Naphthalimide phosphorescence finally exposed in a platinum(II) diimine complex

Unveiling Quantum Coherence Effects in Modulating Electron Transfer in Platinum (II) Donor-Acceptor-Donor Systems

Quantum coherence effects (QCEs), arising from the interference of wave-like amplitudes, are crucial in controlling the electron transfer function of molecular systems. Here, we report a coherence phenomenon associated with charge separation (CS) in a range of Pt (II) cis-acetylide donor-acceptor-donor (D-A-D) systems, where the photogenerated Pt (III) center acts as an acceptor connecting two (R)phenothiazine (R = H or tBu) donors. Femtosecond transient absorption spectroscopy revealed that CS rates in D-A-D systems with double CS paths were accelerated by 4-8 times compared to their donor-acceptor (D-A) counterparts with a single path. An enhancement factor of 2-3 in electronic coupling, within the context of interference between CS paths, is derived, providing a clear signature of QCEs. This enhancementin CS processes closely correlates with the strength of coupling between donors. This study highlights the significant impact of QCEs on the photophysical properties of molecular systems and offers insights into charge and energy transport mechanisms in both natural and artificial systems.

Naphthalimide-Modified Clusters for Red-Emitting Devices with High Color Purity

Conventional fluorescent materials frequently exhibit narrow-band emissions with a small full width at half-maximum (fwhm) due to localized-state characteristics, but electroluminescence is less efficient owing to the utilization of only singlet excitons. In this work, taking advantage of naphthalimide (NAI)-acetylide derivatives with a rigid planar structure and localized transition characteristics, we elaborately designed two mononuclear Pt(II) complexes with weak double emissions of fluorescence and phosphorescence. Taking them as synthetic precursors, we prepared three PtAu2 heteronuclear clusters and successfully attained highly efficient narrow-band red phosphorescence with the fwhm below 30 nm. Both theoretical and experimental results suggest that the phosphorescence of PtAu2 clusters mainly originates from the naphthalimide-localized 3IL (intraligand) triplet state. Solution-processed organic light-emitting diodes (OLEDs) achieved highly efficient narrow-band red electroluminescence with an external quantum efficiency (EQE) of 16.7%. The CIE coordinates of the electroluminescence (0.69, 0.31) closely match the standard red emission for ultrahigh-definition display.

The effect of thionation of the carbonyl group on the photophysics of compact spiro rhodamine-naphthalimide electron donor-acceptor dyads: intersystem crossing, charge separation, and electron spin dynamics

Herein, a spiro rhodamine (Rho)-thionated naphthalimide (NIS) electron donor-acceptor orthogonal dyad (Rho-NIS) was prepared to study the formation of a long-lived charge separation (CS) state via the electron spin control approach. The transient absorption (TA) spectra of Rho-NIS indicated that the intersystem crossing (ISC) occurs within 7-42 ps to produce the 3NIS state via the spin orbit coupling ISC (SOC-ISC). The energy order of 3CS (2.01 eV in n-hexane, HEX) and 3LE states (1.68 eV in HEX) depended on the solvent polarity. The 3NIS state having n-π* character and a lifetime of 0.38 μs was observed for Rho-NIS in toluene (TOL). Alternatively, in acetonitrile (ACN), the long-lived 3CS state (0.21 μs) with a high CS state quantum yield (ΦCS, 97%) was produced with the 3NIS state as the precursor and the CS took 134 ps. On the contrary, in the case of the reference Rho-naphthalimide (NI) Rho-NI dyad without thionation of its carbonyl group, a long-lived CS state (0.94 μs) with a high energy level (ECS = 2.12 eV) was generated even in HEX with a lower ΦCS (49%). In the presence of an acid, the Rho unit in the Rho-NIS adopted an open form (Rho-o) and the 3NIS state was produced within 24-47 ps with the 1Rho-o state as the precursor. Subsequently, slow intramolecular triplet-triplet energy transfer (TTET, 0.11-0.60 μs) produced the 3Rho-o state (9.4-13.6 μs). According to the time-resolved electron paramagnetic resonance (TREPR) spectra of NIS-NH2, the zero-field splitting (ZFS) parameter |D| and E of the triplet state were determined to be 6165 MHz and -1233 MHz, respectively, indicating that its triplet state has significant nπ* character, which was supported by its short triplet state lifetime (6.1 μs).

