Synthesis, Crystal Growth, Structure, Crystalline Perfection, Thermal, Linear and Nonlinear Optical Investigations on 2-Amino-5-Nitropyridine 4-Chlorobenzoic Acid (1:1): A Novel Organic Single Crystal for NLO and Optical Limiting Optical Applications


 2-amino-5-nitropyridine 4-chlorobenzoic acid (1:1) (2A5NP4CBA), a potentially useful organic adduct compound has been synthesized and grown as optically transparent single crystals by conventional slow evaporation solution technique (SEST) for the first time in the literature. The formation of the new crystalline material was confirmed by the single-crystal X-ray diffraction (SXRD) analysis and the crystal structure of the grown crystal was found to be monoclinic. Fourier transform infrared (FTIR) spectrum has been recorded by the KBr pellet technique to determine the various vibrational functional groups in the title material. The powder X-ray diffraction (PXRD) and high-resolution X-ray diffraction (HRXRD) analyses have been carried out and the obtained results reveal that the grown crystal has a single-phase and is free from structural grain boundaries. The obtained less value (32 arc-s) of full width at half maximum (FWHM) for (001) plane indicates the excellent crystalline quality of the title 2A5NP4CBA single crystal. The linear optical properties were evaluated by the UV-vis-NIR absorption and transmittance analyses and the obtained results reveal that the grown crystal possesses more than 70% of optical transmittance window with the cut-off edge at 419 nm. The thermal analysis discloses that the grown crystal possesses good thermal stability of about 187°C. To determine the appropriateness of the grown crystal for the high-power laser application, laser damage threshold (LDT) analysis has been carried out by Nd: YAG laser of a wavelength of 1064 nm. The third-order nonlinear optical coefficients such as nonlinear absorption (β), nonlinear refraction (n 2 ) and nonlinear susceptibility (χ (3) ) have been evaluated by utilizing the single beam Z-scan technique using a solid-state laser of wavelength 532 nm. The calculated ꭓ (3) value is found to be reasonably good compared to other organic single crystals which are reported in the literature. The optical limiting (OL) behavior of the title crystal was evaluated using a solid-state laser at 532 nm and the limiting threshold was found to be 7.8 mW/cm 2 .


Introduction
In the modern world, the development of functional materials with desirable properties is intensely facilitating the fast-growing multi-disciplinary research areas like nonlinear optics (NLO), dielectric, piezoelectric, pyroelectric and ferroelectric, etc [1]. Over the last few decades, the nonlinear optical (NLO) single crystals play an inevitable role in a wide range of technological applications such as lasers, optoelectronics, information processing, optical data storage, optical switching, THz generation and detection, etc [2][3][4]. The rapid advancements in photonic and optoelectronic devices strongly depend on the ease of design and fabrication of the NLO single crystals. From the technological point of view, a single crystal with an inherent high nonlinearity and desired physico-chemical properties (like optical, mechanical, thermal, electrical, chemical stability and laser damage threshold, etc) are mandatory for numerous applications. Also, the grown crystals should possess a large size with high quality and low economical aspects. In this regard, the large demand for growing NLO single crystals is increasing day by day. Therefore, the crystal growth researchers are still putting enormous efforts into the design and development of novel organic, inorganic and semi-organic single crystals to attain high performances in real-time NLO devices.
In general, most of the organic single crystals possess a large NLO coefficient and fast response time due to the presence of inherent features such as π-delocalization length, donor-π-acceptor (D-π-A) molecular structure, multiple hydrogen-bonding interactions and robust NLO chromophores [5][6]. Due to the distinctive features, the organic NLO single crystals could be employed in a wide range of applications such as high-density optical data storage, electro-optical modulation, frequency conversion, laser devices and THz generation and detection, etc [7][8]. Due to the possessing of synthetic flexibility and high nonlinearity, extensive research work is being done by the crystal growers towards the design and development of organic single crystals to satisfy all the technological necessities mentioned earlier.
