Near Bandgap Excitation Inhibits the Interfacial Electron Transfer of Semiconductor/Cocatalyst

Understanding the ultrafast interfacial electron transfer (IET) process is essential for establishing the structure–property relationship of the semiconductor/cocatalyst system for photocatalytic H2 evolution. However, the IET kinetics for the near bandgap excitation has not been reported. Herein, we investigate the IET kinetics of g-C3N4/Pt as a semiconductor/cocatalyst prototype by femtosecond time-resolved diffuse reflectance spectroscopy. We find that the near bandgap excitation of g-C3N4 inhibits the IET of g-C3N4/Pt due to electron deep trapping, resulting in a markedly decreased apparent quantum efficiency for photocatalytic H2 evolution. This work complements the kinetic understanding for the photocatalytic mechanism of the semiconductor/cocatalyst system in its whole light absorption range

Properties of Excited Radical Cations of Substituted Oligothiophenes

Excited-state properties of radical cations of substituted oligothiophenes (nT•+, n denotes the number of thiophene rings, n = 3, 4, 5) in solution were investigated by using various laser flash photolysis techniques including two-color two-laser flash photolysis. nT•+ generated by photoinduced electron transfer to p-chloranil or resonant two-photon ionization (RTPI) by using the first 355-nm ns laser irradiation was selectively excited with the second picosecond laser (532 nm). Bleaching of the absorption of nT•+ together with growth of a new absorption was observed during the second laser irradiation, indicating the generation of nT•+ in the excited state (nT•+*). The D1 state lifetime was estimated to be 34 ± 4, 24 ± 2, and 18 ± 1 ps for 3T•+, 4T•+, and 5T•+, respectively. In the presence of hole acceptor (Q), bleaching of nT•+ and growth of Q•+ were observed upon selective excitation of nT•+ during the nanosecond−nanosecond two-color two-laser flash photolysis, indicating the hole transfer from nT•+(D1) to Q. Recovery of nT•+ was also observed together with decay of Q•+ because of regeneration of nT•+ by hole transfer from Q•+ to nT at the diffusion-limiting rate. It was suggested that the hole transfer rate (kHT) from nT•+(D1) to Q depended on the free-energy change for hole transfer (−ΔG = 1.41−0.46 eV). The estimated kHT faster than the diffusion-limiting rate can be explained by the contribution of the static quenching for the excited species in the presence of high concentration of Q (0.1−1.0 M)

Site-Selective Bimodal Absorption and Emission of Distonic Radical Cation

An acyclic 1,4-distonic dimer radical cation (DAE2•+) was generated from the dimerization of 1,1-bis(4-methoxyphenyl)ethylene radical cation (DAE•+) with the neutral molecule (DAE) in solution. The absorption spectrum of DAE2•+ shows bimodal absorption bands with peaks at 350 and 500 nm corresponding to the 1,1-bis(4-methoxyphenyl)ethyl radical (An2C•CH3) and 1,1-bis(4-methoxyphenyl)ethyl cation (An2C+CH3), respectively. Therefore, DAE2•+ in the ground state has the spin and positive charge localized on the 1- and 4-positions, respectively. The bimodal characteristic emissions by the site-selective excitation of radical and cation sites of DAE2•+ were observed at 77 K, showing that the excitation energy is localized on the radical or cation site of DAE2•+ in the excited state. The interaction between radical and cation sites of DAE2•+ in the ground and excited states are discussed on the basis of the steady-state spectroscopic and transient absorption measurements, as well as theoretical calculations

Diastereochemically Controlled Porphyrin Dimer Formation on a DNA Duplex Scaffold

