Organic thermally activated delayed fluorescence (TADF) compounds used in photocatalysis

Improved transition metal photosensitizers to drive advances in photocatalysis

To function effectively in a photocatalytic application, a photosensitizer's light absorption, excited-state lifetime, and redox potentials, both in the ground state and excited state, are critically important. The absorption profile is particularly relevant to applications involving solar harvesting, whereas the redox potentials and excited-state lifetimes determine the thermodynamics, kinetics, and quantum yields of photoinduced redox processes. This perspective article focuses on synthetic inorganic and organometallic approaches to optimize these three characteristics of transition-metal based photosensitizers. We include our own work in these areas, which has focused extensively on exceptionally strong cyclometalated iridium photoreductants that enable challenging reductive photoredox transformations on organic substrates, and more recent work which has led to improved solar harvesting in charge-transfer copper(i) chromophores, an emerging class of earth-abundant compounds particularly relevant to solar-energy applications. We also extensively highlight many other complementary strategies for optimizing these parameters and highlight representative examples from the recent literature. It remains a significant challenge to simultaneously optimize all three of these parameters at once, since improvements in one often come at the detriment of the others. These inherent trade-offs and approaches to obviate or circumvent them are discussed throughout.

Styrylpyrimidine chromophores with bulky electron-donating substituents: experimental and theoretical investigation

Styrylpyrimidines with bulky 9,9-dimethylacridan, phenoxazine and phenothiazine electron-donating fragments were designed. Thermally activated delayed fluorescence (TADF) properties were expected for these structures. These chromophores exhibit peculiar emission properties. For 9,9-dimethylacridan and phenoxazine derivatives, a single emission highly sensitive to the polarity is observed in solution whereas for phenothiazine derivative a dual emission is observed in solution and is attributed to the coexistence of quasi-axial (Qax) and quasi-equatorial (Qeq) conformers. This study intends to understand through theoretical and experimental works, why the studied chromophores do not exhibit TADF properties, contrary to what was expected. The absence of phosphorescence both at room temperature and 77 K tends to indicate the impossibility to harvest triplet states in these systems. Wave-function based calculations show that for both conformers of the three chromophores the S1-T1 splitting is significantly larger than 0.2 eV. The second triplet state T2 of Qeq conformers is found very close in energy to the singlet S1 state, but S1 and T2 states possess similar charge transfer characters. This prevents efficient spin-orbit coupling between the states, which is consistent with the absence of TADF.

Inverted Lowest Singlet and Triplet Excitation Energy Ordering of Graphitic Carbon Nitride Flakes

In organic light-emitting diodes (OLEDs), only 25% of electrically generated excitons are in a singlet state, S1, and the remaining 75% are in a triplet state, T1. In thermally activated delayed fluorescence (TADF) chromophores the transition from the nonradiative T1 state to the radiative S1 state can be thermally activated, which improves the efficiency of OLEDs. Chromophores with inverted energy ordering of S1 and T1 states, S1 < T1, are superior to TADF chromophores, thanks to the absence of an energy barrier for the transition from T1 to S1. We benchmark the performance of time-dependent density functional theory using different exchange-correlation functionals and find that scaled long-range corrected double-hybrid functionals correctly predict the inverted singlet-triplet gaps of N-substituted phenalene derivatives. We then show that the inverted energy ordering of S1 and T1 is an intrinsic property of graphitic carbon nitride flakes. A design strategy of new chromophores with inverted singlet-triplet gaps is proposed. The color of emitted light can be fine-tuned through flake size and amine substitution on flake vertices.

Controlled Radical Copolymerization toward Well-Defined Fluoropolymers

In the past 80 years, fluoropolymers have found broad applications in both industrial and academic settings, owing to their unique physicochemical properties. Copolymerizations of fluoroalkene feedstocks present an important avenue to obtain high-performance materials by merging intrinsic attributes of fluorocarbons and great versatility of comonomers. Recently, while massive investigations have disclosed the great potentials of precisely synthesized polymers, researchers have made considerable efforts to approach well-defined fluorinated copolymers. This minireview discusses challenges in controlled radical copolymerizations (CRCPs) of fluoroalkenes and provides a concise perspective on recent progress in CRCPs of fluoroalkenes (e.g., tetrafluoroethylene, chlorotrifluoroethylene, hexafluoropropene, perfluoroalkyl vinyl ethers) with non-fluorinated vinyl comonomers, which have enabled on-demand preparations of various main-chain fluoropolymers with predefined molar masses, low dispersities, as well as regulable chemical compositions and sequences. The synthetic advantages of CRCPs will promote controlled and facile access to customized fluoropolymers for high-tech applications such as batteries, coatings and so on.

Tailoring thermally activated delayed fluorescence emitters for efficient electrochemiluminescence with tripropylamine as coreactant

Using a unified metal-free procedure, a selection of Thermally Activated Delayed Fluorescence (TADF) emitters has been synthesized and characterized. Different acceptor and donor moieties have been explored in order to develop red emitting dyes with reduction potentials suitable for the application in ECL using tri-propylamine as coreactant. The most promising compound shows terephthalonitrile as the acceptor and diphenylamines as donors, and it displayed an ECL efficiency that is double the one of the standard [Ru(bpy)3](PF6)2. Based on such findings, a novel water-soluble TADF emitter (Na4[4DPASO3TPN]) has been synthesized and characterized to enable electrochemiluminescence in an aqueous medium.

