Time-resolved Infrared Absorption Research Articles

Excited State Dynamics of Geometrical Evolution of α-Substituted Dibenzoylmethanatoboron Difluoride Complex with Aggregation-Induced Emission Property.

Organic molecules with an aggregation-induced emission (AIE) property have been attracting much attention from the viewpoint of application to solid state emissive materials. For the AIE mechanism, quantum mechanical studies proposed the restriction of the intramolecular motion (RIM) model with the contribution of the conical intersection (CI) and deduced the importance of the restricted access to a conical intersection (RACI) in the potential energy surface (PES). Although these theoretical studies have contributed to the elucidation of AIE phenomena, direct detection of the reaction dynamics is indispensable to clarify the actual PES and the deactivation mechanism. Along this line, we investigated excited state dynamics of the AIE molecule with dibenzoylmethanatoboron difluoride complexes using time-resolved absorption spectroscopies in both visible and infrared (IR) regions. While the reference system of 1,3-bis(4-methoxyphenyl)methanatoboron difluoride (2aBF2) showed strong emission in solution, the methyl-substituted derivative at the α-position of the dioxaborine ring (2amBF2) led to the very weak fluorescence in solution but strong emission in the solid state. Time-resolved visible absorption measurements revealed a peak shift and broadening of the stimulated emission in the solution of 2amBF2, owing to the rapid change of the molecular geometry. With the temporal evolution of time-resolved IR absorption signals and density functional theory (DFT) calculation of these systems, it was deduced that 2amBF2 has two stable geometries, namely, planar and bending, in the S1 state and the bending geometry in the S1 state led to rapid conversion to the S0 state. These results support the RACI model in the aggregated states, leading to the AIE properties.

Laser induced temperature-jump time resolved IR spectroscopy of zeolites.

Combining pulsed laser heating and time-resolved infrared (TR-IR) absorption spectroscopy provides a means of initiating and studying thermally activated chemical reactions and diffusion processes in heterogeneous catalysts on timescales from nanoseconds to seconds. To this end, we investigated single pulse and burst laser heating in zeolite catalysts under realistic conditions using TR-IR spectroscopy. 1 ns, 70 μJ, 2.8 μm laser pulses from a Nd:YAG-pumped optical parametric oscillator were observed to induce temperature-jumps (T-jumps) in zeolite pellets in nanoseconds, with the sample cooling over 1-3 ms. By adopting a tightly focused beam geometry, T-jumps as large as 145 °C from the starting temperature were achieved, demonstrated through comparison of the TR-IR spectra with temperature dependent IR absorption spectra and three dimensional heat transfer modelling using realistic experimental parameters. The simulations provide a detailed understanding of the temperature distribution within the sample and its evolution over the cooling period, which we observe to be bi-exponential. These results provide foundations for determining the magnitude of a T-jump in a catalyst/adsorbate system from its absorption spectrum and physical properties, and for applying T-jump TR-IR spectroscopy to the study of reactive chemistry in heterogeneous catalysts.

Kinetics of formic acid dehydration on Pt electrodes by time-resolved ATR-SEIRAS.

The potential dependence of the rate of dehydration of formic acid to adsorbed CO (COad) on Pt at pH 1 has been studied on a polycrystalline Pt surface by time-resolved surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode (ATR-SEIRAS) with simultaneous recording of current transients after a potential step. A range of formic acid concentrations has been used to obtain a deeper insight into the mechanism of the reaction. The experiments have allowed us to confirm that the potential dependence of the rate of dehydration has a bell shape, going through a maximum around the potential of zero total charge (pztc) of the most active site. The analysis of the integrated intensity and frequency of the bands corresponding to COL and COB/M shows a progressive population of the active sites on the surface. The observed potential dependence of the rate of formation of COad is consistent with a mechanism in which the reversible electroadsorption of HCOOad is followed by its rate-determining reduction to COad.

Direct Observation of the Dynamics of Ylide Solvation by Hydrogen-bond Donors Using Time-Resolved Infrared Spectroscopy.

