Time-resolved Transient Absorption Research Articles

Synergistic Enhancement of Fluorescence Through Plasmon Resonance and Interfacial Charge Transfer by AgNC@AgAux Core-Shell Quantum Dots.

Bimetallic core-shell quantum dots (QDs) hold great promise in elucidating the bimetallic synergism and optoelectronic devices. The synthesis and properties of AgNC@AgAux QDs of core-shell heterostructure are reported. Significantly enhanced photoluminescence emission on these heterostructures is observed. These enhancements are attributed to electron injection and the surface plasmon-induced strong local electric field, which are observed through time-resolved transient absorption spectroscopy. X-ray absorption near edge structure spectra and density functional theory confirms the electron injection from the Ag core to the AgAux shell. On the other hand, the plasmon resonance of the AgNC@AgAux QDs has been studied by finite-element method analysis and time-resolved photoluminescence spectra. There are 94.06 times fluorescence enhancement and 32.40 times quantum yield improvement of oxygen content correlation compared to AgAu3 QDs. It shows a perfect correlation coefficient of 98.85% for the detection of heavy metal Cu2+ ions. Such Bimetallic core-shell heterostructures have great potential for future optoelectronic devices, optical imaging, and other energy-environmental applications.

Rigidly Locked Pyrene Excimers in Planar Chiral Pyrenophanes for Intense and Stable Circularly Polarized Photoluminescence and Electrochemiluminescence.

Aiming at the construction of novel platforms with excellent performances in both circularly polarized photoluminescence (CP-PL) and electrochemiluminescence (CP-ECL), a new family of pyrenophanes with rigidly locked pyrene dimers and varied bridges has been designed and synthesized. Attributed to densely packed pyrene excimers, the resultant pyrenophanes revealed tunable bridge-dependent emission behaviors, as investigated by femtosecond time-resolved transient absorption spectroscopy. More importantly, all these planar chiral pyrenophanes display strong CP-PL with large dissymmetry factor (gPL) values up to 0.034, and such excellent performances remain stable in a wide range of solvents, temperatures, and concentrations. Attractively, the highest gECL values up to 0.014 reported so far has been achieved for the CP-ECL emissions of the chiral pyrenophanes. Attributed to their unique structural features and outstanding CPL performances, these novel pyrenophanes could serve as promising platforms for both fundamental investigations on pyrene excimers and practical applications such as chiral sensing.

A Complete Description of the Ultrafast Proton Transfer Dynamics in the Excited State of D-Luciferin.

Excited-state proton transfer (ESPT) in organic photoacids is a widely studied phenomenon in which D-luciferin is of special mention, considering the fact that apart from its phenolic OH group, the nitrogen atoms at either of the two thiazole moieties could also participate in hydrogen bonding interactions with a proton-donating solvent during ESPT. As a result, several transient species could appear during the ESPT process. We hereby deploy subpicosecond time-resolved fluorescence upconversion (FLUP) and transient absorption (TA) spectroscopic techniques to understand the detailed photophysics of D-luciferin in water as well as in dimethyl sulfoxide (DMSO) and ethanol. These transient-state spectroscopic studies reveal the population of two kinds of hydrogen-bonded (HB) complexes (HBCs) in the excited singlet (S1) state─HBC-I, which is formed at the OH site of the hydroxy benzothiazole moiety with a proton-accepting solvent, and the other one is HBC-II, which is formed at the N atom site of the thiazoline moiety with a proton-donating solvent. This study provides a complete description of the mechanism of the deprotonation process in HBC-I through distinct identification and characterization of the spectroscopic properties and temporal dynamics of those transient species associated with the four stages of the ESPT process as proposed by the Eigen-Weller model. This study also identifies and characterizes HBC-II, which, however, does not participate in the deprotonation process but provides an efficient nonradiative relaxation mechanism via geminate proton recombination.

In-Situ Growth of Metallocluster Inside Heterometal-Organic Cage to Switch Electron Transfer for Targeted CO2 Photoreduction.

