Triplet States of Cyanostar and Its Anion Complexes

The design of advanced optical materials based on triplet
states
requires knowledge of the triplet energies of the molecular building
blocks. To this end, we report the triplet energy of cyanostar (CS) macrocycles, which are the key structure-directing units
of small-molecule ionic isolation lattices (SMILES) that have emerged
as programmable optical materials. Cyanostar is a cyclic pentamer
of covalently linked cyanostilbene units that form π-stacked
dimers when binding anions as 2:1 complexes. The triplet energies, ET, of the parent cyanostar and its 2:1 complex
around PF6– are measured to be 1.96 and
2.02 eV, respectively, using phosphorescence quenching studies at
room temperature. The similarity of these triplet energies suggests
that anion complexation leaves the triplet energy relatively unchanged.
Similar energies (2.0 and 1.98 eV, respectively) were also obtained
from phosphorescence spectra of the iodinated form, I-CS, and of complexes formed with PF6– and
IO4– recorded at 85 K in an organic glass.
Thus, measures of the triplet energies likely reflect geometries close
to those of the ground state either directly by triplet energy transfer
to the ground state or indirectly by using frozen media to inhibit
relaxation. Density functional theory (DFT) and time-dependent DFT
were undertaken on a cyanostar analogue, CSH, to examine
the triplet state. The triplet excitation localizes on a single olefin
whether in the single cyanostar or its π-stacked dimer. Restriction
of the geometrical changes by forming either a dimer of macrocycles,
(CSH)2, or a complex, (CSH)2·PF6–, reduces the relaxation
resulting in an adiabatic energy of the triplet state of 2.0 eV. This
structural constraint is also expected for solid-state SMILES materials.
The obtained T1 energy of 2.0 eV is a key guide line for
the design of SMILES materials for the manipulation of triplet excitons
by triplet state engineering in the future.