Transient Difference Absorption Spectra Research Articles

Bodipy−corrole tetrads as heavy−atom−free triplet photosensitizers: Study of the energy transfer and application in triplet−triplet annihilation upconversion

Free base corrole can generate triplet state without invoking of heavy−atom effect, however, its weak and narrow absorption band in the visible spectral range make corrole not an ideal triplet photosensitizer. Herein, novel Bodipy−corrole tetrads were prepared by attaching visible light-harvesting antennae of Bodipy or electron spin converter with fullerene units (2B−Tur−Cor and B−C60−Tur−Cor) to solve the above problems, which show a stronger and broader absorption band in the visible region, and the maximum molar absorption coefficient is up to 2.58 × 105 M−1 cm−1 at 504 nm for 2B−Tur−Cor. By using of fluorescence spectroscopy, steady-state and time-resolved transient absorption spectroscopic methods, the singlet energy transfer progress from the Bodipy moiety to the corrole moiety was confirmed. Through fluorescence lifetimes and fluorescence excitation spectrum study, the rate constant of the singlet energy transfer was calculated as 9.58 × 107 s−1 for 2B−Tur−Cor and 2.45 × 108 s−1 for B−C60−Tur−Cor, respectively. The TD−DFT computation suggests that the first and second triplet state is localized in the corrole moiety and the Bodipy moiety for 2B−Tur−Cor, respectively, and the triplet state energy was calculated to be 1.37 eV and 1.61 eV, respectively. Nanosecond time−resolved transient difference absorption spectra show that the triplet excited states of these tetrads are localized on corrole unit, the lifetime of 2B−T−Cor is up to 143 μs, however, the lifetime of B−C60−Tur−Cor is only 3.3 μs. Long-lived triplet excited states play important role in applications of the triplet photosensitizers in photocatalysis or other photophysical processes concerning triplet–triplet−energy−transfer. As a proof of concept, these tetrads were used as heavy−atom−free organic triplet photosensitizers for triplet−triplet annihilation based upconversion. The upconversion quantum yield was up to 5.86% for 2B−Tur−Cor, however, it is only 0.52% for B−C60−Tur−Cor.

Truxene-linked coumarin-corrole triad: Synthesis, photophysical properties and application in Triplet−Triplet annihilation upconversion

It is crucial to develop organic triplet photosensitizers (PSs) with excellent performance for upconversion applications. Herein, we synthesized a novel truxene-linked coumarin-corrole triads (C-Trux-Corr) and investigated the photophysical properties by steady-state UV–Vis absorption and fluorescence spectroscopies as well as nanosecond time-resolved transient absorption spectroscopy. The triad displays an efficient intramolecular singlet energy transfer from coumarin moiety to the corrole moiety. Nanosecond time-resolved transient difference absorption spectra indicate that the triplet excited states of C-Trux-Corr are distributed on the corrole moiety. The triplet lifetimes of C-Trux-Corr and Trux-C are 195.2 μs and 45.9 μs, respectively. The triplet state lifetime of C-Trux-Corr is significantly prolonged and it was applied as an organic triplet photosensitizer in the triplet-triplet upconversion system with an upconversion quantum yield of 2.93%, which is higher than that the corrole itself (1.27%) by more than 2-fold.

Synthesis, Electrochemistry, Photophysics, and Photochemistry of a Discrete Tetranuclear Copper(I) Sulfido Cluster.

A tetranuclear copper(I) complex, [Cu4{μ-(Ph2P)2NH}4(μ4-S)](PF6)2 (1), was synthesized. It was found to display intense and long-lived phosphorescence in the solid and solution states. The lowest-energy excited state was assigned as ligand-to-metal charge transfer (LMCT) [S2- → Cu4] mixed with some metal-centered (ds/dp) character. In addition, the phosphorescent state of this complex was found to be quenched by pyridinium acceptors via an oxidative electron-transfer quenching process. An excited-state reduction potential of -1.74 V versus saturated salt calomel electrode was estimated through oxidative quenching studies with a series of structurally related pyridinium acceptors, indicative of its strong reducing power in the excited state. From the transient absorption difference spectrum of the tetranuclear copper(I) sulfido complex and 4-(methoxycarbonyl)-N-methylpyridinium hexafluorophosphate, in addition to the characteristic absorption of the pyridinyl radical at ca. 395 nm, two absorption bands at ca. 500 and 660 nm were also observed. The former was assigned as an LMCT absorption [S2- → Cu4] and the latter as an intervalence charge-transfer transition, associated with the mixed-valence species CuI/CuI/CuI/CuII.

