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1.
Proc Natl Acad Sci U S A ; 115(20): 5077-5082, 2018 05 15.
Article in English | MEDLINE | ID: mdl-29720443

ABSTRACT

From organic electronics to biological systems, understanding the role of intermolecular interactions between spin pairs is a key challenge. Here we show how such pairs can be selectively addressed with combined spin and optical sensitivity. We demonstrate this for bound pairs of spin-triplet excitations formed by singlet fission, with direct applicability across a wide range of synthetic and biological systems. We show that the site sensitivity of exchange coupling allows distinct triplet pairs to be resonantly addressed at different magnetic fields, tuning them between optically bright singlet ([Formula: see text]) and dark triplet quintet ([Formula: see text]) configurations: This induces narrow holes in a broad optical emission spectrum, uncovering exchange-specific luminescence. Using fields up to 60 T, we identify three distinct triplet-pair sites, with exchange couplings varying over an order of magnitude (0.3-5 meV), each with its own luminescence spectrum, coexisting in a single material. Our results reveal how site selectivity can be achieved for organic spin pairs in a broad range of systems.

2.
Nat Commun ; 9(1): 398, 2018 01 26.
Article in English | MEDLINE | ID: mdl-29374170

ABSTRACT

In weakly spin-orbit coupled materials, the spin-selective nature of recombination can give rise to large magnetic-field effects, e.g. on the electro-luminescence of molecular semiconductors. Although silicon has weak spin-orbit coupling, observing spin-dependent recombination through magneto-electroluminescence is challenging: silicon's indirect band-gap causes an inefficient emission and it is difficult to separate spin-dependent phenomena from classical magneto-resistance effects. Here we overcome these challenges and measure magneto-electroluminescence in silicon light-emitting diodes fabricated via gas immersion laser doping. These devices allow us to achieve efficient emission while retaining a well-defined geometry, thus suppressing classical magnetoresistance effects to a few percent. We find that electroluminescence can be enhanced by up to 300% near room temperature in a seven Tesla magnetic field, showing that the control of the spin degree of freedom can have a strong impact on the efficiency of silicon LEDs.

3.
Phys Rev Lett ; 112(12): 126802, 2014 Mar 28.
Article in English | MEDLINE | ID: mdl-24724667

ABSTRACT

We report a new experimental method to measure the localization length of photogenerated carriers in an organic donor-acceptor photovoltaic blend by comparing their dielectric and electron spin-resonance susceptibilities, which are simultaneously measured by monitoring the resonance frequency of a superconducting resonator. We show that at cryogenic temperatures excitons are dissociated into long lived states, but that these are confined within a separation of around 4 nm. We determine the Debye and recombination times, showing the coexistence of a fast electrical response corresponding to delocalized motion, with glasslike recombination kinetics.

4.
Nat Mater ; 11(7): 599-603, 2012 May 13.
Article in English | MEDLINE | ID: mdl-22581313

ABSTRACT

A promising approach to the fabrication of materials with nanoscale features is the transfer of liquid-crystalline structure to polymers. However, this has not been achieved in systems with full three-dimensional periodicity. Here we demonstrate the fabrication of self-assembled three-dimensional nanostructures by polymer templating blue phase I, a chiral liquid crystal with cubic symmetry. Blue phase I was photopolymerized and the remaining liquid crystal removed to create a porous free-standing cast, which retains the chiral three-dimensional structure of the blue phase, yet contains no chiral additive molecules. The cast may in turn be used as a hard template for the fabrication of new materials. By refilling the cast with an achiral nematic liquid crystal, we created templated blue phases that have unprecedented thermal stability in the range -125 to 125 °C, and that act as both mirrorless lasers and switchable electro-optic devices. Blue-phase templated materials will facilitate advances in device architectures for photonics applications in particular.

5.
Nano Lett ; 8(5): 1393-8, 2008 May.
Article in English | MEDLINE | ID: mdl-18363343

ABSTRACT

We develop a simple model that can explain the current-voltage ( J- V) curves of excitonic photovoltaic solar cells, spanning polymer:polymer, polymer:fullerene, and polymer:nanocrystal devices. We show that by subtracting out the dark current, we can explain apparent intensity-dependent characteristics and thus identify geminate recombination as the dominant loss mechanism and establish its electric field dependence. We present an analytic fit to the J- V curves of all measured devices based on a single fitted parameter, the electric field required to split 50% of geminate charge pairs, which we term the critical field. Devices of different material combinations and morphologies can all be described by this method and yield critical fields varying between >1 x 10(8) V/m for blends of poly(9,9'-dioctylfluorene- co-bis- N, N'-(4-butylphenyl)-bis- N, N'-phenyl-1,4-phenylenediamine) (PFB) and poly(9,9'-dioctylfluorene- co-benzothiadiazole) (F8BT) and 8 x 10 (5) V/m for slow-grown blends of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). A comparison with material properties reveals that the primary route to improved photovoltaic materials is enhanced charge delocalization.


