S1 Energy Research Articles

Photophysical and Photochemical Primary Processes of Diphenylacetylene Derivatives and Related Compounds in Liquid Phase

Abstract Photophysical and photochemical primary processes of diphenylacetylene (DPA) derivatives and related compounds have been reviewed. Curious photophysical properties of the low lying excited singlet states of DPA, such as an exceptionally slow S2 → S1 internal conversion, a distinct temperature effect of the S2 lifetime, and a strong excitation energy dependence of the fluorescence yield, were correlated with the S2–S1 energy gap. Contradictions of the assignments of the lowest excited singlet states of DPA were resolved and the mechanism of the S2 → S1 internal conversion was proposed from the comparison of the dynamic behavior of DPA with that of diphenylpolyenes. The small S2–S1 energy gap as well as the large displacement between the potential curves of the upper and lower electronic states can be a reason why the S2 state of the DPA derivatives is exceptionally long-lived. The DPA derivatives of which the intramolecular charge-separated states are formed in polar solvents were used as probe molecules of the solvent–solute interaction. Significant enhancement of the charge recombination of aminophenyl(phenyl)acetylene in protic solvents should be due to the interaction between amino nitrogen atom of the solute molecule and the hydrogen-bonded solvent oligomer.

S1 and S2 States of Apo- and Diapocarotenes

A series of apocarotenes with 5 to 11 conjugated double bonds were synthesized and all-trans isomers were isolated using HPLC techniques. Absorption, fluorescence, and fluorescence excitation spectra were obtained in 77 K glasses. As previously noted for other polyenes and carotenoids, fluorescence spectra of the apocarotenes exhibit a systematic crossover from S1(21Ag) → S0(11Ag) to S2(21Ag) → S0(11Ag) emissions and a sharp decrease in fluorescence yields with increasing conjugation. The apocarotene spectra have sufficient resolution to accurately locate the dominant vibronic bands of the S0(11Ag) → S2(11Bu) and S1(21Ag) → S0(11Ag) transitions, thus leading to an accurate catalog of S1 and S2 electronic energies as a function of conjugation length. We also have obtained the low-temperature absorption and fluorescence spectra of several model polyenes and diapocarotenes. Comparisons between these series allow a systematic exploration of the influence of terminal cyclohexenyl rings on the energies of carot...

Coumarin 153 in the gas phase: optical spectra and quantum chemical calculations

We report the absorption spectrum of Coumarin 153 in the gas phase at 383 K and its fluorescence spectra in a supersonic jet. At excess vibrational energy above 200 cm-1 in the first excited state S1, intramolecular vibrational redistribution leads to emission from interconverting conformers. For the hot isolated molecules, the emission spectrum is dominated by active modes of 350, 810, and 1150 cm-1 and the distributions of oscillator strength for absorption and emission cross at 25420±40 cm-1. This value should be used in solvation studies for the electronic S1 energy of the bare molecule. The oscillator distributions change with temperature indicating a dependence of Franck-Condon factors for high-frequency modes on a low-frequency coordinate. Calculations of electronic structure with density-functional theory show that the second excited state is well separated from the first. It is proposed that spectral changes observed during nonequilibrium polar solvation are caused by intramolecular reorganisation of the julolidine group in the S1 state on the 200 fs time scale.

Meta−Ortho Effect in Organic Photochemistry: Mechanistic and Exploratory Organic Photochemistry

In early research, the meta effect was based on one-electron, Huckel computations and experimental observation, which revealed a selective transmission of electron density to the meta and ortho positions on an aromatic ring in the first excited singlet. However, attention was focused on the meta site. More recent results have confirmed electron density transmission to the ortho site as well. Typical examples involve benzylic cations. Not only are the S1 cations selectively stabilized by meta-methoxy groups compared with para-methoxy substituents but also the corresponding meta-substituted radicals prove of higher energy than the para-substituted counterparts. Additionally, the S0 − S1 energy gap is dramatically smaller for the meta-substituted cations and ion pairs compared with the gap for the para-substituted counterparts. Also, the radicals and radical pairs exhibit much larger ground-state − excited-state energy separations. With a closer approach of surfaces, the excited-state ion pairs have an avenu...

