Photophysical Processes Research Articles

Design of Intramolecular Dihedral Angle between Electronic Donor and Acceptor in Thermally Activated Delayed Fluorescence Molecules.

In order to improve the exciton utilization efficiency (ηexc) of organic light-emitting materials, we addressed the ideal donor-acceptor dihedral angle (θD-A) in the TADF molecule by striking a balance between two photophysical processes. One is the conversion of triplet excitons into singlet excitons, and the other is the radiative process from a low-lying excited state to the ground state. Using a combination of first-principles calculations and molecular dynamics simulations, we investigated the impact of θD-A on the splitting energy and spin-orbit coupling between singlet and triplet excitons as well as the transition dipole moment for carbazole benzonitrile (CzBN) derivatives. By comparison with the reverse intersystem crossing rate (krISC), fluorescence emission rate (kr), and ηexc, we proposed a potential highest ηexc (of 94.4%) with the ideal θD-A of 77° for blue light CzBN derivatives; the calculated results have a good agreement with experimental measurement. The structure-efficiency physical connection between the molecular structure (θD-A) and efficiency provided an ideal parameter for a potential candidate for blue TADF-OLED materials.

Benchmarking DFT functionals for photophysics of pyranoflavylium cations

An intense absorption, phosphorescence, a long triplet excited state lifetime and singlet oxygen generation capabilities are characteristics of pyranoflavylium cations, analogues to pyranoanthocyanidins originated in the maturation process of red wine. Such properties make these compounds potential photosensitizers to be applied in photodynamic therapy. In this context, the photophysical processes underlying that treatment critically depend on the electronic structure of the pyranoflavylium molecules. When employing density functional theory to describe the electronic structure of molecules, the choice of the most suitable functional is not trivial, and benchmark studies are needed to orient practitioners in the field. In this work, a benchmark of seven of the most commonly used density functionals in addressing the photophysical properties of a set of eight pyranoflavylium cations is reported. Ground and excited state geometries, molecular orbitals, and absorption, fluorescence and phosphorescence transition energies were calculated using density functional theory approaches, and evaluated and compared to experimental data and monoreferential wave function-based methodologies. Statistical analysis of the results indicates that global-hybrid functionals allow an excellent description of absorption and emission energies, with errors around 0.05 eV, while range-separated variants led to somewhat larger errors in the range 0.1–0.2 eV. In contrast, range-separated functionals display excellent phosphorescence energies with errors close to 0.05 eV, in this case global-hybrids showing increased discrepancies around 0.5–0.1 eV.

Multimode‐Responsive Luminescence Smart Platform by Single‐Sm3+‐Doped Phosphors

AbstractMultimode‐responsive luminescence materials play pivotal roles both in understanding the fundamental photophysical processes and fabricating smart devices. To obtain inorganic phosphor‐based multi‐responsive materials, however, triple or even quadruple doping ions or a series of hosts with different compositions are frequently used. In this work, a single‐rare‐earth‐doped phosphor, Sr2YGaO5:Sm3+, with multi‐responsive luminescence is first reported. This single‐Sm3+‐doped phosphor exhibits unique excitation‐wavelength, ultraviolet/X‐ray charging, and thermal stimulated luminescence‐color‐changeable features. Further studies confirm that the multimode‐responsive luminescence of Sr2YGaO5:Sm3+ is attributed to superior optical integration of the host (Sr2YGaO5) and dopant (Sm3+) emissions under specific stimulation conditions. Importantly, this study provides a novel concept for designing inorganic phosphor‐based multifunctional optical materials, which would be conducive to the development of smart platforms.

Exciton States of Azobenzene Aggregates: A First‐Principles Study

AbstractInteraction between azobenzene‐containing molecules in supramolecular structures or self‐assembled monolayers (SAMs) results in the formation of molecular exciton states. These states determine photophysical and photochemical processes in such assemblies. Here, using first‐principles quantum chemical calculations, optical spectra and exciton delocalization of the exciton states in model clusters of azobenzene molecules are studied. Specifically, 1D linear chains and 2D SAM‐like arrangements are considered, and the exciton states are computed by means of time‐dependent long‐range corrected density functional theory (TD‐lc‐DFT) and ab initio configuration interaction singles (CIS), for clusters including up to 18 azobenzene molecules. The nature of the exciton states is analyzed using transition density matrix analysis. In addition, a connection to periodic systems is made applying the Bethe–Salpeter equation (BSE)/Green's function many‐body perturbation theory () approach to a selected system. It is found that the brightest excitons are dominated by local excitations. The energetic location of charge transfer states in the electronic spectra of aggregates depends to a large extent on a given method and distance between nearest neighbors. Furthermore, it is analized how an excitonic delocalization pattern changes with varying molecular orientation in the unit cell of SAMs.

