Solid-state Photophysical Properties Research Articles

Pillar[5]arene-Based Ion-Pair Recognition for Encapsulation of a Stilbazolium-Type Dye with Enhanced Photophysical Properties and Nonlinear Optical Activity.

Constructing organic composite materials through molecular recognition has emerged as an important theme in materials science. Here we report an ion-pair recognition system involving the use of a propoxylated pillar[5]arene (PrP5) to modulate the solid-state photophysical properties of dye trans-4'-(dimethylamino)-N-methyl-4-stilbazolium hexafluorophosphate (DMASP). Single crystal X-ray diffraction analysis reveals that the dye guest DMASP is encapsulated by PrP5 to form a 2:1 host-guest complex 2PrP5⸧DMASP in the crystalline state. The macrocyclic skeleton of PrP5 imposes restrictions on the intramolecular motions of the dye guest, leading to a significant enhancement of its fluorescence emission. Additionally, within the 2PrP5⸧DMASP complex crystal structure, DMASP molecules are found to display two possible opposite orientations in the one-dimensional channels formed by PrP5 molecules. This arrangement is believed to alter the overall solid-state packing structure of DMASP, thereby activating its nonlinear optical activity. This work not only reports a novel ion-pair molecular recognition system based on pillararenes but also provides valuable insights into the modulation of the crystalline state photophysical properties of organic dyes via cocrystal engineering.

Four orotate-based lanthanide coordination polymers with rhomboid [Ln4(μ3-OH)2]-based 1D chainlike structure exhibiting luminescence and large magnetocaloric effect

Four isostructural lanthanide coordination polymers with orotic acid (H3OA) as building ligand, {[Ln(HOA)(μ3-OH)(H2O)2]·H2O}n (Ln = Eu (1); Gd (2); Tb (3); Dy (4)), were synthesized under hydrothermal conditions. Single-crystal X-ray diffraction analyses demonstrate that complexes 1–4 have an one-dimensional (1D) chainlike structure consisting of rhomboid [Ln4(μ3-OH)2] unit and HOA2- linker. Magnetic studies indicate that 2 displays a large magnetocaloric effect with magnetic entropy value of 37.11 J kg−1 K−1 at 2.0 K for ΔH = 7.0 T. This is a considerable value among the reported molecular magnetic coolants, and has a great application potential in the field of ultra-low temperature refrigeration. In addition, the solid-state photophysical properties of 1 and 3 exhibit obvious characteristic Eu(III) and Tb(III) photoluminescence emission in the visible region, suggesting that Eu- and Tb-based luminescence were sensitized effectively by the energy transfer from the ligand to the metal centers.

Two temperature-dependent 2D heterometallic Cd(II)–Dy(III) coordination polymers exhibiting slow magnetic relaxation and luminescence properties

Two unique two-dimensional (2D) Cd(II)–Dy(III) heterometallic coordination polymers, {[DyCd(HPMA)(PMA)2(H2O)4]·2H2O}n (1) and [DyCd(PAA)(PMA)2(H2O)3]n (2, PAA– = phenylacetate), were prepared successfully based on the phenylmalonic acid (H2PMA). The construction of two polymers is sensitive to the reaction temperature and can be synthesized directionally at room temperature and 90 °C, respectively. Complexes 1 and 2 display 2D layer structures that include dinuclear [Dy2] cluster node and Dy(III)-based 1D zigzag chain motif severally. The Dy(III)-based units in two structures both are spaced well with the adjacent ones by the diamagnetic Cd(II)-based moieties. The magnetic studies reveal that 1 and 2 both display slow magnetic relaxation with temperature-dependent relaxation peaks producing an effective energy barrier (Δτ) of 74.5 and 32.1 K, separately. Furthermore, the solid-state photophysical properties of 1 and 2 show strong characteristic luminescent emissions of Dy(III) ion in the visible region.