The photoexcitation of α-diazocarbonyl compounds produces singlet carbene intermediates that react with nucleophilic solvent molecules to form ylides. The zwitterionic nature of these newly formed ylides induces rapid changes in their interactions with the surrounding solvent. Here, ultrafast time-resolved infrared absorption spectroscopy is used to study the ylide-forming reactions of singlet carbene intermediates from the 270 nm photoexcitation of ethyl diazoacetate in various solvents and the changes in the subsequent ylide–solvent interactions. The results provide direct spectroscopic observation of the competition between ylide formation and C–H insertion in reactions of the singlet carbene with nucleophilic solvent molecules. We further report the specific solvation dynamics of the tetrahydrofuran (THF)-derived ylide (with a characteristic IR absorption band at 1636 cm–1) by various hydrogen-bond donors and the coordination by lithium cations. Hydrogen-bonded ylide bands shift to a lower wavenumber by −19 cm–1 for interactions with ethanol, −14 cm–1 for chloroform, −10 cm–1 for dichloromethane, −9 cm–1 for acetonitrile or cyclohexane, and −16 cm–1 for Li+ coordination, allowing the time evolution of the ylide–solvent interactions to be tracked. The hydrogen-bonded ylide bands grow with rate coefficients that are close to the diffusional limit. We further characterize the specific interactions of ethanol with the THF-derived ylide using quantum chemical (MP2) calculations and DFT-based atom-centered density matrix propagation trajectories, which show preferential coordination to the α-carbonyl group. This coordination alters the hybridization character of the ylidic carbon atom, with the greatest change toward sp2 character found for lithium-ion coordination.

Selective photoinduced charge separation in perylenediimide-pillar[5]arene rotaxanes

The ability to control photoinduced charge transfer within molecules represents a major challenge requiring precise control of the relative positioning and orientation of donor and acceptor groups. Here we show that such photoinduced charge transfer processes within homo- and hetero-rotaxanes can be controlled through organisation of the components of the mechanically interlocked molecules, introducing alternative pathways for electron donation. Specifically, studies of two rotaxanes are described: a homo[3]rotaxane, built from a perylenediimide diimidazolium rod that threads two pillar[5]arene macrocycles, and a hetero[4]rotaxane in which an additional bis(1,5-naphtho)-38-crown-10 (BN38C10) macrocycle encircles the central perylenediimide. The two rotaxanes are characterised by a combination of techniques including electron diffraction crystallography in the case of the hetero[4]rotaxane. Cyclic voltammetry, spectroelectrochemistry, and EPR spectroscopy are employed to establish the behaviour of the redox states of both rotaxanes and these data are used to inform photophysical studies using time-resolved infra-red (TRIR) and transient absorption (TA) spectroscopies. The latter studies illustrate the formation of a symmetry-breaking charge-separated state in the case of the homo[3]rotaxane in which charge transfer between the pillar[5]arene and perylenediimide is observed involving only one of the two macrocyclic components. In the case of the hetero[4]rotaxane charge separation is observed involving only the BN38C10 macrocycle and the perylenediimide leaving the pillar[5]arene components unperturbed.

Methanol Dehydrogenation on Pt Electrodes: Active Sites and Role of Adsorbed Spectators Revealed through Time-Resolved ATR-SEIRAS

