Odd–even effect controls twist-elasticity of an organic fluorophore in cocrystals prepared using mechanochemistry

Abstract

The odd–even effect is a fascinating phenomenon observed in various systems where properties of a series of compounds exhibit alternating patterns depending on whether a specific parameter (often the number of repeating units or a specific structural feature) is odd or even. This effect is significant in solid-state chemistry as it influences diverse physical properties of materials, viz. melting point, solubility, elastic modulus, intrinsic dissolution rate, thermal expansion, etc. However, the odd–even effect on the photophysical behaviour of organic chromophores during co-crystalization is not explored in the literature. Herein, as a proof-of-concept, we co-crystallized a novel organic chromophore (PDAN-1) with a series of aliphatic dicarboxylic acids and showed that fluorescence emission shows an odd–even alteration on their emission maximum similar to other physical parameters. Our in-depth crystal structure analysis reveals that variation of the dicarboxylic acids affects the twist-elasticity of PDAN-1 and thus results in a change of crystal packing, thereby, the odd–even effect in solid-state fluorescence. Moreover, nanomechanical analysis and melting point measurements compliment our odd–even effect on the cocrystal.

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Introduction

The odd–even effect is observed in various chemical and physical systems where properties of a series of compounds exhibit an alternating pattern based on whether a specific parameter, often the number of repeating units or a structural feature, is odd or even. This effect can manifest in different contexts, including molecular structure, electronic properties, and material behaviour.^1,2 For example, the switching of melting point between odd and even numbered carbon containing α,ω-alkane dicarboxylic acid, and fatty acid was first observed by Baeyer in 1877.³ Thalladi et al.⁴ explained this phenomenon by recommending a “parallelogram-trapezoid model”. Again, the odd carbon components have a higher inherent energy that can be released during a process viz. melting,^5,6 sublimation,⁷ face-selective crystal nucleation⁸ or dissolution.⁹ This model explains higher solubilities and lower melting points of odd carbon chains than their even carbon species.^9,10 Likewise, Mishra et al.¹¹ applied the nanoindentation technique to the crystals of α,ω-alkanedicarboxylic acids having different carbon chain lengths to investigate the variation of elastic modulus (E) as a function of the number of carbon atoms in aliphatic chains (N). They found that E fluctuates between odd and even numbered carbon containing dicarboxylic acids, which exactly matches the trend observed in melting temperature (T_m). An identical trend has also been observed for other properties like volumetric thermal expansions,¹² chromic behavior (photochromism vs. thermochromism vs. mechanochromism),^13,14 intrinsic dissolution rate⁹etc. The odd–even effect holds significant potential for advancing optoelectronics by enabling the design of materials with tailored photophysical properties. By leveraging the odd–even effect, materials can be designed to exhibit specific absorption and emission wavelengths. This is crucial for optoelectronic applications like organic light-emitting diodes (OLEDs) and organic photovoltaics (OPVs), where precise control over light–matter interaction is necessary. The effect essentially influences molecular packing and crystalline structures, which are critical for optoelectronic features in organic semiconductors.

Leveraging the odd–even effect and the crystal engineering concepts, here we investigate the odd–even parity's effect on the solid-state fluorescence of a novel organic chromophore upon co-crystallization. More specifically, as a proof-of-concept, a specifically designed luminescent π-conjugated organic material containing a pyridine group is synthesized and co-crystallized with aliphatic dicarboxylic acids (C2 to C10) using mechanochemistry in order to tune “twist-elasticity” that affects solid-state fluorescence.^13,15 Twist elasticity in organic chromophores refers to the change in the torsional coordinates of a chromophore molecule.^16–18 We envisaged that the odd–even effect would manifest in the chromophore's twist elasticity upon crystalizing with odd and even-numbered dicarboxylic acid. Thus, tailored luminescence can be achieved in the solid-state.

Experimental section

Instrumental details

Powder X-ray diffraction (PXRD). PXRD measurements were performed at room temperature on a Rigaku Ultima IV X-ray powder diffractometer operating with a Cu Kα X-ray source, equipped with a Ni filter to suppress Kβ emission and a D/teX Ultra high-speed position sensitive detector, with a scan range 2θ = 5–50°, step size of 0.02° and scan rate of 10° min⁻¹.