Investigation of the N^C Ligand Effects on Emission Characteristics in a Series of Bis-Metalated [Ir(N^C)2(N^N)]+ Complexes.

A series of bis-metalated phosphorescent [(N^C)2Ir(bipyridine)]+ complexes with systematic variations in the structure and electronic characteristics of the N^C ligands were synthesized and characterized by using elemental analysis, mass spectrometry, NMR spectroscopy and X-ray crystallography. Investigation of the complexes' spectroscopic properties together with DFT and TD DFT calculations revealed that metal-to-ligand charge transfer (MLCT) and intraligand (LC) transition play key roles in the generation of emissive triplet states. According to the results of theoretical studies, the 3LC excited state is more accurate to consider as an intraligand charge transfer process (ILCT) between N- and C-coordinated moieties of the N^C chelate. This hypothesis is completely in line with the trends observed in the experimental absorption and emission spectra, which display systematic bathochromic shifts upon insertion of electron-withdrawing substituents into the N-coordinated fragment. An analogous shift is induced by expansion of the aromatic system of the C-coordinated fragment and insertion of polarizable sulfur atoms into the aromatic rings. These experimental and theoretical findings extend the knowledge of the nature of photophysical processes in complexes of this type and provide useful instruments for fine-tuning of their emissive characteristics.

A close look at polymer degree of crystallinity versus polymer crystalline quality

AbstractIn the broader polymer field, the term ‘crystallinity’ is often used rather loosely. However, increasingly, it becomes critical to clearly distinguish between degree of crystallinity, which provides the fractional amount of crystalline phase in a polymer, and the crystalline quality, which describes the perfection of the crystalline moieties that may form in a polymer. The reason is that these different structural features dictate important properties of plastic materials, including the mechanical properties of commodity polymers and the behavior of macromolecular ferroelectrics; they also determine which photophysical processes occur in semiconducting polymers. Hence, rigor needs to be applied when establishing structure/processing/property interrelations; and it should become a general practice that specific functions are clearly attributed to the degree of crystallinity, the crystalline quality or a combination of the two. In this perspective, in memoriam of Professor Dick Jones, a long‐time member of IUPAC's Polymer Division, we discuss the challenges of identifying—and distinguishing between—these important structural characteristics when using commonly applied measuring techniques and/or theoretical approaches. This task is often elaborate, as small changes in the chemical nature of the polymer and/or processing conditions selected can have drastic effects on both the crystalline quality and the degree of crystallinity, an issue that combined with the general ambiguity of data obtained with methodologies used to characterize polymer structures, theoretically or experimentally based. © 2023 The Authors. Polymer International published by John Wiley & Sons Ltd on behalf of Society of Industrial Chemistry.

Tracking the Intramolecular Charge Transfer Process of 2,6-Substituted D-A BODIPY Derivatives.

Three 2,6-electron donor-substituted boron dipyrromethene (BODIPY) exhibiting an intramolecular charge transfer (ICT) character with large Stokes shift and moderate fluorescence quantum yields were designed and synthesized. Broadband femtosecond transient absorption (fs-TA) spectroscopy measurements were performed to directly detect the CT state in nonpolar or less polar solvents and the charge separation (CS) state in more polar solvents. A solid foundation for the fs-TA assignment can be found in electrolysis experiments. In addition, the ICT character of the newly designed compounds was investigated by density functional theory (DFT) calculations. Meanwhile, the reference compounds without the donor groups were synthesized, and their photophysical behaviors and ultrafast time-resolved spectra confirmed that no ICT process occurred regardless of the nature of the solvent. This work emphasizes the importance of decorating the BODIPY core with electron-donating substituents at 2,6-positions to efficiently adjust its photofunctional behaviors demonstrating the ICT character. Importantly, the photophysical processes could be easily regulated by changing the solvent with different polarities.

