A Novel DR / NIR T-Shaped AIEgen: Synthesis and X-Ray Crystal Structure Study

: We developed a new benzodifuran derivative as the condensation product between 2,6-diamino -4-(4-nitrophenyl)benzo[1,2-b:4,5-b’]difuran-3,7-dicarboxylate and 3-hydroxy-2-naphthaldehyde. The intramolecular hydrogen-bond interactions in the terminal half-salen moieties produce a sterically encumbered highly conjugated main plane and a D-A-D (donor-acceptor-donor) T-shaped structure. The novel AIEgen (aggregation-induced enhanced emission generator) fulfils the requirement of RIR (restriction of intramolecular rotation) molecules. DR / NIR (deep red / near infrared) emission was recorded in solution and in the solid state, with a noteworthy photoluminescence quantum yield recorded on the neat crystals which undergo some mechanochromism. The crystal structure study of the probe from data collected at a synchrotron X-ray source shows a main aromatic plane π -stacked in a columnar arrangement.


Introduction
In recent years, the design and synthesis of organic fluorescent dyes highly performant in the solid state have received researcher's widespread attention [1][2][3][4]. For some applications, the emission in neat or aggregate state is a basic requirement. This is the case in most optoelectronic devices and in fluorescent bioimaging investigation techniques [5][6][7][8]. In the organic fluorophores, traditionally consisting of extended electronic π-systems, a marked fluorescence quenching in the solid state can occur due to the strong intermolecular interactions. The π−π stacking of planar polycyclic skeletons and the dipole−dipole interaction of donor−acceptor frameworks [5,9,10] can induce the aggregation caused quench (ACQ) effect with a significant fluorescent intensity decrease.
Several strategies can be adopted to suppress undesired strong intermolecular interactions. The fundamental issue is designing a conjugate system depressing the non-radiative deactivation processes through restriction of intramolecular rotation (RIR), restriction of intramolecular vibrations (RIV), and restriction of intramolecular motions (RIM) [10,11]. Therefore, the introduction of sterically encumbered substituents or bulky groups [9,12,13] and the construction of highly twisted skeleton [14][15][16][17] are the possible solutions. Unlike the common π-conjugated fluorogens, this kind of molecules undergo aggregation-induced enhanced emission (AIE) effect providing photoluminescence (PL) in the solid state.
Among the AIEgens (aggregation-induced enhanced emission generators) first described by Tang group in 2001 [18], a special class is represented by the DR/NIR (deep red/near-infrared) fluorophores, characterized by a strong emission at the 650-900 nm region [19]. According to the energy-gap Scheme 1. Synthetic route to the target compound NBDF. D-A-D T-shaped molecule schematically represented. M is the plane containing the diamino-benzodifuran rings condensed with the two 3hydroxy-2-naphthaldehyde moieties, i.e., the fragment depicted in red. Scheme 1. Synthetic route to the target compound NBDF. D-A-D T-shaped molecule schematically represented. M is the plane containing the diamino-benzodifuran rings condensed with the two 3-hydroxy-2-naphthaldehyde moieties, i.e., the fragment depicted in red.
Optical observations were performed by using a Zeiss Axioscop polarizing microscope (Carl Zeiss, Oberkochen, Germany) equipped with an FP90 Mettler microfurnace (Mettler-Toledo International INC MTD, Columbus, OH, USA). The decomposition temperatures (5 wt.% weight loss) and phase transition temperatures and enthalpies were measured under nitrogen flow by employ of a DSC/TGA Perkin Elmer TGA 4000 (PerkinElmer, Inc., Waltham, MA, USA), scanning rate 10 • C/min. Absorption and UV-Visible emission spectra were recorded by JASCO F-530 and FP-750 spectrometers (scan rate 200 nm min −1 , JASCO Inc., Easton, MD, USA) and on a spectrofluorometer Jasco FP-750 (excitation wavelengths set at the absorption maxima of the samples, scan rate 125 nm min−1, JASCO Inc., Easton, MD, USA). Thin films of the neat samples and of the polymeric blends (20% wt. in PVK, molecular weight 1100 Da) were prepared using a SCS P6700 spin coater operating at 600 RPM for 1 min.
Photoluminescence quantum efficiency values were recorded on quartz substrates by a Fluorolog 3 spectrofluorometer (Horiba Jobin Instruments SA), within an integrating sphere provided by an optical fiber connection.

