Effects of a phosphonate anchoring group on the excited state electron transfer rates from a terthiophene chromophore to a ZnO nanocrystal.

Terthiophene dyes were synthesized having a carboxylate or a phosphonate moiety at the 2-position which serves as an anchoring group to zinc oxide nanocrystals (ZnO NCs). Electronic absorption and fluorescence measurements, combined with reduction potentials, provided estimates of -1.81 and -1.86 V vs. NHE for the excited state reduction potential of the carboxylate and phosphonate, respectively. Static quenching was observed when the dyes were bound to the surface of acetate-capped ZnO NCs having a diameter of 2.8 nm. Stern-Volmer studies conducted at several dye concentrations established that a minor fraction of the adsorbed dye remained unquenched even at 1 : 1 dye to NC ratios. Adsorption isotherm measurements established that the phosphonate binds more strongly than the carboxylate and that saturation coverage was ∼1.2 dyes per nm2 for both dyes. Ultrafast transient absorption spectroscopic experiments were used to probe excited state dynamics. In the presence of ZnO NCs, disappearance of the singlet excited state of the dye corresponded to appearance of the spectroscopic signature of the oxidized dye with a time constant of 1.5 ± 0.1 and 6.1 ± 0.2 ps, respectively, for the carboxylate and phosphonate dye. The difference in the electron transfer rates was attributed to a larger electronic coupling for the dye having the carboxylate anchoring group.

Zinc oxide nanocrystal quenching of emission from electron-rich ruthenium-bipyridine complexes.

A series of heteroleptic bipyridine ruthenium complexes were prepared using known synthetic methods. Each compound incorporated one electron withdrawing 4,4'-dicarboxylic acid-2,2'-bipyridine and two bipyridines each of which had electron donating dialkylamine substituents in the 4 and 4' positions. The electronic absorption spectra exhibited absorptions that moved to lower energy as the donor ability of the amine substituent increased. Density functional calculations established that the HOMO was delocalized over the metal and two pyridine groups located trans to the pyridines of the dicarboxylic acid bipyridine. The LUMO was delocalized over the dicarboxylic acid bipyridine. Cyclic voltammetry of the deprotonated compounds exhibit one quasi-reversible oxidation and three reductions. Coupled with the emission data, the excited state reduction potentials were estimated to range from -0.93 to -1.03 V vs. NHE. Monodispersed 3.2 nm diameter nanocrystals (NCs) of zinc oxide were found to quench partially the excited state of the dyes via a static quenching electron transfer process involving the formation of a dyad of the complex and the NC. The magnitude of the partial quenching of complexed dyes was correlated to the distribution of band gaps for the NCs, which is an inverse function of diameter. Dyes attached to the NCs on the small end of the particle size distribution had electron transfer rates that were uncompetitive with radiative and nonradiative decay mechanisms.

Heteroleptic platinum(II) isocyanide complexes: convenient synthetic access, polymorphs, and vapoluminescence.

A convenient synthetic scheme was developed to access a series of square-planar Pt(II) complexes of the form (TBA)[Pt(CN)3(CNR)] (TBA = tetra-n-butyl ammonium cation, R = CH3, c-C6H11, p-(C2H5)-C6H4). These heteroleptic complexes were characterized by combustion analyses, single-crystal X-ray structure determinations, HRFAB-MS, (1)H NMR, ATR-IR, solution UV-Vis, and solid-state emission spectroscopies. Surprisingly, each of these complexes exhibit low energy room temperature solid-state luminescence in the absence of intermolecular Pt-Pt interactions. Additionally, the polymorphic behavior of (TBA)[Pt(CN)3(CN-c-C6H11)] was elucidated through a combination of single-crystal X-ray structure determinations, X-ray powder diffraction, and mass uptake experiments. Vapoluminescence and polymorphic transformations of (TBA)[Pt(CN)3(CN-c-C6H11)] were concomitant with water vapor absorption. A new double-salt complex [Pt(terpy)Cl] [Pt(CN)3(CNCH3)] (terpy = 2,2',6',2''-terpyridine) was also synthesized and characterized by combustion analysis, ATR-IR, HRFAB-MS, and solid-state UV-Vis and emission spectroscopies. The vapoluminescent properties of this salt in response to water were examined by mass uptake experiments, ATR-IR, and solid-state emission spectroscopy. A principal component analysis (PCA) of the solid-state emission changes induced by water vapor confirmed the presence of an intermediate emissive species.