DNA-porphyrin conjugates were designed and synthesized for the preparation of the conformationally controlled porphyrin dimer structures constructed on a d(GCGTATACGC)2. Porphyrin derivatives were introduced to the central TATpA sequence where p represents the phosphoramidate for the attachment of the free-base porphyrin (FbP) and zinc-coordinated porphyrin (ZnP), which allows contact of the two porphyrins in the minor groove. The porphyrin dimers were characterized using CD, UV−vis, steady-state, and time-resolved fluorescence spectroscopies, indicating that the porphyrins form face-to-face conformations. Also the co-facial conformation was confirmed by comparison with spectra of the non-self-complementary duplex containing one porphyrin moiety. Introduction of zinc into porphyrin moiety destabilized the duplex formation. Two diastereomers showed different thermal stabilities and affected the conformations of porphyrin dimers. The temperature-dependent assembly and the conformational change of the porphyrin dimer on the duplex DNA were observed in the UV−vis spectra, indicating that the dynamic movement of the porphyrin dimer occurs on the duplex. The results indicate that the porphyrin dimers of DNA-FbP conjugates are overlapped clockwise and are located in the minor groove of the usual B-form DNA backbone. The interaction and conformation of two porphyrin moieties are controlled by the following three factors:  (1) temperature change during and after formation of the duplex porphyrins at lower temperature; (2) diastereochemistry of the phosphoramidates where porphyrins are connected via a linker; and (3) zinc ion coordination that destabilizes the interaction of porphyrins as well duplex formation

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

Shallow Trap State-Induced Efficient Electron Transfer at the Interface of Heterojunction Photocatalysts: The Crucial Role of Vacancy Defects

Constructing vacancies has been demonstrated to be an effective way to modulate charge flow in semiconductor photocatalysts. However, the role of vacancies in the interfacial electron transfer (IET) of heterojunction photocatalysts remains poorly understood, which hinders the general design of heterojunction photocatalysts. Herein, by taking g-C3N4/MoS2 as a heterojunction photocatalyst prototype, we unravel that vacancies play a critical role in the IET of heterojunction photocatalysts. Theoretical simulations, combined with femtosecond time-resolved diffuse reflectance spectroscopy, give a clear physical picture that N vacancy states act as shallow trap states (STSs) for photogenerated electrons and thereby facilitate the IET process due to a large energy difference between STSs and charge separation states. Moreover, the excess electrons left by the loss of N atoms (producing N vacancies) could partially transfer to MoS2 to generate STSs in the forbidden band of MoS2, where the transferred photogenerated electrons could be further trapped to efficiently drive H2 evolution. This work suggests a promising strategy to tune IET of heterojunction photocatalysts for achieving highly efficient photocatalytic reactions

Photoaccelerated Hole Transfer in Oligothiophene Assemblies

A new series of mesitylene-linked oligothiophenes (<i>n</i>T, <i>n</i> is the number of thiophene units), including 2T-M, 3T-M, 4T-M, 4T-M-2T, and 4T-M-3T, was prepared to investigate the intramolecular hole transfer (HT) from the excited radical cation for the first time. The results of spectroscopic and theoretical studies indicated that mesitylene acts as a spacer minimizing the perturbation to the thiophene π-conjugation and increasing the stability of <i>n</i>T radical cations (<i>n</i>T^•+). Femtosecond laser flash photolysis was applied to the FeCl₃-oxidized 4T^•+-M, 4T^•+-M-2T, and 4T^•+-M-3T. Upon 670 nm laser excitation, the transient absorption spectra of 4T^•+-M showed the existence of two species as the D₁ and D₀^hot states. The intramolecular HT processes from excited 4T^•+ with the time constants of 1.6 and 0.8 ps were observed upon excitation of 4T^•+-M-2T and 4T^•+-M-3T, respectively. This is the first capture of such ultrafast processes with the subsequent back HT from the ground-state 2T^•+ or 3T^•+ in <i>n</i>T assemblies. The current findings indicated an accelerated migration of photocarriers (polarons) in thiophene-based p-type semiconductor materials upon irradiation and provided a fresh viewpoint to understand the successive HT in polythiophenes for various organic molecular devices

