Ultrafast Transient Absorption Spectra Research Articles

Insights into excitonic behavior in single-atom covalent organic frameworks for efficient photo-Fenton-like pollutant degradation

The generation of radicals through photo-Fenton-like reactions demonstrates significant potential for remediating emerging organic contaminants (EOCs) in complex aqueous environments. However, the excitonic effect, induced by Coulomb interactions between photoexcited electrons and holes, reduces carrier utilization efficiency in these systems. In this study, we develop Cu single-atom-loaded covalent organic frameworks (CuSA/COFs) as models to modulate excitonic effects. Temperature-dependent photoluminescence and ultrafast transient absorption spectra reveal that incorporating acenaphthene units into the linker (CuSA/Ace-COF) significantly reduces exciton binding energy (Eb). This modification not only enhances peroxymonosulfate adsorption at Cu active sites but also facilitates rapid electron transfer and promotes selective hydroxyl radical generation. Compared to CuSA/Obq-COF (Eb = 25.6 meV), CuSA/Ace-COF (Eb = 12.2 meV) shows a 39.5-fold increase in the pseudo-first-order rate constant for sulfamethoxazole degradation (0.434 min−1). This work provides insights into modulating excitonic behavior in single-atom catalysts via linker engineering for EOCs degradation.

Dimension-Dependent Nonlinear Optical Properties of Stanene Nanosheets.

Stanene (Sn)-based materials, with a graphene-like hexagonal structure, are now attracting considerable interest for potential applications in photoelectricity. However, the nonlinear optical properties of stanene remain largely unexplored. In this work, we prepared different sizes of stanene by liquid phase exfoliation and differential centrifugation methods, including two-dimensional (2D) Sn nanosheets (NSs) and zero-dimensional (0D) Sn nanodots (NDs). Z-scan measurements reveal that 2D Sn NSs exhibited high saturable absorption behavior, while the 0D Sn NDs led to reverse saturable absorption. Femtosecond ultrafast transient absorption spectra revealed that the reverse saturable absorption of Sn NDs is derived from a stronger excited state absorption and faster exciton relaxation dynamics. This research expands the potential applications of stanene in laser shielding and other ultrafast photonics technologies.

Ultrafast transient absorption spectra and kinetics of human blue cone visual pigment at room temperature

The ultrafast photochemical reaction mechanism, transient spectra, and transition kinetics of the human blue cone visual pigment have been recorded at room temperature. Ultrafast time-resolved absorption spectroscopy revealed the progressive formation and decay of several metastable photo-intermediates, corresponding to the Batho to Meta-II photo-intermediates previously observed with bovine rhodopsin and human green cone opsin, on the picosecond to millisecond timescales following pulsed excitation. The experimental data reveal several interesting similarities and differences between the photobleaching sequences of bovine rhodopsin, human green cone opsin, and human blue cone opsin. While Meta-II formation kinetics are comparable between bovine rhodopsin and blue cone opsin, the transition kinetics of earlier photo-intermediates and qualitative characteristics of the Meta-I to Meta-II transition are more similar for blue cone opsin and green cone opsin. Additionally, the blue cone photo-intermediate spectra exhibit a high degree of overlap with uniquely small spectral shifts. The observed variation in Meta-II formation kinetics between rod and cone visual pigments is explained based on key structural differences.

Optical regulation by localized states of Cu2+ ions with highly sensitive to trap density and thermalization in manganese-doped perovskite nanocrystals

Mn2+-doped perovskites nanocrystals (NCs) have already been extensively studied in fascinating optical, electronic, and magnetic properties. Such interesting traits make them paid considerable attention in light emitting diodes, solar cells, etc. However, the underlying thermal and photophysical processes governing the overall charge carrier dynamics in Mn-doped NCs are far from clear. Herein, trap concentration and thermalization-dependent optical properties of Mn–Cu co-doped CsPbCl3 NCs were investigated via using steady-state, time-resolved PL spectra, variable-temperature PL spectra, and ultrafast transient absorption spectra. The combined experimental and theoretical studies reveal that Cu2+, as an effective hole trap, can trap the holes from Mn2+ and emit the holes to Mn2+ level at lighter and higher doping, respectively. Moreover, this hole trap is highly thermally sensitive, which is responsible for the abnormal thermal effect of Mn2+ emission with increasing temperature. These findings demonstrate an approach of charge regulation through ion doping, which is meaningful for fabricating efficient photoelectric materials and devices.

