Phenothiazine-based TADF emitters with dual conformations for single-molecule white OLEDs

Abstract

Donor–π–acceptor (D–π–A) type phenothiazine derivatives, exhibiting quasi-axial and quasi-equatorial conformers with distinct photophysical properties, serve as tunable, multifunctional materials for applications in sensors and optoelectronic devices. Herein, we report two phenothiazine based D–π–A emitters, NTPH and NTPCF, incorporating phenothiazine as the donor and nicotinonitrile as the acceptor, designed as stimuli-responsive functional materials with switchable conformers exhibiting differential optical properties. Single-crystal analyses show that phenothiazine adopts a quasi-axial conformation in NTPCF, while NTPH exhibits polymorphism with quasi-axial (NTPH-B) and quasi-equatorial (NTPH-O) conformers. The quasi-axial conformer of NTPCF exhibits green emission with a fluorescence maximum at 545 nm, while NTPH shows cyan emission with a fluorescence maximum at 463 nm and a shoulder band at around 516 nm. The equatorial conformer of NTPH has a characteristic TADF orange emission with a maximum of 585 nm. Theoretical studies highlight that singlet-triplet energy gap ΔE_ST is significantly influenced by the molecular conformation and the low energy barriers (<3 kcal mol⁻¹) between the axial and equatorial conformers enable dynamic conformational flexibility at room temperature. Mechano-responsive changes in the emission spectra further support the availability of energetically close, quasi-axial and quasi-equatorial conformers, enabling the tuning of TADF in the solid state. NTPCF shows high-contrast, reversible cyan-to-orange mechanochromism (Δλ_em ∼ 135 nm), suitable for anticounterfeiting, while NTPH exhibits irreversible blue-to-orange switching. Solution-processed orange organic light-emitting diodes (OLEDs) based on the neat film of NTPCF achieve a high luminance of 8400 cd m⁻². Additionally, NTPCF-based single-molecule white OLEDs were developed by varying the doping concentration, yielding a cool white OLED with CIE coordinates of (0.33, 0.37), a CRI of 70, and an EQE of 3.36% and a warm white OLED with CIE coordinates of (0.39, 0.45) and an EQE of 4.25%.

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Introduction

Organic light-emitting diodes (OLEDs) have transformed display and lighting technologies owing to their high efficiency, mechanical flexibility, and tunable emission properties. Among the various strategies to improve OLED performance, thermally activated delayed fluorescence (TADF) has emerged as a promising mechanism.^1–3 TADF enables the utilization of both singlet and triplet excitons for light emission via reverse intersystem crossing (RISC), achieving near-unity internal quantum efficiencies without relying on heavy-metal-based phosphors.^4–9 Recent TADF emitters further expand this concept by employing multiple-resonance frameworks, higher-lying excited states, and through-space charge transfer designs, simultaneously enabling high efficiency, color purity, and device stability.^10–15 Traditionally, TADF emitters are designed to emit from a single emissive state, typically the lowest singlet excited state (S₁). However, recent advances have focused on the development of dual-emission TADF materials, compounds capable of emitting from two distinct excited states, such as locally excited (LE) and charge-transfer (CT) states, or exhibiting both fluorescence and room-temperature phosphorescence (RTP).^16,17 Incorporating dual emission within a single TADF emitter holds the potential for achieving richer emission profiles, broader color tunability, and even single-molecule white-light emission.^18,19 This capability is particularly promising for the design of color-tunable OLEDs and the development of simplified white OLEDs, offering potential advantages in terms of device architecture, color stability, and cost efficiency.^20,21

In recent years, stimuli-responsive TADF materials have gained significant attention due to their unique ability to modulate emission properties in response to external stimuli such as temperature,²² UV irradiation,²³ mechanical force like grinding,^24–28 and solvent polarity.²⁹ These materials hold great promise for applications in sensing, imaging and anti-counterfeiting technologies.³⁰ Recent studies have identified several molecules that integrate TADF and mechanochromism, displaying distinct luminescence colour changes under mechanical stress like grinding and shearing.^31–33 These stimuli-induced modifications can alter molecular conformations, excited-state ordering, or singlet-triplet energy gap (ΔE_ST) and may enable partial violation of Kasha's rule by facilitating radiative transitions from multiple excited states. This approach helps in the tuning of TADF and opens pathways for smart, tunable luminescent materials.

A particularly promising application of dual-emission TADF materials is the realization of white-light emission from a single molecular emitter.^34–36 Generally, white OLEDs rely on multiple emissive layers or dopants emitting in complementary spectral regions (typically blue, green, and red), which increases fabrication complexity and often results in color instability over time.³⁷ In contrast, single-component white emitters offer significant advantages, including simplified device architecture, improved color stability, and better color balance.^38,39 Achieving such functionality requires careful molecular design to balance radiative decay from different states, ensure efficient RISC, and maintain high photoluminescence quantum yields (PLQYs) under device operating conditions. While photoluminescent white light emission has been successfully demonstrated in many cases, achieving efficient electroluminescence (EL) from these materials continues to be a major challenge.^18,40–42

