Triplet States of Cyanostar and Its Anion Complexes

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Abstract
The design of advanced optical materials based on tripletstatesrequires knowledge of the triplet energies of the molecular buildingblocks. To this end, we report the triplet energy of cyanostar (CS) macrocycles, which are the key structure-directing unitsof small-molecule ionic isolation lattices (SMILES) that have emergedas programmable optical materials. Cyanostar is a cyclic pentamerof covalently linked cyanostilbene units that form & pi;-stackeddimers when binding anions as 2:1 complexes. The triplet energies, E (T), of the parent cyanostar and its 2:1 complexaround PF6 (-) are measured to be 1.96 and2.02 eV, respectively, using phosphorescence quenching studies atroom temperature. The similarity of these triplet energies suggeststhat anion complexation leaves the triplet energy relatively unchanged.Similar energies (2.0 and 1.98 eV, respectively) were also obtainedfrom phosphorescence spectra of the iodinated form, I-CS, and of complexes formed with PF6 (-) andIO(4) (-) recorded at 85 K in an organic glass.Thus, measures of the triplet energies likely reflect geometries closeto those of the ground state either directly by triplet energy transferto the ground state or indirectly by using frozen media to inhibitrelaxation. Density functional theory (DFT) and time-dependent DFTwere undertaken on a cyanostar analogue, CSH, to examinethe triplet state. The triplet excitation localizes on a single olefinwhether in the single cyanostar or its & pi;-stacked dimer. Restrictionof the geometrical changes by forming either a dimer of macrocycles,(CSH)(2), or a complex, (CSH)(2)& BULL;PF6 (-), reduces the relaxationresulting in an adiabatic energy of the triplet state of 2.0 eV. Thisstructural constraint is also expected for solid-state SMILES materials.The obtained T-1 energy of 2.0 eV is a key guide line forthe design of SMILES materials for the manipulation of triplet excitonsby triplet state engineering in the future.
Funding Information
  • Vetenskapsr?det
  • Energimyndigheten (46526-1)
  • Danmarks Frie Forskningsfond (DFF-0136-00122B)
  • H2020 Marie Sklodowska-Curie Actions (801199)
  • Division of Materials Research (DMR-2118423)

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