Selective near-IR (NIR) excitation (780 nm) of the conjugated supermolecule ruthenium(II) [15-(4'-ethynyl-(2,2';6',2''-terpyridinyl))-bis[(5,5',-10,20-di(2',6'-bis(3,3-dimethylbutoxy)phenyl)porphinato)zinc(II)]ethyne][4'-pyrrolidin-1-yl-2,2';6',2''-terpyridine] bis(hexafluorophosphate) (Pyr(1)RuPZn(2)) in solutions containing N,N-bis(ethylpropyl)perylene-3,4,9,10-tetracarboxylicdiimide (PDI) or tetracene gives rise to a substantial anti-Stokes energy gain (PDI, 0.70 eV; tetracene, 0.86 eV). Experimental data clearly demonstrate that this upconverted fluorescence signal is produced via Pyr(1)RuPZn(2)-sensitized triplet-triplet annihilation (TTA) photochemistry. The TTA process was confirmed by the quadratic dependence of the integrated (1)PDI* emission centered at 541 nm derived from 780 nm laser excitation. The T(1)→T(n) excited state absorption decay of Pyr(1)RuPZn(2), monitored at 900 nm as a function of PDI concentration, revealed Stern-Volmer and bimolecular quenching constants of 10 048 M(-1) and 5.9 × 10(8) M(-1) s(-1), respectively, for the PDI triplet sensitization process. The T(1)→T(n) PDI extinction coefficient at 560 nm (ε(T) = 6.6 × 10(4) M(-1) cm(-1)) was determined through the triplet energy transfer method utilizing anthracene as the donor chromophore. (3)PDI* transient triplet absorption dynamics observed as a function of 485 nm incident nanosecond pump laser fluence demonstrate a bimolecular (3)PDI*-(3)PDI* TTA rate constant (k(TT) = 1.0 ± 0.2 × 10(9) M(-1) s(-1)). The maximum quantum yield of the supermolecule-sensitized PDI upconverted emission (Φ(UC) = 0.0075 ± 0.0002) was determined relative to [Os(phen)(3)][PF(6)](2) at an incident laser power of 22 mW at 780 nm. This study successfully demonstrates NIR-to-visible photon upconversion and achieves a new record anti-Stokes shift of 0.86 eV for sensitized TTA, using the supermolecular Pyr(1)RuPZn(2)sensitizer. The stability of the Pyr(1)RuPZn(2)/PDI chromophore combination is readily apparent as continuous irradiation at 780 nm produces 541 nm centered fluorescence with no significant decrease in intensity measured over time domains exceeding several hours. The molecular components of these NIR-to-vis upconverting compositions illustrate that substantial anti-Stokes energy gains via a TTA process can be effortlessly realized.

Visible light driven water splitting in a dye-sensitized photoelectrochemical cell (DSPEC) based on a phosphonic acidderivatized donor-p-acceptor (D-p-A) organic dye (P-A-p-D) is described with the dye anchored to an FTO|SnO2/TiO2 core/shell photoanode in a pH 7 phosphate buffer solution. Transient absorption measurements on FTO|TiO2|-[P-A-p-D] compared to core/shell, FTO|SnO2/TiO2(3nm)|-[P-A-p-D], reveal that excitation of the dye is rapid and efficient with a decrease in back electron rate by a factor of ~10 on the core/shell. Upon visible, 1 sun excitation (100 mWcm -2 ) of FTO|SnO2/TiO2(3nm)|-[P-A-p-D] in a phosphate buffer at pH 7 with 20 mM added hydroquinone (H2Q), photocurrents of ~2.5 mA/cm 2 are observed which are sustained over >15 min photolysis periods with a current enhancement of ~30-fold compared to FTO|TiO2|-[P-A-p-D] due to the core/shell effect. On surfaces co-loaded with both -[P-A-p-D] and the known water oxidation catalyst, Ru(bda)(pyP)2 (pyP = pyridin-4-methyl phosphonic acid), maximum photocurrent levels of 1.4 mA/cm 2 were observed which decreased over an 10 min interval to 0.1 mA/cm 2 . O2 was measured by use of a twoelectrode, collector-generator sandwich cell and was produced in low Faradaic efficiencies with the majority of the oxidative photocurrent due to oxidative decomposition of the dye. D] 2+ , inset, Fig. 1a. In acetonitrile, no significant reduction in current was observed even after 50 CV scan cycles at 20 mV/s, Fig. S1, highlighting the relative stability of the mono-and dicationic forms of the dye in acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate (TBAP) as the electrolyte.

