Charge Trapping in Semiconductor Photocatalysts: A Time- and Space-Domain Perspective.

Harnessing solar energy to produce value-added fuels and chemicals through photocatalysis techniques holds promise for establishing a sustainable and environmentally friendly energy economy. The intricate dynamics of photogenerated charge carriers lies at the core of the photocatalysis. The balance between charge trapping and band-edge recombination has a crucial influence on the activity of semiconductor photocatalysts. Consequently, the regulation of traps in photocatalysts becomes the key to optimizing their activities. Nevertheless, our comprehension of charge trapping, compared to that of well-studied charge recombination, remains somewhat limited. This limitation stems from the inherently heterogeneous nature of traps at both temporal and spatial scales, which renders the characterization of charge trapping a formidable challenge. Fortunately, recent advancements in both time-resolved spectroscopy and space-resolved microscopy have paved the way for considerable progress in the investigation and manipulation of charge trapping. In this Perspective, we focus on charge trapping in photocatalysts with the aim of establishing a direct link to their photocatalytic activities. To achieve this, we begin by elucidating the principles of advanced time-resolved spectroscopic techniques such as femtosecond time-resolved transient absorption spectroscopy and space-resolved microscopic methods, such as single-molecule fluorescence microscopy and surface photovoltage microscopy. Additionally, we provide an overview of noteworthy research endeavors dedicated to probing charge trapping using time- and space-resolved techniques. Our attention is then directed toward recent achievements in the manipulation of charge trapping in photocatalysts through defect engineering. Finally, we summarize this Perspective and discuss the future challenges and opportunities that lie ahead in the field.

Electron beam-induced white emission from iridium complexes-doped polymer dots.

Radiation detection plays an important role in diverse applications, including medical imaging, security, and display technologies. Scintillators, materials that emit light upon exposure to radiation, have garnered significant attention due to their exceptional sensitivity. Previous research explored polymer dots (P-dots) doped with iridium complexes as nano-sized scintillators for radiation detection, but these were constrained to emitting specific colors like red, green, and blue, limiting their utility. Recently, there has been a breakthrough in the development of white light emitters stimulated by UV-visible light. These emitters exhibit a broad spectral range in the visible wavelength, enhancing contrast and simplifying detection by visible-light sensors. Consequently, the quest for white color scintillators in radiation detection has emerged as a promising avenue for enhancing scintillation efficiency. In this study, we present a novel approach by applying P-dots doped with two iridium complexes to create white light-emitting nano-sized scintillators. These scintillators offer a wider spectral coverage within the visible-light wavelength range. Under UV light (365 nm) excitation, our synthesized P-dots exhibited remarkable white light emission. Moreover, when excited by electron beam irradiation, we observed the clear emission close to white emission which is valuable for improving the detection of radiation.

Sequential C-F Bond Transformation of the Difluoromethylene Unit in Perfluoroalkyl Groups: A Combination of Fine-Tuned Phenothiazine Photoredox Catalyst and Lewis Acid.

A sequential process via photoredox catalysis and Lewis acid mediation for C-F bond transformation of the CF2 unit in perfluoroalkyl groups has been achieved to transform perfluoroalkylarenes into complex fluoroalkylated compounds. A phenothiazine-based photocatalyst promotes the defluoroaminoxylation of perfluoroalkylarenes with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) under visible light irradiation, affording the corresponding aminoxylated products. These products undergo a further defluorinative transformation with various organosilicon reagents mediated by AlCl3 to provide highly functionalized perfluoroalkyl alcohols. Our novel phenothiazine catalyst works efficiently in the defluoroaminoxylation. Transient absorption spectroscopy revealed that the catalyst regeneration step is crucial for the photocatalytic aminoxylation.

Programmed Control of Fluorescence Blinking Patterns based on Electron Transfer in DNA.

Fluorescence imaging uses changes in the fluorescence intensity and emission wavelength to analyze multiple targets simultaneously. To increase the number of targets that can be identified simultaneously, fluorescence blinking can be used as an additional parameter. To understand and eventually control blinking, we used DNA as a platform to elucidate the processes of electron transfer (ET) leading to blinking, down to the rate constants. With a fixed ET distance, various blinking patterns were observed depending on the DNA sequence between the donor and acceptor units of the DNA platform. The blinking pattern was successfully described with a combination of ET rate constants. Therefore, molecules with various blinking patterns can be developed by tuning ET. It is expected that the number of targets that can be analyzed simultaneously will increase by the power of the number of blinking patterns.

