Switchable ambient-temperature singlet-triplet dual emission in nonconjugated donor-acceptor triarylboron-Pt(II) complexes

Dual-Emissive Monoruthenium Complexes of N(CH3)-Bridged Ligand: Synthesis, Characterization, and Substituent Effect

Three monoruthenium complexes 1(PF6)2-3(PF6)2 bearing an N(CH3)-bridged ligand have been synthesized and characterized. These complexes have a general formula of [Ru(bpy)2(L)](PF6)2, where L is a 2,5-di(N-methyl-N'-(pyrid-2-yl)amino)pyrazine (dapz) derivative with various substituents, and bpy is 2,2'-bipyridine. The photophysical and electrochemical properties of these compounds have been examined. The solid-state structure of complex 3(PF6)2 is studied by single-crystal X-ray analysis. These complexes show two well-separated emission bands centered at 451 and 646 nm (Δλmax = 195 nm) for 1(PF6)2, 465 and 627 nm (Δλmax = 162 nm) for 2(PF6)2, and 455 and 608 nm (Δλmax = 153 nm) for 3(PF6)2 in dilute acetonitrile solution, respectively. The emission maxima of the higher-energy emission bands of these complexes are similar, while the lower-energy emission bands are dependent on the electronic nature of substituents. These complexes display two consecutive redox couples owing to the stepwise oxidation of the N(CH3)-bridged ligand and ruthenium component. Moreover, these experimental observations are analyzed by computational investigation.

Conjugation-Modulated Excitonic Coupling Brightens Multiple Triplet Excited States

The design and regulation of multiple room-temperature phosphorescence (RTP) processes are formidably challenging due to the restrictions imposed by Kasha's rule. Here, we report a general design principle for materials that show multiple RTP processes, which is informed by our study of four compounds where there is modulation of the linker hybridization between donor (D) and acceptor (A) groups. Theoretical modeling and photophysical experiments demonstrate that multiple RTP processes can be achieved in sp³ C-linked D-A compounds due to the arrest of intramolecular electronic communication between two triplet states (T₁^H and T₁^L) localized on the donor and acceptor or between two triplet states, one localized on the donor and one delocalized across aggregated acceptors. However, for the sp² C-linked D-A counterparts, RTP from one locally excited T₁ state is observed because of enhanced excitonic coupling between the two triplet states of molecular subunits. Single-crystal and reduced density gradient analyses reveal the influence of molecular packing on the coincident phosphorescence processes and the origin of the observed aggregate phosphorescence. These findings provide insights into higher-lying triplet excited-state dynamics and into a fundamental design principle for designing compounds that show multiple RTP.

Triarylborane-triphenylamine based luminophore for the mitochondria targeted live cell imaging and colorimetric detection of aqueous fluoride

Bioimaging of subcellular organelles such as mitochondria is crucial for detecting physiological abnormalities induced by fluctuations in the levels of various analytes. Herein, we report the design and synthesis of two novel water-soluble cationic Lewis acid triarylborane-triarylamine conjugates 1 and 2. The optical characteristics of 1 and 2 and their precursor compounds BTPA-NMe2 and BTPA-2NMe2 were evaluated, which show similar absorption and fluorescence spectra, with 1 and 2 exhibiting higher quantum yields of 0.73 and 0.64, respectively, than those of the precursors BTPA-NMe2 and BTPA-2NMe2, indicating the partial disruption of the ICT process and the activation of alternative emission bands in 1 and 2. The live cell imaging ability of compound 2 was examined in HeLa cells using a confocal microscope. Moreover, mitochondrial internalisation using compound 2 was effective and it was found to have high photostability under UV light conditions. Furthermore, compound 2 demonstrated an evident colorimetric response with a colour change to dark yellow in aqueous environments, indicating that it could be used for anion sensing. The spectral changes were observed in UV-visible and fluorescence titration experiments, which were strongly supported by DFT calculations. In short, compound 2 synthesized by us can be exclusively utilized for the selective localization of mitochondria with less cytotoxicity and shows excellent colorimetric response to aqueous inorganic fluoride at levels as low as 0.1 ppm with high selectivity.