The dynamics of methanol dehydrogenation to adsorbed CO (COad) on polycrystalline Pt in the hydrogen underpotential adsorption (Hupd) region has been investigated under potentiostatic conditions using time-resolved surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode (ATR-SEIRAS) after a potential step. Our experiments reveal that the electrooxidation of methanol to COad is possible at potentials at least as negative as 0.01 V versus reversible hydrogen electrode and occurs nearly exclusively at (111)- and (100)-oriented defect sites, whereby (111)-oriented defects show higher activity unless blocked by spectator species. Under conditions in which, due to a combination of low methanol concentration and sufficiently negative potential, the formation of COad is very slow, we have been able to determine the singleton frequency of linearly bonded COad (COL) on Pt (2002 cm–1 ± 2 between 0.01 and 0.06 V). Under these conditions, the slow increase in the coverage of COad (θCO), as revealed by the increase of the integrated intensity of the bands corresponding to COL and bridge or multiple-bonded COad (COB/COM), is not accompanied by an increase in the COL stretching frequency (νCOL), which initially remains constant and then increases sharply, an effect that repeats itself several times, giving rise to a staircase-like increase of νCOL before it starts to increase monotonously in parallel with θCO. We attribute this behavior to the progressive population of terraces by COad diffusing from the defect sites where it has been formed. We also detected under these conditions a band at 1677 cm–1, which we attribute to adsorbed formyl (HCOad) and which we suggest is the last intermediate in the oxidation leading from methanol to COad. Our experiments also allow the direct spectroscopic determination of the rate of formation of COad at the limit of zero coverage (θCO → 0), from which Tafel plots revealing the potential dependence of the reaction rate when θCO → 0 can be drawn. These plots, together with the potential dependence of the time elapsed between the first observation in spectra of COB/COM and that of COL, provide evidence of the important role played by adsorbed spectators in determining the reaction rate.

Direct Observation of Reactive Intermediates by Time-Resolved Spectroscopy Unravels the Mechanism of a Radical-Induced 1,2-Metalate Rearrangement

Radical-induced 1,2-metalate rearrangements of boronate complexes are an emerging and promising class of reactions that allow multiple new bonds to be formed in a single, tunable reaction step. These reactions involve the addition of an alkyl radical, typically generated from an alkyl iodide under photochemical activation, to a boronate complex to produce an α-boryl radical intermediate. From this α-boryl radical, there are two plausible reaction pathways that can trigger the product forming 1,2-metalate rearrangement: iodine atom transfer (IAT) or single electron transfer (SET). Previous steady-state techniques have struggled to differentiate these pathways. Here we apply state-of-the-art time-resolved infrared absorption spectroscopy to resolve all the steps in the reaction cycle by mapping production and consumption of the reactive intermediates over picosecond to millisecond time scales. We apply this technique to a recently reported reaction involving the addition of an electron-deficient alkyl radical to the strained σ-bond of a bicyclo[1.1.0]butyl boronate complex to form a cyclobutyl boronic ester. We show that the previously proposed SET mechanism does not adequately account for the observed spectral and kinetic data. Instead, we demonstrate that IAT is the preferred pathway for this reaction and is likely to be operative for other reactions of this type.

Adenine Radical Cation Formation by a Ligand-Centered Excited State of an Intercalated Chromium Polypyridyl Complex Leads to Enhanced DNA Photo-oxidation.

Assessment of the DNA photo-oxidation and synthetic photocatalytic activity of chromium polypyridyl complexes is dominated by consideration of their long-lived metal-centered excited states. Here we report the participation of the excited states of [Cr(TMP)2dppz]3+ (1) (TMP = 3,4,7,8-tetramethyl-1,10-phenanthroline; dppz = dipyrido[3,2-a:2′,3′-c]phenazine) in DNA photoreactions. The interactions of enantiomers of 1 with natural DNA or with oligodeoxynucleotides with varying AT content (0–100%) have been studied by steady state UV/visible absorption and luminescence spectroscopic methods, and the emission of 1 is found to be quenched in all systems. The time-resolved infrared (TRIR) and visible absorption spectra (TA) of 1 following excitation in the region between 350 to 400 nm reveal the presence of relatively long-lived dppz-centered states which eventually yield the emissive metal-centered state. The dppz-localized states are fully quenched when bound by GC base pairs and partially so in the presence of an AT base-pair system to generate purine radical cations. The sensitized formation of the adenine radical cation species (A•+T) is identified by assigning the TRIR spectra with help of DFT calculations. In natural DNA and oligodeoxynucleotides containing a mixture of AT and GC of base pairs, the observed time-resolved spectra are consistent with eventual photo-oxidation occurring predominantly at guanine through hole migration between base pairs. The combined targeting of purines leads to enhanced photo-oxidation of guanine. These results show that DNA photo-oxidation by the intercalated 1, which locates the dppz in contact with the target purines, is dominated by the LC centered excited state. This work has implications for future phototherapeutics and photocatalysis.

