Thermally activated delayed fluorescence (TADF) has recently become an extensively investigated phenomenon due to its high potential for application in organic optoelectronics. Currently, there is still lack of a model describing correctly basic photophysical parameters of organic TADF emitters. This article presents such a photophysical model describing the rates of intersystem crossing (ISC), reverse ISC (rISC), and radiative deactivation in various media and emphasizing key importance of molecular vibrations on the example of a popular TADF dye 9,10-dihydro-9,9-dimethyl-10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-acridine (DMAC-TRZ). The presented experimental and theoretical investigations prove that ISC and rISC can occur efficiently between the singlet and triplet states of the same charge-transfer nature ( 1 CT and 3 CT, respectively). In emitters with the orthogonal donor and acceptor fragments, such spin-forbidden 1 CT ↔ 3 CT transitions are activated by molecular vibrations. Namely, the change of dihedral angle between the donor and the acceptor affords reasonable spin–orbit coupling, which together with a small energy gap and reorganization energy enable 1 CT ↔ 3 CT transition rates reaching 1 × 10 7 s –1 . Evidence of direct 1 CT ↔ 3 CT spin-flip and negligible role of a second triplet state, widely believed as a key parameter in the design of (r)ISC materials, change significantly the current understanding of TADF mechanism. In authors’ opinion, photophysics, and molecular design principles of TADF emitters should be revised considering the importance of vibrationally enhanced 1 CT ↔ 3 CT transitions.

The mechanism of heavy-atom effect on organic blue TADF emitters is investigated using a combined Arrhenius and Marcus theories approach. Basic molecular design principles of such hybrid organic TADF materials are formulated.

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A series of twisted triaryl-s-triazine derivatives are used as acceptor fragments in design of deep-blue TADF emitters for OLED. Comprehensive photophysical investigations indicate high impact of structural relaxation on the TADF color and efficiency.