The evolution toward solid-state inorganic materials marked a pivotal shift in optical material science, driven by the pursuit of enhanced optical stability, functionality, and overall performance. This resulted in the development of technologies such as light-emitting diodes (LEDs), optical sensors, photovoltaics, and display devices. Recently, developments in organic material engineering and computational physics have led to the creation of new classes of solid-state materials, driven by their ease of synthesis and compound tunability. These have the potential to provide compact, tunable, and cost-effective alternatives to conventional lasing gain materials and optoelectronics. However, widespread adoption of organics has not yet been seen due to limited photostability, low quantum yield efficiency of solid-state emission, and robustness of mechanical properties [1–5], among others. To fulfill the rigorous demands of lasing as an application, the material’s optical properties must be tailored to achieve low lasing thresholds [6–8], high quantum yields [9,10], and the ability to form defect-free films or crystals.

Phenylenediamine (PD) stands out among aromatic benzene-based derivatives as an attractive source for colorful optical emission [11]. It has been demonstrated previously in fluorescence-based organic PD-based carbon dots [12,13], molecular conjugates, and polymers [14,15]. The presence of amine groups attached to the benzene rings gives rise to a wide range of potential applications [16–19]. In particular, para-phenylenediamine (p-PD) Schiff bases show extensive application potential [20], mainly concerning the optical sensitivity of the systems utilized in metal ion complexation and biomedical compounds for antibacterial and antifungal purposes [21,22] to name a few. Possessing unique optical properties due to the bridging imine groups to benzene rings [23], that facilitates the formation of π-conjugated aromatic systems with efficient intramolecular charge transfer (CT), enhancing their application spectrum to include molecules with varied emission properties [24–26] among other examples. p-PD carbon quantum dots demonstrate high fluorescence intensity and bathochromic shifts, extending their applicability to a range of optical devices [27]. Introducing an organic material with donor (D) and acceptor (A) blocks plays an important role in the second-order nonlinear optical (NLO) response, such as D-A, A-D-A, and D-D-A [28]. One interesting compound is N,N’-bis(salicylidene)-1,4-phenylenediamine, which was recently investigated for both linear and nonlinear optical properties [29]. However, many aspects of the material were not fully investigated, such as its photostability and crystal structure.

Here, we crystalize N,N’-bis(salicylidene)-1,4-phenylenediamine and show that it displays solid-state bright emission along with robust mechanical and photostability properties. This platform provides a much-needed baseline for the advancement of organic solid-state technologies. Here, we discuss the synthesis of the molecule in question and its molecular and crystallographic characterization. Then we discuss its linear optical properties, lasing properties, and threshold. This is followed by a description of its nonlinear optical properties and Z-scan analysis. Finally, we demonstrate its stability under high-power pump excitation.

Para-phenylenediamine (p-PD) 99%, salicylaldehyde (SA) 98%, ethanol (EtOH), methanol (MeOH) absolute AR, toluene AR, chloroform AR, SU8 3050, rhodamine B (RhB) 95%, rhodamine 6G (Rh6G) 99%, and acetone 90% were purchased from Merck Ltd.

100 mL EtOH and 2 g p-PD were stirred in a 250 mL three-necked flask with a magnetic stirrer. Two equivalents of SA solution were added, and the reaction volume was heated to 90°C. The mixture was stirred for 3 h, and an orange precipitate appeared. The solid precipitate was collected with a silica filter and washed with an excess of EtOH. The powder was dissolved in toluene, and the solution was heated to full dissolution followed by cooling to allow crystal formation.

The optical properties of the samples were measured using several different techniques. Photoluminescence excitation (PLE) spectroscopy was carried out with a Synergy H1 plate reader. Absorbance spectra were captured using a Macy’s 1100 spectrophotometer, equipped with a tungsten lamp source and a silicon photodiode detector. Before testing, the samples were diluted with deionized water. Fluorescence lifetime measurements were conducted using a PicoQuant system with a Taiko picosecond diode as the 375 nm excitation source. Fourier transform infrared spectroscopy (FTIR) analysis was performed using a Nicolet iS10 FTIR spectrometer, equipped with a KBr/Ge beam splitter and a DTGS detector. Solid samples were analyzed following ASTM E1421 standards. For the H1-NMR analysis, dried and purified SU8-MPD powder was dissolved in chloroform and analyzed using a Bruker Ascend 500 high-resolution NMR machine. A 355 nm laser with a maximum pulse energy of 9 mJ, beam diameter of 4 mm, repetition rate of 50 Hz, and pulse duration of 29 ps was used.

