Femtosekunden Stimulierte Raman-Spektroskopie zur Photochemie von o-Nitrobenzaldehyd

Aromatic nitro-compounds are known to photochemically abstract hydrogen atoms from adjacent hydrocarbon moieties. For ortho-substituted nitro-aromatics these abstractions proceed intra-molecularly and trigger secondary processes. Due to these processes aromatic nitro-compounds are used as, e.g., photo-labile protecting groups and as the leaving group in caged compounds. Here, a prototypical nitro-compound, ortho-nitrobenzaldehyde (o-NBA), which is photochemically transformed into ortho-nitrosobenzoic acid is studied. This reaction is known for more than 100 years but is still not totally specified. Tracing these photochemical processes requires high temporal resolution down to about 100 fs and a detection technique that is sensitive to structural changes after starting the photoreaction. Femtosecond vibrational spectroscopy fulfils these conditions in observation of ultrafast chemical processes. So transient IR spectroscopy and the recently developed femtosecond stimulated Raman spectroscopy (FSRS) are used to record the ultafast structural changes in this photoreaction. In addition the kinetics of the o-NBA photoreaction are monitored by means of visible femtosecond absorption spectroscopy. The novel implementation of FSRS uses a white light continuum as the Stokes probe pulse in the stimulated Raman process. The reaction is started by a actinic laser pulse thereby the transient Raman spectra can be recorded in pump-probe fashion and are compared with calculated spectra of possible intermediates of the reaction. With the use of these techniques the photoreaction of NBA into intermediates can be followed and a reaction mechanism is described

A comparative analysis of the UV/Vis absorption spectra of nitrobenzaldehydes.

In a joint experimental and theoretical study, the UV/Vis absorption spectra of the three isomers (ortho, meta, para) of nitrobenzaldehyde (NBA) were analyzed. Absorption spectra are reported for NBA vapors, cyclohexane and acetonitrile solutions. All spectra are poor in vibronic structure and hardly affected in shape by the surroundings (vapor or solution). Moderate solvatochromic shifts of [similar]−0.2 eV are measured. For all isomers vertical transition energies, oscillator strengths, and excited state dipole moments were computed using the MS-CASPT2/CASSCF and CC2 methods. Based on these calculations the experimental transitions were assigned. The spectra of all isomers are characterized by weak (εmax ≈ 100 M−1 cm−1) transitions around 350 nm (3.6 eV), arising from nπ* absorptions starting from the lone pairs of the nitro and aldehyde moieties. The next band of intermediate intensity peaking around 300 nm (4.2 eV, εmax ≈ 1000 M−1 cm−1) is dominated by ππ* excitations within the arene function. Finally, strong absorptions (εmax ≈ 10 000 M−1 cm−1) were observed around 250 nm (5.0 eV) which we ascribe to ππ* excitations involving the nitro and benzene groups

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

AbstractThe impact of varying excitation densities (∼0.3 to ∼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 ∼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

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 similar to 400 nm and similar to 260 nm excitation. The 400 nm excitation promotes NBA to its lowest excited singlet state of n pi* character whereas for 260 nm an upper excited state of pi pi* 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 similar to 100 fs decay component seen in the 260 nm experiment is absent, indicating that this component is due to a pi pi* -> n pi* 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

On the Mechanism of Intramolecular Sensitization of Photocleavage of the 2-(2-Nitrophenyl)propoxycarbonyl (NPPOC) Protecting Group

A spectroscopic study of a variety of covalently linked thioxanthone(TX)-linker-2-(2-nitrophenyl)propoxycarbonyl(NPPOC)-substrate conjugates is presented. Herein, the TX chromophore functions as an intramolecular sensitizer to the NPPOC moiety, a photolabile protecting group used in photolithographic DNA chip synthesis. The rate of electronic energy transfer between TX and NPPOC was quantified by means of stationary fluorescence as well as nanosecond and femtosecond time-resolved laser spectroscopy. A dual mechanism of triplet-triplet energy transfer has been observed comprising a slower mechanism involving the T1(*) state of TX with linker-length-dependent time constants longer than 20 ns and a fast mechanism with linker-length-dependent time constants shorter than 3 ns. Evidence is provided that the latter mechanism is due to energy transfer from the T2(n*) state which is in fast equilibrium with the fluorescent S1(*) state. In the case of direct linkage between the aromatic rings of TX and NPPOC, the spectroscopic properties are indicative of one united chromophore which, however, still shows the typical NPPOC cleavage reaction triggered by intramolecular hydrogen atom transfer to the nitro group