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It has long been recognized that the measured magnitudes of dynamic hyperpolarizabilities (b l values) can depend sensitively on molecular structure.[1] Whether a chromophore with nonlinear optical properties (NLO chromophore) is an electronically asymmetric, dipolar, donor-linker-acceptor (D-L-A) molecule, or an electronically symmetric, yet noncentrosymmetric, D-L-D or A-L-A octopolar structure, oscillator strength and the extent to which charge is redistributed in electronic transitions depend on the degree of coupling of D and A to the conjugated L. In virtually all known NLO chromophores studied to date, the orientations of D, L, and A are not rigidly fixed; hence, the experimentally determined electronic coupling between these units is generally established by the distribution of condensed-phase conformeric populations set by the nature of D-L and L-A connectivity.[2] Further, if one considers octopolar chromophores, the requirement of noncentrosymmetry places further restrictions on important spatial relationships between D, L, and A: nearly all known NLO octopoles have either D 3h or T d symmetry.[3] It has been recognized that D 2 and D 2d symmetries could be exploited in the design of single-oscillator octopolar NLO chromophores, [4] or octopolar compounds that exhibit metal-to-ligand charge-transfer transitions; [5] however, it has been stated that no interesting chromophoric benchmarks yet have such symmetries.[6] The few established NLO octopoles with D 2 symmetry are characterized by weakly coupled oscillators in which 3D charge redistribution occurs by a through-space delocalization mechanism.[7] We report here that strongly coupled D 2 -symmetric oscillators provide an important motif for potent octopolar NLO chromophores and demonstrate the utility of hyper-Rayleigh light scattering (HRS) measurements [8] to interrogate conformeric populations of chromophores having D 2 and D 2d symmetries at ambient temperature in solution.The utility of HRS in probing structure derives from the fact that it is intrinsically sensitive to symmetry; at the molecular level, a hyperpolarizable molecule must be noncentrosymmetric. When a substantial HRS signal is observed from an isotropic solution of molecules, at least a fraction of them must have noncentrosymmetric structures. This property of even-order nonlinear optical probes of electronic structure has been utilized to characterize the electrooptic characteristics of nondipolar chromophores RuR f PZnRu, OsR f PZnOs, OsPZnOs, PZnRuPZn, and PZnOsPZn (Scheme 1).Scheme 1. Structures of investigated compounds.[*] Dr.

A series of mono-, bis-, tris-, and tetrakis-(porphinato)zinc(II) (PZn)-elaborated ruthenium(II) bis(terpyridine) (Ru) complexes has been synthesized in which an ethyne unit connects the macrocycle meso carbon atom to terpyridyl (tpy) 4-, 4′-, and 4″-positions. These supermolecular chromophores, based on the ruthenium(II) [5-(4′-ethynyl-(2,2′;6′,2″-terpyridinyl))-10,20-bis(2′,6′-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-(2,2′;6′,2″-terpyridine) 2+ bishexafluorophosphate (RuPZn) archetype, evince strong mixing of the PZn-based oscillator strength with ruthenium terpyridyl charge resonance bands. Potentiometric and linear absorption spectroscopic data indicate that for structures in which multiple PZn moieties are linked via ethynes to a [Ru(tpy) 2 ] 2+ core, little electronic coupling is manifest between PZn units, regardless of whether they are located on the same or opposite tpy ligand. Congruent with these experiments, pump-probe transient absorption studies suggest that the individual RuPZn fragments of these structures exhibit, at best, only modest excited-state electronic interactions that derive from factors other than the dipole-dipole interactions of these strong oscillators; this approximate independent character of the component RuPZn oscillators enables fabrication of NLO multipoles with extraordinary hyperpolarizabilities.Dynamic hyperpolarizability (β λ ) values and depolarization ratios (ρ) were determined from hyper-Rayleigh light scattering (HRS) measurements carried out at an incident irradiation wavelength (λ inc ) of 1300 nm. The depolarization ratio data provide an experimental measure of chromophore optical symmetry; appropriate coupling of multiple charge-transfer oscillators produces structures having enormous averaged hyperpolarizabilities (β HRS values), while evolving (PF 6 ) 2 possessing a β HRS value at 1300 nm more than a factor of three larger than that determined for any chromophore having octopolar symmetry examined to date. Because NLO octopoles possess omnidirectional NLO responses while circumventing the electrostatic interactions that drive bulk-phase centrosymmetry for NLO dipoles at high chromophore concentrations, the advent of octopolar NLO chromophores having vastly superior β HRS values at technologically important wavelengths will motivate new experimental approaches to achieve acentric order in both bulk-phase and thin film structures.