Tracking the conical intersection dynamics for the photoinduced Jahn-Teller switch of a Mn(iii) complex.

Octahedral Mn(iii) ions predominantly exhibit an axially elongated Jahn-Teller (JT) distortion, which is responsible for their large uniaxial magnetic anisotropy. As a result, they are often used in the synthesis of single-molecule magnets (SMMs). Modulation of the JT distortion using femtosecond laser pulses could offer a route to controlling the magnetisation direction, and therefore is promising for the development of data storage devices that work on ultrafast timescales. Photoinduced switching of the distortion from an axially elongated to an axially compressed structure has been demonstrated for various Mn(iii) complexes. However, the dynamics around the region of the conical intersection for the photoinduced JT switch remains unclear. Here, ultrafast transient absorption spectra were recorded for solutions of tris(2,2,6,6-tetramethyl-3,5-heptanedionato)manganese(iii) (Mn(dpm)3) in ethanol to further explore the dynamics of the photoinduced JT switch. We observe the generation of a vibrational wavepacket on the excited state surface, which has a frequency of approximately 155 cm-1 and encompasses a JT-active vibrational mode. This coherent motion is maintained after passage through the conical intersection back to the ground state, which launches wavepackets along the ground state potential energy surface (PES) with frequencies of approximately 180 and 110 cm-1 that we assign to the elongated and compressed state, respectively. Inspection of the relative phases of the frequencies reveals phase shifts that are consistent with a one-mode reaction coordinate, and passes through the conical intersection at 1/4 and 3/4 of the excited state vibrational period. Our results provide direct insights into the non-adiabatic dynamics of Mn(iii) complexes, which can be used to guide the synthesis of optically controlled SMMs.

Ultrafast Transient Absorption Spectra and Kinetics of Rod and Cone Visual Pigments.

Rods and cones are the photoreceptor cells containing the visual pigment proteins that initiate visual phototransduction following the absorption of a photon. Photon absorption induces the photochemical transformation of a visual pigment, which results in the sequential formation of distinct photo-intermediate species on the femtosecond to millisecond timescales, whereupon a visual electrical signal is generated and transmitted to the brain. Time-resolved spectroscopic studies of the rod and cone photo-intermediaries enable the detailed understanding of initial events in vision, namely the key differences that underlie the functionally distinct scotopic (rod) and photopic (cone) visual systems. In this paper, we review our recent ultrafast (picoseconds to milliseconds) transient absorption studies of rod and cone visual pigments with a detailed comparison of the transient molecular spectra and kinetics of their respective photo-intermediaries. Key results include the characterization of the porphyropsin (carp fish rhodopsin) and human green-cone opsin photobleaching sequences, which show significant spectral and kinetic differences when compared against that of bovine rhodopsin. These results altogether reveal a rather strong interplay between the visual pigment structure and its corresponding photobleaching sequence, and relevant outstanding questions that will be further investigated through a forthcoming study of the human blue-cone visual pigment are discussed.

Trinuclear Re(I)-Coordinated Organic Cage as the Supramolecular Photocatalyst for Visible-Light-Driven CO2 Reduction.

Photocatalytic reduction of excess CO2 in the atmosphere to value-added chemicals by visible light can be an effective solution to fuel shortage and global warming. Considering these issues, we designed and successfully synthesized a trinuclear Re(I)-coordinated organic cage (Re-C4R) as the supramolecular photocatalyst. Photophysical, electrochemical properties, and photocatalytic performance comparison of Re-C4R and its mononuclear analogue Re-bpy are discussed in detail. Notably, the covalent linkage of three Re(I) subunits in Re-C4R leads to TONCO = 691 (per Re(I) site in 4 h) more than three times as much as TONCO = 208 of Re-bpy. Compared to Re-bpy, higher current enhancement in the control CV experiments under CO2 was observed for Re-C4R. CO2 adsorption process can be promoted because of the cryptand structure and multiple amine groups of Re-C4R. Moreover, decay lifetimes of Re-C4R are shorter than those of Re-bpy in the ultrafast transient absorption (TA) and photoluminescence (PL) decay spectra, indicating that the trinuclear cryptate structure of Re-C4R could facilitate electron transfer efficiency during CO2 reduction.

