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.

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.

Anthraquinone-2-Sulfonate as a Microbial Photosensitizer and Capacitor Drives Solar-to-N2O Production with a Quantum Efficiency of Almost Unity.

Semiartificial photosynthesis shows great potential in solar energy conversion and environmental application. However, the rate-limiting step of photoelectron transfer at the biomaterial interface results in an unsatisfactory quantum yield (QY, typically lower than 3%). Here, an anthraquinone molecule, which has dual roles of microbial photosensitizer and capacitor, was demonstrated to negotiate the interface photoelectron transfer via decoupling the photochemical reaction with a microbial dark reaction. In a model system, anthraquinone-2-sulfonate (AQS)-photosensitized Thiobacillus denitrificans, a maximum QY of solar-to-nitrous oxide (N2O) of 96.2% was achieved, which is the highest among the semiartificial photosynthesis systems. Moreover, the conversion of nitrate into N2O was almost 100%, indicating the excellent selectivity in nitrate reduction. The capacitive property of AQS resulted in 82-89% of photoelectrons released at dark and enhanced 5.6-9.4 times the conversion of solar-to-N2O. Kinetics investigation revealed a zero-order- and first-order- reaction kinetics of N2O production in the dark (reductive AQS-mediated electron transfer) and under light (direct photoelectron transfer), respectively. This work is the first study to demonstrate the role of AQS in photosensitizing a microorganism and provides a simple and highly selective approach to produce N2O from nitrate-polluted wastewater and a strategy for the efficient conversion of solar-to-chemical by a semiartificial photosynthesis system.

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.

Chemical design principles of next-generation antiviral surface coatings.

The ongoing coronavirus disease 2019 (COVID-19) pandemic has accelerated efforts to develop high-performance antiviral surface coatings while highlighting the need to build a strong mechanistic understanding of the chemical design principles that underpin antiviral surface coatings. Herein, we critically summarize the latest efforts to develop antiviral surface coatings that exhibit virus-inactivating functions through disrupting lipid envelopes or protein capsids. Particular attention is focused on how cutting-edge advances in material science are being applied to engineer antiviral surface coatings with tailored molecular-level properties to inhibit membrane-enveloped and non-enveloped viruses. Key topics covered include surfaces functionalized with organic and inorganic compounds and nanoparticles to inhibit viruses, and self-cleaning surfaces that incorporate photocatalysts and triplet photosensitizers. Application examples to stop COVID-19 are also introduced and demonstrate how the integration of chemical design principles and advanced material fabrication strategies are leading to next-generation surface coatings that can help thwart viral pandemics and other infectious disease threats.

Selective Electrocatalytic Reduction of Oxygen to Hydroxyl Radicals via 3-Electron Pathway with FeCo Alloy Encapsulated Carbon Aerogel for Fast and Complete Removing Pollutants.

We reported the selective electrochemical reduction of oxygen (O2 ) to hydroxyl radicals (. OH) via 3-electron pathway with FeCo alloy encapsulated by carbon aerogel (FeCoC). The graphite shell with exposed -COOH is conducive to the 2-electron reduction pathway for H2 O2 generation stepped by 1-electron reduction towards to . OH. The electrocatalytic activity can be regulated by tuning the local electronic environment of carbon shell with the electrons coming from the inner FeCo alloy. The new strategy of . OH generation from electrocatalytic reduction O2 overcomes the rate-limiting step over electron transfer initiated by reduction-/oxidation-state cycle in Fenton process. Fast and complete removal of ciprofloxacin was achieved within 5 min in this proposed system, the apparent rate constant (kobs ) was up to 1.44±0.04 min-1 , which is comparable with the state-of-the-art advanced oxidation processes. The degradation rate almost remains the same after 50 successive runs, suggesting the satisfactory stability for practical applications.

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.

Reductive Debromination of Polybrominated Diphenyl Ethers: Dependence on Br Number of the Br-Rich Phenyl Ring.

