CH2 Spacer Research Articles

Morpholine-modified polyacrylamides with Polymerization-Induced emission and its specific detection to Cu2+ ions

In this work, three morpholine-modified polyacrylamide derivatives (MMPAm) were successfully prepared by free radical polymerization of monomers with morpholine moiety. The intramolecular aggregation of morpholine rings on macromolecular backbone gives MMPAm a significant polymerization-induced emission (PIE). Particularly, poly(N-morpholine acrylamide) (PNMPA) has the characteristics of strong fluorescence at 450 nm, and its fluorescence quantum yield reaches 2.87 %. The introduction of morpholine moiety, the length of CH2 spacer between morpholine ring and the backbone and the molecular weight play the important roles in PIE properties of PNMPA. Interestingly, PNMPA can recognize and detect Cu2+ specifically even in the presence of 12 other metal ions by thorough fluorescence quenching, and the detection limit of PNMPA is 17.3 μM. Furthermore, the dynamic quenching of PNMPA by Cu2+ ions and the complexation ratio of 1:2 according to JOB's working diagram were confirmed by fluorescence titration. Under the assistance of EDTA, a reversible detection system for Cu2+ is achieved, and a portable test paper from PNMPA for the detection of Cu2+ was also made. In conclusion, PNMPA is endowed with a significant PIE effect by the intramolecular aggregation of morpholine rings along the backbone in the polymerization of non-fluorescent monomer, and is expected to be a promising material for specific detection to Cu2+ ions.

Aggregation‐induced emission‐active poly (β‐lactam) prepared by Staudinger polymerization as specific probe to Fe3+ ions and its antimicrobial properties

AbstractFor the nontraditional aggregation‐induced emission (AIE) active polymers, the heterocyclic structure on the macromolecular backbone and side chains play a crucial role in its fluorescence generation. In this paper, we facilely prepared a series of poly(β‐lactam) derivatives (PLTM) by Staudinger polymerization. As expected, PLTMs with four‐membered heterocycles along the main‐chain shows a remarkable AIE performance with the fluorescence emission at 520 nm, while no fluorescence was observed for both β‐lactam (repeating unit) and a model polymer for PLTM. The fluorescence properties of PLTMs greatly depend on the intramolecular aggregation of β‐lactam structures, the size of the substituents, and appropriate length of CH2 spacer. In addition, PLTMs can specifically detect Fe3+ ions with a limit of detection (LOD) of 7.2 μM even in the presence of other 13 metal ions. Similar to the biological activity of β‐lactam, AIE‐active PLTMs shows a bactericidal effect on Staphylococcus aureus and Escherichia coli with the minimum inhibitory concentration of about 8 and 4 mg/mL. In brief, the preparation and versatility of PLTMs with β‐lactam structure provide useful references for designing other nontraditional AIE‐active polymers as well as for ion detection and antibacterial processes.

Dielectric relaxation and glassy dynamics in poly(diisopropyl fumarate) and its copolymers with acrylate segments

Dynamical behavior and glass transitions in copolymers of diisopropyl fumarate (DiPF) with 1-adamantyl acrylate (AdA) or n-butyl acrylate (nBA) are investigated by dielectric relaxation spectroscopy (DRS) to elucidate the effect of structural constraint due to the absence of a CH2 spacer on the dynamics. DRS measurements revealed that the two copolymers exhibit three dynamical processes: α-, β-, and γ-processes. The relaxation time of the β-process can be described by the Vogel-Fulcher-Tammann (VFT) law in addition to the α-process. Hence, two glass transition temperatures, Tg, can be evaluated from α- and β-processes. The AdA and nBA fraction dependence of Tgs is consistent with that obtained by dynamical mechanical analysis. The Vogel temperature of the α-process is nearly equal to that of the β-process for the PDiPF homopolymer. With the increase in the AdA fraction, the difference in the Vogel temperature between the α- and β-processes increases. This result suggested that there is an intrinsic correlation between the α- and β-processes for PDiPF, and the correlation changes with the AdA fraction. From the nBA fraction dependence of the β-process, the β-process of PDiPF can be related to the segmental motion of the main chain. Nevertheless, PDiPF exhibits a solid-like property up to the Tg, which is related to the α-process. The physical origins of other processes also were discussed.

