*2.2. PL Properties of PS, PVK, and PFO Blends*

Polymeric blends are a well-known approach leading to an increase in the emission ability. Blended active layers have been demonstrated to be advantageous to fabricate optoelectronic devices such as efficient pure-color LEDs [43–45], guaranteeing easiness of fabrication, high processability, and low-cost. This situation resembles a diluted solid solution of the emitters into amorphous domains preventing aggregation caused quenching (ACQ) effect [46,47]. Different doped films were spin-coated by dissolving the dyes in a non-emissive and non-conductive (PS) or an emissive and conductive (PVK and mostly PFO) amorphous polymeric matrix. These polymers are typically employed in the construction of emissive layers. In natural light, the doped layers are homogeneous and transparent films except for A2-PFO. They display various shades of color, from yellow to red. Different dye percentages were tested in order to get better PL performance. As expected on the base of our previous study [48–51], the best emission for both samples was recorded on the more diluted blends (10 wt%), where the ACQ effect is attenuated.

π

The films turned out to be stable in air for three months at room temperature under natural light, showing identical optical properties both in absorption and emission. In fluorescence, broad emission bands peaked between orange and orange-red are recorded, with Stokes Shift values ranging from 83 to 183 nm. The maximum of the absorption and emission bands of the blends are reported in Table 2.

In Figure 4, the emission spectra of the blend samples are compared with the crystalline ones. The emission spectra of pure PVK and PFO are reported, for comparison, in Figure S4 of Supplementary Materials section.

A2-PVK blend is the most red-shifted emitter with CIE (0.55; 0.44) (see Table 2). Quantitatively, all A1 blends show higher PLQYs respect to A2 blends but in the case of PFO samples, clearly highlighting the difference in the electronic pattern of the two chromophores. In both cases, PLQYs of the PS blends are lower respect to the analogous PVK blends. For an azobenzene material, the PL performance of A1-PVK is a remarkable achievement. As PVK is poorly fluorescent, 40% PLQY is due mostly to the chromophore itself as the ACQ effect is suppressed. The PL performance of the A1-PFO blend is particularly interesting. Polyfluorene and its derivatives (PFs) have recently emerged as the most promising polymeric matrixes due to their PL and electroluminescence efficiencies, good thermal and chemical stability [52]. Poly (9-octylfluorene) itself emits blue light with a large bandgap. The typical approach to realize tuned emission from polyfluorene derivatives is based on the copolymerization of low-bandgap π-conjugated moieties with PFO monomers. Unfortunately, polyfluorides often show both excimer and aggregate formation during thermal annealing. The formation of excimers involves the generation of dimerized units of the polymer that emit light at energies lower than those of the polymer itself. This effect hinders the use of polyfluorenes for most applications [53].

In our case, the simple approach to dissolve a red/orange dopant in PFO produced an efficient (57% PLQY) bright orange/red (see CIE coordinates in Table 2) luminescent blend. The outstanding PL performance of A1-PFO is due to the excellent match between the chromophore and the polymeric matrix, as rationalized by DFT calculations in the Supplementary Materials part. On the contrary, the attempt to homogeneously dissolve A2 in PFO failed. Only in that case, the film was not perfectly homogeneous and transparent, and the emission of the chromophore in the red region is negligible compared to the much higher blue emission of PFO itself (see Figure 4d and Figure S4 in Supplementary Materials).