Single crystal X-ray diffraction. Single crystal X-ray diffraction (SCXRD) data of all the compounds were collected on a Bruker SMART APEX II CCD diffractometer equipped with a graphite monochromator and a Mo Kα fine-focus sealed tube (λ = 0.71073 Å). Data integration was done using SAINT. Intensities for absorption were corrected using SADABS. Structure solution and refinement were carried out using Bruker SHELXTL.¹⁹ The hydrogen atoms were refined isotropically, and all the other atoms were refined anisotropically. C−H hydrogens were fixed using the HFIX command in SHELXTL. Molecular graphics were prepared using X-SEED²⁰ and Mercury licensed version²¹ 4.2.

Thermal analysis. DSC measurements were performed on a Mettler Toledo DSC instrument with a temperature range 25–400 °C and heating rate of 10 °C min⁻¹ on sample on 40 μL aluminium pan with pin-hole lid under an ultra-high pure nitrogen environment purged at 40 mL min⁻¹. For the PDAN-1B two additional heating rates viz. 5 °C min⁻¹ and 20 °C min⁻¹ was used for thermal measurements.

Nanoindentation. Crystals were mounted using cyanoacrylate glue on a stainless-steel disk-shaped sample holder having a smooth surface in such an orientation so that the major faces would be indented. The experiments were carried out using a nanoindenter (Hysitron Triboindenter, TI Premier, Minneapolis, USA) with a three-sided pyramidal Berkovich diamond indenter tip of radius 150 nm having an in situ scanning probe microscopy (SPM) facility. Before nanoindentation, the tip area function was calculated from a series of indentations on a standard fused quartz sample. The indentations were performed under the load control mode fixing the maximum load constant (P_max) at 6 mN. The rates of loading and unloading were both 1200 μN s⁻¹ with 5 s duration and a 2 s holding period was applied at the maximum indentation depth. SPM images of the indentation impressions were captured immediately just after unloading to avoid any time dependent elastic shape recovery of the residual impressions. The obtained P−h curves were analyzed using the standard Oliver−Pharr method. The machine generated reduced modulus values were converted to elastic modulus (E) by assuming the Poisson's ratio of these materials to be 0.33.

Fluorescence lifetime study. The fluorescence decay profile was performed using time-correlated single-photon counting (TCSPC) methods with HORIBA JOBIN YVON (single photon counting controller: Fluorohub, precision photo multiplier power supply: Fluoro3PS). For molecular excitation 371 nm, Nano-LED was used as a light source and an MCP photomultiplier tube (PMT) (Hamamatsu R3809U-50 series) was used as the detector and the decay was collected with 50 ns TAC range. Photoluminescence (PL) decay profiles were collected in a solid state and fitted multi-exponentially as indicated, and the average lifetime was considered for discussion.
Fluorophore synthesis. In a round-bottomed flask, a catalytic amount of piperidine (80 μL) was added to a mixture of biphenyl-4-carboxaldehyde (0.1825 g, 1 mmol) and 3-pyridine acetonitrile (107 μL, 1 mmol) in ethanol and water (1 : 1) mixture (6 mL) (Scheme 1). The mixture was then allowed to reflux at a temperature of about 80 °C; a shiny off-white colored solid product was obtained after 1.5 h of continuous stirring. The product was then filtered with Whatman-41 filter paper, and the residue was washed with adequate water. The solid residue, (2Z)-3-(biphenyl-4-yl)-2-(pyridin-3-yl)-prop-2-enenitrile (hereafter PDAN-1, a crystalline solid residue) was then allowed to dry and kept for crystallization in various organic solvents. On characterizing this crude crystalline powder PDAN-1 using PXRD confirms the material to be phase pure PDAN-1B polymorph (hereafter crude material is described as PDAN-1). After multiple batches of crystallization, block-shaped single crystals of the fluorophore were obtained in an acetonitrile solvent (hereafter termed PDAN-1B). Polymorphic plate-shaped single crystals were obtained from solution crystallization in methanol (hereafter termed PDAN-1P). The plate-shaped crystals were found to be polymorphs of the parent compound crude PDAN-1 and characterized using single crystal X-ray diffraction (SCXRD), powder X-ray diffraction (PXRD) and thermal analysis (differential scanning calorimetry, DSC).

