Broadband spectral probing revealing ultrafast photochemical branching after ultraviolet excitation of the aqueous phenolate anion

Electron photodetachment from the aromatic anion phenolate excited into the π-π* singlet excited state (S(1)) in aqueous solution is studied with ultrafast transient absorption spectroscopy with a time resolution of better than 50 fs. Broad-band transient absorption spectra from 300 to 690 nm are recorded. The transient bands are assigned to the solvated electron, the phenoxyl radical, and the phenolate S(1) excited state, and confirmation of these assignments is achieved using both KNO(3) as electron quencher and time-resolved fluorescence to measure singlet excited state dynamics. The phenolate fluorescence lifetime is found to be short (～20 ps) in water, but the fast decay is only in part due to the electron ejection channel from S(1). Using global target analysis, two electron ejection channels are identified, and we propose that both vibrationally hot S(1) state and the relaxed S(1) state are direct precursors for the solvated electron. Therefore, electron ejection is found just to compete with picosecond time scale vibrational relaxation and electronic radiationless decay channels. This contrasts markedly with <100 fs electron detachment processes for inorganic anions.     

Photochemistry of 2-naphthoyl azide. An ultrafast time-resolved UV-vis and IR spectroscopic and computational study

The photochemistry of 2-naphthoyl azide was studied in various solvents by femtosecond time-resolved transient absorption spectroscopy with IR and UV-vis detection. The experimental findings were interpreted with the aid of computational studies. Using polar and nonpolar solvents, the formation and decay of the first singlet excited state (S(1)) was observed by both time-resolved techniques. Three processes are involved in the decay of the S(1) excited state of 2-naphthoyl azide: intersystem crossing, singlet nitrene formation, and isocyanate formation. The lifetime of the S(1) state decreases significantly as the solvent polarity increases. In all solvents studied, isocyanate formation correlates with the decay of the azide S(1) state. Nitrene formation correlates with the decay of the relaxed S(1) state only upon 350 nm excitation (S(0) → S(1) excitation). When S(n) (n ≥ 2) states are populated upon excitation (λ(ex) = 270 nm), most nitrene formation takes place within a few picoseconds through the hot S(1) and higher singlet excited states (S(n)) of 2-naphthoyl azide. The data correlate with the results of electron density difference calculations that predict nitrene formation from the higher-energy singlet excited states, in addition to the S(1) state. For all of these experiments, no recovery of the ground state was observed up to 3 ns after photolysis, which indicates that both internal conversion and fluorescence have very low efficiencies.     

The direct detection of an aryl azide excited state: an ultrafast study of the photochemistry of para- and ortho-biphenyl azide

Ultrafast laser flash photolysis (266 nm) of para- and ortho-biphenyl azide in acetonitrile produces azide excited states that have broad absorption bands centered at 480 nm. The para-biphenyl azide excited singlet state has a lifetime of 100 fs. The excited-state lifetime of the ortho-azide isomer is 450 +/- 150 fs. Decay of the azide excited states is accompanied by the formation of the corresponding known singlet nitrenes (para, lambdamax = 350 nm, ortho, lambdamax = 400 nm). Singlet para-biphenylnitrene is born with excess energy and undergoes vibrational cooling with a time constant of 11 ps to form the long-lived (tau approximately 9 ns) relaxed singlet nitrene. Singlet ortho-biphenylnitrene decays with a lifetime of 16 ps in acetonitrile at ambient temperature.     

