Interplay of C–H⋯F and halogen bonding interactions for tunable room-temperature phosphorescence in iododiphenylacetylene systems

Abstract

Efficient room-temperature phosphorescence (RTP) often depends on specific intermolecular interactions, such as C–H⋯F and halogen bonding interactions; however, their role in fluorinated diphenylacetylene derivatives remains unclear. In this study, we systematically synthesized a series of iododiphenylacetylene derivatives containing fluorine or methoxy substituents to elucidate the relationships between intermolecular interactions, molecular packing, and RTP properties. Single-crystal X-ray diffraction and cyclic voltammetry were employed to investigate their packing structures and estimate the electronic properties, respectively, and their photophysical properties were examined. Both C–H⋯F and halogen bonding interactions contributed to enhancing RTP; however, C–H⋯F interaction induced by fluorine atoms played a more significant role in increasing the phosphorescence intensity and efficiency. Moreover, the combination of both intermolecular interactions resulted in a synergistic improvement in RTP properties. These findings provide valuable guidelines for designing and developing efficient diphenylacetylene-based RTP materials.

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Introduction

Organic light-emitting materials are commonly applied in various devices, including organic light-emitting diodes (OLEDs),^1–3 organic field-effect transistors,^4–8 organic lasers,^9–11 and sensing devices.^12,13 Among them, phosphorescent materials that utilize triplet excitons offer a higher theoretical efficiency in OLEDs than in their fluorescent counterparts.¹⁴ However, triplet excitons are easily deactivated at room temperature, inhibiting efficient phosphorescence. To overcome this challenge, precious metals, such as Ir and Pt, can be incorporated into the structure of luminophores to promote intersystem crossing via the heavy-atom effect.^15,16 Although this approach has successfully enabled room-temperature phosphorescence (RTP), growing interest in sustainability and cost reduction has prompted the development of metal-free phosphorescent systems, particularly using organic RTP materials.^17,18 Tang et al. reported the phenomenon of aggregation-induced emission, subsequently resulting in the concept of crystallization-induced phosphorescence (CIP).^19–21 They revealed that molecular motion can be suppressed in the crystalline state by leveraging various intermolecular interactions, such as C–H⋯F interaction, halogen bonding, and CH/π interactions. Thus, their study provided valuable design guidelines for the development of efficient RTP molecules.

In addition to CIP, co-crystallization using organic materials has emerged as a promising strategy for achieving efficient RTP.²² For example, Agostino et al. reported that co-crystals formed between diphenylacetylene (DPA) and diiodotetrafluorobenzene (in 1 : 1 or 1 : 2 molar ratios) via C–I⋯π interactions exhibited efficient RTP. In particular, the 1 : 2 co-crystals completely suppressed fluorescence, resulting in enhanced RTP.²³ Additionally, DPA exhibits ultralong phosphorescence when its two aromatic rings adopt a highly twisted conformation.^24,25 These promising phosphorescent properties of DPA inspired us to further explore DPA-based RTP materials.

We previously reported that a fluorinated DPA derivative, compound 1, exhibited efficient RTP in crystalline states (Scheme 1a).²⁶ Because compound 1 comprises iodine and methoxy groups at both longitudinal ends, the efficient RTP was attributed to a combination of intermolecular interactions, such as C–H⋯F interaction and halogen bonding, and the heavy-atom effect of the iodine atom. The valuable role of the heavy-atom effect of iodine is well established; however, the relative contributions of C–H⋯F and halogen bonding interactions to the RTP properties of compound 1 remain unclear, rendering clarification of this issue essential to rationally design efficient DPA-based RTP materials.

Scheme 1 (a) DPA-based RTP molecule previously reported by our group. (b) Target molecules used in this study.

In this study, we synthesized a series of iododiphenylacetylene derivatives, 2a–c, and evaluated their crystal structures and photophysical properties to elucidate the relationships between intermolecular interactions, molecular packing, and RTP properties (Scheme 1b). The target compounds were as follows: 2a, with only an iodine atom (to isolate the heavy-atom effect);²⁷2b, with both an iodine atom and methoxy group (to enable halogen bonding);²⁸ and 2c, with iodine and fluorine substituents on the same aromatic ring (to enable C–H⋯F interaction via fluorine).^29,30 Detailed analyses using single-crystal X-ray diffraction (SCXRD) and the photophysical properties of 2a–c revealed that C–H⋯F interactions via fluorine atoms play a more significant role in enhancing RTP than halogen bonding or the heavy-atom effect alone. Furthermore, the combined presence of C–H⋯F and halogen bonding interactions improved the RTP properties of the DPA derivatives. These findings provide valuable insight into the molecular design of DPA-based RTP materials.

