Potentiometric, electronic structural, and ground- and excited-state optical properties of conjugated bis[(porphinato)zinc(II)] compounds featuring proquinoidal spacer units

We report the synthesis, optical, electrochemical, electronic structural, and transient optical properties of conjugated (porphinato)zinc(II)-spacer-(porphinato)zinc(II) (PZn-Sp-PZn) complexes that possess intervening conjugated Sp structures having varying degrees of proquinoidal character. These supermolecular PZn-Sp-PZn compounds feature Sp moieties {(4,7-diethynylbenzo[c][1,2,5]thiadiazole (E-BTD-E), 6,13-diethynylpentacene (E-PC-E), 4,9-diethynyl-6,7-dimethyl[1,2,5]thiadiazolo[3,4-g]quinoxaline (E-TDQ-E), and 4,8-diethynylbenzo[1,2-c:4,5-c']bis([1,2,5]thiadiazole) (E-BBTD-E)} that regulate frontier orbital energy levels and progressively increase the extent of the quinoidal resonance contribution to the ground and electronically excited states, augmenting the magnitude of electronic communication between terminal (5,-10,20-di(aryl)porphinato)zinc(II) units, relative to that evinced for a bis[(5,5',-10,20-di(aryl)porphinato)zinc(II)]butadiyne benchmark (PZnE-EPZn). Electronic absorption spectra show significant red-shifts of the respective PZn-Sp-PZn x-polarized Q state (S0 --> S1) transition manifold maxima (240-4810 cm(-1)) relative to that observed for PZnE-EPZn. Likewise, the potentiometrically determined PZn-Sp-PZn HOMO-LUMO gaps (E1/2(0/+) - E1/2(-/0)) display correspondingly diminished energy separations that range from 1.88 to 1.11 eV relative to that determined for PZnE-EPZn (2.01 eV). Electronic structure calculations provide insight into the origin of the observed PZn-Sp-PZn electronic and optical properties. Pump-probe transient spectral data for these PZn-Sp-PZn supermolecules demonstrate that the S1 --> S(n) transition manifolds of these species span an unusually broad spectral domain of the NIR. Notably, the absorption maxima of these S1 --> S(n) manifolds can be tuned over a 1000-1600 nm spectral region, giving rise to intense excited-state transitions approximately 4000 cm(-1) lower in energy than that observed for the analogous excited-state absorption maximum of the PZnE-EPZn benchmark; these data highlight the unusually large quinoidal resonance contribution to the low-lying electronically excited singlet states of these PZn-Sp-PZn species. The fact that the length scales of the PZn-Sp-PZn species (approximately 25 angstrom) are small with respect to those of classic conducting polymers, yet possess NIR S1 --> S(n) manifold absorptions lower in energy, underscore the unusual electrooptic properties of these conjugated structures.     

Synthesis of water-soluble poly(p-phenyleneethynylene) in neat water under aerobic conditions via Suzuki-Miyaura polycondensation using a diborylethyne synthon

We report the synthesis, structure, and characterization of a novel ethyne synthon, 1,2-bis(4',4',5',5'-tetramethyl[1',3',2']dioxaborolan-2'-yl)ethyne (B2C2). We demonstrate the utility of B2C2 in the Suzuki-Miyaura polycondensation reaction, synthesizing a water-soluble poly(p-phenyleneethynylene) from [2,5-diiodo-1,4-bis(3-propoxy-sulfonicacid)benzene] sodium salt in neat water under an aerobic atmosphere.     

Molecular design of porphyrin-based nonlinear optical materials

Nonlinear optical chromophores containing (porphyrinato)Zn(II), proquinoid, and (terpyridyl)metal(II) building blocks were optimized in a library containing approximately 10(6) structures using the linear combination of atomic potentials (LCAP) methodology. We report here the library design and molecular property optimizations. Two basic structural types of large beta(0) chromophores were examined: linear and T-shaped motifs. These T-shaped geometries suggest a promising NLO chromophoric architecture for experimental investigation and further support the value of performing LCAP searches in large chemical spaces.     

Ethyne-bridged (porphinato)zinc(II)-(porphinato)iron(III) complexes: phenomenological dependence of excited-state dynamics upon (porphinato)iron electronic structure

We report the synthesis, spectroscopy, potentiometric properties, and excited-state dynamical studies of 5-[(10,20-di-((4-ethyl ester)methylene-oxy)phenyl)porphinato]zinc(II)-[5'-[(10',20'- di-((4-ethyl ester)methylene-oxy)phenyl)porphinato]iron(III)-chloride]ethyne (PZn-PFe-Cl), along with a series of related supermolecules ([PZn-PFe-(L)1,2]+ species) that possess a range of metal axial ligation environments (L = pyridine, 4-cyanopyridine, 2,4,6-trimethylpyridine (collidine), and 2,6-dimethylpyridine (2,6-lutidine)). Relevant monomeric [(porphinato)iron-(ligand)1,2]+ ([PFe(L)1,2]+) benchmarks have also been synthesized and fully characterized. Ultrafast pump-probe transient absorption spectroscopic experiments that interrogate the initially prepared electronically excited states of [PFe(L)1,2]+ species bearing nonhindered axial ligands demonstrated subpicosecond-to-picosecond relaxation dynamics to the ground electronic state. Comparative pump-probe transient absorption experiments that interrogate the initially prepared excited states of PZn-PFe-Cl, [PZn-PFe-(py)2]+, [PZn-PFe-(4-CN-py)2]+, [PZn-PFe-(collidine)]+, and [PZn-PFe-(2,6-lutidine)]+ demonstrate that the spectra of all these species are dominated by a broad, intense NIR S1 --> Sn transient absorption manifold. While PZn-PFe-Cl, [PZn-PFe-(py)2]+, and [PZn-PFe-(4-CN-py)2]+ evince subpicosecond and picosecond time-scale relaxation of their respective initially prepared electronically excited states to the ground state, the excited-state dynamics observed for [PZn-PFe-(2,6-lutidine)]+ and [PZn-PFe-(collidine)]+ show fast relaxation to a [PZn+-PFe(II)] charge-separated state having a lifetime of nearly 1 ns. Potentiometric data indicate that while DeltaGCS for [PZn-PFe-(L)1,2]+ species is strongly influenced by the PFe+ ligation state [ligand (DeltaGCS): 4-cyanopyridine (-0.79 eV) < pyridine (-1.04 eV) < collidine (-1.35 eV) < chloride (-1.40 eV); solvent = CH2Cl2], the pump-probe transient absorption dynamical data demonstrate that the nature of the dominant excited-state decay pathway is not correlated with the thermodynamic driving force for photoinduced charge separation, but depends on the ferric ion ligation mode. These data indicate that sterically bulky axial ligands that drive a pentacoordinate PFe center and a weak metal axial ligand interaction serve to sufficiently suppress the normally large magnitude nonradiative decay rate constants characteristic of (porphinato)iron(III) complexes, and thus make electron transfer a competitive excited-state deactivation pathway.     

