Rational Design of Carbazole- and Carboline-Based Ambipolar Host Materials for Blue Electrophosphorescence: A Density Functional Theory Study

Density functional theory has been employed to design 41 host molecules for blue electrophosphorescence by incorporating electron donor (carbazole (cbz)) and electron acceptor (alpha-carboline (Cb1) units into N-phenyl-carbazole (PhCbz). We have systematically investigated the influence of the number (mono-, di-, and trisubstituted) and positions of Cb1 and Cbz substitution on an array of electronic properties of the designed hosts. The results underline that the substitution of the N-phenyl ring with a carboline unit yields host molecules with low charge injection barriers, balanced charge transport, efficient charge separation, high triplet energy (B-T), and low singlet triplet energy difference (Delta E-ST). For disubstituted hosts, the second subunit can either be Cb1 or Cbz substituted at the 2/7 position of PhCbz, while substituting the 2 and 3 positions of PhCbz with Cbz subunits generates trisubstituted hosts with efficient electronic properties. Thus, our results indicate that both number and position of subunit substitution in PhCbz play a decisive role in designing hosts with appropriate electronic properties. Among the 41 systems considered in the study, we have identified the two most efficient hosts, and their electronic properties are found to be very promising compared to some of the experimentally reported analogous hosts.