Experimental

Materials and methods

The ¹H-NMR (400 MHz) and ¹³C-NMR (100 MHz) spectra were obtained using a Bruker AVANCE III 400 NMR spectrometer in a chloroform-d (CDCl₃) solution. The chemical shifts were reported in parts per million (ppm) using residual protons in the NMR solvent. The ¹⁹F-NMR (376 MHz) spectra were obtained using a Bruker AVANCE III 400 NMR spectrometer in a CDCl₃ solution with CFCl₃ (δ_F = 0 ppm) as the internal standard. All chemicals, including the solvents, were of reagent grade and purified as typical prior to use. Column chromatography was conducted on silica gel (Fujifilm Wako Pure Chemical Corporation, Wakogel® 60 N, 38–100 μm), and thin-layer chromatography (TLC) was performed on silica gel TLC plates (Merck, Silica gel 60F₂₅₄).

Crystal growth

The single crystals of 2a–c were obtained via recrystallisation from a mixed solvent (dichloromethane/methanol = 1 : 1). Approximately 100 mg of 2a–c was dissolved in 5 mL of the mixed solvent and was slowly evaporated over five days in a loosely capped screw vial.

Single-crystal X-ray diffraction

SCXRD was performed using a Rigaku XtaLAB AFC11 diffractometer. The reflection data were integrated, scaled, and averaged using the Rigaku CrysAlisPro program (ver. 1.171.39.43a). Empirical absorption corrections were performed using the SCALE 3 ABSPACK scaling algorithm (CrysAlisPro). The structures were identified using a direct method (SHELXT-2018/2)³¹ and refined using the full-matrix least-squares method (SHELXL-2018/3)³² visualized using Olex2.³³

Cyclic voltammetry

Cyclic voltammetry (CV) measurements were performed using an ECstat-101 potentiostat (EC Frontier, Kyoto, Japan) with glassy carbon, Pt, and Ag/AgCl as the working, counter, and reference electrodes, respectively. Ferrocene (Fc)/ferrocenium (Fc⁺) was used as an external reference, while tetrabutylammonium hexafluorophosphate (Bu₄NPF₆) was used as the supporting electrolyte (0.1 mol L⁻¹). All measurements were performed following argon bubbling for 30 min in a 1 × 10⁻³ mol L⁻¹ acetonitrile solution at a scan rate of 50 mV s⁻¹. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels were estimated from the onset potentials of the oxidation (E_Ox) and reduction (E_Red) waves (versus Fc/Fc⁺) using the following equations: E_HOMO = −4.80 − E_Ox, E_LUMO = −4.80 − E_Red, and ΔE = E_LUMO − E_HOMO.

Photophysical properties

UV-vis absorption spectra were recorded using a JASCO V-530 absorption spectrometer, and the photoluminescence (PL) spectra of the solutions and crystals were recorded using a JASCO FP-6600 fluorescence spectrometer. The absolute quantum yields of the solutions and crystals were measured using a Hamamatsu Photonics Quantaurus-QY system (C11347-01). The PL lifetime was measured using a Hamamatsu Photonics Quantaurus-Tau lifetime spectrometer (C11367-34).

Results and discussion

Single-crystal X-ray diffraction

The packing structures of compounds 2a–c were confirmed using SCXRD. As shown in Fig. 1a, compound 2a crystallized in the monoclinic P2 space group, with eight molecules per unit cell. Although compound 2a is an achiral molecule, it crystallized in the chiral space group P2. Such spontaneous resolution can occur when directional intermolecular interactions, such as C–H⋯F interaction, guide the molecules into a chiral packing arrangement.³⁴ In particular, the DPA cores adopted a zig-zag herringbone-type arrangement. This packing is supported by multiple short contacts between the C–H bonds of one molecule and the aromatic ring of an adjacent molecule with C–H⋯Cg distances of 2.95–2.99 Å (Fig. 1b, Cg = centroid of the aromatic ring). No significant π–π stacking interactions were observed.

