Picosecond Time-resolved Spectroscopy Research Articles

Photodissociation dynamics of m- and p-cresol in the S1 state: Interplay between the mode-randomization and H atom tunneling reaction.

The H atom tunneling dissociation dynamics of the S1 state of meta- or para-cresol has been investigated by using the picosecond time-resolved pump-probe spectroscopy in a state-specific manner. The S1 state lifetime (mainly due to the H atom tunneling reaction) is found to be mode-dependent whereas it quickly converges and remains constant as the rapid intramolecular vibrational energy redistribution (IVR) starts to participate in the S1 state relaxation with the increase of the S1 internal energy (Eint). The IVR rate and its change with increasing Eint have been reflected in the parent ion transients taken by tuning the total energy (hνpump + hνprobe) just above the adiabatic ionization threshold (so that the dissipation of the initial mode-character could be monitored as a function of the reaction time), indicating that the mode randomization rate into the S1 isoenergetic manifolds exceeds the tunneling rate quite early in terms of Eint for m-cresol (≤∼1200cm-1) or p-cresol (≤∼800cm-1) compared to the case of phenol (≤∼1800cm-1). Though the H atom tunneling dynamics of phenol (S1) seems to be little influenced by the methyl substitution on the either m- or p-position, the IVR rate has been found to be strongly accelerated due to the sharply-increasing (S1) density of states with increasing Eint due to the pivotal role of the low-frequency CH3 torsional mode.

πσ*-Mediated Nonadiabatic Tunneling Dynamics of Thiophenols in S1: The Semiclassical Approaches.

The S-H bond tunneling predissociation dynamics of thiophenol and its ortho-substituted derivatives (2-fluorothiophenol, 2-methoxythiophenol, and 2-chlorothiphenol) in S1 (ππ*) where the H atom tunneling is mediated by the nearby S2 (πσ*) state (which is repulsive along the S-H bond extension coordinate) have been investigated in a state-specific way using the picosecond time-resolved pump-probe spectroscopy for the jet-cooled molecules. The effects of the specific vibrational mode excitations and the SH/SD substitutions on the S-H(D) bond rupture tunneling dynamics have been interrogated, giving deep insights into the multidimensional aspects of the S1/S2 conical intersection, which also shapes the underlying adiabatic tunneling potential energy surfaces (PESs). The semiclassical tunneling rate calculations based on the Wentzel-Kramers-Brillouin (WKB) approximation or Zhu-Nakamura (ZN) theory have been carried out based on the ab initio PESs calculated in the (one, two, or three) reduced dimensions to be compared with the experiment. Though the quantitative experimental results could not be reproduced satisfactorily by the present calculations, the qualitative trends among different molecules in terms of the behavior of the tunneling rate versus the (adiabatic) barrier height or the number of PES dimensions could be rationalized. Most interestingly, the H/D kinetic isotope effect observed in the tunneling rate could be much better explained by the ZN theory compared to the WKB approximation, indicating that the nonadiabatic coupling matrix elements should be invoked for understanding the tunneling dynamics taking place in the proximity of the conical intersection.

State-Specific Chemical Dynamics of the Nonvalence Bound State of the Molecular Anions.

