R.
Martín‡
ab,
A.
Sánchez-Oliva‡
b,
A.
Benito
a,
I.
Torres-Moya
bc,
A. M.
Garcia
b,
J.
Álvarez-Conde
d,
J.
Cabanillas-González
*d,
P.
Prieto
*b and
B.
Gómez-Lor
*a
aInstitute of Materials Science of Madrid (ICMM-CSIC), Sor Juana Inés de la Cruz 3, Cantoblanco 28049, Madrid, Spain. E-mail: bgl@icmm.csic.es
bFaculty of Chemical Science and Technology, University of Castilla-La Mancha, Instituto Regional de Investigación Científica Aplicada (IRICA), University of Castilla-La Mancha, 13071 Ciudad Real, Spain. E-mail: Mariapilar.Prieto@uclm.es
cDepartment of Organic Chemistry, Faculty of Chemical Sciences, Campus of Espinardo, University of Murcia, 30100 Murcia, Spain
dMadrid Institute for Advanced Studies, IMDEA Nanociencia, Calle Faraday 9, Cantoblanco, 28049, Madrid, Spain. E-mail: juan.cabanillas@imdea.org
First published on 16th January 2024
Smart materials with stimuli-responsiveness are a subject of great attention nowadays. In this work, we describe two D–π–A naphthalenimide (NI) derivatives, with different N-functionalization prepared by an environmentally benign process. They self-assemble into fluorescent crystals that display optical waveguiding behaviour with low optical loss coefficients. In addition, they present thermal/mechanical stimuli-responsiveness, which is tuned upon substitution at the molecular main core as a result of changes in crystal packing. A cold-crystallization process, as confirmed by DSC and power X-ray analysis, was identified as the underlying cause of the colour change upon heating, which can be reverted due to an amorphization process upon shearing. The combination of the reversible coloured amorphous-crystalline phase transition, together with the light guiding, holds significant promise for practical applications including use in security inks, rewritable materials or modern optics.
Organic crystals possess interesting features for optoelectronic applications in comparison with their inorganic counterparts.3 They are lighter, present different morphologies and very importantly, their properties can be tuned at the molecular level thanks to the opportunities offered by organic synthesis.4 Focusing on their emissive properties, some organic crystals combine high fluorescence quantum yields with notable light waveguiding properties, arising from low optical losses and refractive indexes in the 1.8–2.2 range, and a good photoluminescence efficiency. Up to now, a great number of organic crystals with excellent performance as optical waveguiding materials have been reported,5–7 although recently, additional features have been put into play. This implies the development of more complex materials as doped structures,8 core/shell structures9 or heterostructures,10 and the incorporation of features such as flexibility,11–13 anisotropy,14 chiral elements15 or stimuli-responsiveness.16,17
The combination in a single material of optical waveguiding properties and stimuli-responsiveness arouses much interest due to the potential applications in the optoelectronics and sensors fields. In this sense, Yawen et al. described five cyano-functionalized 1,4-bis((E)-2-(1H-indol-3-yl)vinyl)benzene derivatives, substituted with alkyl chains of variable length. Interestingly, while compounds with short side chains displayed optical waveguiding properties, those with long ones showed thermochromism.18 In another example, Jia and coworkers reported a single organic crystal of a derivative that connects a pyrene unit and a rhodamine B moiety through a CN group. This crystal possessed multiple properties including optical waveguiding and mechanochromism.19
The use of physical stimulation, such as pressure or temperature, to induce changes in the absorption or emission spectra20,21 arouses much interest to develop tuneable optoelectronic devices. These types of switchable material are attractive candidates in fields as varied as sensors,22 recording technologies,23 security inks24 or rewritable paper.25 The extent of the colour changes can vary, ranging from subtle changes to complete colour loss/gain in response to stimuli-induced modifications on molecular conformation, intermolecular interactions, and crystal packing.26–30 A combination of both types of stimulation, i.e., preparation of organic crystals able to offer a response to both mechanical and thermal stimuli in different directions, is even more interesting in the quest for multi-responsive, smart materials.31,32
Although in the last few years impressive advances have been achieved in the development of crystalline organic waveguides and stimuli responsive materials, more detailed studies are required to advance the ad hoc design of smart materials with desired functions, whereby optical properties and response to stimuli are rationalized from the chemical structure of the molecules. Bearing this in mind, we here report a study of two D–π–A systems featuring a triphenylamine derivative joined to two differently substituted 1,8-naphthalimide (NI) acceptors via an ethynyl bridge. Both derivatives lead to the formation of multifunctional structures that exhibit thermochromism, mechanochromism and optical waveguide behaviour. The response to various stimuli depends on the specific substitution of the NI core present in the molecules. Additionally, XRD analysis aids to establish a correlation between the observed structural features and the mechanochromic and waveguiding behaviour.
