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Doping and alloying induce defect states in atomically thin transition metal dichalcogenides (TMDCs), leading to strong carrier–phonon interactions. The robust excitonic behavior of these layered materials can be modified by injecting a high density of charge carriers. However, comprehending the influence of carrier–phonon and carrier–carrier interactions on the optical properties of 2D materials is crucial for their optoelectronic and photonic applications. Here, transient absorption (TA) spectroscopy is employed to demonstrate the modulation of the transient optical behavior of TMDCs through doping and excitation near Mott density. The TA spectra reveal broadening attributed to carrier–carrier and carrier–phonon interactions, with the broadening being particularly pronounced in vanadium (V) doped TMDCs due to the hybridization of defect and exciton transitions. Analysis of TA kinetics suggests the involvement of various carrier species in the carrier dynamics of TMDCs, with the influence of mid‐gap carriers dominating at higher excitation densities. Nonetheless, the presence of strong carrier–phonon coupling in V‐doped TMDCs is demonstrated by temperature‐dependent Raman and photoluminescence spectroscopy. The results reveal that the enhanced coupling between acoustic phonons and carriers can lead to multiphonon emission. The findings of this study hold promise for controlling the optical response of TMDCs in ultrafast optoelectronic applications. [ABSTRACT FROM AUTHOR]

Four organometallic complexes featuring identical conjugated diimine ligands and varying transition metal centers (Rh(III), Ir(III), Ru(II), and Os(II)) were synthesized. A comprehensive investigation of the photophysical properties of these complexes and their corresponding diimine ligands was conducted by UV‐vis absorption, emission, and transient absorption (TA) spectroscopy, as well as optical power limiting (OPL) measurements. The incorporation of fluorene components with alkyl chains enhanced the π‐conjugation of the ligand, prevented intermolecular π‐π stacking, and improved complex solubility. The diimine ligands showed a slight solvation effect in their emission spectra, indicating that the main property of their emissive state was intramolecular charge transfer (CT). According to the valence states of the four complexes, they can be divided into two classes (Rh(III), Ir(III)) and (Ru(II), Os(II)). For TA spectrum, the former group exhibited positive ΔOD values across the 400~800 nm range, while the latter showed large inverted peaks at 420~500 nm due to significant ground‐state absorption in this wavelength range. Additionally, the optical limiting conduct of the complexes observed the order: 1 d>1 a>1 c>1 b. Complex 1 d exhibits powerful reverse saturable absorption (RSA) properties at 532 nm and could probably be used as an wonderful nonlinear absorbing material. [ABSTRACT FROM AUTHOR]

2D lead halide perovskites (LHPs) exhibit outstanding optoelectronic properties, making them utilized in various emerging applications. Understanding their fundamental properties is urgent for improving device performance. Here, the structural domains in 2D PEA2MAn‐1PbnI3n+1 films are studied by temperature‐dependent transient absorption (TA) measurements. For = 1 film at low temperatures, the ground state bleach (GSB) shows obvious splitting when the high‐energy state is resonantly excited, whereas only one GSB exists under low‐energy resonant excitation, indicating that the two split energy states correspond to different structural domains. For = 2 film, similar phenomena are observed, but the energy level difference between the two domains is decreased. With further increase of the inorganic layers number, the two domains can no longer be distinguished. In addition, by changing the organic cations, it is demonstrated that the two structural domains originate from distortions in the inorganic PbI6 octahedral frames. Finally, the possibility that the two energy states are from the formation of polaron states is ruled out by TA measurement on CsPbBr3 QDs. The results provide new insights into the structural domain properties of 2D LHPs, which directly influence the suitability of these materials for future optoelectronic devices. [ABSTRACT FROM AUTHOR]

Nonlinear optical characteristics of chemically prepared silver selenide (Ag2Se, 3.7 nm) and lead selenide (PbSe, 6.3 nm) colloidal quantum dots (QDs) are investigated using the 1030 nm femtosecond pulses. Ag2Se QDs demonstrated two-photon absorption followed by saturable absorption. PbSe QDs initially showed the saturable absorption at small intensities of 1030 nm probe pulses, followed by reverse saturable absorption at stronger excitation. The difference in the positive nonlinear optical absorption mechanisms in these two QD colloids is attributed to their different bandgaps. The studied QD colloids exhibited the Kerr-induced positive nonlinear refraction. Transient absorption studies of the QDs showed that photoexcitation creates a hot exciton that forms band-edge exciton within 1.2 ps after excitation. Most of the electron-hole pairs further recombine through band-to-band recombination with a time constant of 1490 ps. Furthermore, the results from the transient absorption studies indicate the contribution of trap-assisted recombination occurs on a time scale exceeding that of the experimental time window. [ABSTRACT FROM AUTHOR]

While the photophysics of closed-shell organic molecules is well established, much less is known about open-shell systems containing interacting radical pairs. In this work, we investigate the ultrafast excited state dynamics of a singlet verdazyl diradical system in solution using transient absorption (TA) spectroscopy for the first time. Following 510 nm excitation of the excitonic S0 → S1 transition, we detected TA signals in the 530–950 nm region from the S1 population that decayed exponentially within a few picoseconds to form a vibrationally hot S0* population via internal conversion. The dependence of the S1 decay rate on solvent and radical–radical distance revealed that the excited state possesses charge-transfer character and likely accesses the S0 state via torsional motion. The ultrafast internal conversion decay mechanism at play in our open-shell verdazyl diradicals is in stark contrast with other closed-shell, carbonyl-containing organic chromophores, which exhibit ultrafast intersystem crossing to produce long-lived triplet states as the major S1 decay pathway. [ABSTRACT FROM AUTHOR]

