A heteroleptic bis(tributylphosphine) platinum(II)-alkynyl complex (Pt-1) showing broadband visible-light absorption was prepared. Two different visible-light-absorbing ligands, that is, ethynylated boron-dipyrromethene (BODIPY) and a functionalized naphthalene diimide (NDI) were used in the molecule. Two reference complexes, Pt-2 and Pt-3, which contain only the NDI or BODIPY ligand, respectively, were also prepared. The coordinated BODIPY ligand shows absorption at 503 nm and fluorescence at 516 nm, whereas the coordinated NDI ligand absorbs at 594 nm; the spectral overlap between the two ligands ensures intramolecular resonance energy transfer in Pt-1, with BODIPY as the singlet energy donor and NDI as the energy acceptor. The complex shows strong absorption in the region 450 nm–640 nm, with molar absorption coefficient up to 88 000 M⁻¹ cm⁻¹. Long-lived triplet excited states lifetimes were observed for Pt-1–Pt-3 (36.9 μs, 28.3 μs, and 818.6 μs, respectively). Singlet and triplet energy transfer processes were studied by the fluorescence/phosphorescence excitation spectra, steady-state and time-resolved UV/Vis absorption and luminescence spectra, as well as nanosecond time-resolved transient difference absorption spectra. A triplet-state equilibrium was observed for Pt-1. The complexes were used as triplet photosensitizers for triplet–triplet annihilation upconversion, with upconversion quantum yields up to 18.4 % being observed for Pt-1.

Cyclometalated Pt^II complexes that show strong visible-light absorption and long-lived triplet excited states were used as efficient singlet oxygen (¹O₂) sensitizers for the photooxidation of 1,5-dihydroxynaphthalene (DHN). The photooxidation velocity of the cyclometalated Pt^II complex is 4.4 times that of the conventional Ir^III complex triplet photosensitizers that do not show strong absorption of visible light.

C₆₀–bodipy triads and tetrads based on the energy-funneling effect that show broadband absorption in the visible region have been prepared as novel triplet photosensitizers. The new photosensitizers contain two or three different light-harvesting antennae associated with different absorption wavelengths, resulting in a broad absorption band (450–650 nm). The panchromatic excitation energy harvested by the bodipy moieties is funneled into a spin converter (C₆₀), thus ensuring intersystem crossing and population of the triplet state. Nanosecond time-resolved transient absorption and spin density analysis indicated that the T₁ state is localized on either C₆₀ or the antennae, depending on the T₁ energy levels of the two entities. The antenna-localized T₁ state shows a longer lifetime (τ_T=132.9 μs) than the C₆₀-localized T₁ state (ca. 27.4 μs). We found that the C₆₀ triads and tetrads can be used as dual functional photocatalysts, that is, singlet oxygen (¹O₂) and superoxide radical anion (O₂^.−) photosensitizers. In the photooxidation of naphthol to juglone, the ¹O₂ photosensitizing ability of the C₆₀ triad is a factor of 8.9 greater than the conventional triplet photosensitizers tetraphenylporphyrin and methylene blue. The C₆₀ dyads and triads were also used as photocatalysts for O₂^.−-mediated aerobic oxidation of aromatic boronic acids to produce phenols. The reaction times were greatly reduced compared with when [Ru(bpy)₃Cl₂] was used as photocatalyst. Our study of triplet photosensitizers has shown that broadband absorption in the visible spectral region and long-lived triplet excited states can be useful for the design of new heavy-atom-free organic triplet photosensitizers and for the application of these triplet photosensitizers in photo-organocatalysis.

Modular diboronic acid receptors R-1 and S-1 were synthesized and found to be selective fluorescent chemosensors for saccharides. The fluorescence intensity of R-1 and S-1 increased on addition of saccharides due to the formation of intramolecular 1:1 cyclic complexes. In general, R-1 has higher binding constants and produces a higher fluorescent response with saccharides than S-1.

C₆₀-boron dipyrromethene (BODIPY) dyads (C-1 and C-2) were prepared as heavy-atom-free organic triplet photosensitizers for triplet-triplet annihilation (TTA) based upconversion. BODIPY-C≡C-triphenylamine chromophores were used as the light-harvesting antennas and C₆₀ was used as the singlet energy acceptor and the spin convertor to facilitate the intersystem crossing (ISC) of the dyads from the singlet excited state to the triplet excited state. The C₆₀ dyads strongly absorb visible light (molar extinction coefficients of up to 118800 M⁻¹ cm⁻¹ at 539 nm) and have a long-lived triplet excited state (τ_T=32.3 μs). The photophysical properties of the C₆₀-dyads were studied with steady-state and time-resolved spectroscopy. Photoexcitation of the dyad with visible light produces firstly the singlet excited state of the antenna, then the singlet excited state of the C₆₀ unit through intramolecular energy transfer. In turn, the triplet excited state of the C₆₀ unit will be populated because of the intrinsic ISC property of C₆₀. As a proof of concept, the dyads were used as heavy-atom-free organic triplet photosensitizers for TTA-based upconversion.