Nonvalence bound states (NBS) are anionic states where the excess electron is extremely loosely bound to the neutral core through long-range potentials. In contrast to the valence orbitals of which the electron occupancy determines the molecular structure, as well as the chemical reactivity, the nonvalence orbital is quite diffuse and located far from the neutral core. The NBS can be classified into the dipole-bound state (DBS), quadruple-bound state (QBS), or correlation-bound state (CBS) according to the nature of the electron-neutral interaction, although their interaction potentials may cooperatively contribute. The NBS is ubiquitous in nature and has the strong implications in atmospheric, interstellar, or biological chemistry. Accordingly, NBS has long been conceived to play the role of the doorway into the formation of a stable anion or dissociative electron attachment (DEA). Despite intensive and extensive studies, however, the quantum-mechanical nature of NBS is still far from being thorough understanding. Herein, we describe a new aspect of state-specific NBS-mediated chemical dynamics, which has been revealed through a series of recent studies by our group. We have employed picosecond time-resolved pump-probe spectroscopy combined with cryogenically cooled ion trap and velocity-map imaging techniques to study closed-shell anions generated by electrospray ionization. DBS vibrational Feshbach resonances are prepared by the optical excitation of phenoxide, for instance, and their individual lifetimes have been precisely measured in a state-specific manner to reveal the strong mode-dependency of the autodetachment rate. Fermi's golden rule turns out to be extremely useful for a rational explanation of the experiment, although the more sophisticated theoretical model is desirable for the more quantitative analysis. For the DBS of para-chlorophenoxide or para-bromophenoxide where the polarizability of neutral core is substantial, the Fermi's golden rule based on the charge-dipole potential needs to be significantly modified to include the correlation effects to explain the exceptionally slow autodetachment rates. For the QBS of 4-cyanophenoxide, the mode-specific behavior of the quadrupole ellipsoid tensor explains the strong mode-dependent autodetachment rate. Meanwhile, the nonadiabatic transition of the excess electron into the valence orbital can result in stable anion formation or immediate chemical bond rupture. In the DBS of ortho-, meta-, or para-iodophenoxide, the transformation of the loosely bound excess electron into the πσ* antibonding orbital occurs to give I- as a final fragment. The fragmentation mediated by DBS occurs competitively with the concomitant autodetachment, paving a new way of the reaction control by tuning the quantum-mechanical nature of the DBS Feshbach resonance. This experimental observation provides the foremost evidence for the dynamic role of the DBS as a doorway into anion chemistry, such as DEA. The ponderomotive force on the electron in the nonvalence orbital has been demonstrated for the first time in a strong optical field, giving great promise for the manipulation of polyatomic molecules in terms of the spatial location, as well as the AC-Stark control of the chemical reaction.

(Invited) Discovering Bottlenecks and Opportunities in Hybrid Solar Light Harvesting Systems By Ultrafast X-Ray Spectroscopy

A prerequisite for advancing hybrid solar light harvesting systems is a comprehensive understanding of photoinduced electronic dynamics across a wide range of spatial and temporal scales. Time-resolved spectroscopy techniques in the optical domain have proven instrumental in identifying critical timescales of fundamental processes, such as interfacial charge transfer, charge migration, and carrier lifetimes, which are intimately linked to a system's performance. New light source and instrument developments open opportunities to extend such studies into the X-ray regime. Due to their inherent elemental specificity and sensitivity to local electronic environments, time-domain X-ray spectroscopy techniques are uniquely suited to gain a microscopic picture of dynamics from well-defined reporter sites within often complex nanoscale light harvesting architectures. We will present new insights into fundamental photoinduced dynamics in several archetypical heterostructures based on femtosecond and picosecond time-resolved X-ray photoemission spectroscopy (TRXPS).The photoinduced transient charge redistribution in the model hybrid system of nanoporous zinc oxide (ZnO) sensitized by ruthenium bipyridyl chromophores is probed independently from the viewpoints of the molecular electron donor and the semiconductor acceptor.[1] Charge injection from the chromophore into the semiconductor is found to proceed via a transiently populated interfacial charge transfer (ICT) complex on an overall timescale of ~300 ps. Our results show that, even after release from the ICT states into the ZnO conduction band, the injected electrons remain localized within less than 6 nm from the interface, due to enhanced downward band-bending by the photo-injected charge carriers. This spatial confinement suggests that light-induced charge generation and transport in nanoscale ZnO photocatalytic devices proceeds predominantly within the defect-rich surface region, which may lead to enhanced surface recombination and explain their lower performance compared to titanium dioxide (TiO2)-based systems. The quantitative nature and surface sensitivity of TRXPS also provides direct access to the spatial separation of the holes remaining on the bipyridyl chromophores after charge injection from the semiconductor surface with sub-nm precision.Complementary femtosecond and picosecond TRXPS studies of photoinduced dynamics in copper-phthalocyanine(CuPc)-C60 heterojunctions provide a deeper understanding of both exciton migration pathways toward the interface as well as the absolute efficiency by which ICT states dissociate into separate charges.[2] The measurements reveal surprisingly efficient charge generation from triplet excitons and from the lowest lying ICT states, both of which were previously believed to be inactive during light-to-charge conversion.Photoinduced charge transfer dynamics between spherical gold nanoparticles (AuNPs) and a nanoporous film of TiO2 is monitored by picosecond TRXPS, providing direct access to the absolute photon-to-charge conversion efficiency and subsequent electron-hole recombination dynamics.[3] Preliminary results will be presented for the extension of the measurements into the ambient pressure regime, taking a first step toward the study of photoinduced interfacial chemistry by TRXPS in this model system for nanoplasmonic light harvesting applications.[1] S. Neppl et al., “Nanoscale Confinement of Photo-Injected Electrons at Hybrid Interfaces”, J. Phys. Chem. Lett. 12, 11951 (2021).[2] F. Roth et al., “Direct observation of charge separation in an organic light harvesting system by femtosecond time-resolved XPS”, Nat. Commun. 12, 1196 (2021).[3] M. Borgwardt et al., “Photoinduced Charge Carrier Dynamics and Electron Injection Efficiencies in Au Nanoparticle-Sensitized TiO2 Determined with Picosecond Time-Resolved X-ray Photoelectron Spectroscopy”, J. Phys. Chem. Lett. 11, 5476 (2020). Figure 1