Synthesis of NBDF
To 0.453 g (1.00 mmol) of BDF dissolved at 70 • C in 20 mL of glacial acetic acid 0.688 g (4.00 mmol) of 3-hydroxy-2-naphthaldehyde was added under stirring. After 1 hour at boiling temperature, the crude product precipitated. The compound was recovered from the hot solution and washed in hot ethanol twice. Tm = 330 • C; Td = 340 • C. 1

X-Ray Crystallography
Red needles of NBDF were obtained by slow evaporation (~2−3 days) from a solution of TCE and acetic acid (1.5 mM of 1% solution of acetic acid). Crystals grew with morphology of long slender needles and dimensions of 0.07 × 0.1 × 0.6 mm. Crystals required data to be collected with synchrotron radiation (wavelength, λ = 0.7000 Å) from XRD2 beamline at the Elettra Synchrotron Light Source, Trieste Italy. NBDF crystals undergoing solvent loss required quick harvesting in mother liquor. By using a small loop of fine rayon fiber, the selected crystal was dipped in the cryoprotectant Fomblin oil and flash-frozen in a stream of nitrogen at 100 K. Several crystals were scanned in order to find the most suitable for data collection. For the best diffracting crystal, a total of 400-degree crystal rotation data were collected from two hundred images using an oscillation range of 2 • . No crystal decay was detected. Data were processed using XDS (X-ray Detector Software) for processing single-crystal monochromatic diffraction data recorded) with the data collection statistics reported in Table 1 [40,49]. The crystal gave a primitive monoclinic cell of a = 16.41 Å, b = 4.78 Å, c = 25.89 Å, b=91.2 • , V = 2030 Å 3 and P 1 2 1 /m 1 symmetry was confirmed by Laue group analysis from unmerged intensity using POINTLESS 1.11.21 [50] and no data twinning was detected. Data diffraction resembles that of a tiny protein crystal as only the most intense reflections could be collected, especially at low resolution (Rmerge = 0.070). Despite data completeness being only 77% (I/σ(I) = 2.3) at resolution of 0.93 Å structure solution was quickly found by direct methods using SIR2019 [51] and revealed most of the expected NBDF atoms connectivity. The structure solution by direct methods was possible in the space groups P 1 2 1 1, P 1 2 1 /n 1 or P 1 n 1 and revealed an equivalent and unique solution. Final refinement was performed in centrosymmetric space group P 1 2 1 /n 1 in order to achieve better refinement statistics and better ratio of refinement parameters versus number of unique reflections. Structure was anisotropically refined by using full matrix least-squares methods on F 2 against all independent Crystals 2020, 10, 269 5 of 15 measured reflections using SHELXL [52] run under WinGX suite for small molecule single crystal [53]. All the hydrogen atoms were introduced in calculated positions and refined in agreement with a riding model as implemented in SHELXL. Restraints were introduced at last stage of refinement using FRAG, DFIX, and SIMU instructions as implemented in SHELXL [54]. Nitrophenyl group, ester groups and TCE solvent are disordered and refined as mutually exclusive by introducing free variables as implemented in SHELXL. Thermal motion of disordered solvent and the nitro-phenyl group results in somehow high values of refinement parameters (see Table 1). The figures were generated using Mercury CSD 3.6 [55]. Crystallographic data for NDBF were deposited with the Cambridge Crystallographic Data Centre and can be obtained via https://www.ccdc.cam.ac.uk/structures/.