A simple model for a two-stage chemical reaction with diffusion

We analyse the evolution of a two-stage chemical reaction betweentwo neighbouring plumes of reactants. Under the assumption thatthe plumes are approximately Gaussian we derive a system ofordinary differential equations for the total amount, the centroidand the variance of each reactant. We compare the solution ofthese equations with full numerical simulation of the reaction.Excellent agreement is obtained, with solution of the near-Gaussianmodel requiring considerably less computational effort thanthe full simulations. Of key importance is the yield of thereaction, and we discuss this feature in particular.     

Two-photon absorption properties of proquinoidal D-A-D and A-D-A quadrupolar chromophores

We report the synthesis, one- and two-photon absorption spectroscopy, fluorescence, and electrochemical properties of a series of quadrupolar molecules that feature proquinoidal π-aromatic acceptors. These quadrupolar molecules possess either donor-acceptor-donor (D-A-D) or acceptor-donor-acceptor (A-D-A) electronic motifs, and feature 4-N,N-dihexylaminophenyl, 4-dodecyloxyphenyl, 4-(N,N-dihexylamino)benzo[c][1,2,5]thiadiazolyl or 2,5-dioctyloxyphenyl electron donor moieties and benzo[c][1,2,5]thiadiazole (BTD) or 6,7-bis(3',7'-dimethyloctyl)[1,2,5]thiadiazolo[3,4-g]quinoxaline (TDQ) electron acceptor units. These conjugated structures are highly emissive in nonpolar solvents and exhibit large spectral red-shifts of their respective lowest energy absorption bands relative to analogous reference compounds that incorporate phenylene components in place of BTD and TDQ moieties. BTD-based D-A-D and A-D-A chromophores exhibit increasing fluorescence emission red-shifts, and a concomitant decrease of the fluorescence quantum yield (Φ(f)) with increasing solvent polarity; these data indicate that electronic excitation augments benzothiadiazole electron density via an internal charge transfer mechanism. The BTD- and TDQ-containing structures exhibit blue-shifted two-photon absorption (TPA) spectra relative to their corresponding one-photon absorption (OPA) spectra, and display high TPA cross sections (>100 GM) within these spectral windows. D-A-D and A-D-A structures that feature more extensive conjugation within this series of compounds exhibit larger TPA cross sections consistent with computational simulation. Factors governing TPA properties of these quadrupolar chromophores are discussed within the context of a three-state model.     

Dynamics and transient absorption spectral signatures of the single-wall carbon nanotube electronically excited triplet state

We utilize femtosecond-to-microsecond time domain pump-probe transient absorption spectroscopy to interrogate for the first time the electronically excited triplet state of individualized single-wall carbon nanotubes (SWNTs). These studies exploit (6,5) chirality-enriched SWNT samples and poly[2,6-{1,5-bis(3-propoxysulfonic acid sodium salt)}naphthylene]ethynylene (PNES), which helically wraps the nanotube surface with periodic and constant morphology (pitch length = 10 ± 2 nm), providing a self-assembled superstructure that maintains structural homogeneity in multiple solvents. Spectroscopic interrogation of such PNES-SWNT samples in aqueous and DMSO solvents using E(22) excitation and a white-light continuum probe enables E(11) and E(22) spectral evolution to be monitored concomitantly. Such experiments not only reveal classic SWNT singlet exciton relaxation dynamics and transient absorption signatures but also demonstrate spectral evolution consistent with formation of a triplet exciton state. Transient dynamical studies evince that (6,5) SWNTs exhibit rapid S(1)→T(1) intersystem crossing (ISC) (τ(ISC) ~20 ps), a sharp T(1)→T(n) transient absorption signal (λ(max)(T(1)→T(n)) = 1150 nm; full width at half-maximum ≈ 350 cm(-1)), and a substantial T(1) excited-state lifetime (τ(es) ≈ 15 μs). Consistent with expectations for a triplet exciton state, T(1)-state spectral signatures and T(1)-state formation and decay dynamics for PNES-SWNTs in aqueous and DMSO solvents, as well as those determined for benchmark sodium cholate suspensions of (6,5) SWNTs, are similar; likewise, studies that probe the (3)[(6,5) SWNT]* state in air-saturated solutions demonstrate (3)O(2) quenching dynamics reminiscent of those determined for conjugated aromatic hydrocarbon excited triplet states.