Excited-state Processes Research Articles

Explanation for the Multi-Component Scintillation of Cerium Fluoride Through the Equilibrium and Photophysical Investigation of Cerium(III)-Fluoro Complexes.

CeF3 displays favorable scintillation properties, which have been utilized for decades in various solid-state systems. Its emission undergoes multi-component decays, which were interpreted by lattice defects and so-called intrinsic features herein. This study of the complex equilibria in connection with photophysical behavior of the cerium(III)-fluoride system in solution gave us the possibility to reveal the individual contribution of the [CeIIIFx(H2O)9−x]3−x species to the photoluminescence. Spectrophotometry and spectrofluorometry (also in time-resolved mode) were used, and combined with sophisticated evaluation methods regarding both the complex equilibria and the kinetics of the photoinduced processes. The individual photophysical parameters of the [CeIIIFx(H2O)9−x]3−x complexes were determined. For the kinetic evaluation, three methods of various simplifications were applied and compared. The results indicated that the rates of some excited-state equilibrium processes were comparable to those of the emission decay steps. Our results also contribute to the explanation of the multi-component emission decays in the CeF3-containing scintillators, due to the various coordination environments of Ce3+, which can be affected by the excitation leading to the dissociation of the metal-ligand bonds.

Femtosecond Absorption Spectroscopy of Reduced and Oxidized Forms of Cytochrome c Oxidase: Excited States and Relaxation Processes in Heme a and a3 Centers

Excited electronic states and intraheme relaxation processes in the oxidized and reduced forms of mitochondrial cytochrome c oxidase extracted from a beef heart have been investigated by femtosecond absorption spectroscopy. The spectral and kinetic characteristics of short-lived intermediates have been measured from 80 fs to 20 ps after the photoexcitation. It is found that nonradiative electronic relaxation of the excitation energy in heme a, both in the oxidized (Fe(III)a) and reduced (Fe(II)a) forms, occurs successively as three processes, after the end of which heme a is in the ground state with a large store of vibrational energy. The subsequent vibrational relaxation (heme cooling) lasts for several picoseconds. It is found for reduced heme a3 (Fe(II)a3) that the electronic relaxation occurs as a result of two successive stages, which changes to vibrational relaxation in the ground state. The mechanism and dynamics of electronic excitation energy conversion in cytochrome c oxidase are analyzed.

Temperature-driven anion migration in gradient halide perovskites.

Cesium lead halide perovskite films with a systematic change in the halide composition of CsPbBr3-xIx, in which iodide concentration varies from x = 0 to x = 3, provide a built-in gradient band structure. Such a gradient structure allows for the integrated capture of visible photons and directs them to the energetically low-lying iodide rich region. Annealing gradient halide perovskite films at temperatures ranging from 50 °C to 90 °C causes the films to homogenize into mixed halide perovskites. The movement of halide ions during the homogenization process was elucidated using UV-Visible absorbance and X-ray photoelectron spectroscopy. The halide ion movement in CsPbBr3-xIx gradient films was tracked via absorbance changes in the visible region of the spectrum that enabled us to measure the temperature dependent rate constant and energy of activation (74.5 kJ/mol) of halide ion homogenization. Excited state processes of both gradient and homogenized films probed through transient absorption spectroscopy showed the direct flow of charge carriers and charge recombination in both films.

Excited-state relaxation processes of three newly synthesized multi-branched alkyl-triphenylamine end-capped triazines

The excited-state relaxation processes of three newly synthesized multi-branched alkyl-triphenylamine end-capped triazines ATT-(1–3) are characterized in different solvents by steady-state and time-resolved spectroscopy. In toluene, a weakly polar solvent, the emission originates from the intramolecular charge transfer (ICT) state; in tetrahydrofuran (THF), a strongly polar solvent, the existence of a nonradiative channel from ICT to twisted intramolecular charge transfer (TICT) accelerates the relaxation rate of the ICT state. The rate of the evolution process of ATT-(1–3) increases with increasing number of donor branches, which could ascribed to enhancements in the electron donor and acceptor abilities of the triazines.

