Direct observation of reversible electronic energy transfer involving an iridium center

π-Extended ligands with dual-binding behavior: hindered rotation unlocks unexpected reactivity in cyclometalated Pt complexes

Cyclometalated platinum complexes play a crucial role in catalysis, bioimaging, and optoelectronics. Phenylpyridines are widespread cyclometalating ligands that generate stable and highly emissive Pt complexes. While it is common practice to modify these ligands to fine-tune their photophysical properties, the incorporation of polycyclic aromatic hydrocarbons into the ligand's structure has been largely overlooked. This report describes the cyclometalation of naphthalenyl- and anthracenylpyridine ligands, which has resulted in ten new luminescent PtII and PtIV complexes. These species are enabled by a dual-binding behavior discovered in our polyaromatic-containing ligands. The introduction of naphthalenyl and anthracenyl groups unlocks dual binding modes, with the Pt center bonding to either of two distant carbon atoms within the ligand. These complexes exhibit both symmetric structures with two 5-membered metallacycles and asymmetric structures with 5- and 6-membered metallacycles. This work presents a strategy for the regioselective synthesis of Pt complexes with bespoke structures and photophysical properties. Our findings offer new opportunities in platinum chemistry and beyond, with potential implications for materials and technologies.

Thermo-/Mechano-Chromic Chiral Coordination Dimer: Formation of Switchable and Metastable Discrete Structure through Chiral Self-Sorting

Although strong chiral self-sorting often emerges in extended covalent or supramolecular polymers, the phenomenon is generally weak in discrete assemblies (e.g., dimers and oligomers) of small molecules due to the lack of a cooperative growth mechanism. Consequently, chiral self-sorting has been overlooked in the design of switchable and metastable discrete supramolecular structures. Here, we report a butyl-benzo[h]quinoline-based iridium(III) complex (Bu-Ir) with helical chirality at its metal center, which forms preferentially a homochiral dimer and exhibits thermo-/mechano-chromism based on a monomer-dimer transformation. While a five-coordinate monomer is formed in a racemic or an enantiopure Bu-Ir solution at 25 °C, a six-coordinate homochiral dimer complex is formed almost exclusively at low temperatures, with a higher degree of dimerization in enantiopure Bu-Ir solution. Estimation of apparent dimerization binding constants (K) and thermodynamic parameters (ΔH and ΔS) based on variable temperature ultraviolet-visible (UV-vis) and 1H NMR spectra reveals a strong preference for homochiral dimerization (largest known value for the coordination complex, Khomo/Khetero > 50). Notably, crystals of the homochiral dimer are metastable, undergoing a distinct color change upon grinding (from yellow to red) due to mechanical cleavage of coordination bonds (i.e., a dimer to monomer transformation). A comparison with control compounds having different substituents (proton, methyl, isopropyl, and phenyl groups) reveals that Bu-Ir dimerization involves both strong homochiral self-sorting preference and connected thermo-/mechano-chromic behavior, which is based on matched propeller-shaped chirality and subtle steric repulsion between alkyl substituents that render the homochiral dimer switchable and metastable. These findings provide substantial insights into the emergence of dynamic functionality based on the rational design of discrete chiral assemblies.

Metal-Organic Bichromophore Lowers the Upconversion Excitation Power Threshold and Promotes UV Photoreactions

Sensitized triplet-triplet annihilation upconversion is a promising strategy to use visible light for chemical reactions requiring the energy input of UV photons. This strategy avoids unsafe ultraviolet light sources and can mitigate photo-damage and provide access to reactions, for which filter effects hamper direct UV excitation. Here, we report a new approach to make blue-to-UV upconversion more amenable to photochemical applications. The tethering of a naphthalene unit to a cyclometalated iridium(III) complex yields a bichromophore with a high triplet energy (2.68 eV) and a naphthalene-based triplet reservoir featuring a lifetime of 72.1 μs, roughly a factor of 20 longer than the photoactive excited state of the parent iridium(III) complex. In combination with three different annihilators, consistently lower thresholds for the blue-to-UV upconversion to crossover from a quadratic into a linear excitation power dependence regime were observed with the bichromophore compared to the parent iridium(III) complex. The upconversion system composed of the bichromophore and the 2,5-diphenyloxazole annihilator is sufficiently robust under long-term blue irradiation to continuously provide a high-energy singlet-excited state that can drive chemical reactions normally requiring UV light. Both photoredox and energy transfer catalyses were feasible using this concept, including the reductive N-O bond cleavage of Weinreb amides, a C-C coupling reaction based on reductive aryl debromination, and two Paternò-Büchi [2 + 2] cycloaddition reactions. Our work seems relevant in the context of developing new strategies for driving energetically demanding photochemistry with low-energy input light.

