Unity Quantum Yield Research Articles

The UV-visible absorption cross-sections of IONO<sub>2</sub>

Abstract. The UV-visible absorption spectrum of gaseous IONO2 has been measured over the wavelength range 245--415 nm using the technique of laser photolysis with time-resolved UV-visible absorption spectroscopy. IONO2 was produced in situ in the gas phase by laser flash photolysis of NO2/CF3I/N2 mixtures. Post flash spectra were deconvolved to remove contributions to the observed absorption from other reactant and product species. The resulting spectrum attributed to IONO2 consists of several overlapping broad absorption bands. Assuming a quantum yield of unity for IONO2 photolysis, model calculations show that during sunlit hours at noon, 53° N, the first order solar photolysis rate coefficient (J value) for IONO2 is 4.0 x 10-2 s-1.

New Halogenated Phenylbacteriochlorins and Their Efficiency in Singlet-Oxygen Sensitization

Halogenated phenylbacteriochlorins are synthesized with high yields in a two-step procedure. They have strong absorbances in the red and are very stable to air and light at room temperature. Flash photolysis measurements show that the triplet states of these bacteriochlorins have 30 μs lifetimes in deaerated toluene, that are quenched with diffusion-controlled rate constants by molecular oxygen. Time-resolved photoacoustic measurements, with nanosecond and nanocalorie resolution, show that these bacteriochlorins sensitize the formation of singlet oxygen with nearly unity quantum yield. However, singlet-oxygen phosphorescence measurements indicate that physical quenching occurs before the singlet-oxygen molecules diffuse into solution, and nearly half of the sensitized singlet states are lost.

Photocyclization/Cycloreversion Quantum Yields of Diarylethenes in Single Crystals

Photocyclization and photocycloreversion quantum yields of three diarylethene derivatives were determined in the single-crystalline phase. The former yields in the crystalline phase were twice as large as those in solution. This can be attributed to only photoreactive antiparallel conformers being packed in the crystals, while in solution both photoreactive antiparallel and photoinactive parallel conformers coexist in almost equal amounts. The quantum yields in the crystals were found to be extremely high and 1,2-bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene (3a) exhibited the quantum yield of unity (100%). The latter yield of the closed-ring isomer of 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene (1a) was similar to that in solution, while the yields of the closed-ring isomers of 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene (2a) and 3a were higher than those in solution by a factor of 2 to 3. The high yields are attributable to the constrained conformation of the photogener...

Femtosecond photolysis of aqueous HOCl

This paper reports an experimental study of the photolysis of aqueous HOCl using femtosecond pulses at 266 nm. The formation of photoproducts is monitored by transient absorption spectroscopy from 230 to 400 nm. The HOCl molecules dissociate with unity quantum yield to form OH+Cl faster than 1 ps, and as a result of the potential along the HO–Cl reaction coordinate, all excess energy is given to the fragments as translational energy. After dissociation, and solvent cage escape, the majority of the Cl and OH fragments recombine after diffusion on a time scale of 50 ps. The diffusion dynamics is studied using a simple model for diffusive recombination and a more extensive molecular dynamics simulation. A minor fraction of the Cl atoms (∼10%) reacts with HOCl in a diffusion limited reaction to form Cl2+OH.

Detection of O(1D) produced in the photodissociation of O2. II. Identification of the Σu-3 and Πu3 Rydberg states in 105–113 nm

The production of O(1D) from O2 by photodissociation was measured in the 105–113 nm wavelength region. The quantum yields of O(1D) were employed to identify the excited states as Σu-3 or Πu3, where the former correlated with the production of O(1D) and the latter production of O(3P) only. Many bands of E, E′ of Σu-3 and F, F′ Πu3, were found with their vibrational states identified. Several bands had mixed states, such as the (0,0) and (1,0) bands of D′ 3Σu−←X 3Σg− transition which had smaller than unity quantum yields of O(1D) due to the perturbation caused by the nearby states D′ 3Σu+(v′=1,2) at 112.0 and 110.0 nm. The bands of npσu 3Πu state in the region of 105–110 nm are in agreement with the assignments of other works, and a band near 107.3 nm is consistent with the predicted band position of the F 3Πu(v′=6) upper-state.