Results and Discussion
The synthetic route for the benzodifuran precursor BDF (in Scheme 1) followed a reported procedure [40] consisting in the diazotization of 4-nitroaniline and the coupling of the diazonium salt on benzoquinone. By appropriate choice of the substituted cyanoacetate reacting with 4'-nitro-[1,1'-biphenyl]-2,5-dione [40] a wide range of substituted benzodifuran derivatives could be obtained. In our case, after some preliminary tests, the acetyl substituents were chosen as a good compromise between solubility and ability to crystallization. Different reaction conditions were experimented by varying BDF/3-hydroxy-2-naphthaldehyde ratio, the solvent and the operating temperature. The better results were obtained by using four times the stoichiometric amount of the aldehyde in boiling acetic acid. The product was recovered as microcrystalline powder directly from the hot solution with 30% yield, pure enough for the further characterization. Very selectively, the mono reacted products remain in the mother liquors.
A T-shaped molecule (see Scheme 1) can be envisioned as a cruciform structure obtained by assembling the branches to a central core determining the geometry and the extent of the electronic pattern [56].
Compound NBDF is the condensation product between a diamine and a salicylic aldehyde in 1:2 ratio. The probe contains two half-salen [12,[57][58][59] Schiff base sites working as electron-donor arms and an electron-acceptor core. As shown in Scheme 1, the (4-nitrophenyl) benzodifuran unit represents the acceptor moiety due to the strong electron-withdrawing properties of the nitro substituent. To narrow the bandgap achieving red emission the donor naphthol aromatic cores are fused to BDF enlarging the conjugated length and enriching the aromatic pattern [24]. As a result, a highly conjugated main plane (M moiety, in Scheme 1) is produced in the dye.
The intramolecular hydrogen-bond interactions in the half-salen moieties cause the excited-state intramolecular proton-transfer (ESIPT) [60,61] known to lead to emission in solution [46,59,[61][62][63][64][65][66]. The same effect is known to occur in the solid state [12,67] if the intramolecular H-bond produce hindrance to the torsion of sterically encumbered parts of the molecule [57,68,69]. In our case, the acetyl substituents on the furan rings and the bulky naphthol moieties make NBDF a RIR probe. Interestingly, RIR effect has been confirmed effective both in the solid state and in solution, as discussed below.
Identification and purity degree evaluation were assessed by elemental analysis, mass spectrometry and 1 H NMR (see Figure 1). The NMR pattern appears peculiar (spectrum reported in Section 2.1). The rigid H-bonded M plane (in red in Scheme 1) of NBDF probe impedes rotation around single bonds also in solution. Two non-equivalent sides of the molecule are recognizable: part a, more encumbered due to the nitrophenyl on the same side of the acetyl group, and part b (respectively in red and in blue in Figure 1). Because of this, most resonances split. The signals of two protons at 9.47 and 9.76 ppm (same integration) can be attributed to the imine in the side b and in the side a, respectively. The non-equivalent OCH 2 protons appear as two quadruplets at 3.93 and 4.54 nm and the terminal methyl groups as two triplets at 0.87 and 1.08 ppm. In the complicated aromatic pattern couples of protons of the same naphtholic ring are equivalent (integrating for 2 protons) while the signals in the part a differ from those in the side b. The three signals integrating for one proton can be attributed to the single proton on the BDF unit and the two protons in the nitrophenyl ring in orto to BDF. In the p-substituted system, the two protons in orto to nitro group are recorded as one doublet at 8.45 ppm.
nm and the terminal methyl groups as two triplets at 0.87 and 1.08 ppm. In the complicated aromatic pattern couples of protons of the same naphtholic ring are equivalent (integrating for 2 protons) while the signals in the part a differ from those in the side b. The three signals integrating for one proton can be attributed to the single proton on the BDF unit and the two protons in the nitrophenyl ring in orto to BDF. In the p-substituted system, the two protons in orto to nitro group are recorded as one doublet at 8.45 ppm. Concerning the phase behaviour, bright red crystals of NBDF were obtained by slow evaporation of TCE/acetic acid solution at room temperature. Crystals appeared in ~2−3 days from a 1.5 mM of 1% solution of acetic acid and grew with dimensions of 0.07 × 0.1 × 0.6 mm. They were examined by optical observation and DSC/TGA analysis. The compound melts at 330 °C and is thermally stable up to 350 °C under nitrogen flow. The dye is soluble in many organic solvents. In an acid environment the solutions are stable and retain their spectroscopic characteristics up to three months under natural light at room temperature.