A novel hybrid nanocarbon consisting of one rippled and two planar nanosheets has been synthesized and characterized. Two meso pairs of [5]helicenes are formed along the long molecular axis of the hybrid to connect rippled and planar subunits, leading to a stable conformation. It is shown that the hybrid possesses the individual electronic properties of the rippled and planar subunits. Compared to the rippled subunit, such hybrid has a better geometric match to C60 and complexes with C60 in a 1 : 1 stoichiometry, with the association constant on the order of 105 M−1. The hybrid displays unusual anti‐Kasha fluorescence emission. The transient absorption spectroscopy revealed that the singlet fission (SF) from higher level singlet excited states (Sn) is operative in the film of the hybrid. [ABSTRACT FROM AUTHOR]

Quasi‐2D lead‐halide perovskites consist of conducting inorganic layers with tunable thickness (n) separated by large organic spacer cations. Typically, domains with different n and bandgaps are formed within a single film. Here, the crystallization of the films is tuned by mixing Dion‐Jacobson (DJ) with Ruddlesden‐Popper (RP) spacer cations. Compared to the quasi‐2D perovskite film based on solely the DJ type spacer 1,4‐phenylenedimethylammonium (PDMA), a film with less defects and more vertically aligned crystallization is achieved by addition of the RP type spacer propylammonium (PA). As the film structure plays an important role in the photophysics, time‐resolved photoluminescence (TRPL) and femtosecond transient absorption (TA) are used to investigate the impact of mixing these spacer cations on the dynamics of hot carrier cooling, the occurrence and directionality of energy or electron transfer between the different domains, and the exciton and charge carrier dynamics. Exciton transfer from low‐n to high‐n domains occurs at a favorable faster rate for the PDMA‐based film (0.0640 ps−1) compared to the PA‐based film (0.0365 ps−1), while the mixed spacer film demonstrates intermediate behavior (0.0473 ps−1). This study facilitates the design of advanced materials with optimized photophysical characteristics for a next generation of optoelectronic devices. [ABSTRACT FROM AUTHOR]

Visible‐light excitation of a family of bimetallic ruthenium polypyridines with the formula [RuII(tpy)(bpy)(‐CN)RuII(py)4L]n+ (RuRuLn+), where L=Cl−, NCS−, DMAP and ACN, was used to prepare photoinduced mixed‐valence (PI‐MV) MLCT states as models of the photosynthetic reaction center. Ultrafast transient absorption spectroscopy allowed to monitor photoinduced IVCT bands between 6000 and 11000 cm−1. Mulliken spin densities resulting from DFT and (TD)DFT computations revealed the modulation of the charge density distribution depending on the ligand substitution pattern. Results are consistent with PI‐MV systems ranging from non‐degenerate Class II to degenerate Class III or II/III, with electronic couplings between 1000 and 3500 cm−1. These findings guide the control electron localization‐delocalization in charge‐transfer/charge‐separated excited states, like those involved in the photosynthetic reaction center. [ABSTRACT FROM AUTHOR]

Hydrogen fuel produced from water splitting using sunlight remains one of the cleanest and most sustainable energy sources. However, photoelectrochemical hydrogen production faces several challenges, including poor sunlight absorption, deficient charge carrier lifetime and transfer rates, and very sluggish kinetics of the water oxidation half‐reaction. To address these challenges, researchers have concentrated on enhancing the efficiency of photoelectrochemical water splitting. A critical factor in this enhancement is a deeper understanding of the fundamental processes involved in photoabsorption and the subsequent oxidation evolution reaction. The advent of ultrafast lasers has enabled the detailed tracking of charge carriers within materials after sunlight absorption, including their separation and migration to the surface for water oxidation. Ultrafast transient absorption spectroscopy is a powerful technique that allows real‐time observation of the behavior of excited states and charge carriers on femtosecond to nanosecond timescales. Recent studies utilizing ultrafast transient absorption spectroscopy are explored to investigate the dynamics of charge carriers in photoelectrochemical water splitting, providing insights into the mechanisms that lead to the design of enhanced photoanodes with improved efficiency. [ABSTRACT FROM AUTHOR]

Exciton diffusion and carrier transport are two critical factors that determine the efficiency of organic photovoltaics (OPVs). However, the relationship between these two factors has not been extensively studied. Designing non‐fullerene acceptors (NFAs) with efficient diffusion coefficients and high electronic transmittance is a key area of focus. In this study, materials for bulk‐heterojunction (BHJ) and quasiplanar‐heterojunction (Q‐PHJ) devices are synthesized to validate the desired differences in crystallinity. The single crystal of BOBO4Cl‐βδ demonstrated the most compact packing structure, with an improved planar configuration and closer π···π distances, resulting in higher electron mobility and superior exciton diffusion coefficient. Consequently, BOBO4Cl‐βδ‐based devices achieved a power conversion efficiency (PCE) of 17.38% in Q‐PHJ, compared to a lower PCE of 14.75% in BHJ devices. Furthermore, incorporating BOBO4Cl‐βδ into the D18/L8‐BO Q‐PHJ system increased the PCE from 17.98% to 18.81%, one of the highest values recorded for Q‐PHJ devices. This improvement is attributed to strong crystallinity of BOBO4Cl‐βδ, which enhances the packing arrangement and improves the exciton diffusion coefficient. Our work highlights the importance of molecular design with tunable exciton diffusion and carrier transport for BHJ and Q‐PHJ OPV architectures and reveals the relationship between them, which contributes to the achievement of high‐performance NFAs. [ABSTRACT FROM AUTHOR]

While the photophysics of closed-shell organic molecules is well established, much less is known about open-shell systems containing interacting radical pairs. In this work, we investigate the ultrafast excited state dynamics of a singlet verdazyl diradical system in solution using transient absorption (TA) spectroscopy for the first time. Following 510 nm excitation of the excitonic S0 → S1 transition, we detected TA signals in the 530–950 nm region from the S1 population that decayed exponentially within a few picoseconds to form a vibrationally hot S0* population via internal conversion. The dependence of the S1 decay rate on solvent and radical–radical distance revealed that the excited state possesses charge-transfer character and likely accesses the S0 state via torsional motion. The ultrafast internal conversion decay mechanism at play in our open-shell verdazyl diradicals is in stark contrast with other closed-shell, carbonyl-containing organic chromophores, which exhibit ultrafast intersystem crossing to produce long-lived triplet states as the major S1 decay pathway.

