Detailed Photophysical Properties Research Articles

Influential effect of C^N ligands containing O, S, and Se heteroatoms in the color tuning of four phosphorescent Ir(III) complexes

AbstractA group of new iridium(III) complexes were rationally designed and synthesized with 2‐phenylbenzo[d]oxazole (bo), 2‐phenylbenzo[d]thiazole (bt), 2‐(thiophen‐2‐yl)benzo[d]thiazole (thbt), and 2‐(selenophen‐2‐yl)benzo[d]thiazole (sebt) as C^N ligands and bis(diphenylthiophosphoryl)amide (S‐tpip) as ancillary ligand. They are phosphorescent in a wide region of green to red with short lifetimes around 0.31–0.98 μs and quantum yields up to 64.7%. The detailed photophysical properties were investigated by experiment and further supported by density functional theory (DFT) calculation. These research results show that the spectroscopic properties are tuned by the replacement between oxygen, sulfur, and selenium or the change in aromatic ring. It has been demonstrated that the lowest energy‐level absorption and emission are dominated apparently by C^N ligands, and are ascribed to metal‐to‐ligand charge transfer (MLCT) and intra‐ligand charge transfer (ILCT) characters.

Comparison of Reactive Sites in 2(1H)-Quinolone Derivatives for the Detection of Biologically Important Sulfur Compounds.

Novel fluorescent probes based on 2(1H)-quinolone skeleton containing a malonate group (Q1-Q3) were synthesized and proposed for biothiols detection. Their chemical reactivity toward thiols was compared to the reactivity of derivative having a dicyanovinyl group (Q4) as a reactive site. The detailed photophysical properties of these compounds were assessed through the determination of absorption and fluorescence spectra, fluorescence quantum yield, and fluorescence lifetime. In the presence of biothiols, an increase in the fluorescence intensity of compounds Q1-Q3 and a hypsochromic shift in their emission bands were observed. In contrast, the compound with the dicyanovinyl group (Q4) in the presence of biothiols and cyanide ion showed the quenching of fluorescence, while a fluorescence "turn on" effect was observed toward reactive sulfur species.

Cyanopyridone-cored fluorophores with triphenylamine peripheries: From molecular design to OLED fabrication studies

• Seven novel cyanopyridone-based D-A-D type molecules with a fixed triphenylamine donor, were synthesized. • Theoretical, optical, solvatochromic, thermal, electrochemical, electrical, and EL studies were performed to evaluate structure-property relationships. • Doped OLEDs using novel fluorophores as dopants outperform non-doped devices. • OLEDs emit yellow light with maximum efficiencies of 10.72 cd/A, 7.87 lm/W, and 5.32%. Amongst many types of fluorescent organic materials, compounds with π-conjugation between an electron donor (D) and electron acceptor (A) have been intensively investigated as effective organic electroluminescence (EL) device components. Herein, we report the synthesis and structural characterization of a new series of D-A-D configured small molecule-based fluorophores ( TPA-CyP1-7 ) having triphenylamine ( TPA ) as a donor unit and 3-cyanopyridine-2-one ( CyP ) as an acceptor core with different architects for use as efficient yellow light emitters in fluorescent organic light-emitting diodes (OLED). The detailed photophysical, solvatochromic, thermal, electrochemical, and EL properties, including quantum chemical calculations, were systematically investigated to study their relation between structure and properties. All the fluorophores show high fluorescent quantum yields in the solid film state and display high thermal stability with decomposition temperatures above 350 o C. The study reveals that they possess appropriate HOMO and LUMO energies level for effective charge injection. Finally, these yellow luminogens were employed to fabricate new OLED devices as sole emitters and dopants with CBP host materials. Interestingly, the host-guest devices doped with CBP host emitters show a remarkable improvement in the overall device performance. Among them, the TPA-CyP3 -based doped device has achieved a maximum current efficiency ( η c ), high power efficiency ( η p ), and good external quantum efficiency ( η EQE ) of 10.72 cd/A, 7.87 lm/W, and 5.32%, respectively.