Alkynylphosphonium Pt(II) Complexes: Synthesis, Characterization, and Features of Photophysical Properties in Solution and in the Solid State

A series of heteroleptic bis-alkynyl-diimine mononuclear Pt(II) complexes with alkynylphosphonium and di-tert-butyl-2,2'-bipyridine (dtbpy) ligands have been prepared and characterized by spectroscopic methods and single-crystal XRD. The Pt(II) complexes obtained in the present study demonstrate triplet emission in solution, which originates from 3MLCT/3LC states where the nature of the π-conjugated linker in the alkynylphosphonium ligand manages the contributions of each transition, and this conclusion is supported by DFT calculations. Additionally, the presence of the phosphonium group connected to alkynyl through the π-conjugated linker enhances nonlinear optical properties of the Pt(II) complexes increasing two-photon absorption cross section up to 400 GM. In the solid state, the Pt(II) complexes demonstrate emission that is attributed to 3MMLCT transitions due to the presence of Pt-Pt metallophilic interactions, and the reversible assembly and disassembly of these interactions by grinding and solvent treatment are responsible for the mechanochromic luminescence. It has been experimentally shown that stimuli-responsive emission of the Pt(II) complexes is the result of a "monomer/dimer" transformation; this conclusion is confirmed by DFT calculations for discrete complexes and different dimers with or without Pt-Pt interactions.

Luminescent Materials for Volumetric Three-Dimensional Displays Based on Photoactivated Phosphorescence

True three-dimensional (3D) displays are the best display technologies and their breakthrough is primarily due to advancements in display media. In this paper, we propose two luminescent materials for a static volumetric 3D display based on photoactivated phosphorescence. The luminescent materials include (1) dimethyl sulfoxide (DMSO)/1-methyl-2-pyrrolidinone (NMP) or tetramethylene sulfoxide (TMSO) as the solvent and photochemically-deoxygenating reagent; (2) a metal phthalocyanine complex as the sensitizer; (3) a phosphorescent platinum complex as the emitter. The metal phthalocyanine complex, PdPrPc (PdBuPc), absorbs the light beam of 635 nm and the solvent scavenges the sensitized singlet oxygen. Light beams pass through a deoxygenated zone. The phosphorescent emitter, PtNI, absorbs the 440 nm light beam and phosphoresces only in the deoxygenated zone generated by the sensitizer. Phosphorescent voxels and high-contrast 3D images are well-defined at the intersection of 635 and 440 nm light beams.

Platinum(II) diimine complexes containing phenylpyridine ligands decorated with anionic closo-monocarborane clusters [CB11H12]. Dalton Trans 2023;52:3249-3253. [PMID: 36852922 DOI: 10.1039/d3dt00136a] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

We report three Pt(II) diimine complexes containing ancillary ligands of phenylpyridine furnished with anionic closo-monocarborane clusters [CB₁₁H₁₂]^-. Three neutral complexes exhibit intensive phosphorescence in the solid state and complex 1 was used to detect acetonitrile vapor in a quartz plate.

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.