NN–Pt^II–bis(acetylide) complexes [NN = 4,4′-bis(tert-butyl-2,2′-bipyridine) (dbbpy)] with ethynylpyrene (Pt-2) or 4-ethynyl-1,8-naphthalimide ligands (Pt-3) that show long-lived ³IL triplet excited states (τ_T = 118.0 μs) were used as new triplet sensitizers for triplet–triplet annihilation (TTA) based upconversion (UC). UC quantum yields of up to 18.1 % were observed. We found that the triplet–triplet energy-transfer (TTET) processes, which are crucial for the TTA UC, were improved by up to 150-fold and 250-fold with Pt-2 (Stern–Volmer quenching constant K_SV = 5.70 × 10⁵ M^–1) and Pt-3 (K_SV = 9.64 × 10⁵ M^–1), respectively, relative to the model complex dbbpy–Pt^II–bis(phenylacetylide) (Pt-1) (K_SV = 3.80 × 10³ M^–1). The efficient upconversion is attributed to the long-lived triplet excited states as well as the high T₁-state energy level of the sensitizers Pt-2 and Pt-3.

Room-temperature long-lived near-IR phosphorescence of boron-dipyrromethene (BODIPY) was observed (λ_em=770 nm, Φ_P=3.5 %, τ_P=128.4 μs). Our molecular-design strategy is to attach Pt^II coordination centers directly onto the BODIPY π-core using acetylide bonds, rather than on the periphery of the BODIPY core, thus maximizing the heavy-atom effect of Pt^II. In this case, the intersystem crossing (ISC) is facilitated and the radiative decay of the T₁ excited state of BODIPY is observed, that is, the phosphorescence of BODIPY. The complex shows strong absorption in the visible range (ε=53800 M⁻¹ cm⁻¹ at 574 nm), which is rare for Pt^II–acetylide complexes. The complex is dual emissive with ³MLCT emission at 660 nm and the ³IL emission at 770 nm. The T₁ excited state of the complex is mainly localized on the BODIPY moiety (i.e. ³IL state, as determined by steady-state and time-resolved spectroscopy, 77 K emission spectra, and spin-density analysis). The strong visible-light-harvesting ability and long-lived T₁ excite state of the complex were used for triplet-triplet annihilation based upconversion and an upconversion quantum yield of 5.2 % was observed. The overall upconversion capability (η=ε×Φ_UC) of this complex is remarkable considering its strong absorption. The model complex, without the BODIPY moiety, gives no upconversion under the same experimental conditions. Our work paves the way for access to transition-metal complexes that show strong absorption of visible light and long-lived ³IL excited states, which are important for applications in photovoltaics, photocatalysis, and upconversions, etc.

Ru^II–bis-pyridine complexes typically absorb below 450 nm in the UV spectrum and their molar extinction coefficients are only moderate (ε<16 000 M⁻¹ cm⁻¹). Thus, Ru^II–polyimine complexes that show intense visible-light absorptions are of great interest. However, no effective light-harvesting ruthenium(II)/organic chromophore arrays have been reported. Herein, we report the first visible-light-harvesting Ru^II–coumarin arrays, which absorb at 475 nm (ε up to 63 300 M⁻¹ cm⁻¹, 4-fold higher than typical Ru^II–polyimine complexes). The donor excited state in these arrays is efficiently converted into an acceptor excited state (i.e., efficient energy-transfer) without losses in the phosphorescence quantum yield of the acceptor. Based on steady-state and time-resolved spectroscopy and DFT calculations, we proposed a general rule for the design of Ru^II–polypyridine–chromophore light-harvesting arrays, which states that the ¹IL energy level of the ligand must be close to the respective energy level of the metal-to-ligand charge-transfer (MLCT) states. Lower energy levels of ¹IL/³IL than the corresponding ¹MLCT/³MLCT states frustrate the cascade energy-transfer process and, as a result, the harvested light energy cannot be efficiently transferred to the acceptor. We have also demonstrated that the light-harvesting effect can be used to improve the upconversion quantum yield to 15.2 % (with 9,10-diphenylanthracene as a triplet-acceptor/annihilator), compared to the parent complex without the coumarin subunit, which showed an upconversion quantum yield of only 0.95 %.

