Photophysical Processes Research Articles

Single-Color and Two-Color Femtosecond Pump–Probe Experiments on Graphitic Carbon Nitrides Revealing Their Charge Carrier Kinetics

Graphitic carbon nitride related materials have been one type of the most promising photocatalysts. Comprehensive understanding of their genuine charge carrier kinetic aspects will strengthen their photocatalytic applications. Herein, we synthesized two samples with different termination/functionalization groups, CN and NCN, and performed both single-color and two-color femtosecond pump–probe experiments on them. We found that both the recovery kinetics obtained from single-color femtosecond pump–probe experiments (SCFPPE) and the decay kinetics obtained from two-color femtosecond pump–probe experiments (TCFPPE) for CN and NCN display stronger pump fluence dependences, indicating that their genuine charge carrier kinetics should be measured under very low pump fluence. The charge carrier kinetics obtained under higher pump fluence would introduce a dramatically short picosecond component due to carrier–carrier interactions. With the obtained pump fluence-dependent charge carrier kinetics, for the first time, we derived the absorption cross section of CN and NCN at pump wavelength. The derived absorption cross section of CN at ∼398 nm is ∼5.2 × 10–15 cm2, and the derived absorption cross section of NCN at ∼398 nm is ∼8.3 × 10–15 cm2. Under very low pump fluence, we also found that the recovery kinetics of CN and NCN obtained from SCFPPE is obviously faster than their decay kinetics obtained from TCFPPE, suggesting that an additional fast decay photophysical process of CN and NCN might not be detected in TCFPPE. On the base of those femtosecond studies, we proposed a model for CN and NCN to understand how the pump fluence affect their charge carrier kinetics. We also discussed how the introduced termination/functionalization group in CN and NCN affects their genuine charge carrier kinetics so as to modify their photocatalytic hydrogen production activities. FTIR, XPS, XRD, and SEM EDX of CN and NCN were also measured for characterizing their chemical compositions and structures.

Unveiling controlling factors of the S0/S1 minimum-energy conical intersection (3): Frozen orbital analysis based on the spin-flip theory.

Conical intersections (CIs), which indicate the crossing of two or more adiabatic electronic states, are crucial in the mechanisms of photophysical, photochemical, and photobiological processes. Although various geometries and energy levels have been reported using quantum chemical calculations, the systematic interpretation of the minimum energy CI (MECI) geometries is unclear. A previous study [Nakai et al., J. Phys. Chem. A 122, 8905 (2018)] performed frozen orbital analysis (FZOA) based on time-dependent density functional theory (TDDFT) at the MECI formed between the ground and first electronic excited states (S0/S1 MECI), thereby inductively clarifying two controlling factors. However, one of the factors that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy gap became close to the HOMO-LUMO Coulomb integral was not valid in the case of spin-flip TDDFT (SF-TDDFT), which is frequently used as a means of the geometry optimization of MECI [Inamori et al., J. Chem. Phys. 152, 144108 (2020)]. This study revisited the controlling factors using FZOA for the SF-TDDFT method. Based on spin-adopted configurations within a minimum active space, the S0-S1 excitation energy is approximately represented by the HOMO and LUMO energy gap ΔεHL, a contribution from Coulomb integrals JHL″ and that from the HOMO-LUMO exchange integral KHL″. Furthermore, numerical applications of the revised formula at the SF-TDDFT method confirmed the control factors of S0/S1 MECI.

Coherent Dynamics in Solutions of Colloidal Plexcitonic Nanohybrids at Room Temperature

AbstractThe increasing ability to prepare systems with nanoscale resolution and address their optical properties with ultrashort time precision is revealing quantum phenomena with tremendous potential in quantum nanotechnologies. Colloidal plexcitonic materials promise to play a pivotal role in this scenario. Plexcitons are hybrid states originating from the mixing of the plasmon resonances of metal nanostructures with molecular excitons. They allow nanoscale confinement of electromagnetic fields and the establishment of strong couplings between light and matter, potentially giving rise to controllable and tunable coherent phenomena. However, the characterization of the ultrafast coherent and incoherent dynamics of colloidal plexciton nanohybrids remains highly unexplored. Here, two dimensional electronic spectroscopy (2DES) is employed to study the quantum coherent interactions active after the photoexcitation of these systems. By comparing the response of the nanohybrids with the one of the uncoupled systems, the nonlinear photophysical processes at the base of the coherent dynamics are identified, allowing a step forward toward the effective understanding and exploitation of these nanomaterials.