Nonadiabatic Dynamics Simulations for Photoinduced Processes in Molecules and Semiconductors: Methodologies and Applications

Nonadiabatic dynamics (NAMD) simulations have become powerful tools for elucidating complicated photoinduced processes in various systems from molecules to semiconductor materials. In this review, we present an overview of our recent research on photophysics of molecular systems and periodic semiconductor materials with the aid of ab initio NAMD simulation methods implemented in the generalized trajectory surface-hopping (GTSH) package. Both theoretical backgrounds and applications of the developed NAMD methods are presented in detail. For molecular systems, the linear-response time-dependent density functional theory (LR-TDDFT) method is primarily used to model electronic structures in NAMD simulations owing to its balanced efficiency and accuracy. Moreover, the efficient algorithms for calculating nonadiabatic coupling terms (NACTs) and spin-orbit couplings (SOCs) have been coded into the package to increase the simulation efficiency. In combination with various analysis techniques, we can explore the mechanistic details of the photoinduced dynamics of a range of molecular systems, including charge separation and energy transfer processes in organic donor-acceptor structures, ultrafast intersystem crossing (ISC) processes in transition metal complexes (TMCs), and exciton dynamics in molecular aggregates. For semiconductor materials, we developed the NAMD methods for simulating the photoinduced carrier dynamics within the framework of the Kohn-Sham density functional theory (KS-DFT), in which SOC effects are explicitly accounted for using the two-component, noncollinear DFT method. Using this method, we have investigated the photoinduced carrier dynamics at the interface of a variety of van der Waals (vdW) heterojunctions, such as two-dimensional transition metal dichalcogenides (TMDs), carbon nanotubes (CNTs), and perovskites-related systems. Recently, we extended the LR-TDDFT-based NAMD method for semiconductor materials, allowing us to study the excitonic effects in the photoinduced energy transfer process. These results demonstrate that the NAMD simulations are powerful tools for exploring the photodynamics of molecular systems and semiconductor materials. In future studies, the NAMD simulation methods can be employed to elucidate experimental phenomena and reveal microscopic details as well as rationally design novel photofunctional materials with desired properties.

Modulation of Delayed Fluorescence Guided by Conformational Effect-Mediated Thermally Enhanced Phosphorescence in Phenothiazines-Quinoline-Cl Conjugates

Triplet energy harvesting via thermally activated delayed fluorescence (TADF) from pure organic systems has attracted great attention in organic light-emitting diodes, sensing, and photocatalysis. However, the realization of thermally enhanced phosphorescence (TEP)-guided efficient TADF with a high rate of reverse intersystem crossing (kRISC) still needs to be discovered. Herein, we report two phenothiazine-quinoline conjugates (P2QC, P2QMC) comprising two phenothiazine donors covalently attached to the chlorine-substituted quinolinyl acceptor. Spectroscopic analysis in conjunction with quantum chemistry calculations reveals that TEP in P2QC originated due to slow internal conversion from higher-lying triplet to lowest triplet (T2' → T1') of the quasi-axial (QA) conformer and TADF (kRISC = 1.44 × 108 s-1) originated from the quasi-equatorial (QE) conformer caused by a low singlet-triplet gap (ΔES1-T1 = 0.11 eV) and triplet energy transfer from QA to QE owing to the degenerate ground state of the conformers. In contrast, TADF (kRISC = 0.74 × 108 s-1) and dual phosphorescence under ambient conditions are observed in P2QMC. This study provides a sustainable guideline for developing efficient TADF emitters via conformation effects and energy transfer mechanisms.

Exploration of violet-to-blue thermally activated delayed fluorescence emitters based on "CH/N" and "H/CN" substitutions at diphenylsulphone acceptor. A DFT study

The violet-to-blue thermally activated delayed fluorescence (TADF) emitters were created employing several substituents based on 5,5-dimethyl-5,10-dihydropyrido [2,3-b][1,8] naphthyridine-diphenylsulphone (DMDHPN-DPS) called 1a via "CH/N" and "H/CN" substitutions at the diphenylsulphone acceptor (DPS) moiety. The parent compound 1a was selected from our former work after extensive research employing "CH/N" substitution on Dimethyl-acridine (DMAC) donor moiety. There is a little overlap amid the highest occupied molecular orbitals (HOMOs) and lowest un-occupied molecular orbitals (LUMOs) due to the distribution of HOMOs and LUMOs primarily on the DMDHPN donor and the DPS acceptor moieties, respectively. It resulted in a narrower energy gap (∆E ST) between the lowest singlet (S1) and triplet (T1) excited state. In nearly all derivatives, the steric hindrance results in a larger torsional angle (85°-98°) between the plane of the DMDHPN and the DPS moieties. The predicted ΔE ST values of the compounds with "H/CN" substitution were lower than those of the comparable "CH/N" substituents, demonstrating the superiority of the reversible inter-system crossing (RISC) from the T1 → S1 state. All derivatives have emission wavelengths (λ em) in the range of 357-449 nm. The LUMO → HOMO transition energies in the S1 states are lowered by the presence of -CN groups or -N = atoms at the ortho or meta sites of a DPS acceptor unit, causing the λ em values to red-shift. Furthermore, the λ em showed a greater red-shift as there were more-CN groups or -N = atoms. Three of the derivatives named 1b, 1g, and 1h, emit violet (394 nm, 399 nm, and 398 nm, respectively), while two others, 1f and 1i, emit blue shade (449 nm each) with reasonable emission intensity peak demonstrating that these derivatives are effective violet-to-blue TADF nominees. The lower ΔE ST value for derivative 1i (0.01 eV) with λ em values of 449 nm make this molecule the finest choice for blue TADF emitter amongst all the studied derivatives. We believe our research might lead to the development of more proficient blue TADF-OLEDs in the future.

Red-Shifted Emission in Multiple Resonance Thermally Activated Delayed Fluorescent Materials through Malononitrile Incorporation

Multiple resonance thermally activated delayed fluorescent (MR-TADF) materials offer higher color purity than conventional TADF materials but suffer from aggregation-caused quenching (ACQ) and rarely exhibit red emission. Herein, two malononitrile-substituted emitters are synthesized from a quinolino[3,2,1-de]acridine-5,9-dione (QAO) MR-TADF precursor. Both materials maintain MR-TADF, while they display red-shifted fluorescence. They also overcome ACQ, displaying enhanced emission upon aggregation in solution and forming red-emissive J-aggregates in the solid state with photoluminescent quantum yields of 9 and 11%.