B- and N-Embedded π-Conjugation Units Tuning Intermolecular Interactions and Optical Properties of Platinum(II) Complexes

A new series of neutral and cationic platinum(II) complexes containing a B- or N-embedded π-conjugation unit has been prepared. Notably, significantly different intermolecular interactions (Pt-Pt, π-π, head to tail, and head to head) and interesting optical properties exist in these complexes, which can be attributed to the difference in spatial structures and π-electron properties between B- and N-embedded π-conjugation units. Unexpectedly, under a hypoxic atmosphere, N-embedded neutral complex PtNacac can display a distinct dual-emission with both fluorescence and phosphorescence, whereas only a single fluorescence emission was observed in the air, which is different from the B-embedded neutral complex PtBacac with only a single phosphorescence emission at any atmosphere, as well confirmed by lifetime measurement and oxygen sensing experiments. DFT calculations reveal that unusual ligand-to-metal charge transfer (LMCT) excited state character and low spin orbit coupling (SOC) elements can be found in N-embedded complexes due to the strong electron-donating ability of the N-embedded unit. Based on this, as a novel ratiometric oxygen probe with a simple structure, PtNacac can be successfully used to examine intracellular oxygen levels by monitoring both fluorescence and phosphorescence signals via ratiometric photoluminescence imaging and time-resolved luminescence imaging (TRLI) technology. This work provides a completely new idea for designing fluorescence/phosphorescence dual-emissive complexes.

Rollover Cyclometalation as a Valuable Tool for Regioselective C-H Bond Activation and Functionalization

Rollover cyclometalation constitutes a particular case of cyclometallation reaction. This reaction occurs when a chelated heterocyclic ligand loses its bidentate coordination mode and undergoes an internal rotation, after which a remote C-H bond is regioselectively activated, affording an uncommon cyclometalated complex, called "rollover cyclometalated complex". The key of the process is the internal rotation of the ligand, which occurs before the C-H bond activation and releases from coordination a donor atom. The new "rollover" ligand has peculiar properties, being a ligand with multiple personalities, no more a spectator in the reactivity of the complex. The main reason of this peculiarity is the presence of an uncoordinated donor atom (the one initially involved in the chelation), able to promote a series of reactions not available for classic cyclometalated complexes. The rollover reaction is highly regioselective, because the activated C-H bond is usually in a symmetric position with respect to the donor atom which detaches from the metal stating the rollover process. Due to this novel behavior, a series of potential applications have appeared in the literature, in fields such as catalysis, organic synthesis, and advanced materials.

Bridging-arylene effects on spectroscopic and photophysical properties of arylborane–dipyrrinato zinc(ii) complexes

Novel bis(dipyrrinato)zinc(ii) derivatives having 4-[bis(2,4,6-trimethylphenyl)boryl]phenyl (ZnBph) or 4-[bis(2,4,6-trimethylphenyl)boryl]-2,3,5,6-tetramethylphenyl groups (ZnBdu) at the 5-position of the dipyrrinato ligands were designed and synthesized. In ZnBph with the smaller dipyrrinate–arylene and arylene–dimesitylboryl dihedral angles, an intramolecular charge transfer arising from the presence of the vacant p orbital on the boron atom participates in the ππ* excited state in character in contrast to the pure ππ* excited state of ZnBdu. The synergistic ππ*/ILCT excited state was weakly fluorescent, and the fluorescence was enhanced upon binding of fluoride to the boron atom.

Electronic interactions between a bis(dipyrrinato)zinc(ii) and arylborane moieties were successfully tuned by a structure of bridging arylene groups. The excited state of a phenylene bridged complex was best characterized by the synergistic ππ*/ILCT.

Excitation-Wavelength-Dependent and Auxiliary-Ligand-Tuned Intersystem-Crossing Efficiency in Cyclometalated Platinum(II) Complexes: Spectroscopic and Theoretical Studies

