Triplet Population Research Articles

Modelling of organic magnetoresistance as a function of temperature using the triplet polaron interaction

The organic magnetoresistance (OMR) of 50 nm aluminium tris(8-hydroxyquinoline) organic light emitting diodes (OLEDs) has been measured over a range of temperatures and operating voltages. The OMR data for this device, the current change through the device with applied field, have been fitted using the triplet polaron interaction (TPI) model that includes two independent processes, namely the exciton trapping and the triplet polaron interaction. Two Lorentzian functions were used to fit the data, and the prefactors for the two processes were found to scale linearly with the triplet population. Over the whole temperature range the data appears to fall on a single line which covers nearly six orders of magnitude of exciton concentration. This work demonstrates that the magnitude and shape of the OMR can be predicted and will be useful for understanding the fundamental mechanism behind the OMR.

Determining the influence of excited states on current transport in organic light emitting diodes using magnetic field perturbation

The perturbation of the current transport in aluminum tris(8-hydroxyquinoline) $({\text{Alq}}_{3})$-based organic light emitting diodes has been investigated as a function of magnetic field. The change in current, or organic magnetoresistance, with applied field has been fitted using two Lorentzian functions corresponding to polaron trapping by triplets and the interaction between polarons and triplets as suggested in the triplet polaron interaction model. The model has been applied to a number of devices with ${\text{Alq}}_{3}$ thicknesses from 50 to 90 nm and with two different cathodes. In all cases the data could be fitted using just these two processes, the prefactors for which were found to scale linearly with the triplet population over 6 orders of magnitude. This work demonstrates that the magnitude and shape of the organic magnetoresistance can be predicted and illustrates the importance of magnetic field measurements as a tool for understanding the processes affecting current transport in organic devices.

Singlet–triplet quenching in high intensity fluorescent organic light emitting diodes

Electroluminescence (EL) transient turn-on peaks with more than twice the brightness of the steady-state EL are observed in fluorescent organic light emitting diodes with a doped emission layer. By modeling both the singlet and triplet population dynamics, we identify the EL drop after the peak as due to singlet–triplet (ST) quenching. The peak recovery following a double voltage pulse sequence is explained in terms of triplet population relaxation through triplet–triplet quenching. A mechanism of using phosphors in the emission region is proposed for reducing ST quenching.

Physical and chemical problems of the creation of photostable converters of light energy on the basis of dyed polymers

The main factors affecting the degradation of dyed polymeric matrices by the action of light (conversion of light energy into thermal energy, electron phototransfer, population of triplets, aggregation of dyes, oxidation–reduction processes, interaction with oxygen, including singlet) are analyzed and classified. Their dependence on the structure of the dyes, the nature of the polymer, and the method of dyeing of the latter is examined. Criteria for the creation of lightfast dyed matrices are formulated. Special attention is directed to contemporary light-sensitive materials in which the dye is covalently bonded to the polymer and to organic–inorganic compositions based on them.

Theory of the Field-Induced BEC in the Frustrated Spin-12Dimer CompoundBaCuSi2O6

Building on recent neutron and NMR experiments, we investigate the field-induced exotic criticality observed in the frustrated compound BaCuSi2O6 using a frustrated model with two types of bilayers. A semiclassical treatment of the effective hard-core boson model shows that perfect interlayer frustration leads to a 2D-like critical exponent varphi=1 without logarithmic corrections and to a 3D low temperature phase with different but nonvanishing triplet populations in both types of bilayers. These further suggest a simple phenomenology in terms of a field-dependent transverse coupling in the context of which we reproduce the entire field-temperature phase diagram with quantum Monte Carlo simulations.