The nonlinear optical properties of the SA-p-PD compound were investigated through both open-aperture and closed-aperture Z-scan experiments. The 1064 nm, 30 ps, 50 Hz pulses from the laser source were focused using the 400 mm focal length spherical lens on the sample. The measured beam radius at the focal point was 47 μm. The material was dissolved in DMSO, and the suspension was placed in the 2-mm-thick optical cell.

Two-photon fluorescence experiments were carried out using an optical parametric amplifier system (TOPAS) operating in an 800–900 nm tuning range. The 50 fs, 2 kHz output pulses were delivered to the sample with a beam diameter of 1 mm before being collected and analyzed using a spectrometer.

Density functional theory (DFT) was utilized for the theoretical modeling using CP2K software [30]. A triple-3 polarization quality Gaussian basis set (TZVP-MOLOPT-GTH) and BLYP functional with a plane-wave cutoff of 300 Ry were used. The geometry of the compound was first optimized using the Broyden–Fletcher–Goldfarb–Shanno (BFGS) routine [31] and was run until the force on each atom was smaller than 0.03eV·Å−1. After optimizing the structure, the IR frequencies and intensities were calculated using HESSIAN within the Born–Oppenheimer approximation and finite difference method [32] with an increment of 0.001 Bohr. VESTA software was used to draw the product’s atomic structure [33]. To calculate the excited-state lifetime, we used time-dependent DFT (TDDFT). The lifetime was calculated according to [34] τ=(3ϵ0πℏc3)·(∑iωif3μif2)−1,(1)where ωif is the Bohr pulsation between the initial (i) and final (f) states, μif is the transition dipole moment, and ϵ0, c, and ℏ are the vacuum permittivity, the speed of light, and the reduced Planck constant, respectively.

DFT simulations suggest the chemical structure of SA-p-PD, which is shown in Fig. 1(a). The compound has inversion symmetry and consists of three in-plane aromatic rings. The DFT electronic structure reveals a bandgap of 3.21 eV, while TDDFT predicts an excited state lifetime of ∼2ns. The HOMO and LUMO are also shown in Fig. 2(a). The HNMR1 spectrum of the compound is shown in Fig. 1(b). The HNMR1 peaks are located at 7.7, 7.4, and 6.9 ppm (parts per million). The peak at 7.7 ppm is a doublet; therefore, it has one neighbor and is marked as 2 in Fig. 1. The peak at 7.4 ppm (marked as 3 in Fig. 1) is a triplet with two neighbors. The peak at 6.9 ppm is less evident and most likely has both singlet and doublet contributions; therefore, it is related to no neighbor and one neighbor [1 and 2 in Fig. 1(a), respectively]. The integration of those peaks matches the number of protons in the resulting compound. The HNMR1 is similar to that shown in an earlier study [29] except the peak at 13 ppm, which we did not observe.

The latter was dissolved and recrystallized from hot toluene to form the crystals from the powder. Needle-like crystals formed once the saturated solution started to cool down, as shown in the inset of Fig. 1(e). High-resolution XRD revealed the interference patterns of the compounds from single crystals and resolved its crystal structure. The corresponding space group for the crystal is P21, and its extended unit cell appears in Fig. 1(d) as extracted from Mercury software. Simulations of the XRD patterns were obtained from Mercury software for the powder form appearing in Fig. 1(e) (black dotted line), along with the experimental XRD spectrum for the powder (red line). Overlapping the spectra ensures the high purity of the compound and aligns well with the simulation obtained from the resolved crystal structure. Nevertheless, there are some broadening effects and variations in intensity, which could be attributed to the different sizes of the grains of the powder, and some impurities or secondary phases in the sample. Instrumental parameters like beam and detection efficiency also could affect the peaks. The uncharged material with low bipolarity crystallized based on its π stacking forces, allowing the planar molecules to align in parallel one above the other. Furthermore, the compound crystal structure has been resolved for the first time in this work. The powder exhibited high fluorescence properties in its solid form, thus motivating further detailed optical studies. Different crystal planes of multiple unit cells are shown in Fig. 3.

