Transient Absorption Studies Research Articles

Cobalt(III) Carbene Complex with an Electronic Excited-State Structure Similar to Cyclometalated Iridium(III) Compounds.

Many organometallic iridium(III) complexes have photoactive excited states with mixed metal-to-ligand and intraligand charge transfer (MLCT/ILCT) character, which form the basis for numerous applications in photophysics and photochemistry. Cobalt(III) complexes with analogous MLCT excited-state properties seem to be unknown yet, despite the fact that iridium(III) and cobalt(III) can adopt identical low-spin d6 valence electron configurations due to their close chemical relationship. Using a rigid tridentate chelate ligand (LCNC), in which a central amido π-donor is flanked by two σ-donating N-heterocyclic carbene subunits, we obtained a robust homoleptic complex [Co(LCNC)2](PF6), featuring a photoactive excited state with substantial MLCT character. Compared to the vast majority of isoelectronic iron(II) complexes, the MLCT state of [Co(LCNC)2](PF6) is long-lived because it does not deactivate as efficiently into lower-lying metal-centered excited states; furthermore, it engages directly in photoinduced electron transfer reactions. The comparison with [Fe(LCNC)2](PF6), as well as structural, electrochemical, and UV–vis transient absorption studies, provides insight into new ligand design principles for first-row transition-metal complexes with photophysical and photochemical properties reminiscent of those known from the platinum group metals.

Carboranethiol-Protected Propeller-Shaped Photoresponsive Silver Nanomolecule

We report the synthesis, structural characterization, and photophysical properties of a propeller-shaped Ag21 nanomolecule with six rotary arms, protected with m-carborane-9-thiol (MCT) and triphenylphosphine (TPP) ligands. Structural analysis reveals that the nanomolecule has an Ag13 central icosahedral core with six directly connected silver atoms and two more silver atoms connected through three Ag-S-Ag bridging motifs. While 12 MCT ligands protect the core through metal-thiolate bonds in a 3-6-3-layered fashion, two TPP ligands solely protect the two bridging silver atoms. Interestingly, the rotational orientation of a silver sulfide staple motif is opposite to the orientation of carborane ligands, resembling the existence of a bidirectional rotational orientation in the nanomolecule. Careful analysis reveals that the orientation of carborane ligands on the cluster's surface resembles an assembly of double rotors. The zero circular dichroism signal indicates its achiral nature in solution. There are multiple absorption peaks in its UV-vis absorption spectrum, characteristic of a quantized electronic structure. The spectrum appears as a fingerprint for the cluster. High-resolution electrospray ionization mass spectrometry proves the structure and composition of the nanocluster in solution, and systematic fragmentation of the molecular ion starts with the loss of surface-bound ligands with increasing collision energy. Its multiple optical absorption features are in good agreement with the theoretically calculated spectrum. The cluster shows a narrow near-IR emission at 814 nm. The Ag21 nanomolecule is thermally stable at ambient conditions up to 100 °C. However, white-light illumination (lamp power = 120-160 W) shows photosensitivity, and this induces structural distortion, as confirmed by changes in the Raman and electronic absorption spectra. Femtosecond and nanosecond transient absorption studies reveal an exceptionally stable excited state having a lifetime of 3.26 ± 0.02 μs for the carriers, spread over a broad wavelength region of 520-650 nm. The formation of core-centered long-lived carriers in the excited state is responsible for the observed light-activated structural distortion.

Ultrafast carrier dynamics in CdS@CdSe core-shell quantum dot heterostructure

Semiconductor quantum dots (QDs) have aroused great fundamental and technological interest due to their novel size-tunable optical properties, and the resulting potential applications in photonics, photoelectronics and biomedicine. In this study, uniform, ultrasmall, well-defined CdS@CdSe core-shell QDs are successfully synthesized by a hot injection method for ultrafast carrier dynamics activities. Femtosecond transient absorption studies show that the bleach recovery time of the CdS@CdSe QDs increases with the thickness of the CdSe shell. In addition, theoretical calculations were carried out to reveal this characteristic. The slower carrier relaxation of the CdS@CdSe QDs indicates its great potential applications from light-emitting diodes to photodetectors and photovoltaics. In short, this work provides a valuable reference platform for the application of core-shell QD-based heterostructure in optics and optoelectronics.