The magnetic field effect on the transport and efficiency of group III tris(8-hydroxyquinoline) organic light emitting diodes

Magnetoresistance and efficiency measurements of organic light emitting diode structures based on the group III hydroxyquinolates (Mq3) have been made as a function of magnetic field and Mq3 thickness, where M=Al, Ga, and In. For all quinolates, independent of thickness, we observed very similar behavior for the efficiency of the devices, with large increases in efficiency occurring at low values of applied field, which rapidly saturate as the field is increased. The current through these devices is found to be a strong function of both the device thickness and the metal ion. For Alq3 based devices, the current changes appear to correlate strongly with the triplet population in the devices. For Gaq3 and Inq3 devices, the magnetoresistance is found not to correlate with the triplet concentration and this may be evidence that there is little energetic barrier for carrier trapping in these materials. For all materials, a further dependence of the magnetoresistance on applied field was observed, which needs closer investigation.

Modulated Fluorescence Correlation Spectroscopy with Complete Time Range Information

Two methods to combine fluorescence correlation spectroscopy (FCS) with modulated excitation, in a way that allows extraction of correlation data for all correlation times have been developed and experimentally verified. One method extracts distortion-free correlation data from measurements acquired with standard hardware correlators provided the fluorescence does not change systematically within the excitation pulses. This restriction does not apply to the second method, which, however, requires time-resolved acquisition of the fluorescence intensity. Modulation of the excitation in an FCS experiment is demonstrated to suppress triplet population buildup more efficiently than a corresponding reduction in continuous wave excitation intensity (shown for the dye rhodamine 6G in aqueous solution). Excitation modulation thus offers an additional means to optimize the FCS measurement conditions with respect to the photophysical properties of the dyes used. This possibility to suppress photoinduced states also provides a useful tool to distinguish additional processes occurring in the same time regime in the FCS measurements, as demonstrated here for the protonation kinetics of fluorescein at different pH. In general, the proposed concept opens for FCS measurements with a complete correlation timescale in a range of applications where a modulated excitation is either necessary or brings specific advantages.

The maternal and fetal outcome of 122 triplet pregnancies

The wide use of infertility drugs and assisted reproduction has resulted in 4- to 5-fold increase in the incidence of triplet pregnancies, which carry an extremely high risk of maternal complications and adverse perinatal outcome. In Hungary, reduction of multifetal pregnancies is available for all pregnant women with multifetal gestation since 1998. The goal of the procedure is to ensure better outcome for surviving fetuses. Counseling of pregnant patients should include the maternal and fetal risks of triplet gestation without multifetal pregnancy reduction. To assess the risk of maternal complications, stillbirth, perinatal and neonatal mortality rates, and risk of neonatal morbidity in non-reduced triplets in a large case series, representing the Hungarian triplet population. The study population consisted of triplets delivered between July 1st, 1990 and June 30th, 2006, at the 1st Department of Obstetrics and Gynecology. All three fetuses had to be alive on the 18th-week ultrasound scan to be eligible. Out of the 122 cases, 8 (6.6%) ended in midtrimester miscarriage, 114 (93.4%) ended in delivery. There were no maternal deaths. The most common antepartum maternal complications were pregnancy-induced hypertension (16.7%), gestational diabetes mellitus (18.4%), thrombocytopenia (20.2%), anemia (16.7%) and intrahepatic cholestasis (9.7%). Preterm labor requiring tocolysis occurred in 57.9%, preterm premature rupture of membranes in 32.5%. Prophylactic cerclage was performed in 15.8% of cases, and 69.3% of patients received steroid prophylaxis. The mean gestational age at delivery was 32.3 +/- 3.2 weeks. The rates of very early (<28 weeks) and early (<32 weeks) preterm deliveries were 8.8% and 42.1%, respectively. The mean 5-minute Apgar score was 9.2 +/- 0.8, and the mean birth weight at delivery was 1664 +/- 506 g. 38.0% of infants were very low birth weight (<1500 g). Stillbirth, crude perinatal mortality and corrected perinatal mortality rates were 23.4 per thousand, 64.3 per thousand and 27.4 per thousand, respectively. 11.7 per thousand of infants had some major congenital anomaly. 54.4% of infants required ventilation or oxygen therapy or both. The most common neonatal complication were respiratory distress (17.1%), transitory tachypnea (5.2%), sepsis or pneumonia (25.5%), intraventricular hemorrhage (4.3%) and jaundice (11.4%). Both the maternal and neonatal risks should be considered when patients with triplets are counseled before the decision to continue the triplet gestation or to choose multifetal pregnancy reduction is made.