Donor-Acceptor-Donor Triads with Flexible Spacers: Deciphering Complex Photophysics for Targeted Materials Design

The complex photokinetics of donor-acceptor-donor triads with varying flexible spacer lengths (n = 4-10 carbon atoms) are investigated in liquid and solid solution, as well as in crystals, by steady-state and transient fluorescence spectroscopy combined with computational studies. For the short spacer (n = 4) in a liquid solution, dynamic charge-transfer (CT) state formation with subsequent, efficient exciplex emission is observed, effectively competing with quenching through electron transfer (eT) via a radical ion pair. In a solid solution, a fluorescent CT static complex is formed upon freezing for all spacer lengths. This allows the observations of a former seminal report on stimuli-responsive high-contrast fluorescence on/off switching in films of the triads to be reassigned (Adv. Mater. 2012, 24, 5487), now providing a holistic picture on varying spacer length. In fact, external stimuli of the film by modulating the geometry of the CT complex, which results in on/off fluorescence switching (for n > 4) or in a change of the emission color (n = 4). The work thus demonstrates how in-depth analysis of complex photophysics can be put to practical use in materials science.

Anti-Stokes Luminescence in Multi-Resonance-Type Thermally-Activated Delayed Fluorescence Molecules

Photon-upconversion in organic molecular systems is one of the promising technologies for future energy harvesting systems because these systems can generate excitons that possess higher energy than excitation energy. The photon-upconversion caused by absorbing ambient heat as additional energy is particularly interesting because it could ideally provide a light-driving cooling system. However, only a few organic molecular systems have been reported. Here, we report the anti-Stokes photoluminescence (ASPL) derived from hot-band absorption in a series of multi-resonance-type thermally-activated delayed fluorescence (MR-TADF) molecules. The MR-TADF molecules exhibited an anti-Stokes shift of approximately 0.1 eV with a high PL quantum yield in the solution state. The anti-Stokes shift corresponded well to the 1-0 vibration transition from the ground state to the excited singlet state, and we further evaluated a correlation between the activation energy for the ASPL intensity and the TADF process. Our demonstration underlines that MR-TADF molecules have become a novel class of ASPL materials for various future applications, such as light-driving cooling systems.

Solvent effects on the photoinduced charge separation dynamics of directly linked zinc phthalocyanine-perylenediimide dyads: a nonadiabatic dynamics simulation with an optimally tuned screened range-separated hybrid functional

Herein, we have employed a combination of the optimally tuned screened range-separated hybrid (OT-SRSH) functional, the polarizable continuum model (PCM), and nonadiabatic dynamics (NAMD) simulations to investigate the photoinduced dynamics of directly linked donor-acceptor dyads formed using zinc phthalocyanine (ZnPc) and perylenediimide (PDI), in which ZnPc is the donor while PDI is the acceptor. Our simulations aim to analyze the behavior of these dyads upon local excitation of the ZnPc moiety in the gas phase and in benzonitrile. Our findings indicate that the presence of a solvent can significantly influence the excited state dynamics of ZnPc-PDI dyads. Specifically, the polar solvent benzonitrile effectively lowers the vertical excitation energies of the charge transfer (CT) state from ZnPc to PDI. As a result, the energetic order of the locally excited (LE) states of ZnPc and the CT states is reversed compared to the gas phase. Consequently, the photoinduced electron transfer (PET) dynamics from ZnPc to PDI, which is absent in the gas phase, takes place in benzonitrile with a time constant of 10.4 ps. Importantly, our present work not only qualitatively agrees with experimental results but also provides in-depth insights into the underlying mechanisms responsible for the photoinduced dynamics of ZnPc-PDI. Moreover, this study emphasizes the importance of appropriately considering solvent effects in NAMD simulation of organic donor-acceptor systems, taking into account the distinct excited state dynamics observed in the gas phase and benzonitrile. Furthermore, the combination of the OT-SRSH functional, the PCM solvent model, and nonadiabatic dynamics simulations shows promise as a strategy for investigating the complex excited state dynamics of organic donor-acceptor systems in solvents. These findings will be valuable for the future design of novel organic donor-acceptor structures with improved performance.

Uncovering the substituted-position effect on excited-state evolution of benzophenone-phenothiazine dyads

Photofunctional materials based on donor-acceptor molecules have drawn intense attention due to their unique optical properties. Importantly, Systematic investigation of substitution effects on excited-state charge transfer dynamics of donor-acceptor molecules is a powerful approach for identifying application-relevant design principles. Here, by coupling phenothiazine (PTZ) at the ortho-, meta-, and para-positions of the benzene ring of benzophenone (BP), three regioisomeric BP-PTZ dyads were designed to understand the relationship between substituted positions and excited-state evolution channels. Ultrafast transient absorption is used to detect and trace the transient species and related evolution channels of BP-PTZ dyads at excited state. In a non-polar solvent, BP-o-PTZ undergoes the through-space charge transfer process to produce a singlet charge-transfer (1CT) state, which subsequently proceeds the intersystem crossing process and transforms into a triplet charge-transfer (3CT) state; BP-m-PTZ experiences intramolecular charge transfer (ICT) process to generate the 1CT state, which subsequently transforms into the 3CT state by the intersystem crossing (ISC) and finally converts into the local-excited triplet (3LE) state; as for BP-p-PTZ, only 3LE states can be detected after the ISC process from the 1CT state. On the other hand, the twisted ICT states are generated via twisted motion between the donor and acceptor for all BP-PTZ dyads or planarization of the PTZ unit in high polar solvents. The excited-state theoretical calculations unveil that the features of ICT and intramolecular interaction between the three dyads play a decisive role in determining the through-bond charge transfer and through-space charge transfer processes. Also, these results demonstrate that the excited-state evolution channels of PTZ derivatives could be modified by tuning the substituted positions of the donor-acceptor dyads. This study provides a deep perspective for the substitute-position effect on donor-acceptor-type PTZ derivatives.