Impact of the synergistic collaboration of oligothiophene bridges and ruthenium complexes on the optical properties of dumbbell-shaped compounds.

The linear and non-linear optical properties of a family of dumbbell-shaped dinuclear complexes, in which an oligothiophene chain with various numbers of rings (1, 3, and 6) acts as a bridge between two homoleptic tris(2,2'-bipyridine)ruthenium(II) complexes, have been fully investigated by using a range of spectroscopic techniques (absorption and luminescence, transient absorption, Raman, and non-linear absorption), together with density functional theory calculations. Our results shed light on the impact of the synergistic collaboration between the electronic structures of the two chemical moieties on the optical properties of these materials. Experiments on the linear optical properties of these compounds indicated that the length of the oligothiophene bridge was critical for luminescent behavior. Indeed, no emission was detected for compounds with long oligothiophene bridges (compounds 3 and 4, with 3 and 6 thiophene rings, respectively), owing to the presence of the (3)π-π* state of the conjugated bridge below the (3)MLCT-emitting states of the end-capping Ru(II) complexes. In contrast, the compound with the shortest bridge (2, one thiophene ring) shows excellent photophysical features. Non-linear optical experiments showed that the investigated compounds were strong non-linear absorbers in wide energy ranges. Indeed, their non-linear absorption was augmented upon increasing the length of the oligothiophene bridge. In particular, the compound with the longest oligothiophene bridge not only showed strong two-photon absorption (TPA) but also noteworthy three-photon-absorption behavior, with a cross-section value of 4×10(-78) cm(6) s(2) at 1450 nm. This characteristic was complemented by the strong excited-state absorption (ESA) that was observed for compounds 3 and 4. As a matter of fact, the overlap between the non-linear absorption and ESA establishes compounds 3 and 4 as good candidates for optical-power-limiting applications.

M-M bond-stretching energy landscapes for M2(dimen)4(2+) (M = Rh, Ir; dimen = 1,8-diisocyanomenthane) complexes.

Isomers of Ir(2)(dimen)(4)(2+) (dimen = 1,8-diisocyanomenthane) exhibit different Ir-Ir bond distances in a 2:1 MTHF/EtCN solution (MTHF = 2-methyltetrahydrofuran). Variable-temperature absorption data suggest that the isomer with the shorter Ir-Ir distance is favored at room temperature [K = ∼8; ΔH° = -0.8 kcal/mol; ΔS° = 1.44 cal mol(-1) K(-1)]. We report calculations that shed light on M(2)(dimen)(4)(2+) (M = Rh, Ir) structural differences: (1) metal-metal interaction favors short distances; (2) ligand deformational-strain energy favors long distances; (3) out-of-plane (A(2u)) distortion promotes twisting of the ligand backbone at short metal-metal separations. Calculated potential-energy surfaces reveal a double minimum for Ir(2)(dimen)(4)(2+) (∼4.1 Å Ir-Ir with 0° twist angle and ∼3.6 Å Ir-Ir with ±12° twist angle) but not for the rhodium analogue (∼4.5 Å Rh-Rh with no twisting). Because both the ligand strain and A(2u) distortional energy are virtually identical for the two complexes, the strength of the metal-metal interaction is the determining factor. On the basis of the magnitude of this interaction, we obtain the following results: (1) a single-minimum (along the Ir-Ir coordinate), harmonic potential-energy surface for the triplet electronic excited state of Ir(2)(dimen)(4)(2+) (R(e,Ir-Ir) = 2.87 Å; F(Ir-Ir) = 0.99 mdyn Å(-1)); (2) a single-minimum, anharmonic surface for the ground state of Rh(2)(dimen)(4)(2+) (R(e,Rh-Rh) = 3.23 Å; F(Rh-Rh) = 0.09 mdyn Å(-1)); (3) a double-minimum (along the Ir-Ir coordinate) surface for the ground state of Ir(2)(dimen)(4)(2+) (R(e,Ir-Ir) = 3.23 Å; F(Ir-Ir) = 0.16 mdyn Å(-1)).