Effect of solvent on the excited-state photophysical properties of curcumin

Photophysical properties of curcumin, 1,7-bis-(4-hydroxy-3-methoxy phenyl)-1,6-heptadiene-2,5-dione, a pigment found in the rhizomes of Curcuma longa (turmeric) have been studied in different kinds of organic solvent and also in Triton X-100 aqueous micellar media using time-resolved fluorescence and transient absorption techniques having pico and nanosecond time resolution, in addition to steady-state absorption and fluorescence spectroscopic techniques. Steady-state absorption and fluorescence characteristics of curcumin have been found to be sensitive to the solvent characteristics. Large change (delta mu = 6.1 Debye) in dipole moments due to photoexcitation to the excited singlet state (S1) indicates strong intramolecular charge transfer character of the latter. Curcumin is a weakly fluorescent molecule and the fluorescence decay properties in most of the solvents could be fitted well to a double-exponential decay function. The shorter component having lifetime in the range 50-350 ps and percent contribution of amplitude more than 90% in different solvents may be assigned to the enol form, whereas the longer component, having lifetime in the range 500-1180 ps with less than 10% contribution may be assigned to the di-keto form of curcumin. Our nuclear magnetic resonance study in CDCl3 and dimethyl sulfoxide-D6 also supports the fact that the enol form is present in the solution by more than about 95% in these solvents. Excited singlet (S1) and triplet (T1) absorption spectrum and decay kinetics have been characterized by pico and nanosecond laser flash photolysis. Quantum yield of the triplet is low (phi T < or = 0.12). Both the fluorescence and triplet quantum yields being low (phi f + phi T < 0.18), the photophysics of curcumin is dominated by the energy relaxation mechanism via the internal conversion process.     

Femtosecond Time-Resolved Stimulated Raman Spectroscopy of the S(2) (1B(u)) Excited State of beta-Carotene

The electronic and vibrational structure of beta-carotene's early excited states are examined using femtosecond time-resolved stimulated Raman spectroscopy. The vibrational spectrum of the short-lived ( approximately 160 fs) second excited singlet state (S(2),1B(u) (+))of beta-carotene is obtained. Broad, resonantly enhanced vibrational features are observed at approximately 1100, 1300, and 1650 cm(-1) that decay with a time constant corresponding to the electronic lifetime of S(2). The temporal evolution of the vibrational spectra are consistent with significant population of only two low-lying excited electronic states (1B(u) (+) and 2A(g) (-)) in the ultrafast relaxation pathway of beta-carotene.     

Triplet photophysics of gold(III) porphyrins

Gold porphyrins are often used as electron-accepting chromophores in donor-acceptor complexes for the study of photoinduced electron transfer, and they can also be involved in triplet-triplet energy-transfer interactions with other chromophores. Since the lowest excited singlet state is very short-lived (240 fs), the triplet state is usually the starting point for the transfer reactions, and it is therefore crucial to understand its photophysics. The triplet state of various gold porphyrins has been reported to have a lifetime of around 1.5 ns at room temperature and to have a biexponential decay both in emission and in transient absorption with decay times of around 10 and 100 micros at 80 K. In this paper, the triplet photophysics of two gold porphyrins (Au(III) 5,15-bis(3,5-di-tert-butylphenyl)-2,8,12,18-tetraethyl-3,7,13,17-tetramethylporphyrin and Au(III) 5,10,15,20-tetra(3,5-di-tert-butylphenyl)porphyrin) are studied by steady-state and time-resolved absorption and emission spectroscopy over a wide temperature range (4-300 K). The study reveals the existence of a dark state with an approximate lifetime of 50 ns, which was not previously observed. This state acts as an intermediate between the short-lived singlet and the triplet state manifold. In addition, we present DFT calculations, in which the core electrons of the central metal were replaced by a pseudopotential to account for the relativistic effects, which suggest that the lowest excited singlet state is an optically forbidden ligand-to-metal charge-transfer (LMCT) state. This LMCT state is an obvious candidate for the experimentally observed dark state, and it is shown to dictate the photophysical properties of gold porphyrins by acting as a gate for triplet state formation versus direct return to the ground state.     