Photoinduced energy transfer in truxene-linked zinc porphyrin–fullerene-corrole tetrad and its application in triplet−triplet annihilation upconversion

Triplet photosensitizers (PSs) have attracted widespread interest due to their application in photocatalytic organic reactions, photodynamic therapy (PDT), photoinduced hydrogen production from water splitting, and triplet–triplet annihilation (TTA) upconversion. Herein, the triad T-P-Co and tetrad T-C60-P-Co were synthesized, characterized, and applied in triplet–triplet annihilation (TTA) upconversion. The photophysical processes were investigated with steady-state UV–visible absorption and fluorescence spectroscopies, nanosecond time-resolved transient absorption spectroscopy and electrochemical characterization. T-P-Co and T-C60-P-Co show efficient intramolecular singlet–singlet energy transfer from the porphyrin moiety to corrole/fullerene moiety. The energy transfer efficiency is nearly 100% for T-P-Co and 69.4% for T-C60-P-Co, respectively. Electrochemical investigation shows that the photo-induced intramolecular electron transfer of the excited triad T-P-Co and tetrad T-C60-P-Co is thermodynamically prohibited in toluene due to the positive ΔGCS for charge-separation. Nanosecond time-resolved transient difference absorption spectra indicate that the triplet excited states of T-P-Co and T-C60-P-Co are both distributed on the corrole moiety. The triplet lifetimes of T-P-Co and T-C60-P-Co are 86.9 μs and 73.3 μs, respectively. The triad T-P-Co and tetrad T-C60-P-Co were used as triplet photosensitizers for TTA upconversion, and upconversion quantum yields of 0.76% and 0.84% were observed, respectively.

Highly Efficient Triplet Photosensitizers: A Systematic Approach to the Application of Ir(III) Complexes containing Extended Phenanthrolines.

A series of Ir(III) complexes, based on 1,10-phenanthroline featuring aryl acetylene chromophores, were prepared and investigated as triplet photosensitizers. The complexes were synthesized by Sonogashira cross-coupling reactions using a "chemistry-on-the-complex" method. The absorption properties and luminescence lifetimes were successfully tuned by controlling the number and type of light-harvesting group. Intense UV/Vis absorption was observed for the Ir(III) complexes with two light-harvesting groups at the 3- and 8-positions of the phenanthroline. The asymmetric Ir(III) complex (with a triphenylamine (TPA) and a pyrene moiety attached) exhibited the longest lifetime. Red emission was observed for all the complexes in deaerated solutions at room temperature. Their emission at low temperature (77 K) and nanosecond time-resolved transient difference absorption spectra revealed the origin of their triplet excited states. The singlet-oxygen ((1) O2 ) sensitization and triplet-triplet annihilation (TTA)-based upconversion were explored. Highly efficient TTA upconversion (ΦUC =28.1 %) and (1) O2 sensitization (ΦΔ =97.0 %) were achieved for the asymmetric Ir(III) complex, which showed intense absorption in the visible region (λabs =482 nm, ϵ=50900 m(-1) cm(-1) ) and had a long-lived triplet excited state (53.3 μs at RT).

Broadband visible-light-harvesting trans-bis(alkylphosphine) platinum(II)-alkynyl complexes with singlet energy transfer between BODIPY and naphthalene diimide ligands.