Recent studies have demonstrated that TADF molecules with twisted donor–acceptor (D–A) architectures can exhibit dual emission via simultaneous population of CT and LE states, which is usually aided by conformational heterogeneity.^32,43,44 Among various twisted donor motifs, phenothiazine has attracted significant attention due to its non-planar, butterfly-shaped structure, which imparts inherent conformational flexibility and strong electron-donating ability.^45–51 This flexibility gives rise to quasi-axial and quasi-equatorial conformers, whose coexistence significantly influences photophysical behavior.^36,52 Exploiting this conformational duality, phenothiazine-based emitters have also been employed as single-component white-light emitters for OLED applications. Although phenothiazine derivatives have been explored for white-light emission, their application in single-molecule white organic light-emitting diodes (WOLEDs) remains relatively limited, with only a few reported examples to date.^{43,44,53–55}

Not all phenothiazine motifs with conformational flexibility are capable of producing white-light emission.^56,57 Only those engineered with well-separated and balanced emissive processes, often through dual conformers or combined fluorescence/TADF or fluorescence/RTP, can achieve white emission.⁵⁸ Without such carefully optimized structural and photophysical features, these flexible molecules typically yield single-color emission or insufficiently complementary dual emission.^49,59,60 In this context, careful molecular design of phenothiazine-based emitters is important. By adjusting donor–acceptor orientation, controlling the balance of conformers, and choosing suitable host environments, their excited-state behavior can be tuned. These approaches can enable the development of efficient single-molecule WOLEDs and help clarify how the molecular structure affects the emission properties of flexible luminophores.

Herein, we present two strategically designed phenothiazine-based fluorophores, NTPH and NTPCF, structurally well-defined D–π–A molecules featuring phenothiazine as the electron donor and a nicotinonitrile unit bearing two phenyl groups as the electron acceptor. In NTPCF, a trifluoromethyl (–CF₃) group was introduced to enhance the electron-withdrawing ability of the acceptor, thereby fine-tuning its electronic characteristics. Nicotinonitrile derivatives are well known for their excellent electron-transporting and hole-blocking properties, making them highly suitable for OLED applications.⁶¹ The incorporation of a phenyl π-bridging spacer offers optimal separation between the donor and acceptor units, enabling facile bond rotation that limits spatial orbital overlap, thereby resulting in a small ΔE_ST. Furthermore, single-crystal X-ray diffraction (SCXRD) studies enabled the identification of two distinct conformers of NTPH, providing key insights into the role of molecular packing and intermolecular interactions in modulating luminescence changes and the reversible TADF “on–off” switching upon mechanical stimulation (grinding). Notably, NTPCF exhibited high-contrast, reversible mechanochromism, demonstrating potential utility in anticounterfeiting applications. Furthermore, solution-processed OLEDs based on neat films of NTPH and NTPCF without any host were successfully fabricated, and additionally, warm and cool white OLEDs were realized using NTPCF by carefully tuning the doping concentration. These findings represent a compelling direction for multifunctional OLED platforms that integrate stimuli-responsive photophysics, TADF, and dual emission in a single-component system.

Results and discussion

Design and synthesis

N-Phenyl substituted phenothiazine derivatives are known to exhibit polymorphism featuring quasi-axial, quasi-equatorial and intermediate conformers.⁶² These conformers differ in the degree of planarization of the phenothiazine ring and in the dihedral angle between the phenothiazine core and the attached phenyl ring.^63,64 The nature of the N-substituents significantly influences the conformer population, crystal packing, and the resulting photophysical properties.^60,65 Quasi-axial conformers, characterized by a puckered phenothiazine ring, possess an excited singlet state with dominant LE character and typically emit in the short-wavelength region.⁶⁶ In contrast, quasi-equatorial conformers, having a nearly planar phenothiazine ring and an orthogonal N-phenyl substituent, exhibit significant CT character in their lower excited states with very low ΔE_ST, leading to emission at longer wavelengths, either via RTP or TADF.⁴⁹ In D–π–A type phenothiazine derivatives, the strength of the acceptor group can modulate the conformer population, crystal packing and excited state energy levels, resulting in switchable conformers with distinct physical and optical characteristics. In this study, we have designed two such derivatives, NTPH and NTPCF, incorporating phenothiazine as the donor, a nicotinonitrile core bearing two phenyl substituents as the acceptor and a phenyl group serving as the π-bridge (Fig. 1). The acceptor strength is tuned by incorporating an electron withdrawing –CF₃ group on one of the phenyl substituents on nicotinonitrile in NTPCF, which will stabilize the excited state energy levels and alter the optical properties, without significantly perturbing the overall molecular geometry.

Fig. 1 Molecular structures of the synthesized molecules and molecular conformations of corresponding single crystals.

The target molecules, NTPH and NTPCF, were synthesized via following modified procedures, as outlined in the synthetic routes shown in Scheme S1. The key step involves a Buchwald–Hartwig coupling between phenothiazine and the synthetic intermediates NTH-Br or NTCF-Br, yielding the target compounds, NTPH and NTPCF, with yields exceeding 65%. The chemical structures of both molecules were characterized and confirmed via ¹H NMR, ¹³C NMR, and high-resolution mass spectrometry (HRMS) (Fig. S1–S6). Additionally, their molecular structures were unambiguously determined by single-crystal X-ray crystallography (Fig. 1).

Thermal and electrochemical properties

The thermal properties of the molecules were studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under a nitrogen atmosphere (Fig. S7). Both NTPH and NTPCF exhibited good thermal stability, with decomposition temperatures corresponding to a 5% weight loss observed at 326 °C and 312 °C, respectively. DSC measurements of the powdered samples were conducted over a temperature range of 30 °C to 280 °C to explore thermal transitions. The pristine powders of NTPH and NTPCF displayed prominent endothermic peaks at 215 °C and 240 °C, respectively, corresponding to their melting points. Notably, for NTPH, an additional exothermic peak at 144 °C and an endothermic peak at 154 °C were detected prior to melting, suggesting a possible phase transition to a more stable form which is an indication of polymorphism.