Real-time observation of photoionization-induced water migration dynamics in 4-methylformanilide-water by picosecond time-resolved infrared spectroscopy and ab initio molecular dynamics simulations.

A novel time-resolved pump-probe spectroscopic approach that enables to keep high resolution in both the time and energy domain, nanosecond excitation-picosecond ionization-picosecond infrared probe (ns-ps-ps TRIR) spectroscopy, has been applied to the trans-4-methylformanilide-water (4MetFA-W) cluster. Water migration dynamics from the CO to the NH binding site in a peptide linkage triggered by photoionization of 4MetFA-W is directly monitored by the ps time evolution of IR spectra, and the presence of an intermediate state is revealed. The time evolution is analyzed by rate equations based on a four-state model of the migration dynamics. Time constants for the initial to the intermediate and hot product and to the final product are obtained. The acceleration of the dynamics by methyl substitution and the strong contribution of intracluster vibrational energy redistribution in the termination of the solvation dynamics is suggested. This picture is well confirmed by the ab initio on-the-fly molecular dynamics simulations. Vibrational assignments of 4MetFA and 4MetFA-W in the neutral (S0 and S1) and ionic (D0) electronic states measured by ns IR dip and electron-impact IR photodissociation spectroscopy are also discussed prior to the results of time-resolved spectroscopy.

Solvent-Dependent Excited-State Evolution of Prodan Dyes.

Excited-state character and dynamics of two 6-(dimethylamino)-2-acylnaphthalene dyes (Prodan and Badan-SCH2CH2OH) were studied by picosecond time-resolved IR spectroscopy (TRIR) in solvents of different polarity and relaxation times: hexane, CD3OD, and glycerol-d8. In all these solvents, near-UV excitation initially produced the same S1(ππ*) excited state characterized by a broad TRIR signal. A very fast decay (3, ∼100 ps) followed in hexane, whereas conversion to a distinct IR spectrum with a ν(C═O) band downshifted by 76 cm-1 occurred in polar/H-bonding solvents, slowing down on going from CD3OD (1, 23 ps) to glycerol-d8 (5.5, 51, 330 ps). The final relaxed excited state was assigned as planar Me2N → C═O intramolecular charge transfer S1(ICT) by comparing experimental and TDDFT-calculated spectra. TRIR conversion kinetics are comparable to those of early stages of multiexponential fluorescence decay and dynamic fluorescence red-shift. This work presents a strong evidence that Prodan-type dyes undergo solvation-driven charge separation in their S1 state, which is responsible for the dynamic fluorescence Stokes shift observed in polar/H-bonding solvents. The time evolution of the optically prepared S1(ππ*) state to the S1(ICT) final state reflects environment relaxation and solvation dynamics. This finding rationalizes the widespread use of Prodan-type dyes as probes of environment dynamics and polarity.