Reductive debromination has been widely studied for the degradation of polybrominated diphenyl ethers (PBDEs), although the reaction mechanisms are not so clear. In the present study, the photocatalytic degradation and debromination of ten PBDEs were carried out with CuO/TiO2 nanocomposites as the photocatalyst under an anaerobic condition. The pseudo-first-order rate constants were obtained for the photocatalytic debromination of PBDEs, and their relative rate constants ( kR) were evaluated against kR= 1 for BDE209. Unlike the generally accepted summary that kR is dependent on the total Br number ( N) of PBDEs, kR is found to depend on the Br number on a phenyl ring with more Br atoms than the other one. In other words, a phenyl ring substituted by more Br is more reactive for the reductive debromination. The calculated LUMO energies ( ELUMO) of all PBDEs are well correlated to the more reactive phenyl ring with more Br, compared with the N of two phenyl rings. The result was explained by LUMO localization on the Br-rich phenyl ring, suggesting that the reductive debromination occurs on the phenyl ring.

Complete Defluorination and Mineralization of Perfluorooctanoic Acid by a Mechanochemical Method Using Alumina and Persulfate.

Perfluorooctanoic acid (PFOA) is a persistent organic pollutant that has received concerns worldwide due to its extreme resistance to conventional degradation. A mechanochemical (MC) method was developed for complete degradation of PFOA by using alumina (Al2O3) and potassium persulfate (PS) as comilling agents. After ball milling for 2 h, the MC treatment using Al2O3 or PS caused conversion of PFOA to either 1-H-1-perfluoroheptene or dimers with a defluorination efficiency lower than 20%, but that using both Al2O3 and PS caused degradation of PFOA with a defluorination of 100% and a mineralization of 98%. This method also caused complete defluorination of other C3∼C6 homologues of PFOA. The complete defluorination of PFOA attributes to Al2O3 and PS led to the weakening of the C-F bond in PFOA and the generation of hydroxyl radical (•OH), respectively. During the MC degradation, Al2O3 strongly anchors PFOA through COO--Al coordination and in situ formed from Lewis-base interaction and PS through hydrogen bond. Meanwhile, mechanical effects induce the homolytic cleavage of PS to produce SO4•-, which reacts with OH group of Al2O3 to generate •OH. The degradation of PFOA is initiated by decarboxylation as a result of weakened C-COO- due to Al3+ coordination. The subsequent addition of •OH, elimination of HF, and reaction with water induce the stepwise removal of all carboxyl groups and F atoms as CO2 and F-, respectively. Thus, complete defluorination and mineralization are achieved.

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.

Rapid Electron Transfer of Stacked Heterodimers of Perylene Diimide Derivatives in a DNA Duplex.

The preparation of homo- and heterocomplexes composed of the parent perylene diimide (PH) and pyrrolidine-substituted perylene diimide (PN) in DNA and rapid electron transfer in these complexes, which has been analyzed by steady-state fluorescence and femtosecond transient absorption measurements, have been demonstrated. The DNA molecules possessing PH and PN were prepared through a recently developed method that involved the reaction of enzymatically generated abasic sites with the amino groups of the perylene diimide molecules, through which these molecules can be incorporated as base surrogates within the DNA base stack. Melting temperature analysis showed that the PH and PN monomers, and their homo- and heterodimers, contribute to the stabilization of the DNA duplex, and is comparable to that of natural base pairs. Fluorescence measurements showed that PH in a single-stranded oligopyrimidine showed a strong fluorescence, whereas the fluorescence of PH was completely quenched upon pairing with PN (PH/PN) through duplex formation. The transient absorption measurements showed that a rapid electron-transfer reaction in the stacked PH/PN heterodimer occurred on a sub-picosecond timescale, which allowed highly efficient fluorescence quenching. The PH/PN pair, which served as a fluorophore and quencher, were utilized to design molecular beacon probes with a high signal/noise ratio. The PH/PN pair was also capable of forming a stable, stacked dimer structure and induced rapid electron transfer that could serve as a good signal reporter for fluorescent nucleic acid detection.

Aggregation-Induced Singlet Oxygen Generation: Functional Fluorophore and Anthrylphenylene Dyad Self-Assemblies.