The frontier molecular orbital energies (HOMO and LUMO) of NTPH and NTPCF were estimated using cyclic voltammetry (CV). As illustrated in Fig. S8, both compounds exhibited a one-electron redox process in chlorobenzene solution, attributed to the oxidation of the phenothiazine donor unit and the reduction of the nicotinonitrile unit. The HOMO energy levels were determined from the onset oxidation value of the cyclic voltammogram, while the LUMO levels were calculated using the HOMO values and the optical bandgap. The calculated HOMO/LUMO energy levels were −5.08 eV/−2.10 eV for NTPH and −4.80 eV/−1.85 eV for NTPCF. The slightly lower HOMO and LUMO energies in NTPCF are attributed to the presence of the strong electron-withdrawing –CF₃ group in the acceptor unit.

Theoretical investigations

To investigate the conformational flexibility and steric interactions within the donor-phenyl spacer segment of NTPH and NTPCF, relaxed ground state potential energy surface (PES) scans were performed in the gaseous phase. The PES scan involved rotation about the dihedral angle between the phenyl spacer and the phenothiazine donor unit. All calculations were done using the B3LYP/def2-TZVP level of theory,^67–71 incorporating D3BJ dispersion corrections⁷² with RIJCOSX⁷³ and performed using the ORCA 5.0.3 software package.⁷⁴ The singlet (S₁) and triplet (T₁) energies were calculated as adiabatic (relaxed) excitation energies obtained from geometry optimizations of the respective excited states, and the energy difference between them was denoted as ΔE_ST. The computed geometrical parameters (bond lengths, bond angles, and dihedral angles) are presented in Table S6 (SI II) and have been compared with the single-crystal XRD data. The computed results show good agreement with the experimental parameters, confirming their reliability.

The ground-state rotational energy profiles of NTPH and NTPCF from PES scan (Fig. 2A and B) provide quantitative insight into the torsional flexibility of the donor-phenyl spacer linkage. The computed profiles exhibit two nearly symmetric minima at dihedral angles of approximately 90° and 270°, corresponding to quasi-equatorial (orthogonal) conformations, and two maxima near 0° and 180°, representing quasi-axial (planar) geometry. The barrier heights separating these conformers are modest, approximately 2.8 kcal mol⁻¹ for NTPH and 3.1 kcal mol⁻¹ for NTPCF, indicating facile interconversion at room temperature.

Fig. 2 Relaxed PES scan of the ground state (gas phase) as a function of the twist angle between the phenyl spacer and the phenothiazine unit of (A) NTPH and (B) NTPCF. The HOMO and LUMO of quasi-equatorial (orange box) and quasi-axial (blue box) are also given.

Along the rotation coordinate, the degree of conjugation between the donor and spacer changes continuously. In planar geometries (0° and 180°), strong π-conjugation between phenothiazine and the phenyl ring promotes orbital delocalization and a large HOMO–LUMO overlap, yielding higher ΔE_ST values (>0.5 eV). As the torsion increases toward 90° and 270°, the donor and spacer π-planes become nearly orthogonal, suppressing electronic coupling and forming a twisted intramolecular charge-transfer (TICT) configuration. This decoupling reduces the exchange interaction, leading to small ΔE_ST values (∼0.02–0.05 eV). The DFT-derived rotation profiles clearly delineate how conformational motion governs the electronic coupling and photophysical response in these donor–acceptor systems.

The frontier molecular orbitals (HOMO and LUMO) of both conformations of NTPH and NTPCF were visualized to investigate their electronic structures and charge-transfer characteristics. The corresponding molecular orbital plots for both conformers of NTPH and NTPCF are presented in Fig. 2. In the axial conformations of both molecules, the HOMO and LUMO exhibit considerable spatial overlap, resulting in a larger ΔE_ST. In contrast, the quasi-equatorial conformations display a spatial separation between the frontier orbitals with the HOMO localized on the phenothiazine donor moiety and LUMO distributed across the nicotinonitrile acceptor unit. This spatial separation of electron density between the HOMO and LUMO confirms significant intramolecular charge-transfer (ICT) character, which is desirable for TADF behavior as it leads to a reduction in exchange energy and hence a smaller ΔE_ST.

Additionally, we computed the variation in ΔE_ST for each relaxed geometry along the ground-state PES scan, with the values plotted as a function of the donor–acceptor dihedral angle (Fig. 3A and B). Higher ΔE_ST values (>0.6 eV) were observed in the quasi-axial conformations, indicating stronger orbital overlap and reduced charge-transfer character. In contrast, for both NTPH and NTPCF, the minimum ΔE_ST values occur at near dihedral angles of approximately 90° and 270°, corresponding to twisted (quasi-equatorial) conformations, identified as global minima on the relaxed PES. At these twisted geometries, ΔE_ST approaches near-zero values (∼0.02–0.05 eV) in both systems. These results clearly demonstrate that conformational control is crucial for tuning ΔE_ST and that twisted geometries are essential for enabling efficient TADF. Furthermore, the excitation energies of the singlet and triplet states for the quasi-equatorial conformers of NTPH and NTPCF are shown in Fig. 3C. For NTPH, the S₁ and T₁ energies are 2.195 and 2.189 eV, respectively, yielding a small ΔE_ST of 0.006 eV. Similarly, for NTPCF, the S₁ and T₁ energies are 2.062 and 2.057 eV, resulting in a ΔE_ST of 0.005 eV. In comparison, the estimated ΔE_ST values for the quasi-axial conformations are significantly higher, calculated to be 0.59 eV for NTPH and 0.53 eV for NTPCF. To further confirm the TADF characteristic of the molecule we recalculated the lowest singlet and triplet excited-state energies in implicit toluene (CPCM) at the same level of theory as in the gas phase. The singlet–triplet gaps remain near zero in the toluene solvent phase also: for NTPH, S₁ = 2.195 eV, T₁ = 2.189 eV, and ΔE_ST = 0.006 eV and for NTPCF, S₁ = 2.062 eV, T₁ = 2.057 eV, and ΔE_ST = 0.005 eV. These results further confirm that twisted quasi-equatorial geometries are highly favorable for TADF performance due to their minimal singlet–triplet energy gaps.