Isolating Pure Donor and Acceptor Signals by Polarization-Controlled Transient Absorption Spectroscopy.

The optical spectra of molecules are often highly congested, inhibiting definite assignment of features and dynamics. In this work, we demonstrate and apply a polarization-based strategy for the decomposition of time-resolved optical spectra to analyze the electronic structure and energy transfer in a molecular donor-acceptor (D-A) dyad. We choose a dyad with orthogonal transition dipole moments for D and A and high fluorescence quantum yield to show that polarization-controlled ultrafast transient absorption spectra can isolate the pure D and A parts of the total signal. This provides a strategy to greatly reduce spectral congestion in complex systems and thus allows for detailed studies of electronic structure and electronic energy transfer.

Equatorial restriction of the photoinduced Jahn-Teller switch in Mn(iii)-cyclam complexes.

Ultrafast transient absorption spectra were recorded for solutions of [MnIII(cyclam)(H2O)(OTf)][OTf]2 (cyclam = 1,4,8,11-tetraazacyclotetradecane and OTf = trifluoromethanesulfonate) in water to explore the possibility to restrict the equatorial expansion following photoexcitation of the dxy ← dz2 electronic transition, often resulting in a switch from axial to equatorial Jahn-Teller distortion in MnIII complexes. Strong oscillations were observed in the excited state absorption signal and were attributed to an excited state wavepacket. The structural rigidity of the cyclam ligand causes a complex reaction coordinate with frequencies of 333, 368, 454 and 517 cm-1, and a significantly shorter compressed-state lifetime compared to other MnIII complexes with less restricted equatorial ligands. Complementary density functional theory quantum chemistry calculations indicate a switch from an axially elongated to a compressed structure in the first excited quintet state Q1, which is accompanied by a modulation of the axial tilt angle. Computed harmonic frequencies for the axial stretching mode (∼379 cm-1) and the equatorial expansions (∼410 and 503 cm-1) of the Q1 state agree well with the observed coherences and indicate that the axial bond length contraction is significantly larger than the equatorial expansion, which implies a successful restriction of the wavepacket motion. The weak oscillation observed around 517 cm-1 is assigned to a see-saw motion of the axial tilt (predicted ∼610 cm-1). The results provide insights into the structural perturbations to the molecular evolution along excited state potential energy surfaces of MnIII octahedral complexes and can be used to guide the synthesis of optically controlled MnIII-based single-molecule magnets.

Hot-carrier tunable abnormal nonlinear absorption conversion in quasi-2D perovskite

Controlling the high-power laser transmittance is built on the diverse manipulation of multiple nonlinear absorption (NLA) processes in the nonlinear optical (NLO) materials. According to standard saturable absorption (SA) and reverse saturable absorption (RSA) model adapted for traditional semiconductor materials, the coexistence of SA and RSA will result in SA induced transparency at low laser intensity, yet switch to RSA with pump fluence increasing. Here, we observed, in contrast, an unusual RSA to SA conversion in quasi-two-dimensional (2D) perovskite film with a low threshold around 2.6 GW cm−2. With ultrafast transient absorption (TA) spectra measurement, such abnormal NLA is attributed to the competition between excitonic absorption enhancement and non-thermalized carrier induced bleaching. TA singularity from non-thermalized “Fermi Sea” is observed in quasi-2D perovskite film, indicating an ultrafast carrier thermalization within 100 fs. Moreover, the comparative study between the 2D and 3D perovskites uncovers the crucial role of hot-carrier effect to tune the NLA response. The ultrafast carrier cooling of quasi-2D perovskite is pointed out as an important factor to realize such abnormal NLA conversion process. These results provide fresh insights into the NLA mechanisms in low-dimensional perovskites, which may pave a promising way to diversify the NLO material applications.