Theoretical Insights into the Excited State Decays of a Donor-Acceptor Dyad: Is the Twisted and Rehybridized Intramolecular Charge-Transfer State Involved?

The twisted intramolecular charge transfer has been proposed for a number of years and widely accepted to explain the excited-state dynamics of organic molecules. Recently, a new state termed as "twisted and rehybridized intramolecular charge transfer" has been proposed to explain the excited-state dynamics of an aniline-triazine electron donor-acceptor dyad with an alkyne spacer based on ultrafast time-resolved spectroscopy. However, the change of the geometries along the excited-state decay pathway remains unknown. In this study, by optimization of the excited-state geometry of the donor-acceptor dyad and potential energy surface scan along the twisting angle, we successfully reproduce the experimentally observed band in time-resolved infrared absorption spectroscopy. Our calculation results demonstrated that the rehybridization process is not involved and only the twisted intramolecular charge transfer state is formed. Moreover, we located a minimum energy conical intersection between the ground and first excited-state of the donor-acceptor dyad, which is easily reached and corresponding to the primary nonradiative decay pathway of the donor-acceptor dyad. The energy of minimum energy conical intersection is solvent-dependent and consistent with the experimentally observed solvent-dependent lifetime of excited state.

Nitro End Groups: Remarkable Vibrational Reporters for Charge Transfer in the Excited States of Oligo(p-phenyleneethynylene)-Bridged Donor–Acceptor Dyads

The D-π-A dyads [(n-Hex)2N(1,4-C6H4)]CC[(1,4-C6H4)NO2] (1) and [(n-Hex)2N(1,4-C6H4)]CC(1,4-C6H4)CC[(1,4-C6H4)NO2] (2) have been studied by ultrafast time-resolved infrared absorption spectroscopy. After excitation into their singlet charge-transfer (1CT) state, a fast decay (ca. 6 ps for 1 and 1 ps for 2) of the initially populated singlet state into a ground state (major pathway) and a longer lived excited state (possibly the triplet state) is observed. The nitro and alkyne groups were used as vibrational reporters to probe the changes induced by the charge-transfer process. For the first time, we confirm experimentally that these changes are consistent with expectations based on the traditional valence-bond representations of the CT states of these push-pull chromophores. An almost identical charge transfer takes place in the two dyads, despite π-bridges of different lengths between the donor and acceptor groups. Complementary DFT calculations support the experimental assignments and have helped clarify the photophysical behavior of 1 and 2.

Time-Resolved Temperature-Jump Infrared Spectroscopy at a High Repetition Rate.

Time-resolved temperature-jump infrared absorption spectroscopy at a 0.5 to 1 kHz repetition rate is presented. A 1 kHz neodymium-doped yttrium aluminum garnet (Nd:YAG) laser pumping an optical parametric oscillator provided >70 µJ, 3.75 µm pump pulses, which delivered a temperature jump via excitation of the O-D stretch of a D2O solution. A 10 kHz train of mid-infrared probe pulses was used to monitor spectral changes following the temperature jump. Calibration with trifluoroacetic acid solution showed that a temperature jump of 10 K lasting for tens of microseconds was achieved, sufficient to observe fast processes in functionally relevant biomolecular mechanisms. Modeling of heating profiles across ≤10 µm path length cells and subsequent cooling dynamics are used to describe the initial <100 ns cooling at the window surface and subsequent, >10 µs cooling dynamics of the bulk solution.