Ultrafast Dynamics of a Molecular Rotor-Based Bioprobe-PicoGreen: Understanding toward Fibril Sensing Mechanism

A recent report shows that the cyanine-based molecular rotor, PicoGreen, has very strong affinity toward amyloid fibrils and shows large increase in its emission yield upon binding with insulin amyloid fibrils. To gain deeper knowledge about the excited-state molecular processes that are responsible for its amyloid sensing behavior, detailed ultrafast dynamics of PicoGreen in molecular solvents with varying polarity and viscosity have been investigated. Our detailed studies on femtosecond time-resolved emission of PicoGreen show that both polarity and viscosity of the medium a play vital role in the deactivation of its photoexcited state. Detailed analysis of the time-resolved data suggests the formation of the intramolecular charge transfer (ICT) state, which is independent of solvent viscosity, takes place in ultrafast time scales (<2 ps), followed by the formation of a twisted ICT (TICT) state at a longer time scale in polar solvents. The formation of TICT from the ICT state due to the large amplitude torsional motion in its excited state has been supported by viscosity-dependent excited-state dynamics. Finally, the dynamical studies in fibrillar medium demonstrate the retardation in the excited-state torsional motion, which is primarily responsible for its observed large fluorescence enhancement in insulin amyloid fibrils.

β-Cyclodextrin Encapsulated Coumarin 6 on Graphene Oxide Nanosheets: Impact on Ground-State Electron Transfer and Excited-State Energy Transfer.

We report a unique phenomenon of physical adsorption of coumarin 6-β-cyclodextrin (C6-β-CD) inclusion nanostructures on graphene oxide (GO) nanosheets, thus inducing ground-state electron transfer from the C6-β-CD composite to GO. On excitation, the C6-β-CD composite initially transfers energy to the attached GO surface and subsequently collides with similar C6-β-CD@GO adducts leading to dynamic quenching of energy. The ground-state two-electron transfer process has been confirmed by cyclic voltammetry in aqueous medium, whereas the excited-state processes were inferred from steady-state and time-resolved fluorescence spectroscopy. The concept is developed toward conceiving control over the ground-state electron transfer and excited state energy transfer from the C6-β-CD composite by the adsorbed electron accepting medium (GO in this case). The C6-β-CD composite has been prepared to isolate single C6 molecules that readily undergo microcrystal formation in aqueous medium. The results show its potential toward fabrication of energy-harvesting antenna for further applications.

A Novel ESIPT Phthalimide-Based Fluorescent Probe for Quantitative Detection of H2O2.

Hydrogen peroxide (H2O2) is a majority reactive oxygen species (ROS) and acts as an essential role in pathological and physiological processes. Therefore, the development of quantitative detection of methods for H2O2 is necessary. Here, we constructed of a novel simple fluorescence probe for detection of H2O2 based on the excited-state intramolecular proton transfer process. The probe utilized a phthalimide derivative as the fluorophore and selected phenylboronic acid as the recognition site for H2O2. In response to H2O2, the probe exhibited 63-fold fluorescence intensity enhancement, a low detection limit (8.4 × 10–8 M), and large Stokes shift (111 nm). In addition, the probe displayed high selectivity for H2O2 over other ROS. Moreover, the probe was successfully employed for imaging of H2O2 in living cells.

