Current state-of-the-art hybrid polymer:lead chalcogenide nanocrystal solar cells require postdeposition, thin film chemical treatments to remove insulating organic ligands from the nanocrystal surface, which is a kinetically hindered process. This is compounded by the fact that it can be especially difficult to obtain colloidally stable suspensions of PbS nanocrystals ligand exchanged with small ligands, and many atomic ligands require dispersion in solvents that are incompatible with polymer solubility. Herein, we report a novel one-step colloidal ligand exchange process for PbS nanocrystals employing lead iodide (PbI2) or ammonium iodide (NH4I) as surface ligands along with n-butylamine that allow the ligand-exchanged nanocrystals to be suspended in solvents compatible with polymer dissolution. While ligand exchange is shown to be near quantitative for both iodide sources, when compared to NH4I-exchanged PbS nanocrystals, the PbI2-exchanged PbS nanocrystals not only exhibit better colloidal stability but also display superior photovoltaic performance. When the PbI2-passivated PbS nanocrystals are combined with the donor polymer poly[2,6-(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′3′-d]silole)-alt-4,7-(2,1,3-benzothiadiazole)] (Si-PCPDTBT), the optimized hybrid solar cells give a broad spectral response into the NIR, leading to a power conversion efficiency (PCE) of 4.8% under AM 1.5G illumination. Time-resolved photoluminescence and transient absorption spectroscopies suggest that excitonic processes between the PbS nanocrystals and Si-PCPDTBT are more favorable when PbS nanocrystals are ligand exchanged with PbI2, leading to superior device performance.

Singlet fission is a process in which a singlet exciton converts into two triplet excitons. To investigate this phenomenon, we synthesized two covalently linked 5-ethynyl-tetracene (ET) dimers with differing degrees of intertetracene overlap: BET-X, with large, cofacial overlap of tetracene π-orbitals, and BET-B, with twisted arrangement between tetracenes exhibits less overlap between the tetracene π-orbitals. The two compounds were crystallographically characterized and studied by absorption and emission spectroscopy in solution, in PMMA and neat thin films. The results show that singlet fission occurs within 1 ps in an amorphous thin film of BET-B with high efficiency (triplet yield: 154%). In solution and the PMMA matrix the S1 of BET-B relaxes to a correlated triplet pair 1(T1T1) on a time scale of 2 ps, which decays to the ground state without forming separated triplets, suggesting that triplet energy transfer from 1(T1T1) to a nearby chromophore is essential for producing free triplets. In support of this hypothesis, selective excitation of BET-B doped into a thin film of diphenyltetracene (DPT) leads to formation of the 1(T1T1) state of BET-B, followed by generation of both DPT and BET-B triplets. For the structurally cofacial BET-X, an intermediate forms in <180 fs and returns to the ground state more rapidly than BET-B. First-principles calculations predict a 2 orders of magnitude faster rate of singlet fission to the 1(T1T1) state in BET-B relative to that of crystalline tetracene, attributing the rate increase to greater coupling between the S1 and 1(T1T1) states and favorable energetics for formation of the separated triplets.

We developed a simple and robust colloidal route for the installation of CdX2 (X = Cl, Br, I) ligands on the surface of CdSe nanocrystals, which effectively displace the native ligands and form stable suspensions. After colloidal ligand exchange, these nanocrystals can be easily solution cast into nanocrystal films. Photoelectrochemical measurements on solution-cast nanocrystal films reveal a striking influence of surface cadmium halide on photocurrent response, with mildly annealed, CdCl2-treated CdSe nanocrystals showing the greatest enhancement in photocurrent to above band gap illumination. The strong dependence of photoresponse on surface halide is thought to result from ligand-induced changes in the electronic structure of the nanocrystal samples. We arrive at this conclusion using a combination of ultrafast transient absorption, time-resolved photoluminescence, and surface photovoltage spectroscopies, which are being applied together for the first time to investigate nanocrystal trap states. From these measurements, we establish a trend for ligand-related sub-band gap states that accounts for electron and hole trapping at the nanocrystal surface. The nature of the electron and hole traps in the nanocrystal films are dependent on the thermal history of the sample as well as the specific halide surface treatment employed. After subjecting the nanocrystal films to mild thermal annealing, we find evidence that suggests a drastic reduction in electron trap states. Additionally, depending on the surface halide treatment employed, the energy of the hole trap states varies, with CdCl2 treatment resulting in energetically shallow hole trap states, and CdBr2 and CdI2 treatments leading to much deeper hole traps. Thus, judicious choice of cadmium halide surface treatment can be used to manipulate the trap state landscape of these ligand exchanged CdSe nanocrystals.