Among the organic/inorganic compounds, pyridine-based single crystals have attracted crystal growth researchers due to their ability to form ionic co-crystals with many numbers of organic or inorganic counterparts [9]. Most of the pyridine-based compounds have been synthesized through the protonation reaction by forming short/multiple hydrogen bonds between anion and cation. Pyridine is a basic heterocyclic compound that consists of five carbon atoms and one nitrogen atom. The pyridinium nitrogen plays a vital role in various types of reactions such as protonation, alkylation and acylation. In general, the pyridine-based single crystals possess inter and intramolecular charge transfer molecular structure that occurs between the donor and acceptor chromophores resulting in a large molecular polarization which significantly leads to the strong nonlinearity. In the last few decades, extensive research has been made in the synthesis of pyridine-based compounds due to their multifunctional properties including inter and intramolecular charge transfer, electrostatic behavior, hydrogen bond and π-π interactions [10]. Moreover, the pyridine-based compounds are used in a wide variety of medical drugs and biologically active compounds such as antitumor, antibiotic, anticonvulsant, antimicrobial, antibacterial, antiviral agents, etc [11]. In the literature, it is found that the pyridine-based materials serve a wide range of applications including semiconductors, photovoltaic cells, optical data carriers, optical switching, organic light-emitting diodes (OLEDs), etc [12].
Among the pyridine-based materials, the 2-amino-5-nitropyridine (2A5NP) is one of the attractive materials for NLO applications due to its D-π-A charge transfer molecular structure. It consists of electron donor (amino) and electron acceptor (nitro) groups to induce a high NLO response. The pyridine nitrogen and amino group play a role as cationic bonding and proton acceptor. Owing to the interesting molecular structure, the 2A5NP based single crystals are well recognized as NLO building blocks for the organic and semi-organic molecular complexes.
Thus, the 2A5NP derivative single crystals are projected as forefront candidates for the fundamental and applied investigations. Interestingly, the 2A5NP derivative single crystals display promising properties including large nonlinear coefficient (10-40 pm/V), a wide range of transparency (0.4-1.8 µm), high laser damage threshold, phase matching property as well as improved structural stability [13][14]. Structural and NLO properties of typical 2A5NP derivative single crystals have been already explored in the literature [15][16][17][18][19][20]. Among the 2A5NP derivative crystals, 2A5NPDP single-crystal displays multifunctional properties like electrooptic, piezoelectric, thermo-optic and dielectric [21][22]. Due to the charge asymmetry, the large second harmonic generation (SHG) efficiency is observed in 2A5NPP and 2A5NPT single crystals [23,17]. Therefore, substantial research work has been undertaken for designing 2A5NP derivative crystals for their possible usage in photonic and optoelectronic applications. From the above discussion, it is clear that there is plenty of scope for the investigation of the 2A5NP derivative single crystals. Based on the above discussions, 2A5NP4CBA material has been designed and the optically transparent single crystals have been grown for the first time in the literature. In this manuscript, we report the synthesis, solubility, crystal growth, structural, optical (linear and nonlinear) and thermal properties of the 2A5NP4CBA single crystal.

Material synthesis, solubility measurement and crystal growth
The starting raw materials, 2A5NP (Merck 98%) and 4-chlorobenzoic acid (Merck 98%) were taken in the molar ratio of 1:1 for the synthesis of 2A5NP4CBA material. HPLC grade methanol was used as a solvent for the reaction. Then, the calculated amount of raw materials were completely dissolved using a magnetic stirrer. Then, the filtered saturated solution was allowed to dry by heating at room temperature and finally, the yellow color crystalline material was obtained. The pH of the prepared solution was measured to be 2. The reaction scheme of 2A5NP and 4-chlorobenzoic acid is presented in Figure 1. The success of growing optically transparent single crystals with minimized crystal defects mainly depends on the purity of the synthesized materials. Hence, the synthesized crystalline material was recrystallized more than twice by methanol to obtain high purity.

