Solid-State Highly Efficient DR Mono and Poly-dicyano-phenylenevinylene Fluorophores

An efficient deep red (DR)-emitting organic solid based on a dicyano-phenylenevinylene derivative was reported. The structural and spectroscopic properties of the solid have been described in terms of crystallographic data and time-dependent DFT analysis. A noteworthy fluorescence quantum yield of 53% was observed for the brightest emitter cast into solid films. This result can be explained in terms of the aggregation-induced emission (AIE) effect.


Introduction
Deep red/near-infrared (DR/NIR) solid-state emitters have gained much attention as dopants in fabrication of OLED and PLED devices, in optical probes for bioimaging, and in biomedical applications [1][2][3]. In white PLEDs (WPLEDs), white emission has provided the use of polymer-polymer blends or polymer-small-molecule blends as emissive layers, and the role of the red emitting component is relevant and sensitive [4]. Compared with green and blue emitters [5][6][7][8][9][10][11][12][13], the development of red-light emitting materials is far behind in terms of both purity and efficiency, but is highly required for RGB (red-green-blue)-based devices. In particular, for fluorescence bioimaging and assay applications, the optimal materials are DR/NIR photo emitters with high photoluminescence quantum efficiency, large Stoke's shifts (for eliminating self-absorption), good chemical and thermal stability, good solubility and processability [14].
So far, the most reported DR/NIR organic emitters consist of a planar polycyclic skeleton having an extended π-system or a strong donor-acceptor framework featuring a large dipole moment in the excited state [15]. NIR emitters are often designed utilizing intramolecular charge-transfer (ICT) to obtain red-shifted emission. Nevertheless, these materials usually tend to aggregate in highly concentrated solutions and in solid states, and this could result in non-uniform thin films and in concentration quenching (aggregation-caused quenching, ACQ effect), due to the strong dipole-dipole interactions. Most dyes are highly emissive in dilute solution but become weakly luminescent or even non-emissive in the solid phase due to the strong dipole-dipole interactions that cause the The identification and the evaluation of the purity degree were carried out by mass spectrometry and 1 H NMR. Phase behavior was examined by optical observation and DSC and TGA analysis. All materials are thermally stable up to 300 °C under nitrogen flow. The ter-polymer CN-PPV was synthesized by polycondensation reaction between the chromophore CN-PV-NH2 and a mixture of two different acyl dichlorides (dodecanedioyl and suberoyl). This was designed to guarantee a random and hence more soluble amorphous material, in which chromophores are diluted in a processable polymeric system. Based on Tg and ηinh values, the polymer can be considered an oligomer, although no evidence of COOH terminal groups was found in the 1 H NMR spectrum. On the other hand, the low degree of polymerization guarantees good solubility in many common solvents, and therefore easy processability.

Photophysical Properties
Photophysical measurements were performed both in solution and in the solid state, including PLQY measurements. A strong solvatocromic effect was detected for both of the low molecular weight compounds CN-PV-NH 2 and CN-PV-NHMe (see Table 1, columns 2 and 3, and Figure 1).

