Abstract
The photophysical, electrochemical and electroluminescent properties of newly synthesized blue emitters with donor–π–acceptor geometry, namely, 4′-(1-(naphthalen-1-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenyl-(2-[1,1′-biphenyl]vinyl)-4-amine (NSPI-TPA), 4′-(1-(2-methylnaphthalen-1-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenyl-(2-[1,1′-biphenyl]vinyl)-4-amine (MNSPI-TPA), 4-(2-(4′-(diphenylamino)-(2-[1,1′-biphenyl]vinyl)-4-yl)-1H-phenanthro[9,10-d]imidazol-1-yl)-1-naphthalene-1-carbonitrile (SPNCN-TPA) and 4-(2-(4-(9H-carbazol-9-yl)styryl)-1H-phenanthro[9,10-d]imidazol-1-yl)naphthalene-1-carbonitrile (SPNCN-Cz) were analyzed. The conjugation length in the emitters is not conducive to pure emission and hence, a molecular twisting strategy was adopted in NSPI-TPA, MNSPI-TPA, SPNCN-TPA and SPNCN-Cz to enhance pure emission. The emissive state (HLCT) of twisted D–π–A molecules containing both LE and CT (HLCT) states was tuned for high PL (ηPL) (LE) and high exciton utilization (ηs) (CT) efficiencies by replacing triphenylamine (strong donor) with carbazole (weak donor). Among strong donor compounds, namely, NSPI-TPA, MNSPI-TPA and SPNCN-TPA, the SPNCN-TPA-based device exhibited blue emission (451 nm) with CIE coordinates (0.15, 0.08), maximum current efficiency (ηc) of 2.32 cd A−1, power efficiency (ηp) of 2.01 lm W−1 and external quantum efficiency (ηex) of 3.02%. The device with SPNCN-Cz emitter exhibited higher electroluminescence efficiencies than the SPNCN-TPA-based device, with pure blue emission (443 nm, CIE: 0.15,0.07), ηex of 3.15%, ηc of 2.56 cd A−1 and ηp of 2.45 lm W−1.
Highlights
Organic light-emitting diodes (OLEDs) have been widely investigated and tested commercially in recent decades and utilized in at-panel displays.[1]
The increase in conjugation in phenanthrimidazoles in uenced the blue emission,[38,39,40,41,42] and conjugation length was restricted by incorporating a bulky fragment in the core molecule, which resulted in twisted conformation.[43,44,45,46]
In NSPI-TPA, MNSPI-TPA and SPNCN-TPA, the styryl ring is inserted between triphenylamine and naphthonitrilephenanthrimidazole to extend the p-conjugation, which results in the enhancement of PL efficiency while simultaneously maintaining the strength of both donor (LE component) and acceptor (CT component)
Summary
Organic light-emitting diodes (OLEDs) have been widely investigated and tested commercially in recent decades and utilized in at-panel displays.[1]. The increase in conjugation in phenanthrimidazoles in uenced the blue emission,[38,39,40,41,42] and conjugation length was restricted by incorporating a bulky fragment in the core molecule, which resulted in twisted conformation.[43,44,45,46] In line with this discussion and our own research interest, we report donor–spacer–acceptor derivatives, namely, 40-(1-(naphthalen-1-yl)-1H-phenanthro[9,10-d]imidazol2-yl)-N,N-diphenyl-(2-[1,10-biphenyl]vinyl)-4-amine (NSPI-TPA), 40-(1-(2-methylnaphthalen-1-yl)-1H-phenanthro[9,10-d]imidazol2-yl)-N,N-diphenyl-(2-[1,10-biphenyl]vinyl)-4-amine (MNSPI-TPA), 4-(2-(40-(diphenylamino)-(2-[1,10-biphenyl]vinyl)-4-yl)-1H-phenanthro[9,10-d]imidazol-1-yl)-1-naphthalene-1-carbonitrile Higher hPL of SPNCN-Cz lm was observed compared with that of SPNCN-TPA lm; high hPL and high hS enhanced the hex of the SPNCNCz-based device. These results can be used to design low cost uorescent materials via subtle molecular modi cations using the HLCT emissive state principle
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