Crystal Form Diversity of 2-(4-(Diphenylamino)benzylidene) Malononitrile

Abstract

In the present work, we report the synthesis and characterization of 2-(4-(diphenylamino)benzylidene) malononitrile (DPAM) via a piperidine-catalyzed Knoevenagel condensation reaction. Two distinct crystal forms (A-1 and A-2) of this product were obtained by controlling the crystallization conditions, exhibiting orthorhombic and monoclinic crystal systems, respectively. Single-crystal X-ray diffraction revealed that both forms exhibited highly twisted benzene rings, which suppressed exciplex or excimer formation, enhancing luminescence. Crystal A-1, with a higher density, showed stronger hydrogen bonding and more rigid molecular packing, while A-2, with a lower density, exhibited weaker π–π interactions. Both crystals demonstrated high thermal stability. Notably, the A-2 crystal displayed a mechanochromic behavior: grinding or applying pressure induced a structural transformation into A-1, accompanied by a fluorescence shift from red to yellow. This transformation was attributed to increased steric hindrance and changes in molecular packing. This study highlights the relationship between crystal structure and optoelectronic properties, offering insights into the design of organic crystalline materials for applications in pressure sensing, anti-counterfeiting, and information encryption.

1. Introduction

Organic semiconductors have revolutionized modern electronics by offering a unique combination of flexibility, tunability, and cost-effectiveness, bridging the gap between traditional inorganic semiconductors and next-generation optoelectronic technologies [1,2,3,4,5,6,7]. Among these materials, triphenylamine (TPA) derivatives stand out as a cornerstone of organic semiconductor research due to their distinctive star-shaped molecular architecture, characterized by a central nitrogen atom covalently bonded to three phenyl rings. This molecular design not only imparts exceptional electron-donating capabilities but also enables efficient intramolecular charge transfer (ICT), a phenomenon driven by the spatial separation of electron-donating (D) and electron-accepting (A) groups connected through π-conjugated systems [8]. The ICT behavior in TPA derivatives is further amplified by their non-planar conformation, which arises from the steric hindrance between the phenyl rings and the sp³-hybridized nitrogen center. This three-dimensional geometry suppresses molecular aggregation, a common drawback in planar π-conjugated systems, while facilitating efficient charge transport, making TPA derivatives indispensable in organic electronics [9,10]. Their optoelectronic properties, including high hole mobility, broad spectral absorption, and tunable energy levels, are precisely modulated through substituent engineering, where functional groups such as methoxy, cyano, or carbazole are strategically introduced to adjust the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels [11,12]. These attributes have propelled TPA derivatives into diverse applications, ranging from light-emitting diodes (OLEDs) and perovskite solar cells (PSCs) to chemosensors for metal ion detection and bioimaging probes for cellular processes.

The versatility of TPA derivatives is exemplified by their applications in light-emitting diodes (OLEDs) [13,14,15], where they serve as high-performance hole-transporting layers (HTLs) and emissive materials. Their high hole mobility ensures efficient charge injection into emissive layers, while their amorphous nature mitigates crystallization-induced device degradation. In perovskite solar cells (PSCs) [16,17,18,19], TPA derivatives such as spiro-OMeTAD have become industry standards due to their optimal energy level alignment with perovskite absorbers like CH₃NH₃PbI₃. The star-shaped TPA core facilitates isotropic charge transport, while methoxy substituents passivate surface defects at the perovskite/HTL interface, reducing non-radiative recombination. Fluorinated TPA derivatives have further enhanced moisture resistance, addressing one of the critical stability issues of PSCs. Beyond energy conversion, TPA derivatives excel as chemosensors [20,21,22,23,24], leveraging their electron-rich nature to detect metal ions and nitroaromatic explosives through fluorescence quenching or colorimetric changes. The integration of TPA derivatives into organic field-effect transistors (OFETs) highlights their potential in flexible electronics [25,26,27,28]. Asymmetric TPA oligomers with branched alkyl chains demonstrate balanced ambipolar charge transport, maintaining solution processability while achieving switching frequencies of >1 MHz in complementary logic circuits. However, the commercialization of TPA-based technologies faces hurdles, including the scalability of synthesis and environmental durability. In conclusion, TPA derivatives epitomize the synergy between molecular design and functional performance in organic electronics, offering a versatile platform for applications spanning energy, sensing, and biomedicine. While challenges in stability and scalability persist, interdisciplinary advances in chemistry, materials science, and engineering hold the key to unlocking their full potential, positioning TPA-based materials at the forefront of sustainable, next-generation optoelectronic technologies.

