Femtosecond Transient Spectra Research Articles

Fluorescent Probes Based on AIEgen‐Mediated Polyelectrolyte Assemblies for Manipulating Intramolecular Motion and Magnetic Relaxivity

AbstractUniting photothermal therapy (PTT) with magnetic resonance imaging (MRI) holds great potential in nanotheranostics. However, the extensively utilized hydrophobicity‐driven assembling strategy not only restricts the intramolecular motion‐induced PTT, but also blocks the interactions between MR agents and water. Herein, we report an aggregation‐induced emission luminogen (AIEgen)‐mediated polyelectrolyte nanoassemblies (APN) strategy, which bestows a unique “soft” inner microenvironment with good water permeability. Femtosecond transient spectra verify that APN well activates intramolecular motion from the twisted intramolecular charge transfer process. This de novo APN strategy uniting synergistically three factors (rotational motion, local motion, and hydration number) brings out high MR relaxivity. For the first time, APN strategy has successfully modulated both intramolecular motion and magnetic relaxivity, achieving fluorescence lifetime imaging of tumor spheroids and spatio‐temporal MRI‐guided high‐efficient PTT.

Fluorescent Probes Based on AIEgen-Mediated Polyelectrolyte Assemblies for Manipulating Intramolecular Motion and Magnetic Relaxivity.

Uniting photothermal therapy (PTT) with magnetic resonance imaging (MRI) holds great potential in nanotheranostics. However, the extensively utilized hydrophobicity-driven assembling strategy not only restricts the intramolecular motion-induced PTT, but also blocks the interactions between MR agents and water. Herein, we report an aggregation-induced emission luminogen (AIEgen)-mediated polyelectrolyte nanoassemblies (APN) strategy, which bestows a unique "soft" inner microenvironment with good water permeability. Femtosecond transient spectra verify that APN well activates intramolecular motion from the twisted intramolecular charge transfer process. This de novo APN strategy uniting synergistically three factors (rotational motion, local motion, and hydration number) brings out high MR relaxivity. For the first time, APN strategy has successfully modulated both intramolecular motion and magnetic relaxivity, achieving fluorescence lifetime imaging of tumor spheroids and spatio-temporal MRI-guided high-efficient PTT.

Multiexponential dynamics of Mn2+(3d5) excitation in manganese doped ZnCdS quantum dots: Stimulated emission band in femtosecond transient spectra reveals ultrafast nonradiative energy transfer to Mn2+(3d5)

Stimulated emission (SE) 4T1 → 6A1 bands of Mn2+(3d5) in ZnxCd1−xS or ZnxCd1−xS/ZnS quantum dots were detected in the femtosecond transient absorption spectra at a time delay of 120 fs. Multiexponential rise of SE(Mn2+) amplitude due to nonradiative energy transfer to Mn2+(3d5) occurs both in the femtosecond and picosecond time domain. Transient kinetics of SE(Mn2+) band reveals the dynamics of energy transfer to Mn2+(3d5) with time constants τ1 ~ 400–600 fs, τ2 ~ 4–6 ps and τ3 ~ 87–112 ps. A possible mechanism of ultrafast energy transfer to the Mn2+(3d5) center accompanied by spin-flip is discussed.

Achieving naphthalimide-based aggregation-enhanced emission via the fluorophore-linker-aromatic strategy

The fluorophore-linker-aromatic structure has great promise to improve dyes’ performance by regulating the embedded blocks. Inspired by its versatility, here we propose that aggregation-caused quenching can be converted to aggregation-enhanced emission (AEE) via the structure. To verify this speculation, we designed a novel dye with a naphthalimide-hexyl-quinazoline construction. The novel dye showed strengthened emission in aggregates, which is superior to traditional naphthalimide derivatives. The theoretical calculation and femtosecond transient spectra implied that the photoexcitation-induced geometry alteration could be restricted in the aggregated NIQZ, thus producing brighter fluorescence. On the basis of this mechanism, NIQZ was successfully applied to viscous lipid droplets visualization. This is the first work in which the fluorophore-linker-aromatic structure is utilized to fabricate AEE dye.

