Picosecond Laser Flash Photolysis Research Articles

Hydrogen Atom Transfer Reaction through Partial Charge-Transfer Triplet Complex. Chloranil and Mesitylene System

Abstract Mechanism and kinetics of hydrogen atom transfer reaction from mesitylene (M) to triplet chloranil (3CA) in 1,2-dichloroethane (DCE) at room temperature have been studied by means of picosecond (ps) and nanosecond (ns) laser flash photolysis techniques. It was found that the hydrogen transfer resulting in the formation of chloranil semiquinone radical (CAH·) proceeded through a partial charge-transfer (CT) triplet complex between 3CA and M as an intermediate. No ion-pair formation was observed. The triplet complex showed transient absorption bands in the visible and near infrared regions. The association constant (KT=28 dm3 mol−1) and lifetime (τC=32 ns) of the complex were estimated from a kinetic analysis for 3CA quenching. From these results, combined with those of the quantum yield of CAH· formation, the first order rate constants for the hydrogen transfer and intersystem crossing in the triplet complex were estimated to be (1.7 and 1.4)×107 s−1, respectively.

Picosecond and nanosecond laser flash photolysis study of some halogenated benzene derivatives

Picosecond laser photolyses of benzyl chloride, benzyl bromide and benzyl iodide vapour have been examined by monitoring the time profiles of fluorescence and absorption of benzyl radicals formed in the 266 nm photolysis. The decays of fluorescence for these molecules were well represented by two exponential functions. The short-lifetime component has a value of ca. 1 ns, and the longer one 5–9 ns. The time-resolved absorption of benzyl radicals obtained in the photolysis of benzyl chloride (0.5 Torr) showed that the rise time of benzyl radical absorption, 6 ns, could be attributed to the decomposition rate of benzyl chloride molecules under collision-free conditions, while the rise time for benzyl radicals formed in the photolyses of benzyl bromide and benzyl iodide was ca. 1 ns. Thus the photodecomposition rate of benzyl chloride is in good correlation to the value of the slow component and those of benzyl bromide and benzyl iodide to values of short components.

Picosecond Spectroscopy on Reverse Photoreaction from Batho-Intermediate of Bacteriorhodopsin at 6.5 K

The reverse photoreaction from batho-intermediate of bacteriorhodopsin has been investigated by picosecond laser flash photolysis at 6.5 K. Upon exciting batho-intermediate with 694 nm light pulse of 31 ±4 ps width, the increase of absorbance around 570 nm and concomitant decrease of that around 630 nm are observed without delay time. Based on the above result, the models for the primary photochemical reaction of bacteriorhodopsin are discussed.

Picosecond laser photolysis studies upon photochemical isomerization and protolytic reaction of a stilbazolium betaine

The dynamical behavior of a stibazolium betaine (M) and its protonated form (MH +) in their excited singlet states (S 1) was studied using picosecond laser flash photolysis. From the time-resolved transient spectra direct evidence was obtained for the deprotonation of trans-MH + in the S 1 state, competing with isomerization.

A picosecond kinetic study of the excited-state properties of some osmium octaethylporphyrins