Subject(s)
Electric Power Supplies , Electrochemistry/instrumentation , Fullerenes/chemistry , Models, Theoretical , Photochemistry/instrumentation , Computer Simulation , Electric Conductivity , Electrochemistry/methods , Equipment Design , Equipment Failure Analysis , Fullerenes/radiation effects , Light , Photochemistry/methods , Polymers/chemistry , Polymers/radiation effects , Radiation Dosage
6.
Phys Rev Lett ; 96(11): 117403, 2006 Mar 24.
Article in English | MEDLINE | ID: mdl-16605870

ABSTRACT

We have examined the Coulombic interactions at the interface in a blend of two copolymers with intramolecular charge-transfer character and optimized band offsets for photoinduced charge generation. The combination of both time-resolved measurements of photoluminescence, and quantum-chemical modeling of the heterojunction allows us to show that relative orientation across the heterojunction can lead to either a repulsive barrier ( approximately 65 meV) or an attractive interaction which can enhance the charge-transfer processes. We conclude that polymer orientation at the heterojunction can be as important as energy-band offsets in determining the dynamics of charge separation and optical emission.

7.
J Phys Chem B ; 109(21): 10594-604, 2005 Jun 02.
Article in English | MEDLINE | ID: mdl-16852286

ABSTRACT

Atomistic models based on quantum-chemical calculations are combined with time-resolved spectroscopic investigations to explore the migration of electronic excitations along oligophenylenevinylene-based chiral stacks. It is found that the usual Pauli master equation (PME) approach relying on uncoherent transport between individual chromophores underestimates the excitation diffusion dynamics, monitored here by the time decay of the transient polarization anisotropy. A better agreement to experiment is achieved when accounting for excitation delocalization among acceptor molecules, as implemented in a modified version of the PME model. The same models are applied to study light harvesting and trapping in guest-host systems built from oligomers of different lengths.

8.
Proc Natl Acad Sci U S A ; 99(17): 10982-7, 2002 Aug 20.
Article in English | MEDLINE | ID: mdl-12177444

ABSTRACT

The energy-transfer processes taking place in conjugated polymers are investigated by means of ultrafast spectroscopy and correlated quantum-chemical calculations applied to polyindenofluorenes end-capped with a perylene derivative. Comparison between the time-integrated luminescence and transient absorption spectra measured in solution and in films allows disentangling of the contributions arising from intrachain and from interchain energy-migration phenomena. Intrachain processes dominate in solution where photoexcitation of the polyindenofluorene units induces a rather slow energy transfer to the perylene end moieties. In films, close contacts between chains favors interchain transport of the excited singlet species (from the conjugated bridge of one chain to the perylene unit of a neighboring one); this process is characterized by a 1-order-of-magnitude increase in transfer rate with respect to solution. This description is supported fully by the results of quantum-chemical calculations that go beyond the usual point-dipole model approximation and account for geometric relaxation phenomena in the excited state before energy migration. The calculations indicate a two-step mechanism for intrachain energy transfer with hopping along the conjugated chains as the rate-limiting step; the higher efficiency of the interchain transfer process is mainly due to larger electronic coupling matrix elements between closely lying chains.


Subject(s)
Polymers , Energy Transfer , Luminescent Measurements , Models, Theoretical , Photons , Structure-Activity Relationship
9.
Nature ; 413(6858): 828-31, 2001 Oct 25.
Article in English | MEDLINE | ID: mdl-11677602

ABSTRACT

The efficiency of light-emitting diodes (LEDs) made from organic semiconductors is determined by the fraction of injected electrons and holes that recombine to form emissive spin-singlet states rather than non-emissive spin-triplet states. If the process by which these states form is spin-independent, the maximum efficiency of organic LEDs will be limited to 25 per cent. But recent reports have indicated fractions of emissive singlet states ranging from 22 to 63 per cent, and the reason for this variation remains unclear. Here we determine the absolute fraction of singlet states generated in a platinum-containing conjugated polymer and its corresponding monomer. The spin-orbit coupling introduced by the platinum atom allows triplet-state emission, so optically and electrically generated luminescence from both singlet and triplet states can be compared directly. We find an average singlet generation fraction of 22 +/- 1 per cent for the monomer, but 57 +/- 4 per cent for the polymer. This suggests that recombination is spin-independent for the monomer, but that a spin-dependent process, favouring singlet formation, is effective in the polymer. We suggest that this process is a consequence of the exchange interaction, which will operate on overlapping electron and hole wavefunctions on the same polymer chain at their capture radius.