Highly Efficient Electrochemiluminescence from Imidazole-Based Thermally Activated Delayed Fluorescence Emitters

A family of novel thermally activated delayed fluorescence (TADF) emitters has been synthesized by a straightforward and metal-free synthesis, and structurally characterized. In this work we kept the acceptor moiety, 4-(1H-imidazol-1-yl)benzonitrile, fixed and systemically tested different donors to correlate their photophysical and electrochemical properties with their performance in electrochemiluminescence using both benzoyl peroxide as co-reactant and co-reactant free (annihilation) conditions. Some compounds exceeded the efficiency of the standard [Ru(bpy)3 ]Cl2 by up to 28 times with benzoyl peroxide and 38 times in annihilation. Interestingly, we found that the efficiency is mainly dictated by the electrochemical reversibility of the redox processes rather than by the photophysical properties in terms of photoluminescence quantum yields or excited-state lifetime. In addition, the annihilation electrochemiluminescence efficiency strongly depends on the pulse sequence. The imidazole moiety can be conveniently alkylated, thus allowing the insertion of functional groups, such a carboxylic acid, and enabling practical applications.

Novel Triangulenes: Computational Investigations of Energy Thresholds for Photocatalytic Water Splitting

Organic materials with Inverted Singlet-Triplet (INVEST) gaps are interesting for their potential use in photocatalytic small molecule transformations such as the entirely solar-driven water splitting reaction. However, only a few INVEST emitters are thermodynamically able to split water requiring a first singlet excited dark state, S1 , above 1.27 or 1.76 eV, and absorption near solar the maximum, 2.57 eV. These requirements and the INVEST character are key for achieving a long-lived photocatalyst for water splitting. The only known INVEST emitters that conform to these criteria are large triangular boron carbon nitrides with unknown synthesis pathways. Using ADC(2), a quantum-mechanical method, we describe three triangulenes. 3 a is a cyano azacyclopenta[cd]phenalene derivative while 3 b and 3 c are cycl[3.3.3]azine derivatives. 3 b has a previously undescribed disulfide bridge. Overall, 3 a fulfills requirements for photocatalytic four-electron reduction of water while the S1 states of 3 b and 3 c are likely slightly low for the two-electron reduction process. By analyzing impacts of ligands, we find that there are guidelines describing how S1 -S5 energies and oscillator strengths, T1 energies, and ΔES1T1 gaps are affected, requiring deep-learning algorithms for which studies will be presented by us in due time. The impact of ground-state geometries, solvation effects, as well as reduced-cost ADC(2) algorithms on our findings are also discussed.

Organic thermally activated delayed fluorescence material with strained benzoguanidine donor

Organic thermally activated delayed fluorescence (TADF) materials have been widely investigated due to their impressive electronic properties and applied potential for the third generation of organic light-emitting diodes (OLED). We present organic TADF material (4BGIPN) based on the strained benzoguanidine donor and compare it with the benchmark carbazole-based material (4CzIPN). Extended π-conjugation in 4BGIPN material results in yellow-green luminescence at 512 nm with a fast radiative rate of 5.5 × 10-5 s-1 and a photoluminescence quantum yield of 46% in methylcyclohexane solution. Such a nitrogen-rich 4BGIPN material has a significantly stabilized highest occupied molecular orbital (HOMO) at -6.4 eV while the lowest unoccupied molecular orbital (LUMO) at -4.0 eV, indicating potential suitability for application as the electron transport layer or TADF class III emitter in OLEDs.

Excited-state singlet-triplet inversion in hexagonal aromatic and heteroaromatic compounds

The inversion of the energies of the lowest singlet (S1) and lowest triplet (T1) excited states in violation of Hund's multiplicity rule is a rare phenomenon in stable organic molecules. S1-T1 inversion has significant consequences for the photophysics and photochemistry of organic chromophores. In this work, wave-function based ab initio computational methods were employed to explore the possibility of S1-T1 inversion in hexagonal polycyclic aromatic and heteroaromatic compounds. In these molecules, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are two-fold degenerate. The HOMO-LUMO transition gives rise to three singlet and three triplet excited states. While the singlet-triplet energy gap ΔST, defined as the energy difference between the S1 state and the T1 state, is clearly positive for benzene, it is predicted to be close to zero for borazine, the boron nitride analogue of benzene. Although ΔST decreases with increasing size of hexagonal polycyclic aromatics, it remains positive up to circumcoronene (19 rings). However, symmetry-preserving substitution of C-C pairs by B-N groups in the interior, keeping the conjugation of the outer rim intact, results in compounds with robustly negative ΔST. These findings establish the existence of a new family of boron carbon nitrides with inverted singlet-triplet gaps.

Theoretical investigation on the functional group modulation of UV-Vis absorption profiles of triphenylamine derivatives

Understanding the functional group modulation of electronic structure and excitation is pivotal to the design of organic small molecules (OSMs) for photoelectric applications. In this study, we employed density functional theory (DFT) and time-dependent DFT (TDDFT) calculations to explore the unique absorption character of four triphenylamine photosensitizers. The various conformations were investigated given the multiple single bonds in the compounds, and the resemblance in the electronic structure of different conformations is affirmed because the coplanarity and consequent long-range conjugation is maintained regardless of the orientation of the flexible blocks. Six functionals were evaluated, and MN15 was found to successfully reproduce the intense secondary absorption peak for the double 3,4-ethylenedioxythiophene (EDOT) modified sensitizer over B3LYP, PBE0, M062X, CAM-B3LYP, and ωB97XD. The introduction of EDOT gives rise to a new excited state S4, which is a local excitation constrained in the EDOT substituent triphenylamine block. This new excited state S4, in combination with inherent S2 and S3 derived from prototype molecule TPA-Pyc, jointly contributes to the hump of the secondary absorption peak of ETE-Pyc and finally affects the light-harvesting ability of the dye-sensitized TiO2 photoanode. The current findings provide guidance toward the rational design of OSMs with good light-harvest ability.