The optical properties of SA-p-PD are summarized in Fig. 4(a), showing the absorption, PLE, and emission peaks at 375, 525, and 560 nm, respectively. According to the DFT calculations, the bandgap is at 358 nm (3.46 eV), which matches the observed absorption peak. The simulation absorption peak overlaps with the experimental data and is shown in Fig. 4(b). The FTIR spectrum of the compound is shown in Fig. 4(c). The mode at 1600cm−1 is related to aromatic ring C-C stretching modes, and the mode at 2900cm−1 is related to C-H stretching [35,36]. The zoomed region of the graph between 500 and 1750cm−1 is shown in Fig. 4(d). The mode at 1600cm−1 is related to C-C ring stretching. The modes at 900cm−1 and 1350cm−1 are related to C-N stretching. The TDDFT simulation partially agrees with the experimental data, as shown in Fig. 4(d).

Figures 5(a) and 5(b) present the measured photostability and sustained emission during nanosecond laser pulse irradiation at 532 nm with a 10 Hz repetition rate and a pulse energy of 35 mJ. After excitation for 70 min, the emission showed no deterioration in intensity. This is due to the molecular structure of SA-p-PD, being planar and symmetric, and it is ideally suited for robust π-conjugation, which is crucial under such high-energy excitation. This configuration allows for efficient intramolecular charge transfer and helps in dissipating the absorbed energy uniformly across the molecule, thereby minimizing hotspots that could lead to photodegradation. The solid-state nature of SA-p-PD restricts molecular vibrations and rotations, significantly reducing non-radiative relaxation pathways and enhancing stability. Conversely, RH6G exhibits very different behaviors in its crystalline and thin film or dissolved states. As indicated, the crystalline form of RH6G shows poor fluorescence and rapid photodegradation under similar laser excitation. A loss of more than 40% of its fluorescence intensity was recorded for RH6G. This could be attributed to less optimal molecular packing in the crystal lattice, which might not support effective π−π interactions, leading to localized energy states that are more susceptible to photobleaching. By contrast, in a thin film configuration, RH6G molecules possibly achieve a more favorable configuration that allows better electronic interactions and energy delocalization, resulting in brighter fluorescence. However, it is unstable, as can be seen in Fig. 6. The superior stability of SA-p-PD compared to Rh6G is primarily due to its symmetric molecular structure. Rh6G has a non-symmetric structure in its excited state, with energy or electron density localized on the ethoxy center ring [37]. This localization causes the higher energy levels to be more exposed to environmental factors, leading to increased vulnerability to degradation, including oxidation. Furthermore, Rh6G contains more reactive groups, contributing to its instability. In contrast, SA-p-PD’s symmetric excited state ensures a more uniform distribution of energy across the molecule, reducing its exposure to the environment and enhancing its stability.

Figures 7(a), 7(b) and 7(d), 7(e) show the PL intensities at two different spots on the flat sample. At lower laser powers, the PL peaks uniformly near 550 nm, indicative of the material’s intrinsic electronic properties and the fundamental resonance within the organic structure supporting this wavelength. As the power increases, pronounced peaks at 620 nm and 570 nm emerge in spots 1 and 2, respectively. This variation in peak emission demonstrates the directional nature of lasing—where specific paths of light amplification are favored due to the material’s non-uniformity and potential micro-resonances within the non-uniform film. Further exploring the lasing thresholds [Figs. 7(e) and 7(f)], it can be seen that the required energies to initiate lasing differ between 1MW·cm−2 at spot 1 and 3MW·cm−2 for spot 2. This suggests that localized structural differences within the film, such as thickness variations or defects, can significantly impact the pump efficiency, the cavity round-trip loss, and the material gain. Figure 8 consolidates the findings by showing PL at both 570 nm and 620 nm under a unified collection, with lasing thresholds of 2MW·cm−2 and 8MW·cm−2, respectively. The higher thresholds in this configuration hint at an increased energy requirement, possibly due to the decreased light collection efficiency.

