Radical-Enhanced Intersystem Crossing in a Bay-Substituted Perylene Bisimide−TEMPO Dyad and the Electron Spin Polarization Dynamics upon Photoexcitation**

A 4-amino-2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radical was attached to the bay position of perylene-3,4 : 9,10-bis(dicarboximide) (perylenebisimide, PBI) to study the radical-enhanced intersystem crossing (REISC) and electron spin dynamics of the photo-induced high-spin states. The dyads give strong visible light absorption (ϵ=27000 M⁻¹ cm⁻¹at 607 nm). Attaching a TEMPO radical to the PBI unit transforms the otherwise non-radiative decay of S₁ state (fluorescence quantum yield: Φ_F=2.9 %) of PBI unit to ISC (singlet oxygen quantum yield: Φ_Δ=31.8 %, Φ_F=1.6 %). Moreover, the REISC is more efficient as compared to the heavy atom effect-induced ISC (Φ_Δ=17.8 % for 1,8-dibromoPBI). For the dyad, ISC takes 245 ps and triplet state lifetime is 1.5 μs, much shorter than the native PBI (τ_T=126.6 μs). X- and Q-band time-resolved electron paramagnetic resonance spectroscopy shows that the exchange interaction in the photoexcited radical-chromophore dyad is larger than the triplet zero-field splitting (ZFS) and the difference of Zeeman energies of the radical and chromophore. The inversion of electron spin polarization from emissive to absorptive was observed and attributed to the initial completion of the quartet state population and the subsequent depopulation processes induced by the zero-field splitting.     

Long-lived Triplet Excited State of Perylenemonoimide (PMI) in a Diimine Pt(II) Bis-acetylide Complex: Preparation and Study of the Photophysical Property with Transient Spectra

We prepared a N^N Pt(II) bisacetylide complex that has strong absorption of visible light (molar absorption coefficients ϵ=6.7×10⁴ M⁻¹ cm⁻¹ at 570 nm), and the singlet oxygen quantum yield (Φ_Δ) is up to 78 %. Femtosecond transient absorption spectra show the intersystem crossing (ISC) of the complex takes 81.8 ps, nanosecond transient absorption spectra show the triplet excited state lifetime is 7.6 μs. Density functional theory (DFT) computation demonstrated that the S₁ and T₁ states are mainly localized on the perylenemonoimide (PMI) ligands, although the involvement of the Pt(II) centre is noticeable. The complex was used as triplet photosensitizer to generate delayed fluorescence with perylenebisimide (PBI) as the triplet state energy acceptor and emitter, via the intermolecular triplet-triplet energy transfer (TTET) and triplet-triplet annihilation (TTA), the delayed fluorescence lifetime is up to 52.5 μs under the experimental conditions.     

Coordination-Driven Selective Formation of D2 Symmetric Octanuclear Organometallic Cages

The coordination-driven self-assembly of organometallic half-sandwich iridium(III)- and rhodium(III)-based building blocks with asymmetric ambidentate pyridyl-carboxylate ligands is described. Despite the potential for obtaining a statistical mixture of multiple products, D₂ symmetric octanuclear cages were formed selectively by taking advantage of the electronic effects emanating from the two types of chelating sites – (O,O’) and (N,N’) – on the tetranuclear building blocks. The metal sources and the lengths of bridging ligands influence the selectivity of the self-assembly. Experimental observations, supported by computational studies, suggest that the D₂ symmetric cages are the thermodynamically favored products. Overall, the results underline the importance of electronic effects on the selectivity of coordination-driven self-assembly, and demonstrate that asymmetric ambidentate ligands can be used to control the design of discrete supramolecular coordination complexes.     