(Invited) High-Yield and Long-Lived Triplet Excited States of Pentacene Alkanethiolate Monolayer Protected Gold Nanoparticles By Singlet Fission

Photochemical conversion to high-yield and log-lived triplet excited states of organic molecules is an interesting research content. For example, three-dimensional self-assembled monolayers (SAMs) of organic dyes on metal nanoparticles (MNP) are highly promising as photofunctional nanomaterials. A major research topic is light energy conversion via photoinduced processes such as electron and energy transfer. The other application is optical molecular rulers (nanoruler) in the biological fields. In these cases, a serious problem of organic dye-modified SAMs on MNP is the significant deactivation pathway of the excited states of dyes caused mainly by the fast nanosurface energy transfer (NSET). Actually, more than 90% of fluorescence is quenched and formation of the excited triplet states is negligible. Therefore, the formation of high-yield and long-lived triplet excited states on MNP is favorable for various applications relaying on photoinduced processes. Generation of efficient singlet fission (SF) on gold nanoparticles (GNP) is a promising method to overcome the above-mentioned problem. SF is described as a spin-allowed process in which one singlet exciton splits into two triplet excitons.1 In our study, TIPS-pentacene-alkanethiolate monolayer-protected gold nanoparticles with different chain lengths and particle sizes were successfully synthesized to efficiently generate excited triplet states by singlet fission (Fig. 1). Time-resolved spectroscopy revealed high-yield excited triplet states by suppressing ET to the gold surface.2 The mean diameters (R CORE) of TP-C11-S-MPC and TP-C11-L-MPC are 1.65 ± 0.30 nm and 2.13 ± 0.33 nm, respectively. The core of TP-C11-S-MPC and TP-C11-L-MPC contains 158 and 338 Au atoms, respectively. Moreover, based on the values of elemental analysis, there are 51 and 75 TP molecules on one GNP. In addition, the coverage ratios of TP units (γ) to surface Au atoms in TP-C11-S-MPC and TP-C11-L-MPC are 63% and 51%, respectively. Steady-state absorption and fluorescence spectra of TP-Cn-X-MPCs and TP-Ref were measured in toluene. In absorption spectra (Fig. 2A), the spectral shape of TP-C11-S-MPC (spectrum a) matches well with that of TP-Ref (spectrum b).The fluorescence of TP-C11-S-MPC was strongly quenched relative to TP-Ref. To compare the emission spectral shapes, the fluorescence spectra of TP-C11-S-MPC (spectrum a) and TP-Ref (spectrum b) were normalized in Fig. 2B. The fluorescence excitation spectrum was observed (spectrum c in Fig. 2A) to carefully check the fluorescence species of TP-C11-S-MPC. Then, Fig. 2C shows the femtosecond transient spectra of TP-C11-S-MPC. After laser pulse excitation, the triplet–triplet absorption of TP units at ca. 520 nm develops, whereas the singlet–singlet absorption at ca. 630 nm decays. The maximum Φ Τ values attained 172 ± 26% and 157 ± 17% in TP-C11-S-MPC and TP-C7-S-MPC, respectively. [1] Hasobe, T. et al, J. Phys. Chem. A 2016, 120, 1867. [2] Hasobe, T. et al, Angew. Chem. Int. Ed. 2016, 55, 5230. Figure 1

Electron Transfer Mechanism in Organometallic Molecules Studied by Subpicosecond Extended X-ray Absorption Fine Structure Spectroscopy

The mechanism responsible for the redox reaction of [Co(III)(en)3]Ac3 to Co(II) complex has been determined to be intramolecular electron transfer. It was measured in real time by means of subpicosecond extended X-ray absorption fine structure spectra, EXAFS, and optical experiments and supported by density functional theory calculations. The proposed mechanism is based on histograms of bond length changes of the transient structures measured as a function of time, with subpicosecond time and sub-Angstrom resolution and femtosecond transient spectra and kinetics after excitation with a 267 nm femtosecond pulse. Even though four Fe and Co complexes were excited in the charge transfer band and the photoinduced redox reaction proceeds with similar high redox quantum yield, the dominant electron operating mechanism differs: intramolecular for amine metal complexes and intermolecular for oxalate metal complexes. The ligand orientation degree of freedom and counterion effect are proposed to provide tentative explanation for the electron transfer mechanism.

