Transient Absorption Techniques Research Articles

Ultrafast energy quenching mechanism of LHCSR3-dependent photoprotection in Chlamydomonas

Photosynthetic organisms have evolved an essential energy-dependent quenching (qE) mechanism to avoid any lethal damages caused by high light. While the triggering mechanism of qE has been well addressed, candidates for quenchers are often debated. This lack of understanding is because of the tremendous difficulty in measuring intact cells using transient absorption techniques. Here, we have conducted femtosecond pump-probe measurements to characterize this photophysical reaction using micro-sized cell fractions of the green alga Chlamydomonas reinhardtii that retain physiological qE function. Combined with kinetic modeling, we have demonstrated the presence of an ultrafast excitation energy transfer (EET) pathway from Chlorophyll a (Chl a) Qy to a carotenoid (car) S1 state, therefore proposing that this carotenoid, likely lutein1, is the quencher. This work has provided an easy-to-prepare qE active thylakoid membrane system for advanced spectroscopic studies and demonstrated that the energy dissipation pathway of qE is evolutionarily conserved from green algae to land plants.

Linear and Nonlinear Optical Properties of All-cis and All-trans Poly(p-phenylenevinylene).

Poly(p-phenylenevinylene) (PPV) is a staple of the family of conjugated polymers with desirable optoelectronic properties for applications including light-emitting diodes (LEDs) and photovoltaic devices. Although the significant impact of olefin geometry on the steady-state optical properties of PPVs has been extensively studied, PPVs with precise stereochemistry have yet to be investigated using nonlinear optical spectroscopy for quantum sensing, as well as light harvesting for biological applications. Herein, we report our investigation of the influence of olefin stereochemistry on both linear and nonlinear optical properties through the synthesis of all-cis and all-trans PPV copolymers. We performed two-photon absorption (TPA) using a classical and entangled light source and compared both classical TPA and entangled two-photon absorption (ETPA) cross sections of these stereodefined PPVs. Whereas the TPA cross section of the all-trans PPV was expectedly higher than that of all-cis PPV, presumably because of the larger transition dipole moment, the opposite trend was measured via ETPA, with the all-cis PPV exhibiting the highest ETPA cross section. DFT calculations suggest that this difference might stem from the interaction of entangled photons with lower-lying electronic states in the all-cis PPV variant. Additionally, we explored the photoinduced processes for both cis and trans PPVs through time-resolved fluorescence upconversion and femtosecond transient absorption techniques. This study revealed that the sensitivity of PPVs in two-photon absorption varies with classical versus quantum light and can be modulated through the control of the geometry of the repeating alkenes, which is a key stepping stone toward their use in quantum sensing, bioimaging, and the design of polymer-based light-harvesting systems.

Photolysis mechanism of Di(tert‐butylphenyl)iodonium salt using 2‐isopropylthioxanthone as a sensitizer

AbstractDiaryliodonium 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.

Excited-State Charge Transfer in Push-Pull Platinum(II) π-Extended Porphyrins Fused with Pentacenequinone.

Platinum(II) π-extended porphyrins fused with pentacenequinone and dihydropentacene have been successfully synthesized. These porphyrins were investigated using various techniques including absorption, steady-state, and time-resolved phosphorescence spectroscopy and differential pulse voltammetry. UV-vis absorption spectra of pentacenequinone-fused porphyrins (SW-Pt1 and SW-Pt2) showed unusually broad and nontypical absorption patterns. Phosphorescence spectra of SW-Pt1, SW-Pt2, and SW-Pt3 displayed similar emissions in the 704-706 nm region indicating electronic transitions of similar origin; however, the triplet lifetimes were found to be quenched in the case of both SW-Pt1 and SW-Pt2, suggesting the occurrence of excited-state events. Facile reductions were obtained for both the pentacene-quinone-fused monomer (SW-Pt2) and dimer (SW-Pt1) and were identified to be located at the pentacenequinone components. The observed orbital segregations for SW-Pt2 and SW-Pt1 from DFT calculations supported the possibility of charge transfer in these push-pull systems. Interestingly, the established energy level diagram revealed that the charge transfer from the triplet excited Pt porphyrin is thermodynamically an uphill process. Systematic studies involving both femtosecond and nanosecond transient absorption techniques revealed that the singlet excited Pt porphyrins undergo an intermediate charge transfer state prior to populating the triplet state, providing a plausible explanation for phosphorescence quenching. The lifetime of the intermediate charge transfer states was found to be 25.9 and 5.68 ps, respectively, for SW-Pt1 and SW-Pt2.