Effect of Macrocycles on the Photochemical and Electrochemical Properties of Cobalt-Dehydrocorrin Complex: Formation and Investigation of Co(I) Species.

Co(II)-pyrocobester (P-Co(II)), a dehydrocorrin complex, was semisynthesized from vitamin B12 (cyanocobalamin), and its photochemical and electrochemical properties were investigated and compared to those of the cobester (C-Co(II)), the cobalt-corrin complex. The UV-vis absorptions of P-Co(II) in CH2Cl2, ascribed to the π-π* transition, were red-shifted compared to those of C-Co(II) due to the π-expansion of the macrocycle in the pyrocobester. The reversible redox couple of P-Co(II) was observed at E1/2 = -0.30 V vs Ag/AgCl in CH3CN, which was assigned to the Co(II)/Co(I) redox couple by UV-vis, ESR, and molecular orbital analysis. This redox couple was positively shifted by 0.28 V compared to that of C-Co(II). This is caused by the high electronegativity of the dehydrocorrin macrocycle, which was estimated by DFT calculations for the free-base ligands. The reactivity of the Co(I)-pyrocobester (P-Co(I)) was evaluated by the reaction with methyl iodide in CV and UV-vis to form a photosensitive Co(III)-CH3 complex (P-Co(III)-CH3). The properties of the excited state of P-Co(I), *Co(I), were also investigated by femtosecond transient absorption (TA) spectroscopy. The lifetime of *Co(I) was estimated to be 29 ps from the kinetic trace at 587 nm. The lifetime of *Co(I) became shorter in the presence of Ar-X, such as iodobenzonitrile (1a), bromobenzonitrile (1b), and chlorobenzonitrile (1c), and the rate constants of electron transfer (ET) between the *Co(I) and Ar-X were determined to be 2.9 × 1011 M-1 s-1, 4.9 × 1010 M-1 s-1, and 1.0 × 1010 M-1 s-1 for 1a, 1b, and 1c, respectively.

Photoinduced NO-release from polymer dots doped with an Ir(III) complex and N-methyl-N-nitroso-4-aminophenol.

Nitric oxide (NO) is a signaling molecule that plays a variety of functions in the human body, but it is difficult to use it in biological experiments or for therapeutic purposes because of its high reactivity and instability in the biological milieu. Consequently, photocontrollable NO releasers, which enable spatiotemporal control of NO release, have an important role in elucidating the functions of NO. Our group has developed visible-light-controllable NO-releasing molecules that contain a fluorescent dye structure as a light-harvesting antenna moiety and an N-nitrosoaminophenol structure as an NO-releasing moiety. Here, we aimed to construct an NO-generating system employing an intermolecular photoredox reaction between the two separate components, since this would simplify chemical synthesis and make it easier to examine various dyes as antennae. For this purpose, we constructed polymer nanoparticles doped with both N-methyl-N-nitroso-4-aminophenol (NAP, 1) and an Ir(III) antenna complex (2, 3 or 4) in order to dissolve in aqueous solution without a co-solvent. These polymer nanoparticles released NO upon photoirradiation in vitro in the purple (400-430 nm) or blue (400-460 nm) wavelength region to activate the doped Ir(III) complex.

Mesolysis of an asymmetric diphenyldisulfide radical anion studied by γ-ray and pulsed-electron radiolyses.

Diaryldisulfides are known to undergo S-S bond cleavage upon one-electron reduction, which is called mesolysis of radical anions, to form the corresponding arylthiyl radical and anion. In this study, we prepared (4-cyanophenyl)(4'-methoxyphenyl)disulfide (MeOSSCN), and the mesolytic profiles were investigated by γ-ray and pulsed-electron radiolyses in 2-methyltetrahydrofuran. As a result of radiolysis of MeOSSCN at room and lower temperatures, the formation of the methoxythiyl radical was recognized upon mesolysis of the radical anion. This observation indicated that intramolecular electron transfer in the radical anion occurred, and the stepwise mechanism was operative after the attached electron occupied the antibonding σ*-orbital for promoting the S-S bond cleavage. According to the Arrhenius expression for the decay rates of the radical anion, the activation energy and frequency factor were determined. DFT calculations provided the bond dissociation energy and bond length for the S-S bond and charge distribution on the S atoms in the radical anion. The substituent effects on the mesolysis process are discussed.

