Ps In Methanol Research Articles

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.

Dynamics of Excited States of the Carotenoid Peridinin in Polar Solvents: Dependence on Excitation Wavelength, Viscosity, and Temperature

The dynamics of the excited states of the carotenoid peridinin in polar solvents were studied using femtosecond transient absorption spectroscopy in the spectral range of 500−1900 nm. A broadening of the absorption spectrum in polar solvents is caused by a distribution of conformers having different ground-state properties. In addition, the dependence of the peridinin lifetime on the excitation wavelength reveals that two peridinin forms coexist in protic solvents, where a “red”-absorbing form is produced by hydrogen bonding via the carbonyl group. The observed dynamics show that the S1 and intramolecular charge transfer (ICT) states of peridinin are strongly coupled, forming a collective S1/ICT state whose lifetime is determined by the degree of ICT character. In nonpolar solvent, pure S1 character with a lifetime of ∼160 ps is observed, whereas in polar solvents an increase in the ICT character leads to a lifetime as short as 10 ps in methanol and 13 ps in ethylene glycol. In protic solvents, the ICT ch...

Solvation Dynamics of the Excited 1,2-(p-Cyano-p‘-Methoxydiphenyl)-Ethyne†

Solvation dynamics of 1,2-(p-cyano-p‘-methoxydiphenyl)-ethyne (CMPE) in various solvents were investigated by observing its time-resolved fluorescence and transient absorption in the picosecond regime. Anisotropy decays of CMPE in most of the solvents examined were apparently single-exponential, except for n-butanol, in which the decay was nonexponential. Dipole moment of CMPE in the excited state (μe), limiting anisotropy (A0) and rotational diffusion coefficient (D0) were obtained from the anisotropy decays with the theoretical expression based on continuum model combined with a nonlinear least-squares's method. The values of μe were linearly dependent on the solvent polarity except for benzene (9.9 D), and were 3.4 D in diethyl ether, 5.7 D in tetrahydrofuran, 13 D in n-butanol, 17 D in acetonitrile and 42 D in dimethyl sulfoxide. From transient absorption spectral measurements, it was demonstrated that the CT states appeared with time less than 10 ps in acetonitrile, 10 ps in methanol and 20 ps in dio...

Spectroscopic and Dynamic Properties of the Peridinin Lowest Singlet Excited States

Spectroscopic properties as well as excited state dynamics of the carotenoid peridinin in several solvents of different polarities were investigated by time-resolved fluorescence and transient absorption techniques. A strong dependence of the peridinin lowest excited states dynamics on solvent polarity was observed after excitation into the strongly allowed S-2 state. Peridinin relaxes to the ground state within 10 ps in the strongly polar solvent methanol, while in the nonpolar solvent n-hexane a 160 ps lifetime was observed, thus confirming the previous observations revealed by transient absorption spectroscopy in the visible region (Bautista, J. A.; et al. J. Phys. Chem. B 1999, 103, 8751). In addition, the solvent dependence in the near-IR region is demonstrated by a strong negative feature in the transient absorption spectrum of peridinin in methanol, which is not present in n-hexane. This band, characterized by a 1 ps rise time, is ascribed to stimulated emission from an intramolecular charge-transfer (ICT) state. Time-resolved fluorescence data support assignment of this band to the emissive singlet state, whose dynamic characteristics depend strongly on the dielectric strength of the medium. On the basis of all our time-resolved measurements combined with simulations of the observed kinetics, we propose the following model: the initially populated S-2 state decays to the S-1 state within less than 100 fs for both solvents. Then, the population is transferred from the S-1 state to the S-0 and ICT states. The S1 --> ICT transfer is controlled by a solvent polarity dependent barrier. In n-hexane the barrier is high enough to prevent the S-1 --> ICT transfer and only S-1 --> S-0 relaxation characterized by a time constant of 160 ps is observed. An increase of solvent polarity leads to a significant decrease of the barrier, enabling a direct quenching of the S-1 state by means of the S-1 --> ICT transfer, which is characterized by a time constant of 148 ps for tetrahydrofuran, 81 ps for 2-propanol, and 11 ps for the most polar solvent methanol. The ICT state is then rapidly depopulated to the ground state. This relaxation also exhibits solvent dependence, having a time constant of 1 ps in methanol, 2.5 ps in 2-propanol, and 3.5 ps in tetrahydrofuran. (Less)

Single-Pulse Measurements of Fluorescence Lifetimes: The Influence of Solvent on the Isomerization of trans-Stilbene Included in Zeolites