Scheme 1 Schematic showing the synthesis of the fluorophore, PDAN-1 used in this study (top) and the molecule with various torsion angles (bottom).

Result and discussion

As discussed, to establish the odd–even effect in photo-physical property, we considered a crystal engineering approach where a pyridine-based π-conjugated organic fluorophore, PDAN-1, has been synthesized and systematically co-crystallized with aliphatic dicarboxylic acids (C₂ to C₁₀) with increasing chain length (Scheme 2). The as-synthesized PDAN-1 shows sapphire blue color fluorescence under 365 nm UV light. As mentioned earlier, polymorphic screening using liquid-assisted grinding (LAG) and solution crystallization yields two polymorphic forms viz.PDAN-1B and PDAN-1P with different solid-state fluorescence colors. SCXRD analysis shows PDAN-1B form solved in Pc space group with one molecule in the asymmetric unit. Conformational analysis shows that the PDAN-1 molecule is twisted (pyridine ring dihedral angles θi = 22.5°, θo = 25.2° and biphenyl ring dihedral angles θ_A = 28.6°, θ_B = 28.7°) without possible π⋯π stacking interaction (see Scheme 1 and Table 1). Rather, they are connected using C–H⋯π interactions between adjacent molecules with C–H to pyridine ring centroid distance of 2.89 Å. In 3D, the molecules are stacked in a head-to-head arrangement, as shown in Fig. 1. Plate-shaped polymorph, PDAN-1P, solved in the P1 space group containing two molecules in the asymmetric unit. Due to extremely thin plate shape, the crystal shows weak diffraction peaks. We collected a second set of data, however, it did not improve the quality (in the discussion section, we used the parameters of the data from Bruker instrument, designated as PDAN-1P). One of the molecules has nearly planar conformation (pyridine ring dihedral angles θi = 2.0°, θo = 8.3° and biphenyl ring dihedral angles θ_A = 11.4°, θ_B = 14.9°); whereas, the second symmetry independent molecule is with twisted geometry with respect to the biphenyl ring (pyridine ring dihedral angles θi = 5.9°, θo = 5.5° and biphenyl ring dihedral angles θ_A = 58.2°, θ_B = 46.0°). In 3D, the molecules are stacked in a head-to-tail geometry with a slight offset packing having a centroid-to-centroid distance of 3.88 Å between adjacent molecules. The difference in molecular packing of the two polymorphic forms results in a difference in their photo-physical property. Due to relatively high torsional coordinates (twist-elasticity) that hypsochromically (blue) shift the electronic transition, the absence of unfavourable π–π interactions between neighbouring molecules together shifted the emission spectra of PDAN-1B towards the blue region (sapphire blue color fluorescent crystals). On the other hand, relatively planar conformations of the two symmetry-independent molecules with substantial π–π overlap (66%) and a π–π distance of 3.88 Å between anti-parallel dimer along the molecular length together bathchromically (red) shift the electronic transition showing cyan color fluorescence in solid-state for PDAN-1P polymorph. This concept of twist-elasticity is already well explored for polymorphic π-conjugated distyrylbenzene (DSB)^22,23 and cyano-substituted distyrylbenzene (DCS) in recent literature.^24–26

Scheme 2 Molecular structures of PDAN-1 and dicarboxylic acids used in this study.