Spectroscopic Behavior
The compound underwent a spectroscopic analysis by absorption and UV-Visible emission spectrophotometry in TCE solutions and on thin films of finely crumbled spin-coated crystals.
In solution, the absorbance pattern reveals a broad band peaked at 505 with a shoulder at 543 nm. The emission spectrum of the same sample irradiated at the absorbance maximum shows a Concerning the phase behaviour, bright red crystals of NBDF were obtained by slow evaporation of TCE/acetic acid solution at room temperature. Crystals appeared in~2−3 days from a 1.5 mM of 1% solution of acetic acid and grew with dimensions of 0.07 × 0.1 × 0.6 mm. They were examined by optical observation and DSC/TGA analysis. The compound melts at 330 • C and is thermally stable up to 350 • C under nitrogen flow. The dye is soluble in many organic solvents. In an acid environment the solutions are stable and retain their spectroscopic characteristics up to three months under natural light at room temperature.

Spectroscopic Behavior
The compound underwent a spectroscopic analysis by absorption and UV-Visible emission spectrophotometry in TCE solutions and on thin films of finely crumbled spin-coated crystals.
In solution, the absorbance pattern reveals a broad band peaked at 505 with a shoulder at 543 nm. The emission spectrum of the same sample irradiated at the absorbance maximum shows a double peaked band (at 575 and 615 nm, as shown in Figure 2) and 70 nm Stokes shift. PLQY (photoluminescence quantum yield) measured in diluted TCE solution was 2%.
In the solid state, the absorbance of NBDF sample is peaked at 488 nm. As an AIEgen its PL performance in the solid state is intriguing because the compound shows a large part of the emission band in DR region (the peak at 655 nm, see Figure 2) and an appreciable part in the NIR region (the Crystals 2020, 10, 269 8 of 15 hump above 700 nm). PLQY measured on the crystalline thin layer was 18%, considered a good result for a DR/NIR emitter in the solid state [19].
A large Stokes shift was measured in this case (167 nm from the maximum of absorbance, used as excitation wavelength, to the first emission maximum). The intramolecular relaxation process from LE state to the ICT state usually leads to a large Stokes shift [70]. The reabsorption of the emitted photons is avoided in fluorophores with large Stokes shift making them highly efficient. Emission spectra of NBDF both in TCE solution and in the crystalline phase are showed in Figure 2.
Crystals 2020, 10, x FOR PEER REVIEW 7 of 15 double peaked band (at 575 and 615 nm, as shown in Figure 2) and 70 nm Stokes shift. PLQY (photoluminescence quantum yield) measured in diluted TCE solution was 2%.
In the solid state, the absorbance of NBDF sample is peaked at 488 nm. As an AIEgen its PL performance in the solid state is intriguing because the compound shows a large part of the emission band in DR region (the peak at 655 nm, see Figure 2) and an appreciable part in the NIR region (the hump above 700 nm). PLQY measured on the crystalline thin layer was 18%, considered a good result for a DR/NIR emitter in the solid state [19].
A large Stokes shift was measured in this case (167 nm from the maximum of absorbance, used as excitation wavelength, to the first emission maximum). The intramolecular relaxation process from LE state to the ICT state usually leads to a large Stokes shift [70]. The reabsorption of the emitted photons is avoided in fluorophores with large Stokes shift making them highly efficient. Emission spectra of NBDF both in TCE solution and in the crystalline phase are showed in Figure 2. On solutions of NBDF probe was observed the typical behaviour of AIEgens, depending on a different solvent/non-solvent ratio. The emission performance was examined in TCE/hexane. In diluted TCE solutions (0.2 mM) the sample displays poor