Manipulating the Optical Properties of Carbon Dots by Fine‐Tuning their Structural Features

As a new class of sustainable carbon material, "carbon dots" is an umbrella term covering many types of materials. Herein, a broad range of techniques was used to develop the understanding of hydrothermally synthesized carbon dots, and it is shown how fine-tuning the structural features by simple reduction/oxidation reactions can drastically affect their excited-state properties. Structural and spectroscopic studies found that photoluminescence originates from direct excitation of localized fluorophores involving oxygen functional groups, whereas excitation at graphene-like features leads to ultrafast phonon-assisted relaxation and largely quenches the fluorescent quantum yields. This is arguably the first study to identify the dynamics of photoluminescence including Stokes shift and allow the relaxation pathways in these carbon dots to be fully resolved. This comprehensive investigation sheds light on how understanding the excited-state relaxation processes in different carbon structures is crucial for tuning the optical properties for any potential commercial applications.

Photo-induced Electron Transfer or Proton-Coupled Electron Transfer in Methylbipyridine/Phenol Complexes: A Time-Dependent Density Functional Theory Investigation.

It is often difficult to assign the nature of an excited-state process unambiguously based on a limited number of experimental evidence. The methylbipyridine/phenol complex is a classic example, where experimental observations support a proton-coupled electron transfer (PCET) or a photo-induced electron transfer (PET) process. Here, we implemented time-dependent density functional theory calculation to elucidate the nature of the process. We found that PCET is possible only when mediated by a H-bond between methylbipyridine and phenol. However, a conventional PET can occur through π-π stacking interaction between the donor and the acceptor. Thus, the photophysical process in the complex is indeed governed by competition of H-bonding versus π-π interaction. Our calculations including the solvent model based on density (SMD) suggest that π-π stacking is more favorable than H-bonding, and hence, conventional PET is a more favorable excited-state process for the methylbipyridine/methoxyphenol complex than PCET.

(Invited) Excited State Processes in Semiconductor Nanocrystals and Their Relationships with Light-Driven Multi-Electron Catalysis

This presentation will focus on the excited state behavior of semiconductor nanocrystals as light absorbers that, coupled with redox catalysts, drive light-driven multi-electron transfer reactions. Reactions of interest include hydrogen generation, carbon dioxide reduction, nitrogen fixation, and water oxidation. We have demonstrated that nanocrystal excited state behavior, charge transfer dynamics, and surface chemistry play a governing role in the overall photochemistry of nanocrystal-catalyst complexes. This presentation will describe our most recent findings about how the reactions of interest can be driven and controlled through manipulation of nanocrystal excited state dynamics. In particular, the presentation will focus on: (i) measurement (by transient absorption spectroscopy), kinetic modeling, and control of electron transfer kinetics for injection of photoexcited electrons into redox enzymes; and (ii) elucidation of the motion of photoexcited holes on nanocrystal surfaces, using a combination of transient absorption measurements, modeling, and theory, and the implications of this motion on oxidation photochemistry.

A theoretical exploration and regulating about the excited state process for 2‐(4‐(diphenylamino)phenyl)‐3‐hydroxy‐4H‐chromen‐4‐one system

AbstractIn the present work, density functional theory (DFT) and time‐dependent density functional theory (TDDFT) methods have been performed to explore the ground‐state and excited‐state intramolecular hydrogen bonding interactions for 2‐(4‐(diphenylamino)phenyl)‐3‐hydroxy‐4H‐chromen‐4‐one (2‐DHC) system. We demonstrate that the intramolecular hydrogen bond formed in the ground state should be strengthened in the first single excited state by comparing the bond parameters and infrared (IR) vibrational spectra, which provides the possibility for excited‐state intramolecular proton transfer (ESIPT) reaction. The calculations about hydrogen bonding energy further confirm the strengthening hydrogen bond. The intramolecular charge transfer (ICT) around hydrogen bonding moieties upon photoexcitation promotes the ability to attract hydrogen proton, which reveals the ESIPT tendency. In view of four different polarities solvents, we construct the potential energy curves along with ESIPT path and search the transition state (TS) structure for ESIPT reaction. We clarify the ESIPT mechanism for 2‐DHC molecule, based on which we also present the ESIPT reaction could be regulated and controlled by solvent polarity.

Excited-State Triplet Equilibria in a Series of Re(I)-Naphthalimide Bichromophores.

