Picosecond transient spectroscopy

Light-induced intramolecular proton transfer has long been suspected to play an important role in the mechanism for photochromism in both crystals and solutions. One class of compounds that has been extensively studied is the A -salicylidene anilines. The main evidence for invoking an excited-state proton-transfer step in the photochromic mechanism comes from the observation that the colored species exists as the keto amine tautomer (in the ground state) while the bleached form exists as an enol imine (in the ground state). Replacement of the labile hydrogen with a methyl group prevents the photochemical reaction that leads to the colored species. Picosecond transient spectroscopy was used to study iV-salycylidene-o-toluidine (NST) and the related compound 2-(o-hydroxyphenyl)benzothiazole (HBT). ... 

In the systems for femto-picosecond transient spectroscopy it is used a special detection technique, known as pump and probe . The idea behind this technique is to use the same laser source to generate the excitation pulse (PUMP), and the analysis beam (PROBE). The path of the PROBE beam is varied in length by a delay line, i.e. a mobile platform on which are mounted mirrors that reflect the laser beam with high efficiency. The change in the optical path allows the control of the temporal distance between excitation and analysis (Fig. 8.14). 

Sakamoto A, Okamoto H and Tasumi M 1998 Observation of picosecond transient Raman spectra by asynchronous Fourier transform Raman spectroscopy 1998 Appl. Spectrosc. 52 76-81... 

Using picosecond flash spectroscopy Gupta et al. 2k) reported for 2-hydroxyphenylbenzotriazole in ethanol a short-lived transient (6 ps) followed by a transient absorption whose lifetime is estimated to be 600 ps. The authors assigned the short-lived transient to the "vertical singlet" while the long-lived transient is presumably the "proton transferred species". These measurements of transient absorptions with the picosecond flash method confirm our results derived from the fluorescence emission using the phase fluorimetric method. 

The apparatus used for picosecond flash spectroscopy on these systems has been described before(8 10). Figure 3a and b show typical transient absorption data obtained on 2-hydroxybenzophenone and the copolymer. Summary of these spectral data are given in Table 3. The transient observed at the shortest delay time (7ps) is the first excited singlet in all systems. The spectral data (at delay times > 50ps) permit placement of upper limits on triplet yields in CH2CI2 for both 2-hydroxy benzophenone itself and the copolymerized chromophore. 

Stufkens and Hard observed light induced formation of zwitterions and biradicals from the diimine clusters [Os3(CO)io(iPr-AcPy)] and [Os3(CO)io (dmb)] using picosecond UV/vis and nanosecond IR spectroscopies. Picosecond transient spectra indicated generation of the ligand anion radical and evolution over approximately 50 ps to a much longer lived transient that was attributed to an opened cluster with an associated ligand on the diimine metal. Figure 15 shows the proposed evolution of species in the photoreaction [93,94]. 

Serpone et al. have examined colloidal titanium dioxide sols (prepared by hydrolysis of TiCl4) with mean particle diameters of 2.1, 13.3, and 26.7 nm by picosecond transient absorption and emission spectroscopy [5]. Absorption decay for the 2.1 nm sols was found to be a simple first-order process, and electron/hole recombination was 100% complete by 10 ns. For the 13.3 and 26.7 nm sols absorption decay follows distinct second-order biphasic kinetics the decay times of the fast components decrease with increase in particle size. 10 ns after the excitation pulse, about 90% or more of the photogenerated electron/hole pairs have recombined such that the quantum yield of photooxidations must be 10% or less. The faster components are due to the recombination of shallow-trapped charge carriers, whereas the slower components (x > 20 ns) reflect recombination of deep-trapped electrons and holes. 

The Mallouk study stands apart from all other studies of type 1 dyads in that the charge separated state (e.g., 18 in Scheme 9) is detected by picosecond transient absorption spectroscopy. Analysis of the back ET rate data for complexes 20 indicate that JtBET increases as AGBET becomes less negative, consistent with Marcus inverted region behavior (see Fig 2). A more detailed discussion of the back ET rate data is provided below. 

