Picosecond measurements of exciton migration in tetracene crystals doped with pentacene

The kinetics of electronic energy transfer from host to guest have been measured for a tetracene crystal doped with pentacene. With single picosecond pulse excitation and streak camera diagnostics, the fluorescence risetime of the guest is measured to be the same as the decay time of the host. For low pentacene concentrations the exponential decay of the host at 170 K is consistent with a diffusion model for singlet migration from host to guest. At high intensities, no evidence was found for guest saturation because of the dominating effect of bimolecular exciton annihilation in the host.     

Models for intramolecular triplet exciton migration applied to vinyl aromatic polymers

Models for intramolecular triplet exciton migration have been developed for vinyl aromatic polymers. A one-dimensional model which allows only neighbor-to-neighbor migrations yields frequencies which are several orders of magnitude smaller than those predicted either by exchange or dipole-dipole mechanisms. An intramolecular model permitting three translational degrees of freedom predicts triplet exciton hop frequencies on the order of 10⁴ s⁻ in reasonable agreement with either exchange or dipole-dipole mechanisms.     

Conjugation enhancement of intramolecular exciton migration in poly(p-phenylene ethynylene)s

Efficient energy migration in conjugated polymers is critical to their performance in photovoltaic, display, and sensor devices. The ability to precisely control the polymer conformation is a key issue for the experimental investigations and deeper understanding of the nature of this process. We make use of specially designed iptycene-containing poly(p-phenylene ethynylene)s that display chain-extended conformations when dissolved in nematic liquid crystalline solvents. In these solutions, the polymers show a substantial enhancement in the intrachain exciton migration rate, which is attributed to their increased conjugation length and better alignment. The organizational enhancement of the energy transfer efficiency, as determined by site-selective emission from lower energy traps at the polymer termini, is accompanied by a significant increase of the fluorescence quantum yield. The liquid crystalline phase is a necessary requirement for these phenomena to occur, and when the temperature was increased above the nematic-isotropic transition, we observed a dramatic reduction of the energy transfer efficiency and fluorescence quantum yield. The ability to improve the exciton migration efficiency through precise control of the polymer structure with liquid crystalline solutions demonstrates the importance of a polymer's conformation for energy transfer, and provides a way to improve the energy transporting performance of conjugated polymers.     

Dynamics of unidirectional exciton migration to the molecular periphery in a photoexcited compact dendrimer

We have studied dynamical natures of electronic excited states in a compact series of phenylacetylene dendrimers. So as to clarify the mechanism of unidirectional migration of a photogenerated exciton in a compact dendrimer, we theoretically investigated the temporal behavior of the photogenerated exciton in the molecule by numerically solving the time-dependent Schrodinger equation for the electronic excited states. The structure of the dendrimers is optimized in the ground state, and it is fixed during the calculation of the exciton dynamics. The calculated results show that the exciton generated in the dendrimeric framework tends to migrate toward the outside of the molecule rather than the inside, and to itinerate around the periphery via the through-space interaction between the outer crowding benzene units. This is one of the intrinsic properties that originates from a highly branched treelike structure of the compact dendrimers.     

Singlet exciton migration in a random organic two-component system containing physical and chemical traps

Steady-state as well as time-dependent fluorescence studies have been carried out near 80 K on glassy mixed layers of anthracene and phenanthrene doped with tetracene and containing incipient anthracene dimers as physical traps. The efficiency of energy transfer from initially excited anthracene (“donor”) sites to tetracene (“supertrap”) has been measured as a function of donor concentration c_D. The results are interpreted in terms of a superposition of two transfer pathways: (i) Random walk of excitons across the donor ensemble to either tetracene or anthracene dimers and (ii) single step Förster transfer from the excited dimers (excimers) to tetracene. The efficiency of process (i) is in quantitative accord with the predictions of the 3D version of the Loring, Anderson and Fayer theory assuming dipole-dipole coupling.     

Time resolved exciton migration in mixed naphthalene crystals: triplet and singlet critical concentrations

The contrasting temporal characteristics of the triple and singlet transport (below 4 K) correlate with dynamic and quasistatic percolation, respectively. They do not support an Anderson—Mott transition (AMT) for the triplet nor an AMT followed by a kinetic threshold for the singlet critical concentration. The triplet decay-time displays the critical exponent γ.     

