Charge transfer states impact the triplet pair dynamics of singlet fission polymers.

Polymers are desirable optoelectronic materials, stemming from their solution processability, tunable electronic properties, and large absorption coefficients. An exciting development is the recent discovery that singlet fission (SF), the conversion of a singlet exciton to a pair of triplet states, can occur along the backbone of an individual conjugated polymer chain. Compared to other intramolecular SF compounds, the nature of the triplet pair state in SF polymers remains poorly understood, hampering the development of new materials with optimized excited state dynamics. Here, we investigate the effect of solvent polarity on the triplet pair dynamics in the SF polymer polybenzodithiophene-thiophene-1,1-dioxide. We use transient emission measurements to study isolated polymer chains in solution and use the change in the solvent polarity to investigate the role of charge transfer character in both the singlet exciton and the triplet pair multiexciton. We identify both singlet fluorescence and direct triplet pair emission, indicating significant symmetry breaking. Surprisingly, the singlet emission peak is relatively insensitive to solvent polarity despite its nominal "charge-transfer" nature. In contrast, the redshift of the triplet pair energy with increasing solvent polarity indicates significant charge transfer character. While the energy separation between singlet and triplet pair states increases with solvent polarity, the overall SF rate constant depends on both the energetic driving force and additional environmental factors. The triplet pair lifetime is directly determined by the solvent effect on its overall energy. The dominant recombination channel is a concerted, radiationless decay process that scales as predicted by a simple energy gap law.

First Case On-Time Starts Measured by Incision On-Time and No Grace Period: A Case Study of Operating Room Management.

EXECUTIVE SUMMARY: A delay in first case on-time starts (FCOTS) can lead to less operating room (OR) utilization, greater facility costs, and dissatisfaction among staff and patients. FCOTS is usually measured by the patient in-room metric with a small grace period. For this study, the partnering hospital elected to target and improve delays by aggressively defining FCOTS as time of incision with no grace period. Metric standardization, goal setting, and organizational focus contributed to a 9-month implementation plan to improve the newly defined FCOTS metric. The target was achieved during implementation, with 73.6% of first cases starting on time. Annual impact showed 80,587 min, or 1,343 hr, of saved OR time, which led to $771,000 in annual savings for variable OR labor costs. This redefined metric and related interventions contributed to significant reduction in delays and savings to the hospital. Engaged physician leadership played a key role in this improvement initiative, as well. The methods employed here can be used in other hospitals looking to improve FCOTS metrics in their procedural areas.

Influence of Nanostructure on the Exciton Dynamics of Multichromophore Donor-Acceptor Block Copolymers.

We explore the synthesis and photophysics of nanostructured block copolymers that mimic light-harvesting complexes. We find that the combination of a polar and electron-rich boron dipyrromethene (BODIPY) block with a nonpolar electron-poor perylene diimide (PDI) block yields a polymer that self-assembles into ordered "nanoworms". Numerical simulations are used to determine optimal compositions to achieve robust self-assembly. Photoluminescence spectroscopy is used to probe the rich exciton dynamics in these systems. Using controls, such as homopolymers and random copolymers, we analyze the mechanisms of the photoluminescence from these polymers. This understanding allows us to probe in detail the photophysics of the block copolymers, including the effects of their self-assembly into nanostructures on their excited-state properties. Similar to natural systems, ordered nanostructures result in properties that are starkly different than the properties of free polymers in solution, such as enhanced rates of electronic energy transfer and elimination of excitonic emission from disordered PDI trap states.

Fast Singlet Exciton Decay in Push-Pull Molecules Containing Oxidized Thiophenes.

A common synthetic strategy used to design low-bandgap organic semiconductors employs the use of "push-pull" building blocks, where electron -rich and electron-deficient monomers are alternated along the π-conjugated backbone of a molecule or polymer. Incorporating strong "pull" units with high electron affinity is a means to further decrease the optical gap for infrared optoelectronics or to develop n-type semiconducting materials. Here we show that the use of thiophene-1,1-dioxide as a strong acceptor in "push-pull" oligomers affects the electronic structure and carrier dynamics in unexpected ways. Critically, the overall excited-state lifetime is reduced by several orders of magnitude relative to unoxidized analogs due to the introduction of low-energy optically dark states and low-energy triplet states that allow for fast internal conversion and intramolecular singlet fission. We found that the electronic structure and excited-state lifetime are strongly dependent on the number of sequential thiophene-1,1-dioxide units. These results suggest that both the static and dynamical optical properties are highly tunable via small changes in chemical structure that have drastic effects on the optoelectronic properties, which can impact the types of applications that involve these materials.

A design strategy for intramolecular singlet fission mediated by charge-transfer states in donor-acceptor organic materials.