(Invited) New Mixed-Anion Photocatalysts for Water Splitting and CO2 Fixation

Water splitting and CO2 fixation using semiconductor photocatalysts are important reactions from the viewpoint of solar-to-fuel energy conversion toward artificial photosynthesis. Mixed-anion compounds containing more than one anionic species in a single-phase have attracted attention as visible-light-driven photocatalysts, since as compared to oxygen 2p orbital, p orbitals of less electronegative anion can form a valence band that possesses more negative potential. In this talk, recent progress on the development of new mixed-anion photocatalysts that are applicable to water splitting and CO2 fixation will be given. We developed Ta and N-codoped rutile TiO2 for visible-light-driven water oxidation. TiO2:Ta,N powders modified with a RuO2 cocatalyst were active under visible light up to 540 nm for water oxidation to produce O2 in the presence of reversible electron acceptors (IO3 – or Fe3+), while TiO2:N exhibited negligible activity. Results of time-resolved infrared absorption spectroscopy revealed that the codoping Ta with N into TiO2 prolonged the lifetime of photogenerated free electrons, leading to high photocatalytic activity. Overall water splitting into H2 and O2 was achieved using TiO2:Ta,N, in combination with a H2-evolution photocatalyst of SrTiO3:Rh and an Fe3+/Fe2+ or IO3 –/I– redox couple even under simulated sunlight (AM1.5G). A similar result was obtained with rutile TiO2 codoped with N and F. TiO2:Ta,N also served as a very stable photoanode to oxidize water under visible light, and could be coupled to not only a H2 evolution cathode but also a molecular photocathode for CO2 reduction. A photoelectrochemical cell constructed with RuO2/TiO2:Ta,N photoanode and Ru(II)–Re(I) supramolecular photocathode was capable of reducing CO2 into CO using water as the electron source upon visible light, with stoichiometric production of CO and O2. Another interesting example of mixed-anion photocatalyst is an oxyfluoride pyrochlore Pb2Ti2O5.4F1.2. It had been believed that oxyfluorides are unsuitable as visible-light-responsive photocatalysts because of the highest electronegativity of fluorine. Surprisingly, Pb2Ti2O5.4F1.2 had an unprecedented small band gap of ca. 2.4 eV, and worked as a stable photocatalyst for visible-light H2 evolution and CO2 reduction when modified with suitable promoters. Density functional theory calculations indicated that the unprecedented visible-light-response of Pb2Ti2O5.4F1.2 originates from strong interaction between Pb-6s and O-2p orbitals, which is enabled by a short Pb–O bond in the pyrochlore lattice due to the fluorine substitution.

Vibrational Relaxation and Redistribution Dynamics in Ruthenium(II) Polypyridyl-Based Charge-Transfer Excited States: ACombined Ultrafast Electronic and Infrared Absorption Study.

Ultrafast time-resolved electronic and infrared absorption measurements have been carried out on a series of Ru(II) polypyridyl complexes in an effort to delineate the dynamics of vibrational relaxation in this class of charge transfer chromophores. Time-dependent density functional theory calculations performed on compounds of the form [Ru(CN-Me-bpy) x(bpy)3-x]2+ ( x = 1-3 for compounds 1-3, respectively, where CN-Me-bpy is 4,4'-dicyano-5,5'-dimethyl-2,2'-bipyridine and bpy is 2,2'-bipyridine) reveal features in their charge-transfer absorption envelopes that allow for selective excitation of the Ru(II)-(CN-Me-bpy) moiety, the lowest-energy MLCT state(s) in each compound of the series. Changes in band shape and amplitude of the time-resolved differential electronic absorption data are ascribed to vibrational cooling in the CN-Me-bpy-localized 3MLCT state with a time constant of 8 ± 3 ps in all three compounds. This conclusion was corroborated by picosecond time-resolved infrared absorption measurements; sharpening of the CN stretch in the 3MLCT excited state was observed with a time constant of 3.0 ± 1.5 ps in all three members of the series. Electronic absorption data acquired at higher temporal resolution revealed spectral modulation over the first 2 ps occurring with a time constant of τ = 170 ± 50 fs, in compound 1; corresponding effects are significantly attenuated in compound 2 and virtually absent in compound 3. We assign this feature to intramolecular vibrational redistribution (IVR) within the 3MLCT state and represents a rare example of this process being identified from time-resolved electronic absorption data for this important class of chromophores.