Photochemical dynamics of E-iPr-furylfulgide

As an important theoretical step towards unraveling the mechanistic details of the photochemical switching processes in molecules of the fulgide type, we carried out a large-scale, full-dimensional computational study of the ring closure reaction of E-iPr-furylfulgide. Simulated static UV spectra and femtosecond transient spectra are in good agreement with their experimental counterparts. Using surface-hopping photodynamics simulations, we identify three major de-excitation pathways and their interplay. The dominant photochemical pathway (70% of the trajectories) allows for ring closure, while the two minor pathways involve E-Z double bond isomerization rather than cyclization. The relative abundance of the pathways is rationalized by arguments linking structure with dynamics. It should be emphasized, however, that the distinction into three pathways is only a simplified interpretational model, since the actual dynamical trajectories do not strictly follow these idealized pathways but often show mixed behaviour, evolving along two or three of them during the course of the simulation.

Spectral dependence of energy transfer in wild-type peripheral light-harvesting complexes of photosynthetic bacteria

The precise position of the upper exciton component and relevant vibronic transitions of the B850 ring in peripheral light-harvesting complexes from purple photosynthetic bacteria are important values for determining the exciton bandwidth and electronic structure of the B850 ring. To determine the presence of these components in wild-type LH2 complexes the pump-probe femtosecond transient spectra obtained with excitation into the 730–840 nm spectral range are analyzed. We show that at excitation wavelengths less than 780 nm B850 absorption bands are present and that, in accordance with exciton theory, these bands peak further in the blue when the lowest optically allowed transition is more red-shifted.

S1 → Sn and S2 → Sn Absorption of Azulene: Femtosecond Transient Spectra and Excited State Calculations

The transient spectra of azulene in solution have been measured in the spectral region 600−350 nm at room temperature, pumping into the S1 and S2 states with femtosecond pulses and probing with a delayed femtosecond white light continumm. The spectra are a combination of ground-state bleaching, stimulated emission, and excited-state absorption. The latter component gives a direct information on excited states which may be not active in the ground-state absorption. S1 → Sn and S2 → Sn absorptions have been discussed with the help of ab initio calculations of the MCSCF/CAS type with the 6-31G* basis set and including perturbative corrections. The calculated S0 → Sn, S1,eq → Sn, and S2,eq → Sn vertical excitation energies and oscillator strengths are in satisfactory agreement with the experimental results. It has been found that electronic states, weakly active in the ground-state absorption, occur with high intensity in the femtosecond transient spectra, in particular in the energy range 36000−44000 cm-1 above the ground-state energy.

High-density femtosecond transient absorption spectroscopy of semiconductor nanoparticles. A tool to investigate surface quality

Abstract At the high laser excitation intensities used in our experiments, more than 50 electron-hole pairs are formed in colloidal semiconductor nanoparticles used in our studies. At this density of charge carriers, new transient absorptions are observed in the femtosecond transient spectra in the 450 to 700 nm region with unresolved fast rise (<100 fs) and two decay components of 660 fs and >150 ps. The absorption at 510 nm could be quenched with the adsorption of electron acceptors (e.g., benzoquinone, 1,2-naphthoquinone), whereas the low-energy transient absorption was not affected. For CdS NPs, we found that passivation eliminated most of the transient absorption. The transient absorptions are thus proposed to result from either trap-state absorption, trapped dimers (or complexes) and/or Stark-shifted exciton absorption resulting from surface electric field of the uncompensated trapped electron-hole pairs. All these possibilities require effective surface trapping at these high levels of excitation.

Femtosecond processes and ultrafast biological electron transfer

In this paper, we report the calculated femtosecond transient spectra of the primary electron transfer in the bacterial reaction center of Rhodobacter sphaeroides R26. The excitation dependence of the time-resolved spectra is simulated. The effects of vibrational coherence on the time-resolved spectra are discussed. The electron transfer accompanying vibrational relaxation is theoretically investigated using the single-vibronic level electron transfer rate constants. The effects of multi-modes and temperature on the electron transfer rate constants are also reported.