The assembly of monomeric building blocks can manifest the display of new properties, including optical, mechanical, and electrochemical functionalities. In this study, we sought to develop a functional fluorophore self-assembly that can generate reactive oxygen species only when aggregated. With an anthrylphenylene (AP) group, negatively charged and neutral fluorescein units form non-fluorescent H-aggregates in aqueous solution because of the weak intermolecular interaction between the anthracene and fluorescein moieties. In stark contrast, a boron dipyrromethene (BODIPY) and AP dyad produces two-color-emissive aggregates through the formation of an intermolecular charge-transfer (CT) complex between the electron-rich anthracene and electron-deficient BODIPY moieties. Furthermore, to our surprise, the BODIPY and AP dyad aggregates generate singlet oxygen (1 O2 ) and photocytotoxicity upon excitation, indicating that the BODIPY-anthracene CT state favors an intersystem crossing process. Based on X-ray crystallographic analysis, the lattice-like molecular packing between the BODIPY and AP moieties was determined to bring about the unprecedented aggregation-induced 1 O2 generation (AISG).

Donor-Donor'-Acceptor Triads Based on [3.3]Paracyclophane with a 1,4-Dithiafulvene Donor and a Cyanomethylene Acceptor: Synthesis, Structure, and Electrochemical and Photophysical Properties.

Donor-donor'-acceptor triads (1, 2), based on [3.3]paracyclophane ([3.3]PCP) as a bridge, with electron-donating properties (D') using 1,4-dithiafulvene (DTF; TTF half unit) as a donor and dicyanomethylene (DCM; TCNE half unit) or an ethoxycarbonyl-cyanomethylene (ECM) as an acceptor were designed and synthesized. The pulse radiolysis study of 1 a in 1,2-dichloroethane allowed the clear assignment of the absorption bands of the DTF radical cation (1 a.+ ), whereas the absorption bands due to the DCM radical anion could not be observed by γ-ray radiolysis in 2-methyltetrahydrofuran rigid glass at 77 K. Electrochemical oxidation of 1 a first generates the DTF radical cation (1 a.+ ), the absorption bands of which are in agreement with those observed by a pulse radiolysis study, followed by dication (1 a2+ ). The ESR spectrum of 1 a.+ showed a symmetrical signal with fine structure and an ESR simulation predicted that the spin of 1 a.+ is delocalized over S and C atoms of the DTF moiety and the central C atom of the trimethylene bridge bearing the DTF moiety. Pulse radiolysis, ESR, and electrochemical studies indicate that the DTF radical cation of 1 a.+ is more stable than that of 6.+ , and the latter shows a strong tendency to dimerize. This result indicates that the [3.3]PCP moiety as a bridge can stabilize the DTF radical cation more than the 1,3-diphenylpropane moiety because of kinetic stability due to its rigid structure and the weak electronic interaction of DTF and DCM moieties through [3.3]PCP.

Significant structural relaxations of excited [n]cycloparaphenylene dications (n = 5-9).

Hoop-shaped macrocycles such as cycloparaphenylenes ([n]CPPs, where n denotes the number of phenylene rings) have attracted considerable attention in recent years because of their interesting properties arising from the highly strained aromatic structure and radially oriented p-orbitals. While the radical cation and dication states of [n]CPPs have been characterized, there is no information available about their excited states, which are expected to exhibit enhanced redox properties. In this study, we investigated the S1 state of [n]CPP2+ by transient absorption measurements in the visible and near-IR regions. The energy of the transient absorption peak exhibited a linear relationship with the reciprocal of the repeating unit, which indicated that the distribution of the excited state expanded with the size of the ring. In addition, smaller CPP2+s showed longer excited state lifetimes. Theoretical calculations suggested that there was a substantial structural relaxation of the smaller CPP2+s accompanying the changes in the charge distribution. Therefore, it was concluded that the smaller Franck-Condon factor resulting from the considerable structural change and larger S1 energy were responsible for the longer S1 state lifetime of smaller CPP2+s.