The fluorescence lifetime (τf) of trans-stilbene has been evaluated in zeolites Y and ZSM-5 under various conditions, e.g., at low temperature and in the presence of coadsorbed solvents. To make these measurements, an indirect method was developed to measure short-lived (<100 ps) fluorescence lifetimes with a single laser pulse. In dehydrated NaY, τf,TS was 52 ps compared to 66 ps in hexane and 32 ps in methanol. When the NaY sample was subsequently bathed in either water or cyclohexane, τf,TS increased by a factor of 4 and 5, respectively, reflecting decreased mobility of trans-stilbene because of blocking effects of the solvent. However, with small amounts of coadsorbed water equivalent to 13 H2O/supercage, τf,TS decreased by a factor of 2, reflecting increased mobility of trans-stilbene. Small amounts of coadsorbed cyclohexane only increased τf,TS. Values of τf,TS in the solvent-free zeolites were significantly smaller (higher degree of probe mobility) than those published a decade ago and contradicted...

Singlet and Triplet Energy Transfer in the Peridinin−Chlorophyll a−Protein from Amphidinium carterae

The spectroscopic properties of peridinin in solution, and the efficiency and dynamics of energy transfer from peridinin to chlorophyll a in the peridinin−chlorophyll−protein (PCP) from Amphidinium carterae, were studied by steady-state absorption, fluorescence, fluorescence excitation, and fast transient optical spectroscopy. Steady-state measurements of singlet energy transfer from peridinin to chlorophyll revealed an 88 ± 2% efficiency. Fast-transient absorption experiments showed that the excited S1 state of peridinin decayed in 13.4 ± 0.6 ps in methanol and 3.1 ± 0.4 ps in the PCP complex after direct excitation of the carotenoid. The onset of the bleaching of the chlorophyll absorption band at 672 nm, signifying the arrival of the excitation from the carotenoid, occurred in 3.2 ± 0.3 ps. These data show that the primary route of energy transfer from peridinin to chlorophyll in the PCP complex is through the S1 state of peridinin. Nanosecond time-resolved transient optical spectroscopy revealed that chlorophyll triplet states are efficiently quenched by peridinin whose triplet state subsequently decays with a lifetime of 10 ± 1 μs in the PCP complex. Close association between the peridinins and chlorophylls, which is clearly evident in the 3-D structure of the PCP complex, along with proper alignment of pigments and energy state matching are responsible for the high efficiencies of the photochemical processes.

Inner Sphere Reorganization Dynamics Accompanying Charge Transfer in Cyanoterphenyl

Picosecond transient absorption studies of 4‘-n-pentyl-4-cyanoterphenyl (CTP) suggest that the first excited singlet (S1) state exhibits a solvent-dependent dynamic Stokes shift, attributed to intramolecular charge-transfer in the S1 state (in polar solvents) followed by outer sphere reorganization. The degree of charge-transfer character attained by the CTP S1 state is signaled in picosecond time-resolved resonance Raman (TR3) measurements by a frequency shift of the CN-stretch band. No dynamics are observed for this mode frequency within the time resolution of the experiment (less than 1 ps). However, one of the bands in the 1200−1800 cm-1 region of the spectrum is reported to exhibit a solvent-dependent dynamic shift. Analysis of a spectral shift correlation function for this band reveals time constants of 45 ps in methanol, 36 ps in butanol, 21 ps in pentanol, and 18 ps in octanol. The evidence suggests that energy dissipation from the inner sphere coordinate to the solvent bath is affected by the dynamic Stokes shift (i.e., the outer sphere coordinate).

Ultrafast Investigation of Vibrational Relaxation in the S1Electronic State of HITC

