Near-infrared-emitting phthalocyanines. A combined experimental and density functional theory study of the structural, optical, and photophysical properties of Pd(II) and Pt(II) α-butoxyphthalocyanines

The structural, optical, and photophysical properties of 1,4,8,11,15,18,22,25-octabutoxyphthalocyaninato-palladium(II), PdPc(OBu)(8), and the newly synthesized platinum analogue PtPc(OBu)(8) are investigated combining X-ray crystallography, static and transient absorption spectroscopy, and relativistic zeroth-order regular approximation (ZORA) Density Functional Theory (DFT)/Time Dependent DFT (TDDFT) calculations where spin-orbit coupling (SOC) effects are explicitly considered. The results are compared to those previously reported for NiPc(OBu)(8) (J. Phys. Chem. A 2005, 109, 2078) in an effort to highlight the effect of the central metal on the structural and photophysical properties of the group 10 transition metal octabutoxyphthalocyanines. Different from the nickel analogue, PdPc(OBu)(8) and PtPc(OBu)(8) show a modest and irregular saddling distortion of the macrocycle, but share with the first member of the group similar UV-vis spectra, with the deep red and intense Q-band absorption experiencing a blue shift down the group, as observed in virtually all tetrapyrrolic complexes of this triad. The blue shift of the Q-band along the MPc(OBu)(8) (M = Ni, Pd, Pt) series is interpreted on the basis of the metal-induced electronic structure changes. Besides the intense deep red absorption, the title complexes exhibit a distinct near-infrared (NIR) absorption due to a transition to the double-group 1E (π,π*) state, which is dominated by the lowest single-group (3)E (π,π*) state. Unlike NiPc(OBu)(8), which is nonluminescent, PdPc(OBu)(8) and PtPc(OBu)(8) show both deep red fluorescence emission and NIR phosphorescence emission. Transient absorption experiments and relativistic spin-orbit TDDFT calculations consistently indicate that fluorescence and phosphorescence emissions occur from the S(1)(π,π*) and T(1)(π,π*) states, respectively, the latter being directly populated from the former, and the triplet state decays directly to the S(0) surface (the triplet lifetime in deaerated benzene solution was 3.04 μs for Pd and 0.55 μs for Pt). Owing to their triplet properties, PdPc(OBu)(8) and PtPc(OBu)(8) have potential for use in photodynamic therapy (PDT) and are potential candidates for NIR light emitting diodes or NIR emitting probes.     

Transport properties of composite membranes containing silicon dioxide and Nafion

Silicon dioxide (SiO₂) nanoparticles were incorporated into Nafion 115 membranes using the sol–gel method in order to investigate their effect on water retention/transport, proton concentration, effective proton mobility, and proton conductivity. By adjusting the sol–gel reaction time, Nafion/SiO₂ membranes were fabricated with SiO₂ content ranging from 5.9 to 33.3 wt%. Because the density of the membranes decreased with increasing SiO₂ content and because dimensional changes with swelling in water of the composite membranes were less than that of unmodified Nafion 115 despite having increased water content, the theory that rigid scaffolding is formed inside the membrane is supported. Water content increases with increasing SiO₂ content due to void space formed inside the membrane. This increase in water content dilutes the protons in the membrane leading to lower proton concentration and therefore lower proton conductivity. A decreasing effective proton mobility with increasing SiO₂ content, likely due to an increase in the tortuosity of the proton-conducting pathway, also contributes to the decreasing conductivity. However, as evidenced by the similar water vapour permeance values, the SiO₂ nanoparticles do not increase the effective tortuosity of the water vapour transmission pathways.     

Dipole effects on cation-pi interactions: absolute bond dissociation energies of complexes of alkali metal cations to N-methylaniline and N,N-dimethylaniline

Threshold collision-induced dissociation of M (+)( nMA) x with Xe is studied using guided ion beam mass spectrometry, where nMA = N-methylaniline and N, N-dimethylaniline and x = 1 and 2. M (+) includes the following alkali metal cations: Li (+), Na (+), K (+), Rb (+), and Cs (+). In all cases, the primary dissociation pathway corresponds to the endothermic loss of an intact nMA ligand. The primary cross section thresholds are interpreted to yield 0 and 298 K bond dissociation energies (BDEs) for ( nMA) x-1 M (+)-( nMA) after accounting for the effects of multiple ion-neutral collisions, the internal and kinetic energy distributions of the reactants, and the dissociation lifetimes. Density functional theory calculations at the B3LYP/6-31G* level of theory are used to determine the structures of these complexes, which are also used in single-point calculations at the MP2(full)/6-311+G(2d,2p) level to determine theoretical BDEs. The results of these studies are compared to previous studies of the analogous M (+)(aniline) x complexes to examine the effects of methylation of the amino group on the binding interactions. Comparisons are also made to a wide variety of cation-pi complexes previously studied to elucidate the contributions that ion-dipole, ion-induced-dipole, and ion-quadrupole interactions make to the overall binding.     