Photophysical Properties
Photophysical measurements were performed both in solution and in the solid state, including PLQY measurements. A strong solvatocromic effect was detected for both of the low molecular weight compounds CN-PV-NH2 and CN-PV-NHMe (see Table 1, columns 2 and 3, and Figure 1).  In solution, the absorption was recorded in the range from 436 to 470 nm and the emission from 518 to 622 nm with a red-shift effect depending on solvent polarity. In dioxane, the compounds appear as yellow-green emitters. In more polar solvents, such as DMF, we observed a red shift of the emission peak. This is consistent with the existence of a polar excited state, which is stabilized in a In solution, the absorption was recorded in the range from 436 to 470 nm and the emission from 518 to 622 nm with a red-shift effect depending on solvent polarity. In dioxane, the compounds appear as yellow-green emitters. In more polar solvents, such as DMF, we observed a red shift of the emission peak. This is consistent with the existence of a polar excited state, which is stabilized in a polar environment and has an increased probability of decaying to the ground state by non-radiative paths ( Figure 1). Furthermore, quantum efficiency in DMF decreases significantly. In the case of CN-PPV polymer, we do not observe the same solvatochromic effect: the polymer exhibits a strong yellow-green emission both in dioxane and in DMF, with CIE 1931 coordinates of (0.36, 0.57) and a measured PLQY of 26% in dioxane or in DMF solution (see Figure 2).
In solution, PL quantum yields were measured by relative methods, using as standard quinine sulfate QS (UQS 1 4 0.546 in H 2 SO 4 1 N, when excited at 365 nm according to Melhuish [32]). The values appear to be independent of the solvent, and are very similar. All compounds show deep red photoluminescence with a pronounced Stoke's shift both in solution and in the solid state. Remarkably, in the solid state, Stoke's shifts range from 140 to 160 nm. In order to test the effect of a dilution in the solid state, we prepared two types of solid films: by spin-coating a solution of each compound and by microdispersion of the shattered crystals in a polystyrene matrix at 10% by weight (PS-doped film). In some cases, the use of a microdispersion of the crystalline fluorophores in PS represented the right way to prevent the ACQ effect in the solid state [14,24]. In Figure 3, the solid samples on quartz slides are shown.
Molecules 2018, 23, x FOR PEER REVIEW 5 of 14 polar environment and has an increased probability of decaying to the ground state by non-radiative paths ( Figure 1). Furthermore, quantum efficiency in DMF decreases significantly. In the case of CN-PPV polymer, we do not observe the same solvatochromic effect: the polymer exhibits a strong yellow-green emission both in dioxane and in DMF, with CIE 1931 coordinates of (0.36, 0.57) and a measured PLQY of 26% in dioxane or in DMF solution (see Figure 2). In solution, PL quantum yields were measured by relative methods, using as standard quinine sulfate QS (UQS ¼ 0.546 in H2SO4 1 N, when excited at 365 nm according to Melhuish [32]). The values appear to be independent of the solvent, and are very similar. All compounds show deep red photoluminescence with a pronounced Stoke's shift both in solution and in the solid state. Remarkably, in the solid state, Stoke's shifts range from 140 to 160 nm. In order to test the effect of a dilution in the solid state, we prepared two types of solid films: by spin-coating a solution of each compound and by microdispersion of the shattered crystals in a polystyrene matrix at 10% by weight (PS-doped film). In some cases, the use of a microdispersion of the crystalline fluorophores in PS represented the right way to prevent the ACQ effect in the solid state [14,24]. In Figure 3, the solid samples on quartz slides are shown.  All solid compounds are red, both under natural light and UV lamp. CN-PV-NH2 microdispersed in polystyrene (Figure 3, top) showed enhancement of PL emission respect to the pure thin film, reaching the same value when measured in dioxane-diluted solution (see also Figure 2). This is evidence of an ACQ effect in the pure solid film for this compound. The ACQ effect is lowered in the PS-doped film, thanks to the dilution of the fluorophores in the PS matrix. CN-PV-NHMe displays a deep red emission, with a remarkable 53% PLQY in thin film. The same compound dispersed in polystyrene emits at 598 nm with a comparable 50% PLQY, whereas in solution, it shows only a faint emission. The stronger emission observed in the solid state could be ascribable to the presence of cyano substituents on the conjugated system, whose effect is to restrict molecular motions and to stabilize a twisted conformation in the solid. This permits the fluorophore to emit more efficiently in the aggregated state. In fact, the twisted geometry, hindering intermolecular close stacking and intense π-π interaction, may lead to intense aggregation-induced emission effect (AIE) [16]. In order All solid compounds are red, both under natural light and UV lamp. CN-PV-NH 2 micro-dispersed in polystyrene (Figure 3, top) showed enhancement of PL emission respect to the pure thin film, reaching the same value when measured in dioxane-diluted solution (see also Figure 2). This is evidence of an ACQ effect in the pure solid film for this compound. The ACQ effect is lowered in the PS-doped film, thanks to the dilution of the fluorophores in the PS matrix. CN-PV-NHMe displays a deep red emission, with a remarkable 53% PLQY in thin film. The same compound dispersed in polystyrene emits at 598 nm with a comparable 50% PLQY, whereas in solution, it shows only a faint emission. The stronger emission observed in the solid state could be ascribable to the presence of cyano substituents on the conjugated system, whose effect is to restrict molecular motions and to stabilize a twisted conformation in the solid. This permits the fluorophore to emit more efficiently in the aggregated state. In fact, the twisted geometry, hindering intermolecular close stacking and intense π-π interaction, may lead to intense aggregation-induced emission effect (AIE) [16]. In order to verify this hypothesis, we carried out a crystallographic study on CN-PV-NHMe, which shows a higher PLQY in the solid and was easily obtained as single crystals.
In the solid phase, the polymer is also a DR emitter (see Figure 3), but with a PLQY value similar to those of the compounds in solution (Table 1). CN-PPV can be considered, in fact, a diluted system of the fluorophore core. The PLQY value measured for CN-PPV thin film is lower than the value of CN-PV-NHMe and close to the value of CN-PV-NH 2 dispersed in polystyrene; this is not surprising, because in the polymer chain the fluorophore replaces its strong electron donating amino groups with amide moieties, which possess a lower electron-donating ability. This result, however, is not easily rationalizable, because dipole-dipole interactions in the polymeric amorphous sample are conceivably very different from the same interactions acting in the crystalline samples.