1H-NMR (CDCl3, 500 MHz) (δ, ppm): 8.73 (d, J = 7.9 Hz, 1H, HNI), 8.71 (d, 1H, J = 7.8 Hz, HNI), 8.49 (d, 1H, J = 7.9 Hz, HNI), 8.14 (d, 1H, J = 7.9 Hz, HNI), 8.00 (s, 1H, Hp-phenyl), 7.93 (t, 1H, J = 7.9 Hz, HNI), 7.83 (s, 2H, Ho-phenyl).
1H-NMR (CDCl3, 500 MHz) (δ, ppm): 8.67 (d, J = 7.8 Hz, 1H, HNI), 8.58 (d, 1H, J = 7.8 Hz, HNI), 8.42 (d, 1H, J = 7.8 Hz, HNI), 8.05 (d, 1H, J = 7.8 Hz, HNI), 7.85 (t, 1H, J = 7.8 Hz, HNI), 4.16 (t, 2H, J = 6.6 Hz, N–CH2–), 1.73 (m, 2H, –CH2–), 1.40 (m, 2H, –CH2–), 1.35 (m, 2H, –CH2–), 1.29 (m, 6H, –CH2–), 0.87 (t, 3H, –CH3).
M.p.: 239–240 °C. 1H-NMR (CDCl3, 500 MHz) (δ, ppm): 8.88 (d, J = 8.4 Hz, 1H, HNI), 8.73 (d, 1H, J = 8.4 Hz, HNI), 8.64 (d, 1H, J = 8.4 Hz, HNI), 8.02 (s, 1H, p-Ph-CF3), 7.99 (d, 1H, J = 8.4 Hz, HNI), 7.92 (t, 1H, J = 8.4 Hz, HNI), 7.86 (s, 2H, o-Ph-CF3), 7.54 (d, 2H, J = 8.8 Hz, m-N-PhC-≡), 7.35 (t, 4H, m-N-Ph), 7.19 (d, 2H, J = 8.8 Hz o-N-PhC-≡), 7.18 (d, 4H, J = 8.8 Hz, o-N-Ph), 7.13 (t, 2H, J = 8.8 Hz, p-N-Ph). 13C-NMR (δ, ppm): 164.1, 163.8, 148.9, 147.1, 143.0, 136.9, 135.8, 132.8, 132.6, 132.0, 131.1, 129.9, 129.5, 128.2, 126.8, 125.1, 124.0, 123.5, 120.8, 120.2, 100.0. MS (MALDI TOF/TOF, m/z) 676.463, required for C40H22F6N2O2, 676.159.
The synthesis of compound NI2 has been previously reported by some of the authors,34 while compound NI1 was synthesized for the first time in this work, following a similar procedure that involves a two-step process according to the principles of sustainable chemistry.40,41 First, the imide was formed using 4-bromo-1,8-naphthalic anhydride and the corresponding amine followed by a Sonogashira coupling reaction with the alkynyl donor fragment.34 Microwave radiation was used as the energy source, which results in a significant decrease in reaction time and a large increase in yield, and it is carried out with a minimum amount of solvent and using a reusable catalyst (Pd-Encat TPP30) (Scheme 1, see ESI,† Section S2 for NMR and MS details).
Fig. 1 Absorption and emission spectra of the NI compound solution (CHCl3, 1 × 10−5 M) (a) and (b) and in thin films (c) and (d). |
Compound | Solution | Films | ||||
---|---|---|---|---|---|---|
λ abs (nm) | ε (M−1 cm−1) × 104b | λ em (nm) | ϕ | λ abs (nm) | λ em (nm) | |
a Maximum absorption wavelengths measured in 10−5 M CHCl3 solution. b ε stands for molar extinction coefficient at the first and second λabs values, respectively. c Maximum emission wavelength measured in 10−5 M CHCl3 solution (λexc (NI1) = 472 nm and λexc (NI2) = 452 nm). d PL quantum yield measured in CHCl3 using cresyl violet in ethanol (Φ = 0.56) for NI1 and quinine sulphate in 0.1 M H2SO4 (Φ = 0.54) and fluorescein in 0.1 M NaOH (Φ = 0.79) for NI2. e Maximum absorption wavelengths measured in the film. f Maximum emission wavelengths measured in the film (λexc (NI1) = 485 nm and λexc (NI2) = 457 nm). | ||||||
NI1 | 345/472 | 3.3/2.7 | 631 | 0.95 | 350/485 | 649 |
NI2 | 336/452 | 3.1/2.4 | 606 | 0.94 | 340/457 | 585 |
The absorption spectra of both NI derivatives in CHCl3 are similar except for a slight redshift of NI1 with respect to NI2. They are characterized by a band centred at 330–350 nm, which corresponds to a π–π* transition, and another band at longer wavelength (lower energy) that it is indicative of an ICT process from the triphenylamine donor to the NI acceptor core. The presence of two –CF3 groups in the NI1 groups enhances the electron scavenging nature of the NI core, increases the ICT character of this derivative and gives rise to a bathochromic shift (Fig. 1a). A similar trend can be observed in the fluorescence spectra in solution, with the maximum emission wavelength of NI1 red-shifted compared to NI2 (631 vs. 606 nm) (Fig. 1b), when excited at the ICT band (note that excitation at the π–π* energy band does not produce changes in the emission wavelengths, see Fig. S9, ESI†). Remarkably, both compounds exhibited highly emissive properties in solution, with fluorescence quantum yields of 0.94 and 0.95 for NI1 and NI2, respectively (Table 1).