Synthesis and C60 Binding of Aza[10]CPP and N-Methylaza[10]CPP

Within the growing family of strained carbon nanohoops and nanobelts, [10]CPP arguably offers the best compromise between synthetic accessibility and strong binding affinity for C60. In this work, we report the synthesis of two nitrogen-containing analogues of [10]CPP and we systematically compare the structure, optoelectronic properties and C60 binding affinities of this small set of structurally similar macrocycles. While Aza[10]CPP outcompetes the parent compound by approximately one order of magnitude with respect to C60 binding, we found that the reverse was true for the methylaza analogue. Transient absorption studies showed that photo-induced electron transfer occurred readily from [10]CPP and its aza-analogue to an encapsulated C60 guest. Formation of a charge-separated complex was not observed however for the N-methylated derivative. These insights will prove useful for further applications of strained nanohoops in supramolecular chemistry and organic electronics.

Photoexcitation Dynamics and Long-Lived Excitons in Strain-Engineered Transition Metal Dichalcogenides.

Strain-engineering in 2D transition metal dichalcogenide (TMD) semiconductors has garnered intense research interest in tailoring the optical properties via strain-induced modifications of the electronic bands in TMDs, while its impact on the exciton dynamics remains less understood. To address this, an extensive study of transient optical absorption (TA) of both W- and Mo-based single-crystalline monolayer TMDs grown by a recently developed laser-assisted evaporation method is performed. All spectral features of the monolayers as grown on fused silica substrates exhibit appreciable redshifts relating to the existence of strain due to growth conditions. Moreover, these systems exhibit a dramatic slowing down of exciton dynamics (100s of picoseconds to few nanoseconds) with an increase in carrier densities, which strongly contrasts with the monolayers in their freestanding form as well as in comparison with more traditionally grown TMDs. The observations are related to the modifications of the electronic bands as expected from the strain and associated population of the intervalley dark excitons that can now interplay with intravalley excitations. These findings are consistent across both the Mo- and W-based TMD families, providing key information about the influence of the growth conditions on the nature of optical excitations and fostering emerging optoelectronic applications of monolayer TMDs.

Blue or Near-Infrared Light-Triggered Release of Halogens via Blebbistatin Photocage.

Photocages can provide spatial and temporal control to accurately release the various chemicals and bioactive groups when excited by light. Although the absorption spectra of most photocages are in the ultraviolet absorption region, only a few absorb in the visible or near-infrared region. Blebbistatin (Bleb) would release a hydroxyl radical under blue one-photon or two-photon near-infrared light (800 nm) irradiation. In this work, typical chlorine and bromine as leaving groups substituted hydroxyl compounds (Bleb-Cl, Bleb-Br) are synthesized to evaluate the photocage's capability of Bleb's platform. Driven by the excited-state charge transfer, Bleb-Cl and Bleb-Br show good photolysis quantum yield to uncage the halogen anion and the uncaging process would be accelerated in water solution. The photochemical reaction, final product's analysis, and femtosecond transient absorption studies on Bleb-Cl/Bleb-Br demonstrate that Bleb can act as a photocage platform to release the halogen ion via heterolytic reaction when irradiated by blue or near-infrared light. Therefore, Bleb can be a new generation of visible or near-infrared light-triggered photocage.

Direct manipulation of atomic excitation with intense extreme-ultraviolet laser fields

The coherent excitation and manipulation of a two-level system with ultrashort intense extreme-ultraviolet laser fields is investigated theoretically, based on numerically solving the time-dependent Schr\"odinger equation. We are particularly interested in the dynamical phase excursion of the energy states over the course of the interaction and the resulting spectral modifications. Fitting the absorption line shapes with the Fano profile quantifies the asymmetry parameter and the corresponding dipole phase offset, capturing the phase difference of the state coefficients after the interaction. Nonperturbative analytical calculations using rectangular driving pulses are employed, yielding physical insights into the dependence of the dipole phase shift on the external field. The validity of the formulas is validated by comparing their predictions with numerical results, which proves to be robust against the variation of laser parameters. The present investigation of strong-field dressing effects complements recent attosecond transient absorption studies assuming weak excitation, and marks a ubiquitous phenomenon that should be generally considered for the interaction of matter with intense laser fields.