Fig. 1 Crystal structure of 2a: (a) unit cell and (b) molecular packing involving C–H⋯Cg short contacts.

Compound 2b crystallized in an orthorhombic Pca2₁ space group, with four molecules per unit cell (Fig. 2a). Similar to that for 2a, 2b exhibited a herringbone-type packing structure. This arrangement is supported by a C–H/π interaction, characterized by an H⋯Cg distance of 2.83 Å and a C–H⋯Cg angle of 136.4° (Fig. 2b). In addition, halogen bonding connects adjacent molecules via a C–I⋯O contact of 3.23 Å. The geometry of this bond, with a C–I⋯O angle of 172.7° and I⋯O–C angle of 128.2°, confirmed a directional σ-hole interaction. The overall molecular conformation of 2b is highly similar to that of 2a.

Fig. 2 Crystal structure of 2b: (a) unit cell and (b) molecular packing involving the key C–H/π interaction and C–I⋯O halogen bonding.

The crystal structure of compound 2c has already been reported (CCDC 245724), which was determined at 100 K.³⁰ To enable a systematic and direct comparison of the molecular packing and intermolecular interactions at room temperature, we re-determined the crystal structure of compound 2c at 298 K. As a result, compound 2c was found to belong to the monoclinic P2₁/n space group, with four molecules in a unit cell (Fig. 3a). Unlike that for non-fluorinated 2a and 2b, the packing of 2c is dominated by π-stacking interaction and C–H⋯F interaction. The π-stacking occurs between the alkyne moiety of one molecule and the fluorinated aromatic ring of an adjacent molecule, characterized by short contacts between alkyne carbon atoms and the ring centroid (Fig. 3c, C⋯Cg: 3.55–3.64 Å). The molecules are further connected by intermolecular C–H⋯F interaction. This interaction meets the geometric criteria for C–H⋯F interaction, with an H⋯F distance of 2.61 Å (shorter than the sum of the van der Waals radii, 2.67 Å) and C–H⋯F angle of 147.8° (Fig. 3b). For comparison, the previously reported structure of compound 1 (ref. 26) revealed the presence of both C–H⋯F and halogen bonding interactions (Fig. 4a and b). To comprehensively investigate differences in the intermolecular interactions between 1 and 2a–c, Hirshfeld surface analysis was performed to quantify the contribution of each contact type to the total molecular surface (Fig. 5). In 2a, H⋯C contacts, representing contributions from C–H/π systems, were the most significant, accounting for approximately half of the surface. For 2b, while H⋯C contacts remain dominant, significant contributions from O⋯I (halogen bonding) and O⋯H contacts are also observed. In the fluorinated analogue 2c, H⋯F contacts (C–H⋯F interaction) become the primary contributor. Reference compound 1 is attributed to the synergistic effect of two highly directional interactions: O⋯I halogen bonding and significant contribution from C–H⋯F interaction. This unique combination creates a rigid intermolecular network that is absent in the other derivatives.

Fig. 3 Crystal structure of 2c: (a) unit cell, (b) C–H⋯F interaction, and (c) alkyne–arene π–stacking interactions.

Fig. 4 Crystal structure of 1: (a) unit cell and (b) intermolecular interactions, including C–H⋯F interaction and O⋯I halogen bonding, reported in ref. 26.

Fig. 5 Surface contact areas resulting from the Hirshfeld surface analyses of (a) 2a, (b) 2b, (c) 2c, and (d) 1.