Self-Sorting Supramolecular Polymerization: Helical and Lamellar Aggregates of Tetra-Bay-Acyloxy Perylene Bisimide

A new perylene bisimide (PBI), with a fluorescence quantum yield up to unity, self-assembles into two polymorphic supramolecular polymers. This PBI bears four solubilizing acyloxy substituents at the bay positions and is unsubstituted at the imide position, thereby allowing hydrogen-bond-directed self-assembly in nonpolar solvents. The formation of the polymorphs is controlled by the cooling rate of hot monomer solutions. They show distinctive absorption profiles and morphologies and can be isolated in different polymorphic liquid-crystalline states. The interchromophoric arrangement causing the spectral features was elucidated, revealing the formation of columnar and lamellar phases, which are formed by either homo- or heterochiral self-assembly, respectively, of the atropoenantiomeric PBIs. Kinetic studies reveal a narcissistic self-sorting process upon fast cooling, and that the transformation into the heterochiral (racemic) sheetlike self-assemblies proceeds by dissociation via the monomeric state.     

Efficient singlet oxygen generation by luminescent 2-(2′-thienyl)pyridyl cyclometalated platinum(II) complexes and their calixarene derivatives

Luminescent cyclometalated platinum(II) complexes, namely [Pt(Thpy)(PPh₃)X]ⁿ⁺ (HThpy = 2-(2′-thienyl)pyridine; X = Cl⁻ ( 1 ), n = 0; X = CH₃CN ( 2 ), pyridine ( 3 ), n = 1) and [Pt(Thpy)(HThpy)Y] ^{n +} (Y = Cl⁻ ( 4 ), n = 0; Y = pyridine ( 5 ), n = 1), exhibit structured emission with peak maximum at ∼556 and 598 nm in degassed acetonitrile and with emission quantum yield and lifetime of up to 0.38 and 26 μs, respectively. These complexes are efficient photosensitizers of singlet oxygen with yields up to >90%. Complex 5 exhibited photocytotoxicity towards cancer cells and fluorescence microscopic images of cells incubated with 5 reveal substantial uptake at the nucleus and mitochondria.     

Radical-Enhanced Intersystem Crossing in Perylene-Oxoverdazyl Radical Dyads

Attaching stable radicals to organic chromophores is an effective method to enhance the intersystem crossing (ISC) of the chromophores. Herein we prepared perylene-oxoverdazyl dyads either by directly connecting the two units or using an intervening phenyl spacer. We investigated the effect of the radical on the photophysical properties of perylene and observed strong fluorescence quenching due to radical enhanced ISC (REISC). Compared with a previously reported perylene-fused nitroxide radical compound (triplet lifetime, τ_T=0.1 μs), these new adducts show a longer-lived triplet excited state (τ_T=9.5 μs). Based on the singlet oxygen quantum yield (Φ_Δ=7 %) and study of the triplet state, we propose that the radical enhanced internal conversion also plays a role in the relaxation of the excited state. Femtosecond fluorescence up-conversion indicates a fast decay of the excited state (<1.0 ps), suggesting a strong spin-spin exchange interaction between the two units. Femtosecond transient absorption (fs-TA) spectra confirmed direct triplet state population (within 0.5 ps). Interestingly, by fs-TA spectra, we observed the interconversion of the two states (D₁↔Q₁) at ∼80 ps time scale. Time-resolved electron paramagnetic resonance (TREPR) spectral study confirmed the formation of the quartet sate. We observed triplet and quartet states simultaneously with weights of 0.7 and 0.3, respectively. This is attributed to two different conformations of the molecule at excited state. DFT computations showed that the interaction between the radical and the chromophore is ferromagnetic (J>0, 0.05∼0.10 eV).     

Construction of Supramolecular Polymers with Different Topologies by Orthogonal Self-Assembly of Cryptand–Paraquat Recognition and Metal Coordination

Recently, metal-coordinated orthogonal self-assembly has been used as a feasible and efficient method in the construction of polymeric materials, which can also provide supramolecular self-assembly complexes with different topologies. Herein, a cryptand with a rigid pyridyl group on the third arm derived from BMP32C10 was synthesized. Through coordination-driven self-assembly with a bidentate organoplatinum(II) acceptor or tetradentate Pd(BF₄)₂•4CH₃CN, a di-cryptand complex and tetra-cryptand complex were prepared, respectively. Subsequently, through the addition of a di-paraquat guest, linear and cross-linked supramolecular polymers were constructed through orthogonal self-assembly, respectively. By comparing their proton nuclear magnetic resonance (¹H NMR) and diffusion-ordered spectroscopy (DOSY) spectra, it was found that the degrees of polymerization were dependent not only on the concentrations of the monomers but also on the topologies of the supramolecular polymers.     

