Stephen E. Bradforth

Vibrationally resolved spectra of C2–C11 by anion photoelectron spectroscopy

Anion photoelectron spectroscopy has been employed to obtain vibrationally resolved spectra of the carbon molecules C2–C11. The spectra of C−2–C−9 are dominated by linear anion to linear neutral photodetachment transitions. Linear to linear transitions contribute to the C−11 spectrum, as well. From these spectra, vibrational frequencies and electron affinities are determined for the linear isomers of C2–C9 and C11. The term value is also obtained for the first excited electronic state of linear C4. The spectra of C−10 and C−11 show evidence for transitions involving cyclic anions and/or neutrals. Similar types of transitions are identified in the spectra of other smaller molecules, specifically C−6, C−8, and to a lesser extent C−5.

The Transition State of the F + H2 Reaction

The transition state region of the F + H2 reaction has been studied by photoelectron spectroscopy of FH2–. New para and normal FH2–photoelectron spectra have been measured in refined experiments and are compared here with exact three-dimensional quantum reactive scattering simulations that use an accurate new ab initio potential energy surface for F + H2. The detailed agreement that is obtained between this fully ab initio theory and experiment is unprecedented for the F + H2 reaction and suggests that the transition state region of the F + H2 potential energy surface has finally been understood quantitatively.

Photoelectron spectroscopy of CN−, NCO−, and NCS−

The 266 nm photoelectron spectra of CN−, NCO−, and NCS− have been recorded with a pulsed time‐of‐flight photoelectron spectrometer. The photoelectron spectrum of CN− has also been recorded at 213 nm revealing transitions to the A 2Π state as well as the ground X 2Σ+ state of the CN radical. The following adiabatic electron affinities (EAs) are determined: EA(CN)=3.862±0.004 eV, EA(NCO)=3.609±0.005 eV, and EA(NCS)=3.537±0.005 eV. The adiabatic electron affinity of cyanide is in disagreement with the currently accepted literature value. Our measurement of the electron affinity of NCS confirms recent theoretical estimates that dispute the literature experimental value. By Franck–Condon analysis of the vibrational progressions observed in each spectrum, the change in bond lengths between anion and neutral are also determined. For NCO− this yields R0(C–N)=1.17±0.01 A and R0(C–O)=1.26±0.01 A, and for CN− the equilibrium bond length is found to be Re(C–N)=1.177±0.004 A. The gas phase fundamental for CN− is deter...

Efficient Singlet Fission Discovered in a Disordered Acene Film

Singlet exciton fission is a process that occurs in select organic semiconductors and entails the splitting of a singlet excited state into two lower triplet excitons located on adjacent chromophores. Research examining this phenomenon has recently seen a renaissance due to the potential to exploit singlet fission within the context of organic photovoltaics to prepare devices with the ability to circumvent the Shockley-Queisser limit. To date, high singlet fission yields have only been reported for crystalline or polycrystalline materials, suggesting that molecular disorder inhibits singlet fission. Here, we report the results of ultrafast transient absorption and time-resolved emission experiments performed on 5,12-diphenyl tetracene (DPT). Unlike tetracene, which tends to form polycrystalline films when vapor deposited, DPTs pendant phenyl groups frustrate crystal growth, yielding amorphous films. Despite the high level of disorder in these films, we find that DPT exhibits a surprisingly high singlet fission yield, with 1.22 triplets being created per excited singlet. This triplet production occurs over two principal time scales, with ~50% of the triplets appearing within 1 ps after photoexcitation followed by a slower phase of triplet growth over a few hundred picoseconds. To fit these kinetics, we have developed a model that assumes that due to molecular disorder, only a subset of DPT dimer pairs adopt configurations that promote fission. Singlet excitons directly excited at these sites can undergo fission rapidly, while singlet excitons created elsewhere in the film must diffuse to these sites to fission.

Examination of the 2A’2 and 2E‘ states of NO3 by ultraviolet photoelectron spectroscopy of NO−3

The photoelectron spectrum of the NO−3 anion has been obtained at 266 and at 213 nm. The 266 nm spectrum probes the 2A’2 ground state of NO3. The 213 nm spectrum represents the first observation of the 2E‘ lowest‐lying excited state of NO3. The 2A2 band shows vibrational progressions in the ν1 symmetric stretch and the ν4 degenerate in‐plane bend of NO3. Our analysis of this band indicates that the NO3 ground state has a D3h equilibrium geometry and is vibronically coupled to the 2E’ second excited state via the ν4 mode. We also obtain the electron affinity of NO3, 3.937±0.014 eV, and the heat of formation of NO3 at 298 K, 0.777±0.027 eV (17.9±0.6 kcal/mol). The 2E‘ state of NO3 lies 0.868±0.014 eV above the ground state. The 2E‘ band shows complex and extensive vibrational structure. Several possible assignments of this structure are discussed.