We present the synthesis, structural characterization, electronic structure calculations, and the ultrafast and supra-nanosecond photophysical properties of a series of five bichromophores of the general structural formula [Re(5-R-phen)(CO)3(dmap)](PF6), where R is a naphthalimide (NI), phen = 1,10-phenanthroline, and dmap is 4-dimethylaminopyridine. The NI chromophores were systematically modified at their 4-positions with -H (NI), -Br (BrNI), phenoxy (PONI), thiobenzene (PSNI), and piperidine (PNI), rendering a series of metal-organic bichromophores (Re1-Re5, respectively) featuring variability in the singlet and triplet energies in the pendant NI subunit. Five closely related organic chromophores as well as [Re(phen)(CO)3(dmap)](PF6) (Re6) were investigated in parallel to appropriately model the photophysical properties exhibited in the bichromophores. The excited state processes of all molecules in this study were elucidated using a combination of transient absorption spectroscopy and time-resolved photoluminescence (PL) spectroscopy, revealing the kinetics of the energy transfer processes occurring between the appended chromophores. The spectroscopic analysis was further supported by electronic structure calculations which identified the origin of many of the experimentally observed electronic transitions.

Improvement of Photostability and NIR Activity of Cyanine Dye through Nanohybrid Formation: Key Information from Ultrafast Dynamical Studies.

Near-infrared (NIR) light harvesting has enormous importance for different potential applications in the modern era of research. Some NIR cyanine dyes such as IR820 have achieved great success in energy harvesting and cancer therapy. However, their action is limited for low photostability, considerable thermal degradation, short circulation times, and nonspecific biodistribution. Our present study is an attempt to overcome such limitations by attaching a model cyanine dye IR820 with ZnO nanoparticles. We prepared an IR820-ZnO nanohybrid and characterized it using microscopic and optical spectroscopic tools. Thermogravimetric analysis depicted greater thermal stability of the IR820-ZnO nanohybrid compared to free dye. We explored the enhancement in the photostability of IR820 upon nanohybrid formation. We detected generation of photoinduced reactive oxygen species (ROS) such as superoxide, singlet oxygen, and so forth using appropriate molecular probes. The formation of IR820-ZnO nanohybrid reduced production of photoinduced singlet oxygen. However, it revealed an alternative trend in overall ROS formation (increases total ROS) under red light illumination. To correlate the enhanced photostability of IR820 on the ZnO surface, we explored excited-state dynamical processes at the interface in nanohybrids. We illustrated the photoinduced excited-state electron-transfer process from the lowest unoccupied molecular orbital of IR820 to the conduction band of ZnO. This photoelectron-transfer process enhances the production of ROS and decreases the formation of singlet oxygen that altogether leads to improvement in photostability and overall activity. A quencher of singlet oxygen sodium azide (NaN3) was used to further confirm the direct association of singlet oxygen generation with the photostability issue of IR820. Also, ZnO is able to deliver the dye selectively in acidic environment, which suggests its diseased site-specific targeted activity. Our results provide promising improvement for potential use of IR820 through formation of a nanohybrid that could be translated for other NIR cyanine dyes.

Tracking multiple components of a nuclear wavepacket in photoexcited Cu(I)-phenanthroline complex using ultrafast X-ray spectroscopy

Disentangling the strong interplay between electronic and nuclear degrees of freedom is essential to achieve a full understanding of excited state processes during ultrafast nonadiabatic chemical reactions. However, the complexity of multi-dimensional potential energy surfaces means that this remains challenging. The energy flow during vibrational and electronic relaxation processes can be explored with structural sensitivity by probing a nuclear wavepacket using femtosecond time-resolved X-ray Absorption Near Edge Structure (TR-XANES). However, it remains unknown to what level of detail vibrational motions are observable in this X-ray technique. Herein we track the wavepacket dynamics of a prototypical [Cu(2,9-dimethyl-1,10-phenanthroline)2]+ complex using TR-XANES. We demonstrate that sensitivity to individual wavepacket components can be modulated by the probe energy and that the bond length change associated with molecular breathing mode can be tracked with a sub-Angstrom resolution beyond optical-domain observables. Importantly, our results reveal how state-of-the-art TR-XANES provides deeper insights of ultrafast nonadiabatic chemical reactions.