Solvent-Dependent Nanostructures Based on Active π-Aggregation Induced Emission Enhancement of New Carbazole Derivatives of Triphenylacrylonitrile

In the present study, the carbazole and 2,3,3-triphenylacrylonitrile (TPAN) nanostructures (2-CTPAN and 2,2′-CTPAN) have been designed and synthesized by Pd-catalyzed Sonogashira cross-coupling reaction. CTPAN exhibit aggregation-induced emission enhancement (AIEE) behavior in water with high fluorescence quantum yield. Both the compounds show tunable self-assembly in water as well as in N,N-dimethylformamide (DMF) by extended π–π stacking interactions. CTPAN can be self-assembled into spherical particles in water and the structures of these self-assemblies have been investigated using X-ray diffraction. Interestingly, 2-CTPAN and 2,2′-CTPAN form organogels with a critical gelation concentration (CGC) of 11 and 15 mg mL⁻¹, respectively, in DMF and exhibit acicular and rod shaped morphology, respectively. The single-crystal structure of 2-CTPAN shows that the intermolecular C−H⋅⋅⋅π interactions lock the molecular conformation into a staircase-shaped supramolecular assembly. These AIEE active compounds reveal high water dispersibility, strong yellow fluorescence with high quantum yield, promising photostability and excellent biocompatibility, which make them potential bioimaging agents.     

Synthesis and characterization of supramolecular complexes of a tetranuclear metallocycle with platinum(II) bis-ethynylphenylethynylpyridine organometallic complexes

A novel Pt(II) organometallic complex with 4-(4-ethynyl-phenylethynyl)-pyridine (DEBPy-H), namely [Pt(dppp)(DEBPy)₂] (dppp = 1,3-bis-(diphenylphosphino)-propane), has been prepared by two synthetic routes. The DEBPy-H ligand was prepared by a shorter synthetic pathway and in higher yield than the method of previous reports. New Pt(II) organometallic complexes with DEBPy-H and four bidentate ligands [Pt(L–L)(DEBPy)₂] (L–L = 1,2-bis-(diphenylphosphino)-benzene, bis-(dicyclohexyl)-ethane, 4,4′-dimethyl-2,2′-bipyridine, or 5,5′-dimethyl-2,2′-bipyridine) have also been prepared. These five Pt(II) complexes have two pyridyl units as an available coordination site that can operate as a metalloligand in nanoscale tectonics. A supramolecular complex of a tetranuclear metallocycle, [Pt(dppp)(DEBPy)₂]₂[Pt(dppp)]₂(OTf)₄, was synthesized from [Pt(dppp)(DEBPy)₂] and [Pt(dppp)(OTf)₂] as a corner connection through coordination-driven self-assembly. This supramolecular Pt(II) complex exists as a tetranuclear structure of the square type according to the interpretation of the ESI-mass and NMR spectra in solution. Five Pt(II) organometallics demonstrated the formation of similar tetranuclear metallocycles with [Pt(dppp)(OTf)₂], as indicated by their ESI-mass and UV–vis spectra in solution.     

cis‐Bis(dimethyl sulfide)dinitratoplatinum(II) at 295 and 150 K

The title compound, [Pt(NO₃)₂(C₂H₆S)₂], crystallizes in the P2₁/n space group (Z′ = 2), with pseudo‐square‐planar coordination geometry. The complex forms dimers with pseudosymmetry C_i arranged in columns along the b‐axis direction, with Pt...Pt distances of 6.3056 (3) and 4.2382 (2) Å (at 150 K). Each column is surrounded by six other columns in a honeycomb rod‐like packing. The coordination mode of the nitrate ligands is monodentate, with Pt—O—N angles ranging from ∼117 to ∼118° and a tilt between the nitrate ligands and the coordination planes in the range ∼63–70° (at 150 K). The coordination mode of the nitrate ligands is compared with that observed in reported Pt(NO₃)₂L₂ complexes (where L is a ligand with a donor atom from group 15 or 16), all of which are monodentate, with an average Pt—O—N angle of 118 (2)° and a tilt in the range 90 ± 30° (with two exceptions, in which the nitrates are approximately in the coordination plane).     