Exceptionally long-lived luminescence from [Cu(I)(isocyanide)2(phen)]+ complexes in nanoporous crystals enables remarkable oxygen gas sensing.

We report crystalline mixed-ligand copper complexes with phenanthroline and isocyanides with almost millesecond emission lifetimes that are efficient dioxygen sensors. The oxygen sensitivity of the prototype ([Cu(CN-xylyl)(2)(dmp)]tfpb, dmp = 2,9-dimethyl-1,10-phenanthroline; CN-xylyl = 2,6-dimethylphenylisocyanide; tfpb = tetrakis(bis-3,5-trifluoromethylphenylborate) is 38 times better than that of [Ru(phen)(3)]tfpb(2) (phen = 1,10-phenanthroline).

Effect of axially projected oligothiophene pendants and nitro-functionalized diimine ligands on the lowest excited state in cationic Ir(III) bis-cyclometalates.

The novel terthiophene (3T) oligomer 6 and a series of cationic Ir(III) bis-cyclometalates [Ir(C^N)(2)(N^N)]PF(6) 9-12 were prepared. The synthesis, characterization, electrochemical, and photophysical properties are reported. The cyclometalating ligands (C^N) are 2-phenylpyridinato (ppy) or the 3T oligomer (3T-ppy), asymmetrically capped in the 5 and 5" positions with the ppy and mesityl groups. The diimine ligands (N^N) are 2,2'-bipyridine (bpy) or 4-NO(2)-bipyridine (4-NO(2)-bpy). Hybrid metal-organic complexes 11 and 12 bear 3T-pendants ligated through the ppy cap, 10 and 12 contain NO(2) functionalized diimines, whereas 9 contains neither. Structural characterization of 10 by single crystal X-ray diffraction confirms the presence of the NO(2) substituent and pseudo-octahedral coordination geometry about the Ir(III) ion. Cyclic voltammetry highlights the large electron withdrawing effect of the NO(2) substituent, providing an 850 mV shift toward lower potentials for the first diimine centered reduction of 10 and 12. Strong overlap of the intense π → π* absorptions of the 3T-pendants with Ir(III) charge transfer bands is evident in complexes of 11 and 12, precluding the possibility for selective excitation of either chromophore. Photoexcitation (λ(ex) = 400 nm) of the series affords strong luminescence from the 3T oligomer 6 and the unsubstituted 9, with φ(em) = 0.11. In stark contrast the NO(2) and 3T functionalized complexes 10-12 display near total quenching of luminescence. Computations of the ground and excited state electronic structure using density functional theory (DFT) and time-dependent DFT (TD-DFT) indicate that both the NO(2) and 3T substituents play an important role in excited state deactivation of complexes 10-12. A substantial electronic contribution of the NO(2) substituent results in stabilization of the diimine based molecular orbital (MO) and offers an efficient nonradiative decay pathway for the excited state. Spin-orbit coupling effects of the Ir(III) ion lead to efficient population of the low lying, nonluminescent, triplet states centered on the 3T-pendants.

(S)-(-)-1-Phenyl-ethanaminium 4-(4,4-di-fluoro-1,3,5,7-tetra-methyl-3a,4a-diaza-4-borata-s-indacen-8-yl)benzoate.

The title compound, C(8)H(12)N(+)·C(20)H(18)BF(2)N(2)O(2) (-), crystallizes with a significant amount of void space [4.0 (5)%] in the unit cell. The structure displays N-H⋯O hydrogen bonding between the components. The plane formed by the benzoic acid moiety of the BODIPY-CO(2) (-) is twisted by 80.71 (6)° relative to the plane formed by the ring C and N atoms of the tetramethyldipyrrin portion of the molecule.

Optical response of a cyclometalated iridium(III) hydrazino complex to carbon dioxide: generation of a strongly luminescent iridium(III) carbazate.

