Femtosecond studies of the iodine–mesitylene charge‐transfer complex

Femtosecond laser studies have been performed to investigate the initial photodissociation reactions of I2–mesitylene charge transfer complexes. Photodissociation occurs along both the I2–mesitylene ‘‘bond’’ and the I–I bond with a branching ratio of 2:3 for the two reaction coordinates. Following excitation at 400 nm, geminate recombination occurs along both reaction coordinates. The reformed I2–mesitylene complexes are formed vibrationally hot and relax on a time scale of 13 ps. The I–mesitylene spectrum is fully developed within 500 fs of the pump pulse. Approximately 40% of the I–mesitylene complexes undergo geminate recombination on a time scale of 14 ps. Most of the remaining complexes recombine with their original partners on a time scale of 400 ps. The initial anisotropy of the photoproduct absorption is 0.09±0.02. This low anisotropy is a direct result of the geometry of the complex and nature of the electronic transition rather than indicative of ultrafast motion toward an asymmetric transition state preceding dissociation. © 1995 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70671/2/JCPSA6-103-18-7877-1.pd

The ultrafast ground and excited state dynamics of cis-hexatriene in cyclohexane

One- and two-color kinetics have been combined with broadband ultraviolet transient absorption spectroscopy in the 265–300 nm region to elucidate the photophysics of cis-hexatriene in cyclohexane solvent. The lowest singlet excited state, the 2 1A121A1 state, is observed to have a lifetime of 200±50 fs. The ground-state hexatriene is produced vibrationally hot. The excess vibrational energy permits ultrafast isomerization around the C–C single bonds in hexatriene. This results in a dynamic equilibrium of the three cis-hexatriene rotamers, which then relaxes multiexponentially to the room-temperature distribution in which the di-s-trans-Z-hexatriene form predominates. The peak of the mono-s-trans (cZt-HT) population is estimated to be ∼50%. Vibrational cooling results in trapping of a small amount, ∼8%, of cZt-HT that relaxes on a much longer time scale as the barrier to isomerization becomes important. An estimate of the absorption spectrum of cZt-HT is deduced from analysis of the spectral data at 50 ps. © 1997 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70051/2/JCPSA6-107-13-4985-1.pd

The ultrafast photochemical ring-opening reaction of 1,3-cyclohexadiene in cyclohexane

The ring-opening reaction of 1,3-cyclohexadiene in cyclohexane solution and the subsequent photoproduct cooling dynamics have been investigated by using two-color transient absorption kinetic measurements and novel time-resolved absorption spectroscopy in the 260–300 nm spectral region. The initial photoproduct in this reaction, s-cis,Z,s-cis-1,3,5-hexatrienes-cis,Z,s-cis-1,3,5-hexatriene (cZc-HT) is formed on a ∼ 250 fs∼250fs time scale. Spectra deduced for time delays very close to zero, as well as calculated Rice–Ramsperger–Kassel–Marcus unimolecular reaction rates, provide strong evidence that the quantum yield for the reaction is determined before any relaxation occurs on the ground state. Upon formation, the vibrationally excited hexatriene photoproduct is able to isomerize around C–C single bonds freely. As a result, the evolution observed in the transient absorption measurements represents a combination of rotamer population dynamics and thermalization due to energy transfer to the solvent. Three distinct time scales for relaxation are observed. These time scales correspond approximately to the development of an evolving equilibrium of Z-HT rotamers (1–5 ps), vibrational cooling and thermal equilibration with the surroundings (10–20 ps), and activated isomerization of trapped cZt-HT to tZt-HT (≫100 ps).(≫100ps). © 1998 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70793/2/JCPSA6-108-2-556-1.pd

The vibrational relaxation of I2I2 (X 1Σg+)(X1Σg+) in mesitylene

Transient absorption measurements between 400 nm and 570 nm are used to extract information on the vibrational relaxation of iodine in the complexing solvent mesitylene. The well characterized nature of the I2I2-arene complex makes it an excellent prototype for the study of relaxation processes in the presence of weak interactions. The data and analysis presented here demonstrate the rapid nonexponential vibrational relaxation of I2I2 in the interacting solvent mesitylene. The peak of the population distribution has dropped below n = 10n=10 by 11 ps and n = 7n=7 by 15.5 ps. The energy relaxation is characterized by a biexponential decay with time constants of 4.41±0.08 ps4.41±0.08ps and 20.3±0.7 ps.20.3±0.7ps. Quantitative comparisons of relaxation in a variety solvents are made by using a simple time-delay to peak absorption characterization of the relaxation. The initial 4.4 ps decay in mesitylene is significantly faster than the time scales for relaxation in noninteracting hydrocarbon solvents. The difference in the relaxation rate cannot be attributed to a change in vibrational frequency as the vibrational frequency of I2I2 has only a small dependence on the solvent. It is suggested that the vibrational relaxation of I2I2 in mesitylene through the high-lying levels is better characterized as an “intramolecular” vibrational energy redistribution process than relaxation to a solvent bath. The ultrafast vibrational relaxation occurs via the anharmonic coupling of the I–I stretching coordinate and the I-MST stretching coordinate of an I2I2-MST complex. © 1998 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70030/2/JCPSA6-108-12-4992-1.pd

Experimental Verification of the Through-bond Mechanism of Electron Transfer in Bridged Donor-Acceptor Complexes

Two rigid bischromophoric systems featuring identical donor and acceptor moieties have been studied in order to directly determine the relative importance of through-space and through-bond mechanisms for electron-transfer reactions. The two molecular systems studied have the unique feature that the through-bond distance between any pair of atoms in the two molecules is held constant while the spatial distance between the donor and acceptor is changed. Time-resolved laser measurements reveal that back-electron-transfer dynamics following photoexcitation of the ground-state charge-transfer absorption band are the same in the two systems. The results provide direct evidence for the through-bond mechanism of electron transfer in bridged organic donor-acceptor systems

Bacterial and viral superantigens: roles in autoimmunity?

Superantigens are bacterial, viral, or retroviral proteins which can activate specifically a large proportion of T cells. In contrast with classical peptide antigen recognition, superantigens do not require processing to small peptides but act as complete or partially processed proteins. They can bind to major histocompatibility complex class II molecules and stimulate T cells expressing particular T cell receptor V beta chains. The other polymorphic parts of the T cell receptor, which are crucial for classical antigen recognition, are not important for this interaction. When this strategy is used a large proportion of the host immune system can be activated shortly after infection. The activated cells have a wide variety of antigen specificities. The ability to stimulate polyclonal B (IgG) as well as T cell responses raises possibilities of a role for superantigens in the induction of autoimmune diseases. Superantigens have been a great tool in the hands of immunologists in unravelling some of the basic mechanisms of tolerance and immunity