The synthesis of TPA derivatives typically employs Buchwald–Hartwig amination [29] or Ullmann coupling [30] reactions, with substituent engineering (e.g., methoxy, cyano, or carbazole groups) allowing the precise modulation of the highest occupied molecular orbital (HOMO) levels. Despite these advances, challenges persist in balancing solubility, thermal stability, and environmental resistance, necessitating further molecular design innovations.

2-(4-(diphenylamino)benzylidene) malononitrile (DPAM) is a kind of triphenylamine derivative, which was first reported by Kuroda in 2003 [31], as shown in Figure 1. In this molecule, the triphenylamine group acts as the electron-donating unit, while the dicyano group serves as the electron-accepting unit, connected by a vinyl bond [32,33]. However, the relationship between the crystal structure, morphology, and mechanochromic phenomenon of DPAM has not been thoroughly investigated, let alone the exploration of new crystal forms. The crystal form diversity of DMAP was obtained in this study. In addition, the characterization, crystal structure, and mechanochromic phenomenon were studied.

2. Experimental Section

2.1. Materials

4-diphenylaminobenzaldehyde (96%) was purchased from Bidepharm (Shanghai, China), while Energy Chemical (Shanghai, China) supplied the remaining chemicals. All chemicals were used without any further purification. All solvents were purchased with HPLC-grade quality, also from Energy Chemical (Shanghai, China).

2.2. Synthesis of DPAM

2-(4-diphenylamino-benzylidene)-malononitrile (DPAM) was synthesized by a piperidine-catalyzed Knoevenagel condensation reaction between 4-diphenylaminobenzaldehyde and malononitrile. As shown in Scheme 1, 4 drops of piperidine were added to a solution of 4-(diphenylamino) benzaldehyde (2.74 g; 10 mmol) and malononitrile (1.48 mL; 10 mmol) in ethanol (40 mL). The solution was stirred at room temperature for 3 h, and water (40 mL) was added to the reaction mixture. The DPAM product was precipitated as an orange-red crystal (A-2) and obtained in a 95% yield (3.04 g) by filtering and drying at 110 °C for 3 h. The above DPAM product (1.52 g) was completely dissolved in ethanol (30 mL) under reflux, and water (8 mL) was added. Then, the mixture solution was poured into ice water (100 mL) to precipitate a yellow crystal (A-1), which was obtained in a 90% yield (1.37 g) by filtering and drying at 110 °C for 3 h.

A-1: 3.04 g, 95% yield; yellow solid, m.p. 137.6–138.6 °C; ¹H NMR (CDCl₃, 400 MHz): δ = 7.78–7.66 (m, 2H), 7.51 (s, 1H), 7.38 (dd, J = 10.8, 4.9 Hz, 4H), 7.27–7.14 (m, 7H), and 7.00–6.87 (m, 2H); ¹³C NMR (CDCl₃, 101 MHz): δ = 157.88, 153.49, 145.16, 132.98, 129.96, 126.72, 126.13, 122.80, 118.49, 115.18, 114.07, and 75.59; IR (film): γ = 3052 (w), 2651 (w), 2219 (s), 1567 (s), 1506 (s), 1487 (s), 1352 (s), 1191 (s), and 826 (w) cm⁻¹; HRMS (ESI): calcd. for C₂₂H₁₅N₃ [M+H]⁺ 322.1344, found 322.1340; [M+Na]⁺ 344.1164, found 344.1157 (for the results, see Figures S1–S3 in the SI).

