Antonio Maggiore*ab,
Yangyang Qua,
Gilles Clavier
a,
Marco Colellac,
Andrew Danos
cd,
Andrew Monkman
c,
Regis Guillot
e,
Marco Pugliese
b,
C. Tania Prontera
b,
Roberto Giannuzzib,
Fabrizio Marianob,
Sonia Carallob,
Gianluca Accorsi
b,
Vincenzo Maioranob,
Pierre Audebert
a,
Remì Metivier
a and
Fabien Miomandre
a
aUniversité Paris-Saclay, ENS Paris-Saclay, CNRS, PPSM, Gif-sur-Yvette, 91190, France
bCNR-NANOTEC – Institute of Nanotechnology, c/o Campus Ecoteckne, Via Monteroni, Lecce, 73100, Italy. E-mail: antonio.maggiore@cnr.it
cDepartment of Physics, University of Durham, South Road, Durham, DH1 3LE, UK
dSchool of Physical and Chemical Sciences, Queen Mary University of London, London, E1 4NS, UK
eUniversité Paris-Saclay, CNRS, Institut de chimie moléculaire et des matériaux d’Orsay, Orsay, 91405, France
First published on 28th April 2025
In this study, we explore the solid-state photophysical properties of four donor–acceptor–donor′ (D–A–D′) fluorinated benzonitrile compounds, featuring combinations of phenoxazine and carbazole donor units. We have previously shown that these compounds exhibit dual charge transfer thermally activated delayed fluorescence (CT-TADF) emission in solution; here we investigate the steady-state and time-resolved photophysical properties in the solid state. Initial photophysical characterizations were performed in polymethyl methacrylate (PMMA) polymer films, followed by detailed studies of the supramolecular packing in neat films under various preparation conditions, including solvent exposure and thermal treatments. Different polymorphs and amorphous phases were obtained, revealing that the dual TADF emission could be maintained to some extent in the solid-state. Mechanical stress and solvent vapour exposure induced mechanochromic luminescence (MCL), with emission changes correlating to the supramolecular structures. OLED devices were also fabricated, demonstrating similar tuneable emission properties based on preparation techniques. This work highlights the potential of this class of materials towards advanced optoelectronic applications with tuneable and stimuli-responsive emission characteristics.
It has also been shown that D–A and D–A–D materials in the condensed phase can be highly sensitive to external stimuli such as grinding or pressing, exhibiting mechanochromic luminescence (MCL) behaviour, as well as heating and solvent exposure,18,19 all of which impact emission colour either by enacting conformational changes in the molecules or by altering intermolecular interactions between neighbouring molecules.20–23 Mechanochromic luminescence typically starts from a crystalline or aggregated phase that switches to another crystalline or amorphous phase after mechanical stress, with an accompanying change in the emission spectrum, this phenomenon being allowed by the polymorphism of the material. The search for polymorphic crystal forms of such emitters is widely investigated as it enables interesting properties and applications, especially in optoelectronics.24–27 The high sensitivity of some D–A materials to external stimuli, together with their ability to resist aggregation caused quenching (ACQ) phenomena, is driving research to develop new D–A materials with MCL properties,28–30 particularly for sensor development.
In the present work, we investigate the solid-state photophysical properties of a series of four donor–acceptor–donor′ (D–A–D′) fluorinated benzonitrile (BzN, A) compounds (Fig. 1a) substituted with mixed phenoxazine (PhOx) and carbazole (Cz) donor units (D and D′). We have previously shown that these compounds exhibit dual CT-TADF emission in solution, from excited states involving Cz and PhOx.31 In this study, we have investigated in detail the steady-state and time-resolved photophysical properties in polymethylmethacrylate (PMMA) rigid films, as well as in the non-doped condensed phase. We have specifically studied the supramolecular packing obtained in neat films by varying the sample preparation conditions (e.g. type of solvent, solvent saturation conditions and temperature; see the ESI† for more details). This allows us to obtain different polymorphs or amorphous phases with various MCL properties that also translate to OLED emission in non-doped emissive layers.