Tunable structures and magnetic / optical properties of six Cd(II)-based coordination polymers by introducing different para- or dia-magnetic metal ions

One monometallic and five heterometallic coordination polymers (HCPs) based on homophthalic acid (H2HPA), [Cd2(HPA)2(H2O)2]·H2O (1), [CdMn(HPA)2(H2O)2]·H2O (2), [Cd3Co(HPA)4(H2O)4]·2H2O (3), [CdCo(HPA)2(DMSO)(H2O)2] (4), [CdNa2(HPA)2(μ2-H2O)(H2O)] (5) and [Cd2Na2(HPA)3(μ2-H2O)(H2O)2] (6), were synthesized solvothermally. Complexes 1-3 have similar two-dimensional (2D) layer structures except that all or part six-coordinated Cd(II) ions in 1 were replaced by the six-coordinated Mn(II) or Co (II) ion in 2 and 3, respectively. Complex 4 shows an 1D ladderlike structure, and 5 and 6 exhibit different 2D layer structures. The different structures of 3/4 and 5/6 could be assembled directly by controlling the different pH values with other same reaction conditions, respectively. Magnetic studies reveal that the introducing of para-magnetic Mn(II) and Co(II) ions endow antiferromagnetic interarions between the neigboring spins in 2–4, and 2 with isotropic Mn(II) ion displays a significant cryogenic magnetocaloric effect with a maximum −ΔSm value of 19.29 J K−1 kg−1 at 2 K and ΔH = 7 T. In addition, the solid-state photophysical properties of 1, 5 and 6 display blue emissions centered at ca. 453 nm, 448 nm and 428 nm, respectively.

Impact of polymer chain packing and crystallization on the emission behavior of curcumin-embedded poly(L-lactide)s.

The development of biodegradable and biocompatible fluorescent materials with tunable emission in the solid state has become increasingly relevant for smart packaging and biomedical applications. Molecular packing and conformations play a critical role in tuning the solid-state photophysical properties of fluorescent materials. In this work, tunable emission of bioactive curcumin was achieved through the manipulation of the crystallization conditions and the polymorphic form of covalently linked poly(L-lactide) in the curcumin-embedded poly(L-lactide) (curcumin-PLLA). In the melt-crystallized curcumin-PLLA, with the increase in the isothermal crystallization temperature, a bathochromic shift in the fluorescence of curcumin-PLLA was observed due to the change in the intramolecular conjugation length of curcumin. The change in the isothermal crystallization temperature of curcumin-PLLA resulted in the rotation of the terminal phenyl rings of curcumin with respect to the central keto-enol group due to the covalently linked helical PLLA chains. In addition, solvent-induced single crystals and a gel of curcumin-PLLA were prepared and the influence of the polymorphic form of PLLA on the emission behavior of curcumin-PLLA was investigated. The results suggest that the polymer chain packing, crystallization conditions, morphology, and polymorphic form could play an influential role in dictating the fluorescence properties of fluorophore-embedded polymers.

9,9-Bis[4-(N-aryl)phenyl]methylidene-xanthylidene Derivatives Displaying Mechano-, Crystallo-, and Thermochromism.

Tetraphenylethylene (TPE) derivatives bearing a xanthene moiety are of interest because they have novel optical properties. 9,9-Bis[4-(N,N-diphenylamino)phenyl] and 9,9-bis[4-(9-carbazolyl)-phenyl]methylidene-xanthylidenes 3 and 4 were synthesized using Suzuki-Miyaura coupling of 9,9-dibromomethylidene-xanthylidene with the corresponding boronic acids. Diphenylamino derivative 3 exhibits mechanochromism and mechanofluorochromism (MC and MFC) reflected in absorption and fluorescence color changes. In contrast, carbazolyl derivative 4 displays thermo- and crystallo-chromism in addition to MC and MFC in the solid state. Powder X-ray diffraction and single crystal X-ray crystallographic analysis reveal that the solid state photophysical properties of these substances are governed by conformational changes rather by the creation of planar π-conjugation extended geometries.

Impact of Intermolecular Non-Covalent Interactions in a CuI 8 PdII 1 Discrete Assembly: Conformers' Geometries and Stimuli-Sensitive Luminescence Properties.

