posted on 2021-05-26, 11:34authored byJung Min Ha, Hye Beom Shin, Joonyoung Francis Joung, Won Jae Chung, Ji-Eun Jeong, Sangin Kim, Seon Hyoung Hur, Suk-Young Bae, Jun-Yun Kim, Jun Yeob Lee, Sungnam Park, Han Young Woo
A series
of green-emitting fluorophores based on a tetra-azaacene
core is synthesized by introducing nitrile substituents at different
positions. Their molecular structure–optical property relationship
[i.e., vibronic transitions in photoluminescence (PL) and electroluminescence
(EL) spectra] is investigated to obtain a sharp emission where the
vibronic peak ν0–0 should be intensified by
suppressing ν0–n (n = 1, 2, 3...) transitions. The intensity ratios (I0–1/I0–0) of the ν0–1 and ν0–0 vibronic transitions in the PL spectra of DBBNP, DBBNP2CN1, and
DBBNP2CN2 in hexane are 1.13, 0.80, and 0.67, respectively. Theoretical
calculations explain that the CN substitution at positions 8 and 13
in DBBNP2CN2 induces a uniform charge distribution and reduces the
Huang–Rhys factors (HRFs) of the vibrational normal modes coupled
to the electronic transition. The organic light-emitting diode (OLED)
fabricated with DBBNP2CN2 shows a narrower green EL emission at 518
nm with a smaller bandwidth (50 nm) than those of devices adopting
DBBNP or DBBNP2CN1. The careful modification of the molecular structures
and positions of substituents enables us to reduce the HRFs of vibrations
to achieve a narrow emission bandwidth with decreased I0–1/I0–0, which
suggests a design strategy to develop narrowband organic fluorophores
to improve the color purity for wide-gamut OLED displays.