Figure 1. Reaction scheme of 2A5NP with 4-chlorobenzoic acid
The solubility measurement of 2A5NP4CBA material has been carried out by the gravimetric method using three different solvents (methanol, ethanol and water) from room temperature to 55°C. In the beginning, a volume of 100 mL of methanol was transferred into the closed glass beaker and it was kept in a constant temperature bath (CTB). At 35°C, the powder material was slowly added to the glass beaker until it reaches the equilibrium concentration. For every 5°C, the solution was continuously stirred for 2 hours to ensure the equilibrium concentration. The same procedures were followed for ethanol and water solvents at different temperatures. The corresponding solubility diagram of 2A5NP4CBA is shown in Figure 2. It is observed that the title molecule shows increasing solubility in methanol and ethanol solvents with increasing temperature. The flat solubility is observed when using water as a solvent in the temperature range from 35-55°C. Based on the solubility data, it is found that the solubility in methanol is higher than in the ethanol and water solvents. At room temperature, the solubility was found to be 4.7 g/100 mL, 3.4 g/100 mL, 0.3 g/100 mL for methanol, ethanol and water solvents, respectively. Attempts have been made to grow 2A5NP4CBA single crystals using ethanol, methanol solvents by conventional SEST. Optically transparent and good quality single crystals were obtained only in methanol solvent. Particularly, the growth of title crystal was performed only in methanol solvent due to the reasonable solubility compared to the other two solvents.

Figure 3. As grown 2A5NP4CBA adduct crystals
Based on the solubility data, the 2A5NP4CBA crystalline material was dissolved in methanol, and it was continuously stirred for 2 hours to obtain the homogeneous saturated solution. Then, the filtered solution was poured into a well-cleaned petri dish and covered with a thick polythene sheet to control the abundant solvent evaporation. A few hours later, very small pinholes were made on the polythene sheet to achieve the controlled solvent evaporation and it was housed in the CTB at 35°C for the crystal growth. Previously, several growth attempts have been carried out by SEST to obtain the definite morphology single crystal. The spurious nucleation was controlled during the crystal growth process by the controlled growth conditions.
Optically transparent yellow color 2A5NP4CBA single crystals were harvested after 90 days with the maximum crystal dimension of 7×5×5 mm 3 . The photograph of as-grown crystals with definite morphology is shown in Figure 3.

X-ray diffraction (XRD) analysis
The unit cell parameters and crystal structure of the 2A5NP4CBA single crystal were determined using a computer-aided single-crystal X-ray diffractometer (Model: Bruker AXS Kappa APEX II) at 296K. The instrument has the provision of graphite monochromator and MoKα radiation (λ=0.71073Å). The CCD (charge-coupled device) detector was used to collect the intensity of the X-ray diffraction. The 2A5NP4CBA single crystal of dimension 0.3×0.25×0.2 mm 3 was used for the analysis. The data collection, unit cell refinement, absorption correction and data reduction for the entire experiment were performed using APEX2 (Bruker, 2004), SAINT (Bruker, 2004) and SADABS software [24]. The three-dimensional molecular structure of this compound was determined by X-ray crystallography using SHELXS-97 and later refined by SHELXL-16 to a final R-value of 3.4 %.
From the SXRD analysis, it can be declared that the grown 2A5NP4CBA crystal has monoclinic with the centrosymmetric crystal structure. The determined space group, number of molecules per unit cell, molecular weight and density are found to be P21/n, 4, 295.68 g/mol and 1.538 Mg/m 3 , respectively. With reference number 15111154, crystallographic data of the title crystal has been deposited in the Cambridge Crystallographic Database (CCDC). Table 1.
represents the crystallographic data of the title crystal. The asymmetric unit of the title salt comprises 2A5NP (C5 H5 N3 O2) and 4-chlorobenzoic acid (C7 H5 O2 Cl1) are shown in Figure 4. The atomic coordinates and equivalent isotropic displacement parameters, bond lengths and angles, anisotropic displacement parameters, hydrogen coordinates and isotropic displacement parameters and selected torsion angles are given in section S1, electronic supplementary information (ESI) (Tables (S1-S5)). The N3 nitrogen atom act as a donor atom and also forming two hydrogen bond interaction with the nearest molecules in the crystal structure. The N3-H3A…O2 ii and N3-H3B…O2 iii intermolecular hydrogen bond interaction links the molecules into an infinite one-dimensional chain with the descriptor of C(4) running parallel to "b" axis. The O2 oxygen atom acts as an acceptor atom in the crystal structure and it's forming three acceptor hydrogen bond interactions.