Excited-state dynamics of all-trans protonated retinal Schiff base in CRABPII-based rhodopsin mimics

The rhodopsin mimic is a chemically synthetized complex with retinyl Schiff base (RSB) formed between protein and the retinal chromophore that can mimic the natural rhodopsin-like protein. The artificial rhodopsin mimic is more stable and designable than the natural protein and hence has wider uses in photon detection devices. The mimic structure RSB, like the case in the actual rhodopsin-like protein, undergoes isomerization and protonation throughout the photoreaction process. As a result, understanding the dynamics of the RSB in the photoreaction process is critical. In this study, the ultrafast transient absorption spectra of three mutants of the cellular retinoic acid-binding protein II-based rhodopsin mimic at acidic environment were recorded, from which the related excited-state dynamics of the all-trans protonated RSB (AT-PRSB) were investigated. The transient fluorescence spectra measurements are used to validate some of the dynamic features. We find that the excited-state dynamics of AT-PRSB in three mutants share a similar pattern that differs significantly from the dynamics of 15-cis PRSB of the rhodopsin mimic in neutral solution. By comparing the dynamics across the three mutants, we discovered that the aromatic residues near the β-ionone ring structure of the retinal may help stabilize the AT-PRSB and hence slow down its isomerization rate. The experimental results provide implications on designing a rhodopsin-like protein with significant infrared fluorescence, which can be particularly useful in the applications in biosensing or bioimaging in deeper tissues.

Enhancement of ultrafast nonlinear absorption by different substituted positions in pyrene-containing twistacenes isomers

End-substituted planar and twisted isomers (C1 and C2) of pyrene-containing twistacenes were designed and synthesized to investigate the modulation of ectopic substitution on nonlinear optical (NLO) properties. Results of ultrafast transient absorption spectra reveal that C1 exhibits broad-band excited state absorption (ESA), meanwhile C2 displays an additional electronic delocalized state due to charge transfer. The twisted structure caused by large steric hindrance in C2 is thought to be the cause of enhancement according to the density functional theory calculation. Moreover, the evolution of ESA regarding the transition from local excited (LE) to intramolecular charge transfer (ICT) state is observed in the transient spectra. The performance of reverse saturable absorption at 480 nm, 5 ns, is improved by ESA originating from ICT in C2. The extension of π-conjugated system results in a significant enhancement of the two-photon absorption (TPA) cross-section of C1/toluene from 235 to 737 GM at 532 nm. With different substitution positions of terminal group in isomers, TPA and ESA of these chromophores can be dramatically modulated. This work may provide an idea for the optimization of nonlinear functional isomeric materials.

Exciton‐Like and Mid‐Gap Absorption Dynamics of PtS in Resonant and Transparent Regions

AbstractTwo essential ranges of energy within and out of bandgap determine the global optical property of a general semiconductor. Here, exciton‐like inter‐sub‐band resonance absorption and optical bleaching of platinum sulfide (PtS) in the resonant region are found and investigated through the linear optical spectra and ultrafast transient absorption spectra. Besides, free‐carrier absorption and defect‐assisted Auger recombination in the transparent region are identified via the intensity‐ and temperature‐dependent pump–probe experiments. Good experimental and theorical agreements are obtained using the mid‐gap carrier‐capture model considering the complex optical conductivity based on the rate equations. This work facilitates further understanding of the ultrafast photoinduced carrier behavior in PtS, which provides the possibility for future applications in photoelectric devices.

Disentangling Light- and Temperature-Induced Thermal Effects in Colloidal Au Nanoparticles.