Optical and photophysical properties of platinum benzoporphyrins with C2v and D2h symmetry

Platinum(II) mono- and di-benzoporphyrins carrying either ester or imide peripheral groups have been successfully synthesized. These benzoporphyrins possessing C[Formula: see text] and D[Formula: see text] symmetry were investigated using a range of techniques including UV-Vis and phosphorescence spectroscopy, differential pulse voltammetry, and transient absorption spectroscopy covering wide spatial and temporal ranges. Up to three oxidation and three reductions within the accessible potential window of dichlorobenzene were observed. Facile reductions for both mono- and dibenzoporphyrin derivatives, more so with imide peripheral substituents were witnessed. Computational studies were supportive of push-pull type interactions within these molecular systems. The formation of triplet excited states within 5 ps after laser excitation was confirmed by femtosecond transient absorption studies. Nanosecond transient absorption studies coupled with phosphorescence lifetime studies revealed relatively rapid relaxation of the triplet excited states as compared to a controlled platinum porphyrin suggesting push-pull type interactions on a slower time scale.

Ultrafast Collective Excited-State Dynamics of a Virus-Supported Fluorophore Antenna.

Radiation brightening was recently observed in a multifluorophore-conjugated brome mosaic virus (BMV) particle at room temperature under pulsed excitation. On the basis of its nonlinear dependence on the number of chromophores, the origins of the phenomenon were attributed to a collective relaxation. However, the mechanism remains unknown. We present ultrafast transient absorption and fluorescence spectroscopic studies which shed new light on the collective nature of the relaxation dynamics in such radiation-brightened, multifluorophore particles. Our findings indicate that the emission dynamics is consistent with a superradiance mechanism. The ratio between the rates of competing radiative and nonradiative relaxation pathways depends on the number of chromophores per virus. The findings suggest that small icosahedral virus shells provide a unique biological scaffold for developing nonclassical, deep subwavelength light sources and may open new avenues for the development of photonic probes for medical imaging applications.

Impacts of CsPbBr3/PbSe Heterostructures on Carrier Cooling Dynamics at Low Carrier Density

AbstractMetal halide perovskite nanocrystals have recently emerged as a front‐runner material for high‐performance solar cells. However, slowing down the hot carrier (HC) cooling of perovskites at carrier densities comparable to the sun‐illumination level (≈1017 cm−3) is still a thriving challenge. A new strategy is presented to retard the HC cooling via charge localization at the CsPbBr3/PbSe heterostructure interface. Ultrafast transient absorption study reveals two times slower HC relaxation time (from 770 fs to 1.4 ps) and much higher initial HC temperature, TC (1663 K, compared to 900 K) for the heterostructure compared to the pure CsPbBr3 at 3.64 eV excitation, under 1.86 × 1017 cm−3 carrier density. Furthermore, a combination of an electron‐phonon coupling model and first‐principles calculations confirm the slow relaxation through the Klemens channel due to a large energy gap between the longitudinal optical (LO) and longitudinal acoustic (LA) phonon modes. The localization of charge density near the heterojunction is responsible for the up‐conversion of LO modes to the higher energy and retards the HC relaxation. The findings highlight a new protocol for achieving long‐lived HCs in perovskites at low carrier densities, which would benefit the future of HC photovoltaics.

Vibration-Assisted Intersystem Crossing in the Ultrafast Excited-State Relaxation Dynamics of Halocoumarins.