Ruthenium(II) Complexes of Heteroditopic N-Heterocyclic Carbene Ligands: Efficient Catalysts for C-N Bond Formation via a Hydrogen-Borrowing Strategy under Solvent-Free Conditions.

Both imidazol-2-ylidene (ImNHC) and 1,2,3-triazol-5-ylidene (tzNHC) have evolved to be elite groups of N-heterocyclic carbene (NHC) ligands for homogeneous catalysis. To develop efficient ruthenium(II)-based catalysts incorporating these ligands for C-N bond-forming reactions via hydrogen-borrowing methodology, we utilized chelating ligands integrated with ImNHC and mesoionic tzNHC donors connected via a CH2 spacer with a diverse triazole backbone. The synthesized ruthenium(II) complexes 3 are found to be highly efficient for C-N bond formation across a wide range of primary amine and alcohol substrates under solvent-free conditions, and among all of the complexes studied here, catalyst 3a with a mesityl substituent displayed maximum activity. To our delight, catalyst 3a is also effective for the selective mono-N-methylation of various anilines utilizing methanol as a coupling partner, known to be relatively more difficult than other alcohols. Furthermore, complex 3a also delivers various substituted quinolines successfully via the reaction of 2-aminobenzyl alcohol with several secondary alcohols. Importantly, catalyst 3a exhibited the highest activity among the reported ruthenium(II) complexes for both the N-benzylation of aniline [achieving a turnover number (TON) of 50000] and the realization of quinoline 8a by reacting 2-aminobenzyl alcohol with 2-phenylethanol (attaining a TON of 30000).

Crystallization of Long-Spaced Precision Polyacetals I: Melting and Recrystallization of Rapidly Formed Crystallites

Rapidly melt-crystallized polyethylenes with acetal groups (−O–CH2–O−) precisely spaced by 12, 18, 19, or 23 methylene backbone carbons exhibit two reorganizations and three melting endotherms on heating at relatively low heating rates. Real-time wide-angle X-ray diffraction (WAXD) experiments further confirm that these transitions are associated with a reorganization to a different crystalline structure. The differential scanning calorimetry endotherms are associated with melting of the initially formed disordered (or hexagonal) phases, followed by melting of Form I and by melting of Form II crystals with increasing temperature. In parallel with these structural changes, the long spacing undergoes a discrete step increase at the polymorphic transitions. Upon the transition from disordered to Form I crystals, the core crystal thickness increases by one repeating unit, whereas the crystal thickness remains basically unchanged during the transition from Form I to Form II crystallites. The effect of even vs odd CH2 spacer on the observed melting temperatures is prevalent for spacer <10 CH2 units. The even or odd number of methylenes between acetals also affects the type and kinetics of packing assembly in layered crystallites. Although a disordered mesomorphic-like structure develops under fast cooling in odd-spaced polyacetals (PA-19, PA-23), the formation of Form I crystals cannot be bypassed in PA-12 and PA-18. Furthermore, some differences are also found in the WAXD patterns of Form II between odd- and even-spaced polyacetals, denoting that staggering of the acetals in the crystallites is affected by the configuration of consecutive acetals with respect to the methylene sequence.