Tuning Emission Lifetimes of Ir(C^N)2(acac) Complexes with Oligo(phenyleneethynylene) Groups

Emissive compounds with long emission lifetimes (μs to ms) in the visible region are of interest for a range of applications, from oxygen sensing to cellular imaging. The emission behavior of Ir(ppy)₂(acac) complexes (where ppy is the 2-phenylpyridyl chelate and acac is the acetylacetonate chelate) with an oligo(para-phenyleneethynylene) (OPE3) motif containing three para-rings and two ethynyl bridges attached to acac or ppy is examined here due to the accessibility of the long-lived OPE3 triplet states. Nine Ir(ppy)₂(acac) complexes with OPE3 units are synthesized where the OPE3 motif is at the acac moiety (aOPE3), incorporated in the ppy chelate (pOPE3) or attached to ppy via a durylene link (dOPE3). The aOPE3 and dOPE3 complexes contain OPE3 units that are decoupled from the Ir(ppy)₂(acac) core by adopting perpendicular ring-ring orientations, whereas the pOPE3 complexes have OPE3 integrated into the ppy ligand to maximize electronic coupling with the Ir(ppy)₂(acac) core. While the conjugated pOPE3 complexes show emission lifetimes of 0.69-32.8 μs similar to the lifetimes of 1.00-23.1 μs for the non-OPE3 Ir(ppy)₂(acac) complexes synthesized here, the decoupled aOPE3 and dOPE3 complexes reveal long emission lifetimes of 50-625 μs. The long lifetimes found in aOPE3 and dOPE3 complexes are due to intramolecular reversible electronic energy transfer (REET) where the long-lived triplet-state metal to ligand charge transfer (³MLCT) states exchange via REET with the even longer-lived triplet-state localized OPE3 states. The proposed REET process is supported by changes observed in excitation wavelength-dependent and time-dependent emission spectra from aOPE3 and dOPE3 complexes, whereas emission spectra from pOPE3 complexes remain independent of the excitation wavelength and time due to the well-established ³MLCT states of many Ir(ppy)₂(acac) complexes. The long lifetimes, visible emission maxima (524-526 nm), and photoluminescent quantum yields of 0.44-0.60 for the dOPE3 complexes indicate the possibility of utilizing such compounds in oxygen-sensing and cellular imaging applications.

Post-complexation Functionalization of Cyclometalated Iridium(III) Complexes and Applications to Biomedical and Material Sciences

Cyclometalated iridium(III) (Ir(III)) complexes exhibit excellent photophysical properties that include large Stokes shift, high emission quantum yields, and microsecond-order emission lifetimes, due to low-lying metal-to-ligand charge transfer (spin-forbidden singlet-triplet (³MLCT) transition). As a result, analogs have been applied for research not only in the material sciences, such as the development of organic light-emitting diodes (OLEDs), but also for photocatalysts, bioimaging probes, and anticancer reagents. Although a variety of methods for the synthesis and the applications of functionalized cyclometalated iridium complexes have been reported, functional groups are generally introduced to the ligands prior to the complexation with Ir salts. Therefore, it is difficult to introduce thermally unstable functional groups such as peptides and sugars due to the harsh reaction conditions such as the high temperatures used in the complexation with Ir salts. In this review, the functionalization of Ir complexes after the formation of cyclometalated Ir complexes and their biological and material applications are described. These methods are referred to as "post-complexation functionalization (PCF)." In this review, applications of PCF to the design and synthesis of Ir(III) complexes that exhibit blue -red and white color emissions, luminescence pH probes, luminescent probes of cancer cells, compounds that induce cell death in cancer cells, and luminescent complexes that have long emission lifetimes are summarized.