We present temperature-dependent (from room temperature to 80 °C) absorption spectra of Au/SiO2 core–shell nanoparticles (NPs) (core diameter: ∼25 nm) in water in the range from 1.5 to 4.5 eV, which spans the localized surface plasmon resonance (LSPR) and the interband transitions. A decrease in absorption with temperature over the entire spectral range is observed, which is more prominent at the LSPR. These changes are well reproduced by theoretical calculations of the absorption spectra, based on the experimentally measured temperature-dependent real (ε1) and imaginary (ε2) parts of the dielectric constant of Au NPs and of the surrounding medium. In addition, we model the photoinduced response of the NPs over the entire spectral range. The experimental and theoretical results of the thermal heating and the simulations of the photoinduced heating are compared with the ultrafast photoinduced transient absorption (TA) spectra upon excitation of the LSPR. These show that while the latter is a reliable monitor of heating of the NP and its environment, the interband region mildly responds to heating but predominantly to the population evolution of charge carriers.

Optimized Bridgman growth and quality improvement of LiInSe2 crystal by annealing in Li2Se vapor atmosphere

Mid-infrared (MIR) lasers based on nonlinear frequency conversion by nonlinear optical (NLO) crystals are urgently desired for various military and civilian applications. LiInSe2 (LISe) crystal has aroused intense attention due to its remarkable optical properties. However, the presence of point defects in LISe increases absorption loss and degrades the laser damage threshold (LDT), which limits the practical application in high-power MIR lasers. Therefore, it is essential to reveal the defect types and their formation mechanism in LISe, which is beneficial to design a rational strategy to improve the crystal quality. Here, large-sized LISe crystals with a diameter of 26 mm were successfully grown by the optimized Bridgman method using oriented seed crystal. Furthermore, three types of defects, VSe, VLi, and InLi in the as-grown LISe crystals were verified by photoluminescence (PL) spectra. Therefore, we designed the thermal annealing process under Li2Se vapor to compensate for the losses of Li and Se elements and reduce the defect concentration in LISe. Moreover, the evolution of defects in LISe crystals was determined by X-ray photoelectron spectroscopy (XPS), PL and ultrafast transient absorption spectra (TAS). All the experimental results confirm that the defect concentration in LISe greatly decreases and the crystal quality was obviously improved after annealing under Li2Se vapor. More importantly, this work provides a general strategy to reveal the defects in crystals and to improve the crystal quality of MIR chalcogenides for practical applications.

Photoinduced Jahn-Teller switch in Mn(III) terpyridine complexes.

Ultrafast transient absorption spectra were recorded for [Mn(terpy)X3], where X = Cl, F, and N3, to explore photoinduced switching from axial to equatorial Jahn-Teller (JT) distortion. Strong oscillations were observed in the transients, corresponding to a wavepacket on the excited-state potential energy surface with oscillation frequency around 115 cm-1 for all three complexes. Multireference quantum chemistry calculations indicate that the reaction coordinate is a pincer-like motion of the terpyridine ligand arising from bond length changes in the excited state due to the JT switch. We observed long dephasing times of the wavepacket, with times of 620 fs for [Mn(terpy)Cl3], 450 fs for [Mn(terpy)F3], and 370 fs for [Mn(terpy)(N3)3]. The dephasing time of these coherences decreases with an increasing number of vibrational modes at lower energy than the mode dominating the reaction coordinate, suggesting they act as an effective bath to dissipate the excess energy obtained from photoexcitation.

Water-Molecule-Induced Emission Transformation of Zero-Dimension Antimony-Based Metal Halide.

Low-dimensional organic-inorganic metal halides have recently emerged as a class of promising luminescent materials. However, the intrinsic toxicity of lead would strongly hamper future application. Herein, we synthesized a new type of lead-free zero-dimensional (0D) antimony-based organic-inorganic metal halide single crystals, (PPZ)2SbCl7·5H2O (PPZ = 1-phenylpiperazine), which features a broadband emission at 720 nm. Ultrafast transient absorption and temperature-dependent photoluminescence (PL) spectra are combined to investigate the PL mechanism, revealing that self-trapped exciton recombination was involved. Furthermore, it is interesting that (PPZ)2SbCl7·5H2O material shows reversible PL emission transformation between red light (720 nm) and yellow light (590 nm) as water molecules are inserted or removed from the lattice. Such reversible emission transformation phenomenon renders the (PPZ)2SbCl7·5H2O as a potential low-cost water sensing material.

