Well-Ordered 4CzIPN ((4s,6s)-2,4,5,6-Tetra(9-H-carbazol-9-yl)isophthalonitrile) Layers: Molecular Orientation, Electronic Structure, and Angular Distribution of Photoluminescence

Abstract

We fabricated a well-ordered homogeneous monolayer of disk-shaped, carbazolyl dicyanobenzene (CDCB)-based thermally activated delayed fluorescence (TADF) molecule, i.e., 4CzIPN((4s,6s)-2,4,5,6-tetra(9Hcarbazol-9-yl)isophthalonitrile) at room temperature on flat Ag(111), Au(111), and Cu(111) surfaces. The second layer of the 4CzIPN was also found to be well ordered. The electronic states of the well-ordered monolayer and multilayer of 4CzIPN were found to be nearly unchanged from that of the isolated molecule calculated by the density functional theory (DFT), suggesting that the ordered layers retain the TADF properties. Indeed, we demonstrated the delayed fluorescence and the nearly perfect in-plane alignment of the transition dipole moment of a 4CzIPN thin film on glass substrate even in an ambient condition. These results indicate that the well-ordered films of the disc-shaped carbazole-based TADF molecules could potentially be utilized in organic light-emitting diode (OLED) devices with high light outcoupling efficiency.

What Controls the Orientation of TADF Emitters?

“…TADF compounds thus typically possess emissive charge-transfer (CT) states and the direction of their TDM often coincides with (or is very close to) the long molecular axis between donor and acceptor units. Note that there are some exceptions like the well-known 4CzIPN, which is an almost spherical molecule (Hasegawa et al, 2018 ). Nevertheless, the inline alignment of TDM and long molecular axis of TADF emitters holds great potential for manipulating their TDM orientation, and thus the light outcoupling from the OLED, by controlling molecular orientation upon thermal evaporation of films.…”

Molecular Orientation Effects in Organic Light‐Emitting Diodes

“…For example, it has been reported that several linear shaped rod-like and planar disk-shaped TADF emitters can achieve an V H as high as 97 % without considering other fabricationrelated parameters. [67] In practice, as discussed earlier for the conventional fluorescent materials, the intermolecular interactions play a determining role in providing a high degree of internal rigidity, which induces the high aspect ratios typically observed in molecules forming highly oriented films. Liu et al [66] Recently, another molecular design strategy has been focused on engineering intermolecular interactions.…”

“…Unlike the IQE, there is relatively limited literature specifically investigating the molecular orientation effects in TADF emitters. [63,[65][66][67][68]73] Indeed, as discussed earlier, the overall planarity and geometrical shape of the molecule plays crucial roles. For example, it has been reported that several linear shaped rod-like and planar disk-shaped TADF emitters can achieve an V H as high as 97 % without considering other fabricationrelated parameters.…”

“…For example, in the case of TADF molecule (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN),it has been reported a high V H of 0.97 in the neat film, compared to V H = 0.73 when doped in N,N'-dicarbazolyl-3,5-benzene (mCP) and B3PyMPM. [67,74]…”

What controls the orientation of TADF emitters?

“…TADF compounds thus typically possess emissive charge-transfer (CT) states and the direction of their TDM often coincides with (or is very close to) the long molecular axis between donor and acceptor units. Note that there are some exceptions like the well-known 4CzIPN, which is an almost spherical molecule (Hasegawa et al, 2018). Nevertheless, the inline alignment of TDM and long molecular axis of TADF emitters holds great potential for manipulating their TDM orientation, and thus the light outcoupling from the OLED, by controlling molecular orientation upon thermal evaporation of films.…”