A heteroleptic bis(tributylphosphine) platinum(II)-alkynyl complex (Pt-1) showing broadband visible-light absorption was prepared. Two different visible-light-absorbing ligands, that is, ethynylated boron-dipyrromethene (BODIPY) and a functionalized naphthalene diimide (NDI) were used in the molecule. Two reference complexes, Pt-2 and Pt-3, which contain only the NDI or BODIPY ligand, respectively, were also prepared. The coordinated BODIPY ligand shows absorption at 503 nm and fluorescence at 516 nm, whereas the coordinated NDI ligand absorbs at 594 nm; the spectral overlap between the two ligands ensures intramolecular resonance energy transfer in Pt-1, with BODIPY as the singlet energy donor and NDI as the energy acceptor. The complex shows strong absorption in the region 450 nm-640 nm, with molar absorption coefficient up to 88 000 M(-1) cm(-1) . Long-lived triplet excited states lifetimes were observed for Pt-1-Pt-3 (36.9 μs, 28.3 μs, and 818.6 μs, respectively). Singlet and triplet energy transfer processes were studied by the fluorescence/phosphorescence excitation spectra, steady-state and time-resolved UV/Vis absorption and luminescence spectra, as well as nanosecond time-resolved transient difference absorption spectra. A triplet-state equilibrium was observed for Pt-1. The complexes were used as triplet photosensitizers for triplet-triplet annihilation upconversion, with upconversion quantum yields up to 18.4 % being observed for Pt-1.

Thermally Activated Delayed Fluorescence of Fluorescein Derivative for Time-Resolved and Confocal Fluorescence Imaging

Compared with fluorescence imaging utilizing fluorophores whose lifetimes are in the order of nanoseconds, time-resolved fluorescence microscopy has more advantages in monitoring target fluorescence. In this work, compound DCF-MPYM, which is based on a fluorescein derivative, showed long-lived luminescence (22.11 μs in deaerated ethanol) and was used in time-resolved fluorescence imaging in living cells. Both nanosecond time-resolved transient difference absorption spectra and time-correlated single-photon counting (TCSPC) were employed to explain the long lifetime of the compound, which is rare in pure organic fluorophores without rare earth metals and heavy atoms. A mechanism of thermally activated delayed fluorescence (TADF) that considers the long wavelength fluorescence, large Stokes shift, and long-lived triplet state of DCF-MPYM was proposed. The energy gap (ΔEST) of DCF-MPYM between the singlet and triplet state was determined to be 28.36 meV by the decay rate of DF as a function of temperature. The ΔE(ST) was small enough to allow efficient intersystem crossing (ISC) and reverse ISC, leading to efficient TADF at room temperature. The straightforward synthesis of DCF-MPYM and wide availability of its starting materials contribute to the excellent potential of the compound to replace luminescent lanthanide complexes in future time-resolved imaging technologies.

Synthesis and photophysical characterization of porphyrin and porphyrin–Ru(ii) polypyridyl chromophore–catalyst assemblies on mesoporous metal oxides

A layer-by-layer procedure has been used to prepare chromophore–catalyst assemblies consisting of phosphonate-derivatized porphyrin chromophores and a phosphonate-derivatized Ru(II) water oxidation catalyst on the surfaces of SnO2 and TiO2 mesoporous, nanoparticle films. In the procedure, initial surface binding of the phosphonate-derivatized porphyrin is followed in sequence by reaction with ZrOCl2 and then with the phosphonate-derivatized water oxidation catalyst [RuII(2,6-bis-(1-methylbenzimidazole-2-yl)pyridine)(2,2′-bipyridine-4,4′-hydroxymethylphosphonate)(H2O)]2+, [RuII(Mebimpy)(4,4′-(PO(OH)2–CH2)2-bpy)(OH2)]2+. Fluorescence from both the free base and Zn(II) porphyrin derivatives on SnO2 is quenched; substantial emission quenching of the Zn(II) porphyrin occurs on TiO2. Transient absorption difference spectra provide direct evidence for appearance of the porphyrin radical cation on SnO2via excited-state electron injection. For the chromophore–catalyst assembly on SnO2, transient absorption difference spectra demonstrate rapid intra-assembly electron transfer oxidation of the catalyst following excitation and injection by the porphyrin chromophore.

