Picosecond laser photolysis

Note added in typing In a very recent paper (81) Vaida and co-workers have used picosecond laser photolysis to show that, in cyclohexane solution, Cr(CO)5...cyclohexane (Amax 497 nm) is formed within 25 ps of the photolysis of Cr(C0)5 This suggests that, in solution, the primary photoproduct is Cr(C0)5 and that there is essentially no activation energy for the reaction of Cr(C0)5 with the solvent. Clearly, experiments with pulsed KrF lasers on carbonyls in solution and matrix may be very revealing. 

Rebbert, R.E. Lias, S.G. Ausloos, P. Picosecond ultraviloet multiphoton laser photolysis related to radiolysis. J. Photochem. 1975, 4, 121. 

More direct evidence for the intervention of excited states of triplet carbenes in reactions in solution is obtained by spectroscopic studies. Thus, picosecond lasers make it possible to study the quenching of carbene fluorescence by various substrates in solution at room temperature. Diphenylcarbene is generated upon laser photolysis of 30 and a second UV laser pulse is time delayed by 8 ns and is used to excite the carbene, thereby producing the excited triplet DPC (Scheme 9.32). The fluorescence of DPC is then monitored with a streak camera. The fluorescence... 

Exciplexes are defined as molecular complexes which are stable under electronic excitation30. The picosecond (or femtosecond) laser photolysis methods are suitable for investigating the very rapid photo-induced processes related to CT complexes. 

Kurita et al. 2 Takeda et al. 3 and Parthenopoulos and Rentzepis11 used picosecond laser photolysis techniques to study the photochromic processes of furyl fulgide. They found that the excited states of furyl fulgide and its colored form were singlet states and had n,n characteristics. Takeda et al.13 reported from theoretical studies that the values of the oscillator strength and the radiation lifetime (/r) were... 

According to laser photolysis results, the time scale of the coloration of heterocyclic fulgides is shorter than a few nanoseconds and extends to the picosecond time scale. The results indicated that the photoresponse of the photochromic optical disk could be very fast. 

The third possibility is to design photochromic materials in which the photochemical bleaching reaction of the colored form has an activity energy barrier that could act as a threshold. Picosecond laser photolysis studies on furyl fulgides12 demonstrated that the transformation from the excited state of the colored form to the S o potential surface of furyl fulgides has an activity energy barrier. 

S. Kurita, A. Kashiwagi, Y. Kurita, H. Miyasaka, and N. Mataga, Picosecond laser photolysis studies on the photochromism of a furyl fulgide, Chem. Phys. Lett., 171, 553-557 (1990). 

H. Miyasaka, S. Arai, A. Tabata, T. Nobuto, N. Mataga, and M. Irie, Picosecond laser photolysis studies on photochromic reactions of l,2-bis-(2,4,5-trimethyl-3-thienyl)maleic anhydride in solutions, Chem. Phys. Lett. 230, 249-254 (1994). 

Miyasaka H, Masuhara H, Mataga N. (1987) Picosecond 266-nm multiphoton laser photolysis studies on the solvated electron formation process in water and liquid alcohols. Laser Chem 7 119-128. 

Temperature Dependence of Singlet Excitation Energy Migration in Liquid Benzene as Revealed by Picosecond Laser Photolysis... 

A detailed study was carried out on (benzophenonylmethyl)-tri- -butylammonium triphenylbutylborate (BTAB). Nano- and picosecond laser photolysis demonstrated electron transfer from the borate counteranion to the excited triplet state of the benzophenone moiety. This leads to formation of a benzophenone moiety and a boranyl radical that dissociates rapidly to form butyl radicals. In the nonpolar solvent benzene the short lifetime of the triplet state (300 ps) suggests an intra-ion-pair process. The addition of 1 % MeCN caused an increase in the triplet lifetime to 1.2 ns, suggesting formation of a solvent-separated ion pair. For a lO" m solution in neat MeCN triplet decay is a function of tetrabutylammonium triphenyl- -butylborate concentration. 

Figure 17 shows an energetic scheme for the electron transfer processes taking place at the dye-sensitized heterojunction of such a device. Electron injection from the sensitizer s excited state into the conduction band of Ti02 is followed by regeneration of the dye by hole injection into the hole transport material (HTM). Conduction-band electrons in the metal oxide, as well as holes in the organic medium, are then transported by electronic conduction to the anode and the cathode, respectively. Pulsed picosecond laser photolysis has shown that the hole injection from the oxidized dye sensitizer [Ru (dcbpy)2(NCS)2]" into the spiro-MeOTAD... 

Chemiluminescence has been used to measure the relative yields of excited ketones formed from self reaction of alkoxyl and alkylperoxyl radical pairs . In the photochemistry of aryl azides a dehydroazepine is detected by time resolved infra red spectroscopy and flash photolysis at room temperature . Singlet and triplet nitrenes and dehydroazepenes have also been detected in the photochemistry of 3- and 4-nitrophenyl azides . Picosecond and nanosecond laser photolysis of p-nitrophenyl acetate in aqueous media produces a triplet state of the -nitrobenzylanion and CO2 after cleavage of the rnr triplet. Absorption, emission, and reaction kinetics of dimethylsilylene produced by flash photolyses of dodecamethylcycloherasilane is another interesting study 2,... 

Fulgides and related diarylethenes have been investigated extensively because of the long-term thermal stability of their photocyclized colored forms, which could lead to this application in erasable optical recording materials and photoswitchable optical elements. The entrapment of these photochromic molecules in polymer films is necessary for these practical applications. Picosecond laser photolysis was employed to study the electrocyclic reaction of a furylfulgide (18, Figure 8) in polymer solids and revealed that the colored structure was formed with a time constant of ca. lOps irrespective of the nature of the polymer matrix.49... 

Miyasaka, H., Nobuto, T., Itaya, A., Tamai, N. and Irie, M. (1997) Picosecond laser photolysis studies on a photochromic dithienylethene in solution and in crystalline phases. Chem. Phys. Lett., 269, 281-285. 

The present spectroscopy is similar to that of ATR infrared one, however, nanosecond and picosecond time-resolution can be first attained for the present UV-visible spectroscopy. Now this opens a new field of surface photochemistry where spectroscopic and kinetic analysis of reaction will be done in detail as in solution phase photochemistry by transmittance laser photolysis. 

In this chapter results of the picosecond laser photolysis and transient spectral studies on the photoinduced electron transfer between tryptophan or tyrosine and flavins and the relaxation of the produced ion pair state in some flavoproteins are discussed. Moreover, the dynamics of quenching of tryptophan fluorescence in proteins is discussed on the basis of the equations derived by the present authors talcing into account the internal rotation of excited tryptophan which is undergoing the charge transfer interaction with a nearby quencher or energy transfer to an acceptor in proteins. The results of such studies could also help to understand primary processes of the biological photosynthetic reactions and photoreceptors, since both the photoinduced electron transfer and energy transfer phenomena between chromophores of proteins play essential roles in these systems. 

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