A-2: 1.37 g, 90% yield; orange solid, m.p. 139.2–139.9 °C; ¹H NMR (CDCl₃, 400 MHz): δ = 7.78–7.66 (m, 2H), 7.51 (s, 1H), 7.38 (dd, J = 10.8, 4.9 Hz, 4H), 7.27–7.14 (m, 7H), and 7.00–6.87 (m, 2H); ¹³C NMR (CDCl₃, 101 MHz): δ = 157.88, 153.49, 145.16, 132.98, 129.96, 126.72, 126.13, 122.80, 118.49, 115.18, 114.07, and 75.59; IR (film): γ = 3060 (w), 2655 (w), 2219 (s), 1565 (s), 1504 (s), 1486 (s), 1350 (s), 1190 (s), and 825 (w) cm⁻¹; HRMS (ESI): calcd. for C₂₂H₁₅N₃ [M+H]⁺ 322.1344, found 322.1340; [M+Na]⁺ 344.1164, found 344.1157 (for the results, see Figures S1–S3 in the SI).

2.3. Single-Crystal X-Ray Crystallography

Single-crystal X-ray diffraction data were collected using a Bruker D8 Venture Ims3.0 single-crystal X-ray diffractometer (Bruker, Berlin, Germany) with Mo Kα (λ = 0.71073) for cell determination and subsequent data collection. The A-1 crystal was kept at 170.00 K, and the A-2 crystal was kept at 302.00 K during the data collection. Using Olex2 [34], the structure was solved with the SHELXT [35] structure solution program using intrinsic phasing and refined with the SHELXL [36] refinement package using least squares minimization.

2.4. Other Physicochemical Measurements

¹H NMR and ¹³C NMR spectra were recorded with a Bruker AVANCE 400M superconducting NMR spectrometer at 25 °C, with deuterated chloroform as the lock solvent. The working solution concentration for NMR was 20 mg/mL. All chemical shifts are given in the ppm scale and refer to the nondeuterated proportion of the solvent. The infrared (IR) spectra were recorded with a Nicolet iS10 spectrometer with an attenuated total reflectance (ATR) (Thermo Fisher Scientific, Waltham, MA, USA) in the range of 4000 to 400 cm⁻¹ and with a resolution of approximately 4 cm⁻¹. HRMS were obtained using an Agilent 6230 TOF mass spectrometer (Agilent, Waldbronn, Germany) with electron spray ionization (ESI). The thermogravimetric analysis (TG) of the compounds was conducted using a DSC Q100 thermal analysis system (TA Instruments, New Castle, DE, USA). The TG test was performed under a nitrogen atmosphere, heating from 25 °C to 400 °C at a rate of 10 °C·min⁻¹, with weight–temperature curves recorded. The differential scanning calorimetry (DSC) test was performed in the temperature range of 20–200 °C, at a heating rate of 10 °C·min⁻¹, with heat flow–temperature curves recorded. The X-ray powder diffraction data were collected with a Bruker D8 Discover diffractometer with parafocusing Bragg–Brentano geometry and a one-dimensional LynxEye XE-T detector (Bruker, Karlsruhe, Germany) using Cu Kα radiation (λ = 1.5418 Å) and operated at 42 kV and 100 mA at room temperature. The scan 2θ range was from 2° to 50° at a rate of 1°/min.

3. Results and Discussion

3.1. Description of Crystal Structure

The color of a substance is primarily determined by its molecular crystalline structure, as the molecular structure dictates its light absorption and reflection properties. Therefore, studying the relationship between a substance’s color and its molecular crystalline structure helps us gain a deeper understanding of the substance’s physical and chemical properties. To gain insight into the molecular organization of DPAM, the yellow needle-shaped single crystal A-1, suitable for structural analysis, was obtained by the slow evaporation of an ethanol and water mixed solution of DPAM at room temperature for one week. The orange-red block-shaped single crystal A-2, suitable for structural analysis, was obtained by cooling the hot solution of DPAM in ethanol at room temperature for 6 h. The crystal data, collection procedures, and refinement results of A-1 and A-2 are summarized in

Table 1. Crystal data and structural refinement of compounds A-1 and A-2.