Quantum Yields Lower than Unity in Photo- induced Dissociative Electron Transfers: The Reductive Cleavage of Carbon Tetrachloride

It has been shown recently that the electrochemical reduction of carbon tetrachloride in N,N'-dimethylformamide follows a mechanism in which electron transfer and bond cleavage are concerted. We report here results concerning photoinduced electron transfer from the singlet excited state of two aromatic molecules, 2-ethyl-9,10-dimethoxyanthracene and perylene, to CCl4 , which is characterised by a quantum yield of complete quenching fragmentation ranging from 0.7 to 0.8. It is shown that a quantum yield below unity is compatible with a dissociative mechanism and arises from partitioning of the system at the intersection of the product- and ground-state potential energy surfaces. This phenomenon predominates over back electron transfer from the clustered fragments state. The photoinduced reductive cleavage of CCl4 thus provides a clear illustration of the recent theoretical prediction, that photoinduced dissociative electron transfers are not necessarily endowed with a unity quantum yield. This offers an opportunity to estimate the magnitude of the electronic matrix element that couples the fragmented product state and the ground reactant state.

Photoinduced Dissociative Electron Transfer: Is the Quantum Yield Theoretically Predicted to Equal Unity?

An attractive manner of fighting back-electron transfer to the ground state in photoinduced electron transfer reactions is to use a system in which the donor and/or the acceptor in the ion-pair undergoes a rapid fragmentation. Intuitively, it seems that an ideal situation in this respect, leading to a unity quantum yield, should be met when fragmentation and electron transfer are concerted. Accordingly, a quantum yield below 1 would be the signature of a nonconcerted two-step mechanism. It is shown, from first principles, that a purely dissociative photoinduced electron transfer is not necessarily endowed with a unity quantum yield. The reason is that the system partitions between fragmentation and back-electron transfer in the funnel offered by the upper first-order potential energy surface combining the ground state and fragments zero-order surfaces. A semiclassical model is presented, relating the quantum yield to the electronic matrix coupling element, H. Only in the case of a completely nonadiabatic ...

The efficient intramolecular sensitisation of terbium(III) and europium(III) by benzophenone-containing ligands

The europium and terbium complexes of an octadentate macrocyclic ligand incorporating benzophenone have been prepared; excitation of the ketone, which has a triplet quantum yield of unity, results in very efficient sensitisation of the emissive states of the bound metal ions.

Femtosecond Time Resolved Infrared Spectroscopy of the Ethylenic Stretch Vibration During the all-Trans to 13-Cis Isomerization of Bacteriorhodopsin

We report on room temperature infrared transient absorption experiments on Bacteriorhodopsin concerning the early picosecond dynamics of the all trans to 13-cis isomerization of the retinal chromophore. The absorption band of the all-trans chromophore ethylenic stretch vibration bleaches concomitantly to photoexcitation whereas new absorption strength is created redshifted within less than 1 ps. the initial bleach recovers partly according to the non unity quantum yield of the photoisomerization within 2 ps. With a time constant of 3ps the low energy absorption band is downshifled 5–10 cm-1. Thus, vibrational relaxation and cooling processes appear to have ended within a few ps and on the time scale of 20 ps a stationary BR-K difference spectrum is obtained.

Synthesis and Properties of a New Photostable Soluble Perylene Dye: N,N'-Di-(1-Dehydroabietyl) Perylene-3,4,9,10-Bis(Dicarboximide)

N,N'-Di-(-1-Dehydroabietyl) perylene-3,4,9,10-bis(dicarboximide) is prepared in 95% yield by the condensation of perylene 3,4,9,10-tetracarboxylic dianhydride with dehydroabietyl amine. Thermal and photostable bright red-orange dye is measured to have a fluorescence quantum yield of unity, Qf=1, in chloroform solutions. Solubility of the dye is 1.8 g/L in chloroform. The diimide is an ideal reference probe for fluorescence quantum yield measurements in 500—650 nm region and also very interesting dye for chemiluminescent light devices.