Excited-State Dynamics of Graphitic Carbon Nitride Photocatalyst and Ultrafast Electron Injection to a Ru(II) Mononuclear Complex for Carbon Dioxide Reduction

We have previously developed photocatalytic CO2 reduction systems using graphitic carbon nitride (g-C3N4) and a Ru(II) mononuclear complex (e.g., trans(Cl)–[RuII{4,4′-(H2PO3)2bpy}2(CO)2Cl2] bpy = 2,2′-bipyridine, abbreviated as RuP) hybrids and demonstrated its high activities under visible light (λ > 400 nm). To understand the excited-state dynamics of C3N4 and electron-transfer process to RuP, here we examined the photophysical properties of g-C3N4 as well as mesoporous g-C3N4 (mpg-C3N4) by means of time-resolved emission and/or time-resolved infrared absorption (TR-IR) spectroscopy. The emission decay measurements showed that g-C3N4 (as well as mpg-C3N4) has at least three emissive excited states with different lifetimes (g-C3N4; 1.3 ± 0.4, 3.9 ± 0.9, and 15 ± 4 ns at 269 nm photoexcitation) in aqueous suspension. These excited states were not quenched upon addition of a hole scavenger (e.g., disodium dihydrogen ethylenediamine tetraacetate dehydrate) and/or an electron acceptor (RuP), even though phot...

Bimodal Exciplex Formation in Bimolecular Photoinduced Electron Transfer Revealed by Ultrafast Time-Resolved Infrared Absorption.

The dynamics of a moderately exergonic photoinduced charge separation has been investigated by ultrafast time-resolved infrared absorption with the dimethylanthracene/phthalonitrile donor/acceptor pair in solvents covering a broad range of polarity. A distinct spectral signature of an exciplex could be identified in the -C≡N stretching region. On the basis of quantum chemistry calculations, the 4-5 times larger width of this band compared to those of the ions and of the locally excited donor bands is explained by a dynamic distribution of exciplex geometry with different mutual orientations and distances of the constituents and, thus, with varying charge-transfer character. Although spectrally similar, two types of exciplexes could be distinguished by their dynamics: short-lived, "tight", exciplexes generated upon static quenching and longer-lived, "loose", exciplexes formed upon dynamic quenching in parallel with ion pairs. Tight exciplexes were observed in all solvents, except in the least polar diethyl ether where quenching is slower than diffusion. The product distribution of the dynamic quenching depends strongly on the solvent polarity: whereas no significant loose exciplex population could be detected in acetonitrile, both exciplex and ion pair are generated in less polar solvents, with the relative population of exciplex increasing with decreasing solvent polarity. These results are compared with those reported previously with donor/acceptor pairs in different driving force regimes to obtain a comprehensive picture of the role of the exciplexes in bimolecular photoinduced charge separation.

Electron-transfer acceleration investigated by time resolved infrared spectroscopy.

Ultrafast electron transfer (ET) processes are important primary steps in natural and artificial photosynthesis, as well as in molecular electronic/photonic devices. In biological systems, ET often occurs surprisingly fast over long distances of several tens of angströms. Laser-pulse irradiation is conveniently used to generate strongly oxidizing (or reducing) excited states whose reactions are then studied by time-resolved spectroscopic techniques. While photoluminescence decay and UV-vis absorption supply precise kinetics data, time-resolved infrared absorption (TRIR) and Raman-based spectroscopies have the advantage of providing additional structural information and monitoring vibrational energy flows and dissipation, as well as medium relaxation, that accompany ultrafast ET. We will discuss three cases of photoinduced ET involving the Re(I)(CO)3(N,N) moiety (N,N = polypyridine) that occur much faster than would be expected from ET theories. [Re(4-N-methylpyridinium-pyridine)(CO)3(N,N)](2+) represents a case of excited-state picosecond ET between two different ligands that remains ultrafast even in slow-relaxing solvents, beating the adiabatic limit. This is caused by vibrational/solvational excitation of the precursor state and participation of high-frequency quantum modes in barrier crossing. The case of Re-tryptophan assemblies demonstrates that excited-state Trp → *Re(II) ET is accelerated from nanoseconds to picoseconds when the Re(I)(CO)3(N,N) chromophore is appended to a protein, close to a tryptophan residue. TRIR in combination with DFT calculations and structural studies reveals an interaction between the N,N ligand and the tryptophan indole. It results in partial electronic delocalization in the precursor excited state and likely contributes to the ultrafast ET rate. Long-lived vibrational/solvational excitation of the protein Re(I)(CO)3(N,N)···Trp moiety, documented by dynamic IR band shifts, could be another accelerating factor. The last discussed process, back-ET in a porphyrin-Re(I)(CO)3(N,N) dyad, demonstrates that formation of a hot product accelerates highly exergonic ET in the Marcus inverted region. Overall, it follows that ET can be accelerated by enhancing the electronic interaction and by vibrational excitation of the reacting system and its medium, stressing the importance of quantum nuclear dynamics in ET reactivity. These effects are experimentally accessible by time-resolved vibrational spectroscopies (IR, Raman) in combination with quantum chemical calculations. It is suggested that structural dynamics play different mechanistic roles in light-triggered ET involving electronically excited donors or acceptors than in ground-state processes. While TRIR spectroscopy is well suitable to elucidate ET processes on a molecular-level, transient 2D-IR techniques combining optical and two IR (or terahertz) laser pulses present future opportunities for investigating, driving, and controlling ET.