Construction of metal-organic cages (MOCs) with internal modifications is a promising avenue to build enzyme-like cavities and unlocking the mystery of highly catalytic activity and selectivity of enzymes. However, current interests are mainly focused on single-metal-node cages, little achievement has been expended to metallocluster-based architectures, and the in situ endogenous generation of metal clusters. Herein, based on the hard-soft-acids-bases (HSAB), the metallocluster-based heterometallic MOC (Cu3VMOP) constructed of [Cu3OPz3]+ and [V6O6(OCH3)9(SO4)(CO2)3]2- clusters was obtained by one-pot method. In addition, Cu4I4 was generated in situ in the cage to form Cu4I4@Cu3VMOP by the coordination-driven hierarchical self-assembly strategy. As catalysts for CO2 reduction, Cu3VMOP produces HCOOH and CH3COOH as the main reduction product with yield of CH3COOH up to 0.9 mmol g-1, ranking among the highest value of reported materials, whereas Cu4I4@Cu3VMOP exhibited targeted CO2-to-HCOOH conversion with 100 % formic acid selectivity and the yield outperforms that of Cu3VMOP by 5 fold. Theoretical calculations and femtosecond time-resolved transient absorption reveal that endogenous Cu4I4 not only regulates orbital arrangements and enhances localized electron states to generate a long-lived charge-separated state, but also raises *CO coupling energy barrier, resulting in the targeted conversion of CO2 to formic acid.

Concurrent ultrafast twisting and proton transfer photoreactions in new pyrano[2,3-c]pyrazole derivatives.

Pyrano[2,3-c]pyrazole derivatives are a class of compounds exhibiting dual solvent-dependent fluorescence. This interesting and potentially useful optical property is attributed to the excited state intramolecular proton transfer (ESIPT). We have investigated excited state dynamics of these molecules in detail using femtosecond time-resolved fluorescence and transient absorption spectroscopy. We found that when the compounds containing methoxy groups in a phenyl ring are dissolved in a polar protic solvent (methanol), they undergo excited state twisting that competes with the ESIPT reaction. Additionally, the dumping of the tautomer stimulated emission allowed us to populate a short-lived ground-state tautomer and track a ground-state proton transfer (GSIPT) back reaction. We found that the GSIPT decays on the sub-picosecond to picosecond time scale, and a fast process is more pronounced in less polar solvents.

Doublet Spin State Mediated Photoluminescence Upconversion in Organic Radical Donor-Triplet Acceptor Dyads.

Donor-acceptor dyads are promising materials for improving triplet-sensitized photon upconversion due to faster intramolecular energy transfer (ET), which unfortunately competes with charge transfer (CT) dynamics. To circumvent the issue associated with CT, we propose a novel purely organic donor-acceptor dyad, where the CT character is confined within the donor moiety. In this work, we report the synthesis and characterization of a stable organic radical donor-triplet acceptor dyad (TTM-Cz-Per) consisting of the acceptor perylene (Per) linked to the donor (4-N-carbazolyl-2,6-dichlorophenyl)-bis(2,4,6-trichlorophenyl)methyl radical (TTM-Cz). Upon red-excitation of TTM-Cz-Per, the doublet emission of the donor (TTM-Cz) is significantly quenched, and a recorded delayed emission centered at ca. 490 nm was attributed to the fluorescence emission from the Per acceptor. Time-resolved transient absorption spectroscopy suggests doublet-to-triplet energy transfer (DTET) dynamics from the donor to the acceptor as the time constant τ for the donor transient species decreases from 21.47 ns for the TTM-Cz sensitizer to 8.73 ns for TTM-Cz-Per dyad. This process is accompanied by the appearance of a long-lived component with τ = 97.06 ns, which we ascribe to the triplet transient of the acceptor Per. Furthermore, computational results indicate that the DTET is intramolecular as computed spin densities of the quartet state show unpaired electrons of ρ ≈ 1 on the TTM-Cz donor and of ρ ≈ 2 on the acceptor Per. The present study highlights the possibility to employ doublet chromophoric systems for light-harvesting and energy upconversion, which can be further tailored for several optoelectronic applications.

Quantum-State Renormalization in Semiconductor Nanoparticles.