Superfluorescence from optically trapped calcium atoms

We have studied superfluorescence (SF) under highly unfavorable conditions of rapid collisional and radiative distribution in a Doppler-broadened medium. Nanosecond SF pulses at 5.5 \ensuremath{\mu}m were generated on the Ca $4s4p{}^{1}{P}_{1}--3d4s{}^{1}{D}_{2}$ transition from a column of calcium vapor buffered with Ar by optically pumping the ${4s}^{2}{}^{1}{S}_{0}--4s4p{}^{1}{P}_{1}$ transition. The Rabi frequency associated with the intense pump pulse prevents the occurrence of SF while the pump laser is on. As a result, the predicted scaling laws that describe the properties of SF in a transversely excited system, such as peak heights, pulse widths, and delay times, are shown to apply in our situation in which the conditions resemble swept excitation. The delay times were found to be in agreement with a fully quantum mechanical calculation which describes the initiation of SF. Measurements of the densities of the three levels, the absolute SF photon yield, and the spatial distribution of the excited states indicate that the system has a quantum yield of unity. The SF intensity increases with an increase in Ar pressure due to collisional redistribution until the collisional dephasing rate inhibits SF. The conditions describing the transition of SF to amplified spontaneous emission allow us to measure the collisional broadening rate for the SF transition.

Photodissociation dynamcis of ClO radicals in the range (237≤ λ≤270) nm and at 205 nm and the velocity distribution of O( 1D) atoms

Isotopically selected photofragmentation of thermal ClO radicals was studied in the continuous and structured region of the A 2Π ← X 2Π transition in the wavelength range 237< λ<270 nm employing resonance enhanced multiphoton ionisation (REMPI) detection schemes. Photoexcitation of ClO generated Cl( 2P 3/2, 1/2 ) and O( 1D) atoms with a quantum yield of unity. The translational energy of O( 1D) was determined and quantified as a function of the wavelength of the incident radiation. Some experiments on the kinetic energy deposited in Cl( 2P 3/2, 1/2 ) are also reported. From the decrease of the O( 1D) REMPI signal intensity approaching the transition from continuous to structured absorption, the cut-off excitation wavelength for O( 1D) in thermal ClO radicals was evaluated to be 263.4±0.2 nm. In the structured region of the spectrum oxygen atoms are formed in the triplet ground state.

Detection of O(1D) Produced in the Photodissociation of O2 in the Schumann−Runge Continuum

O(1D) produced from O2 by photodissociation in the Schumann−Runge continuum was studied. The metastable atom O(1D) was detected by observing the 762 nm fluorescence from O2(b 1∑g+) generated by the energy transfer process O(1D) + O2 → O + O2(b 1∑g+). A quantum yield of unity was found in the wavelength range 139−175 nm. O(1D) was detected at wavelengths longer than the threshold at 175 nm with reduced quantum yields due to excitations of molecules in the high rotational states. The quantum yields vary from 1 in the range 130−139 nm, resulting from crossing of the state 3∑u- with 3Πu. The absorption spectra for both states were determined. The peaks for maxima and minima absorptions were compared with previous experimental and theoretical results.

