Pulse radiolysis of trans-1,2-bis(4-pyridyl)ethylene in aqueous solution

Abstract

acids, stearic acid having a C18 alkyl chain may fit more smoothly with SMC than arachidic acid having a C20 alkyl chain from a molecular model. Therefore, it is reasonable that the fluorescence decay time of SMC excited at 576 nm becomes shorter in the order stearic acid, arachidic acid, and tripalmitine; in other words, the reaction rate of SSP *-SMC becomes larger in the same order. This result indicates that the volume and rigidity of reaction cage surrounding chromophores is essential to determine the rate of photoisomerization of SSP •*-SMC.On the other hand, the fluorescence decay of SMC on excitation at 314 nm (Figure 9) is slightly faster in aliphatic acids than in tripalmitine. In the foregoing discussion, it is found that when SMC is irradiated with UV laser, the energy relaxation pathway in SMC is predominantly the excitation energy migration and trapping processes instead of the isomerization reaction. Since the excitation energy migration is sensitive to the interchromophore distance, the difference in the fluorescence decays can be interpreted as due to the slightly close average chromophore distance in aliphatic acids compared in tripalmitine.The time-resolved fluorescence spectra (Figure 8) show a large red-shift in the SMC fluorescence band with time; the maximum wavelength is 616 nm at 0 ps and 660 nm at 1 ns. Such a large time-dependent spectral shift is quite common in LB monolayer films containing organic dye chromophores such as rhodamine B.21 Excited singlet dye chromophores in LB films undergo energy migration among energetically different sites and energy trapping by higher aggregates as trap sites. It is recognized that the electronic energy level of dye chromophores is slightly different with each other due to different conformation and different interaction with environmental molecules. In the case of rhodamine B, site-energy distribution arises from different relative angles of the phenyl ring. This interpretation will be adapted to the excitation energy relaxation of merocyanine in LB films. It is possible that chromophores of merocyanine caused by the C-O bond cleavage of spiropyran have different conformations in LB films. This will result in the energy level distribution of merocyanine among different sites. The energy migration among different sites is responsible for the time-dependent red shift of the fluorescence of SMC.Acknowledgment. This work was supported by the Ministry of Education, Science and Culture, Grant-in-Aid for General Scientific Research No. 01470002. We acknowledge The Research Foundation for Opto-Science and Technology (Hamamatsu) and The Mitsubishi Foundation for their financial support.

Visible-Light-Driven Photoreactions of [(bpy)2Ru(II)L]Cl2 in Aqueous Solutions (bpy = Bipyridine, L = 1,2-Bis(4-(4‘-methyl)-2,2‘-bipyridyl) Ethene)

“…Using the lifetimes of 531 and 765 ns, the two excited-state p K a *'s are calculated at 2.7 and 5.8. The Forster's cycle calculation requires the transition energy of the unprotonated and protonated forms for the triplet MLCT state. , Since this data is not available, we have not used this method.…”

“…17 Stilbene-like N-heterocyclic compounds represent such a class of ligands, 18,19 and their photochemistry in aqueous solutions has been extensively studied. [19][20][21][22] These molecules undergo photoaddition and photoreduction reactions involving radical intermediates, where the corresponding hydrocarbons are unreactive. Transition metal complexes containing stilbenelike ligands have been synthesized, and their photophysical and photochemical properties have been reported.…”

“…The characteristics of the MLCT state can be altered by varying the ligands. , Ruthenium complexes containing rigid ligands with extended π-conjugation systems tend to have longer excited-state lifetimes as a result of shorter bond displacements in the excited state . Stilbene-like N-heterocyclic compounds represent such a class of ligands, , and their photochemistry in aqueous solutions has been extensively studied. − These molecules undergo photoaddition and photoreduction reactions involving radical intermediates, where the corresponding hydrocarbons are unreactive. Transition metal complexes containing stilbene-like ligands have been synthesized, and their photophysical and photochemical properties have been reported. 9h,− Several studies have shown that photoreactions including trans−cis isomerization occur under UV-light irradiation in organic solvents.…”

Photochemical one-way adiabatic isomerization of aromatic olefins

“…9a (R = lBu) isomerizes by the one-way mode; however, 9b (R = Ph) isomerizes by a dual mechanism of twoand one-way. Chrysenylethylenes (10)31 and a phenanthrylethylene (11) undergo two-way photo- The rate constants for deactivation of the twisted and transoid triplets, kpd and ktd, can be assumed to be similar to those for stilbene (2 x 107 s-1) and one-way isomerizing olefins (2 X 104 s-1), respectively. Substitution of these values into eq 4 gives a Ktp value of ca.…”

“…Figure11. Potential energy surfaces of the isomerization of 2-styrylanthracene (a) and stilbene (b) in the excited singlet and the triplet states.…”