Figure 9 illustrates the emission characteristics of SA-p-PD when embedded within an SU8 polymer matrix and housed in a planar cavity with silver mirrors, excited by a supercontinuum picosecond laser with wavelengths between 450 nm and 500 nm. This specific configuration, leveraging the reflective capabilities of the silver mirrors, enhances the photon lifetime within the cavity, potentially amplifying the emission intensity through increased photon-matter interactions. In other words, the cavity has smaller losses, hence a lower threshold. The observed enhancement in the emission profile under various excitation powers suggests the formation of a favorable environment for lasing, with thresholds ranging from 0.5 to 5.5kW·cm−2. These preliminary findings indicate a promising direction for further exploration, though a detailed analysis of the spectral behavior and lasing stability remains to be comprehensively investigated.

Also, we have conducted an analysis using the quality factor (Q) of the cavity. This analysis was based on the interference fringes observed in the white light transmission spectrum through the cavity. The quality factor was determined from the contrast between the maxima and minima of the fringes. The visibility (contrast) V was used to calculate the finesse (F) of the cavity [38]: F=πV·(1−V2)−1.(2)

The free spectral range (FSR) was calculated by identifying the wavelength differences between consecutive peaks, and the quality factor (Q) was then determined using Q=Fλ·FSR−1.(3)

The FSR can be calculated according to FSRAvg=ΣΔλpeak(n−1)−1.(4)

For our cavity with two gold or silver mirrors, each 30 nm thick, and a gain material with low absorbance, we found the average quality factor to be approximately 100–200 depending on the fringe wavelength. Furthermore, we have used the full width at half-maximum method and retrieved similar results. This Q value indicates a typical value of such geometry. Figure 9(c) illustrates the emission characteristics of SA-p-PD when embedded within an SU8 polymer matrix and closed in a planar cavity with silver mirrors. Figure 9(d) is the photon counting statistics of the structure, with 2.2 ns for the material without a resonator, 2.1 at 575 nm, and 1.8 at 525 nm peaks, respectively, indicated on the specific counted fringe in the cavity. The experimental lifetime matches the theoretical value calculated in Fig. 1 with 2 ns estimated decay. The difference between the values could be attributed to the calculation accuracy and to the environmental conditions in the experiment, in which there are surrounding molecules and other parameters such as local field and temperature that are different.

Figure 10(a) shows open-aperture Z-scans of the sample in its liquid form, illuminated at a laser pulse energy of 30 μJ. The suspension demonstrated steady growth of saturable absorption, where the transmittance of samples increased as the suspension moved toward the focal plane (z=0). A kinetic model can be used for the analysis of nonlinear absorption in the case of depletion of the ground state [39]. The absorption coefficient in the case of this model can be written as α(z)=α0(1+I(z)/Isat)−1.(5)Here I(z) and Isat are the laser radiation intensity and saturation intensity, respectively, and α0 is the low-intensity linear absorption coefficient. The fitting of the normalized open-aperture Z-scan transmittance is based on this model and is shown in Fig. 10(a) for an incident pulse energy of 30 μJ (solid red curve). The assumption of depletion of the ground state seems reasonable at intensities of (1–5)×1010W·cm−2. The intensity-dependent measurements of normalized transmittance showed that in the investigated range of intensities (2.5×1010 to 5×1010W·cm−2) an open-aperture Z-scan shape was caused by the depletion of the ground state concentration. From this fitting the ratio of I0 and Isat was deduced (I0/Isat=0.2). I0 corresponds to the intensity of laser radiation in the focal plane (5×1010W·cm−2). The corresponding Isat of the studied suspensions was determined to be 2.5×1011W·cm−2.