The gold porphyrin first excited singlet state

Gold porphyrins are often used as electron-accepting chromophores in artificial photosynthetic constructs. Because of the heavy atom effect, the gold porphyrin first-excited singlet state undergoes rapid intersystem crossing to form the triplet state. The lowest triplet state can undergo a reduction by electron donation from a nearby porphyrin or another moiety. In addition, it can be involved in triplet-triplet energy transfer interactions with other chromophores. In contrast, little has been known about the short-lived singlet excited state. In this work, ultrafast time-resolved absorption spectroscopy has been used to investigate the singlet excited state of Au(III) 5,15-bis(3,5-di-t-butylphenyl)-2,8,12,18,-tetraethyl-3,7,13,17-tetramethylporphyrin in ethanol solution. The excited singlet state is found to form with the laser pulse and decay with a time constant of 240 fs to give the triplet state. The triplet returns to the ground state with a life-time of 400 ps. The lifetime of the singlet state is comparable with the time constants for energy and photoinduced electron transfer in some model and natural photosynthetic systems. Thus, it is kinetically competent to take part in such processes in suitably designed supermolecular systems.     

Unravelling the Reaction Mechanism for the Fast Photocyclisation of 2‐Benzoylpyridine in Aqueous Solvent by Time‐Resolved Spectroscopy and Density Functional Theory Calculations

A combined femtosecond transient absorption (fs‐TA) and nanosecond time‐resolved resonance Raman (ns‐TR³) spectroscopic investigation of the photoreaction of 2‐benzoylpyridine (2‐BPy) in acetonitrile and neutral, basic and acidic aqueous solvents is reported. fs‐TA results showed that the nπ* triplet 2‐BPy is the precursor of the photocyclisation reaction in neutral and basic aqueous solvents. The cis triplet biradical and the cis singlet zwitterionic species produced during the photocyclisation reaction were initially characterised by ns‐TR³ spectroscopy. In addition, a new species was uniquely observed in basic aqueous solvent after the decay of the cis singlet zwitterionic species and this new species was tentatively assigned to the photocyclised radical anion. The ground‐state conformation of 2‐BPy in acidic aqueous solvent is the pyridine nitrogen‐protonated 2‐BPy cation (2‐BPy‐NH⁺) rather than the neutral form of 2‐BPy. After laser photolysis, the singlet excited state (S₁) of 2‐BPy‐NH⁺ is generated and evolves through excited‐state proton transfer (ESPT) and efficient intersystem crossing (ISC) processes to the triplet exited state (T₁) of the carbonyl oxygen‐protonated 2‐BPy cation (2‐BPy‐OH⁺) and then photocyclises with the lone pair of the nitrogen atom in the heterocyclic ring. Cyclisation reactions take place both in neutral/basic and acidic aqueous solvents, but the photocyclisation mechanisms in these different aqueous solvents are very different. This is likely due to the different conformation of the precursor and the influence of hydrogen‐bonding of the solvent on the reactions.     

State preparation and excited electronic and vibrational behavior in hemes

The temporally overlapping, ultrafast electronic and vibrational dynamics of a model five-coordinate, high-spin heme in a nominally isotropic solvent environment has been studied for the first time with three complementary ultrafast techniques: transient absorption, time-resolved resonance Raman Stokes, and time-resolved resonance Raman anti-Stokes spectroscopies. Vibrational dynamics associated with an evolving ground-state species dominate the observations. Excitation into the blue side of the Soret band led to very rapid S2 --> S1 decay (sub-100 fs), followed by somewhat slower (800 fs) S1 --> S0 nonradiative decay. The initial vibrationally excited, non-Boltzmann S0 state was modeled as shifted to lower energy by 300 cm(-1) and broadened by 20%. On a approximately 10 ps time scale, the S0 state evolved into its room-temperature, thermal distribution S0 profile largely through VER. Anti-Stokes signals disappear very rapidly, indicating that the vibrational energy redistributes internally in about 1-3 ps from the initial accepting modes associated with S1 --> S0 internal conversion to the rest of the macrocycle. Comparisons of anti-Stokes mode intensities and lifetimes from TRARRS studies in which the initial excited state was prepared by ligand photolysis [Mizutani, T.; Kitagawa, T. Science 1997, 278, 443, and Chem. Rec. 2001, 1, 258] suggest that, while transient absorption studies appear to be relatively insensitive to initial preparation of the electronic excited state, the subsequent vibrational dynamics are not. Direct, time-resolved evaluation of vibrational lifetimes provides insight into fast internal conversion in hemes and the pathways of subsequent vibrational energy flow in the ground state. The overall similarity of the model heme electronic dynamics to those of biological systems may be a sign that the protein's influence upon the dynamics of the heme active site is rather subtle.     