Flash Photolysis of 5‐Methyl‐1,4‐naphthoquinone in Aqueous Solution: Kinetics and mechanism of photoenolization and of enol trapping

Abstract5‐Methyl‐1,4‐naphthoquinone (1) is a remarkable probe to study hydrogen and proton transfer reactions. The photoenol 4‐hydroxy‐5‐methylidene naphthalen‐1 (5H)‐one (2) is formed in the ground state within 2 ps of excitation and with a quantum yield of unity, presumably through a conical intersection of the S0 and S1 hypersurfaces. In aqueous acid, enol 2 is hydrated to 5‐(hydroxymethyl)naphthalene‐1,4‐diol 3 (X  OH, Scheme 1). The rate of hydration of 2 increases linearly with acid concentration from ca. 1.5 × 104 s−1 at pH 6 to reach a maximum value of 9 ×107 s−1 when the remaining carbonyl function is protonated, pKa(2+) = 1.1. Contrary to an earlier suggestion, the rate‐determining step in the acid‐catalyzed hydration of 2 is addition of water to the conjugate acid 2+. Pronounced acceleration of the decay rate of 2 by hydrazoic‐acid buffers indicates competitive trapping of 2+ by the azide ion. In neutral‐to‐weakly‐basic solutions, enol 2 reacts by ionization, pKa(2) = 6.5, and nearly diffusion‐controlled condensation of the carbanionic species 2− with quinone 1. Proto‐nation at the methylidene C‐atom does not compete measurably with protonation on carbonyl O‐atom, despite a Substsial thermodynamic basic for carbon Portoation of ca. 50 kJ mol−1 for 2 and 100 mol−1 for 2−.

Miscellanea. Femtosecond infrared spectroscopy on bacteriorhodopsin using a broad band carbon monoxide laser

AbstractA pump‐probe mid infrared transient absorption apparatus was built, employing a broad band continuous wave carbon monoxide laser and an optical gating technique resulting in femtosecond time resolution.It was used to monitor the all‐trans to 13‐cis isomerization of the retinal chromophore in Bacteriorhodopsin (BR). The C=C in‐phase stretch vibration, located at 1529cm−1 in the light adapted, unphotolyzed state BR570 served as marker band. Photoexcitation leads to a bleach centered at this wavenumber and to a partial recovery corresponding to the non unity quantum yield of the BR photocycle. On the picosecond time scale the high energy shoulder of this bleach is superimposed by newly created IR absorption strength, due to the corresponding vibrational modes of the J‐ and the K‐state.

A Thermal and Photostable Reference Probe for QfMeasurements: Chloroform Soluble Perylene 3,4,9,10-Tetracarboxylic Acid-bis-N, N-Dodecyl Diimide

Abstract The synthesis of a chloroform soluble perylene 3, 4, 9, 10-tetracarboxylic acid-bis-N, N′-dodecyl diimide via condensation of perylene 3, 4, 9, 10-tetracarboxylic dianhydnde and 1, 12-diamino dodecane is described. The thermal analysis is shown thermal stabilities above 400°C. Thermal and photostable dye is measured to have a fluorescence quantum yield of unity, Qf = 1.00, in chloroform solutions. Perylene bis-dodecyl diimide is an ideal reference probe for fluorescence quantum yield measurements in 500–650 nm region and a reliable soluble photosensitizer for solar photochemical applications.

Effects of Asp residues near the L-side pigments in bacterial reaction centers.

The primary photochemistry in Rhodobacter capsulatus reaction centers (RCs) containing the Phe to Asp mutation at L polypeptide residue 121 near the photoactive bacteriopheophytin (BPhL) is characterized using ultrafast transient absorption spectroscopy. At 285 K, initial charge separation from P* proceeds with essentially unity quantum yield in approximately 6 ps to form a transient denoted P+I-. This transient is proposed to involve P+BPhL- and probably P+BChlL- as well (BChlL is the L-side bacteriochlorophyll molecule). P+I- decays in approximately 150 ps both by electron transfer to give P+QA- (approximately 78% yield) and by charge recombination to the ground state (approximately 22% yield). These results indicate that the F(L121)D mutant is closely related, in terms of its electron transfer properties, to previously reported RCs in which BPhL is replaced with a bacteriochlorophyll (beta-type RCs) or a pheophytin. However, the native BPhL pigment is retained in the F(L121)D mutant. We propose that the Asp at L121 raises the free energy of P+BPhL-, thereby giving rise to the altered photochemistry. At 77 K, the P+I- lifetime is shortened slightly to approximately 120 ps and the yield of P+QA- is increased to approximately 88%. This result is somewhat different from that obtained for beta-type RCs at low temperature, where the P+I- lifetime lengthens and the yield of P+QA- diminishes or stays about the same compared to the values near room temperature. We exploit these differences in developing a model for the charge separation process in the F(L121)D mutant. The effects of introducing an Asp near BPhL are compared to those obtained previously in two mutants in which an Asp is introduced near BChlL.