X-ray Crystallography
The molecular structure of CN-PV-NHMe was determined by single crystal X-ray analysis. Compound crystallizes in the monoclinic C2/c space group, with one molecule contained in the asymmetric unit. All bond lengths and angles fall within the normal range. An ORTEP view of the molecular structure is reported in Figure 4. Details of crystallographic data and refinement parameters are reported in Table S1 of the Supplementary Materials.  The molecule is characterized by three phenyl rings mutually twisted each to the other. The mean planes of outlying C1/C6 and C19/C24 phenyl rings are tilted 49.01(18)° and 31.47(16)° with respect to the central C10/C15 mean plane ring. An overall elongated, almost flat, shape of the molecule is observed, with aminic and alifatic groups being quite in-plane. As expected, the planar geometry at the aminic N atoms shows a degree of conjugation with the bound phenyl group. The crystal packing is stabilized by face-to-edge aromatic interactions ( Figure 5). No π-π aromatic stacking is found, due to the twisting of phenyl rings in adjacent molecules ( Figure S1, Supplementary Material). The molecule is characterized by three phenyl rings mutually twisted each to the other. The mean planes of outlying C1/C6 and C19/C24 phenyl rings are tilted 49.01(18) • and 31.47 (16) • with respect to the central C10/C15 mean plane ring. An overall elongated, almost flat, shape of the molecule is observed, with aminic and alifatic groups being quite in-plane. As expected, the planar geometry at the aminic N atoms shows a degree of conjugation with the bound phenyl group. The crystal packing is stabilized by face-to-edge aromatic interactions ( Figure 5). No π-π aromatic stacking is found, due to the twisting of phenyl rings in adjacent molecules ( Figure S1, Supplementary Material). mean planes of outlying C1/C6 and C19/C24 phenyl rings are tilted 49.01(18)° and 31.47(16)° with respect to the central C10/C15 mean plane ring. An overall elongated, almost flat, shape of the molecule is observed, with aminic and alifatic groups being quite in-plane. As expected, the planar geometry at the aminic N atoms shows a degree of conjugation with the bound phenyl group. The crystal packing is stabilized by face-to-edge aromatic interactions ( Figure 5). No π-π aromatic stacking is found, due to the twisting of phenyl rings in adjacent molecules ( Figure S1, Supplementary Material).

Computational Studies
The excitation energies were obtained at the density functional level by using the timedependent perturbation theory approach (TDDFT) with the adiabatic local density approximation, which have high reliability in obtaining accurate predictions for excitation energies and oscillator strengths. Calculations on CN-PV-NHMe in DMF showed the localization of HOMO and LUMO orbitals (see Figure 6).

Computational Studies
The excitation energies were obtained at the density functional level by using the time-dependent perturbation theory approach (TDDFT) with the adiabatic local density approximation, which have high reliability in obtaining accurate predictions for excitation energies and oscillator strengths. Calculations on CN-PV-NHMe in DMF showed the localization of HOMO and LUMO orbitals (see Figure 6).  HOMO is delocalized over the entire conjugated backbone, with only a moderate contribution of the cyano groups. Contrarily, the LUMO is mainly focused on the central part of the molecule and on the electron-withdrawing cyano groups. The main transition, at 487 nm, is the HOMO → LUMO. The second strongest absorption at 394 nm is due to the transition HOMO-1 → LUMO. The predicted absorption maximum is in good agreement with the experimental values (see Tables 1 and 2), the two main absorptions correspond to the HOMO → LUMO and HOMO-1→LUMO transitions. CN-PV-NHMe exhibits hole and electron reorganization energies (HRE and ERE respectively) that are typical for these compounds. DFT calculations make is possible to investigate the first oxidation potential to explain the electron-donating properties of the compound. Withdrawal of the electron from the compound in a redox reaction is performed from its HOMO. The observed trend in oxidation potential for the three compounds indicates destabilization of the HOMO. The intense DR emission of CN-PV-NHMe arises from the D-A-D-A-D motif with strong donor and acceptor functionalities and, at the same time, largely delocalized frontier MOs, giving rise to the high oscillator strength of the emitting state. As reported for similar compounds [14], in our systems, only a minor intramolecular charge transfer character is observed, and the oscillator strength is thus large for the ground to the first excited state transition (f = 2.27) [31]. The large radiative decay and the strong fluorescence, with a quantum yield of about 50% in the solid, could also be attributed to strongly twisted equilibrium structures, as confirmed by the X-ray analysis (Figures 4 and 5).

