Orientation Of Transition Dipoles Research Articles

Polarization-resolved spectroscopy imaging of grain boundaries and optical excitations in crystalline organic thin films.

Exploration of optical properties of organic crystalline semiconductors thin films is challenging due to submicron grain sizes and the presence of numerous structural defects, disorder and grain boundaries. Here we report on the results of combined linear dichroism (LD)/ polarization-resolved photoluminescence (PL) scanning microscopy experiments that simultaneously probe the excitonic radiative recombination and the molecular ordering in solution-processed metal-free phthalocyanine crystalline thin films with macroscopic grain sizes. LD/PL images reveal the relative orientation of the singlet exciton transition dipoles at the grain boundaries and the presence of a localized electronic state that acts like a barrier for exciton diffusion across the grain boundary. We also show how this energy barrier can be entirely eliminated through the optimization of deposition parameters that results in films with large grain sizes and small-angle boundaries. These studies open an avenue for exploring the influence of long-range order on exciton diffusion and carrier transport.

Electrical behaviour of a silicone elastomer under simulated space environment

The electrical behavior of a space-used silicone elastomer was characterized using surface potential decay and dynamic dielectric spectroscopy techniques. In both cases, the dielectric manifestation of the glass transition (dipole orientation) and a charge transport phenomenon were observed. An unexpected linear increase of the surface potential with temperature was observed around Tg in thermally-stimulated potential decay experiments, due to molecular mobility limiting dipolar orientation in one hand, and 3D thermal expansion reducing the materials capacitance in the other hand. At higher temperatures, the charge transport process, believed to be thermally activated electron hopping with an activation energy of about 0.4 eV, was studied with and without the silica and iron oxide fillers present in the commercial material. These fillers were found to play a preponderant role in the low-frequency electrical conductivity of this silicone elastomer, probably through a Maxwell–Wagner–Sillars relaxation phenomenon.

Phosphorescent dye-based supramolecules for high-efficiency organic light-emitting diodes.

Organic light-emitting diodes (OLEDs) are among the most promising organic semiconductor devices. The recently reported external quantum efficiencies (EQEs) of 29-30% for green and blue phosphorescent OLEDs are considered to be near the limit for isotropically oriented iridium complexes. The preferred orientation of transition dipole moments has not been thoroughly considered for phosphorescent OLEDs because of the lack of an apparent driving force for a molecular arrangement in all but a few cases, even though horizontally oriented transition dipoles can result in efficiencies of over 30%. Here we use quantum chemical calculations to show that the preferred orientation of the transition dipole moments of heteroleptic iridium complexes (HICs) in OLEDs originates from the preferred direction of the HIC triplet transition dipole moments and the strong supramolecular arrangement within the co-host environment. We also demonstrate an unprecedentedly high EQE of 35.6% when using HICs with phosphorescent transition dipole moments oriented in the horizontal direction.

Distance and plasmon wavelength dependent fluorescence of molecules bound to silica-coated gold nanorods.

Plasmonic nanoparticles can strongly interact with adjacent fluorophores, resulting in plasmon-enhanced fluorescence or fluorescence quenching. This dipolar coupling is dependent upon nanoparticle composition, distance between the fluorophore and the plasmonic surface, the transition dipole orientation, and the degree of spectral overlap between the fluorophore's absorbance/emission and the surface plasmon band of the nanoparticles. In this work, we examine the distance and plasmon wavelength dependent fluorescence of an infrared dye ("IRDye") bound to silica-coated gold nanorods. Nanorods with plasmon band maxima ranging from 530 to 850 nm are synthesized and then coated with mesoporous silica shells 11-26 nm thick. IRDye is covalently attached to the nanoparticle surface via a click reaction. Steady-state fluorescence measurements demonstrate plasmon wavelength and silica shell thickness dependent fluorescence emission. Maximum fluorescence intensity, with approximately 10-fold enhancement, is observed with 17 nm shells when the nanorod plasmon maximum is resonant with IRDye absorption. Time-resolved photoluminescence reveals multiexponential decay and a sharp reduction in fluorescence lifetime with decreasing silica shell thickness and when the plasmon maximum is closer to IRDye absorption/emission. Control experiments are carried out to confirm that the observed changes in fluorescence are due to plasmonic interactions, is simply surface attachment. There is no change in fluorescence intensity or lifetime when IRDye is bound to mesoporous silica nanoparticles. In addition, IRDye loading is limited to maintain a distance between dye molecules on the surface to more than 9 nm, well above the Förster radius. This assures minimal dye-dye interactions on the surface of the nanoparticles.