Photoinduced Water-Heptazine Electron-Driven Proton Transfer: Perspective for Water Splitting with g-C3N4.

Heptazine-assembled polymeric carbon nitride (CN) materials have fascinated the research community as a photocatalyst for hydrogen evolution while less attention has been devoted to the mechanistic features of the host materials. Using excited-state nonadiabatic dynamics simulations, the molecular-level picture of the decomposition of heptazine hydrogen bonded to water molecule(s) (heptazine-water complex) into heptazinyl and hydroxyl biradical products is revealed. Dynamics simulations show that hydrogen detachment from the water molecule to the heptazine occurs within tens of femtoseconds and suggest that excited-state deactivation via N-H······O-H electron-driven proton transfer (EDPT) is the dominant and most relevant excited-state deactivation process in heptazine-water complexes leading to conical intersection. The observation of photorelaxation-induced water splitting by heptazine is proof of the water-splitting reaction principle, which presents further challenges for computational and experimental investigations of the deactivation of heptazinyl and OH biradical products for efficient hydrogen evolution.

A time‐dependent density functional theory study on the excited state behavior of a novel T2(OH)B molecule

AbstractA novel pH‐sensitive fluorescent probe T2(OH)B was selected to theoretically investigate its excited state hydrogen bonding effects and excited state intramolecular proton transfer (ESIPT) process. First, it was verified that one intramolecular hydrogen bond is formed spontaneously in T2(OH)B itself. Given the geometrical changes, we further confirm that the hydrogen bond should be strengthened in the first excited state. When it comes to the photoexcitation process, we present the charge redistribution around hydrogen bonding moieties facilitate the ESIPT tendency. The increased electronic densities around acceptor promote the attraction of hydrogen protons. The potential energy barrier in the constructed potential energy curves reveals that the ESIPT process of the T2(OH)B system should be ultrafast. And comparing several nonpolar solvents, we deem solvent polarity plays little role in the ESIPT reaction. Furthermore, we also search the S1‐state transition state structure along with the ESIPT path, based on which we simulate the intrinsic reaction coordinate path. We not only confirm the ESIPT mechanism presented in this work but also clarify the ultrafast excited state process and explain previous experiment. We sincerely hope that our theoretical work could guide novel applications based on the T2(OH)B system in future.

Mechanism of Photoinduced Dihydroazulene Ring-Opening Reaction.

The photoinduced ring-opening reaction is a key process in the functioning of dihydroazulene/vinylheptafulvene (DHA/VHF) photoswitches. Over the years, the mechanism of this reaction has been extensively debated. Herein, by means of nonadiabatic trajectory dynamics simulations and quantum chemistry calculations, we present the first detailed and comprehensive investigation on the mechanism of the photoinduced ring-opening reaction of DHA. The results show the crucial role of the excited-state ring planarization process for the bond breaking. Our dynamics simulations show that the DHA ring opening is an ultrafast reaction that does not follow exponential kinetics but exhibits ballistic dynamics. Upon photoexcitation, the planarization occurs within 300-500 fs. This leads to the ring-opening reaction and concurrent decay of the molecule to the ground state within 100 fs through an S1 → S0 internal conversion process toward forming the VHF isomer. These results are consistent with previous ultrafast time-resolved experiments and lead to a thorough understanding of the DHA/VHF photoconversion.