Bichromophoric naphthalene derivatives of diethylenetriaminepentaacetate (DTPA): fluorescent response to H+, Ca2+, Zn2+, and Cd2+

ABSTRACTAs an effort to design selective fluorescent sensors toward Ca2+, Zn2+ and Cd2+, synthetic and fluorometric studies were performed on four bichromophores, each of which consists of two naphthyl or methynaphthyl units (1- and 2-isomers) linked with a diethylenetriaminepentaacetate (DTPA) chain. Every bichromophore exhibits naphthalene-monomer emission at 370 nm and excimer emission at 405 nm. Emission intensities show sensitive pH dependence, from which protonation constants were determined. Fluorometric titrations with the metal ions were performed at the physiological pH and the conditional formation constants were determined. Naphthyl rings define the stoichiometry and stability of the complexes. The insertion of CH2 spacer intensifies the emission and enhances the selective response to metal ions: the excimer emission is strengthened by 70−100 % with Cd2+ coordination, weakened by 60 % with Zn2+, and insensitive to Ca2+. The high response of methylnaphthyl bichromophores to Cd2+ is advantageous in fluorometric analyses.

A series of supramolecular complexes constructed from discrete lanthanide dinuclear butterfly-shaped clusters: Syntheses, structures and properties

Butterfly shaped dinuclear clusters 1–3, with phathaly–based dicarboxylatic group and CH2 spacer, are synthesized by the reaction of Ln(NO3)3 (Ln=Eu3+, Nd3+, Pr3+) with ligand (2-({2-[2-carbosybenzoy])oxy]-ethoxy}carbonyal)benzoic acid) (H2L) in CH3CH2OH, CH3OH, (C2H5)3N and C2H3N. Their formula is identified as [Ln2(μ2-η3-L)2·(CH3OH)4·(η2-NO3)2]·2CH3OH (Ln=Eu(1); Nd(2); Pr(3). The structures of the complexes are characterized by IR spectra, thermal gravimetry (TG), single crystal X-ray diffraction analysis as well as powder X-ray diffraction (PXRD). Structural analyses reveal that they are isomorphous. The thermal behaviors of complexes 1–3 were investigated and the complexes were found to be stable up to about 300°C. Complex 1 exhibited luminescence emission bands at 590, 613, 685 and 697nm at room temperature when excited at 395nm.

Time-evolution study of photoinduced charge-transfer in tertiary amine-fluorophore systems

Photoinduced charge transfer in a fluorescent sensor of organophosphorus nerve agents is examined by solution of the time dependent Schrödinger equation, under the influence of an external electromagnetic field and employing potential energy curves obtained by density functional theory (DFT) and time dependent DFT (TDDFT) calculations for the ground and the excited states, respectively. The systems of interest here consist of a tertiary amine-pyrene molecule, with different numbers of CH2 spacer units between the t-amine and the pyrene. Quenching of pyrene emission has been observed in these systems, to different extent depending on the number of spacer units, which is due to photoinduced electron transfer from the t-amine to pyrene. Here, we present a new approach based on time-evolution for the explanation of the experimental observation of the quenching of pyrene emission. In this work the inversion coordinate at nitrogen is considered as a generalized coordinate for the charge-transfer process. Vibrational levels, dipole moments and dipole transition moments of the ground, excited, and charge transfer electronic states along the inversion coordinate are computed by DFT and TDDFT calculations. The time evolution of the excitation probability between the vibrational levels of the ground state and the first excited state and the charge transfer state has been calculated for different excitation frequencies. The results are in agreement with experimental observations regarding the photoinduced electron transfer and decreasing probability for charge transfer with increasing number of CH2 spacer units between the t-amine and the pyrene.

Comparative Structural Studies of Four Homologous Thioamidic Unclosed Crytpands: Self-Encapsulation of Lariat Arm, Odd–Even Effects, Anomalously Short S···S Chalcogen Bonding, and More

Unclosed cryptands (UCs) 4a–d containing flexible (CH2)n spacers (n = 2–5) and a fixed p-nitrophenyl thioamide substituent (lariat arm) were synthesized and characterized by single crystal X-ray analysis. Comparative analysis of their crystal structures provided an opportunity to recognize that conformation of 4a–d and occurrence of lattice solvent strictly depend on the length and parity of aliphatic linkers. Namely, the degree of self-inclusion of the lariat arm within the macrocyclic cavity was found to increase with greater elongation of the CH2 spacer. Odd-membered UCs (4b and 4d) showed a tendency to crystallize without lattice solvent, while even-membered UCs (4a and 4c) crystallized as various solvates. For two solvates of UC 4c anomalously short and highly directional C═S···S═C chalcogen interactions (dS···S = 3.21–3.35 A) were observed between adjacent UC molecules, forming a dimeric cube-like supramolecular assembly. Packing of dimers and the length of homo-chalcogen interaction were affected b...