Table 1 Structural parameters for different conformation of PDAN-1 in polymorphs and all multicomponent solids: symmetries, torsion angles θ as defined in Scheme 1 and crystal density

Compound	Symmetry	θ i	θ O	θ A	θ B	Density (ρ)
PDAN-1B	Pc	22.5	25.2	28.6	28.7	1.239
PDAN-1P	P1	5.9	5.5	58.2	46.0	1.182
		2.0	8.3	11.4	14.9
PDAN-1P-Rigaku	P1	4.7	7.7	18.6	9.9	1.278
		6.9	4.0	46.5	54.8
PDAN-1·OA (C2)	P1̄	15.4	3.4	39	37.8	1.357
PDAN-1·MA (C3)	C2/c	0.6	3.7	0.4	0.4	1.289
PDAN-1·SA (C4)	P1̄	15.1	13.1	32.2	31.6	1.280
PDAN-1·GA (C5)	C2/c	0.6	4.2	0.8	0.9	1.279
PDAN-1·AA (C6)	P21/n	21.0	29.6	43.7	42.7	1.238
PDAN-1·AZA (C9)	P21/c	25.4	17.7	14.5	16.0	1.235
		6.0	8.7	31.0	31.5
PDAN-1·SEBA (C10)	P21/n	21.9	28.1	43.7	42.2	1.225
PDAN-1·MEA (cis)	P21/c	28.4	3.6	28.7	26.4	1.334
		24.2	4.8	28.5	26.8
		28.9	0.1	27.7	24.4
		25.1	8	25.7	22.7
PDAN-1·FA (trans)	P1̄	15.1	13.0	32.8	32	1.282

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Fig. 1 (a) Head-to-head stacking of molecules observed in PDAN-1B form and head-to-tail stacking arrangement is observed for PDAN-1P form; (b) solution and solid-state fluorescence spectra (normalized) of the two polymorphs; (c) powder material of PDAN-1B shows off-white color under day light and sapphire blue color fluorescence under 365 nm UV light; whereas, PDAN-1P powder material shows yellow color under day light and cyan color fluorescence under 365 nm UV light.

An in-depth mechanical grinding in the presence of catalytic amounts of various liquids shows the generation of phase pure polymorphic forms. PDAN-1P is obtained during mechanochemical grinding in the presence of catalytic amounts of liquids such as acetonitrile, acetone, chloroform, ethyl acetate, THF and toluene, whereas in the presence of methanol, ethanol, dioxane and hexane, results in the formation of PDAN-1B. A comparison of the PXRD patterns of the LAG experiments with the calculated powder patterns of the two polymorphic forms is shown in Fig. 2. In order to investigate possible interconversion between the two polymorphs under mechanochemical conditions, freshly prepared PDAN-1B was ground in the presence of acetone shows complete conversion to PDAN-1P; whereas PDAN-1P form do not convert back to PDAN-1B while ground the transformed powder in presence of EtOH (see ESI† Fig. S1). DSC thermogram shows the enantiotropic nature of PDAN-1B with a polymorphic phase transformation at 148 °C, whereas PDAN-1P exhibits monotropic nature with a single melting endotherm at 149.8 °C (see ESI† Fig. S2). In order to confirm the enantiotropic nature of the polymorphic phase transformation of PDAN-1B material, we have carried out DSC measurements of PDAN-1B under two different heating rates, i.e. 5 °C min⁻¹ and 20 °C min⁻¹ that clearly shows disappearance of the phase transformation event and melting of the PDAN-1B material with a melting onset at 149 °C slightly lower than the PDAN-1P (see ESI† Fig. S2b and c) i.e.PDAN-1P could be considered as thermodynamic form and PDAN-1B as kinetic polymorph. Owing to the higher thermodynamic stability and monotropic nature of the polymorphic PDAN-1P, it was selectively used for the synthesis of all the multi-component solids.

Fig. 2 Stack plot of the PXRD patterns of LAG samples compared with the respective calculated powder patterns of the two polymorphs of PDAN-1.