Molecule | Sample name | Preparation technique |
---|---|---|
4 | 4fDCM | Drop casting from DCM |
4fSVA | Solvent vapour annealing | |
4fDCM150 | Drop casting from DCM + 150 °C | |
4fTOL | Drop casting from toluene | |
5 | 5fDCM | Drop casting from DCM |
5fSVA | Solvent vapour annealing | |
5fDCM150 | Drop casting from DCM + 150 °C | |
5fTOL | Drop casting from toluene | |
6 | 6fDCM | Drop casting from DCM |
6fSVA | Solvent vapour annealing | |
6fDCM150 | Drop casting from DCM + 150 °C | |
6FDCM200 | Drop casting from DCM + 200 °C | |
6fTOL | Drop casting from toluene | |
7 | 7fDCM | Drop casting from DCM |
7fSVA | Solvent vapour annealing | |
7fDCM150 | Drop casting from DCM + 150 °C | |
7fTOL | Drop casting from Toluene |
To prepare the different neat (non-doped) films, we varied the chemical and physical processing conditions such as solvent polarity, temperature and evaporation rate (see the ESI† for more details). Fast solvent evaporation (conducted under a hood) resulted in amorphous films, while slower, controlled evaporation (achieved by saturating the evaporation chamber with the solvent) generally resulted in more ordered or crystalline phases. Specifically:
– Solvent vapour annealing (SVA) is a slow evaporation process where the solvent vapor environment promotes molecular mobility, favouring self-assembly and crystallization.33 The samples obtained by solvent vapour annealing (using as a solvent toluene into a hexane saturated chamber, see the ESI† for details) of particular emitter “X” are labelled as “XfSVA”, (f = from).
– Dropping the solution directly onto the substrate enables us to control the assembly by selecting appropriate solvents. We used two different solvents: DCM (dichloromethane), which has a low boiling point and fast evaporation properties, favouring an amorphous phase, and toluene, which has a high boiling point and high viscosity, favouring high-order molecular self-assembly. The samples obtained by drop casting with DCM are labelled with postscript fDCM, and those from toluene with fTOL.
– In addition, we also used thermal annealing, which facilitates molecular diffusion and nucleation, leading to crystalline phases following initial deposition. This was achieved by heating the drop cast samples from the DCM solution at 150 °C or 200 °C for 15 minutes. The samples obtained by thermally annealing are labelled as fDCM150 (T = 150 °C) and fDCM200 (T = 200 °C).
Images of all prepared samples are shown in the ESI† (Fig. S1–S4).
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Fig. 2 Powder XRD patterns of different forms of the reported molecules: (a) molecule 4, (b) molecule 5, (c) molecule 6, and (d) molecule 7. |
All films prepared via solvent vapor annealing (fSVA) and thermal annealing at 150–200 °C display distinct diffraction peaks, confirming the presence of crystalline phases. Similarly, all fTOL samples exhibit crystalline features, with the exception of 5fTOL. In contrast, fDCM films are X-ray amorphous, consistent with the rapid solvent evaporation hindering molecular organization. Notably, variations in XRD patterns across different processing methods for the same molecule indicate polymorphism or variations in molecular packing driven by distinct crystallization dynamics. Single-crystal X-ray diffraction was also performed on crystals obtained from the initial powders (used for the MCL study in Section 5). For molecule 5, the unit cell contains two distinct molecules and one solvent molecule with different donor–acceptor (D–A) angles, consistent with the presence of multiple charge-transfer (CT) states (Table S1, ESI†). Two polymorphic forms were identified for molecule 6, one solvent-free and one solvated (6 and 6+solv), supporting the hypothesis that polymorphism plays