Near‐IR Phosphorescent Ruthenium(II) and Iridium(III) Perylene Bisimide Metal Complexes

The phosphorescence emission of perylene bisimide derivatives has been rarely reported. Two novel ruthenium(II) and iridium(III) complexes of an azabenz‐annulated perylene bisimide (ab‐PBI), [Ru(bpy)₂(ab‐PBI)][PF₆]₂ 1 and [Cp*Ir(ab‐PBI)Cl]PF₆ 2 are now presented that both show NIR phosphorescence between 750–1000 nm in solution at room temperature. For an NIR emitter, the ruthenium complex 1 displays an unusually high quantum yield (Φ_p) of 11 % with a lifetime (τ_p) of 4.2 μs, while iridium complex 2 exhibits Φ_p<1 % and τ_p=33 μs. 1 and 2 are the first PBI‐metal complexes in which the spin–orbit coupling is strong enough to facilitate not only the S_n→T_n intersystem crossing of the PBI dye, but also the radiative T₁→S₀ transition, that is, phosphorescence.     

Spin–Orbit Charge-Transfer Intersystem Crossing in Anthracene–Perylenebisimide Compact Electron Donor–Acceptor Dyads and Triads and Photochemical Dianion Formation

Perylenebisimide ( PBI )–anthracene ( AN ) donor–acceptor dyads/triad were prepared to investigate spin–orbit charge-transfer intersystem crossing (SOCT-ISC). Molecular conformation was controlled by connecting PBI units to the 2- or 9-position of the AN moiety. Steady-state, time-resolved transient absorption and emission spectroscopy revealed that chromophore orientation, electronic coupling, and dihedral angle between donor and acceptor exert a significant effect on the photophysical property. The dyad PBI-9-AN with orthogonal geometry shows weak ground-state coupling and efficient intersystem crossing (ISC, Φ_Δ=86 %) as compared with PBI-2-AN (Φ_Δ=57 %), which has a more coplanar geometry. By nanosecond transient absorption spectroscopy, a long-lived PBI localized triplet state was observed (τ_T=139 μs). Time-resolved EPR spectroscopy demonstrated that the electron spin polarization pattern of the triplet state is sensitive to the geometry and number of AN units attached to PBI . Reversible and stepwise generation of near-IR-absorbing PBI radical anion ( PBI^−⋅ ) and dianion ( PBI²⁻ ) was observed on photoexcitation in the presence of triethanolamine, and it was confirmed that selective photoexcitation at the near-IR absorption bands of PBI^.− is unable to produce PBI²⁻ .     

Shape‐Persistent (Pt‐salphen)2 Phosphorescent Coordination Frameworks: Structural Insights and Selective Perturbations

The development of molecular frameworks derived from binuclear platinum(II) aromatic Schiff base (salphen) complexes and their supramolecular chemistry have been undertaken. A series of axially rotating (Pt‐salphen)₂ luminophores, tethered in a cofacial manner by a rigid linker (xanthene, 1 ; dibenzofuran, 2 ; biphenylene, 3 ), was synthesized in which the O(salphen) groups are potentially amenable for guest‐binding. The molecular structures of 1 and 3 have been determined by X‐ray crystallography, revealing intra‐ and intermolecular π‐stacking interactions, as well as contrasting syn ( 1 ) and anti ( 3 ) configurations, for the (Pt‐salphen)₂ moiety. All complexes are luminescent in solution at room temperature. Their photophysical and solvatochromic properties have been examined, and the emissions are assigned to mixed triplet O(p)/Pt(d)→π*(diimine) excited states. The red‐shifted fluid emissions and lower quantum yields of 1 and 3 , relative to 2 , are ascribed to enhanced intramolecular π‐stacking interactions. Photophysical changes and selective responses to metal ions (particularly Pb²⁺) have been investigated by using various spectroscopic methods and DFT calculations, and through comparative studies with control complexes. A plausible binding mechanism is proposed based on occupation of the O(salphen)‐binding cavity, which induces perturbation of intramolecular π–π interactions, and hence the self‐quenching and emission properties, of the (Pt‐salphen)₂ unit.     