有机化合物的光物理研究 XI. 芘在不同极性溶剂中的瞬态过程

芘是一种很好的电子转移试剂,它既可以作为电子给体,又可以作为电子受体。因此,在电子转移光化学、光物理中有不少研究文章报导。由于其较强的荧光、较高的量子产率及较长的荧光寿命,因而常被用作荧光探针,研究胶体分子及生物大分子的结构和功能等。利用紫外、红外、电子脉冲辐射和激光光解瞬态光导的测量方法研究芘与一些溶剂的相互作用以及在不同极性溶剂中的光离子化现象已有一些研究工作。本文报导芘在不同极性溶剂中的时间分辨瞬态吸收光谱的研究结果。藉对芘在不同极性溶剂中的时间分辨瞬态吸收光谱的研究,了解芘在不同极性溶剂中的瞬态反应过程,考查其与溶剂间在激光作用下的相互作用。     

Ultrafast study of p-biphenylyldiazomethane and p-biphenylylcarbene

p-Biphenylyldiazomethane was excited by femtosecond pulses of UV light in acetonitrile, in cyclohexane, and in methanol. Ultrafast photolysis produces a singlet excited state of p-biphenylyldiazomethane with lambdamax = 490 nm, and lifetimes of less than 300 fs in acetonitrile, in cyclohexane, and in methanol. The decay of the excited state is accompanied by the growth of transient absorption with lambdamax = 360 nm. The carrier of this transient absorption is attributed to singlet p-biphenylylcarbene, a result that is consistent with the predictions of TD-DFT calculations. The singlet carbene lifetimes are 200 and 77 ps in acetonitrile and cyclohexane, respectively, and are controlled by intersystem crossing to the lower energy triplet state. The transient absorption does not decay to baseline in acetonitrile, because of the formation of nitrile ylide. The equilibrium mixture of singlet and triplet p-biphenylylcarbene reacts with acetonitrile to form a nitrile ylide (lambdamax = 370 nm), and with cyclohexane by C-H insertion 1-20 ns after the laser pulse. The singlet carbene lifetime is only 7.9 ps in methanol, owing to a rapid reaction with the solvent. Reaction with the solvent gives rise, in part, to a p-biphenylylbenzyl cation (lambdamax = 450 nm, tau = 6.3 ps) in methanol.     

Time-resolved two-photon spectroscopy of photosystem I determines hidden carotenoid dark-state dynamics

We present time-resolved fs two-photon pump-probe data measured with photosystem I (PS I) of Thermosynechococcus elongatus. Two-photon excitation (lambda(exc)/2 = 575 nm) in the spectral region of the optically forbidden first excited singlet state of the carotenoids, Car S1, gives rise to a 800 fs and a 9 ps decay component of the Car S1 --> S(n) excited-state absorption with an amplitude of about 47 +/- 16% and 53 +/- 10%, respectively. By measuring a solution of pure beta-carotene under exactly the same conditions, only a 9 ps decay component can be observed. Exciting PS I at exactly the same spectral region via one-photon excitation (lambda(exc) = 575 nm) also does not show any sub-ps component. We ascribe the observed constant of 800 fs to a portion of about 47 +/- 16% beta-carotene states that can potentially transfer their energy efficiently to chlorophyll pigments via the optically dark Car S1 state. We compared these data with conventional one-photon pump-probe data, exciting the optically allowed second excited state, Car S2. This comparison demonstrates that the fast dynamics of the optically forbidden state can hardly be unravelled via conventional one-photon excitation only because the corresponding Car S1 populations are too small after Car S2 --> Car S1 internal conversion. A direct comparison of the amplitudes of the Car S1 --> S(n) excited-state absorption of PS I and beta-carotene observed after Car S2 excitation allows determination of a quantum yield for the Car S1 formation in PS I of 44 +/- 5%. In conclusion, an overall Car S2 --> Chl energy-transfer efficiency of approximately 69 +/- 5% is observed at room temperature with 56 +/- 5% being transferred via Car S2 and probably very hot Car S1 states and 13 +/- 5% being transferred via hot and "cold" Car S1 states.     