a key role in modulating solid-state emission properties (Table S1, ESI†). Molecular packing in the single crystals of compounds 4–7 is illustrated in Fig. S5–S10 (ESI†). Key intermolecular interactions include C–H⋯π, C–H⋯N, C–F⋯H, C–H⋯O, and C–F⋯C. Notably, π–π stacking is observed only for compound 5 (offset stacking between two carbazole units with a π–π distance of 3.702 Å; Fig. S5b and S7, ESI†) and for 6+solv (offset, head-to-tail stacking between carbazoles with a distance of 3.867 Å; Fig. S5d and S9, ESI†). The presence of multiple donor groups and relatively weak directional interactions appears to facilitate conformational adaptability and structural rearrangement, consistent with the MCL behaviour observed in these systems.34–36
Optical and SEM images (Fig. 3 and Fig. S11, ESI†) corroborate the structural data obtained from XRD. Films obtained via fSVA show well-defined surface textures, with crystalline domains clearly observable under both optical and electron microscopy. The fTOL samples, excluding 5fTOL, also display heterogeneous morphologies, characterized by visible crystalline regions. For instance, 4fTOL features faceted multidomain sheets, while 6fTOL exhibits rectangular, flat crystalline domains. In 7fTOL, SEM reveals micrometric, filament-like aggregates not easily detected in optical images. In contrast, fDCM samples appear smooth and homogeneous in both optical and SEM analysis, reflecting their amorphous nature as confirmed by XRD. Similarly, 5fTOL presents a featureless morphology, consistent with its amorphous character. Upon thermal annealing of fDCM films, the formation of crystalline domains becomes evident. Optical and SEM images reveal the emergence of textured crystalline regions in 4fDCM150, 6fDCM150, 6fDCM200, and 7fDCM150. Interestingly, while 5fDCM150 appears featureless in optical microscopy, SEM at 20 μm resolution reveals branched crystalline domains. At higher magnification (4 μm), these appear as elongated, bar-like or rod-like structures (Fig. S11, ESI†).
Comparative analysis of the crystalline morphologies across different preparation methods provides further insight highlighting the sensitivity of molecular organization to processing conditions. Molecule 4 forms elongated, interconnected needle-like crystalline domains in fSVA. In 4fTOL, the crystals appear as large, flat, multifaceted domains with irregular polygonal boundaries. 4fDCM150 exhibits smaller, elongated rectangular flat domains. Molecule 5 forms elongated hexagonal platelets domains in fSVA, characterized by sharp edges and well-defined facets. 5fDCM150 displays a heterogeneous morphology; high-magnification SEM reveals small crystallites with bar-like or branched structures radiating from localized nucleation points (4 μm scale, Fig. S11, ESI†). For molecule 6, fSVA induces elongated spear-like crystallites. 6fTOL shows smaller rectangular faceted crystalline domains, while 6fDCM150 presents a polycrystalline mosaic texture. Upon annealing at 200 °C (6fDCM200), a clear morphological transition occurs, showing interconnected crystalline areas with petal-like features. Molecule 7 displays elongated hexagonal crystalline facets in fSVA. SEM images of 7fTOL reveal fibrous or needle-like crystalline structures. In 7fDCM150, SEM analysis shows parallelogram-shaped crystalline domains.
The combined XRD and microscopy analyses clearly demonstrate that the processing method strongly influences the crystallinity, molecular packing, and surface morphology of 4–7 thin films. The ability of each molecule to adopt multiple packing arrangements under different conditions is closely related to its polymorphic behaviour and is directly linked to the solid-state photophysical properties explored in subsequent sections.