The mechanistic details of a reported allylation reaction are investigated by means of Stern–Volmer experiments and nanosecond transient absorption spectroscopy. Both reference substrates, i.e., an allylic chloride and a trifluoroacetamide, are inefficient quenchers but large quenching rate constants are observed upon the addition of Ni(COD)2 and a bisoxazoline ligand. The large quenching rate constants and absence of observable photoproducts are consistent with a mechanism that operates by energy transfer between the excited-state iridium photosensitizer and the nickel complex. [ABSTRACT FROM AUTHOR]

A novel series of homoconjugated oligomers of azapentacene, linked via the rigid connecting unit bicyclo[2.2.2]octane bridge is presented. The investigation delves into intramolecular singlet fission (iSF) within these dimeric and trimeric frameworks, employing various time‐resolved spectroscopies and quantum chemical calculations. Combining the merits of both through‐space and through‐bond couplings between the constituent chromophores within these rigid homoconjugated structures, iSF proceeds within a few picoseconds, exhibiting a 2–3 orders of magnitude acceleration compared to traditional covalently linked oligomers and achieving an iSF efficiency approaching unity. In the homoconjugated trimer, the larger interplanar angle between neighboring chromophores leads to a relatively slower iSF rate compared to that of the homoconjugated dimer. Furthermore, the active involvement of the charge transfer state is demonstrated to accelerate the iSF process in the trimer, while exerting no influence on the iSF process in the dimer. The study provides novel avenues for investigating the influence of molecular geometry on singlet fission and reveals the crucial role of the interplay between spatial and covalent coupling in facilitating intramolecular singlet fission. [ABSTRACT FROM AUTHOR]

Organic–inorganic hybrid halide perovskites have emerged as the most promising optoelectronic materials for next‐generation solution‐processed devices, primarily attributed to their distinctive properties such as cost‐effectiveness, facile fabrication process, high light absorption coefficient, tunable bandgap, and relatively high charge carrier mobility. Nevertheless, the presence of Pb elements is inevitable in most high‐performance perovskite optoelectronic devices, posing environmental pollution concerns and impeding their commercialization. In this study, two novel non‐toxic bismuth‐ and antimony‐based perovskite materials with environmentally conscious approaches are developed. Their structures and optoelectronic properties have been systematically characterized. Subsequently, photoconductive‐ and photodiode‐type photodetectors are designed and fabricated. Notably, the photodiode configuration exhibits exceptionally low dark current and noise, superior detection sensitivity, fast response speed, and good device stability. Owing to these superior performance metrics, the devices demonstrate great potential for real applications. Furthermore, this study furnishes a theoretical framework and design perspectives for the solution‐processed semiconductor materials. [ABSTRACT FROM AUTHOR]

Singlet fission (SF), as an effective way to break through the Shockley–Queisser limit, can dramatically improve energy conversion efficiency in solar cell areas. The formation, separation, and relaxation of triplet-pair excitons directly affect the triplet yield, especially triplet-pair separation; thus, how to enhance the triplet-pair separation rate becomes one of the key points to improve SF efficiency; the decay mechanism where the singlet state is converted into two triplet states is significant for the study of the SF mechanism. Herein, we employ ultrafast transient absorption spectroscopy to study the singlet-fission process of nano-amorphous 6, 13-bis(triisopropylsilylethynyl)-Pentacene (TIPS-pentacene) films in a diamond anvil cell (DAC). A kinetics model related to the structural geometric details, as well as an evaluation of the pressure manipulation impacts, is demonstrated based on the experimental results. The results indicate that pressure manipulation enhanced the triplet-pair separation rates of SF-based materials according to their structural micro-environmental improvement when compressed in DAC, while the triplet-exciton transportation lifetime is prolonged. This work shows that pressure may effectively optimize the structural disorder of SF materials, which were found to improve triplet-pair separation efficiency and potentially offer an effective way to further improve SF efficiency. [ABSTRACT FROM AUTHOR]

Being a constitutional isomer of the indigo dye, indirubin has been known as a purple colourant for a long time. On the other hand, its structural and photophysical properties as ligand in metal complexes are virtually unexplored. Herein, for the first time, we utilized indirubin as ancillary chelator to develop two new heteroleptic iridium(III) complexes equipped with two 2‐phenylpyridine units as archetypical cyclometalating ligands. These new complexes display fully reversible oxidation and reduction processes, and show a panchromatic absorption extending up to 900 nm in acetonitrile solution at 298 K. Moreover, an unexpected broad and unstructured emission band is detected for both complexes in the near‐infrared region, peaking at approx. 1100 nm and having a lifetime of approx. 60 ps; such an emission is attributed to a ligand‐centred triplet excited state (3LC) located on the indirubin ligand itself, as proved by DFT calculations. These findings may pave the way for further exploration of the indirubin dye as a ligand for different types of metal centres to create efficient near‐infrared triplet emitters without the need for difficult synthetic procedures. [ABSTRACT FROM AUTHOR]

We have synthesized two boron complexes based on N,N‐type bidentate imidazo[1,5‐a]pyridine ligands exhibiting blue emission with good quantum yields in solution. The transient absorption and theoretical studies of the boron complexes indicated the intramolecular charge transfer nature. In addition, one specific complex, 1 a was found to work as a mitochondrial imaging agent. [ABSTRACT FROM AUTHOR]

Diaryliodonium salts (Ar2I+X−) are used as a photosensitive initiator that generates acid or radical species by ultraviolet light irradiation. Recently, sensitization of Ar2I+X− has gained importance owing to the escalating demand for high‐sensitive initiators with longer wavelength absorption such as 365 and 436 nm. However, the mechanism of photolysis of Ar2I+X− has not been strictly elucidated. This paper shows discussions of the details of its mechanism. Herein, we analyzed the photosensitization of Ar2I+X− with 2–isopropylthioxanthone (ITX) based on transient absorption techniques. As a result, it was revealed that electron transfer occurred from a triplet excited state of ITX to Ar2I+X− with an electron transfer rate constant of 4.2 × 109 s−1. Furthermore, high performance liquid chromatography measurements found the quantum yield of the photolysis was determined to be 0.48. [ABSTRACT FROM AUTHOR]