Fig. 3 Calculated ΔE_ST for each conformer obtained from the relaxed potential energy surface scan on the ground state (gas phase) (A) NTPH and (B) NTPCF. Singlet and triplet state energies of NTPH and NTPCF for quasi-equatorial conformation (C). Calculations were done using TDDFT at the B3LYP/def2-TZVP-D3BJ level of theory.

Single crystal analyses

To elucidate the structure–property relationships, we performed an in-depth analysis of SCXRD data, providing detailed insights into molecular packing, conformation, and intermolecular interactions within the crystal lattice. The crystallographic data for NTPCF (Table S1), NTPH-B (Table S2), and NTPH-O (Table S3) are included in the SI I. The ORTEP diagrams of NTPCF and NTPH are given in Fig. S9. Green-emissive, needle-like single crystals of NTPCF were obtained via slow evaporation of its toluene solution, crystallizing in the triclinic system with space group P1 with Z = 4. Notably, the unit cell of NTPCF consists of two twinned axial crystal pairs, each containing two asymmetric units, Ax1 and Ax2, interconnected by intermolecular interactions forming a dimeric unit (Fig. S10). In both Ax1 and Ax2, phenothiazine adopts a quasi-axial conformation, with its central sulphur- and nitrogen-containing ring assuming a boat-like conformation. The torsional angles between the phenothiazine and the benzene ring are 16.71° and 20.34° for Ax1 and Ax2, respectively. The Ax1–Ax2 dimer structure reveals a network of non-covalent interactions including π–π, C–H⋯π, C–H⋯N, C–S and C–N interactions, that contribute to its molecular stability and packing arrangement. A detailed analysis and discussion of these interactions are provided in the SI (Fig. S10). Along the a-axis, the molecules adopt a head-to-head arrangement, while along the c-axis, they form a slip-stacked head-to-tail configuration that helps minimize steric hindrance (Fig. S11). The columns aligned along the a-axis are interconnected through C–H⋯F interactions (2.77–2.95 Å) and π–π stacking (3.58 and 3.67 Å), leading to interdigitated layers and the formation of a corrugated, sheet-like structure.

Slow evaporation of NTPH solution in a 4 : 2 : 1 mixture of dichloromethane, ethyl acetate and acetonitrile yielded two NTPH polymorphs: cyan emissive, needle-like crystals (referred to as NTPH-B) and orange emissive, plate-like crystals (referred to as NTPH-O) (Fig. 4). NTPH-B crystallizes in the triclinic system with space group P1 and Z = 4. It corresponds to the quasi-axial conformer of phenothiazine, adopting a conformation similar to that observed in the green-emissive NTPCF crystals. The structure has two twinned axial conformers, denoted as Ax3 and Ax4, which together form an asymmetrical dimer unit. Ax3 exhibits a nearly zero twist angle between the phenothiazine and the phenyl spacer, whereas Ax4 shows a torsional angle of 14.35°. As shown in Fig. 4A and B, the interface between the symmetric equivalents, Ax2 and Ax3, of NTPH-B adopts a head-to-head parallel arrangement, stabilized by multiple C–H⋯π interactions (2.77–3.51 Å) and π–π stacking interactions (3.49, 3.65, and 3.63 Å). An internal pore is formed through the antiparallel packing of the asymmetric units Ax3 and Ax4, facilitated by additional C–H⋯π interactions (2.99 Å, 3.10 Å and 3.18 Å) (Fig. 4C). This packing arrangement generates a long-range, orderly channel structure, resulting in the formation of slipped, 2D corrugated layers (Fig. S12).

Fig. 4 The crystal packing mode of (B) NTPH-B and (E) NTPH-O. The interactions between (A) axial pairs, Ax3 and Ax4 and (C) two columns of NTPH-B. The interactions between equatorial pairs (D) along the c axis and (F) along the b axis of NTPH-O.

NTPH-O crystallizes in the monoclinic system with space group P2₁/c and Z = 4. It corresponds to the quasi-equatorial conformer of phenothiazine, in which the phenothiazine adopts a planar orthogonal conformation, with a torsional angle of 96.88° relative to the phenyl spacer. Unlike NTPH-B, no asymmetric unit is observed in the unit cell. As shown in Fig. 4E and S13, the molecules in NTPH-O adopt a head-to-tail, slip-stacked antiparallel arrangement, resulting in a tightly packed two-dimensional sheet-like structure. This packing is stabilized by multiple intermolecular interactions (Fig. 4D and F), including C–H⋯π interactions (2.99–3.20 Å). Strong π–π stacking interactions are observed between the phenothiazine and the phenyl rings attached to the nicotinonitrile group (3.46 Å, 3.60 Å, and 3.44 Å). Additionally, a C–H⋯S interaction (3.04 Å) connects the phenothiazine and the phenyl ring, further reinforcing the head-to-tail configuration. A C–H⋯N interaction (2.90 Å) between the phenothiazine and a phenyl hydrogen also contributes to the overall structural stability. The distinct packing patterns observed highlight the crucial role of molecular conformation in modulating spatial arrangements, determining crystal packing motifs, and influencing the photophysical and optical properties of the materials.