A New Entry to Purely Organic Thermally Activated Delayed Fluorescence Emitters Based on Pyrido[2,3‐b]pyrazine‐Dihydrophenazasilines Donor‐Acceptor Dyad

AbstractA new family of thermally activated delayed fluorescence (TADF) emitters based on a twisted donor‐acceptor (D−A) dyad scaffold comprising of dihydrophenazasiline (D) and pyrido[2,3‐b]pyrazine (A) has been developed, and their properties have been investigated. Time‐resolved spectroscopic analysis in matrices revealed the detailed photophysical properties of the D−A compounds. These D−A compounds serve as the emitter for organic light‐emitting diodes (OLEDs), showing a moderate external quantum efficiency (EQE) up to 9% in CBP matrix. Furthermore, theoretical calculations uncovered the excited states nature of the developed TADF emitters.

Aggregation Turns BODIPY Fluorophores into Photosensitizers: Reversibly Switching Intersystem Crossing On and Off for Smart Photodynamic Therapy

Aggregation Turns BODIPY Fluorophores into Photosensitizers: Reversibly Switching Intersystem Crossing On and Off for Smart Photodynamic Therapy

Peri-N-amine-perylenes, with and without phenyl bridge: Photophysical studies and their OLED applications

Donor-bridge-acceptor framework has been studied widely for their detailed photophysical properties with the aim of efficient long range charge transfer. Carbazole attached with electron acceptors has been used for charge transfer properties, thermally activated delayed fluorescence, exciton dynamics etc. In this study, we synthesized donor–acceptor dyads of seconday amine (carbazole and diphenyl amine) attached to perylene through nitrogen, with phenyl (P-Ph-N-CBZ and P-Ph-N-BP) and without phenyl (P-N-CBZ and P-N-BP). Further their detailed photophysical and electroluminescent properties are discussed. P-N-BP is known and is used as standard in this work. DFT studies show nearly orthogonal geometry of P-N-CBZ with the dihedral angle of ∼ 71° between the planes of perylene and carbazole. For P-Ph-N-BP and P-Ph-N-CBZ, the perylene and amines are adopting almost co-planar geometries. Emission spectra of P-Ph-N-CBZ and P-N-CBZ in dichloromethane are structural with clear vibronic features while for P-Ph-N-BP and P-N-BP, the emission is structureless and broad indicating prominent electronic coupling between molecular orbitals of donor and acceptor in the latter case. Fluorescence quantum yields of phenyl-linked dyads are found to be higher than directly linked perylene-amine derivatives. High quantum yield (0.79) in non-polar hexane and polar DMSO (0.66) is observed for P-Ph-N-CBZ. It is interesting to note that the fluorescence lifetimes of these dyads are increasing while fluorescence quantum yields are decreasing with increasing polar solvents, suggesting that the emission has a significant contribution from the 1CT state. Dipole moments estimated from DFT studies and photophysical methods are higher for phenyl-linked dyads than those directly attached which is also consistent with their solvatochromism properties. Weaker electronic coupling in phenyl-linked dyads resulted due to the long distance between donor and acceptor in phenyl-linked dyads. Furthermore application in OLEDs was shown for P-Ph-N-CBZ and P-Ph-N-BP. OLEDs of P-Ph-N-BP showed a peak emission at 568 nm resulting in an amber device. A high luminance of 4.3 × 103 Cd/m2 at a current density of 100 mA/cm2 and a maximum EQE of 4.2% with a low turn on volatage of ∼ 4 V was obtained for these devices. Thus, our results show that weaker charge transfer takes place in perylene based donor–acceptor dyads resulting in bright OLED devices.