3-Substituted 2-Aminonaphthalene Photocages for Carboxylic Acids and Alcohols; Decaging Mechanism and Potential Applications in Synthesis.

3-Hydroxymethyl-2-aminonaphthalene photocage (photoremovable protecting group) 2 was synthesized and transformed to different ethers and esters to investigate the applicability to decage alcohols and carboxylic acids, respectively. The photoelimination of carboxylic acids takes place relatively efficiently (ΦR = 0.11) upon excitation with near-visible light, contrary to the elimination of alcohols. The scope of the decaging of both alcohols and esters was demonstrated on several examples, including aliphatic and aromatic substrates, carbohydrates, and nonsteroidal anti-inflammatory drugs. The photophysical properties of the photocage and its models, methyl ether 4a and acetyl ester 5a, were investigated. The fluorescence quantum yields (Φf = 0.40-0.002) were found to be reversely proportional to the efficiency of elimination of OH, alcohols, or carboxylic acids. The decaging photochemical reaction mechanism was investigated experimentally by transient absorption techniques with time scales from femtoseconds to seconds and computationally on the TD-DFT level of theory. The photoelimination of carboxylates takes place directly in the singlet excited state by a homolytic cleavage producing a radical pair within 1 ns. The subsequent electron transfer gives rise to aminonaphthalene carbocation and the carboxylate. A wide scope of substrates that can be decaged relatively efficiently with near-visible light and the chromo-orthogonal compatibility of aminonaphthalene and aniline derivatives render these photocages potentially applicable in organic synthesis or biology.

Length and Charge of the N-terminus Regulate the Lifetime of the Signaling State of Photoactive Yellow Protein.

Photoactive yellow protein (PYP) is one of the most extensively studied photoreceptors. Nevertheless, the role of the N-terminus in the photocycle and structural transitions is still elusive. Here, we attached additional amino acids to the N-terminus of PYP and investigated the effect of the length and charge of additional N-terminal residues using circular dichroism, two-dimensional nuclear magnetic resonance (2D-NMR), transient absorption (TA), and transient grating (TG) spectroscopic techniques. TA experiments showed that, except for negatively charged residues (5D-PYP), additional N-terminal residues of PYP generally enable faster dark recovery from the putative signaling state (pB2) to the ground state (pG). TG data showed that although the degree of structural changes can be controlled by adjusting specific amino acid residues in the extended N-terminus of N-terminal extended PYPs (NE-PYPs), the dark recovery times of wt-PYP and NE-PYPs, except for 5D-PYP, are independent of the structural differences between pG and pB2 states. These results demonstrate that the recovery time and the degree of structural change can be regulated by controlling the length and sequence of N-terminal residues of PYP. The findings in this study emphasize the need for careful attention to the remaining amino acid residues when designing recombinant proteins for genetic engineering purposes.

An evidence of electron donor behavior of single-walled carbon nanotube in photo-induced electron transfer

The electron transfer reaction between C60 and SWCNTs in D2O was investigated. Repeated laser irradiation of water-soluble C60 encapsulated in γ-cyclodextrin and SWCNTs with carboxymethylcellulose dispersed in D2O resulted in the accumulation of C60 anion radicals and a decrease in the E11 absorption band of SWCNTs, strongly suggesting electron transfer between C60 and SWCNTs. Transient absorption techniques revealed that this electron transfer occurred through the excited triplet of C60.

Ultrafast dynamic processes and nonlinear optical properties of Platinum(II) fluoren-acetylides