Photodissociation of thioglycolic acid studied by femtosecond time-resolved transient absorption spectroscopy

Steady-state and time-resolved spectroscopies were employed to study the photodissociation of both the neutral (HS-CH(2)-COOH) and doubly deprotonated ((-)S-CH(2)-COO(-)) forms of thioglycolic acid (TGA), a common surface-passivating ligand used in the aqueous synthesis and organization of semiconducting nanostructures. Room temperature UV-Vis absorption spectroscopy indicated strong absorption by the S(1) and S(2) excited states at 250 nm and 185 nm, respectively. The spectrum also contained a weaker absorption band that extended to approximately 550 nm, which was assigned to the π(CO) (*)←n(O) transition. Femtosecond time-resolved transient absorption spectroscopy was performed on TGA using 400 nm excitation and a white-light continuum probe to provide the temporally and spectrally resolved data. Both forms of TGA underwent a photoinduced dissociation from the excited state to form an α-thiol-substituted acyl radical (α-TAR, S-CH(2)-CO(●)). For the acidic form of TGA, radical formation occurred with an apparent time constant of 60 ± 5 fs; subsequent unimolecular decay took 400 ± 60 fs. Similar kinetics were observed for the deprotonated form of TGA (70 ± 10 fs radical formation; 420 ± 40 fs decay). The production of the α-TAR was corroborated by the observation of its characteristic optical absorption. Time-resolved data indicated that the photoinduced dissociation of TGA via cleavage of the C-OH bond occurred rapidly (≤100 fs). The prevalence of TGA in aqueous semiconducting nanoparticles makes its absorption in the visible spectral region and subsequent dissociation key to understanding the behavior of nanoscale systems.     

Photophysical Properties of Rufloxacin in Neutral Aqueous Solution

The photophysical properties of rufloxacin, 9-fluoro-2_r3-dihydro-10-(4-methyl-l-pyrazinyl)-7-oxo-7-H-pyri-do[l,2,3-de]-l,4-benzothiazin-6-carboxylic acid, a fluoroquinolone antibacterial drug exhibiting photosensitizing action toward biological substrates, were studied in aqueous solutions at neutral pH. The lowest excited electronic states of the zwitterion were characterized by both experimental techniques and theoretical methods. Steady-state and time-resolved emission, triplet-state absorption and singlet oxygen production were investigated. The results indicate that the lowest excited singlet is a fluorescent, relatively long-lived state (φ_r= 0.075, T_r? 4.5 ns) with an efficient intersystem crossing to the triplet manifold (φ_isc? 0-⁷)- The lowest triplet is a long-lived state (T_T? 10 μs at 295 K in 0.01 M phosphate buffer), with properties that make it a good candidate for being the precursor of the photodecarboxylation of the drug. It is quenched by oxygen at a rate of 1.7 times 10⁹M^-1 s-¹ and singlet oxygen is formed with a quantum yield of 0.32 in air-saturated solutions.     

Imidazole-based excited-state intramolecular proton-transfer (ESIPT) materials: observation of thermally activated delayed fluorescence (TDF)

We report the first observation of thermally activated delayed fluorescence (TDF) from an excited-state intramolecular proton-transfer (ESIPT) molecule, a hydroxyl-substituted tetraphenyl imidazole derivative (HPI-Ac), in degassed solutions as well as in low-temperature organic matrixes. In the absence of oxygen, the blue emission of an identical spectral feature was observed in the nanosecond ( approximately 4.4 ns) and microsecond ( approximately 25 micros) time domains, and the fluorescence intensity increased with temperature. From the temperature dependence of the time-resolved spectra of HPI-Ac, the energy gap between the first-excited singlet state and the lowest triplet state was determined to be 7.6 +/- 0.3 kJ/mol (630 +/- 25 cm-1), and the limiting rate constant of intrinsic reverse intersystem crossing was estimated to be 1.3 (+/-0.5) x 107 s-1.     