Infrared multiphoton dissociation spectroscopy of cationized serine: effects of alkali-metal cation size on gas-phase conformation

The gas-phase structures of alkali-metal cation complexes of serine (Ser) are examined using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser, in conjunction with ab initio calculations. Spectra of Li+(Ser) and Na+(Ser) are similar and relatively simple, whereas Cs+(Ser) includes distinctive new IR bands, and K+(Ser) and Rb+(Ser) exhibit intermediate behavior. Measured IRMPD spectra are compared to spectra calculated at a B3LYP/6-311+G(d,p) level to identify the structures present in the experimental studies. On the basis of these experiments and calculations, the only conformations accessed for the complexes to the smaller alkali-metal cations, Li+ and Na+, are charge-solvated structures involving tridentate coordination to the amine and carbonyl groups of the amino acid backbone and to the hydroxyl group of the side chain, M1[N,CO,OH]. For the cesiated complex, a band corresponding to a zwitterionic structure, ZW[CO2-], is clearly visible. K+(Ser) and Rb+(Ser) exhibit evidence of the charge-solvated analogue of the zwitterions, M3[COOH], in which the metal cation binds to the carboxylic acid group. Calculations indicate that the relative stability of the M3[COOH] structure is very strongly dependent on the size of the metal cation, consistent with the range of conformations observed experimentally.     

Structural and Energetic Effects in the Molecular Recognition of Acetylated Amino Acids by 18-Crown-6

Absolute 18-crown-6 (18C6) binding affinities of four protonated acetylated amino acids (AcAAs) are determined using guided ion beam tandem mass spectrometry techniques. The AcAAs examined in this work include: N-terminal acetylated lysine (N_???CAcLys), histidine (N_???CAcHis), and arginine (N_???CAcArg) as well as side chain acetylated lysine (N_???CAcLys). The kinetic-energy-dependent cross sections for collision-induced dissociation (CID) of the (AcAA)H⁺(18C6) complexes are analyzed using an empirical threshold law to extract absolute 0 and 298?K (AcAA)H⁺?18C6 bond dissociation energies (BDEs) after accounting for the effects of multiple collisions, kinetic and internal energy distributions of the reactants, and unimolecular dissociation lifetimes. Theoretical electronic structure calculations are performed to determine stable geometries and energetics for neutral and protonated 18C6 and the AcAAs as well as the proton bound complexes of these species, (AcAA)H⁺(18C6), at the B3LYP/6-311+G(2d,2p)//B3LYP/6-31?G* and M06/6-311+G(2d,2p)//B3LYP/6-31G* levels of theory. For all four (AcAA)H⁺(18C6) complexes, loss of neutral 18C6 corresponds to the most favorable dissociation pathway. At elevated energies, products arising from sequential dissociation of the primary CID product, H⁺(AcAA), are also observed. Protonated N_???CAcLys exhibits a greater 18C6 binding affinity than other protonated N_???CAcAAs, suggesting that the side chains of Lys residues are the preferred binding sites for 18C6 complexation to peptides and proteins. N_???CAcLys exhibits a greater 18C6 binding affinity than N_???CAcLys, suggesting that binding of 18C6 to the side chain of Lys residues is more favorable than to the N-terminal amino group of Lys.     