Cyclometalated Ir^III complexes with acetylide ppy and bpy ligands were prepared (ppy=2-phenylpyridine, bpy=2,2′-bipyridine) in which naphthal (Ir-2) and naphthalimide (NI) were attached onto the ppy (Ir-3) and bpy ligands (Ir-4) through acetylide bonds. [Ir(ppy)₃] (Ir-1) was also prepared as a model complex. Room-temperature phosphorescence was observed for the complexes; both neutral and cationic complexes Ir-3 and Ir-4 showed strong absorption in the visible range (ε=39600 M⁻¹ cm⁻¹ at 402 nm and ε=25100 M⁻¹ cm⁻¹ at 404 nm, respectively), long-lived triplet excited states (τ_T=9.30 μs and 16.45 μs) and room-temperature red emission (λ_em=640 nm, Φ_p=1.4 % and λ_em=627 nm, Φ_p=0.3 %; cf. Ir-1: ε=16600 M⁻¹ cm⁻¹ at 382 nm, τ_em=1.16 μs, Φ_p=72.6 %). Ir-3 was strongly phosphorescent in non-polar solvent (i.e., toluene), but the emission was completely quenched in polar solvents (MeCN). Ir-4 gave an opposite response to the solvent polarity, that is, stronger phosphorescence in polar solvents than in non-polar solvents. Emission of Ir-1 and Ir-2 was not solvent-polarity-dependent. The T₁ excited states of Ir-2, Ir-3, and Ir-4 were identified as mainly intraligand triplet excited states (³IL) by their small thermally induced Stokes shifts (ΔE_s), nanosecond time-resolved transient difference absorption spectroscopy, and spin-density analysis. The complexes were used as triplet photosensitizers for triplet-triplet annihilation (TTA) upconversion and quantum yields of 7.1 % and 14.4 % were observed for Ir-2 and Ir-3, respectively, whereas the upconversion was negligible for Ir-1 and Ir-4. These results will be useful for designing visible-light-harvesting transition-metal complexes and for their applications as triplet photosensitizers for photocatalysis, photovoltaics, TTA upconversion, etc.

A library of new fluorescent molecular rotors (FMRs) for viscosity sensing was synthesized. The sensitivity of the fluorescence emission toward solvent viscosity and polarity was investigated by using UV/Vis absorption, fluorescence emission spectra, and theoretical calculations. For the new FMRs, red-shifted emissions at 620 nm, Stokes shifts of 170 nm, and up to 40-fold fluorescence enhancement upon increasing the viscosity of solvents were observed (cf. known FMRs with emissions at 491 nm, Stokes shift of 33 nm and fivefold emission enhancement). By using solvents with high viscosity but low polarity, for example, polyethylene glycol (PEG-400) or dimethyl silicone oil, rotors previously identified as non-FMRs with ethylene glycol/glycerol show FMR properties. We found triphenylamine (TPA)-based rotors showed solvent-polarity-dependent emission, but also FMR properties. This is contrary to the known theory of FMRs. One of the TPA-based rotors shows the highest x value of 0.88 (versus 0.6 for known FMRs). The emissive excited states of the FMRs proved to be locally excited (LE) states and twisted intramolecular charge-transfer (TICT) states were identified as dark states through trifluoroacetic acid titration and DFT/time-dependent DFT calculations, which show that the nonradiative decay channel of the excited FMRs is the rotation about the dicyanovinyl C=C double bonds in the S₁ state, not rotation around C–C single bonds. Extension of the π conjugation of the rotors increases the rotation barrier around the C=C double bond in the S₁ state (to ca. 50 kJ mol^–1 or higher), thus the nonradiative channel is blocked and higher fluorescence quantum yields are observed for the new rotors. Our results will be useful for future design of FMRs as fluorescent viscosity sensors.