Due to its numerous applications, triplet formation and resulting phosphorescence remain a frontier area of research for over eight decades. Facile intersystem crossing (ISC) is the primary requirement for triplet formation and observation of phosphorescence. The incorporation of a heavy atom in molecules is one of the common approaches employed to facilitate ISC. A detailed study of the excited state dynamics that governs ISC is necessary to understand the mechanism of heavy atom effect (HAE). Incorporation of iodine at the 3 position of coumarin-1 reduces fluorescence quantum yield (ϕf) drastically as expected, whereas bromine substitution at the same position increased the ϕf. Such a contrasting effect of the two heavy atoms suggests that there are other features yet to be discovered to fully understand the HAE. Detailed steady state and femtosecond transient absorption studies along with theoretical calculations suggest that the C3-X (X = Br, I) bond vibration plays an important role in the ISC process. The study reveals that while in the case of the iodo-derivative there is no energy barrier in the singlet triplet crossing path, there is a barrier in the case of the bromo-derivative, which slows the ISC process. Such an unexpected phenomenon is not limited to halocoumarins as this rationalizes the photobehavior of 1-bromo-/iodo-substituted naphthalenes as well.

Effect of Phenolic Substituent Position in Boron Complexes of Imidazo[1,5‐a]pyridine

AbstractWe have synthesized two boron complexes of 3‐(o‐hydroxyphenyl)imidazo[1,5‐a]pyridine exhibiting blue emission in solution. The transient absorption and theoretical studies of the complexes and their 1‐(o‐hydroxyphenyl) substituted isomers revealed that the phenolic substituent position in boron complex of imidazo[1,5‐a]pyridine affected the excites state dynamics involved in photoluminescence (PL). Although the introduction of phenolic substituent at 1‐position of the boron complex is effective to achieve the strong emission, the introduction of phenolic substituent at 3‐position results in the decreasing PL quantum yield due to intersystem crossing from the singlet excited state to the triplet excited state.

Kinetics of Electron Returns in Successive Two-Photon DNA Repair by (6-4) Photolyase

(6-4) photolyase is a flavoenzyme that catalyzes the chemically challenging repair of carcinogenic (6-4) photoproduct lesions in DNA using sunlight. Nature may have overcome the challenge by combining two distinct successive photoreactions, each starting with electron transfer from an excited flavin to the lesion. We herewith report a transient absorption study of electron returns to flavin following the chemical transformations of the lesion. We have successfully dissected the kinetics of the electron returns that finalize the first and second photoreactions (∼40 μs and ∼200 ns, respectively) in the (6-4) photolyase from Xenopus laevis, corroborating and detailing the two-photon reaction model.

Kinetic evidence for methanol trimer assisted proton transfer: Transient absorption study on excited triplet state of 4-hydroxy-4′-nitrobiphenyl

Hydroxyaromatic compounds have a wide range of applications in catalytic synthesis and biological processes due to their enhanced acidity upon photo-excitation. Most hydroxyaromatic compounds with a medium excited state acidity are unable to deprotonate in non-aqueous solvents such as alcohol due to their short-lived excited singlet states. The nitro group in 4-hydroxy-4′-nitrobiphenyl (NO2-Bp-OH) increases the spin-orbit coupling between excited singlet states and the triplet state, resulting in ultrafast intersystem crossing and the formation of the long-lived lowest excited triplet state (T1) with a high yield. Using transient absorption spectroscopy and kinetic analysis, we discover that, despite its moderate acidity, the T1 state of NO2-Bp-OH (3NO2-Bp-OH) is able to transfer proton to methanol. Following the formation of the hydrogen-boned complex between 3NO2-Bp-OH and three methanol molecules in a consecutive process, proton transfer occurs very fast. This finding suggests that the long lifetime of the photoacid excited state allows for the formation of alcohol oligomer with sufficient basicity to induce photoacid deprotonation.

Temperature driven charge transfer process in quantum confined two-dimensional Mn-doped CsPbBr3 perovskite nanoplatelets.

A temperature dependent transient absorption study has been demonstrated for Mn2+ doped CsPbBr3 nanoplatelets. At 5 K, charge transfer is suppressed due to the trapping of charge carriers in defect states. By contrast, at 300 K, an efficient charge/energy transfer process is observed as the thermally active carriers become de-trapped from the defect states upon strong exciton-phonon coupling.