Synthesis, photophysics, and reverse saturable absorption of platinum complexes bearing extended π-conjugated C^N^N ligands

The synthesis of ligands 1-L-6-L that feature the 6-[9,9-di(2-ethylhexyl)-7-R-9H-fluoren-2-yl]-2,2'-bipyridine (C^N^N) core (R = 4-R'-phenylethynyl with R' = NO2, benzothiazol-2-yl (BTZ), H and OCH3 or R = 4'-BTZ-phenyl or BTZ) and their platinum complexes 1-6 were reported in this paper. The photophysical properties of these ligands and the Pt(II) complexes, including the UV-vis absorption spectra, emission characteristics at room temperature and at 77 K, and the triplet transient difference absorption spectra, were systematically investigated in order to understand the effects of the substituent at the 4-position of the 1-ethynylphenyl component and the extension of π-conjugation between the C^N^N core and the BTZ substituent. Reverse saturable absorption (RSA) of complexes 1-6 was demonstrated at 532 nm using 4.1 ns laser pulses. The UV-vis absorption spectra of 1-L-6-L are featured by strong (1)π,π* transitions in the blue spectral region, and the absorption bands are effectively red-shifted by substitution at the 4-position of the ethynylphenyl motif and by the extended π-conjugation of the linkage. A similar effect was observed for the fluorescence spectra of these ligands in CH2Cl2 at room temperature, but the nature of the fluorescence varies from (1)π,π* fluorescence in 3-L and 6-L, to intraligand charge transfer ((1)ILCT) fluorescence in 1-L, 2-L and 5-L; while 4-L possesses mixed (1)π,π*/(1)ILCT characters. All ligands exhibit moderate triplet transient absorption (TA) in the visible spectral region, with substitution at the 4-position of the ethynylphenyl component broadening of the TA bands, while extended π-conjugation of the linkage inducing red-shifts of the TA bands. For Pt(II) complexes 1-6, their UV-vis absorption spectra constitute red-shifted (1)π,π* transitions and low-energy metal-to-ligand charge transfer ((1)MLCT/(1)ILCT) tails. The emission of these complexes at room temperature in CH2Cl2 predominantly originates from the C^N^N core localized (3)π,π* state, probably mixed with minor (3)MLCT character. 4-Position substitution and extended π-conjugation on the ligands exert a negligible effect on the shape and energy of the emission spectra. Similar to their respective ligands, 1-6 all exhibit broader and red-shifted TA spectra with respect to their ligands and both the 4-position substitution and extended π-conjugation bathochromically shift the TA band maxima. The nonlinear transmission experiments carried out for 1-6 at 532 nm reveal that all complexes exhibit strong reverse saturable absorption (RSA), and the degree of RSA follows this trend: 6 <4 <5 <2 ≤3 <1. The RSA performance is efficiently improved by electron-withdrawing substituents (NO2 and BTZ) and by extending the π-conjugation; while electron-donating substituent (OCH3 in 4) decreases the RSA at 532 nm.

Using C60-bodipy dyads that show strong absorption of visible light and long-lived triplet excited states as organic triplet photosensitizers for triplet–triplet annihilation upconversion

The synthesis of visible light-harvesting C60-bodipy dyads (bodipy = boron-dipyrromethene), and the study of the photophysical properties and the application of dyads as heavy-atom free organic triplet photosensitizers for triplet–triplet annihilation (TTA) based upconversion, are reported. By attaching carbazole units to the π-core of the bodipy chromophore via an ethynyl linker, the absorption wavelength of the antenna in the dyads is readily tuned from 504 nm for the unsubstituted bodipy, to 538 nm (one carbazole unit, ε = 61 800 M−1 cm−1) and 597 nm (two carbazole units, ε = 58 200 M−1 cm−1). Upon photoexcitation at 538 nm (dyad C-1) or 597 nm (dyad C-2), intramolecular energy transfer from the antenna to the C60 unit occurs, and as a result, the singlet excited state of the C60 unit is populated. Subsequently, with the intrinsic intersystem crossing (ISC) of C60, the triplet excited state of the C60 unit is produced (τT up to 24.5 μs). Thus, without the need for any heavy atoms, the triplet excited state of the dyads was populated upon visible light excitation. The population of the C60-localized triplet excited state of the dyads was confirmed by nanosecond time-resolved transient difference absorption spectra and spin density analysis. The dyads were used as triplet photosensitizers for TTA upconversion and upconversion quantum yields of up to 2.9% were observed.

