ortho‐, meta‐, and para‐Dihydroindenofluorene Derivatives as Host Materials for Phosphorescent OLEDs

Abstract

This work reports the first structure-properties relationship study of ortho [2,1-c]-, meta [1,2-a]-, and para [1,2-b]dihydroindenofluorenes, highlighting the influence of bridge rigidification on the electronic properties. This study has made it possible to devise an extended π-conjugated molecule with both a high triplet state energy level and excellent thermal and morphological stability. As a proof of concept, dihydroindenofluorenes were used as the host in sky-blue phosphorescent organic light-emitting diodes (PhOLEDs) with high performance.

Spirobifluorene Regioisomerism: A Structure–Property Relationship Study

“…Indeed, the absorption spectrum of the meta terphenyl (analogue of 1 and 3 but without the bridge) possesses a λ max at 246 nm, strongly blue shifted compared to that of its regioisomer, the para terphenyl (analogue of 2), λ max = 277 nm. [2] This π-conjugation disruption observed for meta terphenyl finds its origin in the shape and distribution of the molecular orbitals involved (small contributions are indeed found on meta carbons). [30] Indeed, it is admitted that there is a better delocalisation of π-electrons following the para/ortho/meta sequence and numbers of studies have tried to elucidate the origin of the restricted π-conjugation between para, ortho and meta substituted oligophenylenes.…”

“…Para, ortho and meta terphenyls, analogue of 1-4 but without any bridge, follow the same trend with E T of 2.55, 2.67 and 2.82 eV respectively. [2,37] Two important features need to be stressed out: (i) The E T of 1 (2.86 eV) is almost identical to that of SBF (E T =2.88 eV) confirming that the pendant phenyl has no influence on the T1 state (Figure 4 bottom), which is a key point for further use as host in blue PhOLED (see below), (ii) The emission from T1 state follows a classical para/ortho/meta sequence (E T increases as follows 2/4/3/1), being hence different to that of S1-S0. Thus, and oppositely to our observations in absorption and fluorescence, the nature of the linkage fully drives the E T values.…”

“…Recently, our groups have reported the remarkable impact of regioisomerism to finely tune the singlet and triplet energies of dihydroindenofluorenes leading to highly efficient optoelectronic devices. [1,2] Similarly, Haley and co-workers have shown the impact of the regioisomerism on the electronic properties of a promising family of antiaromatic indenofluorene derivatives. [3,4] Spiro configured compounds constitute one of the most important class of OSCs.…”

“…Indeed, in order to obtain a high triplet energy (E T ), key feature in the design of host materials for blue PhOLEDs, which are still the weakest link of this technology, it is mandatory to restrict the π-electrons delocalization within the OSC. This π-conjugation disruption has been successfully investigated with ortho linked SBFs (substitution in position 4) [12][13][14][15][16] and meta linked SBFs (substitution in position 3), [2,[17][18][19][20] leading to high performance blue PhOLEDs. However and despite the recent very high efficiency devices obtained by Jiang et al, [17] and by our groups [15] only few examples of 3-and 4-substituted SBFs have been described to date and more importantly no rational structure-properties relationship studies have been reported on SBF regioisomerism.…”

Structure–property relationship of 4-substituted-spirobifluorenes as hosts for phosphorescent organic light emitting diodes: an overview

“…1,2 In such compouds, at least two molecular π-systems with equal or different functions (emission, charge transport) are connected via a shared sp 3 -hybridized atom. Such an architecture has several benefits due to the perpendicular arrangement of the π-systems and the resulting spiroconfigured OSCs present many appealing properties such as high thermal/morphological and emission colour stability, which are key properties for Organic Light-Emitting Diodes (OLED) applications.…”