Singlet Fission and Excimer Formation in Disordered Solids of Alkyl-Substituted 1,3-Diphenylisobenzofurans

We describe the preparation and excited state dynamics of three alkyl derivatives of 1,3-diphenylisobenzofuran (<b>1</b>) in both solutions and thin films. The substitutions are intended to disrupt the slip-stacked packing observed in crystals of <b>1</b> while maintaining the favorable energies of singlet and triplet for singlet fission (SF). All substitutions result in films that are largely amorphous as judged by the absence of strong X-ray diffraction peaks. The films of <b>1</b> carrying a methyl in the para position of one phenyl ring undergo SF relatively efficiently (≥75% triplet yield, Φ_T) but more slowly than thin films of <b>1</b>. When the methyl is replaced with a <i>t</i>-butyl, kinetic competition in the excited state favors excimer formation rather than SF (Φ_T = 55%). When <i>t</i>-Bu groups are placed in both meta positions of the phenyl substituent, SF is slowed further and Φ_T = 35%

EPR Spectroscopy of Radical Ions of a 2,3-Diamino-1,4-naphthoquinone Derivative

We report the electron paramagnetic resonance spectra of the radical cation and radical anion of 1,2,2,3-tetramethyl-2,3-dihydro-1<i>H</i>-naphtho­[2,3-<i>d</i>]­imidazole-4,9-dione (<b>1</b>) and its doubly ¹³C labeled analogue <b>2</b>, of interest for singlet fission. The hyperfine coupling constants are in excellent agreement with density functional theory calculations and establish the structures beyond doubt. Unlike the radical cation <b>1</b>^<b>•+</b>, the radical anion <b>1</b>^{<b>•–</b>} and its parent <b>1</b> have pyramidalized nitrogen atoms and inequivalent methyl groups 15 and 16, in agreement with the calculations. The distinction is particularly clear with the labeled analogue <b>2</b>^{<b>•–</b>}

Self-Assembled Containers Based on Extended Tetrathiafulvalene

Two original self-assembled containers constituted each by six electroactive subunits are described. They are synthesized from a concave tetratopic π-extended tetrathiafulvalene ligand bearing four pyridyl units and <i>cis</i>-M­(dppf)­(OTf)₂ (M = Pd or Pt; dppf = 1,1′-bis­(diphenylphosphino)­ferrocene; OTf = trifluoromethane-sulfonate) complexes. Both fully characterized assemblies present an oblate spheroidal cavity that can incorporate one perylene molecule

Self-Assembled Containers Based on Extended Tetrathiafulvalene

Two original self-assembled containers constituted each by six electroactive subunits are described. They are synthesized from a concave tetratopic π-extended tetrathiafulvalene ligand bearing four pyridyl units and <i>cis</i>-M­(dppf)­(OTf)₂ (M = Pd or Pt; dppf = 1,1′-bis­(diphenylphosphino)­ferrocene; OTf = trifluoromethane-sulfonate) complexes. Both fully characterized assemblies present an oblate spheroidal cavity that can incorporate one perylene molecule

Self-Assembled Containers Based on Extended Tetrathiafulvalene

Two original self-assembled containers constituted each by six electroactive subunits are described. They are synthesized from a concave tetratopic π-extended tetrathiafulvalene ligand bearing four pyridyl units and <i>cis</i>-M­(dppf)­(OTf)₂ (M = Pd or Pt; dppf = 1,1′-bis­(diphenylphosphino)­ferrocene; OTf = trifluoromethane-sulfonate) complexes. Both fully characterized assemblies present an oblate spheroidal cavity that can incorporate one perylene molecule

Covalent Dimers of 1,3-Diphenylisobenzofuran for Singlet Fission: Synthesis and Electrochemistry

The synthesis of covalent dimers in which two 1,3-diphenylisobenzofuran units are connected through one phenyl substituent on each is reported. In three of the dimers, the subunits are linked directly, and in three others, they are linked via an alkane chain. A seventh new compound in which two 1,3-diphenylisobenzofuran units share a phenyl substituent is also described. These materials are needed for investigations of the singlet fission process, which promises to increase the efficiency of solar cells. The electrochemical oxidation and reduction of the monomer, two previously known dimers, and the seven new compounds have been examined, and reversible redox potentials have been compared with results obtained from density functional theory. Although the overall agreement is satisfactory, some discrepancies are noted and discussed

Covalent Dimers of 1,3-Diphenylisobenzofuran for Singlet Fission: Synthesis and Electrochemistry

The synthesis of covalent dimers in which two 1,3-diphenylisobenzofuran units are connected through one phenyl substituent on each is reported. In three of the dimers, the subunits are linked directly, and in three others, they are linked via an alkane chain. A seventh new compound in which two 1,3-diphenylisobenzofuran units share a phenyl substituent is also described. These materials are needed for investigations of the singlet fission process, which promises to increase the efficiency of solar cells. The electrochemical oxidation and reduction of the monomer, two previously known dimers, and the seven new compounds have been examined, and reversible redox potentials have been compared with results obtained from density functional theory. Although the overall agreement is satisfactory, some discrepancies are noted and discussed

C₆₀ Recognition from Extended Tetrathiafulvalene <i>Bis</i>-acetylide Platinum(II) Complexes

The favorable spatial organization imposed by the square planar 4,4′-di­(<i>tert</i>-butyl)-2,2′-bipyridine (dbbpy) platinum­(II) complex associated with the electronic and shape complementarity of π-extended tetrathiafulvalene derivatives (exTTF) toward fullerenes is usefully exploited to construct molecular tweezers, which display good affinities for C₆₀

Arrays of Molecular Rotors with Triptycene Stoppers: Surface Inclusion in Hexagonal Tris(<i>o</i>‑phenylenedioxy)cyclotriphosphazene

A new generation of rod-shaped dipolar molecular rotors designed for controlled insertion into channel arrays in the surface of hexagonal tris­(<i>o</i>-phenylenedioxy)­cyclotriphosphazene (TPP) has been designed and synthesized. Triptycene is used as a stopper intended to prevent complete insertion, forcing the formation of a surface inclusion. Two widely separated ¹³C NMR markers are present in the shaft for monitoring the degree of insertion. The structure of the two-dimensional rotor arrays contained in these surface inclusions was examined by solid-state NMR and X-ray powder diffraction. The NMR markers and the triptycene stopper functioned as designed, but half of the guest molecules were not inserted as deeply into the TPP channels as the other half. As a result, the dipolar rotators were distributed equally in two planes parallel to the crystal surface instead of being located in a single plane as would be required for ferroelectricity. Dielectric spectroscopy revealed rotational barriers of ∼4 kcal/mol but no ferroelectric behavior

Photocurrent Enhanced by Singlet Fission in a Dye-Sensitized Solar Cell

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, <b>1</b>) as the sensitizer is reported here. Self-assembly of the blue-absorbing <b>1</b> 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 <b>1</b> 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 <b>1</b> 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