Understanding the factors affecting the intersystem-crossing (ISC) rate constant (k_ISC) of transition-metal complexes is crucial to material design with tailored photophysical properties. Most of the works on ISC to date focused on the influence by the chromophoric ligand and the understanding of the ISC efficiency were mainly drawn from the steady-state fluorescence to phosphorescence intensity ratio and ground-state calculations, with only a few high-level calculations on k_ISC that take excited-state structural change and solvent reorganization into account for quantitative comparisons with the experimental data. In this work, a series of [Pt(thpy)X)]⁺ complexes were prepared [Hthpy = 2-(2'-thienyl)pyridine, where X = auxiliary ligands] and characterized by both steady-state and time-resolved luminescence spectroscopies. A panel of auxiliary ligands with varying σ-donating/π-accepting character have been used. For comparison, analogues of [Pt(ppy)(P^P)]⁺ (Hppy = 2-phenylpyridine and P^P = diphosphino ligand) were also examined. The [Pt(thpy)(P^P)]⁺ complexes exhibit dual fluorescence-phosphorescence emission, with their ISC rate constants varied with the electronic characteristics of the auxiliary ligand: the more electron-donating ligand induces faster ISC from the S₁ excited state to the triplet manifold. Density functional theory (DFT)/time-dependent DFT calculations of k_ISC(S₁→T₂) at the optimized excited-state geometries give excellent quantitative agreement with the femtosecond time-resolved fluorescence measurements; it was revealed that the more electron-donating auxiliary ligand increases metal contributions to both occupied and virtual orbitals and decreases the energy gap of the coupling excited states, leading to a decrease in the activation energy and an increase in spin-orbit coupling. Furthermore, the ISC rate constants of [Pt(thpy)(P^P)]⁺ complexes are found to depend on the excitation wavelengths. The deviation from Kasha-Vavilov's rule upon photoexcitation at λ_exc < 350 nm is due to the ultrafast S₂ → T₂ and S₂ → T₃ ISCs, as demonstrated by the calculated τ_ISC < 100 fs, giving hints as to why S₂ → S₁ internal conversion (τ_IC ∼ ps) is not competitive with this hyper-ISC.

Aggregation-tuned dual emission of silole derivatives: synthesis, crystal structure, and photophysical properties

Novel silole derivatives are highly emissive in the microcrystalline state and the dual emission can be tuned by the aggregation states.

Synthesis, structure and density functional theory calculations of a novel photoluminescent trisarylborane-bismuth(III) complex

A novel trisarylborane-Bi(III) complex, tris(4-(dimesitylboryl)phenyl)bismuthine [Bi(PhBMes₂ )₃ ], in which (Ph = phenyl, and Mes = mesityl), was synthesized via the reaction of bismuth (III) chloride (BiCl₃ ) with three equivalents of lithiated (4-bromophenyl)- dimesitylborane [BrPhBMes₂ ]. The new trisarylbismuthine was characterized by elemental analysis, ultraviolet-visible (UV-vis) spectroscopy, and NMR (¹ H and ¹³ C) spectroscopy. The molecular structure of Bi(PhBMes₂ )₃ in the solid state was determined using single-crystal X-ray diffraction analysis, which showed short intermolecular C-H···H-C contact. The complex is a fluorescent emitter (λ_max  = 395 nm) at room temperature and a phosphorescent emitter (λ_max  = 423 nm) at 77 K, which displayed a long lifetime of 495 ms. The UV-vis transitions were investigated using density function theory (DFT) and time-dependent (TD)-DFT calculations. Natural bond orbital analysis showed that the bismuth (III) center was mainly Lewis acidic in nature.

Salen-indium/triarylborane triads: synthesis and ratiometric emission-colour changes by fluoride ion binding

Salen-based indium triads, [{(3-tBu)2-(5-Mes2B)2-salen}In-Me] (1) and [{(3-tBu)2-(5-Mes2Bphenyl)2-salen}In-Me] (2), bearing triarylborane (TAB) units were prepared and fully characterised by NMR spectroscopy and elemental analysis. The major absorption bands of 1 and 2 appeared in the region centred at 347 nm and 374 nm, respectively, and the intense emission spectra were observed in the sky blue (λem = 491 nm for 1) and bluish-green (λem = 498 nm for 2) regions, respectively. The solvatochromism effects in various organic solvents and computational calculation results strongly suggested that these absorption and emission features are mainly attributed to intramolecular charge transfer (ICT) transitions between the salen ligand moieties and the TAB units. Furthermore, UV-vis and photoluminescence (PL) titration experiments by the addition of fluoride anions demonstrated ratiometric quenching patterns in both the absorption and emission spectra, indicating that binding of the fluoride anion to the boron centres interrupts these ICT transitions in each compound. Interestingly, both triads exhibited a gradual red-shifted response in each emission spectrum upon the addition of the fluoride anions, resulting in a dramatic colour-change to yellow. The computational calculation results of the S1 states revealed that these emission-colour change properties arise from the elevation of HOMO levels, which are mainly localised on the TAB moieties, resulting from the fluoride anion binding to the borane centres.