A single photoexcited electron-hole pair within a polar semiconductor nanocrystal (SNC) alters the charge screening and shielding within it. Perturbations of the crystal lattice and of the valence and conduction bands result, and the quantum-confinement states in a SNC shift uniquely with a dependence on the states occupied by the carriers. This shifting is termed quantum-state renormalization (QSR). This Perspective highlights QSR in semiconductor quantum wires and dots identified in time-resolved transient absorption and two-dimensional electronic spectroscopy experiments. Beyond the interest in understanding the principles of QSR and energy-coupling mechanisms, we pose the contributions of QSR in time-resolved spectroscopy data must be accounted for to accurately identify the time scales for intraband relaxation of the carriers within SNCs.

Emissive Features of Perylene Diimide Derivative with β-Cyclodextrin: Probing the role of inter and intramolecular interactions.

Perylene diimide (PDI) derivatives have been extensively explored as chromophoric dyes for functional organic materials. Here, the custom synthesized tyrosine appended perylene diimide (PDI-Tyr) derivative has shown strong aggregation in aqueous medium diminishing its emissive features, which was surpassed by the supramolecular interaction with β-cyclodextrin (β-CD). Complex formation between PDI-Tyr and β-CD, proposed from the absorption and emission studies, have been substantiated by the 1H-NMR, ITC and geometry optimization data. Time-resolved emission and transient absorption studies carried out on PDI-Tyr in the absence and presence of β-CD, revealed an excited state intramolecular charge transfer (ICT) process from the tyrosine moiety to the PDI core with a time constant of ~30 ps, and the same gets retarded on encapsulation of the tyrosine groups in the βCD:PDI-Tyr (2:1) complex and the kinetics slows down to ~480ps, reviving the emissive features. The absence of a long lifetime component and the lower emission enhancement in the monomeric condition as compared to the phenyl alanine (PDI-Phe) derivative, emphasizes the major role of the ICT process in PDI-Tyr, which subdue the features of intermolecular aggregation, establishing the tunability of the emissive properties with the customization of intra and intermolecular interactions.

RGB photoluminescence from single-component hydrocarbon single-crystals: Revealing excited-state dynamics in organic semiconductors

The development of single-component organic materials that exhibit tunable red, green and blue (RGB) luminescence under ambient conditions can pave the way of materials with tailored photophysical properties. The optical behavior of such organic materials is largely determined by their excited-state dynamics of the excitons, which are formed when electrons are excited in the material. The excited-state dynamics of an organic molecules can be sensitively tuned, where even marginal variations in crystal morphology and molecular arrangement can drastically modify their optical behavior. Herein, we report a discovery of π-conjugated single-component system that can exhibit the RGB emission. This was realized by altering the morphological crystal dimensionality of a highly tunable single-component hydrocarbon coronene molecule in a single-step crystallization processes into 1D wire, 2D plate, and 3D rod, without introducing additional components or varying external stimuli. Time-resolved photoluminescence (PL) and transient absorption spectroscopy revealed the excited-state absorption (ESA) and self-trapped exciton (STE) formation in the excited electrons in 1D wire crystal plays a key role in emission color change from blue to green. Furthermore, static PL and absorption spectroscopy and single-crystal XRD revealed the dimerization of coronene results in significant reduction of optical band-gap energy and red shift into red emission band. We elucidated the complex relationship between excited-state dynamics and crystal structure of the coronene crystals. Our work presents a novel strategy for tuning the optical properties of single-component organic materials through crystal engineering, offering new possibilities for the development of advanced organic semiconducting devices.

From Molecular to Polymeric Donors: Prolonged Charge Separation in Modular Photoredox-Active Ru(II) Polypyridyl-Type Triads.

In this contribution, the divergent modular synthesis of photoredox-active dyads, triads and a tetrad descending from one ligand precursor is presented by combining "chemistry-on-the-ligand", stepwise complexation and "chemistry-on-the-complex" with minimal synthetic efforts. In the final step, Pd-mediated borylation and subsequent Suzuki-Miyaura cross-coupling was employed to introduce the different (multi)donor moieties at the preassembled P-A dyad subunit. The (spectro-)electrochemical data revealed preserved redox properties of the subunits and minimal driving force for oxidative quenching by the naphthalene diimide-based (NDI) acceptor and, thus, high-energy charge separated (CS) states. Time-resolved transient absorption and emission data revealed the formation of long-lived CS states in the polymer-based triads, i.e., the CS lifetime is extended by 2 orders of magnitude in comparison to the molecular triad. The long-lived CS state (13.2 μs) of the conjugated polycarbazole (Carbn) multidonor demonstrates that the rational modular design and efficient synthesis of advanced photoredox-active assemblies can be readily achieved by late-stage diversification utilizing the "chemistry-on-the-complex" approach.