10.
J Am Chem Soc ; 123(38): 9412-7, 2001 Sep 26.
Article in English | MEDLINE | ID: mdl-11562224

ABSTRACT

The energy gap law established for aromatic hydrocarbons and rare earth ions relates the nonradiative decay rate to the energy gap of a transition through a multiphonon emission process. We show that this energy gap law can be applied to the phosphoresce of a series of conjugated polymers and monomers for which the radiative decay rate has been enhanced through incorporation of a heavy metal. We find that the nonradiative decay rate from the triplet state T(1) increases exponentially with decreasing T(1)-S(0) gap for the polymers and monomers at 300 and 20 K. Comparison of the nonradiative decay of polymers with that of their corresponding monomers highlights the role of electron-lattice coupling.

11.
Science ; 293(5532): 1119-22, 2001 Aug 10.
Article in English | MEDLINE | ID: mdl-11498585

ABSTRACT

Self-organization of liquid crystalline and crystalline-conjugated materials has been used to create, directly from solution, thin films with structures optimized for use in photodiodes. The discotic liquid crystal hexa-peri-hexabenzocoronene was used in combination with a perylene dye to produce thin films with vertically segregated perylene and hexabenzocoronene, with large interfacial surface area. When incorporated into diode structures, these films show photovoltaic response with external quantum efficiencies of more than 34 percent near 490 nanometers. These efficiencies result from efficient photoinduced charge transfer between the hexabenzocoronene and perylene, as well as from effective transport of charges through vertically segregated perylene and hexabenzocoronene pi systems. This development demonstrates that complex structures can be engineered from novel materials by means of simple solution-processing steps and may enable inexpensive, high-performance, thin-film photovoltaic technology.

12.
Ann Behav Med ; 23(2): 133-8, 2001.
Article in English | MEDLINE | ID: mdl-11394555

ABSTRACT

Previous research has shown that social support can have a beneficial impact on coping processes and psychological adjustment in patients with rheumatoid arthritis (RA). The association of individual coping styles and perceived responses from others to one's pain episodes with patients 'distress and disease status over time was investigated. The sample consisted of 42 middle-aged patients with RA who were predominantly White (98%), female (64%), and married (88%). Participants completed surveys and their rheumatologist completed clinical assessments of patient disease status at 2 time points over a 9-month period. Although punishing responses from others (e.g., getting irritated or angry when the patient is in pain) were perceived as relatively infrequent, they were associated with a patient coping style of focusing on and venting of negative emotion as well as elevated negative affect (NA). Findings also indicated that those who perceived punishing responses from close others and coped by venting negative emotions reported increased NA over time and were rated by their rheumatologist as having more severe RA disease status over time. Implications for psychosocial intervention and directions for future research are discussed.


Subject(s)
Adaptation, Psychological , Arthritis, Rheumatoid/psychology , Social Support , Stress, Psychological/etiology , Female , Humans , Male , Middle Aged , Models, Psychological , New York City , Social Perception , Stress, Psychological/prevention & control , Stress, Psychological/psychology
13.
J Microsc ; 202(Pt 2): 433-8, 2001 May.
Article in English | MEDLINE | ID: mdl-11309108

ABSTRACT

Fluorescence scanning near-field optical microscopy (SNOM) is used to investigate binary polyfluorene-based composites of varying composition. The samples investigated contain blends of the polymer poly(9,9'-dioctylfluorene-cobenzothiadiazole), F8BT, with similar polyfluorenes of wider band gap. Images acquired from a film containing 50% by weight F8BT exhibit a high degree of correlation between the topography and fluorescence, with an F8BT-rich phase which protrudes from the surface of the film forming isolated regions with sizes from hundreds of nanometres to several micrometres. A film containing 10% by weight F8BT also has micrometre-size F8BT-rich regions, but also present are small and locally varying proportions of F8BT in the other polyfluorene component phase, indicating a hierarchy of phases within this sample. The fluorescence and topographic images of a third sample studied, containing 90% by weight F8BT, display no correlation, demonstrating that it is not always appropriate to use topographic information to determine the phase structure within polymer blends. The fluorescence SNOM images acquired from these samples are able to assist our understanding of the photovoltaic efficiency of devices fabricated from these films, which are governed by the extent of the interfacial area between these two constituent polymers.