In order to investigate the odd–even effect of dicarboxylic acids on twist-elasticity that induces a change in solid-state fluorescence color, organic fluorophore PDAN-1 (PDAN-1P form) is co-crystallized with aliphatic dicarboxylic acids with increasing chain length, i.e., starting from oxalic acid (OA; C₂ diacid) to sebacic acid (SEBA; C₁₀ diacid). LAG is an efficient mechanochemical method that forms various solid-state phases such as polymorphs, cocrystals, salts, solvates, amorphous materials, etc. and is well explored in literature.^27–31 LAG of PDAN-1 with aliphatic dicarboxylic acids was performed in 2 : 1 stoichiometry (except OA, which results in a 1 : 1 salt cocrystal system) in the presence of the catalytic amount of acetonitrile. The PXRD patterns obtained for the ground materials were found to be different from those of respective starting materials, confirming the formation of a new phase. Based on SCXRD analysis, we confirmed the materials to be cocrystals of PDAN-1 with dicarboxylic acids in 2 : 1 stoichiometry. The experimental PXRD patterns of the mechanochemically synthesized cocrystals nicely match with the respective calculated powder patterns obtained from SCXRD data (see ESI† Fig. S3–13). Even after multiple batches of solution crystallization, we were not able to generate suitable single crystals of PDAN-1·PMA and PDAN-1·SUBE for structural elucidation. However, the formation of the respective cocrystals and their stoichiometry was confirmed based on PXRD analysis (see ESI† Fig. S8 and S9). An extensive polymorph screening for all the synthesized cocrystals was carried out using LAG with various added liquids; however, no new polymorphic phase has been identified during cocrystallization using mechanochemistry and solution crystallization (see ESI† Fig. S14–S22).

Solution crystallization of the PDAN-1 diacid cocrystals from a 3 : 1 mixture of chloroform and acetonitrile afforded block-shaped single crystals of the respective products except for PDAN-1·PMA and PDAN-1·SUBE. Crystal structure analysis shows PDAN-1·OA resulted formation of a 1 : 1 salt cocrystal solved in the P1̄ space group with a protonated PDAN-1 molecule and half molecules each of oxalate and oxalic acid coformer in the asymmetric unit. All other aliphatic diacids resulted formation of cocrystal in 2 : 1 stoichiometry containing one molecule of PDAN-1 and half molecules of diacids, except PDAN-1·AZA that contains two molecules of PDAN-1 and one molecule of AZA in the asymmetric unit. Single crystal structural analysis shows the planar conformation of the even diacids with dihedral angles close to the all-anti conformation. Whereas, in the case of PDAN-1 cocrystals with odd diacids, the dihedral angles of the coformers significantly deviate from the all-anti-conformation (Fig. 3).

Fig. 3 Twist in the molecular geometry and direction of the atoms is shown using thermal ellipsoids from structures collected at 298 K for all the diacid coformers used to prepare cocrystals with PDAN-1 (view along the molecular chain). Even diacids show trans conformation, whereas odd diacids show cis conformation.

This conformational variation of the odd and even diacids induces change in molecular conformation of PDAN-1 that results difference in crystal packing of the resultant PDAN-1 cocrystals. Conformational analysis of the PDAN-1 molecule in all the crystal structures of the dicarboxylic acid salt/cocrystals (see Table 1) nicely agree to twist-elasticity controlled solid-state UV-visible absorption and fluorescence emission. Salt/cocrystals with even dicarboxylic acids show strongly twisted geometry of PDAN-1 molecule as a result blue emitting phase with relatively lower emission maxima (λ_max) was observed. From the molecular packings of the crystal structures it was observed that due to the twisted structure of PDAN-1, side-by-side arrangement is not possible; as a result micro-herringbone structure³² was observed for even diacid cocrystals with an edge-to-face arrangement of the phenyl ring to avoid unfavourable π–π interactions (Fig. 4). Whereas for cocrystals with odd diacids (PDAN-1·MA and PDAN-1·GA) relatively planar geometry of PDAN-1 with substantial π–π overlap³³ (Fig. 4) was observed, hence a bathochromic shift similar to that of PDAN-1P polymorph results, exhibiting green emitting phase. Due to the presence of two symmetry-independent PDAN-1 molecules in PDAN-1·AZA with twisted geometry, the edge-to-face arrangement of the phenyl ring is observed with a relatively small (∼20%) π–π overlap for the pyridine rings of the second independent molecule with nearest neighbor. As a result, PDAN-1·AZA cocrystal exhibits blue fluorescence similar to that of even diacid cocrystals. Cisoid conformation of odd diacids and transoid conformation in even diacids as well as presence of strong acid-pyridine synthon between the diacids and PDAN-1 induces this conformational twist of PDAN-1 in the cocrystal system. As a result, all the synthesized PDAN-1 diacid cocrystals showcases alteration in UV-visible absorption and solid-state fluorescence emission (for both powder samples and single crystals) as shown in Fig. 5 and 6. Crystallographic parameters of all the synthesized cocrystals of PDAN-1 with various diacids are listed in ESI† Table S1.