Platinum-Based Two-Photon Photosensitizer Responsive to NIR Light in Tumor Hypoxia Microenvironment

Platinum-based photosensitizers are promising anticancer agents in photodynamic therapy. The cytotoxic effects primarily arise from the production of singlet oxygen and platination of DNA. However, their efficacy is limited by drug resistance and hypoxic tumor microenvironment. A naphthalimide-modified cyclometalated platinum(II) complex PtPAN [PA = N-(2-(diethylamino)ethyl)picolinamide, N = N-(2'-ethylhexyl)-4-ethynyl-1,8-naphthalimide] is designed to conquer these problems. PtPAN generates ROS efficiently under both normoxia and hypoxia. It does not interact with DNA and shows low cytotoxicity in the dark, while it kills tumor cells via ROS under near-infrared light irradiation; moreover, it inhibits tumor growth in mice at a low light dose with negligible side effects. PtPAN is the first reported platinum-based photosensitizer that is unreactive to DNA in the dark but highly cytotoxic upon near-infrared (NIR) irradiation for oxygen-independent photodynamic therapy. Owing to its two-photon excitation property (λ = 825 nm), PtPAN may be suitable for the treatment of deep solid tumors.

Boosting Sulfides Photooxidation by Fusing Naphthalimide and Flavin together

Efficient and selective photocatalytic conversion of chemicals with visible light and naturally abundant resources has long been desired, but this requires finely designed sensitizers that are capable to convert light...

Ru(ii) and Ir(iii) phenanthroline-based photosensitisers bearing o-carborane: PDT agents with boron carriers for potential BNCT

Four novel transition metal-carborane photosensitisers were prepared by Sonogashira cross-coupling of 1-(4-ethynylbenzyl)-2-methyl-o-carborane (A-CB) with halogenated Ru(ii)- or Ir(iii)-phenanthroline complexes. The resulting boron-rich complexes with one (RuCB and IrCB) or two carborane cages (RuCB2 and IrCB2) were spectroscopically characterised, and their photophysical properties investigated. RuCB displayed the most attractive photophysical properties in solution (λ_em 635 nm, τ_T 2.53 μs, and φ_p 20.4%). Nanosecond time-resolved transient absorption studies were used to explore the ³MLCT nature of the triplet excited states, and the highest singlet oxygen quantum yields (Φ_Δ) were obtained for the mono-carborane-phenanthroline complexes (RuCB: 52% and IrCB: 25%). None of the complexes produce dark toxicity in SKBR-3 cells after incubation under photodynamic therapy (PDT) conditions. Remarkably, mono-carboranes RuCB and IrCB were the best internalised by the SKBR-3 cells, demonstrating the first examples of tris-bidentate transition metal-carborane complexes acting as triplet photosensitisers for PDT with a high photoactivity; RuCB or IrCB killed ∼50% of SKBR-3 cells at 10 μM after irradiation. Therefore, the high-boron content and the photoactive properties of these photosensitisers make them potential candidates as dual anti-cancer agents for PDT and Boron Neutron Capture Therapy (BNCT).

Structure illumination microscopy imaging of lipid vesicles in live bacteria with naphthalimide-appended organometallic complexes

There is a lack of molecular probes for imaging bacteria, in comparison to the array of such tools available for the imaging of mammalian cells. Here, organometallic molecular probes have been developed and assessed for bacterial imaging, designed to have the potential to support multiple imaging modalities. The chemical structure of the probes is designed around a metal-naphthalimide structure. The 4-amino-1,8-naphthalimide moiety, covalently appended through a pyridine ancillary ligand, acts as a luminescent probe for super-resolution microscopy. On the other hand, the metal centre, rhenium(i) or platinum(ii) in the current study, enables techniques such as nanoSIMS. While the rhenium(i) complex was not sufficiently stable to be used as a probe, the platinum(ii) analogue showed good chemical and biological stability. Structured illumination microscopy (SIM) imaging on live Bacillus cereus confirmed the suitability of the probe for super-resolution microscopy. NanoSIMS analysis was used to monitor the uptake of the platinum(ii) complex within the bacteria and demonstrate the potential of this chemical architecture to enable multimodal imaging. The successful combination of these two moieties introduces a platform that could lead to a versatile range of multi-functional probes for bacteria.

