Chimeric behavior of excited thioxanthone in protic solvents: I. Experiments.

The photophysics of thioxanthone (TX) dissolved in methanol (MeOH) and 2,2,2,-trifluoroethanol (TFE) was studied by time-resolved fluorescence and absorption spectroscopy. The spectrally integrated stimulated emission is seen to lose amplitude within ∼5-10 ps. This is much shorter than the fluorescence lifetimes of the compound (2.7 ns for MeOH and 7.6 ns for TFE). The initial reduction in amplitude is attributed to reversible intersystem crossing between the primarily excited (1)ππ* and a triplet (3)nπ* state. The latter one is energetically slightly (∼0.02 eV) above the former one. Addition of the quencher 1-methylnaphthalene (1-MN) reduces the fluorescence lifetime and yields triplet excited 1-MN, giving further evidence for the equilibrium of singlet and triplet excitations. The depopulation of these two states on the nanosecond time scale results in the rise of the lowest triplet state, a (3)ππ* state. Temperature dependencies attribute this to an activated internal conversion process between the two triplet states. Kinetic and energetic parameters derived from the experimental data will be compared with quantum chemical results in the accompanying paper [Rai-Constapel , V. , Villnow , T. , Ryseck , G. , Gilch , P. , and Marian , C. M. J. Phys. Chem. A 2014 , DOI: 10.1021/jp5099415].

Strong impact of the solvent on the photokinetics of a 2(1H)-pyrimidinone.

Pyrimidinones are part of the (6-4) photolesions which may be formed from two pyrimidine bases adjacent on a DNA strand. In relation to the secondary photochemistry of the (6-4) lesion, i.e. its transformation into a Dewar valence isomer, photophysical and photochemical properties of 1-methyl-2(1H)-pyrimidinone (1MP) in water, acetonitrile, methanol, and 1,4-dioxane are reported here. As deduced from steady state fluorescence and femtosecond transient absorption spectroscopy the S1 lifetime of 1MP is strongly affected by the solvent. The lifetimes range from 400 ps for water to 40 ps for 1,4-dioxane. Internal conversion (IC) and intersystem crossing (ISC) contribute to the S1 decay. The solvent effect on the IC rate constant is more pronounced than on the ISC constant. The quantum yields for the consumption of 1MP (values for nitrogen purged solvents) are large for methanol (0.35) and 1,4-dioxane (0.24) and small for acetonitrile (0.02) and water (0.003). Hydrogen abstraction from the solvent by the triplet state of 1MP may rationalize this.

Kasha or state selective behavior in the photochemistry of ortho-nitrobenzaldehyde?

The photochemistry of ortho-nitrobenzaldehyde dissolved in tetrahydrofuran was studied by means of femtosecond UV/Vis and IR spectroscopy. Comparison was made of the spectral and temporal signatures for ~400 nm and ~260 nm excitation. The 400 nm excitation promotes NBA to its lowest excited singlet state of nπ* character whereas for 260 nm an upper excited state of ππ* character is addressed. On the picosecond time scale, the molecule undergoes hydrogen transfer, yielding a ketene intermediate, internal conversion recovering the starting material, and intersystem crossing. Time constants and yields of these processes are virtually not affected by the excitation wavelength. For 400 nm excitation a ~100 fs decay component seen in the 260 nm experiment is absent, indicating that this component is due to a ππ* → nπ* internal conversion. In contrast to its formation, the decay of the ketene intermediate is influenced by the excitation wavelength. This can be attributed to different amounts of vibrational excitation.

Femtosecond spectroscopy on the photochemistry of ortho-nitrotoluene.

The photo tautomerisations of ortho-nitrotoluene (oNT) and its methylated derivative ortho-ethylnitrobenzene (oENB) have been studied by means of femtosecond spectroscopy and (TD)-DFT computations. In UV/Vis transient absorption spectroscopy a band peaking at 400 nm is seen to rise in a bi-modal manner with time constants of 1-10 ps and 1500 ps. Femtosecond stimulated Raman experiments clearly identify aci-nitro forms as the spectroscopic carriers of the 400 nm band. The assignment of the Raman spectra is based on TD-DFT computations. The quantum yields of the aci-nitro forms after 3 ns are 0.08 (oNT) and 0.3 (oENB). The aci-nitro forms are formed via a singlet channel (1-10 ps) and a triplet channel (1500 ps). There are indications that the triplet channel involves a bi-radical intermediate. In between 3 ns and 1 ms the spectrum of the aci-nitro form shifts from 400 to 390 nm. This could indicate a tautomerisation from Z-aci-nitro to an E form.

A 75 MHz light source for femtosecond stimulated raman microscopy.

In femtosecond stimulated Raman microscopy (FSRM) a spectrally broad pulse (Raman probe) and a spectrally narrow pulse (Raman pump) interact in a sample and thereby generate a Raman spectrum of the focal volume. Here a novel light source for FSRM is presented. It consists of an 8-fs laser (repetition rate of 75 MHz) operating as Raman probe. A Yb(3+) based fiber amplifier generates the Raman pump light at 980 nm. The amplifier is seeded by the spectral wing of the 8-fs laser output which ensures synchronisation of pump and probe pulses. Spectral and temporal characteristics of these pulses are reported and simultaneous recording of broadband Raman spectra relying on these pulses is demonstrated.

The ketene intermediate in the photochemistry of ortho-nitrobenzaldehyde.