Synthesis of fluorinated rhodamines and application for confocal laser scanning microscopy

Four different fluorinated fluorescence dyes were prepared by attaching perfluoroalkyl ponytails (including CH2 or C2H4 spacer) to each of the two amine groups of rhodamine (Rh) and characterized with respect to their fluorescence properties in 2,2,2-trifluoroethanol (TFE), N,N-dimethylformamide (DMF), tetrahydrofuran (THF), and toluene. They showed an excellent quantum yield in TFE. Double staining with Rh-C2H4-C10F21 and Rh-DPPE was employed to visualize the distribution of perfluoropalmitic acid in mixed giant unilamellar vesicles with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) observed by confocal laser scanning microscopy.

Σ- versus π-activation of alkynyl benzoates using B(C₆F₅)₃.

We have prepared a range of alkynyl benzoates in high yields and have investigated their reactivities with the strong Lewis acid B(C6F5)3. In such molecules both σ-activation of the carbonyl and π-activation of the alkyne are possible. In contrast to the reactivity of propargyl esters with B(C6F5)3 which proceed via 1,2-addition of the ester and B(C6F5)3 across the alkyne, the inclusion of an additional CH2 spacer switches off the intramolecular cyclization and selective σ-activation of the carbonyl group is observed through adduct formation. This change in reactivity appears due to the instability of the species which would be formed through B(C6F5)3 activation of the alkyne.

Cyclic Voltammetric Investigation of Interactions between Bisnitroaromatic Compounds and ds.DNA

Herein, the cyclic voltammetric (CV) investigations of structurally similar bisnitrocompounds (N3, N4, N5, N6, having differentCH2� spacer length) is presented. CV study offered interesting interactional possibilities of bisnitrocom- pounds with chicken blood ds.DNA at physiological pH 4.7 and human body temperature, 310 K. The results indicated strong interaction by these symmetric molecules with ds.DNA and strength of binding is found to depend on length of CH2 spacer group in their molecular structure. Thermodynamics derived from electrochemical binding parameters also favored the irre- versible interactions. Moreover, threading intercalation mode of binding is suggested based on thermodynamic and kinetic binding parameters extracted from CV studies.

Conjugated Thiophene-Containing Polymer Zwitterions: Direct Synthesis and Thin Film Electronic Properties

We report a direct and facile synthesis of novel conjugated polymeric zwitterions (CPZs) as a simple route to electronically active homopolymers and copolymers containing dipole-inducing pendent zwitterions. Sulfobetaine-containing polythiophenes (PTSB-1 and PTSB-2) and alternating thiophene–benzothiadiazoles (PTBTSB-1 and PTBTSB-2) were prepared and characterized relative to alkylated polymer analogues (POT-a-T and POT-a-BT). The polar zwitterionic side chains make these polymers hydrophilic and salt-responsive, with interesting electronic properties that depend on zwitterion distance from the conjugated polymer backbone (tether length), as characterized by UV–vis absorption and ultraviolet photoelectron spectroscopy (UPS). Close proximity (CH2 spacer) of the sulfobetaine groups to the polymer backbone results in increased ionization potential and enlarged band gaps of 2.19 and 2.04 eV for PTSB-1 and PTBTSB-1, respectively. On Au and Ag surfaces, the zwitterionic pendent groups significantly alter the work function due to the presence of an interfacial dipole, with the largest interfacial dipoles measuring −1.29 eV (PTBTSB-1/Au) and −0.69 eV (PTBTSB-1/Ag).