red emission. Upon incremental addition of hexane to the TCE sample, red fluorescence increases, as can be naked-eye perceived, starting from 60% hexane solution (see Figure 3A). After 80% the sample undergoes precipitation. As expected for AIE undergoing molecules, the aggregation process involving the self-assembly of emissive units improve fluorescence. Hydrogen bonding and van der Waals interactions are involved in the process [71]. On solutions of NBDF probe was observed the typical behaviour of AIEgens, depending on a different solvent/non-solvent ratio. The emission performance was examined in TCE/hexane. In diluted TCE solutions (0.2 mM) the sample displays poor red emission. Upon incremental addition of hexane to the TCE sample, red fluorescence increases, as can be naked-eye perceived, starting from 60% hexane solution (see Figure 3A). After 80% the sample undergoes precipitation. As expected for AIE undergoing molecules, the aggregation process involving the self-assembly of emissive units improve fluorescence. Hydrogen bonding and van der Waals interactions are involved in the process [71].
Finally, it was found that NBDF probe produced some mechanochromic effect. The luminescence properties of compounds able to form strong stacked molecular organization are subject to the molecular packing in the solid state [72,73]. Increase in the crystallite size distribution was expected to produce higher PL intensity because of the stacking of the NBDF moieties hinders non-radiative relaxation pathways. Grinding the very fine powder of the as-synthetized compound some particles fused. The increase in the particle size distribution causes enhanced florescence intensity in NBDF. After grinding (see Figure 3B), the sample shows the same emission color with slight PL increase (about 15%). In the fuming process, the same sample was treated with acetone vapor. We checked several solvents and the most relevant effect was found in acetone. The fumed samples were obtained by fuming the grinded powders for 1 min. Finally, the same sample was kept at 150 • C for 3 min (heating process). Due to the disruption of the molecular packing obtained by grinding and the production of an amount of less structured material, PL performance are expected worsen as a result of the fuming (first) and of the heating (subsequent) process. In fact, the luminescent states checked by PL measurements show a small decrease correspondent to 4%−5% in PL intensity to each of the two stages. Finally, it was found that NBDF probe produced some mechanochromic effect. The luminescence properties of compounds able to form strong stacked molecular organization are subject to the molecular packing in the solid state [72,73]. Increase in the crystallite size distribution was expected to produce higher PL intensity because of the stacking of the NBDF moieties hinders non-radiative relaxation pathways. Grinding the very fine powder of the as-synthetized compound some particles fused. The increase in the particle size distribution causes enhanced florescence intensity in NBDF. After grinding (see Figure 3B), the sample shows the same emission color with slight PL increase (about 15%). In the fuming process, the same sample was treated with acetone vapor. We checked several solvents and the most relevant effect was found in acetone. The fumed samples were obtained by fuming the grinded powders for 1 min. Finally, the same sample was kept at 150 °C for 3 min (heating process). Due to the disruption of the molecular packing obtained by grinding and the production of an amount of less structured material, PL performance are expected worsen as a result of the fuming (first) and of the heating (subsequent) process. In fact, the luminescent states checked by PL measurements show a small decrease correspondent to 4%−5% in PL intensity to each of the two stages.