Anion‐Sensitive 2,4‐Dinitrophenylhydrazone‐Containing Terpyridine Derivative and Its Platinum Chloride Complex

AbstractTerpyridine derivative 1, bearing 2,4‐dinitrophenylhydrazone, and its platinum chloride complex 2 were synthesized and characterized. The UV/Vis, emission, and triplet transient difference absorption spectra of 1 and 2 were investigated. Both 1 and 2 exhibit broad and strong absorption in the visible spectral region (λmax = 404 nm for 1 and 433 nm for 2), which are dominated by the 1π–π* transition of the ligand. When excited at the respective absorption band maximum, 1 and 2 exhibit charge‐transfer fluorescence atapproximately 550 nm. Compounds 1 and 2 also exhibit a broad and strong 3π–π* excited‐state absorption band in the visible region (λmax = 520 nm with ϵT1–Tn = 4.74 × 104 L mol–1 cm–1for 1, and λmax = 590 nm with ϵT1–Tn = 3.45 × 104 L mol–1 cm–1 for 2). The triplet excited states of 1 and 2 are both long‐lived, with lifetimes longer than 2 ms. Compounds 1 and 2 are sensitive to basic anions such as F–, AcO–, and H2PO4–. Upon addition of these anions, the UV/Vis absorption spectra of these two compounds are significantly redshifted, which is reflected by the color change from yellow to purple for 1 and from yellow to blue for 2. These anions also quench the charge‐transfer emission band at 550 nm. Moreover, the triplet transient absorption of 1 and 2 in DMSO solution is quenched upon addition of these anions. These spectroscopic changes are attributed to the deprotonation of the hydrazone NH group, which causes intramolecular electron transfer. The deprotonation mechanism is supported by the upfield shifts of the 1H NMR signals of the 2,4‐dinitrophenylhydrazone group, the imine group, and the phenyl group, following the addition of excess F–.

Organic Triplet Sensitizer Library Derived from a Single Chromophore (BODIPY) with Long-Lived Triplet Excited State for Triplet–Triplet Annihilation Based Upconversion

Triplet-triplet annihilation (TTA) based upconversions are attractive as a result of their readily tunable excitation/emission wavelength, low excitation power density, and high upconversion quantum yield. For TTA upconversion, triplet sensitizers and acceptors are combined to harvest the irradiation energy and to acquire emission at higher energy through triplet-triplet energy transfer (TTET) and TTA processes. Currently the triplet sensitizers are limited to the phosphorescent transition metal complexes, for which the tuning of UV-vis absorption and T(1) excited state energy level is difficult. Herein for the first time we proposed a library of organic triplet sensitizers based on a single chromophore of boron-dipyrromethene (BODIPY). The organic sensitizers show intense UV-vis absorptions at 510-629 nm (ε up to 180,000 M(-1) cm(-1)). Long-lived triplet excited state (τ(T) up to 66.3 μs) is populated upon excitation of the sensitizers, proved by nanosecond time-resolved transient difference absorption spectra and DFT calculations. With perylene or 1-chloro-9,10-bis(phenylethynyl)anthracene (1CBPEA) as the triplet acceptors, significant upconversion (Φ(UC) up to 6.1%) was observed for solution samples and polymer films, and the anti-Stokes shift was up to 0.56 eV. Our results pave the way for the design of organic triplet sensitizers and their applications in photovoltaics and upconversions, etc.

Photophysical Properties of Ruthenium(II) Polypyridyl-Gold(I) Ethynyl Dyads and Triads Containing Mono- or Diethynylphenanthroline Incorporated into Gold(I) Triphenylphosphine Organometallics