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Fig. 4 Normalized fluorescence spectra (λexc = 400 nm) of the different solid forms obtained for (a) molecule 4, (b) molecule 5, (c) molecule 6, and (d) molecule 7. |
Comp. | QY (%) air | λPL (nm) Std. state | FWHM (nm) | Emission onset (eV) | Excitation onset (eV) |
---|---|---|---|---|---|
4SVA | 9.0 | 561 | 112 | 2.51 | 2.36 |
4fDCM150 | 3.3 | 573 | 124 | 2.43 | 2.32 |
4fTOL | 2.7 | 573 | 128 | 2.47 | 2.38 |
4fDCM | 1.0 | 594 | 141 | 2.46 | 2.31 |
5SVA | 7.8 | 567 | 112 | 2.47 | 2.36 |
5fDCM150 | 4.1 | 574 | 121 | 2.47 | 2.37 |
5fTOL | 1.1 | 610 | 140 | 2.44 | 2.35 |
5fDCM | 1.0 | 612 | 149 | 2.44 | 2.34 |
6SVA | 2.4 | 626 | 113 | 2.19 | 2.07 |
6fDCM150 | 7.3 | 598 | 123 | 2.35 | 2.31 |
6fDCM200 | 2.9 | 621 | 125 | 2.25 | 2.16 |
6fTOL | 2.6 | 598 | 116 | 2.33 | 2.15 |
6fDCM | 2.3 | 612 | 132 | 2.36 | 2.35 |
7SVA | 8.5 | 600 | 104 | 2.29 | 2.16 |
7fDCM150 | 2.8 | 613 | 124 | 2.29 | 2.13 |
7fTOL | 9.1 | 585 | 115 | 2.38 | 2.25 |
7fDCM | 1.8 | 620 | 131 | 2.31 | 2.21 |
Notably, each polymorph of compounds 6 and 7 (isolated in the XRD analysis) shows different photophysical behaviour. The emission at the highest energy is observed for 6fDCM150, which also exhibits CIEE properties (Table 2 and Fig. S13a, ESI†). In contrast the emission at lowest energy is detected for 6fSVA, which has the lowest QY in agreement with the energy gap law (Table 2 and Fig. S13a, ESI†). Interestingly, the crystalline 6fSVA film shows a bathochromic shift relative to the amorphous 6fDCM (Fig. S13a and b, ESI†), indicating that this molecular packing strongly stabilizes the CT state, leading to increased non-radiative deactivation paths. Conversely, 7fTOL shows a hypsochromic shift with higher PLQY relative to 7fSVA (Table 2 and Fig. S13a, ESI†). Looking at the different solid samples obtained from the same molecule, it can be seen that the bluer shifted the emission, the higher the QY (Table 2 and Fig. S13, ESI†). This behaviour is also valid for the polymorphs in 7fTOL/7fSVA and 6fDCM150/6fSVA.
Fig. S14 (ESI†) shows the absorption and excitation (PE) spectra of 4–7 in PMMA, and their good match indicates that all transitions involved in the absorption process contribute to the emission output. As previously discussed for the absorption in PMMA, the bands at lower energy are assigned to the CT transitions of PhOx–BzN (CT1) and Cz–BzN (CT2) moieties. Fig. S15 (ESI†) instead shows the PE spectra of all neat films, compared as well to PMMA, while those in Table S3 (ESI†) are reported for their peak maxima (λmax) and the onset for the lower energy bands. We can observe that for all molecules the higher energy peaks (between 363 and 367 nm), are intense and match with both the absorption and PE spectra of 4–7 in PMMA (Fig. S14, ESI†), and thus are assigned to the locally excited state of the donors. On the other hand, in neat films the CT1 and CT2 bands are strongly red-shifted and in most cases are also more intense compared to the isolated molecules in PMMA (Fig. S15 and Table S3, ESI†). Since no short distance π–π interactions were found (only long-distance π–π interactions between two carbazoles were found in 5 and 6+solv crystals Fig. S7 and S9, ESI†), the formation of aggregate emissive species is not orderly in these films. Therefore, the strong red-shift in the CT transitions observed in PE spectra can be attributed to either the presence of different molecular conformations in the solid samples and/or to different polarity and dielectric constants in the solid phases. These factors can produce a redistribution of the electronic density of the molecules that stabilizes the CT states.