Compound	A-1	A-2
Empirical formula	C22H15N3	C22H15N3
Formula weight	321.37	321.37
Temperature (K)	170.0	302.00
Crystal system	Orthorhombic	Monoclinic
Space group	Pbca	P21/c
a (Å)	11.2205(6)	7.0262(9)
b (Å)	14.6394(10)	15.9319(16)
c (Å)	20.9226(16)	16.1075(19)
α (°)	90	90
β (°)	90	95.169(5)
γ (°)	90	90
Volume (Å3)	3436.8(4)	1795.8(4)
Z	8	4
ρcalc (g·cm−3)	1.242	1.189
μ (mm−1)	0.075	0.072
F(000)	1344.0	672.0
Crystal size (mm3)	0.48 × 0.05 × 0.02	0.35 × 0.2 × 0.15
Radiation	MoKα (λ = 0.71073)	MoKα (λ = 0.71073)
2θ range for data collection (°)	4.972 to 54.252	5.078 to 56.686
Index ranges	−14 ≤ h ≤ 13, −18 ≤ k ≤ 17, −26 ≤ l ≤ 26	−9 ≤ h ≤ 9, −21 ≤ k ≤ 21, −21 ≤ l ≤ 21
Reflections collected	23800	37280
Independent reflections	3802 (Rint = 0.0672, Rsigma = 0.0419)	4468 (Rint = 0.0397, Rsigma = 0.0241)
Data/restraints/parameters	3802/0/226	4468/0/226
Goodness-of-fit on F2	1.058	1.163
Final R indexes [I >= 2σ (I)]	R1 = 0.0434, wR2 = 0.0901	R1 = 0.0888, wR2 = 0.2343
Final R indexes [all data]	R1 = 0.0698, wR2 = 0.1058	R1 = 0.1317, wR2 = 0.3143
Largest diff. peak/hole (e.Å−3)	0.13/−0.19	0.20/−0.30

. The A-1 crystal was confirmed as a new crystal form with an orthorhombic crystal system (Figure 2). The results indicate that the A-1 molecular crystal existed in the orthorhombic space group Pbca with a = 11.2205 (6) Å, b = 14.6394 (10) Å, c = 20.9226 (16) Å, and β = 90°. A-2 was a crystal form with a monoclinic crystal system (Figure 3).The results indicate that the A-2 molecular crystal existed in the monoclinic space group P2(1)/c with a = 7.0262 (9) Å, b = 15.9319 (16) Å, c = 16.1075 (19) Å, and β = 95.169 (5)°. The space group of A-2 was one of the most prevalent space groups, first discovered and reported by the Son group in 2009 [33], whereas the crystal structure of A-1 has not been previously reported. A comparison drawn from Table 1 revealed that A-2 had a density of 1.189 g/cm⁻³, whereas A-1 had a density of 1.242 g/cm⁻³. A-2 exhibited a lower density than A-1; the crystal structure of the A-2 compound was, therefore, more porous.

Table 2. Bond lengths for compounds A-1 and A-2.

Bond/Distance (Å)	A-1	A-2
N1-C6	1.4370(18)	1.435(3)
N1-C7	1.4335(17)	1.438(3)
N1-C13	1.3885(18)	1.375(3)
N2-C21	1.146(2)	1.149(4)
N3-C22	1.150(2)	1.148(4)
C1-C2	1.385(2)	1.371(4)
C1-C6	1.388(2)	1.379(4)
C2-C3	1.379(2)	1.386(4)
C3-C4	1.378(2)	1.360(6)
C4-C5	1.386(2)	1.371(5)
C5-C6	1.381(2)	1.380(5)
C7-C8	1.391(2)	1.364(4)
C7-C12	1.389(2)	1.379(4)
C8-C9	1.390(2)	1.388(5)
C9-C10	1.384(2)	1.366(5)
C10-C11	1.379(2)	1.352(5)
C11-C12	1.386(2)	1.387(4)
C13-C14	1.4099(19)	1.408(3)
C13-C18	1.4071(19)	1.401(3)
C14-C15	1.3751(19)	1.370(3)
C15-C16	1.408(2)	1.402(3)
C16-C17	1.402(2)	1.405(3)
C16-C19	1.438(2)	1.432(3)
C17-C18	1.373(2)	1.369(3)
C19-C20	1.358(2)	1.355(4)
C20-C21	1.438(2)	1.423(4)
C20-C22	1.429(2)	1.431(4)

and

Table 3. Bond angles for compounds A-1 and A-2.