A new highly solid-state luminescent phase of a previously reported weakly luminescent CuI 8 PdII 1 dicationic assembly is reported revealing the high geometrical versatility of this moiety that importantly alters its luminescent properties. This very minor new species Bc is based on a different conformer scaffold than the one encountered in the previously reported Bo form and, essentially differs from Bo by displaying shorter CuI -CuI intermetallic distances. DFT calculations allow concluding that the predominance in the solid-state of the weakly luminescent and less stable Bo phase is due to the extra stability induced by a larger number of intermolecular non-covalent π-CH interactions in its crystalline packing and not by the intrinsic stability of the CuI 8 PdII 1 dicationic moiety. Calculations also revealed that a more stable conformation Bcalc is expected in vacuum, which bears a different distribution of CuI -CuI intermetallic distances than the dications in Bo and Bc phases. Taking into account that the geometrical alterations are associated to drastic changes of luminescence properties, this confer to the CuI 8 PdII 1 assembly high potentiality as stimuli-sensitive luminescent materials. Indeed, by applying mechanical or thermal stress to samples of Bo phase, new phases Bg and Bm , respectively, were obtained. Alterations of the solid-state photophysical properties of these new species compared to those recorded for Bo are reported together with a combined experimental and computed study of the structures/properties relationships observed in these phases.

Solid-State Photophysical Properties of Chiral Perylene Diimide Derivatives: AIEnh-Circularly Polarized Luminescence from Vacuum-Deposited Thin Films

Abstract Studies on the optical properties of chiral perylene diimide derivatives, (S,S)- and (R,R)-BPP (BPP = N,N′-bis(1-phenylethyl)perylene-3,4,9,10-tetracarboxylic diimide), through absorption, steady-state and time-resolved fluorescence, and chiroptical spectroscopies revealed that the vacuum-deposited thin films of (S,S)- and (R,R)-BPP exhibit aggregation-induced enhanced (AIEnh) circularly polarized luminescence (CPL), as observed when they are dispersed in KBr and polymer matrices. Fluorescence lifetimes revealed that two emissive species exist in the deposited thin film: the dimer state with a fast decay and the weakly coupled excimer state with a slow decay. Unlike the dilute solutions of chiral BPPs, the deposited thin films displayed circular dichroism (CD) and CPL, indicating solid-state aggregation-induced chiroptical properties of chiral BPPs. Time-dependent density functional theory calculations on isolated monomer and π-stacked dimer suggested that the formation of a chirally twisted molecular arrangement through intermolecular π–π interactions is essential for the chiroptical properties of (S,S)- and (R,R)-BPP in the solid states. Moreover, we fabricated organic light-emitting diodes (OLEDs) using (S,S)- and (R,R)-BPP isomers as CPL emitters. The devices exhibited circularly polarized electroluminescence (CPEL) although the EL dissymmetry factor (gEL) was significantly low, as has been frequently noted in earlier studies.

2‐Aryl‐1H‐benz[de]isoquinolinium Ions: Cationic Dyes Displaying Mechanochromism and Crystallochromism

Abstract2‐Aryl‐5,8‐di‐tert‐butyl‐1H‐benz[de]isoquinolinium tetrafluoroborates were synthesized and crystalized to form yellow to orange crystals. Absorption and fluorescence maxima (λABS and λFL‐SL) of the compounds in CH2Cl2 depend on the electronic and steric properties of the aryl ring substituents and closely match those arising from theoretical calculations. λABS and λFL‐SL values of the compound possessing bulky 2,6‐diisopropylphenyl substituents are the shortest among the substances because of sterically promoted retardation of conjugation between the isoquinolinium and aryl ring planes. In contrast, solid‐state fluorescence maxima (λFL‐SD) of the compounds are not closely correlated with the electronic properties of the aryl substituents. Furthermore, one crystalline form of both the 4‐trifluoromethylphenyl and 4‐nitrophenyl derivatives exhibit mechanochromic and crystallochromic behavior reflected in absorption and fluorescence color changes. X‐ray diffraction and crystallographic analysis reveal that the solid state photophysical properties of these substances are related to different molecular stacking interactions occurring in different crystalline forms.