The excited state dynamics of phenol in water have been investigated using transient absorption spectroscopy. Solvated electrons and vibrationally cold phenoxyl radicals are observed upon 200 and 267 nm excitation, but with formation time scales that differ by more than 4 orders of magnitude. The impact of these findings is assessed in terms of the relative importance of autoionization versus proton-coupled electron transfer mechanisms in this computationally tractable model system.

Oxidative damage to DNA and hole transport between nucleobases in oxidized DNA are important processes in lesion formation for which surprisingly poor thermodynamic data exist, the relative ease of oxidizing the four nucleobases being one such example. Theoretical simulations of radiation damage and charge transport in DNA depend on accurate values for vertical ionization energies (VIEs), reorganization energies, and standard reduction potentials. Liquid-jet photoelectron spectroscopy can be used to directly study the oxidation half-reaction. The VIEs of nucleic acid building blocks are measured in their native buffered aqueous environment. The experimental investigation of purine and pyrimidine nucleotides, nucleosides, pentose sugars, and inorganic phosphate demonstrates that photoelectron spectra of nucleotides arise as a spectral sum over their individual chemical components; that is, the electronic interactions between each component are effectively screened from one another by water. Electronic structure theory affords the assignment of the lowest energy photoelectron band in all investigated nucleosides and nucleotides to a single ionizing transition centered solely on the nucleobase. Thus, combining the measured VIEs with theoretically determined reorganization energies allows for the spectroscopic determination of the one-electron redox potentials that have been difficult to establish via electrochemistry.

Ultrafast transient absorption spectroscopy is used to study charge transfer dynamics in hybrid films composed of the low band gap polymer PCPDTBT and CdSe quantum dots capped with tert-butylthiol ligands. By selectively exciting the polymer, a spectral signature for electrons on the quantum dots appears on ultrafast time scales (≲ 65 fs), which indicates ultrafast electron transfer. From this time scale, the coupling between the polymer chains and the quantum dots is estimated to be J ≳ 17 meV. The reduced quantum dot acceptors exhibit an unambiguous spectral bleach signature, whose amplitude allows for the first direct calculation of the absolute electron transfer yield in a hybrid solar cell (82 ± 5%). We also show that a limitation of the hybrid system is rapid and measurable geminate recombination due to the small separation of the initial charge pair. The fast recombination is consistent with the internal quantum efficiency of the corresponding solar cell. We therefore have identified and quantified a main loss mechanism in this type of third generation solar cell.

Macrocyclic vinyl aromatic polymers, particularly at small degrees of polymerization (DP), exhibit properties that diverge from linear polymers. A series of matched DP linear and macrocyclic polymers are prepared for one such model system, poly(2-vinylnaphthalene), and their spectroscopic properties (electronic absorption, steady-state and picosecond time-resolved emission) are compared. It is found that small macrocycles exhibit pronounced differences in their excited state dynamics compared to their linear analogues: as the DP decreases, the ratio of monomer to excimer emission is strongly enhanced as is the overall emission quantum yield, and the excimer emission is increasingly blue-shifted. Moreover, time-resolved data shows the formation of excimers in the DPn = 12 macrocycle is at least an order of magnitude faster than that for the matching linear polymer but there are at least two excimer populations for the cycles whereas the linear polymer shows only one. It is suggested that the strained macrocycle conformation tends to splay the pendent chromophores pseudoequatorial, leading to a greater number of preconfigured excimer sites, but that these are shallower traps with both faster formation and dissociation time scales. Overall, this leads to longer overall exciton lifetimes, explaining the greater overall monomer emission. By reducing the impact of trapping and potentially keeping the excitons mobile, these results suggest that macrocyclic architectures have potential advantages for light harvesting.

Singlet fission is a process that occurs in select molecular systems wherein a singlet excited state divides its energy to form two triplet excitations on neighboring chromophores. While singlet fission has been largely studied in molecular crystals, colloidal nanoparticles offer the ability to investigate fission using liquid suspensions, allowing questions regarding the importance of molecular arrangement and charge transfer to be assessed. Herein, we report the synthesis of aqueous colloidal nanoparticles of 5,12-diphenyltetracene (DPT), a material recently demonstrated to undergo singlet fission in disordered films. Upon synthesis, nanoparticles display absorption features that lie between those of monomeric DPT and disordered DPT films. These features evolve over a few days in a manner that suggests an increase in the degree of association between neighboring molecules within the nanoparticles. Transient absorption and time-resolved emission experiments indicate that photoexcited DPT nanoparticles undergo fission, but produce a lower triplet yield than disordered films.