Photodynamic therapy applications of Re(I)‐BODIPY functionalized nanoparticles

AbstractOrganometallic Re(I)‐BODIPY compounds have recently received attention as metal‐based photosensitizers for photodynamic therapy (PDT) applications owing to their strong absorption in the visible region. We synthesized and characterized an organometallic [ReI(L)2(CO)3Br] (ReL) complex containing a BODIPY ligand (L) and prepared nanoparticles of the ligand (NPL) as well as the complex (NPReL). The formation of nanoparticles was confirmed by dynamic light scattering (DLS), atomic force microscopy (AFM), transmission electron microscopy (TEM), and energy dispersive X‐ray (EDX) analyses. Single‐crystal structure of ReL showed octahedral environment around Re center, and the BODIPY chromophore was placed at distances of around 12 Å. Detailed photophysical properties of ReL were investigated which showed strong visible absorption at 551 nm (ε = 1.46 × 105 M−1 cm−1) owing to π‐conjugation of BODIPY with the metal center Re(I). Efficient intersystem crossing (ISC) and formation of triplet‐excited states with lifetimes of 0.80 and 0.65 μs for L and ReL, respectively, were confirmed through nanosecond‐laser flash photolysis and density functional theory (DFT) calculations. The photosensitization efficiency of ReL was observed to be ϕΔ = 0.77. In vitro biological studies of NPReL showed negligible toxicity and reactive oxygen species (ROS) generation in the dark as compared with NPL despite the good photosensitization ability of NPL. Therefore, NPReL showed coordination‐enhanced cytotoxicity under irradiation conditions and interestingly found to be nontoxic to normal NIH3T3 cell lines in comparison with L. Flow cytometry studies further indicated that the major cause of cell death by NPReL was via apoptotic pathways (54%), which authenticates its potential as a PDT agent.

Understanding the amyloid sensing mechanism of SYPRO Orange

Amyloid fibrils, an aggregated form of proteins, are responsible for different types of neurological diseases. A constant effort is there for the search of a sensitive and selective probe for amyloid fibrils. Recently, it has been shown that a zwitterionic merocyanine dye, SYPRO Orange, can detect the amyloid fibrils with much higher sensitivity and with much preferred spectroscopic properties than present gold standard amyloid probe, Thioflavin T. In the present work, we have investigated the detailed photophysical properties of SYPRO Orange in different molecular solvents and solvent mixtures with wide range of properties to disentangle the fundamental processes that are responsible for its amyloid sensing activity. It has been shown that SYPRO Orange forms aggregates in water and results in extremely low emission quantum yield. The amyloid induced de-aggregation of SYPRO Orange is primarily responsible for enormous increase in its emission intensity upon binding with amyloid fibrils. Photophysical studies using steady state and time-resolved emission spectroscopy further enlighten us on the role of solvent polarity, solute-solvent H-bonding and solvent viscosity on its sensitivity towards amyloid fibrils. Thus, present results will be quite useful for the use of SYPRO Orange in different other spectroscopic applications.

A novel meso-tetrakis(2,4,6-trimethylphenyl) porphyrinato ([Zn(TMP)(4,4′-bpy)]) complex: Synthesis, characterization, and application

A new metalloporphyrin (4,4′-bipyridine)meso-tetrakis(2,4,6-trimethylphenyl) porphyrinato) zinc(II) ([Zn(TMP)(4,4′-bpy)]) (3) was obtained by reacting meso-tetrakis(2,4,6-trimethylphenyl)porphyrinato) zinc(II) [Zn(TMP)] (2) and 4,4′-bipyridine (4,4′-bpy). The starting material meso-tetrakis(2,4,6-trimethylphenyl)porphyrine (H2TMP, 1) as well as complexes (2 and 3) were characterized by the means of microanalysis, 1H NMR, and ESI-MS techniques. The molecular structure of the final complex (3) was also confirmed by single crystal X-ray crystallography. Detailed photo-physical and electrochemical properties of the complexes were also determined. The oxidative degradation ability of complexes (2 and 3) was evaluated and compared against an organic dye (calmagite). When used as catalyst, complex (2) showed better efficiency ~ 48% than the complex (3) (41%). Overall, this work not only explores the ability of metalloporphyrin to degrade organic dyes in aqueous solution, but also showed that the degradation ability of a metalloporphyrin complex can be fine-tuned by varying axial ligand.

π-Conjugated twin molecules based on 9,9-diethyl-1-phenyl-1,9-dihydrofluoreno[2,3-d]imidazole module: synthesis, characterization, and electroluminescence properties

A series of well-defined, π-conjugated twin molecules by connecting two 9,9-diethyl-1-phenyl-1,9-dihydrofluoreno[2,3-d]imidazole units via an ortho-, meta-, or para-benzene with high quantum efficiency of fluorescence is synthesized and fully characterized. The detailed thermal, electrochemical, photophysical, and electro-luminescence properties have been investigated. The electro-luminescence properties of the obtained materials are studied by fabricated the device using the configuration ITO/HATCN (5 nm)/NPB (20 nm)/TCTA (10 nm)/CBP: 5% doping (20 nm)/TMPYPB (40 nm)/LiQ (1 nm)/Al (80 nm). The doped OLED based on the compound with para-benzene as the emitting layer shows the best performance with the maximum current efficiency of 2.30 cd/A, maximum power efficiency of 1.52 lm/W, located in the deep-blue region with CIE (x, y) value of (0.1572, 0.0587) with relatively lower onset voltage of 4.25 V.