Direct Investigation of Excited C60 Dianion and Its Intramolecular Electron Transfer Behaviors.

For the first time, the dynamics of excited fullerene dianions and associated intramolecular electron transfer (ET) were directly investigated by using femtosecond pump-probe laser flash photolysis on selectively reduced C60, pyrrolidino[60]fullerene (C60H), and dyads including C60-naphthalenediimide (NDI) and C60-pyromellitimide (PI). Upon near-infrared laser excitation, the excited dianion of C60 or C60H displayed two states with lifetimes of less than one and several tens of ps, attributed to prompt internal conversion from the theoretically predicted Sn state. Furthermore, the ET processes from the excited C602- in dyad molecules, including C602--NDI•- and C602--PI•-, were confirmed with varied ET rate constants due to the difference in the driving force for ET. The current findings provide a clear description of the hitherto uncharted excited-state and photoinduced ET characteristics of fullerene dianions, paving the way for photochemical studies of excited multi-ions (excited multi-polarons) and their application in organic semiconducting materials.

Single-Molecule Study of Redox Reaction Kinetics by Observing Fluorescence Blinking.

Recent advances in fluorescence microscopy allow us to track chemical reactions at the single-molecule level. Single-molecule measurements make it possible to minimize the amount of sample needed for analysis and diagnosis. Signal amplification is often applied to ultralow-level biomarker detection. Polymerase chain reaction (PCR) is used to detect DNA/RNA, and enzyme-linked immunosorbent assay (ELISA) can sensitively probe antigen-antibody interactions. While these techniques are brilliant and will continue to be used in the future, single-molecule-level measurements would allow us to reduce the time and cost needed to amplify signals.The kinetics of chemical reactions have been studied mainly using ensemble-averaged methods. However, they can hardly distinguish time-dependent fluctuations and static heterogeneity of the kinetics. The information hidden in ensemble-averaged measurements would be extractable from a single-molecule experiment. Thus, single-molecule measurement would provide unique opportunities to investigate unrevealed phenomena and to elucidate the questions in chemistry, physics, and life sciences. Redox reaction, which is triggered by electron transfer, is among the most fundamental and ubiquitous chemical reactions. The redox reaction of a fluorescent molecule results in the formation of radical ions, which are normally nonemissive. In single-molecule-level measurements, the redox reaction causes the fluctuation of fluorescence signals between the bright ON-state and the dark OFF-state, in a phenomenon called blinking. The duration of the OFF-state (τOFF) corresponds to the lifetime of the radical ion state, and its reaction kinetics can be measured as 1/τOFF. Thus, the kinetics of redox reactions of fluorescent molecules can be accessed at the single-molecule level by monitoring fluorescence blinking. One of the key aspects of single-molecule analysis based on blinking is its robustness. A blinking signal with a certain regular pattern enables single fluorescent molecules to be distinguished and resolved from the random background signal.In this Account, we summarize the recent studies on the single-molecule measurement of redox reaction kinetics, with a focus on our group's recent progress. We first introduce the control of redox blinking to increase the photostability of fluorescent molecules. We then demonstrate the control of redox blinking, which allows us to detect target DNA by monitoring the function of a molecular beacon-type probe, and we investigate antigen-antibody interactions at the single-molecule level. By tracing the time-dependent changes in blinking patterns, redox blinking is shown to be adaptable to tracking the structural switching dynamics of RNA, the preQ1 riboswitch. This Account ends with a discussion of our ongoing work on the control of fluorescent blinking. We also discuss the development of devices that allow single-molecule-level analysis in a high-throughput fashion.

Single-molecule and -particle probing crystal edge/corner as highly efficient photocatalytic sites on a single TiO2 particle.

The exposed active sites of semiconductor catalysts are essential to the photocatalytic energy conversion efficiency. However, it is difficult to directly observe such active sites and understand the photogenerated electron/hole pairs' dynamics on a single catalyst particle. Here, we applied a quasi-total internal reflection fluorescence microscopy and laser-scanning confocal microscopy to identify the photocatalytic active sites at a single-molecule level and visualized the photogenerated hole-electron pair dynamics on a single TiO2 particle, the most widely used photocatalyst. The experimental results and density functional theory calculations reveal that holes and electrons tend to reach and react at the same surface sites, i.e., crystal edge/corner, within a single anatase TiO2 particle owing to the highly exposed (001) and (101) facets. The observation provides solid proof for the existence of the surface junction "edge or corner" on single TiO2 particles. These findings also offer insights into the nature of the photocatalytic active sites and imply an activity-based strategy for rationally engineering catalysts for improved photocatalysis, which can be also applied for other catalytic materials.