Spectroscopic and computational studies of the solid state photophysical properties of a biscoumarin dye

The solid state photophysical properties of the 3,3′-paraphenyl bis[6,8-dimethoxy-2H-chromen-2-one] symmetrical biscoumarin material were investigated by optical spectroscopy techniques and by theoretical calculations. Vibrational analysis using IR absorption and Raman scattering techniques carried out together with DFT theoretical calculations have confirmed the structure of this biscoumarin. The geometry optimization using different functionals reveal a nonplanar equilibrium structure with a dihedral between the phenyl and the pyran rings of about 142°. The UV–Visible absorption measurements and the TDDFT simulation show that this biscoumarin is characterized by a bicomponent feature resulting from ππ* electronic transitions and Intramolecular Charge Transfer (ICT). Solid state photoluminescence showed a bright blue-green emission at 506 nm with a large stokes shift estimated at 146 nm, and the temperature dependence study of this emission reveals two thermal evolution regimes. Finally, these good optical properties, as well as the stability of the emission, make this biscoumarin dye of potential interest for optoelectronic applications.

Derivation-density-engineered molecular packing, emission property, mechanofluorochromism and mechanoluminescence of triarylimidazole-substitued triphenylamines

Three fluorophores IMTPA-1, IMTPA-2 and IMTPA-3 where one, two and three 2, 4, 5-triarylimidazole (IM) moieties are grown up on a triphenylamine (TPA) core are prepared while their solution- and solid-state photophysical properties are investigated. It is found that the derivation density makes very profound impact on solvatochromic behaviors, emission wavelength and intension together with mechanofluorochromic (MFC) and mechanoluminescent activities of these compounds. As the derivation density is increased, solvatochromic effect becomes weaker. Moreover, fluorescence quenching only occurs on IMTPA-3 crystal in comparison of high emission intensity on IMTPA-1 and IMTPA-2 crystals. The three fluorophores are all MFC-active but the emission wavelengths of the amorphous samples are insensitive to the derivation density. However, mechanoluminescence (ML) effect on these crystals are highly derivation-density-dependent, as force strength needed for ML phenomena to occur is in IMTPA-1 ≫ IMTPA-2 ~ IMTPA-3 order while the ML intensity is in the contrary one. In aid of X-ray crystallography and theory calculations, a deep insight of structure-property relationship on this series in view of derivation density is obtained, which is a novel method to present compounds with differentiated MFC and/or ML activity.

Solution and Solid-State Photophysical Properties of Positional Isomeric Acrylonitrile Derivatives with Core Pyridine and Phenyl Moieties: Experimental and DFT Studies.

The compounds I (Z)-2-(phenyl)-3-(2,4,5-trimethoxyphenyl)acrylonitrile with one side (2,4,5-MeO-), one symmetrical (2Z,2′Z)-2,2′-(1,4-phenylene)bis(3-(2,4,5-trimethoxyphenyl)acrylonitrile), II (both sides with (2,4,5-MeO-), and three positional isomers with pyridine (Z)-2-(pyridin-2- 3, or 4-yl)-3-(2,4,5-trimethoxyphenyl)acrylonitrile, III–V were synthetized and characterized by UV-Vis, fluorescence, IR, H1-NMR, and EI mass spectrometry as well as single crystal X-ray diffraction (SCXRD). The optical properties were strongly influenced by the solvent (hyperchromic and hypochromic shift), which were compared with the solid state. According to the solvatochromism theory, the excited-state (μe) and ground-state (μg) dipole moments were calculated based on the variation of Stokes shift with the solvent’s relative permittivity, refractive index, and polarity parameters. SCXRD analyses revealed that the compounds I and II crystallized in the monoclinic system with the space group, P21/n and P21/c, respectively, and with Z = 4 and 2. III, IV, and V crystallized in space groups: orthorhombic, Pbca; triclinic, P-1; and monoclinic, P21 with Z = 1, 2, and 2, respectively. The intermolecular interactions for compounds I–V were investigated using the CCDC Mercury software and their energies were quantified using PIXEL. The density of states (DOS), molecular electrostatic potential surfaces (MEPS), and natural bond orbitals (NBO) of the compounds were determined to evaluate the photophysical properties.