Fig. 4 Change in molecular packing of all the PDAN-1 diacid cocrystals along with their solid-state fluorescence color of the powder materials under 365 nm UV light.

Fig. 5 PDAN-1 diacid cocrystals powder samples and single crystals with their respective optical images of change in fluorescence color observed under 365 nm UV light. For PDAN-1·PMA and PDAN-1·SUBE no single crystals were obtained. MEA represents cis conformation and FA represents trans conformation.

Fig. 6 Alternation in trend of (a) UV-visible absorption wavelength (λ_max/Abs) in nm; (b) fluorescence emission wavelength (λ_max) in nm observed for PDAN-1 cocrystals with increase in chain length of the aliphatic dicarboxylic acids used as coformers.

To further support our hypothesis– “conformational effect of coformers induces molecular twist in PDAN-1 that results alteration in the solid-state fluorescence behavior of the cocrystals”, we have extended our study via co-crystallization of PDAN-1 with maleic acid (MEA; unsaturated cis-diacid) and fumaric acid (FA; unsaturated trans-diacid) which also exhibits identical trend i.e.PDAN-1·MEA with cyan color having comparatively longer emission wavelength (λ_max) (red-shift) similar to all odd diacid cocrystals and PDAN-1·FA with blue emission having relatively shorter wavelength (λ_max) (blue-shift) similar to that of all even diacid cocrystals.

Solution crystallization of 1 : 1 stoichiometric mixture of PDAN-1 and MEA from a mixture of chloroform and acetonitrile (3 : 1) solvent afforded block-shaped single crystals of PDAN-1·MEA. Crystal structure was solved in the P2₁/c space group containing four molecules each of protonated PDAN-1 and maleate anion respectively in the asymmetric unit resulting formation of a molecular salt. Twisted conformation of the PDAN-1 molecules are observed for all the symmetry independent molecules; however, θ_O are comparable (planar pyridine ring with respect to cyano group) to that of cocrystals of odd diacids. Moreover, substantial π–π overlap is observed that supports bathochromic shift of the electronic transition and observed cyan emission for both powder and crystal samples. On the other hand, FA form a 2 : 1 cocrystal containing one molecule of PDAN-1 and half molecule of FA in the asymmetric unit and solved in P1̄ space group. No proton transfer is observed for this multi-component system with twisted conformation of the PDN-1, hence no possible π–π interactions between neighboring PDAN-1 molecules was visible. As a result, PDAN-1·FA cocrystal exhibits blue emission both in powder as well as crystal form with relatively shorter emission wavelength compared to PDAN-1·MEA molecular salt (Fig. 7). The photoluminescence (PL) lifetime measurements of the crystalline solid samples reveal a relatively longer average decay time for the PDAN-1B polymorph (τ_avg = 2.4 ns) compared to PDAN-1P (τ_avg = 1.8 ns). Moreover, the PDAN-1 cocrystal incorporating a conformer with an even number of carbon atoms, namely PDAN-1·OA, displays a significantly shorter average lifetime (τ_avg = 0.7 ns) than the cocrystal containing a conformer with an odd number of carbon atoms, PDAN-1·MA (τ_avg = 4.4 ns) (see ESI† S27). The observed lifetimes, all falling within the nanosecond regime, indicate that the emissive transitions originate from the singlet excited state. A more comprehensive understanding of the underlying photoexcitation mechanisms would require further in-depth investigation, which lies beyond the scope of the present study. Details of the optical measurements and photo-physical properties of all the PDAN-1 multicomponent system are discussed in ESI† Fig. S23–S36.

Fig. 7 Change in molecular packing of PDAN-1·FA salt and PDAN-1·MEA cocrystal along with their solid-state fluorescence color of the powder materials under 365 nm UV light.