Photoinduced charge transfer and excitonic energy transfer in the CuInS2/ZnS/MoS2 heterotrilayer

Photo-induced charge transfer is the key process of many applications such as photovoltaics, photodetection, and light-emitting devices. With the outgrowth of a new class of low-dimensional semiconductors, i.e., monolayer transition metal dichalcogenides and semiconductor nanocrystals, charge transfer at the 2D/0D heterostructures has drawn many efforts because of the outstanding optical and electrical properties. This paper studies the dynamics of excitons of the CuInS2/ZnS/MoS2 heterotrilayer through femtosecond time-resolved transient absorption spectroscopy. The electron and hole transfer are observed by selectively exciting the electrons with tunable pump wavelengths. The exciton lifetimes are obtained on the picosecond scale. This work provides clues on exploring the non-toxic optoelectronic devices based on the CuInS2/ZnS/MoS2 heterotrilayer.

Superior Photostability of the Unnatural Base 6-Amino-5-nitropyridin-2-ol: A Case Study Using Ultrafast Broadband Fluorescence, Transient Absorption, and Theoretical Computation.

6-Amino-5-nitropyridin-2-ol (Z), a nitroaromatic compound and a base for Hachimoji nucleic acids, holds significant potential in expanding the genetic alphabet, as well as in synthetic biology and biotechnology. Despite its promising applications, the spectral characterization and photoinduced properties of Z have remained largely unexplored until now. This study presents a comprehensive investigation into its excited state dynamics in various solvents, utilizing state-of-the-art ultrafast broadband time-resolved fluorescence and transient absorption spectroscopy, complemented by computational methods. The acquired results provide direct experimental evidence that, upon photoexcitation, Z emits prompt fluorescence from a nearly planar structure in its excited state, independent of solvent properties. This state deactivates nonradiatively within sub-picoseconds through internal conversion with a unitary yield, primarily mediated by the rotation of the nitro group. This unusually rapid deactivation pathway entirely excludes the involvement of long-lived nπ* states, triplet states, and photoproducts, which are commonly observed in most nitroaromatic compounds and natural DNA and RNA bases. Our findings underscore that Z, as an unnatural base, exhibits superior photostability compared to canonical natural bases. This provides valuable insights into the photodynamics of nitroaromatic compounds, which is beneficial for strategic substitution design in environmental and biological applications.

Triplet Energy Gap-Regulated Room Temperature Phosphorescence in Host-Guest Doped Systems.

The organic room temperature phosphorescence (RTP) materials via host-guest doped method receive considerable attention in the fields of optoelectronics, bioimaging, and information encryption. Despite many host-guest doped materials with excellent RTP properties have been developed, their luminous mechanism is still limited. Here, a series of host-guest doped materials, using benzophenone as the host and quinone compounds as the guests, were constructed to investigate the effect of the triplet energy gap (ΔET) between the host and guest on triplet states population. The guest's triplet state is proposed to be a "triplet energy reservoir", gathering the triplet excitons to emit RTP when ΔET is large and returning triplet excitons to the host when ΔET is small. By combining the results of steady-state and delayed emission spectra, time-resolved transient absorption spectra, and theoretical calculations, a bidirectional energy transfer process is proved, which are triplet-triplet energy transfer and reverse triplet-triplet energy transfer processes. The thermal equilibrium of these two energy transfer processes can be regulated by the ΔET and temperature. The potential applications of these RTP properties are also realized in data encryption and anti-counterfeiting. This work provides valuable insight into the design of host-guest doped materials based on energy transfer mechanisms.