Purely Electronic and Vibronic Fluorescence Excitation of Single Terrylene Molecules in a Naphthalene Crystal

Fluorescence excitation spectra of spatially resolved single terrylene molecules in a naphthalene crystal were studied at 5 K as a function of exciting laser light intensity. The fully saturated fluorescence detection rate for purely electronic transition (0,0) was found to be about twice smaller than that for the two main vibronic components located 239 and 251 cm−1 above. This relation is fulfilled when the triplet population rate k23 is much weaker than the depopulation rate k31. We propose that the weak fluorescence excitation lines observed at the frequencies of 38, 67, 97, and 145 cm−1 correspond to nontotally symmetric low-frequency vibration modes of terrylene.

Maternal age as a risk factor in triplet pregnancy

Upon the diagnosis of triplet conception, pregnant women should be counseled about the risk of triplet pregnancy and the chances of survival of triplet newborns. This information can help women with triplet gestations to make well-informed decisions whether to continue their pregnancy or to undergo multifetal pregnancy reduction. To assess the maternal age as a risk factor in a large representative sample of the Hungarian triplet population with evaluation of maternal complications and perinatal results. In a retrospective cohort study we analyzed 114 triplet pregnancies which ended in delivery from July 1st, 1990 until June 30th, 2006. In the study population there were 15 patients aged 35 years or more at delivery (13.2%) and 99 women were younger than 35 years (86.8%). The rate of maternal complications was similar in the two groups, and there was no significant difference in gestational age at delivery (33.0 +/- 2.9 vs. 32.2 +/- 3.3 weeks), very early (< 28 weeks) premature deliveries (6.7% vs. 9.1%) and early (< 32 weeks) premature deliveries (32.9% vs. 43.4%), respectively, although all of these variables were more favorable over 35 years of age. The mean birth weight of live born infants (1796 +/- 492 vs. 1664 +/- 506 g, p = 0.064 g) was higher, the prevalence of very-very-low-birth-weight (< 1000 g) infants (6.8% vs. 10.7%) and very-low-birth-weight (< 1500 g) infants (34.1% vs. 38.6%) was lower in the groups of older patients. The 1- and 5-minute Apgar scores of newborns in the > 35-year group were significantly larger (8.4 +/- 0.5 vs. 8.0 +/- 1.0, p = 0.016 and 9.5 +/- 0.7 vs. 9.2 +/- 0.8, p = 0.006, respectively). There was no difference in perinatal results or puerperal complications between the groups. The incidence of sepsis/pneumonia and need for ventilation/oxygen therapy was lower among the infants of the patients over 35 years (6.9% vs. 28.6%, p = 0,011 and 31.0% vs. 58.2%, p = 0,011, respectively). The rate of other neonatal complications was similar in the two groups. Although the incidence of some maternal complications during pregnancy and in the puerperal period is higher at older age, the favorable perinatal results and neonatal morbidity rates make older maternal age a risk reducing (protective) factor in triplet pregnancies.

Photodissociation of F 2 in solid Ar: Electronic state distribution in cage-exit

The role of different electronic states in cage-exit of F atoms for F 2 photodissociation in solid Ar is investigated by nonadiabatic molecular dynamics simulations, using Tully’s “Surface-Hopping” approach. The 36 potential surfaces involved are treated by the diatomics-in-molecules (DIM) method. A simulation of 255 Ar atoms in an FCC structure with periodic boundary conditions is used. The results are: (I) Direct cage-exit events are not observed. At least two collisions of the F atoms with the Ar cage are necessary for cage-exit. (II) The total singlet to triplet population ratio at the instant of cage-exit is approximately the statistical one. (III) Some cage-exit events occur for the attractive states 1 Σ g + , 3Π u but their role is much smaller than for the caged species. (IV) The Λ quantum number distribution at cage-exit is more uniform than for the caged species. Thus, the electronic state distribution for cage-exit differs greatly from that of the caged species.