Solution state photophysical studies

The UV-Vis absorption and steady-state photoluminescence (PL) spectra of NTPCF and NTPH were recorded in dilute toluene solutions (1 × 10⁻⁵ M) at room temperature (Fig. 5A), and key photophysical parameters are summarized in Table S4. The absorption spectra of both compounds display a strong π–π* transition near 300 nm and a broad CT band at around 370 nm. In the PL spectra, both NTPCF and NTPH exhibit dual emission. NTPH shows emission peaks at 420 nm (LE) and 580 nm (CT), while NTPCF exhibits corresponding red-shifted peaks at 440 nm and 608 nm, respectively. The higher-energy LE band is attributed to the quasi-axial conformation of the phenothiazine donor, exhibiting weak CT character, whereas the lower-energy emission corresponds to the quasi-equatorial conformation with a strong CT nature. These assignments are consistent with theoretical predictions, solvatochromic studies (Fig. S14) and previous reports on similar phenothiazine-based donor–acceptor systems and are further supported by our single-crystal structural analyses.^75,76

Fig. 5 (A) Normalized UV/visible absorption spectra and steady-state PL spectra measured in toluene solution (1 × 10⁻⁵ M) at room temperature of NTPH and NTPCF and (B) steady state PL spectra of NTPCF in toluene under argon and oxygen saturated conditions. The transient PL decay profiles of NTPCF in toluene solutions probed at 600 nm, (C) under aerated and degassed conditions and (D) at 77 K and 298 K.

The quasi-equatorial conformations of phenothiazine-based donor–acceptor systems with strong excited state CT character are known to exhibit delayed fluorescence via a TADF mechanism.⁵⁴ Our theoretical calculations on NTPCF and NTPH predicted near zero ΔE_ST values for both derivatives (0.006 eV and 0.005 eV, respectively) in the quasi-equatorial conformers. To confirm the involvement of possible TADF, steady-state PL spectra of NTPCF (Fig. 5B) and NTPH (Fig. S15A) were recorded in dilute toluene solutions (1 × 10⁻⁵ M) under both argon and oxygen saturated conditions. For both molecules, a significant quenching of long-wavelength emission was observed in the presence of oxygen, suggesting the involvement of triplet excited states in the emission process. Transient PL decay measurements were carried out under argon and oxygen atmospheres to further investigate the TADF nature. As shown in Fig. 5C, NTPCF exhibits a biexponential decay at 600 nm with a prompt component of 15 ns and a delayed component of 521 ns.

Similarly, NTPH displays biexponential decay at 580 nm with prompt and delayed lifetimes of 16 ns and 558 ns, respectively (Fig. S15B). In the presence of oxygen, both molecules show complete suppression of the delayed component, confirming that the long-wavelength emission arises from TADF. In contrast, transient PL decay at shorter wavelengths reveals only prompt fluorescence, indicating that TADF is specifically associated with the quasi-equatorial conformation, responsible for long-wavelength emission and the corresponding lifetime values are provided in Table S5. Further evidence for TADF was obtained from temperature-dependent transient PL studies at 77 K and 300 K (Fig. 5D and S15C), which demonstrated an increase in delayed emission with rising temperature, consistent with the thermally activated nature of the delayed fluorescence in both emitters.

The phosphorescence spectra of NTPCF and NTPH were recorded in toluene glass at 77 K and compared with their fluorescence in dilute toluene solutions at 298 K (Fig. S16A and B). Interestingly, both molecules exhibited phosphorescence originating from a localized ³LE state, which was blue-shifted relative to the long wavelength TADF emission assigned to the quasi-equatorial conformer. Notably, the emission from the ³CT state corresponding to the quasi-equatorial band was absent at 77 K, likely due to restricted molecular rotations in the frozen toluene matrix. Similar observations were previously reported for phenothiazine-based donor–acceptor systems, supporting the involvement of a localized ³LE state at lower temperatures.^77,78 At elevated temperatures, increased conformational flexibility facilitates the formation of quasi-equatorial conformers, leading to red-shifted emissions from the ¹CT/³CT states. To further probe the nature of the ³LE emission, we compared the phosphorescence spectra of NTPCF in toluene and in relatively polar 2-methyltetrahydrofuran (2-MeTHF) at 77 K (Fig. S17A). Minimal solvent dependence was observed, reinforcing the localized nature of the ³LE state. Additionally, the phosphorescence spectrum of the isolated acceptor moiety, measured under identical conditions (Fig. S17B), closely matched that of NTPCF in both emission maxima and spectral profile. This spectral overlap strongly suggests that the observed triplet emission in NTPCF arises predominantly from a ³LE state localized on the acceptor unit.