The decay profiles of all compounds at RT in PMMA are shown in Fig. S20 (ESI†), and the decay times of other films (decays in Fig. S22–S36, ESI†) are summarized in Table 3. The prompt fluorescence (PF) part of the decay (nanosecond domain) is fitted with a biexponential function, with the two components associated with PF emission from the 1LE of PhOx and/or the 1CT of BzN–Cz. Similarly, the long-lived component (DF domain) also shows biexponential behavior, but in this case, the two decay times can be attributed to DF arising from the two CT states associated with the PhOx–BzN and Cz–BzN moieties. The power law relationship between DF emission intensity and excitation dose with gradient close to 1 confirms a monomolecular process for all compounds, excluding TTA as the DF mechanism (Fig. S17, S37–S40, ESI†). Both the time-resolved PL emission and decay profiles at RT show that, similar to the solution phase, dual TADF emission channels are present in the solid-state environment. However, the higher energy DF (originating from the Cz–BzN branch) contributes negligibly to the total emission output.
λstd.st. (nm) | λPF (nm) | S1onset (eV) | τ1 (ns) | τ2 (μs) | τ3 (μs) | τ4 (μs) | τDF,av | QY (%) | DF/PF | |
---|---|---|---|---|---|---|---|---|---|---|
λstd.st. = emission maximum of steady-state fluorescence, λPF = emission maximum of prompt fluorescence, S1onset = onset energy of singlet, τPF,av = average decay time of PF, τDF,av = average decay time of DF, *: monoexponential, DF/PF= ratio between DF to PF emission. | ||||||||||
4fDCM | 594 | 605 | 2.41 | 2.2 | 0.3 | 0.9 | 4.0 | 1.3 | 1.0 | 0.03 |
4fTOL | 573 | 602 | 2.43 | 2.6 | 0.1 | 1.2 | — | 0.9 | 2.7 | 0.05 |
4fDCM150 | 573 | 560 | 2.43 | 4.0 | 1.4 | — | — | 1.4* | 3.3 | 0.09 |
4fSVA | 561 | 573 | 2.38 | 4.0 | 1.2 | — | — | 1.2* | 9.0 | 0.08 |
5fDCM | 612 | 600 | 2.36 | 2.2 | 0.2 | 0.8 | 3.7 | 1.4 | 1.0 | 0.17 |
5fTOL | 610 | 585 | 2.40 | 2.0 | 0.1 | 0.6 | 2.6 | 1.2 | 1.1 | 0.07 |
5fDCM150 | 574 | 562 | 2.46 | 4.6 | 0.4 | 2.8 | — | 2.2 | 4.1 | 0.15 |
5fSVA | 567 | 562 | 2.44 | 8.7 | 0.7 | — | — | 0.7* | 7.8 | 0.22 |
6fDCM | 612 | 609 | 2.28 | 3.1 | 0.2 | 1.3 | 4.4 | 1.3 | 2.3 | 0.05 |
6fDCM150 | 598 | 579 | 2.37 | 4.3 | 2.5 | — | — | 2.5* | 7.3 | 0.06 |
6fDCM200 | 621 | 612 | 2.21 | 3.3 | 1.0 | — | — | 1.0* | 2.9 | 0.04 |
6fSVA | 626 | 614 | 2.19 | 3.1 | 1.1 | — | — | 1.1* | 2.4 | 0.04 |
7fDCM | 620 | 611 | 2.30 | 2.2 | 0.03 | 0.3 | 2.0 | 1.6 | 1.8 | 0.05 |
7fSVA | 600 | 577 | 2.35 | 4.6 | 0.3 | 4.9 | — | 4.2 | 8.5 | 0.16 |
7fTOL | 585 | 574 | 2.39 | 5.2 | 0.3 | 5.6 | — | 4.7 | 9.1 | 0.26 |
Time-resolved emission was also collected at 77 K (Fig. S18, ESI†). Similar to RT, at initial times (nanosecond range), the time-resolved PL emission shows a shoulder/band at higher energy (450 nm) attributed to the PF of the 1LE of PhOx and/or the 1CT of BzN–Cz, and a main band at lower energy attributed to the 1CT PF of the PhOx–BzN moieties. With increasing delay time, the emission progressively redshifts and after 100 ns the shoulder at 450 nm disappears, leaving only a DF emission from the 1CT of the PhOx–BzN moiety. With further increasing the delay time, the emission progressively blue shifts and in some cases gains structural features, and since the late emission (TD = 70 ms) has an onset close to the phosphorescence of phenoxazine (Fig. S19, ESI†), it can be attributed to the emission from the phenoxazine fragment 3LE.