Electron transfer dynamics from organic adsorbate to a semiconductor surface: zinc phthalocyanine on TiO2(110)

The femtosecond time evolutions of excited states in zinc phthalocyanine (ZnPC) films and at the interface with TiO2(110) have been studied by using time-resolved two-photon photoelectron spectroscopy (TR-2PPE). The excited states are prepared in the first singlet excited state (S1) with excess vibrational energy. Two different films are examined: ultrathin (monolayer) and thick films of approximately 30 A in thickness. The decay behavior depends on the thickness of the film. In the case of the thick film, TR-2PPE spectra are dominated by the signals from ZnPC in the film. The excited states decay with tau = 118 fs mainly by intramolecular vibrational relaxation. After the excited states cascaded down to near the bottom of the S1 manifold, they decay slowly (tau = 56 ps) although the states are located at above the conduction band minimum of the bulk TiO2. The exciton migration in the thick film is the rate-determining step for the electron transfer from the film to the bulk TiO2. In the case of the ultrathin film, the contribution of electron transfer is more evident. The excited states decay faster than those in the thick film, because the electron transfer competes with the intramolecular relaxation processes. The electronic coupling with empty bands in the conduction band of TiO2 plays an important role in the electron transfer. The lower limit of the electron-transfer rate was estimated to be 1/296 fs(-1). After the excited states relax to the states whose energy is below the conduction band minimum of TiO2, they decay much more slowly because the electron-transfer channel is not available for these states.     

Photophysical Properties of Some Methyl-Substituted Angelicins: Fluorometric and Flash Photolytic Studies

This paper describes the results of a study of the photophysical properties of various methyl-angelicins (MA) in solvents of different polarity and proticity. The behavior of their excited singlet and triplet states was investigated by fluorometry and nanosecond laser flash photolysis. On the basis of semiempirical (ZINDO/S-CI) calculations and the solvent effect on the absorption and fluorescence properties, the lowest excited singlet state (S₁) is assigned to a partially allowed π, π* state. The close lying S₂ state is n,π* in nature. The efficiency of the decay pathways of S₁ (fluorescence, intersystem crossing and internal conversion) strongly depends on the energy gap between the S₁ and S₂ states consistent with the manifestation of “proximity effect.” Thus, MA in cyclohexane decay only through S₁→ S₀ internal conversion, while in acetonitrile and ethanol, where the n, π* state is located at higher energy, their fluorescence and intersystem crossing increase significantly. The lowest excited triplet states (T₁) were characterized in terms of their absorption spectra, decay kinetics, molar absorption coefficients and formation quantum yields. The interaction of T₁ MA with molecular oxygen leads to an efficient formation of singlet oxygen, as evidenced by the appearance of characteristic IR phosphorescence centered at 1269 nm.     

Existence of a new emitting singlet state of proflavine: femtosecond dynamics of the excited state processes and quantum chemical studies in different solvents