Four osmium porphyrin complexes, Os(OEP)LL'((OEP) = octaethylporphyrin), have been investigated by picosecond laser flash photolysis using a double-beam, mode-locked Nd:glass system delivering 6-ps (fwhm) pulses at 527 nm with 1-2 mJ/pulse. Time-resolved excited-state spectra were recorded from the time of photolysis to 5 ns after photolysis. For the four compounds studied (Os(OEP)(P(OMe)/sub 3/)/sub 2/, Os(OEP)CO(py), Os(OEP)NO(OMe), and Os(OEP)O/sub 2/((Me = methyl, py = pyridine, and O/sub 2/ = dioxo)), the initial excited state, S/sub 1/, decayed in less than or equal to8, approx.50, approx.36, and approx.13 ps, respectively. The yields of the later excited state, T/sub 1/, were approx.0.8, unmeasurable, approx.0.6, and approx.1.0, respectively. The T/sub 1/ states lived for times 6.2, 16, 5.5, and 6 ns, respectively. A third relaxation time of approx.15 ps was found for Os(OEP)NO(OMe). The lowest energy excited states for the first two complexes were previously identified as (d,..pi..*) and for the second two as (..pi..,..pi..*) and a red shift of the excited-state absorption of the former with respect to the latter is observed. For the two molecules with lowest energy (..pi..,..pi..*) states, S/sub 1/ and T/sub 1/ are identified as /sup 1/(..pi..,..pi..*) and /sup 3/(..pi..,..pi..*), i.e., singlet and triplet states. For the two moleculesmore » with lowest energy (d,..pi..*) states, spin should not be a good quantum number, and S/sub 1/ is identified as a limited set of (d,..pi..*) states produced after subpicosecond relaxation of the initial photoexcited /sup 1/(..pi..,..pi..*) state.« less

Picosecond pulse radiolysis and laser flash photolysis studies on polymer degradation of polystyrene and poly-α-methylstyrene

Recently early events in photolysis of polystyrene (PSt) solutions in chloroform and in 1,4-dioxane have been made clear by using a nanosecond laser flash photolysis system (Nd-YAG: 265 nm) at Hahn-Meitner-Institut [1]. Early events in radiolysis of PSt solution have also been studied by using a picosecond pulse radiolysis system at Univ. of Tokyo [2]. In the present paper, the difference and the similarity between photolysis and radiolysis of PSt and poly-α-methylstyrene (P-α-MeSt) are discussed. Important results are as follows; 1. (1) In both cases of radiolysis and photolysis, strong absorptions of intramolecular excimer have been observed for PSt ( λmax = 530 nm) and P-α-MeSt ( λmax = 520 nm) solutions in dioxane and cyclohexane, in addition to the monomer and excimer fluorescence of these polymers. In these solutions, those polymers are hardly decomposed. 2. (2) In both radiolysis and photolysis, charge-transfer complexes (PSt +δ, Cl −δ or P-α-MeSt +δ, Cl −δ) have been observed for PSt and P-α-MeSt solutions in chloroform (CHCl 3) and in carbon tetrachloride (CCl 4). In these solutions, polymers are effectively decomposed. The charge-transfer complexes are precursors of macroradicals. 3. (3) The charge-transfer complexes are mainly produced from ion pairs (PST +…Cl − or P-α-MeSt +…Cl −) in the case of radiolysis and from exciplexes of excited polymer molecules with solvent molecules in the case of photolysis. 4. (4) Since the presence of oxygen is prerequisite to main chain cleavage of PSt in CHCl 3 and in CCl 4, it is important to note that oxygen is not involved in initial stages of the main process of macroradical formation.

Squid hypsorhodopsin and bathorhodopsin by a picosecond laser photolysis

The formation of bathorhodopsin (formerly prelumirhodopsin) was first observed by Yoshizawa and Kiti, [ 11 who irradiated cattle rhodopsin at liquid nitrogen temperature (77 K). Afterwards, the spectra of batho-intermediates in various animals were measured by low temperature spectrophotometry [2]. Therefore it is generally believed that bathorhodopsin is the earliest photoproduct in the photobleaching process of rhodopsin. Recently it was found that irradiation of cattle [2] or squid rhodopsin [3] at liquid helium temperature yielded two thermolabile photoproducts hypsorhodopsin @,a,: cattle 430 nm, squid 446 nm) and bathorhodopsin (A,,,: cattle 543 nm, squid 534 nm). Now the question arises: which is an earlier photoproduct of rhodopsin, hypsorhodopsin or bathorhodopsin? In order to elucidate this problem, we tried a picosecond laser flash photolysis of squid rhodopsin using a 347 nm light pulse from a mode locked ruby laser.