Silole-containing linear and hyperbranched polymers: synthesis,thermal stability,light emission,nano-dimensional aggregation,and optical power limiting

Linear polyacetylenes and hyperbranched polyphenylenes carrying 1,2,3,4,5-pentaphenylsilolyl (PS) pendants are designed and synthesized. Homo-polymerization of HC≡CPS, HC≡C(CH₂)₉OPS, and C₆H₅C≡C(CH₂)₉OPS and (co)polycyclotrimerization of (HC≡C)₂PS with 1-octyne are effected by NbCl₅-, WCl₆-, MoCl₅-, and TaCl₅-based catalysts. High molecular weight linear ( 1-3 ) and hyperbranched polymers (6) are obtained in high yields (M_w up to ∼70 × 10³ and yield up to 85%). All the polymers are thermally stable, losing little weight when heated to 350°C. Whereas all the polymers emit faint light when molecularly dissolved, polymers 2 , 3 , and 6 become emissive when aggregated in poor solvents or when cooled to low temperatures. Restricted intramolecular rotations of the phenyl rings upon the axes of the single bonds linked to the silole cores may be responsible for the aggregation- or cooling-induced emission. A multilayer electroluminescent device using 3 as an active layer emits a blue light of 496 nm with maximum brightness, current efficiency, and external quantum yield of 1118 cd/m², 1.45 cd/A, and 0.55%, respectively. Polymers 6 are non-linear optically active and strongly attenuate the optical power of intense laser pulses, whose optical limiting performances are superior to that of C₆₀, a best-known optical limiter.     

Structural Control of (1)A(2u)-to-(3)A(2u) Intersystem Crossing in Diplatinum(II,II) Complexes

Analysis of variable-temperature fluorescence quantum yield and lifetime data for per(difluoroboro)tetrakis(pyrophosphito)diplatinate(II) ([Pt(2)(μ-P(2)O(5)(BF(2))(2))(4)](4-), abbreviated Pt(pop-BF(2))), yields a radiative decay rate (k(r) = 1.7 × 10(8) s(-1)) an order of magnitude greater than that of the parent complex, Pt(pop). Its temperature-independent and activated intersystem crossing (ISC) pathways are at least 18 and 142 times slower than those of Pt(pop) [ISC activation energies: 2230 cm(-1) for Pt(pop-BF(2)); 1190 cm(-1) for Pt(pop)]. The slowdown in the temperature-independent ISC channel is attributed to two factors: (1) reduced spin-orbit coupling between the (1)A(2u) state and the mediating triplet(s), owing to increases of LMCT energies relative to the excited singlet; and (2) diminished access to solvent, which for Pt(pop) facilitates dissipation of the excess energy into solvent vibrational modes. The dramatic increase in E(a) is attributed to increased P-O-P framework rigidity, which impedes symmetry-lowering distortions, in particular asymmetric vibrations in the Pt(2)(P-O-P)(4) core that would allow direct (1)A(2u)-(3)A(2u) spin-orbit coupling.     

Liquid crystalline supramolecular crosslinked polymer complexes of ditopic rylenebisimides and P4VP

Perylenebisimide and naphthalenebisimide (PBI‐PDP and NBI‐PDP) end functionalized with pentadecyl phenol is designed as ditopic hydrogen bonding acceptors to form supramolecular crosslinked network with poly(4‐vinyl pyridine) (P4VP). The pristine PBI‐PDP has been grown as single crystals from DCM‐MeOH (dichloromethane‐methanol) mixture at room temperature, which revealed a P21 space group. Noticeably, the pentadecyl alkyl chain shields the aromatic perylene core on both sides resulting in the absence of π–π interaction in single‐crystal assembly. The naphthalenebisimide derivative exhibits thermotropic liquid crystalline behavior, while both the molecules exhibits lyotropic liquid crystalline phases in tetrahydrofuran (THF), which were characterized using a combination of differential scanning calorimeter, X‐ray diffraction, and polarized light microscopy. The hydrogen‐bonded complex of both the rylenebisimides with P4VP preserves the mesomorphic properties in THF. The electron transport mobility measured by space charge limited current measurements reveals a two orders of magnitude increase in the charge transport in the P4VP complex compared to that of the pristine molecule. The average electron mobility obtained is μ _e,avg: 10^?3 cm²/Vs for P4VP‐PBI compared to μ _e,avg: 10^?5 cm²/Vs for pristine PBI derivative. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 951–959     