No abstract

Integration of photoresponsive chromophores that initiate multistep catalysis is essential in dye-sensitized photoelectrosynthesis cells and related devices. We describe here an approach that incorporates a chromophore assembly surface-bound to metal oxide electrodes for light absorption with an overlayer of catalysts for driving the half-reactions of water splitting. The assembly is a combination of a core-twisted perylene diimide and a ruthenium polypyridyl complex. By altering the connection sequence of the two subunits in the assembly, in their surface-binding to either TiO or NiO, the assembly can be tuned to convert visible light into strongly oxidizing equivalents for activation of an electrodeposited water oxidation catalyst (NiCoO ) at the photoanode, or reducing equivalents for activation of an electrodeposited water reduction catalyst (NiMoS ) at the photocathode. A key element in the design of the photoelectrodes comes from the synergistic roles of the vertical (interlayer) charge transfer and lateral (intralayer) charge hopping in determining overall cell efficiencies for photoelectrocatalysis.

Octopolar D2-symmetric chromophores, based on the MPZnM supermolecular motif in which (porphinato)zinc(II) (PZn) and ruthenium(II) polypyridyl (M) structural units are connected via ethyne linkages, were synthesized. These structures take advantage of electron-rich meso-arylporphyrin or electron-poor meso-(perfluoroalkyl)porphyrin macrocycles, unsubstituted terpyridyl and 4′-pyrrolidinyl-2,2′;6′,2″-terpyridyl ligands, and modulation of metal(II) polypyridyl-to-(porphinato)zinc connectivity, to probe how electronic and geometric factors impact the measured hyperpolarizability. Transient absorption spectra obtained at early time delays (tdelay < 400 fs) demonstrate fast excited-state relaxation, and formation of a highly polarized T1 excited state; the T1 states of these chromophores display expansive, intense T1 → Tn absorption manifolds that dominate the 800–1200 nm region of the NIR, long (μs) triplet-state lifetimes, and unusually large NIR excited absorptive extinction coefficients [ε(T1 → Tn) ∼ 105 M–1 cm–1]. Dynamic hyperpolarizability (βλ) values were determined from hyper-Rayleigh light scattering (HRS) measurements, carried out at multiple incident irradiation wavelengths spanning the 800–1500 nm spectral domain. The measured βHRS value (4600 ± 1200 × 10–30 esu) for one of these complexes, RuPZnRu, is the largest yet reported for any chromophore at a 1500 nm irradiation wavelength, highlighting that appropriate engineering of strong electronic coupling between multiple charge-transfer oscillators provides a critical design strategy to realize octopolar NLO chromophores exhibiting large βHRS values at telecom-relevant wavelengths. Generalized Thomas–Kuhn sum (TKS) rules were utilized to compute the effective excited-state-to-excited-state transition dipole moments from experimental linear-absorption spectra; these data were then utilized to compute hyperpolarizabilities as a function of frequency, that include two- and three-state contributions for both βzzz and βxzx tensor components to the RuPZnRu hyperpolarizability spectrum. This analysis predicts that the βzzz and βxzx tensor contributions to the RuPZnRu hyperpolarizability spectrum maximize near 1550 nm, in agreement with experimental data. The TKS analysis suggests that relative to analogous dipolar chromophores, octopolar supermolecules will be likely characterized by more intricate dependences of the measured hyperpolarizability upon irradiation wavelength due to the interactions among multiple different β tensor components.