Linking ReI and PtII Chromophores with Aminopyridines: A Simple Route to Achieve a Complicated Photophysical Behavior

The bifunctional aminopyridine ligands H₂ N-(CH₂ )_n -4-C₅ H₄ N (n=0, L1; 1, L2; 2, L3) have been utilized for the preparation of the rhenium complexes [Re(phen)(CO)₃ (L1-L3)]⁺ (1-3; phen=phenanthroline). Complexes 2 and 3 with NH₂ -coordinated L2 and L3, respectively, were coupled with cycloplatinated motifs {Pt(ppy)Cl} and {Pt(dpyb)}⁺ (ppy=2-phenylpyridine, dpyb=dipyridylbenzene) to give the bimetallic species [Re(phen)(CO)₃ (μ-L2/L3)Pt(ppy)Cl]⁺ (4, 6) and [Re(phen)(CO)₃ (μ-L2/L3)Pt(dpyb)]²⁺ (5, 7). In solution, complexes 4 and 6 show ³ MLCT {Re}-based emission at 298 K, which changes to the ³ IL(ppy) state at 77 K. The photophysical properties of compounds 5 and 7 display a pronounced concentration dependence, presumably due to the formation of bimolecular aggregates. Analysis of the spectroscopic data, combined with TD-DFT simulations, suggest that unconventional heteroleptic {Re(phen)}⋅⋅⋅{Pt(dpyb)} π-π stacking operates as the driving force for ground-state association. The latter, together with intra- and intermolecular energy-transfer processes, determines the appearance of multiple emission bands and results in nonlinear relaxation kinetics of the excited states.

Synthesis and Dual-Emission Feature of Salen-Al/Triarylborane Dyads

Novel salen-Al/triarylborane dyad complexes were prepared and characterized with their corresponding mononuclear compounds. The UV-vis and photoluminescence experiments for dyads exhibited photoinduced energy transfer from borane to the salen-Al moiety in an intramolecular manner. Theoretical calculation and fluoride titration results further supported these intramolecular energy-transfer features.

Dibenzothiophene-platinated complexes: probing the effect of ancillary ligands on the photophysical performance

Alteration of the ancillary mono- and bidentate ligands was used to influence the photoemission behaviour of a family of cyclometalated platinum complexes.

Triarylborane conjugated dicyanovinyl chromophores: intriguing optical properties and colorimetric anion discrimination

Three new triarylborane conjugated dicyanovinyl chromophores (Mes2B-π-donor-DCV); donor: N-methyldiphenylamine () and triphenylamine ( and [with two BMes2 substitutions]) of type A-D-A (acceptor-donor-acceptor) are reported. Compounds exhibit intense charge transfer (CT) absorption bands in the visible region. These absorption peaks are combination CT bands of the amine donor to both the BMes2 and DCV units. This inference was supported by theoretical studies. Compound shows weak fluorescence compared to and . The discrimination of fluoride and cyanide ions is essential in the case of triarylborane (TAB) based anion sensors as a similar response is given towards both the anions. Anion binding studies of , and showed that fluoride ions bind selectively to the boron centre and block the corresponding CT transition (donor to BMes2) leaving the other CT transition to be red shifted. On the other hand, cyanide ions bind with both the receptor sites and stop both the CT transition processes and hence a different colorimetric response was noted. The binding of F(-)/CN(-) induces colour changes in the visible region of the electronic spectra of and , which allows for the naked-eye detection of F(-) and CN(-) ions. The anion binding mechanisms are established using NMR titration experiments.