The bimetallic nanostructures that mix a plasmonic metal with a transition metal in the form of the core-shell nanoparticles are promising to promote catalytic performance. But it is still unclear how the heat (hot electrons and phonons) transfers on the interface between two metals. We have designed and synthesized Au@Cu bimetallic nanoparticles with Au as core and Cu as shell. By using transient absorption spectroscopy, we find that there are two plasmon induced heat funneling processes from Au core to Cu shell. One is the electron temperature equilibrium (electron heat transfer) with equilibration time of ~560 fs. The other is the lattice temperature equilibrium (lattice heat transfer) with equilibration time of ~13 ps. This plasmon induced heat funneling may be universal in similar bimetallic nanostructures, so our finding could contribute to further understanding the catalytic mechanism of bimetallic plasmonic photothermal catalysis. [ABSTRACT FROM AUTHOR]

Porphyrin‐based metal‐organic frameworks (MOFs) have received great attention for solar photochemistry applications. Manipulating the direction of energy (exciton) migration plays an essential role in boosting the light‐harvesting efficiency of porphyrin‐based MOFs. Here, based on the time‐dependent exciton‐exciton annihilation rate, an optical control of directional exciton migration is demonstrated in cobalt‐porphyrin surface‐supported MOF nanofilms (Co‐TCPP SURMOFs). Utilizing photocarrier densities or lattice temperature as the control knob, the dynamic form of exciton migration in Co‐TCPP SURMOFs can be switched between the nearest‐neighbor 1D interaction and the quasi‐isotropic 2D interaction, which is well explained using a diffusion model of exciton‐exciton scattering. This results provide an optical approach to regulating exciton migration in MOFs, which enhances understanding of the excitonic behavior in artificial light harvesters. [ABSTRACT FROM AUTHOR]

As excellent functional materials, the composite materials based on graphene and its derivatives (GNDs) have lots of important application values in the emerging fields such as solar energy conversion, and the exploration of electron (or energy) transfer properties of those composites is the key for revealing their further applications. In this review, femtosecond transient absorption spectroscopy (TAS) has been introduced as an essential technique to understand the carrier behaviors in GNDs composites, as well as the advancements of TAS. The specific examples of electron transport in various composite materials characterized by TAS are summarized and discussed, which consist of GNDs with semiconductor quantum dots (QDs), organic dyes, organic polymers, perovskites and other materials, respectively. This review provides a deep insight into the electron transfer (ET) kinetics of various GNDs composites from the perspective of TAS. [ABSTRACT FROM AUTHOR]

Significance: The emergence of label-free microscopy techniques has significantly improved our ability to precisely characterize biochemical targets, enabling noninvasive visualization of cellular organelles and tissue organization. However, understanding each label-free method with respect to the specific benefits, drawbacks, and varied sensitivities under measurement conditions across different types of specimens remains a challenge. Aim: We link all of these disparate label-free optical interactions together and compare the detection sensitivity within the framework of statistical estimation theory. Approach: To achieve this goal, we introduce a comprehensive unified framework for evaluating the bounds for signal detection with label-free microscopy methods, including second-harmonic generation, third-harmonic generation, coherent anti-Stokes Raman scattering, coherent Stokes Raman scattering, stimulated Raman loss, stimulated Raman gain, stimulated emission, impulsive stimulated Raman scattering, transient absorption, and photothermal effect. A general model for signal generation induced by optical scattering is developed. Results: Based on this model, the information obtained is quantitatively analyzed using Fisher information, and the fundamental constraints on estimation precision are evaluated through the Cramér–Rao lower bound, offering guidance for optimal experimental design and interpretation. Conclusions: We provide valuable insights for researchers seeking to leverage label-free techniques for non-invasive imaging applications for biomedical research and clinical practice. [ABSTRACT FROM AUTHOR]

Diblock conjugated oligomers are π-conjugated molecules that contain two segments having distinct frontier orbital energies and HOMO-LUMO gap offsets. These oligomers are of fundamental interest to understand how the distinct π-conjugated segments interact and modify their excited state properties. The current paper reports a study of two series of diblock oligomers that contain oligothiophene (Tn) and 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (TBT) segments that are coupled by either ethynyl (-C≡C-) or trans-(-C≡C-)2Pt(II)(PBu3)2 acetylide linkers. In these structures, the Tn segment is electron rich (donor), and the TBT is electron poor (acceptor). The diblock oligomers are characterized by steady-state and time-resolved spectroscopy, including UV-visible absorption, fluorescence, fluorescence lifetimes, and ultrafast transient absorption spectroscopy. Studies are compared in several solvents of different polarity and with different excitation wavelengths. The results reveal that the (-C≡C-) linked oligomers feature a delocalized excited state that takes on a charge transfer (CT) character in more polar media. In the (-C≡C-)2Pt(II)(PBu3)2-linked oligomers, there is weak coupling between the Tn and TBT segments. Consequently, short wavelength excitation selectively excites the Tn segment, which then undergoes ultrafast energy transfer (~1 ps) to afford a TBT-localized excited state. [ABSTRACT FROM AUTHOR]

