Small, apolar aromatic groups, such as phenyl rings, are commonly included in the structures of fluorophores to impart hindered intramolecular rotations, leading to desirable solid-state luminescence properties. However, they are not normally considered to take part in through-space interactions that influence the fluorescent output. Here, we report on the photoluminescence properties of a series of phenyl-ring molecular rotors bearing three, five, six, and seven phenyl groups. The fluorescent emissions from two of the rotors are found to originate, not from the localized excited state as one might expect, but from unanticipated through-space aromatic-dimer states. We demonstrate that these relaxed dimer states can form as a result of intra- or intermolecular interactions across a range of environments in solution and solid samples, including conditions that promote aggregation-induced emission. Computational modeling also suggests that the formation of aromatic-dimer excited states may account for the photophysical properties of a previously reported luminogen. These results imply, therefore, that this is a general phenomenon that should be taken into account when designing and interpreting the fluorescent outputs of luminescent probes and optoelectronic devices based on fluorescent molecular rotors.
We
expose significant changes in the emission color of carbazole-based
thermally activated delayed fluorescence (TADF) emitters that arise
from the presence of persistent dimer states in thin films and organic
light-emitting diodes (OLEDs). Direct photoexcitation of this dimer
state in 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) reveals the significant influence of dimer species
on the color purity of its photoluminescence and electroluminescence.
The dimer species is sensitive to the sample preparation method, and
its enduring presence contributes to the widely reported concentration-mediated
red shift in the photoluminescence and electroluminescence of evaporated
thin films. This discovery has implications on the usability of these,
and similar, molecules for OLEDs and explains disparate electroluminescence
spectra presented in the literature for these compounds. The dimerization-controlled
changes observed in the TADF process and photoluminescence efficiency
mean that careful consideration of dimer states is imperative in the
design of future TADF emitters and the interpretation of previously
reported studies of carbazole-based TADF materials.
Rather than donor–acceptor dihedral angles, the TADF performance of DMAC–BZN positional isomers is instead controlled by differences in acceptor strength arising from π-system electron density – along with a through-space dipole interaction.
This review highlights the up-and-coming pyrrole-BF2 (BOPHY) fluorophores, with a focus on synthetic procedures, photophysical properties – including structure–property analyses – as well as emerging applications.
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