Vibronic dynamics resolved by global and target analysis of ultrafast transient absorption spectra.

We present a methodology that provides a complete parametric description of the time evolution of the electronically and vibrationally excited states as detected by ultrafast transient absorption (TA). Differently from previous approaches, which started fitting the data after ≈100fs, no data are left out in our methodology, and the "coherent artifact" and the instrument response function are fully taken into account. In case studies, the method is applied to solvents, the dye Nile blue, and all-trans β-carotene in cyclohexane solution. The estimated Damped Oscillation Associated Spectra (DOAS) and phases express the most important vibrational frequencies present in the molecular system. By global fit alone of the experimental data, it is difficult to interpret in detail the underlying dynamics. Since it is unfeasible to directly fit the data by a theoretical simulation, our enhanced DOAS methodology thus provides a useful "middle ground" where the theoretical description and the fit of the experimental data can meet. β-carotene in cyclohexane was complementarily studied with femtosecond stimulated Raman spectroscopy (FSRS). The fs-ps dynamics of β-carotene in cyclohexane in TA and FSRS experiments can be described by a sequential scheme S2 → hot S1 → S1' → S1 → S0 with lifetimes of 167fs (fixed), 0.35, 1.1, and 9.6ps. The correspondence of DOAS decaying concomitantly with hot S1 and the Species Associated Difference Spectra of hot S1 in TA and FSRS suggest that we observe here features of the vibrational relaxation and nuclear reorganization responsible for the hot S1 to S1 transition.

2D Bismuthene Metal Electron Mediator Engineering Super Interfacial Charge Transfer for Efficient Photocatalytic Reduction of Carbon Dioxide.

The interfacial charge transfer still limits the photoactivity of artificial Z-scheme photocatalysts although they showed complementary light absorption and a strong photoredox ability. In this study, layered metallene is designed as an efficient electron mediator for constructing a C3N4/bismuthene/BiOCl 2D/2D/2D Z-scheme system. This bismuthene serves as a bridge processing superior charge conductibility, abundant metal-semiconductor contact sites, and the shortened charge diffusion distance, enhancing the photocatalytic CO2 reduction reaction activity and stability. Density functional theory calculations show that the bismuthene creates a built-in electric field and congregates interfacial electrons, which is confirmed by the stable and consistent emission of the ultrafast transient absorption spectra. This work gives new insight into the interface design of Z-scheme photocatalysts by selecting a novel metallene electron mediator.

Room-temperature quaternary alkylammonium passivation toward morphology-controllable CsPbBr3 nanocrystals with excellent luminescence and stability for white LEDs

Colloidal all-inorganic CsPbX3 (X = Cl, Br, I) perovskite nanocrystals (PNCs) have recently shown incomparable superiority and vast application prospects in optoelectronics. However, developing low-cost and high-quality PNCs is the priority on their way to commercialization. Herein, we proposed a room-temperature quaternary alkylammonium (didodecyldimethylammonium bromide, DDAB) passivation strategy without organic polar solvents for synthesizing CsPbBr3 PNCs with controllable morphology via a modified ligand-assisted reprecipitation method. The morphology and size of PNCs were strongly affected by the amount of surface ligands (DDAB and organic acids with different chain lengths), resulting in the shift of emission peak. Benefiting from tight binding of surface ligands, the resulting CsPbBr3 PNCs exhibited bright green emission with near-unit photoluminescence quantum yields (PLQY) and excellent stability against water and air. The ultrafast transient absorption and time-resolved photoluminescence spectra further revealed that the lifetime of trap-assisted non-radiative recombination in PNCs was especially short, which could be attributed to the efficient passivation of surface defect states. Furthermore, light-emitting diodes (LEDs) based on green PNCs showed bright white light emission, and its corresponding color coordinate (0.330, 0.329) was almost consistent with that of the standard white light, which indicated that the as-prepared PNCs had latent practical value in lighting and display technology.