Using Room Temperature Phosphorescence of Gold(I) Complexes for PAHs Sensing

The synthesis of two new phosphane-gold(I)–napthalimide complexes has been performed and characterized. The compounds present luminescent properties with denoted room temperature phosphorescence (RTP) induced by the proximity of the gold(I) heavy atom that favors intersystem crossing and triplet state population. The emissive properties of the compounds together with the planarity of their chromophore were used to investigate their potential as hosts in the molecular recognition of different polycyclic aromatic hydrocarbons (PAHs). Naphthalene, anthracene, phenanthrene, and pyrene were chosen to evaluate how the size and electronic properties can affect the host:guest interactions. Stronger affinity has been detected through emission titrations for the PAHs with extended aromaticity (anthracene and pyrene) and the results have been supported by DFT calculation studies.

Highly photoluminescent poly(norbornene)s carrying platinum–acetylide complex moieties in their side chains: evaluation of oxygen sensing and TTA–UC

Poly(norbornene)s carrying platinum–acetylide complex moieties change their photoluminescence colors in response to oxygen. The polymers serve as excellent sensitizers of TTA–UC with 9,10-diphenylanthracene.

Improving photosensitization for photochemical CO2-to-CO conversion

Inspired by nature, improving photosensitization represents a vital direction for the development of artificial photosynthesis. The sensitization ability of photosensitizers (PSs) reflects in their electron-transfer ability, which highly depends on their excited-state lifetime and redox potential. Herein, for the first time, we put forward a facile strategy to improve sensitizing ability via finely tuning the excited state of Ru(II)-PSs (Ru-1–Ru-4) for efficient CO₂ reduction. Remarkably, [Ru(Phen)₂(3-pyrenylPhen)]²⁺ (Ru-3) exhibits the best sensitizing ability among Ru-1–Ru-4, over 17 times higher than that of typical Ru(Phen)₃²⁺. It can efficiently sensitize a dinuclear cobalt catalyst for CO₂-to-CO conversion with a maximum turnover number of 66 480. Systematic investigations demonstrate that its long-lived excited state and suitable redox driving force greatly contributed to this superior sensitizing ability. This work provides a new insight into dramatically boosting photocatalytic CO₂ reduction via improving photosensitization.

Cyclometalated Ir(iii) complexes towards blue-emissive dopant for organic light-emitting diodes: fundamentals of photophysics and designing strategies

The fundamental photophysics of cyclometalated Ir(iii) complexes and surveys design strategies for efficient blue phosphorescent Ir(iii) complexes are summarised.

AIPE-active Pt(ii) complexes with a tunable triplet excited state: design, mechanochromism and application in anti-counterfeiting

A series of AIPE-active Pt(ii) complexes exhibit tunable triplet excited state properties, mechanochromic behavior and potential application in anti-counterfeiting.

The effects of extended π-conjugation in bipyridyl ligands on the tunable photophysics, triplet excited state and optical limiting properties of Pt(ii) naphthalimidyl acetylide complexes

Pt(ii) complexes that exhibit long-lived triplet excited state lifetimes are promising for optical power limiting materials. The introduction of large π-conjugated substituents can switch the triplet excited state to a long-lived 3π,π* state. Herein, we report four Pt(ii) diimine complexes with high π-conjugation via inserting an aryl group on the diimine ligand. Their photophysical properties were investigated using spectroscopic techniques. All the complexes exhibit strong ground absorption bands in their UV-Vis absorption spectra (maximum peaks ranging from 370 to 530 nm) and long-lived emission and triplet excited states. The insertion of π-conjugated substituents induces a pronounced red-shift in the ground state absorption and longer emission lifetime. Broadband transient absorption spectra in the visible-NIR region and Z-scan properties under 532 nm were carried out on the Pt(ii) diimine complexes, resulting in a remarkably strong reverse saturable absorption at 532 nm for nanosecond laser pulses. Otherwise, the high π-conjugation in the bipyridyl ligand increases the reverse saturable absorption. Therefore, these Pt(ii) diimine complexes with high π-conjugation are excellent candidates for devices that require strong reverse saturable absorption.