Stacked Thiazole Orange Dyes in DNA Capable of Switching Emissive Behavior in Response to Structural Transitions.

Functional nucleic acids with the capability of generating fluorescence in response to hybridization events, microenvironment or structural changes are valuable as structural probes and chemical sensors. We now demonstrate the enzyme-assisted preparation of nucleic acids possessing multiple thiazole orange (TO) dyes and their fluorescent behavior, that show a spectral change from the typical monomer emission to the excimer-type red-shifted emission. We found that the fluorescent response and emission wavelength of the TO dyes were dependent on both the state of the DNA structure (single- or double-stranded DNA) and the arrangement of the TO dyes. We showed that the fluorescent behavior of the TO dyes can be applied for the detection of RNA molecules, suggesting that our approach for preparing the fluorescent nucleic acids functionalized with multiple TO dyes could be useful to design a fluorescence bioimaging and detection technique of biomolecules.

Control of Triplet Blinking Using Cyclooctatetraene to Access the Dynamics of Biomolecules at the Single-Molecule Level.

To explore the dynamics of biomolecules, tracing the kinetics of photo-induced chemical reactions via the triplet excited state (T1 ) of probe molecules offers a timescale that is about 106 times wider than via the singlet excited state (S1 ). Using cyclooctatetraene (COT) as a triplet energy acceptor and at the same time as a photostabilizer, the triplet-triplet energy transfer (TTET) kinetics governed by oligonucleotide (oligo) dynamics were studied at the single-molecule level by measuring fluorescence blinking. TTET kinetics measurement allowed us to access the length- and sequence-dependent dynamics of oligos and realize the single-molecule detection of a model microRNA biomarker. In sharp contrast to the singlet-singlet Förster resonance energy transfer (FRET) that occurs in the 1-10 nm range, TTET requires a Van der Waals contact. The present method is thus a complementary method to FRET and provides direct information on biomolecular dynamics on the µs to ms timescale.

Formation of the Charge-Localized Dimer Radical Cation of 2-Ethyl-9,10-dimethoxyanthracene in Solution Phase.

Although dimer radical ions of aromatic molecules in the liquid-solution phase have been intensely studied, the understanding of charge-localized dimers, in which the extra charge is localized in a single monomer unit instead of being shared between two monomer units, is still elusive. In this study, the formation of a charge-localized dimer radical cation of 2-ethyl-9,10-dimethoxyanthracene (DMA), (DMA)2 .+ is investigated by transient absorption (TA) and time-resolved resonance Raman (TR3 ) spectroscopic methods combined with a pulse radiolysis technique. Visible- and near-IR TA signals in highly concentrated DMA solutions supported the formation of non-covalent (DMA)2 .+ by association of DMA and DMA.+ . TR3 spectra obtained from 30 ns to 300 µs time delays showed that the major bands are quite similar to those of DMA except for small transient bands, even at 30 ns time delay, suggesting that the positive charge of non-covalent (DMA)2 .+ is localized in a single monomer unit. From DFT calculations for (DMA)2 .+ , our TR3 spectra showed the best agreement with the calculated Raman spectrum of charge-localized edge-to-face T-shaped (DMA)2 .+ , termed DT.+ , although the charge-delocalized asymmetric π-stacked face-to-face (DMA)2 .+ , termed DF3.+ , is the most stable structure of (DMA)2 .+ according to the energetics from DFT calculations. The calculated potential energy curves for the association between DMA.+ and DMA showed that DT.+ is likely to be efficiently formed and contribute significantly to the TR3 spectra as a result of the permanent charge-induced Coulombic interactions and a dynamic equilibrium between charge localized and delocalized structures.

Proton Transfer Accompanied by the Oxidation of Adenosine.