An NN Pt^II bis(acetylide) complex was prepared with the rhodamine fluorophore [Pt–Rho, in which NN is dbbpy = 4,4′-bis(tert-butyl-2,2′-bipyridine)]. Complex Pt–Rho shows intense absorption in the visible region (ϵ = 185800 M^–1 cm^–1 at 556 nm) and a fluorescence emission of the ligand (λ_em = 580 nm, Φ_L = 41.1 %, τ_L = 2.50 ns). The long-lived rhodamine-localised intraligand triplet excited state (³IL, τ_T = 83.0 μs) was proposed to be populated upon excitation of the rhodamine ligand, proved by nanosecond time-resolved transient absorption and spin density analysis; the triplet excited state was studied by time-dependent DFT calculations. In comparison, the model complex (dbbpy)Pt^II bis(phenylacetylide) (Pt–Ph) shows weak absorption in the visible region (ϵ = 14700 M^–1 cm^–1 at 424 nm) and a short-lived metal-to-ligand charge-transfer excited state (³MLCT) (τ_T = 1.36 μs). Complex Pt–Rho was used as triplet sensitiser for upconversion based on triplet–triplet annihilation. An upconversion quantum yield of 11.2 % was observed. Our strategy to access the long-lived ³IL excited state of the organic chromophore by metallation with Pt^II will be useful for the preparation of transition metal complexes that show intense absorption of visible light and long-lived triplet excited states.

Fluorescent molecular rotors can be used as molecular sensors for the viscosity of a microenvironment. However, these molecular rotors are limited to 9-(dicyanovinyl)julolidine (DCVJ) and a few derivatives. Furthermore, these traditional rotors show short absorption/emission wavelengths and small Stokes shifts. To address these drawbacks, we have developed a small library of new molecular rotors for viscosity sensing, prepared by incorporating a thiophene unit into the conventional fluorescent molecular rotors with the aim of accessing molecular rotors with redshifted excitation/emission wavelengths and larger Stokes shifts compared with the known rotors. The new rotors show substantially improved photophysical properties. For example, rotor 4 shows absorption/emission wavelengths of 559/697 nm, respectively, and a very large Stokes shift of 138 nm compared with the absorption/emission wavelengths (465/503 nm) and very small Stokes shift (38 nm) of the traditional fluorescent molecular rotor DCVJ. The photophysical properties of the rotors were rationalized by DFT calculations.

Cyclometalated iridium(III) complexes with intense absorption in the visible region (ϵ = 70920 M^–1 cm^–1 at 466 nm) were prepared by incorporating light-harvesting coumarin into the diimine ligand (for comparison: ϵ = 7030 M^–1 cm^–1 at 371 nm for the model diimine complex without light-harvesting ability). The complexes feature long-lived intraligand triplet excited states (³IL, supported by spin density analysis of the triplet state). At room temperature, lifetimes, τ_T, are up to 75.5 μs, as determined by nanosecond time-resolved transient absorption spectroscopy. Despite the weak phosphorescence (Φ_P = 0.6–0.7 %) observed for the complexes, the triplet excited state (T₁) was demonstrated to be efficiently populated upon photoexcitation, by using the complexes as triplet sensitizers in triplet–triplet-annihilation (TTA) upconversion (upconversion quantum yields, Φ_UC, are up to 23.4 %). We propose that weakly phosphorescent or even nonphosphorescent transition metal complexes can be used as triplet sensitizers for TTA upconversion, instead of the currently used phosphorescent complexes. Our design of the light-harvesting Ir^III complexes will be helpful for the preparation of transition metal complexes with intense absorption of visible light and long-lived triplet excited states, which are suitable for applications in areas such as photocatalysis, photovoltaics, and upconversion.

The cover picture shows the triplet-triplet-annihilation (TTA) upconversion between a visible-light harvesting Ir^III complex and 9,10-diphenylanthracene (DPA). In the upper left-hand corner is the qualitative Jablonski diagram describing the photophysical process involved in the TTA upconversion. Although the coumarin-containing Ir^III complexes are weakly phosphorescent, we proved that the long-lived ³IL excited state was populated upon photoexcitation by using time-resolved transient absorption spectroscopy, emission spectroscopy at 77 K, and DFT calculations. The complexes were used as triplet sensitizers for TTA upconversion, and an upconversion quantum yield of up to 23.4 ° was observed. Thus, we propose that the T1 excited state of the triplet sensitizers does not necessarily have to be emissive. Details are discussed in the article by J. Zhao et al. on p. 3165 ff. The cover art was produced by Dr. Xin Liu (Dalian University of Technology).