Two N-(4-ethylnylphenyl)-9H-fluoren-9-imine (EFT)arylacetylides (1, 2) and three EFT based Platinum(II) arylacetylides (3, 4, 5) were synthesized. The ultrafast dynamic processes after excitation and the nonlinear optical properties for picosecond lasers were investigatedusing femtosecond transient absorption and Z-scan techniques. The influence of conjugated length and Pt(II) content percentage on the properties ofthese moleculeswas discussed in detail. For compounds 1 and 2, two dynamic decay processes are observed after excitationby the pump laser beams. The first process is an ultrafast singlet (first singlet excited state, S1S*) → singlet state (ground state, GS), followed by a geometric twisting charge transfer state (CTS*) of the fluoren units relative to the central arylacetylide plane. For 3, 4, and 5, three dynamic decay processes are observed. The first process is the same as those of 1 and 2. However,the geometric twisting of 3, 4, and 5 is easier than 1 and 2 due to the presence of Pt atoms in the molecular skeleton of 3, 4, and 5, which makes a fast S1S* → geometric twisting state (TS*) occurred before the charge transfer occurring, and then a subsequent geometric twisting state (TS*) to geometric twisting charge transfer state (CTS*) occurs. The nonlinear optical properties of these compounds, studied using the open aperture Z-scan technique (λ = 532 nm, pulse width 23 ps), show that the values of the title compounds are around tens of cm/GW, which are comparable to newly reported porphyrin-based MOF materials and much better than Ru(II) terpyridine complexes..The mechanism of the nonlinear optical properties is attributed to the excited state absorption based on S1S*, TS*, and CTS* states, according to the femtosecond transient absorption and Z-scan results. The increase of Pt content percentage does not benefit the nonlinear absorption of Pt-containing compounds for the picosecond pulse laser. This is because the increase of the intersystem crossing rate weakly contributes to the nonlinear optical properties.

Competition of Carrier Kinetics Contributes to Amplified Spontaneous Emission in Quasi-2D/3D (PBA)2MAn–1PbnBr3n+1 Thin Films under Strip Light Mode

Quasi-2D halide perovskites have potential in lasing due to their amplified spontaneous emission (ASE) properties. The ASE of (PBA)2MAn-1PbnBr3n+1 thin films has been confirmed by photoluminescence (PL) testing using stripe light excitation (SLE). The ASE threshold decreases with decreasing environmental temperature (TE) or increasing number of inorganic layers (n). Using the transient absorption technique, the Auger recombination and the cooling process of the high-activity carrier are accelerated with the decrease of n or TE. A new ASE mechanism is proposed where high-activity carriers directly emit photons under photon perturbation from adjacent sites, leading to the accumulation and amplification of emitted photons only in the SLE region for ASE to occur. In addition, the reduction of n promotes light scattering between nano-thin layers, which supports a rapid increase in the ASE signal after the ASE threshold is crossed.

Study on carrier dynamics of perovskite solar cells via transient absorption

Perovskite solar cells (PSCs) are the most promising candidate for next-generation photovoltaics. The carrier dynamics behavior in perovskite is closely related to the performance of the solar cell. Particularly, manipulating the hot carriers has the potential to achieve ultrahigh-efficiency solar cells beyond the conventional Shockley-Queisser efficiency limit. Therefore, it is crucial to study and understand the carrier transport process in perovskite. However, the carrier dynamics behavior in the femtosecond time scale is difficult to be detected. Nowadays, femtosecond transient absorption techniques address this challenge effectively. We summarize the monitoring of hot carriers cooling, trapping of defect states, and charge transfer process after photoexcitation of PSCs by transient absorption and analyze the effective utilization of carriers in these processes. Finally, an outlook that will encounter in its future development is provided.

Efficient Charge Transfer in MAPbI3 QDs/TiO2 Heterojunctions for High-Performance Solar Cells

Methylammonium lead iodide (MAPbI3) perovskite quantum dots (QDs) have become one of the most promising materials for optoelectronics. Understanding the dynamics of the charge transfer from MAPbI3 QDs to the charge transport layer (CTL) is critical for improving the performance of MAPbI3 QD photoelectronic devices. However, there is currently less consensus on this. In this study, we used an ultrafast transient absorption (TA) technique to investigate the dynamics of charge transfer from MAPbI3 QDs to CTL titanium dioxide (TiO2), elucidating the dependence of these kinetics on QD size with an injection rate from 1.6 × 1010 to 4.3 × 1010 s-1. A QD solar cell based on MAPbI3/TiO2 junctions with a high-power conversion efficiency (PCE) of 11.03% was fabricated, indicating its great potential for application in high-performance solar cells.