Pseudospins revealed through the giant dynamical Franz-Keldysh effect in massless Dirac materials

The dynamical Franz-Keldysh effect, indicative of the transient light-matter interaction regime between quantum and classical realms, is widely recognized as an essential signature in wide bandgap condensed matter systems such as dielectrics. In this theoretical study, we applied time-resolved transient absorption spectroscopy to investigate ultrafast optical responses in graphene, a zero-bandgap system. We observed in the gate-tuned graphene that the massless Dirac materials notably enhance intraband light-driven transitions, significantly leading to the giant dynamical Franz-Keldysh effect compared to the massive Dirac materials, a wide bandgap system. In addition, employing the angle-resolved spectroscopy, it is found that the unique polarimetry orientation, i.e., perpendicular polarizations for the pump and the probe, further pronounces the optical spectra to exhibit the complete fishbone structure, reflecting quantum pseudospin natures of Dirac cones. Our findings expand the establishment of emergent transient spectroscopy frameworks into not only zero-bandgap systems but also pseudospin-mediated quantum phenomena, moving beyond dielectrics.

Colloidal Synthesis of Blue-Emitting Cs3TmCl6 Nanocrystals via Localized Excitonic Recombination for Down-Conversion White Light-Emitting Diodes.

Lead-halide perovskite nanocrystals (NCs) have gained significant attention for their promising applications in lighting and display technologies. However, blue-emitting NCs have struggled to match the high efficiency of their red and green counterparts. Moreover, many reported blue-emitting perovskite NCs contain heavy metal lead (Pb), which poses risks to human health and the environment. In this study, we synthesized rare-earth-based Cs3TmCl6 NCs via the hot injection method, which exhibit a broadband blue emission at 440 nm. Combined experimental and theoretical studies indicate that the broadband emission in Cs3TmCl6 arises from self-trapped excitons due to the excited-state structural distortion of the [TmCl6]3- cluster. Furthermore, the ultrafast dynamics of charge carriers were analyzed using time-resolved photoluminescence and transient absorption measurements. Encouraged by the remarkable thermal, light, and water stabilities of Cs3TmCl6 NCs, as evidenced by experimental and theoretical results, a white light-emitting diode was further designed and fabricated using the Cs3TmCl6 NCs as the color converter. The device exhibits outstanding performance, achieving a long half-lifetime of 336 h and a large color-rendering index of 87.0. Combining eco-friendly features and a facile synthesis method, the rare-earth-based Cs3TmCl6 NCs mark a significant breakthrough as a reliable blue emitter, showcasing their future potential in lighting and display applications.

Triad photosensitizers based on iodo-aza-Bodipy and naphthalenediimides (NDI) with broadband absorption: Study of resonance energy transfer and electron transfer

Two organic photosensitizers based on resonance energy transfer (A-1 and A-2) are prepared. Naphthalenediimides was used as intramolecular energy donor and iodo-aza-Bodipy was used as intramolecular energy acceptor/spin converter. A-1 (ε = 42,800 at 618 nm and ε = 59,600 at 674 nm) and A-2 (ε = 33,300 at 514 nm and ε = 68,300 at 674 nm) give the two strong absorption band and both of them show broadband absorption in visible region. The photophysical properties of the compounds were studied with steady state and time-resolved spectroscopy in detail. With steady state and time-resolved spectroscopy, we found that photoexcitation into the energy donor was followed by singlet energy transfer, then via the intersystem crossing (ISC) of the energy acceptor. By nanosecond time-resolved transient absorption spectroscopy and DFT calculation, triplet excited state is localized on the iodo-aza-Bodipy moiety. Quantum yield of singlet oxygen of A-1 and A-2 is 39.7 % and 40.9 %, respectively. Based on fluorescence lifetime quenching, the efficiency of intramolecular energy transfer is estimated to be 15.9 % and 27.7 % for A-1 and A-2. And PET is responsible for the relatively low efficiency, which is identified by the calculation of kcs/kFRET and the Gibbs free energy change (△G0cs). A-1 and A-2 were used for singlet oxygen (1O2) mediated photooxidation of 1,5-dihydroxylnaphthalene and the photosensitizing ability are more efficient than the triplet photosensitizers with the mono-chromophore photosensitizer.

Full Picture of Energy Transfer in a Trivalent Europium Complex with Bidentate β-Diketonate Ligand.