Mechanochemical Syntheses of Isostructural Luminescent Cocrystals of 9-Anthracenecarboxylic Acid with two Dipyridines Coformers

Tuning and controlling the solid-state photophysical properties of organic luminophore are very important to develop next-generation organic luminescent materials. With the aim of discovering new functional luminescent materials, new cocrystals of 9-anthracene carboxylic acid (ACA) were prepared with two different dipyridine coformers: 1,2-bis(4-pyridyl)ethylene and 1,2-bis(4-pyridyl)ethane. The cocrystals were successfully obtained by both mechanochemical approaches and conventional solvent crystallization. The newly obtained crystalline solids were characterized thoroughly using a combination of single crystal X-ray diffraction, powder X-ray diffraction, Fourier-transform infrared spectroscopy, differential thermal analysis, and thermogravimetric analysis. Structural analysis revealed that the cocrystals are isostructural, exhibiting two-fold interpenetrated hydrogen bonded networks. While the O–H···N hydrogen bonds adopts a primary role in the stabilization of the cocrystal phases, the C–H···O hydrogen bonding interactions appear to play a significant role in guiding the three-dimensional assembly. Both π···π and C–H···π interactions assist in stabilizing the interpenetrated structure. The photoluminescence properties of both the starting materials and cocrystals were examined in their solid states. All the cocrystals display tunable photophysical properties as compared to pure ACA. Density functional theory simulations suggest that the modified optical properties result from charge transfers between the ACA and coformer molecules in each case. This study demonstrates the potential of crystal engineering to design solid-state luminescence switching materials through cocrystallization.

Extended π-Systems in Diimine Ligands in [Cu(P^P)(N^N)][PF6] Complexes: From 2,2′-Bipyridine to 2-(Pyridin-2-yl)Quinoline

We describe the synthesis and characterization of [Cu(POP)(1)][PF6], [Cu(POP)(2)][PF6], [Cu(xantphos)(1)][PF6], and [Cu(xantphos)(2)][PF6] in which ligands 1 and 2 are 2-(pyridin-2-yl)quinoline and 2-(6-methylpyridin-2-yl)quinoline, respectively. With 2,2'-bipyridine (bpy) as a benchmark, we assess the impact of the extended π-system on structural and solid-state photophysical properties. The single crystal structures of [Cu(POP)(2)][PF6], [Cu(xantphos)(1)][PF6], and [Cu(xantphos)(2)][PF6] were determined and confirmed a distorted tetrahedral copper(I) coordination environment in each [Cu(P^P)(N^N)]+ cation. The xanthene unit in [Cu(xantphos)(1)][PF6] and [Cu(xantphos)(2)][PF6] hosts the quinoline unit of 1, and the 6-methylpyridine group of 2. 1H NMR spectroscopic data indicate that these different ligand orientations are also observed in acetone solution. In their crystal structures, the [Cu(POP)(2)]+, [Cu(xantphos)(1)]+, and [Cu(xantphos)(2)]+ cations exhibit different edge-to-face and face-to-face π-interactions, but in all cases, the copper(I) centre is effectively protected by a ligand sheath. In [Cu(POP)(2)][PF6], pairs of cations engage in an efficient face-to-face π-stacking embrace, and we suggest that this may contribute to this compound having the highest photoluminescence quantum yield (PLQY = 21%) of the series. With reference to data from the Cambridge Structural Database, we compare packing effects and PLQY data for the complexes incorporating 2-(pyridin-2-yl)quinoline and 2-(6-methylpyridin-2-yl)quinoline, with those of the benchmark bpy-containing compounds. We also assess the effect that Cu⋯O distances in the {Cu(POP)} and {Cu(xantphos)} domains of [Cu(P^P)(N^N)][X] compounds have on solid-state PLQY values.