Computational tools for the simulation and analysis of spin-polarized EPR spectra

The EPR spectra of paramagnetic species induced by photoexcitation typically exhibit enhanced absorptive and emissive features resulting from sublevel populations that differ from thermal equilibrium. The populations and the resulting spin polarization of the spectra are dictated by the selectivity of the photophysical process generating the observed state. Simulation of the spin-polarized EPR spectra is crucial in the characterization of both the dynamics of formation of the photoexcited state as well as its electronic and structural properties. EasySpin, the simulation toolbox for EPR spectroscopy, now includes extended support for the simulation of the EPR spectra of spin-polarized states of arbitrary spin multiplicity and formed by a variety of different mechanisms, including photoexcited triplet states populated by intersystem crossing, charge recombination or spin polarization transfer, spin-correlated radical pairs created by photoinduced electron transfer, triplet pairs formed by singlet fission and multiplet states arising from photoexcitation in systems containing chromophores and stable radicals. In this paper, we highlight EasySpin’s capabilities for the simulation of spin-polarized EPR spectra on the basis of illustrative examples from the literature in a variety of fields ranging across chemistry, biology, material science and quantum information science.

Natural photosensitizer-loaded in micellar copolymer to prevent bovine mastitis: A new post-dipping protocol on milking

Good management practices such as post-dipping applications (post-milking immersion bath) contribute to the dairy cattle health during lactation and minimize the appearance of mastitis (an infection in the mammary gland). The post-dipping procedure is performed conventionally using iodine-based solutions. The search for therapeutic modalities that are not invasive and do not cause resistance to the microorganisms that cause bovine mastitis instigates the interest of the scientific community. In this regard, antimicrobial Photodynamic Therapy (aPDT) is highlighted. The aPDT is based on combining a photosensitizer (PS) compound, light of adequate wavelength, and molecular oxygen (3O2), which triggers a series of photophysical processes and photochemical reactions that generate reactive oxygen species (ROS) responsible for the inactivation of microorganisms. The present investigation explored the photodynamic efficiency of two natural PS: Chlorophyll-rich spinach extract (CHL) and Curcumin (CUR), both incorporated into the Pluronic® F127 micellar copolymer. They were applied in post-dipping procedures in two different experiments. The photoactivity of formulations mediated through aPDT was conducted against Staphylococcus aureus, and obtained a minimum inhibitory concentration (MIC) of 6.8 mg mL−1 for CHL-F127 and 0.25 mg mL−1 for CUR-F127. Only CUR-F127 inhibited Escherichia coli growth with MIC 0.50 mg mL−1. Concerning the count of microorganisms during the days of the application, a significant difference was observed between the treatments and control (Iodine) when the teat surface of cows was evaluated. For CHL-F127 there was a difference for Coliform and Staphylococcus (p < 0.05). For CUR-F127 there was a difference for aerobic mesophilic and Staphylococcus (p < 0.05). Such application decreased bacterial load and maintained the milk quality, being evaluated via total microorganism count, physical-chemical composition, and somatic cell count (SCC).

Smart One-for-All Agent with Adaptive Functions for Improving Photoacoustic /Fluorescence Imaging-Guided Photodynamic Immunotherapy.

Multifunctional phototheranostics that integrate several diagnostic and therapeutic strategies into one platform hold great promise for precision medicine. However, it is really difficult for one molecule to possess multimodality optical imaging and therapy properties that all functions are in the optimized mode because the absorbed photoenergy is fixed. Herein, a smart one-for-all nanoagent that the photophysical energy transformation processes can be facilely tuned by external light stimuli is developed for precise multifunctional image-guided therapy. A dithienylethene-based molecule is designed and synthesized because it has two light-switchable forms. In the ring-closed form, most of the absorbed energy dissipates via nonradiative thermal deactivation for photoacoustic (PA) imaging. In the ring-open form, the molecule possesses obvious aggregation-induced emission features with excellent fluorescence and photodynamic therapy properties. In vivo experiments demonstrate that preoperative PA and fluorescence imaging help to delineate tumors in a high-contrast manner, and intraoperative fluorescence imaging is able to sensitively detect tiny residual tumors. Furthermore, the nanoagent can induce immunogenic cell death to elicit antitumor immunity and significantly suppress solid tumors. This work develops a smart one-for-all agent that the photophysical energy transformation and related phototheranostic properties can be optimized by light-driven structure switch, which is promising for multifunctional biomedical applications.