Bis-Metal Complexes of Doubly N-Confused Dioxohexaphyrins as Potential Near-Infrared-II Photoacoustic Dyes.

We investigated the detailed photophysical properties of a series of bis-metal (Zn and Cu) dioxohexaphyrin complexes as potential second near-infrared (NIR-II)-light responsive dyes. A cisoid-configured 28π-electron-conjugated dioxohexaphyrin analogue (c-3a) containing two peculiar "confused pyrrole" moieties in the framework is identified as a reduced isomer derivative of a transoid 26π-dioxohexaphyrin (t-2a). The symmetry-altered structure of c-3a affords a heteroleptic inner environment within the NNNN/NNOO donor core, which imparts its highly flexible electronic features and nonplanar geometry. The macrocycle c-3a can be transformed into the corresponding 26π-electron congener (c-2a) having a coplanar rectangular structure by unique solvent-mediated redox reactivity. Furthermore, upon metal complexation, saddle-distorted bis-metal complexes (c-M2-2a) were formed as the 26π-conjugated structural isomer of the trans-dioxohexaphyrin species (i.e., t-M2-2a). These isoelectronic dioxohexaphyrins demonstrate precise geometry-dependent photophysical properties. Broad tailing NIR-II absorption, weak emissive character, and rapid-decay of the S1 state are observed for c-Zn2-2a. In contrast, the coplanar t-M2-2a exhibits efficient photoacoustic response upon laser excitation with NIR-II light (λ > 1000 nm). To the best of our knowledge, this is the first example of an expanded porphyrin-based photoacoustic contrast agent responsive to NIR-II light.

Simple diphenylamine based D-π-A type sensitizers/co-sensitizers for DSSCs: a comprehensive study on the impact of anchoring groups.

Herein, we report the design, synthesis and characterization of a new series of simple donor-π spacer-acceptor/anchor (D-π-A) type diphenylamine based metal-free organic dyes possessing three different anchoring groups, viz. 4-aminobenzoic acid (DTP), 2-(4-nitrophenyl)acetonitrile (DTN), and barbituric acid (DTB), connected with 2-(thiophene-2-yl)-acetonitrile, as effective sensitizers and co-sensitizers in Dye Sensitized Solar Cells (DSSCs). They were subjected to photophysical, electrochemical and theoretical studies. The dyes exhibited characteristic λabs and λemi in the range of 445-485 and 545-570 nm, respectively. Both optical and electrochemical band gaps were found to be in the range of 2.2 to 2.35 eV. The driving forces for injection (ΔGinj), recombination (ΔGrec) and regeneration (ΔGreg) processes were evaluated to understand their feasibility. Finally, the DSSC devices were fabricated employing the new dyes as sensitizers as well as co-sensitizers along with the Ru(ii) based N3 dye. Interestingly, DTP carrying 4-aminobenzoic acid as the anchoring group shows the best photoelectrochemical performance, viz. photovoltaic conversion efficiency (PCE) = 4.4%, open circuit potential (VOC) = 0.577 V, and short-circuit current density (JSC) = 9.06 mA cm-2 with a broad incident photon conversion efficiency (IPCE) spectrum. Co-sensitization of the dyes brought about enhanced VOC values, compared to the N3 dye alone. Finally, different interface resistance values obtained from the electrochemical impedance spectroscopy (EIS) circuit fitting were used to study the fundamental processes of energy conversion.