Not only has binding of imidazoles and pyridines to Fe protoporphyrin IX been studied, as discussed in Section 4.1.2, but also photodissociation of axial ligands such as pyridines, imidazoles, or piperidines from six-coordinate, low-spin Fe porphyrins, in which the porphyrin is derived from protoporphyrin IX, or proto- or deuteroporphyrin IX dimethyl ester, has been investigated in nonaqueous solvents using picosecond transient absorption spectroscopy (see Photochemistry of Transition Metal Complexes). It has been shown that photodissociation leads to the formation of five-coordinate complexes, that is, only one ligand appears to be released upon excitation of the six-coordinate complex. ... 

Heilweil, E. J. and Hochstrasser, R. M. (1985). Nonlinear Spectroscopy and Picosecond Transient Grating Study of Colloidal Gold. J. Chem. Phys. 82 4762-4770. 

Enescu and Lindqvist [44] have investigated the deactivation pathways for the complexes of several purine nucleotides with MB+ by means of picosecond transient absorption spectroscopy. They observe rapid biexponential decay of complexes of singlet MB+ with GMP and XMP. The fast decay components (ti < 10 ps) were attributed to solvent relaxation and the slower decay components (tj 50 ps for GMP and 10 ps for XMP) to charge transfer (Eq. 14),... 

The primary intermediates formed from the benzophenone triplet state in the presence of tertiary amines such as dimethylaniline have been observed by picosecond transient absorption spectroscopy [99]. The initial step of the process involves one-electron transfer from amine (dimethylaniline, DMA) to the triplet state... 

Transient spectroscopy experiments were performed with a pump-probe spectrometer [7] based on a home-made original femtosecond Ti saphire pulsed oscillator and a regenerative amplifier system operated at 10 Hz repetition rate. The Tirsaphire master oscillator was synchronously pumped with doubled output of feedback controlled mode-locked picosecond pulsed Nd YAG laser. The pulse width and energy of Ti saphire system after the amplifier were ca. 150 fs and 0.5 mJ, respectively, tunable over the spectral range of 760-820 nm. The fundamental output of the Ti saphire system (790 nm output wavelength was set for present study) splitted into two beams in the ratio 1 4. The more intense beam passed through a controlled delay line and was utilized for sample... 

The vibronic spectra of Do — Di — D2 electronic states recoded by da Silva Filho et al. [45] revealed resolved vibrational structures of the Do and D2 electronic states and a broad and structureless band for the Di state. A slow ( 3-20 ps) and fast k, 200 fs) relaxation components are estimated for the Dq D2 transition in a (femto)picosecond transient grating spectroscopy measurements [16]. The fast component is attributed to the Do D2 transition and a nonradiative relaxation time of 212 fs is also estimated from the cavity ringdown (CRD) spectroscopy data [42]. Electronic structure results of Hall et al. [107] suggest that the nonradiative Do D2 relaxation occurs via two consecutive sloped type CIs [66,108]. We developed a global model PESs for the Do — Di— D2 electronic states and devised a vibronic coupling model to study the nuclear dynamics underlying the complex vibronic spectrum and ultrafast excited state decay of N +[20]. 

E. J. Heilweil R. M. Hochstrasser, Nonlinear spectroscopy and picosecond transient grating study of colloidal gold. /. Chem. Phys. 1985, 82, 179-183. 

In a picosecond transient absorption spectroscopy study of 6-nitro-BIPS derivatives, Krysanov and Alfimov8 identified a transient absorbing at 440 nm and proposed its formation within 8 ps from the excited singlet state SPNo2 of the... 

A transient absorption band, with a profile similar to that of the dimethylani-line radical cation, was observed in polar solvents by time-resolved picosecond absorption spectroscopy. 

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