Isomer-Selective Spectroscopy and Dynamics of Phenol-Arn (n ≤ 5) Clusters.

Structures and ionization-induced solvation dynamics of phenol-(argon)n clusters, PhOH-Arn (n ≤ 5), were studied by using a variety of isomer-selective photoionization and vibrational spectroscopic techniques. Several higher-energy isomers were found and assigned for the first time by systematically controlling the experimental conditions of the supersonic expansion. This behavior is also confirmed for the PhOH-Kr2 cluster. Solvation structures are elucidated by evaluating systematic shifts in the S1 ← S0 origin and ionization energies obtained by resonance-enhanced photoionization, in addition to the OH stretching frequency obtained by IR photodissociation. Isomer-selective picosecond time-resolved IR spectroscopy for the n = 2 clusters revealed that the dynamics for the ionization-induced intermolecular π → H site-switching reaction strongly depends on the initial isomeric structure. In particular, the reaction time for the (1|1) isomer is 7 ps, while that for (2|0) is <3 ps. This difference shows that the switching time is determined by the distance of the reaction coordinate between the initial π-site and the final OH-site.

(Invited) Ultrafast X-Ray Studies of Electron Dynamics Underlying Molecular and Nanoplasmonic Light Harvesting Architectures

Chemically engineered light harvesting architectures based on molecular and nanoplasmonic chromophores in contact with semiconducting electron acceptors provide exciting opportunities for novel photocatalytic and photovoltaic applications. Optimizing their performance, however, often hinges on a deeper understanding of the underlying fundamental electronic and chemical dynamics. Processes evolving on spatial and temporal scales spanning many orders of magnitude have to be connected in order to enable molecular, interfacial, and macroscopic charge and energy transport. We will discuss the application of ultrafast X-ray spectroscopy techniques to gain deep insight into photoinduced electronic dynamics in complex, heterogeneous light-harvesting systems based on organic heterojunctions as well as molecule-semiconductor and metal nanoparticle-semiconductor interfaces.Femtosecond and picosecond time-resolved X-ray photoelectron spectroscopy (TRXPS) is used to study photoinduced charge injection in N3 dye-sensitized films of ZnO nanoparticles. The elemental site-specificity and surface sensitivity of TRXPS provides a comprehensive real-time picture of the interfacial charge dynamics including the locations of intermittently trapped electrons, the emergence and decay of interfacial dipoles, as well as charge-injection induced band-bending dynamics in the semiconductor substrate. Potential consequences of the findings for the design of dye-sensitized nanoporous electrodes are discussed.TRXPS studies of photoinduced dynamics in copper-phthalocyanine(CuPc)-C60 heterojunctions provide a deeper understanding of the predominant energy transport and charge generation mechanisms. Ultrafast X-ray probing captures the actual moment of charge generation from interfacial charge-transfer (ICT) states and provides a quantitative measure of the exciton dissociation efficiency compared to photon energy loss by exciton recombination. The findings complement observations with ultrafast optical techniques, adding important aspects regarding the dynamic pathways and branching ratios that govern charge generation efficiencies in this canonical model system for organic light harvesting concepts.In a third showcase example, photoinduced charge transfer dynamics in a nanoporous film of TiO2 sensitized with gold nanoparticles (AuNPs) is monitored by TRXPS. Direct access to the absolute photon-to-charge conversion efficiency and subsequent electron-hole recombination dynamics is demonstrated with single-electron sensitivity. The results provide important boundary conditions for the design of AuNP-TiO2 based light harvesting systems. Opportunities for extending the measurements toward studies of photoinduced interfacial chemical transformations will be discussed.