Photophysical Properties of Pyrene‐Substituted nido‐Carborate Anion and the Change of Luminescence Species by Counter Cations in Solution and Solid States

AbstractThis study reports unique luminescent properties of pyrene‐substituted nido‐carborate anion (PYnCA) with two counter cations, tetrabutylammonium (NBu4+) and tetraphenylphosphonium (PPh4+). It is revealed that PYnCA shows twisted intramolecular charge transfer (TICT) emission by the change of the dihedral angle between pyrene and the CcageCcage bond in the carborane unit in the excited state. Although the structure of nCA is almost same as that of ortho‐carborane (oC), the photophysical processes of both pyrene‐substituted compounds are completely different. The study also succeeds in modulating the luminescence species by counter cations in both solution and solid states. To the best of the current knowledge, this is the first report that anion‐based luminescent materials can show the different luminescent species not only in solution but also in the mechanochromic behaviors depending on the type of counter cations. This fundamental research on nCA demonstrates anion‐based emitters as a new platform for stimuli‐responsive materials.

Evolution of photoelectric conversion and device stability of PM6:N2200 all-polymer solar cells

In this work, to study the performance degradation process in PM6:N2200 all-polymer solar cells, including the optical absorption, morphology, photo-physical process of the active layer and current–voltage characteristics of the device were analyzed by employing multiple characterization techniques. The results showed that the morphology (phase sizes and phase separation scales) of PM6:N2200 would change with time. Unstable morphology of active layer determined the changeable photoelectric conversion process in operating solar cells. Besides, we found that morphological change of PM6:N2200 had a little influence on the exciton behaviors (diffusion and dissociation) within 40 days. However, it showed an undesired impact on photogenerated charge behaviors (transport and recombination). Unstable morphology of active layer and high charge recombination probability were essential reasons for the performance degradation of polymer solar cells in operating time.

Influence of reducing agent concentration on the luminescence and photophysical processes constant rates of silver clusters in silica-based glass

Series of silica-based glasses doped with antimony was synthesized to host luminescent silver clusters formed via Na+-Ag+ low-temperature ion exchange and heat treatment. Spectroscopic study reveals that increase in antimony concentration affects emission spectra, quantum yields, and fluorescence lifetimes. Photophysical processes rate constants, including radiative and nonradiative transitions and intersystem crossing, were estimated based on experimental spectroscopic characterization. Correlation between photophysical processes rate constants and increase in concentration of antimony was demonstrated.

Aza-Cibalackrot: Turning on Singlet Fission Through Crystal Engineering.

Singlet fission is a photophysical process that provides a pathway for more efficient harvesting of solar energy in photovoltaic devices. The design of singlet fission candidates is non-trivial and requires careful optimization of two key criteria: (1) correct energetic alignment and (2) appropriate intermolecular coupling. Meanwhile, this optimization must not come at the cost of molecular stability or feasibility for device applications. Cibalackrot is a historic and stable organic dye which, although it has been suggested to have ideal energetics, does not undergo singlet fission due to large interchromophore distances, as suggested by single crystal analysis. Thus, while the energetic alignment is satisfactory, the molecule does not have the desired intermolecular coupling. Herein, we improve this characteristic through molecular engineering with the first synthesis of an aza-cibalackrot and show, using ultrafast transient spectroscopy, that singlet fission is successfully "turned on."

Singlet Fission Driven by Excited‐State Intramolecular Proton Transfer (ESIPT+SF): A (TD)DFT Study

AbstractSinglet fission is a photophysical process able to generate two triplet excitons after the absorption of one high‐energy photon. Although this process is expected to boost the efficiency of photovoltaic cells, the instability of all known singlet fission materials hindered its implementation in any practical device. In order to overcome this limitation, we present a novel photophysical process that combines excited‐state intramolecular proton transfer and singlet fission (ESIPT+SF). Together with the suggested mechanism, this study provides a computational analysis identifying possible ESIPT+SF materials.