Strategies to Improve Photostabilities in Ultrasensitive Fluorescence Spectroscopy

Given the particular importance of dye photostability for single-molecule and fluorescence fluctuation spectroscopy investigations, refined strategies were explored for how to chemically retard dye photobleaching. These strategies will be useful for fluorescence correlation spectroscopy (FCS), fluorescence-based confocal single-molecule detection (SMD) and related techniques. In particular, the effects on the addition of two main categories of antifading compounds, antioxidants (n-propyl gallate, nPG, ascorbic acid, AA) and triplet state quenchers (mercaptoethylamine, MEA, cyclo-octatetraene, COT), were investigated, and the relevant rate parameters involved were determined for the dye Rhodamine 6G. Addition of each of the compound categories resulted in significant improvements in the fluorescence brightness of the monitored fluorescent molecules in FCS measurements. For antioxidants, we identify the balance between reduction of photoionized fluorophores on the one hand and that of intact fluorophores on the other as an important guideline for what concentrations to be added for optimal fluorescence generation in FCS and SMD experiments. For nPG/AA, this optimal concentration was found to be in the lower micromolar range, which is considerably less than what has previously been suggested. Also, for MEA, which is a compound known as a triplet state quencher, it is eventually its antioxidative properties and the balance between reduction of fluorophore cation radicals and that of intact fluorophores that defines the optimal added concentration. Interestingly, in this optimal concentration range the triplet state quenching is still far from sufficient to fully minimize the triplet populations. We identify photoionization as the main mechanism of photobleaching within typical transit times of fluorescent molecules through the detection volume in a confocal FCS or SMD instrument (<1-20 ms), and demonstrate its generation via both one- and multistep excitation processes. Apart from reflecting a major pathway for photobleaching, our results also suggest the exploitation of the photoinduced ionization and the subsequent reduction by antioxidants for biomolecular monitoring purposes and as a possible switching mechanism with applications in high-resolution microscopy.

Chlorophyll triplet states associated with Photosystem I and Photosystem II in thylakoids of the green alga Chlamydomonas reinhardtii

The analysis of FDMR spectra, recorded at multiple emission wavelengths, by a global decomposition technique, has allowed us to characterise the triplet populations associated with Photosystem I and Photosystem II of thylakoids in the green alga Chlamydomonas reinhardtii. Three triplet populations are observed at fluorescence emissions characteristic of Photosystem II, and their zero field splitting parameters have been determined. These are similar to the zero field parameters for the three Photosystem II triplets previously reported for spinach thylakoids, suggesting that they have a widespread occurrence in nature. None of these triplets have the zero field splitting parameters characteristic of the Photosystem II recombination triplet observed only under reducing conditions. Because these triplets are generated under non-reducing redox conditions, when the recombination triplet is undetectable, it is suggested that they may be involved in the photoinhibition of Photosystem II. At emission wavelengths characteristic of Photosystem I, three triplet populations are observed, two of which are attributed to the P 700 recombination triplet frozen in two different conformations, based on the microwave-induced fluorescence emission spectra and the triplet minus singlet difference spectra. The third triplet population detected at Photosystem I emission wavelengths, which was previously unresolved, is proposed to originate from the antenna chlorophyll of the core or the unusually blue-shifted outer antenna complexes of this organism.

