## 3 Results and Discussion  
### Host-Guest System  
Based on [PERSON] et al. research on phosphorescent Pt(II) Schiff Base complexes [16], we synthetized [Pt(II)(tetra-_tert_-butylsalophen)] named [Pt] (Scheme 1) which is thermally stable (Td = 390 \({}^{\circ}\)C). Time-dependent _density functional theory_ (TD-DFT) calculations [15] have been performed in order to assign the different electronic transition of [Pt].  
[Pt] exhibits absorption bands in the range of 400-600 nm attributed to metal-ligand-ligand charge transfer (MLLCT). After excitation at \(\lambda_{\text{ex}}\) = 390 nm, an emission band was observed between 575 nm and 750 nm (\(\lambda_{\text{max}}\) = 660 nm) (Figure 2). Alq\({}_{3}\) emission spectrum overlaps with the absorption spectrum of [Pt] and should favor an efficient energy transfer from Alq\({}_{3}\) to [Pt].  
The doping rate of the emissive layer plays a key role in the devices efficiency and color emission. It was demonstrated that the device efficiency increased with a concentration of platinum (II) dopant, up to 5 wt % or 6 wt % depending on the platinum(II) complex [16]. By considering a 6 wt % concentration of [Pt] (with Alq\({}_{3}\) number density \(\sim\)10\({}^{21}\) cm\({}^{-3}\)[17]) and by assuming that the Pt-complexes do not aggregate, the number density of Pt-complex is estimated at about 6.10\({}^{19}\) [Pt]/cm  
Figure 1: Schematic (**a**) and energy band diagram (**b**) of organic light-emitting diodes (OLEDs).  
and the average distance between neighboring complexes at \(\sim\)3 nm. Thereby, the effective distance over which the energy transfers between complexes (estimated at 3 nm) would be higher than the radius of triplet-triplet annihilation (\(<\)1 nm through Dexter mechanism) [18; 19; 20]. Furthermore, the steric hindrance between adjacent molecules induced by the _tert_-butyl substituents of [Pt] tends to limit the formation of [Pt] aggregates and hence reduces aggregation-induced phosphorescence quenching in thin solid film [15; 16]. Consequently, doping rate of [Pt] is fixed at 6 wt % to maximize the performance of the devices.  
The host-guest confinement methodology was used to modulate and to improve the electroluminescent properties of the devices (Figure 3). The monolayer device exhibited a deep red emission band attributed to the [Pt], whereas the green emission of Alq\({}_{3}\) was vanished, implying an efficient energy transfer from the host to the guest. In our previous investigation [15], it was observed that during OLED polarization, some of charges were directly captured by the dye to form an exciton. HOMO/LUMO levels of [Pt] (estimated at \(-\)5.84 eV and \(-\)3.48 eV (Figure 1) respectively from electrochemistry measurements) are within the electronic gap of Alq\({}_{3}\) matrix (with HOMO level: \(-\)5.98 eV and LUMO level: \(-\)3.13 eV); involving that exciton formation, arising from charge transfer mechanism, would be confined on [Pt] (Figure 3) by a charge transfer (CT) process. The emission process can be described by (i) migration of charge carriers (holes and electrons) within the host layer (ii) CT from the host to the guest (iii) exciton formation on [Pt] (iv) radiative recombination of triplet states.  
Figure 2: Normalized absorption (in black) and emission spectra (in red) of [Pt(II)(tetra-_tert_-butylsalophen)] in chloroform; and normalized emission spectra of Alq\({}_{3}\) (in green) and of an Alq\({}_{3}\):[Pt] (in orange) based OLEDs.