The absorption spectra of the two compounds in the film resemble those in solution (Fig. 1c), but important differences emerge from the fluorescence spectra (Fig. 1d). Whereas the introduction of –CF3 groups at the NI core (NI1) leads to a slight bathochromic shift in the solid state with respect to solution (649 vs. 631 nm) ascribed to a change in the dielectric medium, the emission spectra of NI2 films are significantly blue shifted compared to solution (585 vs. 606 nm). These differences in solid state fluorescence denote a likely change in crystal packing modulated by the side substituents, as will be demonstrated (vide infra). Curiously, when comparing the fluorescence spectra of solutions and films, NI1 shows a notable redshift, indicating a strong electronic coupling of the constituent molecular units. Conversely, the emission spectra of NI2 films are blue shifted when compared to its solution state. Probably, in this particular case, the steric hindrance introduced by the flexible alkyl chains impedes the emergence of close π-interactions between molecules in the solid state, which would favour the electronic delocalization.
NI1 showed needle-like crystals between 0.5-1 mm length, appropriate for the study of the optical waveguiding behaviour, when THF is used as a good solvent and hexane as a poor solvent (Fig. S6, ESI†). On the other hand, NI2 formed rectangular slabs appropriate for optical waveguiding studies in THF as a good solvent and methanol as a poor solvent, although they are slightly smaller than the ones for NI1 (Fig. S7, ESI†).
PL microscopy images and spectra were recorded for crystals of NI derivatives to qualitatively evaluate their optical waveguiding behaviour (Fig. 2). Both microscopy images depict bright orange fluorescence from crystals of both NI compounds upon photoexcitation at 355 nm as well as emission from the tips, although slightly weaker than from the bulk, evidencing active optical waveguiding capability.
In addition, in order to evaluate the efficiency of the optical waveguides obtained from NI crystals, the optical loss coefficients were measured upon moving the photoexcitation (355 nm) spot along the length of the crystal while detecting the emission at one of the tips (Fig. 3a). A schematic representation of the set-up used for the measurements can be found in Fig. S8 (ESI†). The fluorescence intensity (Iout) upon moving the pump a distance x with respect to the initial position (Iin) is given by the Lambert–Beer law Iout = Iine−αx, where Iout and Iin are the PL intensities at the output and input, respectively, x is the propagation distance, and α is the absorption coefficient in μm−1, which is related to the optical loss coefficient α′ (dB μm−1) through the equation α′ (dB μm−1) ≈ 4.34α (μm−1). The α and α′ values obtained for NI1 were 2.1 × 10−3 μm−1 and 9.1 × 10−3 dB μm−1, respectively (Fig. 3b). The α′ value is lower than some recently reported in optical waveguiding organic crystals42–44 and on the order of the best derivatives reported recently by our research group45 and by others.46 It is known that the excellent crystallinity improves the exciton-photon coupling and the migration of excitons, favouring the light transmission process.47 In the case of NI2, it was not possible to determine the α′ value because the crystals obtained were not large enough to be studied with our experimental setup.
The outcomes indicate that the presence of diCF3-phenyl substituents contributes to the formation of better quality and larger crystals when compared to the alkyl chains.