Based on the onset of phosphorescence, the ³LE state energies for NTPH and NTPCF were estimated to be 2.56 eV and 2.63 eV, respectively. From spectroscopic measurements, the ¹LE (quasi-axial) and ¹CT (quasi-equatorial) states were estimated to be 3.15 eV and 2.47 eV for NTPH and 3.10 eV and 2.40 eV for NTPCF, respectively. In the quasi-axial conformation, due to puckering, the nitrogen lone pair exhibits reduced localization into the phenyl rings of phenothiazine, resulting in significant HOMO–LUMO overlap (Fig. 2) and enhanced D–A conjugation. This leads to a higher singlet–triplet separation with reduced charge separation. In contrast, the quasi-equatorial conformation with more planar phenothiazine rings allows greater delocalization of the nitrogen lone pair, decreasing HOMO–LUMO overlap and D–A conjugation. This promotes electronic decoupling in quasi-equatorial conformers and facilitates a lower-energy CT state with more efficient charge transfer. Accordingly, the energy difference between the ¹LE and ³LE states (denoted as ΔE_ST,ax) for NTPH and NTPCF was estimated to be 0.63 and 0.51 eV, respectively, in good agreement with the TD-DFT calculations (0.59 eV & 0.53 eV). Since the ³CT emission associated with the quasi-equatorial conformer was not detected under these conditions, experimental determination of the corresponding ΔE_ST values was not feasible. Therefore, we relied on values derived from TD-DFT calculations.

Solid state luminescence & mechanochromism

Due to the dual-conformation flexibility of the phenothiazine moiety, both emitters exhibited stimulus-responsive fluorescence with high contrast and sensitivity. Based on this structural feature, we investigated the mechanochromic behaviour of NTPH and NTPCF in detail. The pristine powder samples of both compounds were obtained by precipitation from dichloromethane and hexane, while the single crystals were prepared as discussed in the previous section. For NTPCF, the pristine powder displayed a broad cyan emission with a maximum at 457 nm, while its single crystal exhibited a red-shifted green emission at 545 nm (Fig. 6A and S18). Upon mechanical grinding of the pristine powder (Fig. 6A and C), the emission further red shifted to orange, with a maximum at 593 nm. The original cyan fluorescence was restored upon fuming with solvent vapors, such as chloroform, dichloromethane or hexane. This fluorescence colour change was reversible and repeatable over multiple cycles.

Fig. 6 Normalized emission spectra and photographs under UV excitation of (A and C) NTPCF and (D and F) NTPH in different solid-states. PXRD patterns of pristine, ground and fumed samples of (B) NTPCF and (E) NTPH and (G) demonstration of rewritable mechanochromism: photographs, under UV light showing, writing and erasing via solvent fuming on a filter paper coated with NTPCF.

In the case of NTPH, the pristine powder showed dual-emission bands with peaks at 463 nm and 516 nm. Mechanical stimuli induced a red shift of the emission maximum to around 585 nm (Fig. 6D and F). Fuming the ground sample of NTPH with chloroform, even for five minutes, couldn't completely reverse the emission of the pristine powder and lead to a very broad emission covering the entire visible region. Among the single crystal polymorphs, NTPH-B exhibited bluish-green emission with dual peaks at 467 nm and 510 nm, whereas NTPH-O emitted an orange fluorescence centered at 585 nm (Fig. 7). Interestingly, the emission profile of NTPH-B matched that of the pristine powder, while NTPH-O mirrored the emission of the ground sample, establishing a clear correlation between mechanochromic behavior and molecular conformation. As illustrated in Fig. 7 and discussed earlier, we can assign that the higher energy bluish-green emission originates from the LE state (quasi axial conformer), while the orange emission band originates from the CT state (quasi-equatorial conformer). Furthermore, the computational studies predicted lower energy barriers between the conformers (<3 kcal mol⁻¹), indicating good conformational flexibility at room temperature. These results underscore the critical role of conformational flexibility in modulating photophysical responses to mechanical stimuli. To further understand the photoluminescence characteristics, absolute photoluminescence quantum yield (PLQY) measurements were performed before and after grinding. For NTPCF, the pristine form exhibited a maximum PLQY of 35%, which decreased to 14% after grinding. In contrast, NTPH showed a PLQY of 15% in its pristine form, which increased to 19% after grinding. These results demonstrate that a subtle modification of the acceptor unit, such as introducing a –CF₃ group, can significantly enhance the quantum yield and influence mechanochromic response.

Fig. 7 Schematic figure illustrating the correlation between the mechanochromic behaviour and molecular conformation of NTPH.

To understand the mechanism of the observed mechanochromism, powder X-ray diffraction (PXRD) analyses were performed on the pristine, ground and fumed powders of both derivatives (Fig. 6B and E). The sharp and intense diffraction peaks observed in the pristine powders of NTPCF and NTPH confirm their crystalline nature. Upon grinding, NTPCF showed a significant loss of peak intensity and broadening of the diffraction pattern, indicating a transition to a semi-crystalline state. In contrast, NTPH retained its crystallinity even after grinding, with the appearance of new peaks, suggesting a crystalline-to-crystalline transformation. For NTPCF, the original crystallinity was fully restored upon fuming the ground sample, as evidenced by the reappearance of sharp diffraction peaks. However, in the case of NTPH, only partial recovery of crystallinity was observed after fuming. These observations are in agreement with the observed changes in their optical properties. Furthermore, we have demonstrated the write-erase-rewrite process using NTPCF coated filter paper, as shown in Fig. 6G. The high-contrast and reversible mechanochromic behaviour of NTPCF highlights its potential for applications in anti-counterfeiting rewritable paper.