Production of a Red Emissive Polymeric Layer
To further explore the potential applications of NBDF probe in the production of emissive layers, the fluorophore was mixed with poly(vinylcarbazole) (PVK), a polymeric conductive matrix often employed in optoelectronic devices. Production of polymeric blends, and more generally deposition of low-weight molecules on conductive substrates [45,57,74], is a functional approach to produce macroscopically processable layers. For ACQ undergoing molecules this is a way to relieve fluorescence quenching effect [45,57]. In our case, the AIE performance of the dye required the employ of a high percentage of NBDF in the amorphous host matrix.
Thin films of NBDF were obtained by the spin-coating of a dispersion of finely shattered crystals (obtained by sonication of a dispersion in hexane) and PVK in chloroform/hexane (3:1) using an SCS P6700 spin-coater operating at 600 rpm. All the blends produced homogeneous microcrystalline layers retaining their optical characteristics up to two months under natural light at room temperature.
Up to 5% wt. the addition of PVK acted as a conductive plasticizer, with no relevant decrease of PLQY. In the larger host percentage blends, from 5% to 25% wt., PLQYs rapidly decreased to about one half (8% PLQY was recorded on the more diluted sample). Not unexpected, both in solid and in

Production of a Red Emissive Polymeric Layer
To further explore the potential applications of NBDF probe in the production of emissive layers, the fluorophore was mixed with poly(vinylcarbazole) (PVK), a polymeric conductive matrix often employed in optoelectronic devices. Production of polymeric blends, and more generally deposition of low-weight molecules on conductive substrates [45,57,74], is a functional approach to produce macroscopically processable layers. For ACQ undergoing molecules this is a way to relieve fluorescence quenching effect [45,57]. In our case, the AIE performance of the dye required the employ of a high percentage of NBDF in the amorphous host matrix.
Thin films of NBDF were obtained by the spin-coating of a dispersion of finely shattered crystals (obtained by sonication of a dispersion in hexane) and PVK in chloroform/hexane (3:1) using an SCS P6700 spin-coater operating at 600 rpm. All the blends produced homogeneous microcrystalline layers retaining their optical characteristics up to two months under natural light at room temperature.
Up to 5% wt. the addition of PVK acted as a conductive plasticizer, with no relevant decrease of PLQY. In the larger host percentage blends, from 5% to 25% wt., PLQYs rapidly decreased to about one half (8% PLQY was recorded on the more diluted sample). Not unexpected, both in solid and in liquid diluted media the emission of the AIEgen weakens. The emission color of the neat compound placed in the DR region with CIE: (0.68; 0.32), see Figure 4. On the other hand, going from the neat crystalline dye (0% PVK blend) up to 25% PVK blend the CIE coordinates gradually undergo blue-shift, so that emission color of 25% blend is very similar to the 30% wt. TCE solution, CIE: (0.58; 0.43) and (0.58; 0.42) respectively ( Figure 4). placed in the DR region with CIE: (0.68; 0.32), see Figure 4. On the other hand, going from the neat crystalline dye (0% PVK blend) up to 25% PVK blend the CIE coordinates gradually undergo blueshift, so that emission color of 25% blend is very similar to the 30% wt. TCE solution, CIE: (0.58; 0.43) and (0.58; 0.42) respectively (Figure 4).

Single-Crystal X-Ray Structure
The molecular structure of NBDF shows a T-shaped pattern recognizable in the two different segments: main plane (M in red in Scheme 1) and nitrophenyl group ( Figure 5). The probe crystallizes in P 1 21/n 1 space group with one molecule in the asymmetric unit, as a combination of two 50% statistically equivalent molecules oriented in opposite directions. A partially occupied molecule of TCE was also detected ( Figure 5). The center of symmetry of P 1 21/n 1 space group is located in the center of the benzene ring of BDF. NBDF shows a main plane corresponding to the BDF core condensed with the two naphthol units, with an average displacement of the atoms of about ~0.03 Å . NBDF shows an intramolecular N--H-O hydrogen bond in the half-salen group (O…N distance = 2.55 Å) enforcing the planarity of the system. The disordered nitro-phenyl group is twisted of ~45° with respect to the main plane, to minimize interactions with the close bulky disordered ethyl-ester group, also out from the main plane. In turn, ethyl-ester group twisted of ~28° with respect to the main plane. This pattern was already observed in BDF molecules [45]. The strictly hindered rotations satisfy the requirements of RIR molecules.
A crystal packing with herringbone arrangement of NBDF molecules can be observed (see Figure  6). NBDF is stabilized by intermolecular Van-der Waals interactions and self-assembled into πstacked columns along the b axis and arranged around the center of symmetry and the symmetry elements of the space group ( Figure 6). Each columnar arrangement is characterized by strong π-π interactions along the main plane and by an average interplanar distance of ~3.5 Å (Figure 6).