Osmium Complex-Chromophore Conjugates with Both Singlet-to-Triplet Absorption and Long Triplet Lifetime through Tuning of the Heavy-Atom Effect

Os(II) complexes showing singlet-to-triplet absorption are of growing interest as a new class of triplet sensitizers that circumvent energy loss during intersystem crossing, and they enable effective utilization of input photon energy in various applications, such as photoredox catalysis, photodynamic therapy, and photon upconversion. However, triplet excited-state lifetimes of Os(II) complexes are often too short (τ < 1 μs) to transfer their energy to neighboring molecules. While the covalent conjugation of chromophores has been known to extend the net excited-state lifetimes through an intramolecular triplet energy transfer (IMET), heavy-atom effects of the central metals on the attached chromophore units have rarely been discussed. Here, we investigate the relationship between the spin-density contribution of the heavy metals and the net triplet excited-state lifetimes for a series of Os(II) and Ru(II) bis(terpyridine) complexes modified with perylene units. Phosphorescence lifetimes of these compounds strongly depend on the lifetimes of the perylenyl group-localized excited states that are shortened by the heavy-atom effect. The degree of heavy-atom effect can be largely circumvented by introducing meta-phenylene bridges, where the perylene unit retains its intrinsic long excited-state lifetime. The thermal activation to the short-lived excited states is suppressed, thanks to sufficient but still small energy losses during the IMET process. Involvement of the metal center was also confirmed by the prolonged lifetime by replacing Os(II) with Ru(II) that possesses a smaller spin-orbit coupling constant. These results indicate the importance of ligand structures that give a minimum heavy-atom effect as well as the sufficient energy gap among the excited states and fast IMET for elongating the triplet excited-state lifetime without sacrificing the excitation energy.

Syntheses and structure of dinuclear metal complexes containing naphthyl-Ir bichromophore

A series of novel metal complexes were synthesized containing an Ir-cyclometalated bichromophore as a visible-light sensitizer. A new bichromophoric unit containing a naphthyl substituent and methyl substituents on the 2-phenylpyridine chelating ligand was synthesized and characterized for the first time. According to the increased crystallinity of the bichromophoric unit, novel Ir-M metal complexes (M = Pd, Mn, and Ir) were synthesized and fully characterized. The novel Ir-Pd complex maintained photocatalytic activity toward styrenes under visible-light irradiation, and polymerization with p-chlorostyrene, copolymerization with styrene and p-chlorostyrene furnished corresponding polymers.

Damming an electronic energy reservoir: ion-regulated electronic energy shuttling in a [2]rotaxane

We demonstrate the first example of bidirectional reversible electronic energy transfer (REET) between the mechanically bonded components of a rotaxane. Our prototypical system was designed such that photoexcitation of a chromophore in the axle results in temporary storage of electronic energy in a quasi-isoenergetic “reservoir” chromophore in the macrocycle. Over time, the emissive state of the axle is repopulated from this reservoir, resulting in long-lived, delayed luminescence. Importantly, we show that cation binding in the cavity formed by the mechanical bond perturbs the axle chromophore energy levels, modulating the REET process, and ultimately providing a luminescence read-out of cation binding. Modulation of REET processes represents an unexplored mechanism in luminescent molecular sensor development.

Delayed emission due to reversible electronic energy transfer (REET) between chromophores in the axle and macrocycle components of a rotaxane is demonstrated. The REET process can be modulated by metal ion binding in the cavity of the rotaxane.