Efficiency Enhancement of Organic Light‐Emitting Diodes Incorporating a Highly Oriented Thermally Activated Delayed Fluorescence Emitter

An organic light‐emitting diode (OLED) with the blue emitter CC2TA showing thermally activated delayed fluorescence (TADF) is presented exhibiting an external quantum efficiency () of 11% ± 1%, which clearly exceeds the classical limit for fluorescent OLEDs. The analysis of the emission layer by angular dependent photoluminescence (PL) measurements shows a very high degree of 92% horizontally oriented transition dipole moments. Excited states lifetime measurements of the prompt fluorescent component under PL excitation yield a radiative quantum efficiency of 55% of the emitting species. Thus, the radiative exciton fraction has to be significantly higher than 25% due to TADF. Performing a simulation based efficiency analysis for the OLED under investigation allows for a quantification of individual contributions to the efficiency increase originating from horizontal emitter orientation and TADF. Remarkably, the strong horizontal emitter orientation leads to a light‐outcoupling efficiency of more than 30%.

Strong anisotropic lifetime orientation distributions of a two-level quantum emitter around a plasmonic nanorod.

Spontaneous emission lifetime orientation distributions of a two-level quantum emitter in metallic nanorod structures are theoretically investigated by the rigorous electromagnetic Green function method. It was found that spontaneous emission lifetime strongly depended on the transition dipole orientation and the position of the emitter. The anisotropic factor defined as the ratio between the maximum and minimum values of the lifetimes along different dipole orientations can reach up to 103. It is much larger than those in dielectric structures which are only several times usually. Our results show that the localized plasmonic resonance effect provides a new degree of freedom to effectively control spontaneous emission by the dipole orientation of the quantum emitters.

A13T Mutation in the Regulatory Light Chain Associated with Cardiac Hypertrophy Imposes Differences in Kinetics of Healthy and Diseased Ventricles

The effect of A13T (alanine to threonine) hypertrophic cardiomyopathy mutation in the myosin regulatory light chain (RLC) was examined in working ex-vivo myofibrils from the hearts of transgenic (Tg) mice. Myofibrillar actin was labeled with a fluorescent dye. A small volume within the I-band (?1 fL), containing on average 3-4 actin molecules, was observed by confocal microscopy. The myofibrils were cross-linked with EDC [1-ethyl-3-[3-(dimethylamino) propyl] carbodiimide] to prevent shortening during muscle contraction. Working myosin cross-bridges cause actin to undergo the cyclic fluctuations of orientation, which were measured by recording the polarization of fluorescent light emitted by the actin-bound fluorophore. The autocorrelation function of fluctuations of polarized fluorescence contains information about the kinetics of motion, which were found to be very different for left vs. right ventricles. The center of distribution of orientations of transition dipoles during contraction were very different for the Wild Type (WT) and mutated (MUT) ventricles, but their skewness and kurtosis were the same. The distribution of orientations measured in contracting WT myofibrils could be fitted by at least two Gaussians reflecting a pre- and post power stroke states of the myosin cross-bridges. However, the distribution of MUT myofibrils showed only one Gaussian relationship suggesting that the hypertrophic phenotype associated with the A13T-RLC mutation might be characterized by a loss of the pre-power stroke state.

