## 3 Results and Discussion
### Photoluminescence (PL)
The different PL characteristics of these two samples match well with different dye compositions as indicated by the TLC results shown in Figure 7c. When both samples were excited using the seventh excitation band-pass filter, a red shift of the emission maxima to 745 nm and 765 nm was observed for the PK1 and BV1, respectively. Under the excitation, using the eighth-to-tenth filters, both PK1 and BV1 samples possess a weak PL with an emission maximum centered around 845 nm. The weak PL under these excitation filters agrees with the weak IRL observed using the same filters as discussed earlier in Figure 3.  
Figure 11: PL spectra of the ink strokes of the writing BP pens of subgroup I, namely (**a**) PE1, (**b**) TO1, (**c**) RX1, and (**d**) RO4 pens under excitation of the ten different preset optical filters of the VSC. Figure 12: PL spectra of the ink strokes of the BP pens of subgroup II, namely (**a**) RE1a, (**b**) PR2, and (**c**) VN1, under excitation of the ten different preset optical filters of the VSC. Figure 13: PL spectra of the ink strokes of the writing BP pens of subgroup III, namely (**a**) PK1 and (**b**) BV1 pens under excitation of the ten different preset optical filters of the VSC. For the ink samples of subgroup IV, the TLC results (Figure 9d) indicated the presence of some dye components in the three samples. This is despite the fact that these samples did not express IRL and their corresponding ink strokes appeared dark (black) under the illumination of the ten preset optical excitation filters of the VSC. Additionally, the same samples showed no appreciable PL properties and no emission peaks could be recorded for them. To investigate this optical behavior, the PL spectra of the dye bands on the TLC plates of the corresponding samples were studied along with reference crystal violet. Figure 14 compares the PL spectra of TLC bands of reference crystal violet and crystal violet band for the RO4 sample (subgroup I) using the ten different preset optical excitation filters of the VSC. The PL spectra of the TLC of reference and sample crystal violet dyes are similar, showing low emission intensity under the first (400-485 nm) band-pass excitation filter. Upon excitation using the second-to-fifth filter, an increased emission could be observed with a maximum emission centered around 660 nm. When the bands were excited using the sixth 515-640 nm band-pass excitation filter, a red shift of the emission maximum to \(\sim\)705 nm was observed. The PL then started to decay from the seventh 545-675 nm band-pass filter until the emission was completely damped using the remaining excitation filters. Figure 15 PL spectra of the TLC bands of (a) D1, (b) D2, and (c) D3 dyes separated from the solvated ink samples of the RO4, PR2, and PK1 pens, respectively. The PL spectra indicate the high PL of the D2, D2, and D3 bands with emission maxima centered around 672 nm, 670 nm, and 690 nm, respectively, using the first (400-485 nm) band-pass excitation filter up to the fifth (485-610 nm) band-pass excitation filter. The optical excitation using the sixth (515-640 nm) band-pass filter resulted in a red shift of emission maxima to 705 nm, 707 nm, and 710 nm, respectively. Using the seventh excitation filter, a further red shift of the emission maxima to 734 nm for both D1 and D2 and to 740 in the case of D3. Upon excitation using band-pass filters with transmission greater than 585 nm, only the D1 sample continued to express significant PL under excitation using the 585-720 nm, 605-730 nm, and 645-800 nm band-pass filters. The D1 dye exhibited higher PL compared to crystal violet, and the high PL remained under the excitation of almost all band-pass bands. These results explain the high-intensity IRL for the samples of the mother Group I (Figure 1) since they contain the D1 dye as indicated by the TLC study (Figure 7a). For the D2 and D3, PL behaviors similar to that of the crystal violet were observed when excited using band-pass filters of wavelengths greater than 400-485 nm.