3DPAFIPN as a halogenated dicyanobenzene-based photosensitizer catalyzed gram-scale photosynthesis of pyrano[2,3-d]pyrimidine scaffolds

Utilizing the Knoevenagel-Michael tandem cyclocondensation reaction of barbituric acid/1,3-dimethylbarbituric acid, malononitrile, and aryl aldehydes, a sustainable methodology for the photosynthesis of pyrano[2,3-d]pyrimidine scaffolds has been devised. The present study expounds on the development of a green radical synthetic approach toward this class of compounds. In this study, a novel halogenated dicyanobenzene-based photosensitizer was utilized in an aqueous solution, exposed to air at room temperature, and activated by a blue LED as a renewable energy source for the purpose of generating energy. The primary aim of this endeavor is to employ a recently developed, easily obtainable, and affordably priced halogenated cyanoarene-based donor-acceptor (D-A). The 3DPAFIPN [2,4,6-tris(diphenylamino)-5-fluoroisophthalonitrile]} photocatalyst, as a thermally activated delayed fluorescence (TADF), is capable of inducing single electron transfer (SET) upon irradiation with visible light, thereby offering a facile and efficient approach with a high degree of effectiveness, energy efficiency, and eco-friendliness. The aforementioned phenomenon facilitates the exploration of the temporal changes that have occurred in the interactions between the surroundings and chemical constituents. The present study aimed to investigate the turnover number (TON) and turnover frequency (TOF) for pyrano[2,3-d]pyrimidine scaffolds. Additionally, it has been demonstrated that gram-scale cyclization is a viable method for utilization in industrial applications.

One-Step Electropolymerization of a Dicyanobenzene-Carbazole-Imidazole Dye to Prepare Photoactive Redox Polymer Films

To the best of our knowledge, this study reports the first direct electropolymerization of a dicyanobenzene-carbazole dye functionalized with an imidazole group to prepare redox- and photoactive porous organic polymer (POP) films in controlled amounts. The POP films were grown on indium-doped tin oxide (ITO) and carbon surfaces using a new monomer, 1-imidazole-2,4,6-tri(carbazol-9-yl)-3,5-dicyanobenzene (1, 3CzImIPN), through a simple one-step process. The structure and activities of the POP films were investigated as photoelectrodes for electrooxidations, as heterogeneous photocatalysts for photosynthetic olefin isomerizations, and for solid-state photoluminescence behavior tunable by lithium-ion concentrations in solution. The results demonstrate that the photoredox-POPs can be used as efficient photocatalysts, and they have potential applications in sensing.

Designing Nonconventional Luminescent Materials with Efficient Emission in Dilute Solutions via Modulation of Dynamic Hydrogen Bonds

Nonconventional luminescent materials (NLMs) which do not contain traditional aromatic chromophores are of great interest due to their unique chemical structures, optical properties, and their potential applications in various areas, such as cellular imaging and chemical sensing. However, most reported NLMs show weak or no emission in dilute solutions, which severely limits their applications. In this work, dynamic hydrogen bonds were utilized to design NLMs with efficient emission in dilute solutions. To further validate the results, polymers P1 and P2 were successfully prepared and investigated. It was found that the luminescence quantum efficiency of P1 and P2 at a concentration of 0.1 mg/mL in water solution was 8.9 and 0.6%, respectively. The high efficiency can be attributed to the fact that polymer P1 has more intra- or intermolecular dynamic hydrogen bonds and other short interactions than P2 in dilute solutions, allowing P1 to achieve the through-space conjugation effect to increase the degree of system conjugation, restrict molecular motion, and decrease nonradiative transitions, which can effectively improve luminescence. In addition, polymer P2 exhibits the characteristics of clustering-triggered emission, excitation wavelength-dependent and concentration-dependent fluorescence properties, excellent photobleaching resistance, low cytotoxicity, and selective recognition of Fe³⁺. The present study investigates the manipulation of luminescence properties of NLMs in dilute solutions through the modulation of dynamic hydrogen bonds. This approach can serve as a semi-empirical technique for designing and building innovative NLMs in the times ahead.

Photoredox-HAT Catalysis for Primary Amine α-C-H Alkylation: Mechanistic Insight with Transient Absorption Spectroscopy

The synergistic use of (organo)photoredox catalysts with hydrogen-atom transfer (HAT) cocatalysts has emerged as a powerful strategy for innate C(sp³)-H bond functionalization, particularly for C-H bonds α- to nitrogen. Azide ion (N₃^-) was recently identified as an effective HAT catalyst for the challenging α-C-H alkylation of unprotected, primary alkylamines, in combination with dicyanoarene photocatalysts such as 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN). Here, time-resolved transient absorption spectroscopy over sub-picosecond to microsecond timescales provides kinetic and mechanistic details of the photoredox catalytic cycle in acetonitrile solution. Direct observation of the electron transfer from N₃^- to photoexcited 4CzIPN reveals the participation of the S₁ excited electronic state of the organic photocatalyst as an electron acceptor, but the N₃^• radical product of this reaction is not observed. Instead, both time-resolved infrared and UV-visible spectroscopic measurements implicate rapid association of N₃^• with N₃^- (a favorable process in acetonitrile) to form the N₆^•- radical anion. Electronic structure calculations indicate that N₃^• is the active participant in the HAT reaction, suggesting a role for N₆^•- as a reservoir that regulates the concentration of N₃^•.

The luminescent and reaction mechanisms of a fluorescent probe for the detection of hypochlorous acid: Insights from theory

We have designed and synthesized a novel fluorescent probe BMH for detection of hypochlorous acid (HClO), which can increase dramatically the fluorescence intensity and had ultrafast response, a low detection limit and a wide pH range of application. In this paper, we further studied its fluorescence quantum yield and photoluminescence mechanism theoretically. The calculated results indicated the first excited states of BMH and BM (it was the oxidized product by HClO) were bright states with large oscillator strengths, however, due to more larger reorganization energy of BMH, the predicted internal conversion rate k_IC of BMH was four orders of magnitude larger than that of BM; moreover, owing to the effect of heavy atom from sulfur atom in BMH, the predicted intersystem crossing rate k_isc of BMH was five orders of magnitude larger than that of BM; meanwhile there was no significant difference found between both the predicted radiative rates k_r, thus the calculated fluorescence quantum yield of BMH was nearly zero and that of BM was more than 90%, the data showed the BMH had no fluorescence but its oxidated produce BM possessed strong fluorescence. In addition, the reaction mechanism of BMH transforming into BM has been investigated too, according to the potential energy profile, we found that the course of BMH converting into BM consisted of three elementary reactions. The research results revealed the solvent effect can decreased the activation energy, which was more favorable for these elementary reactions.