Synthesis and the spectral, electrochemical, and nonlinear optical properties of β-dicyanovinyl-appended 'push-pull' porphyrins.

A series of "push-pull" porphyrins, MTPP(MN)(TPA)2 (M = 2H, CuII, NiII, and ZnII), having triphenylamine (TPA) and dicyanovinyl (DCN) groups at antipodal positions were synthesized and characterised by UV-Vis, fluorescence and NMR spectroscopic techniques, MALDI-TOF mass spectrometry, cyclic voltammetry, DFT, and elemental analysis, which were then further utilized for third-order nonlinear optical measurements under mild conditions using femtosecond laser pulses. Remarkably, MTPP(MN)(TPA)2 (M = 2H, CuII, NiII, and ZnII) exhibited 21-48 nm and 38-80 nm bathochromic shifts in B and Qx(0,0) bands as compared to the corresponding MTPPs (M = 2H, CuII, NiII, and ZnII); the results are consistent with the effect of enhanced resonance due to TPA and -I effect of DCN moieties. In cyclic voltammetry, the push-pull porphyrins exhibited a cathodic shift (0.13-0.51 V) in their first oxidation potential as compared to the precursor owing to the presence of electron-donating TPA groups. The third-order nonlinear optical responses were recorded using a single-beam femtosecond Z-scan technique to retrieve information about the nonlinear absorption and nonlinear refraction of the samples. The two-photon absorption coefficients (β) are in the range of 0.87 × 10-13 to 4.28 × 10-13 m W-1 and the nonlinear refractive index (n2) in the range of 1.21 × 10-19 to 7.36 × 10-19 m2 W-1. The ultrafast absorption dynamics of the ground-state bleaching (GSB) and photo-induced absorption (PIA) are monitored by femtosecond broadband transient absorption studies. The strong nonlinearity of these push-pull porphyrins makes them potential candidates for nonlinear optical and photonic device applications.

Modulating the intersystem crossing mechanism of anthracene carboxyimide-based photosensitizers via structural adjustments and application as a potent photodynamic therapeutic reagent.

Herein, a series of compact anthracene carboxyimide (ACI) based donor-acceptor dyads were prepared by substituting bulky aryl moieties with various electron-donating ability to study the triplet-excited state properties. The ISC mechanism and triplet yield of the dyads were successfully tuned via structural manipulation. Efficient ISC (ΦΔ ≈ 99%) and long-lived triplet state (τT ≈ 122 μs) was observed for the orthogonal anthracene-labeled ACI derivative compared to the Ph-ACI and NP-ACI dyads, which showed fast triplet state decay (τT ≈ 7.7 μs). Femtosecond transient absorption study demonstrated the ultrafast charge separation (CS) and efficient charge recombination (CR) in the orthogonal dyads and ISC occurring via spin-orbit charge transfer (SOCT) mechanism (AN-ACI: τCS = 355 fs, τCR = 2.41 ns; PY-ACI: τCS = 321 fs, τCR = 1.61 ns), while in Ph-ACI and NP-ACI dyads triplet populate following the normal ISC channel (nπ* → ππ* transition), no CS was observed. We found that the attachment of suitable aryl donor moiety (AN- or PY-) to the ACI core can ensure the insertion of the intermediate triplet state, resulting in a small energy gap among charge separated state (CSS) and triplet state, which leads to efficient ISC in these derivatives. The SOCT-ISC-based AN-ACI dyad was confirmed to be a potent photodynamic therapeutic reagent; an ultra-low IC50 value (0.27 nM) that was nearly 214 times lower than that of the commercial Rose Bengal photosensitizer (57.8 nM) was observed.

Molecular rotational conformation controls the rate of singlet fission and triplet decay in pentacene dimers.