Role of solvent H-bonding and polarity on photophysical properties of a benzothiazole-based ratiometric amyloid fibril sensor

Detailed photophysical properties of 2-[2′-Me,4′-(dimethylamino)-phenyl]benzothiazole (2Me-DABT), a potential ratiometric amyloid fibril sensor, have been investigated in different solvents and solvent mixtures using steady state and time-resolved spectroscopic techniques. Our studies show that emission properties of 2Me-DABT are significantly modulated by polarity of solvent media. Extent of solvatochromism (3585 cm−1) shown by 2Me-DABT is quite large and can be used to monitor micropolarity of different complex media. Further, Stokes’ shift (>10,000 cm−1) shown by 2Me-DABT in all studied solvents is also much larger than most common molecular probes. It has also been shown that hydrogen bonding with solvent molecules results in large modulation in photophysical properties of 2Me-DABT. The H-bonding with water molecules induces a large change (39°) in the dihedral angle between benzothiazole and aniline moieties resulting large changes in photophysical properties of 2Me-DABT in polar protic solvents. Photophysical properties reported herein are used to explain observed amyloid sensing behaviour of 2Me-DABT. Detailed quantum chemical calculations are performed to support the experimental results.

Near-infrared excited cooperative upconversion in luminescent Ytterbium(ΙΙΙ) bioprobes as light-responsive theranostic agents

Near-infrared (NIR) Ytterbium(ΙΙΙ) complexes namely [Yb(dpq)(DMF)2Cl3] (1), [Yb(dppz)(DMF)2Cl3] (2), [Yb(dpq)(ttfa)3] (3) and [Yb(dppz)(ttfa)3] (4) based on photosensitizing antenna: dipyrido-[3,2-d:2′,3′-f]-quinoxaline (dpq), dipyrido-[3,2-a:2′,3′-c]-phenazine (dppz) and 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione (Httfa), were designed as NIR bioimaging agents utilizing cooperative upconversion luminescence (CUCL) of Yb(III). Their structures, detailed photophysical properties, biological interactions, photo-induced DNA cleavage, NIR photocytotoxicity and cellular internalization and bioimaging properties were examined. Discrete mononuclear complexes adopt a seven-coordinated {LnN2O2Cl3} mono-capped octahedron (1, 2) and eight-coordinated {LnN2O6} distorted square antiprism geometry (3, 4) with bidentate N, N-donor dpq, dppz and O,O-donor ttfa ligands. The designed Yb(III) probes (3, 4) having advantages of dual sensitizing antennae (dpq/dppz and Httfa) to modulate the desirable optical properties in NIR region for bioimaging in biologically transparent window and light-responsive intracellular damage with spatiotemporal control. The lack of inner-sphere water (q = 0), remarkable photostability, large Stokes' shifts, presence of energetically rightly poised ligand 3T states allows efficient energy transfer (ET) to the emissive 2F5/2 state of Yb(ΙΙΙ). The unique cooperative upconversion luminescence (CUCL) of Yb(III) was observed in 1–4 in the visible blue region (λem = 490 nm) upon NIR excitation at 980 nm, makes them special candidates for NIR-to-visible or NIR-to-NIR cellular imaging probes. The CUCL property of Yb(III) were observed in the discrete mononuclear complexes both in solid state and solution. We elegantly utilized this remarkable property of Yb(III) for cellular imaging application for the first time to the our knowledge including potential uses in CUC/multiphoton excitation microscopy. The complexes exhibit significant binding propensity to DNA, HSA and BSA (K ∼ 105 M−1). They effectively cleave supercoiled (SC) DNA to its nicked circular (NC) form at 365 nm via photoredox pathways. The cellular uptake studies evidently displayed cytosolic and nuclear localization of the complexes. Finally, the capability of Yb(III) complexes usage for PDT were demonstrated through significant near-IR photocytotoxicity at 980 nm CW laser. The results depicted here offers an intelligent strategy towards developing light-responsive highly photostable Yb(III) probes for NIR theranostic application in the biologically transparent phototherapeutic window.

