Rational Combination of π‐Conjugated and Non‐π‐Conjugated Groups Achieving Strong Nonlinear Optical Response, Large Optical Anisotropy, and UV Light‐Switchable Fluorescence

Abstract Combining π‐conjugated and non‐π‐conjugated groups is an important strategy for synthesizing new nonlinear optical (NLO) crystals. However, the second harmonic generation (SHG) response and optical anisotropy can be limited by improper spatial alignment of these functional groups in the crystal structure. In this work, it is revealed that non‐π‐conjugated [NH2SO3] group acts as both hydrogen bond donor and acceptor, effectively regulating the 2D planar structure formed by π‐conjugated [C4N3H6] groups. The resulting organic–inorganic hybrid crystal C4N3H6SO3NH2 exhibits a strong SHG response (2.5 × KDP), large optical anisotropy (0.233@546 nm), and blue‐violet and green fluorescence near 360 and 520 nm, respectively. This work expands the methodology for creating new NLO crystals through organic–inorganic hybridization, while also showcasing the potential of C4N3H6SO3NH2 as a multifunctional optical material.


Table of Contents
Crystal synthesis.Single crystals of C4N3H7SO3NH2 were prepared by evaporation of aqueous solution.2-Aminopyrimidine and HNH2SO3 were mixed in a 1:1 ratio in 5 ml of deionised water.The mixed solution was thoroughly stirred and placed in an oven at 40 °C to slowly evaporate, and colourless solid crystals were obtained after one week.
Luminescent thin films preparation.An aqueous solution of C4N3H6SO3NH2 with a concentration of 10 mg/ml was prepared by dissolving 0.1 g of C4N3H6SO3NH2 powder in 10 ml of deionized water.A PVA solution with a mass fraction of 5 % was prepared by weighing 1 g of PVA and dissolving it in 19 ml of deionized water.0.2 ml of C4N3H6SO3NH2 aqueous solution was mixed with 10 ml of PVA solution to obtain the complex solution.The resulting solution was slowly added drop by drop to the slide surface and dried at room temperature for 24 hours.Complete and separate composite films were collected from the surface of the slides.
Single-Crystal X-ray Diffraction.A micron-level colorless transparent single crystal with good quality was selected under the polarizing microscope, and the data were collected by the Bruker D8 VENTURE diffractometer (Mo-Kα radiation).The SAINT [1] and SADABS [2] programs were used for data integration and absorption correction, and the ShelXT and ShelXL [3] were used to process single crystal data.Then, the PLATON [4] program was used to verify the symmetry of the crystal structure.In general, the position of hydrogen atoms could not accurately determined from XRD.However, they can be figure out if high-quality and high-resolution XRD data were acquired.In this work, H atoms were determined by two ways: (1) The position of H atoms (H3, H4, and H5) from [C4N3H6] groups were handled with AFIX 43 command (suitable for riding model refinement), which means an idealized terminal NH2 with the hydrogen atoms in the plane of the nearest substituent on the N atom.(2) Hydrogen atoms of the amino group in the [SO3NH2] group and the [C4N3H6] group, however, have been detected in the experimental electron density in difference Fourier map.A close examination of the residual electron density could locate two possible H positions near the N atom, and "DFIX 0.86 N1 H1" and "DFIX 0.9 N3 H2" commands were used to restraint the two N-H distances.The crystal data and structure refinement for C4N3H6SO3NH2 are displayed in Table S1.Crystallographic information such as atomic coordinates, bond lengths and angles, and hydrogen bonds for C4N3H6SO3NH2 are listed in Tables S2-S4.
Powder XRD.The phase purity of the grown crystals was checked by the powder XRD measurement.Dandong Haoyuan DX-27mini X-ray diffractometer (Cu Kα, λ = 1.5406Å) was used to collect the XRD data at room temperature.The 2θ range was 10-70°, the scanning step was 0.02°, and the scanning time was 2 s.
Scanning electronic microscope (SEM) images and energy dispersive X-Ray (EDX) microanalysis.SEM images and EDX microanalysis of the crystal were performed on a Hitachi TM4000Plus microscope with an acceleration voltage of 15 kV.The elemental mapping images show the distribution of elements in the crystal.
Thermal analysis.Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were gathered using a NETZSCH STA449F3 thermal analyzer.The crystal samples were heated from room temperature to 800 °C at a rate of 10 °C min -1 under air condition.
Infrared (IR) spectrum.The IR spectrum of C4N3H6SO3NH2 at room temperature was scanned with a Nicolet iS10 spectrometer in the wavelength of 400-4000 cm -1 .The crystal sample and KBr were grounded and pressed into a transparent sheet at a ratio of 1:100.
UV-Vis-NIR diffuse reflectance spectrum.The UV-Vis-NIR diffuse reflectance spectrum of C4N3H6SO3NH2 was collected in a Shimadzu UV-2600i spectrometer in the wavelength range of 200-1100 nm.
Fluorescence spectrometer.The excitation and emission fluorescence spectra of C4N3H6SO3NH2 were analyzed using a fluorescence spectrometer (FS5, Edinburgh, UK).
Birefringence Measurements.The birefringence was measured by interference colour method [5] using the polarizing microscope (ZEISS Axio Scope.A1) equipped with a quartz wedge compensator at 546 nm.The birefringence index was calculated by the formula R = Δn × d, where R, Δn and d were the optical path difference measured by polarizing microscope, the birefringence index and the crystal thickness measured by single-crystal X-ray diffraction, respectively.
Second harmonic generation (SHG) measurement.Under 1064 nm coherent light radiation generated by a Q-switched Nd: YAG laser, the powder SHG effects test was performed by using the modified Kurtz and Perry method. [6]fter grinding and sieving, the polycrystalline powder samples were divided into different particle sizes: 20-38.5, 38.5-55, 55-80, 80-125, 125-160, and 160-200 μm.KH2PO4 (KDP) samples with the same particle size range were used as the standard.
Theoretical calculation.The CASTEP [7] package was used for theoretical calculation based on density functional theory (DFT). [8]The generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) [9] and HSE06 [10] functionals were used to describe the exchange-correlation energy.The basic cutoff energy of a plane wave was identified as 750 eV.The Monkhorst-Pack scheme with a grid density of 4 × 3 × 4 at point k is used for the numerical integration of Brillouin zone for C4N3H6SO3NH2.

Figure S1 .S9Figure S2 .S10Figure S3 .
Figure S1.2D Fingerprint plots for (a) overall interactions and (b−i) individual interactions of atom types in crystal packing of C4N3H6SO3NH2.Here, de and di represent distances from the Hirshfeld surfaces to the nearest nucleus outside and inside the surface, respectively.