Angles of A-1 (°)	A-1	Angles of A-2 (°)	A-2
C7-N1-C6	117.59(11)	C1-N1-C7	117.8(2)
C13-N1-C6	120.80(11)	C13-N1-C1	120.8(2)
C13-N1-C7	121.33(12)	C13-N1-C7	121.3(2)
C2-C1-C6	120.02(15)	C2-C1-N1	120.2(2)
C3-C2-C1	120.27(15)	C2-C1-C6	120.1(3)
C4-C3-C2	119.70(15)	C6-C1-N1	119.7(3)
C3-C4-C5	120.38(15)	C1-C2-C3	119.5(3)
C6-C5-C4	120.07(14)	C4-C3-C2	120.5(4)
C1-C6-N1	120.69(13)	C3-C4-C5	119.9(3)
C5-C6-N1	119.75(13)	C4-C5-C6	120.3(3)
C5-C6-C1	119.55(14)	C1-C6-C5	119.6(3)
C8-C7-N1	119.57(13)	C8-C7-N1	120.2(3)
C12-C7-N1	120.39(13)	C8-C7-C12	119.5(3)
C12-C7-C8	120.03(13)	C12-C7-N1	120.3(3)
C9-C8-C7	119.16(15)	C7-C8-C9	119.9(3)
C10-C9-C8	120.74(15)	C10-C9-C8	120.2(3)
C11-C10-C9	119.75(15)	C11-C10-C9	120.3(3)
C10-C11-C12	120.29(15)	C10-C11-C12	120.0(3)
C11-C12-C7	119.99(15)	C7-C12-C11	120.1(3)
N1-C13-C14	121.47(13)	N1-C13-C14	121.6(2)
N1-C13-C18	120.74(13)	N1-C13-C18	120.8(2)
C18-C13-C14	117.78(13)	C18-C13-C14	117.6(2)
C15-C14-C13	121.14(13)	C15-C14-C13	120.4(2)
C14-C15-C16	121.37(14)	C14-C15-C16	122.4(2)
C15-C16-C19	125.72(14)	C15-C16-C17	116.6(2)
C17-C16-C15	116.85(13)	C15-C16-C19	118.5(2)
C17-C16-C19	117.43(13)	C17-C16-C19	124.9(2)
C18-C17-C16	122.54(14)	C18-C17-C16	121.5(2)
C17-C18-C13	120.31(14)	C17-C18-C13	121.4(2)
C20-C19-C16	131.58(15)	C20-C19-C16	130.9(2)
C19-C20-C21	119.85(15)	C19-C20-C21	125.7(2)
C19-C20-C22	125.64(14)	C19-C20-C22	119.7(2)
C22-C20-C21	114.46(14)	C21-C20-C22	114.6(2)
N2-C21-C20	178.90(19)	N2-C21-C20	179.1(3)
N3-C22-C20	177.76(18)	N3-C22-C20	179.0(3)

list the selected bond distances and angles. As can be observed from Table 2, in both compounds, the carbon-carbon double-bond lengths of C19-C20 increased, and the carbon-carbon single-bond lengths of C20-C21 shortened, while the bond length of the two carbon-nitrogen triple bonds, N2-C21 and N3-C22, decreased. This was attributed to the formation of conjugation through electron delocalization, leading to changes in bond length. In Table 3, the bond angles of the three bonds for A-1 (C7-N1-C6, C13-N1-C6, and C13-N1-C7) and the three bonds for A-2 (C1-N1-C7, C13-N1-C1, and C13-N1-C7) were all approximately 120°, suggesting that the N atom at the center of triphenylamine was close to sp² hybridization. Meanwhile, the bond angle of C22-C20-C21 in both configurations was around 114°, which was less than 120°, indicating that the C20 atom in the double bond did not exhibit standard sp² hybridization. This deviation was likely due to the compression exerted by the adjacent triphenylamine group on the carbon–carbon double bond, resulting in increased steric hindrance.