In the crystal structure, the C9 atom is forming hydrogen bond interaction with the 4-chlorobenzoic acid of O2 atom (C9-H9…O2 iii ). The N3-H3B…O2 iii and C9-H9…O2 iii hydrogen bond interactions together form an R2 1 (6) graph-set ring motif as shown in Figure 5. The C10-H10…O3 iv intermolecular interactions form cyclic centrosymmetric inversion dimers with an R2 2 (10) graph-set ring motif view down "b" axis as shown in Figure 6.   Using the crystallographic data, the packing diagram of the title molecule was simulated by the Mercury 3.9 program [26] and it is presented in Figure 8 (a). Besides, the morphology of the as-grown crystal has been predicted by the SXRD diffractometer (Model: Bruker AXS Kappa APEX II) with MoKα radiation (λ=0.71073Å) at room temperature. Based on the SXRD results, the morphology of the 2A5NP4CBA crystal has been drawn by the WinXMorph software [26] (which is similar to the results obtained from the SXRD) and it is shown in Figure 8 (b). The details of hydrogen bonding for the 2A5NP4CBA are described in Table 2.  To claim phase purity of the crystallinity of title material, the powder XRD analysis has been carried out at room temperature in the 2θ scanning range of 10-50°. The PANAnalytic XPERT PRO X-ray diffractometer with CuKα1 radiation of wavelength 1.54059 Å has been used for this experiment. Also, the PXRD spectrum has been simulated from the SXRD data and the obtained hkl planes were matched by the Mercury 3.9 software [26]. The comparison of the simulated and the experimentally obtained PXRD spectra are presented in Figure 9. It is observed that both the PXRD spectra seem to be almost identical. The well-defined sharp peaks at 2θ angle evidence that the grown crystal has good crystallinity with high purity. The obtained The crystalline superiority of the grown crystal was evaluated by the HRXRD analysis using PANanalytical X'pert PRO MRD high-resolution X-ray diffractometer with CuKα1 radiation. The rocking curve was recorded for as-grown 2A5NP4CBA single crystal along (001) plane and it is presented in Figure 10. The rocking curve shows the single peak with the FWHM of 36 arc-s and it indicates that the grown crystal has good crystalline perfection [27]. The obtained low value of FWHM for the title crystal is found to be almost close to the plane wave a b theory of dynamical X-ray diffraction for ideal crystal [28][29].

FTIR spectroscopy analysis
To identify the presence of multifarious functional groups in the 2A5NP4CBA adduct crystal, the FTIR spectrum was recorded in the scanning range between 4000 to 500 cm -1 . The experiment was done using the KBr pellet method by the Perkin-Elmer instrument at room atmosphere. Figure 11. represents the FTIR spectrum of the title adduct compound. The title molecule is known as an unprotonated molecule where the moieties such as 2A5NP and 4-chlorobenzoic acid bonded together with the interactions of hydrogen bonds. The obtained vibrational bands and their assignments are presented in Table 3.

Linear optical studies
The determination of the linear optical characteristics of the grown crystal helps to identify its utility in optoelectronics and nonlinear optical device applications. This is because the grown crystal could be of utility if it possesses a wide range of optical transmittance without having any absorption at the fundamental wavelength region [31]. Usually, the single crystal with a wide optical window and high transmittance are the essential parameters for the transmission of second and third harmonic generation [32]. The linear optical study also provides important information on the electronic band structure of the grown crystal. Here, the absorption of light involves the excitation of electrons from the ground state to the higher energy states [33].