Studies of T4 Primosome DNA Unwinding by Single-Molecule Fluorescence-Detected Linear Dichroism

The replication machine (replisome) of the T4 bacteriophage is an important model system for studies of DNA synthesis in all higher organisms. The helicase-primase (primosome) complex is one of three major sub-assemblies of the replisome, whose function is to processively unwind double-stranded DNA to expose single-stranded daughter strands for template-directed DNA synthesis. The detailed mechanism by which the primosome complex assembles and performs duplex DNA unwinding is not understood. Recently, we developed and applied single-molecule fluorescence-detected linear dichroism (smFLD), which can detect fluctuations of the transition dipole orientation on microsecond-to-millisecond time scales. We have studied model DNA replication fork constructs with cyanine fluorophores rigidly incorporated at the fork junction to monitor 'breathing' of the backbone as it interacts with the weakly binding T4 hexameric helicase. Our recent results [1] indicate that the time scale and distribution of microsecond breathing fluctuations at the replication fork junction are heavily influenced by the presence of the weakly binding T4 helicase, and suggest that these motions are on the pathway to the formation of the fully assembled DNA-primosome complex. The possible significance of these findings for helicase mechanisms will be discussed.[1] Phelps, C.; Lee, W.; Jose, D.; von Hippel, P. H.; Marcus, A. H. “Single-Molecule FRET and Linear Dichroism Studies of DNA ‘Breathing’ and Helicase Binding at Replication Fork Junctions,” Proc. Nat. Acad. Sci. (in press, accepted July 26, 2013, doi/10.1073/ pnas.1314862110).

The dependence of exciton transport efficiency on spatial patterns of correlation within the spectral bath

Spatial correlations in spectral bath motions have been proposed to explain long-lived coherence in exciton transport. Systems of interest, ranging from photosynthetic complexes to organic photovoltaics, contain inhomogeneous environments. We consider the possibility that the degree of spatial correlation varies throughout an exciton transport system. We model exciton transport in the Fenna–Matthews–Olson complex (FMO), a photosynthetic light-harvesting complex. Although it remains unclear whether significant spatial correlations exist in FMO, its very high exciton transport efficiency makes it an interesting case for studies of exciton transport. We also simulate a highly symmetric ten-site model system. We use an extension of the environment-assisted quantum transport model to simulate transport, allowing the spatial correlation function to vary throughout the system. We demonstrate both via analysis and via simulation that exciton transport efficiency is most sensitive to changes in correlation between the site coupled to the trap and its neighboring sites. This asymmetry in sensitivity is highly robust and appears irrespective of changes in parameters such as transition dipole orientations and initial conditions. Our results suggest that in the design of exciton transport systems, efforts to increase efficiency by controlling spatial correlation should be focused on the region near the site of exciton trapping.

Hydration Dynamics of Aqueous Nitrate

Aqueous nitrate, NO3(-)(aq), was studied by 2D-IR, UV-IR, and UV-UV time-resolved spectroscopies in combination with molecular dynamics (MD) simulations with the purpose of determining the hydration dynamics around the anion. In water, the D3h symmetry of NO3(-) is broken, and the degeneracy of the asymmetric-stretch modes is lifted. This provides a very sensitive probe of the ion-water interactions. The 2D-IR measurements reveal excitation exchange between the two nondegenerate asymmetric-stretch vibrations on a 300-fs time scale concomitant with fast anisotropy decay of the diagonal-peak signals. The MD simulations show that this is caused by jumps of the transition dipole orientations related to fluctuations of the hydrogen bonds connecting the nitrate ion to the nearest water molecules. Reorientation of the ion, which is associated with the hydrogen-bond breaking, was monitored by time-resolved UV-IR and UV-UV spectroscopy, revealing a 2-ps time constant. These time scales are very similar to those reported for isotope-labeled water, suggesting that NO3(-)(aq) has a labile hydration shell.