Hybrid Catalysts for Enantioselective Photo-Phosphoric Acid Catalysis

The syntheses of two novel, organic, and chiral photocatalysts are presented. By combining donor-acceptor cyanoarene-based photocatalysts with a chiral phosphoric acid, bifunctional catalysts have been designed. In preliminary proof-of-concept reactions, their use in both enantioselective energy transfer and photoredox catalysis is demonstrated.

Blending Aryl Ketone in Covalent Organic Frameworks to Promote Photoinduced Electron Transfer

Aryl-ketone derivatives have been acknowledged as promising organic photocatalysts for photosynthesis. However, they are limited by their photostability and have been less explored for photoinduced electron transfer (PET) applications. Herein we demonstrate a novel strategy to cover the shortage of aryl-ketone photocatalysts and control the photoreactivity by implanting symmetric aryl ketones into the conjugated covalent organic frameworks (COFs). To prove the concept, three comparative materials with the same topology and varied electronic structures were built, adopting truxenone knot and functionalized terephthalaldehyde linkers. Spectroscopic investigation and excited carrier dynamics analysis disclosed improvements in the photostability and electronic transfer efficiency as well as the structure-performance relationships toward N-aryl tetrahydroisoquinoline oxidation. This system provides a robust rule of thumb for designing new-generation aryl-ketone photocatalysts.

Allenyl Carbonate as a Butadiene Surrogate in Cobalt-Catalyzed Crotylation of Aldehydes

Allenyl carbonate was used as a 1,3-butadiene surrogate to develop a photocatalytically sustainable protocol for cobalt-catalyzed crotylation of aldehydes. The developed method tolerated a wide range of aromatic and aliphatic aldehydes with retention of functional groups under mild conditions and produced good-to-excellent yields of crotylated secondary alcohols. Based on preliminary mechanistic studies and literature precedents, a plausible mechanism is proposed.

Dibenzodipyridophenazines with Dendritic Electron Donors Exhibiting Deep-Red Emission and Thermally Activated Delayed Fluorescence

The development of deep-red thermally activated delayed fluorescence (TADF) emitters is important for applications such as organic light-emitting diodes (OLEDs) and biological imaging. Design strategies for red-shifting emission include synthesizing rigid acceptor cores to limit nonradiative decay and employing strong electron-donating groups. In this work, three novel luminescent donor-acceptor compounds based on the dibenzo[a,c]dipyrido[3,2-h:20-30-j]-phenazine-12-yl (BPPZ) acceptor were prepared using dendritic carbazole-based donors 3,3″,6,6″-tetramethoxy-9'H-9,3':6',9″-tercarbazole (TMTC), N³,N³,N⁶,N⁶-tetra-p-tolyl-9H-carbazole-3,6-diamine (TTAC), and N³,N³,N⁶,N⁶-tetrakis(4-methoxyphenyl)-9H-carbazole-3,6-diamine (TMAC). Here, dimethoxycarbazole, ditolylamine, and bis(4-methoxyphenyl)amine were introduced at the 3,6-positions of carbazole to increase the strength of these donors and induce long-wavelength emission. Substituent effects were investigated with experiments and theoretical calculations. The emission maxima of these materials in toluene were found to be 562, 658, and 680 nm for BPPZ-2TMTC, BPPZ-2TTAC, and BPPZ-2TMAC, respectively, highlighting the exceptional strength of the TMAC donor, which pushes the emission into the deep-red region of the visible spectrum as well as into the biological transparency window (650-1350 nm). Long-lived emission lifetimes were observed in each emitter due to TADF in BPPZ-2TMC and BPPZ-2TTAC, as well as room-temperature phosphorescence in BPPZ-2TMAC. Overall, this work showcases deep-red emissive dendritic donor-acceptor materials which have potential as bioimaging agents with emission in the biological transparency window.

Visible Light-Mediated Carbamoylation of para-Quinone Methides

We report a photocatalytic approach for the installation of the amide moiety onto para-quinone methides. This transformation features a net reductive approach for the generation of carbamoyl radicals from amide-substituted Hantzsch ester derivatives under transition metal-free conditions. This protocol exhibits wide scope and allows access to diarylacetamides employing a C-C bond formation approach.

Regioselective electrophilic aromatic borylation as a method for synthesising sterically hindered benzothiadiazole fluorophores

Regioselective stepwise phenylation of 4,7-diarylbenzo[c][1,2,5]thiadiazole fluorophores has been achieved through a facile one-pot, three-step synthetic strategy involving sequential borylation, hydroxydechlorination and Suzuki-Miyaura cross-coupling reactions. Crucial to the selectivity was the use of BCl₃ to regioselectively install a boronic acid group in the ortho-position of only one of the diaryl groups. The subsequent introduction of ortho-phenyl groups through Suzuki-Miyaura cross-coupling gave rise to twisted structures with hindered intramolecular rotation, providing a structural lever with which the fluorophore absorption and emission properties could be adjusted.

Blue-to-UVB Upconversion, Solvent Sensitization and Challenging Bond Activation Enabled by a Benzene-Based Annihilator

Several energy-demanding photoreactions require harsh UV light from inefficient light sources. The conversion of low-energy visible light to high-energy singlet states via triplet-triplet annihilation upconversion (TTA-UC) could offer a solution for driving such reactions under mild conditions. We present the first annihilator with an emission maximum in the UVB region that, combined with an organic sensitizer, is suitable for blue-to-UVB upconversion. The annihilator singlet was successfully employed as an energy donor in subsequent FRET activations of aliphatic carbonyls. This hitherto unreported UC-FRET reaction sequence was directly monitored using laser spectroscopy and applied to mechanistic irradiation experiments demonstrating the feasibility of Norrish chemistry. Our results provide clear evidence for a novel blue light-driven substrate or solvent activation strategy, which is important in the context of developing more sustainable light-to-chemical energy conversion systems.