In conclusion, the time-resolved study in polymer shows that the dual TADF emission previously shown in solution remains present in the solid state. However, the higher energy DF emission (originating from the Cz–BzN branch) contributes negligibly to the total emission.
From the time-resolved spectra, it is evident that all samples exhibit typical TADF behaviour. However, while the emission in crystalline samples remains unchanged at different delay times (4fDCM150, 4fSVA, 5fDCM150, 5fSVA, 6fDCM150, 6fDCM200, 6fSVA, 7fTOL, 7fSVA), the amorphous samples show slight spectral evolution over time (4fDCM, 4fTOL, 5fDCM, 5fTOL, 6fDCM, and 7fDCM). Consequently, the amorphous films are fitted with bi- or multi-exponential curves, while the crystalline ones, with few exceptions, are fitted with mono-exponential curves. The laser fluency experiments reported in Fig. S37–S40 (ESI†) again show the expected linear dependence of the DF intensity on the laser power. Based on these observations, we can conclude that the dual emission observed in solution and in the polymer matrix is not as strongly maintained in the non-doped condensed phase. The double exponential decay observed in the amorphous samples is probably caused by the poor homogeneity of the films, which contain molecules with slightly different configurations. In contrast, the ordered packing of crystalline films allows only one molecular conformation to dominate.
The results of electro-optical characterization of the OLEDs are shown in Fig. 6. As expected, devices using a host:guest system as EML perform better than the neat film case. In particular, devices with molecules 4, 5, 6 and 7 dispersed in mCP show a maximum current efficiency of 2.8, 3.4, 6.4 and 7.6 cd A−1, respectively, with luminance that in any case exceeds 1000 cd m−2. Conversely, self-quenching induces detrimental effects on neat film-based devices, for which the performance drops considerably, both in terms of efficiency and luminance. This behaviour is even more evident if the neat film EMLs are annealed after deposition. In that case, although the quantum yield is higher in modified films than in amorphous films (Table 2), we found that OLEDs exposed to the annealing step (and thus also exhibiting aggregation/crystallisation in the EML) generally have much lower efficiencies (see Fig. 6). This may be due to alterations in the morphology of the active layer as a result of heat exposure and the induced supramolecular organisation, which negatively impacts the electrical characteristics of the devices to a greater extent than any increase in PLQY. This also emerges from the current density curves as a function of the applied voltage, where it is clear that in the case of devices with an annealed EML there is a significant increase in passive currents even at low voltage. The external quantum efficiency (EQE) has been estimated for all OLEDs and is shown in Fig. S41 (ESI†).
We then compared the electroluminescence spectra of the devices with an EML in neat film (Fig. 7), with and without thermal annealing. The latter was used to induce/accelerate an eventual transition from the amorphous phase of the film to an aggregate/crystalline phase, as was observed in optical studies of the corresponding films. In all devices exposed to annealing at 150 °C, a shift of the emission towards higher energies can be observed, compared to the spectra of the non-annealed devices. The shift is in line with that observed in the steady-state photoluminescence spectra (Table 2) and suggests the formation of aggregates or crystalline domains whose optical properties are also reflected in electroluminescence.