Electric Field Effect on The Charge Migration of Methylene-Linked Carbazole and Terephthalic Acid Methyl Ester in PMMA Polymer Films

Photoinduced inter- and intramolecular electron transfer reactions of methylene-linked (n) carbazole and terephthalic acid methyl ester (abbreviated as D-(n)-A, n=4 and 12) have been examined by using external electro-modulated fluorescence spectroscopy and photocurrent measurement, which allow one to obtain information about the Stark shift and the efficiency of fluorescence quenching as well as photocarrier generation induced by an electric field. The action spectra of the photocurrent could be successfully measured at various concentrations in the absorption region of carbazole (240–365 nm). The obtained action spectra agree well with the simultaneously measured fluorescence excitation spectra, indicating that photocarrier generation occurs following the relaxations including internal conversion to S1 and excitation energy migration among carbazole chromophores. Furthermore, the efficiency of photocarrier generation of D-(12)-A is higher than that of D-(4)-A, indicating that an intramolecular recombination process plays an important role in charge recombination of a photogenerated ion-pair state.

Low-Temperature Photochemistry of Previtamin D: A Hula-Twist Isomerization of a Triene.

What is the standard course of the cis-trans isomerization of nonpolar polyenes? Many results support a pathway in which only a central CH group rotates out of the plane, while the remaining parts of the molecule reorient within the plane ("hula-twist" mechanism). The intermediate structure through which the molecule passes corresponds to the geometry predicted for the conical intersection of the S1 and S0 potential energy surfaces (see picture).

Photoisomerization Path for a Realistic Retinal Chromophore Model: The Nonatetraeniminium Cation

In this paper, ab initio CASSCF computations are used to investigate the photoisomerization path of the protonated Schiff base (PSB) 4-cis-γ-methylnona-2,4,6,8-tetraeniminium cation: a five conjugated double bond model of the retinal chromophore of rhodopsin (the human retina visual pigment).We show that, after initial skeletal relaxation from the Franck−Condon region (which involves a large increase in the central CC bond length), the system is “trapped” in an energy plateau on the S1 energy surface which may be the origin of the “slow” cis → trans isomerization dynamics observed in retinal PSBs in solution. The energy plateau is absent in shorter retinal chromophore models which have a steeper S1 isomerization path. The rhodopsin cavity (where the native chromophore is embedded) may have the effect of removing the energy plateau from the S1 potential thus dramatically increasing the photoisomerization rate from picoseconds to femtoseconds.

Photophysical Properties and Photobiological Activity of the Furanochromones Visnagin and Khellin

ABSTRACTThe larger photobiological activity of visnagin (VI) versus khellin (KH) toward several living organisms, including fungi, viruses, yeasts and bacteria, induced a detailed investigation of the photophysical properties of these naturally occurring furanochromones, using laser‐flash‐photolysis, photoacoustic calorimetry and fluorescence (steady‐state and time‐resolved) techniques in solvents with different polarity and content of water, including micelles and vesicles. The results have shown that the magnitude of all the three rate constants out of S1 (radiative, kf; internal conversion, kic and intersystem crossing, kisc) for VI and KH strongly depend on the solvent, namely on its hydrogen bonding ability and polarity. The changes of kf and kisc are due to the solvent‐assisted mixing and/or inversion of the two first singlet excited states (1n,π* and 1π,π*), while kic increases with a decrease of the So–S1 energy gap. As a consequence, the quantum yield of triplet formation (φT) strongly decreases from values of ˜0.8 in dioxane to < 0.05 in water for both compounds. The magnitude of solvent polarity/hydrogen bonding ability required, at which the state order is inverted and φT starts to decrease, is greater for VI than for KH and consequently φT (VI) > φT (KH) over a broad range of water content including that appropriate to the environment of the compounds in a living system. These facts account for the larger photobiological activity of VI with respect to KH, regarding both the fungus Fusarium culmorum L. and the wild strain of Escherichia coli, studied by us.

Jet spectroscopy, structure, anomalous fluorescence, and molecular quantum beats of silylidene (H2C=Si), the simplest unsaturated silylene

The jet-cooled B̃ 1B2–X̃ 1A1 spectrum of silylidene, the simplest unsaturated silylene, has been observed for the first time. H2C=Si and D2C=Si have been produced by an electric discharge through tetramethylsilane and tetramethylsilane-d12 vapor diluted in argon at the exit of a supersonic expansion. Rotational analysis of the 000 bands yielded the following substitution structures: rs″(CSi)=1.706(5) Å, rs″(CH)=1.099(3) Å, θs″(HCH)=114.4(2)°, rs′(CSi)=1.815(5) Å, rs′(CH)=1.073(4) Å, and θs′(HCH)=133.7(1)°. The electronic transition consists primarily of strong electronically allowed perpendicular bands, but a weaker system of vibronically induced parallel bands has also been assigned. Transitions involving Δv=2 changes in the ν6 (b2) mode show up prominently in the spectrum, due to a very large change in the vibrational frequency on excitation. Silylidene has very interesting excited state decay dynamics. Anomalous S2−S0 fluorescence is observed due to the very large S2−S1 energy gap. Rotational level specific intensity anomalies are found in the laser induced fluorescence spectra. Collision-free fluorescence decay curves exhibit superimposed quantum beats for almost all the accessible rotational levels in the 000 bands of H2CSi and D2CSi. Density of states arguments lead to the conclusion that most of the beat patterns are due to coupling with high vibrational levels of the ground state, although two examples of hyperfine splittings associated with singlet–triplet interactions have also been found.