Two series of lanthanide complexes with 4-chlorophenylacetate ligand displaying luminescence and significant magnetocaloric effect

Two series of lanthanide complexes based on 4-chlorophenylacetic acid (HCPA), dinuclear [Ln2(CPA)6(DMF)2(H2O)2] (Ln = Eu (1); Gd (2); Tb (3); Dy (4)) and one-dimensional (1D) linear [Ln2(CPA)6(H2O)]n (Ln = Eu (5); Gd (6); Tb (7); Dy (8)), were prepared, which are sensitive to the pH value. The dinuclear structure can be obtained when the pH value was controlled to be about 4.2, while the 1D structure can be formed when the pH value is ca. 5.1. The structure transformation of the dinuclear to 1D analogue at different pH values indicates that the CPA− ligand has a better affinity to replace the H2O/DMF molecule to bridge the neighboring Ln3+ ions to form the 1D structure at a higher pH value (∼5.1). The magnetic studies show that complexes 2 and 6 with isotropic Gd3+ ion display significant cryogenic magnetocaloric effect with a maximum −ΔSm value of 20.27 J kg−1 K−1 at 3 K and 7 T and 22.50 J kg−1 K−1 at 2 K and 7 T, respectively. Furthermore, the solid-state photophysical properties of 1/5 and 3/7 were studied and respectively exhibit strong characteristic emission peaks of Eu3+ and Tb3+ ions in the visible region, indicating that Eu- and Tb-based luminescences are sensitized effectively by the energy transfe from the ligand to the spin centers.

Mononuclear lanthanide(III)-oxamate complexes as new photoluminescent field-induced single-molecule magnets: solid-state photophysical and magnetic properties.

Implementing additional optical (luminescent) properties into the well-known class of single-molecule magnets (SMMs) is considered as a promising route toward obtaining the next generation of optomagnetic materials for quantum information storage and computing. Herein, we report a joint optical and magneto-structural study for the two novel series of lanthanide(iii) complexes of general formula Bu4N[LnIII(HL)4(dmso)]·nH2O where H2L = N-(4-Xphenyl)oxamic acid with X = Cl and n = 2 [Ln = Eu (1_Cl), Gd (2_Cl), Dy (3_Cl), and Tb (4_Cl)] and X = F and n = 3 [Ln = Eu (1_F), Gd (2_F), Dy (3_F), and Tb (4_F)]. All these compounds are mononuclear species with each lanthanide(iii) cation in a low-symmetry nine-coordinate environment (LnO9) which is constituted by four didentate monoprotonated oxamate groups and one dmso molecule. Magnetic measurements show the occurrence of field-induced SMM behavior for the Gd3+ (2_Cl and 2_F), Dy3+ (3_Cl and 3_F), and Tb3+ complexes (4_Cl and 4_F). Solid-state photophysical measurements for the Eu3+ (1_Cl and 1_F) and Tb3+ complexes (4_Cl and 4_F) reveal that both monoprotonated chloro- and fluoro-substituted phenyl(oxamate) ligands are able to sensitize the lanthanide(iii)-based luminescence in the visible region, through an energy transfer process ("antenna effect"), as supported by theoretical calculations for Eu3+ compounds. In particular, 1_Cl and 1_F present a quantum efficiency of approximately 50%, being potentially suitable as efficient light conversion molecular devices (LCMDs).

Molecular Packing and Solid-State Photophysical Properties of 1,3,6,8-Tetraalkylpyrenes.