To determine the linear optical characteristics of the grown crystal, the optical absorption and transmittance analyses have been carried out using the UV-vis-NIR spectrophotometer (Model: Perkin-Elmer Lambda 35) in the scanning range of 200 nm to 1100 nm. The well-polished 2A5NP4CBA single crystal with the crystal thickness of 2 mm was subjected to the absorption and transmittance analysis and the recorded spectra are shown in Figure 12. The resultant transmission spectrum illustrates that the grown crystal has a cut-off edge at 419 nm which facilitates it to be a good candidate for frequency conversion applications. It has been inferred from the figure that the transmittance of the grown crystal has more than 70% in the visible and near-infrared region. The comparison of cut-off wavelength for 2A5NP derivative single crystals is presented in Table 4. Such good optical transmittance in the grown crystal is attributed to the minimized intrinsic structural and crystalline defects like line defects, point defects, vacancies, voids and low angle grain boundaries [34]. If the density of defects is high, the optical loss may be induced in the optical medium owing to the occurrence of increased scattering centers. In the present case, the grown crystal has good optical transmittance and this is due to the occurrence of required optical properties such as less absorption and the less scattering centers within the crystal medium. Hence, it is noticed that the optical transmittance of the grown crystal seems to be quite good and this property leads to the positive sign for its usefulness in the linear and nonlinear optical device applications [35].
The absorption spectrum clearly shows that the grown crystal possesses strong absorption in the scanning range between 200 nm to 419 nm, which is due to the presence of various optically active chromophores such as NO2, NH2, NH + and COOH in the title molecule. These chromophores are also responsible for the appearance of yellow color in the title crystal. The absorption edge in the title molecule is attributed to the π-π* transitions [36]. The optical constants such as the absorption coefficient, bandgap and refractive index play an important role in the fabrication of photonics and optoelectronic device applications. Hence, such optical constants were determined for the 2A5NP4CBA crystal from the UV-Vis NIR analysis using some standard formulae and the details are presented in section S2 (ESI).
Normally, the wide bandgap materials are utilized in the photonic and optoelectronic device applications because of the less chances for the promotion of spontaneous emission or absorption at the fundamental region during the frequency conversion [37]. Based on the Tauc's plot relation, the bandgap of the 2A5NP4CBA crystal was 2.9 eV. The value of 'm' indicates the presence of direct allowed transition in the grown crystal. The refractive index and extinction coefficient of the grown crystal was found to be 1.66 at 532 nm. The Tauc's plot, refractive index spectra of the grown crystal are presented in Figure 13 (a and b), respectively. Thus, the obtained results show that the grown crystal has excellent optical properties such as good optical transmittance (more than 70% in the visible and NIR region), lower cut-off wavelength (419 nm), wide bandgap (2.9 eV) and a direct allowed electronic transition (m=1/2). Hence, the obtained results lead to conclude that the grown crystal can be utilized in the fabrication of linear and nonlinear optical applications.

Thermal analysis
The efficiency of nonlinear optical materials strongly depends on their thermal stability.
Usually, the high-power radiation induces a considerable amount of heat in the NLO medium which reduces the overall performance of the optical device. Hence, the discovery of new thermally stable nonlinear optical materials is always engaged in recent days due to its huge technological importance. The grown 2A5NP4CBA crystal has been subjected to thermal analysis using the instrument (model-Perkin Elmer Diamond TG-DTA) in the temperature range of 30°C to 350°C. A small piece of crystal sample weighing 7.2 mg was placed in the platinum crucible and it was subjected to heat with the heating rate of 10.0°C/min. During the experiment, an inert atmosphere has been maintained using nitrogen gas with a flow rate of 100 ml/min.  The TG spectrum shows that there is a single-stage weight loss occurring in the grown crystal when increasing the temperature upto 350°C. It is observed that there is no peak in the temperature range between 30°C to 140°C which evidences the absence of moisture in the grown crystal. The dissociation occurred in the temperature range between 140°C to 243°C in the form of volatile gaseous products. The sharp endothermic peak was observed in the DTA spectrum at 186°C which reveals the good crystalline quality and phase purity of the grown crystal. The nonexistence of endothermic or exothermic peaks in the temperature range from 30°C to 180°C reveals the absence of any structural changes in the grown crystal.