Powder X-ray diffraction was conducted to understand the phase transitions leading to the colour change. For both compounds, initial amorphous thin layers were observed (Fig. 5). After heating, a crystallization phase transition is evidenced by the presence of sharp and intense reflection peaks in the X-ray diffractogram, suggesting that the colour change observed corresponds to a change between an amorphous-phase and a crystalline phase. Upon shear-induced amorphization, molecules may experience an increase in rotational freedom facilitating their rearrangement to form new intermolecular charge-transfer species or excimers.48 Note that flat NI moieties have a strong tendency to form excimers due to associative interactions between excited and ground state molecules, which could be responsible for the emission red shift in amorphous NI1 and NI2.49,50
Fig. 5 Powder X-ray diffraction pattern for (a) NI1 and (b) NI2 in pristine films and after heating. |
Differential scanning calorimetry (DSC) results further confirmed that a cold-crystallization is taking place when both compounds are heated (Fig. 6 and Fig. S13, ESI†).51,52 As illustrated in Fig. 6, in the first heating cycle, NI2 shows a glass transition at 32.5 °C, followed by a cold-crystallization transition peak at 102.4 °C (4.76 kJ mol−1). The sharp endothermic peak at 127.5 °C corresponds to the melting of the crystalline phase. In the cooling cycle, we only observed a glass transition. NI1 also shows a cold-crystallization at 137.3 °C (1.15 kJ mol−1) before melting at 174.6 °C (Fig. S13, ESI†). A similar thermal behaviour is observed in the second DSC cycle highlighting the reversibility of the process (Fig. S14, ESI†). Thermogravimetric analysis (TGA) shows good thermal stability of both compounds in the range of the phase transition (Fig. S15, ESI†).
Fig. 7 Lateral view of the dimers and columns that emerge as a result of the crystallographic packing of molecules of (a) NI1 and (b) NI2. |
On the other hand, compound NI2 crystallizes in the P21/n space group with two independent molecules in the asymmetric unit (Fig. 7b). In this case, we again observe differences in the torsion angle of the phenyl ring of the triphenylamine and the naphthalimide moiety (which exhibit values much larger than in the previous case, 58.2(3)° and 42.9(4)°), as well as in the disposition of the long alkyl chains, one of them nearly orthogonal to the NI moiety, introducing a high steric hindrance (Fig. 8b).
An analysis of the crystallographic packing shows that the two independent molecules of NI1 organize into two different columns that grow along the b axis. Such an arrangement results in extended stacks of the NI moieties, which are laterally shifted by approximately 43°. This type of aggregate is highly desirable for light transport and photonic applications.53 In contrast, the two crystallographically independent molecules of NI2 organize to form dimers with the NI moieties laterally shifted by around 38° giving rise to an arrangement in which lateral alkyl chains remain interdigitated along the whole structure. Examining the close contacts between adjacent molecules reveals that both structures are stabilized by multiple CH–π and π–π interactions involving the triphenylamine, alkyne groups and NI moiety of neighbouring molecules. Significantly shorter contact distances are observed between NI2 molecules when compared to NI1, pointing to stronger intermolecular interactions, thus explaining the higher transition phase enthalpies recorded for NI2 (4.76 kJ mol−1vs. 1.07 kJ mol−1).
Interestingly, in both cases the powder X-ray diffractograms simulated from the single-crystal data coincide with those obtained from the crystallized films (Fig. S21, ESI†). Note that meniscus-guided processing techniques, such as blade-coating, favour highly aligned films, and therefore, only the most intense peaks are observed.54 This good match allowed us to investigate the origin of the colour changes between the two interconverted phases induced by a thermal or mechanical stimulus. Presumably, during shear-induced amorphization, the distance between the highly emissive NI molecules is reduced, thus promoting intermolecular electronic delocalization. In the case of NI1, the presence of more extended stacks together with the possible planarization of the highly distorted diCF3-phenyl substituents (thus resulting also in an intramolecular contribution to the colour change) is probably responsible for the larger bathochromic shift in this particular case. In contrast, in NI2, where a dimeric packing arrangement is induced due to steric hindrance introduced by the flexible chains, the colour change is notably less pronounced.
Both compounds are strongly fluorescent and present a reversible stimuli-responsive behaviour under thermal and mechanical action. An amorphous-to-crystalline phase transition is induced by heating (cold crystallization), giving rise to a blue shift of the emission colour that can be reverted due to an amorphization process upon shearing.
X-ray analysis of the crystalline packing shows different arrangements for these compounds in which N-substitution plays a crucial role in the final structure.
The combination of the properties of these crystalline organic waveguides with the thermal/mechanical reversible stimuli-responsiveness, may lead to broad applications such as in tunable optics, security inks or rewritable materials.
We hope that these findings will provide important structural clues for the ad hoc design of smart materials with desired properties and functions.
Footnotes |
† Electronic supplementary information (ESI) available. CCDC 2306088 and 2306089. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3tc04100j |
‡ These authors contributed equally to this work. |
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