Investigation into mechanical properties

Having established a direct correlation between emission wavelength and chain length of dicarboxylic acid coformers, we moved to quantify the nanomechanical properties of these diacid cocrystals and check whether there is any trend of their elastic modulus (E) and hardness (H) with λ_max. Nanoindentation is a very useful technique as it can provide E and H values from very small volumes of molecular crystals and yet with high precision.³⁴ By correlating the mechanical properties of organic crystals with their underlying crystal structures, we can rationalize the same in terms of interaction anisotropy,³⁵ phase transformations³⁶ or mass migration.³⁷

We begin with the PDAN-1·OA (C₂) salt cocrystal. The structure consists of strong C–H⋯O hydrogen bonds (2.418 Å, 158.1°) along with the archetypal O–H⋯O (1.575 Å, 174.9°) and N–H⋯O (1.869 Å, 142.0°). Nanoindentation was performed on the major (001) face of the single crystals which confirmed an elastic modulus of 13.8 GPa. When we move to PDAN-1·MA (C₃) the first one among the odd-diacids, which adopt a layered packing in the crystal structure. Interestingly, the O–H⋯O strong hydrogen bond disappears and a C–H⋯N (2.565 Å, 156.56°) appears with the N-atom of the cyano group and there is a remnant O–H⋯N (1.626 Å, 164.75°) and C–H⋯O (2.590 Å, 166.9°) in these crystals. Indentation on the major face (200) reveals an elastic modulus of 9.26 GPa. The next one in the series, PDAN-1·SA (C₄) cocrystal has O–H⋯N (1.551 Å, 171.49°), C–H⋯O (2.449 Å, 149.96°) and C–H⋯N (2.544 Å, 159.65°). A thorough analysis reveals that the comparatively weaker C–H⋯O bond parameters drop sharply from C₃ to C₄, revealing the drop in elastic modulus also from 9.26 GPa to 6.99 Gpa. Alongside, the density of the cocrystals also drops consistently from C₂ (1.357) to C₃ (1.29) to C₄ (1.28). For PDAN-1·GA (C₅), the density remains the same (1.28), while the bond parameters are O–H⋯N (1.683 Å, 172.15°), C–H⋯O (2.565 Å, 144.0°) and C–H⋯N (2.618 Å, 154.47°). The elastic modulus for PDAN-1·GA increases to 8.17 GPa. For PDAN-1·AA (C₆), the bond parameters are O–H⋯N (1.750 Å, 169.85°), C–H⋯O (2.462 Å, 146.78°) and C–H⋯N (2.374 Å, 170.10°). We see that from C₅ to C₆, the strong hydrogen bond (O–H⋯N) weakens leading to reduction in density (from 1.28 to 1.24) as manifested in reduction of elastic modulus from 8.17 GPa to 6.09 GPa. For PDAN-1·PMA (C₇) and PDAN-1·SUBE (C₈), we did not get single crystals suitable for nanoindentation experiment. Again, for PDAN-1·AZA (C₉), the O–H⋯N (1.816 Å, 173.28°) and C–H⋯O (2.602 Å, 162.16°) further weakens, while the C–H⋯N hydrogen bond bifurcates into two (2.716 Å, 174.57° and 2.724 Å, 156.55°) leading to a density of 1.235 and an increase in elastic modulus of 12.17 GPa. This is because upon indenting on the major face (100), the tip faces an increased resistance due to the additional C–H⋯N interaction. For the next cocrystal of the series, PDAN-1·SEBA (C₁₀), there is a small increase in the bond parameters, O–H⋯N (1.672 Å, 174.58°), C–H⋯O (2.456 Å, 143.66°) while the C–H⋯N bond converges (2.339 Å, 173.25°) leading to an elastic modulus of 7.72 GPa. The trend of change in elastic modulus of the PDAN-1 multicomponent solids with the melting point of pure diacids is shown in Fig. 8 (see ESI† Fig. S37 for DSC thermograms). BFDH morphology, representative load-depth curves and scanning probe microscopic (SPM) image of the residual indent impression of all the PDAN-1 cocrystal systems are listed in ESI† Fig. S38–S45. Although the trend of melting points of the cocrystals are similar to that of the individual diacids showing an odd–even effect, the alternation behavior of elastic modulus of the PDAN-1 cocrystals with the melting points is rather unexpected. It is to be noted that generating suitable single crystals of smooth surface free of artifacts for all the cocrystals is challenging. Moreover, major face of indentation of all the cocrystals and salts are not equivalent; therefore, it is difficult to correlate elastic moduli of all the cocrystals with respect to the odd–even parity that we are investigating. In fact, λ_max of PDAN-1 cocrystal with C₈ diacid i.e. SUBE is not following the trend which is difficult to explain, as unfortunately for C₇ and C₈ diacids we could not able to generate suitable single crystals for structure elucidation as well as nanomechanical analysis. Nevertheless, among the remaining systems, when plotted against the wavenumber (which is an energy unit and determined by the reciprocal of wavelength), shows an agreeable odd–even parity with respect to C-atoms in the co-formers.