Energy Transfer Studies in Binary Dye Solution Mixture of Coumarin 440 + Coumarin 540 and its LifetimeCalculations

The sensitized fluorescence emission from the bimixture [Coumarin 440 (donor) + Coumarin 540 (acceptor)] has been measured as a function of dye concentration using a fluorescence spectrophotometer. The concentration of Coumarin 440 (donor) is kept constant at 5×10-5M while the concentration of Coumarin 540 (acceptor) is varied. The spectroscopic parameters for this bimixture have been calculated from the concentration dependence of peak fluorescence intensity curves. Also the data obtained from the Horiba JobinYvon Time Resolved Fluorescence Spectrophotometer was analyzed for the determination of lifetimes at various emission peaks. A comparative study between theoretically calculated lifetimes and experimentally obtained lifetimes shows a good agreement. The results indicate that the energy transfer process between unlike molecules can be studied by lifetime measurements (which could also be determined from the fluorescence emission spectral studies). Many attempts have been made in studies aimed to improve dye lasers efficiency and to extend their spectral range of operation. Energy transfer dye lasers (ETDL) have generated considerable interest as they are useful in obtaining enhanced laser output, wide tuning range, reduced concentration and pump threshold requirements and ultra-short pulse width in contrast to their single dye counterpart [1-7]. Energy Transfer Dye Lasers using numerous donor-acceptor dye pairs have been reported by various investigators duringthe last five decades. Fluorescence energy transfer is the transfer of the excited state energy from a donor (D) to acceptor (A) [8–19]. This transferoccurs without the appearance of photon and is primarily a resultof dipole–dipole interaction between the donor and the acceptor.The rate of energy transfer depends upon the extent of overlapof the emission spectrum of the donor with the absorptionspectrum of the acceptor, the relative orientation of the donorand acceptor transition dipoles and the distance between thesemolecules. The non-radiative energy transfer occurs as a result ofdipole–dipole coupling between the donor and the acceptor,anddoes not involve the emission and reabsorption of photons. In contrast to these trivial factorsnonradiative energy transfer depends upon the molecular detailsof donor–acceptor pairs.Non-radiative energy transfer is effective overdistance ranging of 50A°.The intervening of solvent or other macromoleculeshas little effect on the efficiency of the energy transfer, whichdepends primarily on the D–A distances[18].Dye lasers have some limitations as the dye solution used as an active medium absorbs energy from the excitation source in a very limited range and so the emission band also has these limitations. If a dye laser has to be used as an ideal source its spectral region needs to be extended. In order to extend the spectral region of operation mixtures of different dye solutions/dye molecules embedded in solid matrices arebeingused.The use of such energy transfer in dye lasers is also helpful in minimizing the photoquenching effects and thereby, increasing the laser efficiency. In the present studies the energy transfer mechanism has been investigated in the ethanol solution mixture, i.e. Coumarin 440 and Coumarin 540 from their absorption and emission spectra. The dependence of lifetimes of the dye molecules on their

Quantification of Förster resonance energy transfer by monitoring sensitized emission in living plant cells

Förster resonance energy transfer (FRET) describes excitation energy exchange between two adjacent molecules typically in distances ranging from 2 to 10 nm. The process depends on dipole-dipole coupling of the molecules and its probability of occurrence cannot be proven directly. Mostly, fluorescence is employed for quantification as it represents a concurring process of relaxation of the excited singlet state S1 so that the probability of fluorescence decreases as the probability of FRET increases. This reflects closer proximity of the molecules or an orientation of donor and acceptor transition dipoles that facilitates FRET. Monitoring sensitized emission by 3-Filter-FRET allows for fast image acquisition and is suitable for quantifying FRET in dynamic systems such as living cells. In recent years, several calibration protocols were established to overcome to previous difficulties in measuring FRET-efficiencies. Thus, we can now obtain by 3-filter FRET FRET-efficiencies that are comparable to results from sophisticated fluorescence lifetime measurements. With the discovery of fluorescent proteins and their improvement toward spectral variants and usability in plant cells, the tool box for in vivo FRET-analyses in plant cells was provided and FRET became applicable for the in vivo detection of protein-protein interactions and for monitoring conformational dynamics. The latter opened the door toward a multitude of FRET-sensors such as the widely applied Ca2+-sensor Cameleon. Recently, FRET-couples of two fluorescent proteins were supplemented by additional fluorescent proteins toward FRET-cascades in order to monitor more complex arrangements. Novel FRET-couples involving switchable fluorescent proteins promise to increase the utility of FRET through combination with photoactivation-based super-resolution microscopy.