Solvent-Dependent Photophysics of 1-Cyclohexyluracil: Ultrafast Branching in the Initial Bright State Leads Nonradiatively to the Electronic Ground State and a Long-Lived 1nπ* State

The modified nucleic acid base, 1-cyclohexyluracil, was studied by femtosecond transient absorption spectroscopy in protic and aprotic solvents of varying polarity. UV excitation at 267 nm populates the lowest-energy bright state, a (1)pipi* state, which has a lifetime of 120-270 fs, depending on the solvent. In all solvents, this initial bright state population bifurcates with approximately 60% undergoing subpicosecond nonradiative decay to the electronic ground state and the remaining population branching to a singlet dark state. The latter absorbs between 340 and 450 nm. The latter state is assigned to the lowest-energy (1)npi* state. It decays to the electronic ground state with a lifetime that varies from 26 ps in water to at least several nanoseconds in aprotic solvents. The results suggest that the two nonradiative decay pathways identified for photoexcited uracil in recent quantum chemical calculations (Matsika, S. J. Phys. Chem. A. 2004, 108, 7584) are simultaneously operative in a wide variety of solvent environments. The lowest-energy triplet state was also detected by transient absorption. The triplet population appears in a few picoseconds and is not formed from the thermalized (1)npi* state. It is suggested that high spin-orbit coupling is found only along initial segments of the nonradiative decay pathways. Efficient intersystem crossing prior to vibrational cooling offers a possible explanation for the wavelength-dependent triplet yields seen in single DNA bases.

Transient analysis of triplet exciton dynamics in amorphous organic semiconductor thin films

We study triplet exciton diffusion in the archetype organic material 4,4′-bis(N-carbazolyl)biphenyl (CBP) commonly used as a conductive host in the emissive zone of organic light emitting devices. Using time-resolved spectral decay ensuing from the diffusion of an initially localized triplet population to a spatially separated phosphor doped region, we model the delayed fluorescence and phosphorescence decays based on non-dispersive triplet transport. Fits to the model yield a diffusion coefficient of D = (1.4 ± 0.3) × 10−8 cm2/s, and a triplet–triplet bimolecular quenching rate constant of KTT = (1.6 ± 0.4) × 10−14 cm3/s. The results are extended by doping a wide energy-gap molecule into CBP that serves to frustrate triplet transport, lowering both the diffusion coefficient and annihilation rate. These results are used to model a recently demonstrated white organic light emitting device that depends on triplet diffusion in CBP to excite spatially separate fluorescent and phosphorescent doped regions of the emissive layer. We determine the extent to which diffusion contributes to light emission in this structure, and predict its performance based on ideal lumophores with unity quantum yield.

Triplet-State Conformational Changes in 15-cis-Spheroidene Bound to the Reaction Center fromRhodobacter sphaeroides2.4.1 as Revealed by Time-Resolved EPR Spectroscopy: Strengthened Hypothetical Mechanism of Triplet-Energy Dissipation

Time-resolved EPR spectra of 15-cis-spheroidene bound to the reaction center from Rhodobacter sphaeroides 2.4.1 were recorded at low temperatures. (1) A three-component analysis of the spectral-data matrices by singular-value decomposition followed by global fitting identified the transformation of the triplet carotenoid, (3)Car(I) --> (3)Car(II); during this process, the leak of the triplet population was suggested. A four-component analysis suggested the presence of a representative intermediate, (3)Car(R), that forms a leak channel of the triplet population. (2) A theoretical calculation of the zero-field splitting parameters, |D| and |E|, by the use of a polyene model, showed that the transformation, (3)Car(I) --> (3)Car(R) --> (3)Car(II), accompanies the conformational changes of (0 degrees , 0 degrees , 0 degrees ) --> (+20 degrees , -20 degrees , +20 degrees ) --> (+45 degrees , -40 degrees , +40 degrees ) around the central cis C15=C15', trans C13=C14, and trans C11=C12 bonds, respectively. (3) The initial, rapid decrease followed by the inversion of spin polarization along the z axis of (3)Car was observed, which was correlated with a change in the spin angular momentum. (4) In reference to the binding pocket of the Car, determined by X-ray crystallography, the conformational changes were ascribed to the intrinsic isomerization property of 15-cis (3)Car as well as the Car-peptide intermolecular interaction; a detailed picture was proposed. All of the above results support the mechanism of triplet-energy dissipation proposed previously: the rotational motions around the central double bonds cause a change in the orbital angular momentum and, through the spin-orbit coupling, a change in the spin angular momentum, which enhances the T(1) --> S(0) intersystem crossing dissipating the triplet energy.