Fig. 8 Alternation in trend of elastic modulus (E) observed for PDAN-1 cocrystals plotted against respective melting point and wavenumbers with increase in chain length of the aliphatic dicarboxylic acids used as coformers.

Conclusion

In summary, odd–even parity as a function of emission as well as absorption wavelength (λ_max) has been established for a series of cocrystals of a π-conjugated organic fluorophore with aliphatic dicarboxylic acids as coformer. The strained molecular conformations in odd acids and a trade-off between attractive dispersive forces and O⋯O repulsions resulted difference in twist-elasticity of PDAN-1 molecule in the crystal structure induces variation in crystal packing for the PDAN-1 cocrystals. Also it influences alternation of absorption as well as fluorescence emission behavior considered in this study. Moreover, a similar trend has been observed for odd–even T_m alternation as well as to some extent for elastic modulus. As generation of suitable single crystals sometimes is a challenge; more number of similar systems needs to be considered for systematic study in near future. Finally, for the first time, odd–even diacids as co-formers are used to tune twist-elasticity of a π-conjugated organic fluorophore that triggers switch in UV-visible absorption and fluorescence emission along with mechanical property.

Author contributions

N. K., P. D. and A. G. synthesized all the materials; N. K., P. D., and K. J. K. carried out the characterization and various photophysical studies; I. G. carried out the nanoindentation measurements. The manuscript was written through contributions of all authors.

Data availability

Crystallographic data for PDAN-1B; PDAN-1P; PDAN-1·OA; PDAN-1·MA; PDAN-1·SA; PDAN-1·GA; PDAN-1·AA; PDAN-1·AZA; PDAN-1·SEBA; PDAN-1·MEA; PDAN-1·FA has been deposited at the Cambridge Crystallographic Data Centre (CCDC) under [CCDC No. 2393120–2393130, 2441999]† and can be obtained from [https://www.ccdc.cam.ac.uk]. The datasets supporting this article have been uploaded as part of the ESI.†

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was funded by Science and Engineering Research Board under the Teachers Associateship for Research Excellence (TARE) grant (Project No. TAR/2021/000251) and Polish National Agency for Academic Exchange (Application no. BNI/ULM/2024/1/00042). CMR thanks DST for Swarnajayanti fellowship (DST/SJF/CSA-02/2014-15) and SERB (No: EMR/2021/000492) for funding. PD thanks the UGC for the Junior Research Fellowship (PhD scholar). IG thanks PMRF for fellowship. RT thankfully acknowledge the Sophisticated Analytical Instrumentation Facility (SAIF), GU for the provision of the single crystal X-ray diffractometer and DST-FIST program for supporting the Department of Chemistry, GU for the Rigaku powder X-ray diffractometer, basic instrumentation facility, and infrastructure. RT also thankfully acknowledges the Polish National Agency for Academic Exchange to work as a Ulam NAWA fellow at Faculty of Chemistry, University of Warsaw, Poland. We also thank IISER Kolkata for its instrumentation facilities. The authors also acknowledge Mr Shant Chhetri for his help in collecting the TCSPC data.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and supplementary figures, graphs, and tables. CCDC 2393120–2393130 and 2441999. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5tc00313j

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