The choice of crystallization solvents had a significant impact on the macro-scale morphology of the crystals. The yellow crystal, A-1, belonging to the orthorhombic crystal system, was obtained from a mixed solvent of ethanol and water. In contrast, the orange-red crystal, A-2, which had a monoclinic crystal system, was obtained from pure ethanol. The crystalline structure revealed that the molecule of A-1 was arranged in antiparallel patterns characterized by hydrogen bonds. The highly twisted benzene rings within the crystal structure inhibited the formation of exciplexes or excimers, further promoting increased emissions. The crystalline structure revealed that the molecule of A-2 was arranged in parallel patterns characterized by π…π weaker interactions and hydrogen bonds. This arrangement helped to stabilize the conformations of the molecules, leading to enhanced emissions. The highly twisted stacking architecture resulted in shorter effective conjugation lengths, producing bluer emissions, while the planar conformation increased the effective lengths, yielding redder emissions.

3.2. IR Spectra

Infrared spectroscopy is effective in identifying specific functional groups and molecular vibrations in crystalline materials. The attenuated total reflectance (ATR) is particularly useful for crystal samples without the need to crush or grind them into a powder. This is particularly advantageous for maintaining the integrity and original structure of the samples [37]. The IR spectra of A-1 and A-2 exhibited small differences in the 4000–400 cm⁻¹ region (Figure 4 and

Table 4. The major IR spectra of A-1 and A-2 (cm⁻¹).

Compounds	v (C-H)	v (C≡N)	v (C-H)
A-1	3052	2219	1487
A-2	3060	2219	1486

). Both of the compounds displayed the same a strong absorption band at 2219 cm⁻¹, attributed to the C≡N stretching vibration. But the spectrum of the A-1 crystal displayed a weak absorption band at 3052 cm⁻¹ and 1487 cm⁻¹, attributed to the C-H stretching vibration, while the A-2 crystal exhibited a slightly shifted peak at 3060 cm⁻¹ and 1486 cm⁻¹, indicating a possible interaction with different molecular environments.

3.3. Thermal Properties

Thermal stability is an important property of organic crystal materials [38,39]. The TG/DSC techniques were used to study the thermal properties of compounds A-1 and A-2 in a nitrogen atmosphere. The results are presented in Figure 5 and Figure 6 (see also Figures S4–S7 in the SI). A-1 and A-2 exhibited high thermal stability. It can be observed that the A-1 crystal exhibited significant mass loss for the quickest decomposition only after reaching 319.88 °C, while the A-2 crystal exhibited little mass loss (0.37%) after reaching 175.00 °C, and exhibited significant mass loss for the quickest decomposition after reaching 324.22 °C. The DSC test curves for A-1 and A-2 show that the A-1 crystal exhibited a melting point at around 138.02 °C, and the A-2 crystal exhibited a melting point at around 139.43 °C. The results are consistent with the melting point measurement. From the DSC curves, it can be calculated that the enthalpy of A-1 was 25.4 kJ/mol, while A-2’s enthalpy was 26.1 kJ/mol. The higher enthalpy of compound A-2 suggested superior crystallization behavior, aligning with the observation that A-2’s decomposition temperature (324.22 °C) exceeded A-1’s (319.88 °C). However, as illustrated in Figure 5 and Figure 6, the thermal properties of both crystals exhibited a high degree of similarity. This similarity was primarily attributed to the intermolecular hydrogen bonds, with A-1 possessing hydrogen bonds of 2.609 Å and 2.6577 Å, and A-2 featuring hydrogen bonds of 2.600 Å and 2.693 Å. The comparable hydrogen bonds between the two compounds accounted for their similar thermal properties. Additionally, it is crucial to note that when A-2 was subjected to heating, it did not transition to the A-1 configuration. This was due to the fact that heating caused the compound’s volume to expand and its density to decrease, while A-2’s density was greater than A-1’s, and heating did not induce a conformational change from A-2 to A-1.