Effect of Fluorinated End-Groups on the Exciton Dynamics and Charge Transfer of Non-fused Ring Acceptors

In recent years, the optimization of non-fused ring electron acceptors (NFREAs) has become an important topic, which includes π extension, side-chain engineering, and end-group halogenation. While the performance characterization of different NFREAs, including their morphology and power conversion efficiencies, has been extensively studied, the influence of different optimization methods on their photophysical processes is not clear. Here, we analyze the effect of end-group fluoridation on exciton dynamics and charge transfer (CT) in NFREA-based organic photovoltaics in detail using a novel NFREA Isopropyl-0F and its end-group fluorinated homologue Isopropyl-2F by means of spectroscopy, in order to understand the physical mechanism for the differences in their performance. The transient absorption spectra show that the more ordered molecular arrangement in the Isopropyl-2F-based heterojunction resulted in faster exciton diffusion and higher hole transfer efficiency, which were conducive to CT and reduced recombination loss. These results well explain the improved properties after end-group fluoridation.

Photoinduced Fluorescence Switching in Molecular Aggregates by Topological [2+2] Cycloaddition

AbstractThe optical modulation of fluorescence characteristics in molecular aggregates, which mainly involves diverse molecular stacking and the consequent intermolecular interactions, remains a significant challenge for potential applications in optical anticounterfeiting, data storage, and imaging. Here we successfully realize in situ fluorescence switching in molecular aggregates of thianaphthene‐dioxide derivatives by topological [2+2] photocycloaddition, which represents a promising way to regulate the molecular stacking and alter photophysical processes. Notably, 2‐(3,5‐bis‐trifluoromethylphenyl)benzo[b]thiophene‐dioxide (BTO‐TF) in both crystal and powder forms exhibits a unique switching from an initial nonfluorescent state to a highly fluorescent state (ΦPL=0.46) upon UV irradiation, because of the destruction of the [2+2] cycloaddition process by volume expansion of the photodimer. Furthermore, we demonstrate such a [2+2] photocycloaddition can occur when 2‐(4‐carboxypheny)benzo[b]thiophene‐dioxide (BTO‐OH) is doped within selective polymer matrixes, and can be utilized for the visualization of macrophase separation in polymer blends.

Photoinduced Fluorescence Switching in Molecular Aggregates by Topological [2+2] Cycloaddition.

The optical modulation of fluorescence characteristics in molecular aggregates, which mainly involves diverse molecular stacking and the consequent intermolecular interactions, remains a significant challenge for potential applications in optical anticounterfeiting, data storage, and imaging. Here we successfully realize in situ fluorescence switching in molecular aggregates of thianaphthene-dioxide derivatives by topological [2+2] photocycloaddition, which represents a promising way to regulate the molecular stacking and alter photophysical processes. Notably, 2-(3,5-bis-trifluoromethylphenyl)benzo[b]thiophene-dioxide (BTO-TF) in both crystal and powder forms exhibits a unique switching from an initial nonfluorescent state to a highly fluorescent state (ΦPL =0.46) upon UV irradiation, because of the destruction of the [2+2] cycloaddition process by volume expansion of the photodimer. Furthermore, we demonstrate such a [2+2] photocycloaddition can occur when 2-(4-carboxypheny)benzo[b]thiophene-dioxide (BTO-OH) is doped within selective polymer matrixes, and can be utilized for the visualization of macrophase separation in polymer blends.

The Role of H-Bonds in the Excited-State Properties of Multichromophoric Systems: Static and Dynamic Aspects.

Given their importance, hydrogen bonds (H-bonds) have been the subject of intense investigation since their discovery. Indeed, H-bonds play a fundamental role in determining the structure, the electronic properties, and the dynamics of complex systems, including biologically relevant materials such as DNA and proteins. While H-bonds have been largely investigated for systems in their electronic ground state, fewer studies have focused on how the presence of H-bonds could affect the static and dynamic properties of electronic excited states. This review presents an overview of the more relevant progress in studying the role of H-bond interactions in modulating excited-state features in multichromophoric biomimetic complex systems. The most promising spectroscopic techniques that can be used for investigating the H-bond effects in excited states and for characterizing the ultrafast processes associated with their dynamics are briefly summarized. Then, experimental insights into the modulation of the electronic properties resulting from the presence of H-bond interactions are provided, and the role of the H-bond in tuning the excited-state dynamics and the related photophysical processes is discussed.