Controlled transition dipole alignment of energy donor and energy acceptor molecules in doped organic crystals, and the effect on intermolecular Förster energy transfer

The orientation factor κ(2) ranging from 0 to 4, which depends on the relative orientation of the transition dipoles of the energy donor (D) and the energy acceptor (A) in space, is one of the pivotal factors deciding the efficiency and directionality of resonance energy transfer (RET) in a D-A molecular system. In this work, tetracene (Tc) and pentacene (Pc) are successfully doped in a trans-1,4-distyrylbenzene (DSB) crystalline lattice to form definite D-A mutually perpendicular transition dipole orientations. The cross D-A dipole arrangement results in an extremely small orientation factor, which is about two orders smaller than that in the disordered films. The energy transfer properties from the host (DSB) to the guest (Tc/Pc) were investigated in detail by steady-state as well as time-resolved fluorescence spectroscopy. Our experimental research results show that the small value of κ(2) allows less or partial energy transfer from the host (DSB) to the guest (Tc) in a wide range of guest concentration, with the Förster distance of around 1.5 nm. By controlling the doping concentrations in the Tc and Pc doubly doped DSB crystals, we demonstrate, as an example, for the first time the application of the restricted energy transfer by D-A cross transition dipole arrangement for preparation of a large-size, white-emissive organic crystal with the CIE coordinates of (0.36, 0.37) approaching an ideal white light. In contrast, Tc is also doped in an anthracene crystalline lattice to form head-to-tail D-A transition dipole alignment, which is proved to be highly effective to promote the intermolecular energy transfer. In this doped system, the orientation factor is relatively large and the Förster distance is around 7 nm.

Effect of Thermal Cycling on the Ferroelectric Characteristics of Vinylidene Fluoride-Trifluoroethylene Copolymer Thin Films

Thin films of polyvinylidene (PVDF) and its copolymers containing 50 to 30 mole % of trifluoroethylene (TrFE) demonstrate rapid reversal of polarization under applied electric field, and can be used as ferroelectric materials in non-volatile memory devices. In this study, Fourier-transform infrared transmission (FTIR-TS) and grazing incident reflection absorption (FTIR-GIRAS) spectroscopy techniques were used to examine the changes in Curie transition temperature (Tc), chain and dipole orientation in selected VDF copolymer spun-cast films as a subjected to a reversible thermal-cycling treatment. A reversible solid-to-solid β-crystalline phase transition with thermal hysteresis and a favorable C-F dipole orientation along the electric field direction were observed for the sample subjected to thermal-cycling below its melting point (Tm). An irreversible β-crystalline phase transition with unfavorable C-F dipole orientation was observed for the sample treated to heating-cooling cycle above Tm. The results are reported in detail here.

Determining emissive dipole orientation in organic light emitting devices by decay time measurement

We present a method to detect anisotropy in the distribution of the transition dipole moment in organic light emitting diodes (OLEDs). The method is based on the dependency of the exciton decay rate on the optical environment and the orientation of the dipole transition moment, also called the Purcell effect. We use this method to demonstrate a preferential orientation of the small molecule emitter Ir(MDQ)2(acac) in a TPBi matrix. The outcoupling improvement for OLEDs that could be obtained with perfectly oriented transition dipoles is estimated by simulation. For perfectly planar structures this shows an EQE in air of up to 34%.

Inhibited Spontaneous Emission of Quantum Dots Observed in a 3D Photonic Band Gap

We present time-resolved emission experiments of semiconductor quantum dots in silicon 3D inverse-woodpile photonic band gap crystals. A systematic study is made of crystals with a range of pore radii to tune the band gap relative to the emission frequency. The decay rates averaged over all dipole orientations are inhibited by a factor of 10 in the photonic band gap and enhanced up to 2× outside the gap, in agreement with theory. We discuss the effects of spatial inhomogeneity, nonradiative decay, and transition dipole orientations on the observed inhibition in the band gap.