Trivalent europium (Eu(III)) complexes emit narrow-band luminescence as a result of energy transfer following the photoexcitation of antenna ligands. Bidentate β-diketonates are typically used as antenna ligands; however, their entire energy transfer mechanism to Eu(III) remains unknown. We used time-resolved photoluminescence spectroscopy and femtosecond transient absorption spectroscopy to map the complete intramolecular energy transfer process in the [Eu(hfa)3(TPPO)2] (hfa = hexafluoroacetylacetonate, TPPO = triphenylphosphine oxide) complex; hfa is a β-diketonate antenna ligand. Our analysis provides a "full picture" of energy transfer, tracing each step from initial photoexcitation to final relaxation: intersystem crossing in the ligands, energy transfer from the ligands to the metal center, and multiple internal conversion of Eu(III). We demonstrated that the direct bonds of the bidentate ligands enabled a quick and near-unity-efficiency energy transfer from the triplet state of the ligand to the 5D2 state of Eu(III). We also revealed that the loss of the ligand-centered intersystem crossing process reduces the overall sensitization efficiency. Our findings provide a foundation for the rational design of advanced luminescent materials, paving the way for advances in lanthanide-based luminescence research.

Fast singlet excited-state deactivation pathway of flavin with a trimethoxyphenyl derivative

Incorporation of the trimethoxyphenyl group at position 7 of flavin can drastically change the photophysical properties of flavin. We show unique fast singlet 1(π,π*) excited state deactivation pathway through nonadiabatic transition to the 1(n,π*) excited- state, and subsequent deactivation to the ground electronic state (S0), closing the photocycle. This mechanism explains the exceptionally weak fluorescence and the short excited–state lifetime for the flavin trimethoxyphenyl derivative and the lack of excited triplet T1 state formation. Full recovery of flavin in its ground state takes place within a 15 ps time window after photoexcitation in a polar solvent such as acetonitrile. According to quantum chemical calculations, the C(2)-O distance elongates by 0.16 Å in the 1(n,π*) state, with respect to the ground state. Intermediate–state structures are predicted by theoretical ab initio calculations and their dynamics are investigated using broadband vis-NIR time-resolved transient absorption and fluorescence up-conversion techniques.

Ultrafast Charge Separation Driven by Solvation-Coupled Intramolecular Torsion.

Photoinduced intramolecular charge separation in a pyrene- and triarylamine-based donor-acceptor dyad was studied by polarization-dependent femtosecond time-resolved transient absorption (TA) spectroscopy in polar solvents. Photoexcitation forms an excited state with charge transfer (CT) character due to the intrinsic electronic coupling between the triarylamine and pyrene groups, resulting in ultrafast charge separation (CS) in polar solvents. TA measurements reveal a correlation between the rate of CS and solvation dynamics, which implies that solvation is involved in the CS reaction. In addition, polarization-dependent TA spectroscopy was devoted to tracking the ultrafast anisotropy evolution of the cationic absorption band, which is attributed to intramolecular torsional motion and is proposed to be coupled to diffusive orientational solvent modes. The results therefore reveal that the evolution of the CT state in the condensed phase is driven by solvation-coupled excited-state structural relaxation. In other words, intramolecular torsional motion is directly confirmed to be involved in the reaction coordinate of the CS reaction in a strongly coupled donor-acceptor dyad.

Generation of 1 MHz 15 fs pulses in the near-infrared with high energy and their application to time-resolved spectroscopy.

Ultrashort pulses with a duration of ∼10 fs are crucial to fully resolve nuclear motions in molecules and materials. Here, we employ nonlinear pulse compression using photonic crystal fiber in the near-infrared region to generate ultrashort pulses at a high repetition rate with significant pulse energy. Femtosecond pulses centered around 1200 nm from a cavity-dumped optical parametric oscillator are compressed to a duration of 15 fs. The pulse energy reaches several tens of nanojoules, sufficient for wavelength conversion via nonlinear processes. Second harmonic generation produces clean 13 fs pulses at around 600 nm with pulse energy of 4 nJ. The effectiveness of nonlinear pulse compression using photonic crystal fiber for ultrafast time-resolved spectroscopy is demonstrated through time-resolved fluorescence (photoluminescence) and transient absorption apparatus, utilizing the second harmonic as pump pulses. Specifically, a time resolution of ∼20 fs is achieved in a time-resolved fluorescence experiment, enabling the recording of nuclear wave packets over 1200 cm-1 by spontaneous fluorescence. The high repetition rate at the megahertz level significantly improved the signal quality.