Solid-state transient PL decay measurements of NTPH (Fig. S19A and B) and NTPCF (Fig. 8A and B) were carried out before and after grinding to investigate their excited-state dynamics. Interestingly, the results revealed mechanical stimuli-induced TADF in both emitters, enabling them to perform as TADF on/off systems. In the pristine state, both NTPH and NTPCF showed only nanosecond lifetimes, corresponding to prompt fluorescence. However, upon mechanical grinding, the PL decay spectra of both emitters displayed two components: a nanosecond-scale prompt component and a microsecond-scale delayed component, consistent with TADF behavior. These delayed components match those observed in the long-wavelength region in toluene solution, and the corresponding lifetime values are provided in Table S5. This TADF activation is accompanied by the transition from the quasi-axial to quasi-equatorial state, thereby promoting efficient RISC and delayed fluorescence.

Fig. 8 Transient PL decay spectra of NTPCF (A) before and (B) after mechanical grinding. (C) Steady-state PL spectra of films of PVK doped with varying concentrations of NTPCF (1 wt%, 3 wt%, 5 wt% and 10 wt% and neat), excited at 375 nm. (D) Transient PL decay spectra of NTPCF in the film state; comparison between the neat film and 10 wt% PVK-doped samples.

To understand the excited-state behavior of solution-cast films of NTPCF and NTPH under conditions relevant to OLED applications, both emitters were incorporated into poly(N-vinyl carbazole) (PVK), a widely used OLED host matrix. Steady-state and time-resolved PL measurements were performed at various doping concentrations to evaluate their emission characteristics. At a low doping concentration of 1 wt%, both NTPCF (Fig. 8C) and NTPH (Fig. S20A) exhibited emission contributions from the axial and equatorial conformers, resulting in white-light emission. However, with increasing doping concentrations, the contribution from the axial conformer gradually diminished. At 10 wt% doping, the emission was predominantly attributed to the equatorial conformation. This shift suggests that increased molecular aggregation at higher concentrations facilitates intermolecular energy transfer from the higher-energy quasi-axial to the lower-energy quasi-equatorial state, thereby altering the overall emission profile.

The transient PL decay profiles of NTPCF (Fig. 8D) and NTPH (Fig. S20B) were examined in both neat and 10 wt% PVK-doped films. In both cases, prompt (nanosecond scale) and delayed (microsecond scale) emission components were observed (Table S5). Upon doping, the delayed emission component showed a marked enhancement, from 604 ns to 1.1 µs for NTPCF and from 551 ns to 1.3 µs for NTPH, indicating a significant increase in delayed fluorescence. For 1 wt% PVK-doped NTPCF, the axial conformer displayed a maximum emission at 440 nm with a corresponding lifetime of 130 ps (Fig. S21). Collectively, these findings highlight the pivotal role of conformational dynamics, host–guest interactions, and molecular packing in governing the emission behaviour of NTPCF and NTPH in the film state.

OLED device fabrication

OLED devices were fabricated via solution processing using the architectures shown in Fig. 9, employing NTPH or NTPCF as the emitting layer without a host matrix (Devices A & B). In addition, by utilizing the distinct emission behaviour of NTPCF at different doping concentrations in a poly(N-vinylcarbazole) (PVK) host matrix, we fabricated cool white and warm white OLEDs (Devices C & D). The corresponding energy level diagrams for all four devices are also shown in Fig. 9. The detailed device structures are as follows: (i) Devices A & B: ITO/PEDOT:PSS (40 nm)/neat NTPH or NTPCF (30 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm) and (ii) Devices C & D: ITO/PEDOT:PSS (40 nm)/PVK: NTPCF (1 wt% for Device C and 10 wt% for Device D) (30 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm). In these configurations, PEDOT:PSS and LiF functioned as the hole- and electron-injection layers, respectively. TPBi served as the electron-transporting and hole-blocking layer, while ITO and Al were used as the anode and cathode electrodes, respectively.

Fig. 9 Schematic diagram of device structure and energy level of OLEDs using (A) NTPH (Device A) and NTPCF (Device B) neat films and (B) NTPCF doped with 1 wt% (Device C) and 10 wt% (Device D) in PVK.

The EL spectra of Device A (neat NTPH) and Device B (neat NTPCF) are shown in Fig. 10, with EL maxima at 596 nm and 584 nm, respectively. Devices A and B display orange electroluminescence with CIE coordinates of (0.53, 0.46) and (0.52, 0.48), respectively, which can be attributed to the TADF emission from the quasi-equatorial conformation. Device B shows a lower turn-on voltage (V_t: 4 V) compared to Device A (V_t: 5 V). Moreover, Device B (NTPCF) outperforms Device A (NTPH) in terms of efficiency, achieving a maximum current efficiency (CE) of 5.54 cd A⁻¹, power efficiency (PE) of 2.98 lm W⁻¹, and external quantum efficiency (EQE) of 2.31%, compared to 2.84 cd A⁻¹, 1.49 lm W⁻¹, and 1.39% for Device A. Device B also demonstrates higher luminance, with a maximum value of 8400 cd m⁻² versus 4700 cd m⁻² for Device A.

Fig. 10 (A) EL spectra of Devices A and B (inset: the image of real device illumination during operation). (B) CIE vs. voltage graph of Devices A and B. (C) Current efficiency–power efficiency–luminance curves of Devices A and B. (D) Current density–voltage–luminance curves of Devices A and B.