Anharmonic Vibrational Modes of Nucleic Acid Bases Revealed by 2D IR Spectroscopy

Polarization-dependent two-dimensional infrared (2D IR) spectra of the purine and pyrimadine base vibrations of five nucleotide monophosphates (NMPs) were acquired in D(2)O at neutral pH in the frequency range 1500-1700 cm(-1). The distinctive cross-peaks between the ring deformations and carbonyl stretches of NMPs indicate that these vibrational modes are highly coupled, in contrast with the traditional peak assignment, which is based on a simple local mode picture such as C═O, C═N, and C═C double bond stretches. A model of multiple anharmonically coupled oscillators was employed to characterize the transition energies, vibrational anharmonicities and couplings, and transition dipole strengths and orientations. No simple or intuitive structural correlations are found to readily assign the spectral features, except in the case of guanine and cytosine, which contain a single local CO stretching mode. To help interpret the nature of these vibrational modes, we performed density functional theory (DFT) calculations and found that multiple ring vibrations are coupled and delocalized over the purine and pyrimidine rings. Generally, there is close correspondence between the experimental and computational results, provided that the DFT calculations include explicit waters solvating hydrogen-bonding sites. These results provide direct experimental evidence of the delocalized nature of the nucleotide base vibrations via a nonperturbative fashion and will serve as building blocks for constructing a structure-based model of DNA and RNA vibrational spectroscopy.

Control of orientation of rhodamine 6G in organic–inorganic hybrid film by capillary electrophoresis doping technique

Rhodamine 6G (R6G) molecules were fixed with oriented transition dipole moment in organic–inorganic hybrid film by capillary electrophoresis doping (CED) technique and their orientation distribution was analyzed quantitatively by using single-molecule fluorescence defocused imaging. The CED technique forces R6G molecules to migrate into the hybrid film with the dipole moments aligned parallel to the electric field of CED. An appropriate pre-baking of the film enables stabilization of the oriented molecules inside the film; the doped molecules have the direction of the transition dipole moment parallel with the film plane.

Linear dichroism amplification: Adapting a long-known technique for ultrasensitive femtosecond IR spectroscopy

We demonstrate strong amplification of polarization-sensitive transient IR signals using a pseudo-null crossed polarizer technique first proposed by Keston and Lospalluto [Fed. Proc. 10, 207 (1951)] and applied for nanosecond flash photolysis in the visible by Che et al. [Chem. Phys. Lett. 224, 145 (1994)]. We adapted the technique to ultrafast pulsed laser spectroscopy in the infrared using photoelastic modulators, which allow us to measure amplified linear dichroism at kilohertz repetition rates. The method was applied to a photoswitch of the N-alkylated Schiff base family in order to demonstrate its potential of strongly enhancing sensitivity and signal to noise in ultrafast transient IR experiments, to simplify spectra and to determine intramolecular transition dipole orientations.

Circular Dichroism of a Finite Number of Identical Chromophores in a Helical Arrangement

Compact expressions to calculate the transition energies, absorption line strengths, and rotational line strengths of circular dichroism for the excitonic states in a helical arrangement of N identical chromophores are presented. The absorption spectrum A(ν) and the CD spectrum C(ν) are given in terms of the same function G(ν,α) as A(ν) = a(1)G(ν,0) + a(2)G(ν,α), C(ν) = (s(1) - s(2))G(ν,α) + s(3)(d/dα)G(ν,α). The function G(ν,α) depends only on the helical angle α and the number N of interacting chromophores. An analytical expression can be given when only next-neighbor interactions are considered. All other structural parameters of the system (e.g. orientation of transition dipoles and the translation vector of the helix) enter only into the prefactors a(j) and s(j).