Excited-State Engineering in Heteroleptic Ionic Iridium(III) Complexes

Iridium(III) complexes have assumed a prominent role in the areas of photochemistry and photophysics due to the peculiar properties of both the metal itself and the ligand environment that can be assembled around it. Ir(III) is larger, heavier, and bears a higher ionic charge than its analogue and widely used d⁶ ions such as Fe(II) and Ru(II). Accordingly, its complexes exhibit wider ligand-field d–d orbital splitting with electronic levels centered on the metal, typically nonemissive and photodissociative, not playing a relevant role in excited-state deactivations. In other words, iridium complexes are typically more stable and/or more emissive than Fe(II) and Ru(II) systems. Additionally, the particularly strong heavy-atom effect of iridium promotes singlet–triplet transitions, with characteristic absorption features in the UV–vis and relatively short excited-state lifetimes of emissive triplet levels. Ir(III) is also a platform for anchoring ligands of rather different sorts. Its versatile chemistry includes not only coordination with classic N^∧N neutral ligands but also the binding of negatively charged chelators, typically having a cyclometalating C^∧N anchor. The carbon–metal bond in these systems has some degree of covalent character, but this does not preclude a localized description of the excited states of the related complexes, which can be designated as metal-centered (MC), ligand-centered (LC), or charge transfer (CT), allowing a simplified description of electronic and photophysical properties. The possibility of binding different types of ligands and making heteroleptic complexes is a formidable tool for finely tuning the nature and energy of the lowest electronic excited state of cationic Ir(III) complexes by ligand design. Herein we give an account of our work on several families of iridium complexes typically equipped with two cyclometalating bidentate ligands (C^∧N), in combination with mono or bidentate “ancillary” ligands with N^∧N, C^∧N, and C^∧C motifs. We have explored new synthesis routes for both cyclometalating and ancillary ligands, obtaining primarily cationic complexes but also some neutral or even negatively charged systems. In the domain of the ancillary ligands, we have explored isocyanides, carbenes, mesoionic triazolylidenes, and bis-tetrazolic ligands. For the cyclometalating moiety, we have investigated carbene, mesoionic triazolylidene, and tetrazolic systems. Key results of our work include new strategies to modify both cyclometalating and ancillary ligands by relocating ionic charges, the determination of new factors affecting the stability of complexes, a demonstration of subtle structural effects that strongly modify the photophysical properties, new options to get blue-greenish emitters for optoelectronic devices, and a set of ligand modifications allowing the optimization of electrochemical and excited-state properties to obtain new promising Ir(III) complexes for photoredox catalysis. These results constitute a step forward in the preparation of custom iridium-based materials crafted by excited-state engineering, which is achieved through the concerted effort of computational and synthetic chemistry along with electrochemistry and photochemistry.

Ultrafast excited state relaxation dynamics in a heteroleptic Ir(iii) complex, fac-Ir(ppy)2(ppz), revealed by femtosecond X-ray transient absorption spectroscopy

The experimental and calculation results demonstrate that the ³ML_ppzCT state generated by the spin-forbidden transition rapidly relaxes to ³ML_ppyCT through internal conversion process with a time constant of ∼450 fs.

Anthryl-Appended Platinum(II) Schiff Base Complexes: Exceptionally Small Stokes Shift, Triplet Excited States Equilibrium, and Application in Triplet-Triplet-Annihilation Upconversion

Two anthryl platinum(II) N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-benzenediamine Schiff base complexes were synthesized, with the anthryl attached via its 9 position (Pt-9An) or 2 position (Pt-2An) to the platinum (Pt) Schiff base backbone. The complexes show unusually small Stokes shifts (0.23 eV), representing a very small energy loss for the photoexcitation/intersystem crossing process, which is beneficial for applications as triplet photosensitizers. Phosphorescence of the Pt(II) coordination framework (Φ_P = 11.0%) is quenched in the anthryl-containing complexes (Φ_P = 4.0%) and shows a biexponential decay (τ_P = 3.4 μs/87% and 18.2 μs/13%) compared to the single-exponential decay of the native Pt(II) Schiff base complex (τ_P = 3.7 μs). Femtosecond/nanosecond transient absorption spectroscopy suggests an equilibrium between triplet anthracene (³An) and triplet metal-to-ligand charge-transfer (³MLCT) states, with the dark ³An state slightly lower in energy (1.96 eV for Pt-9An and 1.90 eV for Pt-2An) than the emissive ³MLCT state (1.97 eV for Pt-9An and 1.91 eV for Pt-2An). Intramolecular triplet-triplet energy transfer (TTET) and reverse TTET take 4.8 ps/444 ps for Pt-9An and 55 ps/1.7 ns for Pt-2An, respectively. The triplet-state equilibrium extends the triplet-state lifetime of the complexes to 103 μs (Pt-2An) or 163 μs (Pt-9An), in comparison to the native Pt(II) complex, which shows a lifetime of 4.0 μs. The complexes were used for triplet-triplet-annihilation upconversion with perylene as the triplet acceptor. The upconversion quantum yield is up to 15%, and a large anti-Stokes shift (0.75 eV) is achieved by excitation into the singlet metal-to-ligand charge-transfer absorption band (589 nm) of the complexes (anti-Stokes shift is 0.92 eV with 9,10-diphenylanthracene as the acceptor).