A new ruthenium(II)-gold(I) dyad, [Ru(bpy)(2){5-{(PPh(3))-Au-C[tripe bond]C}-phen}](PF(6))(2) (2), with a different substituted site compared to [Ru(bpy)(2){3-{(PPh(3))-Au-C[triple bond]C}-phen}](PF(6))(2) (1), and a triad, [Ru(bpy)(2){3,6-bis{(PPh(3))-Au-C[triple bond]C}-phen}](PF(6))(2) (3), with an unsymmetric diethynylphenanthroline relative to [Ru(bpy)(2){3,8-bis{(PPh(3))-Au-C[triple bond]C}-phen}](PF(6))(2) (4) have been prepared. These four ruthenium(II)-gold(I) compounds showed typical metal-to-ligand charge-transfer (MLCT) absorption bands in the 400-550 nm region and a lowest energy pi-pi* absorption involved with the gold(I) perturbation in the 300-400 nm region. Broad emission bands assignable to the triplet MLCT transition were definitely observed in all compounds, indicating that the hybrid architecture constructed with Ru(II)-polypyridyl and Au(I)-ethynyl units converts the blue-green gold(I) perturbed pi-pi* phosphorescence into an orange MLCT-based emission. The transient absorption difference spectra of four compounds showed the difference in the electron transfer process between 2 and other compounds 1, 3, and 4 under the excited state. Ru(II)-Au(I) compounds except for 2 receive the supposed charge injection from a ruthenium center to an extended pi-conjugated ethynyl-substituted phenanthroline, which contains one or two gold(I) organometallic unit(s), while 2 undergoes the electron transfer process from the ruthenium center not to the 5-ethynylphenanthroline but to one of the bipyridyl ligands under the excited state. This hypothesis is supported by the deflection of the spots of 2 and [Ru(bpy)(3)](PF(6))(2) from a linear correlation line in a plot of E(0-0) versus DeltaE(1/2), which was based on the electrochemical and emission data of Ru(II)-Au(I) compounds and mononuclear ruthenium(II) polypyridyl complexes.

Boron Dipyrromethene (Bodipy) Phosphorescence Revealed in [Ir(ppy)2(bpy-C≡C-Bodipy)]+

The synthesis, structural characterization, electrochemistry, and molecular photophysics of [Ir(ppy)(2)(bpy-C[triple bond]C-Bodipy)](PF(6)), where ppy is 2-phenylpyridine and bpy-C[triple bond]C-Bodipy is 5-ethynyl-2,2'-bipyridine-8-phenyl-1,3,5,7-tetramethyl-4,4-bis(2,5-dioxaoct-7-ynyl)-4-bora-3a,4a-diaza-s-indacene (4), is presented. Static and dynamic photoluminescence and absorption measurements in conjunction with cyclic voltammetry were employed to elucidate the nature of the intramolecular energy transfer processes occurring in the excited state of the title chromophore. Parallel studies were performed on appropriate model chromophores (2 and 3) intended to represent the photophysics of the isolated molecular subunits, that is, triplet metal-to-ligand-charge-transfer ((3)MLCT) and triplet Bodipy intraligand ((3)IL) excited states, respectively. Upon charge transfer excitation of the title chromophore, the (3)MLCT based phosphorescence readily observed in 2 (Phi(em) = 0.027, tau = 243 ns) is quantitatively quenched resulting from production of the (3)Bodipy excited state through intramolecular triplet-triplet energy transfer. The formation of the (3)Bodipy-localized excited state is confirmed by features in the transient absorption difference spectrum, extended excited-state lifetime (tau = 25 micros), as well the observation of (3)IL Bodipy-based phosphorescence detected at 730 nm at 77 K. The low temperature Bodipy phosphorescence is readily produced in 4 as a result of the internal Ir(III) heavy atom.

Photophysics in Platinum(II) Bipyridylacetylides

The synthesis, structural characterization, photoluminescence, and excited state absorption properties of a series of platinum(II) terpyridyl complexes bearing a bipyridyl acetylide subunit are presented. The [(t)Bu(3)tpyPtC identical withCbpy](+) (1) complex displays a broad and structureless emission profile at room temperature (RT), a lifetime of 5.8 mus, and transient absorption (TA) difference spectra characteristic of a charge transfer (CT) excited state. Upon coordination of Fe(2+) to 1, producing tetranuclear 2, the CT emission was quantitatively quenched presumably through the low-lying iron-based ligand field states present. Surprisingly, the addition of Zn(2+) to solutions of 1 produces a higher energy emissive state with a substantially longer excited state lifetime of 16.1 mus. The combined spectroscopic data measured for the zinc titration product (3) suggests that the overall excited state is dominated by a CT manifold, albeit at higher energy relative to 1. The photophysics of a bis-phosphine complex bearing two trans-disposed bpy-acetylide subunits (4) produced a model chromophore possessing an intraligand triplet excited state with a lifetime of 26 mus at RT. The bipyridyl analogue of 1, (t)Bu(2)bpyPt(C identical withCbpy)(2) (5), was also prepared and its photophysics are consistent with a lowest CT parentage at RT. The 77 K emission spectra measured for complexes 1, 3, 4, and 5 are all consistent with a triplet bpy-acetylide localized excited state; the E(00) energies vary over a modest 344 cm(-1) across the series. However, the shorter 77 K excited state lifetimes observed for 1, 3, and 5 in comparison to 4 suggests that the energetically proximate CT state in the former compounds significantly influences excited state decay at low temperature.