Singlet and Triplet Products of the Geminate Recombination of a Radical Pair with a Single Magnetic Nucleus (I = 1/2)

The double-channel recombination and separation of the photochemically created singlet radical pair is investigated, taking into account the spin conversion in a zero magnetic field and the arbitrary initial distance between the radicals. The quantum yields of the singlet and triplet products and the free radicals production are found analytically, assuming that the recombination of the diffusing radicals occurs at contact. All the yields are related to the singlet and triplet populations of the recombining radical pair, subjected to spin conversion and contact exchange interaction. The general analytical expressions for the quantum yields are specified for the particular limits of the weak and strong exchange. They are greatly simplified in the case of polar solvents, especially at the contact start. A close similarity is obtained with the results of a previously developed incoherent model of spin conversion, provided that the conversion rate is appropriately related to the hyperfine coupling constant.

A Fluorescence Detected Magnetic Resonance Investigation of the Carotenoid Triplet States Associated with Photosystem II of Isolated Spinach Thylakoid Membranes

The carotenoid triplet populations associated with the fluorescence emission chlorophyll forms of Photosystem II have been investigated in isolated spinach thylakoid membranes by means of fluorescence detected magnetic resonance in zero field (FDMR). The spectra collected in the 680-690 nm emission range, have been fitted by a global analysis procedure. At least five different carotenoid triplet states coupled to the terminal emitting chlorophyll forms of PS II, peaking at 682 nm, 687 nm and 692 nm, have been characterised. The triplets associated with the outer antenna emission forms, at 682 nm, have zero field splitting parameters |D| = 0.0385 cm-1, |E| = 0.00367 cm-1; |D| = 0.0404 cm-1, |E| = 0.00379 cm-1 and |D| = 0.0386 cm-1, |E| = 0.00406 cm-1 which are very similar to those previously reported for the xanthophylls of the isolated LHC II complex. Therefore the FDMR spectra recorded in this work provide insights into the organisation of the LHC II complex in the unperturbed environment represented by thylakoid membranes. The additional carotenoid triplet populations, detected by monitoring the chlorophyll emission at 687 and 692 nm, are assigned to carotenoids bound to inner antenna complexes and hence attributed to beta-carotene molecules.

Triplet population and depopulation rates of single terrylene molecules in p-terphenyl crystal

Fluorescence intensity correlation functions of single terrylene molecules in p-terphenyl crystals were measured within the temperature range 5–300 K. The analysis provided the population ( k 23) and depopulation ( k 31) intersystem crossing (ISC) rate constants of the lowest excited triplet state T 1. These rates increase continuously with temperature, without any noticeable change at the 193 K, temperature of the order–disorder phase transition in p-terphenyl crystal. The depopulation channel composed of the exciting light intensity dependent T 1 → T n absorption followed by the T n → S 1 ISC is important at higher temperatures.

Two-step field-induced singlet dissociation in a fluorene trimer

We time resolve the event of singlet separation into free carriers in an oligofluorene using femtosecond field assisted pump-probe technique. Steady state photoinduced absorption (PA) measurements have also been employed to identify triplet absorption $({T}_{1}\ensuremath{-}{T}_{n})$ in the spectral region of $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. As a result of an applied electric field of $2.2\phantom{\rule{0.3em}{0ex}}\mathrm{MV}∕\mathrm{cm}$ singlets are observed to undergo a dissociation process consisting of first singlet splitting into bound polaron pairs and second further dissociation of polaron pairs into free carriers. Subsequent free carrier recombination yields a new population of singlets and triplets. Our results suggest that nongeminate recombination leads to the formation of 35% singlets and 65% triplets.