Electron transfer dynamics and excited state branching in a charge-transfer platinum(II) donor-bridge-acceptor assembly.

A linear asymmetric Pt(ii) trans-acetylide donor-bridge-acceptor triad designed for efficient charge separation, NAP[triple bond, length as m-dash]Pt(PBu3)2[triple bond, length as m-dash]Ph-CH2-PTZ (), containing strong electron acceptor and donor groups, 4-ethynyl-N-octyl-1,8-naphthalimide (NAP) and phenothiazine (PTZ) respectively, has been synthesised and its photoinduced charge transfer processes characterised in detail. Excitation with 400 nm, ∼50 fs laser pulse initially populates a charge transfer manifold stemming from electron transfer from the Pt-acetylide centre to the NAP acceptor and triggers a cascade of charge and energy transfer events. A combination of ultrafast time-resolved infrared (TRIR) and transient absorption (TA) spectroscopies, supported by UV-Vis/IR spectroelectrochemistry, emission spectroscopy and DFT calculations reveals a self-consistent photophysical picture of the excited state evolution from femto- to milliseconds. The characteristic features of the NAP-anion and PTZ-cation are clearly observed in both the TRIR and TA spectra, confirming the occurrence of electron transfer and allowing the rate constants of individual ET-steps to be obtained. Intriguingly, has three separate ultrafast electron transfer pathways from a non-thermalised charge transfer manifold directly observed by TRIR on timescales ranging from 0.2 to 14 ps: charge recombination to form either the intraligand triplet (3)NAP with 57% yield, or the ground state, and forward electron transfer to form the full charge-separated state (3)CSS ((3)[PTZ(+)-NAP(-)]) with 10% yield as determined by target analysis. The (3)CSS decays by charge-recombination to the ground state with ∼1 ns lifetime. The lowest excited state is (3)NAP, which possesses a long lifetime of 190 μs and efficiently sensitises singlet oxygen. Overall, molecular donor-bridge-acceptor triad demonstrates excited state branching over 3 different pathways, including formation of a long-distant (18 Å) full charge-separated excited state from a directly observed vibrationally hot precursor state.

Ultrafast photoinduced charge transport in Pt(II) donor-acceptor assembly bearing naphthalimide electron acceptor and phenothiazine electron donor.