Photophysics of Pt(II) 4,6-diphenyl-2,2′-bipyridyl complexes in solution and in LB film

Two amphiphilic platinum(II) 4,6-diphenyl-2,2′-bipyridyl complexes ( 1 and 2) were synthesized and characterized. LB films of these two complexes were prepared and characterized by AFM technique. Electronic absorption and emission characteristics of these complexes in solutions at room temperature, in glassy solutions at 77 K, and in LB films were studied. The emission energies of the complexes in LB films are similar to those in acetonitrile solutions at room temperature. However, the lifetimes of 1 and 2 in LB films are 4–7 times as long as those in CH 3CN solutions. The triplet transient difference absorption spectra of these complexes exhibit broad absorption in the visible region to the near-IR region. Introducing a hydroxyl substituent on the 4,6-diphenyl-2,2′-bipyridyl ligand favors the formation of intermolecular hydrogen bonding and thus helps the deposition of uniform and stable LB films and increases the self-quenching rate constant for emission of 2 in CH 3CN solution.

Evolution of the Triplet Excited State in PtII Perylenediimides

Here, we present the ultrafast dynamics of a series of metal complexes developed to permit access to the perylenediimide (PDI) triplet manifold that preserves the desirable colorfastness and visible light-absorption properties associated with these dyes. To this end, three Pt(II) complexes each bearing two PDI moieties tethered to the metal center through acetylide linkages emanating from one of the PDI bay positions have been thoroughly examined by static spectroscopic methods, electrochemistry, laser flash photolysis, and ultrafast transient absorption spectrometry. Upon ligation to the Pt(II) center, the bright singlet-state fluorescence (Phi = 0.91, tau = 4.53 ns) of the free PDI-CCH chromophore is quantitatively quenched, and no long wavelength photoluminescence is observed from any of the Pt(II)-PDI complexes in deaerated solutions. Ultrafast transient measurements reveal that upon ligation of PDI-CCH to the Pt(II) center, picosecond intersystem crossing (tau = 2-4 ps) from the (1)PDI excited state is followed by vibrational cooling (tau = 12-19 ps) of the hot (3)PDI excited state, whereas only singlet-state dynamics, including stimulated emission, were observed in the "free" PDI-CCH moiety. In each of the Pt-PDI chromophores, quantitatively similar transient absorption difference spectra were obtained; the only distinguishing characteristic is in their single-exponential lifetimes (tau = 246 ns, 1.0 mus, and 710 ns). These long-lived (3)PDI excited states are clearly poised for bimolecular electron and energy transfer schemes. In the present case, the latter is demonstrated through bimolecular sensitization of singlet oxygen phosphorescence at approximately 1270 nm in aerated dichloromethane solutions, producing reasonable (1)O(2) quantum yields (Phi(Delta) = 0.40-0.55) across this series of molecules.

Structural analysis of ultrafast extended x-ray absorption fine structure with subpicometer spatial resolution: Application to spin crossover complexes

We present a novel analysis of time-resolved extended x-ray absorption fine structure (EXAFS) spectra based on the fitting of the experimental transients obtained from optical pump/x-ray probe experiments. We apply it to the analysis of picosecond EXAFS data on aqueous [Fe(II)(bpy)(3)](2+), which undergoes a light induced conversion from its low-spin (LS) ground state to the short-lived (tau approximately 650 ps) excited high-spin (HS) state. A series of EXAFS spectra were simulated for a collection of possible HS structures from which the ground state fit spectrum was subtracted to generate transient difference absorption (TA) spectra. These are then compared with the experimental TA spectrum using a least-squares statistical analysis to derive the structural change. This approach reduces the number of required parameters by cancellation in the differences. It also delivers a unique solution for both the fractional population and the extracted excited state structure. We thus obtain a value of the Fe-N bond elongation in the HS state with subpicometer precision (0.203+/-0.008 A).