To further enhance the performance of NTPCF and achieve white OLED emission, PVK was employed as a host matrix at doping concentrations of 1 wt% (Device C) and 10 wt% (Device D). As anticipated, the doped devices exhibited improved EQE, PE, and CE compared to those with neat films of NTPH and NTPCF. Device C and Device D exhibited dual complimentary emission, as shown in Fig. 11. At 1 wt% doping (Device C), a blue emission peak at 425 nm, attributed to the quasi-axial conformer, was observed alongside the orange emission at 555 nm from the quasi-equatorial conformer. With an increased doping concentration of 10 wt% (Device D), the blue emission intensity diminished while the orange emission became more prominent. The enhancement of orange emission is attributed to the efficient utilization of both singlet and triplet excitons due to the TADF nature of the quasi-equatorial conformer and efficient energy transfer from the quasi-axial conformer, leading to improved device performance.

Fig. 11 (A) EL spectra of Device C and (B) Device D (inset: the image of real device illumination during operation). (C) Current efficiency–power efficiency–luminance curves of Device C and Device D. (D) Current density–voltage–luminance curves of Device C and Device D.

Device C and Device D both have a turn on voltage of 5 V. Device C achieves a maximum luminance of 2420 cd m⁻² at a current density of 137 mA cm⁻², while Device D reaches 5240 cd m⁻² at 303 mA cm⁻². The maximum EQEs are 3.36% for Device C and 4.55% for Device D. Device C delivers a maximum CE of 7.59 cd A⁻¹ and PE of 3.97 lm W⁻¹, whereas Device D achieves higher values, with a CE of 11.54 cd A⁻¹ and PE of 6.26 lm W⁻¹. At a luminance of 2420 cd m⁻², Device C emits cool white light with CIE coordinates of (0.33, 0.37), a correlated color temperature (CCT) of 5740 K, and a color rendering index (CRI) of 70. In contrast, Device D emits warm white light with CIE coordinates of (0.39, 0.45), a CCT of 4040 K, and a CRI of 63 at 3100 cd m⁻². Furthermore, at practical brightness levels of 500 cd m⁻² and 1000 cd m⁻², the EQEs of Device C are 2.76% and 1.90%, while Device D achieves EQEs of 3.48% and 2.75%, respectively. Further optimization is currently underway in our lab to enhance the efficiency of the devices.

Conclusion

In summary, we have successfully designed and synthesized two novel donor–acceptor emitters, NTPH and NTPCF, incorporating phenothiazine as the electron donor and nicotinonitrile as the electron acceptor, linked via a phenyl spacer. Single-crystal X-ray diffraction and theoretical calculations confirmed the existence of two conformers, quasi-axial and quasi-equatorial, of the phenothiazine unit. Photophysical studies in both solution and solid states demonstrated dual emission behaviour, with transient PL confirming that the quasi-equatorial conformer exhibits TADF, while the quasi-axial conformer shows prompt fluorescence. Theoretical analysis and crystal packing studies highlighted that molecular conformation and packing significantly influence the ΔE_ST and excited-state dynamics. The NTPH-O polymorph, characterized by an orthogonal twisted, quasi-equatorial conformation and dense head-to-tail packing, facilitates stronger intramolecular charge transfer (ICT), leading to a reduced ΔE_ST and enhanced TADF efficiency. In contrast, the NTPH-B polymorph, with its quasi-axial conformation facilitating through-bond π–π interactions, shows weaker ICT and reduced TADF performance. Both emitters exhibited mechanochromic luminescence, with NTPCF demonstrating a reversible spectral shift of 135 nm, making it suitable for anti-counterfeiting applications. Both emitters were successfully used to fabricate solution-processed OLEDs with neat films of NTPCF and NTPH, producing orange electroluminescence, with the NTPCF-based device achieving a luminance of 8400 cd m⁻². The high PLQY of NTPCF also enabled the fabrication of cool and warm white OLEDs by varying the doping concentration in a PVK host matrix. Overall, NTPH and NTPCF represent promising multifunctional materials for applications in OLEDs and security technologies.

Author contributions

P. V. P. and J. J. conceptualized the work, developed the molecular designs and synthetic protocols and wrote the manuscript. P. V. P. carried out the syntheses, characterization, and photophysical studies of solution, powder and film states. K. N. N. U. and V. D. carried out the device fabrication and optimization. S. V. and I. S. D. helped with single crystal analyses. A. A. and M. B. did the theoretical calculations. All authors reviewed and edited the final manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

All supporting data are provided within the article and its accompanying supplementary information (SI). Supplementary information: experimental details, synthetic procedures, structural characterization (NMR, HRMS), theoretical calculations, spectroscopic data, and device performance data. See DOI: https://doi.org/10.1039/d5sc04370k. CCDC 2463502 (NTPCF), 2463503 (NTPH-B) and 2463504 (NTPH-O) contain the supplementary crystallographic data for this paper.^79a–c

Acknowledgements

The authors gratefully acknowledge the funding support from CSIR, DST, SERB, and DCPC, Government of India. Support from the SC-XRD Lab, SAIF, IIT Madras, for assistance with single-crystal X-ray diffraction data collection and structure refinement is also duly acknowledged. Financial support from the CSIR-FTT Project (MLP-0081), DST-AISRF (DST/INT/AUS/P-74/2017), the DST Nanomission Project (DST/NM/TUE/EE-02/2019-1), and the CSIR-NIIST DCPC Center of Excellence (PC-II-25014/3/2022-PC II-CPC, FTS: 3019123) is gratefully acknowledged. J. J. also acknowledges support from SERB under the CRG scheme (CRG/2022/005710).

Notes and references

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