Fluorescência atrasada ativada termicamente: caracterização fotofísica e aplicação de molécula orgânica assimétrica

Abstract

The dissertation explores and characterizes the photophysical properties of an asymmetric emitter of Thermally Activated Delayed Fluorescence (TADF), including electrical characterization for the production of Organic Light Emitting Diodes (OLEDs). The studied molecule is composed of phenoxazine as the donor and dibenzothiophene-S,S-dioxide (DBZ) as the acceptor. The photophysical study enabled the OLED development, emphasizing the 𝑇𝐴𝐷𝐹 effect, which is crucial for maximizing the efficiency of OLEDs by surpassing the statistical spin limit. This mechanism involves the intersystem crossing rates, which enables the excited electrons conversion from non-radiative triplet excited states to radiative singlet states, which requires the energy difference minimization, Δ𝐸𝑆𝑇, between these states. Consequently, the 𝑇𝐴𝐷𝐹 mechanism captures triplet states, allowing OLEDs based on this effect to achieve an internal quantum efficiency (𝐼𝑄𝐸) close to 100%. The remarkable efficiency of 𝑇𝐴𝐷𝐹 is attributed to the near-perpendicular arrangement between the donor (𝐷) and acceptor (𝐴) units, as well as the near-isoenergetic alignment, on the order of energy of (𝐾𝐵𝑇), between the charge transfer (𝐶𝑇) states and the local triplet states. The experimental investigations comprise steady-state and time-resolved photophysical measurements, including time-resolved spectra acquisition conducted both in solutions and in the solid state. Temperature-dependent experiments allowed the observation of thermal effects on delayed fluorescence and the dynamics of triplet and singlet states. This empirical approach led to the identification of key design principles and molecular architectures that facilitate efficient triplet state harvesting and reverse intersystem crossing processes.