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Fig. 7 Comparison between normalized electroluminescence and photoluminescence spectra using: (a) compound 4; (b) compound 5; (c) compound 6; (d) compound 7, before and after thermal annealing. |
(Initial)a λPL, nm | +Grindingb λPL, nm | +DCM vapourc λPL, nm | +Grinding λPL, nm | +TOL vapourd λPL, nm | +Grinding λPL, nm | |
---|---|---|---|---|---|---|
a Initial powder.b Grinding in the mortar.c Exposure to DCM vapour.d Exposure to Toluene vapour. | ||||||
4 cryst. powd. | 577 | #1, 586 | 608 | — | 582 | 601 |
#2, 599 | ||||||
5 cryst. powd. | 578 | 603 | 608 | — | 608 | — |
6 cryst-red powd. | 629 | 611 | 610 | — | 609 | — |
6 orange cryst | 610 | 601 | — | — | 578 | 610 |
7 crystal | 602 | 624 | — | — | — | — |
7 red powder | 621 | 621 | 621 | 621 | 592 | 621 |
The initial crystalline powders of compounds 4 (Fig. 8) and 5§ (Fig. S47, ESI†) show a bright yellow emission similar to that of 4fSVA and 5fSVA. Upon grinding, the emission shifts to red and its brightness decreases, resulting in spectra similar to those of 4fDCM and 5fDCM, indicating the formation of amorphous phases also in the powders. The ground powders of 4 and 5 were then exposed to DCM vapours (+DCM), resulting in a slight red shift of the emission. Subsequent exposure to toluene vapours (+TOL) caused the emission to return to the initial position, similar to that of 4fTOL, indicating the restoration of the crystalline form. However, no spectral shift was observed when 5 (+DCM) was exposed to toluene (+TOL) (Fig. S47, ESI†), consistent with the behaviour of 5fTOL.
Two different forms of compound 6 were observed in the initial powder precipitated from the same batch (Fig. S45, ESI†). One form appeared as a polycrystalline red powder (Fig. S45 and S47, ESI†) with a narrow emission (λmax = 629 nm) similar to both 6fSVA and 6fDCM200 (steady state). The other form appeared as an orange crystal (λmax = 610 nm) (Fig. 8 and Fig. S45, circled, ESI†). These two forms were manually separated with a pair of tweezers. Grinding of the polycrystalline red powder resulted in a broadened and blue-shifted emission (18 nm), indicating the formation of an amorphous phase with emission similar to that of 6fDCM. Grinding of the orange crystals (Fig. 8) resulted in a blue shift (9 nm) together with broadening of the emission. The compound remained crystalline as indicated by the emission matching with 6fDCM150 (Fig. 8c). Finally, exposure of the ground orange crystal to toluene resulted in a hypsochromic shift (23 nm) with a narrowed yellow emission, different from that observed for the drop-cast thin films, indicating the formation of a possible additional polymorph that red-shifts upon grinding.
The initial powder of compound 7 also presented two different forms (Fig. S46, ESI†): one as a multicrystalline orange powder (λmax = 601 nm, Fig. S47, ESI†) and the other as a red powder (λmax = 621 nm, Fig. 7). Grinding the multicrystalline orange powder (similar emission to 7fSVA, Fig. S47, ESI†) resulted in a red-shifted emission similar to 7fDCM, indicating the formation of the amorphous phase. Grinding the red powder (Fig. 8) resulted in a blue-shift of the emission onset, which becomes broader and similar to that of 7fDCM, indicating that the initial powder contained a more ordered phase. Exposure of the ground powder to DCM did not change the emission, consistent with an amorphous material. Finally, exposure to toluene favoured crystallisation, as indicated by the strong blue shift (λmax = 592 nm), matching that of 7fTOL. A second grinding redshifted the emission back to the amorphous phase.
