Picosecond Time-resolved Resonance Raman Spectra Research Articles

Time-resolved resonance Raman scattering of the excited singlet state of 2-(2′-hydroxyphenyl)benzoxazole after excited state intramolecular proton transfer

Picosecond time-resolved resonance Raman spectra after excited state intramolecular proton transfer in 2-(2′-hydroxyphenyl)benzoxazole have been investigated. The two most intense resonance Raman lines belonging to the S 1-keto tautomer are assigned to vibrations with strong ν(CO) and δ(NH) components, respectively. The intensities of corresponding anti-Stokes resonance Raman spectra indicate that the high-frequency modes are not directly vibrationally excited by the reaction. For modes below 1000 cm −1 substantial excess populations were observed. The S 1-keto tautomer is already close to thermal equilibrium after 2 ps and it cools down within about 8 ps.

Time-resolved infrared and resonance Raman studies of benzil. Vibrational analysis and structures of the excited states

Structures of the S1 and T1 states of benzil are examined based on the experimental results from nanosecond time-resolved infrared spectroscopy and picosecond time-resolved Raman spectroscopy. Nanosecond time-resolved infrared spectra of the T1 state of benzil as well as its three isotopically substituted analogues were measured in carbon tetrachloride. The observed infrared bands of T1 benzil were assigned based on the frequency shifts on isotopic (18O, and deuteration) substitutions. The infrared band at 1312 cm−1 is assigned to the CO anti-symmetric stretch vibration. An infrared band that has large contribution from the central C–C stretch is not observed. Picosecond time-resolved resonance Raman spectra of the S1 state of benzil were also measured. It has been reported that after the photoexcitation, the benzil molecule shows an ultrafast conformational change in the S1 state. The observed resonance Raman bands are attributable to the vibrations of the relaxed form of the S1 state. By comparing the Raman and infrared spectra of the S0, S1, and T1 states of benzil, the structures of benzil in the excited states are discussed. Upon going from the S0 state to the S1 or T1 state, the bond order of the CO bond decreases while that of the central C–C bond increases. Although several ground-state bands appear in both the infrared and Raman spectra, there is no band observed simultaneously in the infrared and Raman spectra of the T1 state, except for bands attributable to the phenyl ring vibrations. We conclude that T1 benzil has the inversion center that arises from the trans-planar structure. The spectral pattern of the resonance Raman scattering of the relaxed S1 state is very similar to that of the T1 state. This implies that the molecular structure of the relaxed S1 state is similar to that of the T1 state. The structure of the relaxed form of the S1 state is also considered to be trans-planar.

Effect of Pump and Probe Light Field on Picosecond Time-Resolved Resonance Raman Spectra of S1 trans-Stilbene. Disagreement between Stokes- and Anti-Stokes Scattering Frequencies

Abstract Picosecond time-resolved spontaneous resonance Raman spectra of the first excited singlet (S1) state of trans-stilbene were measured with various power densities of pump or probe light, for both the Stokes and anti-Stokes scattering regions. Careful calibration of the peak positions of the 285 cm-1 Raman bands reveals that the Stokes scattering band and the anti-Stokes scattering band shift to the same direction in absolute wavenumber, or to the opposite directions in Raman shift, when the pump or probe power density is increased. The direction of the wavenumber shift caused by the pump light field is opposite to that caused by the probe. Because of these apparent peak shifts, the Stokes Raman band and the anti-Stokes Raman band are recorded at different Raman shifts. The intrinsic change of the vibrational level spacing and the apparent peak shift caused by the pump or probe light field can be clearly separated by calculating the “symmetric” and “anti-symmetric” components from the Stokes and anti-Stokes peak positions. The experimental results are not explained by the optical Stark effect in its simplest form. Possible mechanisms to account for the results are discussed.

Picosecond time-resolved resonance Raman observation of Iso-CH 2Br–I following A-band photodissociation of CH 2BrI in the solution phase

We present picosecond time-resolved resonance Raman spectra that demonstrate formation of the iso-CH 2Br–I species following A-band excitation of bromoiodomethane in cyclohexane solvent at room temperature. The iso-CH 2Br–I species is produced within several ps and appears to decay with a half-life on the 2–3 ns time scale. We compare these results to previous studies on solution phase photodissociation of dihalomethanes.

A Determination of the Structure of the Intramolecular Charge Transfer State of 4-Dimethylaminobenzonitrile (DMABN) by Time-Resolved Resonance Raman Spectroscopy

Picosecond time-resolved resonance Raman spectra of the A (intramolecular charge transfer, ICT) state of DMABN, DMABN-d6 and DMABN-15N have been obtained. The isotopic shifts identify the νs(ph-N) mode as a band at 1281 cm-1. The ∼96 cm-1 downshift of this mode from its ground state frequency rules out the electronic coupling PICT model and unequivocally supports the electronic decoupling TICT model. However, our results suggest some pyramidal character of the A state amino conformation.

