Rational Synthesis of Tetrahydrodibenzophenanthridine and Phenanthroimidazole as Efficient Blue Emitters and their Applications

• Published in: European journal of organic chemistry 2020-02, Vol.2020 (7), p.834-844
• Main Authors: Thiyagarajan, Manojkumar Dhanthala, Balijapalli, Umamahesh, Nasiri, Sohrab, Volyniuk, Dmytro, Simokaitienec, Jurate, Pathak, Madhvesh, Iyer, Sathiyanarayanan Kulathu, Gražulevičius, Juozas Vidas
• Format: Article
• Language: English
• Subjects: Blue luminophore; Electrochemistry; Electroluminescence; Emitters; Emitters (electron); Energy gap; Ionization potentials; Mathematical analysis; Molecular orbitals; Nitrogen dioxide; Optical properties; Phenananthridines; Phenanthroimidazoles; Photoluminescence; Solvatochromism; Toluene
• ISSN: 1434-193X; 1099-0690
• DOI: 10.1002/ejoc.201901711

Ten luminophores based on tetrahydrodibenzophenanthridine (THDP) and phenanthroimidazole (PI) were designed, synthesized and characterized for their thermal, electrochemical, electro‐optical, charge‐transporting characteristics and electroluminescent properties. The blue luminophores exhibited high photoluminescence quantum yields of 66–93 % in toluene solutions and of 5–59 % in solid films. The highest values were observed for the derivative of THDP and PI containing methoxy group. The compounds showed close values of ionization potentials (5.74–6.11 eV) and electron affinities (2.71–3.06 eV). The selected compounds were tested in electroluminescent devices for the preparation of non‐doped light‐emitting layers. The best device fabricated using derivative of THDP and PI with methoxy groups showed blue electroluminescence with brightness of 10000 cd/m2 at high applied voltages. We performed DFT calculations and observed lowest singlet–triplet gap (ΔEST) values of 0.33 and 0.03 eV, oscillator strength (f) values of 0.034 and 0.008 for CN and NO2 derivatives. Interestingly, we also observed that compounds 3g and 3i showed HOMO and LUMO levels with similar energy gap (Eg) of 3.60 eV and deeper HOMO values of –5.30, –5.33 eV and LUMO values of –1.94, –2.77 eV. Tetrahydrodibenzo[a,i]phenanthridine (THDP) and phenanthroimidazoles (PI) coupled bipolar THDP‐PI derivatives exhibited deep blue emission in the range of 425–435 nm. The highest PLQY value up to 93 % with displaying positive solvatochromic behaviour proving CT character of their emission. Charge mobility of all the derivatives except compound 3a showed hole mobility of 1.3 × 10–4 cm2/V at electric fields higher than 106 V/cm.