Synthesis of C4-Aminated Carbazoles and Their Derivatives via Pd/NBE Chemistry

Carbazole, as one of the most important organic frameworks, has been used in optoelectronic materials and biochemistry. However, the synthesis of C4-substituted carbazole has always been an unsolved problem. This report describes the one-step synthesis of C4-aminated carbazoles and their derivatives through the series reaction of C-H amination and arylation. The substrate scope is wide. C4-Amino carbazoles substituted by C2, C6, C7, and C8 methyl groups, especially carbazole derivatives of fused rings, pyridine, and dibenzofuran, can be synthesized.

Thermally activated delayed fluorescence emitters: a thionation approach toward next-generation photosensitizers

Achieving highly efficient intersystem crossing (ISC) remains a key focus in the design of heavy atom-free photosensitizers (PSs) for various photophysical and photochemical applications. Herein, we report a general and robust molecular design strategy for obtaining photoactivatable heavy atom-free PSs by performing a simple sulfur substitution of carbonyl oxygen atoms of a thermally activated delayed fluorescence (TADF) emitter. This thionation led to a significant fluorescence loss, resulting in an increased ISC transformation. Upon white-light irradiation, the sulfur-substituted TADF compound (S-AIOH-Cz) exhibited a long-lived fluorescence turn-on response, a long-lasting triplet state lifetime and a superior reactive oxygen species (ROS) generation ability, which is desirable for time-resolved fluorescence imaging and photodynamic disinfection against antimicrobial resistance.

Programmable Photocatalytic Activity of Multicomponent Covalent Organic Frameworks Used as Metallaphotocatalysts

The multicomponent approach allows to incorporate several functionalities into a single covalent organic framework (COF) and consequently allows the construction of bifunctional materials for cooperative catalysis. The well-defined structure of such multicomponent COFs is furthermore ideally suited for structure-activity relationship studies. We report a series of multicomponent COFs that contain acridine- and 2,2'-bipyridine linkers connected through 1,3,5-benzenetrialdehyde derivatives. The acridine motif is responsible for broad light absorption, while the bipyridine unit enables complexation of nickel catalysts. These features enable the usage of the framework materials as catalysts for light-mediated carbon-heteroatom cross-couplings. Variation of the node units shows that the catalytic activity correlates to the keto-enamine tautomer isomerism. This allows switching between high charge-carrier mobility and persistent, localized charge-separated species depending on the nodes, a tool to tailor the materials for specific reactions. Moreover, nickel-loaded COFs are recyclable and catalyze cross-couplings even using red light irradiation.

Multi-Resonant Thermally Activated Delayed Fluorescent (MR-TADF) Compounds as Photocatalysts

Donor-acceptor (D-A) thermally activated delayed fluorescent (TADF) compounds, such as 4CzIPN, have become a widely used sub-class of organic photocatalysts for a plethora of photocatalytic reactions. Multi-resonant TADF (MR-TADF) compounds, a subclass of TADF emitters that are rigid nanographene derivatives, such as DiKTa and Mes₃ DiKTa, have to date not been explored as photocatalysts. In this study both DiKTa and Mes₃ DiKTa were found to give comparable or better product yield than 4CzIPN in a range of photocatalytic processes that rely upon reductive quenching, oxidative quenching, energy transfer and dual photocatalytic processes. In a model oxidative quench process, DiKTa and Mes₃ DiKTa gave increased reaction rates in comparison to 4CzIPN, with DiKTa being of particular interest due to the lower material cost (£0.94/mmol) compared to that of 4CzIPN (£3.26/mmol). These results suggest that DiKTa and Mes₃ DiKTa would be excellent additions to any chemist's collection of photocatalysts.

Formation and degradation of strongly reducing cyanoarene-based radical anions towards efficient radical anion-mediated photoredox catalysis

Cyanoarene-based photocatalysts (PCs) have attracted significant interest owing to their superior catalytic performance for radical anion mediated photoredox catalysis. However, the factors affecting the formation and degradation of cyanoarene-based PC radical anion (PC^•‒) are still insufficiently understood. Herein, we therefore investigate the formation and degradation of cyanoarene-based PC^•‒ under widely-used photoredox-mediated reaction conditions. By screening various cyanoarene-based PCs, we elucidate strategies to efficiently generate PC^•‒ with adequate excited-state reduction potentials (E_red^*) via supra-efficient generation of long-lived triplet excited states (T₁). To thoroughly investigate the behavior of PC^•‒ in actual photoredox-mediated reactions, a reductive dehalogenation is carried out as a model reaction and identified the dominant photodegradation pathways of the PC^•‒. Dehalogenation and photodegradation of PC^•‒ are coexistent depending on the rate of electron transfer (ET) to the substrate and the photodegradation strongly depends on the electronic and steric properties of the PCs. Based on the understanding of both the formation and photodegradation of PC^•‒, we demonstrate that the efficient generation of highly reducing PC^•‒ allows for the highly efficient photoredox catalyzed dehalogenation of aryl/alkyl halides at a PC loading as low as 0.001 mol% with a high oxygen tolerance. The present work provides new insights into the reactions of cyanoarene-based PC^•‒ in photoredox-mediated reactions.

N,N'-Diaryldihydrophenazines as a Sustainable and Cost-Effective Alternative to Precious Metal Complexes in the Photoredox-Catalyzed Alkylation of Aryl Alkyl Ketones

An inexpensive and highly efficient metal-free alternative to commonly used Ru- and Ir-based catalysts was proposed. It was shown that the new 2,7-di-tert-butyl-5,10-bis(4-trifluoromethylphenyl)-5,10-dihydrophenazine outcompeted the iridium phenylpyridyl complex in photoredox activity in the alkylation of silyl enol ethers yielding aryl alkyl ketones. The reaction occurred under visible light irradiation at room temperature and was also applicable to drug derivatives (ibuprofen and naproxen). In-depth photophysical, electrochemical, and quantum chemical studies showed that the aforementioned N,N-diaryldihydrophenazine exhibited enhanced properties that were essential for the photoredox catalysis (a long-lived triplet excited state, strong reducing ability, high stability of the radical cations formed in single-electron-transfer event, and chemical inertness of the catalyst with respect to reactants). Importantly, the substituted N,N'-diaryldihydrophenazines could be obtained directly from diaryl amines; a facile, easily handled and scaled-up one-pot synthetic procedure was elaborated.