Mesolysis of Radical Anions of Tetra‑, Penta‑, and Hexaphenylethanes

A central carbon–carbon (C–C) σ bond dissociation of polyphenylethane radical anions (Ph_<i>n</i>E^•‑, <i>n</i> = 3–6), mesolysis, was investigated by the transient absorption measurement during pulse radiolysis of Ph_<i>n</i>E in 2-methyltetrahydrofuran. The charge resonance (CR) band of 1,1,2,2-tetraphenylethane radical anion (1,1,2,2-Ph₄E^•‑) was observed in the near-infrared region immediately after an electron pulse to be attributed to the intramolecular dimer radical anion. The CR band disappeared with simultaneous formation of two absorption bands at 330 and 460 nm corresponding to diphenylmethyl radical and diphenylmethyl anion, respectively, indicating the occurrence of the mesolysis in 1,1,2,2-Ph₄E^•‑. During pulse radiolysis of 1,1,1,2,2,2-hexaphenylethane (Ph₆E), an absorption band of triphenylmethyl radical was observed at 340 nm immediately after an electron pulse. It is suggested that one electron attachment to Ph₆E is followed by the subsequent rapid C–C σ bond dissociation. Formation of intramolecular dimer radical anions in Ph_<i>n</i>E^•‑ such as 1,1,2-triphenylethane (Ph₃E), 1,1,1,2-tetraphenylethane (1,1,1,2-Ph₄E), and 1,1,1,2,2-pentaphenylethane (Ph₅E) was also studied together with the subsequent mesolysis. The mesolysis of Ph_<i>n</i>E^•‑ is discussed in terms of charge delocalization in the intramolecular dimer radical anions and the central C–C σ bond as well as bond dissociation energy of the central C–C σ bond of Ph_<i>n</i>E^•‑

Intramolecular Charge Resonance in Dimer Radical Anions of Di-, Tri-, Tetra-, and Pentaphenylalkanes

Intramolecular dimer radical anions of di-, tri-, tetra-, and pentaphenylalkanes were investigated on the basis of absorption spectral measurements during γ-radiolysis in 2-methyltetrahydrofuran (MTHF) glassy matrix at 77 K and theoretical calculations. The absorption spectrum of 1,1,2,2-tetraphenylethane (1,1,2,2-Ph₄E) radical anion showed two bands in the near-infrared (NIR) region (900–2600 nm). One band observed at shorter wavelength than 2000 nm is assigned to the intramolecular charge resonance (CR) band between two phenyl groups of the 1,1-diphenylmethyl chromophore (1,1-dimer radical anion). The intramolecular CR band of the 1,1-dimer radical anion was observed for various alkanes having 1,1-diphenylmethyl chromophore such as 1,1,1-triphenylmethane (1,1,1-Ph₃M), 1,1,1,1-tetraphenylmethane (1,1,1,1-Ph₄M), and so on. The other intramolecular CR band observed at longer wavelength than 2200 nm is assigned to intramolecular dimer radical anion between two phenyl groups of the 1,2-diphenylethyl chromophore (1,2-dimer radical anion). The intramolecular CR band of the 1,2-dimer radical anion was observed for various alkanes having a 1,2-diphenylethyl chromophore, such as 1,1,2-triphenylethane (1,1,2-Ph₃E), 1,1,2,2-Ph₄E, and 1,1,1,2,2-pentaphenylethane (1,1,1,2,2-Ph₅E) and so on. No dimer radical anion was observed for 1,<i>n</i>-diphenylalkanes (<i>n</i> > 2) without 1,1-diphenylmethyl chromophore. The relationship between the structure and negative charge delocalization over two phenyl groups connected by an sp³ carbon is discussed

Pt–Au Triangular Nanoprisms with Strong Dipole Plasmon Resonance for Hydrogen Generation Studied by Single-Particle Spectroscopy

Three anisotropic Pt-covered, Pt-edged, and Pt-tipped Au triangular nanoprisms (TNPs) were prepared by controlling the overgrowth of Pt as photocatalysts for H₂ generation. With strong electric field and more interface for the hot electrons transfer, the H₂ generation rate of Pt-edged Au TNPs was 3 and 5 times higher than those of Pt-tipped and Pt-covered Au TNPs. Single-particle photoluminescence (PL) spectra and finite-difference-time-domain (FDTD) simulations demonstrated that dipole surface plasmon resonance (DSPR) of Au TNPs enhanced the hot electrons transfer from Au to Pt leading to H₂ generation. SPR bands of Au TNPs depending on the size play an important role on the photocatalytic activity of Pt-edged Au TNPs