Synthesis and photo-physics of red emitting europium complexes: An estimation of the role of ancillary ligand by chemical partition of radiative decay rate

Three highly luminescent Eu(III) complexes of the type [Eu(β-diket.)3(tm-phen)] where β-diket. = hexafluoroacetylacetone (hfaa), btfa; 4,4,4-trifluoro-1-phenyl-1,3-butanedione (btfa), 2-thenoyltrifluoroacetone (tta) and tm-phen = 3,4,7,8-tetramethyl-1,10-phenanthroline have been synthesized and characterized. The detailed photophysical properties of the complexes were analyzed both theoretically and experimentally. The theoretical photophysical properties calculated using the LUMPAC program [1] are in excellent agreement with experimental results. The Judd-Ofelt (J-O) parameters (Ω2 and Ω4), radiative (Arad) and non-radiative (Anrad) decay rates were calculated and discussed. Chemical partition of the radiative decay rates (Arad) of electric-dipole transitions revealed that β-diketone contributes 77 to 84% while tm-phen contributes 22 to 16% towards the Arad. The intramolecular energy transfer (WET) and back-energy transfer (WBT) rate are predicted from the singlet (S1) and triplet (T1) levels to the emissive 5D1 and 5D0 states for Eu(III) and follow the path S0 → S1 → T1 → 5D1 → 5D0 → 7F0,4. The complexes emit typical Eu(III) red emission with long luminescence lifetime (0.75–0.88 ms), show high intrinsic quantum yield (ϕLn) (66–70%) and may be used as one of the red components in light emitting devices.

Recent Advances in Polymer‐Based Metal‐Free Room‐Temperature Phosphorescent Materials

AbstractPolymer‐based materials have evoked increasing attention in the research field of room‐temperature phosphorescence (RTP) because they can not only possess fantastic properties such as good flexibility, easy processing, low cost, high thermal stability and so forth, but also serve as rigid matrices to suppress the nonradiative decay process and thus contribute to phosphorescence emission at room temperature. This Review outlines the development of RTP in organic polymer‐based materials from the perspective of nondoped and doped polymer systems, containing the detailed photophysical properties, luminescent mechanisms and their applications. The conclusion and outlook are presented to point out the advances and challenges for further study of polymer‐based metal‐free RTP materials.

Influence of acceptor strength on photoinduced charge transfer process in a newly designed molecule in bulk solvent and in β-CD microenvironment

We present a detailed photophysical properties of a synthesized charge transfer (CT) molecule (2E, 4E)-ethyl 2-cyano-5-(4-(dimethylamino) phenyl) penta-2, 4-dienoate (ECDPPD) in bulk medium and its spectral modulation inside β-Cyclodextrin cavity. Enhancement of acceptor strength of 4-(dimethylamino) cinnamaldehyde by replacing the moderate acceptor in the form of a Formyl group by two acceptors known to display greater electron withdrawing ability along with extra conjugation which may be responsible for higher dipole moment both in ground as well as in excited state. The observed excited CT process and its solvent polarity dependent emissive properties is due to simply increasing the ‘end-to-end’ length between donor and acceptor along with introduction of two different acceptors in the same site. Low fluorescence quantum yield and shorter fluorescence lifetime is due to the operation of active non-radiative channels through flexible bonds supported by viscosity and temperature dependent spectral measurement. The molecule ECDPPD easily interacts with β-CD with the formation of inclusion complex with a definite stoichiometry where the photophysics are modulated and hence can be used as fluorescence microenvironment reporter for example to evaluate the polarity as well as the micro viscosity of various regions of protein and other bimolecular systems.

Ping-Pong Energy Transfer in a Boron Dipyrromethane Containing Pt(II)-Schiff Base Complex: Synthesis, Photophysical Studies, and Anti-Stokes Shift Increase in Triplet-Triplet Annihilation Upconversion.

A boron dipyrromethane (BDP)-containing Pt(II)-Schiff base complex (Pt-BDP), showing ping-pong singlet-triplet energy transfer, was synthesized, and the detailed photophysical properties were investigated using various steady-state and time-resolved transient spectroscopies. Femtosecond/nanosecond transient absorption spectroscopies demonstrated that, upon selective excitation of the BDP unit in Pt-BDP at 490 nm, Förster resonance energy transfer from the BDP unit to the Pt(II) coordination center occurred (6.7 ps), accompanied by an ultrafast intersystem crossing at the Pt(II) coordination center (<1 ps) and triplet-triplet energy transfer back to the BDP moiety (148 ps). These processes generated a triplet state localized at BDP, and the lifetime was 103.2 μs, much longer than the triplet-state lifetime of Pt-Ph (3.5 μs), a complex without the BDP moiety. Finally, Pt-BDP was used as a triplet photosensitizer for triplet-triplet annihilation (TTA) upconversion through selective excitation of the BDP unit or the Pt(II) coordination center at lower excitation energy. An upconversion quantum yield of up to 10% was observed with selective excitation of the BDP moiety, and a large anti-Stokes shift of 0.65 eV was observed upon excitation of the lower-energy band of the Pt(II) coordination center. We propose that using triplet photosensitizers with the ping-pong energy-transfer process may become a useful method for increasing the anti-Stokes shift of TTA upconversion.

