7 research outputs found
Investigation of Thermally Activated Delayed Fluorescence from a DonorāAcceptor Compound with Time-Resolved Fluorescence and Density Functional Theory Applying an Optimally Tuned Range-Separated Hybrid Functional
Emitters showing thermally activated delayed fluorescence (TADF) in electroluminescent devices rely on efficient reverse intersystem crossing (rISC) arising from small thermal activation barriers between the lowest excited triplet and singlet manifolds. A small donorāacceptor compound consisting of a demethylacridine donor and a methylbenzoate acceptor group is used as a model TADF emitter. The spectroscopic signatures of this system are characterized using a combination of photoluminescence and photoluminescence excitation, and the photoluminescence decay dynamics are recorded between delays of 2 ns and 20 ms. Above T = 200 K, our data provide convincing evidence for TADF at intermediate delays in the microsecond range, whereas tripletātriplet annihilation and slow triplet decay at later times can be observed over the entire temperature range from T = 80 K to room temperature. Moreover, close to room temperature, we find a second and faster up-conversion mechanism, tentatively assigned to reverse internal conversion between different triplet configurations. An interpretation of these experimental findings requires a calculation of the deformation patterns and potential minima of several electronic configurations. This task is performed with a range-separated hybrid functional, outperforming standard density functionals or global hybrids. In particular, the systematic underestimation of the energy of charge transfer (CT) states with respect to local excitations within the constituting chromophores is replaced by more reliable transition energies for both kinds of excitations. Hence, several absorption and emission features can be assigned unambiguously, and the observed activation barriers for rISC and reverse internal conversion correspond to calculated energy differences between the potential surfaces in different electronic configurations
Not the Sum of their Parts: Understanding Multi-Donor Interactions in Symmetric and Asymmetric TADF Emitters
A pair of thermally activated delayed fluorescence (TADF) emitters with symmetric and asymmetric D-A-D structure are investigated. Despite displaying near-identical photoluminescence spectrum and quantum yields, the symmetric material possesses significantly better delayed fluorescence characteristics and OLED performance. Building on a previous study of analogous D-A materials we are able to explain these differences in terms of different strengths of electronic interactions between the two donor units. This interaction lowers the energy of the TADF-active triplet state in the asymmetric molecule, increasing its singletātriplet energy gap and leading to worse performance. This result therefore demonstrates a new strategy to selectively control the triplet states of TADF molecules, in contrast to established control of singlet states using host environment. These results also show that multi-donor TADF emitters cannot be understood simply as the sum of their isolated parts; these parts have different electronic interactions depending on their relative positions, even when there is no scope for steric interaction
Not the sum of their parts : understanding multi-donor interactions in symmetric and asymmetric TADF emitters
A pair of thermally activated delayed fluorescence (TADF) emitters with symmetric and asymmetric D-A-D structure are investigated. Despite displaying near-identical photoluminescence spectrum and quantum yields, the symmetric material possesses significantly better delayed fluorescence characteristics and OLED performance. Building on a previous study of analogous D-A materials we are able to explain these differences in terms of different strengths of electronic interactions between the two donor units. This interaction lowers the energy of the TADF-active triplet state in the asymmetric molecule, increasing its singlet-triplet energy gap and leading to worse performance. This result therefore demonstrates a new strategy to selectively control the triplet states of TADF molecules, in contrast to established control of singlet states using host environment. These results also show that multi-donor TADF emitters cannot be understood simply as the sum of their isolated parts; these parts have different electronic interactions depending on their relative positions, even when there is no scope for steric interaction
Structure Properties Relationship of DonorāAcceptor Derivatives of Triphenylamine and 1,8-Naphthalimide
Solution-processable donorāacceptor molecules
consisting
of triphenylamine core and 1,8-naphthalimide arms were designed and
synthesized by palladium-catalyzed Heck reaction. Dilute solutions
of the synthesized compounds show strong absorption peaks in the visible
wavelength range from 400 to 550 nm, which can be ascribed to the
intramolecular charge transfer. Fluorescence quantum yields of dilute
solutions of the synthesized materials range from 0.45 to 0.70, while
those of the solid samples are in the range of 0.09ā0.18. The
synthesized molecules exhibit high thermal stability with the thermal
degradation onset temperatures ranging from 431 to 448 Ā°C. The
compounds form glasses with glass-transition temperatures of 55ā107
Ā°C. DFT calculations show that HOMO and LUMO orbitals are almost
entirely localized on the donor and acceptor moieties, respectively.
Consequently, the frontier orbital energies for the three synthesized
compounds are similar and practically do not depend on the number
of 1,8-naphthalimide moieties. Ionization potentials of the solid
samples (5.75ā5.80 eV) are comparable. The charge-transporting
properties of the synthesized materials were studied using xerographic
time-of-flight method. Hole mobilities in the layers of the compounds
having one and two 1,8-naphthalimide moieties exceed 10<sup>ā3</sup> cm<sup>2</sup>Ā·V<sup>ā1</sup>Ā·s<sup>ā1</sup> at high electric fields at room temperature. The differences on
the hole mobilities between the three synthesized compounds are discussed
in the frame of Marcus theory by comparing the reorganization energy
and electronic coupling parameters