To understand plasmon-induced charge-transfer mechanisms between a photo-excited gold (Au) nanoparticle and a TiO2 nanoparticle, a Monte Carlo random walk (MCRW) simulation was applied to reproduce the charge recombination kinetics in the nanocrystalline (Au/TiO 2) assemblies reported previously based on transient absorption spectroscopy. The Au/TiO2 assemblies consist of a confined electron diffusion space within a tiny TiO2 nanoparticle, making it possible to study electron diffusion transport through MCRW simulation. In this simulation algorithm, the electron diffusion starts at the coordinate origin of a rectangle, and the next direction of movement is obtained by calculating the coordinate matrix and random offset so that the electron is reflected on three boundaries and absorbed when it reaches the other boundary. By simulation programming, the histogram which indicates the occurrence frequency of the step accumulation number up to the right boundary was obtained. From 100 to 100000 steps under condition of 10000 iteration, that is, changing the steps but keeping the iteration times to ensure that all particles experience absorption in the simulation. Comparing the trace of 106 particles position with that of 104 under 1000 simulations, the electron density was found to saturate other than the region near the right boundary, where electrons disappear by the absorption process during the electron diffusion process. Finally, by fitting curves, it is confirmed that the tendency of the simulated response reproduced the transient absorption kinetics. [ABSTRACT FROM AUTHOR]

Manipulating and optimizing the pathway of the interfacial and band engineering for MoS2 with excellent nonlinear absorption and dynamics are very important for two-dimensional semiconductor optoelectronic device technology in the future. However, the application of MoS2 in many optical devices has been seriously limited by the electron transfer and carrier regulation. Herein, designing Ca2O3 combined with MoS2 was used to solve the problems of the narrow absorption band range and low efficient photoelectric transfer of the MoS2 material. Ga2O3/MoS2 (GMS) nanoheterostructures with a type I energy band arrangement were prepared by a two-step hydrothermal/magnetron sputtering method. The dependence of experimental parameters on the structural characteristics of samples is discussed. The analysis of Raman and X-ray photoelectron spectroscopy (XPS) indicated the electron transfer between MoS2 and Ga2O3. The results of X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) characterization further indicate that the VB edge and CB edge of MoS2 are located within the band gap of Ga2O3, forming an I-type GMS nanoheterostructure. Photoluminescence and ultraviolet absorption results show that the transfer of electrons from Ga2O3 to MoS2 enhances the electron–hole recombination of MoS2. The Z-scan results show that the nonlinear absorption of GMS nanoheterostructures changes from saturable absorption to reverse saturable absorption. The nonlinear absorption properties of GMS nanoheterostructures could be attributed to the competition between the ground- and excited-state absorption. Transient absorption measurements confirmed that the ultrafast, fast, and slow carrier processes of GMS nanoheterostructures were attributed to the redistribution of electrons in the excited state and the bottom of the conduction band, the relaxation of electrons from the excited state to the defect state, and the three processes of interband relaxation, respectively. The research has some repercussions for MoS2 and Ga2O3 nanoheterostructures with rich band gap and interfacial charge transfer engineering and obtaining highly efficient charge transfer photoelectric functional materials, which are promising materials for improving nonlinear optic properties in photonic devices and optoelectronic applications. [ABSTRACT FROM AUTHOR]

Abstract Transient absorption, or pump–probe microscopy is an absorption-based technique that can explore samples ultrafast dynamic properties and provide fluorescence-free contrast mechanisms. When applied to graphene and its derivatives, this technique exploits the graphene transient response caused by the ultrafast interband transition as the imaging contrast mechanism. The saturation of this transition is fundamental to allow for super-resolution optical far-field imaging, following the reversible saturable optical fluorescence transitions (RESOLFT) concept, although not involving fluorescence. With this aim, we propose a model to numerically compute the temporal evolution under saturation conditions of the single-layer graphene molecular states, which are involved in the transient absorption. Exploiting an algorithm based on the fourth order Runge–Kutta (RK4) method, and the density matrix approach, we numerically demonstrate that the transient absorption signal of single-layer graphene varies linearly as a function of excitation intensity until it reaches saturation. We experimentally verify this model using a custom pump–probe super-resolution microscope. The results define the intensities necessary to achieve super-resolution in a pump–probe nanoscope while studying graphene-based materials and open the possibility of predicting such a saturation process in other light-matter interactions that undergo the same transition.

Cumulenes are linear molecules consisting of consecutive double bonds linking chains of sp-hybridized carbon atoms. They have primarily been of interest for potential use as molecular wires or in other nanoscale electronic devices, but more recently, other applications such as catalysis or even light harvesting through singlet fission have been speculated. Despite the recent theoretical and experimental interest, the photoexcitation of cumulenes typically results in quenching on the picosecond timescale, and the exact quenching mechanism for even the simplest of [3]-cumulenes lacks a clear explanation. In this report, we perform transient absorption spectroscopy on a set of model [3]-cumulene derivatives in a wide range of environmental conditions to demonstrate that the planarization of phenyl groups ultimately quenches the excited state. By restricting this intramolecular motion, we increase the excited state lifetime by a few nanoseconds, strongly enhancing photoluminescence and demonstrating an approach to stabilize them for photochemical applications.

We establish a general kinetic scheme for the energy transfer and radical-pair dynamics in photosystem I (PSI) of Chlamydomonas reinhardtii, Synechocystis PCC6803, Thermosynechococcus elongatus and Spirulina platensis grown under white-light conditions. With the help of simultaneous target analysis of transient-absorption data sets measured with two selective excitations, we resolved the spectral and kinetic properties of the different species present in PSI. WL-PSI can be described as a Bulk Chl a in equilibrium with a higher-energy Chl a, one or two Red Chl a and a reaction-center compartment (WL-RC). Three radical pairs (RPs) have been resolved with very similar properties in the four model organisms. The charge separation is virtually irreversible with a rate of ≈900 ns−1. The second rate, of RP1 → RP2, ranges from 70–90 ns−1 and the third rate, of RP2 → RP3, is ≈30 ns−1. Since RP1 and the Red Chl a are simultaneously present, resolving the RP1 properties is challenging. In Chlamydomonas reinhardtii, the excited WL-RC and Bulk Chl a compartments equilibrate with a lifetime of ≈0.28 ps, whereas the Red and the Bulk Chl a compartments equilibrate with a lifetime of ≈2.65 ps. We present a description of the thermodynamic properties of the model organisms at room temperature. [ABSTRACT FROM AUTHOR]