Thermally Activated Delayed Photoluminescence: Deterministic Control of Excited-State Decay

Thermally activated photophysical processes are ubiquitous in numerous organic and metal-organic molecules, leading to chromophores with excited-state properties that can be considered an equilibrium mixture of the available low-lying states. Relative populations of the equilibrated states are governed by temperature. Such molecules have been devised as high quantum yield emitters in modern organic light-emitting diode technology and for deterministic excited-state lifetime control to enhance chemical reactivity in solar energy conversion and photocatalytic schemes. The recent discovery of thermally activated photophysics at CdSe nanocrystal-molecule interfaces enables a new paradigm wherein molecule-quantum dot constructs are used to systematically generate material with predetermined photophysical response and excited-state properties. Semiconductor nanomaterials feature size-tunable energy level engineering, which considerably expands the purview of thermally activated photophysics beyond what is possible using only molecules. This Perspective is intended to provide a nonexhaustive overview of the advances that led to the integration of semiconductor quantum dots in thermally activated delayed photoluminescence (TADPL) schemes and to identify important challenges moving into the future. The initial establishment of excited-state lifetime extension utilizing triplet-triplet excited-state equilibria is detailed. Next, advances involving the rational design of molecules composed of both metal-containing and organic-based chromophores that produce the desired TADPL are described. Finally, the recent introduction of semiconductor nanomaterials into hybrid TADPL constructs is discussed, paving the way toward the realization of fine-tuned deterministic control of excited-state decay. It is envisioned that libraries of synthetically facile composites will be broadly deployed as photosensitizers and light emitters for numerous synthetic and optoelectronic applications in the near future.

Design and Synthesis of Cyclometalated Iridium(III) Complexes-Chromophore Hybrids that Exhibit Long-Emission Lifetimes Based on a Reversible Electronic Energy Transfer Mechanism

We report on the design and synthesis of triscyclometalated iridium (Ir) complexes that contain aryloxy groups at the end of diamino linkers, which exhibit an extraordinarily long-emission lifetime, and were prepared by regioselective substitution reactions of fac-tris-homoleptic cyclometalated Ir complexes, fac-Ir(tpy)₃ (tpy = 2-(4'-tolyl)pyridine). It was found that the Ir(tpy)₃ complex, equipped with approximately one to six 6-N,N-dimethylamino-2-naphthoic acid (DMANA) groups through the appropriate alkyl linkers, exhibited remarkably long-emission lifetimes of up to 216 μs in DMSO/H₂O at room temperature through a reversible electronic energy transfer effect between the Ir complex core and the organic chromophore moieties; however, under the same conditions, the lifetime of fac-Ir(tpy)₃ was 1.4 μs. Regarding the mechanistic aspects, the relationship between the emission lifetimes of the Ir complexes and the structures and numbers of the conjugated chromophores, linker lengths, solvents, positions of the chromophores on the Ir(tpy)₃ core, and related items are discussed.

