The electronic structure and the nature of the P-O bond in PPT

During the past decade, numerous advances have characterized the field of organic semiconductor devices mainly related to the synthetic versatility of organic materials, which can be designed with tuned properties,

including emission energy, charge transport, and morphological stability.

In particular, significant efforts have been made on solid-state lighting applications involving phosphorescent OLEDs (PhOLEDs) for their potential as full-color displays. Taking into account the OLED architectures, homojunction devices represent the commercially most attractive alternative to the traditional heterojunction systems due to their simplicity and ease of processing.

In contrast to the heterojunction structures, where different layers of materials with specific properties allow the charge transport and recombination, the homojunction organic devices (Fig. 1a) are based on organic molecules with the multiple roles of hole/electron transport and light emission. This usually

requires an ambipolar molecular film, allowing high and balanced mobility of both holes and electrons. To accomplish this, the ambipolar organic material is typically composed of three basic building blocks within the same system: a hole transporting molecule (donor), an electron transporting system (acceptor), and a polycyclic aromatic moiety used as spacer (Fig. 1b).

A promising class of ambipolar materials in blue PhOLEDs is represented by derivatives of dibenzothiophene substituted with diphenylphosphine oxide, such as PPT (2,8-bis(diphenylphosphoryl)- dibenzo[b,d]thiophene). PPT is an ambipolar phosphorescent electrontransporting material, with sky-blue emission, high emission efficiency, and characterized by a wide bandgap with high triplet energy level. The PPT structure (Fig. 1c) can be rationalized by the presence of an electron-rich dibenzothiophene core (DBT, Fig. 1d), functionalized by two electronwithdrawing phosphine oxide groups, modeled in the study by the triphenylphosphine oxide (TPPO, Fig. 1e) building block. The coexistence of these two counterparts ensures good electronand hole-transporting properties of the PPT, thus maintaining charge balance in the emissive layer of PhOLEDs.
 

Within this context, we performed a joint experimental and theoretical investigation of the electronic structure of gas phase PPT and TPPO through XPS and NEXAFS at the S and P LII, LIII- edge. The experimental results have

been rationalized by relativistic time dependent density functional theory (TDDFT), which allows the inclusion of the coupling between 1h-1p excited configurations from the 2p degenerate core holes and gives a good account of the relativistic effects (mainly spin− orbit coupling) which are necessary to describe the transitions converging to the LII and LIII-edges.

The calculation of the S 2p and P 2p XP spectra allowed us to analyze the binding energies (BEs) both in terms of the spin−orbit splitting of the 2p coreholes and of the molecular-field splitting of the 2p3/2 levels. A comparison of S 2p and P 2p XP spectra with the two building blocks of PPT (respectively shown in Fig. 2a, b), reveals that both splittings are substantially conserved.

The small increase of the S 2p experimental BEs going from DBT to PPT (Fig. 2a) is a consequence of the

decreased shielding of the electronic charge density on sulfur due to the addition of two electron-withdrawing phosphine oxide moieties in PPT.

The small decrease of the P LII and LIII experimental BEs observed from TPPO to PPT (Fig. 2b) can be instead rationalized by the replacement of a single phenyl ring of TPPO with one condensed ring of the DBT moiety in PPT.

Furthermore, the TDDFT results are accurate enough to provide an unambiguous assignment of all

absorption bands that characterize the below threshold region of the S 2p and

P 2p NEXAFS spectra (Fig. 2c, d). They display strong similarities with those of the PPT building blocks, DBT and TPPO, in line with the similar local environment of the S and P atoms, being little affected by the increased molecular complexity of PPT.

This work sheds light into the structure of the P-O bond in PPT, a highly debated topic in the chemical literature and brings ahead our investigation of the electronic structure of ambipolar molecules and their building blocks by means of photoionization techniques. In this respect, while in our previous works the analysis of the O K-edge regions allows a more straightforward mapping of the O 2p molecular orbitals (MOs), the assignment of the NEXAFS P LII, III- edge features requires a higher level of computation: by including the coupling between different excitation channels arising from the 2p degenerate coreholes and relativistic spin-orbit coupling effects, we obtain a quantitative description on the higher-lying localized σ*(P−O) virtual MOs.

The results here presented indicate: (a) that P 3d atomic orbitals (AOs) are not involved in the formation of the P−O bond, and (b) the energy ordering of P 2p transitions to σ*(P−O) and π*(P−O) virtual states are compatible

with the traditional view of the P−O bond formation through a mechanism of negative hyperconjugation. A further study at the NEXAFS P K-edge would be useful to evaluate in detail the weight of the P 2p AO contributions to the P−O bond and will be the subject of future works.