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.

Palladium(ii) and platinum(ii) complexes of glyoxalbis(N-aryl)osazone: molecular and electronic structures, anti-microbial activities and DNA-binding study

A new family of palladium(ii) and platinum(ii) complexes of redox non-innocent osazone ligands that exhibit moderate antileishmanial activity were isolated.

Long-lived triplet excited state in a platinum(ii) perylene monoimide complex

We report the synthesis and solution based photophysical properties of a new Pt(ii)-terpyridine complex coupled to a perylene monoimide (PMI) chromophoric unit through an acetylene linkage. This structural arrangement resulted in quantitative quenching of the highly fluorescent PMI chromophore by introducing metal character into the lowest energy singlet state, thereby leading to the formation of a long-lived PMI-ligand localized triplet excited state (τ = 8.4 μs). Even though the phosphorescence from this triplet state was not observed, highly efficient quenching of this excited state by dissolved oxygen and the observation of singlet oxygen photoluminescence in the near-IR at 1270 nm initially pointed towards triplet excited state character. Additionally, the coincidence of the excited state absorbance difference spectra from the sensitized PMI ligand using a triplet donor and the Pt-PMI complex provided strong evidence for this triplet state assignment, which was further supported by TD-DFT calculations.

Rise of Conjugated Poly-ynes and Poly(Metalla-ynes): From Design Through Synthesis to Structure-Property Relationships and Applications

Conjugated poly-ynes and poly(metalla-ynes) constitute an important class of new materials with potential application in various domains of science. The key factors responsible for the diverse usage of these materials is their intriguing and tunable chemical and photophysical properties. This review highlights fascinating advances made in the field of conjugated organic poly-ynes and poly(metalla-ynes) incorporating group 4-11 metals. This includes several important aspects of conjugated poly-ynes viz. synthetic protocols, bonding, electronic structure, nature of luminescence, structure-property relationships, diverse applications, and concluding remarks. Furthermore, we delineated the future directions and challenges in this particular area of research.

Cyclometalated N-heterocyclic carbene iridium(iii) complexes with naphthalimide chromophores: a novel class of phosphorescent heteroleptic compounds

A series of cyclometalated N-heterocyclic carbene complexes of the general formula [Ir(C^N)₂(C^C:)] has been prepared. Two sets of compounds were designed, those where (C^C:) represents a bidentate naphthalimide-substituted imidazolylidene ligand and (C^N) = ppy (3a), F2ppy (4a), bzq (5a) and those where (C^C:) represents a naphthalimide-substituted benzimidazolylidene ligand and (C^N) = ppy (3b), F2ppy (4b), bzq (5b). The naphthalimide-imidazole and naphthalimide-benzimidazole ligands 1a,b and the related imidazolium and benzimidazolium salts 2a,b were also prepared and fully characterized. The N-heterocyclic carbene Ir(iii) complexes have been characterized by NMR spectroscopy, cyclic voltammetry and elemental analysis. Moreover, the molecular structures of one imidazolium salt and four Ir(iii) complexes were determined by single-crystal X-ray diffraction. The structures provide us with valuable information, most notably the orientation of the naphthalimide chromophore with respect to the N-heterocyclic carbene moiety. All compounds are luminescent at room temperature and in a frozen solvent at 77 K, exhibiting a broad emission band that extends beyond 700 nm. The presence of the naphthalimide moiety changes the character of the lowest excited state from ³MLCT to ³LC, as corroborated by DFT and TD-DFT calculations. Remarkably, replacing imidazole with a benzimidazole unit improves the quantum yields of these compounds by decreasing the k_nr values which is an important feature for optimized emission performance. These studies provide valuable insights about a novel class of N-heterocyclic carbene-based luminescent complexes containing organic chromophores and affording metal complexes emitting across the red-NIR range.