Impact of intramolecular twisting and exciton migration on emission efficiency of multifunctional fluorene-benzothiadiazole-carbazole compounds

Novel donor-acceptor compounds consisting of singly bonded fluorene (Fl), benzothiadiazole (BT), and carbazole (Cz) functional units in the same molecule were investigated. Analysis of the optical spectra and fluorescence transients of the compounds revealed the domination of intramolecular charge transfer (ICT) states with high fluorescence quantum yield (72%-85%). A similar Cz-Fl-Cz compound exhibiting 100% fluorescence quantum yield and no ICT character was also studied as a reference to reveal the impact of electron-accepting BT groups. Thorough examination of the optical properties of the compounds in different media, i.e., dilute solution and polymer matrix, indicated their twisted conformations due to steric hindrance in the ground state and flattened geometry in the excited state for both reference and ICT compounds. Remarkable fluorescence efficiency losses (amounting to 70%) observed upon casting the molecular solutions into neat films were determined to originate from the low-fluorescent twisted conformers and migration-facilitated exciton quenching. The majority of emission efficiency losses (over 70%) were caused by the twisted conformers, whereas only less than 30% by exciton-migration-induced nonradiative deactivation.     

Fast migration of fluoride ions in growing anodic titanium oxide

The rapid inward migration of fluoride ions in growing anodic titanium oxide under a high electric field has been elucidated by anodizing a Ti–12 at% silicon alloy, where film growth proceeds at nearly 100% efficiency in selected electrolytes. Further, incorporated silicon species in the anodic film are immobile, acting as marker species. The migration rate of fluoride ions is determined precisely by three-stage anodizing, consisting of initial anodic film formation at a constant current density to 50 V in ammonium pentaborate electrolyte, subsequent incorporation of fluoride ions by reanodizing to 55 V in ammonium fluoride electrolyte and, finally, anodizing again in ammonium pentaborate electrolyte at high current efficiency. The resultant films were analyzed by glow discharge optical emission spectroscopy to reveal the depth distribution of fluoride ions and the location of the silicon marker species. The fluoride ions migrate inward at twice the rate of O²⁻ ions. Consequently, anodizing of titanium in fluoride-containing electrolytes develops a fluoride-rich layer that separates the alloy substrate from the anodic oxide, with eventual detachment of the film from the substrate.     

Singlet exciton interactions in crystalline naphthalene

The decay of prompt fluorescence in crystalline naphthalene at 300 K, excited by a picosecond 266 nm pulse, has been studied as a function of excitation intensity. Experimental decay curves can be fitted only when the exponential distribution in depth of excitation and the radial (gaussian) intensity profile of the excitation are both taken into account. From an analysis of the decay at early time (?5 ns) a best fit value of the singlet—singlet annihilation rate constant is found γ_SS = (4 ± 1) × 10^?10 cm³ s^?1. If the reaction is diffusion-limited, this rate implies an average singlet diffusivity D_S = (2 ± 1) × 10^?4 cm² s^?1.     

Cooperative exciton spectra in molecular crystals

It is shown that the singlet exciton spectrum in the crystal is coupled with that of the two-particle excitations (singlet, triplet excitons, magnons, triplet exciton-magnon). Splitting on the excitation spectrum occurs when the composite energy of the two-particle excitations is close to that of the singlet exciton.     

Triplet exciton lifetime in crystalline pyrene

The triplet exciton lifetime in crystalline pyrene is found to be 140 ± 10 msec at room temperature, over 500 times longer than previously reported values. The temperature dependence of the triplet lifetime has been measured over the range 80–300°K. The rate of bimolecular annihilation of triplet excitons in pyrene is found to be independent of temperature over the range 150–300°K. It is concluded that the transfer of triplet energy within the crystal may be described using the monomer exciton model.     

Charge-transfer exciton transitions in anthracene-PMDA crystal

The reflection spectrum of the first singlet charge transfer transition of anthracene-PMDA (pyromellitic dianhydride) crystal has been obtained at temperatures close to 2 K. The spectrum is polarized along the stack axis and consists of a series of vibronic bands, approximately 500 cm^?1 wide, with impressed fine structure arising from intermolecular phonon modes. This fine structure is particularly prominent for the 00 band which starts with a zero-phonon line which is the most dominant feature of the entire spectrum. The zero-phonon line has a reflectivity of approximately 0.37 and a line width less than 5 cm^?1. This is the first time that an absolute reflection spectrum of a charge transfer transition with such vibronic details has been obtained. A preliminary discussion of the nature of the transition is presented.     

Ultrafast relaxation and exciton–exciton annihilation in PTCDA thin films at high excitation densities

Femtosecond pump–probe spectroscopy is applied to thin films of the quasi-one-dimensional organic semiconductor 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA). We present transient absorption spectra over a broad spectral range. Ultrafast intraband relaxation in the S₁ manifold towards the border of the Brillouin zone is shown to depend on temperature and excitation density. The intraband relaxation time is of the order of 100 fs. At high excitation densities (>10¹⁹ cm⁻³), the major de-excitation mechanism for the relaxed excitons is exciton–exciton annihilation. The experimental decay dynamics can be explained very well by two alternative annihilation models: one-dimensional diffusion limited bimolecular recombination or single-step long range Förster-type annihilation. In contrast, a three-dimensional diffusion limited annihilation model is significantly inferior. For all three models, we extract annihilation rates, diffusion constants, diffusion lengths, and Förster radii for room and liquid Helium temperature.     