Overall, the MCL behaviour observed for compounds 4–7 can be rationalized by considering both intramolecular conformational flexibility and supramolecular organization, as revealed by single-crystal XRD, thin-film XRD, and SEM analyses. The D–A torsion angles extracted from the single-crystal data (Table S1, ESI†) vary between polymorphs and influence the electronic coupling between donor and acceptor moieties, thus modulating the energy and efficiency of the CT excited states. Crystalline samples such as fSVA derivatives (excluding 6fSVA), 6fDCM150, and 7fTOL exhibit blue-shifted, narrower, and more intense emission consistent with more twisted D–A conformations and reduced non-radiative decay. This behaviour is likely due to conformational locking and restricted relaxation pathways in ordered molecular environments. In contrast, red-shifted and broader emission with lower PLQYs, as observed for 6fSVA, 6fDCM200, 5fTOL, and the amorphous fDCM samples, is indicative of more planar D–A conformations and enhanced CT stabilization. While in amorphous films this behaviour is likely due to increased conformational freedom, in crystalline samples such as 6fSVA and 6fDCM200, it likely arises from polymorph-specific packing that favours less twisted conformations and promotes non-radiative decay, in line with the energy gap law. SEM analysis of both 6fSVA and 6fDCM200 confirms the presence of well-defined crystalline domains, excluding morphological disorder as the origin of their red-shifted and low-efficiency emission. This further supports the hypothesis that specific polymorphs with more planarized D–A geometries are responsible for enhanced CT stabilization and reduced radiative efficiency, highlighting the complex role of molecular packing in governing excited-state behaviour. SEM analysis of the thin films (Fig. 3 and Fig. S11, ESI†) provides direct insight into their surface morphology. Crystalline samples (fSVA, fTOL, fDCM150) with their long-range molecular order correlate with higher PLQYs and more structured emission. Conversely, amorphous films (fDCM, 5fTOL) appear smooth and featureless, reflecting disordered packing and reduced emission efficiency. Although SEM was not performed on ground powders, the correlation observed in thin films provides a structural framework to interpret the emission shifts seen in MCL experiments (Fig. 8 and S47, ESI†). Mechanical grinding induces amorphization, allowing conformational relaxation and red-shifted emission. Recrystallization via solvent vapor exposure restores surface order and blue-shifted emission, as seen in fSVA, fTOL, and fDCM150 samples.
Together, these results demonstrate a strong correlation between surface morphology, molecular packing, and emission properties, confirming that both intramolecular torsion and supramolecular organization govern the reversible MCL behaviour of these materials.
(b) Neat films were obtained by:
– Solvent vapour annealing (SVA): a few drops of a toluene solution containing the material are deposited onto a quartz substrate, which is placed on a support inside a glass container. This container has a small amount of hexane (approximately one finger's depth) at the bottom, below the support. The film is exposed to the vapours of the organic solvents, causing it to swell as the vapours penetrate it. This swelling allows the molecules within the film to become more mobile and reorganize. As the solvent evaporates, the film solidifies into a crystalline morphology. The degree of reorganization depends on factors such as the type of solvent, the duration of exposure, and the temperature.
– Drop-casting from solutions: in this process, a small amount of a solution containing the desired material is deposited onto the quartz substrate. The solvent is then allowed to evaporate, leaving behind a uniform thin film of the material. Key factors that influence the quality of the film include the concentration of the solution, the nature of the solvent, the substrate surface properties, and the evaporation conditions such as temperature and humidity.
– Thermal annealing: the material, in the form of a thin film is heated to a specific temperature below its melting point and maintained at that temperature for a set period. This heat treatment allows the material's atoms or molecules to move and rearrange, reducing defects and promoting crystallinity. Key parameters that affect the annealing outcome include the annealing temperature, duration, and cooling rate.
(c) Polycrystalline powders: as synthesized powders were collected from the PE/DCM solution.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc04347b |
‡ To allow these smaller samples to fit into the cryostat. |
§ Since 5 has similar emission properties to 4, MCL experiments are described in the ESI.† |
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