Sodium Cation Affinities of Commonly Used MALDI Matrices Determined by Guided Ion Beam Tandem Mass Spectrometry

The sodium cation affinities of six commonly used MALDI matrices are determined here using guided ion beam tandem mass spectrometry techniques. The collision-induced dissociation behavior of six sodium cationized MALDI matrices, Na⁺(MALDI), with Xe is studied as a function of kinetic energy. The MALDI matrices examined here include: nicotinic acid, quinoline, 3-aminoquinoline, 4-nitroaniline, picolinic acid, and 3-hydroxypicolinic acid. In all cases, the primary dissociation pathway corresponds to endothermic loss of the intact MALDI matrix. The cross section thresholds are interpreted to yield zero and 298 K Na⁺−MALDI bond dissociation energies (BDEs), or sodium cation affinities, after accounting for the effects of multiple ion-neutral collisions, the kinetic and internal energy distributions of the reactants, and dissociation lifetimes. Density functional theory calculations at the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G* and MP2(full)/6-311+G(2d,2p)//B3LYP/6-31G* levels of theory are used to characterized the structures and energetics for these systems. The calculated BDEs exhibit very good agreement with the measured values for most systems. The experimental and theoretical Na⁺−MALDI BDEs determined here are compared with those previously measured by cation transfer equilibrium methods.     

Metal cation dependence of interactions with amino acids: bond energies of Cs+ to Gly, Pro, Ser, Thr, and Cys

The interactions of cesium cations with five amino acids (AA) including glycine (Gly), proline (Pro), serine (Ser), threonine (Thr), and cysteine (Cys) are examined in detail. Experimentally, the bond dissociation energies (BDEs) are determined using threshold collision-induced dissociation of the Cs(+)(AA) complexes with xenon in a guided ion beam tandem mass spectrometer. Analyses of the energy-dependent cross sections include consideration of unimolecular decay rates, internal energy of the reactant ions, and multiple ion-neutral collisions. Bond dissociation energies (0 K) of 93.3 ± 2.5, 107.9 ± 4.6, 102.3 ± 4.1, 105.4 ± 4.3, and 96.8 ± 4.2 kJ/mol are determined for complexes of Cs(+) with Gly, Pro, Ser, Thr, and Cys, respectively. Quantum chemical calculations are conducted at the B3LYP, B3P86, MP2(full), and M06 levels of theory with geometries and zero-point energies calculated at the B3LYP level using both HW*/6-311+G(2d,2p) and def2-TZVPPD basis sets. Results obtained using the former basis sets are systematically low compared to the experimental bond energies, whereas the latter basis sets show good agreement. For Cs(+)(Gly), theory predicts the ground-state conformer has the cesium cation binding to the carbonyl group of the carboxylic acid. For Cs(+)(Pro), the secondary nitrogen accepts the carboxylic acid hydrogen to form the zwitterionic structure, and the metal cation binds to both oxygens. Cs(+)(Ser), Cs(+)(Thr), and Cs(+)(Cys) are found to have tridentate binding at the MP2(full) level, whereas the density functional approaches slightly prefer bidentate binding of Cs(+) at the carboxylic acid moiety. Comparison of these results to those for the smaller alkali cations provides insight into the trends in binding affinities and structures associated with metal cation variations.     

Effect of temperature and surfactant structure on the microviscosity at the micelle-water interface: isomeric alkylbenzenesulfonates used as their own probe

We investigated the effect of temperature and surfactant structure on the microviscosity in aqueous micellar solutions formed by isomeric hexadecylbenzenesulfonates (xphiC16, where x=4-6 and indicates the position of the benzene ring [phi] along the alkyl chain) by fluorescence polarization and excimer emission spectroscopy. For a given isomer, the degree of polarization (p) was found to decrease with increasing temperature, with no evidence for changes in micellar structure. etaint/tau ratios, where etaint is the microviscosity of the benzene environment in micelles and tau its natural lifetime, were derived from fluorescence polarization measurements and showed a similar temperature behavior to that observed with the degree of polarization, suggesting that a thermal effect is the determinant factor in the variation of etaint. Interestingly, the microviscosity around the benzene ring was found to depend on the isomer structure in the entire range of temperatures investigated (8-60 degrees C) and is mainly determined by the orientation of the surfactant at the micelle-water interface in which the short alkyl chain is preferentially located at the interface and the long alkyl chain in the micellar core. This micelle conformation was found to prevail in the entire range of temperatures. In contrast to the dependence of p with temperature, excimer to monomer maximum emission ratios (IE/IM) were found to increase with increasing temperature, showing that when IE/IM is high (strong excimer emission), the degree of polarization is low (low microviscosity) and vice versa. Thus, the two independent measurements (IE/IM and p) yield the same information, namely, that the benzene moiety in all xphiC16 aqueous micelles resists both translational and rotational diffusion in a similar manner in the entire range of temperatures investigated (approximately 8-60 degrees C).