Oxidized molecular states are key intermediates in photo‐induced redox reactions, e. g. intermolecular charge transfer between photosensitizer and catalyst in photoredox catalysis. The stability and longevity of the oxidized photosensitizer is an important factor in optimizing the respective light‐driven reaction pathways. In this work the oxidized states of ruthenium(II)‐4H‐imidazole dyes are studied. The ruthenium complexes constitute benchmark photosensitizers in solar energy interconversion processes with exceptional chemical stability, strong visible light absorption, and favourable redox properties. To rationalize the light‐induced reaction in the oxidized ruthenium(III) systems, we combine UV‐vis absorption, resonance Raman, and transient absorption spectroelectrochemistry (SEC) with time‐dependent density functional theory (TDDFT) calculations. Three complexes are compared, which vary with respect to their coordination environment, i. e. combining an 4H‐imidazole with either 2,2'‐bipyridine (bpy) or 2,2′;6′2"‐terpyridine (tpy) coligands, and chloride or isothiocyanate ligands. While all oxidized complexes have similar steady state absorption properties, their excited state kinetics differ significantly; the study thus opens the doorway to study the light‐driven reactivity of oxidized molecular intermediates in intermolecular charge transfer cascades. [ABSTRACT FROM AUTHOR]

In the context of developing transparent near‐IR absorbing dye‐sensitized solar cells, diketopyrrolopyrrole (DPP) cyanine dyes have recently emerged as an alternative to strongly aggregating linear cyanines. In our efforts to increase both the power conversion efficiency (PCE) and the average visible transmittance (AVT), a thienylated version, called TB202, that shows a red‐shifted absorption with respect to our champion dye TB207 was designed. However, the lower energy LUMO level of TB202 brings along a lower driving force (−ΔG) for carrier injection, which we recently identified as the main parameter limiting the PCE to 1.5 % in the best device conditions. In the present paper, we publish a detailed account of the effect of the de‐aggregating cheno‐deoxycholic acid (CDCA) for both TB207 and TB202. Both transient absorption (TAS) and fluorescence up‐conversion (FLUPS) data are presented, which allow to quantitively compare the effect of −ΔG and the CDCA concentration, in terms of the kinetic competition of ensemble averaged carrier injection and monomer‐to‐aggregate energy transfer (ET) rates. A comprehensive picture emerges on how ET is reduced by higher CDCA concentrations, leading in the best device conditions to injection efficiencies in the range of 65 % for TB207 and only 35 % for TB202. [ABSTRACT FROM AUTHOR]

Cumulenes are linear molecules consisting of consecutive double bonds linking chains of sp-hybridized carbon atoms. They have primarily been of interest for potential use as molecular wires or in other nanoscale electronic devices, but more recently, other applications such as catalysis or even light harvesting through singlet fission have been speculated. Despite the recent theoretical and experimental interest, the photoexcitation of cumulenes typically results in quenching on the picosecond timescale, and the exact quenching mechanism for even the simplest of [3]-cumulenes lacks a clear explanation. In this report, we perform transient absorption spectroscopy on a set of model [3]-cumulene derivatives in a wide range of environmental conditions to demonstrate that the planarization of phenyl groups ultimately quenches the excited state. By restricting this intramolecular motion, we increase the excited state lifetime by a few nanoseconds, strongly enhancing photoluminescence and demonstrating an approach to stabilize them for photochemical applications. [ABSTRACT FROM AUTHOR]

Triarylhydrazones represent an attractive class of photochromic compounds offering many interesting features including high molar absorptivity, good addressability, and extraordinary thermal stability. In addition, unlike most other hydrazone‐based photoswitches, they effectively absorb light above 365 nm. However, previously prepared triaryhydrazones suffer from low quantum yields of the Z→E photoisomerization. Here, we have designed a new subclass of naphthoyl‐benzothiazole hydrazones that balance the most beneficial features of previously reported naphthoyl‐quinoline and benzoyl‐pyridine triarylhydrazones. These preserve the attractive absorption characteristics, exhibit higher thermal stability of the metastable form than the former and enhance the rate of the Z→E photoisomerization compared to the later, as a result of the weakening of the intramolecular hydrogen bonding between the hydrazone hydrogen and the benzothiazole moiety. Introducing the benzothiazole motif extends the tunability of the photochromic behaviour of hydrazone‐based switches. [ABSTRACT FROM AUTHOR]

Two-dimensional layered semiconductors have attracted intense interest in recent years. The van der Waals coupling between the layers tolerates stacking various materials and establishing heterostructures with new characteristics for a wide range of optoelectronic applications. The interlayer exciton dynamics at the interface within the heterostructure are vitally important for the performance of the photodetector and photovoltaic device. Here, a heterostructure comprising two-dimensional organic-inorganic Ruddlesden–Popper perovskites and transition metal dichalcogenide monolayer was fabricated and its ultrafast charge separation processes were systematically studied by using femtosecond time-resolved transient absorption spectroscopy. Significant hole and electron transfer processes in the ps and fs magnitude at the interface of the heterostructure were observed by tuning pump wavelengths of the pump-probe geometries. The results emphasize the realization of the exciton devices based on semiconductor heterostructures of two-dimensional perovskite and transition metal dichalcogenide.

Photosensitization of proteins mediated by chromophores is a mechanism commonly employed by nature and mimicked in a broad array of laboratory research and applications. Nature has evolved specialized complexes of proteins and photosensitizers (PS) that assemble to form photoreceptor proteins (PRP). These are used by many organisms in diverse processes, such as energy conversion, protection against photodamage, etc. The same concept has been used in laboratory settings for many applications, such as the stimulation of neurons or the selective depletion of proteins in a signaling pathway. A key issue in laboratory settings has been the relationship between the photooxidation of proteins and conformational changes in host proteins. For several years, we have been interested in creating non-native PRP using porphyrin PS. In this study, we investigated the self-assembled complex between zinc protoporphyrin IX (ZnPPIX) and bovine β-lactoglobulin (BLG) as a model of non-native PRP. Since BLG undergoes a significant conformational transition near physiological pH, the study was carried out at acidic (pH 5) and alkaline (pH 9) conditions where the two conformations are respectively prevalent. We employed a series of steady-state and time-resolved optical spectroscopies as well as gel electrophoresis to experimentally characterize the photosensitization mechanisms and their effect on the host protein. Our results show that ZnPPIX prompts light-dependent modifications of BLG, which appear to be much more significant at alkaline pH. The modifications seem to be driven by photooxidation of amino acid residues that do not lead to the formation of cross-links or protein fragmentation.