Synthesis and photophysical properties of ruthenium(ii) polyimine complexes decorated with flavin

Phosphorescent emission from a flavin localized triplet excited state (³IL) is observed for the first time in a flavin decorated tris(dipyridine) Ru(ii) complex with strong visible light absorption.

Effect of thiophene substitution on the intersystem crossing of arene photosensitizers

Thiophene substitution gives energy level-matched S₁/T₂ states and the ISC is enhanced, which was not observed with phenyl substitution.

Triboluminescence and Metal Phosphor for Organic Light-Emitting Diodes: Functional Pt(II) Complexes with Both 2-Pyridylimidazol-2-ylidene and Bipyrazolate Chelates

We report the utilization of both pyrid-2-yl-imidazolylidene and dianionic bipz chelates as constituents in syntheses of a new series of charge-neutral Pt(II) complexes 1-4, among which complex 4 revealed remarkable triboluminescence, i.e., generation of photoemission upon grinding or cracking of the solid sample. The triboluminescence is found to be sensitive to the subtle changes of the associated substituents of pyrid-2-yl-imidazolylidene chelate, as verified by the disappearance of the triboluminescence for complexes 1-3. Alternatively, the well-ordered solid packing of 3, as indicated by the grazing incidence X-ray scattering experiment, serves as an ideal emitter for the fabrication of highly efficient OLEDs, rendering high external quantum efficienciy (25.9%) and luminesce efficiency (90 cd A^-1) at the practical brightness of 100 cd m^-2. The rather low roll-off in efficiency (24.4%, 85 cd A^-1 at high brightness of 1000 cd m^-2) is attributed to the short excited-state lifetime of 3 (∼800 ns) in the solid state, which in turn is associated with the MMLCT transition character.

Versatile Room-Temperature-Phosphorescent Materials Prepared from N-Substituted Naphthalimides: Emission Enhancement and Chemical Conjugation

Purely organic materials with room-temperature phosphorescence (RTP) are currently under intense investigation because of their potential applications in sensing, imaging, and displaying. Inspired by certain organometallic systems, where ligand-localized phosphorescence ((3) π-π*) is mediated by ligand-to-metal or metal-to-ligand charge transfer (CT) states, we now show that donor-to-acceptor CT states from the same organic molecule can also mediate π-localized RTP. In the model system of N-substituted naphthalimides (NNIs), the relatively large energy gap between the NNI-localized (1) π-π* and (3) π-π* states of the aromatic ring can be bridged by intramolecular CT states when the NNI is chemically modified with an electron donor. These NNI-based RTP materials can be easily conjugated to both synthetic and natural macromolecules, which can be used for RTP microscopy.

Conformation controlled turn on–turn off phosphorescence in a metal-free biluminophore: thriving the paradox that exists for organic compounds

The dual state intense emission, both fluorescence and phosphorescence, of CBIQD by way of conformational regulation–deregulation.

Accessing the Long-Lived Triplet Excited States in Transition-Metal Complexes: Molecular Design Rationales and Applications

Transition-metal complex triplet photosensitizers are versatile compounds that have been widely used in photocatalysis, photovoltaics, photodynamic therapy (PDT) and triplet-triplet annihilation (TTA) upconversion. The principal photophysical processes in these applications are the intermolecular energy transfer or electron transfer. One of the major challenges facing these triplet photosensitizers is the short triplet-state lifetime, which is detrimental to the above-mentioned photophysical processes. In order to address this challenge, transition-metal complexes showing long-lived triplet excited states are highly desired. This review article summarizes the development of this fascinating area, including the molecular design rationales, the principal photophysical properties, and the applications of these complexes in PDT and TTA upconversion.