Photoinduced excited state dynamical behavior and ESIPT mechanism for 2-(2-hydroxy-3,5-dimethyl-phenyl)-benzooxazole-5-carboxylicacid molecule

We perform a theoretical investigation about the excited state intramolecular proton transfer (ESIPT) reaction for a novel 2-(2-hydroxy-3,5-dimethyl-phenyl)-benzooxazole-5-carboxylicacid (HDPBC) system. The ultrafast excited state process is due to the low potential barrier, which should be the reason why only one fluorescence peak can be observed in previous experiment. Comparing the potential energy barriers in different aprotic solvents, we present nonpolar solvents play roles in facilitating ESIPT process for HDPBC molecule. This work not only clarifies the detailed ESIPT mechanism for the novel HDPBC system and explains experimental phenomenon, but also present that solvent effects could control the ESIPT process.

Investigation on the excited state intramolecular proton transfer process for the novel 2‐(3,5‐dichloro‐2‐hydroxy‐phenyl)‐benzooxazole‐5‐carboxylicacid system

AbstractIn view of the importance of excited state hydrogen bonding interactions and excited state proton transfer (ESPT) dynamical behaviors, in this work, we theoretically explore the excited state process for a novel 2‐(3,5‐dichloro‐2‐hydroxy‐phenyl)‐benzooxazole‐5‐carboxylicacid (DHPBC) system. On the basis of discrete Fourier transform (DFT) and time‐dependent density functional theory (TDDFT) methods, we reappear previous experimental reports with checking the reasonability of our theoretical level. We calculate and compare the primary geometrical parameters and hydrogen bonding energies around hydrogen bonding moieties, which reveals that the intramolecular hydrogen bond OH···N of DHPBC is enhanced in the S1 state. The increased electron densities around N atom contribute to attracting proton of hydroxyl upon the photoexcitation, which plays important roles in strengthening hydrogen bond and in facilitating excited state intra‐ or intermolecular proton transfer (ESIPT) process. Via constructing potential energy curves along with the ESIPT path, we verify that the ESIPT process of DHPBC system is ultrafast because of the low potential energy barrier. That should be the reason why the normal emission of DHPBC cannot be detected in previous experiment. This work not only clarifies the excited state hydrogen bonding interactions and fills the vacancy of specific ESIPT mechanism for DHPBC system in experiment, but also explains the fluorescence quenching phenomenon.

Solution-Processed Highly Efficient Bluish-Green Thermally Activated Delayed Fluorescence Emitter Bearing an Asymmetric Oxadiazole-Difluoroboron Double Acceptor.

Difluoroboron (BF2)-containing dyes have attracted great interest owing to their exceptionally high luminescence efficiency and good electron-withdrawing properties. However, only a few reports on difluoroboron-based thermally activated delayed fluorescence (TADF) have been addressed. In this contribution, a novel BF2-containing TADF molecule of BFOXD, which contains two acceptor fragments of oxadiazole (OXD) and BF2 and one donor unit of 9,9-dimethylacridine, was synthesized and characterized. For comparison, the precursor of OHOXD bearing one acceptor unit was also investigated. Both molecules clearly show TADF characteristics with sky-blue emission in solution and film state. Additionally, OHOXD undergoes excited-state intramolecular proton transfer-coupled intramolecular charge transfer processes. Using 9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi) as the host, the organic light-emitting diodes fabricated via a solution process show maximum external quantum efficiency (EQE) of 2.98 and 13.8% for OHOXD- and BFOXD-based devices, respectively. While the bipolar TADF host of 10-(4-((4-(9H-carbazol-9-yl)phenyl)sulfonyl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (CzAcSF) is utilized instead of CzSi, the OHOXD- and BFOXD-based devices exhibit better performances with the maximum EQEs of 12.1 and 20.1%, respectively, which render the most efficient and the bluest emission ever reported for the BF2-based TADF molecules. This research demonstrates that introduction of one more acceptor unit into the TADF molecule could have a positive effect on emission efficiency, which opens a new way to design high-efficiency TADF molecules.