Cyclic voltammetry

CV measurements were conducted in acetonitrile to estimate the electronic properties of compounds 1 and 2a–c (Fig. 6). The energy levels of the frontier orbitals were calculated based on the onset potentials obtained from the cyclic voltammograms (Table 1). The HOMO and LUMO of 2a were estimated to be −6.24 and −2.51 eV, respectively, yielding an energy gap (ΔE) of 3.73 eV. In compound 2b, introducing an electron-donating methoxy group resulted in destabilization of the HOMO level to −5.86 eV, while the LUMO level remained nearly unchanged. Consequently, the ΔE decreased to 3.30 eV. In compound 2c, which contains electron-withdrawing fluorine atoms, both the HOMO and LUMO levels were more stable than those of 2a. However, the LUMO was significantly more stabilized owing to the more extensive delocalization of its orbital, resulting in a decreased ΔE value, similar to that of 2b. Compound 1, which includes both methoxy and fluorine substituents, demonstrated the lowest ΔE value among all the derivatives. This result was attributed to the combined effect of the methoxy group (increasing the HOMO energy) and fluorine atoms (decreasing the LUMO energy).

Fig. 6 Cyclic voltammograms of 1 and 2a–c in an acetonitrile solution (10⁻³ mol L⁻¹).

Table 1 Orbital energies and relative parameters of 1 and 2a–c obtained from the CV results

	HOMO [eV]	LUMO [eV]	ΔE [eV]
1	−6.06	−3.35	2.71
2a	−6.24	−2.51	3.73
2b	−5.86	−2.56	3.30
2c	−6.50	−3.30	3.20

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In addition to the orbital energies derived from the potentials, the peak current intensities also provided key insights. The peak current for fluorinated derivative 2c was significantly higher than those of 2a and 2b. We hypothesize that this is due to the pre-aggregation of 2c in 10⁻³ mol L⁻¹ acetonitrile solution or its adsorption onto the electrode surface, induced by intermolecular interactions with the fluorine atoms. This hypothesis was further supported by a comparison with compound 1. Although compound 1 also contains fluorine atoms, the current was not enhanced. We attribute this difference to the presence of the methoxy group in 1, which likely improves solvation and, thus, counteracts the tendency for aggregation that might otherwise be induced by the fluorine atoms.

Photophysical properties in dilute solutions

UV-vis absorption and PL were measured in a 10⁻⁵ mol L⁻¹ hexane solution (Fig. 7, Table 2). Compound 2a exhibited a maximum absorption wavelength of 289 nm and multiple absorption peaks in the longer-wavelength region (Fig. 7a). The spectral shape of 2b was similar to that of 2a; however, it displayed a longer wavelength shift. This observation was consistent with the decreased ΔE value caused by the methoxy group, as confirmed by the CV results.

Fig. 7 (a) UV-vis absorption and (b) PL spectra of 1 and 2a–c in 10⁻⁵ mol L⁻¹ hexane obtained at a maximum absorption wavelength.

Table 2 Photophysical properties 1 and 2a–c in 10⁻⁵ mol L⁻¹ hexane

	λ abs [nm] (ε [L mol^−1 cm^−1])	λ PL [nm]	Φ PL
a Excited at λabs. b Calculated using an integrating sphere.
1	307 (49.2), 327 (51.6)	344	<0.01
2a	289 (36.3), 297 (28.4), 307 (33.1)	322	<0.01
2b	298 (32.1), 306 (26.7), 317 (29.6)	334	<0.01
2c	291 (58.3), 310 (54.8)	325	<0.01

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The fluorinated compounds 2c and 1 exhibited similar spectral shapes; however, 1 exhibited a longer wavelength shift than 2c, which was analogous to the trend observed between 2a and 2b. Notably, all compounds exhibited near-UV emissions, with an emission maximum at approximately 330 nm following excitation at their maximum absorption wavelength. A correlation between the emission maxima (λ_PL) and ΔE value estimated from CV was also observed. The absolute PL quantum yield (Φ_PL) of all compounds was <0.01, which was too low for observation with the naked eye. This weak emission was likely due to a non-radiative decay pathway involving a transition to a trans-bent state.^35,36