Despite numerous experimental and theoretical studies, the proton transfer accompanying the oxidation of 2'-deoxyadenosine 5'-monophosphate 2'-deoxyadenosine 5'-monophosphate (5'-dAMP, A) is still under debate. To address this issue, we have investigated the oxidation of A in acidic and neutral solutions by using transient absorption (TA) and time-resolved resonance Raman (TR3 ) spectroscopic methods in combination with pulse radiolysis. The steady-state Raman signal of A was significantly affected by the solution pH, but not by the concentration of adenosine (2-50 mm). More specifically, the A in acidic and neutral solutions exists in its protonated (AH+ (N1+H+ )) and neutral (A) forms, respectively. On the one hand, the TA spectral changes observed at neutral pH revealed that the radical cation (A.+ ) generated by pulse radiolysis is rapidly converted into A. (N6-H) through the loss of an imino proton from N6. In contrast, at acidic pH (<4), AH.2+ (N1+H+ ) generated by pulse radiolysis of AH+ (N1+H+ ) does not undergo the deprotonation process owing to the pKa value of AH.2+ (N1+H+ ), which is higher than the solution pH. Furthermore, the results presented in this study have demonstrated that A, AH+ (N1+H+ ), and their radical species exist as monomers in the concentration range of 2-50 mm. Compared with the Raman bands of AH+ (N1+H+ ), the TR3 bands of AH.2+ (N1+H+ ) are significantly down-shifted, indicating a decrease in the bond order of the pyrimidine and imidazole rings due to the resonance structure of AH.2+ (N1+H+ ). Meanwhile, A. (N6-H) does not show a Raman band corresponding to the pyrimidine+NH2 scissoring vibration due to diprotonation at the N6 position. These results support the final products generated by the oxidation of adenosine in acidic and neutral solutions being AH.2+ (N1+H+ ) and A. (N6-H), respectively.

Charge-Separated Mixed Valency in an Unsymmetrical Acceptor-Donor-Donor Triad Based on Diarylboryl and Triarylamine Units.

In this study, we report the generation of new mixed-valence (MV) subspecies with charge-separated (CS) characters from an unsymmetrical acceptor-donor-donor (A-D-D) triad. The triad was synthesized by attaching a dimesitylboryl group (A) to a D-D conjugate that consisted of triarylamine (NAr3) units. The MV radical cation, obtained by chemical oxidation of the triad, exhibited a strong intervalence charge transfer (IVCT) absorption derived from the bis(NAr3)•+ moiety in the near-IR region. The charge-separated MV (CSMV) state, obtained by photoexcitation of the triad, caused a blue shift in IVCT energy in the femtosecond transient absorption spectra, reflecting a bias of positive charge distributions to the D end site. This resulted from increased electron density at the A site and restructuring of the central D site from NAr3 to NAr2 sites. Interestingly, any shift in the IVCT energy that was caused by the polarity of the solvent was minimal, reflecting the unique characteristics of the CSMV state. These findings represent the first detailed analysis of the CSMV state, including a comparison with conventional MV states. Therefore, this work provides new insights into counterion-free MV systems and their applications in molecular devices.

The role of nitrogen defects in graphitic carbon nitride for visible-light-driven hydrogen evolution.

The introduction of nitrogen (N) defects (N vacancies labeled as Vns and cyano groups) has been demonstrated as one of the promising strategies to extend the light absorption range of graphitic carbon nitride (CN), thus improving the photocatalytic activity for hydrogen (H2) evolution. However, the photocatalysis mechanism of such N-deficient CN (DCN) has not been fully understood. In this study, N defects are introduced into CN by a KOH-assisted thermal polymerization method. On the basis of experimental investigations and density functional theory (DFT) calculations, it is found that the extension of the absorption range of DCN is attributed to both the valence band (VB) tailing induced by Vns and bandgap narrowing induced by cyano groups. Moreover, the conduction band (CB) is lowered by the N defects, indicating a reduced driving force for H2 evolution. Transient absorption (TA) spectroscopy reveals that when the electrons in the intrinsic VB of DCN are excited to the CB, the separation efficiency of these electrons and as-generated holes is seriously restricted by their low mobility. While when the electrons in VB tail states (Vn states) are excited to the CB, the separation efficiency of these electrons and as-generated holes could be almost maintained thanks to the improved mobility of the holes. As a result, DCN shows a limited enhancement of the H2 evolution rate compared with CN under visible light irradiation. This work points out that extending the light absorption range of a given photocatalyst by doping (or self-doping) may be accompanied by some negative factors, which restrict the overall photocatalytic activity.

Size-Dependent Relaxation Processes of Photoexcited [ n]Cycloparaphenylenes ( n = 5-12): Significant Contribution of Internal Conversion in Smaller Rings.