14.
Chem Commun (Camb) ; (18): 1778-9, 2001 Sep 21.
Article in English | MEDLINE | ID: mdl-12240311

ABSTRACT

A block copolymer of distyrylbenzene with sexi(ethylene oxide) spacers displays high solid state photoluminescence efficiency (34%). Single layer light-emitting diodes with calcium or aluminium cathodes exhibit luminances over 2000 cd m-2 and efficiencies of 0.5 cd A-1.

15.
Science ; 290(5499): 2123-6, 2000 Dec 15.
Article in English | MEDLINE | ID: mdl-11118142

ABSTRACT

Direct printing of functional electronic materials may provide a new route to low-cost fabrication of integrated circuits. However, to be useful it must allow continuous manufacturing of all circuit components by successive solution deposition and printing steps in the same environment. We demonstrate direct inkjet printing of complete transistor circuits, including via-hole interconnections based on solution-processed polymer conductors, insulators, and self-organizing semiconductors. We show that the use of substrate surface energy patterning to direct the flow of water-based conducting polymer inkjet droplets enables high-resolution definition of practical channel lengths of 5 micrometers. High mobilities of 0.02 square centimeters per volt second and on-off current switching ratios of 10(5) were achieved.

16.
Health Psychol ; 18(1): 54-62, 1999 Jan.
Article in English | MEDLINE | ID: mdl-9925046

ABSTRACT

Laboratory studies show that strategic self-presentations strongly influence private self-evaluations. The present study experimentally manipulated self-presentations of dialysis patients' coping skills in order to influence their adjustment. In all, 42 renal dialysis patients matched for diabetes, gender, and dialysis years were randomly assigned to 3 conditions; adjustment was assessed at baseline, post-intervention, and 1 month follow-up. Patients in a self-presentation condition selectively presented themselves as successful copers in a videotaped interview, ostensibly as part of a training program for new patients. Patients in a problem disclosure condition discussed problems with managing their illness. Control group patients viewed a medical videotape about adjusting to dialysis. Patients in the self-presentation condition reported better adjustment, fewer physical symptoms, and more coping skills 1 month later, compared with patients in the other 2 conditions. In addition, coping skills were shown to mediate the relationship between strategic self-presentation and adjustment.


Subject(s)
Adaptation, Psychological , Attitude to Health , Cognitive Behavioral Therapy , Kidney Failure, Chronic/psychology , Renal Dialysis/psychology , Self Concept , Adult , Analysis of Variance , Cognitive Behavioral Therapy/methods , Cognitive Behavioral Therapy/standards , Cognitive Dissonance , Depression/etiology , Depression/therapy , Female , Humans , Kidney Failure, Chronic/therapy , Male , Middle Aged , Prospective Studies , Sick Role , Treatment Outcome
17.
Ann Behav Med ; 21(1): 12-9, 1999.
Article in English | MEDLINE | ID: mdl-18425649

ABSTRACT

This study compared two interventions designed to decrease denial of sexually transmitted disease (STD)/human immunodeficiency virus (HIV) risk, to increase purchase of and intent to use condoms, and to decrease sexual risk behaviors. One hundred and fifty sexually active male and female undergraduates were assessed at baseline for denial of STD/HIV risk, knowledge of sexual risk behaviors, and self-reports of sexual behavior and were randomly assigned to either a motivational or cognitive intervention or a control condition. After the intervention, subjects were offered the opportunity to purchase condoms and were assessed for denial and intent to use condoms. Two months later, sexual risk behaviors and denial were measured. The motivational approach was most effective in reducing denial and in increasing intent to use condoms immediately following the intervention and in reducing sexual risk behaviors 2 months later.


