentials of indicated genotypes. Genotypes of animal are <i>dTulp¹</i>/<i>CyO</i>, <i>dTulp¹</i>,<i>F-GAL4</i>/<i>dTulp¹</i>, <i>dTulp¹</i>,<i>F-GAL4</i>/<i>dTulp¹</i>;<i>UAS-dTulp_wt/</i>+, <i>dTulp¹</i>,<i>F-GAL4</i>/<i>dTulp¹</i>;<i>UAS-dTulp_mutA</i>/+, <i>dTulp¹</i>,<i>F-GAL4</i>/<i>dTulp¹</i>;<i>UAS-dTulp_mutB</i>/+, and <i>dTulp¹</i>,<i>F-GAL4</i>/<i>dTulp¹</i>;<i>UAS-dTulp_mutAB</i>/+. *<i>p</i><0.05 and **<i>p</i><0.01 compared to <i>dTulp¹/CyO</i>. The number of flies used for quantification of each genotype is indicated in parentheses. All <i>p</i> values were calculated using ANOVA with <i>post-hoc</i> Tukey assay. All error bars represent SEM.</p