Electronic Interactions in Photosynthetic Light-Harvesting Complexes: The Role of Carotenoids

The origin and distance dependence of the electronic interactions which promote energy transfer within photosynthetic light-harvesting complexes is investigated. A model based on localized molecular orbitals is related to canonical molecular orbital calculations, therefore demonstrating its practical utility and allowing us to interpret the results of CAS-SCF calculations of the coupling between donor−acceptor pairs. We then focus on the mechanism of energy transfer involving the carotenoid 21Ag (S1) electronic state: [carotenoid (21Ag) (Car) to carotenoid (21Ag)] and [carotenoid (21Ag) to bacteriochlorophyll (Qy) (Bchl)] interactions. The Car−Car coupling is found to involve reasonably long-range interaction terms, with a primary contribution from dispersion-type interactions, which have an R6 distance dependence. The primary contributor to the Car-Bchl S1 → S1 energy transfer mechanism is suggested to be proportional to the product of dipole−dipole and polarization interactions. In neither case does th...

Deuterium Isotope Effect on 4-Aminophthalimide in Neat Water and Reverse Micelles

Picosecond time-resolved emission of 4-aminophthalimide (4-AP) in neat water and D2O and in the water pool of aerosol OT reverse micelles in n-heptane is reported. It is proposed that in protic solvents 4-AP undergoes ultrafast solvent-mediated proton transfer and the emission originates from the proton-transferred species. The abrupt decrease in the emission quantum yield and lifetime of 4-AP in protic solvents is attributed to the reduction in the S1−S0 energy gap in the proton-transferred species. Nearly 4-fold increase in the emission quantum yield (φf) and lifetime (τf) of 4-AP is observed in neat D2O compared to water. This is ascribed to the retardation of the nonradiative S1 → S0 internal conversion processes on deuteration of the imide proton. In neat water or D2O the solvation dynamics is too fast to be detected using our picosecond setup. However, in the water pool of reverse micelles the relaxation of the water (or D2O) slows down appreciably. The initial component of the solvation dynamics in...

Spectroscopic Properties of Spheroidene Analogs Having Different Extents of π-Electron Conjugation

The spectroscopic properties of spheroidene and a series of spheroidene analogs with extents of π-electron conjugation ranging from 7 to 13 carbon−carbon double bonds were studied using steady-state absorption, fluorescence, fluorescence excitation, and time-resolved absorption spectroscopy. The spheroidene analogs studied here were 5‘,6‘-dihydro-7‘,8‘-didehydrospheroidene, 7‘,8‘-didehydrospheroidene, and 1‘,2‘-dihydro-3‘,4‘,7‘,8‘-tetradehydrospheroidene and taken together with data from 3,4,7,8-tetrahydrospheroidene, 3,4,5,6-tetrahydrospheroidene, 3,4-dihydrospheroidene already published (DeCoster, B.; Christensen, R. L.; Gebhard, R.; Lugtenburg, J.; Farhoosh, R.; Frank, H. A. Biochim. Biophys. Acta 1992, 1102, 107) provide a systematic series of molecules for understanding the molecular features that control energy transfer to bacteriochlorophyll in photosynthetic bacterial light-harvesting complexes. All of the molecules were purified by high-pressure liquid chromatographic techniques prior to the spectroscopic experiments. The absorption spectra of the molecules were observed to red-shift with increasing extent of π-electron conjugation. The room temperature fluorescence data show a systematic crossover from dominant S1 → S0 (21Ag → 11Ag) emission to dominant S2 → S0 (11Bu → 11Ag) with increasing extent of conjugation. The S2 fluorescence quantum yields of all the carotenoids in the series were measured here and indicate that 3,4-dihydrospheroidene with nine carbon−carbon double bonds has an S2 quantum yield of (2.7 ± 0.3) × 10-4 which is the highest value in the series. The lifetimes of the S1 states of the molecules were determined from time-resolved transient absorption spectroscopy and found to decrease as the conjugated chain length increases. The transient data are discussed in terms of the energy gap law for radiationless transitions which allows a prediction of the S1 energies of the molecules. The implications of these results for the process of light harvesting by carotenoids in photosynthesis are discussed.