The ability to advance our understanding of multiple exciton generation (MEG) in organic materials has been restricted by the limited number of materials capable of singlet fission. A particular challenge is the development of materials that undergo efficient intramolecular fission, such that local order and strong nearest-neighbour coupling is no longer a design constraint. Here we address these challenges by demonstrating that strong intrachain donor-acceptor interactions are a key design feature for organic materials capable of intramolecular singlet fission. By conjugating strong-acceptor and strong-donor building blocks, small molecules and polymers with charge-transfer states that mediate population transfer between singlet excitons and triplet excitons are synthesized. Using transient optical techniques, we show that triplet populations can be generated with yields up to 170%. These guidelines are widely applicable to similar families of polymers and small molecules, and can lead to the development of new fission-capable materials with tunable electronic structure, as well as a deeper fundamental understanding of MEG.

Multiphonon relaxation slows singlet fission in crystalline hexacene.

Singlet fission, the conversion of a singlet excitation into two triplet excitations, is a viable route to improved solar-cell efficiency. Despite active efforts to understand the singlet fission mechanism, which would aid in the rational design of new materials, a comprehensive understanding of mechanistic principles is still lacking. Here, we present the first study of singlet fission in crystalline hexacene which, together with tetracene and pentacene, enables the elucidation of mechanistic trends. We characterize the static and transient optical absorption and combine our findings with a theoretical analysis of the relevant electronic couplings and rates. We find a singlet fission time scale of 530 fs, which is orders of magnitude faster than tetracene (10-100 ps) but significantly slower than pentacene (80-110 fs). We interpret this increased time scale as a multiphonon relaxation effect originating from a large exothermicity and present a microscopic theory that quantitatively reproduces the rates in the acene family.

Packing dependent electronic coupling in single poly(3-hexylthiophene) H- and J-aggregate nanofibers.

Nanofibers (NFs) of the prototype conjugated polymer, poly(3-hexylthiophene) (P3HT), displaying H- and J-aggregate character are studied using temperature- and pressure-dependent photoluminescence (PL) spectroscopy. Single J-aggregate NF spectra show a decrease of the 0-0/0-1 vibronic intensity ratio from ~2.0 at 300 K to ~1.3 at 4 K. Temperature-dependent PL line shape parameters (i.e., 0-0 energies and 0-0/0-1 intensity ratios) undergo an abrupt change in the range of ~110-130 K suggesting a change in NF chain packing. Pressure-dependent PL lifetimes also show increased contributions from an instrument-limited decay component which is attributed to greater torsional disorder of the P3HT backbone upon decreasing NF volume. It is proposed that the P3HT alkyl side groups change their packing arrangement from a type I to type II configuration causing a decrease in J-aggregate character (lower intrachain order) in both temperature- and pressure-dependent PL spectra. Chain packing dependent exciton and polaron relaxation and recombination dynamics in NF aggregates are next studied using transient absorption spectroscopy (TAS). TAS data reveal faster polaron recombination dynamics in H-type P3HT NFs indicative of interchain delocalization whereas J-type NFs exhibit delayed recombination suggesting that polarons (in addition to excitons) are more delocalized along individual chains. Both time-resolved and steady-state spectra confirm that excitons and polarons in J-type NFs are predominantly intrachain in nature that can acquire interchain character with small structural (chain packing) perturbations.

Surgical factors affecting return of renal function after partial nephrectomy.

INTRODUCTION: Nephron-sparing surgery is becoming the standard treatment for small renal tumors. In this study, we investigate the relationship between operative factors and recovery of renal function after partial nephrectomy. METHODS: Records of 141 partial nephrectomy patients at the University of Alabama Medical Center at Birmingham between 1999 and 2008 were reviewed retrospectively. Renal function was assessed preoperatively, at 1 day (early) and 6 months (late) postoperatively by calculated creatinine clearance (CC). Anesthesia time, arterial clamp time, use of ice slush, tumor size, and change in hematocrit following surgery were assessed for their impact on change in early and late renal function after adjusting for patient age, gender, race, co-morbidities, preoperative renal function and operative approach. Descriptive statistics are presented for independent predictors and research outcome by time points. Multivariate regression model was used to identify independent predictors of renal function. RESULTS: Increasing anesthesia time, clamp time, and postoperative hematocrit were associated with decreased renal function (CC) at 1 day postoperative. At 6 months, tumor size and change in early postoperative hematocrit predicted a decline in CC. In multivariate analysis, decreased renal function at 6 months was predicted by change in postoperative hematocrit level. CONCLUSION: Long hilar clamp times and anesthesia times adversely affect early postoperative renal function but not late renal function. Intraoperative bleeding adversely affected renal function at both early and late time points. Limiting intraoperative blood loss may be more important than clamp times or renal cooling in the recovery of renal function after partial nephrectomy.