Electron-transfer behaviors between photoexcited metal complex and methyl viologen codoped in ionic nanospheres

In this study, we investigated photo-induced electron transfer (PET) dynamics from [Ru(bpy)3]2+ to MV2+ codoped in ionic nanospheres by picosecond time-resolved luminescence spectroscopy. The luminescence from [Ru(bpy)3]2+ was decreased with an increase of the MV2+ content in nanospheres, suggesting an electron transfer from the 3MLCT excited state of [Ru(bpy)3]2+ to MV2+. The fast decay component with the time-constant of 62 ns was found in the luminescence decay kinetics. This component is associated with a Perrin-type electron transfer and increases with an increase of the MV2+ content, suggesting the efficient Perrin-type electron transfer in the higher MV2+ content. In addition, the decay component of 280∼410 ns depending on a MV2+ concentration is explained by the dynamic-type electron transfer since the inverse lifetime of this component exhibits the linear dependence on the MV2+ content. The Perrin- and dynamic-type electron transfer rates were determined to be 1.4 × 107 s−1 and 7.8 × 10−3 nmol−1 mg μs−1, respectively. The PET efficiency was determined 70 % in ionic nanospheres containing 92 nmol/mg of [Ru(bpy)3]2+ and 152 nmol/mg of MV2+.

Photoinduced Charge Carrier Dynamics and Electron Injection Efficiencies in Au Nanoparticle-Sensitized TiO2 Determined with Picosecond Time-Resolved X-ray Photoelectron Spectroscopy.

Progress in the development of plasmon-enabled light-harvesting technologies requires a better understanding of their fundamental operating principles and current limitations. Here, we employ picosecond time-resolved X-ray photoemission spectroscopy to investigate photoinduced electron transfer in a plasmonic model system composed of 20 nm sized gold nanoparticles (NPs) attached to a nanoporous film of TiO2. The measurement provides direct, quantitative access to transient local charge distributions from the perspectives of the electron donor (AuNP) and the electron acceptor (TiO2). On average, approximately two electrons are injected per NP, corresponding to an electron injection yield per absorbed photon of 0.1%. Back electron transfer from the perspective of the electron donor is dominated by a fast recombination channel proceeding on a time scale of 60 ± 10 ps and a minor contribution that is completed after ∼1 ns. The findings provide a detailed picture of photoinduced charge carrier generation in this NP-semiconductor junction, with important implications for understanding achievable overall photon-to-charge conversion efficiencies.

Photo-activated CO-release in the amino tungsten Fischer carbene complex, [(CO)5WC(NC4H8)Me], picosecond time resolved infrared spectroscopy, time-dependent density functional theory, and an antimicrobial study

Picosecond time-resolved infrared spectroscopy was used to probe the photo-induced early state dynamics preceding CO loss in the Fischer carbene complex, [(CO)5WC(NC4H8)CH3]. Time-dependent density functional theory calculations were employed to help in understanding the photochemical and photophysical processes leading to CO-loss. Electrochemical initiated CO release was quantified using gas chromatography. The potential of [(CO)5WC(NC4H8)CH3], as an antimicrobial agent under irradiation conditions was studied using a Staphylococcus aureus strain.

Excited-state proton transfer in protonated adrenaline revealed by cryogenic UV photodissociation spectroscopy.

We report a comprehensive study of the structures and deactivation processes of protonated adrenaline through cryogenic UV photodissociation spectroscopy. Single UV and double-resonance UV-UV hole burning spectroscopies have been performed and compared to coupled-cluster SCS-CC2 calculations performed on the ground and first electronic states. Three conformers were assigned, two lowest energy gauche conformers along with a higher energy conformer with an extended structure which is indeed the global minimum in solution. This demonstrates the kinetic trapping of this high energy gas phase conformer during the electrospray process. At the band origin of all conformers, the main fragmentation channel is the Cα-Cβ bond cleavage, triggered by an excited state proton transfer to the catechol ring. Internal conversion leading to the water loss channel competes with the direct dissociation and tends to prevail with the increase of excess energy brought by the UV laser. Picosecond time-resolved pump-probe spectroscopy was performed to measure the excited state lifetimes of the three conformers of AdH+, which decay with the increase of excess energy in the ππ* state, from 2 ns at the band origin down to few hundreds of picoseconds 0.5 eV to the blue. Finally, about 0.8 eV above the band origin, the πσ* state is directly reached, leading to the opening of the H-loss channel.