TADF Material Design: Photophysical Background and Case Studies Focusing on CuI and AgI Complexes.

The development of organic light emitting diodes (OLEDs) and the use of emitting molecules have strongly stimulated scientific research of emitting compounds. In particular, for OLEDs it is required to harvest all singlet and triplet excitons that are generated in the emission layer. This can be achieved using the so-called triplet harvesting mechanism. However, the materials to be applied are based on high-cost rare metals and therefore, it has been proposed already more than one decade ago by our group to use the effect of thermally activated delayed fluorescence (TADF) to harvest all generated excitons in the lowest excited singlet state S1 . In this situation, the resulting emission is an S1 →S0 fluorescence, though a delayed one. Hence, this mechanism represents the singlet harvesting mechanism. Using this effect, high-cost and strong SOC-carrying rare metals are not required. This mechanism can very effectively be realized by use of CuI or AgI complexes and even by purely organic molecules. In this investigation, we focus on photoluminescence properties and on crucial requirements for designing CuI and AgI materials that exhibit short TADF decay times at high emission quantum yields. The decay times should be as short as possible to minimize non-radiative quenching and, in particular, chemical reactions that frequently occur in the excited state. Thus, a short TADF decay time can strongly increase the material's long-term stability. Here, we study crucial parameters and analyze their impact on the TADF decay time. For example, the energy separation ΔE(S1 -T1 ) between the lowest excited singlet state S1 and the triplet state T1 should be small. Accordingly, we present detailed photophysical properties of two case-study materials designed to exhibit a large ΔE(S1 -T1 ) value of 1000 cm-1 (120 meV) and, for comparison, a small one of 370 cm-1 (46 meV). From these studies-extended by investigations of many other CuI TADF compounds-we can conclude that just small ΔE(S1 -T1 ) is not a sufficient requirement for short TADF decay times. High allowedness of the transition from the emitting S1 state to the electronic ground state S0 , expressed by the radiative rate kr (S1 →S0 ) or the oscillator strength f(S1 →S0 ), is also very important. However, mostly small ΔE(S1 -T1 ) is related to small kr (S1 →S0 ). This relation results from an experimental investigation of a large number of CuI complexes and basic quantum mechanical considerations. As a consequence, a reduction of τ(TADF) to below a few μs might be problematic. However, new materials can be designed for which this disadvantage is not prevailing. A new TADF compound, Ag(dbp)(P2 -nCB) (with dbp=2,9-di-n-butyl-1,10-phenanthroline and P2 -nCB=bis-(diphenylphosphine)-nido-carborane) seems to represent such an example. Accordingly, this material shows TADF record properties, such as short TADF decay time at high emission quantum yield. These properties are based (i) on geometry optimizations of the AgI complex for a fast radiative S1 →S0 rate and (ii) on restricting the extent of geometry reorganizations after excitation for reducing non-radiative relaxation and emission quenching. Indeed, we could design a TADF material with breakthrough properties showing τ(TADF)=1.4 μs at 100 % emission quantum yield.

Luminescence Investigation of Samarium(III)/Dicyanoaurate(I)-based Coordination Networks with and without Aurophilic Interactions

Lanthanide-containing coordination polymers are interesting optical materials with a wide range of potential applications. In this paper we report on the detailed photophysical properties of aurophilic and nonaurophilic systems composed of Au(CN)2 - and Sm(III) ions. The compounds were probed in the solid state using low-temperature luminescence spectroscopy. The luminescence lifetimes and quantum yields for both systems were obtained and the possibility of energy transfer between Sm(III) and Au(I) was investigated. Excited state properties of the systems were further characterized using DFT calculations. In the nonaurophilic case we observe energy transfer between Sm(III) and Au(I) ions, while in the aurophilic case energy transfer occurs between Au(I) ions.