Fig. 1. (a) The suggested chemical structure of the compound, consisting of three aromatic rings. The DFT-predicted bandgap is ∼3.21  eV, and the TDDFT-predicted excited state lifetime is ∼2  ns. The HOMO and LUMO are highlighted. (b), (c) HNMR1 spectra reveal three peaks at 7.7, 7.4, and 6.9 ppm, which we attributed to the hydrogen atoms labeled by 1, 2, and 3 in (a). (d) Crystal residual from XRD data plotted with CCDC with the main axis. (e) XRD spectrum of the structure simulated in Mercury software from the resolved high resolution XRD and the powder XRD patterns collected, with crystals formed in toluene (inset).

Fig. 2. The reaction path: salicylaldehyde + para-phenylenediamine in an ethanol medium was refluxed to synthesize N,N’-bis(salicylidene)-1,4-phenylenediamine. The resulting compound is shown with diimine groups. Lower panel: an illustration of a Fabry–Perot laser based on a thin film solid light-emitting molecular layer.

Fig. 3. (a)–(c) Crystal structure of SA-p-PD: (a) xy, (b) xz, and (c) yz planes’ respective view of several unit cells. (d) Morphology of the crystal as calculated from BFDH approximation.

Fig. 4. Optical properties of the SA-p-PD compound. (a) The absorption, PLE, and emission peaks are at 375, 525, and 560 nm, respectively. (b) Absorption spectrum according to TDDFT simulation overlayed with experimental data. (c) and (d) FTIR spectrum reveals different modes: C-H stretching at 3000  cm−1, C-C at 1650  cm−1, C-N stretching at 900 and 1350  cm−1, and C-H bending at 750  cm−1.

Fig. 5. (a) PL spectrum before and after 70 min of illumination with nanosecond laser 35 mJ, 10 Hz repetition rate pulses with a wavelength of 532 nm. SA-p-PD and RH6G films, as in legends. (b) Fluorescent intensity as a function of time; SA-p-PD remained stable for 70 min, while RH6G integration decreased to 0.7 from its initial value after 10 min.

Fig. 6. Integration of PL intensity at different times for rhodamine 6G (RH6G) and SA-p-PD. The PL was recorded upon exciting the materials with nanosecond pulsed YAG:Nd laser (Litron Nano S) at a wavelength of 532 nm.

Fig. 7. (a), (b) PL intensities at different laser pump powers for two spots on the sample. At low laser power, the PLs have peaks at ∼550  nm. Increasing the pump power leads to amplified spontaneous emission and lasing, leading to spectral narrowing. (c), (d) Spectral color maps showing the PL intensity spectra for different pump powers (e), (f) Light-light curves indicating the laser thresholds of 1 and 3  MW  ⋅  cm−2, respectively.

Fig. 8. (a), (b) PL intensity at different laser powers. At low laser power, the PL has a peak at nearly 550 nm. On the other hand, at high laser power, the PL reveals a new peak at 620 and 570 nm. (c) and (d) are the integration of the PL at different laser powers. As we can see, the lasing peaks are at the same wavelengths as observed in Fig. 7.

Fig. 9. SA-p-PD emission under different excitation power and 450–500 nm excitation wavelength from a supercontinuum picosecond pulsed laser. The material is inside a planar cavity with silver mirrors, embedded within the SU8 polymer matrix. (a) Transmitted signal from excitation at different powers. (b) White light transmission under white light excitation. (c) Emission spectrum after excitation with a supercontinuum laser at a wavelength of 490 nm. (d) Excited state lifetime of the SA-p-PD with and without a cavity, corresponding to the peaks highlighted in (c) (refer to Table 1).

Fig. 10. (a) Open-aperture Z-scans of the material dissolved in DMSO measured at a laser pulse energy of 30 μJ. (b) Closed-aperture Z-scan of studied suspension using 30 μJ probe pulse. The fitting of the experimental curve using the measured radius of the focused beam at full width at the level of e−1 of maximum (47 μm) is shown as a red solid curve. (c) Two-photon fluorescence intensity versus wavelength. The peak fluorescence is observed for pump wavelengths around 820 nm for an excitation intensity of 50  GW·cm−2 used for all wavelengths. Two-photon fluorescence spectrum for a pumping wavelength of 820 nm (inset). (d) Change in the fluorescence intensity with increasing input power. The fitting of the data confirms a second-order two-photon process (inset).

Fig. 11. (a) Open-aperture Z-scans of the material dissolved in DMSO measured at different energies of laser pulses. (b) Open aperture Z-scan at pulse energy of 30 μJ and two fitting models. (c) Closed-aperture Z-scan of studied suspension using 30 μJ probe pulse. Fittings of the experimental curve at two Rayleigh lengths (5.6 mm and 6.3 nm) are shown by red and blue solid curves, respectively. (d) Two-photon fluorescence spectrum.

Fig. 12. Two-photon fluorescence (TPF) under 1064 nm excitation of 80 MHz modulated femtosecond pulsed laser.

Fig. 13. (a) Simulated polarizability diagonal elements at different wavelengths and (b) hyperpolarizability main tensor elements as estimated from the semi-empirical approach.