Lewis acidity enhancement of triarylborane by appended phosphine oxide groups

A series of mono-, di-, and tri-phosphine oxide substituted triarylboranes, Mes2BAr (1), MesBAr2 (2), and BAr3 (3) (Ar = 4-(Ph2PO)-2,6-Me2-C6H2) were prepared to investigate the effect of a phosphine oxide group (Ph2PO) on Lewis acidity enhancement of triarylboranes. The X-ray crystal structure of 3 revealed peripheral decoration of Ph2PO groups with a C3-axis perpendicular to the trigonal boron center. UV/Vis absorption and PL spectra indicated a significant contribution of π(Mes or phenylene) → pπ(B) charge transfer in the lower-energy electronic transition. The reduction potential measured by cyclic voltammetry showed apparent LUMO stabilization by introduction of phosphine oxide groups, the extent of which gradually increased with the increasing number of phosphine oxide groups. Lewis acidity enhancement was also supported by the gradual increase in fluoride ion affinity in the order 3 > 2 > 1. Theoretical calculations suggest that introduction of a Ph2PO group into a triarylborane significantly enhances the Lewis acidity of the boron center via an inductive electron-withdrawing effect and this effect is additive for multiple phosphine oxide groups.

An insight into fluorescent transition metal complexes

The emission from transition metal complexes is usually produced from triplet excited states. Owing to strong spin-orbit coupling (SOC), the fast conversion of singlet to triplet excited states via intersystem crossing (ISC) is facilitated. Hence, in transition metal complexes, emission from singlet excited states is not favoured. Nevertheless, a number of examples of transition metal complexes that fluoresce with high intensity have been found and some of them were even comprehensively studied. In general, three common photophysical characteristics are used for the identification of fluorescent emission from a transition metal complex: emission lifetimes on the nanosecond scale; a small Stokes shift; and intense emission under aerated conditions. For most of the complexes reviewed here, singlet emission is the result of ligand-based fluorescence, which is the dominant emission process due to poor metal-ligand interactions leading to a small metal contribution in the excited states, and a competitive fluorescence rate constant when compared to the ISC rate constant. In addition to the pure fluorescence from metal complexes, another two types of fluorescent emissions were also reviewed, namely, delayed fluorescence and fluorescence-phosphorescence dual emissions. Both emissions also have their respective unique characteristics, and thus they are discussed in this perspective.

Luminescent copper(i) halide and pseudohalide phenanthroline complexes revisited: simple structures, complicated excited state behavior

Despite their chemical simplicity, copper(i) phenanthroline halides appear to involve multiple states in the emission process and exhibit non-trivial photophysical properties.

Through space charge-transfer emission in lambda (Λ)-shaped triarylboranes and the use in fluorescent sensing for fluoride and cyanide ions

By introducing a twisted and non-conjugated Λ-shaped TB scaffold to triarylboranes, we provide an efficient strategy to develop a new class of organoboron compounds applied as colorimetric and ratiometric fluorescent sensors for fluoride and cyanide.

Computational insights into the photodeactivation dynamics of phosphors for OLEDs: a perspective

In this perspective we highlight recent computational efforts to unravel competing photodeactivation mechanisms of radiative and non-radiative nature of phosphors.

Synthetic control of spectroscopic and photophysical properties of triarylborane derivatives having peripheral electron-donating groups

The spectroscopic and photophysical properties of triarylborane derivatives were controlled by the nature of the triarylborane core (trixylyl- or trianthrylborane) and peripheral electron-donating groups (N,N-diphenylamino or 9H-carbazolyl groups). The triarylborane derivatives with and without the electron-donating groups showed intramolecular charge-transfer absorption/fluorescence transitions between the π orbital of the aryl group (π(aryl)) and the vacant p orbital on the boron atom (p(B), π(aryl)-p(B) CT), and the fluorescence color was tunable from blue to red by the combination of peripheral electron-donating groups and a triarylborane core. Detailed electrochemical, spectroscopic, and photophysical studies of the derivatives, including solvent dependences of the spectroscopic and photophysical properties, demonstrated that the HOMO and LUMO of each derivative were determined primarily by the nature of the peripheral electron-donating group and the triarylborane core, respectively. The effects of solvent polarity on the fluorescence quantum yield and lifetime of the derivatives were also tunable by the choice of the triarylborane core.

Novel fluorescent probes based on intramolecular charge- and proton-transfer compounds

Excited-state intramolecular proton-transfer (ESIPT) compounds and intramolecular charge-transfer (ICT) compounds have attracted attention due to their interesting and even unique emission properties. The intense and environment-sensitive emission showed by some members of the two families has been exploited in fluorescent probes for various forms of environmental sensing. In this paper, we summarize our recent study on the utilization of novel ESIPT and ICT compounds with intense emission as fluorescent probes after an introduction to the ESIPT and/or ICT processes and related photophysics mechanism.