[ n]Cycloparaphenylenes ([ n]CPPs; n, number of phenyl rings) have gained considerable attention because they exhibit interesting properties owing to their highly strained structure and radially oriented p orbitals. Recently, [ n]CPPs with n ≥ 5 have been synthesized, but the ring-size dependence of the deactivation processes of the excited states has not been explained particularly for smaller [ n]CPPs ( n ≤ 7). In the present study, we characterized the deactivation processes of [ n]CPPs (5 ≤ n ≤ 12) using transient absorption spectroscopy at sub-pico-, sub-nano-, nano-, and microsecond time scales. Although the fluorescence quantum yield increased with the ring size, the longest S1-state lifetime was observed with [8]CPP, and both the decrease and increase of the ring size resulted in the decrease of the lifetime. Characterization of the intersystem crossing and internal conversion processes explained unique ring-size dependence of the deactivation processes of [ n]CPPs, i.e., the enhanced radiation rate of the larger CPP and the fast internal conversion rate of smaller CPP dominate their S1-state lifetimes.

Faster Electron Injection and More Active Sites for Efficient Photocatalytic H2 Evolution in g-C3 N4 /MoS2 Hybrid.

Herein, the structural effect of MoS2 as a cocatalyst of photocatalytic H2 generation activity of g-C3 N4 under visible light irradiation is studied. By using single-particle photoluminescence (PL) and femtosecond time-resolved transient absorption spectroscopies, charge transfer kinetics between g-C3 N4 and two kinds of nanostructured MoS2 (nanodot and monolayer) are systematically investigated. Single-particle PL results show the emission of g-C3 N4 is quenched by MoS2 nanodots more effectively than MoS2 monolayers. Electron injection rate and efficiency of g-C3 N4 /MoS2 -nanodot hybrid are calculated to be 5.96 × 109 s-1 and 73.3%, respectively, from transient absorption spectral measurement, which are 4.8 times faster and 2.0 times higher than those of g-C3 N4 /MoS2 -monolayer hybrid. Stronger intimate junction between MoS2 nanodots and g-C3 N4 is suggested to be responsible for faster and more efficient electron injection. In addition, more unsaturated terminal sulfur atoms can serve as the active site in MoS2 nanodot compared with MoS2 monolayer. Therefore, g-C3 N4 /MoS2 nanodot exhibits a 7.9 times higher photocatalytic activity for H2 evolution (660 µmol g-1 h-1 ) than g-C3 N4 /MoS2 monolayer (83.8 µmol g-1 h-1 ). This work provides deep insight into charge transfer between g-C3 N4 and nanostructured MoS2 cocatalysts, which can open a new avenue for more rationally designing MoS2 -based catalysts for H2 evolution.

Facet Effects of Ag3 PO4 on Charge-Carrier Dynamics: Trade-Off Between Photocatalytic Activity and Charge-Carrier Lifetime.

Silver phosphate (Ag3 PO4 ) is a promising visible-light-driven photocatalyst with a strong oxidation power and exceptionally high apparent quantum yield of O2 evolution. Although engineering Ag3 PO4 facets is widely known to enhance its photocatalytic activity, most studies have explained its facet effect by calculating surface energies. Herein, the charge carrier dynamics in three kinds of Ag3 PO4 crystals (mixed facets, cubic, and tetrahedral structures) were first investigated using single-particle photoluminescence microscopy and femtosecond time-resolved diffuse reflectance spectroscopy. As a result, we clarified that the disagreement between the photocatalytic activities (dye degradation and O2 evolution) of different Ag3 PO4 facets are the consequence of trade-off between catalytic activity and lifetime of photogenerated charge carriers in addition to surface energy.

The Development of Functional Mesocrystals for Energy Harvesting, Storage, and Conversion.

Higher-ordered semiconductors have attracted extensive research interest as an adopted engineering for active solar energy harvesting, storage, and conversion. It is well-known that the effective separation and anisotropic migration of photogenerated charges are the basic driven force required for superior efficiency. However, the morphology and stoichiometric variation of these semiconductors play essential roles in their physicochemical properties of bulk and surface, especially for efficient interparticle or interfacial charge transfer. To this point, the strategy of controlling the topotactic transformation toward superstructures with optimized functionality is preferable for a wide range of optoelectronic and catalytic engineering applications. In this Minireview, we provide an overview of the crystal orientation, synthetic engineering, functional applications, and spatial and temporal charge dynamics in TiO2 mesocrystals and others. The viewpoint of in-depth understanding of the structure-related kinetics would offer an opportunity for design of versatile mesocrystal semiconductors sought-after for potential applications.