A wavelength-resolved (λpump = 295, 285, 270, and 267 nm) photodissociation study of para-methylthiophenol (p-MePhSH) in ethanol solution has been performed using femtosecond transient absorption (TA) spectroscopy, and the results compared with those from studies of the corresponding photodissociation in cyclohexane solution at 270 nm. Anisotropy spectra are used to identify the electronic character of the initially populated excited state(s). S–H bond fission is found to occur via the dissociative S2(11πσ*) state, which can be populated directly, or by ultrafast nonradiative transitions from the S3(21ππ*) state, or by very efficient tunneling from the S1(11ππ*) state, depending on the excitation wavelength, in line with conclusions from previous gas-phase studies of this same molecular photodissociation (Oliver, T. A. A.; King, G. A.; Tew, D. P.; Dixon R. N.; Ashfold, M. N. R. J. Phys. Chem. A 2012, 116, 12444). p-MePhS radicals are observed on a time scale faster than the instrument response at all wavelengths, but the available time resolution affords a rare opportunity to explore the branching between different electronic states of a product (the à and X̃ states of the p-MePhS radical in this case). The present study provides estimates of this branching in the products formed immediately after the first pass through the conical intersection (CI) between the S2 and S0 states. At 270 nm, for example, we identify a marked population inversion in the radical products, in contrast to the reported gas phase behavior. The finding that the contrast in branching ratio is largest between cyclohexane solution and the gas phase, with ethanol being intermediate, can be rationalized by recognizing the differing distributions of the S–H torsion angle (relative to the ring plane) in a room temperature solution compared with those in a jet-cooled molecular beam. The available time resolution also allows exploration of the electronic quenching of nascent à state radicals as solvent motion encourages recrossing of the S2/S0 CI. The average separation distance, ⟨r0⟩, between the H + p-MePhS products arising in successful dissociation events is seen to increase with decreasing photolysis wavelength. This finding accords with the previous gas phase results, which determined that most of the excess energy following population of the dissociative S2 state (directly, or by ultrafast coupling from the S3 state) is released as product translation, and the expectation that ⟨r0⟩ should scale with the total kinetic energy release. The present work also confirms that geminate recombination of the H + p-MePhS products leads not just to reformation of parent p-MePhSH molecules but also to alternative adducts wherein the H atom bonds to the benzene ring. Analysis of the present data and results of high level ab initio calculations together with recent UV-IR pump–probe measurements (Murdock, D.; Harris, S. J.; Karsili, T. N. V.; Greetham, G. M.; Clark, I. P.; Towrie, M.; Orr-Ewing, A. J.; Ashfold, M. N. R. J. Phys. Chem. Lett. 2012, 3, 3715) allows identification of the likely adduct structures.

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, DPT’s 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.

The ICN photodissociation reaction is the prototype system for understanding energy disposal and curve crossing in small molecule bond-breaking. The wide knowledge base on this reaction in the gas phase makes it an excellent test case to explore and understand the influence of a liquid solvent on the photo-induced reaction dynamics. Molecular dynamics simulations that include surface-hopping have addressed numerous aspects of how the solvent should influence non-adiabatic transitions and energy flow and ultimately determine product branching for this reaction system. In this paper, we report femtosecond transient absorption work directly combined with new molecular dynamics simulations that make direct connection with the spectroscopic observables. The full spectral evolution after initiating ICN photodissociation at 266 nm in water and ethanol is recorded with unprecedented time resolution, fast enough to see the nascent products emerge before interacting with the solvent cage. Use of a 266 nm pump maximizes the probability of subsequent caging on the upper diabat while launching large rotational energy release for trajectories emerging on the lower diabat. The 2D dataset yields a map of the different products and how they interconvert. In particular, information on the branching ratio and spectral evolution of the product bands is revealed as the products relax their electronic and rotational degrees of freedom. An evolution from rotationally hot gas-phase like CN (sharp band, at 390 nm) to equilibrated and solvated CN radicals (broad, at 326 nm in water and 415 nm in ethanol) is clearly observed in both solvents, and signals assignable to I* are also captured. The non-adiabatic molecular dynamics simulations focus on identifying when trajectories curve cross, filtering the trajectory ensemble into spectroscopically distinct sub-populations and analyzing the rotational energy for the CN product population. The experimental results, taken together with the MD simulations, establish the initial surface crossing probability and suggest multiple passes through the curve crossing region determine the final product yields and provide a source of freshly torqued CN radicals that continues to top up the population of rotationally hot photoproduct over the first few picoseconds.