Three pentacene dimers have been synthesized to investigate the effect of molecular rotation and rotational conformations on singlet fission (SF). In all three dimers, the pentacene units are linked by a 1,4-diethynylphenylene spacer that provides almost unimpeded rotational freedom between the pentacene- and phenylene-subunits in the parent dimer. Substituents on the phenylene spacer add varying degrees of steric hindrance that restricts both the rotation and the equilibrium distribution of different conformers; the less restricted conformers exhibit faster SF and more rapid subsequent triplet-pair recombination. Furthermore, the rotational conformers have small shifts in their absorption spectra and this feature has been used to selectively excite different conformers and study the resulting SF. Femtosecond transient absorption studies at 100 K reveal that the same dimer can have orders of magnitude faster SF in a strongly coupled conformer compared to a more weakly coupled one. Measurements in polystyrene further show that the SF rate is nearly independent of viscosity whereas the triplet pair lifetime is considerably longer in a high viscosity medium. The results provide insight into design criteria for maintaining high initial SF rate while suppressing triplet recombination in intramolecular singlet fission.

Two-photon brightness of highly fluorescent imidazopyridine derivatives: Two-photon and ultrafast transient absorption studies

Imidazopyridine derivatives are unnatural purines isosteres and have been idealized and synthesized in the last decades, given their great applicability in several science areas. For instance, they may present potential to be used as fluorescent bioprobes on DNA or RNA spectroscopic studies. Furthermore, their synthesis can be performed to incorporate different peripheral structures in the central imidazopyridine scaffold, modeling and increasing the optical properties. Aiming applications as fluorescent probes as molecular liquids at the therapeutic window, thanks to the considerable emissive characteristics of this class of compound, optical spectroscopic studies considering nonlinear optical properties were performed on two distinct classes of N3-MEM-3H-imidazo[4,5-b]pyridine derivatives. The presented results exposed exceptionally high fluorescence quantum yields and two-photon absorption effects at the therapeutic window. The two-photon absorption spectra revealed a particular contribution of a higher excited state in some imidazopyridine derivatives containing naphthalene group. The excited state was confirmed by femtosecond time resolved spectroscopy. The obtained two-photon brightness shows substantial values for all compounds, with the potential to be used as fluorescent probes induced by two-photon excitation.

In-Depth Studies of Ground- and Excited-State Properties of Re(I) Carbonyl Complexes Bearing 2,2':6',2″-Terpyridine and 2,6-Bis(pyrazin-2-yl)pyridine Coupled with π-Conjugated Aryl Chromophores.

In the current work, comprehensive photophysical and electrochemical studies were performed for eight rhenium(I) complexes incorporating 2,2′:6′,2″-terpyridine (terpy) and 2,6-bis(pyrazin-2-yl)pyridine (dppy) with appended 1-naphthyl-, 2-naphthyl-, 9-phenanthrenyl, and 1-pyrenyl groups. Naphthyl and phenanthrenyl substituents marginally affected the energy of the MLCT absorption and emission bands, signaling a weak electronic coupling of the appended aryl group with the Re(I) center. The triplet MLCT state in these complexes is so low lying relative to the triplet 3ILaryl that the thermal population of the triplet excited state delocalized on the organic chromophore is ineffective. The attachment of the electron-rich pyrenyl group resulted in a noticeable red shift and a significant increase in molar absorption coefficients of the lowest energy absorption of the resulting Re(I) complexes due to the contribution of intraligand charge-transfer (ILCT) transitions occurring from the pyrenyl substituent to the terpy/dppy core. At 77 K, the excited states of [ReCl(CO)3(Ln-κ2N)] with 1-pyrenyl-functionalized ligands were found to have predominant 3ILpyrene/3ILCTpyrene→terpy character. The 3IL/3ILCT nature of the lowest energy excited state of [ReCl(CO)3(4′-(1-pyrenyl)-terpy-κ2N)] was also evidenced by nanosecond transient absorption and time-resolved emission spectroscopy. Enhanced room-temperature emission lifetimes of the complexes [ReCl(CO)3(Ln-κ2N)] with 1-pyrenyl-substituted ligands are indicative of the thermal activation between 3MLCT and 3IL/3ILCT excited states. Deactivation pathways occurring upon light excitation in [ReCl(CO)3(4′-(1-naphthyl)-terpy-κ2N)] and [ReCl(CO)3(4′-(1-pyrenyl)-terpy-κ2N)] were determined by femtosecond transient absorption studies.