Neil A. Anderson

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...

Ultrafast and long-lived photoinduced charge separation in MEH-PPV/nanoporous semiconductor thin film composites

Photoinduced charge separation and recombination dynamics were investigated in composite materials of conjugated polymer deposited on nanocrystalline metal oxide thin film. Using femtosecond IR transient absorption spectroscopy, the electron transfer from the excited state of poly[2-methoxy-5-(2-ethyl-hexyloxy)-(phenylene vinylene)] (MEH-PPV) to SnO2 and TiO2 nanoporous thin films is shown to occur with timescales of 800 and <100 fs, respectively. Negligible carrier recombination is observed for MEH-PPV/SnO2 within 1 ns. Microsecond charge recombination dynamics were measured using step-scan FTIR. Long-lived charge separation is observed in both systems, persisting for microseconds to seconds.

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.

The internal conversions of trans- and cis-1,3,5-hexatriene in cyclohexane solution studied with sub-50 fs UV pulses

Abstract Subpicosecond internal conversion dynamics of cis - and trans -1,3,5-hexatriene in cyclohexane were studied using sub 50 fs ultraviolet pulses. This time resolution allows direct observation of the 1B (S 2 ) and 2A (S 1 ) lifetimes. The 1B lifetime is 55±20 fs for trans -hexatriene, and ⩽50 fs for cis -hexatriene. The subsequent 2A lifetime of trans -hexatriene is found to be 190±30 fs, and is controlled by the rate of intramolecular vibrational energy redistribution. The 2A lifetime in cis -hexatriene is 250±30 fs, and is affected by modification of the excited state potential energy surface from coupling with the solvent.

Subpicosecond photoinduced electron transfer from a conjugated polymer to SnO2 semiconductor nanocrystals

Photoinduced electron transfer from the conjugated polymer poly[2-methoxy, 5-(2 � -ethyl-hexyloxy)-p-phenylenevinylene] (MEH-PPV) to a SnO2 nanoporous thin 5lm was studied using ultrafast IR spectroscopy. The absorption signals observed using this technique correspond to electron transfer, minimizing signal contamination from other processes. The electron transfer from polymer to nanocrystal was found to occur on an 800 fs timescale. The fast and e9cient charge separation is attributed to the large interface area and favorable energetics for transfer. Charge separation is long-lived, persisting for microseconds. ? 2002 Elsevier Science B.V. All rights reserved.

Ultrafast mid-IR detection of the direct precursor to the presolvated electron following electron ejection from ferrocyanide

Abstract Excitation of Fe(CN) 6 4− complex in D 2 O solution with a 267 nm ultrashort laser pulse results in rapid charge-transfer-to-solvent, and subsequent electron hydration. Mid-IR transient absorption spectroscopy is used to investigate the ejection and earliest steps in electron solvation. A broad, intense absorption signal is observed with instrument-response-limited rise and 170±20 fs single exponential decay at all probe wavelengths studied, stretching from ∼3 to 7 μm. No significant solvent isotope dependence in the time-evolution was seen. The spectral and temporal characteristics of the mid-IR signal provide strong evidence that it arises from a step in solvation immediately preceding formation of the presolvated electron.

Ultrafast electron transfer dynamics from molecular adsorbate to semiconductor nanoparticles

Electron transfer (ET) dynamics between molecular adsorbates and semiconductor nanoparticles has been a subject of intense recent interest because of relevance to many applications of nanomaterials, such as dye-sensitized solar cells, molecular electronics and sensors. However, it is still unclear how the charge transfer rate depends on the properties of molecules and semiconductors. In this paper we examine electron injection from Ru and Re polypyridyl complexes to metal oxide (TiO2, SnO2 and ZnO) nanocrystalline thin films. Adsorbates with different energetics and electronic coupling are compared to identify molecular properties that influence ET dynamics. Different semiconductor nanomaterials are compared to understand the dependence on conduction band composition and energetics. ET dynamics were found to be biphasic consisting of ultrafast (<100fs) and slower components, with varying partitioning between them and rates of slow components. These kinetics can be well described by a two-state injection model, which includes injection from both unthermalized and thermalized excited states and competition between electron injection and intramolecular relaxation from the unthermalized state. The dependence of ET rates on various molecular and semiconductor properties is also discussed.

Direct observation of ultrafast excited state dynamics in condensed phase photochemistry and photobiology

Tunable, ultrashort, sub 40 fs, pulses have been used to investigate the excited state photochemistry and photophysics of vitamin B12 and the B12 coenzymes 5’-deoxyadenosylcobalamin and methylcobalamin.

Mid-IR detection of a precursor to the presolvated electron,

Ultrafast transient absorption spectroscopy probing in the mid-IR is used to investigate electron ejection from Fe(CN)64−. A broad absorption is seen corresponding to a precursor to the presolvated electron. This signal decays within 150±20 fs.

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.