A system pairing the luminescent core of [Ir(ppy)(2)L(2)](+) (ppy = 2-phenylpyridine) with simple hydrazino ancillary ligands (L = N(2)H(4)) has been prepared for the direct optical detection of carbon dioxide (CO(2)). Silver-assisted and silver-free techniques were used for the successful introduction of N(2)H(4) into the [Ir(ppy)(2)Cl](2) coordination sphere at room temperature to give the corresponding biscyclometalated iridium(III) hydrazino species as either a CF(3)SO(3)(-) (OTf(-), 2a) or Cl(-) (2b) salt. The silver-free route was accomplished by the direct replacement of the ligated Cl(-) using a slight excess of hydrazine. The luminescence profile of the cationic iridium(III) hydrazino complex 2a (λ(max) = 501 nm) undergoes a red shift (λ(max) = 524 nm), accompanied by a change in the peak shape during exposure to CO(2) in solution. The spectral changes observed are attributed to the formation of the corresponding neutral carbazate species Ir(ppy)(2)(H(2)NNHCOO) (3) and are not consistent with protonation of the ligated hydrazine. Conversion of the hydrazino species to the carbazate species is solvent-dependent and irreversible. The hydrazino and carbazate species have been structurally characterized by single-crystal X-ray diffraction; both compounds exhibit long-lived and intense room temperature luminescence in solution with τ = 1.56 and 1.80 µs and φ(em) = 0.42 and 0.45, respectively.

Tuning electron transfer rates via systematic shifts in the acceptor state density using size-selected ZnO colloids.

We report direct measurements of the influence of the available density of acceptor states on the rate of near-barrierless electron transfer between a dye sensitizer and an oxide semiconductor. The electron donor was the excited state of a zinc porphyrin, and the acceptors were a series of size-selected ZnO nanocrystals. The available density of states was tuned by controlling the relative position of the ZnO band edge using quantum confinement. The resulting change in the rate was consistent with a simple model of the state density as a function of energy above the ZnO band edge.

Oxygen gas sensing by luminescence quenching in crystals of Cu(xantphos)(phen)+ complexes.

We have shown that crystals of the highly emissive copper(I) compounds [Cu(POP)(dmp)]tfpb, [Cu(xantphos)(dmp)]tfpb, [Cu(xantphos)(dipp)]tfpb, and [Cu(xantphos)(dipp)]pftpb, (where POP = bis[2-(diphenylphosphino)phenyl]ether; xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene; dmp = 2,9-dimethyl-1,10-phenanthroline; dipp = 2,9-diisopropyl-1,10-phenanthroline (dipp); tfpb(-) = tetrakis(bis-3,5-trifluoromethylphenylborate); and pftpb = tetrakis(pentfluorophenyl)borate) are oxygen gas sensors. The sensing ability correlates with the amount of void space calculated from the crystal structures. The compounds exhibit linear Stern-Volmer plots with reproducible K(SV) constants from sample to sample; these results reinforce the observations that the sensing materials are crystalline and the sensing sites are homogeneous within the crystals. The long lifetime (∼30 µs), high emission quantum yield (ϕ = 0.66), appreciable K(SV) value (5.65), and very rapid response time (51 ms for the 95% return constant) for [Cu(xantphos)(dmp)]tfpb are significantly better than those for the [Cu(NN)(2)]tfpb complexes studied previously and compare favorably with [Ru(4,7-Me2phen)(3)](tfpb)(2), (K(SV) = 4.76; 4,7-Me(2)phen = 4,7-dimethyl-1,10- phenanthroline). The replacement of precious metals (like Ru or Pt) with copper may be technologically significant and the new compounds can be synthesized in one or two steps from commercially available starting materials. The strictly linear Stern-Volmer behavior observed for these systems and the absence of a polymer matrix that might cause variability in sensor to sensor sensitivity may allow a simple single-reference point calibration procedure, an important consideration for an inexpensive onetime limited use sensor that could be mass produced.

Mono- and bis-tolylterpyridine iridium(III) complexes.