Light- and pH-regulated Water-soluble Pseudorotaxanes Comprising a Cucurbit[7]uril and a Flavylium-based Axle

A linear double pyridinium-terminated thread comprising a central chalcone moiety is shown to provide two independent binding sites with similar affinity for cucurbit[7]uril (CB7) macrocycles in water as judged from NMR, UV-Visible and fluorescence spectroscopies. Association results in [2] and [3]pseudorotaxanes, which are both pH and photosensitive. Switching from the neutral chalcone to the cationic flavylium form upon irradiation at 365 nm under acidic conditions provided an enhanced CB7 association (K_1:1 increases from 1.2×10⁵ M⁻¹ to 1.5×10⁸ M⁻¹), limiting spontaneous on-thread cucurbituril shuttling. This co-conformational change in the [2]pseudorotaxane is reversible in the dark with k_obs=4.1×10⁻⁴ s⁻¹. Threading the flavylium moiety into CB7 leads to a dramatic increase in the fluorescence quantum yield, from 0.29 in the free axle to 0.97 in the [2]pseudorotaxane and 1.0 in the [3]pseudorotaxane.     

Neutron Scattering Studies of Structure and Self-Assembly of Star-Shaped Polymers with Fullerene Centres in Solutions

Protonated star-shaped polystyrenes with single and double fullerene C₆₀ core and the hybrid stars with pairs of polar and non-polar arms (tertbuthylmetacrylate, polystyrene) have been studied in deuterated toluene (20 °C) by small-angle neutron scattering at low and moderate polymer concentrations (c₁ ∼ 1 g/dl, c₂ ∼ 3–6 g/dl) to evaluate the peculiarities of fullerene centre action on polymers self-assembly in solutions. As we found, the cores composed of two fullerenes, linked via Si(CH₃)₂-bridge, induce stars' anisotropic interactions and association into chain-like structures (correlation radius ∼400–600 nm). Meanwhile, the single-core stars of polystyrene and hybrids organize globular clusters (size ∼ 10³ nm) those geometry do not change significantly by polymer content variation.     

Phosphorescent self-assembled Pt(II) tetranuclear metallocycles

A series of rigid Pt(II) diimine diacetylide complexes and their corresponding metallocyclic derivatives were synthesized through coordination-driven self-assembly. The photophysical properties of these complexes have been studied in detail, revealing exceptionally high RT phosphorescence quantum yields and lifetimes when the excited state becomes localized on the π-conjugated bridging-ligand following intramolecular charge-transfer sensitization.     

Metal-Templated Design of Chemically Switchable Protein Assemblies with High-Affinity Coordination Sites

To mimic a hypothetical pathway for protein evolution, we previously tailored a monomeric protein (cyt cb₅₆₂) for metal-mediated self-assembly, followed by re-design of the resulting oligomers for enhanced stability and metal-based functions. We show that a single hydrophobic mutation on the cyt cb₅₆₂ surface drastically alters the outcome of metal-directed oligomerization to yield a new trimeric architecture, (TriCyt1)_3. This nascent trimer was redesigned into second and third-generation variants (TriCyt2)₃ and (TriCyt3)₃ with increased structural stability and preorganization for metal coordination. The three TriCyt variants combined furnish a unique platform to 1) provide tunable coupling between protein quaternary structure and metal coordination, 2) enable the construction of metal/pH-switchable protein oligomerization motifs, and 3) generate a robust metal coordination site that can coordinate all mid-to-late first-row transition-metal ions with high affinity.