The effect of constitutional and conformational isomerization on the electrical properties of diblock molecular diode

The ultimate goal of this paper is to introduce new links between the chemical response of the molecule to the electric field and the physics of cell involving the molecule. Density functional theory (DFT) calculations were preformed to investigate the effect of constitutional and conformational isomerization on the electrical properties of the dipyrimidinyl-diphenyl dithiol ( DPDPh1) as a diblock molecular diode. The non-equilibrium Green’s function approach combined with DFT (NEGF-DFT) has been used to compute the current–voltage characteristics in order to support the obtained results. The calculations could differentiate among the isomers regarding their rectification efficiency. Moreover, plots of the global electrophilicity index and the tendency of constitutional isomers to receive charge from the circuit against applied voltage show I–V curve feature. The conformational analysis has been done through examining the effect of structural twist on the current, dipole moment, HOMO energy, and molecular gain of the DPDPh1 molecule. The rectification is slightly affected by twisting because of the limited change in molecular polarization. However, the results demonstrated that the insertion of a CH 2-spacer or twisting the donor and acceptor parts of the DPDPh1 molecule by 90° generates an Aviram–Ratner-like diode where the HOMO and LUMO are localized on the donor and acceptor subunits, respectively.

Glucopyranoside-Incorporated N-Heterocyclic Carbene Complexes of Silver(I) and Palladium(II): Efficient Water-Soluble Suzuki−Miyaura Coupling Palladium(II) Catalysts

Glucopyranoside-incorporated silver(I) bis(carbene) complexes have been prepared through the reactions of silver oxide with related imidazolium salts. The palladium(II) bis(carbene) complexes were synthesized from their corresponding silver(I) complexes via the technique of transmetalation. In solution, both trans-anti and trans-syn rotamers are observed for these square-planar Pd(II) complexes. In the solid state, the complex [1-methyl-3-(methyl 2,3,4-tri-O-benzoyl-6-deoxy-α-d-glucopyrano-6-yl)imidazol-2-ylidene]2PdBr2 adopts a trans-anti conformation, while the complexes [1-ethyl-3-(methyl 2,3,4-tri-O-benzoyl-6-deoxy-α-d-glucopyrano-6-yl)imidazol-2-ylidene]2PdBr2 and [1-n-butyl-3-(methyl 2,3,4-tri-O-benzoyl-6-deoxy-α-d-glucopyrano-6-yl)imidazol-2-ylidene]2PdBr2 adopt a trans-syn conformation. The steric congestion between the two glucopyranoside groups in the last two trans-syn compounds are perhaps relaxed through tilting of the imidazole rings and rotation of the flexible CH2 spacer, which connects the imidazole ring and the glucopyranoside group. The palladium(II) complexes were further hydrolyzed to produce water-soluble hydrolysates. Preliminary catalytic studies show that these hydrolyzed palladium precatalysts are efficient in the Suzuki−Miyaura coupling reaction between phenylboronic acid and a hydrophilic substrate, 2-chlorobenzoic acid, in aqua medium. In the presence of tetra-n-butylammonium bromide (TBAB), these precatalysts also display efficient catalytic properties in the reaction of phenylboronic acid with both the hydrophilic substrate 1-chloro-3-nitrobenzene and the hydrophobic substrate 2-chlorophenol. In all cases, the catalytic systems are stable, which allow them to be reused for at least three cycles. This catalytic system also facilitates the separation of products from the catalyst and, therefore, is environmentally benign.

Copper(I) Cyanide Coordination Polymers Constructed from Bis(Pyrazole-1-yl)alkane Ligands: Observation of the Odd−Even Dependence in the Structures