3.4. X-Ray Powder Diffraction

X-ray powder diffraction represents a pivotal method in the analysis of crystal structures. The powder diffraction results for A-1 and A-2 are presented in Figure 7 and Figure 8, respectively. Additionally, the numerical values of the five most significant 2θ angles for both compounds are tabulated in

Table 5. The major angles of 2θ of A-1 and A-2 (°).

2θ (°)	1	2	3	4	5
A-1	7.780	12.359	13.779	15.622	19.942
A-2	7.820	12.382	13.815	15.633	19.983

. Upon examining the data, it was evident that the 2θ angles of A-2 were slightly larger compared with those of A-1. Furthermore, the difference in peak intensity between 2θ₁ and 2θ_2-5 for A-2 was larger than that for compound A-1, indicating that A-2 was not adequately ground during powder diffraction, resulting in a certain degree of orientation in the crystal.

3.5. Mechanochromic Phenomenon

In recent years, mechanochromically luminescent crystalline organic materials, which can reversibly alter the color of their solid-state emissions in response to mechanical stimuli, have garnered significant attention due to their diverse potential applications [40,41,42,43,44]. These materials have a wide range of applications in fields such as rewritable paper, anti-counterfeiting ink, pressure sensing, and optical sensors. After grinding the orange-red A-2 crystal in a mortar for 10 min, its color changed to yellow, as shown in Figure 9. Under excitation with UV light at 254 nm, the fluorescence color of A-2 shifted from red to yellow. This change may be attributed to alterations in the twisting angles of the three benzene rings within its crystalline structure due to the grinding process, resulting in the formation of the yellow crystal form of A-1. Furthermore, when the A-2 crystal was subjected to a pressure of 10 MPa for 15 min, affording the yellow crystal A-1, the enhanced fluorescence property was also observed under 254 nm UV light. The mechanism behind the mechanically induced change in the emission color was attributed to the transition of the crystal form. To verify whether compound A-2 truly transformed into A-1 after being subjected to high pressure, we conducted X-ray powder diffraction tests on A-2 after thorough grinding (Figure 10). Upon comparing Figure 10 with Figure 7, it became evident that the powder diffraction pattern of A-2 after high pressure closely resembled that of A-1, suggesting that A-2 underwent a transformation into compound A-1 upon extensive grinding. This transformation occurred due to an increase in the compound’s density after high pressure, leading to a shift from a configuration with low symmetry to one with high symmetry. Given that the density of A-2 was lower than that of A-1, it transitioned to the configuration of A-1 upon high pressure. Additionally, high pressure disrupted the electron delocalization of A-2, leading to an increased energy difference between the HOMO and LUMO orbitals. Consequently, the emission wavelength shortened, resulting in a shift in fluorescence from red to yellow.

4. Conclusions

In summary, 2-(4-diphenylamino-benzylidene)malononitrile (DPAM) was synthesized in a high yield via a piperidine-catalyzed Knoevenagel condensation reaction. The crystal form diversity of DMAP was observed through the careful selection of crystallization solvents. Two distinct crystal forms (A-1 and A-2) of this product were obtained, exhibiting orthorhombic and monoclinic crystal systems, respectively. A systematic study encompassing the preparation process, spectroscopic data, thermal stability, single-crystal X-ray diffraction, and mechanochromism of the ICT-type organic crystal material DPAM allowed us to derive significant structure–property relationships. Based on the single-crystal X-ray diffraction analysis, the highly twisted benzene rings within these crystal structures inhibited the formation of exciplexes or excimers, thereby promoting enhanced emissions. The results of the mechanochromism study indicate that the mechanism underlying the mechanically induced changes in emission color was attributed to the transition of the crystal form. The results contribute to our understanding of the relationships between molecular structure, weak interactions, orientation growth processes, and self-assembly morphology. This research presents a promising organic crystalline material for applications in pressure sensing, anti-counterfeiting, and information encryption.