Materials and Methods
All starting products were commercially available. The chlorides were distilled before use. 2-(2-Ethylhexyloxy)-5-methoxyterephthalaldehyde was obtained as described in [25]. Optical observations were performed by using a Zeiss Axioscop polarizing microscope equipped with a FP90 Mettler heating stage. Phase transition temperatures and enthalpies were measured using a DSC scanning calorimeter Perkin Elmer Pyris 1 at a scanning rate of 10 • C/min, under nitrogen flow. The decomposition temperature (T d ) is assumed as the temperature where is recorded the 5 wt. % weight loss. T d were determined by thermogravimetric analysis under nitrogen flow, using a Perkin Elmer TGA 4000. 1 H NMR spectra were recorded with Bruker Avance II 400 MHz apparatus. Mass spectrometry measurements were performed using a Q-TOF premier instrument (Waters, Milford, MA, USA) equipped by an electrospray ion source and a hybrid quadrupole-time of flight analyzer. Mass spectra were acquired in positive ion mode, in 50% CH 3 CN solution, over the 400-800 m/z range. Instrument mass calibration was achieved by a separate injection of 1 mM NaI in 50% CH 3 CN. Data were processed by using MassLynx software (Waters). UV-Visible and fluorescence spectra were recorded with JASCO spectrometers. Jasco F-530 (scan rate 200 nm min −1 ) and on a spectrofluorimeter Jasco FP-750 (excitation wavelengths set at the absorption maxima of the samples, scan rate 125 nm min −1 ). For the polymer CN-PPV, Inherent viscosity (η inh ) at 25.0 • C in N-methyl pyrrolidinone (NMP) was measured with an Ubbelohde viscometer. The molecular mass and its distribution was obtained with a Lab Flow 2000 GPC/HPLC apparatus with polystyrene samples as references and tetrahydrofuran (THF) as the eluent.
Thin films of the amino precursor, the fluorophore and the polymer were obtained by the spin-coating technique of a micro-dispersion of shattered crystals (10% by weight) and low weight commercially available polystyrene (PS) in acetone/dichloromethane (1:2) by a SCS P6700 spin-coater operating at 800 RPM. Thin films of the sample without polystyrene were obtained by casting from a chloroform solution (about 30% by weight).

Preparation of CN-PV-NO 2
2-(2-Ethylhexyloxy)-5-methoxyterephthalaldehyde (7.60 g, 0.0260 mmol) [25] and piperidine (2 mL) were dissolved in pyridine (20 mL). The solution was warmed at 40 • C and then 2-(4-nitrophenyl)acetonitrile (16.2 g, 0.102 mol) was added in portions over 1 h. After this time, the solution color turned to red and the formation of a solid occurred. After one more hour, the reaction was stopped and the suspension was poured in 40 mL of ethanol at room temperature. The bright red solid was then recovered by filtration. The compound was recrystallized from ethanol in needle-shaped, dark red crystals. Yield: 55%; mp: 232 • C; T d = 352 • C. 1

Preparation of CN-PV-NH 2
To CN-PV-NO 2 (1.00 g, 0.700 mmol) dissolved in DMF (20.0 mL) 5.60 mL of a 1.20 M Na 2 S·9H 2 O aqueous solution was added and the color turned from red to black. After 15 min, water (1.00 mL) was added and the system kept at 150 • C for 15 min. The mixture was slowly cooled to room temperature, leading to the formation of a red crystalline solid. The compound was recovered by filtration and washed with slightly acid water (acetic acid, pH about 5) and then with water. The red crystalline product was crystallized again from chloroform/ethanol. Yield: 90%; mp: 190 • C; T d = 337 • C. 1

Preparation of CN-PV-NHMe
To 1.35 g (5.37 mmol) of CN-PV-NH 2 , suspended in 10 mL of dry THF, 2.79 g (16.0 mmol) of terzbutyl carbonate (tBOC) was added under stirring at room temperature. After 6 hours, the solvent was removed in vacuo and the crude product washed with hot hexane three times. The compound was employed in the following step without any further purification.
To CN-PV-NHtBOC dissolved in 200 mL of boiling acetone, 8.10 g (145 mmol) of KOH powder was slowly added, followed by a first aliquot of 7.70 mL of ICH 3 . Then a second aliquot of ICH 3 (7.70 mL) was added dropwise after 15 min. The reaction was kept to the boil under stirring for 15 min. The mixture was cooled at room temperature, filtered and the solution concentered in vacuo to 50 mL and 100 mL of heptane added, with the precipitation of a red-orange solid ensuing. The product was recovered by filtration and crystallized from acetone/heptane. Yield: 70%; mp: 160 • C; T d = 367 • C. 1