Halogen Bond-Driven Aggregation-Induced Emission Skeleton: N-(3-(Phenylamino)allylidene) Aniline Hydrochloride.

Aggregation-induced emission (AIE) is a unique photophysical process, and its emergence brings a revolutionary change in luminescence. However, AIE-based research has been limited to a few classical molecular skeletons, which is unfavorable for in-depth studies of the photophysical characteristics of AIE and the full exploitation of their potential values. There is an urgent need to develop new skeletons to rise to the challenges of an insufficient number of AIE core structures and difficult modification. Here, we report a novel dumbbell AIE skeleton, in which two phenyls are connected through (E)-3-iminoprop-1-en-1-amine. This skeleton shows extremely strong solid-state emission with an absolute quantum yield up to 69.5%, a large Stokes shift, and typical AIE characteristics, which well resolves the challenge of difficult modification and low luminous efficiency of the traditional AIE skeletons. One-step reaction, high yield, and diversified modification endow the skeleton with great scalability from simple to complicated, or from symmetrical to asymmetrical structures, which establishes the applicability of the skeleton in various scenarios. These molecules self-assemble into highly ordered layer-, rod-, petal-, hollow pipe-, or helix-like nanostructures, which contribute to strong AIE emission. Crystallographic data reveal the highly ordered layer structures of the aggregates with different substituents, and a novel halogen bond-driven self-assembly mechanism that restricts intramolecular motion is clearly discovered. Taking advantage of these merits, a full-band emission system from green to red is successfully established, which displays great potential in the construction of light-emitting films and advanced light-emitting diodes. The discovery of this AIE skeleton may motivate a huge potential application value in luminescent materials and lead to hitherto impossible technological innovations.

Role of Carbonyl Distortions Facilitating Persistent Room-Temperature Phosphorescence

A decrease in the room-temperature phosphorescence (RTP) yield and RTP lifetime with increasing temperature is often explained by the increased nonradiative transition from the lowest triplet excited state (T1) caused by molecular vibrations of the chromophores. Here, we report a positive contribution of molecular vibrations and the distortion of vanillic acid (VA) dispersed in amorphous insulating polymer hosts to phosphorescence characteristics. We investigated triplet generation as well as photophysical processes from T1 of VA dispersed in poly(methyl methacrylate) (PMMA) and poly(vinyl alcohol) (PVA). Comparisons between optically measured data and calculation-based data, regarding the phosphoresce rate (kp) and the rate constant of the nonradiative transition (knr) from T1, reveal that kp and knr of the dispersed VA negligibly changed in PMMA or PVA, indicating that intermolecular processes between VA and PMMA are related to a large RTP quenching of VA in PMMA. Vibrational out-of-plane distortion of the carbonyl moiety of VA induced ππ*–nπ* mixing between the high-order singlet excited state and the ground state, mainly enhancing kp compared with knr of VA. Although vibrations are often reported to quench RTP, this report suggests that some distortions induced by vibrations of other chromophores contribute to RTP enhancement of molecular solid materials.

Influence of Excited-State Delocalization on Singlet Fission: Tuning Triplet-Pair-State Emission in Thin Films

The coherent distribution of an electronic excitation over multiple organic molecules in the solid state, namely excited-state delocalization, plays an important role in photophysical processes such as singlet fission. However, experimental studies of the influence of excited-state delocalization on singlet fission have been challenging mainly for two reasons. First, there is no easy way of measuring the excited-state delocalization, and second, tracking the resulting changes for singlet fission is demanding due to the triplet-pair state, which is a crucial intermediate in singlet fission, being an optically dark state and hence hard to access experimentally. Binary systems offer a way to adapt the growth conditions of a singlet fission material, which enables tuning of the excited-state delocalization, possibly due to the impact of structural disorder on exciton localization. By varying the growth conditions, we demonstrate that emission from the triplet-pair state via Herzberg–Teller coupling is detectable in films with low growth rates of the singlet fission material, while the triplet-pair state shows no luminescence in the other cases due to triplet dissociation outcompeting the luminescent decay. With this we find that triplet-pair state luminescence correlates with higher excited-state delocalization.