The condensed phase relaxation dynamics of electronically excited 1,1‘,3,3,3‘,3‘-hexamethylindotricarbocyanine iodide HITC (a cyanine dye) has been examined by transient bleach/stimulated emission experiments. These measurements were performed using tunable pump and probe laser pulses with ∼200 fs time resolution. The dynamics observed was assigned to vibrational relaxation in the S1 state of HITC. Solvation effects make a negligible contribution to these experiments because the dipole moment of HITC only changes by a small amount when the S1 ← S0 transition is excited. Experiments performed with variable wavelength pump and probe pulses show that vibrational relaxation is faster at high energies in the S1 state. At low energies in the S1 state the vibrational relaxation times depend on the solvent. The measured relaxation times at low energies are1.7 ps in acetonitrile, 3.2 ps in dimethyl sulfoxide, 2.4 ps in methanol, 3.5 ps in ethanol, 7.1 ps in 1-butanol, and 6.4 ps in ethylene glycol. These results show that the vibrational relaxation rate decreases with solvent viscosity and increases with solvent dipole moment. To explain these observations, we propose that the torsional motion associated with isomerization in the S1 state of HITC is responsible for vibrational deactivation at low energies. Rotation about the central C−C bond in the HITC polyene chain produces an internal charge transfer state, creating a large dipole moment along the long axis of the molecule. Thus, the torsional vibration generates an oscillating dipole which can couple to the dipole moments of the solvent molecules, providing a mechanism for energy exchange between HITC and the solvent. The rate of energy exchange will increase with the solvent dipole moment and decrease with the solvent viscosity because highly viscous solvents hinder the torsional vibration and, so, reduce the magnitude of the induced dipole moment in HITC.

Time-Resolved Measurement of O2(1.SIGMA.+g) in Solution. Phosphorescence from an Upper Excited State

The second excited state 1 Σ g + of O 2 has been detected for the first time in time-resolved measurements at room temperature in solution. The lifetime τ Σ of O 2 ( 1 Σ g + ) was determined via the 1 Σ g + → 1 Σ g - phosphorescence to be 130±10 ns in CCl 3 . In addition rate constants of O 2 ( 1 Σ g + ) quenching by several solvent molecules M have been measured in diluted solution in CCl - , allowing the estimation of values of τ Σ in pure solvents of about 20 ps in methanol, 80 ps in cyclohexane, 120 ps in acetone, 130 ps in acetonitrile and in benzene, and 1.2 ns in chloroform

Cis-stilbene photochemistry: direct observation of product formation and relaxation through two-color UV pump-probe Raman spectroscopy

The formation of hot trans-stilbene by photoisomerization of cis-stilbene, and its subsequent vibrational cooling, have been monitored by picosecond two-color UV pump-probe anti-Stokes Raman spectroscopy. Pairs of pump (295 nm) and probe (278 nm) pulses at a 500 Hz repetition rate interrogate flowing samples of cis-stilbene in cyclohexane or methanol, and the anti-Stokes Raman scattering due mainly to the trans-stilbene product is detected. Hot ground-state trans-stilbene appears promptly within our 10 ps experimental time resolution, and the anti-Stokes intensity decays with a time constant of 39 ± 10 ps in cyclohexane and 17 ± 5 ps in methanol. The decay of the anti-Stokes intensity is accompanied by a shift of the C=C stretching vibrations by about 20 cm−1 to higher frequencies. The question of whether or not the initial vibrational distribution is statistical is addressed, but the present data do not allow firm conclusions to be drawn.

Picosecond ground-state rotational diffusion of merocyanine 540 in polar solvents

The ground-state rotational diffusion of merocyanine 540 in polar solvents was studied by using picosecond polarized transient bleaching. The rotational diffusion time τ rot was obtained from fits of a single exponential to the absorption anisotropy. The value of τ rot was 78±5 ps in acetone, 100±24 ps in methanol, 190±19 ps in dimethylformamide, 270±00 ps in ethanol, 490±50 ps in 1-propanol, 500±10 ps in dimethyl sulfoxide, and 700±100 ps in 1-butanol. The dependence of the observed rotational diffusion time on the solvent's shear viscosity was the same in protic and aprotic solvents. The reorientational dynamics of this dye in these solvents can be explained by hydrodynamics for a prolate ellipsoid using stick boundary conditions

Pinacyanol as a mode-locking saturable absorber for Rhodamine dye lasers

The absorption recovery time of the ground state of pinacyanol (S 1 → S 0) can be varied through the choice of solvent from 10 ps in methanol to 300 ps in glycerol. We show that mode-locking of Rhodamine 6G and Rhodamine B dye lasers can be achieved using methanol, DMSO or glycerol solutions.

Reorientation of hydrogen-bonded solvents around a large dipole

Using picosecond spectroscopy, we find rise times τR ≲ 20 ps for p-dimethylaminobenzonitrile S2 → S0 fluorescence in the weakly hydrogen-bound alcohols 2,2,2-trifluoroethanol and 2-fluoroethanol. Since we previously obtained τR ≈ 40 ps in methanol and ethanol, this provides evidence that hydrogen-bonding-retarded solvent reorientation largely governs the fluorescence in these solvents.