Excited-state properties of asymmetric pentaazadentate expanded porphyrins

The research results of the excited-state properties of asymmetric pentaazadentate ex-panded-porphyrins are reported. The results show that the emission spectra appear in the 770-820 nm region in accord with the different substituents on the benzene ring. The quantum yield of fluorescence is less than 0.02, and the lifetime of the first singlet excited state (S1) is shorter than 1ns. There are excellent linear relationships between the energy of the S1 and the Hammett constants as well as the quantum yield of fluorescence and the electrophilic substituent constants σ+. The energies of the lowest excited triplet state (T1) of the complexes are in the range of 94 -130kJ/mol. The lifetime of T 1 is as long as tens of microseconds. The T1-Tn transient absorption spectra appear in 450-550 nm region. And the quantum yield of title compounds for generating singlet oxygen is as high as 0.9.

HIGH TEMPERATURE EXPANSION OF STRING FREE ENERGY IN HYPERBOLIC SPACE

The high temperature behavior of the open bosonic string free energy in the space S1 ⊗ HN with vanishingly small curvature is investigated. The leading term of the high temperature expansion of the one-loop free energy, near the Hagedorn instability, is obtained. The problem of ir regularization of thermodynamical quantities is pointed out. For minimally coupling quantum fields related to the normal modes of strings, the results are similar to the ones valid for Rindler space. In the lower mass string states regime a connection with the quantum corrections to the black hole entropy is outlined.

Measurements of the subpicosecond relaxation rates of the first excited singlet states of some pseudoazulenes in solution

The subpicosecond population decay times of the S1 states of a number of pseudoazulenes have been measured in solvents of different polarity, using a two-photon, two-color, pump-probe method. The nonradiative decay rates of the S1 states vary exponentially with the S1–S0 energy gap, and the results are interpreted in terms of the energy gap law formalism of radiationless transition theory. The behavior of the excited states of the pseudoazulenes is compared with that of azulene and its derivatives.

The application of the energy gap law to the S1 energies and dynamics of carotenoids

The energy gap law for radiationless transitions set forth by Englman and Jortner (Mol. Phys. 18 (1970) 145) has been evaluated for use in deducing the S1 energies of carotenoids. A simultaneous knowledge of the dynamics and energies of the S1 states of carotenoids are available for only a few of these molecules owing to the lack of detectable S1 state fluorescence in many cases. All the available data where the S1 dynamics and energies of carotenoids are simultaneously known were fit by the energy gap law expression in its full exponential form. The parameters derived from the computer optimization suggest that the weak coupling limit form of the energy gap law is valid for describing the relationship between the dynamics and energetics of carotenoid molecules. The optimal fitting parameters were then used in conjunction with the energy gap law expression to calculate the S1 energies of several biologically important carotenoids from the lifetimes of their S1 states. Particularly important is the S1 energy of β-carotene determined in this analysis to be 14 100 cm−1.

Enzymes and proteins from organisms that grow near and above 100 degrees C.

Microorganisms that can grow at and above 100 degrees C were discovered a decade ago, and about 20 different genera are now known. These so-called hyperthermophiles are the most ancient of all extant life; all but two genera are classified as Archaea. All have been isolated from geothermal heated environments including deep-sea hydrothermal vents. This group includes some methanogenic and sulfate-reducing species, but the majority are strictly anaerobic heterotrophs that utilize complex peptide mixtures as sources of energy, carbon, and nitrogen. Only a few species are saccharolytic. Most of the hyperthermophiles absolutely depend on the reduction of elemental sulfur (S0) to H2S for significant growth, a property that severely limits their large-scale culture in conventional fermentation systems. Consequently, most physiological and metabolic studies have focused on those that can also grow in the absence of S0, including species of the Archaea, Pyrococcus and Thermococcus, and the bacterium Thermotoga. The fermentative pathways for the metabolism of both peptides and carbohydrates in the Archaea appear to depend upon enzymes that contain tungsten, an element seldom used in biological systems. The mechanisms of S0 reduction and energy conservation remain unclear. Enzymes purified from the S0-reducing hyperthermophiles include proteases, amylolytic-type enzymes, hydrogenases, redox proteins, various ferredoxin-linked oxidoreductases, dehydrogenases, and DNA polymerases, some of which are active up to 140 degrees C. However, complete amino acid sequences are known for only a handful of these proteins, and the three-dimensional structure of only one hyperthermophilic protein has been determined. Potential mechanisms by which proteins and various biological cofactors and organic intermediates are stabilized at extreme temperatures are only now beginning to emerge.

