We report the effect of crystal structure and crystallite grain size on singlet fission (SF) in polycrystalline tetracene, one of the most widely studied SF and organic semiconductor materials.

In order to elucidate the mechanism of singlet fission in thin films of 1,3-diphenylisobenzofuran (1) we have performed ultrafast transient absorption spectroscopy as a function of sample temperature and excitation fluence on polycrystalline thin films composed of two polymorphs. Our earlier investigations revealed that films enriched in a particular polymorph of 1 displayed near 200% efficiency for triplet formation at 77 K, while films composed primarily of a second polymorph had a very low triplet quantum yield. Present data confirm the triplet yield disparities in the two polymorphs and demonstrate the distinct fates of the initially prepared singlets in films of different structure. Singlet fission is inhibited in the more stable polymorph due to rapid excimer formation and trapping. The less stable polymorph undergoes highly efficient singlet fission with a dominant time constant of 10-30 ps and without strong thermal activation. Transient absorption measurements with varying excitation fluence indicate that singlet-singlet annihilation is a primary competitor of singlet fission at higher fluence and that fission from higher-lying states can also contribute to the triplet formation process. Measurements employing different excitation energies and sample temperatures reveal the role that trapping processes play in attenuating the triplet quantum yield to produce the complex temperature dependence of the singlet fission yield. The rate constants for singlet fission itself are essentially temperature independent.

Polycrystalline thin films of 1,3-diphenylisobenzofuran (1) with a morphology referred to here as α exhibit highly efficient singlet fission, producing two triplet states for every absorbed photon at 77 K, and about 1.4 triplet states per absorbed photon at room temperature. However, the triplet yield depends strongly on the specific crystalline form of 1, and for the morphology referred to as β the triplet yields are roughly an order of magnitude smaller. In this study, α, β, and mixed α/β films of 1 are prepared by thermal evaporation and solution drop-casting, and the structural and photophysical differences that may account for the very different triplet quantum yields are explored. The crystallites of 1 in thin films have been identified with two bulk crystal polymorphs grown from solution and structurally characterized. Analysis of absorption spectra of the films reveals a 600 cm–1 blue shift in the onset and a unique spectral profile for the form α crystallites as compared to form β. Intermolecular interactions between columns of slip-stacked molecules are different in the two polymorphs, and this likely gives rise to the much smaller triplet quantum yield for β-1.

We report an investigation of structure and photophysics of thin layers of cibalackrot, a sturdy

Investigations of singlet fission have accelerated recently because of its potential utility in solar photoconversion, although only a few reports definitively identify the role of singlet fission in a complete solar cell. Evidence of the influence of singlet fission in a dye-sensitized solar cell using 1,3-diphenylisobenzofuran (DPIBF, 1) as the sensitizer is reported here. Self-assembly of the blue-absorbing 1 with co-adsorbed oxidation products on mesoporous TiO2 yields a cell with a peak internal quantum efficiency of ∼70% and a power conversion efficiency of ∼1.1%. Introducing a ZrO2 spacer layer of thickness varying from 2 to 20 Å modulates the short-circuit photocurrent such that it is initially reduced as thickness increases but 1 with 10-15 Å of added ZrO2. This rise can be explained as being due to a reduced rate of injection of electrons from the S1 state of 1 such that singlet fission, known to occur with a 30 ps time constant in polycrystalline films, has the opportunity to proceed efficiently and produce two T1 states per absorbed photon that can subsequently inject electrons into TiO2. Transient spectroscopy and kinetic simulations confirm this novel mode of dye-sensitized solar cell operation and its potential utility for enhanced solar photoconversion.

Two isomers of both the lowest excited singlet (S1) and triplet (T1) states of the directly para, para'-connected covalent dimer of the singlet-fission chromophore 1,3-diphenylisobenzofuran have been observed. In one isomer, excitation is delocalized over both halves of the dimer, and in the other, it is localized on one or the other half. For a covalent dimer in solution, such "excitation isomerism" is extremely rare. The vibrationally relaxed isomers do not interconvert, and their photophysical properties, including singlet fission, differ significantly.

No abstract

The pr?bl~m of e!ectromagnetic s.cattering by circular wire loops loaded with lumped impedances alo?g the Wire IS considered. A solutiOn to the appropriate integral equation is oliiained by a Fourier senes method. The theory is valid for loops in arbitrary impressed fields, and for both monostatic and bistatic scattering. Previously obtained solutions for the loop antenna, the loaded loop antenna, and the unloaded loop scatterer are contained in the solution as special cases. . IntroductionThe electromagnetic theory of circular wire loops has received considerable attention in the past because (a) it is a relatively simple geometry for analysis, and (b) it is a prototype of loop scatterers and antennas in generaL Pocklington [1897] first considered the loop in a plane-wave field and obtained the current for broadside incidence. He also discussed the generalization to arbitrary angles of incidence, and Oseen [1913] carried out the details. Hallen [1938] considered the loop antenna excited by a lumped generator, and obtained a formal solution for the current. However, because of an apparent singularity in the Fourier expansion he was unable to obtain numerical results except for small loops. Storer [1956] reconsidered Hallen's solution and, by approximating the series by an integral, obtained numerical results for both the current and the input admittance of the loop. Kouyoumjian [1956] used a variational procedure to analyze the loop in a plane-wave field, and Weston [1957] considered the same problem by an alternative approach. Both of these latter solutions agree with Oseen's solution. All of the above authors considered a complete loop (unloaded), and used the thin-wire approximation.A loaded loop is formed by placing one or more lumped admittance elements in a series with the wire loop. The loaded loop antenna has been analyzed by lizuka [1965]. The general problem of determining the current on the loop and the field produced when the loaded loop is placed in an arbitrary applied field is considered in this paper. Previous analyses of loaded scatterers have been made for center-loaded dipoles [Hu, 1958] and for small scatterers [Harrington, 19621 analysis Is available in the literature [Harrington, 1964]. For reasons discussed in section 8, the solution is given in terms of admittance parameters for an N-port system. Current on a LoopConsider a perfectly conducting scatterer in a field produced by external sources. The problem can be represented by the integral equation [Harrington, 1961] -~X ~°C~) =!!X Is J n:, ~') . j(()dS' !: on s (1) where S is the scatterer surface, a. is the unit normal outward from S, E 0 is the field due to all external sources, J is the current on the scatterer' and r is the tensor Green's function relating E to j in free space. For a thin-wire circular loop, (1) can lie approximated by [Storer, 1956; Kouyoumijian, 1956] where the coordinate system is defined in figure L Here /(cp) is the loop current at any angle q,, E$(b, q,, where 1J = v' J-L/E and k = wv' EJ-L = 27T/A.. ...