The first intermediate of the photochemical transformation of ortho-nitrobenzaldehyde to ortho-nitrosobenzoic acid in acetonitrile solvent has been characterized by femtosecond spectroscopy and time-dependent density functional theory (TDDFT) calculations. Femtosecond stimulated Raman spectroscopy (FSRS) indicates that this intermediate adopts a ketene structure. This assignment is supported by the TDDFT results. A kinetic analysis of FSRS and transient absorption data points to two channels for the formation of the ketene. For the predominating first channel the formation takes 0.4 ps. For the second channel it is much slower and takes 220 ps. We assign the first channel to a reaction via an excited singlet state. The second one might involve a triplet state.

Ultrafast changes of molecular crystal structure induced by dipole solvation.

Femtosecond photoexcitation of organic chromophores in a molecular crystal induces strong changes of the electronic dipole moment via intramolecular charge transfer as is evident from transient vibrational spectra. The structural response of the crystal to the dipole change is mapped directly for the first time by ultrafast x-ray diffraction or diffuse scattering. Changes of diffracted and transmitted x-ray intensity demonstrate an angular rearrangement of molecules around excited dipoles following the 10 ps kinetics of charge transfer and leaving lattice plane spacings unchanged. Transient x-ray scattering is governed by solvation, masking changes of the chromophore molecular structure.

On the unusual fluorescence properties of xanthone in water.

Photo-excited xanthone is known to undergo ultrafast intersystem crossing (ISC) in the 1 ps time domain. Correspondingly, its fluorescence quantum yield in most solvents is very small ( approximately 10(-4)). Surprisingly, the quantum yield in water is 100 times larger, while ISC is still rapid ( approximately 1 ps), as seen by ultrafast pump probe absorption spectroscopy. Temperature dependent steady state and time resolved fluorescence experiments point to a delayed fluorescence mechanism, where the triplet (3)npi* state primarily accessed by ISC is nearly isoenergetic with the photo-excited (1)pipi* state. The delayed fluorescence of xanthone in water decays with a time constant of 700 ps, apparently by internal conversion between the (3)npi* state and the lowest lying triplet state (3)pipi*.

Magnetic field and temperature dependencies shed light on the recombination kinetics of a transition metal donor/acceptor system.

The radical pair recombination of an intramolecular electron-transfer system containing a transition metal moiety has been addressed by femtosecond spectroscopy. The radical pair is formed by ultrafast electron transfer (90 fs) from a ferrocene residue to a photoexcited Nile blue moiety. Its recombination proceeds on the picosecond time scale in a multiexponential fashion. The kinetic pattern is a manifestation of spin processes competing with electron transfer. Magnetic field effects on these kinetics allow one to disentangle the two contributions. Their temperature dependencies yield the activation parameters of the two processes. The discussion focuses on the mechanism of electron spin relaxation. Strong evidence for the Orbach/Kivelson mechanism will be given.

Photoswitchable elements within a peptide backbone-ultrafast spectroscopy of thioxylated amides.

A series of thioxo compounds, thioacetamide, N-methylthioacetamide, a cyclic thioxoamide [(S)-5-thioxopyrrolidine-2-carboxylic acid ethyl ester], two thioxylated dipeptides (Ala-Psi[CS-NH]-Ala and Phe-Psi[CS-NH]-Ala) and a thioxylated dodecapeptide (Lys-Glu-Thr-Ala-Ala-Ala-Lys-Phe-Glu-Arg-Gln-His-Psi[CS-NH]-Nle-Asp-Ser-Ser-Thr-Ser-Ala-Ala, or [thioxo-His(12)]-S-peptide; Nle = norleucine) are investigated by ultrafast spectroscopy in the visible and near UV. The different molecules show very similar absorption dynamics featuring a rise of a strong visible absorption band on the subpicosecond and picosecond time scale. The decay of the visible absorption occurs within 150-600 ps. The observations are interpreted by the ultrafast formation of triplet states and their decay on the subnanosecond time scale. Comparison with published IR experiments on N-methylthioacetamide indicates that the cis-trans isomerization around the thioxopeptide bond is terminated within less than 1 ns.

Excited-state dynamics of bacteriorhodopsin probed by broadband femtosecond fluorescence spectroscopy.

The impact of varying excitation densities (approximately 0.3 to approximately 40 photons per molecule) on the ultrafast fluorescence dynamics of bacteriorhodopsin has been studied in a wide spectral range (630-900 nm). For low excitation densities, the fluorescence dynamics can be approximated biexponentially with time constants of <0.15 and approximately 0.45 ps. The spectrum associated with the fastest time constant peaks at 650 nm, while the 0.45 ps component is most prominent at 750 nm. Superimposed on these kinetics is a shift of the fluorescence maximum with time (dynamic Stokes shift). Higher excitation densities alter the time constants and their amplitudes. These changes are assigned to multi-photon absorptions.

Magnetic field effect on picosecond electron transfer

The recombination dynamics of a transition metal redox system monitored by femtosecond pump-probe spectroscopy are shown to be sensitive to high magnetic fields at times shorter than 10 picoseconds. The effect, based on coherent population beats of different spin states, is quantitatively accounted for and allows direct access to spin relaxation rates far beyond the time resolution of the fastest electron paramagnetic resonance technique. The presence of this ultrafast magnetic field effect helps in understanding complex reaction schemes in transition metal chemistry, which occur in a wide range of fields, such as bioinorganic chemistry and catalysis.