Nonlinear optical properties and transient absorption in Hibiscus Sabdariffa dye solution probed with femtosecond pulses

Highly conjugated organic molecules are attractive chromophores for nonlinear optics due to their significant third-order nonlinear response near the absorption bands located in the broad spectral range from 200 to 900 nm. In this work, we report on the wavelength-dependence of one- and two-photon absorption processes in Hibiscus sabdariffa organic dye excited by femtosecond (fs) laser pulses centered at 1030 nm and 515 nm. Z-scan and transient absorption (TA) techniques have been employed to investigate the absorption and relaxation processes in the target dye. The dye is found to have a significant negative nonlinear absorption coefficient (β) of value ∼ −2.7 × 10−11 cm/W at 515 nm probing laser pulses, while it has a positive value equal to +2.1 × 10−13 cm/W when 1030 nm laser pulses were employed. The TA decay measurements using 40 fs laser pulses, centered at 515 nm wavelength, reveal that the dye exhibits a short relaxation time of ∼3 ps attributed to the involvement of the excited state proton transfer (ESPT) process in the Hibiscus dye solution. Moreover, our time-integrated fluorescence measurements revealed that no fluorescence emission could be detected. The application of ultrashort pulses of ∼40 fs duration enabled the tracking of ESPT in the Hibiscus dye, which was not detected in previous studies that employed laser pulses of picoseconds and nanoseconds duration.

Ultrafast Electron Transfer Dynamics of Organic Polymer Nanoparticles with Graphene Oxide.

We prepared organic polymer poly-3-hexylthiophene (p3ht) nanoparticles (NPs) and graphene oxide (GO)/reduced graphene oxide (RGO) composites p3ht NPs-GO/RGO by using the reprecipitation method. We demonstrated that GO/RGO could improve the ordering and planarity of p3ht chains as well as the formation of p3ht NPs, and confirmed the effects of GO/RGO on the fluorescence and carrier transport dynamics of p3ht NPs by using femtosecond fluorescence upconversion and transient absorption (TA) techniques. Ultrafast electron transfer (∼1 ps) between GO/RGO and p3ht NPs quenched the fluorescence of p3ht NPs, indicating excellent properties of p3ht NPs-GO/RGO as the charge transfer complexes. Efficient electron transfer may promote the applications of p3ht NPs-GO/RGO composites in organic polymer solar cells and photocatalysis. Moreover, RGO had stronger interfacial interactions and more matched conduction band energy levels with p3ht NPs than GO did, which implied that p3ht NPs-RGO might have greater application values than p3ht NPs-GO.

Identification of Ultraviolet Photoinduced Presolvated Electrons in Water as the Reducing Agent in the Photoreduction of Graphene Oxide

Light-induced reduction of graphene oxide (GO) is a promising and an ingenious way of producing reduced GO (rGO) because it avoids the use of harsh chemicals. However, to date, the physicochemical mechanisms underlying photoreduction remain controversial. For example, there is no consensus on whether GO is reduced in water via direct excitation using ultraviolet (UV) laser light or via radiologically produced solvated electrons. Because of the photoinduced solvated electrons, the excited state of GO exhibits similar electronic absorption responses in the visible and near-infrared (NIR) wavelength range; therefore, distinguishing GO photoreduction dynamics from solvated electron dynamics via transient electronic absorption techniques is challenging. In this work, we used femtosecond-stimulated Raman spectroscopy (FSRS) to understand the ultrafast photoreduction dynamics of GO dispersion in water. Raman pump wavelength was selected to prevent the resonance of the excited-state Raman signal of water while obtaining the fingerprint Raman signals of GO. In contrast to the results of previous studies, our FSRS results indicate that upon UV excitation, the reduction of GO is triggered by the precursor of the solvated electron (presolvated electron) in 300 fs.

Transient optical absorption technique to test timing properties of LYSO:Ce scintillators for the CMS Barrel Timing Layer

A novel technique for testing the timing properties of scintillators is presented. The technique is based on transient absorption (TA) induced in a scintillating material by a selective excitation of the activator ion. A figure of merit to assess the timing properties of scintillators is suggested. This parameter was estimated for a set of cerium doped lutetium–yttrium oxyorthosilicate (LYSO:Ce) bars, which have been fabricated for Barrel Timing Layer sensor of Compact Muon Solenoid detector (CMS BTL) and exhibited different timing properties, and compared with the results obtained by conventional coincidence time resolution (CTR) measurements. The figure of merit applied for the tested bars shows a strong correlation (Pearson's correlation coefficient R = 0.95) with the CTR. These results suggest that the TA technique could be used as an experimental method to expand in a complementary way the extensive qualification procedure of LYSO:Ce crystals that will be performed for the production of the CMS BTL detector.

Direct Tracking of Charge Carrier Drift and Extraction from Perovskite Solar Cells by Means of Transient Electroabsorption Spectroscopy.