Plasmon-dependent photophysical preparation of reversible Au@safranine T core-shell nanostructures with edit and erase features

Core-shell materials have superior properties over their single-component counterparts, and thus how to develop a new type of core-shell structure deserves much hard work. Herein, a plasmon-dependent photophysical (PDPP) process was developed to prepare Au@Safranine T core-shell nanostructures with reversible properties, wherein Safranine T (ST) was at first adsorbed on the surface of negatively charged gold nanospheres (AuNSs) through the electrostatic attraction and then controllably deposited by adjusting plasmonic illumination. The whole PDPP process was monitored in real-time via dark field microscopy (DFM) imaging system, verifying that plasmon-dependent light-matter interaction occurs. The as-prepared Au@ST nanostructures were then successfully applied for a reversible process of edit and erase that was suitable for encryption application with nanosized precision. This research showed that the PDPP process has high promise for developing precise structures of new materials, particularly those through nanoprocessing and nanooperation.

Highly Efficient Triplet–Triplet Annihilation Upconversion with a Thermally Activated Delayed Fluorescence Molecule as Triplet Photosensitizer

Thermally activated delayed fluorescence (TADF) molecules have great potential for practical applications as triplet photosensitizers, owing to their long triplet lifetimes, almost negligible energy loss in intersystem crossing, and excellent light-harvesting ability. Recently, several triplet–triplet annihilation (TTA) upconversion systems using the TADF photosensitizer have been reported. To provide comprehensive dynamic information for designing an optimized system in the future, more experiments with the new TADF photosensitizers are necessary. Herein, we conducted a new TTA upconversion system with a recently developed TADF molecule, DMACPDO, as the triplet photosensitizer and 9,10-diphenylanthracene (DPA) as the annihilator. The overall upconversion yield was determined to be relatively high, e.g., 22.3% in toluene, 21.9% in benzene, and 8.1% in chlorobenzene. By examining elementary photophysical and photochemical processes with time-resolved transient absorption and fluorescence emission spectroscopy, we revealed the influence of solvent polarity and viscosity on each step. Our conclusion highlights the elusive solvent effect in TTA upconversion application of the TADF photosensitizer.

Transient Energy-Resolved Photoluminescence Study of Excitons and Free Carriers on FAPbBr3 and FAPbBr3/SnO2 Interfaces

Lead bromide perovskites have a larger band gap and are significantly more stable than their iodine counterparts, offering the perspective for higher voltage, tandem photovoltaics exceeding the Shockley–Queisser limit, and shorter time to deployment of photovoltaics. However, their efficiencies still need to be rivaling the iodine ones. Herein, the photophysics of FAPbBr3 and the ones behind electron transfer from FAPbBr3 to SnO2, one of the most effective electron transporting materials (ETMs), are reported. Time- and energy-resolved photoluminescence studies revealed the existence of two emitting states in the perovskite, which were assigned to bounded excitons and free carriers. SnO2 extracted electrons from excitons and free carriers, with a selectivity related to the SnO2 surface treatment. This new insight helps explain SnO2’s unique qualities as an ETM to produce photovoltaics with reduced voltage losses. Furthermore, this study illustrates the importance of performing time- and energy-resolved photoluminescence to capture the intricacies of the photophysical process.

