Stuart H. Pullen

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-hexatriene (cZc-HT) is formed on a ∼250 fs 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 a...

Femtosecond transient absorption study of the ring-opening reaction of 1,3-cyclohexadiene

Abstract Deep ultraviolet femtosecond transient absorption measurements of the photoisomerization reaction of 1,3-cyclohexadiene to form cis-1,3,5-hexatriene demonstrate that the rate of the photochemical ring-opening reaction is ⩾ 1 ps −1 . The subsequent kinetics observed in picosecond resonance Raman and transient absorption measurements represents the time scales for vibrational and conformational relaxation of the photoproduct.

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 1A1 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.

On the structure of iodine charge-transfer complexes in solution

Abstract Femtosecond transient absorption studies of charge-transfer complexes of I2 with hexamethylbenzene have been performed in a series of noncomplexing solvents. Anisotropy measurements of the bleach of the charge-transfer absorption band indicate that the geometry and electronic structure of the complex is dependent upon the solvent environment. The results are interpreted as favoring an oblique, nearly axial, geometry in alkanes and a resting geometry in chlorinated methanes.

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 ...

The vibrational relaxation of I2 (X 1Σg+) 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 I2-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 I2 in the interacting solvent mesitylene. The peak of the population distribution has dropped below n=10 by 11 ps and n=7 by 15.5 ps. The energy relaxation is characterized by a biexponential decay with time constants of 4.41±0.08 ps and 20.3±0.7 ps. 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...

Femtosecond Studies of Isomerization and Energy Relaxation in Small Polyenes

The reaction dynamics of 1,3-cyclohexadiene (CHD) and both cis- (cHT) and trans- (tHT) 1,3,5-hexatriene were studied using ultrafast absorption spectroscopy. Kinetics on the red edge of the ground state absorption spectrum show that ‘hot’ cHT is formed from CHD within 400 fs. The hexatrienes recover on a similar time scale.