The relationship between the photophysical properties and molecular orientation of 1,3,6,8-tetraalkylpyrenes in the solid state is described herein. The introduction of alkyl groups with different chain structures (in terms of length and branching) did not affect the photophysical properties in solution, but significantly shifted the emission wavelengths and fluorescence quantum yields in the solid state for some samples. Pyrenes bearing ethyl, isobutyl, or neopentyl groups at the 1-, 3-, 6-, and 8-positions showed similar emission profiles in both the solution and solid states. In contrast, pyrenes bearing other alkyl groups exhibited an excimer emission in the solid state, similar to that of the parent pyrene. On studying the photophysical properties in the solid state with respect to the obtained crystal structures, the observed solid-state photophysical properties were found to depend on the relative position of the pyrene chromophores. The solid-state photophysical properties can be controlled by the alkyl groups, which provide changing crystal packing. Among the pyrenes tested, 1,3,6,8-tetraethylpyrene showed the highest fluorescence quantum yield of 0.88 in the solid state.

Exploring the Relationship between Intermolecular Interactions and Solid-State Photophysical Properties of Organic Co-Crystals

Five new binary co-crystals have been prepared through molecular self-assembly of π-electron-rich molecules, such as phenanthrene, benzo[c]cinnoline, and phenazine, in the presence of π-electron-de...

Crystalline triphenylamine-substituted arenes: solid-state packing and luminescence properties

The crystal packing and solid state photophysical properties of twisted propeller-shaped triphenylamine (T) incorporated aromatic hydrocarbons [ArT where Ar = benzene (Ph), naphthalene (N), anthracene (A), phenanthrene (Phe), pyrene (Py) and perylene (Pe)] are investigated. The qualitative crystal structure and quantum theory of atoms-in-molecules (QTAIM) analyses revealed the dominant role of intermolecular C–H⋯C interactions in governing the three-dimensional arrangement in ArT crystals. Hirshfeld surface analyses revealed that the triphenylamine-substituted arene (ArT) crystals possess a herringbone motif with ρ (%C–H⋯C/%C⋯C) values varying from 10.4 in PeT to 101.2 in AT. A blue shift of ca. 10–28 nm in the emission maxima for PhT, NT, AT, PheT and PyT is observed in the solid state in comparison to that in dichloromethane while the solid state emission maxima is red shifted (ca. 25–71 nm) compared to that in hexane. A 2.40-fold enhancement in the quantum yield of AT is observed in the solid state relative to the solution state (dichloromethane). Solvent polarity dependent absorption and emission measurements of ArT derivatives along with Lippert–Mataga analyses evidenced the presence of charge transfer interactions between Ar and T units. A precise and systematic understanding of the influence of the crystal structure on the photophysical properties is quintessential to achieve highly efficient organic light emitting diodes and other optoelectronic devices in the near future.

Construction of Coordination Polymers from Semirigid Ditopic 2,2′-Biimidazole Derivatives: Synthesis, Crystal Structures, and Characterization

Eight coordination polymers (CPs), {[Ag(L1)]ClO4}n (1), {[Ag(L2)1.5]ClO4·C2H3N}n (2a), {[Ag(L2)]ClO4}n (2b), [Zn(L1)Cl2]n (3), {[Zn(L2)Cl2]·CHCl3}n (4), {[Cu(L1)2Cl]Cl·H2O}n (5), [Cu2(L2)(μ-Cl)2]n (6), and [Cu4(L2)(μ-Cl)4]n (7) were synthesized via self-assembly of corresponding metal ions and biimidazole based ditopic ligands, 1,1′-bis(pyridin-3-ylmethyl)-2,2′-biimidazole L1 and 1,1′-bis(pyridin-4-ylmethyl)-2,2′-biimidazole L2. These ligands possess conformational flexibility and two pairs of coordination sites: pyridine nitrogen (NPy) atoms and imidazole nitrogen (NIm) atoms. Depending on the metal center in CPs, the biimidazole compounds act as tetra- (1, 7), tri- (2a), or bidentate (2a, 2b, 3–6) ligands binding to the metal either via NPy or NIm, or both. All these CPs were structurally fully characterized with single crystal X-ray structure, mass spectrometry, and NMR spectroscopy. The solid state photophysical properties and thermal stabilities of the CPs were also briefly studied in the solid state.