Heavy Atom Effect in Halogenated mCP and Its Influence on the Efficiency of the Thermally Activated Delayed Fluorescence of Dopant Molecules

In this study, we explore the impact of halogen functionalization on the photophysical properties of the commonly used organic light-emitting diode (OLED) host material, 1,3-bis(N-carbazolyl)benzene (mCP). Derivatives with different numbers and types of halogen substituents on mCP were synthesized. By measuring steady-state and transient photoluminescence at 6 K, we study the impact of the type, number, and position of the halogens on the intersystem crossing and phosphorescence rates of the compounds. In particular, the functionalization of mCP with 5 bromine atoms results in a significant increase of the intersystem crossing rate by a factor of 300 to a value of (1.5 ± 0.1) × 1010 s–1, and the phosphorescence rate increases by 2 orders of magnitude. We find that the singlet radiative decay rate is not significantly modified in any of the studied compounds. In the second part of the paper, we describe the influence of these compounds on the reverse intersystem crossing of the 7,10-bis(4-(diphenylamino)phenyl)-2,3-dicyanopyrazino-phenanthrene (TPA-DCPP), a TADF guest, via the external heavy atom effect. Their use results in an increase of the reverse intersystem crossing (RISC) rate from (8.1 ± 0.8) × 103 s–1 for mCP to (2.7 ± 0.1) × 104 s–1 for mCP with 5 bromine atoms. The effect is even more pronounced for the mCP analogue containing a single iodine atom, which gives a RISC rate of (3.3 ± 0.1) × 104 s–1. Time-dependent DFT calculations reveal the importance of the use of long-range corrected functionals to predict the effect of halogenation on the optical properties of the mCP, and the relativistic approximation (ZORA) is used to provide insight into the strength of the spin–orbit coupling matrix element between the lowest-lying excited singlet and triplet states in the different mCP compounds.

Fit parameters

Figure S23 .
Figure S23.Photoluminescence at 6 K and 300 K, which shows fluorescence (330 -400 nm) and phosphorescence (400 -600 nm) emission of a drop cast film of 0.1 wt% a) mCP, b) mCP-Br and c) mCP-Br5, doped in PMMA, excited at 315 nm.The observed trends and features are the same as for the 1 wt.% film.

Table S1 .
Biexponential decay fit parameters of the transient PL decay of 1 wt.% mCP derivatives doped in PMMA thin films at 6 K with emission wavelength at 360 nm.

Table S2 .
Biexponential decay fit parameters of the transient PL decay of 10 wt.% TPA-DCPP thin films doped in mCP and mCP derivatives with emission wavelength at 630 nm.

Table S4 .
Biexponential decay fit parameters of the transient PL decay of 10 wt.% 4CzIPN thin films doped in mCP and mCP derivatives with emission wavelength at 550 nm.

Table S5 .
Summary of photophysical properties and calculated RISC rate of doped thin films of 10 wt.% of 4CzIPN in different mCP analogues.