Panchromatic Light-Absorbing [70]Fullerene-Perylene-BODIPY Triad with Cascade of Energy Transfer as an Efficient Singlet Oxygen Sensitizer.

A panchromatic light-absorbing [70]fullerene-perylene-BODIPY triad (C70-P-B) was synthesized and applied as a heavy atom-free organic triplet photosensitizer for photooxidation. The photophysical processes were comprehensively investigated by the methods of steady-state spectroscopy, time-resolved spectroscopy, as well as theoretical calculations. C70-P-B shows a strong absorption ability from 300-620 nm. Efficient cascading intramolecular singlet-singlet energy transfer in C70-P-B was confirmed by the luminescence study. The backward triplet excited state energy transfer from C70 moiety to perylene then occurs to populate 3perylene*. Thus, the triplet excited states of C70-P-B are distributed on both C70 and perylene moiety with lifetimes of 23 ± 1 μs and 175 ± 17 μs, respectively. C70-P-B exhibits excellent photooxidation capacity, and its yield of singlet oxygen reaches 0.82. The photooxidation rate constant of C70-P-B is 3.70 times that of C70-Boc and 1.58 times that of MB, respectively. The results in this paper are useful for designing efficient heavy atom-free organic triplet photosensitizers for practical application in photovoltaics, photodynamic therapy, etc.

Elucidating the Origin of Plasmon-Generated Hot Holes in Water Oxidation

Plasmon-generated hot electrons in metal/oxide heterostructures have been used extensively for driving photochemistry. However, little is known about the origin of plasmon-generated hot holes in promoting photochemical reactions. Herein, we discover that, during the nonradiative plasmon decay, the interband excitation rather than the intraband excitation generates energetic hot holes that enable to drive the water oxidation at the Au/TiO2 interface. Distinct from lukewarm holes via the intraband excitation that only remain on Au, hot holes from the interband excitation are found to be transferred from Au into TiO2 and stabilized by surface oxygen atoms on TiO2, making them available to oxidize adsorbed water molecules. Taken together, our studies provide spectroscopic evidence to clarify the photophysical process for exciting plasmon-generated hot holes, unravel their atomic-level accumulation sites to maintain the strong oxidizing power in metal/oxide heterostructures, and affirm their crucial functions in governing photocatalytic oxidation reactions.

Energy Component Analysis for Electronically Excited States of Molecules: Why the Lowest Excited State Is Not Always the HOMO/LUMO Transition

The ability to tune excited-state energies is crucial to many areas of molecular design. In many cases, this is done based on the energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). However, this viewpoint is incomplete neglecting the many-body nature of the underlying excited-state wave functions. Within this work, we highlight the importance of two crucial terms, other than orbital energies, that contribute to the excitation energies and show how to quantify them from quantum chemistry computations: a Coulomb attraction and a repulsive exchange interaction. Using this framework, we explain under which circumstances the lowest excited state of a molecule, of either singlet or triplet multiplicity, is not accessed via the HOMO/LUMO transition and show two paradigmatic examples. In the case of the push-pull molecule ACRFLCN, we highlight how the lowest triplet excited state is a locally excited state lying below the HOMO/LUMO charge transfer state due to enhanced Coulomb binding. In the case of the naphthalene molecule, we highlight how the HOMO/LUMO transition (the 1La state) becomes the second excited singlet state due to its enhanced exchange repulsion term. More generally, we explain why excitation energies do not always behave like orbital energy gaps, providing insight into photophysical processes as well as methodogical challenges in describing them.

Sulfur-Decorated Nonaromatic Amine Emitters Towards Efficient Triplet Exciton Utilization in Organic Light-Emitting Diodes.