Preparation of CN-PPV
To 0.160 g (0.60 mmol) of dodecandioic dichloride and 0.126 g (0.60 mmol) of suberic dichloride dissolved in 5.0 mL of THF a solution of CN-PV-NH 2 (0.607 g, 1.20 mmol) dissolved in 5 mL of dry pyridine was added. The reaction was kept under vigorous stirring at reflux for 2 h. The solution was poured into 150 mL of water and a solid product was recovered by filtration. The red crude product was dissolved in hot DMF (50 mL) and precipitated again in 400 mL of water, recovered by filtration and dried at 120

PLQY Measurements
Photoluminescence efficiency measurements were conducted with a setup similar to the one suggested by de Mello et al. [33]. This setup takes into account not only the exciting laser and direct photoluminescence spectrum but also the photoluminescence that originates due to the scattering effect of the integrating sphere that is part of the setup. The setup consists of a 376 nm laser whose emission does not overlap with the emitted photoluminescence spectrum, an integrating sphere (Stellarnet Inc, Tampa, FL, USA) and a photospectrometer (BLACK Comet Stellarnet Inc, Tampa, FL, USA). The emission was measured at five different points on the sample.

X-ray Crystallography
Single crystals of CN-PV-NHMe were obtained by slow evaporation of acetone/heptane solution at ambient temperature. It was not possible to obtain very good quality crystals. One selected crystal was mounted in flowing N 2 at 173 K on a Bruker-NoniusKappaCCD diffractometer equipped with Oxford Cryostream apparatus (graphite monochromated MoK α radiation, λ = 0.71073 Å, CCD rotation images, thick slices, ϕ and ω scans to fill asymmetric unit). Semi-empirical absorption corrections (SADABS [34]) were applied. The structure was solved by direct methods (SIR97 program [35]) and anisotropically refined by the full matrix least-squares method on F 2 against all independent measured reflections using SHELXL-2016 [36] and WinGX software [37]. The H atoms at aminic groups were located in difference Fourier maps and freely refined with U iso (H) equal to 1.2U eq of the carrier atom. All the other hydrogen atoms were introduced in calculated positions and refined according to the riding model with C-H distances in the range 0.95-1.00 Å and with U iso (H) equal to 1.2U eq or 1.5U eq (C methyl ) of the carrier atom. Difference Fourier maps showed two disordered positions of the branched aliphatic group that refined with occupancy factors 0.596 and 0.404. Some restraints were introduced in the last stage of refinement to model the disorder using DFIX, SAME and SIMU commands of SHELXL program. Crystal data and structure refinement details are reported in Table  S1 (Supplementary Materials). The figures were generated using ORTEP-3 [38] and Mercury CSD 3.9 [39] programs.
Crystallographic data were deposited at Cambridge Crystallographic Data Centre with assigned number CCDC 1843139. These data can be obtained free of charge from www.ccdc.cam.ac.uk/structures.

Computational Studies
All quantum calculations were carried out using the Jaguar program [40], as implemented in the Schrödinger Material Science Suite [41]. Geometry optimizations were performed with the B3LYP functional and the 6-31G** basis set in which the transition metals are represented using the Los Alamos LACVP** basis that includes relativistic effective core potentials. The energies of the optimized structures are re-evaluated by additional single-point calculations on each optimized geometry using Dunning's correlation-consistent triple-ζ basis set42 cc-pVTZ(-f), which includes a double set of polarization functions. Vibrational frequency calculation results based on analytical second derivatives at the B3LYP/6-31G**(LACVP**) level of theory were used to confirm proper convergence to local minima and to derive the zero-point-energy (ZPE) and entropy corrections at room temperature where unscaled frequencies were used.

Conclusions
A new DR dicyano-phenylenevinylene emitter and the ter-polymer containing the same fluorophore unit have been synthesized. A detailed study of PL properties in solution and in the solid-state compounds has been carried out. A bright red emission for the fluorophore was recorded with 50% PLQY in the solid state. Interestingly, no ACQ effect was detected on the fluorophore. A detailed X-ray investigation correlated with TD-DFT analysis was presented in order to explore this behavior. In the solid state of CN-PV-NHMe (film or doped PS film), owing to the restriction of intramolecular motions induced by the cyano substituents, the radiative channel was activated, causing intense aggregation-induced emission (AIE). Regarding the polymer, although a noteworthy quantum efficiency was not observed, this was a way of exploiting the advantages to covalently bond the fluorophores into an easily processable material retaining medium DR photoluminescence.