Visible light-induced charge transfer dynamics were investigated in a novel transition metal triad acceptor-chromophore-donor, (NDI-phen)Pt(II)(-C≡C-Ph-CH2-PTZ)2 (1), designed for photoinduced charge separation using a combination of time-resolved infrared (TRIR) and femtosecond electronic transient absorption (TA) spectroscopy. In 1, the electron acceptor is 1,4,5,8-naphthalene diimide (NDI), and the electron donor is phenothiazine (PTZ), and [(phen)Pt(-C≡C-Ph-)], where phen is 1,10-phenanthroline, represents the chromophoric core. The first excited state observed in 1 is a (3)MLCT/LL'CT, with {Pt(II)-acetylide}-to-phen character. Following that, charge transfer from the phen-anion onto the NDI subunit to form NDI(-)-phen-[Pt-(C≡C)2](+)-PTZ2 occurs with a time constant of 2.3 ps. This transition is characterised by appearance of the prominent NDI-anion features in both TRIR and TA spectra. The final step of the charge separation in 1 proceeds with a time constant of ∼15 ps during which the hole migrates from the [Pt-(C≡C)2] subunit to one of the PTZ groups. Charge recombination in 1 then occurs with two distinct time constants of 36 ns and 107 ns, corresponding to the back electron transfer to each of the two donor groups; a rather rare occurrence which manifests that the hole in the final charge-separated state is localised on one of the two donor PTZ groups. The assignment of the nature of the excited states and dynamics in 1 was assisted by TRIR investigations of the analogous previously reported ((COOEt)2bpy)Pt(C≡C-Ph-CH2-PTZ)2 (2), (J. E. McGarrah and R. Eisenberg, Inorg. Chem., 2003, 42, 4355; J. E. McGarrah, J. T. Hupp and S. N. Smirnov, J. Phys. Chem. A, 2009, 113, 6430) as well as (bpy)Pt(C≡C-Ph-C7H15)2, which represent the acceptor-free dyad, and the chromophoric core, respectively. Thus, the step-wise formation of the full charge-separated state on the picosecond time scale and charge recombination via tunnelling have been established; and the presence of two distinct charge recombination pathways has been observed.

Determination of the Rate Constant for the OH(X2Π) + OH(X2Π) → H2O + O(3P) Reaction Over the Temperature Range 295 to 701 K

The rate constant for the radical-radical reaction OH(X(2)Π) + OH(X(2)Π) → H2O + O((3)P) has been measured over the temperature and pressure ranges 295-701 K and 2-12 Torr, respectively, in mixtures of CF4, N2O, and H2O. The OH radical was produced by the 193 nm laser photolysis of N2O. The resulting O((1)D) atoms reacted rapidly with H2O to produce the OH radical. The OH radical was detected by high-resolution time-resolved infrared absorption spectroscopy using a single Λ-doublet component of the OH(1,0) P1e/f(4.5) fundamental vibrational transition. A detailed kinetic model was used to determine the reaction rate constant as a function of temperature. These experiments were conducted in a new temperature controlled reaction chamber. The values of the measured rate constants are quite similar to the previous measurements from this laboratory of Bahng and Macdonald (J. Phys. Chem. A 2007 , 111 , 3850 - 3861); however, they cover a much larger temperature range. The results of the present work do not agree with recent measurements of Sangwan and Krasnoperov (J. Phys. Chem. A 2012 , 116 , 11817 - 11822). At 295 K the rate constant of the title reaction was found to be (2.52 ± 0.63) × 10(-12) cm(3) molecule(-1) s(-1), where the uncertainty includes both experimental scatter and an estimate of systematic errors at the 95% confidence limit. Over the temperature range of the experiments, the rate constant can be represented by k1a = 4.79 × 10(-18)T(1.79) exp(879.0/T) cm(3) molecule(-1) s(-1) with a uncertainty of ±24% at the 2σ level, including experimental scatter and systematic error.

Determination of Midgap State Energy Levels of an Anatase TiO2 Nanocrystal Film by Nanosecond Transient Infrared Absorption – Excitation Energy Scanning Spectra

Midgap levels for wide gap TiO2 have become increasingly important because they can be used to capture solar light efficiently for photocatalysis as demonstrated by black TiO2 in a recent paper [Chen, X., et al. Science 2011, 331 (6018), 746–750]. However, a method for systematically characterizing the midgap state energy levels is still lacking. We proposed an optical method, i.e., transient infrared (IR) absorption – excitation energy scanning spectrum, by recording nanosecond time-resolved transient IR absorption from the excited electrons either in the conduction band or at the excited localized states below the conduction in combination with midgap excitation energy scanning. We demonstrate that both the electron trap states beneath the Fermi level and those excited localized states below the conduction band as well as the Fermi level of TiO2 nanoparticles can unambiguously be determined by this method, which has great potential for characterizing the midgap trap states of various semiconductor nanom...