Subpicosecond pump–probe measurements of the electronic relaxation rates of the S1 states of azulene and related compounds in polar and nonpolar solvents

The lifetimes of the S1 states of azulene, azulene-d8, 4,6,8-trimethylazulene, and guaiazulene (1,4-dimethyl-7-isopropylazulene) have been measured in three nonviscous solvents of different polarity and structure using a two-photon, two-color, pump–probe method with subpicosecond time resolution. A significant solvent effect is measured. The rate constants for S1■S0 internal conversion in all four compounds in all three solvents exhibit one common S1–S0 energy gap law correlation, indicating that variations in the electronic relaxation rates are governed exclusively by changes in the Franck–Condon factors for the transition. No effect is observed when the exciting wavelength is changed, indicating that vibrational relaxation is occurring on a time scale which is faster than that of electronic relaxation in these systems. No significant deuterium isotope effect is measured in azulene, indicating that high frequency C–H(D) stretching vibrations do not act as significant accepting modes in the radiationless transition.

P-fluorotoluene. I. Methyl (CH3 and CD3) internal rotation in the S1 and S states

Supersonic jet S1-S0 spectroscopy (resonance-enhanced multiphoton ionization, fluorescence excitation, and dispersed single vibronic level fluorescence) has been used to determine the S1 and S0 internal rotation energy level structure of p-fluorotoluene with a CD3 methyl rotor as well as to extend observations of the CH3 rotor structure. The observed rotor energy levels 2≤m≤8 for both species in both states are fit by a simple sixfold hindered rotor Hamiltonian for which the rotor inertial constants B and the internal rotation potential energy barriers V6 are evaluated. V6 may be obtained independently from B by observations of ΔE3, the observed splitting of the 3a″1 and 3a″2 rotor levels. Numerical solution of the wave equation shows that the perturbation theory relationship V6=−2ΔE3 holds well for any reasonable B value. Correspondingly, the B constant may be obtained from other level energies without appreciable sensitivity to (reasonably) assumed barrier heights. Earlier microwave and S1-S0 fluorescence results are combined with the present work to produce a set of preferred values for these constants. The values in cm−1 for the S0 state are B=5.46 (2.82) and V6=−4.77 (−4.77) for CH3 (CD3) rotors. The S1 values are B=4.90 (2.54) and V6=−33.0 (−25.2). The 20% barrier height reduction occurring on transformation from a CH3 to a CD3 rotor is similar to that observed in other systems. Calculation implies that the staggered conformer is the minimum energy configuration for both electronic states. Many of the S1-S0 rotor transitions are forbidden, and a discussion is given of induced intensity mechanisms that involve coupling of internal rotation to overall rotation or coupling of internal rotation to electronic motion. Substantial energy-level perturbations often occur for states with m≥5. A survey of B values and hindered rotation constants for 30 species with methyl rotors attached to aromatic rings reveals some general correlations.

Cyclodextrins as modifiers of the luminescence characteristics of aflatoxins

The fluorescence properties in solution of the four main aflatoxins, B 1, B 2, G 1 and G 2, in the presence of various cyclodextrin derivatives were studied. A preliminary study was made in order to determine the absorbance and fluorescence spectra, relative quantum fluorescence yields (φ F), Stokes shifts, E S1 (0-0) energy levels and low-temperature (77 K) data for these aflatoxins. The effect of cyclodextrins on the fluorescence emission was then investigated. A substantial enhancement of the fluorescence emission of aflatoxins with an unsaturated furan ring (B 1 and G 1) in the presence of aqueous solutions of α,β-heptakis-di- O-methyl-β-cyclodextrin and hydroxypropyl-β-cyclodextrin was observed. Surprisingly, γ-cyclodextrin was inefficient in this respect. Conversely, neither aflatoxin B 2 or G 2, with a saturated furan ring, showed such an interaction and their emission characteristics remained constant in the presence of cyclodextrins. From a mechanistic point of view, the selectivity of the interaction seems to involve partially a non-inclusion process, because of the high molar ratio of cyclodextrin to aflatoxin needed for the fluorescence enhancement. The correlation between the behaviour in the presence of cyclodextrins and the solvent effect on the furan-unsaturated aflatoxins is discussed.