The first structure report of trichlorido[4'-(p-tolyl)-2,2':6',2''-terpyridine]iridium(III) dimethyl sulfoxide solvate, [IrCl(3)(C(22)H(17)N(3))] x C(2)H(6)OS, (I), is presented, along with a higher-symmetry setting of previously reported bis[4'-(p-tolyl)-2,2':6',2''-terpyridine]iridium(III) tris(hexafluoridophosphate) acetonitrile disolvate, [Ir(C(22)H(17)N(3))(2)](PF(6))(3) x 2 C(2)H(3)N, (II) [Yoshikawa, Yamabe, Kanehisa, Kai, Takashima & Tsukahara (2007). Eur. J. Inorg. Chem. pp. 1911-1919]. For (I), the data were collected with synchrotron radiation and the dimethyl sulfoxide solvent molecule is disordered over three positions, one of which is an inversion center. The previously reported structure of (II) is presented in the more appropriate C2/c space group. The iridium complex and one PF(6)(-) anion lie on twofold axes in this structure, making half of the molecule unique.

The influence of internal charge transfer on nonradiative decay in substituted terthiophenes.

Photophysical data for a series of end substitued 3',4'-dibutyl-2,2':5',2''-terthiophenes are reported. Static absorption and fluorescence, quantum yields, time-resolved fluorescence, and time- and frequency-resolved pump-probe spectra are applied to investigate excited state relaxation in bromo, nitro, and tricyanovinyl substituted species in a variety of solvents. The effect of solvent polarizability and end-group substitution is discussed in the context of charge transfer in the excited state and its impact on nonradiative decay rates. In solution at room temperature, both symmetric and asymmetric addition of electron withdrawing end groups generate an excited state with substantial charge transfer character. Solvent polarizability has a significant influence on the excited state dynamics in the charge transfer compounds. Examples include a 20-fold reduction in the intersystem crossing rate going from hexane to toluene and an order of magnitude increase in the internal conversion rate between toluene and acetone. The results demonstrate that the impact of the substituents on intramolecular charge transfer, and the resulting amplification of the interactions between the excited state(s) and the local molecular environment, can dramatically change the excited state relaxation dynamics in substituted terthiophenes.

Inefficient crystal packing in chiral [Ru(phen)(3)](PF(6))(2) enables oxygen molecule quenching of the solid-state MLCT emission.

The molecular oxygen quenching of the solid-state emission from pure crystals of Delta-Ru(phen)(3)(PF(6))(2), Lambda-Ru(phen)(3)(PF(6))(2), and racemic Ru(phen)(3)(PF(6))(2) (phen = 1,10-phenanthroline) was studied by emission spectroscopy. Crystals of the pure enantiomers exhibit significant and nearly identical emission-intensity quenching [0.36(2) and 0.33(2), respectively] in the presence of air [where the fraction quenched is (I(nitrogen) - I(air))/I(nitrogen)]; in comparison, the racemic compound shows a much lower value [0.05(2)]. The large difference in the quenching behavior is a result of major structural differences between the two chiral salts and the racemic salt. The chiral compounds crystallize in the space groups P4(1) and P4(3), respectively, with toluene and acetonitrile molecules in the lattice that can be partially removed to create void-space channels. These open channels allow the diffusion of oxygen molecules within the crystals and enable efficient emission quenching that is not possible in the closely packed racemic salt. Lifetime measurements, thermal gravimetric analysis, and single-crystal X-ray structure determinations support these conclusions.

Concurrent sensing of benzene and oxygen by a crystalline salt of tris(5,6-dimethyl-1,10-phenanthroline)ruthenium(II).

The complex [Ru(5,6-Me2Phen)3]tfpb2 has been examined as a solid-state benzene and oxygen sensor. The crystalline solid undergoes a reversible vapochromic shift of the emission lambda max to higher energy in the presence of benzene. Additionally, in the presence of oxygen the solid exhibits linear Stern-Volmer quenching behavior. When simultaneously exposed to benzene vapor and oxygen the crystals uptake benzene which inhibits the diffusion of oxygen in the lattice; very little quenching is observed. However, when benzene is removed from the carrier gas, partial loss of benzene occurs and oxygen diffusion is restored resulting in quenching of the emission. The practicality of this crystalline solid as a benzene sensor was investigated by examination of a lower concentration of benzene vapor (0.76%).