Kerr-gated picosecond time-resolved resonance Raman spectroscopic probing of the excited states in ?-[Ru(bipy)2dppz](BF4)2 (bipy = 2,2?-bipyridyl, dppz = dipyrido[3,2-a :2?,3?-c ]phenazine)

Kerr-gated picosecond time-resolved resonance Raman spectra of an enantiomerically pure ruthenium(II) diimine intercalator complex in water and acetonitrile are reported, affording the first direct temporally resolved evidence that its excited state is structurally different in the two solvents. Copyright © 2000 John Wiley & Sons, Ltd.

Photoinduced Solvent Ligation to Nickel(II) Octaethylporphyrin Probed by Picosecond Time-Resolved Resonance Raman Spectroscopy

Pump/probe picosecond time-resolved resonance Raman spectra of nickel(II) octaethylporphyrin (NiOEP) in pyridine were observed in the time range −5 to +1000 ps. The spectra demonstrate generation of the vibrationally and electronically excited (d,d) state (B1g) immediately after the excitation to the ππ* state of the macrocycle, subsequent vibrational relaxation, and the formation of six-coordinate exciplex, B1g(L)2 (L = pyridine). Singular value decomposition analysis was applied to a series of picosecond time-resolved Raman spectra to investigate whether a five-coordinate complex was generated prior to the formation of the six-coordinate species. The results indicate insignificant population of the five-coordinate species preceding the formation of B1g(L)2 and suggest concerted coordination of two solvent molecules to axial positions of the (d,d) excited NiOEP. The formation process of the B1g(L)2 species is discussed.

The picosecond transient Raman spectra of S1 excited oligophenyls, oxazols and stilbene derivatives

Picosecond time-resolved resonance Raman spectra of electronically excited laser-dye molecules, oligophenyls, oxazols, and stilbene derivatives in solution have been obtained. The spectral profiles of several transients are observed to change on the picosecond timescale.

Vibrational mode-selective effects in the picosecond time-resolved resonance Raman spectrum of singlet excited trans-stilbene

Picosecond time-resolved resonance Raman spectra of singlet excited trans-stilbene have been obtained using several photolysis (pump) and Raman excitation (probe) wavelengths. Vibrational mode-selective changes in peak wavenumbers and bandwidths are dependent on the sample temperature, the pump wavelength, and the solvent. The effects are greatest for the band at ≈ 1570 cm −1, attributed to the olefinic bond stretching mode. The peak wavenumber of this band decreases and the band broadens as the temperature is raised. Excess energy of excitation results in an initial decrease of peak wavenumber and increase in width followed by an increase in peak wavenumber and decrease in width as the delay between pump and probe increases further, with a similar time dependence for peak position as for width. Fits to the sum of two exponentials give time constants in the range 1 to 5 ps for the initial and 9 to 12 ps for the later stages of the evolution. There is an similarity between the temperature and excess energy dependence, but bandwidth is more (× 2.9) sensitive than peak wavenumber to excess energy. The mode-selective changes and dynamics are interpreted with a model involving intramolecular vibrational relaxation and reorientation of the solvent cage in response to microdielectric stabilisation forces and quasi-equilibration with bulk solvent. The role of vibrationally “'hot” low-wavenumber modes is clarified.

Solvent effects on the ag carbon-carbon double bond stretching mode in the 21Ag- excited state of .beta.-carotene and two derivatives: picosecond time-resolved resonance Raman spectroscopy

Picosecond time-resolved resonance Raman spectra in the C{double bond}C stretching region are presented for {beta}-carotene and two of its derivatives, {beta}-apo-8{prime}-carotenal and ethyl {beta}-apo-8{prime}-carotenoate. The solvent effects on the Franck-Condon-active a{sub g} C{double bond}C stretching mode in the {sup 1}A{sub g}{sup {minus}} ground state (S{sub 0}) and the 2{sup 1}A{sub g}{sup {minus}} excited state (S{sub 1}) of each carotenoid are described. The C{double bond}C stretching frequencies in S{sub 1} are affected by the solvent and show a correlation with the absorption maxima of the S{sub 2} ({sup 1}B{sub u}{sup +}) {l arrow} S{sub 0} transition, while those in S{sub 0} are not significantly affected. These results are interpreted in terms of the vibronic coupling among the S{sub 0}, S{sub 1}, and S{sub 2} electronic states, the solvent effect on the energy of the S{sub 1} and S{sub 2} states, and the structures of carotenoid molecules.

Frequencies of the Franck-Condon active a g CC stretching mode in the 2 1A −g excited state of carotenoids

Picosecond time-resolved resonance Raman spectra (CC stretching region) of the 2 1A − g excited state (S 1) have been measured for β-carotene, three derivatives of β-carotene (β-apo-8′-carotenal, ethyl β-apo-8′-carotenoate and canthaxanthin), and five carotenoids extracted from photosynthetic bacteria (neurosporene, spheroidene, spheroidenone, rhodopin and spirilloxanthin). In all of these carotenoids, the frequency of the Franck-Condon active a g CC stretching mode in S 1 is much larger than that found in the 1A − g ground state (S 0). The frequencies in both S 0 and S 1 correlate well with the absorption maxima for all carotenoids examined except β-apo-8′-carotenal and ethyl β-apo-8′-carotenoate, in which cases the S 1 frequencies are smaller than expected. These results are discussed with respect to the previous studies of the shorter polyenes and a vibronic coupling mechanism.