Singlet fission (SF), as an effective way to break through the Shockley–Queisser limit, can dramatically improve energy conversion efficiency in solar cell areas. The formation, separation, and relaxation of triplet-pair excitons directly affect the triplet yield, especially triplet-pair separation; thus, how to enhance the triplet-pair separation rate becomes one of the key points to improve SF efficiency; the decay mechanism where the singlet state is converted into two triplet states is significant for the study of the SF mechanism. Herein, we employ ultrafast transient absorption spectroscopy to study the singlet-fission process of nano-amorphous 6, 13-bis(triisopropylsilylethynyl)-Pentacene (TIPS-pentacene) films in a diamond anvil cell (DAC). A kinetics model related to the structural geometric details, as well as an evaluation of the pressure manipulation impacts, is demonstrated based on the experimental results. The results indicate that pressure manipulation enhanced the triplet-pair separation rates of SF-based materials according to their structural micro-environmental improvement when compressed in DAC, while the triplet-exciton transportation lifetime is prolonged. This work shows that pressure may effectively optimize the structural disorder of SF materials, which were found to improve triplet-pair separation efficiency and potentially offer an effective way to further improve SF efficiency.

Cyclic azobenzene‐BODIPY hybrids were synthesized via cyclization by 1) acid‐catalysed condensation of azobenzene‐bridged dipyrroles with 3,5‐di‐tert‐butylbenzaldehyde, 2) oxidation with DDQ, and 3) metalation with BF3 ⋅ Et2O. The structures of many cyclic hybrids have been confirmed by single crystal X‐ray analysis. The absorption spectra of the hybrids reveal the effective cyclic conjugation. The ultrafast measurements reveal that the photoexcited decays of these cyclic hybrids depend upon the ring size and connectivity. [ABSTRACT FROM AUTHOR]

To develop semiconductor detectors based on TlBr with excellent energy resolution at high yields, a variety of characterization methods for TlBr crystals are necessary. Among the various methods based on excitation by photons or ionizing radiation, we chose transient absorption measurements after the irradiation of pulsed electron beams to analyze the recombination dynamics of electron-hole pairs generated by the electron beams. The decay behavior of the transient absorption depended on the pulse intensity and was satisfactorily fitted with a decay function based on bimolecular recombination dynamics. The bimolecular recombination coefficients obtained from the fitting were similar for the samples cut from different parts of a crystal boule. Assuming that the recombination occurred through Shockley-Read-Hall bimolecular recombination via electronic levels of defects within the bandgap, the result indicates that the concentrations of the defects responsible for the recombination were similar for the samples out from different parts of the crystal boule. [ABSTRACT FROM AUTHOR]

Ultrafast charge carrier dynamics of green‐light emitting InP/ZnSe/ZnS core–shell quantum dots (QDs) with three different ZnSe thickness of 2.0 nm (gQD‐20), 2.4 nm (gQD‐24), or 3.3 nm (gQD‐33) are interrogated by femtosecond transient absorption (TA) and femtosecond stimulated Raman spectroscopy (FSRS). Auger‐type cooling of hot electron into the band‐edge level of InP is clearly manifested in the TA spectra, giving its time constant of ≈222, 300, or 349 fs for gQD‐20, gQD‐24, or gQD‐33, respectively, indicating that the state‐filling to the 1S(e) of InP is significantly slowed down with the increase of the mid shell thickness. The global analysis of the whole TA spectra turns out to be extremely useful to explain the dynamic behavior of the heterogeneous ensemble of single and/or bi‐ (or multi‐) excitons in the presence of the Stark‐shift. The hole‐relaxation dynamics into the band‐edge 1S (h) of the core InP is revealed in the bleaching dynamics of the FSR bands corresponding to the phonon mode of InP or ZnSe, giving the estimated lifetime of 1.0–1.4 ps for gQD‐20. The hole‐relaxation dynamics by the phonon emission seem to be little sensitive to the ZnSe shell thickness. [ABSTRACT FROM AUTHOR]

Phenol blue (PhB) is a dye that exhibits positive solvatochromism which is also nonfluorescent due to its ultrashort excited state lifetime. We have conducted femtosecond time‐resolved transient absorption (TA) spectroscopic measurements of a PhB solution using two distinct excitation wavelengths, at shorter and longer wavelength sides of the visible absorption band. The results revealed that a long‐lived heterogeneity exists in protic solvents whereas, in aprotic solvents, the system rapidly returns to the original thermal equilibrium on the ultrafast timescale. Interestingly, the lifetime of the heterogeneity is longer for methanol (MeOH) compared to ethanol (EtOH) solution suggesting stronger interaction between PhB and MeOH. Additionally, since the excited state lifetime of PhB was still in the sub‐picosecond domain even in a polymer matrix, we suggest a nonradiative decay mechanism through high‐frequency vibrational modes without involving large structural reconfiguration such as twisting of the benzoquinone (BzQ) moiety. [ABSTRACT FROM AUTHOR]

