posted on 2024-01-17, 10:31authored byAlexandre Malinge, Shiv Kumar, Dongyang Chen, Eli Zysman-Colman, Stéphane Kéna-Cohen
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.