Effect of Molecular Conformation Restriction on the Photophysical Properties of N^N Platinum(II) Bis(ethynylnaphthalimide) Complexes Showing Close-Lying 3MLCT and 3LE Excited States

Using naphthalimide (NI), complexes (Pt-PhNI and Pt-PhMeNI) based on the N^N platinum(II) bis(phenylacetylide) coordination framework were prepared, in which there are two close-lying triplet states, i.e., the metal-to-ligand-charge-transfer (³MLCT) and the NI localized emissive state (³LE). Pt-PhNI has better electronic communication between the Pt coordination center and the NI moiety, whereas in Pt-PhMeNI, they are more isolated by orthogonal geometry. For Pt-PhMeNI, the S₀ → ¹MLCT and S₀ → ¹LE absorption bands are separated by 5655 cm^-1, while they are more overlapped in Pt-PhNI. The ³MLCT → S₀ and ³LE → S₀ dual phosphorescence emissions were observed for both Pt-PhNI (in toluene) and Pt-PhMeNI (in benzonitrile). The molecular conformation tunes the ³MLCT/³LE state population ratio, and the orthogonal geometry makes the ³LE state in Pt-PhMeNI basically a dark state (in toluene). Switching of the relative energy levels of the ³MLCT/³LE states by variation of the solvent polarity and temperature was achieved. For Pt-PhMeNI, the energy level of ³MLCT state is higher in a polar solvent; thus, the ³MLCT emission decreases, while the phosphorescence lifetime is prolonged from 9.5 μs (in toluene) to 58 μs (in benzonitrile) because of the different equilibria with the nonemissive ³LE state. Conversely, increasing the temperature enhances the upward transition from the nonemissive ³LE state to the emissive ³MLCT state; as such, the phosphorescence of Pt-PhMeNI was intensified at higher temperature (which is unusual), and the phosphorescence lifetime decreased from 58 μs (298 K) to ca. 5 μs (348 K). The ultrafast intersystem crossing (ca. 0.5 ps) and intramolecular triplet-triplet energy transfer (3-11 ps) were studied by femtosecond transient absorption spectroscopy. These results are useful for an in-depth understanding of the photophysics of multichromophore transition-metal complexes and for the design of external stimuli-responsive sensing materials, for instance, temperature or microenvironment sensing materials.

Photofunctions of iridium(iii) complexes in vesicles: long-lived excited states and visible-light sensitization for hydrogen evolution in aqueous solution

Utilization of DPPC vesicles allows water-insoluble photoactive Ir(iii) complexes to be dispersed in bulk aqueous solution.

Coordination mode-induced isomeric cyclometalated [Ir(tpy)(nbi)Cl](PF6) complexes: distinct luminescence, self-assembly and cellular imaging behaviors

Two isomeric Ir(iii) complexes Ir-O and Ir-R arising from the different coordination mode of a naphthalene-containing ligand, show distinct luminescence, self-assembly ability and cellular imaging behaviors.

Designed Long-Lived Emission from CdSe Quantum Dots through Reversible Electronic Energy Transfer with a Surface-Bound Chromophore

The size-tunable emission of luminescent quantum dots (QDs) makes them highly interesting for applications that range from bioimaging to optoelectronics. For the same applications, engineering their luminescence lifetime, in particular, making it longer, would be as important; however, no rational approach to reach this goal is available to date. We describe a strategy to prolong the emission lifetime of QDs through electronic energy shuttling to the triplet excited state of a surface-bound molecular chromophore. To implement this idea, we made CdSe QDs of different sizes and carried out self-assembly with a pyrene derivative. We observed that the conjugates exhibit delayed luminescence, with emission decays that are prolonged by more than 3 orders of magnitude (lifetimes up to 330 μs) compared to the parent CdSe QDs. The mechanism invokes unprecedented reversible quantum dot to organic chromophore electronic energy transfer.

Exploiting the benefit of S0→ T1 excitation in triplet-triplet annihilation upconversion to attain large anti-stokes shifts: tuning the triplet state lifetime of a tris(2,2'-bipyridine) osmium(ii) complex

Os(ii) complexes are particularly interesting for triplet-triplet annihilation (TTA) upconversion, due to the strong direct S₀→ T₁ photoexcitation, as in this way, energy loss is minimized and large anti-Stokes shift can be achieved for TTA upconversion. However, Os(bpy)₃ has an intrinsic short T₁ state lifetime (56 ns), which is detrimental for the intermolecular triplet-triplet energy transfer (TTET), one of the crucial steps in TTA upconversion. In order to prolong the triplet state lifetime, we prepared an Os(ii) tris(bpy) complex with a Bodipy moiety attached, so that an extended T₁ state lifetime is achieved by excited state electronic configuration mixing or triplet state equilibrium between the coordination center-localized state (³MLCT state) and Bodipy ligand-localized state (³IL state). With steady-state and time-resolved transient absorption/emission spectroscopy, we proved that the ³MLCT is slightly above the ³IL state (by 0.05 eV), and the triplet state lifetime was prolonged by 31-fold (from 56 ns to 1.73 μs). The TTA upconversion quantum yield was increased by 4-fold as compared to that of the unsubstituted Os(ii) complex.