Electron photodetachment from the aromatic anion phenolate excited into the π−π* singlet excited state (S1) in aqueous solution is studied with ultrafast transient absorption spectroscopy with a time resolution of better than 50 fs. Broad-band transient absorption spectra from 300 to 690 nm are recorded. The transient bands are assigned to the solvated electron, the phenoxyl radical, and the phenolate S1 excited state, and confirmation of these assignments is achieved using both KNO3 as electron quencher and time-resolved fluorescence to measure singlet excited state dynamics. The phenolate fluorescence lifetime is found to be short (∼20 ps) in water, but the fast decay is only in part due to the electron ejection channel from S1. Using global target analysis, two electron ejection channels are identified, and we propose that both vibrationally hot S1 state and the relaxed S1 state are direct precursors for the solvated electron. Therefore, electron ejection is found just to compete with picosecond time scale vibrational relaxation and electronic radiationless decay channels. This contrasts markedly with <100 fs electron detachment processes for inorganic anions.

Vertical ionization energies of the nucleosides cytidine and deoxythymidine in water, the lowest ones amounting in both cases to 8.3 eV, are obtained from photoelectron spectroscopy measurements in aqueous microjets. Ab initio calculations employing a nonequilibrium polarizable continuum model quantitatively reproduce the experimental spectra and provide molecular interpretation of the individual peaks of the photoelectron spectrum, showing also that lowest ionization originates from the base. Comparison of calculated vertical ionization potentials of pyrimidine bases, nucleosides, and nucleotides in water and in the gas phase underlines the dramatic effect of bulk hydration on the electronic structure. In the gas phase, the presence of sugar and, in particular, of phosphate has a strong effect on the energetics of ionization of the base. Upon bulk hydration, the ionization potential of the base in contrast becomes rather insensitive to the presence of the sugar and phosphate, which indicates a remarkable screening ability of the aqueous solvent. Accurate aqueous-phase vertical ionization potentials provide a significant improvement to the corrected gas-phase values used in the literature and represent important information in assessing the threshold energies for photooxidation and oxidation free energies of solvent-exposed DNA components. Likewise, such energetic data should allow improved assessment of delocalization and charge-hopping mechanisms in DNA ionized by radiation.

The ICN photodissociation reaction is the prototype system for understanding energy disposal and curve crossing in small molecule bond-breaking. The wide knowledge base on this reaction in the gas phase makes it an excellent test case to explore and understand the influence of a liquid solvent on the photo-induced reaction dynamics. Molecular dynamics simulations that include surface-hopping have addressed numerous aspects of how the solvent should influence non-adiabatic transitions and energy flow and ultimately determine product branching for this reaction system. In this paper, we report femtosecond transient absorption work directly combined with new molecular dynamics simulations that make direct connection with the spectroscopic observables. The full spectral evolution after initiating ICN photodissociation at 266 nm in water and ethanol is recorded with unprecedented time resolution, fast enough to see the nascent products emerge before interacting with the solvent cage. Use of a 266 nm pump maximizes the probability of subsequent caging on the upper diabat while launching large rotational energy release for trajectories emerging on the lower diabat. The 2D dataset yields a map of the different products and how they interconvert. In particular, information on the branching ratio and spectral evolution of the product bands is revealed as the products relax their electronic and rotational degrees of freedom. An evolution from rotationally hot gas-phase like CN (sharp band, at 390 nm) to equilibrated and solvated CN radicals (broad, at 326 nm in water and 415 nm in ethanol) is clearly observed in both solvents, and signals assignable to I* are also captured. The non-adiabatic molecular dynamics simulations focus on identifying when trajectories curve cross, filtering the trajectory ensemble into spectroscopically distinct sub-populations and analyzing the rotational energy for the CN product population. The experimental results, taken together with the MD simulations, establish the initial surface crossing probability and suggest multiple passes through the curve crossing region determine the final product yields and provide a source of freshly torqued CN radicals that continues to top up the population of rotationally hot photoproduct over the first few picoseconds.