Heteroleptic cationic iridium(III) complexes bearing naphthalimidyl substituents: synthesis, photophysics and reverse saturable absorption

Three heteroleptic cationic iridium(iii) complexes containing a cyclometalating 2-[3-(7-naphthalimidylfluoren-2'-yl)phenyl]pyridine ligand and different diimine (N^N) ligands (N^N = 2,2'-bipyridine (bpy, ), 1,10-phenanthroline (phen, ), and 5,5'-bis[7-(benzothiazol-2'-yl)fluoren-2'-yl]-2,2'-bipyridine (BTF-bpy, )) were synthesized and characterized. The photophysics of these complexes was systematically investigated via spectroscopic methods and by time-dependent density functional theory (TDDFT). All complexes possess a very weak charge-transfer tail at ca. 450-570 nm; and two intense absorption bands in the region of 290-350 nm and 350-450 nm, respectively. The emission of in CH2Cl2 emanates predominantly from the C^N ligand-localized (3)π,π* state. These emitting excited states also give rise to broadband triplet excited-state absorption in the visible to the near-IR region (i.e. 420-800 nm for and , and 460-800 nm for ). The kinetics of fs transient absorption (TA) reveals that the lowest singlet excited-state lifetimes of these complexes vary from 1.43 ps to 142 ps. The stronger excited-state absorption of compared to their respective ground-state absorption in the visible spectral range leads to strong reverse saturable absorption (RSA) at 532 nm for ns laser pulses. The trend of transmission signal decrease follows > > . Extending the π-conjugation of the N^N ligand increases the strength of RSA. In addition, the naphthalimidyl (NI) substitution at the cyclometalating ligand dramatically increases the triplet excited-state lifetimes and broadens the triplet excited-state absorption to the NIR region compared to the respective Ir(iii) complexes with a benzothiazolyl substituent on the cyclometalating ligand.

The effect of the regioisomeric naphthalimide acetylide ligands on the photophysical properties of N^N Pt(II) bisacetylide complexes

Two N^N Pt(II) bis(acetylide) complexes Pt-1 and Pt-2 with regioisomeric amino NI acetylide ligands (L-1 and L-2, L-1 = 5-amino-4-ethylnaphthaleneimide; L-2 = 3-amino-4-ethylnaphthaleneimide) were prepared. The photophysical properties of the complexes were studied by steady state and time-resolved spectroscopy. The two complexes with regioisomeric ligands (Pt-1 and Pt-2) show different photophysical properties such as maximal absorption wavelength (485 nm vs. 465 nm), triplet excited state lifetimes (23.7 μs vs. 0.9 μs), and different solvent-polarity dependences of the emission properties. The absorption of the complexes is red-shifted as compared with the previously reported Pt(II) complex containing the 4-ethylnaphthaleneimide ligand. The two complexes with regioisomeric NI ligands were used as triplet photosensitizers for triplet-triplet annihilation (TTA) upconversion; drastically different upconversion quantum yields (15.0% vs. 1.1%) were observed. Our results are useful for designing new visible light-harvesting Pt(II) bisacetylide complexes as triplet photosensitizers which can be used in areas such as photocatalysis, photodynamic therapy and TTA upconversion.

Triangular platinum(II) metallacycles: syntheses, photophysics, and nonlinear optics

Three triangular platinum(II) diimine metallacycles incorporating large cyclic oligo(phenylene-ethynylene) (OPE) bisacetylide ligands are synthesized, and their photophysical properties are studied. Two types of triplet excited states with ligand/metal-to-ligand charge-transfer and acetylide-ligand-centered characteristics respectively, are exhibited by these complexes depending on the size (conjugation length) and electronic features of the cyclic OPE ligands. When the energy levels of the two excited states are close to each other, the lowest triplet state is found to switch between the two in varied solvents, resulting from their relative energy inversion induced by solvent polarity change. Density functional theory and time-dependent density functional theory calculations provide corroborative evidence for such experimental conclusions. More importantly, the designed metallacycles show impressive two-photon absorption (2PA) and two-photon excitation phosphorescing abilities, and the 2PA cross section reaches 1020 GM at 680 nm and 670 GM at 1040 nm by two different metallacycles. Additionally, pronounced reverse saturable absorptions are observed with these metallacycles by virtue of their strong transient triplet-state absorptions.