An anionic iridium(iii) complex as a visible-light absorbing photosensitizer

A new anionic Ir(iii) photosensitizer bearing coumarin dyes has been developed and applied to the visible-light-driven hydrogen generation.

Luminescent Iridium Complexes Used in Light-Emitting Electrochemical Cells (LEECs)

Cationic iridium(III) complexes represent the single largest class of emitters used in light emitting electrochemical cells (LEECs). In this chapter, we highlight the state-of-the-art emitters in terms of efficiency and stability in LEEC devices, highlighting blue, green, yellow/orange, red and white devices, and provide an outlook to the future of LEECs.

Triplet-Triplet Annihilation Photon Upconversion in Polymer Thin Film: Sensitizer Design

Efficient visible-to-UV photon upconversion via triplet-triplet annihilation (TTA) is accomplished in polyurethane (PU) films by developing new, powerful photosensitizers fully functional in the solid-state matrix. These rationally designed triplet sensitizers feature a bichromophoric scaffold comprising a tris-cyclometalated iridium(III) complex covalently tethered to a suitable organic small molecule. The very rapid intramolecular triplet energy transfer from the former to the latter is pivotal for achieving the potent sensitizing ability, because this process out-competes the radiative and nonradiative decays inherent to the metal complex and produces long-lived triplet excitons localized with the acceptor moiety readily available for intermolecular transfer and TTA. Nonetheless, compared to the solution state, the molecular diffusion is greatly limited in solid matrices, which even creates difficulty for the Dexter-type intramolecular energy transfer. This is proven by the experimental results showing that the sensitizing performance of the bichromophoric molecules strongly depends on the spatial distance separating the donor (D) and acceptor (A) units and that incorporating a longer linker between the D and A evidently curbs the TTA upconversion efficiency in PU films. Using a rationally optimized sensitizer structure in combination with 2,7-di-tert-butylpyrene as the annihilator/emitter, the doped polyurethane (PU) films demonstrate effective visible-to-UV upconverted emission signal under noncoherent-light irradiation, attaining an upconversion quantum yield of 2.6%. Such quantum efficiency is the highest value so far reported for the visible-to-UV TTA systems in solid matrices.

Regioselectivity in C-H activation: reagent control in cyclometallation of 2-(1-naphthyl)-pyridine

2-(1-Naphthyl)-pyridine () possesses sp(2) C-H bonds in both the γ- and δ-positions and is therefore a suitable substrate for studying the cyclometallation selectivity with different reagents and conditions. Such selectivity studies are reported. Based on deuterium-exchange experiments it is concluded that cycloruthenation with RuCl2(p-cymene) dimer is reversible with kinetic and thermodynamic preference for γ-substitution. Electrophilic cycloborylation, on the other hand, shows unusual δ-substitution. The previously published cyclopalladation and cycloauration of the substrate was studied in detail and was shown to be irreversible; they proceed under kinetic control and give γ- and δ-substitution for palladium and gold, respectively.

Harnessing Reversible Electronic Energy Transfer: From Molecular Dyads to Molecular Machines

Reversible electronic energy transfer (REET) may be instilled in bi-/multichromophoric molecule-based systems, following photoexcitation, upon judicious structural integration of matched chromophores. This leads to a new set of photophysical properties for the ensemble, which can be fully characterized by steady-state and time-resolved spectroscopic methods. Herein, we take a comprehensive look at progress in the development of this type of supermolecule in the last five years, which has seen systems evolve from covalently tethered dyads to synthetic molecular machines, exemplified by two different pseudorotaxanes. Indeed, REET holds promise in the control of movement in molecular machines, their assembly/disassembly, as well as in charge separation.

