Vibrationally Assisted Direct Intersystem Crossing between the Same Charge-Transfer States for Thermally Activated Delayed Fluorescence: Analysis by Marcus–Hush Theory Including Reorganization Energy

Abstract

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.

Understanding the internal heavy-atom effect on thermally activated delayed fluorescence: application of Arrhenius and Marcus theories for spin–orbit coupling analysis

“…The analysis of molecular vibrations and inhomogeneity reveals why the effect of heavy halogens on the 3 CT → 1 CT spin–flip is so peculiar. Within our previously developed TADF model, in DMAC-TRZ, 19 the 3 CT → 1 CT transition is efficient in rotamers with various θ -deviation due to non-zero SOC, low energy gap and reorganization energy. Surprisingly, in θ -rotamers, the increase in the atomic number of the substituent results in the decrease of SOC (Fig.…”

Ambient Room Temperature Phosphorescence and Thermally Activated Delayed Fluorescence from a Core-Substituted Pyromellitic Diimide Derivative

“…Phosphorescence from a lower-lying charge-transfer state was not observed because of the inherently low oscillator strength of the 3 CT excited state . Since the locally excited triplet state is higher in energy as compared to the 1 CT state ( E 3LE – E 1CT = 0.2 ± 0.04 eV), the fast delayed fluorescence component of CzPhPmDI -PMMA hybrids arise from the reverse intersystem crossing from the 3 CT → 1 CT state due to an expected small energy gap between the 1 CT and 3 CT states and a hyperfine coupling mechanism is envisioned to promote this spin-flipping process. − Current understanding of the TADF mechanism, however, suggests a vibronic coupling between two closely residing 3 CT and 3 LE states is crucial in order to have a faster reverse intersystem crossing. − Therefore, the exact role of the higher-lying 3 LE state which presumably mediates the ISC/RISC between the 1 CT and 3 CT states in CzPhPmDI , is currently being investigated. Nonetheless, the excited state dynamics of CzPhPmDI is particularly different from our previous studies on donor–acceptor naphthalene diimide derivatives ( CzPhNDI ), where the RISC was observed between the 1 CT and 3 LE states, as the 3 LE state was found to be the lowest triplet (T 1 ) state centered at the acceptor (NDI) unit, resulting in enlarged Δ E ST and slow TADF lifetime (Figure S6).…”

Host‐Dopant Interaction between Organic Thermally Activated Delayed Fluorescence Emitter and Host Material: Insight into the Excited State

“…Bryce and co-workers [83] underlined the complexity of vibronic couplings in the mediation of RISC and revealed the important role of vibrational modes in RISC. Very recently, Illia E. Serdiuk and co-workers [84] argued that the ISC and RISC between 1 CT and 3 CT can be effective due to the vibrationally enhanced SOC, very small energy gap, and small reorganization energy and that the RISC should be described by the three-level model. Thus, efforts from both theoretical calculation and experiment are still needed to provide more insights into TADF mechanism.…”