The ejection distribution of solvated electrons generated by the one-photon photodetachment of aqueous I− and two-photon ionization of the solvent

The ultrafast dynamics following one-photon UV photodetachment of I− ions in aqueous solution are compared with those following two-photon ionization of the solvent. Ultrafast pump–probe experiments employing 50 fs ultraviolet pulses reveal similar and very rapid time scales for electron ejection. However, the electron ejection process from water pumped into the conduction band and from iodide ions detached at threshold are readily distinguishable. The observed picosecond timescale geminate recombination and electron escape dynamics are reconstructed using two different models, a diffusion-limited return of the electron from ∼15 A to its parent and a competing kinetics model governed by the reverse electron transfer rate. We conclude that the “ejected” electron in the halide detachment is merely separated from the halogen atom within the same solvent shell. The assignment of detachment into a contact pair is based on the recombination profile rather than by the postulate of any new spectral absorption due...

Singlet and Triplet Excitation Management in a Bichromophoric Near-Infrared-Phosphorescent BODIPY-Benzoporphyrin Platinum Complex

Multichromophoric arrays provide one strategy for assembling molecules with intense absorptions across the visible spectrum but are generally focused on systems that efficiently produce and manipulate singlet excitations and therefore are burdened by the restrictions of (a) unidirectional energy transfer and (b) limited tunability of the lowest molecular excited state. In contrast, we present here a multichromophoric array based on four boron dipyrrins (BODIPY) bound to a platinum benzoporphyrin scaffold that exhibits intense panchromatic absorption and efficiently generates triplets. The spectral complementarity of the BODIPY and porphryin units allows the direct observation of fast bidirectional singlet and triplet energy transfer processes (k(ST)((1)BDP→(1)Por) = 7.8 × 10(11) s(-1), k(TT)((3)Por→(3)BDP) = 1.0 × 10(10) s(-1), k(TT)((3)BDP→(3)Por) = 1.6 × 10(10) s(-1)), leading to a long-lived equilibrated [(3)BDP][Por]⇌[BDP][(3)Por] state. This equilibrated state contains approximately isoenergetic porphyrin and BODIPY triplets and exhibits efficient near-infrared phosphorescence (λ(em) = 772 nm, Φ = 0.26). Taken together, these studies show that appropriately designed triplet-utilizing arrays may overcome fundamental limitations typically associated with core-shell chromophores by tunable redistribution of energy from the core back onto the antennae.

Flowing liquid sample jet for resonance Raman and ultrafast optical spectroscopy

A wire-guided, gravity-driven jet apparatus is described that produces optically stable thin films of liquids flowing at rates suitable for high repetition rate spectroscopy. Unlike conventional free-flowing jets, the design works well for low viscosity solvents including water and aqueous solutions of proteins. The construction of the wire guide, jet nozzle, and flow system is described. A stable water film whose thickness can be varied from 6 to 100 μm is demonstrated that has been employed in resonance Raman and femtosecond transient absorption experiments.

Experimental and theoretical studies of the F+H2 transition state region via photoelectron spectroscopy of FH−2

The transition state region of the F+H2 reaction is studied by photoelectron spectroscopy of FH2−. The photoelectron spectra consist of overlapping electronic bands with different angular distributions. The ground state band shows partially resolved features which differ depending on whether the anion is made from normal or para hydrogen. This dependence on the anion nuclear spin statistics implies that these features are due to progressions in bending levels of the neutral FH2 complex. In order to confirm this, and to determine the sensitivity of the photoelectron spectrum to the bend potential near the F+H2 transition state, three‐dimensional simulations of the FH2− photoelectron spectrum were performed assuming various potential energy surfaces for the F+H2 reaction. We found that the London–Eyring–Polanyi–Sato surface proposed by Takayanagi and Sato gave better agreement than either the T5a or 5SEC surfaces. From the higher energy band, we can extract information on the F+H2 excited electronic states,...

Improving Open Circuit Potential in Hybrid P3HT:CdSe Bulk Heterojunction Solar Cells via Colloidal tert-Butylthiol Ligand Exchange

Organic ligands have the potential to contribute to the reduction potential, or lowest unoccupied molecular orbital (LUMO) energy, of semiconductor nanocrystals. Rationally introducing small, strongly binding, electron-donating ligands should enable improvement in the open circuit potential of hybrid organic/inorganic solar cells by raising the LUMO energy level of the nanocrystal acceptor phase and thereby increasing the energy offset from the polymer highest occupied molecular orbital (HOMO). Hybrid organic/inorganic solar cells fabricated from blends of tert-butylthiol-treated CdSe nanocrystals and poly(3-hexylthiophene) (P3HT) achieved power conversion efficiencies of 1.9%. Compared to devices made from pyridine-treated and nonligand exchanged CdSe, the thiol-treated CdSe nanocrystals are found to consistently exhibit the highest open circuit potentials with V(OC) = 0.80 V. Electrochemical determination of LUMO levels using cyclic voltammetry and spectroelectrochemistry suggest that the thiol-treated CdSe nanocrystals possess the highest lying LUMO of the three, which translates to the highest open circuit potential. Steady-state and time-resolved photoluminescence quenching experiments on P3HT:CdSe films provide insight into how the thiol-treated CdSe nanocrystals also achieve greater current densities in devices relative to pyridine-treated nanocrystals, which are thought to contain a higher density of surface traps.