A modular approach was proposed for the preparation of chiral fluorescent molecular sensors, in which the fluorophore, scaffold, and chirogenic center can be connected by ethynyl groups, and these modules can easily be changed to other structures to optimize the molecular sensing performance of the sensors. This modular strategy to assembly chiral sensors alleviated the previous restrictions of chiral boronic acid sensors, for which the chirogenic center, fluorophore, and scaffold were integrated, thus it was difficult to optimize the molecular structures by chemical modifications. We demonstrated the potential of our new strategy by the preparation of a sensor with a larger scaffold. The photoinduced electron-transfer (PET) effect is efficient even with a large distance between the N atom and the fluorophore core. Furthermore, the rarely reported donor-PET (d-PET) effect, which was previously limited to carbazole, was extended to phenothiazine fluorophore. The contrast ratio, that is, PET efficiency of the new d-PET sensor, is increased to 8.0, compared to 2.0 with the previous carbazole d-PET sensors. Furthermore, the ethynylated phenothiazine shows longer excitation wavelength (centered at 380 nm) and emission wavelength (492 nm), a large Stokes shift (142 nm), and high fluorescence quantum yield in aqueous solution (Φ=0.48 in MeOH/water, 3:1 v/v). Enantioselective recognition of tartaric acid was achieved with the new d-PET boronic acid sensors. The enantioselectivity is up to 10 (ratio of the binding constants toward D- and L-tartaric acid, k_D/k_L). A consecutive fluorescence enhancement/decrease was observed, thus we propose a transition of the binding stoichiometry from 1:1 to 1:2 as the analyte concentration increases, which is supported by mass spectra analysis. The boronic acid sensors were used for selective and sensitive recognition of disaccharides and glycosylated steroids (ginsenosides).

[(Aryl-ppy)Pt(acac)] (ppy = 2-phenylpyridine, acac = acetylacetonato) derivatives with triphenylamine (TPA) substituents on the ppy ligand have been prepared. The TPA fragment is either directly cyclometallated (Pt-1) or attached to the ppy ligand through a C–C single bond (Pt-2) or a novel α-diketo group (Pt-3). All the complexes show room-temperature phosphorescence in fluid solution with emission bands in the range of 530–590 nm, which are red-shifted relative to the model complex [ppyPt(acac)] (λ_em = 486 nm). This emission colour tuning effect is attributed to either an elevated HOMO energy caused by electron-donating TPA substituents on the ppy ligand or a decreased LUMO energy caused by the electron-trap effect of electron-withdrawing substituents; both result in a smaller HOMO–LUMO energy gap and thus red-shifted emission. The complexes show extended luminescence lifetimes (τ = 3.0–5.5 μs) relative to the parent complex [ppyPt(acac)] (τ = 2.6 μs). The luminescent oxygen-sensing properties of the complexes were studied in solution and polymer films. White light emission was observed with an OLED device fabricated with complex Pt-3 with CIE coordinates of (0.32, 0.32).

Pyrene-containing cyclometallated Pt^II complexes, with the pyrene moiety directly cyclometallated (Pt-1) or connected to a 2-phenylpyridine (ppy) ligand through a C–C (Pt-2) or C≡C bond (Pt-3), and a control complex with a phenyl group attached to the ppy ligand (Pt-4) have been prepared. Room-temperature deep-red/near-IR (NIR) phosphorescence emission (650–800 nm) was observed for Pt-1, Pt-2 and Pt-3, whereas Pt-4 showed emission at 528 nm. We found that Pt-2, in which the pyrene moiety is not directly cyclometallated, shows intense pyrene-based phosphorescence, which contrasts with a previous report that direct cyclometallation is necessary for the observation of the phosphorescence of pyrene in cyclometallated complexes. Besides the phosphorescence emission in the deep-red/near-IR range, a fluorescence emission band at higher energy was observed. Thus, these complexes can be described as unichromophore multi-emissive materials. Normal ³MLCT/³IL emission at 528 nm was observed for Pt-4. The UV/Vis absorption and phosphorescence emissions of the complexes were rationalized by DFT/TDDFT calculations. Theoretical calculations propose pyrene-localized T₁ states (³IL) for Pt-1, Pt-2 and Pt-3, which is supported by the experimental results. The complexes were used in luminescent O₂-sensing experiments. These studies will be helpful in the development of room-temperature phosphorescent materials and their application as luminescent molecular sensing or electroluminescent materials are promising.

A colorimetric sensor for fluoride ions based on a new sensing mechanism is reported. The colorimetric sensor contains an isomerizable enol–keto moiety as the recognition site and phenothiazine as chromogenic center. A color change visible to the naked eye is observed upon addition of fluoride ions to the solution of sensor 1 in aprotic solvents such as CHCl₃ and MeCN. The sensor shows no colorimetric response for other halide ions. Enol-keto tautomerization is proposed to be responsible for the anion sensing of 1, based on UV/VIS absorption, ¹H-NMR, and single-crystal structure analysis.