Excimers and Exciplexes of Conjugated Polymers

Observations of intermolecular excimers in several pi-conjugated polymers and exciplexes of these polymers with tris(p-tolyl) amine are reported. It is shown that the luminescence of conjugated polymer thin films originates from excimer emission and that the generally low quantum yield is the result of self-quenching. Thus, in sufficiently dilute solution, the "single-chain" emission has a quantum yield of unity. Exciplex luminescence and exciplex-mediated charge photogeneration have much higher quantum yields than the excimer-mediated photophysical processes. These results provide a basis for understanding and controlling the photophysics of conjugated polymers in terms of supramolecular structure and morphology.

Insights into the factors controlling the rates of the deactivation processes that compete with charge separation in photosynthetic reaction centers

This article addresses the factors that underlie the unity quantum yield of charge separation in the bacterial photosynthetic reaction center (RC). Of particular interest are the factors that suppress charge recombination and other deactivation processes which could compete effectively with charge separation. Studies on mutant RCs with enhanced charge recombination show that rates of decay processes can be increased tenfold or more upon a relatively small fractional increase in the free energy gap to the ground state. This behavior is opposite to that expected on the basis of Franck-Condon effects, where an increase in Δ G should decrease the deactivation rate. In the mutants, it is proposed that the enhanced deactivation rates of the intermediate charge-separated states result from (i) increased quantum mechanical mixing with electronic states at slightly higher energy that have inherently strong deactivation properties, and (ii) increased thermal repopulation of such strongly deactivated states. The thermal repopulation mechanism is proposed to play an important role in the heterodimer mutants studied here. This mechanism requires that the free energy gaps between the charge-separated and strongly deactivated states must decrease with temperature to be consistent with the weakly temperature-dependent deactivation behavior. Potential mediating states include the excited dimeric primary electron donor and a state involving the oxidized dimer and the reduced accessory bacteriochlorophyll molecule. It is concluded that increasing the free energy of charge-separated intermediates in order to minimize the Franck-Condon factors can enhance the deactivation rates and thus reduce the quantum yield of charge separation.

The near U.V. absorption spectra of several aliphatic aldehydes and ketones at 300 K

Abstract Absorption cross-sections are reported for acetaldehyde, propionaldehyde, isobutyraldehyde, n -butyraldehyde, acetone, 2-butanone, 2-pentanone, and 3-pentanone at wavelengths greater than 200 nm, a temperature of 300 K, and a spectral resolution of 0.50 nm. In general, the results are in good agreement with previous measurements of the spectra of these molecules, but are of higher precision than past measurements. For acetaldehyde, propionaldehyde, isobutyraldehyde, and acetone the cross-section results are combined with previous experimental measurements of the primary quantum yield for photolysis to estimate photodissociation rate constants for a representative set of atmospheric conditions. For the other four carbonyl compounds, upper limits for the photodissociation rate constant are calculated assuming a primary quantum yield of unity at all wavelengths. For acetaldehyde, photolysis is slow in comparison with reaction with hydroxyl radical. For the larger aldehydes and for acetone, photolysis and reaction with OH radical take place at comparable rates. For larger ketones, if the quantum yields for photolysis are unity, as assumed in our calculations, the rate of loss of the compounds by photodissociation could be important compared to reaction with hydroxyl radical.