Purely organic compounds served as promising materials for organic electronics have intrigued extensive research focuses owing to their unique photophysical and electronic properties. In the field of organic light-emitting diodes (OLEDs), the utilization of triplet excitons is of great significance for realizing high-performance devices. In contrast to the traditional aromatic amine-based counterparts, sulfur atom with a high-level outer orbit simultaneously provides powerful electron-donating ability and striking spin-orbit coupling effect to facilitate the utilization of theoretically spin-forbidden triplet excitons, demonstrating great prospect in constructing highly emissive purely organic emitters for OLED applications. Herein, we summarize the currently developed sulfur-decorated nonaromatic amine-based emitters exhibiting attractive photoelectronic characteristics, and gain insight into the understanding of molecular design and photophysical processes of these emitters, providing new perspectives for enriching the existing luminescent material systems and designing high-performance emitters.

Simultaneous Photoluminescence and Photothermal Investigation of Individual CH3NH3PbBr3 Microcrystals

The photoluminescence (PL) of CH3NH3PbBr3 (MAPbBr3), from thin films to nanoparticles, has been widely studied, providing information about charge carrier dynamics. However, the other energy dissipative channel, nonradiative relaxation, has not been thoroughly investigated due to a lack of proper technology. In this work, we simultaneously investigated the PL and photothermal (PT) properties of single MAPbBr3 microcrystals (MCs) by a home-built PL and PT microscope. In addition to the direct observation of the heterogeneity of the PL and PT images and kinetics of different MCs, we demonstrated the variation in the absorption of single MAPbBr3 MCs, which was believed to be constant. We also proved that more absorbed energy dissipated from the nonradiative channel at higher heating power. These results show that PL and PT microscopy is an effective and convenient method to investigate the charge carrier behaviors of optoelectronic materials at the single particle level for a deep understanding of their photophysical processes.

Localized surface plasmon controlled chemistry at and beyond the nanoscale

Gaining valuable insight into chemistry-related fields, such as molecular and catalytic systems, surface science, and biochemistry, requires probing physical and chemical processes at the sub-nanoscale level. Recent progress and advancements in nano-optics and nano-photonics, particularly in scanning near-field optical microscopy, have enabled the coupling of light with nano-objects using surface plasmons with sub-nanoscale precision, providing access to photophysical and photochemical processes. Herein, this review highlights the basic concepts of surface plasmons and recent experimental findings of tip-assisted plasmon-induced research works and offers a glimpse into future perspectives.

Control over rotary motion and multicolour switching in 3-hydroxyphthalimide fluorophores: An interplay between AIE and ESIPT

In this paper, we demonstrate the ability of N-substituted 3-hydroxyphthalimides with pyridine and isoquinoline rings to perform multicolour switching by different stimuli. Restriction of the C–N rotary motion of the heterocycles allows control over their photoluminescence performance (PLQY = 54–65%) in water-induced aggregation by an additive interplay of excited-state proton-mediated processes in the aggregates. Restriction of the rotations by the affinity of the heterocyclic nitrogen atoms to Zn2+ and switching to new emission bands (PLQY = 8–29%) in solution and solid state were also achieved. Protonation of the pyridine/isoquinoline rings allows the formation of intramolecular hydrogen bonds with the imide carbonyl groups, resulting in restricted rotations. The protonated forms switch to strong blue emission due to local excitation of the protonated heterocycles, which are unable to undergo proton transfer. Understanding the interplay between both photophysical processes would allow the switching of emissions by multistimuli.

Nanoparticles for combined photo- and chemodynamic therapy ofcancer cells involving endogenous glutathione depletion.

Background: Reactive oxygen species (ROS) are powerful weapons for various anticancer therapies. However, high glutathione (GSH) levels in cancer cells can significantly reduce the efficacy of such therapies. Methods: In this study, pH-responsive fluorescein-encapsulated zeolitic imidazolate framework-8 nanoparticleswere synthesized for ROS-mediated combination therapy. Results: Upon blue light activation, fluorescein displayed a high singlet oxygen photogeneration ability for photodynamic therapy. Concurrently, accumulated Zn2+ from degraded zeolitic imidazolate framework-8 stimulated simultaneous ROS generation and GSH depletion, thereby successfully inducing chemodynamic therapy. This triggered a cascade of photo-physical and chemical processes culminating in the localized generation of ROS, ultimately breaking the intracellular redox equilibrium. Conclusion: This nanoformulation canpotentially be used for light-activated ROS-mediated therapy for the management of superficial tumors.