Proflavine (3,6-diaminoacridine) shows fluorescence emission with lifetime, 4.6 ± 0.2 ns, in all the solvents irrespective of the solvent polarity. To understand this unusual photophysical property, investigations were carried out using steady state and time-resolved fluorescence spectroscopy in the pico- and femtosecond time domain. Molecular geometries in the ground and low-lying excited states of proflavine were examined by complete structural optimization using ab initio quantum chemical computations at HF/6-311++G** and CIS/6-311++G** levels. Time dependent density functional theory (TDDFT) calculations were performed to study the excitation energies in the low-lying excited states. The steady state absorption and emission spectral details of proflavine are found to be influenced by solvents. The femtosecond fluorescence decay of the proflavine in all the solvents follows triexponential function with two ultrafast decay components (τ(1) and τ(2)) in addition to the nanosecond component. The ultrafast decay component, τ(1), is attributed to the solvation dynamics of the particular solvent used. The second ultrafast decay component, τ(2), is found to vary from 50 to 215 ps depending upon the solvent. The amplitudes of the ultrafast decay components vary with the wavelength and show time dependent spectral shift in the emission maximum. The observation is interpreted that the time dependent spectral shift is not only due to solvation dynamics but also due to the existence of more than one emitting state of proflavine in the solvent used. Time resolved area normalized emission spectral (TRANES) analysis shows an isoemissive point, indicating the presence of two emitting states in homogeneous solution. Detailed femtosecond fluorescence decay analysis allows us to isolate the two independent emitting components of the close lying singlet states. The CIS and TDDFT calculations also support the existence of the close lying emitting states. The near constant lifetime observed for proflavine in different solvents is suggested to be due to the similar dipole moments of the ground and the evolved emitting singlet state of the dye from the Franck-Condon excited state.     

2-Naphthyl(carbomethoxy)carbene revisited: combination of ultrafast UV-vis and infrared spectroscopic study

Ultrafast laser flash photolysis (310 nm) of methyl 2-napthyldiazoacetate (2-NpCN2CO2CH3) in acetonitrile or cyclohexane produces a diazo excited state which absorbs broadly in the visible region (tau = 300 fs). The decay of the excited diazo compound is accompanied by growth of the vibrationally excited singlet 2-naphthyl(carbomethoxy)carbene ((1)NpCCO2CH3). The singlet carbene absorbs at 360 and 470 nm. In acetonitrile these bands do not decay over 3 ns, but they do decay by approximately 50% of their original intensity in cyclohexane in 3 ns. It is concluded that (1)NpCCO2CH3 has a singlet ground state in acetonitrile but a triplet ground state in cyclohexane. Related experiments reveal a singlet ground state in Freon-113 and chloroform. This interpretation is supported by ultrafast IR spectroscopy, which confirms that only (1)NpCCO2CH3 is formed within 50 ps of the laser pulse rather than a singlet-triplet equilibrium mixture of carbene. The planar singlet relaxes to the preferred perpendicular singlet over a few tens of picoseconds, as evidenced by a red shift of the carbonyl stretching vibration. Although our data agrees with previous studies, its interpretation is somewhat altered.     

The excited-state chemistry of protochlorophyllide a: a time-resolved fluorescence study.

The excited-state processes of protochlorophyllide a, the precursor of chlorophyll a in chlorophyll biosynthesis, are studied using picosecond time-resolved fluorescence spectroscopy. Following excitation into the Soret band, two distinct fluorescence components, with emission maxima at 640 and 647 nm, are observed. The 640 nm emitting component appears within the time resolution of the experiment and then decays with a time constant of 27 ps. In contrast, the 647 nm emitting component is built up with a 3.5 ps rise time and undergoes a subsequent decay with a time constant of 3.5 ns. The 3.5 ps rise kinetics are attributed to relaxations in the electronically excited state preceding the nanosecond fluorescence, which is ascribed to emission out of the thermally equilibrated S(1) state. The 27 ps fluorescence, which appears within the experimental response of the streak camera, is suggested to originate from a second minimum on the excited-state potential-energy surface. The population of the secondary excited state is suggested to reflect a very fast motion out of the Franck-Condon region along a reaction coordinate different from the one connecting the Franck-Condon region with the S(1) potential-energy minimum. The 27 ps-component is an emissive intermediate on the reactive excited-state pathway, as its decay yields the intermediate photoproduct, which has been identified previously (J. Phys. Chem. B 2006, 110, 4399-4406). No emission of the photoproduct is observed. The results of the time-resolved fluorescence study allow a detailed spectral characterization of the emission of the excited states in protochlorophyllide a, and the refinement of the kinetic model deduced from ultrafast absorption measurements.