Subject(s)
Cognitive Behavioral Therapy , Denial, Psychological , Motivation , Students/psychology , Unsafe Sex/prevention & control , Unsafe Sex/psychology , Adolescent , Adult , Awareness , Cognitive Dissonance , Condoms/statistics & numerical data , Female , Follow-Up Studies , HIV Infections/prevention & control , HIV Infections/psychology , Health Knowledge, Attitudes, Practice , Humans , Male , Peer Group , Sexually Transmitted Diseases/prevention & control , Sexually Transmitted Diseases/psychology , Social Desirability
18.
J Pers Soc Psychol ; 73(3): 560-73, 1997 Sep.
Article in English | MEDLINE | ID: mdl-9294902

ABSTRACT

Chronic illness places considerable burdens on patients and their interpersonal relations with families. In this study, patients' perceptions of family and medical staff expectations regarding responsibility for care and routine functions were examined. The authors hypothesized that a patient's perceived inability to meet others' expectations about coping with illness would lead to poorer adjustment. Forty-two chronically ill patients were assessed prospectively for perceptions of others' expectations, social support, and psychological adjustment. Findings confirmed that expectations predicted subsequent decreases in psychological adjustment over a 3-month period, even when social support was controlled. A test of the reverse hypothesis showed that poorly adjusted patients did not misperceive others' expectations. Theoretical interpretations of the findings and their relation to social support research are discussed.


Subject(s)
Chronic Disease/psychology , Interpersonal Relations , Social Adjustment , Social Support , Adult , Aged , Cross-Sectional Studies , Depression/psychology , Family/psychology , Female , Humans , Male , Middle Aged , Prospective Studies , Quality of Life , Surveys and Questionnaires
19.
Adv Perit Dial ; 13: 155-7, 1997.
Article in English | MEDLINE | ID: mdl-9360672

ABSTRACT

The purpose of our study was to determine whether albumin influenced patients' depression or whether depression influenced patients' albumin. Patients from a tertiary care university medical hospital were assessed for both serum albumin and depression [Beck Depression Inventory (BDI)] at two time points separated by 6 months. Data were collected for 72 patients (43 male, 29 female; mean age 54 years). The sample consisted of 32 hemodialysis and 40 peritoneal dialysis patients. The outcome measures were changes in depression and albumin over time. Regression analysis indicated that all three Time 1 measures of BDI, BDICOG (BDI cognitive), and BDISOM (BDI somatic) significantly predicted decreases in albumin from Time 1 to Time 2 (beta = -0.22, p < 0.002; beta = -0.17, p < 0.015; beta = -0.23, p < 0.002, respectively). However, Time 1 measures of albumin did not predict changes in BDI, BDICOG, or BDISOM (beta = -0.04, p < 0.738; beta = -0.08, p < 0.375; beta = -0.07, p < 0.618, respectively). Thus depression at Time 1 predicted decreases in albumin from Time 1 to Time 2. The reverse effect that albumin influences depression from Time 1 to Time 2 was not found. In conclusion, this study suggests that depression influences the nutritional status indicated by albumin levels. Thus poor nutritional status may mediate the relation between depression and mortality in end-stage renal disease (ESRD).


Subject(s)
Depression/blood , Kidney Failure, Chronic/blood , Serum Albumin/analysis , Adult , Aged , Aged, 80 and over , Depression/diagnosis , Depression/etiology , Female , Humans , Kidney Failure, Chronic/psychology , Kidney Failure, Chronic/therapy , Male , Middle Aged , Peritoneal Dialysis , Prospective Studies , Psychiatric Status Rating Scales , Renal Dialysis
20.
Ann Behav Med ; 19(4): 344-7, 1997.
Article in English | MEDLINE | ID: mdl-9706360

ABSTRACT

Excessive fluid intake in hemodialysis patients can lead to serious cardiovascular complications. However, previous studies have not investigated factors that affect fluid adherence over time. The influence of three sets of factors--attributions, health beliefs, and negative emotions--was examined to determine their influence on changes in fluid adherence over time. We assessed patient's fluid-intake changes across two time periods, as well as their absolute level. The results indicated that attributions, while predicting absolute fluid adherence, did not predict changes in fluid adherence. On the other hand, health beliefs predicted changes in fluid adherence but not absolute levels. Negative emotions predicted neither absolute nor changes in fluid adherence. It is suggested that attributions are more responsible for maintenance and control of stable fluid adherence levels, while health beliefs are more responsible for motivational factors related to changes in fluid adherence over time.


Subject(s)
Internal-External Control , Kidney Failure, Chronic/psychology , Patient Compliance/psychology , Renal Dialysis/psychology , Sick Role , Adult , Aged , Anger , Depression/psychology , Drinking , Female , Humans , Kidney Failure, Chronic/therapy , Male , Middle Aged , Motivation , Weight Gain
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