Photophysical properties of the crystals

The emission spectra of 1 and 2a–c were measured in the crystalline state (Fig. 8a, Table 3). All compounds exhibited dual emission maxima at approximately 400 and 500 nm. Time-resolved measurements revealed that the emission at approximately 400 nm had nanosecond or less lifetimes, which is characteristic of fluorescence (τ_flu) (Fig. S5–S7). Conversely, the emission at approximately 500 nm exhibited microsecond-range lifetimes, corresponding to phosphorescence (τ_phos). To further compare the fluorescence and phosphorescence of these crystals, the fluorescence intensities (I_flu) were normalized (Fig. 8b, inset). Compound 2a exhibited extremely weak phosphorescence intensity (I_phos) with respect to the fluorescence intensity (I_flu), whereas 2b showed a slight improvement. Conversely, the fluorinated compounds 2c and 1 exhibited significantly enhanced I_phos, with phosphorescence intensity exceeding that of fluorescence. The PL quantum efficiency (Φ_PL) for 1 and 2c were higher than those for 2a and 2b, indicating that introducing fluorine atoms enhanced both I_phos and the overall PL efficiency. The ratio of phosphorescence-to-fluorescence intensities (I_phos/I_flu) for compounds 1 and 2a–c, as well as their intermolecular interactions, are summarized in Fig. 8b. Based on the Hirshfeld surface analysis (Fig. 5), the improvement in I_phos from 2a to 2b was attributed to the restriction of molecular motion due to O⋯I halogen bonding. The further enhancement observed for 2c can be explained by the presence of C–H⋯F interactions.

Fig. 8 (a) PL spectra excited at 360 nm. (b) Phosphorescence intensity of 1 and 2a–c in the crystalline state.

Table 3 Photophysical properties 1 and 2a–c in the crystalline state

	λ PL [nm]	τ flu [ns]	τ phos [μs]	Φ PL
a Excited at 350 nm. b Calculated using an integrating sphere.
1	387, 405, 559	1.1	8.6	0.12
2a	387, 405, 559	<1.0	2.3	0.01
2b	387, 482, 581	<1.0	5.2	0.03
2c	392, 493, 520	1.2	5.9	0.06

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To provide theoretical support for these findings, density functional theory calculations were performed based on the single-crystal geometry to estimate the frontier orbital energies in the solid state (see SI, Fig. S15 and Table S4 for details). Ultimately, the synergistic effect of both C–H⋯F and halogen bonding interactions in compound 1 resulted in the most significant overall improvement in its RTP properties.

Conclusions

This study systematically examined the influence of intermolecular interactions, particularly C–H⋯F and halogen bonding interactions, on the RTP properties of DPA derivatives. We synthesized a series of DPA derivatives (2a–c) with varying substituents to demonstrate that both types of interactions contributed to RTP enhancement. In particular, C–H⋯F interaction involving fluorine atoms is a primary factor for improving both phosphorescence intensity and efficiency. Detailed crystallographic analyses confirmed that the fluorinated derivatives (1 and 2c) formed strong intermolecular C–H⋯F interactions, which effectively restricted molecular motion and facilitated efficient RTP. Although C–H⋯F interaction alone had a moderate effect, its combination with halogen bonding produced a synergistic enhancement in RTP behavior. These results deepen our understanding of how non-covalent interactions modulate the photophysical properties of organic crystals and provide valuable design strategies for the development of high-performance, metal-free phosphorescent materials.

Author contributions

M. M.: data curation, investigation, validation, visualization, writing (original draft preparation), and writing (review and editing); M. Y., T. K.: investigation and writing (review and editing); S. Y.: conceptualization, data curation, investigation, validation, visualization, funding acquisition, writing (original draft preparation), writing (review and editing), and project administration.

Conflicts of interest

The authors declare that they have no conflicts of interest.

Data availability

Supplementary information: The SI provides experimental details, NMR spectra of the new compounds, PL spectra, single-crystal X-ray diffraction data, and quantum chemical calculations. See DOI: https://doi.org/10.1039/D5CE00690B.

CCDC 2125408–2125410 (2a–2c) contain the supplementary crystallographic data for this paper.^37a–c

The crystallographic data were deposited in the Cambridge Crystallographic Data Centre (CCDC) database (CCDC 2125408 for 2a, 2125409 for 2b, and 2125410 for 2c). The crystal structure of 1 has already been reported by our group (CCDC 2116930).²⁶

Acknowledgements

This work was supported by the research equipment shared through the MEXT Project for Promoting Public Utilization of Advanced Research Infrastructure (Program for Supporting Introduction of the New Sharing System; Grant Number JPMXS0421800222).

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Footnote

† Dedicated to Professor Resnati, celebrating a career in fluorine and noncovalent chemistry on the occasion of his 70th birthday.

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