The presence of surface trap states (STSs) is one of the key factors to affect the electronic and optical properties of quantum dots (QDs), however, the exact mechanism of how STSs influence QDs remains unclear. Herein, we demonstrated the impact of STSs on electron transfer in CdSe QDs and triplet-triplet energy transfer (TTET) from CdSe to surface acceptor using femtosecond transient absorption spectroscopy. Three types of colloidal CdSe QDs, each containing various degrees of STSs as evidenced by photoluminescence and X-ray photoelectron spectroscopy, were employed. Time-resolved emission and transient absorption spectra revealed that STSs can suppress band-edge emission effectively, resulting in a remarkable decrease in the lifetime of photoelectrons in QDs from 17.1 ns to 4.9 ns. Moreover, the investigation of TTET process revealed that STSs can suppress the generation of triplet exciton and effectively inhibit band-edge emission, leading to a significant decrease in TTET from CdSe QDs to the surface acceptor. This work presented evidence for STSs influence in shaping the optoelectronic properties of QDs, making it a valuable point of reference for understanding and manipulating STSs in diverse QDs-based optoelectronic applications involving electron and energy transfer. [ABSTRACT FROM AUTHOR]

The realization of the potential of hybrid inorganic organic systems requires an understanding of the coupling between the constituents: its nature and its strength. The observation of hybrid optical transitions in the monolayer WS2/terrylene hybrid is reported. The first‐principle calculations, linear optical, and transient absorption spectroscopy are employed to investigate the optical spectrum of the hybrid, which exhibits a new transition that does not appear in the constituents' spectra. The calculations indicate type II level alignment, with the highest occupied level of terrylene in the gap of WS2. Exploiting state‐resolved transient absorption, the response of the hybrid interface to optical excitation is selectively probed. The dynamics reveal rapid hole transfer from WS2 to the terrylene layer, with a decay time of 88 ps. This hole transfer induces a bleach of the hybrid transition, which indicates that terrylene contributes to its initial state. Based on this, the hybrid resonance energy, and on our calculations, we assign the hybrid feature to a transition from the highest occupied molecular orbital of terrylene to the conduction band of WS2 close to the Γ point. The results indicate that the conditions for strong electronic coupling are met in this hybrid system. [ABSTRACT FROM AUTHOR]

Monolayer transition metal dichalcogenides (TMDCs) have become a promising platform in valleytronics due to possessing the regulatable valley degrees of freedom. While, as a result of the rapid intervalley scattering, it is difficult to measure the PL valley polarization of monolayer TMDCs at room temperature, which limits their application in valleytronics devices. Here, we report a room temperature photoluminescence (PL) valley polarization up to 3.73% in an "optical bilayer" WS2 formed by transferring monolayer WS2 onto flat Ag film. Furthermore, in the transient absorption (TA) measurements, a remarkably long valley depolarization lifetime is found. Thus, we demonstrate the valley properties of such "optical bilayer" WS2 resemble actual bilayer WS2, in which the robust valley polarization can be attributed to the phonons depletion effect and the blocked interlayer hopping processes. These peculiar valley features in "optical bilayer" WS2 provide a particularly simple method to enhance valley control at room temperature. [ABSTRACT FROM AUTHOR]

Photosensitization of proteins mediated by chromophores is a mechanism commonly employed by nature and mimicked in a broad array of laboratory research and applications. Nature has evolved specialized complexes of proteins and photosensitizers (PS) that assemble to form photoreceptor proteins (PRP). These are used by many organisms in diverse processes, such as energy conversion, protection against photodamage, etc. The same concept has been used in laboratory settings for many applications, such as the stimulation of neurons or the selective depletion of proteins in a signaling pathway. A key issue in laboratory settings has been the relationship between the photooxidation of proteins and conformational changes in host proteins. For several years, we have been interested in creating non-native PRP using porphyrin PS. In this study, we investigated the self-assembled complex between zinc protoporphyrin IX (ZnPPIX) and bovine β-lactoglobulin (BLG) as a model of non-native PRP. Since BLG undergoes a significant conformational transition near physiological pH, the study was carried out at acidic (pH 5) and alkaline (pH 9) conditions where the two conformations are respectively prevalent. We employed a series of steady-state and time-resolved optical spectroscopies as well as gel electrophoresis to experimentally characterize the photosensitization mechanisms and their effect on the host protein. Our results show that ZnPPIX prompts light-dependent modifications of BLG, which appear to be much more significant at alkaline pH. The modifications seem to be driven by photooxidation of amino acid residues that do not lead to the formation of cross-links or protein fragmentation. [ABSTRACT FROM AUTHOR]

The aggregation of perylene diimide (PDI) and its derivatives strongly depends on the molecular structure and therefore has a great impact on the excited states. By regulating the molecular stacking such as monomer, dimer, J- and/or H-aggregate, the formation of different excited states is adjustable and controllable. In this study, we have synthesized two kinds of PDI derivatives: undecane-substituted PDI (PDI-1) and diisopropylphenyl-substituted PDI (PDI-2), and the films are fabricated with spin-coating method. By employing photoluminescence, time-resolved photoluminescence, and transient absorption spectroscopy, the excited-state dynamics of two PDI amorphous films have been investigated systematically. The result reveals that both films form excimers after photoexcitation mainly due to the stronger electronic coupling among molecule aggregate in the amorphous film. It should be noted that the excited state dynamics in PDI-2 shows a singlet fission like process, which is evidenced by the appearance of triplet state absorption. This study provides the dynamics of excited state in amorphous PDI films, and paves the way for better understanding and adjusting the excited state of amorphous PDI films. [ABSTRACT FROM AUTHOR]

Two-dimensional layered semiconductors have attracted intense interest in recent years. The van der Waals coupling between the layers tolerates stacking various materials and establishing heterostructures with new characteristics for a wide range of optoelectronic applications. The interlayer exciton dynamics at the interface within the heterostructure are vitally important for the performance of the photodetector and photovoltaic device. Here, a heterostructure comprising two-dimensional organic-inorganic Ruddlesden–Popper perovskites and transition metal dichalcogenide monolayer was fabricated and its ultrafast charge separation processes were systematically studied by using femtosecond time-resolved transient absorption spectroscopy. Significant hole and electron transfer processes in the ps and fs magnitude at the interface of the heterostructure were observed by tuning pump wavelengths of the pump-probe geometries. The results emphasize the realization of the exciton devices based on semiconductor heterostructures of two-dimensional perovskite and transition metal dichalcogenide. [ABSTRACT FROM AUTHOR]