The simplified formula to extract the negative value of nonlinear absorption is T(z)=1−q(2−3/2). Here q=βI0Leff/(1+z2z0−2), where β is a nonlinear absorption coefficient, I0 is the intensity of laser radiation at the focal plane, Leff is an effective length, which at small linear absorption can be replaced by the thickness of the cell containing the suspension, and z0 is a Rayleigh length of the focused radiation (5.6 mm in this case). This relation can also be applied to the calculation of the negative value of the nonlinear absorption coefficient in the case of nonlinear absorption, with the substitution of a plus instead of a minus in the above formula. Then the fitting of this formula with the experimental data determined the nonlinear absorption coefficient related to the saturable absorption (βSA). This value was determined to be βSA=−2×10−11cm⋅W−1. These different nonlinear absorptions at several energies are presented in Fig. 11(a), and in Fig. 11(b), different models of fitting are presented to achieve the optimal parameters.

The closed-aperture Z-scans of the samples showed the positive sign of the nonlinear refractive index measured at a pulse energy of 30 μJ. The minimum followed by a peak transmission with the Z-scan position corresponds to the self-focusing of the probe pulses in the studied suspension. In the case of the laser pulse energies and pulse repetition rates used, the only process observed in this liquid was Kerr-induced positive nonlinear refraction. The small size of the closed aperture (1.5 mm) excluded the asymmetric pattern with larger peaks concerning the smaller valley of this CA Z-scan. This shape of the CA Z-scan at larger aperture sizes highlights the growing influence of the negative nonlinear absorption analyzed in this subsection. This experimental dependence was fitted using the standard Eq. (6) [40]. This allows the extraction of the nonlinear absorption coefficient and nonlinear refractive index (γ), T=1+2(−ρx2+2x−3p)·((x2+9)(x2+1))−1Δφ0.(6)Here x=z/z0, where z0 is the Rayleigh length of the focused radiation given by z0=π(w0)2/λ, w0 is the beam waist radius at 1/e of the FWHM (42.5 μm in this case), ρ=β/2kγ, Δφo=kγI0Leff, and k=2π/λ, where λ is the wavelength of probe pulses (1064 nm), I0 is the intensity of probe pulses in the focal plane [5×1010W⋅cm−2 in the case shown in Fig. 10(b)], Leff=(1−exp(−α0L))/α0 is the effective length of the sample, α0 is the linear absorption coefficient, and L is the thickness of the studied suspension inserted in the 2-mm-thick cell. The Rayleigh length and correspondingly the beam waist radius can be determined from the closed-aperture curve by applying the relation for the distance between the valley and peak (ΔZ≈1.7z0) [41] in the case when the Kerr-related nonlinearity showed a prevailing influence over other nonlinear optical processes. The fitting of the experimental curve using the measured radius of the focused beam at full width at the level of 1/e of intensity distribution (47 μm) is shown as a red solid curve. The determination of the Rayleigh length from the experimentally measured closed-aperture Z-scan (z0exp≈6.3mm) allowed the adjustment of the fitting curve [see also the fittings in Fig. 11(c) for 5.6 and 6.3 mm Rayleigh lengths]. This curve gives a better match with the experimental data. The corresponding value of γ retrieved from the fitting procedure in this case was 5×10−16cm2⋅W−1. Two-photon fluorescence (TPF) under 1064 nm excitation of 80 MHz modulated femtosecond pulsed laser is shown in Fig. 12.

The two-photon nonlinear properties of the compound were examined by illuminating the material with tunable femtosecond pulses from 800 to 900 nm. Figure 10(c) shows the total two-photon fluorescence intensity as a function of wavelength, with a maximum value observed around 820 nm. A peak pump intensity of approximately 50GW·cm−2 was used for all the applied wavelengths. Increasing the wavelength beyond 820 nm results in a significant decrease in the fluorescence intensity. The inset in Fig. 10(c) illustrates the spectral emission profile between 500 and 700 nm, with a significant emission peak centered at 560 nm and a full width at half maximum (FWHM) of 60 nm. Figure 10(d) shows the change in fluorescence intensity as a function of input power at a wavelength of 840 nm. The fit to the data shown in the inset confirms the second-order two-photon absorption process. The inset in Fig. 10(d) presents a log-log plot of fluorescence intensity versus pump power, with a slope of 2.08. This slope is a critical indicator of a two-photon absorption process, confirming the nonlinear behavior of the fluorescence emission.