The best perovskite solar cells currently demonstrate more than 25% efficiencies, yet many fundamental processes that determine the operation of these devices are still not fully understood. In particular, even though the device performance strongly depends on charge carrier transport across the perovskite layer to selective electrodes, information about this process is still very controversial. Here, we investigate charge carrier motion and extraction from an archetypical CH3NH3PbI3 (MAPI) perovskite solar cell. We use the ultrafast electric-field-modulated transient absorption technique, which allows us to evaluate the electric field dynamics from the time-resolved electroabsorption spectra and to visualize the motion of charge carriers with subpicosecond time resolution. We demonstrate that photogenerated holes drift across the mesoporous TiO2/perovskite layer during hundreds of picoseconds. On the other hand, their extraction into the spiro-OMeTAD hole transporting layer lasts for more than 1 nanosecond, suggesting that the hole extraction is limited by the perovskite/spiro-OMeTAD interface rather than by the hole transport through the perovskite layer. Additionally, we use the ultrafast time-resolved fluorescence technique that reveals fluorescence decay during tens of picoseconds, which we attribute to the spatial separation of electrons and holes.

Nonlinear optical absorption and two-photon luminescent properties of FAPbBr3 nanocrystals

Typical lead-based halide perovskite has been extensively explored as a light-harvesting and emission material for solar cell and light-emitting diodes in the past ten years. But it remains challenging to improve the optoelectronic characteristics and clarify the photophysical mechanism with the interaction of laser. Herein, the nonlinear optical absorption and two-photon luminescent properties of FAPbBr3 (FA=CH(NH2)2+) nanocrystals (NCs) are investigated by means of open-aperture Z-scan, steady-state/time-resolved photoluminescence (PL), and transient absorption (TA) techniques. When the laser excitation wavelength varies from 475 to 532 nm in open-aperture Z-scan measurement, strong saturated absorption can be found and its saturation intensity increases. Compared with normal luminescence, the PL emission peak has a red shift of 6 nm and increased PL lifetime under 800 nm femtosecond laser excitation, and the linewidth changes smaller considerably from 114 to 94 meV. This difference is mainly attributed to the different relaxation pathways. The TA spectra show the PL peak positions and full width at half maximum different features under 400 or 800 nm femtosecond excitation on the time scale. Due to different selection rules, the 400 or 800 nm femtosecond excitations can induce electrons populated different states and then relax to the same lowest excited state. Hence, we can estimate that different states induce the difference in steady-state PL, TRPL, and TA decays. These results may contribute to the application of perovskite NCs for multifunctional photonic sources and nanolaser devices under strong laser field.

Ultrafast photoinduced carrier dynamics in single crystalline perovskite films

Carrier dynamic processes in CsPbBr3 single crystalline perovskite, including hot carrier cooling, defect state trapping and carrier recombination, were studied using micro-region transient absorption technique.

Broadband transient absorption spectroscopy using an incoherent white-light source as probe.

Time-resolved spectroscopy and, in particular, transient absorption methods have been widely employed to study the dynamics of materials, usually achieving time resolution down to femtoseconds with measurement windows up to a few nanoseconds. Various techniques have been developed to extend the measurement duration up to milliseconds and beyond to permit probing slower dynamics. However, most of these either demand complicated and expensive equipment or do not provide broadband spectral coverage. This paper proposes a transient absorption technique in which an ultra-short pulse laser and a broadband incoherent continuous-wave light source are employed as pump and probe, respectively. Detection of the transient probe transmission is performed in a time-resolved fashion with a fast photodiode after a monochromator and the data is recorded with an oscilloscope. The time resolution is determined by the electronic bandwidth of the detection and acquisition devices and is ∼1 ns, with a measurement duration window of up to milliseconds and a spectral resolution of <2 nm covering from 0.4 to 2 µm. In addition, the setup can be employed to measure time- and spectrally-resolved photoluminescence.

Multi-exciton properties of InP/ZnS core-shell quantum dots

The multi-exciton properties of InP/ZnS core-shell quantum dots were investigated with the excitation of 400 nm fs pulses by transient absorption technique. By fitting experimental results, the lifetimes of multi-exciton were obtained. The lifetime of 4-exciton was about 3.65 ps, 3-exciton was about 9.7 ps and bi-exciton was about 43 ps respectively. The relationship between the Auger recombination rate of multi-exciton and the number of excitons in InP/ZnS quantum dots obeyed cubic statistics. The obtained first-order Auger recombination rate constant was about 1/88 ps−1. Under incident intensities of 1.8 × 105 W/m2, up to four excitons was observed, and average 3.4 excitons were generated per InP/ZnS quantum dot.