The mixture of CuCl2·2H2O and K3[Fe(CN)6] reacts with a series of flexible bis(pyrazole-1-yl)alkane ligands, Pz-(CH2)n-Pz (n = 1 for bpzm, n = 2 for bpze, n = 3 for bpzp, and n = 4 for bpzb), under hydrothermal conditions to produce four new copper(I) cyanide complexes: {[Cu2(CN)2(bpzm)]}n (1), {[Cu2(CN)2(bpze)]}n (2), {[Cu2(CN)2(bpzp)]}n (3), and {[Cu(CN)(bpzb)]}n (4), respectively. The X-ray analyses reveal that 1 exhibits a novel three-dimensional (3D) honeycomb-like framework, 2 shows a 2-fold interpenetration two-dimensional (2D) network, 3 exhibits a 3D structure, and 4 forms a 2D grid-like structure. An interesting odd−even phenomenon is observed in the complexes: when n is odd (n is the number of the CH2 spacer of ligands), the resulting complex exhibits a 3D framework, whereas when n is even, a 2D structure is generated. It could be explained that the numbers have a close relationship with dihedral angles among pyrazole rings of the ligands which have a great effect on the structures. Luminescent properties of the four complexes have been investigated in the solid state and 1 and 3 exhibit strong emissions, while 2 and 4 generate weak or no emission.

A new stable CNHC⁁CH⁁CNHCN-heterocyclic dicarbene ligand: its mono- and dinuclear Ir(i) and Ir(i)–Rh(i) complexes

The ligand [(1,3-phenylene)bis(methylene)]bis(1-ethyl-imidazol-2-ylidene) ( 2) (abbreviated as EtCNHC--CH--CNHC with CH = central aromatic ring, -- = CH2 spacer) has been synthesised, and isolated, by deprotonation of [(1,3-phenylene)bis(methylene)]bis(1-ethyl-imidazolium) dichloride, Et(CHimid.--CH--CHimid.)Cl2 ( 1) with LiN(SiMe3)2. This new, remarkably stable NHC dicarbene ligand is quantitatively protonated with HBr to form [(1,3-phenylene)bis(methylene)]bis(1-ethyl-imidazolium) dibromide Et(CHimid.--CH--CHimid.)Br2 ( 3) and reprotonated slowly enough in MeOH to allow observation of the monoprotonated intermediate 4. Dicarbene 2 reacts with S8 to give the dithione compound [(1,3-phenylene)bis(methylene)]bis(1-ethyl-imidazol-2-thione) Et[(SC)--CH--(CS)] ( 6). Reaction of 2 with [IrCl(CO)(PPh3)2] and [Ir(mu-Cl)(cod)]2 yielded the mononuclear [Ir(CO)(PPh3)2Et(CHimid.--CH--CNHC)(PF6)](PF6) ( 8) and [IrCl(cod)Et(CHimid.--CH--CNHC)](PF6) ( 10) complexes, respectively, and the dinuclear complexes [{Ir(CO)(PPh3)2}2Et(mu-CNHC--CH--CNHC)](PF6)2 ( 7) and [{IrCl(cod)}2Et(mu-CNHC--CH--CNHC)] ( 9), respectively, in which 2 acts as a bridging, non-pincer type ligand. Only one other Ir(I) complex has been reported before containing a CNHC--CH--CNHC ligand. The structures of 6 and 9 were determined by X-ray diffraction and depict different conformations of the (1,3-phenylene)bis(methylene) (or xylylene) moiety. The mononuclear Ir complex 9 reacted smoothly with [Rh(mu-Cl)(cod)]2 and Cs2CO3 to generate a rare example of a heteronuclear NHC complex, [IrRhCl2(cod)2Et(mu-CNHC--CH--CNHC)] ( 11).

Demonstration of bidirectional photoinduced electron transfer (PET) sensing in 4-amino-1,8-naphthalimide based thiourea anion sensors

The thiourea based 4-amino-1,8-naphthalimide molecules 1-5 were designed as fluorescent anion sensors and their photophysical properties investigated upon recognition of biologically relevant anions such as acetate, dihydrogen phosphate and fluoride in DMSO. Synthesised in a single step from their respective aniline precursors, 6-9, these molecules were designed on the fluorophore-spacer-receptor principle, where in the case of sensors 1-3 the thiourea anion recognition moieties were connected to the fluorophore via the 4-amino moiety, while sensors 4 and 5 had the thiourea moieties connected to the 'imide' via a CH2 spacer. The current study showed that 1-5 operated as photoinduced electron transfer (PET) sensors, as no significant changes were observed in their absorption spectra, while their fluorescence emissions were quenched upon recognition of ions such as AcO(-), H2PO4(-) and F(-), which demonstrates that bidirectional PET sensing occurs in such naphthalimide based anion sensors.