Reduced band gap dithieno[3,2-b:2',3'-d]pyrroles: new n-type organic materials via unexpected reactivity.

Direct addition of tetracyanoethylene to N-(p-hexylphenyl)dithieno[3,2-b:2',3'-d]pyrrole yields not only the aromatic mono- and bis-tricyanovinyl-substituted products but also a quinoidal product with dicyanomethylene groups. The analogous reaction with dithieno[3,2-b:2',3'-d]thiophene yields exclusively the aromatic mono-tricyanovinyl product. The aromatic and quinoidal products possess red-shifted absorptions, increased electron affinities, and favorable pi-stacking motifs in comparison to the unsubstituted oligomers.

Tetra-n-butyl-ammonium bis-(1,1-dicyano-ethyl-ene-2,2-dithiol-ato)platinum(II).

In the title compound, (C(16)H(36)N)(2)[Pt(C(4)N(2)S(2))(2)], the Pt(II) center adopts a distorted square-planar geometry due to the 4-membered chelate rings formed by coordination to the S atoms of the 1,1-dicyano-ethyl-ene-2,2-dithiol-ate (i-mnt) ligands [bite angle 74.35 (4)°]. The bond distances in the coordinated i-mnt ligands indicate some delocalization of the π-system.

Tetra-n-butyl-ammonium bis-(2,2-dicyano-ethyl-ene-1,1-dithiol-ato)palladium(II).

In the title compound, (C(16)H(36)N)(2)[Pd(C(4)N(2)S(2))(2)], the Pd(II) center adopts a distorted square-planar geometry due to the four-membered chelate rings formed by coordination of the 2,2-dicyano-ethyl-ene-1,1-dithiol-ate (i-mnt) ligands [bite angle 75.0159 (17)°]. The bond distances in the coordinated i-mnt ligands indicate some delocalization of the π-system.

Selective low-temperature syntheses of facial and meridional tris-cyclometalated iridium(III) complexes.

We have developed a selective low-temperature synthesis of fac and mer tris-cyclometalated Ir(III) complexes. The chloro-bridged dimers [Ir(CwedgeN)2Cl]2 (CwedgeN = cyclometalating ligand) are cleaved in coordinating solvents like acetonitrile to give neutral Ir(CwedgeN)2(NCCH3)Cl species which in turn are reacted with AgPF6 to give hexafluorophosphate salts of the bis-acetonitrile species [Ir(CwedgeN)2(NCCH3)2]PF6 for CwedgeN = 2,2'-thienylpyridine (thpy) and 2-phenylpyridine (ppy). These bis-acetonitrile complexes are excellent starting materials for the synthesis of tris-Ir(III) complexes. The complexes of the general formula fac-Ir(CwedgeN)3 were synthesized with the ligands thpy and ppy at 100 degrees C in o-dichlorobenzene from the corresponding [Ir(CwedgeN)2(NCCH3)2]PF6 complexes. The reaction of [Ir(CwedgeN)2(NCCH3)2]PF6 with thpy at room temperature did not give the expected tris complex but instead gave [Ir(thpy)2(N,S-thpy)]PF6, with the third chelating ligand complexed through the sulfur atom of the thiophene ring. [Ir(thpy)2Cl]2, [Ir(ppy)2Cl]2, Ir(thpy)2(NCCH3)Cl, [Ir(thpy)2(NCCH3)2]PF6, [Ir(ppy)2(NCCH3)2]PF6, and [Ir(thpy)2(N,S-thpy)]PF6 were structurally characterized by X-ray crystallography. Additionally, hydroxy-bridged dimers, [Ir(CwedgeN)2(OH)]2, were synthesized as starting materials for the selective synthesis of mer-Ir(CwedgeN)3 complexes at 100 degrees C in o-dichlorobenzene. A mechanism is proposed that may account for the selectivity observed in the formation of the mer-Ir(CwedgeN)3 and fac-Ir(CwedgeN)3 isomers in previous studies and the studies presented here.