Efficient charge generation from triplet excitons in metal-organic heterojunctions

The success of many emerging molecular electronics concepts hinges on an atomistic understanding of the underlying electronic dynamics. We employ picosecond time-resolved x-ray photoemission spectroscopy (tr-XPS) to elucidate the roles of singlet and triplet excitons for photoinduced charge generation at a copper-phthalocyanine--${\mathrm{C}}_{60}$ heterojunction. Contrary to common belief, fast intersystem crossing to triplet excitons after photoexcitation is not a loss channel but contributes to a significantly larger extent to the time-integrated interfacial charge generation than the initially excited singlet excitons. The tr-XPS data provide direct access to the diffusivity of the triplet excitons ${D}_{\mathrm{CuPc}}=(1.8\ifmmode\pm\else\textpm\fi{}1.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{s}$ (where CuPc is copper-phthalocyanine) and their diffusion length ${L}_{\mathrm{diff}}=(8\ifmmode\pm\else\textpm\fi{}3)\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$.

Real-time probing of charge-transfer induced interfacial fields in a dye-semiconductor system using time-resolved XPS

Photo-induced charge carrier dynamics and transient interfacial fields at the interface between N3 polypyridine complexes and films of nanocrystalline ZnO are probed by picosecond time-resolved X-ray photoelectron spectroscopy.

Controlling Photosynthetic Excitons by Selective Pigment Photooxidation.

As a basis of photosynthesis, photoinduced oxidation of (bacterio)chlorophyll molecules in the special reaction center complexes has been a subject of extensive research. In contrast, the generally harmful photooxidation of antenna chromoproteins has received much less attention. Here, we have established the permanent structural changes in the LH2 antenna bacteriochlorophyll-protein complex from a sulfur photosynthetic purple bacterium Ectothiorhodospira haloalkaliphila taking place at physiological conditions upon intense optical irradiation. To this end, a crystal structure of the LH2 complex from E. haloalkaliphila was first resolved by X-ray diffraction to 3.7 Å, verifying a great similarity with the earlier structure from Phaesporillum molischianum. Analysis of the various steady-state and picosecond time-resolved optical spectroscopy data and related model simulations then confirmed that the major spectral effects observed-bleaching and blue-shifting of the B850 exciton band and correlated emergence of a higher-energy C700 exciton band-are associated with photooxidation of increasing numbers of B850 bacteriochlorophylls into 3-acetyl-chlorophylls, with no noticeable damage to the pigment-binding protein scaffold. A prospective noninvasive method for an in situ optical control of excitons by selective photooxidation of pigment chromophores was thus revealed and demonstrated in a structurally well-defined native system.

Identifying complex Fermi resonances in p-difluorobenzene using zero-electron-kinetic-energy (ZEKE) spectroscopy.

The vibrations of the ground state cation ( 2B2g) of para-difluorobenzene (pDFB) have been investigated using zero-electron-kinetic-energy (ZEKE) spectroscopy. A comprehensive set of ZEKE spectra were recorded via different vibrational levels of the S1 state (<00 + 1300 cm-1). The adiabatic ionization energy for pDFB was measured as 73 869 ± 5 cm-1. Use of different intermediate levels allows different cationic vibrational activity to be obtained via the modification of the Franck-Condon factors for the ionization step, allowing the wavenumbers of different vibrational levels in the cation to be established. In addition, assignment of the vibrational structure in the ZEKE spectra allowed interrogation of the assignments of the S1 ← S0 transition put forward by Knight and Kable [J. Chem. Phys. 89, 7139 (1988)]. Assignment of the vibrational structure has been aided by quantum chemical calculations. In this way, it was possible to assign seventeen of the thirty vibrational modes of the ground state pDFB+ cation. Evidence for complex Fermi resonances in the S1 state, i.e., those that involve more than two vibrations, was established. One of these was investigated using picosecond time-resolved photoelectron spectroscopy. In addition, we discuss the appearance of several symmetry-forbidden bands in the ZEKE spectra, attributing their appearance to a Rydberg state variation of an intrachannel vibronic coupling mechanism.