Attachment of 4-methoxy benzyl units to a tripodal fluoroionophore shows reversal of output functionality with Cu(II) input

Syntheses of three tris-(2-aminoethyl)amine, tren based tripodal fluoroionophores ( L 2 , L 3 , and L 5 ), are reported. These fluoroionophores are designed based on the fluorophore–spacer–receptor format (choice of fluorophore in all three cases is anthryl unit). In L 2 , three anthracene moieties are attached to the three arms of tren via –CH 2-spacer whereas L 3 and L 5 have p-nitro benzyl and p-methoxy benzyl substitutions, respectively, on L 2 , which are in close proximity to the photoinduced electron transfer (PET) center. All three fluoroionophores show appreciably lower fluorescence compared to anthracene due to effective PET process in these systems but the quantum yield varies depending upon the nature of substitution at the PET center. In the cases of L 2 and L 5 different amounts of fluorescence recovery are observed in the presence of different cation inputs whereas L 3 is almost inactive toward cation sensing. Detailed fluorescence emission studies on L 2 and L 5 in the presence of different cation inputs showed that L 5 having N4 donor sets bearing three p-methoxy benzyl units attached to the three nitrogen centers involving photoinduced electron transfer process is a viable candidate for enhancement of fluorescence with Cu(II) input. In the absence of p-methoxy benzyl units at the nitrogen centers' resulting system, L 2 shows quenching of fluorescence with the Cu(II) under same experimental conditions.

Comparison of Interfacial Electron Transfer through Carboxylate and Phosphonate Anchoring Groups†

The effects of anchoring groups on electron injection from adsorbate to nanocrystalline thin films were investigated by comparing injection kinetics through carboxylate versus phosphonate groups to TiO2 and SnO2. In the first pair of molecules, Re(LA)(CO)3Cl (ReC1A) and Re(Lp)(CO)3Cl (ReC1P), [LA=2,2'-bipyridine-4,4'-bis-CH2-COOH, Lp=2,2'-bipyridine-4,4'-bis-CH2-PO3H2], the anchoring groups were insulated from the bipyridine ligand by a CH2 group. In the second pair of molecules, Ru(dcbpyH2)2(NCS)2 (RuN3) and Ru(bpbpyH2)2(NCS)2 (RuN3P), [dcbpy=2,2'-bipyridine-4,4'-biscarboxylic acid, bpbpy=2,2'-bipyridine-4,4'-bisphosphonic acid], the anchoring groups were directly connected to the bipyridine ligands. The injection kinetics, as measured by subpicosecond IR absorption spectroscopy, showed that electron injection rates from ReC1P to both TiO2 and SnO2 were faster than those from ReC1A. The injection rates from RuN3 and RuN3P to SnO2 films were similar. On TiO2, the injection kinetics from RuN3 and RuN3P were biphasic: carboxylate group enhances the rate of the <100 fs component, but reduces the rate of the slower components. To provide insight into the effect of the anchoring groups, the electronic structures of Re-bipyridyl-Ti model clusters containing carboxylate and phosphonate anchoring groups and with and without a CH2 spacer were computed using density functional theory. With the CH2 spacer, the phosphonate group led to a stronger electronic coupling between bpy and Ti center than the carboxylate group, which accounted for the faster injection from ReC1P than ReC1A. When the anchoring groups were directly connected to the bpy ligand without the CH2 spacer, such as in RuN3 and RuN3P, their effects were 2-fold: the carboxylate group enhanced the electronic coupling of bpy pi* with TiO2 and lowered the energy of the bpy orbital. How these competing factors led to different effects on TiO2 and SnO2 and on different components of the biphasic injection kinetics were discussed.