The sharp endothermic peak was observed at 186°C in the DTA curve which coincides with the weight loss observed in the TG curve. This is an indication of the simultaneous melting and complex decomposition of the 2A5NP4CBA crystal. The obtained thermal stability is quite good compared to some 2A5NP derivative single crystals and the details are presented in Table   4. The good thermal stability of the grown crystal is due to the presence of multiple hydrogen bond interactions between 2A5NP and 4-chlorobenzoic acid moieties.

Laser damage threshold (LDT) analysis
For the selection of an effective crystal for nonlinear optical applications, the crystal should have the ability to withstand high-power lasers. It is one of the major criteria is considered for NLO device fabrication. In addition, the investigation of the LDT for the newly grown single crystal with high power laser becomes crucial for the crystal growers to perform as potential material for the NLO and optoelectronic device applications. In general, the origin of the LDT is a complex process and majorly depends on the material's nature and various parameters of the experimental setup. When the laser beam is irradiated onto the crystal surface, the part of incident light induces the rise in temperature by the absorption and it leads to the temperature gradient. If the temperature gradient occurs, the corresponding thermal expansion coefficient is also induced in the crystal sample. Due to the anisotropic nature of the thermal expansion coefficient, the damage is induced in the case of light-absorbing optical materials [17].
Hence, it can be correlated that the thermal effect is one of the major reasons to induce the damage in the 2A5NP4CBA crystal with the exposure of a high-power laser beam. In another way, material's intrinsic factors like multi-photon absorption, electron avalanche effect, photochemical dissociation and photo-ionization may be responsible for the light-induced laser damage threshold [23]. Apart from that, the laser-induced damage can be persuaded by a great number of intrinsic factors like crystal quality, thermal stability, the density of crystal defects and the nature of the crystal surface. The experimental parameters like the wavelength of the laser, pulse width, repetition rate, intensity of the laser, longitudinal and transverse mode structures and beam size also influence the development of damage in the crystal sample [42]. Apart from that, the laser-induced damage may occur due to the existence of various nonlinear optical factors such as the self-focusing and multi or two-photon absorption behavior of the material [43][44].  Here we noticed that the size of the damaged pattern increases when increasing the intensity of the laser radiation. Based on the above laser parameters, the grown crystal withstands the laser radiation upto 10 mJ. The crystal's LDT was determined by the following expression [45]. focal point which is measured by the following expression.
where λ is the wavelength of the laser, f is the focal length of the convex lens and d is the diameter of the laser beam. From the above relation, the value of ωo was calculated and it was found to be 0.237 mm.  Table 5. Based on the obtained result, the significant large LDT can be attributed to the good quality and the stable molecular structure of the 2A5NP4CBA single crystal. Hence, the result shows that the grown crystal is a potential candidate for the high-power laser and NLO device applications.

Nonlinear optical analyses
The fabrication of optoelectronic devices strongly depends on the development of efficient NLO single crystals. For the usage of the grown crystal to the application, it is important to determine the quantitative information about their third-order nonlinear characteristics such as nonlinear absorption coefficient (β), nonlinear refractive index (n 2 ) and nonlinear susceptibility (χ (3) ). Numerous characterization techniques such as four-wave mixing [48], nonlinear interferometry [49], nonlinear ellipse rotation [50], beam-distortion [51] and the single beam Z-scan [52][53] are available to determine β and n2 of the optical materials.
However, the single-beam Z-scan is a widely accepted technique by the scientific community because of its reliability, simplicity and high accuracy [53][54].