Trifluoromethylated thermally activated delayed fluorescence molecule as a versatile photocatalyst for electron-transfer- and energy-transfer-driven reactions

In this study, we propose that the trifluoromethylated thermally activated delayed fluorescent molecule 4[Cz(CF₃)₂]IPN is a versatile organic photocatalyst that can be used for electron-transfer-driven reactions requiring a photocatalyst with high oxidizing power and energy-transfer-driven reactions that require an Ir photocatalyst. 4[Cz(CF₃)₂]IPN was used in radical reactions via electron transfer and dearomative cycloaddition reactions via energy transfer.

The Interplay between ESIPT and TADF for the 2,2'-Bipyridine-3,3'-diol: A Theoretical Reconsideration

Organic molecules with excited-state intramolecular proton transfer (ESIPT) and thermally activated delayed fluorescence (TADF) properties have great potential for realizing efficient organic light-emitting diodes (OLEDs). Furthermore, 2,2'-bipyridine-3,3'-diol (BP(OH)₂) is a typical molecule with ESIPT and TADF properties. Previously, the double ESIPT state was proved to be a luminescent state, and the T₂ state plays a dominant role in TADF for the molecule. Nevertheless, whether BP(OH)₂ undergoes a double or single ESIPT process is controversial. Since different ESIPT channels will bring different TADF mechanisms, the previously proposed TADF mechanism based on the double ESIPT structure for BP(OH)₂ needs to be reconsidered. Herein, reduced density gradient, potential energy surface, IR spectra and exited-state hydrogen-bond dynamics computations confirm that BP(OH)₂ undergoes the barrierless single ESIPT process rather than the double ESIPT process with a barrier. Moreover, based on the single ESIPT structure, we calculated spin-orbit coupling matrix elements, nonradiative rates and electron-hole distributions. These results disclose that the T₃ state plays a predominant role in TADF. Our investigation provides a better understanding on the TADF mechanism in hydrogen-bonded molecular systems and the interaction between ESIPT and TADF, which further provides a reference for developing efficient OLEDs.

Intramolecular Hydrogen Bonding in Thermally Activated Delayed Fluorescence Emitters: Is There Evidence Beyond Reasonable Doubt?

Intramolecular hydrogen bonding between donor and acceptor segments in thermally activated delayed fluorescence (TADF) materials is now frequently employed to─purportedly─rigidify the structure and improve the emission performance of these materials. However, direct evidence for these intramolecular interactions is often lacking or ambiguous, leading to assertions that are largely speculative. Here we investigate a series of TADF-active materials incorporating pyridine, which bestows the potential ability to form intramolecular H-bonding interactions. Despite possible indications of H-bonding from an X-ray analysis, an array of other experimental investigations proved largely inconclusive. Instead, after examining computational potential energy surfaces of the donor-acceptor torsion angle we conclude that the pyridine group primarily alleviates steric congestion in our case, rather than enabling an H-bond interaction as elsewhere assumed. We suggest that many previously reported "H-bonding" TADF materials featuring similar chemical motifs may instead operate similarly and that investigation of potential energy surfaces should become a key feature of future studies.

Photochemistry Journey to Multielectron and Multiproton Chemical Transformation

The odyssey of photochemistry is accompanied by the journey to manipulate "electrons" and "protons" in time, in space, and in energy. Over the past decades, single-electron (1e^-) photochemical transformations have brought marvelous achievements. However, as each photon absorption typically generates only one exciton pair, it is exponentially challenging to accomplish multielectron and proton photochemical transformations. The multistep differences in thermodynamics and kinetics urgently require us to optimize light harvesting, expedite consecutive electron transfer, manipulate the interaction of catalysts with substrates, and coordinate proton transfer kinetics to furnish selective bond formations. Tandem catalysis enables orchestrating different photochemical events and catalytic transformations from subpicoseconds to seconds, which facilitates multielectron redox chemistries and brings consecutive, value-added reactivities. Joint efforts in molecular and material design, mechanistic understanding, and theoretical modeling will bring multielectron and proton synthetic opportunities for fuels, fertilizers, and chemicals with enhanced versatility, efficiency, selectivity, and scalability, thus taking better advantage of photons (i.e., sunlight) for our sustainable society.

Polymer dots and glassy organic dots using dibenzodipyridophenazine dyes as water-dispersible TADF probes for cellular imaging

Fluorescence imaging of living cells is key to better understanding cellular morphology and biological processes. Water-dispersible nanoparticles exhibiting thermally activated delayed fluorescence (TADF) have recently emerged as useful probes for time-resolved fluorescence imaging (TRFI), circumventing interference from biological autofluorescence. Many existing approaches, however, require TADF dyes with specific structural features, precluding many high-performance TADF materials from being used in this application. Here, we describe the synthesis of two TADF emitters based on the rigid and strongly electron-withdrawing dibenzo[a,c]dipyrido[3,2-h:2'-3'-j]phenazine-12-yl (BPPZ) motif, and demonstrate two parallel approaches for the encapsulation of these fluorophores to yield water-dispersible nanoparticles suitable for TRFI. First, fluorescent polymer dots (Pdots) were formed by dye encapsulation within cell-penetrating amphiphilic copolymers. Glassy organic nanoparticles (g-Odots) were also prepared, giving nanoparticles with higher photoluminescence quantum yields and improved colour purity. Both approaches yielded nanoparticles suitable for imaging, with reasonable uptake and cytotoxicity on the timescale of standard imaging experiments using human cervical (HeLa) and liver (HepG2) cancer cell lines. This work demonstrates two flexible strategies for preparing water-dispersible TADF nanoparticles for TRFI, both of which should be readily adaptable to nearly any existing hydrophobic TADF dye.