Iridium-Based High-Sensitivity Oxygen Sensors and Photosensitizers with Ultralong Triplet Lifetimes

The photophysics of a series of bichromophoric molecules featuring an intramolecular triplet energy transfer between a triscyclometalated iridium(III) complex and covalently linked organic group are studied. By systematically varying the energy gap (0.1-0.3 eV) between the donor (metal complex) and acceptor (pyrene unit), reversible triplet energy transfer processes with equilibrium constant K ranging from ca. 500 to 40 000 are established. Unique photophysical consequences of such large K values are observed. Because of the highly imbalanced forward and backward energy transfer rates, triplet excitons dominantly populate the acceptor moiety in the steady state, giving rise to ultralong luminescence lifetimes up to 1-4 ms. Because the triscyclometalated Ir and triplet pyrene groups both impart relatively small nonradiative energy loss, decent phosphorescence quantum yields (Φ = 0.1-0.6) are attained in spite of the exceptionally prolonged excited states. By virtue of such precious combination of long-lived triplet state and high Φ, these bichromophoric molecules can serve as highly sensitive luminescent sensors for detecting trace amount of O2 and as potent photosensitizers for producing singlet oxygen even under low-oxygen content conditions.

Chromophore-labelled, luminescent platinum complexes: syntheses, structures, and spectroscopic properties

Ligands based upon 4-carboxamide-2-phenylquinoline have been functionalised with naphthyl, anthracenyl and pyrenyl chromophores. The pyrene appended Pt(ii) complex shows excited state equilibration and a long emission lifetime.

High performance optical oxygen sensors based on iridium complexes exhibiting interchromophore energy shuttling

Reversible electronic energy transfer is used for sensing oxygen traces and results in very high sensitivity.

Metal controlled regioselectivity in the cyclometallation of 2-(1-naphthyl)-pyridine

Cyclometallations of 2-(1-naphthyl)-pyridine with gold and palladium salts proceed with completely orthogonal regioselectivities.

Triplet state formation in homo- and heterometallic diketopyrrolopyrrole chromophores

The synthesis, structural characterization, and excited-state dynamics of series of diketopyrrolopyrrole (DPP) bridged homodinuclear Ir(III) and heterodinuclear Ir(III)/Pt(II) complexes is described. Steady-state and time-resolved photoluminescence along with transient absorption measurements were used to probe the nature of the emissive and long-lived excited states. Upon excitation into the (1)DPP ligand-localized excited state in the presence of coordinated Ir(III) or Pt(II) metal centers, the intersystem crossing is enhanced, leading to a quenching of the (1)DPP fluorescence and the formation of the long-lived (τ ≈ 30-40 μs) (3)DPP excited state in all instances.

Tuning the emission lifetime in bis-cyclometalated iridium(III) complexes bearing iminopyrene ligands

Bis-cyclometalated Ir(III) complexes with the general formula Ir(ppz)2(X^NPyrene), where ppz = 1-phenylpyrazole and X^NPyrene is a bidentate chelate with X = N or O, are reported. Modifications on the ancillary ligand containing pyrene drastically affect the emission lifetimes observed (0.329 to 104 μs). Extended emission lifetimes in these complexes compared to model complexes result from reversible electronic energy transfer or the observation of dual emission containing along-lived pyrene ligand-centered triplet ((3)LC) component. A combination of steady-state and time-resolved spectroscopic techniques are used to observe reversible electronic energy transfer in solution between the iridium core and pyrene moiety in the complex [Ir(ppz)2(NMe^NCH2Pyr)][PF6] (2), where NMe^NCH2Pyr = N-(pyren-1-ylmethyl)-1-(pyridin-2-yl)ethaneimine. Studies on [Ir(ppz)2(NMe^NCH2Pyr)][PF6] in a poly(methyl methacrylate) (PMMA) film reveal that reversible energy transfer is no longer effective, and instead, dual emission with a long-lived (3)LC component from pyrene is observed. Dual emission is observed in additional cyclometalated iridium complexes bearing pyrene-containing ancillary ligands N^NPyrene and O^NPyrene when the complexes are dispersed in a PMMA film.