Single-Crystal X-Ray Structure
The molecular structure of NBDF shows a T-shaped pattern recognizable in the two different segments: main plane (M in red in Scheme 1) and nitrophenyl group ( Figure 5). The probe crystallizes in P 1 2 1 /n 1 space group with one molecule in the asymmetric unit, as a combination of two 50% statistically equivalent molecules oriented in opposite directions. A partially occupied molecule of TCE was also detected ( Figure 5). The center of symmetry of P 1 2 1 /n 1 space group is located in the center of the benzene ring of BDF. NBDF shows a main plane corresponding to the BDF core condensed with the two naphthol units, with an average displacement of the atoms of about~0.03 Å. NBDF shows an intramolecular N--H-O hydrogen bond in the half-salen group (O . . . N distance = 2.55 Å) enforcing the planarity of the system. The disordered nitro-phenyl group is twisted of~45 • with respect to the main plane, to minimize interactions with the close bulky disordered ethyl-ester group, also out from the main plane. In turn, ethyl-ester group twisted of~28 • with respect to the main plane. This pattern was already observed in BDF molecules [45]. The strictly hindered rotations satisfy the requirements of RIR molecules.
A crystal packing with herringbone arrangement of NBDF molecules can be observed (see Figure 6). NBDF is stabilized by intermolecular Van-der Waals interactions and self-assembled into π-stacked columns along the b axis and arranged around the center of symmetry and the symmetry elements of the space group ( Figure 6). Each columnar arrangement is characterized by strong π-π interactions along the main plane and by an average interplanar distance of~3.5 Å (Figure 6).

Conclusions
We examined the PL performance of a novel AIEgen. The central BDF scaffold forms a single main plane with two half-salen terminal naphthol groups. The probe shows a T-shaped D-A-D pattern highly encumbered, undergoing RIR effect both in solution and in the solid state. Crystal structure analysis revealed a strong intramolecular H-bond and the ability to form π-stacked columns. The emission pattern of the neat compound is placed in the DR/NIR region with noteworthy PLQY in the solid state. The neat crystals undergo mechanochromism depending on the crystallite size distribution. In solution, NBDF keeps the typical behaviour of AIEgens compliant with the selfassembly of emissive units. Finally, the probe turned out to be potentially useful in building concentered dye-doped emissive layers. Emission CIE coordinates (0.68; 0.32) place the probe in the highly sought-after groups of solid-state DR fluorophores.

Conclusions
We examined the PL performance of a novel AIEgen. The central BDF scaffold forms a single main plane with two half-salen terminal naphthol groups. The probe shows a T-shaped D-A-D pattern highly encumbered, undergoing RIR effect both in solution and in the solid state. Crystal structure analysis revealed a strong intramolecular H-bond and the ability to form π-stacked columns. The emission pattern of the neat compound is placed in the DR/NIR region with noteworthy PLQY in the solid state. The neat crystals undergo mechanochromism depending on the crystallite size distribution. In solution, NBDF keeps the typical behaviour of AIEgens compliant with the self-assembly of emissive units. Finally, the probe turned out to be potentially useful in building concentered dye-doped emissive layers. Emission CIE coordinates (0.68; 0.32) place the probe in the highly sought-after groups of solid-state DR fluorophores.