Photoexcitation dynamics and energy engineering in supramolecular doping of organic conjugated molecules

Doping and blending strategies are crucial means to precisely control the excited states and energy level in conjugated molecular systems. However, effective models and platforms are rarely proposed to systematically explore the effects of the formation of trapped doped centers on heterogeneous structures, energy level and ultrafast photophysical process. Herein, for deeply understanding the impact of molecular doping in film energy levels and photoexcitation dynamics, we set a supramolecular N-B coordination composed by the conjugated molecules of pyridine functionalized diarylfluorene (host material), named as ODPF-Phpy and ODPF-(Phpy)2, and the molecule of tris(perfluorophenyl)borane (BCF) (guest material). The generation of the molecular-level coordination bond increased the binding energy of N atoms and tuned the band-gap, leading to a new fluorescent emission center with longer excitation wavelength and emission wavelength. The intermolecular Förster resonance energy transfer (FRET) in blending films make it present inconsistent fluorescent behaviors compared to that in solution. The charge transfer (CT) state of N-B coordinated compounds and the changed dielectric constant of blending films resulted in a large PL spectra red-shift with the increased dopant ratio, causing a wide-tunable fluorescent color. The excited state behaviors of two compounds in blending system was further investigated by the transient absorption (TA) spectroscopy. Finally, we found supramolecular coordination blending can effectively improve the films’ photoluminescence quantum yield (PLQY) and conductivity. We believe this exploration in the internal coordination mechanisms would deepen the insights about doped semiconductors and is helpful in developing novel high-efficient fluorescent systems.

Detection of the Dark States in Thermally Activated Delayed Fluorescence (TADF) Process of Electron Donor-Acceptor Dyads: Insights from Optical Transient Absorption Spectroscopy.

The photophysical processes involved in the electron donor-acceptor thermally activated delayed fluorescence (TADF) emitters are complicated and controversial. The recent consensus is that at least three states are involved, i. e. the singlet charge transfer state (1 CT), the triplet localized excited state (3 LE) and the triplet CT state (3 CT). It is clear the very often used steady state and time-resolved luminescence spectroscopic methods are unable to present direct evidence for the dark states, i. e. the 3 LE and 3 CT states, as well as the interconversion of these states. Concerning this aspect, the femtosecond-nanosecond transient absorption spectroscopic methods are in particular interests. Both the emissive state and the dark state can be detected in these spectra, and interconversion of the states involved in TADF process can be also revealed. This review article focuses on the recent development of using the transient absorption spectra to study the photophysics of the TADF emitters.

Extending multi-layer energy-based fragment method for excited-state calculations of large covalently bonded fragment systems.

Recently, we developed a low-scaling Multi-Layer Energy-Based Fragment (MLEBF) method for accurate excited-state calculations and nonadiabatic dynamics simulations of nonbonded fragment systems. In this work, we extend the MLEBF method to treat covalently bonded fragment ones. The main idea is cutting a target system into many fragments according to chemical properties. Fragments with dangling bonds are first saturated by chemical groups; then, saturated fragments, together with the original fragments without dangling bonds, are grouped into different layers. The accurate total energy expression is formulated with the many-body energy expansion theory, in combination with the inclusion-exclusion principle that is used to delete the contribution of chemical groups introduced to saturate dangling bonds. Specifically, in a two-layer MLEBF model, the photochemically active and inert layers are calculated with high-level and efficient electronic structure methods, respectively. Intralayer and interlayer energies can be truncated at the two- or three-body interaction level. Subsequently, through several systems, including neutral and charged covalently bonded fragment systems, we demonstrate that MLEBF can provide accurate ground- and excited-state energies and gradients. Finally, we realize the structure, conical intersection, and path optimizations by combining our MLEBF program with commercial and free packages, e.g., ASE and SciPy. These developments make MLEBF a practical and reliable tool for studying complex photochemical and photophysical processes of large nonbonded and bonded fragment systems.

Monodispersed aluminum in carbon nitride creates highly efficient nitrogen active sites for ultra-high hydrogen peroxide photoproduction

Monodispersed metal sites are commonly considered as active centers that dominate the high catalytic activity and selectivity of a catalyst. Photocatalysis is a complex series of photophysical and electrocatalytic process, in which, it still remains challenges to obtain a well-defined profile on the functions of the monodispersed metal sites. Here we report that monodispersed aluminum (Al) metal sites are not active centers but they create abundant nitrogen non-metal active centers in carbon nitrides. Density functional theory (DFT) calculations show that Al metal sites serve as the bridges among isolated carbon nitride layers to establish diverse transfer channels for electrons, thus achieving rapid carrier separation. These findings provide a new perspective on understanding the important role of monodispersed metal sites for highly effective catalytic reactions.