Polarizability and hyperpolarizability estimation is shown in Fig. 13. The calculation is based on MOPAC code [42]. The diagonal elements of polarizability showed biaxial property, with high birefringence between the axes. The latter is expected from elongated planar molecules, and it means that the excited dipoles have significant polarizability along the resonant axes than in other planes. Furthermore, the excitation dipoles along the x-axis show resonant behavior around 500 nm, thus approaching the limits of semi-empirical calculation that diverges near the absorption lines. Moreover, the highest hyperpolarizability components are related to x-polarized plane wave excitations that are aligned with the molecular main axis.

The molecular symmetry appears in the presented Schiff-based molecule with the A-D-A order placing this molecule as a highly stable fluorescent material. The significant delocalization of electronic excitations along the symmetric plane, and with efficient solid-state packing has enabled this configuration to distribute CT along the molecule efficiently preventing rapid bleaching. Furthermore, π stacking alignment in the solid-state has intensified the emission from the substance without observation of quenching due to close packing. As a result, we can conclude that this solid-state effective optical configuration emphasizes a new set of optical materials to be investigated for prolonged durable emission and stabilized organic solids.

In this study, we synthesized and characterized a para-phenylenediamine Schiff base, exploring its potential for applications in solid-state organic materials and laser technology. The SA-p-PD compound was prepared through a condensation reaction of para-phenylenediamine with salicylaldehyde, resulting in a solid precipitate that demonstrated notable fluorescent properties with an emission maximum located at 560 nm and significant absorbance around 380 nm. Detailed molecular and crystallographic analyses, including HNMR1, FTIR, and XRD, affirmed the structural integrity and optical activity of the compound. We investigated both the linear and nonlinear optical properties of SA-p-PD, revealing a two-photon absorption cross-section of 2×10−11cm2·W−1·GM (GM, Goeppert Mayer unit) and a critical lasing threshold observed at 1MW·cm−2, reducible to 0.6kW·cm−2 in an optimized resonant cavity. Other nonlinear optical characteristics determined during these studies are the saturable absorption-induced nonlinear absorption coefficient (−2×10−11cm⋅W−1), saturated intensity (2.5×1011W·cm−2), and Kerr-induced nonlinear refractive index (5×10−16cm2⋅W−1).

Furthermore, the compound displayed sustained emission and minimal photobleaching under rigorous femtosecond laser excitation, indicating its high photostability. The exceptional nonlinear optical properties of the para-phenylenediamine Schiff base are significantly influenced by its symmetric and planar molecular structure, which facilitates effective electron delocalization. The A-D-A configuration of the molecule ensures a uniform distribution of the electronic cloud across the structure, enhancing the material’s ability to engage in two-photon absorption and other nonlinear interactions. The planar imine bridged structure of the para-phenylenediamine Schiff base enhances nonlinear optical performance by promoting π-electron delocalization, crucial for efficient two-photon absorption and harmonic generation. This structural symmetry stabilizes the electron distribution, reducing relaxation losses and enabling sustained high-intensity interactions, vital for advanced photonic applications. Such features highlight the potential of similar configurations in nonlinear optics, warranting further exploration. This electron delocalization is critical as it minimizes energy localization, reducing potential sites of photochemical damage and contributing to the material’s photostability. Such properties are essential for applications requiring high-intensity laser excitations, where nonlinear optical responses can be exploited to achieve efficient frequency conversion and photostable lasing. Consequently, the intrinsic structural symmetry supports robust photon-electron interactions and promotes sustained emission characteristics, positioning this compound as a versatile candidate for advanced optoelectronic applications. This opens the door for using SA-p-PD and other potential isomers in advanced photonic technologies, offering another material for enhancing the performance and functionality of organic-based lasers and optoelectronic devices.

Acknowledgment. We thank Al-Zahrawi Foundation for hosting our activities and providing support to the TRDC research team.