To determine the third-order nonlinearity of 2A5NP4CBA single crystal, a diode laser of wavelength 532 nm has been used with the power of 100 mW. The sample size should be considered less than the Rayleigh length (ZR) during the experiment. It is found that the crystal thickness (L) is less than the ZR, and hence basic criteria are found to be satisfied for the Z-scan measurement. The value of L and ZR are calculated to be 0.83 mm and 1.32 mm, respectively. In the case of open aperture measurement, the entire transmitted light obtained through the sample was recorded without placing any aperture at the detector. Accordingly, the interesting nonlinear effects like self-focusing and self-defocussing behavior of the nonlinear medium can be identified while the signature of n2 is found to be positive and negative, respectively [55]. The normalized open aperture spectrum of the 2A5NP4CBA crystal is shown in Figure 16 (a). Here, the transmittance is found to be maximum at the focus (Z=0) when increasing optical intensity, which indicates the nonlinear saturable absorption (SA) and positive absorption coefficient of the grown crystal. Also, the maximum transmittance (peak) at focus (Z=0) in the open aperture spectrum reveals the photon absorption in the 2A5NP4CBA single crystal [56].
The recorded closed aperture spectrum is presented in Figure 16  To determine absolute third-order nonlinear parameters such as n2, β and χ (3) of the grown crystal, the standard nonlinear transmission equations have been used and the details are described in the literature [40,58]. The calculated value of n2, β and χ (3) values are found to be 6.042×10 -8 cm 2 /W, 0.019×10 -4 cm/W and 2.958×10 -6 esu, respectively. The obtained χ (3) is found to be good compared to some know NLO single crystal and the details are presented in Table 6.
Hence, the obtained result leads to conclude that the grown crystal is a potential competitor in optoelectronic and NLO fields.
In recent days, extensive research is being carried out to develop efficient optical limiters for the protection of optical elements from high-intensity light [59]. This interesting phenomenon can be observed in NLO materials where the transmitted intensity of the optical medium remains constant at a particular point with increasing intensity or fluence [60]. In the present work, a similar Z-scan setup with a solid-state laser of wavelength 532 nm was used to determine the optical limiting property of the 2A5NP4CBA single crystal. The intensity of the incident laser was systematically increased using a polarizer-analyzer (PA) setup and the corresponding transmitted intensity of light was recorded through the aperture by the photodetector. The measured OL response of the title crystal is shown in Figure 17. Interestingly, the intensity of the transmitted light is linearly increased at low input power and then it saturates beyond the 7.8 mW/cm 2 . This result indicates the existence of noticeable optical limiting characteristics of the 2A5NP4CBA single crystal. This is due to the strong nonlinearity of the grown title single crystal which may be originated by its molecular structure.
Also, it can be correlated with the existence of third-order nonlinear properties such as negative refraction and strong saturable absorption of the 2A5NP4CBA single crystal. In general, the origin of the optical limiting is a complex process, however, it may be correlated with important factors such as nonlinear optical refraction (or thermal effect), nonlinear optical absorption and nonlinear optical scattering [61]. Hence, the obtained result satisfies the essential requirement of optical limiters.

S2. Determination of linear optical parameters:
The optical absorption coefficient of the 2A5NP4CBA single crystal was calculated by the following equation. where, α is the optical absorption coefficient, T is the optical transmittance and t is the thickness of the sample. The bandgap was calculated by the following Tauc's plot relation.
where,  is the frequency of incident photons, A is optical constant. The tauc's plot was drawn between the photon energy (hν) versus (αhν) 2 and the bandgap of the grown 2A5NP4CBA crystal was found to be 2.9 eV. In equation 2, the value of m describes the electronic structure of the 2A5NP4CBA molecule. Here, the value of m is found to be 1/2, which indicates the direct allowed transition behavior of the title crystal. The reflectance (R) was evaluated using the following relation.
The parameters such as α and wavelength (λ) were used to calculate the extinction coefficient (K) of the 2A5NP4CBA single crystal and the relevant equation is given below.
The refractive index ( 0 n ) was calculated by the following equation.