Spectral Oscillation in Optical Frequency-Resolved Quantum-Beat Spectroscopy With a Few-Cycle Pulse Laser

Impulsive stimulated resonant Raman scattering was induced by a 5.7 fs visible pulse from a non-collinear optical parametric amplifier. The pulse duration corresponded to 2.8 cycles of the central wavelength of the laser spectrum at 610 nm. The induced spectral change was time-resolved with a 0.8 fs delay time step and the effective absorbance change DeltaA(lambda,t) was calculated from the normalized transmittance change DeltaT(lambda,t)/T. The observed optical and vibrational 2-D spectrum was analyzed to separate the electronic and vibrational contributions to the transient difference absorption spectrum. The probe wavelength dependence of the vibrational amplitude was explained in terms of the coupling between the laser field, Stokes field, and the vibrational coordinates. Simultaneous measurement of both the time and wavelength dependence of the vibrational amplitudes clearly showed that the broadband spectra of the components corresponding to the laser and Stokes lights are oscillating in pi out of phase with the corresponding vibrational frequency. The probe delay time and probe wavelength dependence of the oscillation in the absorbance change induced by the femtosecond laser were fully analyzed. The results were explained by the imaginary and real part of the third-order susceptibility, respectively.

4′-(5′′′-R-Pyrimidyl)-2,2′:6′,2′′-terpyridyl (R = H, OEt, Ph, Cl, CN) Platinum(II) Phenylacetylide Complexes: Synthesis and Photophysics

A series of new square-planar 4'-(5'''-R-pyrimidyl)-2,2':6',2''-terpyridyl platinum(II) phenylacetylide complexes ( 1a- 5a) bearing different substituents (R = H, OEt, Ph, Cl, CN) on the pyrimidyl ring have been synthesized and characterized. The electronic absorption, photoluminescence, and triplet transient difference absorption spectra were investigated. All of the complexes exhibit broad, moderately strong absorption between 400 and 500 nm that can be tentatively assigned to the metal-to-ligand charge transfer ( (1)MLCT) transition, possibly mixed with some ligand-to-ligand charge transfer ( (1)LLCT) character. Photoluminescence arising from the (3)MLCT state was observed both in fluid solutions at room temperature and in a rigid matrix at 77 K. The (1)MLCT/ (1)LLCT absorption bands and the (3)MLCT emission bands for 1a- 5a red-shift in comparison to those of the corresponding 4'-toly-2,2':6',2''-terpyridyl platinum(II) phenylacetylide complex. In addition, the energies of the (1)MLCT/ (1)LLCT absorption and the (3)MLCT emission bands exhibit a linear correlation with the Hammett constant (sigma p) of the 5'''-substituent on the pyrimidyl ring. The lifetime of the (3)MLCT emission at room temperature is governed by the energy gap law. The triplet transient difference absorption spectra of 1a- 5a exhibit a broad absorption band from 500 to 800 nm, and a bleaching band between 420 and 500 nm. Complex 5a, which contains the -CN substituent, exhibits a lower-energy triplet absorption band at 785 nm and a shorter lifetime (130 ns) in CH 3CN than 2a, which has the -OEt substituent, does (lambda T1-Tn (max) = 720 nm, tau T = 660 ns). The triplet excited-state absorption coefficients at the band maxima for 1a- 5a vary from 36 600 L.mol (-1).cm (-1) to 115 090 L.mol (-1).cm (-1), and the quantum yields of the triplet excited-state formation range from 0.19 to 0.66. All complexes exhibit a moderate nonlinear transmission for nanosecond laser pulses at 532 nm. Moreover, these complexes can generate singlet oxygen efficiently in air-saturated CH 3CN solutions, with the singlet oxygen generation quantum yield (Phi Delta) varying from 0.24 to 0.46.