Energy transfer dynamics in strongly inhomogeneous hot-dense-matter systems.

Direct measurements of energy transfer across steep density and temperature gradients in a hot-dense-matter system are presented. Hot-dense-plasma conditions were generated by high-intensity laser irradiation of a thin-foil target containing a buried metal layer. Energy transfer to the layer was measured using picosecond time-resolved x-ray emission spectroscopy. The data show two x-ray flashes in time. Fully explicit, coupled particle-in-cell and collisional-radiative atomic kinetics model predictions reproduce these observations, connecting the two x-ray flashes with staged radial energy transfer within the target.

UV-Photochemistry of the Disulfide Bond: Evolution of Early Photoproducts from Picosecond X-ray Absorption Spectroscopy at the Sulfur K-Edge.

We have investigated dimethyl disulfide as the basic moiety for understanding the photochemistry of disulfide bonds, which are central to a broad range of biochemical processes. Picosecond time-resolved X-ray absorption spectroscopy at the sulfur K-edge provides unique element-specific insight into the photochemistry of the disulfide bond initiated by 267 nm femtosecond pulses. We observe a broad but distinct transient induced absorption spectrum which recovers on at least two time scales in the nanosecond range. We employed RASSCF electronic structure calculations to simulate the sulfur-1s transitions of multiple possible chemical species, and identified the methylthiyl and methylperthiyl radicals as the primary reaction products. In addition, we identify disulfur and the CH2S thione as the secondary reaction products of the perthiyl radical that are most likely to explain the observed spectral and kinetic signatures of our experiment. Our study underscores the importance of elemental specificity and the potential of time-resolved X-ray spectroscopy to identify short-lived reaction products in complex reaction schemes that underlie the rich photochemistry of disulfide systems.

Unraveling the Ultrafast Exciton Relaxation and Hidden Energy State in CH3NH3PbBr3 Nanoparticles

Recently organic–inorganic lead-halide perovskite nanoparticles (NPs) have been attractive as low-cost and high-conversion-efficient solar cells and light-emitting diode. The generation of the exciton and dissociation into free carriers are quite important for primary photoelectric conversion processes. In this study, we have examined the initial exciton dynamics of CH3NH3PbBr3 (MAPbBr3) NPs by femtosecond transient absorption spectroscopy and picosecond time-resolved luminescence spectroscopy. The ultrafast exciton quenching with a time constant of 200 fs was observed, which may be related with longitudinal optical phonon and/or lurching motion of MA cation in the excited state. In addition, higher electronic state with a short lifetime was clearly detected by the excitation intensity dependence of time-resolved luminescence and transient absorption spectra. These findings of MAPbBr3 NPs are very important not only for understanding the generation of charge carrier but also for constructing the high-effi...

A meso-meso β-β β-β Triply Linked Subporphyrin Dimer.

A meso-meso β-β β-β triply linked subporphyrin dimer 6 was synthesized by stepwise reductive elimination of β-to-β doubly PtII -bridged subporphyrin dimer 9. Dimer 6 was characterized by spectroscopic and electrochemical measurements, theoretical calculations, and picosecond time-resolved transient absorption spectroscopy. X-ray diffraction analysis reveals that 6 has a bowl-shaped structure with a positive Gaussian curvature. Despite the curved structure, 6 exhibits a remarkably red-shifted absorption band at 942 nm and a small electrochemical HOMO-LUMO gap (1.35 eV), indicating an effectively conjugated π-electronic network.