Porphyrin boxes constructed by homochiral self-sorting assembly: optical separation, exciton coupling, and efficient excitation energy migration

meso-Pyridine-appended zinc(II) porphyrins Mn and their meso-meso-linked dimers Dn assemble spontaneously, in noncoordinating solvents such as CHCl3, into tetrameric porphyrin squares Sn and porphyrin boxes Bn, respectively. Interestingly, formation of Bn from Dn proceeds via homochiral self-sorting assembly, which has been verified by optical separations of B1 and B2. Optically pure enantiomers of B1 and B2 display strong Cotton effects in the CD spectra, which reflect the length of the pyridyl arm, thus providing evidence for the exciton coupling between the noncovalent neighboring porphyrin rings. Excitation energy migration processes within Bn have been investigated by steady-state and time-resolved spectroscopic methods in conjunction with polarization anisotropy measurements. Both the pump-power dependence on the femtosecond transient absorption and the transient absorption anisotropy decay profiles are directly associated with the excitation energy migration process within the Bn boxes, where the exciton-exciton annihilation time and the polarization anisotropy rise time are well described in terms of the F?rster-type incoherent energy hopping model by assuming a number of hopping sites of N = 4 and an exciton coherence length of L = 2. Consequently, the excitation energy hopping rates between the zinc(II) diporphyrin units have been estimated for B1 (48 ps)(-1), B2 (98 +/- 3 ps)(-1), and B3 (361 +/- 6 ps)(-1). Overall, the self-assembled porphyrin boxes Bn serve as a well-defined three-dimensional model for the light-harvesting complex.     

Photochromic polypeptides

Two photochromic polypeptides were synthesized by reaction of 1-(4-iodobutyl)-3,3- dimethylindolindolino-6′-nitrobenzospiropyran with poly-L -tyrosine; their molar contents on photochromic units were 27.3 and 44.7%. The spectra of the photo-induced merocyanines and their decoloration kinetics were compared with these of the monomeric model compound, obtained by reaction of the same N-(4-iodobutyl)-indolinospiropyran derivative with N-acetyltyrosine methyl ester. Different types of solvents have been examined, mainly dimethylformamide and pyridine, acetone and tetrahydrofuran, and methanol and ethylene glycol. The polypeptides showed a much less pronounced solvatochromism than their model; on the other hand, their absorption spectra presented two absorption maxima instead of one for the model. These differences in photochromic behavior were interpreted on the basis of the solvatation of the polymeric chain. Inverse photochromism was observed for polypeptide P₂ as well as for the model in ethylene glycol solution; this effect is due to a higher merocyanine content at the thermal equilibrium spiropyran ? merocyanine in high polar solvent.     

Triplet exciton mobilities in weak donor-acceptor complexes

The coupling between neutral and charge-transfer triplet excitons is considered from the viewpoint of its possible influence on triplet exciton migration in alternating stack donor-acceptor complexes. The results are applied for the anthracene-TCNB crystal.     

Fast Li+ migration in LiPON electrolytes doped by multi-valent Fe ions

Achieving superior ionic conductivity of Li PON solid electrolyte films is critical for the solid-state thinfilm batteries with high energy density. Here we describe a method of preparing Li PON with promising ionic migration capability and high work function by systematically tailoring the concentration of Fe ions doping. Fe-doped LiPON exhibits excellent ionic conductivity(1.08 × 10^-5S cm^-1, nearly 10 times higher than the pristine LiPON), low ionic activation energy, and...     

One-dimensional exciton diffusion in perylene bisimide aggregates

The dynamics and mobility of excitons in J-aggregates of perylene bisimides are investigated by transient absorption spectroscopy with a time resolution of 50 fs. The transient spectra are compatible with an exciton delocalization length of two monomers and indicate that vibrational and configurational relaxation processes are not relevant for the spectroscopic properties of the aggregates. Increasing the pump pulse energy and in that way the initial exciton density results in an accelerated signal decay and pronounced exciton-exciton annihilation dynamics. Modeling the data by assuming a diffusive exciton motion reveals that the excitons cannot migrate freely in all three directions of space but their mobility is restricted to one dimension. The observed anisotropy supports this picture and points against direct Fo?rster-transfer-mediated annihilation between the excitons. A diffusion constant of 1.29 nm(2)/ps is deduced from the fitting procedure that corresponds to a maximal exciton diffusion length of 96 nm for the measured exciton lifetime of 3.6 ns. The findings indicate that J-aggregates of perylene bisimides are promising building blocks to facilitate directed energy transport in optoelectronic organic devices or artificial light-harvesting systems.